Umlagerung von 5-Amino-5-X-pentadienalen zu 2-Aminopyrylium-Salzen
摘要:
Various 2-aminopyrylium salts 7 (X = Cl, Br, I) are available in a simple one-pot procedure by reacting 'push-pull' enynes 5 with equivalent amounts of HCl, HBr, or HI. On the other hand, reaction of NF or AcOH with 'push-pull' enynes 5 is considerably slower so that an excess of HF or AcOH I is needed for the reaction to 7 (X = F, AcO). The 2-aminopyrylium salts 7 are the key intermediates in the postulated rearrangement of 5-amino-5-halogeno-pentadienals 6 to 5-halogenopenta-2,3-dienamides 8 (Scheme 1, bottom), which is vinylogous to the well-known rearrangement of 3-amino-3-X-propenals 2 to 3-X-propenamides 3 (Scheme I, top).
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.