Improved synthesis of mercapto C-nucleoside possessing p-phenyl thiol as base using a lithiated coupling reaction

We have developed a new route for synthesizing mercapto C-nucleoside possessing a phenyl thiol group, using organometallic reagents. Duplexes incorporating redox-active nucleobase analogues display a high melting temperature under oxidation condition. Originally, we had anticipated the production of mercapto C-nucleoside using a Friedel-Crafts coupling reaction via bis(toluoyl) protected ribose and tert-butyl phenyl sulfide in the presence of Lewis acid. However, an undesired coupling compound was formed by cleavage of the S-tert-butyl group of S-tert-butyl phenyl sulfide by Lewis acids (BF₃ Et₂O, SnCl₄). The highly stereoselective synthesis of mercapto C-nucleoside was, however, achieved by the addition of p-(tert-butyl)thiophenyllithium to a disiloxane-protected 2-deoxyribonolactone. This route showed moderately good yield at all steps. The tert-butyl moiety coupled to the sulfur atom at the phenyl group was converted to a 2-nitrophenylsulfenyl (Nps) group, and the Nps group was easily cleaved by ethanethiol to afford the desired compound and its disulfide dimer.