Selenoxides formation

Holmes has also reported an interesting route to synthesise ketene a 0-acetals starting with phenylselenyl acetal 286. In the example, transacetalization of 285 with aminoalcohol derivative 284 results phenylselenyl A, 0-acetal 286. Selenoxide formation and syn-elimination... 

Sulfonesand sulfonic esters can also be alkylated in the a position if strong enough bases are used. Alkylation at the a position of selenoxides allows the formation of alkenes, since selenoxides easily undergo elimination (17-11). 

Like amine oxide elimination, selenoxide eliminations normally favor formation of the E-isomer in acyclic structures. In cyclic systems the stereochemical requirements of the cyclic TS govern the product composition. Section B of Scheme 6.21 gives some examples of selenoxide eliminations. 

In the case of the most reactive compounds, substitution at the carbon atom of diselenonium and ditelluronium dications is also a possible pathway. For example, formation of diselenide 117 from selenoxide 115 was explained by demethylation of intermediate dication 116 with trifluoroacetate anion.126 Dealkylation of salt 118, which is stable up to —20°C, leads to formation of nitrilium salt 119. The latter is transformed to amide 120 upon hydrolysis.64 Dealkylation of intermediate diselenonium dication 122 was suggested as the key step in the oxidative synthesis of 1,2,4-diselenazolidines 123 from eight-membered heterocycles 121 (Scheme 46).127... 

The only example of an organic dicationic system with participation of oxygen was described for the selenuranium dication 145 prepared by reaction of selenoxide 146 with triflic anhydride.144 X-ray analysis of this dication confirmed formation of a hypervalent structure with an almost collinear... 

On the other hand, optically active telluroxides have not been isolated until recently, although it has been surmised that they are key intermediates in asymmetric synthesis.3,4 In 1997, optically active telluroxides 3, stabilized by bulky substituents toward racemization, were isolated for the first time by liquid chromatography on optically active columns.13,14 The stereochemistry was determined by comparing their chiroptical properties with those of chiral selenoxides with known absolute configurations. The stability of the chiral telluroxides toward racemization was found to be lower than that of the corresponding selenoxides, and the racemization mechanism that involved formation of the achiral hydrate by reaction of water was also clarified. Telluroxides 4 and 5, which were thermodynamically stabilized by nitrogen-tellurium interactions, were also optically resolved and their absolute configurations and stability were studied (Scheme 2).12,14... 

Furthermore, the first catalytic synthesis of allenes with high enantiomeric purity [15c, 25] was applied recently to the pheromone 12 by Ogasawara and Hayashi [26] (Scheme 18.7). Their palladium-catalyzed SN2 -substitution process of the bromo-diene 16 with dimethyl malonate in the presence of cesium tert-butanolate and catalytic amounts of the chiral ligand (R)-Segphos furnished allene 17 with 77% ee. Subsequent transformation into the desired target molecule 12 via decarboxylation and selenoxide elimination proceeded without appreciable loss of stereochemical purity and again (cf. Scheme 18.5) led to the formation of the allenic pheromone in practically the same enantiomeric ratio as in the natural sample. 

It is interesting to discern the different migratory aptitudes displayed by spirocyclic ketones on exposure to hydrogen peroxide [252] (Eq. 226,227). Although participation of the selenium atom to direct the attack of the peroxide has been formulated, the results are also consistent with an electronic explanation. Thus, the rapid formation of selenoxide renders the spirocyclic center acceptor-like through polarity alternation, and the migration of the methylene group becomes more favorable. 

Formation of CK-configurated cyclobutanones has also been observed with 2-methylcyclopen-tanone and 2-methylcyclohexanone/8 However, stereoreversed eyclobutanone formation can be achieved by opening the intermediate oxaspiropentane with sodium phenyl selenide, oxidation of the resulting / -hydroxy selenide with 3-chloroperoxybenzoic acid and subsequent rearrangement in the presence of pyridine/18 Thus, from one oxaspiropentane 8, either stereoisomeric eyclobutanone cis- or lrans-9 was produced. The stereoreversed eyclobutanone formation proceeds from a stereohomogenous / -hydroxy selenoxide and is thought to be conformationally controlled. 

Many examples of natural furans are recorded as having been prepared from five-membered heterocycles such as 2(5H)-furanones (butenolides), which are reduced to furans with diisobutylaluminum hydride. The facile elimination of selenoxides derived from a-phenylseleneyl-y-lactones with formation of endocyclic a,/3-unsaturated butenolides is reported (75JOC542) as a useful route to 2,4- and 2,3,4-substituted furans via their corresponding butenolides. The mixture of dihydrofurans obtained from the tosylhydrazone of tetrahydro-2-furanone (Scheme 88) was oxidized to furans by 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (66CJC1083). 

Selenides (290) and (292) were oxidized to the unstable selenoxides (298) and (299) by aqueous hydrogen peroxide under controlled conditions or by sodium metaperiodate. When selenoxide (298) was subjected to the Pummerer reaction using acetic anhydride in the presence of N- phenylmaleimide, a mixture of exo/endo adducts (144) was obtained in 58% yield, indicating the transient formation of the selenolo[3,4-c]thiophene (143 Scheme 100) <77H(6)1349). 

Phenyl methyl selenium dihydroxide, (C6H5)(CH3)Se(OH)2.— The dibromide (10 grams) is triturated with a suspension of 12 grams of silver oxide in 100 c.c. of water until the yellow colour disappears. Filtration, evaporation and desiccation yield about 8 c.c. of the hydroxide as a viscous oil. When heated at 100° C. at 15 mm., or at 170° C. at 760 mm., it decomposes, yielding phenyl methyl selenide, diphenyl diselenide and formaldehyde. The formation of formaldehyde is in agreement with the view that phenyl methyl selenoxide may exist in two isomeric forms, which, being unstable, decompose thus ... 

Similar reactions were also achieved by the formation of diastereomeric optically active selenoxides as intermediates in the elimination reaction. Optically active ferrocenyl diselenide 19 was used in selenenylations of alkynes generating vinyl selenides of type 164. Oxidation of the selenides was performed with mCPBA under various reaction conditions which afforded the corresponding chiral selenoxides, which, after elimination, afforded axial chiral allenecarboxylic ester derivatives 165 in high enantioselectivities (R = Me 89% ee, R=Et 82% ee, R = C3H7 85% ee) (Scheme 47)>85 87... 

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