Synthesis of diastereomeric bis-unsaturated prostaglandins

First Total Synthesis of Phytoprostanes with Prostaglandin-Like Configuration, Evidence for Their Formation in Edible Vegetable Oils and Orienting Study of Their Biological Activity

Phytoprostanes (PhytoP) are natural products, which form in plants under oxidative stress conditions from α-linolenic acid. However, their epimers with relative prostaglandin configuration termed phytoglandins (PhytoG) have never been detected in Nature, likely because of the lack of synthetic reference material. Here, the first asymmetric total synthesis of such compounds, namely of PhytoGF_1α (9-epi-16-F_1t -PhytoP) and its diastereomer ent-16-epi-PhytoGF_1α (ent-9,16-diepi-16-F_1t -PhytoP), has been accomplished. The synthetic strategy is based on radical anion oxidative cyclization, copper(I)-mediated alkyl-alkyl coupling and enantioselective reduction reactions. A UHPLC-MS/MS study using the synthesized compounds as standards indicates PhytoG formation at significant levels during autoxidation of α-linolenic acid in edible vegetable oils. Initial testing of synthetic PhytoGs together with F₁ -PhytoP and 15-F_2t -IsoP derivatives for potential interactions with the PGF_2α (FP) receptor did not reveal significant activity. The notion that PUFA-derived oxidatively formed cyclic metabolites with prostaglandin configuration do not form to a significant extent in biological or food matrices has to be corrected. Strong evidence is provided that oxidatively formed PhytoG metabolites may be ingested with plant-derived food, which necessitates further investigation of their biological profile.

Preparation and Chiral HPLC Separation of the Enantiomeric Forms of Natural Prostaglandins

Four enantiomeric forms of natural prostaglandins, ent-PGF2α ((−)-1), ent-PGE2 ((+)-2) ent-PGF1α ((−)-3), and ent-PGE1 ((+)-4) have been synthetized in gram scale by Corey synthesis used in the prostaglandin plants of CHINOIN, Budapest. Chiral HPLC methods have been developed to separate the enantiomeric pairs. Enantiomers of natural prostaglandins can be used as analytical standards to verify the enantiopurity of synthetic prostaglandins, or as biomarkers to study oxidation processes in vivo.

Regio- and Stereoselective Reactions of 17-Phenyl-18,19,20-trinorprostaglandin F(2)(alpha) Isopropyl Ester

Novel prostaglandin F(2)(alpha) derivatives, functionalized at C13 and C14, have been prepared. 17-Phenyl-18,19,20-trinorprostaglandin F(2)(alpha) isopropyl ester [(15S)-1] and its epimer [(15R)-1] were stereoselectively epoxidized, using Sharpless conditions, to produce each of the four diastereomeric epoxides (15S)-2, (15S)-3, (15R)-2, and (15R)-3. Treatment of the four epoxides with LiOH stereospecifically-produced the pentahydroxy substituted analogues 12 and 13. Alternatively, epoxides 2 and 3were allowed to react with thiophenolate ion. The attack of the sulfur nucleophile on the epoxide occurred at either C13 or C14 depending on the stereochemistry of the epoxide and of C15.

The single prostacyclin receptor of gel-filtered platelets provides a correlation with antiaggregatory potency of PGI2 mimics

Gel-filtered human platelets (GFP) display only a single binding site for [3H]-PGI2: KD = 61nM, 234 fmol/10(8) platelets (1410 sites/platelet). Platelet-rich plasma (PRP) displays the same receptor density but the KD value increases to 123 nM due to protein binding of PGI2 which lowers its effective concentration. The [3H]-PGI2/GFP binding assay has been used to evaluate the molecular basis of aggregation inhibition for prostacyclin analogs and mimics, three PGE type structures, and PGD2. Antiaggregatory IC50s and radioligand binding IC50s correlate for PGE2, E1, and six PGI2 analogs. PGD2, and to a lesser extent 6-oxo-PGE1, display greater antiaggregatory potency than expected based on PGI2-binding site affinity data.

Mass spectrometric fragmentation patterns for the syn and anti isomers of PGE2 and PGD2-methyloxime methyl esters and their analogs

A systematic study of the mass spectral fragmentation of the methyl ester-methyloxime-trimethylsilyl ether derivatives of D and E prostaglandins and selected omega-chain analogs is presented. Fragments from the omega-chain analogs are shifted the appropriate mass when compared with the parent PGD2 or PGE2. NMR data of the methyloxime methyl ester of PGE2 have permitted assignment of the syn and anti isomers (relative to the alpha chain) to the fast and slow eluting gas chromatographic peaks, respectively.

Temperature and field dependent 1H-NMR relaxation data as probes of prostaglandin motional parameters in aqueous media

Proton resonance correlation times (tau eff) for PGF2 alpha and a more rigid analog have been derived from the field-strength dependence of spin-lattice relaxation times ( T1D ) using 200 and 500 MHz observation. Those hydrogens showing tau eff less than the value calculated for whole molecule tumbling (which applies for H-5----H-15) also show a significantly greater temperature dependence for T1D at 500 MHz. Minor wagging may occur at the C-7 and C-10 methylenes , and gradually increasing segmental motion is observed toward both side chain termini. A current model for the aqueous geometry of PGF2 alpha is developed from this data and studies of relaxation rate changes upon specific deuteration.

On the multiplicity of platelet prostaglandin receptors. II. The use of N-0164 for distinguishing the loci of action for PGI2, PGD2, PGE2 and hydantoin analogs

The omega-chain variant analogs of prostacyclin (PGI2) and PGD2 in which n-amyl side-chain has been replaced by a cyclohexyl group have been prepared and their cardiovascular activities have been compared to those of BW-245C(Fig. 1) a potent anti-aggregatory vasodilator bearing a cyclohexyl-terminated side-chain on a hydantoin skeleton. The cyclohexyl group has little effect on PGI2, but converts PGD2 to a long lasting hypotensive agent and increases the platelet anti-aggregatory potency of PGD2 by a factor of 8. The prostaglandin antagonist N-0164 selectively blocks the anti-aggregatory actions of PGD2, cyclohexyl-PGD2, and BW-245C; with essentially no effect on PGI2, cyclohexyl-PGI2 and PGE2 at comparably effective doses. The latter observation is contrary to an earlier report by MacIntyre, but supports the view that the anti-aggregatory effect of high doses of PGE2 (EC50=50 microM) is mediated by the PGI2 receptor. The hydantoin acts at the platelet PGD2 receptor.

High-field 1H NMR studies of prostaglandin H2 and its decomposition pathways

Prostaglandin H2 displays at 500 MHz a detailed 1H-NMR in which all methylene groups are non-equivalent in C6D6 solution. The spectrum was assigned by analogy to isosteric structures. The dissymmetric perturbation and steric hindrance of the bicyclo [2.2.1] core caused by the side-chains provides a rationale for the selective fragmentations which PGH2 undergoes. Purified PGH2 is considerably more robust than previous literature accounts suggest. The following transformations were monitored by 1H-NMR: 1) O-O bond cleavage by Ph3P , 2) aqueous media fragmentation to PGE2 and PGD2, 3) base catalyzed fragmentation to ketoaldehydes , and 4) thermolysis attempts.

A single platelet aggregometric assay for pro-and antiaggregatory prostanoids

A method is described for characterizing disaggregatory and proaggregatory prostanoids in a single assay. This "post-addition assay" consists of adding the test agent to PRP containing PGI2 (4-6 nM) 90 sec after an ADP challenge. The relative potencies of known inhibitors of ADP-induced aggregation were identical to the relative disaggregatory potencies determined by post-addition. The proaggregatory relative potencies for 16, 16-Me2-PGE2,PGE2,PGH2 and 11 alpha,9 alpha-epoxymethano-15 alpha-hydroxy-prostadienoic acid were 5.8 :2 : 1.8 : 1, respectively, revealing the significant thrombosis promoting activity of PGE2 and its analogs.

Omega chain methylated analogs of PGF2 alpha and PGE2

Our previously published prostaglandin (PG) synthesis route, in which the omega-chain is added in the penultimate step, provides facile access to a wide variety of omega-chain variant PG analogs. Each series requires only the synthesis of the appropriate methylated acylphosphonate for the Emmons' condensation. The syntheses of analogs bearing the following methylation patterns are detailed: 15-Me; 17,17-(Me) 2; 17, 17, 20-(Me) 3; 18, 18, 20-(Me) 3; 15, 18, 18, 20-(Me) 4; and 15-OMe-18, 18, 20- (Me) 3. The well-known 16., 16-dimethyl prostaglandins have also been prepared by this sequence. The synthesis of 16, 16-tetramethylene-PG analogs is also described.

Synthesis of macrolide prostaglandin analogs

Prostaglandin analogs of the E- and F2 alpha-functional type, which are constrained to conformations in which the side-chains are close in space and specifically aligned in the terminal portions by covalent bonding, have been synthesized. These analogs are 1, (omega-1)-macrolides. The syntheses proceeded from aldehyde intermediate I via the Emmon's condensation with dimethyl n-(dimethyl-t-butylsilyloxy)2-oxoalkylphosphonate anions (II a or b). The macrolide closures were performed using 2, 2'-dipyridyl disulfide. For the synthesis of 9-ketoprostaglandin macrolides, a free 9-hydroxy is available for oxidation after macrolide closure, so long as the 9-position is protected as the acetate rather than benzoate. Chiroptical data revealed that the conformations of the macrolide prostaglandins are unchanged (relative to the natural unconstrained prostaglandins) in the vicinity of the five-membered ring and the allyl alcohol unit by the formation of the macrolide linkage.

Molecular basis for prostaglandin potency. III. Tests of the significance of the "hairpin conformation" in biorecognition phenomena

Prostaglandin analogs of the PGF2 alpha, 15-epi-PGF2 alpha, and PGE2 type bearing the following methyl substitution patterns -- 15-Me, 16, 16-(Me)2, 17, 17-(Me)2, and 18, 18, 20-(Me)3 -- and analogs constrained to "hairpin" alignment [via 1, (omega-1)-olide formation] and to "non-hairpin" arrangements [via 1, 9- and 1, 15-olide formation] are compared in the following biological assays: contraction of uterine and gastro-intestinal smooth muscle strips, luteolytic antifertility potency in the hamster, binding affinity to two different PGF2 alpha-receptor preparations from bovine corpora lutea, binding to the PGE-specific receptors from rat kidney and liver, inhibition of ADP- induced aggregation of human platelet-rich-plasma, and the effect on rat blood blood pressure. The methylated prostaglandins were also concerted to the corresponding prostacyclins and examined as to action on the platelet and on rat blood pressure. All evidence points to topographically distinct receptors for F2 alpha-, E- and I2- type prostaglandins. Cross-reactivity is reduced in most of the analogs examined. Independent of the target organ or tissue, the receptors show common features based on the functional class of PG recognized. "Hairpin" alignment improves binding (and potency) only for the PGF2 alpha specific assays. PGE-specific binding and potency is disrupted to an increasing extent as the chain branching point is moved further from the 15-hydroxyl center. In contrast 16, 16-dimethylation is particularly disruptive for the PGI2/E1 platelet receptor interaction.

Methyl ethers of prostaglandins F2 alpha and I2

Regiospecific monomethyl prostaglandin f2 alpha ethers (at 0-9, 0-11, and 0-15) have been prepared by total synthesis. The 9, 15-bis-ether was also prepared. The 11- and 15-monoethers have been converted to the corresponding prostacyclins. Nuclear Magnetic Resonance (NMR) spectral comparisons indicate conformational changes associated with ether formation; nonetheless, the PGF2 alpha monoethers all retain significant biological activity: 3-420% of natural PGF2 alpha. The 9- and 15- menthyl ethers show increased selectivity for luteolytic activity as measured in the hamster antifertility (HAF) assay. In contrast the prostacyclin ethers are essentially devoid of agonist activity on both the platelet and vasculature. Prostacyclin diastereomers [5a] also lack activity and it appears that any modification at or of the C-11 or C-15 functions reduces receptor binding by at least a factor of 100.

On the multiplicity of platelet prostaglandin receptors. I. Evaluation of competitive antagonism by aggregometry

Methods for the evaluation of competitive interactions at receptors associated with platelet activation and inhibition using aggregometry of human PRP have been developed. The evidence supports the suggestion that PGE1 and PGI2 share a common receptor for inhibition of platelet reactivity, but only a portion (if any) of the aggregation stimulation associated with PGE2 is the result of PGE2 binding (without efficacy) to this receptor. PGE2 (at .3-20 microM) is an effective antagonist of PGE1, PGI2, and PGD2 producing a shift of about one order of magnitude in the IC50-values obtained from complete aggregation inhibition dose response curves. The antagonism of PGD2 inhibition is particularly notable, 80 nM PGE2 levels are detectable. This and other actions of PGE2 indicate another platelet receptor for PGE2. PGE1 acts at both the PGE2 and PGI2 receptor. Other substances showing PGI2-like actions only at high doses (1-30 microM), display additive responses with PGI2 indicative of decreased affinity for the I2/E1 receptor and the absence of PGE2-like aggregation stimulation activity. PGI2 methyl ester has intrinsic inhibitory action not associated with in situ ester hydrolysis. The methyl ester is dissaggregatory showing particular specificity for inhibition of release and second wave aggregation.

Synthesis and chiroptical characterization of prostacyclin diastereomers

In the mercuri- and halo-cyclizations of PGF2 alpha methyl ester and its 11,15-bis(alpha- ethoxyethyl)-ether (or other protected forms) the exo-PGI1 derivative predominates independent of reagent and degree of protection of the PGF2 alpha sample used. Diastereomerically pure samples of exo- and endo-PGI1 and prostacycline (PGI2) were prepared. PGI0 epimers were prepared: catalytic hydrogenation of PGI2 Me ester provides exclusively the endo isomer. PGI2 methyl ester was found to be stable to extensive chromatography on silica, and to storage for at least a year in anhydrous ethanol at -20 degrees C. At pH 7.4 in 2:1 H2O:EtOH, the ester has a half-life in excess of 5 hr at 25 degrees C. A reproducible small scale (0.4-3 mg) synthesis of prostacyclin uses a modification of Whittaker's iodocyclization followed by DBN treatment. This procedure, developed with 15-3H-PGF2 alpha, proved widely applicable to PGF2 alpha analogs and diastereomers. The following prostacyclins (in the Me ester and Na salt forms) bearing the 5-en-6-yl ether unit were prepared in this way: ent-PGI2, rac-PGI2, 15-epi-PGI2, ent-15-epi-PGI2, 11-epi-PGI2, 8,9,12-epi-PGI2, E-PGI2, 13,14-dihydro-PGI2, and 13,14-dihydro-15-epi-PGI2. NMR comparisons for the methyl esters reveal that of the resonances (H-5,9,UU, 15) that appear at delta 4.0 +/- 0.6 ppm, the most deshielded is H-9 so long as the 5.6-olefin is Z. The 8R,9S-6,9-oxido-Z-5,6-ene unit is most readily characterized by its strong positive dichroic absorption at 210-230 nm. CD spectroscopy not only serves to confirm the presence of this unit in analogs, but also can be used for quantitative analysis of PGI2 solutions and for monitoring the rate of hydrolytic cleavage of these enol ethers.

Aureobasidium pullulans metabolism of prostaglandins of the A-type

Aureobasidium pullulans, originally introduced as an inadvertent contaminant in solutions used for evaluating the stability of prostaglandins, proved to lead to the rapid disappearance of the cyclopentenone unit of PGA2 (as monitored by circular dichroic spectroscopy). The cyclopentenone unit is converted, in various metabolites, to a 9-keto, 9 alpha or 9 beta-hydroxy group lacking the ring unsaturation. The major EtoAc-soluble 9-hydroxy metabolite (Compound-I) was shown to be 9 alpha, 15 alpha-dihydroxy-2, 3, 4, 5-tetranor-13-trans-prostenoic acid. Similar tetranor 9-hydroxy metabolites with one additional degree of unsaturation, and with a 9 beta-hydroxy group, also occur but these have not been fully characterized. Only two of the wide range of 9-keto metabolites are fully characterized by mass spectral (MS) data: 9, 15-oxo-2, 3, 4, 5-tetranorprostanoic acid and 9, 15-oxo-2, 3, 4, 5-tetranor-13-trans-prostenoic acid. The water soluble metabolites have not been characterized further. The fully characterized metabolites together with MS data from mixtures of minor metabolites indicate that A. pullulans can perform the following transformation: beta-oxidation, dehydrogenation at C-15, reduction of the enone carbon-carbon double bonds (both delta 10,11 and delta 13,14), reduction of the 9-ketone, and possibly migration of the cyclopentyl double bond (delta 10, 11 leads to delta 11, 12). A. pullulans metabolizes 15-epimeric PGA2 equally readily with the production of similar products. PGA1 affords less 9-keto metabolites with compound I constituting 33% of the product by HPLC analysis. A. pullulans displays some enantioselectivity, PGA2 and 15-epi-PGA2 are each metabolized more rapidly than their enantiomers. Other prostaglandins appear to be less readily metabolized.