Enol-acetate nucleosides synthesis

Other pyrimidine keto-nucleosides, such as 47a and 47b, could be treated with acetic anhydride-pyridine without glycosylic cleavage.33 The stability of 47a and 47b in this medium permitted elaboration of a new, mild, regioselective synthesis of enol acetates, The reaction of 4 -ketonucleosides of uracil with acetals of N,N-dimethylformamide led,33 after addition of acetic anhydride, to the corresponding enol acetate-nucleosides 77a and 77b. 

Enol acetates of oxo nucleosides are readily accessible by reaction of the nucleoside with DMF acetal and then with acetic anhydride (eq 16). However, this method is limited to compounds with an oxo group in the 4 -position and with a free CO-NH group in the pyrimidine ring. This reaction is the first synthesis of oxo nucleoside enol acetates by direct enolization. Previous methods... 

Various kinds of chiral acyclic nitrones have been devised, and they have been used extensively in 1,3-dipolar cycloaddition reactions, which are documented in recent reviews.63 Typical chiral acyclic nitrones that have been used in asymmetric cycloadditions are illustrated in Scheme 8.15. Several recent applications of these chiral nitrones to organic synthesis are presented here. For example, the addition of the sodium enolate of methyl acetate to IV-benzyl nitrone derived from D-glyceraldehyde affords the 3-substituted isoxazolin-5-one with a high syn selectivity. Further elaboration leads to the preparation of the isoxazolidine nucleoside analog in enantiomerically pure form (Eq. 8.52).78... 

---