Total synthesis of (–)-C₃₄-botryococcene, the principal triterpenoid hydrocarbon of the freshwater alga Botryococcus braunii

Abstract

The hydrocarbon (–)-C34-botryococcene 1 has been synthesized in enantiomerically pure form by coupling the alkylcopper species 41 to each terminus of diiodide 37. The former was prepared from (2S)3-hydroxy-2-methylpropionate 2 which was first converted into the alcohol 7. Coupling of the dianion of 7, obtained by deprotonation with the Schlosser base, with tosyl ester 11 gave the alcohol 12 and thence iodide 39. The central segment 37 of the botryococcene chain was synthesized from (2R)-3-hydroxy-2-methylpropionate 15 and entailed a novel rearrangement of the derived αβ-unsatured MOM ester 23 to establish the quaternary allylic centre at C-13. The pair of stereoisomeric hydroxy esters 24 and 25 obtained from this rearrangement were independently advanced to stereochemical convergence at 29, which was subsequently transformed into diiodide 37. The route of synthesis confirms the previous assignment of absolute configuration made to botryococcene 1 at five of the six stereogenic centres, i.e. (3S,7S,10R,16S,20S).