Oxidation-reduction mechanism epimerization

Dihydrocorynantheine was obtained via similar steps from normal cyanoacetic ester 319 (172). Stereoselective transformation of the alio cyanoacetic ester 315 to the normal stereoisomer 319 was achieved by utilizing a unique epimerization reaction of the corresponding quinolizidine-enamine system (174). Oxidation of alio cyanoacetic ester 315 with lead tetraacetate in acetic acid medium, followed by treatment with base, yielded the cis-disubstituted enamine 317, which slowly isomerized to the trans isomer 318. It has been proved that this reversible eipmerization process occurs at C-15. The ratio of trans/cis enamines (318/317) is about 9 1. The sodium borohydride reduction of 318 furnished the desired cyanoacetic ester derivative 319 with normal stereo arrangement. The details of the C-15 epimerization mechanism are discussed by B rczai-Beke etal. (174). 

As shown in Fig. 2, two mechanisms involving an intermediate oxidation may be written for the epimerization at C-4". In the first one (A), the oxidation results in an a-D-xj/lo-hexopyranosyl-4-ulose derivative (96), which is then attacked by a hydride ion from the opposite side of the carbonyl group a change in conformation of the enzyme-intermediate complex seems necessary for such a process. The mechanism depicted under (B) postulates oxidation at C-3", and the resulting hexopyranosyl-3-ulose derivatives (54 and 97) then achieve equilibrium through the common enediol intermediate (98) before undergoing reduction at C-3". Compound 98 may also be formed from the hexopyranosyl-4-ulose ester 96, and in such a manner, both of the pathways may be linked. 

Enzymes thus far found to catalyze isomerization or epimerization by oxidation and reduction require the cofactor NAD, which is often very tightly bound, not being removed by dialysis, but only by treatment with charcoal. Enzymes for which a keto-enol mechanism has been suggested do not usually involve NAD. The two mechanisms should also be distinguishable by the nonoccurrence or occurrence, respectively, of hydrogen exchange with the solvent. 

Methylation of a phenolic -OH group in (S)-3 -hydroxy-JV-methyl-coclaurine by SAM gives (S)-reticuline through the usual Sn2 pathway, and epimerization of the chirality center forms (i )-reticuline. The epimerization is a two-step process, the first an oxidation of the tertiary amine to an intermediate iminium ion and the second a hydride reduction of the iminium ion. The mechanism of the oxidation step is not yet known, but the reduction of the iminium ion requires NADPH as cofactor (Figure 25.11). 

---