Ribosylation of the trimethylsilyl derivative of imidazole-2-thione using either stannic chloride or silver perchlorate as catalyst resulted in the formation of the acylated derivatives of 1-(.beta.-D-ribofuranosyl)imidazole-2-thione (3c) and 1,3-di-(.beta.-D-ribofuranosyl)imidazole-2-thione (4c) with the latter predominating (4c:3c .apprx. 2:1). The diribosylated nucleoside 4c was the N,N-disubstituted product rather than the N,S-disubstituted product by 1H NMR and 13C NMR spectroscopy. Employment of the iodine-catalyzed fusion procedure reversed the aforementioned product ratios and provided the monoriboside 3c in excellent yield. When the trimethylsilyl derivative of 2-methylthioimidazole was reacted with 2,3,5-tri-O-benzoyl-D-ribofuranosyl bromide in acetonitrile, the major product was 1,3-di-(2,3,5-tri-O-benzoyl-.beta.-D-ribofuranosyl)-imidazole-2-thione (4b). The formation of 4b in this reaction is thought to arise via the Hilbert-Johnson mechanism. Heterocycles containing the thioureylene group constitute the majority of known effective antithyroid agents. Methimazole (1-methylimidazole-2-thione, Tapazole), one of the drugs currently employed in the treatment of hyperthyroidism, possesses this characteristic structural feature.
