C-nucleoside studies. Part II. Pentofuranosylethynes from 2,3-O-isopropylidene-D-ribose

Ethynylmagnesium bromide reacted with 2.3-O-isopropylidene-D-ribose (1) in tetrahydrofuran to give, in 70% yield, 1,2-dideoxy-4,5-O-isopropylidene-D-allo- hept-1-ynitol (3), which was converted into its 7-trityl ether (4). The structures of the ethynes (3) and (4) were shown by conversion into 1,2-dideoxy-4,5-O-isopropylidene-7-O-trityl-D-allo-hept-1-enitol (7) which, by hydrolytic and ozonolytic cleavage, afforded allose, identified by reduction to allitol. Treatment of the trityl ether (4) with toluene-p-sulphonyl chloride in pyridine yielded 2,3-O-isopropylidene-5-O-trityl-a-D-ribofuranosylethyne (9), acidic hydrolysis of which gave crystalline-a-D-ribofuranosylethyne (11). 3-O-Benzoyl-1,2-dideoxy-4,5-O-isopropylidene-6-O-methylsulphonyl-7-O- trityl-D-allo-hept-1-ynitol (14), on reaction with sodium methoxide, gave 2,3-O-isopropylidene-5-O-trityl-a-L-lyxofuranosylethyne (17). The ethynes (3) and (4) were oxidised, at the position a to the triple bond by manganese dioxide; reduction of the resulting hemiacetals (29b) and (34b) with sodium borohydride yielded mainly the D-altro-isomers (5) and (6). Oxidation of the ethyne (3) with sodium periodate afforded crystalline 5,6-dideoxy-2,3-O- isopropylidene-L-ribo-hex-5-ynofuranose (31b), which was also one of the products of oxidation with manganese dioxide. Other transformations are described, including the synthesis of 2,3-O-isopropylidene-5-O-trityl-ß-D- ribofuranosylethyne (20) and the corresponding ß-L-lyxo-isomer (35).