Total synthesis of (+)-dihydrocuscohygrine and cuscohygrine.

The first enantioselective synthesis of (+)-dihydrocuscohygrine 1 and cuscohygrine 2 is presented. 1 was obtained in nine steps and 30% overall yield with a ruthenium-catalyzed tandem ring rearrangement metathesis key step starting from enantiomerically pure cycloheptene-1,3,5-triol derivative 6. The unknown absolute configuration of natural dihydrocuscohygrine 1 could be determined as (S,S)-(-). Cuscohygrine 2 was obtained by Jones oxidation of 1 in quantitative yield but unfortunately with complete epimerization.