1. Field of the Invention
The present invention relates generally to improved methods of making glycoside compounds using non-cryogenic techniques.
2. Description of the Background Art
Glycosides are compounds containing a carbohydrate and a non-carbohydrate residue in the same molecule. The carbohydrate residue, also referred to as the glycone, is attached by an acetyl linkage via a carbon atom to the non-carbohydrate residue of aglycone. Glycosides are compounds that are useful, inter alia, as intermediates in a variety of chemical processes and as pharmaceutically active compounds.
Glycosides are difficult to synthesize. In one widely used synthesis route, which involves reaction of a protected sugar with a metalated aromatic via nucleophilic addition, hydrogen abstraction to the lactone tends to occur. Only with careful attention to conditions for the addition of the sugar and metalated aromatic, and selection of appropriate hydroxyl protecting groups, have these synthesis routes produced an acceptable yield. Rosenblum, S. B. and Bihovsky, R., J. Am. Chem. Soc. 1990, 112, 2746–2748. One approach to address this problem, has been to perform batch processes under cryogenic conditions. However, in these synthesis routes, failure to use cryogenic conditions results in side reactions that produce unacceptable amounts of impurities.
In one conventional batch method, in a first reaction vessel under cryogenic conditions, a halogenated aromatic reactant is lithiated using an organo lithium reagent in the presence of a solvent to form a lithiated anion species. Under cryogenic conditions, the lithium anion species is transferred to a second reaction vessel. A reactant having a carbonyl functional group is coupled to the lithiated anion species in the presence of a solvent to form a protected form of the glycoside product. The protected form is then deprotected by reaction with methanesulfonic acid in methanol to form the desired glycoside end product. The cryogenic conditions are maintained by using a liquid nitrogen blanket. Liquid nitrogen is typically a temperature of about −196° C. The transfer line for the lithiated anionic species is maintained at about −78° C. or below. See, for example, Manabe, S. and Ito, Y.; Ogawa, T., Chem. Lett. 1998, 919–920; and Sollogoub, M., et al., Tetrahedron Lett. 2002, 43, 3121–3123.
The lithiating and the coupling steps are each highly exothermic reactions and proceed very quickly. For this reason the conventional lithiating and coupling steps must be performed under cryogenic conditions at about −80° C. and under a nitrogen blanket. The yield for this process is about 70%. The following Scheme A is representative of the conventional method:
The definitions of the symbols used and terms used in Scheme A are as follows:TMS is trimethyl silane; THF is tetrahydrofuran; R1 is hydrogen, NO2, OR4, a halogen, or a substituted or non-substituted alkyl, aryl, or heterocycle, wherein R4 is a substituted or non-substituted alkyl or aryl; R2 is a substituted or non-substituted alkyl group; X1 is a heteroatom; and PG is a protective group.
In the conventional method shown in Scheme A, cryogenic conditions (i.e., temperatures of about −78° C.) are necessary because of the exothermic and rapid nature of the lithiating and coupling steps. Allowing these reactions to proceed at normal temperatures results in formation of excessive amounts of impurities. Specifically, the lithiated anion species is unstable and has a tendency to undergo nucleophilic substitution with BuBr, a side product of reaction. To avoid formation of this impurity, transfer of the lithiated anion species to a reaction vessel must be performed, also under cryogenic conditions. Thus, it is necessary to pre-cool the lithiated anion species and cold trace the transfer line to the second reaction vessel. Coupling of the lithiated anion species to the carbonyl reactant must also be performed under cryogenic conditions. As a result, the conventional batch process requires two cryogenic reactors, one for synthesis of the lithiated starting material, and one for the coupling reaction.
Failure to maintain these stringent cryogenic conditions results in low yields and the presence of impurities. Even with temperatures as low as −10° C., a calculated yield is only about 30%. The reaction profile for reactions at these temperatures using the conventional method suggests that there is less than 50% of the desired product in the reaction mixture.
Drawbacks of the conventional methods of making the glycosides of interest include those typically associated with cryogenic processes in general. Namely, it is necessary to bring starting materials to cryogenic conditions before reacting. The lithiated anion intermediate must also be cryogenically stored and cold traced in the transfer to the coupling reaction vessel. The coupling reaction vessel must also be maintained under cryogenic conditions. This involves elevated costs related to specially designed reaction vessels and transfer lines capable of withstanding the very low temperatures involved.
Cryogenic reactions are also of concern from a safety perspective, as using liquid nitrogen as a coolant can cause bums upon contact with skin and also poses a risk of explosion if not used in conjunction with pressure relief equipment. Additionally, the addition of reagent must be closely regulated to avoid an increase in temperature which risks low yield and formation of undesirable side products, as discussed infra. Slow addition of reagent results can increase reaction time necessary to obtain end product.
Accordingly, there is a present need for a method of making glycosides that does not require costly cryogenic reaction vessels, transfer lines, or pre-cooling of starting materials, that reduces risk of formation of undesirable side products, and that provides an acceptable yield of product.
Known techniques for optimizing wet chemistry processes include close regulation of temperature, pressure, mixing conditions, relative volumes of reactants, catalysts, and the like. In order to achieve or better regulate these optimization parameters, microreactors have been developed. A microreactor is a miniaturized reaction system containing one or more reaction channels having sub-millimeter dimensions. Examples of microreactors include those disclosed in U.S. patent application Ser. No. 20010029294, U.S. Pat. Nos. 5,534,328, 5,690,763, and E.P.O. Patent No. EP 1123734.
Microreactors vary in structure but generally include a series of stacked plates having openings for fluid transfer. The microreactors will have an input port for introduction of reactants and an output port for discharge of a chemical product. In addition, heat transfer fluid pathways are also incorporated into the design to perform a heat exchange function. Using microreactors allows transfer of heat and mass much more quickly than in standard wet chemistry methods, resulting in improved control of rates of reaction and addition of reactants. Specifically, using microreactors allows more accurate control over the reaction temperature of exothermic or endothermic reactions. Additionally, reactants can be heated or cooled immediately upon entry into the reactor. Although microreactors have been used to provide more accurate control of temperature per se, they have not traditionally been used to perform those reactions which must be performed under cryogenic conditions. Furthermore, to date, methods of making glycosides using microreactor technology have not been developed.
It would be a useful advantage to have methods of making glycoside compounds that include a process which consumes reactive reagents as they are made and that does not require cryogenic conditions to be performed.