Dithioacetals, hydrolysis

These diverse results can be explained either by the variability of the substrates, or by the influence of minor experimental modifications. Particularly, dichloro-methane is the solvent used wherever an alcohol is selectively oxidized, while acetonitrile is the main solvent when a selective dithioacetal hydrolysis is achieved. The presence of water in the reaction media seems to play no role as a selective dithioacetal hydrolysis can be observed under anhydrous reaction conditions after an aqueous work-up.39... 

It is generally beheved that selectivity of hydrolytic enzymes strongly depends on the proximity of the chiral center to the reacting carbonyl group, and only a few examples of successful resolutions exist for compounds that have the chiral center removed by more than three bonds. A noticeable exception to this rule is the enantioselective hydrolysis by Pseudomonasfluorescens Hpase (PEL) of racemic dithioacetal (5) that has a prochiral center four bonds away from the reactive carboxylate (24). The monoester (6) is obtained in 89% yield and 98% ee. 

BF3 Et20, EtSH, 25°, 40 min, 80-90% yield. Addition of sodium sulfate prevents hydrolysis of a dithioacetal group present in the compound replacement of ethanethiol with ethanedithiol prevents cleavage of a dithiolane group. 

The aldehyde function at C-85 in 25 is unmasked by oxidative hydrolysis of the thioacetal group (I2, NaHCOs) (98 % yield), and the resulting aldehyde 26 is coupled to Z-iodoolefin 10 by a NiCh/CrCH-mediated process to afford a ca. 3 2 mixture of diaste-reoisomeric allylic alcohols 27, epimeric at C-85 (90 % yield). The low stereoselectivity of this coupling reaction is, of course, inconsequential, since the next operation involves oxidation [pyridinium dichromate (PDC)] to the corresponding enone and. olefination with methylene triphenylphosphorane to furnish the desired diene system (70-75% overall yield from dithioacetal 9). Deprotection of the C-77 primary hydroxyl group by mild acid hydrolysis (PPTS, MeOH-ClHhCh), followed by Swem oxidation, then leads to the C77-C115 aldehyde 28 in excellent overall yield. 

The Nucleotide Sequence in Deoxypentosenucleic Acids. Part III. The Nature of the End Groups Produced by Alkaline Hydrolysis of Calf Thymus aldehydo Apurinic Acid Di(carboxymethyl) Dithioacetal, A. S. Jones, D, S. Letham, and M. Stacey,/. Chem. Soc., (1956) 2584-2586. 

Partial hydrolysis of dithioacetals has been found useful for the preparation of anomers not obtained by the foregoing methods and also for preparation of fu-ranosidic 1-thioglycosides. 

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]. 

These problems were circumvented by protecting the C(4),C(5) diol prior to Wittig olefination step (Figure 3). Thus, treatment of 10b (a mixture of pyranose and furanose anomers prepared by hydrolysis of 8 with aqueous trifluoroacetic acid) with excess EtSH and concentrated HCI (as solvent) at provided dithioacetal 9 in 50% yield, along with 25% of a mixture of thiopyranosides and thiofuranosides that was recycled to 10b in high yield by treatment with HgCIa and CaCOa in aqueous CH3CN. Finally, the diol unit was protected as a cyclohexylidene ketal, and then the thioacetal was hydrolyzed under oxidative conditions to arrive at the key aldehyde intermediate 3. 

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