Biomimetic synthesis of the tetracyclic core of berkelic acid

Acid-catalyzed condensation of 2,6-dihydroxybenzoic acid 3 with ketal aldehyde 14 in methanol at 25 degrees C, followed by CH2N2 esterification, gave a 4:1:4:1 mixture of diastereomers 15b-18b in 60% yield. Equilibration of this mixture with TFA in CDCl3 provided tetracycle 15b (83% yield) with the complete skeleton of berkelic acid. A similar condensation at 0 degrees C afforded 15b-18b and a reduction product 19b, which was probably formed by a 1,5-hydride shift.     

Facile and highly stereoselective synthesis of the tetracyclic erythrinane core

A highly stereoselective synthesis of the tetracyclic core of the Erythrina alkaloids is reported through the application of a Meyers bicyclic lactam template.     

Synthetic studies on (+)-manzamine A: stereoselective synthesis of the tetracyclic core framework

The stereoselective synthesis of the tetracyclic intermediate 3 for (+)-manzamine A (1) has been achieved. The key features of this stereoselective synthesis of 3 are the Rh-catalyzed asymmetric hydrogenation and a diastereoselective intermolecular Diels-Alder reaction. The 8-membered ring is efficiently constructed utilizing our Ns-strategy.     

RCM-mediated stereoselective synthesis of three novel tetrahydroisoquinoline tetracyclic core frameworks

Three novel tetrahydroisoquinoline tetracyclic core frameworks were stereoselectively synthesized. The key steps included a ring-closing metathesis (RCM)-mediated cyclization and a subsequent intramolecular S_N2 N(O)-substituted reaction. This simple method for tetracyclic core synthesis facilitates the further exploration of the chemical space of tetrahydroisoquinoline alkaloids.     

Novel approach to the lundurine alkaloids: synthesis of the tetracyclic core

The tetracyclic core of the lundurine family of alkaloids has been synthesized by a novel approach that features a double ring-closing olefin metathesis to form the five-and eight-membered rings.     

Synthesizing the tetracyclic core of nanolobatolide

A concise synthetic pathway that enables the stereoselective construction of the tetracyclic core of nanolobatolide has been developed by applying a tandem ring-closing metathesis (RCM) reaction of dienynes, a Eu(fod)(3)-catalyzed intermolecular Diels-Alder reaction, and a biomimetic epoxide opening reaction as key steps.     

Diastereoselective synthesis of pyrrolidines using a nitrone/cyclopropane cycloaddition: synthesis of the tetracyclic core of nakadomarin A

The synthesis of the tetracyclic core of nakadomarin A is described. The core contains all the heterocycles and the required stereocenters found in the natural product and provides a promising route to the target itself. The strategy utilizes a general, diastereoselective pyrrolidine synthesis that proceeds via a homo 3 + 2 dipolar cycloaddition. The scope of this methodology is also described. [structure: see text]     

Synthetic studies on (+)-naphthyridinomycin: stereoselective synthesis of the tetracyclic core framework

[reaction: see text] The stereoselective synthesis of the tetracyclic intermediate 21 for (+)-naphthyridinomycin (1) has been accomplished. The convergent synthesis used the Ugi 4CC reaction with the amine derivative 10. The key features of the stereoselective synthesis of 21 were the intramolecular Mizoroki-Heck reaction, an aromatic-aldehyde cyclization, and a stereoselective hydroboration.     

A synthesis of the tetracyclic carboskeleton of isaindigotidione

An efficient synthesis of the unique indolizino[7,6-c]quinoline carboskeleton of isaindigotidione has been achieved. This strategy employed l-proline and isatin as the main building blocks in the construction of the framework. Four transformations occurred in a one-pot operation to furnish the tetracyclic nucleus.     

A convergent approach to the tetracyclic core of atisane diterpenes

Atisane diterpenes are biologically interesting natural products. We report here a convergent approach to construct the tetracyclic core of the atisane skeleton. The two segments were assembled through a Wittig reaction, while an intramolecular Aldol condensation served as the key step to furnish the tetracyclic skeleton.     

Synthesis of the tetracyclic core of chlorospermines

Chlorospermines A and B are biologically interesting acridone natural products and recently isolated from Glycosmis chlorosperma.We report here a convergent approach to construct the tetracyclic core of the natural products.The two fragments are assembled together through Sonogashira coupling,and a cis-triene intermediate was prepared by using hydrosilylation/desilylation.A 6p-electrocyclization/aromatization sequence served as the key step of the synthesis,which formed the tetrasubstituted arene motif in one pot.     

Synthesis of the tetracyclic core of the bisabosquals

HCl-catalyzed deprotection and cyclization of 8b provided tricycle 9b cleanly. Epoxidation of 9b afforded tetracycle 13 with the wrong stereochemistry at the tertiary alcohol. Selective elimination of the tertiary alcohol to give the exocyclic methylene compound, alkene cleavage to form the ketone with OsO₄ and NaIO₄, and addition of MeMgBr to the ketone from the least hindered face gave tertiary alcohol 16 with the tetracyclic core of bisabosqual A (1).     

Stereoselective construction of the tetracyclic core of Cryptotrione

An efficient stereoselective approach to the tetracyclic core of Cryptotrione, involving an asymmetric Michael addition, ring-closing metathesis, and subsequent cyclopropanation, is described.     

Expedient synthesis of pyrrolo[1,2-a]indoles: preparation of the core of yuremamine

Pyrrolo[1,2- a]indoles are conveniently prepared from tetrahydro-1,2-oxazines, which in turn are generated through the reaction of nitrones with 1,1-cyclopropanediesters. The synthetic route proves to be highly diastereoselective and provides access to the core of the recently discovered pyrrolo[1,2- a]indole natural product yuremamine.     

Expedient synthesis of 1,2,3-triazole-fused tetracyclic compounds by intramolecular Huisgen (‘click’) reactions on carbohydrate-derived azido-alkynes

An efficient, practical and convenient synthesis of 1,2,3-triazole-fused tetracyclic compounds was achieved by intramolecular 1,3-dipolar cycloaddition of carbohydrate-derived azido-alkynes.     

Synthesis of the alkenyl-substituted tetracyclic core of the bisabosquals

HCl-catalyzed deprotection and cyclization of benzylic alcohol 15 cleanly provided tricycle 16 by a cis-selective intramolecular Diels-Alder reaction. Acetylation of the phenol, bis epoxidation, and base-catalyzed hydrolysis and cyclization afforded tetracycle 19 with the bisabosqual skeleton, but the opposite stereochemistry at the tertiary alcohol stereocenter. Selective dehydration of the tertiary alcohol to form the exocyclic alkene, ozonolysis, reductive deoxygenation of the side chain epoxide, and addition of MeMgBr to the ketone from the less hindered face gave tertiary alcohol 24 with the tetracyclic core of bisabosqual A (1).     

Synthesis of the alkenyl-substituted tetracyclic core of the bisabosquals

HCl-catalyzed deprotection and cyclization of benzylic alcohol 15 cleanly provided tricycle 16 by a cis-selective intramolecular Diels-Alder reaction. Acetylation of the phenol, bis epoxidation, and base-catalyzed hydrolysis and cyclization afforded tetracycle 19 with the bisabosqual skeleton, but the wrong stereochemistry at the tertiary alcohol. Selective dehydration of the tertiary alcohol to form the exocyclic alkene, ozonolysis, reductive deoxygenation of the side chain epoxide, and addition of MeMgBr to the ketone from the less hindered face gave tertiary alcohol 24 with the tetracyclic core of bisabosqual A (1).     

A convenient synthesis of a novel fused tetracyclic heterocoumarin

New tetracyclic 6H‐[1]benzopyrano[3,4‐e]pyrazolo[1,5‐a]pyrimidin‐6‐ones ( 4a‐e ) have been synthesized through the condensation under acidic conditions of [1]benzopyrano[4,3‐c][1,5]‐benzodiazepin‐7(8H)‐one ( 1 ) and a series of 3,4‐disubsituted 5‐amino‐1H‐pyrazoles 3a‐e .