Chiral cations in 4+3 cycloadditions

The reaction of the chiral acetal 4 with furan and cyclopentadiene in the p resence of a Lewis acid gives 4+3 cycloadducts in very good yields. Diaster eoselectivity in the case of furan is exceptionally high, while that for th e reaction with cyclopentadiene is lower. The sense of diastereoselection i n the case of furan is opposite to that which would be predicted on the bas is of mechanistic models for the origin of stereoselectivity in many reacti ons of chiral acetals mediated by Lewis acids. (C) 1999 Elsevier Science Lt d. All rights reserved.