Catalytic Enantioselective Claisen Rearrangements of O-Allyl β-Ketoesters

Substrate-Controlled Diastereo- and Enantiodivergent Synthesis of Bis-Spirocyclopropyloxindoles from Available Isatin as a Single Starting Material

The first diastereo- and enantiodivergent asymmetric synthesis of new bis-spirocyclopropyloxindole scaffolds has been accomplished from the readily available isatin as a single starting material. Four rel-(1R,2R,3R), rel-(1S,2S,3R), rel-(1R,2R,3S), and rel-(1S,2S,3S) configurations of desired products were constructed in excellent enantiopurity via a simple switch in substrates using the chiral auxiliary-controlled method. The absolute configuration of cycloadducts with three contiguous quaternary/tertiary stereogenic centers was confirmed through X-ray diffraction analysis. A facile synthesis of versatile precursor 3-chlorooxindoles was also introduced.

[3,3]-Sigmatropic Rearrangements of Naphthyl 1-Propargyl Ethers: para-Propargylation and Catalytic Asymmetric Dearomatization

The para-Claisen rearrangement of aryl 1-propargyl ethers involves two-step [3,3]-sigmatropic rearrangements and dearomatization process, which has high activation barriers and is of challenge. Here we discovered thermal para-Claisen rearrangement of naphthyl 1-propargyl ethers, and it enabled the formation of formal para-C-H propargylation products upon rearomatization. Chirality transfer occurred if optically active propargyl ethers were employed, leading to the construction of aryl/propargyl-containing stereogenic centers. Moreover, catalytic asymmetric dearomatization of naphthyl 1-propargyl ethers with different substitution at para-position gave access to benzocyclohexenones bearing all-carbon quaternary stereocenters. The reaction was accelerated by a chiral N,N'-dioxide/Co(OTf)₂ complex catalyst to achieve high yields (up to 98 %) and high enantioselectivities (up to 93 % ee). The DFT calculations and experimental results provided important clues to clarify the para-Claisen rearrangement process as well as the chiral induction and remote delivery.

Construction of Vicinal Quaternary Carbon Stereocenters Through Diastereo- and Enantioselective Oxidative 1,6-Conjugate Addition

The asymmetric construction of vicinal quaternary carbon stereocenters with at least one moiety in acyclic systems is a formidable challenge. We disclose a solution involving diastereo- and enantioselective oxidative 1,6-conjugate addition. The practical asymmetric cross-dehydrogenative coupling of 2,2-diarylacetonitriles and diverse α-substituted cyclic 1,3-dicarbonyls proceeds, for vicinal quaternary carbon stereocenters with one center in acyclic systems, in excellent yields and stereoselectivities. The generality of the approach is further demonstrated by the stereoselective creation of vicinal quaternary carbon stereocenters with both centers in acyclic systems using acyclic β-ketoesters as coupling partners. Computational studies elucidate the origins of both diastereo- and enantioselectivity.

Catalytic Enantioselective Desymmetrizing Fischer Indolization through Dynamic Kinetic Resolution

The first catalytic enantioselective Fischer indolization of prochiral diketones containing enantiotopic carbonyl groups is developed and shown to proceed through dynamic kinetic resolution (DKR). Catalyzed by the combination of a spirocyclic chiral phosphoric acid and ZnCl₂ (Lewis acid assisted Brønsted acid), this direct approach combines 2,2-disubstituted cyclopentane-1,3-diones with N-protected phenylhydrazines to furnish cyclopenta[b]indole derivatives containing an all-carbon quaternary stereocenter with good to excellent enantioselectivities.

A 1,3,2-Diazaphospholene-Catalyzed Reductive Claisen Rearrangement

1,3,2-Diazaphospholenes (DAPs) are an emerging class of organic hydrides. In this work, we exploited them as efficient catalysts for very mild reductive Claisen rearrangements. The method is tolerant towards a wide variety of functional groups and operates at ambient temperature. Besides being enantiospecific for substrates with existing stereogenic centers, the diastereoselectivity can be switched by varying solvents and DAP catalysts. The reaction kinetics show direct rearrangements of O-bound phospholene enolates and provide a proof-of-principle for catalytic enantioselective reactions.

Enantioselective Synthesis of Vicinal All-Carbon Quaternary Centers via Iridium-Catalyzed Allylic Alkylation

The development of the first enantioselective transition-metal-catalyzed allylic alkylation providing access to acyclic products bearing vicinal all-carbon quaternary centers is disclosed. The iridium-catalyzed allylic alkylation reaction proceeds with excellent yields and selectivities for a range of malononitrile-derived nucleophiles and trisubstituted allylic electrophiles. The utility of these sterically congested products is explored through a series of diverse chemo- and diastereoselective product transformations to afford a number of highly valuable, densely functionalized building blocks, including those containing vicinal all-carbon quaternary stereocenters.

The Cation-π Interaction in Small-Molecule Catalysis

Catalysis by small molecules (≤1000 Da, 10(-9)  m) that are capable of binding and activating substrates through attractive, noncovalent interactions has emerged as an important approach in organic and organometallic chemistry. While the canonical noncovalent interactions, including hydrogen bonding, ion pairing, and π stacking, have become mainstays of catalyst design, the cation-π interaction has been comparatively underutilized in this context since its discovery in the 1980s. However, like a hydrogen bond, the cation-π interaction exhibits a typical binding affinity of several kcal mol(-1) with substantial directionality. These properties render it attractive as a design element for the development of small-molecule catalysts, and in recent years, the catalysis community has begun to take advantage of these features, drawing inspiration from pioneering research in molecular recognition and structural biology. This Review surveys the burgeoning application of the cation-π interaction in catalysis.

Evolution of a Unified Strategy for Complex Sesterterpenoids: Progress toward Astellatol and the Total Synthesis of (-)-Nitidasin

Astellatol and nitidasin belong to a subset of sesterterpenoids that share a sterically encumbered trans-hydrindane motif with an isopropyl substituent. In addition, these natural products feature intriguing polycyclic ring systems, posing significant challenges for chemical synthesis. Herein, the evolution of our stereoselective strategy for isopropyl trans-hydrindane sesterterpenoids is detailed. These endeavors included the synthesis of several building blocks, enabling studies toward all molecules of this terpenoid subclass, and of advanced intermediates of our initial route toward a biomimetic synthesis of astellatol. These findings provided the basis for a second-generation and a third-generation approach toward astellatol that eventually culminated in the enantioselective total synthesis of (-)-nitidasin. In particular, a series of substrate-controlled transformations to install the ten stereogenic centers of the target molecule was orchestrated and the carbocyclic backbone was forged in a convergent fashion. Furthermore, the progress toward the synthesis of astellatol is disclosed and insights into some observed yet unexpected diastereoselectivities by detailed quantum-mechanical calculations are provided.

Gold(I)-Catalyzed Desymmetrization of 1,4-Dienes by an Enantioselective Tandem Alkoxylation/Claisen Rearrangement

An enantioselective alkoxylation/Claisen rearrangement reaction was achieved by a strategic desymmetrization of 1,4-dienes under the catalysis of (S)-DTBM-Segphos(AuCl)2/AgBF4. This reaction system was highly selective for the formation of 3,3-rearrangement products, providing cycloheptenes with various substitutions in good yield and good to excellent enantioselectivity. This transformation was further extended to bicyclic ring substrates, providing the opportunity to easily assemble 5,6- and 6,7-fused ring systems.

Aromatic Claisen Rearrangements of O-prenylated tyrosine and model prenyl aryl ethers: Computational study of the role of water on acceleration of Claisen rearrangements

LynF, an enzyme from the TruF family, O-prenylates tyrosines in proteins; subsequent Claisen rearrangements give C-prenylated tyrosine products. These reactions in tyrosines and model phenolic systems have been explored with DFT and SCS-MP2 calculations. Various ab initio benchmarks have been computed (CBS-QB3, MP2, SCS-MP2) to examine the accuracy of commonly used density functionals, such as B3LYP and M06-2X. Solvent effects from water were considered using implicit and explicit models. Studies of the ortho-C-prenylation and Claisen rearrangement of tyrosine, and the Claisen rearrangement of α,α-dimethylallyl (prenyl) coumaryl ether establish the energetics of these reactions in the gas phase and in aqueous solution.

Catalytic asymmetric Claisen rearrangement of enolphosphonates: construction of vicinal tertiary and all-carbon quaternary centers

A Cu-catalyzed enantioselective Claisen rearrangement of easily-accessible enolphosphonates using commercially available (R, R )-PhBOX as the chiral ligand was developed. A wide range of rearrangement products with contiguous tertiary and all carbon quaternary centers were obtained in excellent yields and stereoselectivities. The α -ketophosphonate products could be easily transformed into other functional groups in high efficiency.

The chemistry of the polycyclic polyprenylated acylphloroglucinols

With their fascinating biological profiles and stunningly complex molecular architectures, the polycyclic polyprenylated acylphloroglucinols (PPAPs) have long provided a fertile playing field for synthetic organic chemists. In particular, the recent advent of innovative synthetic methods and strategies together with C-C bond-forming reactions and asymmetric catalysis have revitalized this field tremendously. Consequently, PPAP targets which once seemed beyond reach have now been synthesized. This Review aims to highlight the recent achievements in the total synthesis of PPAPs, as well as notable methods developed for the construction of the bicyclo[3.3.1] core of these chemically and biologically intriguing molecules.