Dienone phenol rearragment

Rearragment

1. BY Dr . G.C.Wadhawa DEPARTMENT OF CHEMISTRY K. B. P . College,Vashi,Navimumbai Dienone -phenol rearrangement

2. The acid- and base-catalyzed or photochemically-induced migration of alkyl groups in cyclohexadienones is knownas the dienone-phenol rearrangement, and is widely used for the preparation of highly substituted phenols. In 1893,A . Andreocci described the rearrangement of santonin to desmotroposantonin upon acidic treatment, but it was only in 1930 that the starting material and the product of this rearrangement were carefully characterized.1,3 The term “ dienone -phenol rearrangement” was introduced by A.L. Wilds and C. Djerassi Dienone -phenol rearrangements require only moderately strong acidic media (e.g., H2SO4 in acetic acid, acetic anhydride, Lewis acidic clay , etc.), and they are considerably exothermic due to the formation of very stable aromatic compounds.

3. Mechanism: Most dienone -phenol rearrangements involve acid catalysis and the products appear to be the result of sigmatropic [1,3 ]-migrations of C-C bonds. The [1,3]-alkyl migrations are actually the result of two subsequent [1,2]-alkyl shifts as was demonstrated by 14C isotope labeling studies. Depending on the nature of the migrating groups, other rearrangements such as [1,2], [1,3], [1,4], [1,5], [3,3], [3,4], and [3,5] can also take place. When the migrating group is benzyl, the products predominantly arise from [1,5]-migrations, and the rate of these rearrangements is several orders of magnitude greater than for simple alkyl groups. If the migrating group is allyl , crotyl , or propargyl , then the main course of the rearrangement takes place via [3,3]-shifts rather than [1,2]-shifts. The scheme below depicts the mechanism of the acid-catalyzed rearrangement of p- cyclohexadienone to the corresponding 3,4-disubstituted phenol as well as the rearrangement of a bicyclic dienone via two subsequent [1,2]-shifts.

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