Phenylselenyl halides

It has been shown that phenylselenyl halides easily reacted with 0-allyl oximes 221 to give cyclic iminium salts 222, which by reaction with water afforded isoxazolidines 223 in moderate to good yields (equation 96) . Compounds 222 can be reduced in situ by sodium borohydride to produce Ai-alkyl-substituted isoxazolidines 224 in 50-95% yields . ... 

The simple treatment of 4-alkcnols with electrophiles, e.g., mercury(II) salts, iodine, A -bromosue-cinimide, A -iodosuccinimide or phenylselenyl halides, affords the corresponding tetrahydrofurans with the cyclization proceeding under moderate to high stereocontrol, depending on the substitution pattern, the nature of the substituents and the reaction conditions1-3,116 162-164... 

Conversion of ketones into a,(3-unsaturated ketones has been effected by bromination-dehydrobromination, although a better method involves a-phenyl-seleno ketones as intermediates. These are normally obtained by reaction of the enolate of the ketone with a phenylselenyl halide or diphenyl diselenide at low temperature. Oxidation with hydrogen peroxide, sodium periodate or other oxidant gives the selenoxide which immediately undergoes syn p-elimination to form the a, -unsaturated ketone. The process is tolerant of many functional groups, such as... 

The reaction of enolates with phenylselenyl halides is very fast, even at —78 °C, and the kinetically generated enolates react without rearrangement to the more stable isomer. Unsymmetrical ketones may therefore be converted into one or other of the two alternahve a,(3-unsaturated ketones. 2-Methylcyclohexanone, for example, gave 2-methyl-2-cyclohexenone or 6-methyl-2-cyclohexenone selectively by way of the corresponding thermodynamic or kinetic enolate respectively. 

Phenylselenyl halides, phenylselenyl trifluoroacetate, and phenylselenyl acetate add to alkenes ... 

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... 

Scheme 38. Phenylselenyl Sulfate Catalyzed One-Pot Conversions of Vinyl Halides into Alkoxy Acetals and of Styrene Derivatives into 1,1- and 1,2-Dialkoxy Alkanes...

Ito, Y, Ogawa, T, An efficient approach to stereoselective glycosylation of A-acetylneuraminic acid use of phenylselenyl group as a stereocontrolling auxiliary. Tetrahedron Lett., 28, 6221-6224, 1987. Lemieux, R U, Hendriks, K B, Stick, R V, James, K, Halide ion catalyzed glycosidation reactions. Syntheses of a-linked disaccharides, J. Am. Chem. Soc., 97, 4056-4062, 1975. 

Homogeranic acid (203) has been synthesised by conjugate addition of prenylmagnesium halide to g-isopropenyl-g-propio-lactone (204). Homofarnesylic acid has been made by a similar route. The acid (203) reacts with phenylselenyl chloride or with... 

The transformation of saturated ketones into a,/8-unsaturated ketones is often carried out by the introduction of an a-seleno-substituent, followed by oxidation and elimination. An alternative method for selenation of ketones (and esters) has been described wherein the lithium enolate reacts with selenium metal (rather than a selenyl halide) followed by methyl iodide (Scheme 29). Yields are comparable to the classical method. Phenylselenyl chloride reacts with enamines derived from aldehydes to give a-phenylselenoaldehydes, and hence o(,/3-unsaturated aldehydes by oxidative elimination [equation (21)]. ... 

Good yields (60—97%) of olefins can be obtained by the action of RSe on /3-heterosubstituted alkyl halides,and a-phenylselenyl enones result from the... 

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