Wide deviations (enhanced stabilities) from a linear relationship between the corresponding stability constants (given as Gibbs standard free energies of complexing) for gaseous Li+ and H+ adducts of monodentate diazines and pyridines are found for pyridazine, 2-fluoropyridine, and 1 Jbnaphthyridine. These results are interpreted to indicate the formation of a bidentate ring structure for the latter three compounds (and others of similar structures) with Li+ but not with H+. Ab initio calculations of structures and energies show the most stable forms of all of the H+ adducts to be monodentate, whereas threeand four-membered-ring bidentate structures are confirmed for the Li+ adducts of the three above-mentioned compounds. The charge distributions obtained support the previous conclusion that bonding to H+ is largely covalent, whereas that to Li+ is largely ionic.

As a result of a combined theoretical and experimental study, we describe a two-step protocol for the preparation of an optically pure, multifunctional, cyclopentanic core shared by a number of natural products. This process is based on a hitherto unreported Ti(III)-mediated diastereoselective cyclization in which the hydroxy-directed template effect played by the Ti(III) species was found to be crucial for the stereoselective outcome of the reaction. The viability of this concept was confirmed with the first protecting-group free synthesis of three enantiopure chokols, namely, chokols K, E, and B.

Highly efficient preparations of anticancer β-elemene and other bioactive elemanes were carried out using the natural product germacrone as a renewable starting material. The syntheses were achieved in only 3-5 steps with excellent overall yields (43-54%). An enantioselective approach to these molecules is also described.

Treatment of germacrone (1) with different electrophiles, and of its epoxy derivatives germacrone-4,5-epoxide (2), germacrone-1,10-epoxide (3) and isogermacrone-4,5-epoxide (4) with Brönsted/Lewis acids and Ti(III), gives rise to a great structural diversity. Thus, by using a maximum of two steps, the production of more than 40 compounds corresponding to 14 skeletons is described. Computational calculations rationalizing the structural divergence produced are also described. Finally, since some of the compounds generated are bioactive natural sesquiterpenes, the mechanisms of formation of these substances will provide new insights in their biosynthesis.

Regioselective halogenation of the terminal isopropylidene unit of different acyclic polyolefinic polyprenoids (farnesyl acetate, geranylgeranyl acetate, squalene, etc.) using NCS/catalytic polymer-supported selenenyl bromide is described; good to excellent yields are obtained (68-96%). The first applications of this protocol include the concise synthesis of bioactive terpenoids 1-3.