A concise enantioselective synthesis of the chlorosulfolipid malhamensilipin A

Dinuclear Titanium(III)-Catalyzed Radical-Type Kinetic Resolution of Epoxides for the Enantioselective Synthesis of cis-Glycidic Esters

Glycidic esters represent pivotal constituents in synthetic chemistry, offering enhanced versatility for tailoring toward a diverse array of molecular targets in comparison with simple epoxides. While considerable progress has been made in the asymmetric synthesis of trans- and trisubstituted glycidic esters, achieving enantioselective preparation of cis-glycidic esters has remained a long-standing challenge. Here, we demonstrate a selectivity-predictable modular platform for the asymmetric synthesis of cis-glycidic esters via a novel dinuclear (salen)titanium(III)-catalyzed radical-type kinetic resolution (KR) approach. This radical KR protocol operates under mild conditions and demonstrates a wide substrate scope, facilitating the synthesis of alkyl- and aryl-substituted cis-glycidic esters with high levels of regioselectivity and enantioselectivity, along with hydroxy ester byproducts representing synthetically valuable motifs as well. This study presents a unique exploration of radical-type KR applied to epoxides, effectively overcoming the steric challenges inherent in conventional nucleophilic-type methodologies typically employed in epoxide chemistry.

Stereospecific syn-dihalogenations and regiodivergent syn-interhalogenation of alkenes via vicinal double electrophilic activation strategy

Whereas the conventional anti-dihalogenation of alkenes is a valuable synthetic tool with highly predictable stereospecificity, the restricted reaction mechanism makes it challenging to alter the diastereochemical course into the complementary syn-dihalogenation process. Only a few notable achievements were made recently by inverting one of the stereocenters after anti-addition using a carefully designed reagent system. Here, we report a conceptually distinctive strategy for the simultaneous double electrophilic activation of the two alkene carbons from the same side. Then, the resulting vicinal leaving groups can be displaced iteratively by nucleophilic halides to complete the syn-dihalogenation. For this purpose, thianthrenium dication is employed, and all possible combinations of chlorine and bromine are added onto internal alkenes successfully, particularly resulting in the syn-dibromination and the regiodivergent syn-bromochlorination.

The Rise of Trichlorides Enabling an Improved Chlorine Technology

Chlorine plays a central role for the industrial production of numerous materials with global relevance. More recently, polychlorides have been evolved from an area of academic interest to a research topic with enormous industrial potential. In this minireview, the value of trichlorides for chlorine storage and chlorination reactions are outlined. Particularly, the inexpensive ionic liquid [NEt₃ Me][Cl₃ ] shows a similar and sometimes even advantageous reactivity compared to chlorine gas, while offering a superior safety profile. Used as a chlorine storage, [NEt₃ Me][Cl₃ ] could help to overcome the current limitations of storing and transporting chlorine in larger quantities. Thus, trichlorides could become a key technique for the flexibilization of the chlorine production enabling an exploitation of renewable, yet fluctuating, electrical energy. As the loaded storage, [NEt₃ Me][Cl₃ ], is a proven chlorination reagent, it could directly be employed for downstream processes, paving the path to a more practical and safer chlorine industry.

Asymmetric Total Synthesis of Clionastatins A and B

Herein we report the first total synthesis of polychlorinated steroids clionastatins A and B, which was accomplished asymmetrically by means of a convergent, radical fragment coupling approach. Key features of the synthesis include an Ireland-Claisen rearrangement to introduce the C5 stereocenter (which was ultimately transferred to the C10 quaternary stereocenter of the clionastatins via a traceless stereochemical relay), a regioselective acyl radical conjugate addition to join the two fragments, an intramolecular Heck reaction to install the C10 quaternary stereocenter, and a diastereoselective olefin dichlorination to establish the synthetically challenging pseudoequatorial dichlorides. This work also enabled us to determine that the true structures of clionastatins A and B are in fact C14 epimers of the originally proposed structures.

The Recurring Roles of Chlorine in Synthetic and Biological Studies of the Lissoclimides

Halogenated natural products number in the thousands, but only in rare cases are the evolutionary advantages conferred by the halogens understood. We set out to investigate the lissoclimide family of cytotoxins, which includes several chlorinated members, because of our long-standing interest in the synthesis of chlorinated secondary metabolites.Our initial success in this endeavor was a semisynthesis of chlorolissoclimide (CL) from the commercially available sesquiterpenoid sclareolide. Featuring a highly selective and efficient-and plausibly biomimetic-C-H chlorination, we were able to access enough CL for collaborative studies, including X-ray cocrystallography with the eukaryotic ribosome. Through this experiment, we learned that CL's chlorine atom engages in a novel halogen-π dispersion interaction with a neighboring nucleobase in the ribosome E-site.Owing to the limitations of our semisynthesis approach, we established an analogue-oriented approach to access numerous lissoclimide compounds to both improve our understanding of structure-activity relationships and to learn more about the halogen-π interaction. In the course of these studies, we made over a dozen lissoclimide-like compounds, the most interesting of which contained chlorine-bearing carbons with unnatural configurations. Rationalizing the retained potency of these compounds that appeared to be a poor fit for the lissoclimide binding pocket, we came to realize that the chlorine atoms would engage in these same halogen-π interactions even at the expense of a chair to twist-boat conformational change, which also permitted the compounds to fit in the binding site.Finally, because neither of the first two approaches could easily access the most potent natural lissoclimides, we designed a synthesis that took advantage of rarely used terminal epoxides to initiate polyene cyclizations. In this case, the chlorine atom was incorporated early and helped control the stereochemical outcome of the key step.Over the course of this project, three different synthesis approaches were designed and executed, and our ability to access numerous lissoclimides fueled a range of collaborative biological studies. Further, chlorine played impactful roles throughout various aspects of both synthesis and biology. We remain inspired to learn more about the mechanism of action of these compounds and to deeply investigate the potentially valuable halogen-π dispersion interaction in the context of small molecule/nucleic acid binding. In that context, our work offers an instance wherein we might have gained a rudimentary understanding of the evolutionary importance of the halogen in a halogenated natural product.

Construction of sulfur-containing moieties in the total synthesis of natural products

This review surveys the total syntheses of sulfur-containing natural products where sulfur atoms are introduced with different sulfurization agents to construct related sulfur-containing moieties.

Neighboring Effect of Intramolecular Chlorine Atoms on Epoxide Opening Reaction by Chloride Anions

We investigated the diastereoselectivity of ring openings for chloro vinyl epoxides with various chlorination reagents. In the chlorinolysis reactions using vinyl epoxides having an allyl alcohol, inversion:retention ratios varied depending on the chloride sources. In limited cases, the increase in retention ratio was consistent with the intervention of chloronium ions. In contrast, all vinyl epoxides bearing an α,β-unsaturated ester gave only the inversion products. These results suggest the electron-withdrawing property suppressed the chloronium ions.

Total Synthesis as a Vehicle for Collaboration

"Collaboration" is not the first word most would associate with the field of total synthesis. In fact, the spirit of total synthesis is all-too-often reputed as being more competitive, rather than collaborative, sometimes even within individual laboratories. However, recent studies in total synthesis have inspired a number of collaborative efforts that strategically blend synthetic methodology, biocatalysis, biosynthesis, computational chemistry, and drug discovery with complex molecule synthesis. This Perspective highlights select recent advances in these areas, including collaborative syntheses of chlorolissoclimide, nigelladine A, artemisinin, ingenol, hippolachnin A, communesin A, and citrinalin B. The legendary Woodward-Eschenmoser collaboration that led to the total synthesis of vitamin B₁₂ is also discussed.

Organoselenium-catalyzed enantioselective syn-dichlorination of unbiased alkenes

The enantioselective dichlorination of alkenes is a continuing challenge in organic synthesis owing to the limitations of selective and independent antarafacial delivery of both electrophilic chlorenium and nucleophilic chloride to an olefin. Development of a general method for the enantioselective dichlorination of isolated alkenes would allow access to a wide variety of polyhalogenated natural products. Accordingly, the enantioselective suprafacial dichlorination of alkenes catalyzed by electrophilic organoselenium reagents has been developed to address these limitations. The evaluation of twenty-three diselenides as precatalysts for enantioselective dichlorination is described, with a maximum e.r. of 76:24 Additionally, mechanistic studies suggest an unexpected Dynamic Kinetic Asymmetric Transformation (DyKAT) process may be operative.

Stereochemical Revision, Total Synthesis, and Solution State Conformation of the Complex Chlorosulfolipid Mytilipin B

Chlorosulfolipids constitute a structurally intriguing and synthetically challenging class of marine natural products that are isolated from mussels and freshwater algae. The most complex structure from this family of compounds is currently represented by Mytilipin B, isolated in 2002 from culinary mussel Mytilus galloprovincialis, whose initially proposed structure was shown to be incorrect. In this study, we present the synthesis of four diastereomers which allowed the reassignment of eight stereocenters and the stereochemical revision of Mytilipin B, along with the determination of the dominant solution-state conformation.

Enantioselective Synthesis of β-Allenoates via Phosphine-Catalyzed and ZnI2-Promoted Preparation of Oxazolidines and Propargylamines Using Chiral Amines, 1-Alkynes, and Propiolates

Diphenylphosphinoethane (DPPE)-catalyzed and ZnI₂-promoted in situ formation of oxazolidine, alkynyl zinc, and propargylamine intermediates from 1-alkynes, chiral (S)-diphenyl(pyrrolidin-2-yl)methanol, and propiolates gave the corresponding chiral (R)-β-allenoates in 40-72% yield with up to >99% ee. The intermediate propargylamine was isolated in 50% yield and converted to give the β-allenoate 10aa in 68% yield and 96% ee upon reaction with ZnI₂. The results are discussed considering a mechanism involving oxazolidine and iminium ions formed in situ followed by addition of alkynyl zinc complex to produce the propargylamine that gives the corresponding allenoate via a 1,5-hydrogen shift in the presence of ZnI₂.

Second Generation of Aldol Reaction

Since the discovery of the Mukaiyama aldol reaction more than 40 years ago, several landmark publications have inspired researchers in the field. The Mukaiyama AR is one of the most significant named reactions in organic synthesis. In the past few decades, development of the modern AR has been at the forefront in addressing the challenges of regio-, chemo-, diastereo-, and enantioselectivity in organic synthesis. All of these selectivity challenges maybe present in a single pair of reactants, thus controlling the outcome of such a process has great practical value. More than 10 years ago, our group became involved in this iconic carbon-carbon bond-forming process and attempted to very closely investigate all possible features of the AR to solve several issues still encountered by chemists, most notably the selectivity challenges mentioned above. In this context, our group initiated the second generation of the AR based on a Lewis or Brønsted acid-catalyzed process in conjunction with the use of a "super silyl" (tris(trimethylsilyl)silyl) directing group, which has demonstrated unrivalled properties in controlling the outcome of the AR. Using the extraordinary power of the super silyl group, we were able to develop new methods and concepts that broadly impacted the ability to control the selectivity attributes and thus allowed for a highly stereoselective construction of polyketide, halogenated polyketide, polypropionate, and polyol scaffolds through inter- and/or intramolecular aldolization protocols. Our diastereoselective ARs of super silyl enol ethers and aldehydes have shown great efficiency and modularity in producing exclusively and preferentially syn- or anti-adducts, creating up to four new adjacent stereocenters in a one-pot sequential manner and under mild reaction conditions. The super silyl-directed AR does not only provide a solution to stereochemistry control challenges, but also offers an efficient, modular and high yielding technique toward nontrivial construction of complex architectures with unprecedented ease. We believe that the new Lewis- or Brønsted-acid-catalyzed super-silyl-directed AR processes chronicled in our laboratories have come to maturity and now offer a "road map" for strategic stereoselective synthesis of polyketide-like units. Herein we report our recent achievements in the diastereoselective C-C bond formation, through the super-silyl-directed AR, toward the synthesis of complex and sophisticated hydroxy aldehydes. We would like to note that due to the extremely broad range of work reported in this field, only stereoselective AR involving aldehyde-derived super SEEs will be discussed in this Account.

Synthesis of malhamensilipin A exploiting iterative epoxidation/chlorination: experimental and computational analysis of epoxide-derived chloronium ions

We report a 12-step catalytic enantioselective formal synthesis of malhamensilipin A (3) based upon an iterative epoxidation/chlorination strategy.

We report a 12-step catalytic enantioselective formal synthesis of malhamensilipin A (3) and diastereoisomeric analogues from (E)-2-undecenal. The convergent synthesis relied upon iterative epoxidation and phosphorus(v)-mediated deoxydichlorination reactions as well a titanium-mediated epoxide-opening to construct the C11–C16 stereohexad. The latter transformation occurred with very high levels of stereoretention regardless of the C13 configuration of the parent epoxide, implicating anchimeric assistance of either the γ- or δ-chlorine atoms, and the formation of chloretanium or chlorolanium ions, respectively. A computational analysis of the chloronium ion intermediates provided support for the involvement of chlorolanium ions, whereas the potential chloretanium ions were found to be less likely intermediates on the basis of their greater carbocationic character.

Triphosgene-pyridine mediated stereoselective chlorination of acyclic aliphatic 1,3-diols

We describe a strategy to chlorinate stereocomplementary acyclic aliphatic 1,3-diols using a mixture of triphosgene and pyridine. While 1,3-anti diols readily led to 1,3-anti dichlorides, 1,3-syn diols must be converted to 1,3-syn diol monosilylethers to access the corresponding 1,3-syn dichlorides. These dichlorination protocols were operationally simple, very mild, and readily tolerated by advanced synthetic intermediates.

Catalytic Enantioselective Dihalogenation and the Selective Synthesis of (-)-Deschloromytilipin A and (-)-Danicalipin A

A titanium-based catalytic enantioselective dichlorination of simple allylic alcohols is described. This dichlorination reaction provides stereoselective access to all common dichloroalcohol building blocks used in syntheses of chlorosulfolipid natural products. An enantioselective synthesis of ent-(-)-deschloromytilipin A and a concise, eight-step synthesis of ent-(-)-danicalipin A are executed and employ the dichlorination reaction as the first step. Extension of this system to enantioselective dibromination and its use in the synthesis of pentabromide stereoarrays relevant to bromosulfolipids is reported. The described dichlorination and dibromination reactions are capable of exerting diastereocontrol in complex settings allowing X-ray crystal structure analysis of natural and unnatural diastereomers of polyhalogenated stereohexads.

Stereoselective Halogenation in Natural Product Synthesis

At last count, nearly 5000 halogenated natural products have been discovered. In approximately half of these compounds, the carbon atom to which the halogen is bound is sp(3) -hybridized; therefore, there are an enormous number of natural products for which stereocontrolled halogenation must be a critical component of any synthesis strategy. In this Review, we critically discuss the methods and strategies used for stereoselective introduction of halogen atoms in the context of natural product synthesis. Using the successes of the past, we also attempt to identify gaps in our synthesis technology that would aid the synthesis of halogenated natural products, as well as existing methods that have not yet seen application in complex molecule synthesis. The chemistry described herein demonstrates yet again how natural products continue to provide the inspiration for critical advances in chemical synthesis.

Bioisosteric Exchange of Csp3 -Chloro and Methyl Substituents: Synthesis and Initial Biological Studies of Atpenin A5 Analogues

Asymmetric synthesis and initial biological studies of two analogues of a naturally occurring chlorinated antifungal agent, atpenin A5, are described. These analogues were selected on the basis of Cl→CH3 or H3 C→Cl exchanges in the side-chain of atpenin A5. The interchange of chloro and methyl substituents led to complex II inhibitors with equal IC50 values. This suggests that Cl↔Me bioisosteric exchange can be realized in aliphatic settings.

Synthesis and Biological Evaluation of Bromo- and Fluorodanicalipin A

We disclose the syntheses of (+)-bromodanicalipin A as well as (±)-fluorodanicalipin A. The relative configuration and ground-state conformation in solution of both molecules was secured by J-based configuration analysis which revealed that these are identical to natural danicalipin A. Furthermore, preliminary toxicological investigations suggest that the adverse effect of danicalipin A may be due to the lipophilicity of the halogens.

Biological Investigations of (+)-Danicalipin A Enabled Through Synthesis

A total synthesis of the chlorosulfolipid (+)-danicalipin A has been accomplished in 12 steps and 4.4% overall yield. The efficient and scalable synthesis enabled in-depth investigations of the lipid's biological properties, in particular cytotoxicity towards various mammalian cell lines. Furthermore, the ability of (+)-danicalipin A to increase the uptake of fluorophores into bacteria and mammalian cells was demonstrated, indicating it may enhance membrane permeability. By comparing (+)-danicalipin A with racemic 1,14-docosane disulfate, and the diol precursor of (+)-danicalipin A, we have shown that both chlorine and sulfate functionalities are necessary for biological activity.

Enantioselective divergent syntheses of several polyhalogenated Plocamium monoterpenes and evaluation of their selectivity for solid tumors

The family of polyhalogenated monoterpenes from Plocamium counts over a hundred known members. Using glyceraldehyde acetonide as a chiral-pool precursor, an enantioselective and divergent strategy was developed that provides a blueprint for the synthesis of many of the small yet complex acyclic members of this family. The broad applicability of this approach is demonstrated with the short, eight-step synthesis of four natural products and three analogues. These syntheses are the first of any members of the acyclic polyhalogenated Plocamium monoterpenes and permitted the evaluation of their selectivity against a range of tumor cell lines.

General approach to the synthesis of the chlorosulfolipids danicalipin A, mytilipin A, and malhamensilipin A in enantioenriched form

A second-generation synthesis of three structurally related chlorosulfolipids has been developed. Key advances include highly stereocontrolled additions to α,β-dichloroaldehydes, kinetic resolutions of complex chlorinated vinyl epoxide intermediates, and Z-selective alkene cross metatheses of cis-vinyl epoxides. This strategy facilitated the synthesis of enantioenriched danicalipin A, mytilipin A, and malhamensilipin A in nine, eight, and 11 steps, respectively.

Approaches to the chemical synthesis of the chlorosulfolipids

Since the initial discovery of the chlorosulfolipids in 1969, the chemical synthesis community largely ignored these compounds for nearly four decades, perhaps because they contain a high density of chlorine atoms, which suggested that these molecules and any projected synthetic intermediates might be unstable. Beginning in 2008, a sudden flurry of synthesis activity by several research groups, including our own, appeared in the literature. In this Account, we highlight our work from the last several years on the chemical synthesis of the chlorosulfolipids. Our work in this area began with attempts to stereoselectively generate the abundant dichloroalcohol functional group arrangements in these natural targets. In these early studies, we learned that many polychlorinated intermediates were far more stable than anticipated. We also developed a method for the diastereoselective dichlorination of allylic alcohol derivatives that permitted access to the syn,syn-dichloroalcohol stereotriad found in several chlorosulfolipids. Concurrently, we investigated an approach to mytilipin A that included multiple intermediates bearing aldehydes with β-leaving groups, but this route proved intractable. However, we leveraged what we had learned from this approach into our first success in this area: we synthesized danicalipin A via a route that introduced all of the polar functional groups using alkene oxidation reactions. By adapting this relatively general strategy, we completed an enantioselective synthesis of malhamensilipin A. This body of work also resulted in the full stereochemical elucidation of danicalipin A and the structural revision of malhamensilipin A. Finally, with the advent of Z-selective alkene cross metathesis, we developed a second-generation synthesis that featured this strategy in place of a poorly performing Wittig olefination that plagued our first approach. In addition to this new convergent step, we developed a reliable protocol for diastereoselective addition to highly sensitive α,β-dichloroaldehydes and a method for kinetic resolution of complex vinyl epoxides. Altogether, these advances led to a synthesis of enantioenriched mytilipin A in only eight steps. In the context of this work, we discovered a number of highly stereoselective reactions that might offer new, broadly applicable lessons in acyclic stereocontrol. Moreover, this research testifies to the stability of polychlorinated molecules and should inspire confidence in the use of aliphatic chlorides in other applications, including in discovery chemistry.

Synthesis of β-hydroxy-α-haloesters through super silyl ester directed syn-selective aldol reaction

Super silyl haloesters including chloro- and bromoacetate were synthesized and utilized for aldol reactions to give syn-β-hydroxy-α-haloacetates in good yields with high diastereoselectivities. β-Hydroxy-α-fluoroacetate was obtained by lithiation of super silyl bromofluoroacetate. Sequential Darzens reactions provided cis-glycidic esters in moderate yields.

A synthesis of the chlorosulfolipid mytilipin A via a longest linear sequence of seven steps

Magnificent seven: The chlorosulfolipid mytilipin A was synthesized in racemic form in seven steps and in enantioenriched form in eight steps. Key transformations include a highly diastereoselective bromoallylation of a sensitive α,β-dichloroaldehyde, a kinetic resolution of a vinyl epoxide, a convergent and highly Z-selective alkene cross-metathesis, and a chemoselective and diastereoselective dichlorination of a complex diene.

β-Siloxy-α-haloketones through highly diastereoselective single and double mukaiyama aldol reactions

Double-action haloketones: A super silyl group enabled the first highly diastereoselective Mukaiyama aldol reactions of α-chloro- and α-fluoroketones with a wide range of aldehydes, providing anti-β-siloxy-α-haloketones. This process is compatible with one-pot double-aldol methodology and allows for rapid access to new halogen-modified polyketide fragments bearing up to four contiguous stereocenters.

Total synthesis and stereochemical revision of the chlorinated sesquiterpene (±)-gomerone c

Revised: the total synthesis of gomerone C results in revision of the stereochemical assignment at C3. The synthetic strategy relies on a late-stage Conia-ene reaction, which efficiently forms the bicyclo[3.2.1]octane containing the bridgehead chloride and generates an exocyclic olefin, which can be used as a flexible handle for further elaboration. The two contiguous quaternary centers are installed by means of a Diels-Alder reaction.

Chlorination of aliphatic primary alcohols via triphosgene-triethylamine activation

Activation of primary aliphatic alcohols with triphosgene and triethylamine mixtures afforded either alkyl chloride or diethylcarbamate products, and the switch in selectivity appeared to be driven by sterics. The reaction conditions to achieve this highly useful transformation were unexceptionally mild and readily tolerated by a wide range of sensitive functionalities.

Mollenyne A, a long-chain chlorodibromohydrin amide from the sponge Spirastrella mollis

The structure of mollenyne A, a cytotoxic nitrogenous halogenated long-chain carboxamide from the sponge Spirastrella mollis, was elucidated by integrated spectroscopic analysis, including CD, and chemical conversion.

Rapid, one-pot synthesis of β-siloxy-α-haloaldehydes

The Mukaiyama cross-aldol reaction of α-fluoro-, α-chloro-, and α-bromoacetaldehyde-derived (Z)-tris(trimethylsilyl)silyl enol ethers is described, furnishing anti-β-siloxy-α-haloaldehydes. A highly diastereoselective, one-pot, sequential double-aldol process is developed, affording novel β,δ-bissiloxy-α,γ-bishaloaldehydes. Reactions are catalyzed by C(6)F(5)CHTf(2) and C(6)F(5)CTf(2)AlMe(2) (0.5-1.5 mol %) and provide access to halogenated polyketide fragments.

A High-Throughput Screen for Directed Evolution of the Natural Product Sulfotransferase LipB

In this article, the authors describe a colorimetric, high-throughput assay suitable for optimizing the activity of the recently discovered sulfotransferase LipB, by directed evolution. Crucially, LipB uses para-nitrophenol sulfate as donor in the sulfation of the nucleoside antibiotic liposidomycin B-I and other acceptor surrogates. Thus, using a robotic liquid-handling device, crude cell extracts were prepared from an Escherichia coli strain that overproduced LipB in wells of a microplate, and production of para-nitrophenol at 405 nm was monitored spectrophotometrically. Enzyme activity could be detected only in the presence of both LipB substrates and overexpressed LipB. The screen displays a suitable standard deviation for directed evolution and importantly is not limited to the natural desulfo-liposidomycin acceptor. The authors plan to use the screen to identify LipB variants with altered acceptor specificity and promiscuity for use in sulfation of natural products and other small-molecule therapeutics.