Baeyer-Villiger oxidation aldol reaction

Table 5. Direct catalytic asymmetric aldol reactions promoted by heteropolyme-tallic asymmetric catalyst and following Baeyer-Villiger oxidations.

Catalysis in reaction systems with undissolved substrates and products is not restricted to biocatalysis. High yields in sobd-state synthesis, sohd-to-sohd reactions, and solvent-free systems have also been reported for aldol condensation, Baeyer-Villiger oxidation, oxidative coupling of naphthols, and condensation of amines and aldehydes [1, 2]. 

Many enzymes are both active and stable in carbon dioxide and have been used to conduct a number of reactions. Several different types of reactions have been examined, including hydrolysis (Lee et al., 1993 Randolph et al., 1985 Zheng and Tsao, 1996), oxidation (Hammond et al., 1985 Randolph et al., 1988), and esterification/transesterification (Kamihira et al., 1987 Nakamura et al., 1986 Rantakyla and Aaltonen, 1994), but there are other types of reactions that would make worthwhile investigations in carbon dioxide. These include preparation of amides, reduction of ketones, preparation of cyanohydrins from aldehydes, aldol reactions, hydroxylation reactions, and Baeyer-Villiger oxidation. 

Two different modes of reaction have been reported for steroidal A -3-ketones 20). Potassium persulphate in sulphuric acid gave the 4 0xa"3 ketone (23), considered to arise by an initial Baeyer-Villiger oxidation to the unsaturated lactone (21) as an enol lactone of the C(g)-aldehyde (22) this could suffer further degradation through a second Baeyer-Villiger attack on the aldehyde group [64], Other. workers [6 ] used peroxytrifluoroacetic acid and obtained the 5a-carboxy--4"Oxa 3-ketone (25) and the bridged product (26), apparently derived by an internal aldol condensation of the intermediate lactone-aldehyde (24). 

Several of the aldol products obtained were readily converted to their corresponding esters by Baeyer-Villiger oxidation. These results also are summarized in Table 16. Ester 66 was further transformed into key epothilone A intermediate 69 and also a key synthetic intermediate 70 for bryostatin 7. What is the mechanism of these direct catalytic asymmetric aldol reactions using LLB-II It is apparent that self-assembly of LLB and KOH occms, because of the formation of a variety of aldol products in high ee and yields. In addition, the NMR and LDI-TOF(-i-)MS spectra of LLB KOH show the occurrence of rapid exchange between Li and K. We have already found that LPB[LaK3tris(binaphthoxide)] itself is not a useful catalyst for aldol reactions, and that the complexes LPB KOH or LPB LiOH give rise to much less satisfactory results. 

The use of polymer-supported bismuth catalysts for organic synthesis is highly attractive since this approach combines the advantages of the relative nontoxicity of bismuth salts with their easy separation and recyclability after reaction [131]. For example, microencapsulated Bi(0Tf)3 xH20 in polystyrene has been successfully reported for the methoxymethylation of alcohols, allylation of aldehydes, Michael-type and aldol reactions as well as Baeyer-Villiger oxidations [132]. 

SCHEME 8.37. The Shibasaki direct catalytic asymmetric aldol reaction using heteropolymetallic asymmetric catalyst and following Baeyer-Villiger oxidation. 

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