Discovery and bioinspired total syntheses of unprecedented sesquiterpenoid dimers unveiled bifurcating [4 + 2] cycloaddition and target differentiation of enantiomers

[4 + 2] cycloaddition has led to diverse polycyclic chiral architectures, serving as novel sources for organic synthesis and biological exploration. Here, an unprecedented class of cadinane sesquiterpene [4 + 2] dimers, henryinins A-E (1-5), with a unique 6/6/6/6/6-fused pentacyclic system, were isolated from Schisandra henryi. The divergent total syntheses of compounds 1-5 and their enantiomers (6-10) were concisely accomplished in eight linear steps using a protection-free approach. Mechanistic studies illustrated the origin of selectivity in the key [4 + 2] cycloaddition as well as the inhibition of reaction pathway bifurcation via desymmetrization. The chemical proteomics results showed that a pair of enantiomers shared common targets (PRDX5 C100 and BLMH C73) and had unique targets (USP45 C588 for 4 and COG7 C419 for 9). This work provides experimental evidence for the discovery of unprecedented cadinane dimers from selective Diels-Alder reaction and a powerful strategy to explore the biological properties of natural products.

Influence of Water and Enzyme on the Post-Transition State Bifurcation of NgnD-Catalyzed Ambimodal [6+4]/[4+2] Cycloaddition

The enzyme NgnD catalyzes an ambimodal cycloaddition that bifurcates to [6+4]- and [4+2]-adducts. Both products have been isolated in experiments, but it remains unknown how enzyme and water influence the bifurcation selectivity at the femtosecond time scale. Here, we study the impact of water and enzyme on the post-transition state bifurcation of NgnD-catalyzed [6+4]/[4+2] cycloaddition by integrating quantum mechanics/molecular mechanics quasiclassical dynamics simulations and biochemical assays. The ratio of [6+4]/[4+2] products significantly differs in the gas phase, water, and enzyme. Biochemical assays were employed to validate computational predictions. The study informs how water and enzyme affect the bifurcation selectivity through perturbation of the reaction dynamics in the femtosecond time scale, revealing the fundamental roles of condensed media in dynamically controlling the chemical selectivity for biosynthetic reactions.

Post-transition state bifurcations induce dynamical detours in Pummerer-like reactions

A post-transition state bifurcation (PTSB) involved in a Pummerer-type rearrangement is characterized using density functional theory (DFT) calculations on potential energy stationary points and direct dynamics simulations. A sensitivity of the ratio of products produced via this PTSB to solvent dielectric constant is revealed and implications of such a dependence for selectivity control of organic reactions are discussed.

Interruption of Formal Schmidt Rearrangement/Hosomi-Sakurai Reaction of Vinyl Azides with Allyl/Propargylsilanes

An interrupted Schmidt rearrangement/Hosomi-Sakurai reaction of N-tosyl and S-substituted vinyl azides is reported. With BF₃·Et₂O as a Lewis acid promoter, the denitrogenative fragmentation of vinyl azides facilitates the generation of the N/ S-stabilized carbocations, which further undergo nucleophilic addition by allyl/propargylsilanes for C-C bond formation. As a result, a variety of homoallylic/allenylic amines can be afforded in good yields under mild reaction conditions with well-defined functional-group tolerance.

Post-transition state bifurcations gain momentum – current state of the field

The existence of post-transition state bifurcations on potential energy surfaces for organic and biological reaction mechanisms has been known for decades, but recently, new reports of bifurcations have been occurring at a much higher rate. Beyond simply discovering bifurcations, computational chemists are developing techniques to understand what aspects of molecular structure and vibrations control the product selectivity in systems containing bifurcations. For example, the distribution of products seen in simulations has been found to be extremely sensitive to the local environment of the reacting system (i.e. the presence of a catalyst, enzyme, or explicit solvent molecules). The outlook for the future of this field is discussed, with an eye towards the application of the principles discussed here by experimental chemists to design a reaction setup to efficiently generate desired products.

Response to "Comment on 'Analyses of bifurcation of reaction pathways on a global reaction route map: A case study of gold cluster Au5"' [J. Chem. Phys. 143, 177101 (2015)]

The existence of a valley-ridge transition (VRT) point along the intrinsic reaction coordinate does not always indicate the existence of two minima in the product side, but VRT is a sign of bifurcating nature of dynamical trajectories running on the potential energy surface. It is demonstrated by molecular dynamics simulations.

Prediction of enhanced solvent-induced enantioselectivity for a ring opening with a bifurcating reaction path

A fully atomistic molecular dynamics simulation predicts enhance induction of enantiomeric excess in the products of a reaction with a bifurcating reaction coordinate, when run in a chiral solvent.