Protecting groups dithiolanes

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... 

The reaction of aldehydes or ketones with ethane-1,2-dithiol or propane-1,3-dithiol to form 1,3-dithiolanes or 1,3-dithianes is an important reaction, as these compounds under suitable conditions are acyl anion equivalents (see Section 5.9, p. 626). These cyclic dithioacetals have been less used as protective groups, though when required are formed in high yield in the presence of boron trifluoride-etherate.138... 

Removal of ethylenethioketal protecting group.1 A variety of 1,3-dithiolanes (ethylenethioketals) react smoothly with chloramine-T in aqueous methanol or ethanol to give the corresponding carbonyl compounds in good to excellent yield. For example, treatment of (1) with chloramine-T in aqueous methanol/ethanol yields fluorenone (2)... 

Intermolecular electron transfer reactions of S-heterocycles 02ACR247. Removing dithiolane- and oxathiolane-protecting groups using CeCl3 H20/Nal as an efficient water-tolerant Lewis acid 03SL2101. 

The practical use of 1,3-dithioles as synthons in organic synthesis is small whereas 1,3-dithiolanes are quite often used as protecting groups for aldehydes and ketones <92HOU(El4a/3)403). TTF and... 

In addition to acetal formation with simple alcohols, (oxoalkyl)phosphonic diesters afford dioxolanes or dithiolanes from 1,2-diols or 1,2-dithiols". It is possible to transpose 0X0 protecting groups as illustrated in equation 15 ( = 0,1 or 2 X, Y = O, S or NH)" ... 

Chlorination of Sulfides. The treatment of 1,3-oxathioacetals or dithioacetals with NCS in the presence of MeOH, EtOH, 1,2-ethanediol, or 1,3-propanediol results in the clean conversion to the corresponding acetal or cyclic acetal. The protecting group conversion occurs quickly and in excellent yield. For example, the reaction of 2-phenyl-1,3-dithiolane with 1 equiv of NCS and 3 equiv of 1,2-ethanediol in dichloromethane proceeds readily to afford 2-phenyl-1,3-dioxolane in almost quantitative yield (eq 20).3 ... 

The mild oxidizing properties of IBX enhance its utility for the oxidation of compounds with sensitive functional groups such as thioethers and amines. Oxidation with IBX occurs cleanly in the presence of thioethers and 1,3-dithiolane protecting groups. For exan5)le, oxidation of thiochroman-4-ol, even with 10 equiv of IBX is achieved cleanly to give thiochroman-4-one with no detection of the analogous sulfoxide or sulfone by-products (eq 2). ... 

Protective Groups and Their Removal - Lactones and esters may be protected as IjS-d-tth-iolanes. The ester or lactone is treated with bis (dimethyl-aluminum) 1,2-ethanedithlolate, and deprotection is achieved with mercuric oxide and BF3 Et20 in 15% aqueous THF. The dithiolanes were shown to be stable to acetic acid, to KOH-aqueous methanol, and to LiAlHi, or methyl-lithium in ether. 

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. 

Open chain and cyclic thioacetals 1,3-Dithiolane and 1,3-dithiane derivatives are versatile intermediates in the synthesis and interconversion of monocarbonyl and 1,2-dicarbonyl compounds. Protection of carbonyl groups as their open-chain and cyclic thioacetals is an important method in the synthesis of organic molecules. Thioacetals are stable... 

From the viewpoint of atom economy [155], dithiolane or dithiane fluorodesulfuration chemistry suffers from a drawback - as a result of oxidation of the sulfur-ous protecting (and activating) group a relatively large part (by molecular mass) of the starting material is lost and cannot be recovered or recycled. 

---