Selenols synthesis

Synthesis of thiols, selenols, sulfides, selenides, sulfoxides, sulfones, and se-lenones of heterocyclic series 98JCS(P1)1973, 99JCS(P1)641. 

An alternative procedure has been proposed for the direct synthesis of Fmoc-SeC-(Mob)-OH that is based on activation of Fmoc-Ser-OAL as the O-tosylate followed by reaction with 4-methoxybenzyldiselenide reduced in situ with hypophosphoric add to the selenol. 59 Selective reductive cleavage of the allyl ester generates Fmoc-SeC(Mob)-OH in good yields. 

Acyl radical cyclization to cyclohexanones.n Acyl radical cyclization to five-membered rings is well known, but this reaction is also useful for synthesis of substituted cyclohexanones as shown by a recent synthesis of the a-methylenecyclo-hexanone 3. Thus treatment of the selenol ester 1 with Bu3SnH and AIBN in C6H6 at 80° provides a 1 1 mixture of cyclohexanones 2 in 91 % yield. Oxidation of 2 with... 

Early syntheses of SeMet were tedious, non-stereospecific or limited to small-scale preparations. Klosterman and Painter (1947), for example, first reacted 5-((3-bromoethyI)- hydantoin with benzyl selenol to yield y-benzylselenoho-mocysteine. The latter was converted to the sodium salt of DL-selenohomo-cysteine with sodium in liquid ammonia, and reacted with methyl iodide to yield DL-SeMet. Plieninger (1950) obtained DL-SeMet by the reaction of sodium selenomethyl mercaptide with a-amino-y-butyrolactone in an inert solvent at 170°C. A synthesis of DL-SeMet from acrolein was also described (Zdansky, 1968). The first stereospecific synthesis of L-SeMet via esters of tosylated homoserine was reported by Pande et al. (1970). 

Hydrazine155,156 or samarium diiodide157-162 are other useful reagents for the synthesis of selenolates, but electrochemical methods163,164 have also been employed for their selective synthesis from diselenides. Starting from elemental selenium, selenolates 80 can easily be prepared by insertion reactions of selenium into organolithium... 

The most common reaction of selenolates is the nucleophilic substitution of halide compounds. For the synthesis of selenides 82 the selenolates 80 can be reacted with halides. Nucleophilic selenium species can also be reacted with a variety of other electrophilic compounds. The reaction with acid chlorides, chloroformates, or carbamoyl chlorides leads to the corresponding selenoesters 83 (R" = R ), selenocarbonates 83 (R" = OR ), or selenoamides 83 (R" = NR 2), respectively (Scheme 17).166... 

Seleno esters are also produced in good yield by reactions of dimethy-laluminim methanoselenolate103 (CH3)2AlSeCH3, with the protected C-ribofuranosyl acetate 269. Treating the selenol ester 270 with either cuprous or mercuric chloride produced the (Gensler) lactone.110 271, a product useful in the synthesis of various C-nucleoside antibiotics.111,112... 

Nicolaou [46] has also exploited a seleniiun Unker cleaved using radical conditions in the solid-phase synthesis of 2-deoxyglycosides, 2-deoxy-orthoesters and 2,3-allyl orthoesters. Resin-bound tributyltin selenolate 251 was conveniently synthesized in two steps from a selenenyl bromide resin. Treatment of 251 with a trichloroacetimidate donor gave resin-bound se-... 

The nucleophilicity of selenolates is significantly higher than that of the corresponding thiolates. This property allows selenolates to be useful reagents in organic synthesis. Moreover, removal of the selenium moiety from the reaction products is easier because the selenium - carbon bond is weaker than the sulfur-carbon bond [3 b]. 

Reaction of diphenyl diselenide or dimethyl diselenide with hydrazine hydrate and sodium hydroxide generates the corresponding selenolates smoothly in solvents like DMF or diethyl ether and in the presence of tetrabutylammonium chloride as a phase-transfer catalyst [13]. The selenolates react with organic halides to give various selenides (Scheme 9). Similar conditions have been applied to the synthesis of aryl vinyl selenides from diaryl diselenides and acetylene [14]. 

The reaction with a, -epoxy carbonyl compounds 12 leads to the corresponding reductive ring-opened products, -hydroxy carbonyl compounds 13, in good yields (Scheme 22). Electrochemically generated benzeneselenolate [21,22] and sodium phenylseleno(triethoxy)borate (1) [35, 36], have been applied for this type of reaction as a nucleophilic selenolate. In the latter case, the reaction mechanism was suggested as shown in Scheme 23 [36]. Reaction of 1 with 12 first produces the ring-opened adduct 14, which is then reacted with an excess amount of 1 to produce the final product 13. This method is important as a simple synthetic procedure to aldols and -hydroxy esters that are rather difficult to obtain by other methods. The reactions have been extended to the reduction of more functionalized a, -epoxy carbonyl compounds [37] and have been successfully applied for the synthesis of several natural products [38]. 

Treatment of a, -unsaturated carbonyl compounds 18 with nucleophilic selenium species affords -seleno carbonyl compounds 19 in good yields via Michael addition (Scheme 27) [46]. This reaction has been applied to protect a, -unsa-turated lactones [47], in natural product synthesis [48], and in asymmetric Michael additions in the presence of an alkaloid [49]. Michael addition also proceeds with selenolates that are prepared from diphenyl diselenide by cathodic reduction [22], reduction with the Sm-Me3SiCl-H20 system [19], and reduction with tributyl phosphine [25]. 

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