Total Synthesis of (-)-Enigmazole A

Total Synthesis of (-)-Enigmazole A by the Macrocyclization/Transannular Pyran Cyclization Strategy

An 18-step synthesis of (-)-enigmazole A is herein disclosed. The present synthesis is based on a modular assembly of three building blocks of similar complexity, a macrocyclic ring-closing metathesis to forge the 18-membered macrocyclic skeleton, and a desilylative transannular oxa-Michael addition for stereoselective construction of the 2,6-cis-substituted tetrahydropyran ring.

Gold-catalyzed cyclization and cycloaddition in natural product synthesis

Covering: 2016 to mid 2023Transition metal catalysis, known for its remarkable capacity to expedite the assembly of molecular complexity from readily available starting materials in a single operation, occupies a central position in contemporary chemical synthesis. Within this landscape, gold-catalyzed reactions present a novel and versatile paradigm, offering robust frameworks for accessing diverse structural motifs. In this review, we highlighted a curated selection of publications in the past 8 years, focusing on the deployment of homogeneous gold catalysis in the ring-forming step for the total synthesis of natural products. These investigations are categorized based on the specific ring formations they engender, accentuating the prevailing gold-catalyzed methodologies applied to surmount intricate challenges in natural products synthesis.

Total Synthesis of (-)-Enigmazole B

Total synthesis of (-)-enigmazole B was achieved for the first time. Highlights of the present synthesis include an olefin cross-metathesis and hemiacetalization/intramolecular oxa-Michael addition sequence for accessing an (E)-configured enol tosylate, a Sonogashira cross-coupling to assemble all the carbon atoms of the target natural product, a remarkably chemo- and regioselective Au-catalyzed intramolecular alkyne hydroalkoxylation for the construction of the dihydropyran ring, and a Yamaguchi macrolactonization to close the 18-membered macrolactone skeleton.

Direct synthesis of tetrahydropyran-4-ones via O3ReOH-catalyzed Prins cyclization of 3-chlorohomoallylic alcohols

A new variation of Prins cyclization to directly and stereoselectively synthesize cis-2,6-disubstituted tetrahydropyran-4-ones from 3-chlorohomoallylic alcohols and aldehydes catalyzed by perrhenic acid is reported. The reaction is generally compatible with a range of aliphatic and aromatic aldehydes and 24 examples of tetrahydropyran-4-one products have been prepared in moderate to good yields. This methodology highlights the use of simple starting materials and commercially available aqueous perrhenic acid as a catalyst for Prins cyclization reactions to directly synthesize 2,6-disubstituted tetrahydropyran-4-ones.

The phosphate ester group in secondary metabolites

Covering: 2000 to mid-2021The phosphate ester is a versatile, widespread functional group involved in a plethora of biological activities. Its presence in secondary metabolites, however, is relatively rare compared to other functionalities and thus is part of a rather unexplored chemical space. Herein, the chemistry of secondary metabolites containing the phosphate ester group is discussed. The text emphasizes their structural diversity, biological and pharmacological profiles, and synthetic approaches employed in the phosphorylation step during total synthesis campaigns, covering the literature from 2000 to mid-2021.

1,3-Bifunctional Nucleophilic Allylation Reagents: Preparative Methods and Synthetic Applications

1,3-Bifunctional nucleophilic allylation reagents play an important role in organic synthesis. In this short review, we summarize the methods for the preparation of 1,3-bifunctional reagents and their reactions with various electrophiles. Synthetic applications of these reagents in the context of complex molecule synthesis are also discussed.

1 Introduction

2 Reagent Synthesis

2.1 Symmetrical Reagents

2.2 Unsymmetrical Reagents

2.2.1 Bis-silane and Silyl-stannane Reagents

2.2.2 Bis-boron and Silyl-boron Reagents

3 Synthetic Applications

3.1 Allylation of Aldehydes

3.2 Allylation of Ketones

3.3 Allylation of Imines

3.4 Allylation of Other Electrophiles with 1,3-Bifunctional Allylation Reagents

4 Summary

Lessons from Natural Product Total Synthesis: Macrocyclization and Postcyclization Strategies

Macrocyclic natural products are plentiful in the bacteria, archaea, and eukaryote domains of life. For the significant advantages that they provide to the producing organisms, evolution has learned how to implement various types of macrocyclization reactions into the different biosynthetic pathways and how to effect them with remarkable ease. Mankind greatly benefits from nature’s pool, not least because naturally occurring macrocycles or derivatives thereof serve as important drugs for the treatment of many serious ailments.

In stark contrast, macrocyclization reactions are usually perceived as difficult to accomplish by purely chemical means. While it is true that ring closure necessarily entails an entropic loss and may result in the buildup of (considerable) ring strain that must be compensated for in one way or the other, it is also fair to note tremendous methodological advances during the last decades that greatly alleviated this traditional “macrocycle challenge”. It is therefore increasingly possible to explore the advantages provided by large as well as medium-size ring systems in a more systematic manner. This venture also holds the promise of increasing the “chemical space” amenable to drug development to a considerable extent.

In consideration of this and other important long-term perspectives, it is appropriate to revisit the current state of the art. To this end, a number of vignettes are presented, each of which summarizes a total synthesis project targeting macrocyclic natural products of greatly different chemotypes using a variety of transformations to reach these goals. Although we were occasionally facing “dead ends”, which are also delineated for the sake of a complete picture, these case studies illustrate the notion that the formation of a certain macrocyclic perimeter is (usually) no longer seriously limiting. In addition to substantial progress in the “classical” repertoire (macrolactonization and macrolactamization (pateamine A, spirastrellolide, and belizentrin)), various metal-catalyzed reactions have arguably led to the greatest leaps forward. Among them, palladium-catalyzed C–C bond formation (roseophilin and nominal xestocyclamine A) and, in particular, alkene and alkyne metathesis stand out (iejimalide, spirastrellolide, enigmazole, ingenamine, and sinulariadiolide). In some cases, different methods were pursued in parallel, thus allowing for a critical assessment and comparison.

To the extent that the macrocyclic challenge is vanishing, the opportunity arises to focus attention on the postmacrocyclization phase. One may stipulate that a well-designed cyclization precursor does not only ensure efficient ring closure but also fosters and streamlines the steps that come after the event. One way to do so is dual (multiple) use in that the functional groups serving the actual cyclization reaction also find productive applications downstream from it rather than being subject to simple defunctionalization. In this context, better insight into the conformational peculiarities of large rings and the growing confidence in their accessibility in a stereochemically well defined format rejuvenate the implementation of transannular reactions or reaction cascades that can lead to rapid and substantial increases in molecular complexity. The examples summarized herein showcase such possibilities, with special emphasis on tranannular gold catalysis and the emerging ruthenium-catalyzed trans-hydrometalation chemistry for the selective functionalization of alkynes.

Unified Total Synthesis of (-)-Enigmazole A and (-)-15-O-Methylenigmazole A

The total synthesis of cytotoxic marine phosphomacrolides, (-)-enigmazole A and (-)-15-O-methylenigmazole A, is described in detail. The 2,6-cis-substituted tetrahydropyran ring was efficiently elaborated by using a tandem olefin cross-metathesis/intramolecular oxa-Michael addition reaction. The 18-membered macrolactone skeleton was forged via a Au-catalyzed propargylic benzoate rearrangement/macrocyclic ring-closing metathesis sequence. Late-stage diversification of a common intermediate enabled unified total synthesis of (-)-enigmazole A and (-)-15-O-methylenigmazole A.

Evolution of Anion Relay Chemistry: Construction of Architecturally Complex Natural Products

Multicomponent union tactics in which three or more fragments are rapidly connected are highly prized in the construction of architecturally complex natural products. Anion Relay Chemistry (ARC), a multicomponent union tactic, has just such potential to elaborate structurally diverse scaffolds in a single operation with excellent stereochemical control. Conceptually, the ARC tactic can be divided into two main classes: "Through-Bond," by the relay of negative charge through the bonding system of a molecule; and "Through-Space," by the migration of negative charge across space by a transfer agent. "Through-Space" Anion Relay Chemistry, the focus of this Account, can be further subdivided into two types: Type I ARC, originated from the Tietze-Schaumann-Smith coupling reaction, which for the first time permits controllable Brook rearrangements to construct unsymmetrical adducts, and as such has been successfully employed in the total syntheses of diverse natural products, including the mycoticins, bryostatin 1, spongistatins, rimocidin, indolizidine alkaloids, and enigmazole A; and Type II ARC, central to which is the design of novel bifunctional linchpins that enable rapid assembly of linear and cyclic fragments with diverse architectural features, ranging from polyols, spiroketals, and polyenes to polypropionate scaffolds. Recently, the Type II ARC tactic has been exploited as the key construction tactic in the total syntheses of the spirastrellolides, the cryptocarya acetates, secu'amamine A, mandelalide A, and nahuoic acid C_i (B_ii). This Account will present the evolution of both the Type I and Type II Anion Relay tactics, in conjunction with some prominent applications.

Merging Asymmetric [1,2]-Additions of Lithium Acetylides to Carbonyls with Type II Anion Relay Chemistry

An enantioselective three-component coupling reaction has been developed, enabling the union of a variety of lithium acetylides and electrophiles exploiting an achiral linchpin via an anionic reaction cascade. This Type II Anion Relay Chemistry tactic is initiated via an enantioselective [1,2]-carbonyl addition exploiting BINOL catalysis to access an enantioenriched alkoxide intermediate. Migration of charge across the linchpin via a [1,4]-Brook rearrangement with electrophile capture affords a three-component propargyl ether adduct. Herein, we report the development, scope, and limitations of this reaction sequence.

Generation of Dithianyl and Dioxolanyl Radicals Using Photoredox Catalysis: Application in the Total Synthesis of the Danshenspiroketallactones via Radical Relay Chemistry

Visible-light-induced generation of dithianyl and dioxolanyl radicals via selective hydrogen atom transfer (HAT) has been achieved. This radical relay tactic enables remote C(sp³)-H functionalization to permit rapid access to polyol and spiroketal segments, and in turn has been exploited as a key synthetic construct in the total synthesis of the danshenspiroketallactones. The conformational stability of the danshenspiroketallactones has also been defined via experiments and DFT calculations.

Synthesis of Vinyl Cyclopropanes via Anion Relay Cyclization

A method where an allyl alcohol is formed from a Tsuji-Trost allylation between a vinyl epoxide and an acyl containing nucleophile is described. Subsequently, a retro-Claisen condensation is utilized as a means of through-space anion relay. The anion relay results in the formation of a reactive carbanion and simultaneously activates an allylic alcohol toward intramolecular Tsuji-Trost cyclopropanation. Hence, in one pot, Tsuji-Trost allylation, retro-Claisen activation, and Tsuji-Trost cyclopropanation are combined to access synthetically useful vinyl cyclopropanes from vinyl epoxides using a mild and operationally simple procedure.

Total Synthesis of the Marine Phosphomacrolide, (-)-Enigmazole A, Exploiting Multicomponent Type I Anion Relay Chemistry (ARC) in Conjunction with a Late-Stage Petasis-Ferrier Union/Rearrangement

An effective late-stage large-fragment union/rearrangement exploiting the Petasis-Ferrier protocol, in conjunction with multicomponent Type I Anion Relay Chemistry (ARC) to access advanced intermediates, permits completion of a convergent, stereocontrolled total synthesis of the architecturally complex phosphomacrolide (-)-enigmazole A (1).

Total Synthesis of (-)-Enigmazole A

Total synthesis of (-)-enigmazole A, a marine macrolide natural product with cytotoxic activity, has been accomplished. The tetrahydropyran moiety was constructed by means of a domino olefin cross-metathesis/intramolecular oxa-Michael addition of a δ-hydroxy olefin. After coupling of advanced intermediates, the macrocycle was formed through gold-catalyzed rearrangement of a propargylic benzoate, followed by ring-closing metathesis of the resultant α,β-unsaturated ketone.

Synthetic Access to the Mandelalide Family of Macrolides: Development of an Anion Relay Chemistry Strategy

The mandelalides comprise a family of structurally complex marine macrolides that display significant cytotoxicity against several human cancer cell lines. Presented here is a full account on the development of an Anion Relay Chemistry (ARC) strategy for the total synthesis of (-)-mandelalides A and L, the two most potent members of the mandelalide family. The design and implementation of a three-component type II ARC/cross-coupling protocol and a four-component type I ARC union permits rapid access respectively to the key tetrahydrofuran and tetrahydropyran structural motifs of these natural products. Other highlights of the synthesis include an osmium-catalyzed oxidative cyclization of an allylic 1,3-diol, a mild Yamaguchi esterification to unite the northern and southern hemispheres, and a late-stage Heck macrocyclization. Synthetic mandelalides A and L displayed potent cytotoxicity against human HeLa cervical cancer cells (EC50, 1.3 and 3.1 nM, respectively). This synthetic approach also provides access to several highly potent non-natural mandelalide analogs, including a biotin-tagged mandelalide probe for future biological investigation.

Pot-Economy Autooxidative Condensation of 2-Aryl-2-lithio-1,3-dithianes

The autoxidative condensation of 2-aryl-2-lithio-1,3-dithianes is here reported. Treatment of 2-aryl-1,3-dithianes with n-BuLi in the absence of any electrophile leads to condensation of three molecules of 1,3-dithianes and formation of highly functionalized α-thioether ketones orthothioesters in 51-89% yields upon air exposure. The method was further expanded to benzaldehyde dithioacetals, affording corresponding orthothioesters and α-thioether ketones in 48-97% yields. The experimental results combined with density functional theory studies support a mechanism triggered by the autoxidation of 2-aryl-2-lithio-1,3-dithianes to yield a highly reactive thioester that undergoes condensation with two other molecules of 2-aryl-2-lithio-1,3-dithiane.

Design, Synthesis, and Application of Polymer-Supported Silicon-Transfer Agents for Cross-Coupling Reactions with Organolithium Reagents

The initial design, synthesis, and validation of polymer-supported siloxane transfer agents have been achieved that permit the direct use of organolithium reagents in the palladium-catalyzed cross-coupling reactions. Through rational design, two generations of polymer support were developed that significantly simplify product purification and the transfer agent recycling.

Type II Anion Relay Chemistry: Conformational Constraints To Achieve Effective [1,5]-Vinyl Brook Rearrangements

The design, synthesis, and evaluation of bifunctional linchpins, conformationally anchored on six-membered rings to achieve efficient [1,5]-Brook rearrangements involving vinyl silanes have been achieved. The restrained linchpins were subsequently exploited in type II anion relay chemistry (ARC) to permit both alkylations and Pd-mediated cross-coupling reactions (CCR) of sp² stabilized carbanions. DFT calculations were then employed to understand the mechanism and reactivity trends of [1,4]- and [1,5]-vinyl Brook rearrangements to provide insight on the role of the required copper reagent and the substrate geometry.

Total Synthesis of (-)-Mandelalide A Exploiting Anion Relay Chemistry (ARC): Identification of a Type II ARC/CuCN Cross-Coupling Protocol

Anion relay chemistry (ARC), an effective, multicomponent union tactic, was successfully employed for the total synthesis of the highly cytotoxic marine macrolide (-)-mandelalide A (1). The northern hemisphere was constructed via a new type II ARC/CuCN cross-coupling tactic, while the southern hemisphere was secured via a highly efficient four-component type I ARC union. Importantly, the synthesis of 1 showcases ARC as a rapid, scalable coupling strategy for the union of simple readily available building blocks to access diverse complex molecular fragments with excellent stereochemical control.