Concise Total Syntheses of Pyr­ido[4,3-b]carbazole Alkaloids Using Copper-Mediated 6π-Electrocyclization

Nitrogen-to-functionalized carbon atom transmutation of pyridine

The targeted and selective replacement of a single atom in an aromatic system represents a powerful strategy for the rapid interconversion of molecular scaffolds. Herein, we report a pyridine-to-benzene transformation via nitrogen-to-carbon skeletal editing. This approach proceeds via a sequence of pyridine ring-opening, imine hydrolysis, olefination, electrocyclization, and aromatization to achieve the desired transmutation. The most notable features of this transformation are the ability to directly install a wide variety of versatile functional groups in the benzene scaffolding, including ester, ketone, amide, nitrile, and phosphate ester fragments, as well as the inclusion of meta-substituted pyridines which have thus far been elusive for related strategies.

Exploiting an intramolecular Diels-Alder cyclization/dehydro-aromatization sequence for the total syntheses of ellipticines and calothrixin B

The tetracyclic and pentacyclic skeletons of pyrido and quinolinocarbazole alkaloids have been synthesized via a unified strategy. The prominent key step involved a Diels-Alder intramolecular cyclization/dehydro-aromatization sequence. From these carbazole-lactam cores, linear syntheses have been developed for ellipticines and calothrixin B.

Transition-Metal Catalysis of Triene 6π Electrocyclization: The π-Complexation Strategy Realized

Triene 6π electrocyclization, wherein a conjugated triene undergoes a concerted stereospecific cycloisomerization to a cyclohexadiene, is a reaction of great historical and practical significance. In order to circumvent limitations imposed by the normally harsh reaction conditions, chemists have long sought to develop catalytic variants based upon the activating power of metal-alkene coordination. Herein, we demonstrate the first successful implementation of such a strategy by utilizing [(C₅ H₅ )Ru(NCMe)₃ ]PF₆ as a precatalyst for the disrotatory 6π electrocyclization of highly substituted trienes that are resistant to thermal cyclization. Mechanistic and computational studies implicate hexahapto transition-metal coordination as responsible for lowering the energetic barrier to ring closure. This work establishes a foundation for the development of new catalysts for stereoselective electrocyclizations.

A metal-free aromative cascade for the synthesis of diverse heterocycles

A metal-free aromative cascade has been developed for the synthesis of diverse heterocycles from readily accessible hydroxy/aminochalcones and acid/alkyl halides. The cascade being by a base-mediated intramolecular aldol cyclization/dehydration sequence to provide a triene, which sets the stage for a 6π-electrocyclization/oxidative aromatization to access diverse heterocyclic scaffolds.

Total Synthesis of Olivacine and Ellipticine via a Lactone Ring-Opening and Aromatization Cascade

Effective preparation of olivacine and ellipticine via late-stage D-ring cyclization is described. Key features of the synthetic routes include trifluoroacetic acid-mediated formation of a lactone that is fused to a tetrahydrocarbazole derivative and its one-pot two-step ring opening and aromatization mediated by para-toluenesulfonic acid and palladium on carbon, respectively.

Revisiting Furodiindolines: One-Pot Synthesis of Furodiindolines Using Indole 2,3-Epoxide Surrogates and Their Synthetic Applications

Furodiindolines have emerged as versatile intermediates in various transformations, which are easily obtained from indole 2,3-epoxide surrogates via a one-pot procedure and allowed access to heterocyclic compounds with structural diversity and complexity. Four types of transformations from the furodiindolines have been achieved: (i) dehydrative rearrangement to afford 2,3'-biindoles, (ii) hydrolysis/cyclization to give 3,4-disubstituted quinolines, (iii) ring-opening/cyclization to give pyrroloindolines, and (iv) aminal cleavage to give a 3,3-disubstituted 2-oxindole.

Synthesis and Activity against Mycobacterium tuberculosis of Olivacine and Oxygenated Derivatives

The tetracyclic pyrido[4,3-b]carbazole olivacine and four of its oxygenated derivatives have been synthesized by a late-stage palladium-catalyzed Heck-type cyclization of the pyrrole ring as a key step. In a test for the inhibition of the growth of Mycobacterium tuberculosis, 9-methoxyolivacine showed the most significant inhibitory activity against Mycobacterium tuberculosis, with an MIC₉₀ value of 1.5 μM.

Concise Syntheses of Hyrtioreticulins C and D via a C-4 Pictet-Spengler Reaction: Revised Signs of Specific Rotations

The first syntheses of hyrtioreticulins C and D via a Pictet-Spengler reaction at the C-4 position of the indole rings are described. The synthesis proceeds in only two steps from commercially available starting materials. In this Communication, the structures of the natural products were confirmed. Furthermore, we revise the signs of the specific rotations of hyrtioreticulins C and D.