Evaluation of (+)-Sparteine-like Diamines for Asymmetric Synthesis

Enantioselective Synthesis of Phosphine Boranes via Crystallization-Induced Dynamic Resolution of Lithiated Intermediate by Understanding the Underlying Epimerization Process

Described herein is the successful crystallization-induced dynamic resolution (CIDR) of an α-lithiated phosphine borane utilizing the easily accessible and inexpensive ligand (R,R)-TMCDA. Starting from the essential P-prochiral building block dimethyl phenyl phosphine borane we were able to obtain phosphine boranes in yields up to 80 % and e.r. up to 98 : 2 by crystallization of the lithiated intermediate prior to the trapping reaction. NMR-based deuterium labeling experiments indicate that the epimerization in solution is based on the intermolecular proton transfer between nonlithiated phosphine borane and the corresponding lithiated intermediate, rendering the presence of the remaining starting compound in an optimized solvent mixture the main factor for successful enantioselective synthesis. Quantum chemical calculations using different model systems based on solid state structures confirm these experimental results. By gaining insights into the epimerization mechanism, essential principles for CIDR of lithiated phosphine boranes are elucidated that may be expanded to other important P-stereogenic compounds and simple chiral amines.

Stereodynamic Strategies to Induce and Enrich Chirality of Atropisomers at a Late Stage

Enantiomers, where chirality arises from restricted rotation around a single bond, are atropisomers. Due to the unique nature of the origins of their chirality, synthetic strategies to access these compounds in an enantioselective manner differ from those used to prepare enantioenriched compounds containing point chirality arising from an unsymmetrically substituted carbon center. In particular stereodynamic transformations, such as dynamic kinetic resolutions, thermodynamic dynamic resolutions, and deracemizations, which rely on the ability to racemize or interconvert enantiomers, are a promising set of transformations to prepare optically pure compounds in the late stage of a synthetic sequence. Translation of these synthetic approaches from compounds with point chirality to atropisomers requires an expanded toolbox for epimerization/racemization and provides an opportunity to develop a new conceptual framework for the enantioselective synthesis of these compounds.

Synthesis of N-Arylcytisine Derivatives Using the Copper-Catalyzed Chan-Lam Coupling

N-Arylcytisine derivatives are quite rare. We report here a practical methodology to obtain these compounds. Using the copper-catalyzed Chan-Lam coupling, we synthesized new N-arylcytisine derivatives at room temperature, in air and using inexpensive phenylboronic acids. Cytisine and 3,5-dihalocytisines can act as substrates, and among the products, the p-Br-derivative 2r was used as a substrate to obtain biaryl derivatives under Pd-coupling conditions; ester 2j was converted into its acid and amide derivatives using classical carbodiimide conditions. This shows that the Chan-Lam cross-coupling reaction can be included as a versatile synthetic tool in the derivatization of natural products.

Shaping Molecular Landscapes: Recent Advances, Opportunities, and Challenges in Dearomatization

Dearomatization is a fundamental chemical transformation, and it underlies some of the most efficient tactics for generating three-dimensional complexity from basic two-dimensional precursors. The dearomative toolbox, once restricted to only a handful of reactions, has begun to grow more sophisticated as novel methods are added, introducing more functionality under milder conditions and with more control over chemo-, regio-, and stereoselectivity than ever before. Over the past two decades, major developments in dearomative processes have bolstered significant total-synthesis endeavors and greatly expanded the scope and complexity of chemical building blocks accessible from feedstock arenes. In this Perspective, we highlight some of the recent advances and key challenges that remain in this vibrant area of organic chemistry.

Regio- and stereoselective intermolecular carbolithiation reactions

Carbolithiation of alkenes provides a powerful tool for the construction of up to two novel carbon–carbon or carbon–heteroatoms bonds in a regio- and stereo-controlled manner. The synthetic potentiality of this reaction is highlighted.

Semisynthesis of novel N-acyl/sulfonyl derivatives of 5(3,5)-(di)halogenocytisines/cytisine and their pesticidal activities against Mythimna separata Walker, Tetranychus cinnabarinus Boisduval, and Sitobion avenae Fabricius

BACKGROUND

To discover novel natural product-based pesticidal agents for crop protection, a series of N-acyl/sulfonyl derivatives of 5(3,5)-(di)halogenocytisines/cytisine were prepared by structural modifications of cytisine. Their pesticidal activities were evaluated against three typically crop-threatening agricultural pests, Mythimna separata Walker, Tetranychus cinnabarinus Boisduval, and Sitobion avenae Fabricius.

RESULTS

Compound 5f exhibited the promising pesticidal activities against three tested pests. All N-phenylsulfonylcytisine derivatives showed potent acaricidal activity. Compound 5j exhibited 2.5-fold more potent acaricidal activity than cytisine, and showed good control effects. Intermediates 2, and 3/3' displayed pronounced aphicidal activity. Some interesting results of structure-activity relationships were also obtained.

CONCLUSION

These results demonstrate that compounds 5f and 5j could be further modified as pesticidal agents. © 2019 Society of Chemical Industry.

Highly efficient biocatalytic desymmetrization of meso carbocyclic 1,3-dicarboxamides: a versatile route for enantiopure 1,3-disubstituted cyclohexanes and cyclopentanes

Highly efficient biocatalytic desymmetrization of meso carbocyclic 1,3-dicarboxamides to enantiopure 1,3-disubstituted cyclohexanes and cyclopentanes was realized.

Readily Accessible 1,2-Amino Ether Ligands for Enantioselective Intramolecular Carbolithiation

A new class of chiral 1,2-amino ether ligands, readily accessible from naturally occurring α-amino- or α-hydroxy acids, was found to provide high levels of both conversion and stereocontrol (up to 95:5 er) in intramolecular carbolithiation reactions, outperforming the benchmark ligand (-)-sparteine. The ligand could be used in a substoichiometric amount (0.25 equiv) without significant loss of enantioselectivity.

Stereocontrolled Total Synthesis of (-)-Stemaphylline

Homologation of readily available α-boryl pyrrolidines with metal carbenoids is especially challenging even when good leaving groups (Cl- ) are employed. By performing a solvent switch from Et2 O to CHCl3 , efficient 1,2-metalate rearrangement of the intermediate boronate occurs with both halide and ester leaving groups. The methodology was used in the total synthesis of the Stemona alkaloid (-)-stemaphylline in just 11 steps (longest linear sequence), with high stereocontrol (>20:1 d.r.) and 11 % overall yield. The synthesis also features a late-stage lithiation-borylation reaction with a tertiary amine containing carbenoid.

Enantioselective carbolithiation of S-alkenyl-N-aryl thiocarbamates: kinetic and thermodynamic control

Carbolithiation of vinyl thiocarbamates may proceed with the same sense of enantioselectivity with either (−)-sparteine and (+)-sparteine surrogate ligands.

Revisiting the sparteine surrogate: development of a resolution route to the (-)-sparteine surrogate

The improved performance of the sparteine surrogate compared to sparteine in a range of applications has highlighted the need to develop an approach to the (-)-sparteine surrogate, previously inaccessible in gram-quantities. A multi-gram scale, chromatography-free synthesis of the racemic sparteine surrogate and its resolution via diastereomeric salt formation with (-)-O,O'-di-p-toluoyl-l-tartaric acid is reported. Resolution on a 10.0 mmol scale gave the diastereomeric salts in 33% yield from which (-)-sparteine surrogate of 93 : 7 er was generated. This work solves a key limitation: either enantiomer of the sparteine surrogate can now be readily accessed.

Asymmetric lithiation trapping of N-Boc heterocycles at temperatures above -78 °C

The asymmetric lithiation trapping of N-Boc heterocycles using s-BuLi/chiral diamines at temperatures up to -20 °C is reported. Depending on the N-Boc heterocycle, lithiation is accomplished using s-BuLi and (-)-sparteine or the (+)-sparteine surrogate in the temperature range -50 to -20 °C for short reaction times (2-20 min). Subsequent electrophilic trapping or transmetalation-Negishi coupling delivered functionalized N-Boc heterocycles in 47-95% yield and 77:23-93:7 er. With N-Boc pyrrolidine, trapped products can be generated in ∼90:10 er even at -20 °C.

Total synthesis of hygrolines and pseudohygrolines

A concise two-step synthesis of all four diastereoisomeric hygrolines ((-)-hygroline (1), (+)-hygroline (2), (-)-pseudohygroline (3), (+)-pseudohygroline (4)) has been developed based on the (-)-sparteine (5)- or (+)-sparteine surrogate 11-mediated enantioselective lithiation of N-Boc pyrrolidine (6), followed by reaction of the chiral anion with (S)- or (R)-propylene oxide. Reduction of the resulting N-Boc amino alcohols furnished hygrolines and pseudohygrolines in 30% to 56% overall yields with dr's > 95:5.

Readily accessible chiral at nitrogen cage structures

The reaction of glycine-N-methyl amide with paraformaldehyde in the presence of ytterbium triflate (1 mol%) leads to a novel cage structure 6 which is chiral at nitrogen. Single crystal X-ray analysis and DFT calculations suggest this cage structure is rigid and adopts a single low energy conformation. Use of single enantiomer α-amino amides results in two diastereomeric tertiary amines that differ in their absolute configuration at nitrogen. These diastereoisomers interconvert under acidic conditions but are configurationally stable under basic conditions and can be readily separated by either crystallisation or column chromatography. By reacting racemic chiral α-amino amides a third diastereomeric cage can also be isolated through this reaction protocol. Preparation of mixed cages by reacting two different α-amino amides is also possible allowing for greater structural diversity in the products to be attained. Preliminary mechanistic studies show that all three methylene units in the cage structure are labile and can be replaced under acidic reaction conditions.

Inter- and intramolecular enantioselective carbolithiation reactions

In this review we summarize recent developments in inter- and intramolecular enantioselective carbolithiation reactions carried out in the presence of a chiral ligand for lithium, such as (-)-sparteine, to promote facial selection on a C=C bond. This is an attractive approach for the construction of new carbon-carbon bonds in an asymmetric fashion, with the possibility of introducing further functionalization on the molecule by trapping the reactive organolithium intermediates with electrophiles.

Synthesis of the azaphilones (+)-sclerotiorin and (+)-8-O-methylsclerotiorinamine utilizing (+)-sparteine surrogates in copper-mediated oxidative dearomatization

Enantioselective syntheses of the azaphilone natural products (+)-sclerotiorin and (+)-8-O-methylsclerotiorinamine that possess the natural R-configuration at the quaternary center are reported. The syntheses were accomplished using copper-mediated asymmetric dearomatization employing bis-μ-oxo copper complexes prepared from readily available (+)-sparteine surrogates. Of note, site-selective O-methylation of a vinylogous pyridone was used to access the isoquinolin-6(7H)-one core of (+)-8-O-methylsclerotiorinamine.

Optically active (aR)- and (aS)-linear and vaulted biaryl ligands: deracemization versus oxidative dimerization

The copper-mediated deracemization of the C2-symmetric vaulted biaryl ligands VANOL and VAPOL has been investigated. In the course of the studies that have led to a more reliable procedure for this process, an unprecedented oxidative dimerization of these ligands has been uncovered. The structures of these oxidative dimerization products were elucidated by a series of NMR experiments, and these assignments were supported by other spectroscopic techniques as well as their chemical reactivity. This oxidative dimerization process was not observed for the linear biaryl ligands BANOL and BINOL, although the new deracemization procedure was effective for the generation of BINOL with high optical purity. The (aS)-enantiomers of BINOL, VANOL and VAPOL were accessible with a copper complex of (-)-sparteine, and the (aR)-enantiomeric series were accessible with a copper complex of O'Brien's diamine. Both (-)-sparteine and O'Brien's diamine give higher optical purities with VANOL and VAPOL than with BINOL, and this is consistent with the steric congestion present in the matched and mismatched copper complexes of these diamines with the biaryl ligands.

Structural features and reactivity of (sparteine)PdCl2: a model for selectivity in the oxidative kinetic resolution of secondary alcohols

The chiral ligand (-)-sparteine and PdCl(2) catalyze the enantioselective oxidation of secondary alcohols to ketones and thus effect a kinetic resolution. The structural features of sparteine that led to the selectivity observed in the reaction were not clear. Substitution experiments with pyridine derivatives and structural studies of the complexes generated were carried out on (sparteine)PdCl(2) and indicated that the C(1) symmetry of (-)-sparteine is essential to the location of substitution at the metal center. Palladium alkoxides were synthesized from secondary alcohols that are relevant steric models for the kinetic resolution. The solid-state structures of the alkoxides also confirmed the results from the pyridine derivative substitution studies. A model for enantioinduction was developed with C(1) symmetry and Cl(-) as key features. Further studies of the diastereomers of (-)-sparteine, (-)-alpha-iso- and (+)-beta-isosparteine, in the kinetic resolution showed that these C(2)-symmetric counterparts are inferior ligands in this stereoablative reaction [Mohr, J. T., Ebner, D. C., and Stoltz, B. M. Org. Biomol. Chem. 2007, 5, 3571-3576].

Dynamic thermodynamic resolution: advantage by separation of equilibration and resolution

In the investigation of a chemical reaction, researchers typically survey variables such as time, temperature, and stoichiometry to optimize yields. This Account demonstrates how control of these variables, often in nontraditional ways, can provide significant improvements in enantiomeric ratios for asymmetric reactions. Dynamic thermodynamic resolution (DTR) offers a convenient method for the resolution of enantiomeric products in the course of a reaction. This process depends on an essential requirement: the equilibration of the penultimate diastereomers must be subject to external control. As a general case, the reaction of A(R), A(S) with B under the influence of the chiral species, L*, gives resolved products C(R) and C(S). In the first step of dynamic resolution under thermodynamic control, the enantiomeric reactants A(R) and A(S) and L* form the diastereomers A(R)/L* and A(S)/L*. The equilibrium between A(R) and A(S) can be rapid, slow, or not operative, and L* can represent a ligand, an auxiliary, or a crystallization process that provides a chiral environment. Second, the populations of the diastereomers are controlled, usually by thermal equilibration. Finally, the reaction of the diastereomers with a reagent B provides the enantiomeric products C(R) and C(S). The control of the diastereomeric equilibrium distinguishes DTR from other resolution techniques. By contrast, physical resolutions separate thermodynamically stable, nonequilibrating diastereomers, and dynamic kinetic resolutions utilize kinetic control for reactions of rapidly equilibrating diastereomers. The dynamic thermodynamic resolutions discussed in this Account illustrate cases of significantly improved enantioselectivities using this technique. Although many of the well-recognized cases come from organolithium chemistry, the principles are general, and we also present cases facilitated by other chemistries. This approach has been used to control enantioselectivities in a number of different reactions, with improvements in enantiomeric ratios up to 99% from essentially racemic reactants.