Diels-Alder Cycloaddition: (+)-Armillarivin (Banwell), Gelsemiol (Gademann), (+)-Frullanolide (Liao), Myceliothermophin A (Uchiro), Peribysin E (Reddy), Caribenol A (Li/Yang)

Martin G. Banwell of the Australian National University prepared (Org. Lett. 2013, 15, 1934. DOI: 10.1021/ol400583c) the enantiomerically-pure diol 1 by fermentation of the aromatic precursor. Diels-Alder addition of cyclopentenone 2 proceeded well at elevated pressure to give 3, the precursor to (+)-Armillarivin (4).

Karl Gademann of the University of Basel found (Chem. Eur. J. 2013, 19, 2589. DOI: 10.1002/chem.201203746) that the Diels-Alder addition of 6 to 5 proceeded best without solvent and with Cu catalysis, to give 7. Reduction under free radical conditions led to Gelsemiol 8.

Chun-Chen Liao of the National TsingHua University carried out (Org. Lett. 2013, 15, 1584. DOI: 10.1021/ol4003724) the diastereoselective addition of 10 to 9. A later oxy-Cope rearrangement established the octalin skeleton of (+)-Frullanolide 12.

D. Srinivasa Reddy of CSIR-National Chemical Laboratory devised (Org. Lett. 2013, 15, 1894. DOI: 10.1021/ol400555m) a strategy for the construction of the angularly-substituted cis-fused aldehyde 15, based on Diels-Alder cycloaddition of 14 to the diene 13. Further transformation led to racemic Peribysin-E (16). An effective enantioselective catalyst for dienophiles such as 14 has not yet been developed.

Hiromi Uchiro of the Tokyo University of Science prepared (Tetrahedron Lett. 2012, 53, 5167. DOI: 10.1016/j.tetlet.2012.07.058) the bicyclic core of Myceliothermophin A (19) by BF₃^.Et₂O-promoted cyclization of the tetraene 17. The single ternary center of 17 mediated the formation of the three new stereogenic centers of 18, including the angular substitution.

En route to Caribenol A (22), Chuang-Chuang Li and Zhen Yang of the Peking University Shenzen Graduate School assembled (J. Org. Chem. 2013, 78, 5492. DOI: 10.1021/jo4006156) the triene 20 from two enantiomerically-pure precursors. Inclusion of the radical inhibitor BHT sufficed to supress competing polymerization, allowing clean cyclization to 21. Methylene blue has also been used (J. Am. Chem. Soc. 1980, 102, 5088. DOI: 10.1021/ja00535a047) for this purpose.