Pentannulation of Heterocycles by Virtue of Precious Metal Catalysis

Brønsted Acid-Catalyzed Dehydrative Nazarov-type Cyclization of CF3-Substituted 3-Indolylallyl Alcohols: Divergent Synthesis of 1-Trifluoromethylated Cyclopenta[b]indoles

An expedient and efficient synthetic method for the divergent synthesis of 1-trifluoromethylated cyclopenta[b]indoles that relies on Brønsted acid-catalyzed dehydrative Nazarov-type cyclization of CF3-substituted 3-indolylallyl alcohols is described. Two classes of 1-trifluoromethylated cyclopenta[b]indoles can be easily accessed simply by changing the NH-protecting group of indoles. With arylsulfonyl protected 3-indolylallyl alcohols as starting materials, the reaction provided the arylsulfonyl protected 1-trifluoromethylated cyclopenta[b]indoles in good to excellent yields, whereas pivaloyl (Piv) protected substrates led to the formation of NH-free 1-trifluoromethylated cyclolopenta[b]indoles with another alkenyl isomer. This protocol features tunable chemoselectivity, operational simplicity, excellent functional group compatibility, and mild metal-free conditions.

Phosphine-catalysed denitrative rearomatising (3 + 2) annulation of α,β-ynones and 3-nitroindoles

We describe a metal-free strategy to access various α-arylidene cyclopenta[b]indoles via phosphine-catalysed (3 + 2) annulation of α,β-ynones and 3-nitroindoles. For the first time, the rearomatisation of the indole nucleus was observed in such an annulative transformation. The method was extended to the synthesis of an antimalarial natural product, bruceolline E.

Cycloisomerization of π-Coupled Heteroatom Nucleophiles by Gold Catalysis: En Route to Regiochemically Defined Heterocycles

Since the dawn of millennium, catalytic gold chemistry is at the forefront to set off diverse organic reactions via unique activation of π-bonded molecules. Within this purview, cycloisomerization of heteroatom nucleophiles linked to a π-system is one of the well recognized chemistry for the construction of numerous heterocyclic cores. Though the rudimentary aspects of this transformation are reviewed by several groups in different timeline, a holistic view on regiochemistry of such reactions went largely overlooked. Hence, this account emphasizes the gold catalyzed regioselective cycloisomerization of structurally distinctive π-connected hetero-nucleophiles leading to different heterocycles documented in the last two decades. From an application perspective, this account also highlights those methodologies which find a role in the total synthesis of natural products. Wherever appropriate, mechanistic details and contributing factors for selectivity are also discussed.

Palladium-catalyzed diastereo- and enantioselective formal [3 + 2] cycloaddition of vinyl cyclopropanes with cyclic 1-azadienes

We have developed highly efficient and stereoselective Pd-catalyzed formal [3 + 2] cycloadditions of vinyl cyclopropanes with cyclic 1-azadienes.

Organocatalytic Strategies for the Synthesis of Cyclopenta-Fused Arenes and Heteroarenes

Cyclopentanoids are omnipresent in natural products and pharmaceutically relevant compounds. Among them, cyclopenta-fused arenes and heteroarenes possess impressive biological properties and play significant role in materials science. Consequently, several notable methods have been developed for their synthesis over the years. In this review, we mainly described metal-free and organocatalytic approaches that led to the construction of pentannulated arenes and heteroarenes.

Recent metal-catalysed approaches for the synthesis of cyclopenta[b]indoles

The cyclopenta[b]indole scaffold is ubiquitously present in several bioactive natural products and pharmaceutically interesting compounds. Of the numerous methods known for the synthesis of cyclopenta-fused indoles, this review highlights only the metal-catalysed approaches reported from the year 2015 onwards. This review encompasses our own efforts leading to the synthesis of cyclopentannulated indoles, in addition to the seminal contributions of several other researchers.

Recent advances in the synthesis of analogues of phytohormones strigolactones with ring-closing metathesis as a key step

In this paper, we synthesized and evaluated the biological activity of structural analogues of natural strigolactones in which the butenolide D-ring has been replaced with a γ-lactam. The key step to obtain the α,β-unsaturated-γ-lactam was an RCM on suitably substituted amides. Strigolactones (SLs) are plant hormones with various developmental functions. As soil signaling chemicals, they are required for establishing beneficial mycorrhizal plant/fungus symbiosis. Beside these auxinic roles, recently SLs have been successfully investigated as antitumoral agents. Peculiar to the SL perception system is the enzymatic activity of the hormone receptor. SARs data have shown that the presence of the butenolide D-ring is crucial to retain the biological activity. The substitution of the butenolide with a lactam might shed light on the mechanism of perception. In the following, a dedicated in silico study suggested the binding modes of the synthesized compounds to the receptor of SLs in plants.

Brønsted Acid-Mediated Cycloaromatization of 1H-Indol-2-yl Propargyl Benzoates to 7H-Benzo[c]carbazoles

A synthetic method for the efficient assembly of benzo[c]carbazole derivatives that relies on silica gel-activated benzoic acid-mediated cycloaromatization of 1H-indol-2-yl propargyl benzoates under atmospheric conditions is described. Robust with a variety of substitution patterns tolerated, the reaction provides a one-step strategy to construct a member of the N-heterocycles family in good to excellent yields. A tentative mechanism is proposed in which the cycloaromatization process is thought to involve a Brønsted acid-mediated formal 1,3-acyloxy migration/6π-electrocyclization pathway.