; Thermal reaction of alkynyl propargyl sulfides 1 in the presence of n-BuN 3 followed by moist silica gel treatment afforded 4-methylenecyclobutenones 4 in moderate yields. Photogeneration of allenylketenes 5 from 4 in the presence of amine or methanol gave 3,4-pentadienamides 6 or methyl 3,4-pentadienates 7, respectively. Similar reaction in the presence of aldimines afforded unsaturated δ-lactams
Alkynyl propargyl sulfides underwent [3,3] sigmatropic rearrangement followed by ring closure to afford isolable 4-methylenecyclobutenethiones, and an intermediary allenylthioketene 2 was isolated by heating t-butyldimethylsilylethynyl propargyl sulfide.