Ketals reduction

Bartels, B. Hunter, R. A selectivity study of activated ketal reduction with Me2S-BH3. 

Reduction of bicyclic ketals. Reductive cleavage of the bicyclic ketal 1 in the presence of a Lewis acid favors ds-products, regardless of the reductant. The highest dr-selectivity is obtained with (C2H5)3SiH and TiCl4. DIBAL shows high... 

Codeine has also been prepared in 70% overall yield, again without purification of intermediate compounds, from dihydrothebainone (132) by the route (132) — (137) shown in Scheme 4. The initial product of the action of bromine and then alkali on dihydrothebainone is the 1,7-dibromo-derivative of dihydro-codeinone, which can be reduced to dihydrocodeinone (133). This may be converted into 7-bromodihydrocodeinone dimethyl ketal (136), which on treatment with potassium t-butoxide in DMSO at 120 °C is converted exclusively into thebaine, but at 60 °C the product is codeinone dimethyl ketal (137), which can be hydrolysed to codeinone (131).154 The process has obvious value in the possible synthesis of codeine via dihydrothebainone, for which a patent has been filed covering a process that proceeds from the reduced isoquinoline (138) 155 the conversion of A-formylnordihydrothebainone into dihydrothebainone by hydrolysis and reductive methylation and by ketalization, reduction, and hydrolysis has been reported.156... 

Preparative Methods prepared in two steps from commercially available L-gulonolactone via ketalization and oxidative cleavage with sodium periodate at pH 5.5. Also obtained from l-ascorbic acid via (i) ketalization, reduction (lithium aluminum hydride) and oxidative cleavage (sodium periodate), or (ii) ketalization and oxidative fragmentation using hydrogen peroxide and hypochlorous acid. ... 

Ketalization, reduction with lithium dimethoxyaluminum hydride, and mild acid hydrolysis gave the keto amine 404, which exhibited strong Bohlmann bands. Fischer condensation using formic acid on the o-... 

Scheme 17.14. Stereospecific ketal reduction by a distant hydride transfer promoted by the alkoxide site. (Adapted with permission from Ref. 65.)...

A variant in the production of an intermediate in the Robinson synthesis, the tricyclic ABC diketone (18), was developed by Banerjee and co-workers [635, 636] (Scheme 58). The triester (22) was obtained from a-ethoxycarbonylcyclohexanone (21) by the Michael reaction with methyl acrylate, alkaline cleavage, and esterification, and it was then cyclized by Dieckmann s method with subsequent bromination and dehydrogenation to give the unsaturated keto diester (23). The addition of cyanoacetic ester gave compound (26) from which the keto triester (25) was obtained by methylation, acid hydrolysis, and esterification. The latter, by Dieck-mann cyclization and hydrolysis, gave the BC fragment (24). Selective ketalization, reduction, and hydrolysis of the ketal led to the hydroxy-ketone (27). The trans-B/C linkage present in it required the protection... 

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