Methyl 2-Cyano-5-(hydroxymethyl)-6,7-dihydroxy-5. trpyrrolo[l,2-8 ]imidazole-3-carboximidate (29). To a solution of 25 (1 g, 1.8 mmol) in 12 mL of anhydrous MeOH was added NaOH (0.12 g, 2 mmol), and the mixture was stirred at 25 °C for 2 h under nitrogen. The solution was neutralized with Dowex 50 resin (H+) and filtered, and the filtrate was concentrated under reduced pressure. The gummy residue obtained was triturated with dry ether, filtered, and dried to give 0.47 g of product that was homogenous on TLC (CHCl3/MeOH, 4:1). Crystallization from MeOH gave 29 (0.3 g, 64%) as a white powder: mp 182-85

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The reaction of 2-0-acetylated and 2-0-benzoylated glycosides 3a,b/4a,b with silylated p-acetylguanine 7 selectively gave "'-guanine nucleosides 8a,b/Sa,b under kinetically controlled conditions (SnC14/CH3CN, room temperature), whereas 2-0-benzoylated glycosides 3b/4b selectively gave the isomeric Ns-guanine nucleosides lob/ 1 lb under thermodynamically controlled conditions (TMSOTf/(CH,Cl),, reflux). Unambiguous assignment of nucleoside structure was accomplished after hydrolysis (NH3/MeOH) of the initial products to the known nucleosides &, SC, lOc, and l l c followed by 'H and 13C NMR spectral analysis. The described procedures provide the best method to date for the selective synthesis of either N'or NB-guanine nucleosides from a common substrate. As part of a program directed toward the synthesis of the nucleoside antibiotics amipurimycin (1) and miharamycin (2),l we required a method for the synthesis ofNg-pyranosyl-2-aminopurines from their corresponding glycosidic precursors. In this context a coupling procedure that involved the use of a readily available silylated guanine derivative in conjunction with a Lewis acid catalyst seemed most appropriate even though the regioselectivity of this reaction (ie. Wvs Ng-glycosylation) is not generally high with this base.24 Once the desired Ng-nucleoside is (1) Amipurimycin: Goto, T.; Toya, Y.; Ohgi, T.; Kondo, T. Tetrahedron Lett. 1982, 1271. Miharamycin: Seto, H.; Koyama, M.; Ogino, H.; Teuruoka, T.; Inouye, S.; Otake, N. Ibid. 1983, 1805. ( 2 ) For a review of developments in the area of nucleoside synthesis, see.: Dekker, C. A.; Goodman, L. In The Carbohydrates, 2nd Ed.; Pipman, W., Horton, D., Herp, A., Eds.; Academic: New York, 1970. And more recently: Vorbrtiggen, H. NATO Adv. Study Zmt. Ser., Ser. A 1979,26, 35. 0022-3263/88/1953-1294$01.50/0 obtained, however, plodification of the8 guanine moiety to give the 2-aminopurine could then follow well, established procedure^.^ Herein we report on work which has cul-(3) (a) Lichtenthaler, F. W.; Voss, P.; Heerd, A. Tetrahedron Lett. 1974,2141. (b) Vorbniggen, H.; Krolikiewicz, K.; Bennua, B. Chem. Ber. 1981,114,1234. (c) Morr, M. Liebigs Ann. Chem. 1982,666. The NMR data reported in this paper seems to indicate that a (2.5:l) mixture of Ng:W regioisomers waa obtained rather than the reported anomeric mixture of Ng-nucleosides-see Table I. (d) For a related "transnucleosidation", see: Azuma, T.; Isono, K. Chem. Pharm. Bull. 1977,25, 3347. (4) Previous approaches to nucleosides (and nucleotides) that also employed guanine derivatives directly for the coupling include: (a) Imai, K.; Nohara, A.; Honjo, M. Chem. Pharm. Bull. 1966,14, 1377. (b) Nomura, H.; Suhara, I.; Uno, N. Ibid, 1967, 15, 1258. (c) Furukawa, Y.; Honjo, M. Zbid. 1968, 16, 1076. (d) Furukawa, Y.; Imai, K.; Honjo, M. Tetrahedron Lett. 1968,4655. (e) Suzaki, S.; Yamazaki, A.; Kamimura, A.; Mitaugi, K.; Kumashiro, 1. Chem.

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