Omega-side Chain Research Articles

Cross Metathesis as a General Strategy for the Synthesis of Prostacyclin and Prostaglandin Analogues

[reaction: see text] A cross metathesis (CM) approach has been successfully applied to introduce fully functionalized omega-side chain appendages of various prostacyclin and prostaglandin analogues, resulting in high (E)-selectivities for the C13-C14 double bond and leading to the total syntheses of isocarbacyclin, 15R-TIC, carbacyclin, and PGF(2)(alpha) and the formal syntheses of 15-deoxy-TIC and PGJ(2).

Fully Stereocontrolled Syntheses of 3-Oxacarbacyclin and Carbacyclin by the Conjugate Addition-Azoalkene-Asymmetric Olefination Strategy

A fully stereocontrolled synthesis of 3-oxacarbacyclin (3) and a formal synthesis of carbacyclin (2) are described. The syntheses are based on the conjugate addition-azoalkene-asymmetric olefination strategy. Its key features are (1) the stereoselective establishment of the complete omega-side chain of 2 and 3 through conjugate addition of the enantiopure C13-C20 alkenylcopper derivative 10 to the enantiopure C6-C12 bicyclic azoalkene 9 and (2) the 5E-stereoselective construction of the alpha-side chain through a Horner-Wadsworth-Emmons olefination of the bicyclic ketone 7 with the chiral lithium phosphonoacetate 26 with formation of ester E-27. The allylic alcohol 6 serves at late stage as the joint intermediate in the synthesis of 2 and 3.

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.

Fully Stereocontrolled Total Syntheses of the Prostacyclin Analogues 16S-Iloprost and 16S-3-Oxa-Iloprost by a Common Route, Using Alkenylcopper-Azoalkene Conjugate Addition, Asymmetric Olefination, and Allylic Alkylation

In this article we describe fully stereocontrolled total syntheses of 16S-iloprost (16S-2), the most active component of the drugs Ilomedin and Ventavis, and of 16S-3-oxa-iloprost (16S-3), a close analogue of 16S-2 having the potential for a high oral activity, by a new and common route. The key steps of this route are (1) the establishment of the complete C13-C20 omega side chain of the target molecules through a stereoselective conjugate addition of the alkenylcopper derivative 9 to the bicyclic C6-C12 azoalkene 10 with formation of hydrazone 8, (2) the diastereoselective olefination of ketone 7 with the chiral phosphoryl acetate 39, and (3) the regio- and stereoselective alkylation of the allylic acetate 43 with cuprate 42. These measures allowed the 5E,15S,16S-stereoselective synthesis of 16S-2 and 16S-3, a goal which had previously not been achieved. Azoalkene 10 was obtained from the achiral bicyclic C6-C12 ketone 11 as previously described by using as key step an enantioselective deprotonation. The configuration at C16 of omega-side chain building block 9 has been installed with high stereoselectivity by the oxazolidinone method and that at C15 by a diastereoselective oxazaborolidine-catalyzed reduction of the C13-C20 ketone 23 with catecholborane. Surprisingly, a high diastereoselectivity in the reduction of 23 was only obtained by using 2 equiv of oxazaborolidine 24. Application of substoichiometric amounts of 24 resulted in irreproducible diastereoselectivities ranging from very high to nil.

Access to Isocarbacyclin Derivatives via Substrate-Controlled Enolate Formation: Total Synthesis of 15-Deoxy-16-(m-tolyl)- 17,18,19,20-tetranorisocarbacyclin

[reaction: see text] We describe a convergent and flexible synthesis of 15-deoxy-16-(m-tolyl)-17,18,19,20-tetranorisocarbacyclin (15-deoxy-TIC), a simple isocarbacyclin derivative. The synthesis takes advantage of two key step reactions: a regioselective deprotonation of the described ketone under substrate control which is then trapped, as the enol triflate, to generate the C6-C9alpha endocyclic double bond, followed by an sp2-sp3 Pd-catalyzed cross-coupling reaction (C5-C6) with a suitable primary alkyl Grignard reagent. Introduction of the C13-C14 (E)-double bond in the omega-side chain is performed by the Julia-Kocieñski olefination.

A Convergent Synthesis of the 11-Oxa Prostaglandin Analogue AL-12182

[reaction: see text] The 11-oxa prostaglandin analogue AL-12182 1 has potent topical ocular hypotensive activity. A convergent and concise general synthesis of this class of prostanoid was developed employing a stereoselective coupling reaction between a tetrahydrofuran core electrophile and a nucleophilic omega side chain component, providing a route that should be suitable for commercial scale production. The tetrahydrofuran core was assembled from dimethyl d-malate using a stereoselective beta-hydroxy ester dianion alkylation reaction.

Asymmetric synthesis of the highly potent anti-metastatic prostacyclin analogue cicaprost and its isomer isocicaprost.

An asymmetric synthesis of the anti-metastatic prostacyclin analogue cicaprost and a formal one of its isomer isocicaprost by a new route are described. A key step of these syntheses is the coupling of a chiral bicyclic C6-C14 ethynyl building block with a chiral C15-C21 omega-side chain amide building block with formation of the C14-C15 bond of the target molecules. A highly stereoselective reduction of the thereby obtained C6-C21 intermediate carrying a carbonyl group at C15 of the side chain was accomplished by the chiral oxazaborolidine method. The chiral phosphono acetate method was used for the highly stereoselective attachment of the alpha-side chain to the bicyclic C6-C21 intermediate carrying a carbonyl group at C6. Asymmetric syntheses of the bicyclic C6-C14 ethynyl building blocks were carried out starting from achiral bicyclic C6-C12 ketones by using the chiral lithium amide method. In the course of these syntheses, a new method for the introduction of an ethynyl group at the alpha-position of the carbonyl group of a ketone with formation of the corresponding homopropargylic alcohol was devised. Its key steps are an aldol reaction of the corresponding silyl enol ether with chloral and the elimination of a trichlorocarbinol derivative with formation of the ethynyl group. In addition, a new aldehyde to terminal alkyne transformation has been realized. Its key steps are the conversion of an aldehyde to the corresponding 1-alkenyl dimethylaminosulfoxonium salt and the elimination of the latter with a strong base. Two basically different routes have been followed for the synthesis of the enantiomerically pure C15-C21 omega-side chain amide building block. The first is based on the chiral oxazolidinone method and features a highly stereoselective alkylation of (4R)-N-acetyl-4-benzyloxazolidin-2-one, and the second encompasses a malonate synthesis of the racemic amide and its efficient preparative scale resolution by HPLC on a chiral stationary phase containing column.

Preparation of isoprostanes and neuroprostanes.

A new approach to isoprostanes and neuroprostanes featuring cis-dialkyl stereochemistry at the cyclopentane ring has been developed by employing an intramolecular cross-coupling reaction of an alkyl iodide and a tethered alkenylsiloxane for stereoselective installation of a functionalized omega-side chain.

Pharmacological and clinical properties of beraprost sodium, orally active prostacyclin analogue

Prostacyclin is an endogeneous eicosanoid synthesized by vascular endothelial cells, and has potent inhibitory effects on platelet adhesion/aggregation and vasoconstriction. However, its therapeutic use is restricted by its extremely short half-life. Beraprost sodium (beraprost) is the first orally active prostacyclin analogue developed by TORAY Industries, Inc. Beraprost possesses a phenol moiety instead of the exo-enol ether moiety, which is the cause of the instability of prostacyclin, and has a modified omega-side chain that contributes to dissociating antiplatelet action from adverse reactions. In 1992, beraprost was approved as a drug for chronic arterial occlusion. Beraprost is now widely used clinically as "Dorner" or "Procylin". The indication for "primary pulmonary hypertension" was also approved in 1999. Recently in Europe, a placebo controlled trial named "Beraprost et Claudication Intermittent-2 (BERCI-2)" was performed, and it was reported that beraprost improved the walking distances of the patients. Beraprost has a variety of biological activities such as antiplatelet effects, vasodilation effects, antiproliferative effects on vascular smooth muscle cells, cytoprotective effects on endothelial cells and inhibitory effects on the production of inflammatory cytokines. On the basis of basic and clinical research, it has been suggested that beraprost is also effective for many intractable diseases. We expect that the relationship between reduced prostacyclin level and these diseases would be clarified and the beneficial effects of beraprost would be demonstrated by controlled clinical trials in the future.

Research and development of beraprost sodium, a new stable PGI2 analogue

A novel prostaglandin I2 (PGI2) analogue, beraprost sodium, is the first launched drug as an orally active PGI2. PGI2 was discovered in 1976, and has attracted much attention as a medicine for cardiovascular diseases such as strokes and heart attacks because of its potent antiplatelet and vasodilating effect. However, PGI2 is extremely unstable for the use as practical medicines. Thus, stable PGI2 analogues have been explored by a large number of researchers in the world. Just after the discovery of PGI2, we started a research on chemically and metabolically stable PGI2 derivatives with longer duration of action and less adverse reaction. We invented a novel class of stable PGI2, 5,6,7-trinor-4,8-inter-m-phenylenePGI2 analogues that have the phenol moiety instead of the enolether moiety of PGI2. Further efforts were devoted to enhance the efficacy of the PGI2 analogues and to eliminate their side effects, and an orally active analogue, beraprost sodium, was obtained. In order to establish the synthetic route of beraprost sodium, various novel processes were invented, including ortho-selective metalation of bromoanisoles by means of Grignard reagents, copper-catalyzed SN2' cyclization to prepare cyclopenta[b]benzofuran, and stereo-selective elongation of the omega-side chain by Prins reaction. Beraprost sodium inhibit platelet aggregation induced by adenosine 5'-diphosphate (ADP), collagen and arachidonic acid. It was shown that the drug has a potent antiplatelet effect both in vitro and ex vivo in human and several animal species. In clinical studies, beraprost sodium exerted a marked effect to improve arteriosclerosis obliterans. No serious adverse effects related with the drug have been reported. It was highly evaluated as an orally active PGI2 by pharmaceutical companies overseas as well, and now clinical trials are under way in U.S.A. and Europe.

Bisallylic hydroxylation and epoxidation of polyunsaturated fatty acids by cytochrome P450.

Polyunsaturated fatty acids can be oxygenated by cytochrome P450 to hydroxy and epoxy fatty acids. Two major classes of hydroxy fatty acids are formed by hydroxylation of the omega-side chain and by hydroxylation of bisallylic methylene carbons. Bisallylic cytochrome P450-hydroxylases transform linoleic acid to 11-hydroxylinoleic acid, arachidonic acid to 13-hydroxyeicosa-5Z,8Z,11Z,14Z-tetraenoic acid, 10-hydroxyeicosa-5Z,8Z,11Z,14Z-tetraenoic acid and 7-hydroxyeicosa-5Z,8Z,11Z,14Z-tetraenoic acid and eicosapentaenoic acid to 16-hydroxyeicosa-5Z,8Z,11Z,14Z,17Z-pent aenoic acid, 13-hydroxyeicosa-5Z,8Z,11Z,14Z,17Z-pent aenoic acid and 10-hydroxyeicosa-5Z,8Z,11Z,14Z,17Z-pent aenoic acid as major metabolites. The bisallylic hydroxy fatty acids are chemically unstable and decompose rapidly to cis-trans conjugated hydroxy fatty acids during acidic extractive isolation. Bisallylic hydroxylase activity appears to be augmented in microsomes induced by the synthetic glucocorticoid dexamethasone and by some other agents, but the P450 gene families of these hydroxylases have yet to be determined. The fatty acid epoxides, which are formed by cytochrome P450, are chemically stable, but are hydrolyzed to diols by soluble epoxide hydrolases. Epoxidation of polyunsaturated fatty acids is a prominent pathway of metabolism in the liver and the renal cortex and epoxy-genase activity appears to be under homeostatic control in the kidney. Many arachidonate epoxygenases have been identified belonging to the CYP2C gene subfamily. Epoxygenases have also been found in the central nervous system, endocrine organs, the heart and endothelial cells. Epoxides of arachidonic acid have been found to exert pharmacological effects on many cells.

FTIR spectral study of intramolecular hydrogen bonding in thromboxane A2 receptor agonist (U-46619), prostaglandin (PG)E2, PGD2, PGF2 alpha, prostacyclin receptor agonist (carbacyclin), and their related compounds in dilute CCl4 solution: structure-activity relationships.

FTIR spectra measurements and full optimization curve analysis of their spectra were done to obtain parameters of the OH and C = O stretching vibration bands for intramolecular hydrogen bondings in thromboxane (TX)A2 receptor partial agonist (CTA2), prostaglandin (PG)E2, PGD2, PGF2 alpha, prostacyclin (PGI2) receptor agonist (carbacyclin), and their related compounds in dilute CCl4 solutions. For CTA2, PGE2, PGD2, and PGF2 alpha, cyclic intramolecular hydrogen bonds involving a 15-membered ring similar to that observed for the TXA2 receptor agonist (U-46619) were found between a carboxyl group of the alpha-side chain and a 15-hydroxyl group of the omega-side chain. The arrangement of these side chains was P-shaped, and the percentage of the intramolecular hydrogen-bonded molecules with the 15-membered ring in CCl4 solution showed a high value of ca. 80% for these compounds. In addition, it was found that the cyclic intramolecular hydrogen bonds involving the 13-, 12-, and 12-membered rings in PGE2, PGD2, and PGF2 alpha, respectively, are formed between the carboxyl group of the alpha-side chain and the 11-, 9-, and 9-hydroxyl groups of a cyclopentane ring, respectively, although the percentages of the intramolecular hydrogen-bonded molecules with these membered rings are very small. It was also found that the hydrogen bond is more easily formed in the order of the 11-, 9-, and 15-hydroxyl groups. For carbacyclin, the cyclic intramolecular hydrogen bond involving the 13-membered ring was found between the carboxyl group of the alpha-side chain and the 11-hydroxyl group. The percentage of the intramolecular hydrogen-bonded molecules showed the value of 58% for carbacyclin. On the basis of information on the side-chain conformations in CCl4, we examined the structure-activity relationships for U-46619 in place of TXA2, PGE2, PGD2, PGF2 alpha, and carbacyclin in place of PGI2.

Conformation of receptor-associated PGI2: An investigation by molecular modeling

To elucidate the conformation of receptor-associated prostacyclin (PGI2), we first performed structure-activity correlation analysis of over 200 PGI2 analogues and derived from this analysis several crucial features pertaining to structural requirements for PGI2 activity [Ah-lim Tsai and Kenneth K. Wu, Eicosanoids, 2 (1989) 131-143]. These structural features proved to be useful guidelines for selecting 'model molecules' for further investigations by molecular mechanics. By properly selecting four analogues with either rigid or uniquely oriented alpha-side chain structure for geometric fitting, we succeeded in maximally minimizing the degree of freedom of the carboxylate terminus of PGI2. We were able to define the spatial relationship among the four critical functional groups, i.e., C1-COOH, C6a-O, C11-OH and C15-OH. More information is needed, however, to define the geometry of the omega-side chain, particularly for the moiety beyond C15. Nevertheless, results from structure-activity correlation analysis and molecular modeling provide useful information regarding the conformation of receptor-associated PGI2, which assumes an 'elongated' conformation instead of the traditional 'hairpin' structure.

Phorbol ester-induced expression of the common, low-affinity binding site for primary prostanoids in vascular smooth muscle cells.

We showed in an earlier study (Hanasaki, K., and Arita, H. (1989) Biochim. Biophys. Acta 1013, 28-35) that there is a common, low-affinity binding site for primary prostanoids in cultured vascular smooth muscle cells (VSMC). This site, called the "primary prostaglandin (PG) site," can be evaluated by radioreceptor assay using [3H]PGF2 alpha and [3H]PGE1. Comparison of the capacity of several PGF2 alpha analogs to displace both radioligand bindings indicated strict requirements of the 15-hydroxy group as well as the 13,14-double bond in the omega-side chain of prostaglandins for recognition of this site. Treatment of VSMC with phorbol 12-myristate 13-acetate (PMA), a known protein kinase C activator, led to concentration- and time-dependent increases in the binding activities of [3H] PGF2 alpha as well as [3H]PGE1, which could be completely suppressed by the addition of protein kinase C inhibitor, H-7. The PMA effects could be mimicked by phorbol 12,13-dibutylate, but not by inactive phorbol ester. Scatchard analyses revealed an approximately 8-fold increase in the binding density with unaltered binding affinity after PMA treatment. This expression of the primary PG site was blocked by the addition of cycloheximide and actinomycin D. In contrast, PMA did not affect the binding activity for the thromboxane A2/prostaglandin H2 receptor in VSMC. These results suggest that the expression of the primary PG site is regulated by a protein kinase C-dependent mechanism in VSMC.

Degradation and isomerization of nileprost, 5-cyano-16-methyl-prostacyclin, in aqueous solution.

Nileprost is a new prostacyclin analogue stabilized by introduction of the cyano group at its 5-position. The acidic and alkaline degradation and the structure determination of the degradation products were investigated. The degradation of nileprost is very slow in comparison with that of prostacyclin. Although prostacyclin is easily decomposed to 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) through the hydrolysis of a vinyl ether moiety, nileprost gives little of such hydrolysis product but many isomers and dehydrates. The structures of the products were determined by high-performance liquid chromatography (HPLC), negative ion fast atom bombardment mass spectra (N-FABMS) and nuclear magnetic resonance (NMR) studies. It was found that the vinyl ether moiety of nileprost is converted from 5Z form to 5E form in both media, and that the omega-side chain also undergoes transfer of hydroxyl group or dehydration in acid medium. These results indicate that introduction of the cyano group into the 5-position of prostacyclin is extremely effective for the stabilization of the vinyl ether moiety. Furthermore, on the basis of the structural elucidation, the reaction mechanism of nileprost in both media was clarified.

Thromboxane A2 receptor antagonists. II. Synthesis and pharmacological activity of 6,6-dimethylbicyclo(3.1.1)heptane derivatives with the benzenesulfonylamino group.

Various stereoisomers based on the alpha- and omega-side chain ring junctions of 6,6-dimethylbicyclo[3.1.1]heptane were synthesized. Their sodium salts 12, 18, 20, 30 and 37 were examined in vitro for their inhibitory activity toward aggregation of rabbit platelet-rich plasma and rat washed platelets. Their potency was very high and the partial agonist effect was small. The differences of the side chain ring junctions did not affect the activity very much. Homologation in the omega-side chain (as in 47) decreased the activity.

ChemInform Abstract: Structural Requirements for the Inhibition of 5‐Lipoxygenase by 15‐Hydroxyeicosa‐5,8,11,13‐tetraenoic Acid Analogues.

The structural requirements for inhibition of RBL-1 (rat basophilic leukemia) 5-lipoxygenase by 15-hydroxyeicosa-5,8,11,13-tetraenoic acid (15-HETE, 1) were studied by systematic chemical modifications of the molecule at the hydroxyl and carboxyl groups, the double bonds, and the carboxylate and omega side chains. The most potent inhibitors were analogues that contained a 5,8-cis,cis-diene system and acted as alternate substrates for the enzyme. However, several analogues in which the 5,8-diene had been reduced were also found to inhibit the enzyme. Inhibition of 5-lipoxygenase by 15-hydroxyeicosa-11,13-dienoic acid (15-HEDE) analogues was optimal in compounds that generally contained a free carboxyl group, a carboxylate side chain of nine carbons, an omega side chain of five or six carbons, a cis,trans- or trans,cis-11,13-diene or 11,13-diyne system, and a 15-hydroxyl group. Conversion of 15-HEDE to its 16-membered lactone reduced but did not eliminate 5-lipoxygenase inhibitory activity. In contrast, a 3- to 10-fold enhancement of activity occurred when 5,15-diHETE (58) or 5-HETE (56) were cyclized to their respective delta-lactones. Molecular modeling of 15-HEDE analogues, modified in the C11-C15 region, showed that inactive analogues protrude into regions in space not occupied by active analogues. These structural studies indicate that multiple regions are important for 5-lipoxygenase inhibition by both 15-HETE and 15-HEDE analogues and that no single region plays a predominant role in inhibition.

Radioimmunoassay of 11-dehydrothromboxane B 2 in human plasma and urine

Because of the discrepancy between the capacity of platelets to synthesize thromboxane B 2 ex vivo and the actual synthetic rate in vivo, measurement of thromboxane B 2 in plasma is highly influenced by sampling-related artifacts. We have developed and validated a radioimmunoassay for a major enzymatic derivative of thromboxane B 2 with an extended plasma half-life i.e., 11-dehydrothromboxane B 2. The binding of the tracer is displaced by as low as 1 pg/ml of the homologous ligand, with a high degree of specificity for the open ring structure as well as for the omega side-chain. This method can detect changes in the plasma concentration and urinary excretion of 11-dehydrothromboxane B 2 associated with simulated short-term increases of thromboxane B 2 secretion in the human circulation.

Inhibition of prostaglandin biosynthesis by SQ 28,852, A 7-oxabicyclo-[2.2.1]heptane analog

7-Oxabicyclo[2.2.1]heptane analogs of prostaglandin (PG) H 2 can act as thromboxane (Tx) A 2 receptor antagonists or agonists, PGI 2 and/rr PGD 2 receptor agonists, or exhibit a mixture of the above activities. SQ 28,852, a new analog with a hexyloxymethyl omega side chain, is a potent inhibitor of PG synthesis. SQ 28,852 inhibited collagen and arachidonic acid (AA)-induced platelet aggregation and TxB 2 and PGE 2 formation, but did not block platelet aggregation induced by ADP or the TxA 2 mimics, 9,11-azoPGH 2, SQ 26,655, and U-46,619. It also blocked conversion of AA to TxB 2, PGE 2, and 6-ketoPGF 1α by microsomal preparations of human platelets, bovine seminal vesicles, and bovine aortas, respectively, but did not inhibit the conversion of PGH 2 to TxA 2 by the platelet microsomal preparation. SQ 28,852 (p.o.) protected mice against the lethal effects of AA (75 mg/kg, i.v.). The I 50 values for SQ 28,852, indomethacin and aspirin were 0.025, 0.05 and 15 mg/kg, respectively. Neither SQ 28.852 nor indomethacin protected mice from death caused by 9,11-azoPGH 2. SQ 28,852 (0.01 to 1 mg/kg, i.v.) inhibited AA-induced bronchoconstriction in anesthetized guinea pigs for at least 60 min. As an inhibitor of AA-induced bronchoconstriction, SQ 28,852 was 16- and 45-times more potent than indomethacin at 3 and 60 min after i.v. administration, respectively. SQ 28,852 did not inhibit brochoconstriction induced by histamine or 9.11-azoPGH 2, indicating its specificity of action in vivo . SQ 28,852 is the first example of a new class of cyclooxygenase inhibitors whose structure is similar to that of the naturally occurring endoperoxide, PGH 2.

Synthesis and biological evaluation of .omega.-homologues of prostaglandin E1

The synthesis and biological activities of some compounds with novel modifications of the omega side chain of prostaglandin E1 (PGE1) are described. The preparation of (+/-)-omega-Me-PGE1 (3) (+/-)-omega-Pr-PGE1 (5), and (+/-)-omega-Bu-PGE1 (6) is outlined. The compounds were evaluated for in vitro smooth muscle stimulating activity on isolated gerbil colon preparations, for hypotensive action in anesthetized rats, and for gastric antisecretory effects in histamine-stimulated Heidenhain pouch dogs. Structural changes in the omega position of the noncarboxyl side chain of PGE1 influenced the biological potency of the resulting compound when compared to the reference standard PGE1 (2). The homologues demonstrated interesting separation of biological activities; for example, 4 showed potent hypotensive activity (84% of PGE1, it showed very low smooth muscle stimulating activity. Compound 3 possessed the same order of potency as 2 in the gastric antisecretory assay.