5 cloaddition of 1 to maleic anhydride to form the first example of 2-i~ othiazoline.~~,~~ The purpose of this research was to determine the regiospecificity of the cycloaddition reaction between 1 and unsymmetrical acetylenes and to understand this regiospecificity and the stability of 1 by the use of CND0/2 molecular orbital calculations.
