2-Alkylidene-1, 3, 4-thiadiazoline 11a gave thioketone 16 (X= S) and acetylene derivative 17 on thermolysis via thiocarbonyl ylide, quantitatively. Pyrolysis of selenium analogue gave similar results. Allyl substituted 2-alkylidene-1, 3, 4-thiadiazolines 11a and 11b resulted in the novel formation of thiiranimine derivatives 19a and 19b and bicyclo [3.1. 0] thiahexane derivatives 20a and 20bvia azathioallyl intermediates. Thiiranimine derivatives 19a and ...