Miriam Inbar Leighton Group Literature Seminar whitethorn 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 overlap after spin inversion to singlet state cast of the diradical determines stereochemistry of the proceeds
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 magnification 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 record in Diels-Alder reactions as electron deficient dienophiles LUMO energy of the allene is displace by EWG Largest LUMO coefficient rests on C2 Diels-Alder will take place at the internal C=C bond
Allenic Esters Generally Show Endo Selectivity
R
CO2R1 R CO2R1 endo R H H Me Me R1 Et Et Me Me Conditions Benzene,...If you want to get a full essay, order it on our website: Ordercustompaper.com
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