the present method provides a simple and effective method for synthesizing peptides in the n - to - c direction , based on amino acid t - butyl esters , as shown in fig1 . this method takes advantage of the availability of a number of amino acid t - butyl esters , and a list of currently comrnercially available amino acid t - butyl esters appropriate for reverse peptide synthesis is given in table 1 . this list provides suitable derivatives to prepare l peptides containing all the standard l amino acids except asp , cys , and ser , as well as a number of non - standard and d amino acids . additional suitable side chain protected amino acid tbu esters can be prepared by standard methods ( bodanszky , m ., peptide chemistry : a practical textbook . second , revised ed . 1993 , berlin , heidelberg : springer - verlag at 217 ). a number of conditions are expected to affect both coupling efficiency and quality ( racemization ) of the final product . syntheses can be monitored for overall yield and purity by hplc and for racemization using marfey &# 39 ; s reagent ( marfey , calsberg res . commun ., 49 , 591 ( 1984 ); adamson et al ., anal . biochem ., 202 , 202 ( 1992 )). in normal ( c - to - n ) peptide synthesis , an excess of the activated carboxyl component is used to drive the reaction to completion . in n - to - c synthesis , the carboxyl group is anchored and cannot be generated in excess . this problem can be solved by using carbodiimides , or other coupling reagents , which can be used in the presence of the carboxyl component , and an excess of the amine component . a second difficulty concerns which protection chemistry to use for the carboxyl group of the amino component . one recent report ( johansson et al ., cited above ( 2000 )) describes the use of amine components with a silyl - protected carboxyl group , the use of a photolabile attachment to the solid support , and the use of hatu , ( o -( 7 - azabenzotriazol - 1 - yl )- n , n , n ′, n ′- tetramethyluronium hexafluorophosphate ; or tatu ( o - benzotriazol - 1 - yl - n , n - n ′, n ′- tetramethyluronium tetrafluoroborate ) as coupling agents . hbtu , and pybop can also be used as coupling agents . in the present method , these reagents can be used in about 2 to 10 - fold molar excess over resin attached functional groups . bases such as 2 , 4 , 6 - trimethylpyridine ( tmp ), diisopropyl amine ( diea ), and n - methylmorpholine ( nmm ) can be used at about 2 to 10 - fold molar excess . the amino acid concentration can be used in about 2 to 10 - fold molar excess as discussed above . mixtures of ch 2 cl 2 / dmf can be employed as solvents . a free n - terminal amino group is generated by cleavage of the immobilized peptides using glu or gln linkers using the suc / glu / gln linker strategy , or from peptides synthesized using the dde or urethane strategies ( fig8 – 10 ), or from peptide synthesized using the bal ( fig1 ) strategy if a strong acid - sensitive capping group for the secondary amine attachment is used , such as an fmoc or cbz capping / protecting group . attaching the nascent peptide to the resin using a glu / gln linker strategy , yields product peptides and peptide mimetics that all have a glu or gln residue at their n - terminus . however , it can be preferable to have product peptide without a linker on the n - terminus , for example comprising a free n - terminus . in accord with the present method , peptides / mimetics can now be synthesized with a wide variety of n - terminal substituents , or without a substituent , or as cyclic peptides . the methods described herein are focused on both resin attachment strategies , and c - terminal functional group elaboration strategies . the urethane strategy described by felix and merrifield , by letsinger , cited above , and by r . leger et al ., tetrahedron lett ., 39 , 4174 ( 1998 ), is also useful with the present t - butyl ester based synthesis and was successfully implemented for the t - butyl ester based inverse peptide synthesis . using this strategy peptides and peptide mimetics are obtained with a free n - terminus and with all side chains deprotected during the tfmsa / tfa cleavage reaction conditions . a dde based strategy has not been described previously for inverse peptide synthesis . the dde functional group is used to protect amines in organic chemistry ( t . w . green and p . g . m . wuts , protecting groups in organic synthesis , 3d ed . j . wiley & amp ; sons ( 1999 ) and references therein ). dde based resins have been described in the literature for anchoring and protecting amines for solid phase chemistry , and are commercially available ( novabiochem ). see , e . g ., m . e . attardi et al . tetrahedron lett ., 41 , 7391 ( 2000 ); s . r . chhabra et al ., tetrahedron lett ., 41 , 1095 ( 2000 ); ibid . at 1099 . in the attachment strategies referred to above , based on the use of a succinyl / glu / gln linker strategy , a urethane attachment strategy , or a dde attachment strategy , the attached amine of the n - terminal amino acid is rendered unreactive by the attachment method . an alternative to this type of approach is to attach a primary amine to suitably reactive solid phase benzylic carbon , to generate an anchored secondary amine . the benzylic secondary amine is suitable for further reaction , such as acylation , and eventual cleavage of the benzylic attachment to provide the desired product . this approach was reported for use in preparing fully protected peptide ethyl esters using a photocleavable nitrobenzylhalide resin ( m . renil et al ., tetrahedron lett ., 35 , 3809 ( 1994 )). in this approach an amino acid ethyl ester was attached to a nitrobenzylhalide resin , and standard boc based c - to - n synthesis cycles used to elaborate the peptide chain . photolytic cleavage of the benzylic resin - amide attachment provided the fully protected peptide esters . benzylic attachment has been used to anchor amino acid ester monomers as intermediates in benzodiazepine library synthesis by ellman and coworkers ( c . g . boojamra et al ., j . org . chem ., 60 , 5742 ( 1995 )). in this approach an amino acid ester was loaded onto a 4 - oxy - 2 , 6 - dimethoxybenzaldehyde ( dimethoxy ) based resin by reductive amination , followed by elaboration into a benzodiazepine , which was finally cleaved from the resin by acidolysis . benzylic attachment has also been investigated as a way to anchor various amino acid esters for c - to - n based synthesis of c - terminally modified peptides ( backbone amide linker ( bal ) approach ) ( k . jensen et al ., j . amer . chem . soc ., 120 , 5441 ( 1998 ); g . t . bourne et al ., j . org . chem ., 64 , 3095 ( 1999 ); v . boas et al ., j . comb . chem ., 4 , 223 ( 2002 ); j . alsina et al ., j . org . chem ., 64 , 8761 ( 1999 ). patents have been issued on certain aspects of this technology ( see , e . g ., u . s . pat . nos . 6 , 566 , 494 and 5 , 917 , 015 ). approaches for loading the first residue onto the resin for the bal attachment strategy include the synthesis of a suitable amino acid ester - linker construct , which is then attached to a suitable resin ( k . jensen et al ., j . amer . chem . soc ., 120 , 5441 ( 1998 ); g . t . bourne et al ., j . org . chem ., 64 , 3095 ( 1999 ); v . boas et al ., j . comb . chem ., 4 , 223 ( 2002 )). for combinatorial library synthesis direct attachment of an amino acid ester to a suitable resin would be preferable . one approach to direct attachment is through direct displacement of on - resin benzyl halides or other leaving groups ( m . renil et al ., tetrahedon lett ., 35 , 3809 , ( 1994 ); k . ngu et al ., tetrahedron lett ., 38 , 973 ( 1997 ); b . raju et al ., op . cit ., 38 , 4965 ( 1997 )). however , loading by reductive amination of an amine or an amino acid ester onto a benzaldehyde based resins is a generally preferred approach , using suitable mild borohydride based reducing agents such as nabh ( oac ) 3 ( boojamra et al ., cited above ; t . a . okayama et al ., org . lett ., 2 , 1787 ( 2000 )) or nabh 3 cn ( g . t . bourne et al ., tetrahedron lett ., 40 , 7271 ( 1999 )). as discussed above , there are certain advantages to an n - to - c based synthesis strategy for c - terminally modified peptide mimetics . however , the ability to selectively modify the n - terminus of a solid phase peptide chain is also desirable . combining amino acid t - butyl ester based ispps with a bal attachment strategy provides both n and c - termini of the resin bound peptide / mimetic for modification ( fig1 – 12 ). in this approach , the first amino acid t - butyl ester is attached directly onto a benzaldehyde or benzyl halide based resin , followed by capping of the secondary amine attachment with a suitable acyl , urethane , ureido , or sulfonyl group , and then followed ispps cycles using the t - butyl ester based approach described herein . using a suitable protecting group to cap the secondary amine ( eg . fmoc ) allows the secondary amine to be selectively revealed for further modification at any point in peptide / mimetic synthesis . protection with cbz or other moderate acid - stable protecting groups ( cl - cbz is more acid stable than cbz ( m . bodanszky et al ., the practice of peptide synthesis , berlin / heidelberg , springer - verlag ( 1994 )) and preferable in this application ), which is stable to t - butyl ester based ispps , will be removed during cleavage of the peptide / mimetic from the resin to provide a peptide / mimetic with a free n - terminus . finally , moderate and strong acid - stable capping groups ( acyl , sulfonamido , ureido , etc .) will remain in the product peptides , and can be included to provide additional diversity in product peptides / mimetics . these linking methods ( suc / glu / gln , dde , urethane , bal ) provide several approaches for anchoring a peptide / mimetic during t - butyl ester based inverse solid phase peptide synthesis , and can provide different products with a significant level of versatility . for example , the suc / glu / gln method can provide peptides / mimetics with an n - terminal succinyl , glu , or gln residue . the urethane strategy can provide peptides with a free amino terminus and with sidechain protecting groups removed . the dde strategy for inverse peptide synthesis can provide peptides with a free n - terminus , and either with or without side chain and a carboxy terminal protecting group . the backbone attachment ( bal ) strategy has been used previously for normal direction peptide synthesis , but not for ispps . as shown in fig1 , the backbone attachment strategy can provide both n and c termini for further elaboration , and also for cyclization , and can provide a highly versatile approach to peptide mimetic synthesis . during the course of peptide synthesis using the described methods , the extent of coupling is currently qualitatively assessed by testing for free carboxyl groups using a malachite green test ( attardi et al ., tet . lett ., 41 , 7391 ( 2000 )). for the bal attachment method , loading and capping reactions can be qualitatively assessed using a chloranil based test for secondary amines ( j . blake et al ., int . j . peptide protein res ., 7 , 495 ). in some cases , sequence data from peptides might be useful and approaches for c - terminally sequencing peptides are now available ( samyn et al ., anal . biochem ., 72 , 1389 ( 2000 )). for the suc / glu / gln linker strategy , these different resins have been tested — hydroxymethyl polystryrene and pam resin ( 4 - hydroxymethylphenylacetamido - methyl polystyrene ) and mbha ( 4 - methyl - benzhydryl amino polystyrene ). to provide the initial carboxyl group , a succinyl linker was used in preliminary studies . some loss of polypeptide was observed during tfa ( trifluoroacetic acid ) deprotection of t - butyl esters on hydroxymethyl polystyrene resin , but not significantly with pam resin , and pam resin was used in subsequent studies . mbha forms an amide linkage with the glu linker that cleaves to an n - terminal glutamine ( glu ) residue on the free peptide . the coupling strategy uses a ten - fold excess of base ( dipea or tmp ), a five - fold excess of coupling reagent ( dcc / hobt , hbtu , or hatu ) and a ten - fold molar excess of nh 2 - a - c ( o )— otbu . excess coupling reagent and amine can be washed away after coupling is complete . a number of such coupling reagents are known . three have been tested to date : ( o -( 7 - azabenzotriazol - 1 - yl )- n , n , n ′, n ′- tetramethyluronium hexafluorophosphate )( hatu ), dicyclohexylcarbodiimide ( dcc )/ hydroxybenztriazol ( hobt ) and o - benzotriazol - 1 - yl - n , n , n ′, n ′- tetramethyl uranium hexafluorophosphate ( hbtu ). the coupling methods have been tested by comparison of syntheses of the succinylated peptide suc - ala - leu - pro - phe ( fig2 ). starting with commercial pam resin , the resin was first succinylated using succinic anhydride to provide a carboxyl group for subsequent coupling steps . succinylation was selected for initial work since many protease related synthetic peptides are prepared with an n - terminal succinyl group . succinylated resin was then subjected to hbtu - mediated coupling of l - ala - otbu ( l - ala - t - butyl ester ) with diea ( diisopropylethylamine ) as base . subsequently , the t - butyl ester was deprotected with 25 % tfa / dcm ( 1 × 5 seconds , filter , then 1 × 30 min ). this process was repeated with each subsequent amino acid t - butyl ester ( ala , pro , phe to give suc - ala - leu - pro - phe ; seq id no : 1 ). finally , the peptide was liberated from the polymer using tfmsa ( trifluoromethanesulfonic acid )/ tfa cleavage . the final product ( fig2 ) was the desired product , and its identity was confirmed by hplc - ms comparison with commercially available authentic suc - ala - leu - pro - phe ( seq id no : 1 ). using the marfey &# 39 ; s reagent based method cited above , hbtu / diea and hatu / tmp coupling procedures have been compared for synthesis of the test peptide suc - ala - leu - pro - phe ( seq id no : 1 ) as summarized in table 2 . with both coupling procedures & gt ; 20 % racemization of the first residue attached to a succinyl linker is observed . hatu / tmp provides less racemization in general than hbtu or dcc / hobt , and hatu shows & gt ; 5 % racemization of subsequent residues even for the more difficult praline coupling . the observation of high racemization of only the first residue following the succinyl linker suggested that a protected α - amino group in a linker might reduce racemization of the first added residue . z - glu - otbu ( z = cbz ) was therefore tested as a linker . z - glu - otbu was coupled to pam resin using hatu - based coupling protocol , followed by the standard deprotection and coupling procedure . this modification resulted in & lt ; 5 % racemization of the residues in synthesized peptides . peptides synthesized with this linker strategy have a glu residue as the n - terminus ( glu linker strategy ). to avoid the possibility of esterolytic cleavage of the resin - glu attachment , we have also tested the use of an amide attachment strategy by coupling z - glu - otbu to mbha resin , a resin generally used in normal direction peptide synthesis to general c - terminally amidated peptides . cleavage from this resin provides an n terminal gln residue ( i . e ., amidated glu side chain ) ( gln linker strategy ). this linker strategy also provided peptides with low (& lt ; 5 %) racemization in test syntheses . the increased stability of an amide link to the resin may prove useful for some carboxyl group modification methods . one advantage of both the glu and gin linker strategies over the succinyl linker strategy is that they provide the free n - terminal α - amino group of the terminal glu or gln residue , which could then be used to attach a multiplicity of peptides to deriviatized substrates such as to silicon wafers or glass slides comprising free cho groups , to yield “ chip ” libraries useful for very high throughput screening ( cf ., macbeath and schreiber , science 289 , 1760 – 176 ( 2000 )). mbha - z - glu - otbu resin ( linked between the y - carboxyl of z - glu - otbu and the amino of mbha resin ) was employed . mbha is a 4 - methyl - benzhydryl amine polystyrene resin . the coupling method used is the hatu / tmp - mediated activation . details of this method are described below . mbha resin ( 1 . 0 equiv .) is first washed with dmf , 20 % diea / dmf , dmf and dcm . to this pre - washed resin , a solution of z - glu - otbu ( 5 . 0 equiv .) wherein z is benzyloxycarbonyl , hatu ( 5 . 0 equiv .) and tmp ( 5 . 0 equiv .) in dmf is added . the suspension is stirred at room temperature for 3 h . the resin is then filtered and washed with dmf and dcm . the resulting resin is treated with a solution of acetic anhydride ( 3 . 0 equiv .) and diea ( 3 . 0 equiv .) in dmf for 30 min ( to cap any remaining on - resin amine groups ). the resin is filtered and washed with dmf and dcm . above resin is treated with 25 % tfa in dcm ( 1 × 5 s ) and 50 % tfa in dcm ( 30 min × 1 ). the resin is then washed with dcm × 3 , nmp × 2 , dcm × 3 , dried . the z - glu - linked resin is treated with a solution of an amino acid otbu ester ( 5 . 0 equiv . usually hcl salt ), hatu ( 5 . 0 equiv .) and tmp ( 10 . 0 equiv ., 5 equiv . if amino acid is free base and not hcl salt ) in dmf . the suspension is stirred at room temperature for 2 h . the resin is then filtered and washed with dmf and dcm . a small amount of resin sample ( 1 ˜ 5 mg ) is removed and subjected to a color test reaction using 1 ml of 0 . 25 % malachite green ( in ethanol ) along with 1 drop of triethylamine . a double coupling is performed if the testing shows positive ( green or blue on resin ). if the testing gives negative result ( no color on resin ), then repeat step 2 and step 3 until reaching the desired length of peptide . before cleavage , the c - terminal protecting group ( t - bu ) was removed using the identical procedure described in step 2 . after being washed and dried , the deprotected peptide - resin is treated with a mixture of tfa ( 100 μl ) and tfmsa ( 10 μl ) for about 1 h . the solution ( crude product ) is collected by filtration , dried under a nitrogen stream and was analyzed by hplc and lc / ms . the attachment and ispps strategy are outlined in fig3 . hydroxymethyl polystyrene resin ( advanced chemtech ) ( 100 mg ) ( 0 . 1 mmol ) was converted to the chloroformate by treating with 10 equivalents of phosgene in dcm for 30 min and then drying under vacuum . the first amino acid was then loaded onto the resin by adding a solution of 10 equivalents of amino acid t - butyl ester and 5 equivalents of dipea in dmf to the dried resin , and stirring for 4 hrs . to assess loading efficiency , phe was used as the first residue , and after loading and washing the resin was treated with 10 % tfmsa / tfa for 1 hr . total phe attached to the resin was quantitated by hplc . for ispps , the first residue was deprotected with 50 % tfa / dcm , and synthesis cycles performed as outlined in table 3 . b determined by hplc analysis of the crude product at 260 nm on a hewlett - packard series 1050 system equipped with a c18 column ( solvent miser , 2 . 1 × 250 mm , 5 . 0 μm particles ). compounds were separated by gradient elution ; 0 % of solvent b ( 0 . 1 % tfa in 70 % aqueous acetonitrile ) in solvent a ( 0 . 1 % tfa in water ) for 1 min , then 0 % to 100 % of solvent b in solvent a in 10 min , then 0 % to 100 % of solvent c ( 0 . 095 % tfa in acetonitrile ) in solvent b in 5 min . these peptides were analyzed for amino acid racemization using marfey &# 39 ; s reagent . the observed racemization of individual amino acids was less than 2 % ( table 5 ). to demonstrate the utility of this approach for c - terminally modified peptide mimetic synthesis , several peptide mimetic classes were synthesized using this approach . a peptide trifluoromethylketone was synthesized following the procedures described herein and in w . g . gutheil et al ., chem . pharm . bull . ( tokyo ), 50 , 688 ( 2002 ) ( example 4 ( c )). a peptide boronic acid was also synthesized by coupling dl - boroala ( see , a . pechenov et al ., biochem ., 42 , 579 ( 2003 ) and example 4 ( a ), below ) to the resin in the last synthesis cycle . peptide boronic acids have been of high interest as potential drugs and velcade , recently approved for treatment of multiple myeloma , is a peptide boronic acid . this strategy should be readily applicable to peptide aldehydes and chloromethyl ketones . peptide aldehydes and chloromethyl ketones are versatile synthetic intermediates and can be elaborated into a wide variety of functional groups . see example 4 ( b ) and ( d ), above . introduction of common inhibitor functional groups , such as peptide aldehyde , chloromethyl ketone , fluoromethyl ketone , and boronic acid , onto the c terminus of solid phase attached to n - to - c peptide chains in preliminary studies , solution phase methods have been used to synthesize a representative example of each of these inhibitor classes . for solid phase syntheses , two distinct strategies are used to prepare this set of four inhibitor classes . in the case of boronic acids and trifluoromethylketones , suitably derivatized inhibitor monomers must be prepared and introduced into the peptide chain . in the case of chloromethylketones and aldehydes , direct chemical modification of the c terminus can provide the desired functional group . the chemistry required to prepare boroala and several other hydrophobic amino acid analogs such as boroval is well known ( kettner & amp ; shenvi , j . biol . chem ., 259 , 15106 ( 1984 ); matteson & amp ; sadhu , u . s . pat . no . 4 , 525 , 309 ( 1985 )). see fig3 . boro - ala can be introduced onto the c - terminus of a peptide using the standard procedures for adding an amino acid to a peptide chain . peptide boronic acids were synthesized based on the approach outlined in fig5 . the boroala - pinacol derivative was synthesized as described previously ( fig4 ). resin loaded with phe - ala - gly or ans - leu - glu was coupled with boroala - pinacol ( hcl salt ) using the standard hatu / tmp coupling protocol , followed by cleavage with 10 % tfmsa / tfa to give the corresponding peptide boronic acids . as shown in fig6 , several strategies for the synthesis of peptide aldehydes have been described , including from amino alcohols by oxidation ( thompson , biochem ., 12 , 47 ( 1973 ); okura & amp ; swem , tetrahedron , 34 , 1651 ( 1978 )), by reduction of the amino acid methyl esters with diisobutylaluminum hydride ( dibal ) ( ito et al ., biochem . biophys . res . comm ., 49 , 343 ( 1975 )); gorenstein & amp ; shaw , biochem ., 21 , 4679 ( 1982 )), or by reduction of weinreb n - methoxy - n - methylcarboxamides with lialh 4 ( fehrentz & amp ; castro , synthesis , 676 ( 1983 )). the best solution phase method tested so far is reduction of weinreb amides . this method works well in the presence of boc and cbz groups , but may be incompatible with most esters , for example in amino acid side chain protecting groups for glu and asp . the successful pfitzner - moffat oxidation ( dcc / cl 2 hco 2 h / dmso ) of peptide trifluoroamino alcohols suggests this strategy could also work for synthesis of peptide aldehydes as outlined in fig6 . this approach was tested , and was successful in generating an on - resin aldehyde . two problems appeared to affect the yield of the peptide aldehyde product . the first was competition between n and o coupling with the amino alcohol monomer . n coupling resulted in the desired final product , whereas o coupling gave a labile ester intermediate which was cleaved during the resin cleavage reaction to provide the starting peptide . the second problem was that the product peptide aldehyde was somewhat labile to the tfmsa / tfa cleavage conditions . to avoid n and o competition , the oh group in the amino alcohol monomer can be protected by a suitable protecting group . use of t - butyl has been reported ( k . j . jensen et al ., j . amer . chem . soc ., 120 , 5541 ( 1998 )). however , the t - butyl group was difficult to incorporate into an amino alcohol precursor , presumably because of electrostatic interaction with the protonated free amino group . good results were however obtained with acid catalyzed ( 5 % tfa / ch 2 cl 2 ) addition of dihydropyran to the amino alcohol to give the tetrahydropyran ( thp ) adduct of the oh functional group , which is a very easy one pot reaction , followed by drying under vacuum . for the t - butyl ether protecting group , tfa deprotection provides the peptide alcohol ( as cited above ). for thp , 5 % tfa / 10 % meoh / 85 % ch 2 cl 2 gave good results for peptide alcohol synthesis . to avoid the problem of aldehyde degradation under tfmsa / tfa cleavage conditions , the aldehyde can be protected by treatment with a 20 - fold excess of ethylene glycol in 5 % tfa / dcm followed by cleavage from the resin provided the protected aldehyde ( gln - phe - lys - ala - ch ( och 2 ) 2 ( seq id no : 4 ) as the major peptide product ([ m + h ]= 522 . 8 ). impurities were : a ) gln - phe - lys ([ m + h ]= 421 . 5 ) due to incomplete coupling of the amino alcohol , b ) gln - phe - lys - ala - ch 2 oh ([ m + h ]= 486 . 6 ) ( seq lid no : 4 ), due to incomplete oxidation of the amino alcohol , and c ) an unknown impurity ([ m + h ]= 574 . 8 ). reaction of on resin peptide aldehyde with nitroethane and dithioethane have also been tested and give complete conversion of the aldehyde to the expected derivatives ( fig6 ). in fig6 , z = benzyloxycarbonyl , •= resin , and z - glu - otbu is phch 2 oc (═ o ) nhch ( ch 2 ch 2 co 2 h )- co 2 tbu . a large number of commercially available nucleophiles ( cf . anion of nitroethane ) can be reacted with on - resin aldehydes to provide peptide aldehyde adduct libraries of tremendous diversity ( fig6 ). dithiane and other derivatives provide the further possibility of acyl - anion equivalent chemistry , which would allow pepide aldehydes to be further elaborated using various alkylating agents into peptide mimetic ketones ( see , seebach et al ., j . org . chem ., 40 , 231 ( 1975 ); hase et al ., aldrichimica acta , 14 , 73 ( 1981 ). the classic procedure for preparing peptide trifluoromethylketones is shown in fig7 ( imperiali & amp ; abeles , tet . lett ., 22 , 135 ( 1986 )). oxidization of the acyl trifluoromethyl aminoalcohol can be performed with kmno 4 , but can also be performed with a modified pfitzner - moffat ( carbodiimide / cl 2 hco 2 h / dmso ) procedure ( fearon et al ., j . med . chem ., 30 , 1617 ( 1987 )) ( fig7 ), or with a dess - martin periodinane ( 1 , 1 , 1 - triacetoxy - 2 , 1 - benzoxiodol - 3 -( 3h )- one ) procedure ( edwards et al ., u . s . pat . no . 5 , 194 , 588 ( 1993 ); dess & amp ; martin , j . org . chem ., 48 , 4155 ( 1983 )). two peptide trifluoromethylketones were synthesized using the approach outlined in fig7 . the precursor aminotrifluoromethylalcohol was synthesized as described previously by a . pechenov et al ., biochemistry , 42 , 579 ( 2003 ). resin loaded with tyr - ala - phe or tyr - gly - orn was coupled with this aminotrifluoromethylalcohol using the hatu / tmp coupling procedure to give the corresponding peptide trifluoromethylalcohols . oxidation of the on resin trifluoromethylalcohols was performed by pfitzner - moffat oxidation to give the corresponding peptide trifluoromethylketones , which were then cleaved from the resin using 10 % tfmsa / tfa . peptide trifluoromethylketones were detected in lc / ms as their hydrates . the lack of detectable trifluoromethylalcohols indicated quantitative oxidation . there are additional approaches for preparing perfluoroalkyl ketones which involve the addition of perfluoroalkyl anions to suitable amino acid derivatives , such as addition of trifluoromethyl zinc to amino acid aldehydes and addition of pentafluoroethyl lithium to protected amino acid esters ( r . cregge et al ., j . fluorine chem ., 88 , 71 ( 1998 )). peptide chloromethylketones are traditionally synthesized by coupling a carboxylic acid to diazomethane using an isobutylchloroformate mixed anhydride procedure , followed by treatment with hcl ( fig8 ) ( schoellmann & amp ; shaw , biochem ., 2 , 252 ( 1963 ); green & amp ; shaw , j . biol . chem ., 256 , 1923 ( 1981 )). coupling can be accomplished for solid phase chloromethyl ketone synthesis using the hatu / tmp coupling method , or other coupling reagents . conversion to the chloromethyl ketone on resin can be effected with dilute hcl in suitable solvents such as either , dcm , dmf , thf , etc . although chloromethyl ketones are unlikely to have clinical application , given their alkylating capability , they form covalent adducts with their targets and have a number of biochemical applications in the study of structure / function relationships in proteins . chloromethyl ketones are also excellent synthetic intermediates for elaboration into a number of interesting peptide mimetic classes , such as hydroxyethylene inhibitors which are effective inhibitors of hiv protease ( cf . dreyer et al ., biochem ., 31 , 6646 ( 1992 ); knovalinka et al ., eur . j . biochem ., 250 , 559 ( 1997 )), and the alzheimer &# 39 ; s disease - related β - secretase enzyme ( shearman et al ., biochem ., 39 , 8698 ( 2000 ), among others . peptide hydroxamic acids can be synthesized by coupling o - t - butyl hydroxylamine to the c - terminal carboxy group as the last step . the o - t - butyl protecting group is analogous to the t - butyl ester used to protect amino acids in this strategy , and is removed under acidic cleavage conditions . peptide hydroxamic acids were synthesized based on the approach outlined in fig9 . resin loaded with the phe - ala - gly or phe - leu - val was coupled with o -( t - butyl ) hydroxylamine ( hcl salt ) using hatu / tmp , followed by cleavage with 10 % tfmsa / tfa to give the corresponding hydroxamic acids . o -( t - butyl ) hydroxylamine was found to give a better yield and purity of the final peptide hydroxamic acid than the use of unprotected hydroxylamine . the cleaved peptide trifluoromethylketones , peptide boronic acids , and peptide hydroxamic acids were obtained in high purity (& gt ; 70 % in hplc ). hplc purity and molecular weight confirmation of the peptides / peptide mimetics are given in table 4 . two additional strategies for the solid phase attachment of nascent peptide chains to the solid phase have been developed . these strategies allow inverse peptide synthesis of a peptide chain to be performed using the present t - butyl ester based inverse peptide synthesis method , followed by chemical cleavage from the resin . in examples 1 – 2 above , the product peptide always has a succinyl , glu or gln residue as its n - terminus . two other attachment strategies have been developed for use in the present t - butyl ester based inverse peptide synthesis method , which permit the synthesis of peptides / mimetics with virtually any n - terminal substituents , including free nh 2 . dde resin is commercially available and has been described in the literature ( s . r . chhabra et al ., tetrahedron lett ., 41 , 1095 ( 2000 )). the dde functional group reacts with amines as shown in fig1 . after reactions on the dde - amine conjugate the amine can be released by treatment with hydrazine , and the peptide deprotected . using commercially available dde resin from novabiochem , an amino acid t - butyl ester can be attached to the resin simply by incubating the amino acid ester with the resin in the presence of a suitable base . the present t - butyl ester based inverse peptide synthesis cycles can then be used to elaborate a peptide chain on the resin . the t - butyl c - terminal carboxyl protecting group can be removed before cleavage from the resin , or left intact if a carboxy group protected peptide is desired . cleavage of the peptide product using hydrazine provides a side chain and optionally , a carboxy group - protected peptide / mimetic with a free n - terminal amine ( step ( c )). treatment of these peptides / mimetics off - resin with tfmsa / tfa , as shown in fig1 , provides the side chain - and carboxy group - deprotected peptide . several side chain and c - terminally protected and deprotected peptides were synthesized using this method in good yield and purity . analysis of product peptides for racemization using marfey &# 39 ; s reagent as cited above revealed & lt ; 3 % racemization at all residues . a trifluoromethyl ketone synthesis from the trifluoromethyl amino alcohol has been successful with this strategy , demonstrating that this attachment strategy is stable to on resin oxidation . ( see example 4 ( c ).) dde resin ( r - dde ) ( 0 . 87 mmole / g , 100 – 200 mesh ) was purchased from calbiochem - novabiochem ag ( switzerland ), and amino acid t - butyl esters were purchased from bachem ag ( king of prussia , pa .). hatu ( o -( 7 - azabenzotriazol - 1 - yl )- n , n , n ′, n ′- tetramethyluronium hexafluorophosphate ), tmp ( 2 , 4 , 6 - trimethylpyridine ), anhydrous nmp ( n - methypryrrolidone ), tfa ( trifluoroacetic acid ), tfmsa ( trifluoromethanesulfonic acid ), and hydrazine were from aldrich ( milwaukee , wis ., usa ). anhydrous dmf ( n , n - dimethyformamide ), and dcm ( dichloromethane ) were from acros ( new jersey , usa ). marfey &# 39 ; s reagent ( n α -( 2 , 4 - dinitro - 5 - fluorophenyl )- l - alaninamide ) was from sigma ( st . louis , mo ., usa ). the overall synthesis strategy is outlined in fig1 . to load the first amino acid , dde resin ( r - dde ) was swollen in dmf for 2 hours and then washed with dmf . to the resin was added a freshly prepared solution of an amino acid t - butyl ester ( aa - otbu • hcl ) ( 5 eq ) and tmp ( 10 eq ) in dmf , and mixed for 2 hours . the resin was filtered and washed with dmf , and another solution of freshly prepared aa - otbu • hcl ( 5 eq ) and tmp ( 10 eq ) in dmf was again added to the resin and mixed for 12 hours . the resin was then washed and dried . inverse peptide synthesis cycles performed as summarized in table 2 , using an hatu / tmp based coupling method ( a . johansson et al ., j . comb . chem ., 2 , 496 ( 2000 ); w . g . gutheil et al ., chem . pharm . bull . ( tokyo ), 50 , 688 ( 2002 )). peptide - resin samples ( 10 mg ) were treated with 5 % hydrazine / dcm ( 400 μl ) for 60 min . the cleavage solution was filtered , dried , and analyzed by hplc and lc / ms . off - resin treatment of a protected peptide tyr - ala - val - lys ( z )- otbu ( seq id no : 5 ) with 10 % tfmsaitfa for 30 minutes was used to quantitatively deprotect peptide to give h 2 n - tyr - ala - val - lys - oh ( seq id no : 5 ) ( scheme 3 ). hplc was performed on a hewlett - packard series 1050 system equipped with a diode array detector and a c18 column ( solvent miser , 2 . 1 × 250 nm , 5 . 0 μm particles ). compounds were separated by gradient elution ; 100 % solvent a ( 0 . 1 % tfa in water ) for 1 min , then 0 % to 100 % of solvent b ( 0 . 1 % tfa in 30 : 70 water : acetonitrile ) in 10 min , and then 0 % to 100 % of solvent c ( 0 . 095 % tfa in acetonitrile ) in 5 min . lc / ms was carried out on thermoquest ( finnigan ) system equipped with atmospheric - pressure ionization ( api ) electro spray source . the degree of racemization of amino acids in product peptides was determined using marfey &# 39 ; s reagent . a 2 μl aliquot of a 50 mm solution of peptide was hydrolyzed with 100 μl 6 n hcl for 4 hours at 110 ° c . in sealed vials , and the hydrolyzed mixture dried under vacuum . to this was added 14 . 3 μl ( 5 eq ) of a 1 % solution of marfey &# 39 ; s reagent in acetone , 4 μl of 1 m nahco 3 and 6 μl of water per amino acid , and the mixture was kept at 35 – 40 ° c . for 90 min . the reaction was quenched by the addition of 4 μl of 1 m hcl ( per amino acid ). solvent was removed under vacuum and the residue dissolved in 400 μl of 1 : 1 water : acetonitrile . a 10 μl injection was made for hplc analysis ( detection at 340 nm ). the same procedure except for hydrolysis was followed for standards ( 50 mm solution of amino acids ). the percentage of d - diastereomer for each amino acid in each hydrolyzed sample determined by comparison of peak areas . 3 . solid - phase synthesis of two peptide trifluoromethylketones ; tyr - ala - val - nhch ( ch 3 ) c ( o ) cf 3 and gly - val - nhch ( ch 3 ) c ( o ) cf 3 to demonstrate the potential of the dde resin based attachment strategy for peptide mimetic syntheses , two peptide trifluoromethylketones were synthesized based on the approach outlined in fig1 . r - dde - tyr - ala - val and r - dde - gly - val were subjected to hatu / tmp coupling with racemic 1 - trifluoromethyl - 2 - amino - 1 - propanol ( hcl • nh 2 ch ( ch 3 ) ch ( oh ) cf 3 ) in dmf for 6 hours ( b . imperiali et al ., tetrahedron lett ., 27 , 135 ( 1986 )). this procedure was repeated once , and the resins washed and dried to provide r - dde - tyr - ala - val - nhch ( ch 3 ) ch ( oh ) cf 3 and r - dde - gly - val - nhch ( ch 3 ) ch ( oh ) cf 3 . r - dde - tyr - ala - val - nhch ( ch 3 ) ch ( oh ) cf 3 and r - dde - gly - val - nhch ( ch 3 ) ch ( oh ) cf 3 were treated with a solution of dcc / chcl 2 cooh / dmso / toluene ( 10 eq / 1 eq / 0 . 2 ml / 0 . 2 ml ) for 18 hours . after filtration and washing this procedure was repeated once . after filtration , washing , and drying the resins were treated with 400 μl 5 % hydrazine / dcm for 60 min . the cleavage solution was filtered , dried and analyzed by hplc and lc / ms . seven short peptides were synthesized using this strategy and analyzed for degree of amino acid racemization using marfey &# 39 ; s reagent . the degree of racemization was less than 3 % in all cases . all seven peptides were obtained in good yields and purity ( table 6 ). off - resin treatment of a protected peptide with 10 % tfmsa / tfa for 30 minutes can be used to quantitatively deprotect peptide , as demonstrated by off - resin treatment of tyr - ala - val - lys ( z )- otbu to give tyr - ala - val - lys - oh ( fig1 , table 6 entry 7b ). an effort to perform side chain deprotection on - resin gave a complex mixture of products . fluoromethylketones are of interest as inhibitors of serine , cysteine , and aspartic acid proteases , including elastase , cathepsin b , renin , and hiv protease . to demonstrate the potential of the dde resin based attachment strategy for peptide mimetic syntheses , two peptide trifluoromethylketones were synthesized . r - dde - tyr - ala - val and r - dde - gly - val were coupled with an aminotrifluoromethylalcohol to give the corresponding resin attached peptide trifluoromethylalcohols . oxidation of the alcohol by pfitzner - moffat oxidation gave the corresponding resin attached peptide trifloromethylketones . after cleavage the peptide trifluoromethylketones were obtained in good yield and high purity . in lc / ms they were detected as their hydrates . the lack of detectable peptide trifluoromethylalcohol indicated quantitative oxidation . as discussed and referenced above , a number of studies demonstrate that amines and amino acid esters can be attached to a solid support via a benzylic attachment . these attached amines are amenable to elaboration by suitable synthetic methods , followed by cleavage from the solid support to give the desired products . further recent studies demonstrate that an attached amino acid can be extended by normal ( n - to - c ) peptide synthesis cycles , with the peptide chain anchored through the backbone amide of the c - terminal residue ( backbone amide linker strategy ( bal )). see fig1 . this allows an attached peptide to be synthesized in the classic c - to - n direction , with the free c - terminus available for elaboration into suitable peptide mimetic functional groups , and provides an alternative to the inverse ( n - to - c ) synthesis strategy . however , the normal direction bal strategy suffers from several limitations — primarily from the limitation that the peptide chain is synthesized in the c - to - n direction , away from the c - terminal functional group . for split - pool combinatorial peptide mimetic synthesis followed by iterative deconvolution to obtain an optimized agent ( s ), which is arguably one of the better approaches to combinatorial optimization ( d . a . konings et al ., j . med . chem ., 40 , 4386 ( 1997 )), it is the last residues added to a molecule which are optimized first . in the bal approach with c - to - n synthesis cycles these are the residues furthest away from the c - terminal functional group . it seems most reasonable when optimizing a c - terminal peptide mimetic for a specific application to optimize the residues closest to the c - terminus first . by combining the bal attachment strategy with t - butyl ester based inverse peptide synthesis would provide a method for the synthesis of c - terminally modified peptides comprising a wide variety of capping groups at the n - terminus , including various n - terminal capping groups such as acyl , ureido , sulfamoyl , etc ., as well as cyclic peptides and the like . two phenoxybenzaldehyde resins for bal attachment have been found suitable for amino acid t - butyl ester based ispps ( commercially available from novabiochem ), a mono - methoxy benzaldehyde resin ( mm , above ), and a no - methoxy benzaldehyde resin ( nm , above ). as shown in fig1 , an amino acid t - butyl ester is loaded onto the benzaldehyde resins by reductive amination . treatment with acetic anhydride / dipea caps the secondary amine easily as an acetyl derivative on the monomethoxy resin , but capping is more difficult on the no - methoxy resin , and more stringent conditions are often required such as the use of acyl fluorides as has been observed in other studies of acylation of benzylically attached amines ( g . t . bourne et al ., j . org . chem ., 64 , 3095 ( 1999 )). in principle , a wide variety of acyl groups can be used to cap the secondary amine , including n - protected amino acids , acyl groups , ureido groups , and sulfonyl groups , etc ., and this can potentially be used to provide a great variety of diversity in peptides / mimetics accessible with this approach . after loading and acylation the peptide chain can be extended in the inverse direction using the tbu ester approach . for t - butyl ester based ispps reactions on the monomethoxy resin , poor yields of the product peptides were observed when 50 % tfa / ch 2 cl 2 was used in the deprotection reaction , and resin fragments were also present in the peptide products . a study was therefore made of yield and purity of a model tripeptide ( ac - phe - ala - val ) as a function of % tfa in the tfa / ch 2 cl 2 deprotection reagent , with the results summarized in table 7 . excellent results were obtained in a window of from 20 – 30 % tfa . at lower % tfa than 20 %, incomplete deprotection of tbu esters resulted in deletion peptides , whereas at higher than 30 % tfa in deprotection reactions , the resin attachment is prematurely cleaved resulting in decreased yields of the final product , and the optimal % tfa for otbu deprotection on the monmethoxy resin based on this study is 25 %. the acid stability of the attachment to the no - methoxy resin is higher than for the mono - methoxy resin , and 50 % tfa can be used with the no - methoxy resin without adverse consequences . for both mono and no - methoxy resins , peptide is released from the resin with 10 % tfmsa / tfa . other standard cleavage methods for boc chemistry compatible attachment strategies are also expected to work for releasing product peptides for both mono - and no - methoxy resin based backbone attachment strategies , such as 95 % tfa / h 2 o and hf . these examples demonstrate that irreversible capping reactions of the backbone linked secondary amine with acyl ( acetyl ) groups provide the acyl capped peptide product after cleavage from the resin . in contrast , urethane ( fmoc and cl - cbz ) groups were cleaved from the peptides during cleavage of the peptides from the resin . it is expected that essentially all acyl groups will provide irreversible capping groups , whereas all urethanes will provide cleavable capping groups . it is desirable to have several different irreversible capping groups to provide additional diversity in product peptide mimetics for screening . benzenesulfonyl groups are one such capping group of interest . in preliminary studies it has been observed that use of a sulfonyl capping group on the mono - methoxy resin leads to a resin attachment linkage which is unstable to tfa deprotection conditions for inverse peptide synthesis . it is expected that a no - methoxy substituted resin will be more stable , and will allow peptides with a sulfonyl capping group to be synthesized . another capping group of interest are ureas , which can be introduced by treating the secondary amine after loading the first amino acid with an isocyanate . for n - terminal capping with substituted urea groups , the resin loaded with the first residue ( l - phe ) will be treated with a ten molar excess ( 10 ×) of benzylisocyanate in the presence of dipea overnight in dmf , and then washed and dried . the resin will be tested for the absence of secondary amino groups with the chloranil test and for the absence of carboxyl groups with the malachite green test . the resin will be treated with tfa to deprotect the t - butyl ester , and the resin tested for the absence of secondary amine and the presence of carboxyl group . the product bznhco — nh - phe will be cleaved from the resin , and yield , identity , and purity determined by hplc and lc / ms ( fig1 ). a series of tripeptide ureas will be made with several isocyanates and amino acids to determine the scope and generality of this method . for n - terminal capping with substituted suflamoyl groups , a similar strategy to that described for peptide ureas will be followed ( fig1 ). preliminary studies on a mono - methoxy resin demonstrate that a sulfamoylated secondary amine linkage is unstable to tfa , so it is proposed to test the more acid stable no - methoxy resin in this application . preliminary studies will use benzene sulfonyl chloride as the sulfamoylating agent . a series of sulfamoylating agents will be used to synthesize a series of peptide sulfonamides to determine the scope and generality of this method . all cited publications , patent applications , and patents are incorporated by reference herein as though fully set forth .