C-O Ring Natural Products: (-)-Serotobenine (Fukuyama-Kan), (-)-Aureonitol (Cox), Salmochelin SX (Gagné), Botcinin F (Shiina), (-)-Saliniketal B (Paterson), Haterumalide NA (Borhan)

Tohru Fukuyama of the University of Tokyo and Toshiyuki Kan of the University of Shizuoka devised (J. Am. Chem. Soc. 2008, 130, 16854 DOI: 10.1021/ja807676v) the chiral auxiliary-directed Rh-mediated cyclization of 1, setting the two stereogenic centers of 2 with high stereocontrol. The ester 2 was carried on to the indole alkaloid (-)-Serotobenine (3).

In the course of a synthesis of (-)-Aureonitol (6), Liam R. Cox of the University of Birmingham developed (J. Org. Chem. 2008, 73, 7616 DOI: 10.1021/jo801338t) the diastereoselective intramolecular addition of an allyl silane 4 to give the tetrahydrofuran 5. In analogy to what is known about the intramolecular ene reaction, the diastereocontrol observed for this cyclization may depend on the allyl silane being Z.

Michel R. Gagné of the University of North Carolina found (J. Am. Chem. Soc. 2008, 130, 12177 DOI: 10.1021/ja8041564) that the Ni-catalyzed coupling of organozinc halides could be extended to glycosyl halides such as 7. This opened ready access to C-alkyl and C-aryl glycosides, including Salmochelin SX 10.

Isamu Shiina of the Tokyo University of Science established (Org. Lett. 2008, 10, 3153 DOI: 10.1021/ol801066y) that the acid-mediated cyclization of the Sharpless-derived epoxide 10 proceeded with clean inversion, to give 11. The highly-substituted tetrahydropyran core 11 was then elaborated to the antifungal Botcinin F (12).

Ian Paterson of Cambridge University optimized (Org. Lett. 2008, 10, 3295 DOI: 10.1021/ol801148d) the Pd-catalyzed spirocyclization of the ene diol 13, leading to 14, the enantiomerically-pure bicyclic core of (-)-Saliniketal B (15).

Haterumalide NA (18) presented the particular challenge of assembling the geometrically-defined chloroalkene, in addition to closing the macrolide ring. Babak Borhan of Michigan State University addressed (J. Am. Chem. Soc. 2008, 130, 12228 DOI: 10.1021/ja8043695) both of these challenges together, electing to employ a chlorovinylidene chromium carbenoid, as developed by Falck and Mioskowski, to effect the macrocyclization of 16 to 17.