Previously unknown 1-oxa-2,4-diazahexatrienes 1a,b and c, substituted with alkoxy groups, have been prepared by carboxylation of N-alkylideneisoureas 2 with chloroformates 5. Xray analyses of compounds 1b and c show nonplanar, openchain structures. In contrast, thiocarboxylation of 2 with thiochloroformate 6 leads to the spiro compounds 7-ring, as verified by spectroscopic data and X-ray analysis. A tetradonor-substituted 1-oxa-2,4-diazahexatriene 1d was obtained from the reaction of the 1-oxa-3-aza-butadiene 10 with the triply donor-substituted carbenium ion 9. Attempts at further chain elongation of 1a by an alkylation/condensa-

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