rings6 - 6-MEMBERED RING FORMATION 150Six Membered Rings1....

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Unformatted text preview: 6-MEMBERED RING FORMATION 150Six Membered Rings1. Diels-Alder Reaction2. o-Quinodimethanes3. Intramolecular ene reaction4. Cation olefin cyclizations5. Robinson annulationDiels-Alder ReactionACIEE1984, 23, 876; ACIEE1977, 16, 10; Organic Reactions1984, 32, 1W. Carruthers Cycloadditions Reactions in Organic Synthesis(Pergamon Press, Oxford) 1990- reaction of a 1,3-diene with an olefin to give a cyclohexene.- thermal symmetry allowed pericyclic reaction- diene must reaction is an s-cis conformation- highly stereocontrolled process- geometry of starting material is preserved in theproduct- possible control of 4 contiguous stereocenters in one stepAABBYYXXBBXXAAYY+BBYYAAXX+- Alder Endo Rule: In order to maximize secondary orbital interactions, the endo TS isfavored in the D-A rxn.Tetrahedron1983, 39, 2095XXYBBAAYBBXXAAYYOOOHHOOOOOOOOOHHexoendominormajorOrientation RulesXY+YX+XYmajorminor6-MEMBERED RING FORMATION 151Y+Y+YmajorminorXXX- when both the diene and dienophile are "unactivated" the D-A rxn is sluggish- D-A rxns with electron rich dienes and electron defficient dienophiles work the best.Some electron deficient dienophiles are quinones, maleic ahydride, nitroalkenes, ,-unsaturated ketones, esters and nitriles.- D-A rxns with electron deficient dienes and electron rich dienophiles also work well.These are refered to as reverse demand D-A rxns.- D-A rxns are sensitive to steric effects of the dienephiles, particularly at the 1- and 2-postions. Steric bulk at the 1-position may prevent approach of the dienophile while stericbulk at the 2-position may prevent the diene from adopting the s-cis conformation.- The D-A rxn is promoted by Lewis acids (TiCl4, BF3AlCl3, AlEt2Cl, SnCl4,...)- The D-A rxn is promoted by high pressure (1 kbar ~ 14200 psi) Synthesis1985, 1.OMeOMeO+5 KbarOOMeMeOJACS1986,108, 3040OMe+HNHBOCO20 kbars, 50CEu(fod)3OOMeNHBOCH+OOMeNHBOCHSynthesis1986, 928- The D-A rxn is usually insenstive to solvent effects, except for water. ACR1991, 24, 159O+isooctaneHO+Oendoexo4 : 1krel=1H2Okrel= 70020 : 1JACS1980, 102, 7816TL1983, 24, 1901TL1984, 25, 1239CO2-Na+CHOH2OCO2HOHCH+CO2HOHCH(70%)(15%)JOC1984, 49, 5257JOC1985, 50, 1309Tetrahedron1986, 426-MEMBERED RING FORMATION 151Y+Y+YmajorminorXXX- when both the diene and dienophile are "unactivated" the D-A rxn is sluggish- D-A rxns with electron rich dienes and electron defficient dienophiles work the best.Some electron deficient dienophiles are quinones, maleic ahydride, nitroalkenes, ,-unsaturated ketones, esters and nitriles.- D-A rxns with electron deficient dienes and electron rich dienophiles also work well.These are refered to as reverse demand D-A rxns.- D-A rxns are sensitive to steric effects of the dienephiles, particularly at the 1- and 2-postions. Steric bulk at the 1-position may prevent approach of the dienophile while stericbulk at the 2-position may prevent the diene from adopting the s-cis conformation....
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This note was uploaded on 03/11/2012 for the course CHEM 233 taught by Professor Staff during the Fall '10 term at Vanderbilt.

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rings6 - 6-MEMBERED RING FORMATION 150Six Membered Rings1....

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