Enzymatic Synthesis of New Pyridine Nucleosides. Clitidine and Its Amide Derivative

Two β-NAD-analogs were prepared utilizing a base-exchange reaction catalyzed by pig-brain NADase. These analogs were proved to contain a methyl 1,4-dihydro-4-iminonicotinate and a 1,4-dihydro-4-iminonicotinamide moiety. By successive hydrolytic degradations of these analogs, 1,4-dihydro-4-iminopyridine β-ribosides with 3-carboxyl and 3-carbamoyl groups were prepared in good yields. The former compound was identical with clitidine, a toxic pyridine nucleoside recently isolated from a toadstool. The present synthesis confirmed the 1,4-dihydro-4-imino β-riboside structure of clitidine. Preliminary biological tests showed that the 3-carbamoyl compound was more toxic than clitidine, suggesting that the amide derivative of clitidine is an essential toxic substance in the toadstool.