C-Nucleoside studies. Part I. Synthesis of [2,3,5-tri-O-benzyl-α(and β)-D-ribofuranosyl]ethyne

Ethynylmagnesium bromide reacted with 2,3,5-tri-O-benzyl-D-ribofuranose (1) in tetrahydrofuran to give, in quantitative yield, a mixture of 4,5,7-tri-O-benzyl-1,2-dideoxy-D-altro- and D-allo-hept-1-ynitol [(2) and (7)] in the ratio 7:3. Treatment of the mixture with toluene-p-sulphonyl chloride in pyridine afforded (2,3,5-tri-O-benzyl-ß-D-ribofuranosyl)ethyne (13) (52%) and its a-anomer (16) (13%), together with 4,5,7-tri-O-benzyl-1,2-dideoxy-3,6-bis-O-p-tolylsulphonyl-D-altro-hep-1-ynitol (4) (16%). 2,3,5-Tri-O-benzyl-D-ribofuranosyl chloride (12) and ethynylmagnesium bromide gave the ethynes (13) and (16) in 8 and 63% yield, respectively. Hydrogenation of the ethynes (13) and (16) (Pd-C) afforded ß-D-ribofuranosylethane (15) and the crystalline anomer (18), respectively, together with 5,6,7-trideoxy-L-ribo-heptitol (27), characterised as the tetra-acetate (28). The ß-ethyne (13), on treatment with benzyl azide, was converted into 1-benzyl-4(and 5)-(2,3,5-tri-O-benzyl-ß-D-ribofuranosyl)-1,2,3-triazole [(30) and (31)] in approximately equal amounts.