Abstract
Aurilol (1), intricatetraol (3), and enshuol (5), marine halogen-containing members of a family of squalene-derived triterpene polyethers named oxasqualenoids, were isolated from the sea hare Dolabella auricularia, the red alga Laurencia inricata, and omaezakiana Masuda, respectively. Although their planar structures and partial configurations were elucidated by spectroscopic and chemical analyses, until now their entire configurations had not been determined. Many other types of oxasqualenoids have also been isolated from both marine and terrestrial organisms; however, it is often difficult to determine their stereostructures even by modern highly advanced spectroscopic methods, especially in the case of acyclic systems that include stereogenic quaternary carbon centers. Such systems expose the technical limitations of the current highly advanced NMR spectroscopic methods used for the structural elucidation of diverse and complex natural products. Herein, we report the total assignments of the previously incomplete stereostructures of 1, 3, and 5 to 2, 4, and 6, respectively, through the first asymmetric total syntheses featuring regioselective 5-exo and 6-endo cyclizations of bishomoepoxy alcohols.
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