Pd-catalyzed enantioselective aerobic oxidation of secondary alcohols: Applications to the total synthesis of alkaloids

Enantioselective syntheses of the alkaloids (-)-aurantioclavine, (+)-amurensinine, (-)-lobeline, and (-)- and (+)-sedamine are described. The syntheses demonstrate the effectiveness of the Pd-catalyzed asymmetric oxidation of secondary alcohols in diverse contexts and the ability of this methodology to set the absolute configuration of multiple stereocenters in a single operation. The utility of an aryne C-C insertion reaction in accessing complex polycyclic frameworks is also described

Enantioselective Synthesis of Caprolactam and Enone Precursors to the Heterocyclic DEFG Ring System of Zoanthenol

The enantioselective synthesis of both caprolactam and enone synthons for the DEFG ring system of zoanthenol are described. The evolution of this approach proceeds first through a synthesis using the chiral pool as a starting point. Challenges in protecting-group strategy led to the modification of this approach beginning with (±)-glycidol. Ultimately, an efficient approach was developed by employing an asymmetric hetero-Diels–Alder reaction. The caprolactam building block can be converted by an interesting selective Grignard addition into the corresponding enone synthon. Addition of a model alkyne provides support for the late-stage addition of a hindered alkyne to the caprolactam building block

Palladium-Catalyzed Enantioselective Oxidation of Alcohols: A Dramatic Rate Acceleration by Cs_2CO_3/t-BuOH

The addition of Cs_2CO_3 and t-BuOH provides a dramatic rate acceleration in the palladium-catalyzed aerobic oxidative kinetic resolution of secondary alcohols while maintaining the selectivity of the process

Palladium-Catalyzed Enantioselective Oxidation of Alcohols: A Dramatic Rate Acceleration by Cs_2CO_3/t-BuOH

The addition of Cs_2CO_3 and t-BuOH provides a dramatic rate acceleration in the palladium-catalyzed aerobic oxidative kinetic resolution of secondary alcohols while maintaining the selectivity of the process

The Palladium-Catalyzed Aerobic Kinetic Resolution of Secondary Alcohols: Reaction Development, Scope, and Applications

The first palladium-catalyzed enantioselective oxidation of secondary alcohols has been developed, utilizing the readily available diamine (-)-sparteine as a chiral ligand and molecular oxygen as the stoichiometric oxidant. Mechanistic insights regarding the role of the base and hydrogen-bond donors have resulted in several improvements to the original system. Namely, addition of cesium carbonate and tert-butyl alcohol greatly enhances reaction rates, promoting rapid resolutions. The use of chloroform as solvent allows the use of ambient air as the terminal oxidant at 23 °C, resulting in enhanced catalyst selectivity. These improved reaction conditions have permitted the successful kinetic resolution of benzylic, allylic, and cyclopropyl secondary alcohols to high enantiomeric excess with good-to-excellent selectivity factors. This catalyst system has also been applied to the desymmetrization of meso-diols, providing high yields of enantioenriched hydroxyketones

Quantitative Prediction of Rate Constants for Aqueous Racemization To Avoid Pointless Stereoselective Syntheses

Racemisation has a large impact upon the biological properties of molecules but the chemical scope of compounds with known rate constants for racemisation in aqueous conditions was hitherto limited. To address this remarkable blind spot, we have measured the kinetics for racemisation of 28 compounds using circular dichroism and 1H NMR spectroscopy. We show that rate constants for racemisation (measured by ourselves and others) correlate well with deprotonation energies from quantum mechanical (QM) and group contribution calculations. Such calculations thus provide predictions of the second-order rate constants for general-base-catalyzed racemisation that are usefully accurate. When applied to recent publications describing the stereoselective synthesis of compounds of purported biological value, the calculations reveal that racemisation would be sufficiently fast to render these expensive syntheses pointless