Phenylseleno esters

Reductive decarbonylation and decarboxylation can be carried out by (TMSlsSiH using acyl chlorides, phenylseleno esters, or N-hydroxypyridine-2-thione esters. Examples are shown in Reactions (17)-(19). Hydrolysis of the methyl ester followed by decarbonylation at the C2 position of hexahydropyrro-loindole (+)-17 afforded the desired tricycle (+)-18 in 84% yield and >99% ee. ... 

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. ... 

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. 

Acyl radicals from RCOSeCfH. Acyl radicals generated from phenylseleno-esters can participate in inter- and intramolecular alkene addition. The latter tandem... 

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. 

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. 

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. 

Depending on the sequence selected, this epoxide can be converted into either the ( Z)- or ( )-isomer of 3-deuterio-PEP. Opening of the epoxide with diisobutyl-aluminum phenylselenide in hexane affords selectively (3 1) the 2-hydroxy-3-phenylseleno ester. Phosphorylation, oxidation to the selenoxide, and syn-elimination then give the Z-isomer. 

The preparation of optically pure cyclohexanones by radical-induced cyclization of phenylseleno esters onto a terminal double bond has been described. The method was also extended to the production of bicyclo derivatives such as 81 from seleno ester 80. 

Using (TMS)3SiH as the mediator, phenylseleno esters can be conveniently used as a precursor of acyl radicals. An example is the key step for the enantioselective synthesis of the nonisoprenoid sesquiterpene (—)-kumausallene, obtained by radical cyclization at low temperature in a 32 1 mixture in favor of the 2,5-cis diastereoisomer (eq 28). ... 

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. 

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