CHIRAL SYNTHESIS VIA ORGANOBORANES .29. A GENERAL-SYNTHESIS OF ALPHA-CHIRAL MONOSUBSTITUTED ACETYLENES AND THEIR TRIMETHYLSILYL DERIVATIVES FROM ENANTIOMERICALLY PURE BORONIC ESTERS

The procedure for the synthesis of RC = CH by the iodination of [R3BC = CH]-Li+ is impractical for the synthesis of the corresponding chiral derivatives, R*C = CH, 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 RC = CR' and R*C = CR respectively from LiC = CR, but fail for the syntheses of either RC = CH or R*C = CH, in reasonable yield, from LiC = CH. Fortunately, this difficulty can be circumvented by utilizing LiC = CSiMe3. Indeed, treatment of enantiomerically pure monoalkylthexylborinates, R*ThxBOCH3, readily prepared from enantionerically pure boronic esters, with LiC = CSiMe3 forms an ate complex which readily undergoes the desired iodine-induced rearrangement, forming alpha-chiral (trimethylsilyl)acetylenes, R*C = CSiMe3. The (trimethylsilyl)acetylenes are easily desilylated to afford the corresponding alpha-chiral terminal acetylenes, R*C = CH, in yields of approximately 70% and essentially 100% enantiomeric excess (greater-than-or-equal-to 99%). These intermediates, R*C = CSiMe3 and R*C = CH, can be readily converted by simple procedures into a wide variety of pure enantiomers: R*CH = CH2, R*CH2CHO, R*CO2H, R*CH2CO2H, R*COCO2R, etc. Since both (+)- and (-)-alkylboronic esters are now readily available in essentially 100% enantiomeric purity, it is now possible to synthesize (+)- and (-)-alpha-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.