Thioacetals, formation

Thiadiazole, formation, 312, 313, 445 Thiazole formation bromoester, 298-304 bromoketone, 565 Thiazolidinone formation beta lactam, disulfide cleavage, 552 nitrile addition, 301 Thiehopyridine formation, 586 Thienothiazinol formation, 593 Thioacetal formation, 130, 185, 248 glyoxylate, 355 ethyl mercaptopropionate, 447 Thioacid formation, thioformamidoyl chloride, 184... 

When relais substance 431 was selectively dehydrated via the triflate of the secondary alcohol the path toward coiianin (21) was opened (Scheme 50). The authors used the fact that neighboring hydroxy groups enhance the reducing power of borohydrides to selectively reduce one of the lactones to the hemiacetal 434. Reduction to tetrahydrofuran 435 was achieved by thioacetal formation with... 

Fig. 6.3 Interaction between phthalic dialdehyde, mercaptan group (OPA reagents) and primary amine (I) thioacetal formation (II) formation of fluorescent complex between hemithioacetal and primary amine (III).120 Reproduced with permission...

Thioacetal formation with subsequent desulfurization with hydrogen and Raney nickel gives us an additional method for converting carbonyl groups of aldehydes and ketones to —CH2— groups ... 

Show how you might use thioacetal formation and Raney nickel desulfurization to convert (a) cyclohexanone to cyclohexane and (b) benzaldehyde to toluene. 

The presence of dithiol centres at the active sites of a variety of additional enzymes has been proposed on the basis of inhibition studies. For example, the investigations on aldehyde dehydrogenase represent one of the earliest uses of arsenite as a dithiol diagnostic reagent The overall data strongly support the presence of a polythiol site as a general feature of aldehyde oxidases, but its functional role has not been established. Because of the ease of thioacetal formation a dithiol would make a chemically attractive... 

Hoechst workers [140] used steps similar to those employed in the (128) to (129) transformation to convert the enone (130), prepared from the hemiacetal (131) by thioacetal formation, Moffat oxidation and olefln bond migration, to the aldehyde (132) which was then elaborated to afford ( )-l l-deoxy-PGE2. 

Thioacetal formation is useful in organic synthesis because a thioacetal is desulfurized when it reacts with H2 and Raney nickel. Desulfurization replaces the C—S bonds with C—H bonds. 

Thus, thioacetal formation followed by desulfurization provides a way to convert a carbonyl group into a methylene group. 

Miscellaneous Reactions. Y(OTf)3 also catalyzes acetal and thioacetal formation, quinoline formation, 5-benzylation of... 

In Section 17-5 we reviewed methods by which carbonyl compounds can be reduced to alcohols. Reduction of the C=0 group to CH2 (deoxygenation) also is possible. Two ways in which this may be achieved are Clemmensen reduction (Section 16-5) and thioacetal formation followed by desulfurization (Section 17-8). This section presents a third method for deoxygenation—the Wolff-Kishner reduction. 

Thiols are the sulfur analogs of alcohols (Section 15.11). The sulfur atom of a thiol is a better nucleophile than the oxygen atom of an alcohol. Thus, thiols react with aldehydes or ketones to form thioacetals or thioketals by a mechanism similar to that described for acetals and ketals. These sulfur derivatives form in high yield because the equdibrium constant for thioacetal formation is much greater than that for acetal formation. We use Lewis acids such as BFj or ZnCl2 rather than protic acids to catalyze the formation of the thioacetal. Both 1,2-ethanedithiol and 1,3-propanedithiol are used to form cyclic thioacetals and thioketals. 

Reduction. Raney Nickel treatment of thioacetals is a standard method for carbony 1-to-methylene reduction." In the presence of Triethylamine, (1) reduces azides to primary amines (eq 14). Under the acidic conditions employed for thioacetal formation, this reduction does not occur (eq 8). " Reduction of peptidic disulfides to dithiols can be conveniendy accomplished with (1). ... 

H"], HSCH2CH2SH, (-H2O) Cyclic thioacetal formation Ethylene thioglycol can be used to convert an aldehyde or ketone into a cyclic thioacetal. 

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