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-2H-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-2H-indol-2-ones 9a and 9b , respectively (Scheme 3). The diazo compounds 5–8 reacted with 1,3-thiazole-5 (4H)-thiones 10 and thiobenzophenone ( 13 ) to give 6-oxa-1,9-dithia-3-azaspiro4.4]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 N₂ 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 .