Furopyridines. XX. Wittig-Horner reaction of a phosphonate of Reissert analogues of furo[3,2-c]-, -[2,3-c]- and [3,2-b]pyridines

Article abstract of DOI:10.1002/jhet.5570340121

Furo[3,2-c]- (1a), -[2,3-c]- (1b) and -[3,2-b]pyridine (1c) were reacted with isopropyl chloroformate and trimethyl phosphite to give dimethyl 5-isopropoxycarbonyl-4,5-dihydrofuro[3,2-c]pyridine-4-phosphonate (2a), dimethyl 6-isopropoxycarbonyl-6,7-dihydrofuro[2,3-c]pyridine-7-phosphonate (2b) and dimethyl disopropoxycarbonyl-4,7-dihydrofuro[3,2-b]pyridine-7-phosphonate (2c) as unstable syrups. Reaction of 2b and 2c with n-butyllithium and then with benzaldehyde, p-methoxybenzaldehyde, p-cyanobenzaldehyde or propionaldehyde afforded the normal Wittig reaction products 5b-H, 5b-OMe, 5b-CN, 5b-Et, 5c-H, 5c-H, 5c-OMe and 5c-CN, except for 2b with propionaldehyde. While, the same reactions of compound 2a and the reaction of 2b with propionaldehyde afforded the unexpected products, 5-isopropoxycarbonylfuro[3,2-c]pyridinio-4-aryl-(or ethyl)methoxides 3a-H, 3a-OMe, 3a-CN and 3a-Et, 4-(1'-aryl(or ethyl)-1'-hydroxymethyl)furo[3,2-c]pyridines 4a-H, 4a-OMe, 4a-CN and 4a-Et accompanying formation of the normal products. Treatment of the normal Wittig reaction products with lithium diisopropylamide and then with acetone gave the derivatives alkylated at the 2- or the benzylic positions.