“…513, 514 Debromoflustramines A 754 and B 755, the latter of which is a metabolite of Flustra foliacea, 515 have been synthesized using a strategy that can be applied to other alkaloids in this group. 516 A detailed account of the convergent synthesis of bryostatin 2 756, which is a cytotoxic macrolide from Bugula neritina, 517 has been published. 518…”
“…513, 514 Debromoflustramines A 754 and B 755, the latter of which is a metabolite of Flustra foliacea, 515 have been synthesized using a strategy that can be applied to other alkaloids in this group. 516 A detailed account of the convergent synthesis of bryostatin 2 756, which is a cytotoxic macrolide from Bugula neritina, 517 has been published. 518…”
“…[143,144] A conjugate addition of a prenylmagnesium bromide specie to 2-hydroxyindolenines 130 to give the C 3 -epimeric lactone 131. Decyanation of the resulting acyano-g-lactones with wet alumina in refluxing THF, followed by N-deprotection and allylation, gave compounds 133, which, upon N-methyl insertion under the appropriate conditions, afforded the desired target natural compounds.…”
Research on natural products containing hexahydropyrrolo[2,3-b]indole (HPI) has dramatically increased during the past few years. Newly discovered natural products with complex structures and important biological activities have recently been isolated and synthesized. This review summarizes the structures, biological activities, and synthetic routes for natural compounds containing HPI, emphasizing the different strategies for assembling this motif. It covers a broad range of molecules, from small alkaloids to complex peptides.
“…The stereochemistry of the epimers was determined by silylating the intermediate enolate with excess TBDMSCl (3 equiv), resulting in two conjugate addition products (42% yield, 20a : 21 , 6.8:1 ratio) and the aromatic nitrile 22 (11% yield). The diastereomers were separated by radial chromatography, providing pure samples of the major and minor isomers resulting from chelation-controlled and uncomplexed conjugate addition reactions, respectively (vide infra). 2 …”
Addition of Grignard reagents to the unsaturated oxo−nitrile 10 (4-hydroxy-4-methyl-6-oxocyclohex-1-enecarbonitrile) provides conjugate addition products with virtually complete stereocontrol.
Mechanistic evidence supports a chelation-controlled conjugate addition via alkylmagnesium
alkoxide intermediates. Diverse Grignard reagents having sp3-, sp2-, and sp-hybridized carbons
react with comparable efficiency, with even sterically demanding nucleophiles adding with complete
stereocontrol. Unsaturated oxo−nitriles that are incapable of chelation afford diastereomeric
conjugate addition products through an unusual boatlike transition state. Collectively, these
reactions illustrate the complementary stereoselectivity of chelation-controlled conjugate additions
to hydroxylated, unsaturated oxo−nitriles and stereoelectronically controlled conjugate additions
to enones.
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