Mesoionic compounds. Part 10. A study of the reactions of 3-(3-pyridyl)sydnone with some nucleophilic reagents

Abstract

Nucleophilic addition of organo-magnesium and -lithium reagents to 3-(3-pyridyl)sydnone (1) afforded 4-substituted-3-(sydnon-3-yl)-1,4-dihydropyridines (7), accompanied by lesser amounts of the 1,2-dihydropyridine isomers (8). The mixtures were separated by t.l.c. and the isomers examined by u.v. spectroscopy. The structures of the pure dihydropyridines (7, R = Et, Buⁿ, or PhCH₂) were further established by ¹H n.m.r. spectroscopy. Sodium borohydride in aprotic or protic solvents reduced (1) to 3-(sydnon-3-yl)-1,4-dihydropyridine (10) or 3-(sydnon-3-yl)-1,4,5,6-tetrahydropyridine (11) respectively. 1-Alkyl-3-(sydnon-3-yl)pyridinium iodides (12a and 12b) with potassium cyanide gave the respective 1-alkyl-4-cyano-3-(sydnon-3-yl)-1,4-dihydropyridines (13a) and (13b). Methylmagnesium bromide favoured formation of [3-(3-pyridyl)sydnon-4-yl]magnesium bromide (3a) over dihydropyridine adducts (4a) and (5a) and was employed in the preparation of alcohols (14)–(23) which were dehydrated with trifluoroacetic acid to their respective (3-pyridyl)sydnon-3-yl-substituted alkenes (27)–(31).