Catalytic Enantioselective Birch–Heck Sequence for the Synthesis of Tricyclic Structures with All-Carbon Quaternary Stereocenters

Ni-Catalyzed Enantioselective Intramolecular Mizoroki-Heck Reaction for the Synthesis of Phenanthridinone Derivatives

A Ni-catalyzed enantioselective intramolecular Mizoroki-Heck reaction has been developed to transform symmetrical 1,4-cyclohexadienes with attached aryl halides into phenanthridinone analogues containing quaternary stereocenters. Herein, we report important advances in reaction optimization enabling control of unwanted proto-dehalogenation and alkene reduction side products. Moreover, this approach provides direct access to six-membered ring heterocyclic systems bearing all-carbon quaternary stereocenters, which have been much more challenging to form enantioselectively with nickel-catalyzed Heck reactions. A wide range of substrates were demonstrated to work in good to excellent yields. Good enantioselectivity was demonstrated using a new synthesized chiral iQuinox-type bidentate ligand (L27). The sustainability, low price of nickel catalysts, and significantly faster reaction rate (1 h) versus that of a recently reported palladium-catalyzed reaction (20 h) make this process an attractive alternative.

Catalytic Enantioselective Birch-Heck Sequence for the Synthesis of Phenanthridinone Derivatives with an All-Carbon Quaternary Stereocenter

Novel phenanthridinone analogues with an all-carbon quaternary stereocenter have been enantioselectively synthesized using the Birch-Heck sequence. Flat phenanthridinone structures have extensive bioactivity but consequently also suffer from poor therapeutic selectivity. The addition of a quaternary center to the phenanthridinone skeleton has the potential to generate more complex analogues with improved selectivity. Unfortunately, no general synthetic pathway to such derivatives exists. Herein we report a four-step process that transforms inexpensive benzoic acid into 22 different quaternary carbon-containing phenanthridinone analogues with a variety of substituents on all three rings: alkyl groups at the quaternary center; methyl, methoxymethyl, or para-methoxybenzyl on the amide nitrogen; and halogen and methyl substituents on the aryl ring. Good to very good enantioselectivity was demonstrated in the key intramolecular desymmetrizing Mizoroki-Heck reaction. Transformations of the Heck reaction products into molecules with potentially greater therapeutic relevance were also accomplished.

Recent developments in asymmetric Heck type cyclization reactions for constructions of complex molecules

Intramolecular carbometallation-initiated asymmetric transformations are a general and powerful approach for the construction of carbo- and heterocyclic systems with one and more stereocenters. In addition, the newly developed multiple cascade reactions are an attractive strategy for increasing the molecular complexity in one step. In recent years, great progress has been made in this area with the use of various palladium and nickel complexes with P- and N-donor chiral ligands. This review highlights recent developments in intramolecular asymmetric Heck reactions, reductive Heck reactions and various types of cascade transformations (intramolecular Heck/Heck, Heck/nucleophilic trapping, Heck/Tsuji-Trost, Heck/Suzuki-Miyaura, Heck/Sonogashira, and Heck/carbonylation) in the synthesis of complex molecules over the past 5 years. A number of examples from before 2016 are included as background information. Particular attention is paid to the use of inexpensive nickel complexes as highly efficient catalysts for a number of asymmetric reactions considered here. A perspective on current challenges and potential future developments in the field of asymmetric Heck type cyclizations is also provided.

Construction of Spirofused Tricyclic Frameworks by NHC-Catalyzed Intramolecular Stetter Reaction of a Benzaldehyde Tether with a Cyclic Enone

Various benzaldehyde tethers with a cyclic enone were prepared from commercially available 2-hydroxybenzaldehydes via a three-step sequence involving triflate formation, Sonogashira cross-coupling, and regioselective hydrogenation. These substrates were then exposed to an N-heterocyclic carbene, whereupon intramolecular Stetter reaction proceeded smoothly to give various spirofused tricyclic 1,4-diketones in 30-87% yields. Furaldehyde and nicotinaldehyde derivatives also participated in the reaction under the Stetter conditions.

Manganese-Catalyzed Divergent Markovnikov Addition and [2+2+2] Cycloaddition of 2-Carbonyl Indanone with Terminal Alkyne

Herein, we report on manganese-catalyzed regioselective Markovnikov addition and [2+2+2] cycloaddition of 2-carbonyl indanones with terminal alkynes. This method provides an efficient approach for the construction of all-carbon quaternary centers and complex polycyclic hydrocarbon frameworks by the formation of new carbon-carbon single bonds in a regio- and stereoselective manner. Preliminary mechanistic experiments involving deuterium labeling, kinetic, catalytic, and stoichiometric reactions with plausible intermediates were performed to shed light on the reaction mechanism.

Access to chiral tetrahydrofluorenes through a palladium-catalyzed enantioselective tandem intramolecular Heck/Tsuji–Trost reaction

A palladium-catalyzed enantioselective tandem reaction of 2,5-cyclohexadienyl-substituted aryl iodides and carbon or heteroatom nucleophiles has been successfully established by using a chiral H₈-BINOL-based phosphoramidite ligand.