Zum photochemischen Verhalten von Sydnonen und 1,3,4-Oxadiazolin-2-onen

The photochemical behaviour of several 3,4-disubstituted sydnones (cf. Scheme 5) in dioxane solution was investigated. The pure sydnones give, as was already reported [2-6], 2,4,5-trisubstituted 1,2,3-triazoles 2 (cf. Scheme 1) in 25-30% yield. In the presence of dipolarophiles (cf. Scheme 1) pyrazole derivatives 3 or 4 are formed which can be taken as a proof for the formation of nitril-imines 5 as primary products in sydnone photochemistry. Since irradiation of 2-[15N]-3,4-diphenyl- sydnone (2-[15N]-15) in dioxane leads to the formation of 1,3-[15N]-2,4,5-triphenyl-1,2,3-triazole (1,3-[15N]-22; cf. Scheme 10), nitril-imine formation must be induced by the creation of a bond between N(2) und C(4) in the excited sydnones (cf. Scheme 22). The irradiation of sydnones in dioxane solution in the presence of carboxylic acids yields N'-acylhydrazides in 50-70% (cf. Scheme 14), the formation of which can be explained by addition of the acids to the nitril-imines and rearrangement of the primarily formed anhydride monohydrazones (Scheme 15). By analogy, the formation of 1-benzoyl-2-(t-butyl)-4-phenyl-1,2-diazetidin-3-one (14; Scheme 4 ) during the photolysis of 3-(t-butyl)-4-phenylsydnone (12) in benzene solution (cf. [5b]) may also be explained: Sydnone 12 undergoes two different photoreactions leading by loss of carbon dioxide to the corresponding nitril-imine 5 (R = t-C4H9, R' = C6H5) and by loss of isobutylene to a 1,2,3-oxadiazolin-5-one of type 48 or 49 which isomerizes to yield diazophenylacetic acid (51; cf. Scheme 17). Reaction of both products (5 and 51) results in the formation of the N'-acylhydrazide 52 (Scheme 18) which may cyclize after loss of nitrogen to yield the diazeti- dinone 14 in a carbene type reaction. The triazoles 2 are formed photochemically from the corresponding 1,2-bisazo-ethylenes 64 (Scheme 26) which arise from a direct 'head-to-head' dimerization of the nitril-imines 5. This type of reaction seems to be common to all nitrilium betaines (Schema 30). The photolysis of the 2,4-disubstituted 1,3,4-oxadiazolines 76 does not lead to nitril-imines 5 (cf. [11]). On the contrary, loss of carbon monoxide induces the formation of azoketones 77 (cf. Scheme 31), which may be photo-reduced to yield hydrazides or may undergo cleavage of the N,acyl bond to form derivatives of 1,2-diketones (cf. Scheme 32)