Stereodivergent Synthesis of Spiroaminals via Chiral Bifunctional Hydrogen Bonding Organocatalysis

Spiroaminals represent novel structural motifs prevalent in diverse natural products and biologically active molecules. Achieving their enantioselective synthesis is a highly desirable and challenging task in synthetic endeavors due to their intricate molecular frameworks. Herein, we accomplished the first stereodivergent construction of spiroaminals using chiral bifunctional organocatalyzed intramolecular 1,2-addition followed by an oxa-Michael addition cascade in a high atom and step economical pathway. A proper modulation of the cinchona-derived squaramide catalysts efficiently provided access to all the possible stereoisomers with high yield, diastereoselectivity, and excellent enantioselectivity while displaying a broad substrate tolerance. Additionally, we validated the scalability of the reaction and demonstrated the synthesis of variable spiroaminal scaffolds, confirming the viability of our protocol.

Enantioselective synthesis of spiro-N,O-ketals via iridium and Brønsted acid co-catalyzed asymmetric formal [4+2] cycloaddition

We present an iridium and Brønsted acid co-catalyzed enantioselective formal [4+2] cycloaddition reaction of cyclic enamides with 2-(1-hydroxyallyl)phenols. This method yields a wide range of N-unsubstituted spiro-N,O-ketals, with good efficiency (up to 94%) and excellent enantioselectivities (most >95% ee). The protocol features easy scale-up and facile product derivatization.

Facile synthesis of the spiro-pyridoindolone scaffold via a gold-catalysed intramolecular alkynol cyclisation/hydroindolylation

A simple approach for the synthesis of pyridoindolone scaffolds with a spiroannulated tetrahydrofuran ring is described. The overall process comprises intramolecular sequential gold-catalysed 5-endo-dig alkynol cycloisomerization and subsequent addition of indole C2 to the in situ generated oxocarbenium cation.

Preparation and Application of α-Imino Ketones through One-Pot Tandem Reactions Based on Heyns Rearrangement

α-Imino ketone is a useful building block for the preparation of α-amino ketones and α-amino alcohols. However, its preparation has been seldomly seen. Herein, a metal-free and operationally simple strategy has been developed to generate α-imino ketones with high regioselectivity. Meanwhile, the method allowed for the preparation of various N,O-ketals with high regioselectivities and diastereoselectivities through cascade reactions in one pot.

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.

Ruthenium-Catalyzed Intramolecular Double Hydrofunctionalization of Alkynes. Synthesis of Spirocyclic Hemiaminal Ethers and Their Lewis Acid-Mediated Cleavage/Nucleophilic Addition

[RuCl₂(p-cymene)]₂/AgNO₃-catalyzed intramolecular double hydrofunctionalization of internal alkynes having nitrogen and oxygen nucleophilic groups at appropriate positions provided a series of spirocyclic hemiaminal ether derivatives in good to excellent yields. The product spiro-hemiaminal ethers underwent Lewis acid-mediated chemoselective cleavage, and in situ-generated iminium/oxocarbenium ions could be trapped with nucleophiles to afford a range of nitrogen and oxygen heterocycles.

Strategies and methodologies for the construction of spiro-γ-lactams: an update

This review gives an insight into the most recent synthetic methodologies towards spiro-γ-lactams, a class of compounds that are present in a wide range of synthetic bioactive and naturally occurring molecules.

An Apparent Umpolung Reactivity of Indole through [Au]-Catalysed Cyclisation and Lewis-Acid-Mediated Allylation

The sequential functionalization of indole C2 and C3 in an umpolung fashion was executed with a predesigned substrate and choice of reagents. The developed method comprises gold-catalysed alkynol cycloisomerisation/intramolecular addition of C2 of indole and subsequent BF₃ ⋅OEt₂ -mediated regioselective C3 allylation, resulting in the synthesis of the functionalized indoloisoquinolinone scaffold. The reaction involves 5-endo-alkynol cycloisomerisation and the dearomative addition of indole C2 to the intermediate oxocarbenium cation, which results in two equilibrating fused and spiropentacyclic intermediates, which upon treatment with allyl silane in the presence of BF₃ ⋅OEt₂ , undergo selective indole C3 allylation. Other nucleophiles, such as hydride, azide and indole, were also found to be compatible with this process.

Sustainable gold catalysis: synthesis of new spiroacetals

Various [O,O]- and [N,O]-spiroacetals were synthesized by gold-catalyzed spirocyclization of suitable functionalized alkynes. Whereas simple spiroacetals with two heteroatoms were readily obtained by regioselective cyclization of acetylenic diols or aminoalcohols, hitherto unknown spirocyclic isoxazolidines and pyrazolidines bearing three heteroatoms were formed by three-component coupling of alkynols, aldehydes, and protected hydroxylamine or hydrazine derivatives. The sustainability of these spirocyclizations was improved by using recyclable gold catalysts in water or nanomicelles as reaction medium.