C-O Ring Construction: (+)-Varitriol (Liu), (+)-Isatisine A (Panek), (+)-Herboxidiene / GEX1A (Ghosh), (-)-Englerin A (Chain), Platensimycin (Lear/Wright)

En route to (+)-varitriol (4), Xue-Wei Liu of Nanyang Technological University coupled (Org. Lett. 2011, 13, 42. DOI: 10.1021/ol102473k) the glycosyl fluoride derived from 1 with the alkynyl fluoroborate salt 2 to give 3.

James S. Panek of Boston University condensed (Org. Lett. 2011, 13, 502. DOI: 10.1021/ol102848w) the enantiomerically-pure allyl silane 6 with the aldehyde 5 to give the tetrahydrofuran 7. Further elaboration led to (+)-Isatisine A (8), the only alkaloid so far isolated from the roots and leaves of the traditional Asian folk medicine Isatis indigotica.

Arun K. Ghosh of Purdue University effected (Org. Lett. 2011, 13, 66. DOI: 10.1021/ol102549a) oxidative ring expansion of the enantiomerically-pure furan 9 to give, after reduction, the enone 10. This established the tetrahydropyran of (+)-Herboxidiene (11), also known as GEX1A.

William J. Chain of the University of Hawaii observed (J. Am. Chem. Soc. 2011, 133, 6553. DOI: 10.1021/ja201921j) unusual diastereoselectivity in the conjugate addition of the enone 12 to the enantiomerically-pure aldehyde 13. Although eight diastereomers could have been formed, the reaction mixture was 2/3 the diastereomer 14. Reductive cyclization (SmI₂) of 14 then led to (-)-Englerin A (15).

Martin J. Lear of the National University of Singapore cyclized (Org. Lett. 2010, 12, 5510. DOI: 10.1021/ol102390t) the enantiomerically-pure lactol 16 to 17 with catalytic Bi(OTf)₃. Dennis L. Wright of the University of Connecticut prepared (Org. Lett. 2011, 13, 2263. DOI: 10.1021/ol2005775) 21 by dipolar cycloaddition of 20 to 19. Both 17 and 21 were carried on via intramolecular alkylation toward Platensimycin (18).