Nucleosides, preparation from glycals synthesis

The observation that, under acidic conditions, 2,3-dihydro-4if-pyran will substitute at position 9 of purine derivatives led to the suggestion that nucleosides might be prepared directly from glycals. Although the conditions employed during synthesis of the tetrahydropyranyl derivatives were unsuitable for interaction between 6-chloropurine and 3,4-di-O-acetyl-D-arabinal, when these two compoimds were fused together in the presence of p-toluenesulfonic acid, a mixture of 6-chloro-9-(3,4-di-0-acetyl-2-deoxy-a- and -/3-D-er2/iAro-pentopyranosyl)purines resulted, from which the known a-D anomer was isolated in pure form. Alternatively, an unsaturated nucleoside was obtained from tri-O-acetyl-D-glucal by this method (see p. 91). 

There has been a full account of the synthesis of the 2 -stannylated alkene 135 (X = SnBua) by base-induced stannyl migration from C-6 (see Vol. 32, p. 275), and the application of this compound to the preparation of the alkenyl halides 135 (X = Cl, Br, I), and products with carbon substituents at C-2 through Stille couplings. Reaction of di-O-acetyl-L-rhamnal with silylated thjmiine gave the 2 -enopyranosyl nucleoside by allylic rearrangement, as a mixture of anomers. A paper discussing a glycal substituted at C-3 with a nucleobase is mentioned in Chapter 10, and a 3 -ene derived from thymidine is mentioned in Section 17. 

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