Pinacol Cyclization Research Articles

Bu(3)SnH-Mediated Pinacol Coupling of 1,5- and 1,6-Dicarbonyl Compounds: Synthetic and Mechanistic Studies.

A new method is described for the intramolecular pinacol coupling of 1,5- and 1,6-dicarbonyl compounds, employing Bu(3)SnH as the stoichiometric reductant. The key steps in this pinacol cyclization are the addition of a tin ketyl to a carbonyl group and a subsequent intramolecular S(H)2 reaction. The isolation of 1,3-dioxa-2-stannolanes, along with other product and labeling studies, provides strong support for the proposed homolytic substitution step, which distinguishes the pinacol cyclization from other reductive cyclizations of tin ketyls, all of which proceed through abstraction of hydrogen from Bu(3)SnH in the final step. An interesting consequence of the S(H)2 pathway is very high cis selectivity in the cyclization of 1,5-dicarbonyl compounds. Mechanistic studies furnish evidence that the steps that precede homolytic substitution, including C-C bond formation, are reversible under the reaction conditions.

Practical synthesis of a differentially protected myo-inositol

An enantiospecific synthesis of 3,4,5-tri- O-benzyl-6- O-triisopropylsilyl-D- myo-inositol from D-xylose is reported. The synthesis features a diastereofacially selective SmI 2-promoted pinacol cyclization.

Asymmetric Total Synthesis of Taxol.

Asymmetric total syntheses of Taxol and of 8-demethyltaxoids 24-27 from the 8-membered ring compounds 29 and 12 respectively were completed via successive formation of the BC ring system by intramolecular aldol reaction, then the ABC ring system utilizing an intramolecular pinacol cyclization. The conversion of the tricyclic compound 43 to 7-TES baccatin III (49) was carried out by way of a newly devised method of constructing the oxetane ring. The dehydration condensation between a derivative of N- benzoylphenylisoserine and 49, followed by deprotection afforded the antitumor agent Taxol.

Synthesis of ABCtaxoid ring systems via a convergent strategy

Intermediates 4 and 5have been coupled via a Shapiro reaction to afford compound 6 which has been further elaborated to the ABCtaxcid systems 11and 12via a McMurry pinacol cyclization to 11followed by oxidation to12.

Synthesis of 4, 8-dihydroxy-3-methyl-5,6-benzo-2-oxabicyclo[2.2.2]oct-5-ene

The title compound having the same relative configurations as granaticin( 1 ) and U-58,431( 2 ) was prepared either by dehydrogenative cyclization of the precursor 9a or by pinacol cyclization of the dicarbonyl compound 15 .