Novel synthetic strategy toward abietane and podocarpane-type diterpenes from (−)-sclareol: synthesis of the antitumor (+)-7-deoxynimbidiol

A new route to abietane and podocarpane-type terpenoids from labdane diterpenes is reported. The key step is the transformation of β-ketoester 9 into the corresponding O-acetylsalicylate ester 18, via a manganese(III)-based oxidative free-radical cyclization carried out in Ac2O. Utilizing this, the synthesis of the antitumor (+)-7-deoxynimbidiol (5) from (−)-sclareol (11) has been achieved. (+)-Nimbidiol (6) and the natural terpenoid 20 have also been synthesized.