Acetone diethyl dithioacetal

D-Galactose was converted by ethanethiol and hydrochloric acid into crystalline D-galactose diethyl dithioacetal, which was acetonated with acetone-zinc chloride. The product (15) was reduced to the L-fu-citol derivative (16) with Raney nickel. The overall yield of 16 was 29%, and it was characterized as the crystalline 6-p-toluenesulfonate 17. Oxidation of 16 by the Pfitzner-Moffatt reagent55 proceeded readily and, after O-deacetonation, and purification of the product by chromatography on a column of silica gel, L-fucose (18 13% overall yield from D-galactose) and L-fucitol (19 1% yield) were isolated (see Scheme 3). 

H. Zinner, G. Rembarz, and H. Klocking, Isopropyliden-verbindungen der D-arabinose-mercaptale, Chem. Ber. 90 2688 (1957) P. A. J. Gorin, Acetonation of aldose diethyl dithio-acetals. Can. J. Chem. 43 21X18 (1965) D. G. Lance and J. K. N. Jones, Acetonation of D-xylose diethyl dithioacetal, Can. J. Chem. 45 1533 (1967). 

Acid-catalyzed acetonation of L-sorbose gave a mixture of di-O-iso-propylidene and mono-O-isopropylidene derivatives, easily separated by the solubility of the former in benzene. Isopropylidenation of 2-acetamido-2-deoxy-D-xylose diethyl dithioacetal gave the 4-j5- -iso-propylidene derivative accompanied by some 3-0- 1-methoxy-1-methyl-ethyl ) -derivative (/i) when copper(Il) sulphate was used as catalyst with either propanone - 2,2-dimethoxypropane or propanone - 2-meth-oxypropene. The 3,4--0-isopropylidene derivative was obtained when sulphuric acid was used as catalyst in propanone - 2,2-dimethoxypropane. The former was considered the kinetic, and the latter the thermodynamic product. 3,4-0-Isopropylidene-L-fucopyranose... 

Further examples of rearrangements involving cyclic sulphonium ions have been reported. Thus 2,3,5-tri-0-methyl-4-0-toluene-/>-sulphonyl-D-ribose diethyl dithioacetal (184) and dibenzyl dithioacetal (185) gave 4- S -ethyl-2,3,5-tri-0-methyl-4-thio-L-lyxose (186) and benzyl 2,3,5-tri-0-methyl-l,4-dithio-a-L-lyxo-furanoside (187) (Scheme 46) when heated either in aqueous pyridine or with sodium iodide in acetone similar rearrangements to the 4-thiofuranosides were observed with 2,3,5-tri-0-methyl-4-0-toluene-p-sulphonyl-D-xylose and -lyxose... 

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