C-O Ring Construction: Sauropus hexoside (Xie/Wu), (+)-Ipomeamarone (Usuki), Decytospolide A (Fujioka), Cytospolide P (Goswami), (+)-Didemniserinolipid B (Tong), Gymnothelignan N (She)

A range of biological activity was observed for the group of 3,6-anhydro-2-deoxy hexosides, of which 3 is representative, isolated from Sauropus rostratus. Wei-Jia Xie and Xiao-Ming Wu of China Pharmaceutical University prepared (Org. Lett. 2014, 16, 5004. DOI: 10.1021/ol5022838) 3 by the dealkylative cyclization of 1 to 2.

(+)-Ipomeamarone (6) is a phytoalexin isolated from mold-damaged sweet potatoes. Yoshinosuke Usuki of Osaka City University assembled (Chem. Lett. 2014, 43, 1882. DOI: 10.1246/cl.140811) 6 by the diastereoselective cyclization of 4 to tetrahydrofuran 5.

Hiromichi Fujioka of Osaka University protected (Org. Lett. 2014, 16, 3680. DOI: 10.1021/ol501463p) the enone of 7 by the conjugate addition of triphenylphosphine. Diastereoselective reduction of the other ketone followed by deprotection of the enone and cyclization led to Decytospolide A (9).

En route to Cytospolide P (12), Rajib Kumar Goswami of the Indian Association for the Cultivation of Science had planned (J. Org. Chem. 2014, 79, 7689. DOI: 10.1021/jo501184t) the ring-closing metathesis of 10. This failed, but cyclization of the corresponding silyl ether was successful with the second-generation Hoveyda catalyst.

Rongbiao Tong of the Hong Kong University of Science and Technology set (J. Org. Chem. 2014, 79, 6987. DOI: 10.1021/jo501142q) the absolute configuration of (+)-Didemniserinolipid B (15) by Sharpless asymmetric osmylation of the alkene 13. Oxidative Achmatowicz rearrangement/bicycloketalization then delivered 14.

Xuegong She of Lanzhou University observed (Org. Lett. 2014, 16, 4440. DOI: 10.1021/ol501960j) remarkable diastereoselectivity in the reductive cyclization of 16 to 17. Oxidation of 17 led to regioselective cyclization to Gymnothelignan N (18).