The aminopentadienal rearrangement

Contrary to the rearrangement of 3-amino-3-X-propenals, which easily gives 3-X-propenamides at Io cv temperature, the postulated rearrangement (Scheme I) of the vinylogous 5-amino-5-X-pentadienals 2 normally stops at the leve l of (2H-pyran-2-ylidene)ammonium salts 4. The main reason is that salts of type 4 are highly delocalized low-energy. charged species which makes addi tion 4-5 of weak nucleophiles difficult. In this paper, the first examples of the so-called 'aminopentadienal' rearrangement art: reported. Ring-openi ng 4 - 6 is facilitated by nucleophilic counter ions like X = PhO (see Sche me 4) or by adding an excess of 'nucleophilic auxiliaries' such as Et3N or EtOH (see Scheme). In a quite interesting sequence of steps, 5-phenoxy-5-(p yrrol-1-yl)penta-2,4-dienal (2g; X=PhO) is easily transformed into 5H-pyrro lo[1,2-a]azepin-5-one (9) (Scheme 5).