On heating the sydnones l a -h with isopropylidenecyclobutenone (2) in refluxing toluene dihydrodiazepin-5-ones (3a -h) are obtained in the case of N-phenyl-and dialkylsydnones. The Nalkyl-C-hydrogensydnones, however, yield two regioisomeric products, the dihydrodiazepin-5-ones 3d,e as well as the corresponding diazepin-4-ones 4d,e. The frontier molecular orbital model, as applied to sydnones, is critically examined. Extension of the simple model to the calculation of selected encounter complexes is presented. In this way not only the preferred formation of diazepin-5-ones (3) is correctly predicted but the ratio of 3:4 is also satisfactorily reproduced. X-ray diffraction has been used to detcrminc the structure of 4d which is between a boat and a chair and shows a very short NN single bond. ly3-Dipolar cycloadditions with cyclobutenes provide access to seven-membered heterocycles. With diazoalkanes ' "), mi inch none^^^^')^ and nitrile ylides ' ), resp., dihydrodiazepines and dihydroazepines can be synthesized. Whereas the synthesis of six-membered heterocycles from mesoionic compounds has been in~estigated""~ only one example of an addition of a sydnone ring to a derivative of cyclobutene has so far been described in the reaction of N-phenylsydnone with cyclooctatetraene Padwa and Lim") isolated a by-product whose formation can be interpreted as an addition to the valence tautomer bicyclooctatriene. At present Regitz et al. are synthesizing diazepines by adding sydnones to tri-tert-butyl(tert-butoxycar~nyl)cyclobutadiene8b'. We now report on the cycloaddition of the sydnones la-h to isopropylidenecyclobutenone (2) (Scheme 1).
Scheme
210ChemInform Abstract The N-phenyl and dialkyl sydnones (I) react with the cyclobutenone (II) on heating in toluene to give the dihydrodiazepin-5-ones (III) whereas the analogues (V) together with their isomeric 4-ones (VI) are obtained from the reaction of the N-alkyl C-hydrogen sydnones (IV). The frontier MO model, as applied to sydnones, is critically examined. Extension of the simple model to the calculation of selected encounter complexes is presented. In this way, not only the preferred formation of diazepin-5-ones (III) is correctly predicted but the ratio of (V):(VI) is also satisfactorily reproduced. -The structure of (VIa) is determined by X-ray analysis (P21/c; Z=4).
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