Different enynols and propargyl derivatives were sumitted to radical hydrostannylation (Bu 3 SnH/ AIBN), Pd(0)-catalyzed hydrostannylation [Bu 3 SnH/Pd(0)], and stannylcupration [Bu 3 Sn(R)CuCNLi 2 ] conditions. Except for the radical stannylation reaction, high regio-and stereoselective formation of vinyl-and dienylstannanes are obtained. Results are tentatively explained in terms of steric Interactions between the alkyne or enyne substituents and the palladium or cuprate moieties in the different reaction intermediates.

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Hemicalide is a novel marine metabolite polyketide distinguished by a unique mechanism of action. Because of insufficient quantities of purified material, this natural product has evaded complete stereochemical assignments. Recently, we have determined the relative stereochemistry of the C8-C13 hexad by synthesizing the C1-C13 fragment. Presently, we report the assignment of the C17-C25 δ-lactone fragment. NMR analysis of authentic hemicalide along with a computational conformation study allowed us to reduce the number of putative relative isomers from 16 to 4. Concise syntheses of the four candidate diastereomers were achieved using a common strategy based on a Dias aldehyde allylation reaction, an intramolecular Horner-Wadsworth-Emmons olefination, and a dihydroxylation reaction. Finally, thorough NMR comparisons enabled us to deduce the relative stereochemistry of the C1-C17 fragment with high certainty.

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The challenging determination of the relative stereochemistry of a complex natural polyketide portion was achieved. After careful NMR analysis, a concise synthesis of a set of possible relative diastereomers (only 6 diastereomers out of the 32 initially envisioned) has been carried out using a common strategy based on enantioselective aldol reactions. With a high predictability, final NMR comparison established the relative stereochemistry of the C1-C17 fragment of this natural product.

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.

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