text: In order to remove the C15 alcohol group, Enol 15 was converted to tosylate  16  (TsCl, pyridine) and then to mercaptoester 17 (sodium benzylmercaptide) which was then reduced to unsaturated ester 18 by Raney nickel and hydrogen. Further reduction with hydrogen / palladium on carbon afforded the saturated ester 19. Alkaline ester hydrolysis to carboxylic acid 20 was accompanied by epimerization at C14.
  This particular compound was already known from strychnine degradation studies.  Until now all intermediates were racemic but chirality was introduced at this particular stage via chiral resolution using quinidine.
  The C20 carbon atom was then introduced by acetic anhydride to form enol acetate 21 and the free aminoketone 22 was obtained by hydrolysis with hydrochloric acid. Ring VII in intermediate 23 was closed by selenium dioxide oxidation, a process accompanied by epimerization again at C14.
  The formation of 21 can be envisioned as a sequence of acylation, deprotonation, rearrangement with loss of carbon dioxide and again acylation: