Strong nucleophiles are needed for inducing ring opening 3→4 since the pyrylium salt intermediates 3, formed from 5-X-substituted 5-aminopenta-2,4-dienals 2 upon treatment with acid, are quite unreactive due to "aromatic" stabilization. However, this allows an easy access to a variety of 2-aminopyrylium salts 3 from "push-pull" enynes 1. X=OAc, F, Cl, Br, I, OPh.
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 level 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 addition 4-5 of weak nucleophiles difficult. In this paper, the first examples of the so-called 'aminopentadienal' rearrangement art: reported. Ring-opening 4 - 6 is facilitated by nucleophilic counter ions like X = PhO (see Scheme 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-(pyrrol-1-yl)penta-2,4-dienal (2g; X=PhO) is easily transformed into 5H-pyrrolo[1,2-a]azepin-5-one (9) (Scheme 5).
Flitsch, Wilhelm; Gesing, Ernst Rudolf F., Chemische Berichte, 1983, vol. 116, # 3, p. 1174 - 1182
作者:Flitsch, Wilhelm、Gesing, Ernst Rudolf F.
DOI:——
日期:——
FLITSCH W.; GESING E. R., TETRAHEDRON LETT., 1979, NO 47, 4529-4532