Acyl selenides

OOJA11009). Similarly, aryl acyl selenide 111 gave a mixture of diastereoisomers 112 and 113. 

The acyl selenide 19 affords the decarbonylated )S-lactam in good yield. A N-hydroxypyridine-2-thione ester 20 is used in the key step to construct the chiral cis-cyclopropane structure in compounds designed as antidopaminergic agents. The observed high cis selectivity is due to the hydrogen abstraction from the sterically demanding (TMSlsSiH, which occurs from the less-hindered side of the intermediate cyclopropyl radical. 

The use of (TMSlsSiH with acyl selenides can also yield new C-C bond formation, as shown with the a,/l-unsaturated lactam ester (Reaction 67). The resulting ketone can be envisaged as potentially useful for the synthesis of 2-acylindole alkaloids. Both the effects of H-donating ability and steric hindrance by the silicon hydride are evident. 

For the route A, acyl radicals donors like iS are readily generated from acyl selenides (ISa) or acyl cobalt derivatives (iSb) and radicals acceptors 2S are usually multiple bonds as in methyl vinyl ketone (2Sa) -although some homolytic substitutions are possible. On the other hand, nitriles GSal are useful acceptors (3S) in radical cyclisations and 4Sa is an obvious synthon equivalent of radical donor 4S (See Table 7.2). 

Selenenyl groups can be abstracted from acyl selenides to generate radicals on reaction with stannyl radicals.201 202 203 Normally, some type of stabilization of the potential reaction site is necessary. Among the types of selenides that are generated by selenenyl abstraction are x-sclcncnyl cyanides and a-selenenyl phosphates. 

Pyridine-2-thione-A-oxycarbonyl (PTOC) derivatives of carboxylic esters 53 were developed by Barton et al. and serve as a convenient source of acyloxyl radicals, which upon decarboxylation provide specific routes to free radicals (equation 82). This process can also proceed by a radical addition (equation 83). Acyl selenides (54) are a convenient source of acyl radicals, which can undergo decarbonylation also giving specific free radicals (equation 84). ° ... 

Radical cyclization of acyl selenides 52 (Equation 19) leads to oxepanones 53, the main product (>95% in the mixture) being f-isomer and the best yields being obtained with (TMS)3SiH <1996JOC2252>. 

The products here are acyl selenides, which can be reduced to the corresponding aldehydes in high yields by a Pd-catalyzed reduction with tributyltin hydride... 

Acyl selenides serve as acyl radical precursors when treated with tin radicals [43], In the following reaction (Scheme 6.25), a cydoheptanone fused to an indanol skeleton is prepared via a three-component reaction [44], The sequential addition of an acyl radical to two molecules of methyl acrylate followed by a 7-endo-type radical addition account for the annulation. 

Acyl selenides. The triethylammonium salt of carboxylic acids reacts with C6H5SeCl and Bu3P in THF to form selenoesters in 62-85% yield. These products on reaction with Bu3SnH (AIBN) furnish acyl radicals. 

Intramolecular addition of acyl radicals to enoates proceeds in high yields. The acyl radicals are generated conveniently from acyl selenides. The application of this methodology in the stereoselective synthesis of cis-2,5-disubstituted tetrahydrofuran-3-ones is illustrated [95TL31],... 

An intramolecular acyl radical cyclization of acyl selenide 1024 uses a (Z)-vinylogous sulfonate to control rotamer population, affording ry -2,6-disubstituted tetrahydropyran-4-one 1025, a key intermediate during synthesis of the tetrahydropyran unit of mucocin (Equation 399) <1997TL5249>. This methodology is also applicable to the synthesis of polycyclic ethers <1996JOC4880>. 

TMS)3SiH-mediated acyl radical reactions of phenylseleno esters have been utilized for the diastereoselective synthesis of cyclic ethers by Evans s group. Equation 44 shows that the acyl selenide 83 affords the cA-disubstituted tetrahydrofuran 84 in good yield on exposure at room temperature to (TMS SiH and the Et3 B/02 combination as the initiator via a 5-exo-trig cyclization97 98. 

Intramolecular radical substitution of pyrroles and indoles has been well studied this is exemplified in Schemes 90 <1997TL7937> and 91 <2000TL10181>. Intramolecular radical acylation of l-(-halogenoalkyl)-2-methylsulfonyl-5-substituted pyrroles leads to bicyclic ketones with displacement of the sulfonyl moiety <2000TL3035>. Similar cyclizations can be achieved using acyl selenide precursors to generate an acyl radical... 

Cyclization of acyl radicals can be carried out in high yields from acyl selenide precursors 991 without using high pressures of CO (Scheme 190) <2001TL7887>. 

Acyl radicals are very useful synthetic intermediates. Their preparation is not simple since the corresponding halides are highly electrophilic and cannot be used as radical precursors. Organocobalt compounds were proposed as suitable source of acyl radicals [44]. However, the use of acyl selenides proved to be more general [45, 46]. These radical precursors can be efficiently prepared from the corresponding carboxylic acids and esters [47]. Acyl phenyl selenides should be preferred, when possible, relative to acyl methyl selenides due to the consumption of two equivalents of tin hydride with this last system (Scheme 1) [4]. Acyl selenides have found many applications in tandem radical additions to alkenes. Examples of intermole-cular [Eq. (18)] [48,49] and intramolecular reactions [Eq. (19)] [50a] are reported. The enoyl selenide 68 give the unsaturated acyl radicals 69. This intermediate... 

Radical cycUzations. Closure of carbocycles and heterocycles can be achieved by this method using HSi(SiMe3)3 and an initiator (e.g., AIBN, EtjB) to act on halides, thionocarbonates, or acyl selenides. ... 

Roger and Mathvink reported on the extensive synthesis of ketones based on the acyl transfer reaction of acyl selenides to alkenes using tin hydride as the radical mediator (Scheme 4-24) [46]. A radical arising from the addition of an acyl radical to alkenes abstracts hydrogen from the tin hydride with the liberation of a tin radical, thus creating a chain. The addition process is in competition with de-carbonylation. In this regard, aroyl, vinylacyl, and primary alkylacyl radicals are most suitable for this reaction and secondary and tertiary acyl radicals are inferior. 

The photolysis of a-phenylselenoacetate and related compounds in the presence of an alkene and CO leads to acyl selenides via group transfer carbonylation. The mechanism of this three-component coupling reaction involves the addition of an a-(alkoxycarbonyl)methyl radical to an alkene, the trapping of the produced alkyl free radical by CO, and termination of the reaction by a phenylselenenyl group transfer from the starting material (Scheme 4-43) [73]. 

The radical carbonylation of alkyl and aryl radicals and the cyclization of the resulting acyl radicals onto tcrz-butyl sulfides leads to the formation of y-thiolac-tones with expulsion of the tert-butyl radical (Scheme 4-52) [89]. This process is applicable to a range of substituted 4-rerr-butylthiobutyl bromides and iodides giving moderate to excellent yields of the corresponding thiolactones. Using acyl selenide/tin hydride chemistry and competition kinetic methods, the rate constant for the cyclization was determined to be 7.5x10 s at 25 °C [89]. 

Alkyl sulfides, aryl sulfides, alkyl selenides and acyl selenides... 

Photolysis of methyl a-(phenylseleno)acetate in the presence of diallyl ether and carbon monoxide leads to the acyl selenides via radical cyclization and group transfer carbonylation (Scheme 15.49) [124]. 

Treatment of 12 with (Me3Si)3SiH and Et3B predominantly alforded cyclic ethers with cis stereochemistry [17], Construction of oxepines (n = 2) proceeded at low concentration. Additionally, the reaction at low temperature was effective to suppress decarbonylation of the intermediary acyl radical derived from acyl selenide (Scheme 10). 

The importance of desulfurization and deselenation by radical processes has increased significantly in recent years. Generally, the reductive cleavage of the C-Se bond is more efficient than that of the C-S bond. Two examples are reported in Eqs. (4) and (5) [20, 21]. Cleavage of the C-Se bond in acyl selenides and seleno-carbonates provides a very mild method for the generation of acyl radical [10]. 

Acyl radical cyclization of ) -alkoxyacrylates provides five-, six-, and seven-mem bered oxacyclic ketones with high stereoselectivity [45], In a key step in the synthesis of (—)-kumausallene (55) by Evans, the tetrahydrofuran-3-one 69 was obtained stereoselectively from the acyl selenide 68 [46] (Scheme 24). 

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