Route for Optically Active Cyclopropanols Utilizing Organoboranes

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Terminal alkenylboronic acids were easily es. terified with tartaric esters or amides. The chirally modified boronic esters were then cyclopropanated by the Simmons-Smith procedure with high "diastereoface differentiation. Thus produced cyclopropylboronate derivatives could then be oxidized under mild conditions to the corresponding optically active cyclopropanols. Internal alkenylboronic esters, on the other hand, could not be cyclopropanated smoothly by the Simmons-Smith reaction and some other carbenoid reactions. By these results, it was suggested that the a-alkyl substituent of these alkenyboronic esters might cause steric interference in the transition state geometry. The optically active 'cyclopropanols-obtained above were converted to the ringopened products without touching the asymmetric carbon center at the beta position by first converting to its silyl ether and then subjecting to the acetoxymercuration-demercuration reaction. Furthermore, the optically active cyclopropylboronate intermediates can be converted to various types of synthetically useful compounds not only by the oxidation reaction but also by variety of known reactions of organoboranes. To demonstrate it, here, the onecarbon hbmologation reaction with chlbromethyl-lithium was applied. This reaction followed by oxidative work-up afforded the corresponding optically active cyclopropylmethanol derivatives. These compouids have, previously, been shown to be convertible to threo-2-methyl-1, 3-alkanediols by the oxymercuration-demercuration reaction which proceeds regioselective and stereospecific manner. Therefore, this asymmetric cyclopropanation reaction of alkenylboronic ester derivatives provides optically active cyclopropanols and cyclopropylmethanols of high synthetic potential.