Dithioacetals dialdoses

Aldos-2-uk>ses, which have foe ketonic function at C-2, undergo mercaptaladon under the usual conditions, to afford only l,l-(dialkyl dithioacetal) derivatives. The ketone function in these compounds is distinctly unreactive toward thiols in acidic medium, probably as a consequence of electronic and steric effects [42] (Scheme 11). Examples of l,2-bis(dialkyl dithioacetal) derivatives have been prepared by indirect methods [43]. Dialdoses, on the other hand, in which the two carbonyl groups are separated by the sugar backbone, readily undergo mercaptalation at both carbonyl centers to afford bis(dialkyl dithioacetal) derivatives [44]. 

Dialdoses [bis(dithioacetals)] D- threo- Tetrodi-aldose methyl 116.0... 

Finally MacDonald and Fischer have prepared the free ajyZo-pento-dialdose (8) by the sulfone-degradation method. The diethyl dithioacetal of scyllo-inosose (12) was oxidized to the corresponding disulfone which, on treatment with ammonia, gave the dialdose (8). The structure was proved by converting the compound into the bis(ethylene dithioacetal) derivative, which was identical with that obtained from a /io-pentodialdose prepared by a previous method. ... 

MacDonald and Fischer prepared D- fluco-hexodialdose (27) by two different methods. By use of the Nef reaction, 6-deoxy-l,2-0-isopropyli-dene-6-nitro-n-glucose (26) was converted into the dialdose (27), which was isolated from the aqueous solution as its insoluble, crystalline bis-(diethyl dithioacetal) (in 48% yield, after recrystallization). It is of interest to note that the starting material (26) is one of the two isomeric 6-deoxy-6-nitro sugars which Grosheintz and Fischer prepared from 1,2-0-iso-propylidene-a-D-x2/lo-pentodialdo-l, 4-furanose (7). Thus, the synthesis discussed here is an ascent from a pentodialdose to a hexodialdose. 

Wolfrom and coworkers were able to interrupt the borohydride reduction of aldonio acid lactones at the aldehyde stage. This technique was applied to n-glucurono-6,3-lactone (28), which, under controlled conditions (at a pH of about 5 and temperatures of — 1° to -t-4°), afforded the dialdose in 10% yield, isolated as its bis(diethyl dithioacetal). The pH employed seemed to be very important. Under the experimental conditions used, no dialdose was obtained at pH 3 or 6, and no dialdose could be isolated from the products of the reduction of 1,2-0-isopropylidene-r)-glucurono-6,3-lactone. 

D-fiflwco-Hexodialdose and n-manno-hexodialdose have been reported to give crystalline bis (diethyl dithioacetal) derivatives, by the addition of ethanethiol to the dialdose dissolved in cold concentrated hydrochloric acid. The difficultly soluble dithioacetal readily precipitated after a short time. Similarly, the crystalline bis (ethylene dithioacetal) of xylo-pento-dialdose was prepared. A liquid bis(diethyl acetal) of erj/tliro-tetrodialdose and of the isopropylidene acetal of n-i/ireo-tetrodialdose " have also been reported. 

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