ASYMMETRIC SYNTHESIS--STRUCTURE, STEREOCHEMISTRY AND NMR

在开发制备手性醇的方法和制造

In developing methodologies to prepare chiral alcohols and to make tentative assignments of their absolute stereochemistry (based on their method of preparation) we have continued to examine the enantioselective hydrolysis of esters. These hydrolyses are catalyzed by enzymes in the mold Rhizopus nigricans, and the configuration of the alcohol formed can be predicted from considerations of the effective sizes of substituents on the carbinol carbon. These studies were extended to a series of conjugated allylic alcohols where it was found that one chiral alcohol had formed with an unanticipated configuration. The absolute stereochemistry of this compound was rigorously established by an x-ray crystallographic structure determination. In order to identify other alcohols that form with unexptected configurations, we undertook a series of studies of the relation between the sign of the DC curves of the corresponding p-bromobenzoates and the absolute stereochemistry of these molecules. These studies demonstrated that DC measurements can be used for independent assignments of configuration. Compounds in which these assignments and those based on the microbial method differ require further study. While preparing a series of medium-ring 2-bromo-2-cycloalkenyl acetates, we modified the reaction conditions of a silver catalyzed solvolysis reaction from that described in the literature. Our modification altered the geometry about the double bond of the compound formed from that previously reported. It is now possible to prepare either cis- or trans-2-bromocycloalkenes from the same starting material by adjusting the reaction conditions. These cycloalkenes were used to show that three-bond proton-carbon coupling constants, obtained using a recently developed 2D NMR technique could be used to assign the gometry about similarly tri-substituted double bonds. These assignments previously have been very difficult to make.