FACIAL SELECTIVITY IN THE REACTIONS OF 1,3-DIPOLES WITH CIS-3,4-DIMETHYL-1-METHOXYCARBONYL AND TRANS-3,4-DIMETHYL-1-METHOXYCARBONYL CYCLOBUTENES

Face selectivity of the regiospecific reactions of 1,3-dipoles [diazom ethane (5), 3,4-dihydroisoquinoline-N-oxide (6) and nitrile oxides (7) ] with trans-3,4-dimethyl-1-methoxycarbonyl cyclobutene (4) is control led by steric factors with dominant formation of the anti diastereoiso mer (anti/syn: 82:18 for 5, 54:46 for 6 and approximate to 74:26 for 7 ) through the sterically less crowded transition state. On passing to cis-3,4-dimethyl-1-methoxycarbonyl cyclobutene (3b) the expected incre ase in diastereoselectivity is observed only in the case of the reacti on of 3b with 3,4-dihydroisoquinoline-N-oxide (anti/syn = 94:6) while face selectivity decreases in the reaction of 3b with diazomethane (an ti/syn = 72:28) and there is a reversal of face selectivity in the rea ctions of 3b with nitrile oxides (anti/syn = 42:58). In the latter rea ction the dominant syn diastereoisomer is formed via the sterically mo re congested TS. A syn orienting electronic effect (not yet clearly id entified) is evidently present in the reactions of 3b, with 1,3-dipole s and in the reaction of 3b with nitrile oxides it is strong enough to overcome relevant anti orienting-steric effects. (C) 1997 Elsevier Sc ience Ltd.