Acetal selenoxide

The preparation of the requisite y-keto-p-toluenesulfonate rac-35 as homo-Favorskii precursor commenced with commercially available 2,5-dihy-droanisole (36) that was protected and epoxidized to acetal rac-31 (Scheme 11). Regioselective opening of the epoxide with p-chlorophenylse-lenide followed by sequential oxidation to the selenoxide and thermal elimination generated an allylic alcohol that was protected to give pivaloate rac-38. 

Bromination of norperistylane-5,ll-dione (835) gives rapidly and quantitatively the Cj -symmetric dibromide 836a. Similarly, reaction with phenylselenyl chloride delivers 836b . Decomposition of the bis(selenoxide) in glacial acetic acid led to diketo diacetate 837. This product enters into twofold exchange reactions with representative nucleophiles. 

DIHYDROXYLATION Osmium tetroxide-Dihydroquinine acetate. Osmium tetrox-ide-Diphenyl selenoxide. Osmium tetroxide-Trimethylamine N-oxide-Pyiidine. Thallium(l) acetate-iodine. Triphenylmethylphosphonium permanganate. 

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). 

In a further development of this approach, the synthesis of cr,/J-acetylenic acyl silanes has been achieved as shown in Scheme 4514. Oxidation of the 3-selenenyl allenyl ethers (16) with m-chloroperbenzoic acid at —78 °C gave the corresponding unstable selenoxides, which underwent in situ [2,3] sigmatropic shift producing acetals (17). Loss of selenenyl ester on work-up gave the cr,/J-acetylenic acyl silanes in ca 50% yields. 

Further, medium-sized lactones have been prepared by a thermal elimination-Claisen rearrangement sequence, of unsaturated selenoxide cyclic acetals (equation 198)710. The reaction affords reasonable yields of these useful lactones upon treatment with DBU and a siloxy species at 185 °C. The reaction has been used as the key step in the synthesis of (-l-)-laurencin, which contains an 8-membered cyclic ether moiety711. 

E2 elimination reactions occur preferentially when the leaving groups are in an anti copla-nar arrangement in the transition state. However, there are a few thermal, unimolecular sy -eliminations that produce alkenes. For example, pyrolysis of several closely related amine oxides, sulfoxides, selenoxides, acetates, benzoates, carbonates, carbamates and thio-carbamates gives alkenes on heating (Scheme 4.10). The syn character of these eliminations is enforced by a five- or six-membered cyclic transition states by which they take place. 

An intramolecular version of this Claisen rearrangement was used for the synthesis of a natural 10-membered lactone, phoracantholide J (8). The synthesis involves preparation of 6 by known reactions. Treatment of 6 with C MsSeBr in the presence of Hiinig s base gives the cyclic acetal 7 as a mixture of diastereoisomers in 71% yield. Decomposition of the selenoxide results in the rearranged y,B-unsaturated lactone 8 as the major product. The ready formation of a 10-membered ring is noteworthy because yields of rings of this size are low on lactonization of 0-hydroxy acids. 

Moreover, tram-33 is also obtained in 77% yield on reaction of acetoxymethy] methyl selenide with cyclohcxene in the presence of hydrogen peroxide 7 or on heating cyclohexene with an excess of dimethyl selenoxide in a mixture of acetic acid and chloroform 8. 

Electrophilic addition of PhSeCl to alkene 319 is highly stereo- and regioselective (steric effects) and provides adduct 338 if carried out in the presence of an acetate nucleophile. Methanolysis followed by treatment with formalin and protection of the endo alcohol gives ketone 339. Low-temperature oxidation of the selenide 339 generates a selenoxide, which does not... 

Lithium enolates derived from methyl ketones and acetates successfully add to vinyl selenoxides - and to vinyl selenones to give cyclopropyl ketones and esters (Scheme 110). The a-lithioalkyl selenoxide or selenone intermediates presumably exchange to the lithium enolates which, after displacement of the seleno or selenoxy group, lead to the observed products. 

Although the preference for selenoxide elimination to take place away from a 3-oxygen substituent is very marked, the elimination will still occur towards the oxygen substituent if there is no alternative. This has been used in elegant syntheses of ketene acetals derived from allylic alcohols, which are useful Claisen rearrangement precursors (Scheme 36)" and for the synthesis of phenylseleno methyl ketones. ... 

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