Reaktion von 2‐Diazopropan mit 1,3‐Thiazol‐5(4 H )‐thionen

Reaction of 2‐Diazopropane and 1,3‐Thiazole‐5(4 H )‐thiones At 0°, 1,3‐thiazoles 1 and 2‐diazopropane in pentane or MeOH undergo a smooth 1,3‐dipolar cycloaddition to give spirocyclic 2,4‐dihydro‐1,3,4‐thiadiazoles 6 ( Scheme 3 ). Elimination of N 2 occurs already at 20°, yielding spirocyclic thüranes 7 . At room temperature, the latter decompose slowly to give 4,5‐dihydro‐5‐isopropylidene‐1,3‐thiazoles 8 . The desulfurization 7 → 8 can be accelerated by heating or by addition of trimethyl phosphite. The intermediate of the N 2 elimination of 6 is a thiocarbonyl ylide D which can be trapped with the reactive dipolarophiles thiobenzophenone and fumarodinitrile ( Scheme 4 ). In the absence of these trapping reagents, D undergoes ring closure to thüranes 7 . The cycloaddition of D and a second molecule of 1 , the so‐called Schönberg reaction, is not observed.