Hydroxyl groups epimerisation

Asymmetric epoxidation of 10a under standard conditions yields the crystalline epoxy alcohol 2a in 95% ee (91% chemical yield). Treatment of 9a with thioanisol in 0.5N NaOH, in rerf-butyl alcohol solution, gives -after protection of the hydroxyl groups as benzyl ethers- the sulfide a (60% overall yield) through an epoxide ringopening process involving a Payne rearrangement. Since the sulfide could not be hydrolysed to the aldehyde 7a without epimerisation at the a-position, it was acetoxylated in 71% yield under the conditions shown in the synthetic sequence (8a... 

Primary BAs, cholic acid (CA), and chenodeoxycholic acid (CDCA), are synthesised via the 5/3-saturation of the cholesterol double bond by enzymes of the hepa-tocyte microsomal fraction, epimerisation of the 3/j-hydroxyl group to the 3a-con-figuration, and further insertion of a 7 -hydroxyl group, with or without a further 12a-hydroxyl group. After shortening of the side chain by three carbons, oxidation of the terminal carbon of the side chain occurs to form the carboxylic group [3]. Alternative metabolic sequences add to the complexity of this metabolic pathway (Fig. 5.4.2). 

As before, ring opening of the lactone 98 required careful control to prevent epimerisation, and indeed migration of Boc to the primary hydroxyl group of 99, but NaOMe in methanol at -78 °C gave the alcohol 99 essentially quantitatively. 

The enantioselective ctT-dihydroxylation of benzothiophenes and benzofnrans in the heterocyclic ring, by Pseudomonas putida, is analogous to well-known conversions of simple benzenoid compounds, bnt in the heterocyclic context, hydroxyl groups introduced at an a-carbon easily epimerise. Indole gives indoxyl probably via dehydration of an intermediate 2,3-diol. In contrast, cw-dihydroxylation of quinolines, or of 2-phenylpyridines, takes place selectively in the benzene ring. ... 

EDCI), the nitrogen and hydroxyl groups were protected as the N,0-acetal 1233 by reaction of 1232 with 22-dimethoxylpropane in acetone in the presence of trifluoroborane etherate. Finally, reduction of the N-methoxyamide with lithium aluminium hydride afforded Gamer s aldehyde in 88% overall yield for the 4-step sequence, Epimerisation of the stereogenic centre is negligible under these conditions. 

P. J. L. Daniels et synthesised 2 -epi-gentamicin Ci 225 and 2"-deoxy-gentamicin C2 226. Penta-N-acetylgentamicin Cj is mesylated at the 2 -hydroxyl group. Through solvolysis of the mesylate by participation of the vicinal trans acet-amido group, epimerisation at the 2"-C atom takes place. 

Access to p-mannosides can be achieved by epimerising the C-2 hydroxyl group of a P-glucoside, however this can be synthetically time consuming [35]. A more efficient alternative to the deprotection, oxidation, reduction methodology was developed by Lichtenthaler and co-workers and focused on the use of 2-oxo bromide 17 as the glycosyl donor [23]. Access to 2-oxo bromide 17 can be achieved from acetobromoglucose 18 in a simple 4-step synthetic sequence with an overall yield of 54% (Scheme 8.5). 

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