Stereoselective epoxidation of cis-3,4-disubtituted-(CH2X)-cyclobutenes with dimethyldioxirane and peroxy acids. Experimental and computational evidence for a syn-orienting electrostatic effect

The epoxidation reactions of a series of cis-3,4-disustituted-(CH2X)-cyclob utenes 1-8 with dimethyldioxirane (DMD) and mCIPBA have been investigated w ith both reagents. A remarkable syn diastereoselectivity in the formation o f the epoxide has been observed for substrates bearing electron withdrawing substituents. Transition structures for epoxidations of 3,4-dimethylcyclob utene (1), diastereoisomeric 3, 5-dioxa-4-thia-bicyclo[5.2.0]non-8-ene-4-ox ides 7 and 8, and 3,4-bis(mesyloxymethyl)-1-cyclobutene (5) with dioxirane and peroxyformic acid have been located with the B3LYP/6-31G* method. Exper imental dominant syn facial selectivity is rationalized mostly as a result of an electrostatic attractive interaction involving the peroxo oxygens of the oxidizing reagents and the positively charged homoallylic hydrogens of the olefins. (C) 1999 Elsevier Science Ltd. All rights reserved.