Applications of Nonenzymatic Catalysts to the Alteration of Natural Products

Preparation of Antiproliferative Terpene-Alkaloid Hybrids by Free Radical-Mediated Modification of ent-Kauranic Derivatives

A convenient strategy for molecular editing of available ent-kauranic natural scaffolds has been developed based on radical mediated C-C bond formation. Iodine atom transfer radical addition (ATRA) followed by rapid ionic elimination and radical azidoalkylation were investigated. Both reactions involve radical addition to the exo-methylenic double bond of the parent substrate. Easy transformations of the obtained adducts lead to extended diterpenes of broad structural diversity and artificial diterpene-alkaloid hybrids possessing lactam and pyrrolidine pharmacophores. The cytotoxicity of selected diterpenic derivatives was examined by in vitro testing on several tumor cell lines. The terpene-alkaloid hybrids containing N-heterocycles with unprecedented spiro-junction have shown relevant cytotoxicity and promising selectivity indexes. These results represent a solid basis for following research on the synthesis of such derivatives based on available natural product templates.

Recent Advances in Transition-Metal-Free Late-Stage C-H and N-H Arylation of Heteroarenes Using Diaryliodonium Salts

Transition-metal-free direct arylation of C-H or N-H bonds is one of the key emerging methodologies that is currently attracting tremendous attention. Diaryliodonium salts serve as a stepping stone on the way to alternative environmentally friendly and straightforward pathways for the construction of C-C and C-heteroatom bonds. In this review, we emphasize the recent synthetic advances of late-stage C(sp2)-N and C(sp2)-C(sp2) bond-forming reactions under metal-free conditions using diaryliodonium salts as arylating reagent and its applications to the synthesis of new arylated bioactive heterocyclic compounds.

Contemporary advancements in the semi-synthesis of bioactive terpenoids and steroids

Many natural products have intriguing biological properties that arise from their fascinating chemical structures. However, the intrinsic complexity of the structural skeleton and the reactive functional groups on natural products pose tremendous challenges to chemical syntheses. Semi-synthesis uses chemical compounds isolated from natural sources as the starting materials to produce other novel compounds with distinct chemical and medicinal properties. In particular, advancements in various types of sp3 C-H bond functionalization reactions and skeletal rearrangement methods have contributed to the re-emergence of semi-synthesis as an efficient approach for the synthesis of structurally complex bioactive natural products. Here, we begin with a brief discussion of several bioactive natural products that were obtained via a semi-synthetic approach between 2008 and 2015 and we then discuss in-depth contemporary advancements in the semi-synthesis of bioactive terpenoids and steroids reported during 2016-2020.

Ethanol: Unlocking an Abundant Renewable C2 -Feedstock for Catalytic Enantioselective C-C Coupling

With annual production at >85 million tons/year, ethanol is the world's largest-volume renewable small molecule carbon source, yet its use as a C2 -feedstock in enantioselective C-C coupling is unknown. Here, the first catalytic enantioselective C-C couplings of ethanol are demonstrated in reactions with structurally complex, nitrogen-rich allylic acetates incorporating the top 10 N-heterocycles found in FDA-approved drugs.

Phenotypic screen identifies calcineurin-sparing FK506 analogs as BMP potentiators for treatment of acute kidney injury

Acute kidney injury (AKI) is a life-threatening disease with no known curative or preventive therapies. Data from multiple animal models and human studies have linked dysregulation of bone morphogenetic protein (BMP) signaling to AKI. Small molecules that potentiate endogenous BMP signaling should have a beneficial effect in AKI. We performed a high-throughput phenotypic screen and identified a series of FK506 analogs that act as potent BMP potentiators by sequestering FKBP12 from BMP type I receptors. We further showed that calcineurin inhibition was not required for this activity. We identified a calcineurin-sparing FK506 analog oxtFK through late-stage functionalization and structure-guided design. OxtFK demonstrated an improved safety profile in vivo relative to FK506. OxtFK stimulated BMP signaling in vitro and in vivo and protected the kidneys in an AKI mouse model, making it a promising candidate for future development as a first-in-class therapeutic for diseases with dysregulated BMP signaling.

Boronic Acid-Catalyzed Regioselective Koenigs-Knorr-Type Glycosylation

Boronic acid-catalyzed regioselective Koenigs-Knorr-type glycosylation is presented. The reaction of an unprotected or partially protected glycosyl acceptor with a glycosyl halide donor in the presence of silver oxide and a low catalytic amount of imidazole-containing boronic acid was found to proceed smoothly, which enables construction of a 1,2-trans glycosidic linkage with high regioselectivities. This is the first example of the use of a boronic acid catalyst to initiate regioselective glycosylation via the activation of cis-vicinal diols in glycosyl acceptors.

Site-Selective Acylations of α- and β-Hydroxyamides in Complex Molecules: Application of Template-Driven Acylation to Disaccharides and a Glycopeptide

Site-selective acylations of α-and β-hydroxyamides in complex polyols are described. The combination of a pyridine aldoxime ester and Zn(OTf)₂ facilitates the acylation of two types of N-glycolyl disaccharides, namely, Gal-GlcNGc and Neu5Gc-Gal, both of which are partial structures of polysaccharides responsible for biological actions, with highly site-selective modifications achieved. Furthermore, biotinylation, one of the most important techniques in chemical biology, is used to site-selectively acylate the β-hydroxyl group in a glycopeptide.

Boronic Acid-Catalyzed Final-Stage Site-Selective Acylation for the Total Syntheses of O-3'-Acyl Bisabolol β-D-Fucopyranoside Natural Products and Their Analogues

The first concise total syntheses of O-3'-senecioyl α-bisabolol β-D-fucopyranoside (4a) and O-3'-isovaleroyl α-bisabolol β-D-fucopyranoside (4b) were achieved through final-stage site-selective acylation via the activation of cis-vicinal diols by imidazole-containing boronic acid catalysts as a key step. This synthetic method was also effective for the syntheses of unnatural analogues with modified acyl side chains or carbohydrate moiety.

Tunable Artificial Enzyme-Cofactor Complex for Selective Hydrolysis of Acetals

Enzymes frequently use unimpressive functional groups such as weak carboxylic acids for efficient, highly selective catalysis including hydrolysis of acetals and even amides. Much stronger acids generally have to be used for such purposes in synthetic systems. We report here a method to position an acidic group near the acetal oxygen of 2-(4-nitrophenyl)-1,3-dioxolane bound by an artificial enzyme. The hydrolytic activity of the resulting artificial enzyme-cofactor complex was tuned by the number and depth of the active site as well as the hydrophobicity and acidity of the cofactor. The selectivity of the complex was controlled by the size and shape of the active site and enabled less reactive acetals to be hydrolyzed over more reactive ones.

Hybrid Peptide-Thiourea Catalyst for Asymmetric Michael Additions of Aldehydes to Heterocyclic Nitroalkenes

Bifunctional organocatalysis combining covalent and noncovalent activation is presented. The hybrid peptide-thiourea catalyst features a N-terminal proline moiety for aldehyde activation and a thiourea unit for electrophile activation. This catalyst effectively promotes asymmetric Michael additions of aldehydes to challenging but biologically relevant heterocycle-containing nitroalkenes. The catalyst can be used under solvent-free conditions. Spectroscopic and density functional theory studies elucidate the catalyst structure and mode of action.

Synthesis of Complex Molecular Systems-The Foreseen Role of Organic Chemists

How to control the self-assembly of complex molecular systems is unknown. Yet, these complex molecular systems are fundamental for advances in material and biomedical sciences. A step forward is to transform one-step self-assembly into multistep synthesis involving covalent and noncovalent reactions. Key to this approach is to explore the chemical space at the frontiers of advanced covalent synthesis and supramolecular chemistry. Herein, we describe a selection of such reported cases and provide a guide for current limitations and insights for future directions. This outlook is meant to trigger collaborations between synthetic organic and supramolecular chemists, to expand the repertoire of organic syntheses working with supramolecular assemblies and thereby join forces to achieve stepwise emergence of molecular complexity in supramolecular systems.

New Strategies in the Efficient Total Syntheses of Polycyclic Natural Products

Polycyclic natural products are an inexhaustible source of medicinal agents, and their complex molecular architecture renders challenging synthetic targets where innovative and effective approaches for their rapid construction are urgently required. The total synthesis of polycyclic natural products has witnessed exponential progression along with the emergence of new synthetic strategies and concepts, such as sequential C-H functionalizations, radical-based transformations, and functional group pairing strategies. Our group exerts continued interest in the construction of bioactive and structurally complex natural products as well as evaluation of the mode of action of these molecules. In this Account, we will showcase how these new synthetic strategies are employed and guide our total synthesis endeavors.During the last two decades, a series of remarkable advances in C-H functionalization have led to the emergence of many new approaches to directly functionalize C-H bonds into useful functional groups. These selective transformations have provided a great opportunity for the step- and atom-economical construction of key fragments in complex molecule synthesis. We recently furnished the total syntheses for polycyclic natural products: incarviatone A, chrysomycin A, polycarcin V, and gilvocarcin V by employing a multiple C-H bond functionalization strategy. The polysubstituted benzene or naphthalene skeleton was constructed through sequential and site-selective C-H functionalizations from readily available simple starting materials, which reduced the number of steps and streamlined synthesis.Recently, we have also completed the total syntheses for a number of skeletally diverse tetracyclic Isodon diterpenoids inspired by their biogenesis and radical-based retrosynthetic disconnections. Radical transformations are strategically and tactically utilized in our syntheses, and radical-based reactions, including organo-SOMO catalysis, Birch reduction, regioselective 1,6-dienyne reductive cyclization, visible-light-mediated Schenck ene reaction, and photoradical-mediated late-stage skeletal rearrangement, play significant roles in our synthetic endeavors. Protecting-group-free and scalable syntheses are also built into our work to achieve the "ideal" synthesis. Furthermore, our synthetic work reveals that late-stage skeletal rearrangement through a photo radical process is possible in a biological setting in complement with nature's carbocation chemistry in complex natural product biosynthesis.Lycopodium alkaloids are a large family of structurally unique polycyclic natural products with impressive biological activities. Owing to their fascinating polycyclic architectures and diverse biological activities, these alkaloids have continued to serve as targets as well as inspirations for the synthetic community for decades. To access these bioactive natural products or natural product-like molecules for biological exploration and drug discovery, we applied a novel functional group pairing strategy to furnish the total syntheses for several Lycopodium alkaloids and obtained numerous skeletally diverse compounds with structural complexity comparable to natural products.

Site-switchable mono-O-allylation of polyols

Site-selective modification of complex molecules allows for rapid accesses to their analogues and derivatives, and, therefore, offers highly valuable opportunities to probe their functions. However, to selectively manipulate one out of many repeatedly occurring functional groups within a substrate represents a grand challenge in chemistry. Yet more demanding is to develop methods in which alterations to the reaction conditions lead to switching of the specific site of reaction. We report herein the development of a Pd/Lewis acid co-catalytic system that achieves not only site-selective, but site-switchable mono-O-allylation of polyols with readily available reagents and catalysts. Through exchanging the Lewis acid additives that recognize specific hydroxyls in a polyol substrate, our system managed to install a versatile allyl group to the target in a site-switchable manner. Our design demonstrates remarkable scope, and is amenable to the direct derivatization of various complex, bioactive natural products.

Selective manipulation of one functional group, out of many repeatedly occurring in a substrate, represents a grand challenge in chemistry. Here, the authors report a Pd/Lewis acid cocatalytic system that achieves not only site-selective, but also site-switchable mono-O-allylation of polyols.

A Pyridine-Acetylene-Aniline Oligomer: Saccharide Recognition and Influence of this Recognition Array on the Activity as Acylation Catalyst

In order to create new functions of foldamer-type hosts, various kinds of recognition arrays are expected to be developed. Here, a pyridine-acetylene-aniline unit is presented as a new class of a saccharide recognition array. The conformational stabilities of this array were analyzed by DFT calculation, and suggested that a pyridine-acetylene-aniline oligomer tends to form a helical structure. An oligomer of this array was synthesized, and its association for octyl β-D-glucopyranoside was confirmed by ¹ H NMR measurements. UV/Vis, circular dichroism, and fluorescence titration experiments revealed its high affinity for octyl glycosides in apolar solvents (K_a =10⁴ to 10⁵  M^-1 ). This oligomer was relatively stable under basic conditions, and therefore this array was expected to be applied to the derivatization of saccharides. A 4-(dialkylamino)pyridine attached pyridine-acetylene-aniline oligomer proved to catalyze the acylation of the octyl glucoside.

Asymmetric Catalysis Mediated by Synthetic Peptides, Version 2.0: Expansion of Scope and Mechanisms

Low molecular weight synthetic peptides have been demonstrated to be effective catalysts for an increasingly wide array of asymmetric transformations. In many cases, these peptide-based catalysts have enabled novel multifunctional substrate activation modes and unprecedented selectivity manifolds. These features, along with their ease of preparation, modular and tunable structures, and often biomimetic attributes make peptides well-suited as chiral catalysts and of broad interest. Many examples of peptide-catalyzed asymmetric reactions have appeared in the literature since the last survey of this broad field in Chemical Reviews (Chem. Rev. 2007, 107, 5759-5812). The overarching goal of this new Review is to provide a comprehensive account of the numerous advances in the field. As a corollary to this goal, we survey the many different types of catalytic reactions, ranging from acylation to C-C bond formation, in which peptides have been successfully employed. In so doing, we devote significant discussion to the structural and mechanistic aspects of these reactions that are perhaps specific to peptide-based catalysts and their interactions with substrates and/or reagents.

DMSO-Enabled Selective Radical O-H Activation of 1,3(4)-Diols

Control of selectivity is one of the central topics in organic chemistry. Although unprecedented alkoxyl-radical-induced transformations have drawn a lot of attention, compared to selective C-H activation, selective radical O-H activation remains less explored. Herein, we report a novel selective radical O-H activation strategy of diols by combining spatial effects with proton-coupled electron transfer (PCET). It was found that DMSO is an essential reagent that enables the regioselective transformation of diols. Mechanistic studies indicated the existence of the alkoxyl radical and the selective interaction between DMSO and hydroxyl groups. Moreover, the distal C-C cleavage was realized by this selective alkoxyl-radical-initiation protocol.

Site-selective C-H hydroxylation of pentacyclic triterpenoids directed by transient chiral pyridine-imino groups

Pentacyclic triterpenoids (PTs) constitute one of the biggest families of natural products, many with higher oxidation state at the D/E rings possess a wide spectrum of biological activties but are poorly accessible. Here we report a site-selective C-H hydroxylation at the D/E rings of PTs paving a way toward these important natural products. We find that Schönecker and Baran's Cu-mediated aerobic oxidation can be applied and become site-selective on PT skeletons, as being effected unexpectedly by the chirality of the transient pyridine-imino directing groups. To prove the applicability, starting from the most abundant triterpenoid feedstock oleanane, three representative saponins bearing hydroxyl groups at C16 or C22 are expeditiously synthesized, and barringtogenol C which bears hydroxyl groups at C16, C21, and C22 is synthesized via a sequential hydroxylation as the key steps.

Mapping and Exploiting the Promiscuity of OxyB toward the Biocatalytic Production of Vancomycin Aglycone Variants

Vancomycin is one of the most important clinical antibiotics in the fight against infectious disease. Its biological activity relies on three aromatic cross-links, which create a cup-shaped topology and allow tight binding to nascent peptidoglycan chains. The cytochrome P450 enzymes OxyB, OxyA, and OxyC have been shown to introduce these synthetically challenging aromatic linkages. The ability to utilize the P450 enzymes in a chemo-enzymatic scheme to generate vancomycin derivatives is appealing but requires a thorough understanding of their reactivities and mechanisms. Herein, we systematically explore the scope of OxyB biocatalysis and report installation of diverse diaryl ether and biaryl cross-links with varying macrocycle sizes and compositions, when the enzyme is presented with modified vancomycin precursor peptides. The structures of the resulting products were determined using one-dimensional/two-dimensional nuclear magnetic resonance spectroscopy, high-resolution mass spectrometry (HR-MS), tandem HR-MS, and isotopic labeling, as well as ultraviolet-visible light absorption and fluorescence emission spectroscopies. An exploration of the biological activities of these alternative OxyB products surprisingly revealed antifungal properties. Taking advantage of the promiscuity of OxyB, we chemo-enzymatically generated a vancomycin aglycone variant containing an expanded macrocycle. Mechanistic implications for OxyB and future directions for creating vancomycin analogue libraries are discussed.

Chrysomycin A Derivatives for the Treatment of Multi-Drug-Resistant Tuberculosis

Tuberculosis (TB) is a life-threatening disease resulting in an estimated 10 million new infections and 1.8 million deaths annually, primarily in underdeveloped countries. The economic burden of TB has been estimated as approximately 12 billion USD annually in direct and indirect costs. Additionally, multi-drug-resistant (MDR) and extreme-drug-resistant (XTR) TB strains resulting in about 250 000 deaths annually are now widespread, increasing pressure on the identification of new anti-TB agents that operate by a novel mechanism of action. Chrysomycin A is a rare C-aryl glycoside first discovered over 60 years ago. In a recent high-throughput screen, we found that chrysomycin A has potent anti-TB activity, with minimum inhibitory concentration (MIC) = 0.4 μg/mL against MDR-TB strains. However, chrysomycin A is obtained in low yields from fermentation of Streptomyces, and the mechanism of action of this compound is unknown. To facilitate the mechanism of action and preclinical studies of chrysomycin A, we developed a 10-step, scalable synthesis of the isolate and its two natural congeners polycarcin V and gilvocarcin V. The synthetic sequence was enabled by the implementation of two sequential C-H functionalization steps as well as a late-stage C-glycosylation. In addition, >10 g of the advanced synthetic intermediate has been prepared, which greatly facilitated the synthesis of 33 new analogues to date. The structure-activity relationship was subsequently delineated, leading to the identification of derivatives with superior potency against MDR-TB (MIC = 0.08 μg/mL). The more potent derivatives contained a modified carbohydrate residue which suggests that further optimization is additionally possible. The chemistry we report here establishes a platform for the development of a novel class of anti-TB agents active against drug-resistant pathogens.

Enantioselective Hydroxylation of Benzylic C(sp3)-H Bonds by an Artificial Iron Hydroxylase Based on the Biotin-Streptavidin Technology

The selective hydroxylation of C–H bonds is of great interest to the synthetic community. Both homogeneous catalysts and enzymes offer complementary means to tackle this challenge. Herein, we show that biotinylated Fe(TAML)-complexes (TAML = Tetra Amido Macrocyclic Ligand) can be used as cofactors for incorporation into streptavidin to assemble artificial hydroxylases. Chemo-genetic optimization of both cofactor and streptavidin allowed optimizing the performance of the hydroxylase. Using H₂O₂ as oxidant, up to ∼300 turnovers for the oxidation of benzylic C–H bonds were obtained. Upgrading the ee was achieved by kinetic resolution of the resulting benzylic alcohol to afford up to >98% ee for (R)-tetralol. X-ray analysis of artificial hydroxylases highlights critical details of the second coordination sphere around the Fe(TAML) cofactor.

Late-Stage Diversification of Natural Products

Late-stage diversification of natural products is an efficient way to generate natural product derivatives for drug discovery and chemical biology. Benefiting from the development of site-selective synthetic methodologies, late-stage diversification of natural products has achieved notable success. This outlook will outline selected examples of novel methodologies for site-selective transformations of reactive functional groups and inert C-H bonds that enable late-stage diversification of complex natural products. Accordingly, late-stage diversification provides an opportunity to rapidly access various derivatives for modifying lead compounds, identifying cellular targets, probing protein-protein interactions, and elucidating natural product biosynthetic relationships.

Base- and Catalyst-Induced Orthogonal Site Selectivities in Acylation of Amphiphilic Diols

Seeking to selectively functionalize natural and synthetic amphiphiles, we explored acylation of model amphiphilic diols. The use of a nucleophilic catalyst enabled a remarkable shift of the site selectivity from the polar site, preferred in background noncatalyzed or base-promoted reactions, to the apolar site. This tendency was significantly enhanced for organocatalysts comprising an imidazole active site surrounded by long/branched tails. An explanation of these orthogonal modes of selectivity is supported by competitive experiments with monoalcohol substrates.

Catalysis-Enabled Access to Cryptic Geldanamycin Oxides

Catalytic, selective modifications of natural products can be a fertile platform for not only unveiling new natural product analogues with altered biological activity, but also for revealing new reactivity and selectivity hierarchies for embedded functional groups in complex environments. Motivated by these intersecting aims, we report site- and stereoselective oxidation reactions of geldanamycin facilitated by aspartyl-peptide catalysts. Through the isolation and characterization of four new geldanamycin oxides, we discovered a synergistic effect between lead peptide-based catalysts and geldanamycin, resulting in an unexpected reaction pathway. Curiously, our discoveries would likely not have been possible absent the attractive noncovalent interactions intrinsic to both the catalysts and the natural product. The result is a set of new "meta" catalytic reactions that deliver both unknown and previously incompletely characterized geldanamycin analogues. Enabled by the catalytic, site-selective epoxidation of geldanamycin, biological assays were carried out to document the bioactivities of the new compounds.

Recent trends in catalytic sp3 C-H functionalization of heterocycles

Heterocycles are a ubiquitous substructure in organic small molecules designed for use in materials and medicines. Recent work in catalysis has focused on enabling access to new heterocycle structures by sp³ C-H functionalization on alkyl side-chain substituents-especially at the heterobenzylic position-with more than two hundred manuscripts published just within the last ten years. Rather than describing in detail each of these reports, in this mini-review we attempt to highlight gaps in existing techniques. A semi-quantitative overview of ongoing work strongly suggests that several specific heterocycle types and bond formations outside of C-C, C-N, and C-O have been almost completely overlooked.

Cobalt(III)-Catalyzed C-H Amidation of Dehydroalanine for the Site-Selective Structural Diversification of Thiostrepton

Thiostrepton is a potent antibiotic against a broad range of Gram-positive bacteria, but its medical applications have been limited by its poor aqueous solubility. In this work, the first C(sp2 )-H amidation of dehydroalanine (Dha) residues was applied to the site selective modification of thiostrepton to prepare a variety of derivatives. Unlike all prior methods for the modification of thiostrepton, the alkene framework of the Dha residue is preserved and with complete selectivity for the Z-stereoisomer. Additionally, an aldehyde group was introduced by C-H amidation, enabling oxime ligation for the installation of an even greater range of functionality. The thiostrepton derivatives generally maintain antimicrobial activity, and importantly, eight of the derivatives displayed improved aqueous solubility (up to 28-fold), thereby addressing a key shortcoming of this antibiotic. The exceptional functional group compatibility and site selectivity of CoIII -catalyzed C(sp2 )-H Dha amidation suggests that this approach could be generalized to other natural products and biopolymers containing Dha residues.

Dehydration reactions in polyfunctional natural products

Here, we review methods for chemical dehydration of alcohols to alkenes and discuss the potential of late-stage functionalization by direct, site- and chemo-selective dehydration of complex molecular substrates.

Iron- and cobalt-catalyzed C(sp3)–H bond functionalization reactions and their application in organic synthesis

This review highlights the developments in iron and cobalt catalyzed C(sp³)–H bond functionalization reactions with emphasis on their applications in organic synthesis, i.e. natural products and pharmaceuticals synthesis and/or modification.

Facile access to evodiakine enabled by aerobic copper-catalyzed oxidative rearrangement

Oxidation as a fundamentally important method for the synthesis of complex structures is difficult to achieve in a selective manner. Evodiakine, a complex natural product possessing an unprecedented ring system (6/5/5/7/6), has a high oxidation state without a practical solution. Herein, we report the first synthesis of evodiakine via aerobic copper-catalyzed late-stage functionalization of evodiamine.

Selective deoxygenation of gibberellic acid with fluoroarylborane catalysts

Reductive late-stage functionalization of gibberellic acid is reported using three fluoroarylborane Lewis acids; (B(C₆F₅)₃, B(3,5-C₆H₃(CF₃)₂), and B(2,4,6-C₆H₂F₃)₃) in combination with a tertiary silane and a borane (HBCat) reductant. In each case, C-O bond activation occurs, and different products are obtained depending on the reductant and catalyst employed.

Site-Selective and Stereoselective O-Alkylation of Glycosides by Rh(II)-Catalyzed Carbenoid Insertion

Carbohydrates are synthetically challenging molecules with vital biological roles in all living systems. Selective synthesis and functionalization of carbohydrates provide tremendous opportunities to improve our understanding on the biological functions of this fundamentally important class of molecules. However, selective functionalization of seemingly identical hydroxyl groups in carbohydrates remains a long-standing challenge in chemical synthesis. We herein describe a practical and predictable method for the site-selective and stereoselective alkylation of carbohydrate hydroxyl groups via Rh(II)-catalyzed insertion of metal carbenoid intermediates. This represents one of the mildest alkylation methods for the systematic modification of carbohydrates. Density functional theory (DFT) calculations suggest that the site selectivity is determined in the Rh(II)-carbenoid insertion step, which prefers insertion into hydroxyl groups with an adjacent axial substituent. The subsequent intramolecular enolate protonation determines the unexpected high stereoselectivity. The most prevalent trans-1,2-diols in various pyranoses can be systematically and predictably differentiated based on the model derived from DFT calculations. We also demonstrated that the selective O-alkylation method could significantly improve the efficiency and stereoselectivity of glycosylation reactions. The alkyl groups introduced to carbohydrates by OH insertion reaction can serve as functional groups, protecting groups, and directing groups.

Site-selective acylation of natural products with BINOL-derived phosphoric acids

The site-selective acylation of a steroidal natural product 19-hydroxydehydroepiandrosterone catalyzed by 1,1'-Bi(2-napthol)-derived (BINOL) chiral phosphoric acids (CPA's) is described. Systematic variation and multivariate linear regression analysis reveal that the same steric parameters typically needed for high enantioselectivity with this class of CPAs are also required for site-selectivity in this case. Density functional theory calculations identify additional weak CH-π interactions as contributors to site discrimination. We further report a rare example of site-selective acylation of phenols through the evaluation of naringenin, a flavonoid natural product, using CPA catalysis. These results suggest that BINOL-derived CPA's may have broader applications in site-selective catalysis.

Highly efficient asymmetric bioreduction of 1-aryl-2-(azaaryl)ethanones. Chemoenzymatic synthesis of lanicemine

Different ketoreductases (KREDs) have been used to promote a highly selective reduction of several 1-aryl-2-(azaaryl)ethanones (azaaryl = pyridinyl, quinolin-2-yl), the corresponding secondary alcohols being obtained with very high yields and enantiomeric excesses (ee > 99%). The absolute configuration of each optically active alcohol has been assigned by means of modified Mosher and Kelly methods, two shielding effects being evaluated: (1) the Mosher phenyl ring effect on the azaaryl protons and (2) the one of the azaaryl ring on the Mosher methoxy group. In addition, the biologically active amine lanicemine has been synthesized from (R)-1-phenyl-2-(pyridin-2-yl)ethanol, thus proving the utility of the secondary alcohols here prepared.