Small Ring Compounds in Organic Synthesis II by A. de Meijere, P. Binger, H.M. Büch, A. Krief PDF

By A. de Meijere, P. Binger, H.M. Büch, A. Krief

ISBN-10: 3540166629

ISBN-13: 9783540166627

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In the latter case the dimerisation of the diene was observed, especially with nonreactive dienophiles. While these reactions show the stereospecificity usually observed in the Diels-Alder reaction (see Scheme 79e, f), their regioselectivity is significantly different from the one usually reported for 1-substituted dienes which are known to give predominantly the ortho-type adduct 215) In the case of allylidene cyclopropanes a reversed tendency was found and the recta-type adduct was mainly 55 Alain Krief formed 102) (Scheme 81).

While these reactions show the stereospecificity usually observed in the Diels-Alder reaction (see Scheme 79e, f), their regioselectivity is significantly different from the one usually reported for 1-substituted dienes which are known to give predominantly the ortho-type adduct 215) In the case of allylidene cyclopropanes a reversed tendency was found and the recta-type adduct was mainly 55 Alain Krief formed 102) (Scheme 81). The regioselcctivity is high in the case of ¢,13-unsaturated esters, ketones, and aldehydes and somewhat less pronounced with nitroethylcne.

In some cases the complete isomerisation requires treatment of the mixture formed in the above reaction with potassium tert-butordde in THF. The reaction seems to occur via a selenonium ylide rather than via a 13-elimination reaction promoted by the direct attack of the tert-butoxide anion on the 13-hydrogen of the selenonium salt, since it has been shown in a separate experiment 23) that the reaction does not occur when a diphenylselenonium salt (unable to produce the expected intermediate) is used instead of the phenyl-methyl or dimethyl selenonium analogs.

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Small Ring Compounds in Organic Synthesis II by A. de Meijere, P. Binger, H.M. Büch, A. Krief


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