Cross metathesis as a general strategy for the synthesis of prostacyclin and prostaglandin analogues

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.

Concise, scalable and enantioselective total synthesis of prostaglandins

Prostaglandins are among the most important natural isolates owing to their broad range of bioactivities and unique structures. However, current methods for the synthesis of prostaglandins suffer from low yields and lengthy steps. Here, we report a practicability-oriented synthetic strategy for the enantioselective and divergent synthesis of prostaglandins. In this approach, the multiply substituted five-membered rings in prostaglandins were constructed via the key enyne cycloisomerization with excellent selectivity (>20:1 d.r., 98% e.e.). The crucial chiral centre on the scaffold of the prostaglandins was installed using the asymmetric hydrogenation method (up to 98% yield and 98% e.e.). From our versatile common intermediates, a series of prostaglandins and related drugs could be produced in two steps, and fluprostenol could be prepared on a 20-gram scale.

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.

Biochemical basis for the regulation of biosynthesis of antiparasitics by bacterial hormones

Diffusible small molecule microbial hormones drastically alter the expression profiles of antibiotics and other drugs in actinobacteria. For example, avenolide (a butenolide) regulates the production of avermectin, derivatives of which are used in the treatment of river blindness and other parasitic diseases. Butenolides and γ-butyrolactones control the production of pharmaceutically important secondary metabolites by binding to TetR family transcriptional repressors. Here, we describe a concise, 22-step synthetic strategy for the production of avenolide. We present crystal structures of the butenolide receptor AvaR1 in isolation and in complex with avenolide, as well as those of AvaR1 bound to an oligonucleotide derived from its operator. Biochemical studies guided by the co-crystal structures enable the identification of 90 new actinobacteria that may be regulated by butenolides, two of which are experimentally verified. These studies provide a foundation for understanding the regulation of microbial secondary metabolite production, which may be exploited for the discovery and production of novel medicines.

Synthetic approaches to isocarbacyclin and analogues as potential neuroprotective agents against ischemic stroke

Isocarbacyclin is a valuable synthetic analogue of prostacyclin with potential neuroprotective effects for the treatment of ischemic stroke. Herein, we describe the synthesis of isocarbacyclin and bicyclic analogues in only 7-10 steps, with the ω-side chain diversified at a late stage. A combination of new reaction design, function-oriented synthesis, and late-stage diversification led to a series of compounds that were tested for their neuroprotective activities. Efforts toward the synthesis of tricyclic analogues of isocarbacyclin, using the same combination of metal-catalyzed reactions, is also described.

A C-H oxidation approach for streamlining synthesis of chiral polyoxygenated motifs

Chiral oxygenated molecules are pervasive in natural products and medicinal agents; however, their chemical syntheses often necessitate numerous, wasteful steps involving functional group and oxidation state manipulations. Herein a strategy for synthesizing a readily diversifiable class of chiral building blocks, allylic alcohols, through sequential asymmetric C-H activation/resolution is evaluated against the state-of-the-art. The C-H oxidation routes' capacity to strategically introduce oxygen into a sequence and thereby minimize non-productive manipulations is demonstrated to effect significant decreases in overall step-count and increases in yield and synthetic flexibility.

Modular enantioselective synthesis of 8-aza-prostaglandin E1

We report herein for the first time the enantioselective synthesis of 8-aza-PGE1. The synthesis used the cross olefin metathesis reaction to connect the 5-vinyl-γ-lactam subunit, prepared from (R)-malic acid via the Ley's sulfone-based α-amidalkylation protocol (dr = 6.8:1), with the chiral pre-ω-chain. The latter was synthesized in high enantioselectivity from (E)-2-octenol by the Sharpless asymmetric epoxidation and the titanocene-mediated epoxide opening. This modular approach is quite concise and flexible, and requires only eight steps from commercially available reagents.

Stereocontrolled organocatalytic synthesis of prostaglandin PGF2α in seven steps

Prostaglandins are hormone-like chemical messengers that regulate a broad range of physiological activities, including blood circulation, digestion and reproduction. Their biological activities and their complex molecular architectures have made prostaglandins popular targets for synthetic organic chemists for over 40 years. Prostaglandin analogues are widely used as pharmaceuticals and some, such as latanoprost, which is used to treat glaucoma, have become billion-dollar drugs. Previously reported syntheses of these compounds are quite lengthy, and every chemical step costs time and energy, generates waste and is accompanied by material losses. Using a new bond disconnection, here we report a concise synthesis of the most complex prostaglandin, PGF2α, with high levels of control of relative and absolute stereochemistry, and fewer steps. The key step is an aldol cascade reaction of succinaldehyde using proline organocatalysis to create a bicyclic enal in one step and an enantiomeric excess of 98%. This intermediate bicyclic enal is fully primed with the appropriate functionality for attachment of the remaining groups. Access to this bicyclic enal will not only render existing prostaglandin-based drugs more affordable, but will also facilitate the rapid exploration of related chemical structures around the ubiquitous five-membered ring motif, such as potentially therapeutic prostaglandin analogues.

Synthesis of caeliferins, elicitors of plant immune responses: accessing lipophilic natural products via cross metathesis

A cross metathesis (CM)-based synthesis of the caeliferins, a family of sulfooxy fatty acids that elicit plant immune responses, is reported. Unexpectedly, detailed NMR spectroscopic and mass spectrometric analyses of CM reaction mixtures revealed extensive isomerization and homologation of starting materials and products. It is shown that the degree of isomerization and homologation in CM strongly correlates with substrate chain length and lipophilicity. Side-product suppression requires appropriate catalyst selection and use of 1,4-benzoquinone as a hydride scavenger.

Asymmetric Synthesis of 3-Oxa-15-deoxy-16-(m-tolyl)-17,18,19,20-tetranorisocarbacyclin and Its Neuroprotective Analogue 15-Deoxy-16-(m-tolyl)-17,18,19,20-tetranorisocarbacyclin Based on the Conjugate Addition–Azoalkene–Asymmetric Olefination Strategy

A fully stereocontrolled synthesis of 3-oxa-15-deoxy-16-(m-tolyl)-17,18,19,20-tetranorisocarbacyclin (3-oxa-15-deoxy-TIC, 7 b) and a formal one of 15-deoxy-16-(m-tolyl)-17,18,19,20-tetranorisocarbacyclin (15-deoxy-TIC, 7 a) are described. 15-Deoxy-TIC is specific for the neuronal prostacyclin receptor (IP2) and exhibits neuroprotective activities, and the new 3-oxa-15-deoxy-TIC is expected to be metabolically more stable than 15-deoxy-TIC. The syntheses of 7 a and 7 b are based on the convergent conjugate addition-azoalkene-asymmetric olefination strategy. Key building blocks are the readily available bicyclic azoalkene 14 and the alkenylcopper derivative 15. The stereoselective conjugate addition of 15 to 14 gave hydrazone 13, which was stereoselectively converted to the bicyclic ketone 11. The key steps for the construction of the alpha side chain of 7 a and 7 b and the regioselective introduction of the endocyclic Delta6,6a double bond are: 1) a highly selective asymmetric olefination of ketone 11 with the chiral Horner-Wadsworth-Emmons reagent 28 and 2) a regioselective deconjugation of the alpha,beta-unsaturated ester (E)-10 with the chiral lithium amide 29, which gave the beta,gamma-unsaturated ester anti-9 with high selectivity. The homoallylic alcohol 8 served at a late stage as the joint intermediate in the syntheses of 7 a and 7 b. While an etherification of 8 furnished, after hydrolysis and deprotection, 3-oxa-15-deoxy-TIC, its alkylation afforded alcohol 37, the known precursor for the synthesis of 15-deoxy-TIC.

Novel synthetic strategies for the preparation of prostacyclin and prostaglandin analogues ? off the beaten track

The recent increase in activity in the fields of neuroscience and life sciences has been mirrored by the design and synthesis of novel chemically and metabolically stable prostaglandin and prostacyclin analogues. Consequently, convenient and practical access to these important classes of compounds is greatly coveted. Various strategies for the preparation of prostacyclin, prostaglandin and isoprostane analogues are discussed, with particular focus on novel approaches developed in our own laboratories.

Exploration of ω-side chain addition strategies for the syntheses of isocarbacyclin and 15R-16-(m-tolyl)-17,18,19,20-tetranorisocarbacyclin

We describe alternative access to prostacyclin analogues by means of two omega-side chain addition strategies: Grignard reagent addition to an alpha,beta-unsaturated Weinreb amide, followed by diastereoselective reduction of the corresponding enone system, and implementation of Seebach's alkylation chemistry. These strategies have led to the syntheses of biologically active prostacyclin analogues isocarbacyclin and 15R- 16-(m-tolyl)- 17,18,19,20-tetranorisocarbacyclin (15R-TIC), with modest to excellent diastereoselectivity.