Pyrrolopyrimidine nucleosides II. the total synthesis of 7‐β‐D‐ribofuranosylpyrrolo[2,3‐d]pyrimidines related to toyocamycin

The first synthesis of a 7‐β‐D‐ribofuranosylpyrrolo[2,3‐d]pyrimidine by direct ribosidation of a preformed pyrrolo[2,3‐d]pyrimidine has now been accomplished via the fusion procedure. Subsequent functional group transformations furnished the 6‐methyl‐thio derivative of the nucleoside antibiotic toyocamycin. Preparation of the 1‐, 3‐ and 7‐methyl isomers of 4‐amino‐5‐cyano‐6‐methylthiopyrrolo[2,3‐d]pyrimidine was accomplished and has provided an unequivocal assignment for the actual site of ribosidation by a comparison of ultraviolet absorption spectra. Factors utilized for the assignment of anomeric configuration are discussed.