Dithioacetals reduction

The a-oxoketone dithioacetal reduction is reported [9] to follow different pathways, depending on the nature of reducing agents and reaction condition (Chart 25.8). The electrophilic reducing agent, 9-BBN effects the conjugate reduction to afford the p-methylthioketone [10]. 

The dithioacetal is stable to catalytic reduction (H2/Pd-C, CH3OH-HOAC, 12 h, the conditions used to cleave a p-nitrobenzyl carbamate). ... 

Enzymic syntheses are considered next. Xylitol is a substrate for sheep-liver L-iditol dehydrogenase, a NAD-linked enzyme. 1-Deoxy-D-xylitol, prepared by Raney nickel reduction of D-xylose diethyl dithioacetal in a 27% overall yield from D-xylose, was also reported31 to be a substrate, although with a higher Km and lower Vmax. The product was assumed to be l-deoxy-D-f/ireo-pentulose because of the appearance of a yellowish fluorescent spot when a chromatogram was sprayed with acidic 3,5-aminobenzoic acid, resembling that formed from 1 -deoxyfructose. There was no more-rigorous characterization. 

This procedure has been recently applied to the synthesis of L-lyxitol and the polyhydroxylated chain of amphotericin Interesting results have also been obtained in the reduction of ) -oxo derivatives of dithioacetal monoxides. In the reaction sequence of equation 322 two successive asymmetric inductions are involved. After the first reaction, involving acylation of the carbanion, a diastereoisomeric mixture in a 65 35 ratio is produced. When this mixture is reduced with NaBH4 in MeOH-conc. aqueous solution of ammonia, among four possible diastereoisomeric alcohols, the stereoisomer 523 is obtained with a stereoselectivity of 98% . Guanti and coworkers have found that the LiAlH4 reduction of the same substrates at — 78° in THF/ether leads to 523 with a stereoselectivity 99 i6i3.6i4... 

Nitroketene dithioacetal reacts with anthranilic esters to afford quinolone derivatives, which are converted into diazepinones by reductive cyclization. The review by Kolb covers synthetic application of nitroketene dithioacetal for heterocyclic compounds (see Scheme 10.26).145... 

INVERSE ELECTRON-DEMAND DIELS-ALDER CYCLOADDITION OF A KETENE DITHIOACETAL. COPPER HYDRIDE-PROMOTED REDUCTION OF A CONJUGATED ENONE. 9-DITHIOLANOBICYCLO[3.2.2]NON-6-EN-2-ONE FROM TROPONE... 

The dithioacetal of D-xylose generates a primary tosylate 48 that can undergo a 1,5-elimination under basic conditions giving the corresponding 2,5-anhydro-pentose dithioacetal 49. Hydrolysis of the dithioacetal and NaBH4 reduction furnishes the corresponding 2,5-anhydropentitol 5083 (Scheme 19). 

The reaction of methylenesulphones with allyl halides in the presence of quaternary ammonium salts produces the 1-allyl derivatives [52], unlike the corresponding reaction in the absence of the catalyst in which the SN- product is formed (Scheme 6.5). In contrast, alkylation of resonance stabilized anions derived from allyl sulphones produces complex mixtures [51] (Scheme 6.6). Encumbered allyl sulphones (e.g. 2-methylprop-2-enyl sulphones) tend to give the normal monoalkyl-ated products. Methylene groups, which are activated by two benzenesulphonyl substituents, are readily monoalkylated hydride reduction leads to the dithioacetal and subsequent hydrolysis affords the aldehyde [61]. 

The higher homologue of fluoral, pentafluoropropionaldehyde, prepared by reduction of ethyl pentafluoropropionate, is a precursor of diethyl tetrafluoroketene dithioacetal, which is a versatile building block for the synthesis of trifluoromethyl-substimted functional compounds, such as trifluoromethyl lactones, lactams, and heterocyclics, as exemplified in the Figure 2.53. ... 

Padwa synthesized the oxo-brevicomin analogue 174 in 60% yield as a 2 1 mixture of the exo and endo isomers. Dithioacetalization followed by Raney Ni reduction of the dithioketal produced the exo- (175) and endo-brevicomin in good... 

Williams and J. K. N. Jones have shown that the dithioacetals of aldopentoses and aldohexoses may be separated as their acetates.472 In certain cases, however, isomers were not separated (for example, D-altrose and D-talose), but reduction of the dithioacetal to the 1-de-oxyalditol permitted resolution of the acetates. The method is not suitable for quantitative determinations, but it does preserve the... 

Cleavage of the dithioacetal groups from the products, followed by reduction of the resultant carbonyl derivatives (46, 49, 52) with sodium borohydride leads,68 with the three compounds (45, 48, and 51), to 1,4-anhydro-L-ribitol (2,5-anhydro-D-ribitol) (47), 1,4-anhydro-L-xylitol (2,5-anhydro-D-xylitol) (50), and 1,4-anhydro-D-arabinitol (2,5-anhydro-D-lyxitol) (53), identified by comparison with their enantiomorphs, 1,4-anhydro-D-ribitol,69 1,4-anhydro-D-xylitol,70 and 1,4-anhydro-L-arabinitol. 71... 

We have discussed previously (see Section 2.4) some aspects of the dithioacetalization step involved in this strategy. The presence of the sulfur atoms offers another possibility as a methylene unit results from desulfurization by Raney nickel, dissolved metal or LiAlH4/ZnCl2 reduction [281]. 

Dithioacetals (see also dithianes and dithiolanes) alkylation of 98 as acyl anion equivalents 75 carbanions of 87,97-102 cleavage of 14-18,98,102 desulfurization of 78 metal-catalysed coupling 127 reaction with Grignard reagents 127 reductive lithiation of 89 synthesis of 12-19,97-102 Dithioacids synthesis of 40... 

The conversion of dithioesters into aldehydes is also easily realized through reduction to a mixed dithioacetal by tributyltin hydride [264]. 

The use of reductive lithiation to prepare a carbanion from a dithioacetal is due to Cohen [311] ... 

Whereas ethyl l-thio-d-D-arabinofuranoside cannot be prepared directly, the 5-O-benzoyl diethyl dithioacetal gave 38% of ethyl 5-O-benzoyl-l-thio-0-D-arabinofuranoside, which was debenzoylated to the desired product. The 5-benzoate of ethyl 1-thio-a-D-ribofuranoside was similarly prepared.88 Two other compounds, ethyl 2-acetamido-2-deoxy-l-thio-/3-L-arabino-furanoside72 and ethyl 2-acetamido-2-deoxy-l-thio-a-D-xylofuranoside,73 were prepared from the corresponding d-galacto and d-gluco analogs by periodate oxidation, and subsequent borohydride reduction of the product. 

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