Beta-phenylethylamines and the isoquinoline alkaloids

Base-Promoted Synthesis of Isoquinolines through a Tandem Reaction of 2-Methyl-arylaldehydes and Nitriles

A convenient method for preparing 3-aryl isoquinolines via a base-promoted tandem reaction is presented. Simply combining commercially available 2-methyl-arylaldehydes, benzonitriles, NaN(SiMe3)2, and Cs2CO3 enabled the synthesis of a variety of isoquinolines (23 examples, ≤90% yield). Among the syntheses of isoquinolines, the transition metal-free method described here is straightforward, practical, and operationally simple.

Electrocatalysis as a key strategy for the total synthesis of natural products

Electrochemical strategies have been a powerful approach for the synthesis of valuable intermediates, in particular heterocyclic motifs. Because of the mild nature, a wide range of nonclassical bond disconnections have been achieved via in situ-generated radical intermediates in a highly efficient manner. In particular, anodic electrochemical oxidative strategies have been utilized for the total synthesis of many structurally intriguing natural products. In this review article, we have discussed a number of total syntheses of structurally intriguing alkaloids and terpenoids in which electrochemical processes play an important role as a key methodology.

Copper-Catalyzed Aza-Sonogashira Cross-Coupling To Form Ynimines: Development and Application to the Synthesis of Heterocycles

Nitrogen‐substituted alkynes, such as ynamines and ynamides, are versatile synthetic building blocks. Ynimines bearing additional nucleophilic and electrophilic centers relative to ynamines and ynamides are expected to have high synthetic potential. However, their chemical reactivity remains unexplored owing mainly to the lack of synthetic accessibility. We report herein a versatile copper‐catalyzed synthesis of ynimines from readily available O‐acetyl ketoximes and terminal alkynes. A wide range of O‐acetyl ketoximes derived from diaryl ketones, aryl alkyl ketones and dialkyl ketones underwent cross‐coupling with a diverse set of terminal alkynes to afford the ynimines in good to excellent yields. An unprecedented [5+1] heteroannulation reaction exploiting the reactivity of the ynimine generated in situ was subsequently developed for the synthesis of medicinally important heterocycles, including isoquinolines, azaindoles, azabenzofurans, azabenzothiophenes and carbolines.

Recent advances in directed sp2 C-H functionalization towards the synthesis of N-heterocycles and O-heterocycles

Heterocyclic compounds are widely present in the core structures of several natural products, pharmaceuticals and agrochemicals, and thus great efforts have been devoted to their synthesis in a mild and simpler way. In the past decade, remarkable progress has been made in the field of heterocycle synthesis by employing C-H functionalization as an emerging synthetic strategy. As a complement to previous protocols, transition metal catalyzed C-H functionalization of arenes using various directing groups has recently emerged as a powerful tool to create different classes of heterocycles. This review is mainly focussed on the recent key progress made in the field of the synthesis of N,O-heterocycles from olefins and allenes by using nitrogen based and oxidizing directing groups.

Rh(III)-Catalyzed three-component cascade annulation to produce the N-oxopropyl chain of isoquinolone derivatives

Developing powerful methods to introduce versatile functional groups at the N-substituents of isoquinolone scaffolds is still a great challenge. Herein, we report a novel three-component cascade annulation reaction to efficiently construct the N-oxopropyl chain of isoquinolone derivatives via rhodium(iii)-catalyzed C-H activation/cyclization/nucleophilic attack, with oxazoles used both as the directing group and potential functionalized reagents.

Rhodium-catalyzed synthesis of substituted isoquinolones via a selective decarbonylation/alkyne insertion cascade of phthalimides

A rhodium-catalyzed decarbonylation/alkyne insertion cascade of phthalimides has been established. The reaction can be carried out in an operationally simple manner and provides expedient access to a series of isoquinolones in moderate to good yields. This reaction proceeded through a sequential decarbonylation/alkyne insertion/intramolecular annulation procedure and featured good functional group tolerance, ample substrate scope, and the construction of C-C and C-N bonds in one pot.

Nickel-catalyzed oxidative C-H/N-H annulation of N-heteroaromatic compounds with alkynes

The reaction of N-heteroaromatic compounds, such as 2-aryl-pyrrole, benzimidazole, imidazole, indole, and pyrazole derivatives, with alkynes in the presence of a catalytic amount of a nickel complex results in C-H/N-H oxidative annulation. The reaction shows a high functional group compatibility. While both Ni(0) and Ni(ii) complexes show a high catalytic activity, Ni(0) is proposed as a key catalytic species in the main catalytic cycle. In the case of the Ni(ii) system, the presence of a catalytic amount of a strong base, such as KOBu t , is required for the reaction to proceed. In sharp contrast, a base is not required in the case of the Ni(0) system. The proposed mechanism is supported by DFT studies.

Cobalt-catalyzed cyclization of benzamides with alkynes: a facile route to isoquinolones with hydrogen evolution

The reaction of benzamides with alkynes assisted by an 8-aminoquinoline ligand in the presence of Co(OAc)2·4H2O and pivalic acid under an air atmosphere provided isoquinolone derivatives in good to excellent yields. In this reaction, the active Co(iii) species is regenerated by the reaction of Co(i) species with pivalic acid under an air atmosphere with hydrogen evolution. The proposed mechanism was supported by competition experiments, deuterium labelling studies, radical scavenger experiments and kinetic studies.

Co-catalyzed ortho-C–H functionalization/annulation of arenes and alkenes with alkynylsilanes: access to isoquinolone and pyridone motifs

A method for cobalt-catalyzed ortho-C–H functionalization annulation of arenes and alkenes with alkynylsilanes assisted by 8-aminoquinolyl auxiliary has been described. Alkynylsilanes were employed as the coupling partners to react with a broad range of benzamides and acrylamides, affording the corresponding isoquinolone and pyridone derivatives in moderate to high yields. It is worth noting that the silyl group in the final products can be retained or removed by switching the reaction conditions.

A method for cobalt-catalyzed ortho-C–H functionalization annulation of arenes and alkenes with alkynylsilanes assisted by 8-aminoquinolyl auxiliary.

Iron/zinc-catalyzed benzannulation reactions of 2-(2-oxo-alkyl)benzketones leading to naphthalene and isoquinoline derivatives

An efficient method for the synthesis of poly-substituted naphthalenes/isoquinolines via Fe(iii)/Zn(ii)-catalyzed [4 + 2] cycloaddition of 2-(2-oxo-alkyl)benzketones is described.

N-Tosylhydrazone directed annulation via C–H/N–N bond activation in Ru(ii)/PEG-400 as homogeneous recyclable catalytic system: a green synthesis of isoquinolines

A green and sustainable methodology for the synthesis of isoquinolines using Ru(ii)/PEG-400 as homogeneous recyclable catalytic system and N-tosylhydrazone, a rarely explored directing group has been reported.

Recent advances in the Overman rearrangement: synthesis of natural products and valuable compounds

This review documents the reports since 2005 on the Overman rearrangement, an important C–N bond forming reaction that has been profoundly used in the synthesis of natural products, synthetic intermediates, building blocks and valuable compounds.

Expeditious Synthesis of Isoquinolones and Isocoumarins with a Vinyl Borane as an Acetylene Equivalent

An innovative and simple expeditious synthesis of 3,4-unsubstituted isoquinolones and isocoumarins starting from safe and easy to handle two-carbon acetylene equivalent was developed. The synthetic potential of this new method was further demonstrated in the facile total synthesis of two naturally occurring alkaloids: corydaldine and doryanine.

Engineering biosynthesis of the anticancer alkaloid noscapine in yeast

Noscapine is a potential anticancer drug isolated from the opium poppy Papaver somniferum, and genes encoding enzymes responsible for the synthesis of noscapine have been recently discovered to be clustered on the genome of P. somniferum. Here, we reconstitute the noscapine gene cluster in Saccharomyces cerevisiae to achieve the microbial production of noscapine and related pathway intermediates, complementing and extending previous in planta and in vitro investigations. Our work provides structural validation of the secoberberine intermediates and the description of the narcotoline-4'-O-methyltransferase, suggesting this activity is catalysed by a unique heterodimer. We also reconstitute a 14-step biosynthetic pathway of noscapine from the simple alkaloid norlaudanosoline by engineering a yeast strain expressing 16 heterologous plant enzymes, achieving reconstitution of a complex plant pathway in a microbial host. Other engineered yeasts produce previously inaccessible pathway intermediates and a novel derivative, thereby advancing protoberberine and noscapine related drug discovery.

Cobalt-Catalyzed Cyclization of N-Methoxy Benzamides with Alkynes using an Internal Oxidant through C-H/N-O Bond Activation

The cyclization of substituted N-methoxy benzamides with alkynes in the presence of an easily affordable cobalt complex and NaOAc provides isoquinolone derivatives in good to excellent yields. The cyclization reaction is compatible with a range of functional group-substituted benzamides, as well as ester- and alcohol-substituted alkynes. The cobalt complex [Co(III) Cp*(OR)2 ] (R=Me or Ac) serves as an efficient catalyst for the cyclization reaction. Later, isoquinolone derivatives were converted into 1-chloro and 1-bromo substituted isoquinoline derivatives in excellent yields in the presence of POCl3 or PBr3 .

RhI/RhIII catalyst-controlled divergent aryl/heteroaryl C-H bond functionalization of picolinamides with alkynes

The ability to establish switchable site-selectivity through catalyst control in the direct functionalization of molecules that contain distinct C-H bonds remains a demanding challenge that would enable the construction of diverse scaffolds from the same starting materials. Herein we describe the realization of this goal, namely a divergent heteroaryl/aryl C-H functionalization of aromatic picolinamide derivatives, targeting two distinct C-H sites, either at the pyridine ring or at the arene unit, to afford isoquinoline or ortho-olefinated benzylamine (or phenethylamine) derivatives. This complementary reactivity has been achieved on the basis of a RhIII/RhI switch in the catalyst, resulting in different mechanistic outcomes. Notably, a series of experimental and DFT mechanistic studies revealed important insights about the mechanism of the reaction and reasons behind the divergent regiochemical outcome.

A room-temperature protocol to access isoquinolines through Ag(i) catalysed annulation of o-(1-alkynyl)arylaldehydes and ketones with NH4OAc: elaboration to berberine and palmatine

A silver catalysed protocol for the synthesis of a wide range of isoquinolines from o-(1-alkynyl)arylaldehydes has been developed under mild conditions and elaborated to the synthesis of berberine and palmatine.

Preparation of amino-substituted indenes and 1,4-dihydronaphthalenes using a one-pot multireaction approach: total synthesis of oxybenzo[c]phenanthridine alkaloids

Allylic trichloroacetimidates bearing a 2-vinyl or 2-allylaryl group have been designed as substrates for a one-pot, two-step multi-bond-forming process leading to the general preparation of aminoindenes and amino-substituted 1,4-dihydronaphthalenes. The synthetic utility of the privileged structures formed from this one-pot process was demonstrated with the total synthesis of four oxybenzo[c]phenanthridine alkaloids, oxychelerythrine, oxysanguinarine, oxynitidine, and oxyavicine. An intramolecular biaryl Heck coupling reaction, catalyzed using the Hermann-Beller palladacycle was used to effect the key step during the synthesis of the natural products.

Protecting-group-free total synthesis of isoquinoline alkaloids by nickel-catalyzed annulation of o-halobenzaldimine with an alkyne as the key step

An efficient short total synthesis of benzo[c]phenanthridine alkaloids including oxyavicine, oxynitidine, and oxysanguinarine is described. Thus, N-methyl-o-bromobenzaldimines 1 b-d undergo regioselective cyclization with 4-(benzo[d][1,3]dioxol-5-yl)but-3-yn-1-ol (2 b) in the presence of [Ni(cod)(2)] (cod=1,5-cyclooctadiene). In situ oxidation of the resultant isoquinolinium salts gives isoquinolinone derivatives 5 b-d with benzo[d][1,3]dioxol-5-yl substitution at the C(3) atom and a (CH(2))(2)OH group at the C(4) atom. Later, oxidation of the alcohol group in 5 b-d to the aldehyde moiety followed by acid-catalyzed cyclization and dehydration completes the total syntheses to give oxyavicine, oxynitidine, and oxysanguinarine in 67, 65, and 60 % yields, respectively. The synthesis requires four steps from o-bromobenzaldehyde derivatives. Transformations of these alkaloids to the other alkaloids in this family are also discussed herein.