Starting from L-alanine, L-phenylalanine, L-leucine, L-norleucine, or L-isoleucine, 2-diazo-3-oxo-4-phthalimido-alkanoates 8 were prepared in three steps. Considerable racemization occurred at the stage of the 3-oxo-4-phthalimido-alkanoates 7. Dirhodium tetraacetate effectively catalyzed the intramolecular formation of carbonyl ylides 9, which in the absence of a trapping reagent underwent a [3+3] cycloaddition reaction to form the dimers 10. Carbonyl ylides 9 underwent [3+2] cycloaddition reactions with several electron-deficient alkenes and alkynes to give oxygen and nitrogen containing multicyclic systems 12-16. The keto group of the α-oxy-β-ketoester moiety of cycloadducts 2 and 12 is easily hydrated to give the gem-diol.

SummaryMethionine, S-benzylcysteine and S-allylcysteine were converted into 2-diazo-3-oxo-4-phthalimidocarboxylic esters 8a–c in three steps. Upon rhodium-catalysed dediazoniation, two intramolecular carbenoid reactions competed, namely the formation of a cyclic sulfonium ylide and that of a six-ring carbonyl ylide. The S-methyl and S-benzyl ylides 12a and b could be isolated, while S-allyl ylide 12c underwent a [2,3]-sigmatropic rearrangement. The short-lived carbonyl ylides derived from methionine and S-benzylcysteine formed head-to-tail dimers by a [3 + 3]-cycloaddition and could be trapped with external dipolarophiles, while the S-allyl derivative 14c yielded the pentacyclic compound 17 by an intramolecular [3 + 2]-cycloaddition reaction.

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