Dictyoptesterols A–C, ∆22-24-oxo cholestane-type sterols with potent PTP1B inhibitory activity from the brown alga Dictyopteris undulata Holmes

Three new cholestane-type sterols bearing an unusual ∆22–24-oxo side chain, namely, dictyoptesterols A–C (1–3), were isolated from the brown alga Dictyopteris undulata Holmes, together with five known strutural analogues (4–8). Their structures were elucidated on the basis of by extensive spectroscopic analysis. The absolute configurations of the steroidal nuclei of the new compounds were proposed by a comparison of NMR data with those of related known compounds as well as biogenetic considerations. All of the isolates were evaluated in vitro for their potential to inhibit protein tyrosine phosphatase-1B (PTP1B) activity. The results showed that compounds 1–5 exhibited different levels of PTP1B inhibitory activities with IC50 values ranging from 3.03 ± 0.76 to 15.01 ± 2.88 μM. In particular, compounds 3 and 4 showed promising inhibitory effects towards PTP1B with IC50 values of 3.03 ± 0.76 and 3.72 ± 0.40 μM, respectively, when compared to the positive control oleanolic acid (IC50, 2.83 ± 0.39 μM). The chemotaxonomic significance of these isolated ∆22–24-oxo cholestanes has also been discussed.