Diels-Alder Cycloaddition: Fawcettimine (Zhai), Sculponeatin N (Zhai), Elansolid B1 (Kirschning), Frondosin A (Wright), Kingianin H (Parker), Rufescenolide (Snyder)
En route to Fawcettimine (4), Hongbin Zhai of Lanzhou University found (Org. Lett. 2014, 16, 196. DOI: 10.1021/ol403185g) that microwave irradiation improved the efficiency of the cycloaddition of the enone 1 with butadiene 2 to give 3. Takuya Kurahashi and Seijiro Matsubara of Kyoto University developed (Org. Lett. 2014, 16, 2594. DOI: 10.1021/ol500625r) a Ru complex that promoted the cycloaddition of butadiene with cyclic enones.
In the course of a synthesis of Sculponeatin N, Professor Zhai employed (Org. Lett. 2014, 16, 216. DOI: 10.1021/ol403208g) the silyl diene 5. After intramolecular cycloaddition, protonation of the resulting allyl silane with concomitant alkene migration led to the adduct 6.
On the way to Elansolid B1 (10), Andreas Kirschning of Leibnitz Universitšt Hannover oxidized (Org. Lett. 2014, 16, 568. DOI: 10.1021/ol403441c) the alcohol 8 to the enone, that cyclized to 9. Under the influence of MgBr2, the cyclization proceeded with remarkable diastereocontrol.
Dennis L. Wright of the University of Connecticut began (J. Am. Chem. Soc. 2014, 136, 4309. DOI: 10.1021/ja413106t) the synthesis of Frondosin A (13) by preparing the secondary ether 11 in high ee. Diels-Alder cycloaddition of tetrabromocyclopropene gave, after exposure of the initial adduct to water, the dibromoenone 12.
Kathlyn A. Parker of Stony Brook University explored (J. Org. Chem. 2014, 79, 919. DOI: 10.1021/jo402082y) the amine radical cation promoted intermolecular Diels-Alder cycloaddition of the bicyclooctadiene 14. Three readily-separated diastereomeric dimers were observed. The diol 15, the precursor to Kingianin H (16), was the major product.
Scott A. Synder of Scripps/Florida described (J. Org. Chem. 2014, 79, 88. DOI: 10.1021/jo4023167) the interesting oxidative coupling of 17 with 18. The product 19 was readily carried on to Rufescenolide (20).