Phenylselenide

Reaction of 13 [R = H, C(4-CH3OC6H4)(C6H5)2] with morpholine gives the expected 3 -deoxy-3 -morpholino-2, 2 -anhydrouridines 14 in only 10% and 45%, as well as the 2, 3 -ene-2 -mor-pholino-3-phenylselenides 15 in 45% and 25% yield, respectively105. 

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... 

Glycals are also available from 2-deoxy sugars by acid- or base-induced eliminations ofanomeric substituents. These methods are limited by the availability ofthe 2-deoxy sugars, for which the glycals themselves are the most obvious synthetic precursors. However, examples of these methods (Scheme 5.43) are in the direct preparation oftri-O-benzyl-D-glucal (14) from 2-deoxy-tri-O-benzyl-D-glucopyranose (13) via its 1-O-mesylate [117], and di-O-benzyl-D-ribal (16) from the phenylselenide 15 via oxidation to the selenoxide followed by elimination [118]. 

In a rare example of the use of phenylselenides as radical precursors in the generation of alkene radical cations by the fragmentation approach, Giese and coworkers generated a thymidine C3/,C4/ radical cation by expulsion of diethyl phosphate. Trapping experiments were conducted with methanol and with allyl alcohol (Scheme 16), when nucleophilic attack was followed by radical cyclization [66]. 

Selenium-catalysed epoxidations have been carried out using a perfluo-roalkylated phenylselenide together with hydrogen peroxide, as shown in Scheme 9.6 [13]. The catalyst could be recycled 10 times with no loss of activity, with the toxic selenide immobilized in the perfluorinated solvent. 

The introduction of the double bond of rac-14 was performed by conversion of rac-13 into its a-phenylselenide, subsequent peroxide oxidation, and elimination. Following the synthesis reported by Mehta and Srinivas, an alkylative enone transposition was used as the last step towards irradiation... 

It is not surprising that chloro esters 1, 2 readily add thiols, catalyzed by sodium thiolates or triethylamine, to give the corresponding 2-(r-organylthiocy-clopropyl)-2-chloroacetates 85,86 (Scheme 22) [15 b, 22b, 27]. This reaction with thiophenol has been used to quantify the Michael reactivity of 1-Me, 2-Me, 3-X in comparison to simple acrylates (see above). With an excess of PhSH, the nucleophilic substitution of the chlorine in 85 a (but not in 85h) proceeded to give the corresponding bis(phenylthio) derivative in 63% yield [15bj. Alkali thiolates (e.g. NaSMe, NaSBn) add smoothly onto 1-Me, 2c-Me and 2p-Me at - 78 °C, because at this temperature subsequent nucleophilic substitution of the chlorine is much slower [7l, 9]. The Michael additions of sodium phenylselenide and sodium arylsulfenates onto 1-Me and their synthetic utility have been discussed above (see Table 1). 

Tris[(2-perfluorohexyl)ethyl]tin hydride has three perfluorinated segments with ethylene spacers and it partitions primarily (> 98%) into the fluorous phase in a liquid-liquid extraction. This feature not only facilitates the purification of the product from the tin residue but also recovers toxic tin residue for further reuse. Stoichiometric reductive radical reactions with the fluorous tin hydride 3 have been previously reported and a catalytic procedure is also well established. The reduction of adamantyl bromide in BTF (benzotrifluoride) " using 1.2 equiv of the fluorous tin hydride and a catalytic amount of azobisisobutyronitrile (AIBN) was complete in 3 hr (Scheme 1). After the simple liquid-liquid extraction, adamantane was obtained in 90% yield in the organic layer and the fluorous tin bromide was separated from the fluorous phase. The recovered fluorous tin bromide was reduced and reused to give the same results. Phenylselenides, tertiary nitro compounds, and xanthates were also successfully reduced by the fluorous fin hydride. Standard radical additions and cyclizations can also be conducted as shown by the examples in Scheme 1. Hydrostannation reactions are also possible, and these are useful in the techniques of fluorous phase switching. Carbonylations are also possible. Rate constants for the reaction of the fluorous tin hydride with primary radicals and acyl radicals have been measured it is marginally more reactive than tributlytin hydrides. ... 

Dinitrophenyl phenylselenide, C6H.eSeC6H.3(N02) 2, no description of compd is found in abstract (Ref 6)... 

Diisobutylaluminum phenylselenide, /-Bu2AlSeC6H5 (1). This selenoaluminum reagent is prepared by addition of (C6H5Se)2 (1 equiv.) to DIBAH (2 equiv.) in hexane.1... 

Oxophilic phosphomum compound 47 forms from tributylphosphine (46) and the phenylselenide 45, and this then suffers nucleophilic attack by alcohol 18. The resulting free selenium nucleophile 48 displaces phosphine oxide 50 with the formation of phenylsele-nide 51. Phenylselenide 51 is oxidi/ed by hydrogen peroxide to phenylselenoxide 52, which at room temperature undergoes an elimination reaction to (-)-A9tl2)-capnellene (1). The mechanism is similar to that of the Cope elimination, proceeding via a cyclic transition state. 

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>. 

In Kuehne s total synthesis of vincadifformine (89), the pentacyclic natural product was obtained when tetracyclic phenylselenide 88 was treated with 2.5 equivalents of n-Bu SnH and the radical initiator AIBN in refluxing benzene [52]. The stereochemistry of the phenylselenyl substituent in the tetracyclic precursor had no impact on the product yield. After the tertiary radical intermediate 90 was generated (Scheme 17), the 5-exo-trig cyclization was prohibited... 

One of the first examples of radical cydization reactions in the total syntheses of indole alkaloids was Stork s approach towards ( )-gelsemine (55) [58] featuring a mixed acetal 6-exo radical cydization as the pivotal step (Scheme 23). Thus, exposure of cyclopentene bromide 117 to standard radical cydization conditions led to the cii-fiised bicyclic ester 118. A relatively dilute concentration (0.02 M) was needed to minimize possible intermolecular reactions although the intramolecular reaction was kinetically more favored. Diastereomeric phenylselenides were easily obtained by treating 118 with LDA and quenching the enolate with diphenyl diselenide. The a,P-unsaturated ester 119 was secured when the selenide underwent... 

The generation of specific radicals/lesions within DNA is often achieved upon photolyzing adequately substituted derivatives such as ferf-butyl ketone [reaction (66)] or phenylselenide derivatives [reaction (67)]. These and other reactions are discussed in Chapter 10. 

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-... 

In these experiments, the C(4 ) radical has been generated upon photolysis of the corresponding phenylselenide. The reducing agent in these experiments is as yet not ascertained. The base propenal was formed in high yields. This is in contrast to radiolytic experiments, where even with a large excess of (V- as a potential... 

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