The resolution of the enantiomers of (+/-)-β-keto ester 6 using (R,R)-cycloheptane-1,2-diol as a chiral auxiliary for acetal formation was carried out with the enantiomerically pure (8aR)- and (8aS)-decahydro-5,5,8a-trimethyl-2-oxo-naphthalene-1-methanols 7 and (8aR)- and (8aS)-decahydro-5,5,8a-trimethyl-2-oxo-naphthalene-1-carboxylates 6 being obtained. Both (10R)- and (10S)-15,16-epoxy-8(17),13(16),14-labdatriene 4 were synthesized from (8aR)- and (8aS)-hydroxy ketone (7), respectively, with the absolute configuration of the natural (-)-4 unequivocally being established as (5R,9R,10R). On the other hand, both (10R)- and (10S)-15,16-epoxy-7,13(16),14-labdatriene 5 were synthesized from (8aR)- and (8aS)-β-keto ester 6, respectively, and the relative structure of natural (+)-5 unequivocally determined as depicted in Scheme 1.