(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 19b were