Many biologically active substances are composed of sesquiterpene units linked to aromatic structures, especially substituted phenols. Here, we describe an efficient synthetic approach to this class of natural product from commercially available substances in a short sequence. The key transformations involve allylic substitution reactions using a palladium or copper catalyst and titanocene‐catalyzed epoxypolyene cyclization reactions via radicals. The polycyclic core structures are accessed with high chemo‐ and stereocontrol. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Dedicated to Professor Manfred T. Reetz on the occasion of his 60th birthdayOver the last years radicals have been used increasingly in multistep syntheses due to the mild reaction conditions, high functional group tolerance, and broad accessibility of interesting structures, often obtained in sequential transformations.[1] In this context we wish to report our first results on a novel radical tandem reaction [1b] featuring an unprecedented formal homolytic substitution reaction at a Ti-O bond for the formation of tetrahydrofurans. The planned sequence is shown in Scheme 1. The initial step is based on the titanocene-mediated opening of 1 described by Nugent and RajanBabu, [2] which we have developed into a catalytic reaction, [3] to give the radicals cis-and trans-2. The second and conceptually novel step of our tandem reaction constitutes the attack of the tertiary radical 2 on the Ti-O bond. Mechanistically, this can be viewed as a homolytic substitution of the [Cp 2 TiCl] radical.[4] Our work therefore introduces metal-oxygen bonds as a very useful class of radical traps. As a consequence of this homolytic substitution the redox catalyst [Cp 2 TiCl] is regenerated. To the best of our knowledge, this concept of a catalytic redox isomerization is unknown in the literature. Because the second cyclization is counterintuitive, a supposedly strong Ti-O bond is cleaved and a C-O bond formed, we analyzed the transformation of model system 4 by density functional theory (DFT) calculations as shown in Scheme 2.[5] As expected [1c, 6] both the ring opening of 4 (À8.3 kcal mol À1 ) and the 5-exo cyclization of 5 (À12.2 kcalmol À1 for trans-6; À10.6 kcal mol À1 for cis-6) are exothermic. Despite this thermodynamic preference for trans-6 the formation of cis-6 should be kinetically favored according to the Beckwith-Houk rules [6] and our own calculations on related systems. A detailed analysis of the transition-state structures of titanocene-mediated cyclizations will be published in due course.The key steps of our tandem reaction, formation of cis-7 and trans-7 complexed to [Cp 2 TiCl], were also found to be exothermic by À12.2 and À4.6 kcal mol À1 . Thus, breaking of the TiÀO bond is predicted to be possible! The low energy of formation of trans-7 compared to that of cis-7 can be attributed to the strain in the resulting trans-fused bicyclo[3.3.0] system. The product complexes [7·TiCp 2 Cl] shown in Scheme 2 are slightly more stable (À31.1 and À25.1 kcalmol À1 ) than the separated molecules. Since entropy effects favor the dissociation of the product complex, DG will most likely be negative and the catalyst will be regenerated as desired for efficient catalysis.The relative energies of the transition-state structure, 6, and 7 were also confirmed by MP2 calculations. The transition-state structure for the formation of [cis-7·Cp 2 TiCl] is shown in Figure 1. This final ring closure has a low barrier (+ 11.4 kcal mol À1 ) and should therefore be viable even at low temperatures. The transition state exhibits similar TiÀO...

The generation and addition reactions of metal bound radicals derived from normal and meso epoxides by electron transfer from titanocene(III) reagents is described. The control of enantioselectivity and diastereoselectivity of these transformations is investigated by variation of the ligands of the metal complex. The reaction can lead to unprecedented and highly selective reactions, in which synthetically useful alcohols may be prepared. The synthesis presented also circumvents the use of toxic metals. Another advantage is that there is no loss of two functional groups as usually observed in reductive radical chain reactions.

The chromone "altechromone A" was synthesized as a substructure in the course of natural product synthesis. Its architecture was verified by X-ray analysis, but spectroscopic data showed a strong deviation from the reported data. By comparison with the synthesized isomers the structure of altechromone A was revised.