Epoxyolefin cyclizations

Smith and Dieter have demonstrated that /8y-unsaturated diazo-ketones, previously employed in the synthesis of cyclopentenones, may possess considerable potential as polyene cyclization initiators (Scheme 26) and Sutherland et al. have reported an efficient synthetic route to the aldehyde (117) capable of Wittig-Schlosser condensation affording substrates for epoxyolefin cyclization. ... 

Epoxyolefin Cyclization. Among many metal triflates, gallium triflate is shown to be a highly effective catalyst for epoxyolefin cyclization. Geraniolene oxide undergoes isomerization and cyclization to yield a mixture of cyclized products as well as rearranged products (eq 15). Product composition is mainly influenced by the nature of the solvent and the substrate concentration. ... 

SCHEME 25.12. Concept of the Cp2TiCl-mediated epoxyolefin cyclization. 

SCHEME 25.74. Synthesis of (-)-siccanin by Ti(III)-catalyzed epoxyolefin cyclization. 

The cyclization of epoxyolefins has been the subject of intense study ever since the discovery that these reactions are involved in the biosynthesis of many terpenes, including cholesterol. An early example is the cyclization of geraniolene oxide 25 using BF3Et20 to give a mixture of acyclic and cyclic products (Scheme 6) [57,58]. 

Pettersson L, Frejd T (2001) Cyclizations versus rearrangements in the reactions of some epoxyolefins with Lewis acids. J Chem Soc Perkin Trans 1 789... 

Scheme 9 Uses of a transition-metal centered radical for the cyclization of an epoxyolefin.32...

W. A. Nugent and T. V. RajanBabu, Transition metal-centered radicals in organic synthesis— Ti(lH)-induced cyclization of epoxyolefins, J. Am. Chem. Soc. 770 8561 (1988). 

In 1988, Nugent and RajanBabu introduced the Cp2TiCl-induced radical cyclization of epoxyolefins and subsequently GansSuer et al. developed the catalytic version of the reaction, two major advances in the application of... 

The rhodium(II) radical [Rh(dmgH)2PPh3], formed by laser photolysis of the corresponding dimer, is oxidized by a range of cobalt(II) complexes which have coordinated halides. " Electron transfer is believed to occur through a ligand-mediated pathway and the reaction rate is dependent upon the choice of halide. In an examination of the Ti(III)-induced cyclization of epoxyolefins, a mechanism has been invoked which involves a metal-centered radical. ... 

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