Transition metal-catalyzed oxidations of bishomoallylic alcohols

In recent years, new transition metal-catalyzed reactions for chemo- and stereoselective oxidations of bishomoallylic alcohols have been developed. The role of transition metals in this catalysis is connected with (i) activation of a primary oxidant (e.g. molecular oxygen, hydrogen peroxide, or tert-butyl hydroperoxide-reactivity), (ii) direction of the alkenol oxidation into a specific reaction channel (chemoselectivity), and (iii) control of the facial selectivity of pi-bond oxygenation (stereoselectivity). The most important products which originate from these reactions are functionalized tetrahydrofurans which serve as valuable building blocks for the synthesis of e.g. polyether antibiotics or acetogenin-derived natural products. The selective formation of tetrahydropyrans in this type of chemistry is to date restricted to a structurally narrow range of substrates and the use of strongly Lewis-acidic oxidation catalysts. Likewise, only a few methods have been described so far which allow transition metal-catalyzed conversion of a bishomoallylic alcohol into an epoxyalcohol. In many other instances, the Lewis-acidity of the transition metal catalyst, which is required for peroxide or molecular oxygen activation, seems to suffice for converting an epoxyalcohol into secondary products such as functionalized tetrahydrofurans. Chemoselective oxidation of alkenols into gamma,delta-unsaturated carbonyl compounds is until today still dominated by oxochromium(VI)-mediated methods. Only a few transition metal-catalyzed alternatives have been developed for this purpose so far. In view of the diversity of this chemistry, it is the aim of this review to organize the principles of transition metal-catalyzed oxidations of bishomoallylic alcohols and to present its state of the art in modern organic synthesis. An emphasis has been laid on the diastereoselective formation of functionalized cyclic ethers. (C) 2002 Elsevier Science B.V. All rights reserved.