Selenoxides cyclization

No oxidizing agent is required for the sulfuric acid promoted cyclization of iV,iV-diphenyl-hydroxylamine to carbazole (13CB3304). The parallel conversion of diphenyl sulfoxide and diphenyl selenoxide to dibenzothiophene (23CB2275) and dibenzoselenophene (39CR(199)53l) is effected by treatment with sodamide. 

Phenylselenoetherification (8, 26-28). This cyclization has been described in detail.6 The 16 examples reported indicate that the reaction is applicable to unsaturated primary, secondary, and tertiary alcohols as well as to phenols. The most important use is for synthesis of allylic ethers by syn-selenoxide elimination, which proceeds selectively away from the oxygen. The value of this methodology for synthesis of natural products is illustrated by a synthesis of a muscarine analog (1), outlined in equation (I). 

Sulfuric acid promotes cyclization of (V./V-diphenylhydroxylamine (14) to carbazole (11) (13CB3306). The parallel conversions of diphenyl sulfoxide (15) and diphenyl selenoxide to dibenzothiophene (10) (23CB2275) and dibenzoselenophene (39CR(i99)53i) are effected by sodamide. 

Dibenzoselenophene has been obtained by the sodamide promoted cyclization of diphenyl selenoxide (153) (34CR(l99)53l), and via diazotization of o-aminodiphenyl selenide (154) (39JCS151). 

Selenothiolactonization Cycloalkenethiolcarboxylic acids react with 1 to form S-acyl phenylselenosulfides (2), which undergo cyclization to phenylseleno-thiolactones (3) when refluxed in benzene in the presence of AIBN. The C6H5Se group of 3 is selectively oxidized by m-C 1C6H4C03H, and subsequent selenoxide elimination results in thiolactones (4). 

The first synthesis of (R)-4,5-dihydro-37/-dinaphtho[2,l-f l, 2 -i ]selenepin oxide 110 has been achieved from (R)-(+)-l,l -bi-2-naphthol, which in turn was obtained by resolution of raol,l -bi-2-naphthol. Palladium-catalyzed alkoxy carbonylation of the alcohol 108 gave a dimethyl ester which was then reduced by LiAlfLi, and the resultant diol converted to key intermediate chloride 109. Cyclization with sodium selenide gave a novel enantiomerically pure selenide, which upon oxidation yielded the desired selenoxide 110 <2000SC2975>. 

A very similar cyclization has been described using elemental selenium and LiHBEt3, as reducing agent in 95% yield. Although the oxidation step was exactly the same, a study of the reduction of selenoxide 110 to selenide 109 has been detailed <2003BCJ381>. 

Utilization of a selenium-initiated electrophilic cyclization (lactonization, etherification) in conjunction with a [2,3] sigmatropic selenoxide rearrangement provided a convenient, stereoselective access to highly substituted bicyclic lactones or ethers (entries 6-8)25-29. The electrophilic addition of phenylselenenyl halogenides to dienes can occur in a 1,2-(anti)-or 1,4-fashion furnishing allylic selenides in both cases. 

More recently, it has been reported [7] that similar properties were observed for Se-trifluoromethylated dibenzoselenonium salts. Se-(Trifluoromethyl) dibenzoselenophenium triflate (4) was successfully synthesized in high yield by the direct fluorination of an equimolar mixture of the selenide 5 and triflic acid (TfOH) with diluted molecular fluorine. The same salt was also obtained by cyclization of the selenoxide 6 with triflic anhydride (Tf2 0) - a reaction which probably occurs via the selenonium salt 7 (Scheme 1). 

The selenoxide produced by oxidation of Y-(2-phenylselenylethyl)benzamide with MCPBA undergoes cyclization to furnish 4,5-dihydrooxazoles in high yield <92TL4017>. An optically active amide bearing a 2-pyridylseleno group on the p carbon (186) is cyclized without racemization (Equation (25)). 

Treatment of 860 with tri- -butyltin hydride and AIBN under high dilution conditions leads to cyclized product 861 as a 3 1 mixture of EjZ isomers (60-71%). Conversion of the TMS-olefin to an aldehyde (863), phenylselenation, reduction of the aldehyde, and acetylation furnishes 864. Oxidation and subsequent elimination of the selenoxide followed by reduction of all carbonyl groups with lithium aluminum hydride gives the natural product 850. 

The reaction between an aryl vinyl selenoxide and ketone lithium enolates provides a convenient preparation of cyclopropyl ketones/ Further details have appeared concerning the bicycloannulation of cyclohexenones by reaction of a -enolates with vinylphosphonium salts, to form tricyclo[3.2.1.0 ]octan-6-ones [equation (43)]/ The method has been used to synthesize the trachylobane carbon skeleton. The reaction can also be carried out with vinyl sulphones, but with this reagent HMPA is required for the cyclization to be successful. ... 

A related solid-phase method for preparing flavones was also reported by Cao and co-workers. The polystyrene selenium bromide resin functioned as a Lewis acid and promoted concomitant loading and cyclization of chalcones 326 to supported flavones 327. Selenoxide elimination upon treatment with hydrogen peroxide released the corresponding flavones 328 (Scheme 3.56). 

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