Synthesis of 2,3-Ring Fused Pyrroles via Cu-Catalyzed 5-exo-dig Annulation of Alkyne-Tethered Enaminones

Synthesis of pyrimidine-fused skeletons through copper-catalyzed consecutive Sonogashira coupling and aminocyclization

A copper-catalyzed sequence involving a Sonogashira coupling reaction and 5-exo-dig aminocyclization between a terminal alkyne and a 2-(2-bromophenyl)pyrimidine analog based on six-membered rings is presented. This protocol provides a controlled and modular approach to access a variety of synthetically useful pyrimidine-fused skeletons with high efficiency, broad substrate scope, and excellent functional group compatibility. Under acidic conditions, the (Z)-configuration of products spontaneously converts into (E)-9-benzylidene-1,9-dihydro-11H-pyrazolo[4',3':4,5]pyrimido[2,1-a]isoindol-11-ones.

4,5,6,7-Tetrahydroindol-4-Ones as a Valuable Starting Point for the Synthesis of Polyheterocyclic Structures

This review focuses on the synthesis of polyheterocyclic structures with a variety of medicinal and optoelectronic applications, starting from readily available 4,5,6,7-tetrahydroindol-4-one analogs. First, routes toward the 4,5,6,7-tetrahydroindol-4-one starting materials are summarized, followed by synthetic pathways towards polyheterocyclic structures which are categorized based on the size and attachment point of the newly formed (hetero)cyclic ring.

Catalytic Acceptorless Dehydrogenation of Amino Alcohols and 2-Hydroxybenzyl Alcohols for Annulation Reaction under Neutral Conditions

A base-free and acceptorless Ru-catalyzed dehydrogenative approach has been developed for the synthesis of N-heterocycles by using 1,3-dicarbonyls and amino alcohols through a domino sequential enamine formation and intramolecular oxidative cyclization strategy. This unified approach is also applicable for the synthesis of O-heterocycles involving 2-hydroxybenzyl alcohol as a coupling reactant via consecutive C-alkylation and intramolecular cyclization steps. The present protocol is general for the synthesis of varieties of biologically important scaffolds, such as tetrahydro-4H-indol-4-one, 3,4-dihydroacridin-1(2H)-one, and tetrahydro-1H-xanthen-1-ones derivatives using a single catalytic system, viz. RuH₂CO(PPh₃)₃. Environmentally benign H₂O and H₂ are the only byproducts in this domino process. Moreover, RuH₂CO(PPh₃)₃-catalyzed C3-alkylation of tetrahydro-4H-indol-4-one using alcohol as a alkylating partner is also described in this report. For the first time, a solvent-free gram-scale reaction for the acceptorless dehydrogenative annulation has been demonstrated. A plausible mechanism for the Ru-catalyzed base-free and acceptorless dehydrogenative annulation of amino alcohols or 2-hydroxybenzyl alcohols has been provided with several experimental investigations and spectroscopic evidence.

Pd-catalyzed arylation/aza-Michael addition cascade to C2-spiroindolines and azabicyclo[3.2.2]nonanones

A palladium-catalyzed arylation/aza-Michael addition cascade reaction of β-substituted cyclic enones and 2-haloanilines has been reported. Using 1 mol% Pd(PPh₃)₄ as a catalyst, C2-spiroindolines are accessed via an intermolecular vinylogous arylation of β-alkyl cyclic enones and 2-haloanilines followed by an intramolecular aza-Michael addition. The functional group tolerance of this transformation is examined by 18 examples in up to 93% yield. In the second part, we developed an α'-arylation/aza-Michael addition cascade strategy to construct azabicyclo[3.2.2]nonanones catalyzed by Pd(MeCN)₂Cl₂·PPh₃. This study provides a quick route to complex and useful spiro- and bridged-heterocycles from readily available starting materials in good yields with high regioselectivity.