Nucleosides and nucleotides. XLIII. On the stereoselectivity of alkyl addition reaction of pyrimidine 2'-ketonucleosides.

The β-face of a 2'-ketonucleoside is sterically more hindered than the α-face because of the bulky nucleobase on the β-side at the 1'-position. However, on the reaction of 4-ethoxy-1-[3, 5-O-(1, 1, 3, 3-tetraisopropyl-1, 3-disiloxanediyl)-β-D-erythro-2-pentofuranos-2-ulos-1-yl]-2(1H)-pyrimidinone (6d) with methylmagnesium bromide, the β-methyl addition product (8d) was obtained in a considerable amount. It was found that Lewis basicity of the 2-carbonyl oxygen in the pyrimidine moiety, which is influenced inductively by the substituent at the 5-position, governs the stereoselectivity of the methyl addition reaction.