Phenylseleno oxidative elimination

Selenolate 1 [1,2], prepared from diphenyl diselenide by reduction with NaBH4 in ethanol, is not reactive enough towards esters and lactones. However, application of more rigorous conditions, i. e. at 110-120 °C in dry DMF for lactones 15, affords the corresponding ring-opened products, cu-phenylselenyl carboxylic acids 16, in good yields [44]. Compounds 16 can be easily converted to co-ole-finic methyl esters 17 by treatment with diazomethane followed by oxidative elimination of the phenylseleno group (Scheme 25). By contrast, the uncomplex-... 

The addition of PhSeCl to enone (—)-322 (obtained by saponification of 320) exclusively gives adduct 354. After oxidative elimination of the phenylseleno group and subsequent reduction, endo alcohol 355 is obtained in high yield. Applying the reaction sequence described in Scheme 13.92, 355 is converted into derivatives of 2,5-anhydro-3-deoxy-L-vy/6>-hexuronate 356 and 2,5-anhydro-4-deoxy-D-xy/o-hexonic acid 357 (Scheme 13.93). Inversion of configuration of alcohol 355 via... 

Addition of PhSeBr to enone (—)-322 gives adduct 361, which undergoes oxidative elimination of the phenylseleno group to give bromoenone 362. Ketone reduction and protection of the endo alcohol furnishes 363, and the double hydroxylation of its chloralkene unit generates 364 after acetylation. Baeyer-Villiger oxidation yields uronolactone 365, which was converted into 3-deoxy-a-D-araZ mt -hexopyranoside [178] and into methyl 4-deoxyhexonate 368. Finally, it was converted into 4-deoxy-D-fyxo-hexose derivative 369 (Scheme 13.94) [179]. 

The procedures used most often for preparation of arylsulfonyl acetylenes Involve oxidation of the corresponding ethynyl thio ether. The thio ethers are usually obtained via a two-step sequence beginning with two-fold thiophenoxide displacement of chloride ion from cis-l,2-dichloroethylene, followed by elimination with n-butyl1ithium in the resultant cis-1,2-bisarylthioethylene. Less well known methods involve diazotization of 4-arylsulfonyl-5-aminoisoxazoles, dehydrobrpmination of cis- and trans-2-bromovinyl phenyl sulfone with fluoride ion, and oxidative elimination of - phenylseleno)vinyl sulfones. The method described here, which bypasses the need for strongly basic conditions, is adapted from the work of Bhattacharya, Josiah, and Walton. The simplicity and mildness of the method suggest that it may be broadly useful. 

Trimethyl 3-(phenylseleno)orthopropanoate 41 is utilized as an intermediate in the preparation of a-methylene-y-butyrolactones 42 via Claisen ortho ester rearrangement with allylic alcohols followed by oxidative elimination of phenylselenol233. 

Phenylseleno)-9-borabicyclononane prepared by the reaction of 9-BBN with benzeneselenol (Eq. 32.6) undergoes a novel conjugate addition to a variety of a, (3-unsaturated ketones to alford P-selenoboron enolates, which are converted to unsaturated ketols (Eq. 32.7) on treatment with aldehydes and subsequent oxidative elimination [17]. 

Tomoda and his group have reported two procedures for the preparation of l-(phenylseleno)acetylenes from terminal acetylenes which tolerate diverse functionality. Miura and Kobayashi have shown that terminal acetylenes (284) can be transformed into the acetylenic sulphones (287) by the addition of phenyl aryl selenosulphonates (285) and subsequent oxidative elimination of phenyl-selenenic acid from the vinyl sulphones (286). ... 

Greico has further applied the double oxidative elimination of selenium to form the two olefinic bonds in tuberiferine (88) at the same time. He also points out that an a-phenylseleno substituent on an a-methylbutyrolactone can survive a number of synthetic transformations, and thereby act as a protected form of the a-methylenebutyrolactone. Total syntheses of the related lactones ( )-confertin (89)105 and ( )-damsin (90) have been reported. Although the Minato-Horibe sequence for the introduction of the a-methylene group into a butyrolactone was successful in the latter case, it failed in the case of confertin and hence an alternative sequence was used involving condensation of the lactone with dimethyl carbonate, followed by enolate reduction and oxidation. 

Stereochemical outcome of the route hinged on an unprecedented phenylseleno-amidation of the trichloroacetimidate derivative of 279, which gave a 15 1 mixture of traw-oxazoline 280 and its cis isomer in 63% yield. Acidic hydrolysis followed by oxidative elimination of the selenium substituent yielded alkene 281, from which the 3-hydroxypyrrolidine 282 was readily prepared. Two different sets of transformations, both involving cfr-dihydroxylation of the double bond, were applied to 282 to complete divergent pathways to (+ )-239 and (+ )-240. A fiirther interesting twist to this methodology provided access to the minor Castanospermum alkaloid ( + )-7-deoxy-6-epicastanospermine (242) (Scheme 38, bottom line) (47). When the bis-... 

Selenothiolactonization Cycloalkenethiolcarboxylic acids react with 1 to form S-acyl phenylselenosulfides (2), which undergo cyclization to phenylseleno-thiolactones (3) when refluxed in benzene in the presence of AIBN. The C6H5Se group of 3 is selectively oxidized by m-C 1C6H4C03H, and subsequent selenoxide elimination results in thiolactones (4). 

From a- or 0- substituted aldehydes or ketones by elimination reactions Benzeneselenenyl trichloride, 27 Methanesulfonyl chloride-4-Di-methylaminopyridine, 176 9-(Phenylseleno)-9-borabicyclo-[3.3.1]nonane, 245 By oxidation of allylic substrates Pyridinium chlorochromate-Benzo-triazole, 262 By other methods Alumina, 14... 

The reaction of cyclohexene with AT-(phenylseleno)phthalimide in the presence of (S,S)-hydrobenzoin in methylene chloride afforded two diastereomeric oxyselenides (38 and the (li , 2R) diastereomer) in a 1 1 ratio. Compound 38 was separated and converted into the olefin 39 via selenoxide elimination. The second PhSeOTf promoted oxyselenenylation reaction gave only the cis fused bicyclic dioxane 40. Oxidation and subsequent elimination provided the olefin 41. This is the key intermediate for the syntheses of the cyclitols 42 and 43, that were obtained from a series of classical reactions as indicated in the Scheme. Oxyselenenylation reactions have also been employed to promote glycosylation reactions [43]. 

Sonoda and co-workers have found a novel selenoxide anfz-elimination by oxidation of (Z)-l,2-bis(phenylseleno)-l-alkenes. The oxidation of (Z)-l,2-bis(phenylseleno)-l-octene with mefa-chloroperbenzoic acid (MCPBA) and the subsequent work-up afforded the corresponding acetylenic phenyl selenoxide in 67% yield (Scheme 6). This result indicates that the anfz-elimination of... 

From enolates via selenoxidesA Lithium enolates derived from ketones, lactones, and esters react with PhSe-SePh or with phenylselenyl bromide or chloride (PhSeX) to form a-(phenylseleno)carbonyl compounds. These can be oxidized to the corresponding selenoxides with subsequent 5yn-elimination of benzeneselenic acid to form enones. 

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