Synthesis of prostaglandin analogues, latanoprost and bimatoprost, using organocatalysis via a key bicyclic enal intermediate

Total Synthesis of (15R)- and (15S)-Prostaglandin A2

From both pharmaceutical and structural perspectives, the large family of prostaglandins represent a truly remarkable class of natural products. Prostaglandin A2 is a tissue hormone naturally found in human seminal plasma and in the sea whip Plexaura homomalla with yet poorly understood biological or therapeutic effects. Herein, a novel strategy for the stereoselective construction of both naturally occurring prostaglandin A2 epimers and first insights into their functional effects on the major inhibitory neurotransmitter γ-aminobutyric acid (GABA) type A receptors (GABAAR) are provided. The synthesis of both epimers was achieved in only 11 steps starting from commercially available 2,5-dimethoxy-tetrahydrofuran employing an organocatalytic domino-aldol reaction, a Mizoroki-Heck reaction, a Wittig reaction as well as an oxidation-decarboxylation sequence. The (15R)-epimer significantly reduced GABA-induced currents through GABAA receptors while its (15S)-epimer did not show any significant effect. These data suggest that (15R)-PGA2 might serve as a novel scaffold for the development of selective GABAA receptor modulators.

A concise and scalable chemoenzymatic synthesis of prostaglandins

Prostaglandins have garnered significant attention from synthetic chemists due to their exceptional biological activities. In this report, we present a concise chemoenzymatic synthesis method for several representative prostaglandins, achieved in 5 to 7 steps. Notably, the common intermediate bromohydrin, a radical equivalent of Corey lactone, is chemoenzymatically synthesized in only two steps, which allows us to complete the synthesis of prostaglandin F2α in five steps on a 10-gram scale. The chiral cyclopentane core is introduced with high enantioselectivity, while the lipid chains are sequentially incorporated through a cost-effective process involving bromohydrin formation, nickel-catalyzed cross-couplings, and Wittig reactions. This cost-efficient synthesis route for prostaglandins holds the potential to make prostaglandin-related drugs more affordable and facilitate easier access to their analogues.

Organocatalyst-mediated, pot-economical total synthesis of latanoprost

The enantioselective total synthesis of latanoprost, an antiglaucoma agent, has been accomplished with excellent diastereo- and enantioselectivities in a pot-economical manner using six reaction vessels. An enantioselective Krische allylation was conducted in the first pot. In the second pot, olefin metathesis, silyl protection, and hydrogenolysis proceeded efficiently. In the third pot, an organocatalyst-mediated Michael reaction proceeded with excellent diastereoselectivity. The fourth pot involved a substrate-controlled Mukaiyama intramolecular aldol reaction and elimination of HNO2 to afford a methylenecyclopentanone, also with excellent diastereoselectivity. The fifth pot involved a Michael reaction of vinyl cuprate. In the sixth pot, three reactions, a cis-selective olefin metathesis, diastereoselective reduction, and deprotection, afforded latanoprost. Nearly optically pure latanoprost was obtained, and the total yield was 24%.

De Novo Synthesis of Dihydrobenzofurans and Indolines and Its Application to a Modular, Asymmetric Synthesis of Beraprost

Dihydrobenzofurans and indolines are important constituents of pharmaceuticals. Herein, we describe a novel strategy for their construction in which the aromatic ring is created de novo through an inverse-electron demand Diels-Alder reaction and cheletropic extrusion sequence of a 2-halothiophene-1,1-dioxide with an enol ether/enamide, followed by aromatization. Unusually, the aromatization process proved to be highly challenging, but it was discovered that treatment of the halocyclohexadienes with a base effected an α-elimination-aromatization reaction. Mechanistic investigation of this step using deuterium-labeling studies indicated the intermediacy of a carbene which undergoes a 1,2-hydrogen shift and subsequent aromatization. The methodology was applied to a modular and stereoselective total synthesis of the antiplatelet drug beraprost in only 8 steps from a key enal-lactone. This lactone provided the core of beraprost to which both its sidechains could be appended through a 1,4-conjugate addition process (lower ω-sidechain), followed by de novo construction of beraprost's dihydrobenzofuran (upper α-sidechain) using our newly developed methodology. Additionally, we have demonstrated the breadth of our newly established protocol in the synthesis of functionalized indolines, which occurred with high levels of regiocontrol. According to density-functional theory (DFT) calculations, the high selectivity originates from attractive London dispersion interactions in the TS of the Diels-Alder reaction.

Asymmetric Organocatalytic Syntheses of Bioactive Compounds

Background:

The total syntheses of complex natural products have evolved to include new methodologies to save time, simplifying the form to achieve these natural compounds.

Objective:

In this review, we have described the asymmetric synthesis of different natural products and biologically active compounds of the last ten years until the current day.

Results:

An asymmetric organocatalytic reaction is a key to generate stereoselectively the main structure with the required stereochemistry.

Conclusion:

Even more remarkable, the organocatalytic cascade reactions, which are carried out with high stereoselectivity, as well as a possible approximation of the organocatalysts activation with sub-strates are also described.

Development of a new approach for the synthesis of (+)-15-deoxy-Δ12,14-prostaglandin J2 methyl ester based on the [2+2]-cycloadduct of 5-trimethylsilylcyclopentadiene and dichloroketene

This paper described a first example of the application of the “abnormal” lactone of 3-oxabicyclo[3.3.0]oct-6-en-2-one topology in targeted prostaglandin synthesis.

Nickel Mediated Enantioselective Photoredox Allylation of Aldehydes with Visible Light

Here we report a practical, highly enantioselective photoredox allylation of aldehydes mediated by chiral nickel complexes with commercially available allyl acetate as the allylating agent. The methodology allows the clean stereoselective allylation of aldehydes in good to excellent yields and up to 93% e.e. using a catalytic amount of NiCl 2 (glyme) in the presence of the chiral aminoindanol-derived bis(oxazoline) as the chiral ligand. The photoredox system is constituted by the organic dye 3DPAFIPN and a Hantzsch's ester as the sacrificial reductant. The reaction proceeds under visible light irradiation (blue LEDs, 456 nm) at 8-12 °C with excellent stereoselectivities. Compared to other published procedures, no metal reductants (such as Zn or Mn), additives (e.g. CuI) or air-sensitive Ni(COD) 2 are necessary for this reaction. Accurate DFT calculations and photophysical experiments have clarified the mechanistic picture of this stereoselective allylation reaction showing a key role played by Hantzsch's ester for the turnover of the catalyst.

A unified strategy to prostaglandins: chemoenzymatic total synthesis of cloprostenol, bimatoprost, PGF2α, fluprostenol, and travoprost guided by biocatalytic retrosynthesis

Development of efficient and stereoselective synthesis of prostaglandins (PGs) is of utmost importance, owing to their valuable medicinal applications and unique chemical structures. We report here a unified synthesis of PGs cloprostenol, bimatoprost, PGF_2α, fluprostenol, and travoprost from the readily available dichloro-containing bicyclic ketone 6a guided by biocatalytic retrosynthesis, in 11-12 steps with 3.8-8.4% overall yields. An unprecedented Baeyer-Villiger monooxygenase (BVMO)-catalyzed stereoselective oxidation of 6a (99% ee), and a ketoreductase (KRED)-catalyzed diastereoselective reduction of enones 12 (87 : 13 to 99 : 1 dr) were utilized in combination for the first time to set the critical stereochemical configurations under mild conditions. Another key transformation was the copper(ii)-catalyzed regioselective p-phenylbenzoylation of the secondary alcohol of diol 10 (9.3 : 1 rr). This study not only provides an alternative route to the highly stereoselective synthesis of PGs, but also showcases the usefulness and great potential of biocatalysis in construction of complex molecules.

Lactones in the Synthesis of Prostaglandins and Prostaglandin Analogs

In the total stereo-controlled synthesis of natural prostaglandins (PGs) and their structural analogs, a vast class of compounds and drugs, known as the lactones, are encountered in a few key steps to build the final molecule, as: δ-lactones, γ-lactones, and 1,9-, 1,11-, and 1,15-macrolactones. After the synthesis of 1,9-PGF2α and 1,15-PGF2α lactones, many 1,15-lactones of E2, E3, F2, F3, A2, and A3 were found in the marine mollusc Tethys fimbria and the quest for understanding their biological role stimulated the research on their synthesis. Then 1,9-, 1,11-, and 1,15-PG lactones of the drugs were synthesized as an alternative to the corresponding esters, and the first part of the paper describes the methods used for their synthesis. The efficient Corey procedure for the synthesis of prostaglandins uses the key δ-lactone and γ-lactone intermediates with three or four stereocenters on the cyclopentane fragment to link the PG side chains. The paper describes the most used procedures for the synthesis of the milestone δ-Corey-lactones and γ-Corey-lactones, their improvements, and some new promising methods, such as interesting, new stereo-controlled and catalyzed enantioselective reactions, and methods based on the chemical/enzymatic resolution of the compounds in different steps of the sequences. The many uses of δ-lactones not only for the synthesis of γ-lactones, but also for obtaining 9β-halogen-PGs and halogen-substituted cyclopentane intermediates, as synthons for new 9β-PG analogs and future applications, are also discussed.

Total Synthesis of Thromboxane B2 via a Key Bicyclic Enal Intermediate

A 12-step asymmetric synthesis of thromboxane B₂ (TxB₂) from 2,5-dimethoxytetrahydrofuran is described. The synthesis employs our organocatalytic aldol reaction of succinaldehyde to give a key bicyclic enal intermediate. From here, the synthetic strategy involves a conjugate addition of an alkenyl side chain to the bicyclic enal, Baeyer-Villiger oxidation, and a highly Z-selective Wittig olefination of a hemiacetal. Key to success was minimizing redox operations and the manipulation of functional groups in the correct order.

Asymmetric Intramolecular Rauhut-Currier Reaction and Its Desymmetric Version via Double Thiol/Phase-Transfer Catalysis

An asymmetric intramolecular Rauhut-Currier reaction of linear bis(enones) has been achieved via double activation catalysis of thiols and phase transfer substances, furnishing both enantioenriched cyclohexene and cyclopentene derivatives (up to 95% ee). Furthermore, the desymmetric version of prochiral substrates was developed under similar catalysis, producing the frameworks bearing an additional tertiary or even quaternary stereogenic center with moderate to excellent diastereo- and enantioselectivity (up to 95% ee, >19:1 dr).

Prostaglandin Total Synthesis Enabled by the Organocatalytic Dimerization of Succinaldehyde

Prostaglandins have been attractive targets in total synthesis for over 50 years, resulting in the development of new synthetic strategies and methodologies that have served the broader chemical community. However, these molecules are not just of academic interest, a number of prostaglandin analogues are used in the clinic, and some are even on the WHO list of essential medicines. In this personal account, we describe our own approach to the family of prostaglandins, which centers around the synthesis of a key enal intermediate, formed from the l-proline catalysed dimerization of succinaldehyde. We highlight the discovery and further optimization of this key reaction, its scale up, and subsequent application to a range of prostaglandins.

Pot and time economies in the total synthesis of Corey lactone

The Corey lactone is a highly versatile intermediate for the synthesis of a variety of prostaglandin hormones that natively control a multitude of important physiological processes. Starting from commercially available compounds, we herein disclose a time-economical, one-pot enantioselective preparation of the Corey lactone by virtue of a new diphenylprolinol silyl ether-mediated domino Michael/Michael reaction to afford the substituted cyclopentanone core in a formal (3 + 2) cycloadditive fashion. More broadly, this work advances the on-demand, gram-scale synthesis of high-value targets involving chemically orthogonal transformations, whereby distinct reactions of acids, bases, organometalics, reductants and oxidants can be carried out in a single reaction vessel in a sequential fashion.

Access to a Key Building Block for the Prostaglandin Family via Stereocontrolled Organocatalytic Baeyer-Villiger Oxidation

A new protocol for the construction of a crucial bicyclic lactone of prostaglandins using a stereocontrolled organocatalytic Baeyer-Villiger (B-V) oxidation was developed. The key B-V oxidation of a racemic cyclobutanone derivative with aqueous hydrogen peroxide has enabled an early-stage construction of a bicyclic lactone skeleton in high enantiomeric excess (up to 95 %). The generated bicyclic lactone is fully primed with two desired stereocenters and enabled the synthesis of the entire family of prostaglandins according to Corey's route. Furthermore, the reactivity and enantioselectivity of B-V oxidation of racemic bicyclic cyclobutanones were evaluated and 90-99 % ee was obtained, representing one of the most efficient routes to chiral lactones. This study further facilitates the synthesis of prostaglandins and chiral lactone-containing natural products to promote drug discovery.

Reoptimization of the Organocatalyzed Double Aldol Domino Process to a Key Enal Intermediate and Its Application to the Total Synthesis of Δ12 -Prostaglandin J3

Re-investigation of the l-proline catalyzed double aldol cascade dimerization of succinaldehyde for the synthesis of a key bicyclic enal intermediate, pertinent in the field of stereoselective prostaglandin synthesis, is reported. The yield of this process has been more than doubled, from 14 % to a 29 % isolated yield on a multi-gram scale (32 % NMR yield), through conducting a detailed study of the reaction solvent, temperature, and concentration, as well as a catalyst screen. The synthetic utility of this enal intermediate has been further demonstrated through the total synthesis of Δ12 -prostaglandin J3 , a compound with known anti-leukemic properties.

Synthesis of Alfaprostol and PGF2α through 1,4-Addition of an Alkyne to an Enal Intermediate as the Key Step

The veterinary drug Alfaprostol and prostaglandin PGF_2α have been synthesized in just nine steps. The strategy involved the conjugate addition of an alkyne to a bicyclic enal, available in three steps by a proline-catalyzed aldol reaction of succinaldehyde. In the case of Alfaprostol, this resulted in the shortest synthesis reported to date. For PGF_2α, this approach improved our previous route by making the 1,4-addition and ozonolysis more operationally simple.

Enantio- and Diastereoselective Synthesis of Latanoprost using an Organocatalyst

An enantioselective total synthesis of latanoprost was achieved. Its chiral cyclopentane core structure was constructed through an organocatalyst-mediated [3+2]-cycloaddition reaction, and chirality in the ω-side chain was generated by prolinate-anion-mediated α-aminoxylation of an aldehyde. Highly diastereoselective domino acetalization and an oxy-Michael reaction were key steps for the generation of C9 chirality.

Recent advances in asymmetric total synthesis of prostaglandins

Prostaglandins (PGs) are a series of hormone-like chemical messengers and play a critical role in regulating physiological activity. The diversified therapeutic activities and complex molecular architectures of PGs have attracted special attention, and huge progress has been made in asymmetric total synthesis and discovery of pharmaceutically useful drug candidates. In the last 10 years, several powerful syntheses have emerged as new solutions to the problem of building PGs and represent major breakthroughs in this area. This review highlights the advances in methodologies for the asymmetric total synthesis of prostaglandins. The application of these methodologies in the syntheses of medicinally useful prostaglandins is also described. The study has been carefully categorized according to the key procedures involved in the syntheses of various prostaglandins, aiming to give readers an easy understanding of this chemistry and provide insights for further improvements.

Enantioselective Addition of Alkynes to α,α-Dichlorinated Aldehydes

Enantioselective addition of terminal alkynes to α,α-dichlorinated aldehydes employing Zn(OTf)₂/NME is disclosed. The propargylic alcohols obtained are accessed in good yields and high enantioselectivity from easily accessible α,α-dichloroaldehydes. The method opens new strategic opportunities for the synthesis of chlorinated natural products, such as the chlorosulfolipids.

Catalytic discrimination between formyl groups in regio- and stereoselective intramolecular cross-aldol reactions

Catalytic discrimination between inequivalent formyl groups was achieved using an aniline-type acid-base catalyst for the regio-, diastereo-, and enantioselective intramolecular cross-aldol reactions of enolizable dials. Although l-proline gave a mixture of the regio- and stereoisomeric products in the presence of an N-containing 1,6-dial, the aniline-type catalyst afforded anti-3,4-disubstituted pyrrolidine in high regio-, and stereoselectivity beyond the background reaction, which led to the regioisomeric 2,3-disubstituted products. The mild reactivity of the aniline-type amine facilitated catalytic discrimination between the inequivalent formyl groups. Kinetic isotope effect studies and reductive amination experiments suggested that the regioselectivity was controlled under the enamine-forming steps.

Microwave assisted aminocatalyzed [3 + 2] annulation between α-iminonitriles and succinaldehyde: synthesis of pyrrole-3-methanols and related polycyclic ring systems

A quick and highly efficient method for the synthesis of substituted pyrrole-3-methanols from α-iminonitriles and succinaldehyde under microwave irradiation is reported with good to high yields (up to 75%).

Application of a High-Throughput Enantiomeric Excess Optical Assay Involving a Dynamic Covalent Assembly: Parallel Asymmetric Allylation and Ee Sensing of Homoallylic Alcohols

Parallel synthesis and high-throughput ee screening.

Asymmetric Ir-catalyzed C–C coupling of primary alcohols with allyl-acetates, as described by Krische, to form chiral secondary homo-allylic alcohols were performed in parallel as a means to optimize the ee values thereof. Specifically, approximately 400 examples of this reaction were performed by varying the catalyst, added acids and bases, and starting reactants, to form 4-phenyl-1-butene-4-ol (1). The ee values for the transformations were determined in a high-throughput fashion using a 4-component assembly that creates a circular dichroism signal indicative of the extent of asymmetric induction. Further, a parallel and rapid quantitative TLC method measures the yield of each reaction, revealing which reactions give reliable ee values in the CD-based assay. Overall, the nearly 200 reactions whose ee values were determined could be quantitated in under two hours. Using a combination of the TLC method to measure yield with the CD-assay to measure ee values, several trends in reaction conditions were revealed. For example, it was found that the cyclometalated iridium catalyst modified by BINAP and m-nitro-p-cyano-benzoic acid delivered adduct 1 with the highest levels of enantiomeric enrichment (94%), whereas the corresponding SEGPHOS-modified catalyst gave a comparable yield but lower ee (91%). Most importantly, this study shows that supramolecular assemblies can report hundreds of ee values in a rapid and reliable fashion to analyze parallel synthesis routines.