Squalenoids synthesis

In the synthesis of the squalenoid glabrescol (72 originally attributed structure), containing five adjacent (all cis) THF rings, the necessary precursor of the polyepoxide cascade, the pentaepoxide 71, was achieved by epoxidation of each of the trisubstituted double bonds of the known (R)-2,3-dihydroxy-2,3-dihydrosqualene (70) by the Shi epoxidation approach (Scheme 8.18) [34]. Treatment of 71 with CSA at 0 °C and subsequent purification by column chromatography provided the pure polycyclic ether 72 by a cascade process reasonably initiated by the free secondary alcohol functionality [35a]. 

Grosa, G., Viola, F., Ceruti, M., Brusa, P, Delprino, L., Dosio, F., and Cattel, L., Synthesis and biological activity of a squalenoid maleimide and other classes of squalene derivatives as irreversible inhibitors of 2,3-oxidosqualene cyclase, Eur. J. Med. Chem., 29, 17, 1994. 

Xiong ZM, Corey EJ (2000) Simple Total Synthesis of the Pentacyclic Cs-Symmetric Structure Attributed to the Squalenoid Glabrescol and Three Cs-Symmetrie Diastere[Pg.165]

Squalenoid triterpenes are particularly rare in dictyocer-atid sponges, the first and maybe the only known example being a 7,18-diketosqualenoid isolated from an Indonesian Hyrtios erectus. The absolute configtiration was shown to be R,R by synthesis (Williams, Tahir, and Andersen, 1999 Enders and SchiiReler, 2002). 

Little work has been devoted to the class Gastropoda, from which only a few derivatives have been isolated. Limatu-lone is a bicydic triterpene of the squalenoid type produced by the limpet Collisella limatula for protection against predation by crabs and fish (Albizati, Pawhk, and Faulkner, 1985 Pawlik, Albizati, and Faulkner, 1986). Despite the presence of four asymmetric carbon atoms, limatulone is optically inactive and it was determined by total synthesis that natural limatulone contains both the meso derivative and the racemic mixture (Mori, Takikawa, and Kido, 1993). 

In Corey s total synthesis of the antiviral squalenoid venustatriol (71), three different asymmetric epoxidations were used (Scheme 9.8) [82]. Che-moselective epoxidation of the allylic alcohols in farnesol (65) and geraniol (69) [74] were carried out, along with the epoxidation of the trishomoallylic alcohol 67. The last of these proceeded with high diastereoselectivity in the presence of d-(-)-DET (47) and TrOOH as the oxidant [82]. 

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