**1. Introduction**

Natural products are promising sources for the discovery of novel agents/active templates for the development of effective agents against a variety of human diseases [1]. Due to their great structural diversity and wide range of bioactivities [2], diterpenoids have been a constant focus of drug discovery. Among the naturally occurring cyclic diterpenoids (e.g., abietane, labdane and clerodane diterpenoids), tricyclic abietane diterpenoids are of particular significance. These diterpenoids were reported to be present in species of Lamiaceae, Cupressaceae, Pinaceae and Euphorbiaceae as well as in several higher plants [3,4] and were shown to possess antitumor, antioxidant, antibacterial and anti-inflammatory effects [5]. Sugiol, an abietane diterpenoid previously isolated from *Metasequoia glyptostroboides* (Cupressaceae), has been developed as an advanced multimodal anti-inflammatory disease targeting tool [6]; Euphelionolides A, D, I and L, four *ent*-abietane diterpenoids isolated from *Euphorbia helioscopia* (Euphorbiaceae), were demonstrated to be effective free-radical scavengers acting via various reaction pathways [7]; 6-hydroxy-5,6-dehydrosugiol, a derivative of sugiol isolated from the stem bark of *Cryptomeria japonica*, was shown to be a potent androgen receptor antagonist in PCa cells [8].

The roots of *Euphorbia fischeriana* Steud have been used as a traditional Chinese medicine for the treatment of lymphoid tuberculosis and ringworm [9]. Previous phytochemical studies showed that polycyclic diterpenoids including *ent*-abietanes, *ent*-atisanes, *ent*-kauranes, *ent*-isopimaranes and *ent*-pimaranes possessing a common 6/6/6-tricyclic ring are the major constituents of *E. fischeriana* [10,11]. In our earlier study, a series of *ent*abietane diterpenoids with significant cytotoxicity have been isolated from this plant [12]. As part of our continuing efforts toward novel antitumor diterpenoids, the chemical constituents of the roots of *E. fischeriana* were reinvestigated. As a result, two previously undescribed *ent*-abietanes and two known analogues were isolated from the roots of *E. fischeriana*. The new compounds showed significant cytotoxicity against human prostate

cancer C4-2B and C4-2B/ENZR cell lines. Herein, the details of isolation, structure elucidation and cytotoxicity of these compounds are described.
