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Monday, December 14, 2009
Douglass F. Taber
University of Delaware

Organocatalytic C-C Ring Construction: (+)-Ricciocarpin A (List) and (-)-Aromadendranediol (MacMillan)

Yoshiji Takemoto of Kyoto University designed (Org. Lett. 2009, 11, 2425. ) an organocatalyst for the enantioselective conjugate addition of alkene boronic acids to γ-hydroxy enones, leading to 1 in high ee. Attempted Mitsunobu coupling led to the cyclopropane 2, while bromoetherification followed by intramolecular alkylation delivered the cyclopropane 3.

Jeffrey W. Bode of the University of Pennsylvania demonstrated (Org. Lett. 2009, 11, 677. ) a remarkable dichotomy in the reactivity of N-heterocyclic carbenes. A triazolium precatalyst combined 4 and 5 to give 6, whereas an imidazolium precatalyst combined 4 and 5 to give 7.

Xinmiao Liang of the Dalian Institute of Chemical Physics and Jinxing Ye of the East China University of Science and Technology devised (Org. Lett. 2009, 11, 753. ) a Cinchona-derived catalyst that converted the prochiral cyclohexenone 8 into the diester 10 in high ee. Rich G. Carter of Oregon State University found (J. Org. Chem. 2009, 74, 2246. ) a simple sulfonamide-based proline catalyst that effected the Mannich condensation of the prochiral ketone with ethyl glyoxalate 12 and the amine 13, leading to the amine 14.

In the first pot of a concise, three-pot synthesis of (-)-oseltamivir, Yujiro Hayashi of the Tokyo University of Science combined (Angew. Chem. Int. Ed. 2009, 48, 1304. ) 15 and 16 in the presence of a catalytic amount of diphenyl prolinol TMS ether to give an intermediate nitro aldehyde. Addition of the phosphonate 17 led to a cyclohexenecarboxylate, that on the addition of the thiophenol 18 equilibrated to the ester 19. Ying-Chun Chen of Sichuan University used (Org. Lett. 2009, 11, 2848. ) a related diaryl prolinol TMS ether to direct the condensation of the readily-prepared phosphorane 20 with the unsaturated aldehyde 21 to give the cyclohexenone 22. Armando Córdova of Stockholm University also used (Tetrahedron Lett. 2009, 50, 3458. ) diphenyl prolinol TMS ether to mediate the addition of 24 to 23. The subsequent intramolecular aldol condensation proceeded with high diastereocontrol, leading to 25.

Benjamin List of the Max-Planck Institut, Mülheim employed (Nat. Chem. 2009, 1, 225. ) a MacMillan catalyst for the reductive cyclization of 26. Subsequent epimerization and Tishchenko hydride transfer then proceeded with high diastereoselectivity, leading directly to the liverwort-derived (+)-Ricciocarpin A (28).

David W. C. MacMillan of Princeton University devised (Angew. Chem. Int. Ed. 2009, 48, 4349. ) a remarkable one-pot three-component assembly of the lactone 32. Cross-metathesis of 29 with 30 gave a keto aldehyde. Direct addition of the furan 31 and an chiral imidazolidinone catalyst effected conjugate addition to the unsaturated aldehyde. A third catalyst, (S)-proline, then mediated intramolecular aldol condensation. The crystalline lactone 32 was readily carried on to (-)-Aromadendranediol (33).

D. F. Taber, Org. Chem. Highlights 2009, December 14.