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-The nitrile N-oxide 4 undergoes a reversible cycloaddition to the carbonyl group of the l-oxa-3-azoniabutatriene salts 3a-c to give the geminal dioxy substituted 2-azoniaallene salts Sa-c. Azibenzil 7 and 3a,d afford the 2-azoniaallene salts 8a,d. With the nitrone 11 the cumulene 36 reacts to yield the open-chain addition product 12. The salt 3a reacts with two equivalents of diazofluorene to give the p-lactamium salt 13, which can be hydrolyzed to afford the N-formyl-B-la&am 15. K-ray structural analyses of 5b and 8d confirm the proposed constitutions.' 1,3-Dipolar cycloaddition reactions 2 to isocyanates have been used to synthesize a variety of heterocycles and other compounds.3'5 Usually, the 1,3-dipole adds across the C=N double bond of the isocyanate. Thus, benzonitrile oxide is reported '-8 to react with phenyl isocyanate to yield 2,4-diphenyl-1,2,5-oxadiazole-3-one. On the other hand, nitrile oxides add to the C=O double bond rather than to the N=C bond of trichloroacetyl isocyanate.g Similarly, c-ketocarbenes react across the C=N double bond of phenyl iSOcyanate to give N-phenyl-1,3-oxazole-2-ones. 10,ll From the reaction of diazoacetophenone with thiobenzoyl isocyanate compound 1 was 0btained.l' A related reaction was observed for chlorosulfonyl isocyanate.13 Nitrones usually react with the C-N double bond of isocyanates.3112-18 An interesting Beckmann-type rearrangement has recently been described for reactions of chlorosulfonyl isocyanate with nitr0nes.l'