Violations. How Nature Circumvents the Woodward-Hoffmann Rules and Promotes the Forbidden Conrotatory 4n + 2 Electron Electrocyclization of Prinzbach's Vinylogous Sesquifulvalene

A deconstruction-reconstruction strategy for pyrimidine diversification

Structure-activity relationship (SAR) studies are fundamental to drug and agrochemical development, yet only a few synthetic strategies apply to the nitrogen heteroaromatics frequently encountered in small molecule candidates1-3. Here we present an alternative approach in which we convert pyrimidine-containing compounds into various other nitrogen heteroaromatics. Transforming pyrimidines into their corresponding N-arylpyrimidinium salts enables cleavage into a three-carbon iminoenamine building block, used for various heterocycle-forming reactions. This deconstruction-reconstruction sequence diversifies the initial pyrimidine core and enables access to various heterocycles, such as azoles4. In effect, this approach allows heterocycle formation on complex molecules, resulting in analogues that would be challenging to obtain by other methods. We anticipate that this deconstruction-reconstruction strategy will extend to other heterocycle classes.

Modular synthesis of 1,2-azaborines via ring-opening BN-isostere benzannulation

1,2-Azaborines represent a unique class of benzene isosteres that have attracted interest for developing pharmaceuticals with better potency and bioavailability. However, it remains a long-standing challenge to prepare monocyclic 1,2-azaborines, particularly multi-substituted ones, in an efficient and modular manner. Here we report a straightforward method to directly access diverse multi-substituted 1,2-azaborines from readily available cyclopropyl imines/ketones and dibromoboranes under relatively mild conditions. The reaction is scalable, shows a broad substrate scope, and tolerates a range of functional groups. The utility of this method is demonstrated in the concise syntheses of BN isosteres of a PD-1/PD-L1 inhibitor and pyrethroid insecticide, bifenthrin. Combined experimental and computational mechanistic studies suggest that the reaction pathway involves boron-mediated cyclopropane ring-opening and base-mediated elimination, followed by an unusual low-barrier 6π-electrocyclization accelerated by the BN/CC isomerism. This method is anticipated to find applications for the synthesis of BN-isostere analogues in medicinal chemistry, and the mechanistic insights gained here may guide developing other boron-mediated electrocyclizations.

A 21st Century View of Allowed and Forbidden Electrocyclic Reactions

In 1965, Woodward and Hoffmann proposed a theory to predict the stereochemistry of electrocyclic reactions, which, after expansion and generalization, became known as the Woodward-Hoffmann Rules. Subsequently, Longuet-Higgins and Abrahamson used correlation diagrams to propose that the stereoselectivity of electrocyclizations could be explained by the correlation of reactant and product orbitals with the same symmetry. Immediately thereafter, Hoffmann and Woodward applied correlation diagrams to explain the mechanism of cycloadditions. We describe these discoveries and their evolution. We now report an investigation of various electrocyclic reactions using DFT and CASSCF. We track the frontier molecular orbitals along the intrinsic reaction coordinate and modeled trajectories and examine the correlation between HOMO and LUMO for thermally forbidden systems. We also investigate the electrocyclizations of several highly polarized systems for which the Houk group had predicted that donor-acceptor substitution can induce zwitterionic character, thereby providing low-energy pathways for formally forbidden reactions. We conclude with perspectives on the field of pericyclic reactions, including a refinement as the meaning of Woodward and Hoffmann's "Violations. There are none!" Lastly, we comment on the burgeoning influence of computations on all fields of chemistry.

Dihydrophenanthrene Open-Shell Singlet Diradicals and Their Roles in the Mallory Photocyclization Reaction

A computational study (ωB97X-D/6-31G(d)) of the Mallory photocyclization reaction has revealed that the well-established dihydrophenanthrene (DHP) intermediates can adopt either closed-shell (CS) or open-shell-diradical (OS) singlet ground states. A detailed study of the properties of DHPs allowed their classifications as OS, borderline-OS, borderline-CS, or CS intermediates. The triplet electronic state and higher energy CS* isomer of all the OS singlet diradicals were computationally located, and the expected relationship between the diradical index, yo, and the triplet energy and the OS-CS* energy gaps was established. The importance of aromaticity in stabilizing the OS singlet diradicals was confirmed by using the Harmonic Oscillator Model of Aromaticity (HOMA). The thermal decompositions of DHPs by cycloreversions to regenerate the Mallory starting materials were also studied. The cycloreversion mechanism was described as a homolytic cleavage characterized by an anchimeric assistance continuum promoted by bis-β-homolytic cleavage.

Higher-Order Electrocyclizations in Biological and Synthetic Processes

In general, electrocyclizations follow the Woodward-Hoffmann's rules of conservation of orbital symmetry. These rules have been extensively verified in low-order processes, both in thermal and photochemical reactions, up to eight π-electrons. However, when the number of π-electrons in the system increases, some deviations of that general rules can be found. This focused review highlights the main features of reported higher-order electrocyclizations involving 10, 12, 14, 16 and 18 π-electrons. Some of these examples constitute useful intermediates in the synthesis of biologically active compounds. When computational studies were not included in the reported examples, DFT calculations have been performed to be included in this review. Analysis of the respective pericyclic topologies shows the importance of computational tools for understanding the selectivity observed experimentally.

Enantioselective Synthesis of Tropane Scaffolds by an Organocatalyzed 1,3-Dipolar Cycloaddition of 3-Oxidopyridinium Betaines and Dienamines

Tropane alkaloids constitute a compound-class which is structurally defined by a central 8-azabicyclo[3.2.1]octane core. A diverse bioactivity profile combined with an unusual aza-bridged bicyclic framework has made tropanes molecules-of-interest within organic chemistry. Enantioselective examples of (5+2) cycloadditions between 3-oxidopyridinium betaines and olefins remain unexplored, despite 3-oxidopyridinium betaines being useful reagents in organic synthesis. The first asymmetric (5+2) cycloaddition of 3-oxidopyridinium betaines is reported, affording tropane derivatives in up to quantitative yield and with excellent control of peri-, regio-, diastereo-, and enantioselectivity. The reactivity is enabled by dienamine-activation of α,β-unsaturated aldehydes combined with in situ formation of the pyridinium reaction-partner. A simple N-deprotection protocol allows for liberation of the tropane alkaloid motif, and synthetic elaborations of the cycloadducts demonstrate their synthetic utility to achieve highly diastereoselective modification around the bicyclic framework. DFT computations suggest a stepwise mechanism where regio- and stereoselectivity are defined during the first bond-forming step in which the pyridinium dipole exerts critical conformational control over its dienamine partner. In the second bond-forming step, a kinetic preference toward an initial (5+4) cycloadduct was identified; however, a lack of catalyst turn-over, reversibility, and thermodynamic bias favoring a (5+2) cycloadduct rendered the reaction fully periselective.

Organocatalytic Enantioselective Thermal [4 + 4] Cycloadditions

Chiral eight-membered carbocycles are important motifs in organic chemistry, natural product chemistry, chemical biology, and medicinal chemistry. The lack of synthetic methods toward their construction is a challenge preventing their rational design and stereoselective synthesis. The catalytic enantioselective [4 + 4] cycloaddition is one of the most straightforward and atom-economical methods to obtain chiral cyclooctadiene derivatives. We report the first organocatalytic asymmetric [4 + 4] cycloaddition of 9H-fluorene-1-carbaldehydes with electron-deficient dienes affording cyclooctadiene derivatives in good yields and with excellent control of peri-, diastereo-, and enantioselectivities. The reaction concept is based on the aminocatalytic formation of a polarized butadiene component incorporated into a cyclic extended π-system, with restricted conformational freedom, allowing for a stereocontrolled [4 + 4] cycloaddition. FMO analysis unveiled that the HOMO and LUMO of the two reacting partners resemble those of butadiene. The methodology allows for the construction of cyclooctadiene derivatives decorated with various functionalities. The cyclooctadienes were synthetically elaborated, allowing for structural diversity demonstrating their synthetic utility for the formation of, for example, chiral cyclobutene- or cyclooctane scaffolds. DFT computational studies shed light on the reaction mechanism identifying the preference for an initial but reversible [4 + 2] cycloaddition delivering an off-cycle catalyst resting state, from which catalyst elimination is not possible. The off-cycle catalyst-bound intermediate undergoes a retro-[4 + 2] cycloaddition, followed by a [4 + 4] cycloaddition generating a cycloadduct from which catalyst elimination is possible. The reaction pathway accounts for the observed peri-, diastereo-, and enantioselectivity of the organocatalytic [4 + 4] cycloaddition.

Prediction of High-Yielding Single-Step or Cascade Pericyclic Reactions for the Synthesis of Complex Synthetic Targets

Pericyclic reactions, which involve cyclic concerted transition states without ionic or radical intermediates, have been extensively studied since their definition in the 1960s, and the famous Woodward-Hoffmann rules predict their stereoselectivity and chemoselectivity. Here, we describe the application of a fully automated reaction-path search method, that is, the artificial force induced reaction (AFIR), to trace an input compound back to reasonable starting materials through thermally allowed pericyclic reactions via product-based quantum-chemistry-aided retrosynthetic analysis (QCaRA) without using any a priori experimental knowledge. All categories of pericyclic reactions, including cycloadditions, ene reactions, group-transfer, cheletropic, electrocyclic, and sigmatropic reactions, were successfully traced back via concerted reaction pathways, and starting materials were computationally obtained with the correct stereochemistry. Furthermore, AFIR was used to predict whether the identified reaction pathway can be expected to occur in good yield relative to other possible reactions of the identified starting material. In order to showcase its practical utility, this state-of-the-art technology was also applied to the retrosynthetic analysis of a natural product with a relatively high number of atoms (52 atoms: endiandric acid C methyl ester), which was first synthesized by Nicolaou in 1982 and provided the corresponding starting polyenes with the correct stereospecificity via three pericyclic reaction cascades (one Diels-Alder reaction as well as 6π and 8π electrocyclic reactions). Moreover, not only systems that obey the Woodward-Hoffmann rules but also systems that violate these rules, such as those recently calculated by Houk, can be retrosynthesized accurately.

Enantioselective Total Synthesis of Dysiherbols A, C, and D

Herein, we report the enantioselective total synthesis of dysiherbols A, C, and D, a unique group of 6/6/5/6/6 pentacyclic quinone/hydroquinone sesquiterpenes, featuring a photo-induced quinone-alkene [2 + 2] cycloaddition and a tandem [1,2]-anionic rearrangement/cyclopropane fragmentation as key elements. Based on our total synthesis, the originally proposed structures of dysiherbols C and D have been revised. Detailed computational studies were carried out to gain deep insight into the unprecedented [1,2]-anionic rearrangement, which revealed that the transformation, albeit a symmetry-forbidden process, proceeded through a concerted manner owing to the release of high ring-strain energy and the evolution of local aromaticity in the transition state. Taking all, the present work offers a mechanistically interesting and synthetically useful approach to accessing dysiherbols and related congeners.

Understanding chemistry: from "heuristic (soft) explanations and reasoning by analogy" to "quantum chemistry"

"Soft theories," i.e., "heuristic models based on reasoning by analogy" largely drove chemistry understanding for 150 years or more. But soft theories have their limitations and with the expansion of chemistry in the mid-20th century, more and more inexplicable (by soft theory) experimental results were being obtained. In the past 50 years, quantum chemistry, most often in the guise of applied theoretical chemistry including computational chemistry, has provided (a) the underlying "hard evidence" for many soft theories and (b) the explanations for chemical phenomena that were unavailable by soft theories. In this publication, we define "hard theories" as "theories derived from quantum chemistry." Both soft and hard theories can be qualitative and quantitative, and the "Houk quadrant" is proposed as a helpful categorization tool. Furthermore, the language of soft theories is often used appropriately to describe quantum chemical results. A valid and useful way of doing science is the appropriate use and application of both soft and hard theories along with the best nomenclature available for successful communication of results and ideas.