Intricatetraol (1), a member of a family of squalene-derived triterpene polyethers named oxasqualenoids, [1] was isolated from the red alga Laurencia intricata by Suzuki et al. in 1993. A crude fraction containing intricatetraol (1) as the major component exhibited cytotoxic activity against P388 "leukemia cells" with an IC 50 value of 12.5 mg mL À1 .[2] The structural analysis was mainly carried out by NMR spectroscopic methods. Although it was found that the molecule has C 2 symmetry, the cis configuration within the tetrahydrofuran ring, the R configuration at C11 (C14), the relative configurations at C6 and C7 (C18 and C19), C10 and C11 (C14 and C15), and at bromine-bonded C3 (C22) remained to be determined.There are many other types of oxasqualenoids; however, it is often difficult to determine their configurations even by highly advanced spectroscopic methods, especially for the acyclic portions that include stereogenic quaternary carbon centers, such as C6-C7 (C18-C19) and C10-C11 (C14-C15) in 1. These difficulties have prompted attempts to determine the configurations of oxasqualenoids by chemical synthesis. [3] Furthermore, the presence of the vicinal bromochloro functionality in 1 makes the problem of stereochemical assignment even more inaccessible. An enantioselective method for the synthesis of the vicinal bromochloro functionality, [4] which also occurs in the marine polyhalogenated monoterpene halomon (2), a promising anticancer agent, [5] has never been developed. Herein we report the total assignment of the configuration of (+)-intricatetraol (1) as 3 through the first asymmetric total synthesis of this natural product. The synthesis features the enantioselective construction of the vicinal bromochloro functionality through a pathway in which the configuration is secured.Suzuki et al. suggested structure 4, which disregards the configuration at C3 (C22), as a possibility on the basis of the hypothetical biogenesis, [2] and recently we [6] and Ujihara [7] independently confirmed this assignment. Our retrosynthetic analysis of the target compound 4 is depicted in Scheme 1. We envisaged that it would be efficient to dimerize by olefin metathesis the functionalized fragment 5, which represents half of the molecule, because of the C 2 symmetry of the natural product. The R or S configuration at the carbon atom attached to bromine in 5 could be introduced through epoxide chemistry. The desired trihydroxytetrahydrofuran 6 might be constructed by the stereospecific and stereoselective oxacycScheme 1. Retrosynthetic analysis of the target compound 4. AD= asymmetric dihydroxylation, AE = asymmetric epoxidation.[*] Prof.