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Cycloaddition Reactions of Allenes: Examples in Natural Product Synthesis Miriam Inbar Leighton Group Literature Seminar May 4 2007 Summary I. II. [2+2] Cycloadditions of Enones and Allenes [4+2] Cycloadditions with Allenes as Dienophiles III. [4+2] Cycloadditions with Allenes as Dienes IV. Rh-catalyzed [5+2] Cycloadditions V. Allenic Nazarov Cyclizations [2+2] Photocycloadditions of Allenes and Enones O hv O H diradical intermediate O 55% • Diradical intermediate forms from a triplet enone • Triplet diradical cyclizes to product after spin inversion to singlet state • Configuration of the diradical determines stereochemistry of the product O Me H H H O H 70% Me H O H Corey et al. J. Am. Chem. Soc., 1964, 86, 5570-5583 Synthesis of periplanone B O H hv [2+2] H 72%, 2:1 dr O KH, 18-crown-6 [3,3] H 75% 1) 175oC, toluene 2) hv 15:1 trans:cis O O MgBr HO 63% O O O periplanone B Schreiber, S.L. et al. J. Am. Chem. Soc., 1984, 106, 4038 Synthesis of (±)-Steviol CHO hv MeO2C MeO2C 45%, 14:1 dr OMs CHO 1) NaBH4, EtOH 2) MsCl, pyr MeO2C H H Me O [2+2] addition occurs preferentially from less-hindered top face Ziegler, Frederick and Kloek, James A. Tetrahedron, 1977, 33, 373-380 Synthesis of (±)-Steviol OH OMs aq. acetone 2,6-lutidine MeO2C solvolytic ring expansion MeO2C (±)-Steviol OMs OH MeO2C MeO2C A substantial amount of a second rearrangement product was also observed. Ziegler, Frederick and Kloek, James A. Tetrahedron, 1977, 33, 373-380 [4+2] Cycloadditions of Allenes H H 2 1 H EWG LUMO • Allenes usually participate in Diels-Alder reactions as electron deficient dienophiles • LUMO energy of the allene is lowered by EWG • Largest LUMO coefficient rests on C2 • Diels-Alder will take place at the internal C=C bond Allenic Esters

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