Photochemistry of the amide system: furancarboxanilide

On the Mechanism of an Asymmetric α,β-Unsaturated Carboxylic Acid Hydrogenation: Application to the Synthesis of a PGD2 Receptor Antagonist

Ruthenium complexes employing axially chiral ligands were found to be effective asymmetric hydrogenation catalysts for the reduction of alpha,beta-unsaturated ene acid 1-E to give 2, a prostaglandin D2 (PGD2) receptor antagonist. With [(S-BINAP)Ru(p-cymene)Cl2]2 (3, S-BINAP = (S)-(+)-2,2'-bis(diphenylphospino)-1,1'-binapthyl), it was discovered that low hydrogen pressures (<30 psi) were essential to achieve high enantioselectivities (92% ee). A detailed mechanistic study was undertaken to elucidate this pressure dependence. It was determined that compound 1-E is in a ruthenium-catalyzed equilibrium with endocylic isomer 1-Endo and in photochemical equilibrium with Z isomer 1-Z. Each isomer could be hydrogenated to give 2, albeit with different rates and enantioselectivities. Hydrogenation of 1-Endo with 3 was found to give 2 in high enantiomeric excess, regardless of pressure and at a rate substantially faster than that of hydrogenation of 1-E and 1-Z. In contrast, isomers 1-E and 1-Z exhibited pressure-dependent enantioselectivities, with higher enantiomeric excesses obtained at lower pressures. A rationale for this pressure dependence is described. Deuterium labeling studies with 1-Endo and tiglic acid were used to elucidate the mechanism of hydride insertion and product release from ruthenium. Under neutral conditions, protonolysis was the major pathway for metal-carbon cleavage, while under basic conditions, hydrogenolysis of the metal-carbon bond was predominant.

An aldol-based approach to the asymmetric synthesis of L-callipeltose, the deoxyamino sugar of L-callipeltoside A

[reaction: see text] The L-callipeltose subunit of L-callipeltoside A has been synthesized in 10 steps and 13% overall yield from D-threonine. The key steps are a highly diastereoselective Felkin anti aldol addition to a methyl ketone and a selective methylation of a secondary alcohol in the presence of a secondary carbamate.

Transannular Diels-Alder studies on the asymmetric total synthesis of chatancin: the pyranophane approach

[reaction: see text] A pathway is proposed for the biosynthesis of (+)-chatancin and (+)-sarcophytin linking these tetracycles to cembranoids by a pyranophane transannular Diels-Alder reaction. Preliminary synthetic results in this direction to reach macrocyclic dienedione 28 from farnesol are reported. Major synthetic steps include a Prins reaction, two enantioselective hydrogenations, and a macrocyclization via a beta-ketosulfoxide Michael-addition on an enone.

Enantioselective Photocyclization of N-Alkylfuran-2-carboxyanilides to trans-Dihydrofuran Derivatives in Inclusion Crystals with Optically Active Host Compounds Derived from Tartaric Acid

The steric course of the photocyclization reaction of the title achiral compounds (1) to optically active trans-dihydrofuran derivatives (2) was controlled by carrying out the reaction in inclusion crystals (4) with optically active hosts (3) derived from tartaric acid. The mechanism of the enantioselective reaction of 1 in 4 was studied by X-ray structural analysis of 4. In some cases, the steric course of the photoreaction was different depending on whether 4 was prepared by recrystallization or by mixing of 1 and 3.