Directed ortho -Metalation of Aryl Amides, O- Carbamates, and Methoxymethoxy Systems: Directed Metalation Group Competition and Cooperation

The Versatile and Strategic O-Carbamate Directed Metalation Group in the Synthesis of Aromatic Molecules: An Update

The aryl O-carbamate (ArOAm) group is among the strongest of the directed metalation groups (DMGs) in directed ortho metalation (DoM) chemistry, especially in the form Ar-OCONEt2. Since the last comprehensive review of metalation chemistry involving ArOAms (published more than 30 years ago), the field has expanded significantly. For example, it now encompasses new substrates, solvent systems, and metalating agents, while conditions have been developed enabling metalation of ArOAm to be conducted in a green and sustainable manner. The ArOAm group has also proven to be effective in the anionic ortho-Fries (AoF) rearrangement, Directed remote metalation (DreM), iterative DoM sequences, and DoM-halogen dance (HalD) synthetic strategies and has been transformed into a diverse range of functionalities and coupled with various groups through a range of cross-coupling (CC) strategies. Of ultimate value, the ArOAm group has demonstrated utility in the synthesis of a diverse range of bioactive and polycyclic aromatic compounds for various applications.

Ring Walking Mediated by Ni-Ni Species as a Vehicle for Enabling Distal C(sp2)-H Functionalization of Aryl Pivalates

Herein, we report the utilization of Ni-Ni species as a manifold for enabling a "ring-walking" event by dynamic translocation of the metal center over the arene backbone. Experimental and computational studies support a translocation occurring via a 1,2-hydride shift. The synthetic applicability of the method is illustrated in a series of C-C bond formations that occur at distal C(sp2)-H sites of simple aryl pivalates.

Total Synthesis and Structural Plasticity of Kratom Pseudoindoxyl Metabolites

Mitragynine pseudoindoxyl, a kratom metabolite, has attracted increasing attention due to its favorable side effect profile as compared to conventional opioids. Herein, we describe the first enantioselective and scalable total synthesis of this natural product and its epimeric congener, speciogynine pseudoindoxyl. The characteristic spiro-5-5-6-tricyclic system of these alkaloids was formed through a protecting-group-free cascade relay process in which oxidized tryptamine and secologanin analogues were used. Furthermore, we discovered that mitragynine pseudoindoxyl acts not as a single molecular entity but as a dynamic ensemble of stereoisomers in protic environments; thus, it exhibits structural plasticity in biological systems. Accordingly, these synthetic, structural, and biological studies provide a basis for the planned design of mitragynine pseudoindoxyl analogues, which can guide the development of next-generation analgesics.

Transition Metal-Free Synthesis of Halobenzo[b]furans from O-Aryl Carbamates via o-Lithiation Reactions

A straightforward and transition metal-free one-pot protocol to synthesize halobenzo[b]furans has been developed employing simple and easily available starting materials such as O-aryl carbamates and alkynylsulfones. The fine-tuning of the different steps involved was key to achieving a successful one-pot procedure. Initially, a directed ortho-lithiation process, which uses the carbamate as the directed metalation group, was crucial in providing access to O-2-alkynylaryl N,N-diethyl carbamates by a direct alkynylation of the o-lithiated carbamate, with arylsulfonylalkynes as electrophilic reagents. Cyclization of the generated o-alkynylaryl carbamates was successfully accomplished through a strategy involving in situ carbamate alkaline hydrolysis under conventional heating or microwave irradiation, coupled with a subsequent heterocyclization step delivering the desired benzo[b]furans. A wide variety of new halobenzo[b]furans has been synthesized and their utility has been demonstrated by their further transformation.

Carbamates: A Directing Group for Selective C-H Amidation and Alkylation under Cp*Co(III) Catalysis

The selective reactivity of carbamate and thiocarbamate toward alkylation and amidation is reported under stable, high-valent, cost-effective cobalt(III) catalysis. This method reveals the wide possibility of designing a different branch of synthetically challenging yet highly promising asymmetric catalysts based on BINOL and SPINOL scaffolds. Late-stage C-H functionalization of l-tyrosine and estrone was also achieved through this approach. The mechanistic study shows that a base-assisted internal electrophilic substitution mechanism is operative here.

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.

Synthesis of Functionalized Pyrazin-2(1 H)-ones via Tele-Nucleophilic Substitution of Hydrogen Involving Grignard Reactants and Electrophiles

The reaction of 6-chloro-1-methylpyrazin-2(1 H)-one with Grignard reactants followed by quenching with different electrophiles gave access to a variety of 3,6-difunctionalized 1-methylpyrazin-2(1 H)-ones. This regioselective three-component reaction represents the first example of a tele-nucleophilic substitution of hydrogen (S_N^H) in which the anionic σ^H adduct is quenched by electrophiles (other than a proton) before elimination takes place. Quenching the reaction with iodine (I₂) or bromine (Br₂) provides an alternative reaction pathway, yielding a 3-functionalized 6-chloro-1-methylpyrazin-2(1 H)-one or 5-bromo-6-chloro-1-methylpyrazin-2(1 H)-one, respectively. The halogens present offer opportunities for further selective transformations.

Bacterial Catabolism of Biphenyls: Synthesis and Evaluation of Analogues

A series of alkylated 2,3-dihydroxybiphenyls has been prepared on the gram scale by using an effective Directed ortho Metalation-Suzuki-Miyaura cross-coupling strategy. These compounds have been used to investigate the substrate specificity of the meta-cleavage dioxygenase BphC, a key enzyme in the microbial catabolism of biphenyl. Isolation and characterization of the meta-cleavage products will allow further study of related processes, including the catabolism of lignin-derived biphenyls.

Directed ortho-Metalation and Anionic ortho-Fries Rearrangement of Polycyclic Aromatic O-Carbamates: Regioselective Synthesis of Substituted Chrysenes

A general method for the regioselective synthesis of a series of ortho-substituted chrysenyl N, N-diethyl- O-carbamates by the directed ortho-metalation (D oM) strategy is reported. The starting O-carbamates were prepared from the corresponding chrysenols, available by oxidative photochemical cyclization or directed remote metalation tactics. Chrysen-1-yl and chrysene-3-yl ring site selectivity of directed ortho-metalation (D oM) and anionic ortho-Fries rearrangement (A oF) protocols, with s-BuLi/TMEDA, followed by electrophilic quench using a selection of electrophiles, were observed, leading to new chrysenyl derivatives. 5-Chrysenyl N,N-diethyl- O-carbamate underwent instant A oF rearrangement even at -100 °C to furnish chrysenyl o-hydroxycarboxamide. Iterative D oM reactions were carried out to gain insight into the regioselectivity factors.

1,5-O → N Carbamoyl Snieckus-Fries-Type Rearrangement

The reaction of o-lithiated O-aryl N,N-diethylcarbamates with (hetero)aromatic nitriles gives rise to functionalized salicylidene urea derivatives in high yields through a new 1,5-O → N carbamoyl migration. This Snieckus-Fries-type rearrangement nicely complements previously known O → C and O → O related shifts. In addition, when dimethylmalononitrile is used as the electrophilic partner, the carbamoyl shift is preferred over the expected transnitrilation reaction.