1,2-Fused pyridinium salts, synthesis

Quinolizinium and other fused pyridinium salts are formed when a-methylheterocycles react with 2,4,6-triphenylpyrylium, which thus behaves as a C3-synthon <96MC99>. Pyrylium salts also feature in a stereocontroUed route to conjugated dienynes which has led to a synthesis of Carduusyne A, a marine metabolic fatty acid <96TL1913> and in the formation of pyridinium containing crown ethers <96LA9S9>. 

A surprising synthesis of 1,2-fused pyridinium salts (type 230) is exemplified by the formation of derivative 252 from dihydrofuran 251 (83CL21). Amide 253 undergoes, in analogy with homoberbine syntheses, twofold cyclization to achieve enamine 254 (03WO84963). 

The electronic structure and degree of aromaticity of 1,3-azaphospholes has received attention from the theoreticians. An alternative synthesis of the phospha-indolizine system (201), from the reaction of pyridinium salts with phosphorus trichloride, has undergone further development " and has also been applied in the synthesis of fused 1,4,2-diazaphospholes, e.g., (234) ", 2 (235). The 1,2,4,3-triazaphosphole system (236) has been isolated as an... 

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