Sigmatropic carbon shift

These are suprafacial sigmatropic shifts of order [1,5] and should occur with retention of configuration at the migrating carbon. This stereochemical course has been established for the 1,5-alkyl shift that converts 16 to 17. The product which is isolated, 18, results from a subsequent 1,5-hydrogen shift, but this does not alter the stereochemistry at the migrating... 

The dienol is unstable, and two separate processes have been identified for ketonization. These are a 1,5-sigmatropic shift of hydrogen leading back to the enone and a base-catalyzed proton transfer which leads to the / ,y-enone. The deconjugated enone is formed because of the kinetic preference for reprotonation of the dienolate at the a carbon. Photochemical deconjugation is a synthetically useful way of effecting isomerization of a,) -unsaturated ketones and esters to the j ,y-isomers. 

The rearrangement step itself is a ground-state thermal process and may be classified as a [1,4] sigmatropic shift of carbon across the face of a 2-oxybutenyl cation. The... 

Fivefold degenerate reversible [3,3]-sigmatropic shifts were first reported in 1988116,117 in the CPD-amidine system 257, where AG g = 117 to 120 kJmol-1 (equation 88) (for aza-Cope rearrangements see Section IV.E.2). In addition, a slow accumulation of a colored by-product was observed at elevated temperatures. This was identified as a product of a novel intramolecular carbon to nitrogen 1,4-shift of the methoxycarbonyl... 

A very remote secondary H/D isotope effect has been measured for the 2 + 2-cycloaddition of TCNE to 2,7-dimethylocta-2,fran -4,6-triene. The reaction of nitric oxide with iV-benzylidene-4-methoxyaniline to produce 4-methoxybenzenediazonium nitrate and benzaldehyde is thought to proceed via a 2 + 2-cycloaddition between nitric oxide and the imine double bond. A novel mechanism for the stepwise dimerization of the parent silaethylene to 1,3-disilacyclobutane involves a low-barrier [1,2]-sigmatropic shift. Density functional, correlated ab initio calculations, and frontier MO analysis support a concerted 2 + 2-pathway for the addition of SO3 to alkenes. " The enone cycloaddition reactions of dienones and quinones have been reviewed. The 2 + 2-photocycloadditions of homochiral 2(5H)-furanones to vinylene carbonate are highly diastereoisomeric. ... 

Outline the structural and stereochemical consequences of the thermally allowed [1,3]- and [l,5]-sigmatropic shifts of the bridging carbon in each of the following structures ... 

An extensive review of the use of density functional theory calculations on electrocyclic reactions, cycloadditions, and sigmatropic shifts has been published.3 A cycloaddition model for the transformation of graphite into the carbon cages of fullerenes is described.4... 

Diphenylpyrrolidine (77) catalyses the enantioselective cy-chlorination of aldehydes.299 Mechanistic and computational studies suggest that - in contrast to pre- viously proposed mechanisms involving direct formation of the carbon-electrophile bond - iV-chlorination occurs first, followed by a 1,3-sigmatropic shift of chlorine to the enamine carbon. The product iminium ion is then hydrolysed in the ratedetermining step. 

A [1,5]-sigmatropic shift of hydrogen from an A-methyl group to the carbonyl carbon atom in protonated 3-(A,A-dialkylhydrazono)-l,l,l-trifluoroalkan-2-ones has been found to be a key step for the acid-catalysed cyclization of these ketones to 6-trifluoromethyl-3,6-dihydro-2//-[l,3,4]oxadiazines.58 Gas-phase kinetic studies on interconversions of monodeuterocyclohexadienes have given activation parameters in reasonable agreement with previous experimental and theoretical data.59... 

Quinolylimino-l,2,3-dithiazoles 62 undergo by thermolysis an unusual rearrangement to give imidazoquinoli-nethiones 63 (Equation 10) delivery of a sulfur atom to the quinoline 2-position appears to be intramolecular and possibly involves a [1,3] sigmatropic shift of a carbon-sulfur bond <2000J(P1)555>. 

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