Enolate compounds oxidative coupling

Reaction of compound 134, either with sodium carbonate or potassium /tz -butoxide, leads in moderate yields to the enolized bicyclic compound 135 along with a dimer resulting from the oxidative coupling of the initial enolate of substrate 134 (Scheme 24) <2005T1693>. 

The stemona alkaloid stemonamide (49) was synthesized starting from a-stannyl acetate 47 and 2-stannyl pyrrolidine 48. The oxidative coupling of stannyl acetate 47 with acetylenic silyl enol ether affords the functionalized C-7 unit which corresponds to the side arm of the pyrrolidine ring. Then, introduction of the C-7 unit to the pyrrolidine ring is performed by the oxidative generation of acyliminium ion. The carbon skeleton of stemonamide was thus constructed efficiently as shown in Scheme 19 by employing organotin compounds. ... 

In a series of papers on the total syntheses of alkaloids, Baran and coworkers have recently reported that enolates of carbonyl compounds undergo oxidative coupling with indoles and pyrroles in the presence of oxidants such as copper(II) and iron(III) salts . A detailed study of the oxidative cyclization reported in equation 15 has shown that 26 is converted into 27 with the highest yields when Fe(acac)3 is the oxidant, presumably due to its high redox potential (+1.1 V vs. the ferrocenium/ferrocene couple in THF solution ), which is the most positive among all the oxidizing agents tested for the transformation. 

Copper(II) salts are efficient one-electron oxidants for the generation of radicals from lithium enolates [1]. This concept was successfully applied for the oxidative coupling of ketones or amides 36 to afford the corresponding 1,4-dicarbonyl compounds 37 in good yield [17]. If an optically active imidazolidinone is used as chiral auxiliary, the reaction exhibits an excellent simple and induced diastereoselectivity (Scheme 12). 

In a related study, intramolecular oxidative coupling of enolate derivatives has been investigated by Schmittel and co-workers [167]. The intermolecular version of this reaction provides a useful route to 1,4-dicarbonyl compounds but typically suffers from low levels of stereoselectivity. Furthermore, in mixed systems, the desired heterocoupling products are often accompanied by appreciable amounts of homocoupling products. It was hoped that the use of a single metal for both enolate precursors with concomitant intramolecularization of the bond-forming event might overcome some of these problems. 

Both 3-aIkoxycarbonyl- and 3-acylfurans can be prepared by oxidative coupling of enol ethers with 1 3-dicarbonyl compounds in the presence of Mn(III) and subsequent treatment of the resulting 2,3-dihydrofurans with acids (Scheme 25) <87CL223,88SC1841,93S833). 

Generation of a-Acyl Radicals. As a one-electron oxidant, Ce can promote the formation of radicals from carbonyl compounds. In the presence of interceptors such as butadiene and alkenyl acetates, the a-acyl radicals undergo addition. The carbonyl compounds may be introduced as enol silyl ethers, and the oxidative coupling of two such ethers may be accomplished. Some differences in the efficiency for oxidative cyclization of, s-, and ,f-unsaturated enol silyl ethers using CAN and other oxidants have been noted (eq 14). ... 

Phenols and enolizable ketones that cannot undergo Q , -dehy-drogenation may afford intermolecular products arising from either C-C or C-0 coupling on treatment with DDQ in methanol. 2,6-Dimethoxyphenol, for example, results predominantly in oxidative dimerization (eq 21), while the hindered 2,4,6-tri-r-butyl-phenol generates the product of quinone coupling (eq 22). Various other unusual products have been observed on DDQ oxidation of phenols and enolic compounds, their structure being dependent on that of the parent compound. ... 

Treatment of O-silyl enols with silver oxide leads to radical coupling via silver enolates. If the carbon atom bears no substituents, two such r -synthons recombine to symmetrical 1,4-dicarbonyl compounds in good vield (Y. Ito, 1975). 

A total synthesis of functionalized 8,14-seco steroids with five- and six-membered D rings has been developed (467). The synthesis is based on the transformation of (S)-carvone into a steroidal AB ring moiety with a side chain at C(9), which allows the creation of a nitrile oxide at this position. The nitrile oxides are coupled with cyclic enones or enol derivatives of 1,3-diketones, and reductive cleavage of the obtained cycloadducts give the desired products. The formation of a twelve-membered ring compound has been reported in the cycloaddition of one of the nitrile oxides with cyclopentenone and as the result of an intramolecular ene reaction, followed by retro-aldol reaction. 

A number of compounds react rapidly with DDQ at room temperature. They include allylic and benzylic alcohols, which can thus be selectively oxidized, and enols and phenols, which undergo coupling reactions or dehydrogenation, depending on their structure. Rapid reaction with DDQ is also often observed in compounds containing activated tertiary hydrogen atoms. The workup described here can be used in all these cases. 

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