hide
Free keywords:
POLYPHENOL OXIDASE
Chemistry; cycloadditions; enzyme catalysis; oxidases; quinones; tandem reactions;
Abstract:
The first successful combination of enzymatic with nonenzymatic transformations in a domino reaction sequence is reported. In the presence of oxygen the enzyme tyrosinase, from mushrooms, hydroxylates phenols to catechols. These intermediates are oxidized in situ further to ortho quinones, which subsequently undergo a Diels-Alder reaction with inverse electron demand in the presence of different dienophiles The transformation is best carried out in chloroform with purified enzyme immobilized on glass beads. Under these conditions the desired cycloadducts are formed in moderate to high yields which depend on the steric demands and the electronic nature of the substituents of the electron-poor diene and the electron-rich dienophile. By means of this one-pot three-step reaction cascade highly functionalized bicyclo[2.2.2]octenes are available in an efficient manner from simple phenols and olefins. The enzyme-initiated domino reaction can conveniently be coupled to a Sc(OTf)(3)-catalyzed allylation of the 1,2-diketone unit of the cycloadducts in which Sn(All)(4) is employed as allyl donor. This combination of a three-step tandem reaction with a fourth consecutive transformation yields bicyclic alpha-hydroxy ketones with an average yield per step of up to 94%.
