Cascade Syntheses Routes to the Centrocountins

The Pictet-Spengler Reaction Updates Its Habits

The Pictet-Spengler reaction (P-S) is one of the most direct, efficient, and variable synthetic method for the construction of privileged pharmacophores such as tetrahydro-isoquinolines (THIQs), tetrahydro-β-carbolines (THBCs), and polyheterocyclic frameworks. In the lustro (five-year period) following its centenary birthday, the P-S reaction did not exit the stage but it came up again on limelight with new features. This review focuses on the interesting results achieved in this period (2011-2015), analyzing the versatility of this reaction. Classic P-S was reported in the total synthesis of complex alkaloids, in combination with chiral catalysts as well as for the generation of libraries of compounds in medicinal chemistry. The P-S has been used also in tandem reactions, with the sequences including ring closing metathesis, isomerization, Michael addition, and Gold- or Brønsted acid-catalyzed N-acyliminium cyclization. Moreover, the combination of P-S reaction with Ugi multicomponent reaction has been exploited for the construction of highly complex polycyclic architectures in few steps and high yields. The P-S reaction has also been successfully employed in solid-phase synthesis, affording products with different structures, including peptidomimetics, synthetic heterocycles, and natural compounds. Finally, the enzymatic version of P-S has been reported for biosynthesis, biotransformations, and bioconjugations.

Aza-Annulation of 1,2,3,4-Tetrahydro-β-carboline Derived Enaminones and Nitroenamines: Synthesis of Functionalized Indolizino[8,7-b]indoles, Pyrido[1,2-a:3,4-b']diindoles, Indolo[2,3-a]quinolizidine-4-ones and Other Tetrahydro-β-carboline Fused Heterocycles

Aza-annulation of novel 1,2,3,4-tetrahydro-β-carboline derived enaminones and nitroenamines with various 1,2- and 1,3-bis electrophiles, such as oxalyl chloride, maleic anhydride, 1,4-benzoquinone, 3-bromopropionyl chloride, itaconic anhydride, and imines (from formaldehyde and primary amines), has been investigated. These methodologies provide simple one-step pathways for efficient construction of highly functionalized tetrahydro-β-carboline 1,2-fused, five- and six-membered heterocyclic frameworks, such as indolizino[8,7-b]indoles, pyrido[1,2-a:3,4-b']diindoles, indolo[2,3-a]quinolizidines, and pyrimido[1',6':1,2]pyrido[3,4-b]indoles, which are core structures of many naturally occurring indole alkaloids with diverse bioactivity.

The 6π-azaelectrocyclization of azatrienes. Synthetic applications in natural products, bioactive heterocycles, and related fields

Covering: 2006 to 2018 The application of the 6π-azaelectrocyclization of azatrienes as a key strategy for the synthesis of natural products, their analogs and related bioactive or biomedically-relevant compounds (from 2006 to date) is comprehensively reviewed. Details about reaction optimization studies, relevant reaction mechanisms and conditions are also discussed.

Cascade reaction based synthetic strategies targeting biologically intriguing indole polycycles

In this review, cascade reaction based synthesis strategies delivering biologically intriguing indole polycycles are presented.

One-Pot Synthesis of Chiral Tetracyclic Dibenzo[ b, f][1,4]oxazepine-Fused 1,2-Dihydropyridines (DHPs) under Metal-Free Conditions

An efficient protocol for the catalytic asymmetric synthesis of new dibenzo[ b, f][1,4]-oxazepine-fused 1,2-dihydropyridines (DHPs) has been described under metal-free conditions. This reaction proceeds through proline-catalyzed direct Mannich/cyclization between seven-membered dibenzo[ b, f][1,4]-oxazepine-imines and aqueous glutaraldehyde, followed by IBX-mediated site-selective dehydrogenative oxidation in one-pot operation with high yields (up to 92%) and excellent enantioselectivity (up to >99:1 er).

Convenient construction of tetrahydrochromeno[4',3':2,3]indolizino[8,7-b]indoles and tetrahydroindolizino[8,7-b]indoles via one-pot domino reaction

The functionalized tetrahydrochromeno[4′,3′:2,3]indolizino[8,7-b]indoles were conveniently synthesized in high yields by one-pot domino reaction of tryptamines, alkyl propiolates and 2-aryl-3-nitro-2H-chromenes. Under similar conditions, the one-pot reaction of tryptamines, alkyl propiolates and β-nitroalkenes resulted in functionalized tetrahydroindolizino[8,7-b]indoles. The reaction mechanism involved sequential generation of β-enamino ester, Michael addition, Pictet–Spengler reaction and annulation process. The reaction showed high atomic economy and met the goals of sustainable chemistry.

The functionalized tetrahydrochromeno[4′,3′:2,3]indolizino[8,7-b]indoles were conveniently synthesized in high yields by one-pot domino reaction of tryptamines, alkyl propiolates and 2-aryl-3-nitro-2H-chromenes.

Synthesis of Novel Benzochromenes Using Triarylphosphines, Alkyl X-Phenylpropiolates and 2-Hydroxy-1-Naphthaldehyde

The reactions of a series of alkyl X-phenylpropiolates with 2-hydroxy-1-naphthaldehyde and triphenylphosphine led to benzochromenes in moderate yields. When the reactions were performed in the presence of bis(4-methoxyphenyl)phenylphosphine instead of triphenylphosphine, the rate and yields of the reactions increased. Reaction yields were enhanced for alkyl X-phenylpropiolates when X was an electron-deficient substituent such as CF3 or Cl, thus showing that the electronic effect of substituents plays an important role in the reactions.

Metal-Free Cascade Approach toward Polysubstituted Indolizines from Chromone-Based Michael Acceptors

An efficient cascade transformation toward indolizine-based molecules has been developed. This process leads to the rapid construction of two C-N bonds and one C-C bond without the need of any metal catalysis. The approach involves easily accessible chromone-based Michael acceptors and propargylamine derivatives as starting materials. This cascade constitutes a novel and very competitive alternative to the well reported strategies using pyridine or pyrrole derivatives for accessing the indolizine ring with substituents at uncommon C-positions.

A general catalytic reaction sequence to access alkaloid-inspired indole polycycles

A general two-step catalytic reaction sequence affording a range of indole alkaloid-inspired complex molecular frameworks is presented.

Facile construction of novel heterocyclic compounds: three-component, one-pot synthesis of 2-hydroxybenzoyl-1,2-dihydropyridine-3-carboxylates, ketones, pyridone-3-carboxylates and benzopyrido-1,3-oxazole-4-carboxylates

A facile method has been developed for the preparation of novel heterocyclic compounds facilitated by 3-oxobutanoates and 1,3-diketones possessing a trihalo group. This three-component, one-pot protocol provided heterocyclic compounds without a catalyst.

The asymmetric hetero-Diels-Alder reaction in the syntheses of biologically relevant compounds

The hetero-Diels-Alder reaction is one of the most powerful transformations in the chemistry toolbox for the synthesis of aza- and oxa-heterocycles embodying multiple stereogenic centers. However, as compared to other cycloadditions, in particular the dipolar cycloadditions and the Diels-Alder reaction, the hetero-Diels-Alder reaction has been much less explored and exploited in organic synthesis. Nevertheless, this powerful transformation has opened up efficient and creative routes to biologically relevant small molecules and different natural products which contain six-membered oxygen or nitrogen ring systems. Recent developments in this field, in particular in the establishment of enantioselectively catalyzed hetero-Diels-Alder cycloadditions steered by a plethora of different catalysts and the application of the resulting small molecules in chemical biology and medicinal chemistry research, are highlighted in this Minireview.

An enantioselective inverse-electron-demand imino Diels-Alder reaction

The imino Diels-Alder reaction is an efficient method for the synthesis of aza-heterocycles. While different stereo- and enantioselective inverse-electron-demand imino Diels-Alder (IEDIDA) reactions have been reported before, IEDIDA reactions including electron-deficient dienes are unprecedented. The first enantioselective IEDIDA reaction between electron-poor chromone dienes and cyclic imines, catalyzed by zinc/binol complexes is described. The novel reaction provides a facile entry to a natural product inspired collection of ring-fused quinolizines including a potent modulator of mitosis.

One-pot synthesis of 6,11-dihydro-5H-indolizino[8,7-b]indoles via sequential formation of β-enamino ester, Michael addition and Pictet–Spengler reactions

One pot sequential reaction of generation of β-enamino ester, Michael addition and Pictet–Spengler reaction.

Discovery of inhibitors of the Wnt and Hedgehog signaling pathways through the catalytic enantioselective synthesis of an iridoid-inspired compound collection

Cousins you can count on: An iridoid-inspired compound collection was synthesized efficiently by the resolution of cyclic enones in an asymmetric cycloaddition with azomethine ylides. The collection contained novel potent inhibitors of the Wnt and Hedgehog signaling pathways.