Phenylselenide group

A concise free radical cyclization process has been applied to the synthesis of new cyclopentanone-annulated azepines 204 from chiral vinylogous amides (Scheme 26). The free radical was generated from the phenylselenide group in 203 (made in turn by N-acylation of 202) using Bu3SnH and l,l -azobis(cyclohexanecarbonitrile) (ACN), as the initiator <2004SL1917>. 

For studying mechanistic aspects of DNA strand breakage starting from the C(4 ) radical, ODNs substituted by a phenylselenide group at C(4 ) were photo-... 

The oxidation of higher alkenes in organic solvents proceeds under almost neutral conditions, and hence many functional groups such as ester or lac-tone[26,56-59], sulfonate[60], aldehyde[61-63], acetal[60], MOM ether[64], car-bobenzoxy[65], /-allylic alcohol[66], bromide[67,68], tertiary amine[69], and phenylselenide[70] can be tolerated. Partial hydrolysis of THP ether[71] and silyl ethers under certain conditions was reported. Alcohols are oxidized with Pd(II)[72-74] but the oxidation is slower than the oxidation of terminal alkenes and gives no problem when alcohols are used as solvents[75,76]. 

Substitution at the SiH moiety has been carried out with alkylthio groups, such as MeS and i-PrS. Tn s(alkylthio)silanes, (RSlsSiH, are radical-based reducing agents which can effect the reduction of bromides, iodides, xanthates, phenylselenides, and isocyanides in toluene, using AIBN as the initiator at 85... 

With a-phenylselenolactones, endocyclic elimination is preferred if the syn elimination is possible (equation 44), perhaps because the conformation leading to the exocyclic product involves greater dipole-dipole interaction between the selenoxide and the carbonyl group. However, a-methylenelactones are obtained if syn elimination cannot be achieved (equation 45). The conjugate addition of sodium phenylselenide has been recommended for the protection of ot-methylenelactones, the double bond being reintroduced by selenoxide elimination. ... 

The total synthesis of Stemona alkaloid (-)-tuberostemonine was accomplished by P. Wipf. Late in the synthesis, the introduction of an ethyl sidechain was required. This could be achieved in a novel four-step sequence. First, the allyl sidechain was introduced by the Keck radical allylation. To this end, the secondary alkyl phenylselenide substrate was treated with allyltriphenyltin in the presence of catalytic amounts of AIBN. This was followed by the introduction of a methyl group onto the lactone moiety. The allyl group then was transformed into the desired ethyl group as follows the terminal double bond was isomerized to the internal double bond by the method of R. Roy. This was followed by ethylene cross metathesis and catalytic hydrogenation to provide the desired ethyl sidechain. 

S-ethyl as LGC.114 First, the bromide donor 77 was activated over the phenylselenide acceptor 78 in the presence of AgOTf/y-collidine to give the desired disaccharide 79 in 60% yield. The latter was then directly activated over the SEt acceptor 80 by the addition of Ag0Tf/K2C03 to produce the desired trisaccharide 81 in 81% yield. Apparently, the trisaccharide 81 can be directly activated for subsequent glycosyl-ations. A similar activation sequence was reported by Ley s group.116... 

Adding appropriate nucleophiles with leaving group capabilities, such as phenylselenide (SePh) or dimethylamide (NMe2) gives access to precursors of a variety of modified cyclo-propylideneacetates bearing almost any substituent (X = OSiRj, N3, F, H, SMe, SPh). - ° The chiral methyl 2-cyclopropylidene-2-(phenylsulfinyl)acetate can be prepared from methyl 2-cyclopropylidene-2-(phenylsulfanyl)acetate under strictly anhydrous conditions. This bisac-ceptor-substituted methylenecyclopropane can be isolated, but it rapidly attracts water upon standing in air at room temperature. ... 

The presence of the bidentate chelate in allylation reactions was supported by C NMR studies with the anti halide and phenylselenide substrates in the presence of MgBr2-OEt2 [5]. The introduction of a bulky protecting group such as tert-butyldimethylsilyl ether (TBS) on the hydroxyl function led to the syn allylated product (Scheme 2) [4] and was thus shown to prevent chelation of the bidentate Lewis acid in favor of monodentate complex formation. C NMR studies with TBS ether in the presence of MgBr2 OEt2 validated this finding [5]. 

Another method for the conversion of an alkene into an allylic alcohol, but with a shift in the position of the double bond, proceeds from the corresponding p-hydroxyselenide. The p-hydroxyselenide can be obtained from the epoxide by reaction with phenylselenide anion or directly from the alkene by addition of phenylselenenic acid, phenylselenenyl chloride in aqueous MeCN, or by acid-catalysed reaction with A-phenylseleno-phthalimide. The hydroxyselenide does not need to be isolated, but can be oxidized directly with tert-BuOOH to the unstable selenoxide, which spontaneously eliminates phenylselenenic acid to form the E-allylic alcohol. For example, 4-octene gave 5-octen -ol (6.15). Elimination takes place away from the hydroxy group to give the allylic alcohol no more than traces... 

Modifications at the 4a position starting with the 4a-hydroxy derivative [1, 57], with suitable protection of the remaining hydroxy groups lead via a 4a-phenylselenide to a 3-hydro-4a-dehydro-3-hydroxy derivative containing the 4-exocyclic double bond [58]. Further manipulation via the 4a-bromide leads to avermectin B with the correct stereochemistry of the crucial hexahydrobenzo-furan moiety, thus adding to the choice of suitable intermediates for total... 

Sodium phenylselenide generated by this route has been shown to react smoothly with a variety of organic substrates capable of undergoing nucleophilic substitution, including halides, epoxides, and sulfonates. The yields of selenide products are generally excellent and the mild conditions employed ensure that other potentially sensitive functional groups such as THP ethers, ketones, and dioxanes remain intact. 

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