Syntheses of (+)-Complanadine A and Lycodine Derivatives by Regioselective [2 + 2 + 2] Cycloadditions

Asymmetric Total Synthesis and Structure Elucidation of Huperzine H

A first asymmetric total synthesis of huperzine H has been achieved in a 12% overall yield from commercially available (+)-pulegone. The key steps of this synthesis include a highly diastereoselective Mukaiyama-Michael addition reaction of a pyrrole bearing a silyl enol ether and an intramolecular S_N2 cyclization reaction with iodinated pyrrole acting as an effective nucleophile for the formation of the nine-membered ring. As a result, the relative and absolute stereochemistry of huperzine H is established.

One-Carbon Insertion and Polarity Inversion Enabled a Pyrrole Strategy to the Total Syntheses of Pyridine-Containing Lycopodium Alkaloids: Complanadine A and Lycodine

Complanadine A and lycodine are representative members of the Lycopodium alkaloids with a characteristic pyridine-containing tetracyclic skeleton. Complanadine A has demonstrated promising neurotrophic activity and potential for persistent pain management. Herein we report a pyrrole strategy enabled by one-carbon insertion and polarity inversion for concise total syntheses of complanadine A and lycodine. The use of a pyrrole as the pyridine precursor allowed the rapid construction of their tetracyclic skeleton via a one-pot Staudinger reduction, amine-ketone condensation, and Mannich-type cyclization. The pyrrole group was then converted to the desired pyridine by the Ciamician-Dennstedt rearrangement via a one-carbon insertion process, which also simultaneously introduced a chloride at C3 for the next C-H arylation. Other key steps include a direct anti-Markovnikov hydroazidation, a Mukaiyama-Michael addition, and a Paal-Knorr pyrrole synthesis. Lycodine and complanadine A were prepared in 8 and 11 steps, respectively, from a readily available known compound.

Highly Selective Room-Temperature Suzuki-Miyaura Coupling of Bromo-2-sulfonyloxypyridines for Unsymmetrical Diarylpyridines

A new and mild synthetic approach has been developed for the synthesis of pharmaceutically important unsymmetrical diarylpyridines via chemoselective Suzuki-Miyaura coupling reactions of bromo-2-sulfonyloxypyridines. Most reactions allow for facile access to aryl-2-sulfonyloxypyridines at room temperature in yields of 5-99% with excellent chemoselectivity in the presence of Pd(OAc)₂ (2.0 mol %) and Ad₂BnP (2.4 mol %). The second arylation of the remaining tosyl or triflyl group in the monoarylpyridine derivatives obtained was successfully accomplished for the synthesis of unsymmetrical 2,3-, 2,4-, 2,5-, and 2,6-diarylpyridine derivatives. Furthermore, a one-pot synthesis of unsymmetrical diarylpyridines starting from bromo-2-sulfonyloxypyridine was accomplished to demonstrate the practical convenience. Finally, with this method, an antibacterial agent, a topoisomerase inhibitor, and etoricoxib, a nonsteroidal anti-inflammatory drug, were successfully synthesized from the corresponding bromo-2-hydroxypyridines in overall yields of 80, 86, and 49%, respectively.

Air-Stable CpCoI-Phosphite-Fumarate Precatalyst in Cyclization Reactions: Comparing Different Methods of Energy Supply

The robust CoI precatalyst [CpCo(P{OEt}3)(trans-MeO2CHC=CHCO2Me)] was investigated in cyclotrimerizations, furnishing benzenes and pyridines from triynes, diynes and nitriles, comparing the influence of different ways of energy supply; namely, irradiation and conventional (thermal) or microwave heating. The precatalyst was found to work under all conditions, including the possibility to catalyze cyclotrimerizations at room temperature under photochemical conditions at longer reaction times. Performance of the reactions in a microwave reactor proved to be the most time-efficient way to rapidly assemble the expected reaction products; however, careful selection of reaction conditions can be required. The synthesis of pyridines and isoquinolines successfully involved the utilization of versatile functionalized nitriles, affording structurally interesting reaction products. Comparison with the known and often applied precatalyst CpCo(CO)2 demonstrated the significantly higher reactivity of the CpCoI-phosphite-olefin precatalyst.

Synthesis of 3-(aminomethyl)pyridine by traceless C3-selective umpolung of 1-amidopyridin-1-ium salts

A natural product inspired rapid access of 3-(aminomethyl)pyridine by one-pot reaction of 1-amidopyridin-1-ium salt with aminal followed by reductive cleavage of the N-N bond is developed. This C3-selective formal C-H activation of pyridine features a traceless umpolung of the 1-amidopyridin-1-ium salt toward a Mannich type C-C bond formation of the in situ generated 1-amido-2-dialkylamino-1,2-dihydropyridine intermediate.

Synthetic approaches towards alkaloids bearing α-tertiary amines

Alkaloids account for some of the most beautiful and biologically active natural products. Although they are usually classified along biosynthetic criteria, they can also be categorized according to certain structural motifs. Amongst these, the α-tertiary amine (ATA), i.e. a tetrasubstituted carbon atom surrounded by three carbons and one nitrogen, is particularly interesting. A limited number of methods have been described to access this functional group and fewer still are commonly used in synthesis. Herein, we review some approaches to asymmetrically access ATAs and provide an overview of alkaloid total syntheses where those have been employed.

Complanadine A, a selective agonist for the Mas-related G protein-coupled receptor X2

The first biological target for the natural product complanadine A has been determined. The pseudosymmetric alkaloid functions as a selective agonist for the Mas-related G protein-coupled receptor X2 (MrgprX2), a G protein-coupled receptor that is highly expressed in neurons. Given the potential of MrgprX2 to function as a modulator of pain, complanadine A represents a new chemical probe to selectively interrogate the physiological function of MrgprX2 as well as a potential lead for the development of antihyperalgesics for the treatment of persistent pain. While complanadine A possess agonistic activity the related natural product lycodine, representing half of complanadine A, lacks activity providing a cursory description of the structural requirements for agonistic activity.