A stannous chloride-induced deacetalisation–cyclisation process to prepare the ABC ring system of 'upenamide

Traceless Solid-Phase Synthesis of [6,7,8 + 5,6,7]-Fused Molecular Frameworks

We report two synthetic strategies for traceless solid-phase synthesis of molecular scaffolds comprising 6- to 8-membered rings fused with 5- to 7-membered rings. Traceless synthesis facilitated preparation of target molecules without any trace of polymer-supported linkers. The cyclization proceeded via acid-mediated tandem N-acylium ion formation followed by the nucleophilic addition of O- and C-nucleophiles. The presented synthetic strategy enabled, through the use of simple building blocks without any conformational preferences, the evaluation of the predisposition of different combinations of ring sizes to form fused ring molecular scaffolds. Compounds with any combination of [6,7 + 5,6,7] ring sizes were accessible with excellent crude purity. The 8-membered cyclic iminium was successfully fused only with the 5-membered cycle and larger fused ring systems were not formed, probably due to their instability.

A concise Diels-Alder strategy leading to congeners of the ABC ring system of the marine alkaloid 'upenamide

A second-generation approach to the BC spirocycle of 'upenamide is reported. Central to the synthesis is an endo selective Diels-Alder reaction between 1-(t-butyldimethylsiloxy)-1,3-butadiene and bromomaleic anhydride followed by a radical mediated allylation of the ring fusion bromide. Functional group manipulation provides three (9-11) advanced synthetic intermediates ready for coupling with the remaining half (DE bicycle) of 'upenamide.

Expedient metal-free synthesis of 1,3-oxazinen-4-ones

1,3-Oxazinen-4-ones are medicinally important scaffolds which have traditionally been accessed using a hetero-Diels-Alder approach or more recently using a cobalt-catalyzed three-component cycloaddition. Herein we report a novel strategy to access this scaffold which allows for the rapid and high yielding synthesis of 1,3-oxazinen-4-ones under ambient temperature and pressures with improved substrate scope.

Direct imine acylation for molecular diversity in heterocyclic synthesis

Imines and carboxylic acids have been directly coupled using propylphosphonic acid anhydride and NEt(i-Pr)2 to give N-acyliminium ions, which were intramolecularly trapped with oxygen, nitrogen, sulfur, and carbon nucleophiles to provide a wide range of structurally diverse heterocycles.

Direct imine acylation: synthesis of the proposed structures of 'upenamide

The synthesis of the two proposed structures of macrocyclic marine natural product 'upenamide is reported. The key step utilizes direct imine acylation (DIA) with a protected β-hydroxy acid to construct the key tricyclic ABC ring system. The macrocyclization was completed in the final step using a Stille cross-coupling reaction.

The First Synthesis of the ABC-Ring System of ‘Upenamide

The first synthetic route to the spirooxaquinolizidinone core (ABC core) of the macrocyclic marine alkaloid 'upenamide (1) has been developed. All five stereocenters were introduced with complete stereocontrol. The hydroxyl group at C-11 was introduced by a regio- and stereoselective SeO(2)-mediated allylic oxidation. The spirocyclic skeleton was formed by a stannous chloride induced deacetalization-bicyclization procedure. Further stereocenters were introduced by an enzymatic desymmetrization and by incorporation of an (S)-malic acid derived building block.