| Abstract: | Rearrangement of 5-Amino-5-X-pentadienals to 2-Aminopyrylium Salts 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 HF or AcOH with `push-pull' enynes 5 is considerably slower so that an excess of HF or AcOH 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-2,4-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 1, top). |
