Synthesis of methylene-expanded oxetanocin isonucleosides in both enantiomeric forms

We report a novel route to isonucleosides of the 'methylene-expanded' oxetanocin class, in both the D-and L-enantiomeric forms, e.g., compounds L-(+)-2a, D-(-)-2a, and L-(-)-2b, beginning with the simple, known mono-p-bromobenzyl ether 3 of the very inexpensive 2-butene-1,4-diol. Sharpless asymmetric epoxidation of 3 gave either (-) or (+)-4 depending on the chirality of the tartrate used. The p-bromobenzyl ether was used since the epoxide product is crystalline and can be recrystallized to high optical purity. Opening of the epoxide with vinylmagnesium bromide gave the 1,3-diol 5, the primary alcohol of which was protected as the silyl ether 6. Treatment of 6 with iodonium bis(sym-collidine) perchlorate afforded the desired 5-(iodomethyl)tetrahydrofuran-3-ol 8 with loss of the bromobenzyl cation in the key step in the synthetic scheme. This iodide 8 was then converted into the bis(silyloxy)-protected alcohol 15 by acetylation to give the acetate 10, displacement of iodide with acetate, hydrolysis, and selective protection of the primary alcohols. The alcohol 6 could also be converted into 15 via initial acetylation and then iodocyclization to give 10. The diol 5 could also be converted into 15 by a similar route involving bis-acetylation and iodocyclization followed by functional group transformations. The tosylate of 15 was displaced with the anion of adenine or thymidine to give, alter final desilylation, the desired isonucleosides-the D-adenosine analogue (-)-2a and the L-adenosine and thymidine analogues (+)-2a and(-)-2b. All of the stereochemistry of the final products is derived from the first step of the synthesis, namely, the Sharpless asymmetric epoxidation of 3. The biological activity of the new compounds L-(+)-2a and L-(-)-2b against HIV was determined in the anti-HIV drug-testing system of the National Cancer Institute. The adenosine analogue L-(+)-2a was inactive in this screen, while the thymidine analogue L-(-)-2b showed moderate anti-HIV activity (IC50 > 2 x 10(-4) M, EC50 = 8 x 10(-7) M, TI50 > 250).