Epoxygeranyl acetate, cyclization

Barrero, Oltra and coworkers reported on the use of epoxygeranyl acetate in titanocene-mediated cyclizations and found that the termination of the reaction took place via a /i-hydride elimination after trapping of the radical by the second equivalent of Cp2TiCr [94,95]. This finding together with Takahashi s tandem cyclization [96] (see below) marks the first example of extremely interesting developments in epoxypolyene cyclizations via radicals that are discussed separately in the following section. 

Scheme 19 Dependence of the cyclization of epoxygeranyl acetate on the presence of additives...

Finally, Fuse et al. recently proposed the use of EtsB together with 2,6-lutidine hydrochloride or 2,4,6-collidine hydrochloride to improve the capacity of the system for regenerating titanocene(III) from Cp2Ti(Cl)H in the [Ti ]-catalyzed cyclization of 6,7-epoxygeranyl acetate [70]. 

The Cp2TiCl-mediated simple 6-endo cyclization of epoxygeranyl acetate was used by Nakai et al. in the preparation of synthons for building the A and C rings of paclitaxel [105], and a similar cychzation of epoxygeranylace-... 

The method was later extended to the synthesis of a number of meroter-penoids from epoxygeranyl carbonates or acetates in a two-step approach combining titanocene catalysis with Stifle reactions (carbonates) [108,109] or copper-catalyzed allylic substitutions (acetates) [110-112], The cyclizations... 

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