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The cycloaddition reactions of three cyclic imidate esters, 2-ethoxypyrrolin-5-one (6), 2-ethoxyisoindol-3-one (7) and 2-ethoxy-1H-indol-3-one (8) with various 1,3-dipoles were investigated. Dipolarophile 6 added only to nitrile oxides; 8 added to nitrile oxides and to nitrilimines; 7 failed to undergo cycloaddition reactions with either nitrile oxides or nitrilimines.

2-Ethoxy- and 2-(ethylthio)-1-azetines readily undergo 1,3-dipolar cycloadditions with nitrile oxides and nitrile ylides to give stable 4,5-bicyclic cycloadducts. With nitrilimines, however, the expected 1,3-dipolar cycloadducts and/or unexpected ring-opened products, namely 1,2,4-triazoles, are formed depending on the nature of the nitrilimine N-substituent. In contrast, the azetines fail to react with nitrile sulphides, azomethine ylides, nitrones, aryl azides, and various dienes. X-ray crystallographic data on the nitrile oxide, nitrile ylide, and nitrilimine cycloadducts, and on the 1,2,4-triazoles, are presented. Also a mechanistic rationale for triazole formation is offered.

4,5-Dihydro-2-metyloxazole 1a and 4,5-dihydro-2,4,4-trimethyloxazole 1b, which are examples of cyclic imidate esters, undergo 1,3-dipolar cycloaddition reactions with benzonitrile N-oxide, to give the appropriate 5,6-dihydro-7a-methyl-3-phenyl-7aH-oxazolo[3,2-d]-1,2,4-oxadiazole (2a and 2b respectively). 4,5-Dihydro-2-methylthiazole 1c gives the thia analogue 2c. With alkanoyl and aroyl cyanide N-oxides (RCOCNO), 4,5-dihydro-2-methyloxazole 1a and 4,5-dihydro-2-methylthiazole 1c give the open-chain compounds, RCOXCH2CH2N(CN)COMe (X = O and S respectively).