The design and synthesis of a tricyclic single-nitrogen scaffold that serves as a 5-HT2C receptor agonist

Carbene Formation or Reduction of the Diazo Functional Group? An Unexpected Solvent-Dependent Reactivity of Cyclic Diazo Imides

The control of the reactivity of diazo compounds is commonly achieved by the choice of a suitable catalyst, e.g. via stabilization of singlet carbenes or radical intermediates. Herein, we report on the light-promoted reactivity of cyclic diazo imides with thiols, where the choice of solvent results in two fundamentally different reaction pathways. In dichloromethane (DCM), a carbene is formed initially and engages in a cascade C-H functionalization/thiolation reaction to deliver indane-fused pyrrolidines in good to excellent yields. When switching to acetonitrile solvent, the carbene pathway is shut down and an unusual reduction of the diazo compound occurs under otherwise identical reaction conditions, where the aryl thiol acts as reductant. A combined set of experimental and computational studies was carried out to obtain mechanistic understanding and to support that indane formation proceeds via the insertion of a triplet carbene, while the reduction of diazo imides proceeds via an electron transfer process.

Electrochemical Cyclization of Alkynyl Enaminones: Controllable Synthesis of Indeno[1,2-c]pyrroles or Indanones

We report an electrochemical intramolecular [3 + 2] cyclization of alkynyl enaminones in a user-friendly undivided cell under constant current conditions without an oxidant and catalyst, and indeno[1,2-c]pyrrole derivatives could be obtained in good to excellent yields. Notably, preliminary substituent-controlled selective transformation is also achieved under electrocatalysis alone, and indeno[1,2-c]pyrrole (R⁴ ≠ H) or indanone derivatives (R⁴ = H) could be prepared directly under electrocatalysis without adding a base and heating process.

Construction of indeno[1,2-b]pyrroles via chemoselective N-acylation/cyclization/Wittig reaction sequence

An efficient protocol for the chemoselective construction of the indeno[1,2-b]pyrroles and rearranged indeno[1,2-b]pyrrole derivatives is reported via an N-acylation/cyclization/Wittig reaction. Extensive mechanistic investigations revealed that the initially formed crucial spiro-indene-1,2'-[1,3,4]oxadiazol intermediate further reacts with phosphine to generate betaine, thus predominately resulting in the aforementioned heteroarenes proceeding by a Wittig reaction.

Generation of Donor/Donor Copper Carbenes through Copper-Catalyzed Diyne Cyclization: Enantioselective and Divergent Synthesis of Chiral Polycyclic Pyrroles

The generation of metal carbenes from readily available alkynes represents a significant advance in metal carbene chemistry. However, most of these transformations are based on the use of noble-metal catalysts and successful examples of such an asymmetric version are still very scarce. Here a copper-catalyzed enantioselective cascade cyclization of N-propargyl ynamides is reported, enabling the practical and atom-economical construction of diverse chiral polycyclic pyrroles in generally good to excellent yields with wide substrate scope and excellent enantioselectivities (up to 97:3 e.r.). Importantly, this protocol represents the first copper-catalyzed asymmetric diyne cyclization. Moreover, mechanistic studies revealed that the generation of donor/donor copper carbenes is presumably involved in this 1,5-diyne cyclization, which is distinctively different from the related gold catalysis, and thus it constitutes a novel way for the generation of donor/donor metal carbenes.

Gold-catalyzed cascade cyclization of N-propargyl ynamides: rapid access to functionalized indeno[1,2-c]pyrroles

A new gold-catalyzed cascade cyclization of N-propargyl ynamides involving a presumable vinyl cation intermediate has been developed, allowing the efficient and practical synthesis of various functionalized indeno[1,2-c]pyrroles.

A regioselective double Stille coupling reaction of bicyclic stannolanes

A regioselective double Stille coupling reaction was explored using bicyclic stannolanes that were easily prepared from the radical cascade reaction of β-amino-α-methylene esters. Various 1-bromo-2-iodoarenes underwent the double coupling reaction to afford benzoisoindole derivatives in a regioselective manner, where the carbon attached to the iodine selectively coupled with the vinylic carbon, and then the carbon attached to bromine coupled with the alkyl carbon. The combination of intra- and intermolecular coupling reactions provided hexahydroindeno[1,2-b]pyrrole derivatives in good yields. The yields were further improved in the presence of excess amounts of CsF. An attempt to identify the reaction intermediate was made wherein the decomposition of the stannolanes with aqueous HCl and HBr afforded trigonal bipyramidal (TBP) pentacoordinated tin complexes, as confirmed by microanalyses and (119)Sn NMR. Using DCl for the decomposition selectively introduced a deuterium to the E-position of the exomethylene unit. The complexes smoothly underwent the intramolecular Stille coupling reaction in the presence of both a palladium catalyst and DABCO, affording hexahydroindeno[1,2-b]pyrroles in good yields. These results suggest that the double coupling reaction progresses through a TBP tin complex, promoting the second intramolecular coupling reaction between the aryl halide and Csp(3)-tin bond.

Cascade bicyclizations of o-alkynyl aldehydes with thiazolium salts: a new access toward poly-functionalized indeno[2,1-b]pyrroles

A new cascade bicyclization of o-alkynyl aldehydes with thiazolium salts is described, in which 25 examples of densely functionalized indeno[2,1-b]pyrroles are achieved in a functional-group-compatible manner. Thiazole carbenes generated in situ from thiazolium salts play dual roles as reaction partners and as NHC catalysts. The synthetic utility of these bicyclization reactions results in subsequent C-C bond-forming events to rapidly build up molecular complexity.

Identification and characterization of pyrrolidine diastereoisomers as potent functional agonists and antagonists of the human melanocortin-4 receptor

A series of trans-4-phenylpyrrolidine-3-carboxamides were synthesized and characterized as potent ligands of the human melanocortin-4 receptor. Interestingly, a pair of diastereoisomers 13b displayed potent functional agonist and antagonist activity, respectively. Thus, the 3S,4R-pyrrolidine 13b-1 possessed a Ki of 1.0 nM and an EC50 of 3.8 nM, while its 3R,4S-isomer 13b-2 exhibited a Ki of 4.7 and an IC50 of 64 nM. Both compounds were highly selective over other melanocortin receptor subtypes. The MC4R agonist 13b-1 also demonstrated efficacy in a diet-induced obesity model in rats.