Phenylseleno group

Treatment of tetrahydroberberine (26) with sodium benzenethiolate (48) or -selenolate (49) in the presence of ruthenium catalyst afforded the C-14—N bond cleavage products 51 or 52 with a phenylthio or phenylseleno group at C-14 (Scheme 12). The latter was converted to the 10-membered amino olefin 53 on treatment with m-chloroperbenzoic acid. 

Finally, it must be mentioned that phenylselenation of some diolefins may provide a suitable method for the construction of heterocycles containing two phenylseleno groups. For instance, 3 reacts142 with lV-(phenylseleno)phthalimide (NPSP) in the presence of cyanamide (FENCN) to give the regioisomeric 9-azabicyclo[3.3.1]- and 9-azabicyclo [4.2.1]-nonanes, 173 and 174, as the result of a combined process of inter- and intramolecular nucleophilic addition of cyanamide (equation 140). 

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]. 

Secondary alkyl selenides are reduced by (TMS)3SiH, as expected in view of the affinity of silyl radicals for selenium-containing substrates (Table 4.3) [40]. Reaction (4.23) shows the phenylseleno group removal from the 2 position of nucleoside [50]. Similarly to 1,3-dithiolanes and 1,3-dithianes, five- and six-membered cyclic selenoacetals can be monoreduced to the corresponding selenides in the presence of (TMS)3SiH [51]. The silicon hydride preferentially approached from the less hindered equatorial position to give transicis ratios of 30/70 and 25/75 for the five-membered (Reaction 4.24) and six-membered cyclic selenoacetals, respectively. 

Activation of alkenes towards nucleophilic attack by a variety of nucleophiles employing A-phenylselenophthalimide or A-phenylselenosuccinimide as an electrophilic source of the phenylseleno group. 

In an analogous sequence using the corresponding selenides, the yield of the substitution of the bromine for the phenylseleno group is higher. This result depends on the... 

The same reaction, but in the presence of triflic acid, results in /i-acetamido selenides by a mv-addition. In the case of a terminal olefin, the phenylseleno group is introduced mainly at the terminal carbon atom.13... 

Cyclization of some jS-dicarbonyl compounds. In the presence of this reagent and a catalyst (12, />-TsOH, Znl2), /J-dicarbonyl compounds containing an alkenyl substituent undergo cyclization with incorporation of a phenylseleno group. 

The formation of bicyclo compounds by the radical cyclization of alkenes with strategically placed phenylseleno groups, promoted by Bu3SnH, occurs in high yields and gives... 

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],... 

Renaud, P. and Abazi, S. (1996) Use of 0,Se-acetals for radical-mediated phenylseleno group transfer reactions. 

Transfer of the phenylthio(phenylseleno) group to aldehydes and ethers... 

DIB in combination with sodium azide and diphenyl disulphide or diphenyl diselenide can replace the methinic hydrogen of aliphatic, aromatic and heteroaromatic aldehydes by the phenylthio or phenylseleno group. The corresponding esters were obtained in moderate (for unsaturated aldehydes) to very good yields [109] ... 

The radical addition of selenomalonate 149 or the corresponding malononitriles are excellent substrates for phenylseleno group transfer reactions to alkenes and alkynes.263 Malononitrile 150 can be used for annulation and cyclization reactions (Scheme 40) 264,265... 

A photosensitized activation of carbon-selenium bonds was also used for performing phenylseleno group transfer reactions. This process involves a photosensitized electron transfer (PET) as the initial step in the reaction sequence. Fragmentation affords a radical and phenylselenolate, which is oxidized to diphenyl diselenide in the presence of oxygen. The cyclized radical is then trapped by diphenyl diselenide to afford the final product. This process is quite general for intramolecular radical reactions.70,266... 

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). 

Reductive removal of the phenylseleno group of 110 and 111 by triphenyl-tin hydride led to the 2 -deoxy-/3-D-disaccharides 112 and 113. Although these methodologies were reported several years ago, they have not often been employed to prepare a- to /3-linked 2-deoxy disaccharides. 

Selenium electrophiles add to conjugated dienes, only forming 1,2-adducts. Although Markovnikov addition is seen, ordy a few examples have been reported. With allenes the additions are reg ospecific, with the phenylseleno group usually adding to the jp-carbon. Unfortunately, all four stereoisomers of halide attack are seen with unsymmetricaUy substituted allenes. ... 

Epoxide cleavage, Sodium selenophenolate reacts with the furanose 1, prepared from D-glucose, to form the diselenide 3 in quantitative yield. This reaction involves formation and cleavage of the epoxide 2, which can be also prepared by treatment of 1 with NaH. The epoxide is cleaved regiospecifically by attack from the less hindered side at C4. The phenylseleno group can be removed reductively by Raney nickel. The product was used to prepare isoepiallomuscarine (5). 

The reaction is rapid even at low temperature and is generally carried out at — 78 °C. The kinetic enolates are produced from unsymmetrical ketones under these conditions and the phenylseleno group is introduced at the less substituted position usually with high regio- and diastereoselectivity. Thus, the reaction of tricyclic ketone 11 with lithium diisopropylamide followed by reaction with phenylselenenyl chloride gives a mixture of a-selenenylated enones 12 in 3 1 ratio9 . 

A variety of unsaturated alcohols undergo addition of the phenylseleno group followed by intramolecular ether formation (phenylseleno-etherification) on treatment with phenylselenenyl chloride to give product 1125. 

The reaction presumably proceeds via the intermediacy of an initially formed episelenonium species which is captured intramolecularly by the hydroxy group leading to the formation of the (phenylseleno(ethers 10 in high yield. Assuming an SN2 mode of ring closure, the phenylseleno group is trans relative to the ether oxygen. The procedure has been successfully applied to a variety of substrates (Table 2). 

This very mild lactonization procedure is general (see Table 7) and affords phenylselenolactones with a tram relationship between the phenylseleno group and the lactone functionality52, 53. 

Interestingly, the presence of silica gel in the reaction mixture increases the yields of the cyclic products the phenylseleno group is tram to the C—N bond (c/s ring fusion) in the. Y-hctcrocy-cles given in Table 856. 

One of the most important addition reactions which involves nitrogen nucleophiles is the azidoselenenylation of alkenes because both the azido and the phenylseleno group can give rise to several useful transformations. These compounds were first prepared by Krief from the reaction of alkenes with PhSeBr followed by sodium azide in DMF [54]. Direct azidoselenenylation of alkenes can be effected with PhSeCl and sodium azide in DMSO [55]. Under these conditions the reaction is stereospecific but it is not regiospecific. Similarly, the reaction of exocyclic alkenes with N-(phenylseleno)phthalimide and azido-trimethylsilane in methylene chloride (Scheme 11) gives rise to a 1 1 mixture of the regioisomers 67 and 68 [56]. 

In a related investigation Tom described carbocyclization reactions which occur by titanium tetrachloride promoted transfer of the phenylseleno group in a-phenylseleno alkenyl ketones [113]. As indicated in Scheme 34, the reaction of 224 with titanium tetrachloride produces the titanium enolate and the phenylseleno group adds to the carbon-carbon double bond. The intermediate 225 then... 

Selenolate 1 [1,2], prepared from diphenyl diselenide by reduction with NaBH4 in ethanol, is not reactive enough towards esters and lactones. However, application of more rigorous conditions, i. e. at 110-120 °C in dry DMF for lactones 15, affords the corresponding ring-opened products, cu-phenylselenyl carboxylic acids 16, in good yields [44]. Compounds 16 can be easily converted to co-ole-finic methyl esters 17 by treatment with diazomethane followed by oxidative elimination of the phenylseleno group (Scheme 25). By contrast, the uncomplex-... 

Other selenoacetals have been prepared by means of radical phenylseleno group transfer reactions and by trapping of radicals with diphenyl diselenide, these procedures will be discussed later in this chapter (Sects. 3.1 and 5.2). 

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