Allo-yohimbane

3-butadiene to afford 3. This was treated with hydrazine hydrate followed by treatment of the resulting hydrazone with sodium hydride in dimethylsulfoxide to produce the vinyl ether 4. Acetylation of 4 followed by hydrolysis in THF in the presence of 1 N HCl and the oxidation of the resulting hemiacetal with Jones reagent gave 5 in 63% overall yield from 

Deacetylation of 5 followed by condensation with tryptamine in a mixture of THF and diisopropylamine afforded 6, which was cleaved with HIO4. Subsequent reduction of the resulting aldehyde with sodium borohydride gave 7. This was treated with PhjP and CCU 

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In a previously described approach, a largely similar strategy was employed in order to generate enantio-selectively (—)-allo-yohimbane [84]. Yohimbane alkaloids such as reserpine and yohimbine were of long-standing interest from pharmacological and synthetic points of view, and the first enantio-selective (—)-alloyohimban synthesis was reported some time ago by Isobe et al. [88]. 

As summarised here for the chemo-enzymatic approach of (—)-allo-yohimbane, synthon was once again the key lactone 1 (derived enzymatically from the above-mentioned meso-diacetate). Through condensation with tryptamine, a lactam was obtained which was then sequentially converted into (—)-allo-yohimban, indicating the high potential of this chemo-enzymatic approach for production of optically pure alkaloids of the terpenoid indoles. 

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