Conservation of the Planar Chiral Information in the Tandem Oxy-Cope/Ene Reaction

Evidence for Dearomatizing Spirocyclization and Dynamic Effects in the Quasi-stereospecific Nitrogen Deletion of Tetrahydroisoquinolines

Selectivity in organic chemistry is generally presumed to arise from energy differences between competing selectivity-determining transition states. However, in cases where static density functional theory (DFT) fails to reproduce experimental product distributions, dynamic effects can be examined to understand the behavior of more complex reaction systems. Previously, we reported a method for nitrogen deletion of secondary amines which relies on the formation of isodiazene intermediates that subsequently extrude dinitrogen with concomitant C-C bond formation via a caged diradical. Herein, a detailed mechanistic analysis of the nitrogen deletion of 1-aryl-tetrahydroisoquinolines is presented, suggesting that in this system the previously determined diradical mechanism undergoes dynamically controlled partitioning to both the normal 1,5-coupling product and an unexpected spirocyclic dearomatized intermediate, which converges to the expected indane by an unusually facile 1,3-sigmatropic rearrangement. This mechanism is not reproduced by static DFT but is supported by quasi-classical molecular dynamics calculations and unifies several unusual observations in this system, including partial chirality transfer, nonstatistical isotopic scrambling at the ethylene bridge, the isolation of spirocyclic dearomatized species in a related heterocyclic series, and the observation that introduction of an 8-substituent dramatically improves enantiospecificity.

Bioinspired Total Synthesis of Homodimericin A

Homodimericin A is a remarkable fungal metabolite. This highly oxygenated racemic unsaturated polyketide poses a significant synthetic challenge owing to its sterically demanding central cagelike core containing eight contiguous stereogenic centers (including three quaternary stereocenters) and several carbonyl functionalities. On the basis of its proposed biogenetic synthesis, we designed a total synthesis of homodimericin A that proceeds in seven steps and features a double Michael reaction, an intramolecular Diels-Alder reaction, and an ene reaction.

Base-mediated cascade rearrangements of aryl-substituted diallyl ethers

Two base-mediated cascade rearrangement reactions of diallyl ethers were developed leading to selective [2,3]-Wittig-oxy-Cope and isomerization-Claisen rearrangements. Both diaryl and arylsilyl-substituted 1,3-substituted propenyl substrates were examined, and each exhibits unique reactivity and different reaction pathways. Detailed mechanistic and computational analysis was conducted, which demonstrated that the role of the base and solvent was key to the reactivity and selectivity observed. Crossover experiments also suggest that these reactions proceed with a certain degree of dissociation, and the mechanistic pathway is highly complex with multiple competing routes.

Synthesis, structure and reactivity of [15]-macrodilactones

Synthesis and characterization of some new [15]-macrodilactones revealed that the interplay between three planar units, a stereogenic center and a hinge atom generated a planar chirality that governs their molecular topology.

Origin of Diastereoselectivity in the Tandem Oxy-Cope/Claisen/Ene Reaction: Experimental and Theoretical Studies of the Ring Inversion Mechanism

We report herein a detailed investigation into the reaction mechanism of the oxy-Cope/Claisen/ene reaction. A series of chiral substrates was prepared, subjected to the tandem sequence, and the enantiomeric excess of the final products was evaluated. The observed conservation of enantiomeric excess was taken as evidence that the ring inversion of the intermediary enol ether does not occur. DFT calculations were used to map out the potential energy surface for the reaction and evaluate the relative energies of the ring inversions relative to those of the Claisen and ene reactions. Transition state energies thus obtained were found to support the presence of a high-energy transition state for the ring inversion of B to D provided R(1) not equal H. In addition, the calculations lent further support to the hypothesis that the selectivity of the transannular ene reaction is under Curtin-Hammett control.