Xylogranatins A−D: Novel Tetranortriterpenoids with an Unusual 9,10-<i>s</i><i>eco</i> Scaffold from Marine Mangrove<i> Xylocarpus </i><i>g</i><i>ranatum</i>

Four novel tetranortriterpenoids, xylogranatins A−D (1−4), with an unusual 9,10-seco skeleton were isolated from the seeds of a Chinese marine mangrove Xylocarpus granatum. Their structures were determined by spectroscopic and chemical means. Xylogranatin A (1) featured by a unique 1,9-oxygen bridge was confirmed by single-crystal X-ray diffraction, and xylogranatin D (4) with an unprecedented skeleton of C-30−C-9 linkage was postulated biogenetically from 3 via an α-hydroxyl ketone rearrangement and was chemically mimicked.