Silver-catalyzed benzannulation, part 1: Total synthesis of (7S,10R)-2,15-dihydroxycalamene, (7S,10R)-2-Hydroxy-15-calamenenal and (7S,10R)-2-hydroxy-15-calamenenoic acid

This paper reports the development of a silver-catalyzed 6-endo-dig cyclisation of 5-alkoxy-1,5-enynes enabling benzannulation of (–)-menthone with construction of the calamenene skeleton as a single isomer. The total syntheses of (7S,10R)-2,15-dihydroxycalamenene (6 steps, 39.6%), (7S,10R)-2-hydroxy-15-calamenenal (7 steps, 10.7%) and (7S,10R)-2-Hydroxy-15-calamenenoic acid (9 steps, 33.6%) were achieved from (–)-menthone via a common intermediate. Analysis of the spectroscopic data as well as the specific rotations allowed for confirmation of the stereochemistry at C1 and C4 of the natural compounds isolated from Tarenna madagascariensis as (7R,10S)-2,15-dihydroxycalamenene and (7R,10S)-2-hydroxy-15-calamenenal, and the carboxylic acid isolated from Alpinia oxymitra was confirmed as (7S,10R)-2-hydroxy-15-calamenenoic acid.