Synthesis of 1,2-Dihydropyridines, 2,3-Dihydro-4(1H)-pyridinone, and 1,2,3,4-Tetrahydropyridinesvia N-AcylN,O-Hemiacetal Formation

New procedures are described for the synthesis of α,β-ethylenic and acetylenic aldehydes from 2-butene- and 2-butyne-1,4-diol, respectively (see Scheme 1). These are applied to the preparation of a particular δ-acetylamino-α,β-ethylenic aldehyde ((E)-5) as well as of its acetylenic analogue 15. On heating in the presence of a silyl enol ether, the former undergoes a complete dehydrative cyclization affording the N-acetyl-1,2-dihydropyridine 19. The addition of HCl to aldehyde (E)-5 results in the production of the 4-chloro-1,2,3,4-tetrahydropyridine 22 which is hydrolyzed to the corresponding alcohol 23 on silica gel. Similarly, the addition of HCl or HBr to the δ-acetyl-amino-α,β-acetylenic aldehyde 15 leads to the previously unknown 4-halo-1,2-dihydropyridines 26; these are easily hydrolyzed to the 2,3-dihydro-4(1H)-pyridinone 27. The ring-forming process involves a N-acyl N,O-hemiacetal as intermediate which is eventually dehydrated.