Novel Sesquiterpene Skeletons by Multiple Wagner–Meerwein Rearrangements of a Longipinane-1,9-diol Derivative

The tricyclic sesquiterpene (1R,3R,4S,5S,7S,8S,­9S,10R,11R)-7,8-diangeloyl­oxylongipinan-1,9-diol, or rasteviol (7), underwent multiple Wagner–Meerwein molecular rearrangements and several hydride shifts when treated with Et2O–BF3 to generate the six new compounds (1R,3R,4S,5R,7S,­8S,9S,10R,11S)-7,8-diangeloyloxy-1,9-epoxyjiquilpane (8), (1R,3R,4S,5R,7R,­8S,9S,11S)-8-angeloyloxy-1,7-epoxyzamor-10­(14)-ene (11), (2S,3R,4R,5R,6R,­7R,8S,9S,10S)-7,8-diangeloyloxy-6,9-epoxyjanitziane (14), (4R,5R,7S,8S,9S,­10S,11S)-7,8-diangeloyloxy-9-hydroxyjiquilp-3­(15)-ene (16), (2S,3S,5R,7S,­8R,10S,11R)-7,8-diangeloyloxyiratzian-9-one (18), and (2S,3S,5R,10S,11R)-8-angeloyloxyiratzi-7-en-9-one (22), of which 8, 11, 14, and 18 possess new hydrocarbon skeletons. Their structures were determined by 1D and 2D NMR in combination with single-crystal X-ray diffraction analyses of derivatives 10, 15, 20, and 21, which allowed confirmation of their absolute configurations by means of the Flack and Hooft parameters. In addition, some reaction mechanism information was gained from deuterium labeling experiments.