Furopyridines. XX. Wittig‐horner reaction of a phosphonate of reissert analogues of furo[3,2‐c]‐, ‐[2,3‐c]‐ and ‐[3,2‐b]pyridines

Abstract

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 4‐isopropoxycarbonyl‐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‐cyanobenzalde‐hyde 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‐isopropoxycar‐bonylfuro[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.