Thiol-groups Dithiols

A carbonyl group can be protected as a sulfur derivative—for example, a dithio acetal or ketal, 1,3-dithiane, or 1,3-dithiolane—by reaction of the carbonyl compound in the presence of an acid catalyst with a thiol or dithiol. The derivatives are in general cleaved by reaction with Hg(II) salts or oxidation acidic hydrolysis is unsatisfactory. The acyclic derivatives are formed and hydrolyzed much more readily than their cyclic counterparts. Representative examples of formation and cleavage are shown below. 

Dithioketals, especially the cyclic dithiolanes and dithianes, are also useful carbonyl-protecting groups. These can be formed from the corresponding dithiols by Lewis acid-catalyzed reactions. The catalysts that are used include BF3, Mg(03SCF3)2, Zn(03SCF3)2, and LaCl3.100 S-Trimethylsilyl ethers of thiols and dithiols also react with ketones to form dithioketals.101... 

A consequence of the existence of this equilibrium was the formation of a monosulfide (114) in the reaction of the simple dehydrocyclodipeptide (113) with an alanine or phenylalanine residue, as shown in Scheme 36. The reaction could have proceeded by replacement of the OMe by SH, followed by protonation of the exocyclic double bond and intramolecular attack by the thiol group. Alternatively, the m-dithiol could have been in equilibrium with the /rani-dithiol in the latter, a traw.v-annular attack could have generated the monosulfide. 

Turk and Hollocher (1992) have attempted to trap nitroxyl using the dithiol, dithiothreitol, during the reduction of NO by purified nitric oxide reductase from Pa. denitrificans. Because the dithiol served in this system as both electron donor to the enzyme and trap for nitroxyl, the ratio of thiol groups oxidized to NO consumed would have been 1 if nitroxyl trapping did not occur and 3 if nitroxyl... 

Thioacetals and -ketals. A1C13 is an effective catalyst for the condensation of aldehydes and ketones with thiols or dithiols. The reaction of thiols with carbonyl groups with an a-proton results in low yields but the reaction with dithiols proceeds in high yield.8... 

The design of these dithiol reagents is based on two requirements (i) a low value of pKg ( 7 to 8) of their thiol groups and (ii) a high reduction potential. The reactivity of a thiol is influenced... 

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