Construction of Stereogenic α,α-Disubstituted Cycloalkanones via 1° Amine Thiourea Dual Catalysis: Experimental Scope and Computational Analyses

Syntheses of (+)-costic acid and structurally related eudesmane sesquiterpenoids and their biological evaluations as acaricidal agents against Varroa destructor

Synthesis of (+)-costic acid, isolated from Dittrichia viscosa (L.) W. Greuter as a natural acaricidal sesquiterpenoid, was achieved in 16 steps from (R)-carvone with an overall yield of 4.8%, involving the radical cyclization of selenoester to construct a decalone framework as the key step. Other structurally related natural products, (+)-costal, (+)-costol, and (+)-β-selinene, were also synthesized. The acaricidal activities of these four natural products and some synthetic intermediates were also evaluated against Varroa destructor. Among them, (+)-costal especially exhibited potent acaricidal activity.

Enantioselective total syntheses of six natural and two proposed meroterpenoids from Psoralea corylifolia

The first enantioselective total syntheses of six natural and two proposed meroterpenoids isolated from Psoralea corylifolia have been achieved in 7-9 steps from 2-methylcyclohexanone. The current synthetic approaches feature a high level of synthetic flexibility, stereodivergent fashion and short synthetic route, thereby providing a potential platform for the preparation of numerous this-type meroterpenoids and their pseudo-natural products.

Asymmetric Alkylation of Cyclic Ketones with Dehydroalanine via H-Bond-Directing Enamine Catalysis: Straightforward Access to Enantiopure Unnatural α-Amino Acids

The growing importance of structurally diverse and functionalized enantiomerically pure unnatural amino acids in the design of drugs, including peptides, has stimulated the development of new synthetic methods. This study reports the challenging direct asymmetric alkylation of cyclic ketones with dehydroalanine derivatives via a conjugate addition reaction for the synthesis of enantiopure ketone-based α-unnatural amino acids. The key to success was the design of a bifunctional primary amine-thiourea catalyst that combines H-bond-directing activation and enamine catalysis. The simultaneous dual activation of the two relatively unreactive partners, confirmed by mass spectrometry studies, results in high reactivity while securing high levels of stereocontrol. A broad substrate scope is accompanied by versatile downstream chemical modifications. The mild reaction conditions and consistently excellent enantioselectivities (>95 % ee in most cases) render this protocol highly practical for the rapid construction of valuable noncanonical enantiopure α-amino-acid building blocks.

Chiral Thioureas-Preparation and Significance in Asymmetric Synthesis and Medicinal Chemistry

For almost 20 years, thioureas have been experiencing a renaissance of interest with the emerged development of asymmetric organocatalysts. Due to their relatively high acidity and strong hydrogen bond donor capability, they differ significantly from ureas and offer, appropriately modified, great potential as organocatalysts, chelators, drug candidates, etc. The review focuses on the family of chiral thioureas, presenting an overview of the current state of knowledge on their synthesis and selected applications in stereoselective synthesis and drug development.

Straightforward access to densely substituted chiral succinimides through enantioselective organocatalyzed Michael addition of α-alkyl-cyclic ketones to maleimides

A simple, cheap and unchanged organocatalytic system provides efficient access to densely substituted chiral succinimides bearing QCC–TCC vicinal stereocenters from unreactive α-alkyl cyclic ketones and maleimides.

[2,3]-Wittig Rearrangement as a Formal Asymmetric Alkylation of α-Branched Ketones

The enantioselective [2,3]-Wittig rearrangement of cinnamyloxycyclopentanone derivatives was performed in the presence of a Cinchona-based primary amine. The described method provides synthetically valuable α-hydroxy ketones with quaternary stereogenic centers in excellent enantiomeric purities. Relying on the X-ray crystal structure of the product and the DFT calculations, we propose that the rearrangement is promoted by an intramolecular hydrogen bond between the substrate and the catalyst.

Direct Asymmetric Alkylation of Ketones: Still Unconquered

The alkylation of ketones is taught at basic undergraduate level. In many cases this transformation leads to the formation of a new stereogenic center. However, the apparent simplicity of the transformation is belied by a number of problems. So much so, that a general method for the direct asymmetric alkylation of ketones remains an unmet target. Despite the advancement of organocatalysis and transition-metal catalysis, neither field has provided an adequate solution. Indeed, even use of an efficient and general stoichiometric chiral reagent has yet to be reported. Herein we describe the state-of-the-art in terms of direct alkylation reactions of some carbonyl groups. We outline the limited progress that has been made with ketones, and potential routes towards ultimately achieving a widely applicable methodology for the asymmetric alkylation of ketones.

Computational Insights into the Central Role of Nonbonding Interactions in Modern Covalent Organocatalysis

The flexibility, complexity, and size of contemporary organocatalytic transformations pose interesting and powerful opportunities to computational and experimental chemists alike. In this Account, we disclose our recent computational investigations of three branches of organocatalysis in which nonbonding interactions, such as C-H···O/N interactions, play a crucial role in the organization of transition states, catalysis, and selectivity. We begin with two examples of N-heterocyclic carbene (NHC) catalysis, both collaborations with the Scheidt laboratory at Northwestern. In the first example, we discuss the discovery of an unusual diverging mechanism in a catalytic kinetic resolution of a dynamic racemate that depends on the stereochemistry of the product being formed. Specifically, the major product is formed through a concerted asynchronous [2 + 2] aldol-lactonization, while the minor products come from a stepwise spiro-lactonization pathway. Stereoselectivity and catalysis are the results of electrophilic activation from C-H···O interactions between the catalyst and the substrate and conjugative stabilization of the electrophile. In the second example, we show how knowledge and understanding of the computed transition states led to the development of a more enantioselective NHC catalyst for the butyrolactonization of acyl phosphonates. The identification of mutually exclusive C-H···O interactions in the computed major and minor TSs directly resulted in structural hypotheses that would lead to targeted destabilization of the minor TS, leading to enhanced stereoinduction. Synthesis and evaluation of the newly designed NHC catalyst validated our hypotheses. Next, we discuss two works related to Lewis base catalysis involving 4-dimethylaminopyridine (DMAP) and its derivatives. In the first, we discuss our collaboration with the Smith laboratory at St Andrews, in which we discovered the origins of the regioselectivity in carboxyl transfer reactions. We disclose how different Lewis base catalysts (NHC or DMAP) can lead to different regiomeric products as a result of differing magnitudes of aromatic and C-H···O interactions present in the respective transition states. In the second example, we discuss the mechanism and origins of the stereoselectivity of a reaction catalyzed by a planar-chiral 4-(pyrrolidino)pyridine derivative, namely, the coupling of ketenes with cyanopyrrole. We discovered that the chiral base mechanism is operative, in contrast to the originally proposed Brønsted acid mechanism. The selectivity is determined by the ease with which the major and minor TSs can realize strong stabilizing C-H···N interactions between the pyrrole cyano group and the catalyst. These interactions induce increased catalyst distortion in the minor TS, thereby leading to enantioselectivity. Finally, we discuss our computations related to amine-based organocatalysis in collaboration with the Carter laboratory at Oregon State. We probed the mechanism and stereoselectivity of a bifunctional amine thiourea-catalyzed Michael reaction. Our computations led to the design of an improved catalyst. However, synthesis and tests revealed that this catalyst was prone to degradation to side products that also catalyze the reaction, ultimately reducing the observed enantioselectivity. Lastly, we discuss our study of the mechanism and stereoselectivity of a proline sulfonamide-catalyzed Robinson annulation, in which we discovered that the enantioselectivity is controlled by the first Michael step but the diastereoselectivity is controlled by the following Mannich step.