Abstract
Cyclopropane compounds are highly reactive due to their strained structure and are widely used as synthetic intermediates. Thus, many reports on methods to stereoselectively synthesize cyclopropane compounds have been reported. We have attempted the cyclopropanation reaction between α-pyridinium acetates bearing a (-)-8-phenylmenthyl group as a chiral auxiliary and monosubstituted (R) methylidenemalononitriles. As a result, reactions proceeded with complete trans selectivity and high diastereoselectivity (R=Ph, y. 97%, 83:17). Under the assumption that the suppression of the reverse reaction of the initial Michael reaction would lead to higher selectivity, we decided to examine the 8-phenylmenthylamine group. Imination of (-)-8-phenylmenthone followed by reduction with Na/EtOH at reflux provided 8-phenylmenthylamine with high diastereoselectivity (up to 93:7). Chloroacetylation of 8-phenylmenthylamine followed by recrystallization provided the diastereomerically pure chloroacetate. Treatment of the chloroacetate with pyridine at reflux provided the pyridinium salt. The cyclopropanation reaction using this pyridinium salt proceeded with good yield, though the diastereoselectivity turned out to be low (62:38). Currently, we are trying to determine the absolute stereochemistry of each diastereomer and to optimize reaction conditions. The reaction mechanism is also under investigation.
