Zur Reaktionsweise von Enaminen mit Cyclopropenonen V. Einsatz von β‐Carbonyl‐enaminen
Authors: Bilinski Vanda , Dreiding André S. 
Published: December 2004

Diphenylcyclopropenone (10) was heated with five different β‐carbonyl‐enamines, namely 4‐pyrrolidino‐pent‐3 E‐en‐2‐one (12), 4‐dimethylamino‐pent‐3E‐en‐2‐one (13), 4‐dimethylamino‐but‐E‐en‐2‐one (14), 3‐dimethylamino‐1‐phenyl‐prop‐E‐en‐1‐one (15) and ethyl 3‐pyrrolidino‐isocrotonate (16). The resulting reactions were more sluggish than those of 10 with ordinary enamines. The main reaction (between 10 and 69% yield) was in all cases a ‘C, N‐insertion’. The major products were: from 12: an inseparable mixture of 4‐methyl‐6‐oxo‐2,3‐diphenyl‐hepta‐2E, 4E‐dienoic acid pyrrolidide (17) and its 2Z, 4E‐stereomer (18); from 13: 4‐methyl‐6‐oxo‐2, 3‐diphenyl‐hepta‐2E‐dienoic acid dimethylamide (19) and its 2Z, 4E‐stereomer (20); from 14: 6‐oxo‐2, 3‐diphenyl‐hepta‐2E, 4E‐dienoic acid dimethylamide (21); from 15: 6‐oxo‐2,3,6‐triphenyl‐hexa‐2E, 4E‐dienoic acid dimethylamide (22); and from 16: 5‐ethoxycarbonyl‐4‐methyl‐2, 3‐diphenyl‐penta‐2E, 4E‐dienoic acid pyrrolidide (23) and its 2Z, 4E‐stereomer (24). The constitutions of 17 to 24 were derived mainly from spectral properties.

For these products the E‐configuration at the 4,5‐double bond was assigned on the assumption that the insertion of the side‐chain (cyclopropenone carbons) between the enamine carbon and nitrogen atoms occurred with retention of configuration, as had been concluded earlier. This was confirmed in the cases of 21 and 22 by the trans‐coupling between HC(4) and HC(5) in the 1HNMR. spectrum, the educts (14 and 15) having the E‐configuration. The configurational difference between the stereomeric products 17/18, 19/20 and 23/24 was, therefore attributed to the 2,3‐double bond. This was confirmed by aqueous acid treatment in the case of the pair 19/20 : The 2E‐configuration for 19 followed from its conversion to 4‐acetonyl‐4‐methyl‐ 2,3‐diphenyl‐isocrotonolactone (25) and the 2Z‐configuration of 20 by its conversion first to a mixture ol two diastereomers of (presumably) 1‐acetyl‐4‐dimethylaminocarbonyl‐2‐methyl‐3‐ phenyl‐l,4‐dihydronaphthalene (27a) and then, under more drastic conditions, to 6‐methyl‐11H‐benzo[a]fluorene (26).The structures of 25 and of 26 were derived from their spectral properties, and that of the 27a‐mixture was made probable by the plausibility of its intermediacy on the way to 26. A pathway for the conversion of 20 to 27a (scheme 3) and of the latter to 26 (scheme 4)is proposed.

In the case of the reaction of 10 with 12, two stereomeric basic by‐products were isolated (combined yield 150/,). Their structures as traw‐ and cis‐4‐acetonyl‐4,5‐diphenyl‐3‐pyrrolidinocyclopent‐2‐en‐ones (30 and 31) were deduced from their spectral properties and from those of their hydrochlorides 32 and 33. The enamino‐ketone function was found to be resistant to a number of reagents, among which were excess sodium borohydride, which converted 30 to the secondary alcohol 34, and excess methyllithium, which converted 31 to the tertiary alcohol 35.

A mechanism (called ‘rearrangement’) is proposed (scheme 5) for the formation of the enaminoketones (such as 30 and 31), which proceeds via the same ammonio‐enolate intermediate (36) which plays a role in the formation of the major products, the amides (such as 17 to 24). It is suggested (scheme 6) that the 3‐membered ring of the ammonio‐enolate 40 may open in three ways, one of which leads to the amides and another to the enamino‐ketones.