Samir Z. Zard

The invention of radical reactions : Part XV.1Some mechanistic aspects of the decarboxylative rearrangement of thiohydroxamic esters

Abstract Esters (mixed anhydrides) derived from aliphatic or alicyclic carboxylic acids (RCO2H) and thiohydroxamic acids 2 or 3 undergo a thermally or photochemically induced radical chain reaction to give sulphides 4 with loss of carbon dioxide. On irradiation at low-temperature however, the chain reaction is essentially supressed. Under these conditions moderate to good yields of dimers R-R are obtained from primary acids. The mechanistic and synthetic implications of these observations are discussed.

On the mechanism of the deoxygenation of secondary alcohols by the reduction of their methyl xanthates by tin hydrides

Abstract Two alternate proposals for the mechanism of reduction of xanthates by tributylstannane have been examined. Evidence has been secured that under normal reduction conditions the thiocarbonyl group is attacked reversibly. At a high enough temperature the carbon radical fragments to give eventually the reduction product. Under modified conditions, where no reducing agent (Sn-H) is present, the radical formed eliminates methylthiotributylstanne and affords the thiocarbonyloxy radical observed in the e.s.r. spectrum.

The invention of new radical chain reactions. part X1: High yield radical addition reactions of αβ-unsaturated nitroolefins. An expedient construction of the 25-hydroxy-vitamin D3 side chain from bi

Abstract Radicals derived from thiohydroxamic esters 3 readily add to nitroolefins 5 (Z = NO2) to give good yields of α-nitrosulphides. These adducts, where the structure permits, are easily oxidised to carboxylic acids 8 by treatment with alkaline hydrogen peroxide. Reductive cleavage to the corresponding aldehydes or ketones 9 is efficiently carried out by the action of TiCl3. Addition of methyl magnesium iodide to the methyl ketone derived from 3α-acetoxy 11-oxo cholanic acid gives steroid 10 possessing the 25-hydroxycholesterol side chain of the vitamin D3 metabotites. Radical additions to 1-phenylthio-2-nitropropene 11 have been briefly studied.

Radical decarboxylative addition onto protonated heteroaromatic (and related) compounds

Abstract Radicals generated by irradiation of the esters of N-hydroxy-2-thiopyridone with tungsten light add efficiently to protonated heteroaromatic compounds containing a basic methine nitrogen. Other compounds with a similar functionality also show radicophilicity under the same conditions.

A New Radical Allylation Reaction of Dithiocarbonates

A radical allylation reaction without tin: The xanthate group in aliphatic xanthates can be replaced by an allyl unit [Eq. (a)]. This radical chain reaction is propagated by ethyl radicals generated by extrusion of sulfur dioxide from ethanesulfonyl radicals, which are themselves derived from allyl ethyl sulfone.

Concise syntheses of L-selenomethionine and of L-selenocystine using radical chain reactions

Abstract L-Selenomethionine and L-selenocystine were prepared in high overall yields from protected L-glutamic and L-aospartic acid derivatives respectively. Irradiation of the mixed anhydrides (esters) derived from 4 (e.g. 15 ) in the presence of dimethyldiselenide provided the protected L-selenomethionine 16 directly. We have shown that triselenocyanide Se3(CN)2 can serve as an efficient selenocyanating agent for radicals; the selenocyanide group is a good precursor for the diselenide moiety of L-selenocystine.

A New Radical Vinylation Reaction of Iodides and Dithiocarbonates

Heavy metals can be avoided with a new radical equivalent of the Heck reaction for aliphatic derivatives in which iodide and dithiocarbonate substituents are replaced by substituted vinyl groups [Eq. (1)]. The reaction enables the introduction of vinyl fragments into a variety of functionalized substrates.

Radical decarboxylative bromination of aromatic acids

Abstract Thiohydroxamic esters (mixed anhydrides) of aromatic and α,β-unsaturated carboxylic acids undergo clean decarboxylative bromination on treatment with bromotrichloromethane in the presence of a radical initiator.

New decarboxylative chalcogenation of aliphatic and alicyclic carboxylic acids.

Abstract Thiohydroxamic-carboxylic mixed anhydrides can be decarboxylated in the presence of diaryl disulphides, diselenides and ditellurides to give sulphides, selenides and tellurides respectively in reasonable to good yield.

Radical addition to vinyl sulphones and vinyl phosphonium salts

Abstract Alkyl radicals, derived from decarboxylation of carboxylic acids, add readily to phenyl vinyl sulphone and vinyl phosphonium bromide. The adducts may be further converted into a variety of useful synthetic intermediates.