Bridgehead Lithiation-Substitution of Bridged Ketones, Lactones, Lactams, and Imides: Experimental Observations and Computational Insights

Regiodivergent Ring-Expansion of Oxindoles to Quinolinones

The development of divergent methods to expedite structure-activity relationship studies is crucial to streamline discovery processes. We developed a rare example of regiodivergent ring expansion to access two regioisomers from a common starting material. To enable this regiodivergence, we identified two distinct reaction conditions for transforming oxindoles into quinolinone isomers. The presented methods proved to be compatible with a variety of functional groups, which enabled the late-stage diversification of bioactive oxindoles as well as facilitated the synthesis of quinolinone drugs and their derivatives.

Hetero Diels-Alder Reactions with a Dicationic Urea Azine Derived Azo Dienophile and Their Use for the Synthesis of an Electron-Rich Pentacene

Herein, the first hetero Diels-Alder (DA) reactions with a stable, dicationic urea azine derived azo dienophile, synthesized by two-electron oxidation of a neutral urea azine are reported. Several charged DA products were synthesized in good yield and fully characterized. The DA adduct of anthracene is in thermal equilibrium with the reactants at room temperature, and the reaction enthalpy and entropy were determined from the temperature-dependent equilibrium constant. Furthermore, base addition to solutions of the pentacene DA product led to deprotonation, cleavage of the N-N bond, and formation of an electron-rich 6,13-bisguanidinyl-substituted pentacene. The redox and optical properties of this new pentacene derivative were studied. Furthermore, the dication resulting from its two-electron oxidation was synthesized and fully characterized. The results disclose a new elegant route to electron-rich pentacene derivatives.

Rapid and Accurate Prediction of pKa Values of C-H Acids Using Graph Convolutional Neural Networks

The ability to estimate the acidity of C-H groups within organic molecules in non-aqueous solvents is important in synthetic planning to correctly predict which protons will be abstracted in reactions such as alkylations, Michael additions, or aldol condensations. This Article describes the use of the so-called graph convolutional neural networks (GCNNs) to perform such predictions on the time scales of milliseconds and with accuracy comparing favorably with state-of-the-art solutions, including commercial ones. The crux of the method is to train GCNNs using descriptors that reflect not only topological but also chemical properties of atomic environments. The model is validated against adversarial controls, supplemented by the discussion of realistic synthetic problems (on which it correctly predicts the most acidic protons in >90% of cases), and accompanied by a Web application intended to aid the community in everyday synthetic planning.

Collective Total Synthesis of PPAPs: Total Synthesis of Clusianone via Intramolecular Cyclopropanation

The total synthesis of clusianone was accomplished through the stereoselective construction of a bicyclo[3.3.1]nonane derivative via a three-step sequence which has been utilized for the total syntheses of nemorosone garsubellin A and hyperforin: intramolecular cyclopropanation formation of a geminal dimethyl group and regioselective ring opening of cyclopropane. Further elaboration including chemo- and stereoselective hydrogenation to generate the C7 stereogenic center and cross-metathesis to construct prenyl groups in the side-chains was employed to complete the total synthesis of clusianone.

Adventures in bridgehead substitution chemistry: synthesis of polycyclic polyprenylated acylphloroglucinols (PPAPs)

The polycyclic polyprenylated acylphloroglucinol (PPAP) family of natural products includes important compounds with notable biological activities, such as garsubellin A, hyperforin and clusianone. The synthesis of these complex, bridged, highly oxidized and substituted systems presents a formidable challenge to synthetic chemists. This feature article describes how the use of unconventional bridgehead substitution chemistry has enabled the synthesis of these natural products and their analogues.

Stereoselective total synthesis of nemorosone

The highly stereoselective total synthesis of nemorosone via a new approach to the bicyclo[3.3.1]nonane-2,4,9-trione core which features intramolecular cyclopropanation of an α-diazo ketone, stereoselective alkylation at the C8 position, and regioselective ring-opening of cyclopropane is described. The total synthesis of nemorosone includes chemo- and stereoselective hydrogenation directed by the internal alkene.

The SAMP alkylation: a computational study

In a computational study of a stereoselective C-C bond formation, the SAMP alkylation, a previously proposed S(E)2'-front mechanism is evaluated taking into account all current experimental evidence. Using semiempirical, density functional and perturbation theoretical methods, the structure of the key intermediate is revealed and the metalloretentive nature of the mechanism is explained. The experimental ee values of a range of reactions with different electrophiles and carbonyl sources can be correlated with calculated differences in activation energies. Furthermore, it can be concluded that the selectivity derives from the internal stabilization of the transition state 3_syn (corresponding to an electrophilic attack from above the lithiohydrazone plane) by electrophile-lithium interactions. The fast computational approach can be used best as a screening method which excludes less promising candidates to guide this synthetic method.

Oxygen activation by P450(cin): Protein and substrate mutagenesis

A conserved threonine found in the majority of cytochromes P450 (P450s) has been implicated in the activation of dioxygen during the catalytic cycle. P450(cin) (CYP176A) has been found to be an exception to this paradigm, where the conserved threonine has been replaced with an asparagine. Prior studies with a P450(cin) N242A mutant established that the Asn-242 was not a functional replacement for the conserved threonine but was essential for the regio- and stereocontrol of the oxidation of cineole. To explore further how P450(cin) controls the activation of the dioxygen in the absence of the conserved threonine, two concurrent lines of investigation were followed. Modification of P450(cin) indicated that the Thr-243 was not involved in controlling the protonation of the hydroperoxy species. In addition, the N242T mutant did not enhance the rate and/or efficiency of catalytic turnover of cineole by P450(cin). In parallel experiments, the substrate cineole was modified by removing the ethereal oxygen to produce camphane or 2,2-dimethylbicyclo[2.2.2]octane (cinane). An analogous experiment with P450(EryF) showed that a hydroxyl group on the substrate was vital, and in its absence catalytic turnover was effectively abolished. Catalytic turnover of P450(cin) with either of these alternative substrates (camphane or cinane) revealed that in the absence of the ethereal oxygen there was still a significant amount of coupling of the NADPH-reducing equivalents to the formation of oxidised product. Again the substrate itself was not found to be important in controlling oxygen activation, in contrast to P450(EryF), but was shown to be essential for regio- and stereoselective substrate oxidation. Thus, it still remains unclear how dioxygen activation in the catalytic turnover of cineole by P450(cin) is controlled.