Syntheses of lin-benzoinosine, lin-benzoxanthosine, and lin-benzoguanosine (substituted 3-ribosylimidazo- [4,5-g]quinazolin-8-ones) are reported. Ribosidation of the mercuric salt of 6-(ethylthio)imidazo[4,5-g]quinazolin-&one gave a common intermediate in which the ethylthio group was displaced by ammonia to give lin-benzoguanosine or was reductively removed to give lin-benzoinosine. lin-Benzoinosine was oxidized by xanthine oxidase to give lin-benzoxanthosine. Investigations uti:lizing 3-@-~-ribofuranosyl-8-aminoimidazo[4,5-g]quinazoline (lb)' (lin-benzoadenosine)2 and a, R:H b, R : A-Cl-ribofuranose c, R : A-D-r i bof uranosyl 5'-phosphateits phosphate derivatives3s4 have shown that they are effective coenzymes for a variety of enzyme reaction^.^ The fluorescence proper tie^,^-^ the 2.4-A lateral extension of the purine nucleus, and the unaltered loci of interaction make this system uniquely analogous to adenosine and its phosphate derivatives. We have previously reported the synthesis of lin-benn,oguanine (2a)9 and lin-benzohypoxanthine (3a),]-O the latter a precursor of 1. We report here" the preparation of lin-benzoguanosine (2b), linbenzoinosine (3b), and lin-benzoxanthosine (4b) by a divergent synthetic pathway. This pathway and an alternative one, also included, make available a wide variety of ribosylpurine analogues for discerning the role of purine (1) Leonard, N. J.; Sprecker, M. A,; Morrice, A. G. J . Am. Chem. SOC. 1976, 98, 3987.(2) The prefix lin refers to the linear disposition of the three rings in 1-4; "benzo" in the trivial name refers to the additional ring which, only when central, contains no nitrogen. This terminology is adaptable to derivatives similarly related to all purines. The other parts of the names followed accepted IUPAC--IUB nomenclature.(3) Leonard, N. J.; Scopes, D. I. C.; VanDerlijn, P.; Barrio, J. R. Biochemistry 1978, 17, 36'77. (4) Scopes, D. I. C.; Barrio, J. R.; Leonard, N. J. Science 1977,195,296. (5) VanDerLijn, P.; Barrio, J. R.; Leonard, N. J. Proc. Natl. Acad. Sci. (6) Schmidt, M. d.; Truex, L. T.; Leonard, N. J.; Scopes, D. I. C.; Barrio, (7) Kauffman, R. F.; Lardy, H. A,; Barrio, J. R.; Barrio, M. d. C. G.; (8) Thomas, R. W.; Leonard, N.