Synthesis of (4-nitro-1 H -indol-6-yl)- and (4-amino-1 H -indol-6-yl)phosphonic acid derivatives

A convenient methodology for the synthesis of previously unreported (1H-indol-6-yl)phosphonic acid derivatives has been described. Reaction of (4-methyl-3,5-dinitrophenyl)phosphonates with dimethylformamide dimethyl acetal leads to the respective enamines that can be readily converted into indoles by the Batcho–Leimgruber synthetic protocol. Proper choice of reducing agent for the reductive cyclization of intermediate enamines allows to selectively obtain (4-nitro-1H-indol-6-yl)- and (4-amino-1H-indol-6-yl)phosphonates.