α-Glucosidase inhibitory and tyrosinase inhibitory activities of alkaloids from <i>Stephania kwangsiensis</i>

From the tubers of <i>Stephania kwangsiensis</i>, two undescribed alkaloids (7 <i>R</i>, 14<i>S</i>)-thaicanine α-<i>N</i>-oxide (<b>1</b>) and (6<i>S</i>, 6a<i>S</i>, 7 <i>R</i>)-sukhodianine α-<i>N</i>-oxide (<b>2</b>) were isolated, adding 14 alkaloids which had been reported earlier. The structural definition of <b>1</b>-<b>16</b> was achieved by analysing mass spectrum and nuclear magnetic resonance data, as well as comparing the calculated ECD spectra with experimental ones. <i>N</i>-<i>O</i>-Dimethylthaicanine (<b>12</b>) and polycarpine (<b>14</b>) displayed favourable α-glucosidase inhibitory activities <i>in vitro</i> with the IC₅₀ values of 3.001 ± 0.019 mmol/L, 2.132 ± 0.009 mmol/L, respectively. Dactycapnine B (<b>8</b>) demonstrated tyrosinase inhibitory activity with an IC₅₀ value of 2.353 ± 0.136 mmol/L.