Diazo‐Transfer Reaction with Diphenyl Phosphorazidate

Diphenyl phosphorazidate (DPPA) was used as the azide source in a one‐pot synthesis of 2,2‐disubstituted 3‐amino‐2 H ‐azirines 1 ( Scheme 1 ). The reaction with lithium enolates of amides of type 2 , bearing two substituents at C(2), proceeded smoothly in THF at 0°; keteniminium azides C and azidoenamines D are likely intermediates. Under analogous reaction conditions, DPPA and amides of type 3 with only one substituent at C(2) gave 2‐diazoamides 5 in fair‐to‐good yield ( Scheme 2 ). The corresponding 2‐diazo derivatives 6–8 were formed in low yield by treatment of the lithium enolates of N , N ‐dimethyl‐2‐phenylacetamide, methyl 2‐phenylacetate, and benzyl phenyl ketone, respectively, with DPPA. Thermolysis of 2‐diazo‐ N ‐methyl‐ N ‐phenylcarboxamides 5a and 5b yielded 3‐substituted 1,3‐dihydro‐ N ‐methyl‐2 H ‐indol‐2‐ones 9a and 9b , respectively ( Scheme 3 ). The diazo compounds 5–8 reacted with 1,3‐thiazole‐5 (4 H )‐thiones 10 and thiobenzophenone ( 13 ) to give 6‐oxa‐1,9‐dithia‐3‐azaspiro[4.4]nona‐2,7‐dienes 11 ( Scheme 4 ) and thiirane‐2‐carboxylic acid derivatives 14 ( Scheme 5 ), respectively. In analogy to previously described reactions, a mechanism via 1,3‐dipolar cycloaddition, leading to 2,5‐dihydro‐1,3,4‐thiadiazoles, and elimination of N 2 to give the ‘thiocarbonyl ylides’ of type H or K is proposed. These dipolar intermediates with a conjugated CO group then undergo either a 1,5‐dipolar electrocyclization to give spirohetrocycles 11 or a 1,3‐dipolar electrocyclization to thiiranes 14 .