Limonoids with Diverse Oxidation Patterns of C‑12 Indicating a Complete Ring C-<i>seco</i> Biogenetic Pathway from <i>Munronia unifoliolata</i>

Munrolins A–Q (1–17), 17 new ring C-seco limonoids with diverse oxidative patterns of C-12, together with nine known analogues (18–26), were isolated from the CH2Cl2 extract of Munronia unifoliolata. The planar structures were elucidated by a combination of 1D and 2D NMR and HR-MS analyses, while the absolute configurations were confirmed by ECD calculations and single-crystal X-ray diffraction. Munrolins A (1) and B (2) were first identified as ring C-seco limonoids with a 12,13-ether bridge moiety. Munrolins C–J (3–10) have a rare reduced primary alcohol fragment, while munrolin Q (17) has an unusual ketal fragment formed by dehydration of C-12/14. These limonoids with diverse alcohol and aldehyde type C11/12 branches may be generated through different degrees of reduction after the Baeyer–Villiger oxidation at the C ring, as key intermediates to supplement the biogenetic pathway of ring C-seco limonoids. Compounds 11, 19, and 26 could reverse the multidrug resistance of MCF-7/doxorubicin cells with reversal fold values of 5.2, 4.5, and 18.3, respectively.