Phenylselenyl bromide, reaction with

Alkylthiocyanates and alkylselenocyanates are obtained by treatment of trialkylboranes with potassium thiocycanate (260) and sodium selenoisocyanate (261), in the presence of iron(III) compounds, respectively. Unsymmetrical trialkylboranes react preferentially at the more highly branched alkyl group. Alkenylphenyl selenides are obtained by the reaction of alkenylboronic acids with phenylselenyl bromide (262). 

Interestingly, bis(methylthio)-l-nitroethylene (380) reacts with dimetallic zinc-copper species leading to the corresponding exo-methylene cycloalkenes, such as 381 (Scheme 100) . / -Disubstitutcd nitroolefins are especially difficult to prepare by nitroaldol condensation. The addition of zinc-copper reagents to nitroolefins followed by a reaction with phenylselenyl bromide produces, after IFO, oxidation, EtZ mixtures of -disubstituted nitroalkenes, such as 382 (Scheme 100) . 

The cyclohexene 121, which was readily accessible from the Diels-Alder reaction of methyl hexa-3,5-dienoate and 3,4-methylenedioxy-(3-nitrostyrene (108), served as the starting point for another formal total synthesis of ( )-lycorine (1) (Scheme 11) (113). In the event dissolving metal reduction of 121 with zinc followed by reduction of the intermediate cyclic hydroxamic acid with lithium diethoxyaluminum hydride provided the secondary amine 122. Transformation of 122 to the tetracyclic lactam 123 was achieved by sequential treatment with ethyl chloroformate and Bischler-Napieralski cyclization of the resulting carbamate with phosphorus oxychloride. Since attempts to effect cleanly the direct allylic oxidation of 123 to provide an intermediate suitable for subsequent elaboration to ( )-lycorine (1) were unsuccessful, a stepwise protocol was devised. Namely, addition of phenylselenyl bromide to 123 in acetic acid followed by hydrolysis of the intermediate acetates gave a mixture of two hydroxy se-lenides. Oxidative elimination of phenylselenous acid from the minor product afforded the allylic alcohol 124, whereas the major hydroxy selenide was resistant to oxidation and elimination. When 124 was treated with a small amount of acetic anhydride and sulfuric acid in acetic acid, the main product was the rearranged acetate 67, which had been previously converted to ( )-lycorine (108). 

The transylidation reaction of phenylselenyl bromide with two moles of alkylidenetriphenylphos-phorane yields 1-phenylselenoalkylidenetriphenylphosphoranes (equation 28). ... 

When iVj-acyl-tryptophans are exposed to strong acid, the indolium cation is trapped by cyclisation involving the side-chain nitrogen. Comparable tricycles result from phenylselenylation of protected tryptophan or reaction with 4-methyl-l,2,4-triazoline-3,5-dione, ° or dimethyl(succinimido)sulfonium chloride (a CH2SMe group ends up at the indole C-3)." If M-bromosuccinimide is employed, the initially formed 3-bromo-tricycle loses hydrogen bromide to produce an aromatic indole. ... 

The treatment of a,p-unsaturated ketones with organocopper reagents provides another method to access specific enolates of unsymmetrical ketones. Lithium dialkylcuprates (see Section 1.2.1) are used most commonly and the resulting enolate species can be trapped with different electrophiles to give a,p-dialkylated ketones (1.27). Some problems with this approach include the potential for the intermediate enolate to isomerize and the formation of mixtures of stereoisomers of the dialkylated product. The intermediate enolate can be trapped as the silyl enol ether and then regenerated under conditions suitable for the subsequent alkylation. Reaction of the enolate with phenylselenyl bromide gives the a-phenylseleno-ketone 12, from which the p-alkyl-a,p-unsaturated ketone can be obtained by oxidation and selenoxide elimination (1.28). 

The sequence provides a method for converting a, -unsaturated ketones into (3-alkyl derivatives by alkylation with an organocuprate (see Section 1.2.1) and reaction of the intermediate copper enolate with phenylselenyl bromide, followed by oxidative elimination (6.52). 

Phenylselenyl bromide is an alternative reagent for allylic oxidation of alkenes. Addition takes place, and reaction of the adduct with acetic acid, followed by oxidation, generates the allylic acetate. 

Among the applications of allyl silanes reported this year their reaction with phenylselenyl chloride and subsequent oxidation of the resulting allyl selenides to allylic alcoholshas been used in the sequence outlined in Scheme IS. 3-Bromoallyltrimethylsilane therefore behaves as a hydroxypropenyl synthon. Nitroalkenes react with allylsilanes to give, after hydrolysis, y,S-unsaturated ketones, and a synthesis of substituted cyclopentenones based on this procedure has been described.Allyl trimethylsilyl ethers undergo a palladium-promoted coupling with aryl iodides to give /3-aryl-a,/3-unsaturated ketones, and palladium also catalyses the reaction of various aryl bromides with trimethylsilyl acetylene to produce ethynylated aromatics. 

The 3 -bromo-2 ,3 -dideoxydidehydrouridine derivative (70) has been prepared as indicated briefly in Scheme 10. The thionyl bromide reaction also gave some of the 2 -bromo-2. 3 -dideoxy-3 -phenylselenyl product with D-xy/o-configuration, which on elimination gave the 2 -bromoalkene. 0... 

The selenyl cluster [Ru6(Ai6-C)(SePh)(CO)i5] is obtained from treating [Ru6( 6-G)(CO)i6] with phenylselenyl chloride, thermolysis affording Ru6(Ai6-C)(M-SePh)2(CO)i4 223, and reaction with allyl bromide giving Ru6(M6-C)-( -SePh)( -77 -C3Hs)(GO)i4 224. ... 

Selenides eliminate readily without a base. They are generally prepared from enolate anions by reaction with diphenyldiselenide or phenylselenyl bromide to give phenylselenides. The phenylselenides are oxidized with sodium periodate, hydrogen peroxide, or peracids to the selenoxides, which eliminate even at room temperature to afford the a,p-unsamrated ketones and esters [107]. 

Allyl alcohols readily react with trichloroacetonitrile to give the corresponding trichloroacetimidates 145. Activation of the double bond with electrophilic reagents results in ring closure to yield oxazolines 146. The most commonly employed electrophiles include iodine, iodine monochloride, phenylselenyl chloride, and mercuric trifluoroacetate. Other nitriles including cyanogen bromide and N,N-dimethylcyanamide can also be used. Since oxazolines readily hydrolyze to amides, the net effect of this reaction sequence is to produce p-amino alcohols 147 from an allyl alcohol. This strategy has been employed in numerous total syntheses of natural products. Examples are listed in Table 8.18 (Fig. 8.7 Scheme 8.43). ° ... 

There are various ways to generate and synthesize selenium electrophiles and some of these compounds are commercially available. The addition reaction can also be dependent on the counterion X of these reagents and several protocols have been developed to exchange the counterions. The most commonly used electrophile is the phenylselenyl electrophile and compounds like phenylselenenyl chloride 6 and phenylselenenyl bromide 7 are commercially available. They can also be easily generated from diphenyl diselenide 8 by treatment with sulfuryl chloride or elementary chlorine or bromine, respectively. Diselenides in general are very versatile precursors for selenium electrophiles. For addition reactions using external nucleophiles the use of selenenyl halides can lead to complications, because chloride or bromide ions can also act as nucleophiles and lead to undesired side-products. An... 

Vinyllithiums of type 663 (R2 = R3 = H) reacted with primary alkyl bromides, carbonyl compounds, carbon dioxide, DMF, silyl chlorides, stannyl chlorides, disulfides and phenylselenyl bromide142,970-979. Scheme 173 shows the synthesis of dihydrojasmone 669 from the corresponding 1,4-diketone. a-(Phenylsulfanyl)vinyllithium 665, prepared from phenyl vinyl thioether, reacted with hexanal and the corresponding adduct 666 was transformed into its acetoacetate. This ester 667 underwent a Carrol reaction to produce the ketone 668, which was transformed into the cyclopentenone 669 by deprotection either... 

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