Diastereoselective synthesis of a taxane precursor

Calculations on the Ruthenium-Catalyzed Diene and Dienyne Ring-Closing Metathesis Reactions in the Synthesis of Taxol Derivatives

Density-functional and semiempirical calculations (M06, M06L, and PM6) on intermediates in the ring-closing metathesis (RCM) reactions in the synthesis of Taxol derivatives give results in excellent agreement with the results of previous experimental work. The results suggest that the degree of steric overloading plays a decisive role in determining the outcome (ene-ene or ene-yne-ene metathesis). Due to the rigidity of the Taxol skeleton being formed in the ene-yne-ene cascade reaction, the transition states in its final ene-ene metathesis reaction stage are particularly sensitive to steric effects. Thus, the reaction is predicted to be preferred for one diastereomer of the precursor in which the diol functionality is protected with a compact cyclic carbonate moiety, whereas the use of a bulkier benzoate-protecting group results in activation barriers for Taxol formation that are prohibitive. The reason why one diastereomer of the carbonate-protected precursor undergoes formation of a tricycle via an ene-yne-ene RCM cascade, whereas the other diastereomer undergoes cyclooctene formation via an ene-ene RCM, likely lies in the orientation of the pseudoaxial methyl group on the cyclohexene ring, which in the latter case would unfavorably point toward the reactive center of the Ru-complex, leading to Taxol formation.

Study of Cascade Ring-Closing Metathesis Reactions en Route to an Advanced Intermediate of Taxol

A highly functionalized intermediate in the synthesis of Taxol has been synthesized, which features the tricyclic core and the required oxygen substituents at C1, C2, C7, C10, and C13. The key step, a ring-closing dienyne metathesis (RCDEYM) reaction, has been thoroughly optimized to favor the tricyclic product over the undesired bicyclic product resulting from diene metathesis.

Cascade Metathesis Reactions for the Synthesis of Taxane and Isotaxane Derivatives

Tricyclic isotaxane and taxane derivatives have been synthesized by a very efficient cascade ring‐closing dienyne metathesis (RCDEYM) reaction, which formed the A and B rings in one operation. When the alkyne is present at C13 (with no neighboring gem‐dimethyl group), the RCEDYM reaction leads to 14,15‐isotaxanes 16 a,b and 18 b with the gem‐dimethyl group on the A ring. If the alkyne is at the C11 position (and thus flanked by a gem‐dimethyl group), RCEDYM reaction only proceeds in the presence of a trisubstituted olefin at C13, which disfavors the competing diene ring‐closing metathesis reaction, to give the tricyclic core of Taxol 44.

Enantioselective synthesis of a taxol C ring

An enantioselective synthesis of a C ring of taxol has been accomplished. The key step is an oxidative cleavage of a derivative of the Wieland–Miescher ketone. A first attempt of a Shapiro reaction modelling the coupling of the C ring with the A fragment of taxol was also successful.

Ring-closing metathesis in the synthesis of BC ring-systems of taxol

BC ring-systems of taxol with different or no protecting group for the C1,C2-diol moiety have been efficiently synthesized. The eight-membered B ring is formed by a ring-closing metathesis reaction (RCM) between the C10 and C11 carbon atoms. The influence of the 1,2-diol protecting group on the RCM reaction has been studied in detail.