A Novel Ring Enlargement of 2H-Azirine-3-methyl(phenyl)amines via Amidinium-Intermediates: A New Synthetic Approach to 2,3-Dihydro-1,3,3-trimethylindol-2-one

2,2,N-Trimethyl-N-phenyl-2H-azirin-3-amine (1a) was prepared by successive treatment of 2,N-dimethyl-N-phenylpropanamide (18) with phosgene, triethylamine, and sodium azide. Reaction of 1a in THF solution with boron trifluoride gave 2-amino-1,3,3-trimethyl-3H-indolium tetrafluoroborate (19) in high yield.The latter reacted with acetic anhydride in pyridine to give a mixture of N-(2,3-dihydro-1,3,3-trimethylindol-2-yliden)acetamide (22) and 2,3-di- hydro-1,3,3-trimethylindol-2-one (21). On hydrolysis with aqueous HCl, 22 was converted to 21.The molecular structures of 19 and 22 were established by X-ray crystal structure determination