ZWITTERIONS AS INTERMEDIATES IN 1,3-DIPOLAR CYCLOADDITIONS OF ELECTROPHILIC AZIDES TO 2-ALKYLIDENETETRAHYDROIMIDAZOLES AND 2-ALKYLIDENEDIHYDROBENZIMIDAZOLES

Cyclic ketene N,N-acetals derived from imidazolidine (4a, c) and 2,3-d ihydrobenzimidazole (6a-c) add methanesulphonyl azide (2a) or picryl a zide (2f) to afford the zwitterions 5 and 7, respectively. The structu re of 7d is elucidated by X-ray crystallography. Reversibility of form ation and thermal stability of the N-sulphonyl zwitterions depend on t he substitution pattern at the carbon atom to which the triazenide moi ety is attached: In the case of a pair of geminal methyl groups (5a, 7 a) formation is irreversible and decomposition by cyclisation and subs equent reactions occurs above -20 degrees C, while in presence of a si ngle alkyl group (7c, d) these processes require heating to 80 degrees C and are accompanied by partial reversion to 2a and ketene N,N-aceta ls (6b, c). Cyclisation of the zwitterions yields intermediate spirocy clic [3 + 2] cycloadducts, which may undergo [3 + 2] cycloreversion in to N-sulphonylimine 13 and diazo compound 14 or extrude molecular nitr ogen to furnish ring-expanded 2-(sulphonylinino)piperazine derivatives (9, 11).