A-Phenylseleno ester

Acyl radicals can be generated and they cyclize in the usual manner. A polyene-cyclization reaction generated four rings, initiating the sequence by treatment of a phenylseleno ester with Bu3SnH/AIBN to form the acyl radical, which added to the first alkene unit. The newly formed carbon radical added to the next alkene, and so on. Acyl radicals generated firom Ts(R)NCOSePh derivatives cyclize to form lactams. ... 

Displacement of activated halides. a-Halo esters, amides, and acetals undergo radical allylation, which is initiated by AIBN or light. A related transformation, such as displacement of a-seleno esters (e.g., that of p-hydroxy-a-phenylseleno esters) requires a Lew is acid (e.g., MejAl) to give a high anh-selectivity. 

Comasseto et al. reported the smooth reductive deselenation of a-phenylseleno esters in the presence of 2.5 mol% of 211 (Scheme 7.33) [160]. In this highly unusual reaction, a putative mechanistic pathway involves a tellurolate (from NaBH -reduction) that attacks at selenium in a nncleophilic substitution reaction with the ester enolate as leaving group. 

The methanesulfonates (481) of a-hydroxy esters can be converted to the deoxygenated esters (482) in 70 88% yields by indirect electrolysis with PhSeSePh as a recyclable reagent in a divided cell (Scheme 166). The procedure involves the formation of a-phenylselenoester by substitution of the a-methylsulfonyloxyl group with the PhSe followed by displacement of the CK-phenylseleno group with PhSe . The electrolysis is performed in a DMF-NaCl04-(Pt/C) system in the presence of PhSeSePh and ethyl malonate at 50 °C [567]. 

By treatment with NaBH4 in aqueous NaOH in the presence of dithienyl ditelluride, a-phenylseleno carboxylic esters and malonates, as well as a-phenylseleno carboxylic acids, are deselenylated to the corresponding seleno-lree acids in good yields. The selective removal of the seleno group without dealkylation of the ester moiety is achieved by the methods depicted in the scheme. 

OrPhenybelenenylation of a,f -unsaturated esters.3 (F)-3r,/MJnsaturated esters are converted to a-phenylseleno-a,0-unsaturated esters in 20-65% yield by reaction with LDA followed by C6H5SeBr. The reaction is related to the reaction of a,/ -enones with pyridine and C H5SeCl (9, 28-29), and is also believed to involve conjugate addition of the base followed by selenenylation of the enolate. 

P,y-Unsaturated a-amino esters. y-Phenylseleno-a, (3-unsaturated esters on treatment with NCS (3 equiv.), N,N-diisopropylethylamine (6 equiv.), and an alkyl carbamate (3 equiv.) in methanol at 25° rearrange to derivatives of p,y-unsaturated a-amino acids, generally in 60-80% yield.2... 

Free-radical cyclization of phenyl selenide 15 to indolizidinone 16 represented a key step in the total synthesis of (—)-slaframine (equation 52). The two pairs of diastereomers were first separated and then hydrolyzed to the corresponding alcohols in 76% overall yield77. (TMS)3SiH-mediated acyl radical reactions from phenylseleno esters 17 have recently been utilized for the stereoselective synthesis of cyclic ethers78. In fact, the experimental conditions reported in equation 53 are particularly good for both improving cis diastereoselectivity and suppressing decarbonylation. 

Another useful reaction for the difunctionalization of olefins involves a group transfer carbonylation starting from a a-(phenylseleno)carbonyl (or related derivatives) and a terminal alkene under 80 atm of CO. An alkyl radical is first formed by the photocleavage of a C-SePh bond. The addition of this radical to the olefin, followed by the incorporation of CO and radical coupling with PhSe-, gave substituted selenoesters via a three-component coupling reaction [74], The intermolecular formation of C—C bonds via phenylseleno group transfer has been likewise adopted in the reaction between ester-substituted O,Se-acetals and an olefin [75],... 

TMS)3SiH-mediated acyl radical reactions of phenylseleno esters have been utilized for the diastereoselective synthesis of cyclic ethers by Evans s group. Equation 44 shows that the acyl selenide 83 affords the cA-disubstituted tetrahydrofuran 84 in good yield on exposure at room temperature to (TMS SiH and the Et3 B/02 combination as the initiator via a 5-exo-trig cyclization97 98. 

Another synthetic route to ( )-retronecine (27) has been developed by Vedejs and Martinez.11 The protected hydroxy-lactam (22) was prepared from 2-methoxy-1-pyrroline by known methods. The key ylide intermediate (24) was then generated from the salt (23) by desilylation with caesium fluoride (Scheme 6). This ylide (24) reacted with methyl acrylate in a 1,3-dipolar cycloaddition to afford the unsaturated pyrrolizidine (25) in 57% yield from the lactam (22). Catalytic hydrogenation of the ester (25) gave an unstable endo-product, which epimerized to the exo-form (26). Introduction of the 1,2-double-bond into (26) was carried out by insertion and thermal elimination of a phenylseleno-group.12 Reduction then yielded ( )-retronecine (27). 

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. 

Scheme 2. Chelation-controlled radical reduction and allylation of a-halo- and a-phenylseleno-yS-alkoxy esters...

The phenylseleno group can be preset quencher) at low temperatures to gise s Cleavage of oximes. Carbonyl cot Cleavage of stabilized Wit tig reage ucts therefore, a-keto esters and 2.3-di... 

The most useful method is reaction of ketone (and ester) hthium enolates, usually prepared by deprotonation of ketones with LDA, with either Ce H5 SeBr or Ce Hs SeCl. Enol acetates can be converted into a-phenylseleno ketones by reaction with phenylselenenyl trifluoroacetate, prepared in situ by treatment of CeHsSeCl or CeHsSeBr with silver trifluoroacetate or by conversion to the lithium enolate and reaction with CeHsSeBr. It is sometimes possible to obtain isomeric a-phenylseleno ketones by use of these two methods (equations 1 and II). 

The introduction of a phenylseleno group on the a-carbon followed by peroxide oxidation to give the triethyl ester of 2-phosphonopropenoic acid is an alternative to the piperidine-catalysed condensation of triethyl phosphonoacetate with formaldehyde as examples of the conversion of an a-sp -carbon into an a-sp -carbon. 

For example, when a benzene solution of methyl a-(phenylseleno)acetate 63 and 1-octene (50 equiv) was irradiated using a 500 W xenon lamp (>300 nm) for 20 h at 60 °C under 80 atm of CO pressure, selenol ester 64, the three-component coupling product, was formed with 58% yield. In order to irradiate under CO pressure, an autoclave equipped with quartz windows was employed in this work. The reaction pathways shown in Scheme 18 may account for the formation of the selenol esters (1) the photoinduced homolysis of methyl a-(phenylseleno)acetate 63 takes place (2) the (methoxycarbonyl)methyl radical 65 generated adds to... 

Most of the published syntheses of necine bases have been directed towards fully saturated pyrrolizidine derivatives. Robins and Sakdarat have developed a method for the conversion of saturated pyrrolizidine esters into their 1,2-didehydro-derivatives. Thermal elimination of a phenylseleno-group was used to introduce the unsaturation, and ( )-supinidine (17) was synthesized using this technique. The ester (5) is most conveniently prepared by the two-step stereospecific route of Pizzorno and Albonico. Phenylselenenylation of the lithium enolate derived from the ester (5) gave the phenylseleno-ester (15), which was reduced to the corresponding alcohol (16) (Scheme 5). syn-Elimination of the selenoxide derived from (16) gave ( )-supinidine (17). Each step proceeded in about 60% yield. None of the isomeric 1,8-didehydro-base was detected. 

In contrast to this sequence of events, when the C-2 position is occupied by an ester, a phenylthio, or a phenylseleno group, a preliminary intramolecular cyclization occurs prior to the addition of... 

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