Seleno-lactonization

The metal-catalyzed conversion of 2,3-allenoates to A -butenolides has proven to be superior to the acid-catalyzed cyclization of the corresponding carboxylic acids. The reaction of the esters can be effected by Au(m) catalysts in improved yields <2005TL7431>, and furan-2(5//)-ones functionalized in the 4-position can be prepared by iodo- or seleno-lactonization in moderate to good yields (Scheme 33) <2006T4444, 2005EJO3942>. The cycloisomerization of optically active 2,3-allE itoic acids, on the other hand, can proceed with complete conservation of stereochemical information when copper(i) chloride is used as a catalyst <2006S3711>. 

Renaud and co-workers used 78 for the synthesis of (-)-phaseolinic acid (6) and (-)-pertusarinic acid (8) (Scheme 12) [32, 33]. Radical addition of dimethyl phenylselenomalonate to 78 proceeded with rearrangement of the bicyclics to yield the seleno-acetal 79 [34]. After reductive deselenylation and Baeyer-Villiger oxidation treatment of 80 with BU4NI and BBr3 led to a simultaneous cleavage of the ether, the lactone, and the methyl ester func-... 

Scheme 49 Synthesis of furanose-fused C-glycosyl a-hydrazino lactones from phenyl-1-seleno glycosyl donors...

Seleno esters are also produced in good yield by reactions of dimethy-laluminim methanoselenolate103 (CH3)2AlSeCH3, with the protected C-ribofuranosyl acetate 269. Treating the selenol ester 270 with either cuprous or mercuric chloride produced the (Gensler) lactone.110 271, a product useful in the synthesis of various C-nucleoside antibiotics.111,112... 

A large amount of woik has been accomplished using mixed sulfiir/selenium reagents, such as seleno-sulfides and selenosulfones. One example of selenosulfide addition via radicals is selenotbiolactoniza-tion. When selenosulfide (15) is treated with AIBN, a mixture of y-seleno-substituted thiolactones is produced (equation 9). Although these lactonizations result in mixtures of stereoisomers, they usually can be separated. In addition to these reports, selenosulfones have been used to form allenic sulfones, alkynic sulfones and (phenylsulfonyl)(Uenes (S< me4). ... 

Selenenyl halides are relatively stable, though moisture sensitive, compounds that are generally prepared by the reactions shown in Scheme 7 and behave as electrophihc selenium species. " They react with ketones and aldehydes via their enols or enolates to afford a-seleno derivatives (e.g. (17) in equation 11). Similar a-selenenylations of /3-dicarbonyl compounds, esters, and lactones can be performed, although the latter two types of compounds require prior formation of their enolates. Moreover, the a-selenenylation of anions stabilized by nitrile, nifro, sulfone, or various types of phosphorus substituents has also been reported (equation 12). In many such cases, the selenenylation step is followed by oxidation to the selenoxide and spontaneous syn elimination to provide a convenient method for the preparation of the corresponding a ,/3-unsaturated compound (e.g. 18 in equation 11). Enones react with benzeneselenenyl chloride (PhSeCl) and pyridine to afford a-phenylselenoenones (equation 13). 

Generation of seleno carboxylates occurs instantaneously on treatment of selenenyl halides with carboxylic acids and, consequently, the phenylseleno-lactonization of unsaturated carboxylic acids by phenylselenenyl chloride may be formally considered as intramolecular addition of phenylselenenyl carboxylate to the C-C double bond. The reaction of 4-cyclohep-tene-l-carboxylic acid with phenylselenenyl chloride to give lactone 40 is a good example. 

Treatment of a, -unsaturated carbonyl compounds 18 with nucleophilic selenium species affords -seleno carbonyl compounds 19 in good yields via Michael addition (Scheme 27) [46]. This reaction has been applied to protect a, -unsa-turated lactones [47], in natural product synthesis [48], and in asymmetric Michael additions in the presence of an alkaloid [49]. Michael addition also proceeds with selenolates that are prepared from diphenyl diselenide by cathodic reduction [22], reduction with the Sm-Me3SiCl-H20 system [19], and reduction with tributyl phosphine [25]. 

The above-mentioned radical C-glycosylation has been applied for the synthesis of complex compounds. O Scheme 42 shows two such examples of the radical C-glycosylation of man-nosyl bromide with complex olefins such as unsaturated ketone and unsaturated lactone. This highlights the mildness of the reaction and these conditions have compatibility with many functional groups [68]. It is noteworthy that an acceptor bearing seleno and chloro groups can be employed without interference under the radical conditions. In both reactions, the a-C-gly-cosidic products were exclusively obtained, although in moderate yields. 

The elimination is completely chemoselective with l-(l-hydroxyalkyl)-8-lactones and with methyl P-hydroxy-a-selenoalkylcarboxylates (precursors of ( )-a,p unsaturated methyl carboxylates Scheme 39, b and Scheme 170), but not with the other stereoisomer which produces a mixture of (Z)-a,p un-saturated methyl carboxylates (by formal selenenic elimination) and a-seleno-a,p-unsaturated methyl carboxylates (by formal elimination of water). 

P-Hydroxy selenides are conveniently prepared from epoxides by treatment with sodium phenylse-lenide (Scheme 32) and by the addition of benzeneselenenic acid and its derivatives to alkenes (Scheme 33), - -" although in some cases these reactions are not regioselective. Useful phenylseleno -etherification and -lactonization reactions have been developed which can be regioselective (equation 42 and Schemes 34 and 35). -" " Selenide- and selenoxide-stabilized carbanions have been used in addition reactions with aldehydes and ketones, - and the reduction of a-seleno ketones also provides a route to P-hydroxy selenides. ... 

Similarly, the vinyl 2-phenylseleninyl derivative formed by oxidation of the corresponding seleno compound (71) thermally rearranges to afford the y, -unsaturated ten-membered ring lactone (Scheme 25) <83Mi 922-05 >. 

A potentially important method for the synthesis of 6>-unsaturated esters from saturated lactones has been reported by two groups. This entails nucleophilic ring opening of the lactone by sodium phenyl selenoate followed by oxidative removal of the selenium. (Scheme 34). 

The Sn2 cleavage of esters and lactones is achieved with sodium phenyl selenide generated by the reaction between sodium hydride and benzene selenol, instead of the more usual sodium borohydride reduction of diphenyldiselenide. When generated under the former conditions the phenyl selenide anion is a potent nucleophile and good yields of seleno-acids are obtained from a wide range of esters and lactones (Scheme 3). ... 

The readily available E-ethylidene-y-butyrolactone (55) may serve as precursor for the synthesis of 2-methoxycarbonyl-l,3-dienes (56).The procedure (Scheme 26) involves ring opening of the lactone with potassium or sodium selenoate, and thus extends Liotta and Smith s synthesis of a>-olefinic esters described last year. 

The ring opening of lactones by sodium phenyl selenoate leading to oj-unsaturated esters, which was reported last year (2, 205), has been applied to a synthesis of 1-substituted 2-methoxycarbonylbuta-1,3-dienes from a-alkyl-idene-butyrolactones yields overall are around 40—50%. 

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