Harzianone (2), a new harziane diterpene, was isolated from an alga-endophytic isolate of Trichoderma longibrachiatum. The structure and absolute configuration of 2 were unambiguously identified by NMR and mass spectrometric methods as well as quantum chemical calculations. The absolute configuration of harziandione (1) was supported by calculation of optical rotation, and the structure of isoharziandione was revised to 1 on the basis of (13)C NMR data comparison and calculation.

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