our synthesis was initiated with chromone 12 which was prepared in greater than 50 gram batches and purified by recrystallization ( figure 2 ) . utilizing diisopropylsilyl ditriflate to activate chromone 12 to a siloxybenzopyrylium species , vinylogous addition of 2-trimethylsiloxyfuran was achieved which was followed by hydrogenation to afford chromone lactone 13 on adecagram scale ( 89 % yield , dr > 10:1 ) after purification by trituration ( supplementary information s4s19 ) . further treatment of 13 with nah in thf led to the production of epi - blennolide c 14 through dieckmann cyclization . due to the propensity of thevinylogous acid moiety of tetrahydroxanthone 14 to both epimerization and tetrahydroxanthone - to - chromone lactone rearrangement , we utilized the neutral methylating reagent trimethylsilyldiazomethane to obtain the desired o - methylated product 16 in 61 % yield along with the separable isomer 15 ( 20 % yield ) . after phenol - directed ortho - iodination of 16 , the derived aryl iodide 17 was subjected to kinetic acylative resolution using birman 's catalyst   on a gram scale ( s factor > 200 ) . chiral monomer ( )-17 was isolated in 46 % yield ( 99 % ee ) and the o - acylated product ( + ) -18 was obtained in 46 % yield ( 99 % ee ) after recrystallization . after obtaining the two enantiopure tetrahydroxanthone monomers , our initial plan was to employ our stannane dimerization strategy which successfully led to syntheses of secalonic acids a and d. however , after evaluation of various conditions , we observed only trace stannylation of iodides 17 or 18 ( table 1 ) which may be due to steric hindrance of the aryl iodide substrate . gratifyingly , iodide ( )-17 was found to be stable under basic conditions and to produce an inseparable mixture of dimeric products ( dr = 88 : 12 ) under one - pot suzuki coupling conditions using pd(oac)2/sphos and bis ( pinacolato ) diboron . we subsequently attempted to establish the stereochemistry of the major atropisomer by nmr analysis and by acid - mediated deprotection of both atropisomers 19 and 20 to 1 and 23 ( figure 4 ) . however , both efforts failed as the axial chirality center is remote from existing stereocenters leading to almost identical h nmr spectra for both a tropisomers of rugulotrosin a. fortunately , a single crystal of the major atropisomer ( )-19 was obtained by recrystallization ( ch2cl2/meoh ) from its mixture with 20 . x - ray crystal structure analysis of ( )-19 ( ccdc#1022610 ) was achieved which verified that the relative stereochemistry was identical to that found in natural rugulotrosin a. during our evaluation of dimerization conditions , we found that water was essential and that the ratio of atropisomers was dependent on ligands for palladium rather than solvent , temperature , or reaction time . accordingly , we focused our efforts on the influence of ligands on atropisomeric selectivity ( table 1 ) . after extensive ligand screening , we found that monophosphines were crucial for the success of there action , and that bidentate ligands such as xantphos or dpephos ( not shown ) failed to provide the desired dimeric products . besides sphos(l1 ) , the related ligand ruphos ( l2 ) provided dimeric products in even higher atropselectivity ( 93:7 ) . interestingly , we found when the 2 ' position of the monophophine ligand did not contain an oxygen atom ( e.g. carbon atom in xphos l2 or a nitrogen atom ( davephos , not shown ) ) , dimeric products were obtained in trace amounts . after experiments with the achiral ligand l4 , which provided similar results to sphos , we anticipated that related chiral ligands may achieve efficient control of atropselectivity . we began this investigation with ( r ) and ( s)-bi - dime ligands l5 , which recently have shown excellent atropselectivities in biaryl couplings . however , neither ligand was able to perform the one - pot dimerization of ( )-12 . gratifyingly , chiral versions of the sphos ligands , s - cy - mop ( l6 ) and r - cy - mop ( l7 ) , both successfully produced dimeric products . the ligand r - cy - mop provided a satisfactory dr ( 95.5:4.5 ) and the mismatched ligand s - cy - mop afforded a lower dr ( 81:19 ) , both favoring the same major atropisomer . even with the increased bulk of ligand l8 , the dr did not noticeably improve . in order to enhance product yield , we prepared the buchwald 3rd generation precatalysts with sphos , r - cy - mop , and s - cy - mop . indeed , use of the precatalyst increased yields for these cases and maintained at ropselectivity . use of bis ( neopentyl glycolato ) diboron ortetrahydroxydiboron as boron transfer reagents led only to trace amounts of dimeric products . the acylated , enantiopure monomer ( + ) -18 was also investigated in the suzuki dimerization which generated a separable 3:1 mixture of atropisomersin 36 % yield ( figure 3 ) . fortunately , an x - ray crystal structure of the minor atropisomer ( )-22 ( ccdc#1022611 ) was obtained to verify relative and absolute stereochemistry . introduction of a bulk yester onto the secondary alcohol of the monomer ( e.g. ocoph ) did not affect the ratio of atropisomers . after obtaining and establishing stereochemistry for several protected dimers , we anticipated that we could access all atropisomers and enantiomers of rugulotrosin a. dimer ( )-19 was treated with 3 m hcl / meoh for 10 min which afforded rugulotrosin a ( ( )-1 ) in 89 % yield ( figure 4 , a ) . in a similar manner , ( + ) -rugulotrosin a was obtained from acidic hydrolysis of ( + ) -21 ( figure 4 , b ) . with both enantiomers of rugulotrosin a in hand , natural rugulotrosin a was unambiguously assigned as ( + ) -1 though comparison of rotation data . similarly , atrop - rugulotrosin a ( )-23 and entatrop rugulotrosin a ( + ) -23 were synthesized using similar protocols from the protected precursors ( )-22 and ( + ) -22 ( figure 4 , c and d ) . to verify the stability of the axially chiral biaryl moiety , we heated rugulotrosin a ( + ) -1 in toluene ( 1 mg / ml ) at 100 c for 12 h and 150 c for 3 h. under both conditions , we did not observe formation of atrop rugulotrosin . this is supported by a high barrier of rotation of rugulotrosin a , calculated as 47.7 kcal / mol using the b3lyp/6 - 31g(d ) level of theory ( see supplementary information s52s58 ) . in order to rationalize the axial chirality selectivity observed in the dimerization , we modeled the geometry of the expected intermediate diaryl palladium ( ii ) complexes using sphos ( l1 ) as ligand ( figure 5 ) . conformational analysis was carried out by systematic six - fold rotation about the c - pd bonds , followed by optimization of candidate structures at the b3lyp / lanl2dz level of theory . the three lowest energy structures 24a  24c all have dihedral angles ( c1 , c2 , c2 , c1 ) for the monomer fragments which would afford atropisomer ( )-19 ( precursor to rugulotrosin ) after stereospecific reductive elimination ( figure 5,a ) . the next two higher energy structures 25a and 25b display a reversed dihedral angle which should lead to atropisomer 20 ( table 1 ) . space filling models of structures 24a - c and 25a - b ( supplementary information s34s52 ) show a more crowded arrangement of the latter , in part because of steric interactions between the methyl ester and the cyclohexyl group . the distances between the hydrogen of the methyl ester to the hydrogen on the cyclohexyl of the sphos ligand are 2.4 and 2.5  for conformers 25a and 25b , respectively . based on literature precedent , we assume that these catalyst - reactant complexes are under thermodynamic equilibration ; additional computational studies will be required to predict barriers to interconversion . it thus appears that product stereochemistry is determined by reactant and catalyst assembly in the pd ( ii ) complex ; this provides a basis for prediction of stereochemistry in future atropselective couplings . after assigning the absolute configuration of rugulotrosin a , we employed our library of synthetic rugulotrosins to determine whether penicillium nov . ( mst - f8741 ) was capable of producing atrop - rugulotrosin a. to this end , separate 21 day fermentations of mst - f8741 were extracted with either mecn ( figure 4,e(a ) ) or meoh ( figure 4 , e(b ) ) and were subsequently analyzed using an hplc - dad method optimized for the resolution of rugulotrosin a ( figure 4 , e(c , d ) ) and atrop - rugulotrosin a ( figure 4 , e(e ) ) . to further enhance sensitivity , these analyses were also carried out using hplc - dad - esim sprotocols augmented by single ion extraction ( sie ) ( supplementary information , s22 ) . as these analytical studies failed to detect any trace of atrop - rugulotrosin in mst - f8741 extracts , we conclude that the rugulotrosin biosynthetic pathway present in penicillium nov . a had previously been reported to exhibit antibacterial activity , we also tested the synthetic rugulotrosins ( ( + ) -1 , ( )-1 , ( )-23 , and ( + ) -23 ) against several strains of gram - positive and gram - negative bacteria . consistent with earlier reports,(+)-1 exhibited antibacterial activity against bacillus subtilis ( atcc 6633 ) ( ic50 = 2.1 m ) and staphylococcus aureus ( atcc 25923 ) ( ic50 = 6 m ) . of note , the enantiomer ( )-1 was also active against b. subtilis ( atcc 6633 ) ( ic50 = 0.7 m ) and s. aureus ( atcc 25923 ) ( ic50 = 18 m ) , whereas the antibacterial activity of the atropisomer ( )-23 was weak and limited to b. subtilis ( atcc 6633 ) ( ic50 = 10 m ) and ( + ) -23 lacked any appreciable antibacterial activity . none of the rugulotrosins ( + ) -1 , ( )-1 , ( )-23 , or ( + ) -23 exhibited activity against the gram - negative bacteria escherichia coli ( atcc 25922 ) and pseudomonas aeruginosa ( atcc 27853 ) , nor did they exhibit cytotoxicity ( ic50>30 m ) against human colon ( sw620 ) and lung ( nci - h460 ) cancer cells . in conclusion , we have developed a concise , atropselective approach to rugulotrosin a and stereoisomers through point - to - axial chirality transfer which has facilitated determination of the absolute configuration of rugulotrosin a. computational studies modelling the geometry of intermediate diaryl palladium ( ii ) complexes have provided a rationale for the atropselectivity observed in the key suzuki dimerization . these studies highlight the utility of sphos and related bulky monophosphine ligands in palladium couplings both to bring monomers in proximity and enhance remote steric effects leading to atropselectivity . through hplc analysis of fungal extracts and synthetic samples , . moreover , the atropisomers and enantiomers of rugulotrosin a were found to have different activities against gram - positive bacteria illustrating the importance of stereochemistry on target selectivity . further studies regarding atropselective syntheses of dimeric natural products are currently under investigation and will be reported in due course .