Generalization of the aminopentadienal rearrangement

Contrary to the rearrangement of 3-amino-3-X-prop-2-enals 2 (R = H),which e asily give 3-X-prop-2-enamides 3 at low temperature, the postulated rearran gement (Scheme 1) of the vinylogous 5-amino-5-X-penta-2,4-dienals 6(R = H)n ormally stops at the level of 2-aminopyrylium salts 7. The main reason is t hat the charge in salts of type 7 is highly delocalized, leading to low-ene rgy species, which make addition of weak nucleophiles difficult. In this pa per, two concepts for increasing the chances of the 'aminopentadienal rearr angement' 6 --> -->, 8 are presented and substantiated by typical experimen ts. On one side, the easily available 2-aminopyrylium chlorides 7 (X = Cl) are reacted with a twofold excess of secondary amines (Scheme 2) to give 5- (dialkylamino)penta-2,4-dienamides of type 9 and 10. On the other hand, aft er replacing the amino groups of 6 by PhO and EtO groups the corresponding 5-chloro-5-phenoxy-(13b) and 5-chloro-5-ethoxypenta-2,4-dienals (13a) easil y rearrange at low temperature to give 5-chlorapenta-2,4-diene-1-carboxylat es 18a and 18b, respectively, which are now obviously lower in energy than the corresponding pyrylium-salt intermediates 16 (Scheme 4).