Convenient New Synthesis of 3-Substituted β-Lactams Formally Derived from 1-(Aminomethyl)cyclopropanecarboxylic Acids

1,3-Dipolar cycloaddition of N-benzyl-C-(methoxycarbonyl)- nitrone (3a), N-benzyl-C-phenylnitrone (3b), N-benzyl-C-cyanonitrone (3c), N-(p-methoxybenzyl)-C-cyanonitrone (3d), N-phenyl- (3e) and N-(2-pyridyl)-C-methylnitrones (3f) to bicyclopropylidene (2) gave the corresponding cycloadducts 5af in 100, 95, 94, 100, 93 and 71% yields, respectively. Treatment of these bisspirocyclopropanated isoxazolidines with trifluoroacetic acid in acetonitrile furnished the corresponding 3-spirocyclopropanated β-lactams 7af in 78, 75, 75, 94, 96 and 96% yields, respectively. The structures of the cycloadduct 5b and of the β-lactam 9a were proved by X-ray crystal structure analyses. Thus, this new method furnishes compounds with a 5-azaspiro[2.3]hexan-4-one skeleton in 6894% overall yield in two simple steps. β-Lactams 7a, 7b, and 7d were converted into their N-acyl derivatives 9a, 9b, 9d and 11 in 44, 28, 39 and 78% yields, respectively. Heating of the β-lactams 9b and 11 with tert-butyl glycinate (12) or with tert-butyl (S)-phenylalaninate (14) in DMF led to ringopening of the β-lactam moiety to give β-dipeptides 15, 16 and 17 in 61, 84 and 79% yields, respectively, while β-lactam 9a gave the amide 13 (51% yield). β-Lactams 7e and 7f turned out not to be transformable into such peptide products