Huang-Minlon reduction structure

Isolated from Veratrum album subsp. lobelianum-, containing one ace-toxy, one hydroxy, and one ketone group the authors (144) assumed a similarity with tomatillidine, for which, however, a new structural formula was proposed later (172) , Huang-Minlon reduction of deacetylveralodisine led to veralosidine (144). 

The Cram modification has not been greatly exploited in multistep synthetic endeavors, possibly because of the above-mentioned need to preform and isolate the hydrazone intermediate coupled with very slow addition of the hydrazone. This latter requirement is particularly inconvenient when milligram quantities are involved. The aldehyde in a triterpenoid (partial structure 18) was successfully reduced to the corresponding methyl group using the Cram method, albeit in low yield (32%). Notably, the hydrazone (lO mg) was not added slowly. The Cram process was not successful for the reduction of the cyclo-butanone (19) (the Huang-Minlon process succeeded), or the pyrrole aldehydes (20) and (21). ... 

A modified Huang-Minlon procedure, followed by N-methylation and amide reduction, yielded ( )-(5-A6-dihydrodesoxycodeine methyl ether 52, which after resolution proved identical to material derived from natural sources45e-f via manipulation of 3-thebainone, and consequently served as a synthetic relay. (It is within an obscure portion of this series of rigorous structural studies and proofs that Evans later intercepted Gates route and thus formalized his synthesis). 

---