Dethioacetalization reaction

Deterioration, failures due to, 26 982 Deterioration technologies, 15 468470 Dethioacetalization reaction, microwaves in, 16 559-561... 

Scheme 2.2-28 Solid-state dethioacetalization reaction on clayfen.

BnPh3PHS05 was used for deprotection of oximes and semicarbazones to their parent carbonyl compounds under microwave irradiation. Similarly, dethioacetalization of 1,3-dithiolanes and 1,3-dithianes by BnPhsPHSOs in aprotic solvents was also reported . In both these reactions a catalytic amount of bismuth chloride was necessary. 

The approach has found further applications in cleavage of benzyl esters, dethioacetalization, deoximation, and desilylation reactions [34],... 

BTI is a very efficient reagent for the regeneration of aldehydes and ketones from their dithiacetals. As usual, reaction conditions are very mild, with yields impressively high. A great variety of substrates have in this way undergone dethioacetalization, so that BTI has become the reagent of choice for this transformation. The fate of the sulphur-containing moiety has not been identified. 

Dethioacetalization.6 This reaction can be effected with QH5I(OCOCF3)2 (1) in aqueous methanol (9 1) or acetonitrile at 25° without effect on ester, nitrile, hydroxyl groups, or unsaturated bonds. 

Another synthesis of (-)-allosamizoline (8) was also carried out using the glucoseamine derivative 44 by an intramolecular cycloaddition of a nitrile oxide to an olefin as a key step (Scheme 5). ° lodination of 44 followed by reductive 3-elimination using zinc in THF afforded the 5-enofuranose 46, whose reaction with ethanethiol in cone. HCl followed by silylation with TBSOTf afforded 47. Dethioacetalization of 47 with HgCl2-CaC03 followed by treatment of the resulting aldehyde with NH2OH afforded the oxime 50, which underwent intramolecular cycloaddition to produce the isoxazoline 51. Alternatively, treatment... 

The completion of the total synthesis of FK506 (1) is described in Scheme 34. The coupling of the two segments 222 and 230 was accomplished by phosphine oxide-mediated HW olefination. The addition of 222 to 230 afforded a separable 1 1 mixture, and the less polar diastereomer yielded ( )-olefin 231. After selective removal of the TES group, esterification of 231 with (S)-A-Boc-pipecolic acid (232) under DCC-DMAP conditions followed by dethioacetalization afforded aldehyde 233. The Evans aldol reaction of 233 with 218d installed a glycolate unit, and hydrolysis of the chiral auxiliary followed by TES protection provided 234 ready for macrocyclization. The macrolactamization of 234 was effectively... 

Deacetalization and dethioacetalization. Dimethyl acetals, 4-p-methoxy-phenyl-l,3-dioxolanes are cleaved withDDQ in the presence of water. For deprotection of thioacetals under mild conditions photochemical assistance seems advantageous. The same reaction principle of deacetalization can be extended to ether exchange. Thus alcohol protection is possible by mixing with 2,2-dimethoxypropane in the presence of DDQ, and proximal diols are converted to acetonides. Replacement of anomeric arylmethoxyl groups by this method complements other glycosylation procedures. 

Dethioacetalization. The mild conditions (aqueous THF at room temperature) are favorable for regeneration of the carbonyl group from a monothioacetal or dithioacetal. The AgC104-l2 system is even better, but it suffers from potential explosion hazard on a large-scale reaction. 

Dethioacetttlization, Thioacetals are dethioacetalized to the parent carbonyl compound by thallium(lll) nitrate in methanol at 25° in 5-30 minutes yields are 75-99%. About 2.2 molar equiv. of TTN are required for complete reaction. The present method appears to be preferable to the use of thallium(III) tri-fluoroacetate for this purpose. ... 

Thus a common intermediate (140) for the synthesis of gibberellins Aj5 (135) and A37 (141) was derived from (136) as shown in Scheme 23. From this intermediate (140), gibberellins A15 (135) and A37 (141) were efficiently prepared by reduction, Wittig reaction, and demethylation or by Wittig reaction, dethioacetalization, reduction, and demethylation, respectively, as shown in Scheme 24 108). 

Sulphur.—Thioacetals and Thioketals. The widespread utilization of lithiated thio-acetals and thioketals as acyl carbanion equivalents (see Section on Li) renders information about other reactions of such species important in general synthetic methodology. Several methods for dethioacetalization have appeared, and in-clude transthioacetalization using glyoxalic acid in acetic acid, and oxidative hydrolysis using S02Cl2-wet silica gel or Tl nitrate. 

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