New cembrane-type diterpenoids from the South China Sea soft coral Sinularia crassa and their α-glucosidase inhibitory activity.

A detailed chemical research of the soft coral Sinularia crassa collected from the South China Sea yielded a series of cembrane-type diterpenoids, including four new cembranoids, namely sinulacrassins A - C (1, (+)-2, (+)-4) and ent-xishaflavalin G ((+)-3), along with five known analogs ((-)-3, 5-8). Their structures were elucidated by detailed spectroscopic analysis, chemical methods, and the comparison with those literature data. The absolute configuration of 1 was established by time-dependent density functional theory electronic circular dichroism (TDDFT/ECD) calculation, and the absolute configuration of (+)-2 was determined using the modified Mosher's method. The bioassay results revealed that (+)-2 and 5 were novel α-glucosidase inhibitors with IC50 values of 10.65 ± 0.16 and 30.31 ± 1.22 µM, respectively. In addition, (+)-2 and 5 were nontoxic towards human normal hepatocyte (LO2) cells at 100 µM. The present results highlighted the unusual coexistence of α and ß configurations of C-1 in cembranoids from soft coral in the Order Alcyonacea, and provided new chemotype for the development of α-glucosidase inhibitors used in anti-diabetes treatment.