Mercaptals acetals

A method of degradation with excellent preparative possibilities has been devised by MacDonald and Fischer (200), The higher-carbon sugar first is condensed with ethyl mercaptan to form the bis(ethylthio)acetal (mercaptal) (I) (Chapter IV). Oxidation of the mercaptal with perpro-pionic acid then leads to the bis(ethylsulfonyl) compound (II), which is smoothly degraded by aqueous ammonia to the next lower aldose and bis(ethylsulfonyl)methane. 

Mercaptals, CH2CH(SR)2, are formed in a like manner by the addition of mercaptans. The formation of acetals by noncatalytic vapor-phase reactions of acetaldehyde and various alcohols at 35°C has been reported (67). Butadiene [106-99-0] can be made by the reaction of acetaldehyde and ethyl alcohol at temperatures above 300°C over a tantala—siUca catalyst (68). Aldol and crotonaldehyde are beheved to be intermediates. Butyl acetate [123-86-4] has been prepared by the catalytic reaction of acetaldehyde with 1-butanol [71-36-3] at 300°C (69). 

Thioketals are readily formed by acid-catalyzed reaction with ethane-dithiol. Selective thioketal formation is achieved at C-3 in the presence of a 6-ketone by carrying out the boron trifluoride catalyzed reaction in diluted medium. Selective protection of the 3-carbonyl group as a thioketal has been effected in high yield with A" -3,17-diketones, A" -3,20-diketones and A" -3,l 1,17-triones in acetic acid at room temperature in the presence of p-toluenesulfonic acid. In the case of thioketals the double bond remains in the 4,5-position. This result is attributed to the greater nucleophilicity of sulfur as compared to oxygen, which promotes closure of intermediate (66) to the protonated cyclic mercaptal (67) rather than elimination to the 3,5-diene [cf. ketal (70) via intermediates (68) and (69)]." " ... 

This ester is converted in high yield to the y -ketosulfoxide by means of the dimethyl sulfoxide anion. Heating in acetic acid then produces a hemi-mercaptal acetate, which on treatment with Raney nickel gives the desired 21-acetoxy-20-ketone in a 50-70% overall yield from the -ketosulfoxide. A 17a-hydroxyl may also be present during the sequence. 

The mercaptals obtained by the acid catalyzed reaction of J3-ketoesters, e.g., ethyl acetoacetate, with methyl thioglycolate (73) undergo the Dieckmann cyclization with alcoholic potassium hydroxide at lower temperatures to give ethyl 3-hydroxy-5-methyl-2-thiophenecarboxylate (74) in 75% yield. ° Besides ethyl acetoacetate, ethyl a-ethylacetoacetate, ethyl benzoyl acetate, and ethyl cyclopentanonecarboxylate were also used in this reaction/ It is claimed that /8-diketones, hydroxy- or alkoxy-methyleneketones, or /8-ketoaldehyde acetals also can be used in this reaction. From acetylacetone and thioglycolic acid, 3,5-dimethyl-2-thiophenecarboxyl-ic acid is obtained. ... 

Further evidence for a pseudooxazolone intermediate is the fragmentation of 72 by ethyl mercaptan in hydrobromic acid-acetic acid [Eq. (33)]. The mercaptal 74 hydrolyzes readily in acetic acid to give the 2-0X0 acid. 

Ketene Acetals, Ketene Mercaptals, and Ketene Aminals... 

Acetamido-5-methylbenzaldehyde trimethylene mercaptal. Into a 25-ml round-bottomed flask are placed 300 mg of the foregoing amino compound, 5 ml of acetic anhydride and 3 ml of pyridine. The solution is heated on a steam bath for 5 minutes, allowed to cool and poured into 50 ml of water, whereupon the product precipitates. Sodium carbonate is added until the solution is basic. The product is collected by filtration, washed with water and dried under vacuum (320 mg, 90%). Further recrystallisation affords fine, white crystals, m.p. 159-160°C. 

Schmidt and Wernicke74 have described a synthesis of digitalose starting with D-fucose dibenzyl mercaptal, which on condensation with acetone yielded the 4,5-isopropylidene derivative (LXXIV). Elimination of the dibenzyl mercaptal residues with mercuric chloride and cadmium carbonate in methanol gave 4,5-isopropylidene-D-fucose dimethylacetal (LXXV), from which the 2-benzyl ether was obtained on treatment with sodium and benzyl chloride. Methylation produced 2-benzyl-3-methyl-4,5-iso-propylidene-D-fucose dimethyl acetal (LXXVI), from which the iso-propylidene group was eliminated on treatment with methanolic hydrogen chloride, which also effected glycopyranoside formation (LXXVII). The... 

Preparation. The reagent is prepared in 78 % yield by oxidation of formaldehyde dimethyl mercaptal with 30 % hydrogen peroxide in acetic acid. ... 

Cleavage of streptomycin with ethyl mercaptan and hydrogen chloride yielded streptidine and a new derivative of streptobiosamine, ethyl thio-streptobiosaminide hydrochloride diethyl mercaptal. Treatment of methyl streptobiosaminide hydrochloride dimethyl acetal with ethyl mercaptan and hydrogen chloride readily yielded the same mercaptal hydrochloride. It thus was apparent that the relationship between these two compounds resided simply in the replacement of methoxy by thio-ethoxy groups on the same carbon skeleton. Acetylation of the mercaptal yielded two anomeric forms of ethyl tetraacetylthiostreptobiosaminide diethyl mercaptal. ... 

However, solutions of ethyl tetraacetylthiostreptobiosaminide diethyl mercaptal and methyl tetraacetylstreptobiosaminide dimethyl acetal in high concentrations (about 50%) in s /m-tetrachloroethane showed... 

Such a di-thio-ether is known as a mercaptal and this particular one is known as di-thio acetal (p. 117). Analogous compounds obtained from ketones, e.g., from acetone, are called mercaptols. 

D-glucose mixed acetal, at least in the anomeric form employed, is not the probable intermediate in the formation of ethyl a-thio-D-glucofuran-oside from D-glucose diethyl mercaptal but that the mixed acetal is the probable intermediate in the formation of ethyl j3-D-galactofuranoside from D-galactose diethyl mercaptal. 

Mercaptal The sulphur equivalent of an acetal, RCH(SR)2, sometimes called thioacetal. The RCH... 

The Action of Lead Tetra-Acetate on Sugar Mercaptals, E. J. Bourne, W. M. Corbett, M. Stacey, and R. Stephens, Chem. Ind. (London), 106-107 (1954). 

Starting from hydroxybenzaldehydes, several workers prepared independently phenylcarbamates the phenyl group of which contains cyclic acetal (1,3-dioxolane) and cyclic mercaptal (1,3-dithiolane) groups (Nikles et al., 1966 Nikles, 1969 Durden and Weiden, 1969 Bachmann and Legge, 1968). Of the former, 2-(l,3-dioxolan-2-yl)-phenyl N-methylcarbamate, dioxocarb (22), and of the latter, 2-(l,3-dithiolan-2-yl)-phenyl N-methylcarbamate, (23) attained practical importance. 

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