Chemical Transformations of Ajmalicine: Structure and Stereochemistry of Some Interchangeable Transformation Products

Article abstract of DOI:10.1021/jo00137a016

Several chemical transformations were carried out with ajmalicine (1), a heteroyohimbine alkaloid, with a view to studying the reactivity of the enolic ester system occurring in the heterocyclic ring.Ajmalicine hemiacetal (2) and ajmalicine 17-methyl acetal (6) were prepared by treating 1 with aqueous and methanolic sulfuric acid, respectively.C-17 epimerization of 6 was observed in polar aprotic solvents, but this isomerization in 2 was prevented due to hydrogen bonding between vicinal carbomethoxy and hydroxy groups.Ajmalicial (12), the aldehydic derivative of 1, was prepared by following a new one-step pathway.Both cyclic (12) and open (14) forms of ajmalicial, a hemiacetal, were isolated and characterized.The cyclic form (12) was found to exist in an all-chair conformation (11).But for the open form (14) the most stable conformation (15) demands that the D ring must occur in a twist-boat form.This spatial orientation facilitated an electrophilic attack by the C-16 aldehyde group at C-7.The product was shown to be a new isomer (16) of ajmalicial having a 5,16-secoajmaline skeleton.The preferred conformation of 16 is 17.This new isomer was susceptible to protic solvents.LAH reduction of 1 afforded ajmalicinol (23).The structures and conformations of all these products could be established from spectral analyses including 13C NMR spectra.