RESUMO
Details in developing a stereodivergent approach to the lepadin family and establishing an entry to both C2,8a-syn and C2,8a-anti relative stereochemical manifolds through a common intermediate are described here. This works paves a foundation for constructing all members of the lepadin family, which consists of three subsets based on an array of interesting relative configurations. These efforts underline the prominence of aza-[3 + 3] annulation as a unified strategy in alkaloid synthesis.
RESUMO
[3 + 3] Annulations of exo-cyclic vinylogous amides and urethanes with vinyl iminium salts are described here. We observed an intriguing dichotomy in their reaction pathways. For pyrrolidine- and azepane-based vinylogous amides or urethanes, aza-[3 + 3] annulation would dominate to give tetrahydroindolizidines, whereas, unexpectedly, for piperidine-based vinylogous amides or urethanes, carbo-[3 + 3] annulation was the pathway, leading to hexahydroquinolines. The origin for such a contrast is likely associated with a switch in the initial reaction pathway between C-1,2-addition and C-1,4-addition.
RESUMO
A detailed account regarding a formal [3 + 3] cycloaddition method using 4-hydroxy-2-pyrones and 1,3-diketones is described here. This formal cycloaddition reaction or annulation reaction is synthetically useful for constructing 2H-pyranyl heterocycles. The usage of alpha,beta-unsaturated iminium salts is significant in controlling competing reaction pathways to give exclusively 2H-pyrans. Most significantly, experimental evidence is provided to support the mechanism of this reaction that involves a sequential Knoevenagel condensation and a reversible 6pi-electron electrocyclic ring-closure of 1-oxatrienes.
Assuntos
Química Orgânica/métodos , Cetonas/química , Piranos/síntese química , Pironas/química , Alcenos/química , Catálise , Cristalografia por Raios X , Ciclização , Iminas/química , Indicadores e Reagentes , Espectroscopia de Ressonância Magnética , Estrutura Molecular , Oxirredução , Sais/síntese química , EstereoisomerismoRESUMO
Evidence is described here to support that a highly stereoselective 6pi-electron electrocyclic ring closure of 1-azatrienes is a key step in formal [3 + 3] cycloaddition or annulation reactions of chiral vinylogous amides with alpha,beta-unsaturated iminium salts. This would represent the first highly stereoselective 6pi-electron electrocyclic ring closure of 1-azatrienes. We have also unambiguously demonstrated that these specific ring closures are reversible, leading to the major diastereomer that is also thermodynamically more stable, and that a rotation preference likely also plays a role. A synthetic application is illustrated here to stereoselectively transform the resulting dihydropyridines to cis-1-azadecalins with unique anti relative stereochemistry at C2 and C2a, leading to synthesis of epi isomers of (-)-pumiliotoxin C.
Assuntos
Alcaloides/síntese química , Venenos de Anfíbios/síntese química , Naftalenos/síntese química , Quinolinas , Alcaloides/química , Alcenos/química , Venenos de Anfíbios/química , Compostos Aza/síntese química , Compostos Aza/química , Ciclização , Naftalenos/química , EstereoisomerismoRESUMO
Complex piperidinyl heterocycles (for example, 2) were accessed by using a novel intramolecular formal [3+3] cycloaddition reaction of vinylogous amides tethered with enals (for example, 1). This method has been applied to a formal total synthesis of (+)-gephyrotoxin (3).