Total Synthesis of (+)-Erogorgiaene Using Lithiation–Borylation Methodology, and Stereoselective Synthesis of Each of Its Diastereoisomers

Peptidic Boronic Acid Plasmodium falciparum SUB1 Inhibitors with Improved Selectivity over Human Proteasome

Plasmodium falciparum subtilisin-like serine protease 1 (PfSUB1) is essential for egress of invasive merozoite forms of the parasite, rendering PfSUB1 an attractive antimalarial target. Here, we report studies aimed to improve drug-like properties of peptidic boronic acid PfSUB1 inhibitors including increased lipophilicity and selectivity over human proteasome (H20S). Structure-activity relationship investigations revealed that lipophilic P3 amino acid side chains as well as N-capping groups were well tolerated in retaining PfSUB1 inhibitory potency. At the P1 position, replacing the methyl group with a carboxyethyl substituent led to boralactone PfSUB1 inhibitors with remarkably improved selectivity over H20S. Combining lipophilic end-capping groups with the boralactone reduced the selectivity over H20S. However, compound 4c still showed >60-fold selectivity versus H20S and low nanomolar PfSUB1 inhibitory potency. Importantly, this compound inhibited the growth of a genetically modified P. falciparum line expressing reduced levels of PfSUB1 13-fold more efficiently compared to a wild-type parasite line.

Autoxidation of a C2-Olefinated Dihydroartemisinic Acid Analogue to Form an Aromatic Ring: Application to Serrulatene Biosynthesis

Dihydroartemisinic acid (DHAA) is a plant natural product that undergoes a spontaneous endoperoxide-forming cascade reaction to yield artemisinin in the presence of air. The endoperoxide functional group gives artemisinin its biological activity that kills Plasmodium falciparum, the parasite that causes malaria. To enhance our understanding of the mechanism of this cascade reaction, 2,3-didehydrodihydroartemisinic acid (2,3-didehydro-DHAA), a DHAA derivative with a double bond at the C2-position, was synthesized. When 2,3-didehydro-DHAA was exposed to air over time, instead of forming an endoperoxide, this compound predominantly underwent aromatization. This olefinated DHAA analogue reveals the requirement of a monoalkene functional group to initiate the endoperoxide-forming cascade reaction to yield artemisinin from DHAA. In addition, this aromatization process was exploited to illustrate the autoxidation process of a different plant natural product, dihydroserrulatene, to form the aromatic ring in serrulatene. This spontaneous aromatization process has applications in other natural products such as leubethanol and erogorgiaene. Due to their similarity in structure to antimicrobial natural products, the synthesized compounds in this study were tested for biological activity. A group of the tested compounds had minimum inhibitory concentration (MIC) values ranging from 12.5 to 25 μg/mL against the bacterial pathogen Staphylococcus aureus and the fungal pathogen Cryptococcus neoformans.

Developing a Methodology for Catalytic Asymmetric Crotylation of Aldehydes

Asymmetric crotylation has firmly earned a place among the set of valuable synthetic tools for stereoselective construction of carbon skeletons. For a long time the field was heavily dominated by reagents bearing stoichiometric chiral auxiliaries, but now catalytic methods are gradually taking center stage, and the area continues to develop rapidly. This account focuses primarily on preformed organometallic reagents based on silicon and, to some extent, boron. It narrates our endeavors to design new and efficient chiral Lewis base catalysts for the asymmetric addition of crotyl(trichloro)silanes to aldehydes. It also covers the development of a novel protocol for kinetic resolution of racemic secondary allylboronates to give enantio- and diastereomerically enriched linear homoallylic alcohols. As a separate topic, cross-crotylation of aldehydes by using enantiopure branched homoallylic alcohols as a source of crotyl groups is discussed. Finally, the synthetic credentials of the developed methodology are illustrated by total syntheses of marine natural products, in which crotylation plays a key role in setting up stereogenic centers.

1 Introduction

2 Pyridine N-Oxides as Lewis Base Catalysts

3 Bipyridine N,N′-Dioxides as Lewis Base Catalysts

4 Chiral Allylating Reagents

5 Synthetic Applications

6 Concluding Remarks

Total Synthesis of (+)-Erogorgiaene and the Pseudopterosin A-F Aglycone via Enantioselective Cobalt-Catalyzed Hydrovinylation

Due to their pronounced bioactivity and limited availability from natural resources, metabolites of the soft coral Pseudopterogorgia elisabethae, such as erogorgiaene and the pseudopterosines, represent important target molecules for chemical synthesis. We have now developed a particularly short and efficient route towards these marine diterpenes exploiting an operationally convenient enantioselective cobalt-catalyzed hydrovinylation as the chirogenic step. Other noteworthy C-C bond forming transformations include diastereoselective Lewis acid-mediated cyclizations, a Suzuki coupling and a carbonyl ene reaction. Starting from 4-methyl-styrene the anti-tubercular agent (+)-erogorgiaene (>98 % ee) was prepared in only 7 steps with 46 % overall yield. In addition, the synthesis of the pseudopterosin A aglycone was achieved in 12 steps with 30 % overall yield and, surprisingly, was found to exhibit a similar anti-inflammatory activity (inhibition of LPS-induced NF-κB activation) as a natural mixture of pseudopterosins A-D or iso-pseudopterosin A, prepared by β-D-xylosylation of the synthetic aglycone.

Temporary or removable directing groups enable activation of unstrained C-C bonds

Carbon-carbon (C-C) bonds constitute basic skeletons in most organic molecules. One can imagine that selective manipulation of C-C bonds would provide a direct approach to edit or alter molecular scaffolds but has been an ongoing challenge. Due to the kinetic inertness of C-C bonds, the common strategies of activating these bonds by transition metals rely on either the use of highly strained substrates or the assistance of a permanent directing group (DG), in which strain relief or formation of stable metallocycles becomes the driving force. To allow more common and less strained compounds utilized as substrates for C-C activation, the use of temporary and removable DGs has emerged as an attractive strategy in the past two decades. A variety of C-C bonds in unstrained or less strained organic molecules now can be converted to more reactive metal-carbon bonds, and further downstream transformations have led to diverse synthetic methods. This review highlights the development of catalytic approaches that can activate unstrained C-C bonds enabled by temporary or removable DGs. The content is mainly divided based on the nature of the DGs: temporary and removable. Applications of these methods in syntheses of natural products or bioactive molecules are also discussed.

Asymmetric Total Syntheses of Di- and Sesquiterpenoids by Catalytic C-C Activation of Cyclopentanones

To show the synthetic utility of the catalytic C-C activation of less strained substrates, described here are the collective and concise syntheses of the natural products (-)-microthecaline A, (-)-leubehanol, (+)-pseudopteroxazole, (+)-seco-pseudopteroxazole, pseudopterosin A-F and G-J aglycones, and (+)-heritonin. The key step in these syntheses involve a Rh-catalyzed C-C/C-H activation cascade of 3-arylcyclopentanones, which provides a rapid and enantioselective route to access the polysubstituted tetrahydronaphthalene cores presented in these natural products. Other important features include 1) the direct C-H amination of the tetralone substrate in the synthesis of (-)-microthecaline A, 2) the use of phosphoric acid to enhance efficiency and regioselectivity for problematic cyclopentanone substrates in the C-C activation reactions, and 3) the direct conversion of serrulatane into amphilectane diterpenes by an allylic cyclodehydrogenation coupling.

Transition-metal-free radical relay cyclization of vinyl azides with 1,4-dihydropyridines involving a 1,5-hydrogen-atom transfer: access to α-tetralone scaffolds

The remote C(sp³)–H functionalization enabled by a radical-mediated 1,5-hydrogen-atom transfer (HAT) process using vinyl azides and 1,4-dihydropyridines as precursors has been described.

A general asymmetric copper-catalysed Sonogashira C(sp3)-C(sp) coupling

Continued development of the Sonogashira coupling has made it a well established and versatile reaction for the straightforward formation of C-C bonds, forging the carbon skeletons of broadly useful functionalized molecules. However, asymmetric Sonogashira coupling, particularly for C(sp³)-C(sp) bond formation, has remained largely unexplored. Here we demonstrate a general stereoconvergent Sonogashira C(sp³)-C(sp) cross-coupling of a broad range of terminal alkynes and racemic alkyl halides (>120 examples) that are enabled by copper-catalysed radical-involved alkynylation using a chiral cinchona alkaloid-based P,N-ligand. Industrially relevant acetylene and propyne are successfully incorporated, laying the foundation for scalable and economic synthetic applications. The potential utility of this method is demonstrated in the facile synthesis of stereoenriched bioactive or functional molecule derivatives, medicinal compounds and natural products that feature a range of chiral C(sp³)-C(sp/sp²/sp³) bonds. This work emphasizes the importance of radical species for developing enantioconvergent transformations.

Total synthesis of the plant alkaloid racemic microthecaline A: first example of a natural product bearing a tricyclic quinoline-serrulatane scaffold

The first total synthesis of racemic microthecaline A, a quinoline serrulatane alkaloid, isolated from the Australian desert plant Eremophila microtheca is described. The natural product was synthesized in ten steps, starting from ethyl 4-bromo-6-methoxy-8-methylquinoline-3-carboxylate in 8% overall yield.

First total synthesis of quinoline-serrulatane alkaloid microthecaline A.

Broadly Applicable Stereoselective Syntheses of Serrulatane, Amphilectane Diterpenes, and Their Diastereoisomeric Congeners Using Asymmetric Hydrovinylation for Absolute Stereochemical Control

A stereogenic center, placed at an exocyclic location next to a chiral carbon in a ring to which it is attached, is a ubiquitous structural motif seen in many bioactive natural products, including di- and triterpenes and steroids. Installation of these centers has been a long-standing problem in organic chemistry. Few classes of compounds illustrate this problem better than serrulatanes and amphilectanes, which carry multiple methyl-bearing exocyclic chiral centers. Nickel-catalyzed asymmetric hydrovinylation (AHV) of vinylarenes and 1,3-dienes such as 1-vinylcycloalkenes provides an exceptionally facile way of introducing these chiral centers. This Article documents our efforts to demonstrate the generality of AHV to access not only the natural products but also their various diastereoisomeric derivatives. Key to success here is the availability of highly tunable phosphoramidite Ni(II) complexes useful for overcoming the inherent selectivity of the chiral intermediates. The yields for hydrovinylation (HV) reactions are excellent, and selectivities are in the range of 92-99% for the desired isomers. Discovery of novel, configurationally fluxional, yet sterically less demanding 2,2'-biphenol-derived phosphoramidite Ni complexes (fully characterized by X-ray) turned out to be critical for success in several HV reactions. We also report a less spectacular yet equally important role of solvents in a metal-ammonia reduction for the installation of a key benzylic chiral center. Starting with simple oxygenated styrene derivatives, we iteratively install the various exocyclic chiral centers present in typical serrulatane [e.g., a (+)- p-benzoquinone natural product, elisabethadione, nor-elisabethadione, helioporin D, a known advanced intermediate for the synthesis of colombiasin and elisapterosin] and amphilectane [e.g., A-F, G-J, and K,L pseudopterosins] derivatives. A concise table showing various synthetic approaches to these molecules is included in the Supporting Information. Our attempts to synthesize a hitherto elusive target, elisabethin A, led to a stereoselective, biomimetic route to pseudopterosin A-F aglycones.

Enantiospecific sp2 -sp3 Coupling of ortho- and para-Phenols with Secondary and Tertiary Boronic Esters

The coupling of ortho- and para-phenols with secondary and tertiary boronic esters has been explored. In the case of para-substituted phenols, after reaction of a dilithio phenolate species with a boronic ester, treatment with Ph3 BiF2 or Martin's sulfurane gave the coupled product with complete enantiospecificity. The methodology was applied to the synthesis of the broad spectrum antibacterial natural product (-)-4-(1,5-dimethylhex-4-enyl)-2-methyl phenol. For ortho-substituted phenols, initial incorporation of a benzotriazole on the phenol oxygen atom was required. Subsequent ortho-lithiation and borylation gave the coupled product, again with complete stereospecificity.

Stereospecific functionalizations and transformations of secondary and tertiary boronic esters

This feature article discusses the range of stereospecific transformations available to enantioenriched boronic esters, and their applications in synthesis.

Catalytic activation of carbon-carbon bonds in cyclopentanones

In the chemical industry, molecules of interest are based primarily on carbon skeletons. When synthesizing such molecules, the activation of carbon-carbon single bonds (C-C bonds) in simple substrates is strategically important: it offers a way of disconnecting such inert bonds, forming more active linkages (for example, between carbon and a transition metal) and eventually producing more versatile scaffolds. The challenge in achieving such activation is the kinetic inertness of C-C bonds and the relative weakness of newly formed carbon-metal bonds. The most common tactic starts with a three- or four-membered carbon-ring system, in which strain release provides a crucial thermodynamic driving force. However, broadly useful methods that are based on catalytic activation of unstrained C-C bonds have proven elusive, because the cleavage process is much less energetically favourable. Here we report a general approach to the catalytic activation of C-C bonds in simple cyclopentanones and some cyclohexanones. The key to our success is the combination of a rhodium pre-catalyst, an N-heterocyclic carbene ligand and an amino-pyridine co-catalyst. When an aryl group is present in the C3 position of cyclopentanone, the less strained C-C bond can be activated; this is followed by activation of a carbon-hydrogen bond in the aryl group, leading to efficient synthesis of functionalized α-tetralones-a common structural motif and versatile building block in organic synthesis. Furthermore, this method can substantially enhance the efficiency of the enantioselective synthesis of some natural products of terpenoids. Density functional theory calculations reveal a mechanism involving an intriguing rhodium-bridged bicyclic intermediate.

Diborane-Mediated Deoxygenation of o-Nitrostyrenes To Form Indoles

A mild, transition metal-free, diborane-mediated deoxygenation of nitro groups was discovered that in situ generates nitrosoarene reactive intermediates. This new reactivity mode of B2pin2 was leveraged to construct indoles from o-nitrostyrenes through a reductive-cyclization reaction that exhibits a Hammett ρ-value of +0.97 relative to σpara values. Our new deoxygenation reaction is efficient, practical, and scaleable, enabling access to a broad range of indoles.

Gold-Catalyzed Proto- and Deuterodeboronation

A mild gold-catalyzed protodeboronation reaction, which does not require acid or base additives and can be carried out in "green" solvents, is described. As a result, the reaction is very functional-group-tolerant, even to acid- and base-sensitive functional groups, and should allow for the boronic acid group to be used as an effective traceless directing or blocking group. The reaction has also been extended to deuterodeboronations for regiospecific ipso-deuterations of aryls and heteroaryls from the corresponding organoboronic acid. Based on density functional theory calculations, a mechanism is proposed that involves nucleophilic attack of water at boron followed by rate-limiting B-C bond cleavage and facile protonolysis of a Au-σ-phenyl intermediate.

Base-promoted silver-catalyzed protodeboronation of arylboronic acids and esters

The first base-promoted silver-catalyzed efficient protodeboronation of arylboronic acids and esters in aqueous media was developed.

Lithiation-borylation methodology and its application in synthesis

Developing new methods that enable the synthesis of new and complex molecules with complete control of their 3-D shape is central to the advancement of synthetic chemistry with applications spanning from medicine to materials. Our approach consists of the iterative combination of small building blocks through the use of boron chemistry to essentially "grow" molecules. This approach, which we term assembly-line synthesis (ALS), resembles the way that nature assembles natural products (e.g., the polyketide synthase machinery) and has the advantage that many structural variations can be easily introduced and the products can be evaluated in structural or biological contexts. Chiral boronic esters have been recognized as valuable building blocks due to their unique chemical properties. They are both chemically and configurationally stable, and they can be prepared with very high levels of enantioselectivity. Additionally they undergo a broad array of transformations that lead to the stereocontrolled formation of C-C and C-X (X = heteroatom) bonds. This versatility makes boronic acids ideal building blocks for iterative molecular assembly. A powerful reaction platform for chemical diversification using chiral boronic esters is their homologation using lithium carbenoids via 1,2-metalate rearrangement. In the 1980s, Matteson described the use of boronic esters bearing a chiral diol in a two-step homologation process with dichloromethyl lithium and Grignard reagents (substrate-controlled approach). We have focused on reagent control and have found that Hoppe's chiral lithiated carbamates can be used as carbenoid equivalents in conjunction with achiral boronic esters. This reagent-controlled process offers many advantages due to the easy access of both the chiral lithiated carbamates and stable boronic esters. The carbamates can be derived from primary or secondary alcohols, and a broad range of functionalized boronic esters and boranes can be employed. Multiple homologations can be carried out in a one-pot sequence thereby streamlining the process to a single operation. This methodology has enabled the synthesis of many molecules containing multiple contiguous stereogenic centers with exquisite 3-D control. In this Account, we trace our own studies to establish the lithiation-borylation methodology and describe selected synthetic applications.

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.

Synthesis of hydroxyphthioceranic acid using a traceless lithiation-borylation-protodeboronation strategy

In planning organic syntheses, disconnections are most often made adjacent to functional groups, which assist in C-C bond formation. For molecules devoid of obvious functional groups this approach presents a problem, and so functionalities must be installed temporarily and then removed. Here we present a traceless strategy for organic synthesis that uses a boronic ester as such a group in a one-pot lithiation-borylation-protodeboronation sequence. To realize this strategy, we developed a methodology for the protodeboronation of alkyl pinacol boronic esters that involves the formation of a boronate complex with a nucleophile followed by oxidation with Mn(OAc)3 in the presence of the hydrogen-atom donor 4-tert-butylcatechol. Iterative lithiation-borylation-protodeboronation allows the coupling of smaller fragments to build-up long alkyl chains. We employed this strategy in the synthesis of hydroxyphthioceranic acid, a key component of the cell-wall lipid of the virulent Mycobacterium tuberculosis, in just 14 steps (longest linear sequence) with full stereocontrol.

Protodeboronation of ortho- and para-phenol boronic acids and application to ortho and meta functionalization of phenols using boronic acids as blocking and directing groups

The first metal-free thermal protodeboronation of ortho- and para-phenol boronic acids in DMSO was developed. The protodeboronation was successfully applied to the synthesis of ortho- and meta-functionalized phenols using the boronic acid moiety as a blocking group and a directing group, respectively. Mechanistic studies suggested that this protodeboronation proceeds through the coordination of water to the boron atom followed by σ-bond metathesis.

Programmed synthesis of a contiguous stereotriad motif by triple stereospecific reagent-controlled homologation

All distinct diastereoisomers of a contiguous stereotriad motif were separately targeted by a triple chain extension of B-phenethyl boronic esters using four unique presentation sequences of enantiomorphs of 1-[(2)H]-1-chloro-2-(1,3-dioxolan-2-yl)ethyllithium. The (R)- or (S)-configured chloroalkyllithium reagents were generated by sulfoxide-lithium exchange from the appropriate scalemic p-tolyl chloroalkyl sulfoxides using phenyllithium (THF, -78 °C). Stereotriad synthesis was accomplished in a single reaction vessel [7-19% yield, typical dr ≥ 74 (target):26 (∑ all other isomers)] and implemented by a simple algorithm consisting of reagent charging and temperature cycling events.