Enantioselective Route to 5-Methyl- and 5,7-Dimethyl-6,7-dihydro-5H-dibenz[c,e]azepine: Secondary Amines with Switchable Axial Chirality

Organocatalytic diastereo- and atropo-selective construction of eight-membered bridged (hetero)biaryls via asymmetric intramolecular [3 + 2] cycloaddition

An unprecedented and straightforward route for the asymmetric construction of privileged atroposelective bridged (hetero)biaryl eight-membered scaffolds has been accomplished through chiral phosphoric acid catalyzed asymmetric intramolecular [3 + 2] cycloaddition of innovative (hetero)biaryl aldehydes with 3-aminooxindole hydrochlorides. A class of eight-membered bridged (hetero)biaryl lactones fused to spiro[pyrrolidine-oxindole] derivatives, possessing both chiral C-C/C-N axes and multiple contiguous stereocenters, were obtained in good yields with excellent enantioselectivities and diastereoselectivities in one step through this direct strategy. In addition, the good scalability and derivatization of the title compounds demonstrated their synthetic utility.

Organocatalytic Asymmetric Synthesis of Carbo- and Oxacyclic Seven-Membered Bridged Biaryls via Nucleophile-Dependent Switchable Domino Processes

We disclose herein a highly diastereo- and enantioselective divergent synthesis of seven-membered biaryl-bridged carbo- and oxacyclic frameworks by utilizing the catalytic ability of bifunctional hydrogen-bonding squaramide organocatalysts. Starting with the same biaryl substrate bearing two distinct acceptor sites and by choosing soft or hard nucleophiles, we readily accessed the dibenzocycloheptanes or 5,7-dihydrodibenzo[c,e]oxepines bearing multiple elements of chirality via a domino 1,4/1,2-addition or 1,2/oxa-Michael addition sequence, respectively.

5,5,7,7-Tetrametyl-6,7-dihydro-5H-dibenzo[c,e]azepine

5,5,7,7-Tetrametyl-6,7-dihydro-5H-dibenzo[c,e]azepine has been synthesized as a possible pro-chiral (or tropos) unit for the construction of a chiral catalyst and as a molecular chirality sensor for the absolute configuration assignment by chiroptical spectroscopy. A straightforward synthetic strategy for the preparation of the title compound in high overall yield through sequential addition of the four methyl groups on benzylic positions has been described. A VT-NMR study was used to determine the rotational barrier of the aryl–aryl bond in this biphenylazepine, revealing its torsional flexibility at room temperature, which makes the biphenylazepine suitable as both a chirality probe and a tropos moiety in chiral ligands.

Atroposelective Access to 1,3-Oxazepine-Containing Bridged Biaryls via Carbene-Catalyzed Desymmetrization of Imines

We disclose herein an atroposelective synthesis of novel bridged biaryls containing medium-sized rings via N-heterocyclic carbene organocatalysis. The reaction starts with addition of the carbene catalyst to the aminophenol-derived aldimine substrate. Subsequent oxidation and intramolecular desymmetrization lead to the formation of 1,3-oxazepine-containing bridged biaryls in good yields and excellent enantioselectivities. These novel bridged biaryl products can be readily transformed into chiral phosphite ligands. Preliminary density function theory calculations suggest that the origin of enantioselectivity arises from the more favorable frontier molecular orbital interactions in the transition state leading to the major product.

Catalytic asymmetric synthesis of medium-sized bridged biaryls

Despite the persistent presence of medium-sized (seven- to nine-membered) scaffolds in natural products and biologically active molecules, their asymmetric syntheses have always been considered a formidable task; therefore, they have remained underdeveloped when compared to the enantioselective synthesis of five- and six-membered ring scaffolds. One important class of such medium-sized ring frameworks includes seven- to nine-membered biaryl bridged carbo- and heterocycles. These medium-ring-sized biaryl frameworks possess more configurational stability than the related smaller ring structures and are common features of valuable natural products, bioactive compounds, chiral catalysts, and molecular motors. Due to these exciting properties and broad applications, over the last few years, the catalytic enantioselective synthesis of medium-sized bridged biaryls has seen an upsurge. This highlight article describes the development of organocatalysed and transition-metal catalysed transformations for procuring seven-, eight-, and nine-membered bridged biaryls bearing a chiral axis/one or more asymmetric carbon centres.

Synthesis of Hydroxylated Polycyclic Pyrrolo/Indolo[1,2-a]quinoxaline-Fused Lactam Derivatives via PhI(OAc)2-Promoted 1,2-Bond Migration and Solvent Insertion

PhI(OAc)₂-mediated ring expansion via 1,2-bond migration and concurrent solvent insertion generate hydroxylated polycyclic pyrrolo/indolo[1,2-a]quinoxaline-fused lactam derivatives in a highly diastereoselective fashion from spiro-fused quinoxalines in good-to-excellent yield. X-ray crystal structure analysis reveals that the polycyclic lactams are nonplanar molecules devoid of any symmetry as they possess one or two axially chiral biaryl or N-arylindolyl bridges along with one chiral center at the bridgehead carbon.

Asymmetric synthesis of dibenzo[b,d]azepines by Cu-catalyzed reductive or borylative cyclization

A copper-catalyzed asymmetric intramolecular reductive cyclization for the synthesis of dibenzo[b,d]azepines is described. Use of 2'-vinyl-biaryl-2-imines as substrates and in situ formed [Cu^I/(Ph-BPE)] as the catalyst enables the synthesis of 7-membered bridged biarylamines containing both central and axial stereogenic elements in high yields (up to 98%) and with excellent diastereo- and enantioselectivities (>20 : 1 d.r., up to 99% ee). Moreover, the same catalyst was found to facilitate a related borylative cyclization to afford versatile boronic ester derivatives. Both reactions proceed under mild conditions (rt) and are applicable to a variety of substituted aromatic and heterocyclic derivatives.

Catalytic Asymmetric Synthesis of Silicon-Stereogenic Dihydrodibenzosilines: Silicon Central-to-Axial Chirality Relay

A Rh-catalyzed asymmetric synthesis of silicon-stereogenic dihydrodibenzosilines featuring axially chiral 6-membered bridged biaryls is demonstrated. In the presence of a Rh^I catalyst with a chiral diphosphine ligand, a wide range of dihydrodibenzosilines containing both silicon-central and axial chiralities are conveniently constructed in excellent enantioselectivities via dehydrogenative C(sp³ )-H silylation. Absolute configuration analysis by single-crystal X-ray structures revealed a novel silicon central-to-axial chirality relay phenomenon, which we believe will inspire further research in the field of asymmetric catalysis and chiroptical materials.

Asymmetric Reductive Amination/Ring-Closing Cascade: Direct Synthesis of Enantioenriched Biaryl-Bridged NH Lactams

We report here a Ru-catalyzed enantioselective synthesis of biaryl-bridged NH lactams through asymmetric reductive amination and a spontaneous ring-closing cascade from keto esters and NH₄OAc with H₂ as reductant. The reaction features broad substrate generality and high enantioselectivities (up to >99% ee). To showcase the practical utility, a highly enantioselective synthesis of 5-ethylindolobenzazepinone C, a promising antimitotic agent, has been rapidly completed. Furthermore, the amide group in the products enables versatile elaborations through directed C-H functionalization.

Atropselective Synthesis of N,C-Bis(diphenylphosphanes) from Bridged 2-Arylindoles Based on Effective Point-to-Axial Asymmetric Inductions after an Unusual Dilithiation ⊥

An asymmetric methanolysis of glutaric anhydride and 6 ensuing steps gave veratrol-annulated dimethylcyclo-heptenone diastereomers with 99% ee; ring closures occurred by Friedel-Crafts acylations of carboxylic acids obtained by stereospecific hydrogenolyses of a pair of diastereomeric δ-lactones. The mentioned cycloheptenones and Ph-NH-NH₂ underwent Fischer indole syntheses providing the tetracyclic indoles cis- and trans-14a, respectively. Double lithiations with BuLi and quenchings with ClPPh₂ furnished the diphosphanes cis- and trans-15 with perfect (P)- and (M)-atropselectivity, respectively.

Intramolecular asymmetric reductive amination: synthesis of enantioenriched dibenz[c,e]azepines

An enantioselective synthesis of dibenz[c,e]azepines containing both central and axial chiralities through a one pot N-Boc deprotection/intramolecular asymmetric reductive amination sequence has been achieved with generally excellent enantiocontrol (up to 97% ee).

An Ir-catalyzed intramolecular asymmetric reductive amination (ARA) of bridged biaryl derivatives has been described. Using this unprecedented approach, synthetically useful dibenz[c,e]azepines containing both central and axial chiralities are obtained with excellent enantiocontrol (up to 97% ee). This methodology represents a rare example of enantioselective chemocatalytic synthesis of chiral dibenz[c,e]azepines featuring a broad substrate scope, and their synthetic utilities are exhibited by derivatizing the products into a chiral amino acid derivative and chiral phosphoramidite ligands, which display excellent enantiocontrol in Rh-catalyzed asymmetric hydrogenation of α-dehydroamino acid derivatives. Remarkably, our method is also applicable to enantioselectively synthesize an allocolchicine analogue.

Novel binaphthyl and biphenyl α- and β-amino acids and esters: organocatalysis of asymmetric Diels-Alder reactions. A combined synthetic and computational study

Asymmetric catalysis of the Diels-Alder reaction between cyclopentadiene and cinnamaldehydes has been studied using as catalysts a range of novel α- and β-aminoacids and aminoesters with binaphthyl and biphenyl backbones, providing enantioselectivities of up to 62% ee. B3LYP/6-31G* calculations, including free energy corrections, have been carried out on a binaphthyl catalyst example to identify transition state structures and to aid in the identification of major enantiomers. The calculated product ratios agree well with the experimental data; the transition states identified involve preferential approach of cyclopentene along a trajectory adjacent to the acid/ester group. The four lowest energy transition states display a stabilizing dipolar interaction between the carbonyl group oxygen atom and a terminal proton of the diene unit.

Axial stereocontrol in tropos dibenz[c,e]azepines: the individual and cooperative effects of alkyl substituents

6,7-Dihydro-5H-dibenz[c,e]azepines, a class of secondary amine incorporating a centre-axis chirality relay, can be prepared from N-(2-bromobenzyl)-N-(1-arylalkyl)methanesulfonamides via Pd-catalysed intramolecular direct arylation, and methylated at C(7) via the 5,7-trans diastereoselective addition of methylmagnesium bromide to the derived N-benzylazepinium tetraphenylborate. Using these methods, the 4,5-dimethylated and 4,5,7-trimethylated homologues 13 and 14 were obtained and shown by ¹H NMR spectroscopy to be axially biased in opposite senses, as defined by the respective pseudoaxial or pseudoequatorial orientation of the 5-substituent in the preferred conformers, while retaining their tropos nature (the Arrhenius activation energy, E_A, for the conformational exchange process in 14 was estimated to be 57 kJ mol^-1 using 2D-EXSY NMR spectroscopy at 233-248 K). These results serve to illustrate how substituent effects might be exploited in new designs of bridged biaryl ligand in which tropos dynamics operate in combination with a pre-existing axial stereochemical bias.

Di(1-naphthyl) methanol ester of carboxylic acids for absolute stereochemical determination

The absolute stereochemistry of chiral carboxylic acids is determined as a di(1-naphthyl)methanol ester derivative. Computational scoring of conformations favoring either P or M helicity of the naphthyl groups, capable of exciton-coupled circular dichroic coupling, leads to a predicted stereochemistry for the derivatized carboxylic acids.

Biocatalytic Routes to Enantiomerically Enriched Dibenz[c,e]azepines

Biocatalytic retrosynthetic analysis of dibenz[c,e]azepines has highlighted the use of imine reductase (IRED) and ω-transaminase (ω-TA) biocatalysts to establish the key stereocentres of these molecules. Several enantiocomplementary IREDs were identified for the synthesis of (R)- and (S)-5-methyl-6,7-dihydro-5H-dibenz[c,e]azepine with excellent enantioselectivity, by reduction of the parent imines. Crystallographic evidence suggests that IREDs may be able to bind one conformer of the imine substrate such that, upon reduction, the major product conformer is generated directly. ω-TA biocatalysts were also successfully employed for the production of enantiopure 1-(2-bromophenyl)ethan-1-amine, thus enabling an orthogonal route for the installation of chirality into dibenz[c,e]azepine framework.

New biphenyl iminium salt catalysts for highly enantioselective asymmetric epoxidation: role of additional substitution and dihedral angle

New biaryl iminium salt catalysts for enantioselective alkene epoxidation containing additional substitution in the heterocyclic ring are reported. The effects upon conformation and enantioselectivity of this additional substitution, and the influence of dihedral angle in these systems, has been investigated using a synthetic approach supported by density functional theory. Enantioselectivities of up to 97% ee were observed.

Asymmetric epoxidation using iminium salt organocatalysts featuring dynamically controlled atropoisomerism

Introduction of a pseudoaxial substituent at a stereogenic center adjacent to the nitrogen atom in binaphthyl- and biphenyl-derived azepinium salt organocatalysts affords improved enantioselectivities and yields in the epoxidation of unfunctionalized alkenes. In the biphenyl-derived catalysts, the atropoisomerism at the biphenyl axis is controlled by the interaction of this substituent with the chiral substituent at nitrogen.