Nucleosides, preparation from glycals

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). 

As illustrated in Schemes 4.1.4 through 4.1.9, the use of pyrimidinyl mercuric acetates have been instrumental in the development of C-nucleosides. Although the comparison in Scheme 4.1.9 illustrates the utility of aryl iodides in addition to aryl mercury compounds, the nature of the specific reagents were quite different. As shown in Scheme 4.1.11, Zhang, etal.,10 utilized pyrimidinyl iodides in the preparation of C-nucleosides from furanose glycals. As illustrated, the product mixture composition was highly susceptible to the specific reaction conditions used. Thus, of the two products shown, either is available in comparable yields. 

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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