Isomerisation enolisation

Once regioselective and stereoselective controls have been exerted, the cis-decalins must be isomerised to rrans-decalins, the configuration present in the target molecules. Since frans-decalins are thermodynamically more stable than the corresponding cis-decalins, it is possible to isomerise the latter through enolisation, a process that can be favored by the presence of a carbonyl group near to the centre to be inverted. 

Selection then indicates that the lowest energy pathway for the isomerisation of aldoses and ketoses by the enolisation route likewise involves a ct5-enediol(ate). 

Enolisation.— The isomerisation of androst-5-ene-3,17-dione (314) into the 4-ene-3,17-dione (316) proceeds through the enol (315), and is subject to rate-controlling removal of 4-H, whether catalysed by acid, base, or enzyme. Isotopic labelling of 4/5-H has indicated differing stereoselectivities in hydrogen removal according... 

Deoxy-D-glycero-hexo-2,3-diulose is a typical product of degradation and 4-0-substituted derivatives of fructose (e.g. lactulose, which arises by isomerisation of lactose). 1,2-Enolisation of 4-deoxy-D-g lycero-hexo-2,3-diulose yields, via the appropriate l-ene-l,2-diol, unstable 4-deoxyhexos-3-ulose, which splits into formic acid and 1,2-enediol of 3-deoxy-D-glycero-pentulose in neutral or alkaline solutions by a reverse Claisen condensation mechanism. This sugar can be oxidised to 3-deoxy-D-glycero-pentos-2-ulose or may dehydrate to form 3,4-dideoxypentosulose (Figure 4.32). Other reaction products are isosaccharinic acids. 

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