Chiral Synthesis via Organoboranes. 29. A General Synthesis of α-Chiral Monosubstituted Acetylenes and Their Trimethylsilyl Derivatives from Enantiomerically Pure Boronic Esters

Article abstract of DOI:10.1021/jo00004a030

The procedure for the synthesis of by the iodination of is impractical for the synthesis of the corresponding chiral derivatives, , due to the unavailability of the required R*3B compounds.RThxBOMe and R*ThxBOCH3, now readily available by established procedures, serve handily for the syntheses of and respectively from , but fail for the syntheses of either or , in reasonable yield, from .Fortunately, this difficulty can be circumvented by utilizing .Indeed, treatment of enantiomerically pure monoalkylthexylborinates, R*ThxBOCH3, readily prepared from enantiomerically pure boronic esters, with forms an ate complex which readily undergoes the desired iodine-induced rearrangement, forming α-chiral (trimethylsilyl)acetylenes, .The (trimethylsilyl)acetylenes are easily desilylated to afford the corresponding α-chiral terminal acetylenes, , in yields of ca. 70percent and essentially 100percent enantiomeric excess (<*>99percent).These intermediates, and , can be readily converted by simple procedures into a wide variety of pure enantiomers: R*CH=CH2, R*CH2CHO, R*COOH, R*CH2COOH, R*COCOOR, etc.Since both (+)- and (-)-alkylboronic esters are now readily available in essentially 100percent enantiomeric purity, it is now possible to synthesize (+)- and (-)-α-chiral monosubstituted acetylenes and their trimethylsilyl derivatives in very high enantiomeric purities.This provides the first general, efficient synthesis of these valuable synthons in such high enantiomeric purities.