Enantiospecific Total Synthesis of (−)-4-Thiocyanatoneopupukeanane^†

Enantiospecific synthesis of the natural enantiomer of the marine sesquiterpene (−)-4-thiocyanatoneopupukeanane (6) is described. The bicyclo[2.2.2]octanecarboxylate 14, obtained from (R)-carvone via Michael−Michael reaction, was transformed into neopupukeananedione 12 by employing rhodium acetate catalyzed intramolecular C−H insertion of the diazo ketones 16 or 19 as the key reaction. Regioselective deoxygenation of the C-2 ketone transformed the dione 12 into neopupukean-4-one 10. Alternately, the keto ester 18 was also transformed into neopupukean-4-one 10 via regioselective deoxygenation of the ketone in 18 followed by intramolecular rhodium carbenoid C−H insertion of the diazo ketone 31. Finally, neopupukean-4-one 10 was transformed into (−)-4-thiocyanatoneopupukeanane 6 via the alcohol 32 and the mesylate 33.