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Main observation and conclusion: Two new bis‐quinolizidine alkaloids, neopetrosiasins A (1) and B (2), possessing cis‐ and trans‐quinolizidine nuclei, one known related analogue petrosin (3) and three known xestospongins (4—6), were isolated from the South China Sea sponge Neopetrosia chaliniformis. The structures of new compounds were unambiguously determined by extensive spectroscopic and single‐crystal X‐ray analyses. In bioassay, compounds 4—6 exhibited moderate cytotoxicity against several human cancer cells. [ABSTRACT FROM AUTHOR]

• New cembranoids were isolated and identified from the Hainan soft coral Sarcophyton crassocaule. • A pair of enantiomers were purified by chiral HPLC. • The absolute configurations were determined by TDDFT-ECD calculation, modified Mosher's method, and/or Snatzke's method. The determination of the stereochemistry of cembrane type diterpenoids is always a challenging task due to the flexibility of the 14-membered macrocycle. In this paper, three new cembranoids, ximaocembrols A–C (1 – 3), were isolated from the Hainan soft coral Sarcophyton crassocaule , and ximaocembrol C was further separated by chiral HPLC to its corresponding enantiomers (±)- 3 , respectively. The absolute configurations of these new compounds were successfully elucidated by using a series of reliable approaches, including extensive spectroscopic analysis, TDDFT-ECD calculation, modified Mosher's method, and Snatzke's method. [ABSTRACT FROM AUTHOR]