1,5-Sigmatropic shift of hydrogen

A similar analysis of the 1,5-sigmatropic shift of hydrogen leads to the opposite conclusion. The relevant frontier orbitals in this case are the hydrogen Is orbital and ij/j of the pentadienyl radical. The suprafacial mode is allowed whereas the antarafacial mode is forbidden. The suprafacial shift corresponds to a favorable six-membered ring. 

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. 

Another 1,7-dipolar cyclization leading to 1,2-benzodiazepines, but with the difference that a C-C bond is formed, is the ring closure of the nitrile imines 15 to give 17 by a [1,5]-sigmatropic shift of hydrogen in the intermediates 16. The nitrile imines are generated from the (2-vinylphenyl)hydrazonoyl chlorides 14."8 120... 

It resembles both a cycloaddition and a [1, 5] sigmatropic shift of hydrogen. It is a symmetry allowed process shown as follows ... 

Ene reaction (Hydro-allyl addition)-A reaction of an allylic compound with an olefin which resembles both a cycloaddition and a [1, 5]-sigmatropic shift of hydrogen. In this reaction an alkene having an allylic hydrogen atom reacts thermally with a dienophile (enophile) C = C, C = 0, N = N, N = 0, C = S... 

In recent years some applications of retro-ene reactions in the generation of thio-carbonyls have appeared. The retro-ene reaction is a symmetry-allowed process which resembles both [2 + 4] cycloreversion and a [1,5] sigmatropic shift of hydrogen. It usually requires high temperatures, therefore FVT has become very widespread in these reactions. 

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... 

Suprafacial [1, 5]-sigmatropic shifts of hydrogen and silyl substituents are allowed, generally facile, and proceed without inversion of stereochemistry on the migrating atom. A well-known example is the 1,5-shift of an allylsilyl group in silylcyclopentadienes, which proceeds with retention of configuration281. 

Other synthetic work includes a new synthesis of ellipticine (137), in which the critical stage involves a 1,5-sigmatropic shift of hydrogen in a readily synthesised intermediate (136), followed by thermal cyclization (Scheme 23) and the synthesis of 9,10,11-trimethoxyellipticine by the modified Cranwell-Saxton route. 

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