Synthesis of methylene-expanded 2′,3′-dideoxyribonucleosides

A method for the preparation of methylene-expanded 2',3'-dideoxyribonucleosides is reported. The very inexpensive starting material levoglucosenone 8 was converted into the known mixture of alcohols 12ab which were converted into the required silyl ether alcohol 26 in six steps via either of two routes. The first involved a one-step acetylation and opening of the anhydro sugar bridge to give the triacetates 20ab which were reduced with triethylsilane and silyl triflate to afford the diacetates 21ab, both of which gave 26 after further functional group conversions. The second route entailed a simple acetylation of 12ab followed by reduction with triethylsilane and silyl triflate to give the monoacetates 19ab, both converted via straightforward chemistry into 26. Mesylation of the alcohol of 26 furnished the mesylate 27. Alkylation of adenine with the mesylate 27 afforded the silyl ether 28 which was deprotected to give the desired modified dideoxy nucleoside 7a. Alkylation of 2,6-diaminopurine 38 with the mesylate gave the protected diaminopurine nucleoside 39. Upon acetylation, it produced a mixture of di- and monoacetates 40-41, the latter of which was transformed into the desired guanosine analogue 7e. Thus, two new nucleoside analogues 7ae were prepared from levoglucosenone 8.