Phenylselenic acid

Taylor and Flood could show that polystyrene-bound phenylselenic acid in the presence of TBHP can catalyze the oxidation of benzylic alcohols to ketones or aldehydes in a biphasic system (polymer-TBHP/alcohol in CCI4) in good yields (69-100%) (Scheme 117) °. No overoxidation of aldehydes to carboxylic acids was observed and unactivated allylic alcohols or aliphatic alcohols were unreactive under these conditions. In 1999, Berkessel and Sklorz presented a manganese-catalyzed method for the oxidation of primary and secondary alcohols to the corresponding carboxylic acids and ketones (Scheme 118). The authors employed the Mn-tmtacn complex (Mn/168a) in the presence of sodium ascorbate as very efficient cocatalyst and 30% H2O2 as oxidant to oxidize 1-butanol to butyric acid and 2-pentanol to 2-pentanone in yields of 90% and 97%, respectively. This catalytic system shows very good catalytic activity, as can be seen from the fact that for the oxidation of 2-pentanol as little as 0.03% of the catalyst is necessary to obtain the ketone in excellent yield. 

Ni(OH)2-electrode 48) % Phenylselenic acid Selenium dioxide > Manganese dioxide... 

The cyclohexene 121, which was readily accessible from the Diels-Alder reaction of methyl hexa-3,5-dienoate and 3,4-methylenedioxy-(3-nitrostyrene (108), served as the starting point for another formal total synthesis of ( )-lycorine (1) (Scheme 11) (113). In the event dissolving metal reduction of 121 with zinc followed by reduction of the intermediate cyclic hydroxamic acid with lithium diethoxyaluminum hydride provided the secondary amine 122. Transformation of 122 to the tetracyclic lactam 123 was achieved by sequential treatment with ethyl chloroformate and Bischler-Napieralski cyclization of the resulting carbamate with phosphorus oxychloride. Since attempts to effect cleanly the direct allylic oxidation of 123 to provide an intermediate suitable for subsequent elaboration to ( )-lycorine (1) were unsuccessful, a stepwise protocol was devised. Namely, addition of phenylselenyl bromide to 123 in acetic acid followed by hydrolysis of the intermediate acetates gave a mixture of two hydroxy se-lenides. Oxidative elimination of phenylselenous acid from the minor product afforded the allylic alcohol 124, whereas the major hydroxy selenide was resistant to oxidation and elimination. When 124 was treated with a small amount of acetic anhydride and sulfuric acid in acetic acid, the main product was the rearranged acetate 67, which had been previously converted to ( )-lycorine (108). 

The conversion of the cycloadduct 546 to ( )-crinamine (376), ( )-6-hy-droxycrinamine (379), (+)-criwelline (398), and (+)-macronine (401) commenced by reduction of 546 with NaBH4 followed by acetylation to produce solely the acetate 554, which arose by hydride attack on the hindered neopentyl ketone exclusively from the convex face syn to the aryl substituent (Scheme 50) (216). The stereo- and regioselective addition of the elements of PhSeOMe to the carbon-carbon double bond of 554 followed by oxidation of the intermediate methoxy selenide and elimination of phenylselenous acid gave the allylic ether... 

Electron-deficient alkenes, either these with conjugating substituents or of enones, undergo Michael-like additions with regiospecific introductions of the nucleophiles, and this approach has been successful for the formation of cyclopropyl derivatives—notably from some nucleosides. An example involves the addition of the anion of bis(phenylsulfonyl)methane to the phenylselenone 173 which gives the adduct 174 in 35% yield. This reaction presumably occurs by Michael addition to C-2 followed by Sn2 ring-closure reaction at C-3 with displacement of phenylselenic acid. Reductive desulfonylation affords the 2,3,-dideoxy-2,3 -cyclopropayuridine 175.199... 

Sharpless has shown that phenylselenic acid catalyzes the epoxidation of olefins with hydrogen peroxide or f-BuOOH [41]. However, the toxicity of selenium compounds precludes many applications of this catalytic epoxidation. To avoid contamination of the reaction products with selenium compounds polystyrene-bound phenylselenic acid has been used [42]. An alternative to the solid phase chemistry is the immobilisation of the selenium catalyst in the fluorous phase... 

The antineoplastic agent (— )-aplysistatin (998), isolated from the sea hare, is synthesized from (7 )-malic acid via butyrolactone 987 by a biomimetic brominative cyclization of the homogeranyl-alkylated lactone 996 (Scheme 147) [215]. The cyclization occurs in poor yield, and affords a 19 81 mixture of isomeric dihydroaplysistatins (997), the desired isomer being the minor component. The mixture is converted to (— )-aplysistatin (998) and ( + )-12-epiaplysistatin (999) by phenylselenation of the lactone enolates followed by oxidative elimination of phenylselenic acid. The mixture (12 88) is separated by HPLC to give the pure compounds. 

Holmes and co-workers have also applied the Claisen rearrangement to the synthesis of mediumsized lactams. The thermal elimination of phenylselenic acid from the selenoxide (366) gave the unisolated ketene aminal (367), which rearranged to the unsaturated nine-membered lactam (368) (Scheme 56) <92TL6857, 96JCS(P1)123>. 

Thionyl chloride undergoes sonolysis at the S-Cl bond. The chlorine atoms react with bis(trialkylsilyl) ethers to give trialkylsilyl chlorides in good yields. A similar mechanism can be operative in the transformation of phenylselenous acid to phenylselenium trichloride, obtained in 88% yield in 15 min in pentane. ... 

Tayl983 Taylor, R.T. and Rood, L.A., Polystyrene-Bound Phenylselenic Acid Catalytic Oxidations of Olefins, Ketones, and Aromatic Systems, J. Org. Chem., 48 (1983) 5160-5164. 

Grieco, P.A. Miyashita, M. J. Org. Chem., 1974,39,120. a-Phenylselenation can also be accomplished with PhSeSePh, Se02, and an acid catalyst Miyoshi, N. Yamamoto, T. Kambe, N. Murai, S. Sonoda, N. Tetrahedron Lett., 1982, 23, 4813. 

In a closely related asymmetric reaction, the required absolute stereochemistry at C-4 was established via a Michael addition of a cuprate reagent to a dihydropiperidinone (Scheme 12). The stereochemistry at C-3 was introduced in the form of piperidinone 61, a compound readily available from (5)-glutamic acid. Protection of both the amino and alcohol functionalities was achieved using standard reaction conditions to give 62. Introduction of the A -double bond was accomplished via phenylselenation of the lithium... 

Suggest a possible reason why the phenylselenation reaction of step o required acid catalysis. 

The first example of a catalytic approach to the selenium promoted conversion was reported by Torii, who described an oxyselenenylation-deselenenylation process using catalytic amounts of diphenyl diselenide [115]. The electrophilic species was produced from the diselenide by electrochemical oxidation in the presence of the alkene 233 in methanol or in water. As indicated in Scheme 36, the addition product is electrochemically oxidized to afford the selenoxide which by elimination gives the allylic ether or alcohol 234 and the phenylselene-nic acid which continues the cycle by adding again to the alkene 233. 

CSA is also the acid of choice for use in phenylselenation reactions. It has been used as an acid catalyst in hydroxysele-nation reactions of alkenes (eq 8) and organoselenium-induced cyclizations (eq 9) using N-Phenylselenophthalimide (NPSP). ... 

The traditional approach to preparing benzoselenazoles has been to react the o-amino-phenylselenate as the zinc salt ( ) with an acid chloride <79HC(34)217>. The synthesis of the parent benzoselenazole (4) from formic acid, however, has been more difficult. By using the N,N-dimethylchloroiminium chloride (Scheme 11) a 66% yield of benzoselenazole (4) was obtained... 

Cyclic phenylselenated products are obtained when this reaction is applied to alkenes containing hydroxy, benzamido, enolizable ketones and carboxylic acids as remote functional groups. For example, the alkenol derivative 542 reacts with diphenyl diselenide and (diacetoxyiodo)benzene in acetonitrile to furnish C-glycoside 543 in moderate yield (Scheme 3.215) [603]. 

Addition of 2,4-dimethylbenzenesulfenyl chloride to the glycal derived from tetra-O-acetyl-neuraminic acid methyl ester, followed by reaction with sodium methylthiolate, furnished the 2-methylthio-3-thiosialic acid derivative 14. Its use as a glycosyl donor is referred to in Chapter 4, and the azido phenylselenation of a furanoid glycal is covered in Chapter 9. 

D-glutamic acid [148], Phenylselenation of enolate generated from 171 proceeded highly diastereoselectively and led to 172. 

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