Highly functionalized BC ring-systems of Taxol ® having the required chemistry for the C1, C2 and C8 centers have been synthesized using a ring-closing metathesis (RCM) reaction as the key step. Silylene 26 and acetonide 27 were obtained in excellent yields with Schrock's and our recently reported catalyst. In the case of carbonate 23, a trans cyclooctene was formed when using Grubbs' catalyst, indicating that RCM does not always proceed to completion of thermodynamic equilibrium.

13 C NMR longitudinal relaxation times (T 1 ) have been determined for both annonacin (1) and squamocin (2) in the absence and presence of Ca 2+ ions. These data are used to assess structural changes that accompany complexations. Even though acetogenins of the Annonaceae present no ionophoric effects in biological studies with living cells, we assume that they have a role in the bioavailability of the cations in the cell membranes, due to both their lipophilic and polar natures. These results also show differences in the stoichiometry of the complexes of a mono-tetrahydrofuranic acetogenin and a bis-tetrahydrofuranic acetogenin with Ca 2+ ions.

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