Sulfur compounds reductive cleavage

Two methods for the formation of aldehydes embody C—C bond formation and then its cleavage by a reverse aldol reaction after reduction, and a third involves similar formation and breakage of a C—P bond. Reaction of esters with the sulfur compound (55 Scheme 26) affords ketones (56) which can be reduced with NaBHa at 0 °C to the alcohols (57). Simply warming this alcohol, or better, treating it with aqueous potassium carbonate, gives the aldehyde in about 95% yield for aromatic compounds but somewhat less for aliphatic dialdehydes can be obtained from diacids.Acylmalonates (58 Scheme 27)... 

The target compound 8 is derived from a known carboxylic acid 54 [85], which is obtained via reductive cleavage of the carbon-sulfur bond of bicyclic amide 53 whose carboxylic acid congener is an intermediate of the Merck s approach [85]. The compound 53 is obtained through amidation and subsequent cyclization of 5y -a-amino nitrile syn-Sl. The compound syn-51 should be prepared stereoselectively by Strecker reaction of a-amino aldehyde 51 that can be readily prepared from L-cysteine. 

The compound 53 was treated with Zn in acetic acid (AcOH) to effect the reductive cleavage of the carbon-sulfur bond. After filtration of the reaction mixture to remove excess Zn, the filtrate containing thiol amide was directly treated with HCI (110 C, 2 h) to give carboxylic acid 54 in a high yield (87%, two operations). 

Methods for the preparation of organic sulfur compounds by alteration of sulfur-containing groups already present include reduction, oxidation, and cleavage. The preparation of thiocarbonic acid and its derivatives is treated at the end of the present Chapter. For the rearrangement of sulfur compounds see the Section on p. 1074. 

Compounds 296 and 297 had previously been synthesized by our group from artemisin (2) by an alternative route [49]. This synthesis required reductive cleavage of the C(6)-0 bond, a transformation that was achieved by epimerization at C(6) in an acidic medium followed by reductive treatment with Zn in acetic acid and esterification of the resulting acid with MeOH/sulfuric acid to give hydroxy ester 299. Hydrogenation and acidic treatment of this intermediate afforded the same compounds (296 and 297). Compound 296 has been used as the starting materid in the synthesis of several 8,12-sesquiterpenolides. 

Intramolecular addition of thiolate, dithiocarboxylate, and enedithiolate sulfurs to unsaturated carbon-carbon bonds also provides a convenient way to thiophenes. Thus, reductive cleavage of the thioester group of the compound (11) followed by intramolecular addition of the thiolate sulfur to the triple bond affords the thiophene (12) directly (Equation (2)) <93SL209>. In this way, a number of alkyl-substituted and six-membered ring annelated thiophenes were synthesized in good overall yields. 

Graphite reacts with alkali metals - potassium, cesium and rubidium - to form lamellar compounds with different stoichiometries. The most widely known intercalate is the potassium-graphite which has the stoichiometry of CgK. In this intercalate the space between the graphite layers is occupied by K atoms. CgK functions as a reducing agent in various reactions such as reduction of double bonds in a,fl-unsaturated ketones [19], carboxylic acids and Schiff bases alkylation of nitriles [20], esters and imines [21] reductive cleavage of carbon-sulfur bonds in vinylic and allylic sulfones [22]. The detailed reaction mechanism of CgK is not known, and the special properties which are ascribed to the intercalate come either from the equilibrium between K+/K [23], or topochemical observations (the layer structure) [24]. 

Raney-nickel as a reagent for reductive cleavage of chal-cogen atoms from sulfur, selenium and tellurium compounds. Phosphorus, Sulfur Silicon Relat. Elem. 1978 4(3) 259-265. 

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