Selenides, vinyl synthesis

The use of the Wittig reaction and its modifications for the synthesis of vinylchalcogenides, ketene chalcogenoacetals and related species has been reviewed. New developments on this topic include the synthesis of a series of polymer-supported alkyltriphenylphosphoranes (46) which have been utilised in the preparation of vinylic selenides. The synthesis of vinylic selenides, such as 1-chloro-l-phenylselenoalkenes, has also been accomplished in a one-pot reaction which proceeds through the formation of selenoylphosphorane (47) intermediates. ... 

Synthesis of a-Selenoalkyl Metals by Addition of Organometallics to Vinyl Selenides, Vinyl Selenoxides and Vinyl Selenones... 

Metalloamides are suitable for the synthesis of 1-metallovinyl aryl selenides (Scheme 44) and 1-metalloallenyl phenyl selenides (Scheme 45) from the corresponding vinyl or allenyl selenides. The synthesis of the latter organometallics is even more conveniently achieved by metallation of l-phe-nylseleno-l-propyne or by a two-step, one-pot sequence of reactions fiom 2-chloroallyl phenyl selenide (Scheme 45). 

A solid-phase synthesis of 3-substituted isoxazoles 31 in good yields and purities was achieved by 1,3-DC of polymer-supported vinyl selenide with in situ generated nitrile oxides treatment of intermediate isoxazolines 30 with an excess of hydrogen peroxide resulted in the release of isoxazoles 31 while the use of Mel/Nal led to 3-substituted 5-iodoisoxazolines... 

In the reactions with mono- and 1,2-disubstituted olefins, however, no ene product was obtained. This limitation has been overcome by the use of vinylic sulfides and selenides instead of mono- and 1,2-disubstituted olefins. With these substrates, the ene products are obtained with comparably high enantioselectivity and high diastereoselectivity [15]. The synthetic utility of the vinylic sulfide and selenide is shown in the synthesis of enantiopure (R)-(-)-ipsdienol, an insect-aggregation pheromone (Scheme 8C.4) [16]. 

I - Alkyny lphenyl selenides. Benzeneselenenyl bromide and silver nitrite (1 equiv.) rcncl to form a reagent (presumably C6H5SeN02) that converts 1-alkynes into 1-nlkynyl phenyl selenides (equation I).1 The same reaction has been effected in generally lower yields with phenyl selenocyanate catalyzed by Cu(I).2 The products tire useful intermediates for regio- and stereoselective synthesis of vinyl selenides. 

Homopropargylic alcohols are readily available substrates that can be used for the synthesis of 7-lactones. Cul-catalyzed selenation with PhSeBr at the alkyne terminus affords alkynyl aryl selenides. These react with an excess of /i-toluenesulfonic acid monohydrate, in dichloromethane at 60°C, to form a selenium-stabilized vinyl cation intermediate. The cation is then intramolecularly trapped by the tethered hydroxyl group to afford a cyclic selenoketene acetal, which readily adds a molecule of water to give the 7-lactone products (Scheme 55) <2006SL587>. 

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 regiochemistry of the hydrozirconation of acetylenic selenides with the Schwartz reagent [Cp2Zr(H)Cl] is dependent on the nature of the substituents. For simple acetylenic selenides (R = Ar, n-Bu), ( j-j9-zirconated vinyl selenides were formed exclusively. The reaction with electrophilic reagents has allowed the stereoselective synthesis of ( j-2-halovinylaryl selenides [75], (Ej-2-aryl-vinyl aryl selenides [76], ( , j-l-arylselanylbutadienes [77] and ( j-2-butyltel-luranylvinyl selenides [78] (Scheme 55). 

Dabdoud et al. have shown that the hydrozirconation of alkyn-l-ylbutyl selenides was complete with two equivalents of Cp2Zr(H)Cl. The nature of the products, formed after reaction with BuTeBr, is dependent on the substituent R [78] (Scheme 56, reaction 1). With R=Ph, the ketene butylselanyl(butyl-telluranyl)acetal was obtained exclusively. In the other cases (R = alkyl, MeOCH2), small amounts of (Rj-(2-butyltelluranyl)vinyl butyl selenides were also produced. The hydrozirconation of substituted acetylenic butyl selenides could be considered as a convenient approach for the synthesis of... 

The synthetic utility of the vinylic sulfide and selenide approach is exemplified by the synthesis of enantio-pure (i )-(-)-ipsdienol, an insect aggregation pheromone (Sch. 13) [54], Kabat and Uskokovic have demonstrated the asymmetric catalytic synthesis of la,25-dihydroxyvitamin D3 (la,25(OH)2D3) A-ring synthon by means of a glyoxylate-ene reaction catalyzed by BINOL-Ti complex (1) (Sch. 14) [55]. 

Synthesis of a-selenovinyl metals (1 -seleno-1-alkenyl metals) by metallation of vinyl selenides 2.6.2.23 Synthesis of a-metalloalkyl selenoxides, selenones and selenonium salts... 

Synthesis of a-selenovinyl metah (I-seteno-l-olkenyl metals) by metaUation of vinyl selenides... 

J Synthesis of a>Selenoalkyl Metals by Addition of Organometalllcs to Vinyl Selenides,... 

The reaction involving the alkylation of p-hydroxyalkyl selenides to give p-hydroxyalkylselenonium salts which are then cyclized with a base is by far the most general. It allows Ae synthesis of a large variety of epoxides such as tenninal, a,a- and a,p-disubstituted, tri-33-> and tetra-substituted, 3,i88 as oxaspiro[2.0.n]-hexanes, -heptanes and -octanes (Scheme 161, g Scheme 162, d Scheme 164, d Scheme 165, b) - and vinyl oxiranes (Schemes 166 and 181)33 -239 from both p-hydroxy-alkyl methyl33- 3 3 22>.222j36,263 phenyl selenides. ... 

---