Generalization of the Aminopentadienal Rearrangement
Contrary to the rearrangement of 3-amino-3-X-prop-2-enals 2 (R H), which easily give 3-X-prop-2-enamides 3 at low temperature, the postulated rearrangement (Scheme 1) of the vinylogous 5-amino-5-X-penta-2,4dienals 6 (R H) normally stops at the level of 2-aminopyrylium salts 7. The main reason is that the charge in salts of type 7 is highly delocalized, leading to low-energy species, which make addition of weak nucleophiles difficult. In this paper, two concepts for increasing the chances of the aminopentadienal rearrangement 6 33 8 are presented and substantiated by typical experiments. 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, after 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) easily rearrange at low temperature to give 5-chloropenta-2,4-diene-1-carboxylates 18a and 18b, respectively, which are now obviously lower in energy than the corresponding pyrylium-salt intermediates 16 (Scheme 4).