CA1186306A - Optically active 16-methoxy-16-methyl-prostaglandin e derivatives and a process for preparing them - Google Patents

Abstract

ABSTRACT
The present invention relates to an optically active 16-methoxy-16-methyl prostagrandin E1 derivative of formula I

I

wherein R stands for a (C1-C4)alkyl group or a non-toxic pharmaceutically acceptable cation having a chirality at C16 corresponding to that at C3 of the stereoisomer of 3-methoxy-3-methyl-2-oxo-heptylphosphonic acid dimethyl ester which has an[.alpha.]? = + 41.2° (C = 1% in CHCl3) and having a chirality at C15 corresponding to that of the more polar stereoisomer of 9-acetoxy -15-hydroxy-16-methoxy-16-methyl-11.alpha.- [(tetrahydro-1H-pyran-2-yl)oxy] prosta-13 (E) -ene-1-oic acid methyl ester which is the second eluted product in a chromatographic separation of stereoisomers on silica gel using sequentially a mixture of petroleum ether/ethyl ether 8:2 (v/v) and petroleum ether/ethyl ether 6:4 (v/v) as the eluent, and to a process for preparing the same.
The compound is useful as gastroprotective agents.

Description

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This is a divisional application of serial number 349,411 filed on April 9, 1980.
The present invention relates to new 16-methoxy-16-methyl prostaglandin El derivatives useful as gastroprotective agents, to a process for preparing ~hem.
The novel compounds which form the first object of the present invention are 16-methoxy-16-methyl prostaglandin El derivatives of the following general forrnula , -CH2 \ 5 \ Cl12 / 3 2 \ 2 / ~ C00R

CH ~ 14 \ 15 / I \ C~2 \ C~l2 OH

wherein R stands for a (Cl 4) alkyl group or a non-toxic pharmaceutically accept-able cation, such as Na~, Kf, the ammonium cation and its org~nic derivativcs.
In the above formula, dot~ed lines are used to indica~e that a particular substituent lies below the plane of the molecule as drawn (~ configuratlon) while a heavy solid line is employed for a substituent which lies above the plane of ~he molecule as drawn (~ configuration).
The prostaglandin-like compounds o the above formula possess two chiral centers on the lower side chain i.e. at C-15 and C-16. Therefore four different isom~rs of formula I may be prepared characterized by the following combinations of configurational assignments at C-15 and C-16(15-R, 16-S), 20(15-S, 16-R), (15-R, 16-R) and (15-S, 16-S).

- 1 - ~ ;.

The compounds of the present invention is one of the above isomers.
~he absolute configuration has not yet been assigned with certainty, the isomer of the invention has a chirality at C-16 corresponding to that at C3 of the stereoisomer of 3-methoxy-3-methyl-2-oxoheptylphosphonic acid dimethyl ester which has an E~]DO ~ ~41.2 (C = 1% in CHC13) and has a chirality at C-15 corresponding to that of the more polar stereoisomer of 9-acetoxy-15-hydroxy-16-methoxy-16-methyl-11~-~(tetrahydro-lH-pyran-2-yl) oxy]pros~a-13(E)-ene-l-oic acid methyl ester which is the second eluted product in a chromatographic separation of stereoisomers on silica gel using sequentially a mixture of petroleum e~her/ethyl ether 8:2(v/v) and petroleum ether/ethyl ether 6:~(v/v) as the eluent.
According ~o the present invention these compounds are prepared by (A) splitting off the protective group at C-15 and, where present, at C-ll of the compounds of the foliowing formula VIII

~ 7 5 3 )~ CH ~ CH2\ / C 2\2 C~2 CH2 CH2 12 CH 16l 18 ~0 OR 13 0~

2 VII:[

where:in R stands for a (Cl-C~) alkyl group or a non-toxic pharmaceut:ically acceptable catlon, R2 stands ~or a hydrogen atom or a protecting group o~ the hydroxy function, and R~ stands ~or a protecting group of the hydroxy function, which have a chirality at C-16 and C-15 as defined above, by mild hydrolysis, or (B) separating the compounds of formula I of the desired chirality from a mixture of these compounds and their optical isomers having the same con-figuration at C-16 and the opposite configuration at C-15.
The compounds which are ~he object of the present invention are endowed with a remarkable anti-secretory activity, particularly when they are administered by the oral route, and show, even at very low oral doses, outstanding cytoprotective effects.
Prostaglandins make up a class of natural substances which are being investigated in dep~h because they possess different pharmacological actions (abortifacient, anti-secretory, hypotensive, bronchodilator) and also because they are involved in mary biological processes (W. Losert et al., Arzneim. Forsch. Drug Res., 25, No. 2, page 135, 1975). Therefore vast literature exists in this field and there are also a number of patents and patent applications claiming classes of "synthetic" prostaglandins which differ from the naturally occurring ones in the structure of the cyclopentane ring and/or one or both of the side chains (see for instance United Kingdom patents 1.409.841, 1.506.816, and 1.345.934, Belgian patent 827.529 and Unite~ States patent 4.029.698).

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The-same prostaglandins of formula I above which are the object of the present invention, even though complete-ly novel ~ , are encompassed by the general formulae of Belgian Patent 837.865 and of U.K. Patent 1.495.152.
5 However, in the above Belgian Patent, 16-methyl-16-metho-xy-prostaglandin Elderivatives having a saturated bond at C-5 are never described, while in U.K. Patent 1.495.152 only two 16,16-dimethyl prostaglandins E are prepared wherein, moreover, the methylene group at position 17 is 10 replaced with an oxygen atom which! in its turn, bears a propyl or pentyl group, and a cis-5-double bond is always present.
~ The compounds o the present inventlon are prepared accor-'il; ding to known methods which are commonly employed in this t 15 field and are exhaustivel~ described in Belgian Patent 837.865. The starting compound is a cyclopentane aldehyde ~i of the formula ')! , OR

~ ~ CH / \ CH / ~

OR" ~HO II
wherein R is as described above and Rl and R2, each inde-pendently, represent hydrogen or a protecting group of the hydroxy function. Preferably for the scope. of the present invention, Rl represents a (C2 ~)aliphatic acyl and R2 is hydrogen or a tetr~hydro-lH-pyran-2-yl radical.
The starting cyclopentane aldehyde may be prepared through methods described in the literature (see for instance Bel-gian Patents 807.161 and 837.855).

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-. ~5-The process for prepari.ng the prostaglandin-like compounds of the present invention comprises as the first step the ,- condensation between the aldehyde of formula II above and : a phosphonate reagent of the following g~neral formula R O O OCH3 ::
\ P-cH2-co-f-cH2-cH2 CH2 3 lo wherein R3 is an alkyl group of from 1 to 4 carbon atoms, to yield an intermediate compound of the formula .. . \ / H2 \ / 2 \ / CooR

2 ~ / C ~ ~ C}12 \ / CH3 wherein R is as defined above, Rl and R~, each independently are hydrogen or protecting groups of the hydroxy Eunction~
and preferab].y Rl is a ~C2 ~)allphatic acyl ancl ~2 is hydro-gen or the tetrahydro lH-pyran-2-yl radlcal.
The condensation is carried out substantially under the same conditions which are described in the chemical litera-ture concerning synthesis of prostaglandins from cyclopenta-ne aldehyde precursors and phos~horous reagents. In practi-ce it Ls carried out in the presence o~ an inert organic solvent such as for instance tetrahydrofuran, dimethoxy-ethane, benzene, dioxane, and the like, at a temperature comprised between O and 80 C.

;3~6 For carrying out the condensation, the phosphonate reaction partner has to be transformed into the corresponding anion and to this purpose about one equimolecular propor~ion ~calculated on the phosp}lonate of for-mula III) of an alkali metal hydride is employed. The phospho~e offormula III possesses a chiral center ~which is indicated in the above formula III
by an asterisk) at C3 and has an [~]D20 value of +41.2 (C = 1% in C~IC13) when R3 is methyl. The condensation between ~he aldehyde of formula II and an optically active form of the phosphonate III yields a compound of formula IV
having a given configuration at C-16 (R or S).
The second step of the reaction pathway comprises reduction of the 15-keto compound to the corresponding 15-hydroxy derivati~e ~y means of the commonly employed reducing agents, namely mixed metal hydrides, such as sodium borohydride, ~inc borohydride, diphenyl tinhydride or lithium trialkyl borohydrides.
Considering that the reduction of the oxo group at C-15 causes the introduction of a further chiral center, a mixture of two products of formula V is obtained having the same configuration at C-16 (R or S) and opposite configuration (R and S) at C-15.

O,, Rl CH2 C~l2 ~ CH / R

~ ~ CH \ IOCH3 / CH2 / Cl13 OR2 ~CII C~l-C-C~l2 CH2 3~6 .

The mixture of the two isomers of formula V thus obtained may be employed as such in the subsequent reaction steps or it may be spearated into the two single isomers which will then undergo, separately, the same reactions Thus, in the formercase a mix~ure of end products o~ formula I
is obtained from which the desired isomer may be separated while in the latter the use of a single isomer with given absolute configurations at C-15 and C-16, leads to only one of the possible isomers of formula I.
It has been noted that when the reduction is carried out on a compound of formula IV wherein R2 is hydrogen, the two C-15 isomers are obtained in quite different amounts.
Sometimes one of the C-15 isomers, and generally the most polar one, is obtained in very little amounts. Surprisingly it has been found that if the reduction is carried out on a compound of formula IV wherein R2 is a protecting group of the hydroxy function and, preferably, the tetrahydro-lH-pyran-2-yl radical, the two isomers are obtained in approximately the same ratio. In this case, the subsequent separation according to known methods such as, for instance, silica gel column chromatography or preparative thin layer chromatography using silica gel plates, ylelds the two single products of formula V wherein the hydroxy group at position ll is prot~cted, pre~erably, as tetrahydro-1ll-pyran-2-yl ether. I-~owever, :it is possib]e, before submitting the mixture to chromatographic separation, to restore the free hydroxy function at position 11.
According to a preferred embodiment oE this invention, a molar proportion of the compound of formula IV wherein Rl represents a protecting group of the hydroxy function and, preferably, a (C2 ~) aliphatic acyl, and R2 is hydrogen, is reacted with about 2-3 equimolecular proportions of 2,3-dihydropyran in the presence of an anhydrous inert organic solvent, such as benzene, and of a catalytical amount of p-toluenesulfonic acid. The ~6~0~

reac~ion runs at room temperature and takes from abou-t 5 to about 20 minutes.
A compound of formula IV is thus obtained wherein R is as defined above, Rl is a protecting group of the hydroxy function and preferably a (C2 4) aliphatic acyl group and R2 is the tetrahydro-1ll-pyran-2-yl radical.
The subsequent reduction of the oxo group at C-].5 gives the corresponding compound of formula V (as a mixture of the two possible isomers at C-15) wherein R2 is the tetrahydro-lH-pyran-2-yl radical. IE desired, the mixture thus obtained may be separated into the single C-15 isomer according to the procedures described above. The mixture, or the single isomers, are then further processed to yield the end produc~s of formula I. The reaction steps which, starting from a compound of formula V, lead to a final compound I, comprise protecting the hydroxy groups at position 15 and, when R2 is hydrogen , at position 11 of the compounds of the above formula V by reaction with an appropriate protecting agent, preferably 3,4-dihydro-2H-pyran, hydrolys-ing the obtained 11, 15-protected compound of formula V in mild conditions, for instance with sodium or potassiwn carbonate if Rl is a ~C2 4) aliphatic acyl group, to restore the free hydroxy group at position 9, then oxidizing said hydroxy group to oxo by common oxidation procedures (Eor instance w:ith the Collins reagent i.e. the complex pyridine/chrom:ium oxide) ancl ei.nally removing the protecting groups at posit:ion 15 and, where presen~,at position 11.
When the protecting groups of the hydroxy functions at positions 11 and 15 are tetrahydro-lH-pyran-2-yl radicals, their removal is preferably carried out through acid hydrolysis with a mixture acetic acid: water:
tetrahydrofuran = lg:ll:3 (v/v/v) at a temperature of 40-45C, If these reactions are carried out on a mixture of compounds of formula V having the same absolute configuration at C-16 and opposite config-urations at C-lS, a mixture of two compouncds of formula I, isomers at C-15, ~163~$i is obtained. This mixture may be separated in~o the single isomers by means of the usual chromatographic tech1liques illustrated above.
The starting phosphorous reagents of formula III are prepared by condensing a methylphosphonic acid lower alkyl es~er of formula Vl (R30) \ o P-CH VI
(R30) wherein R3 represents a (Cl 4) alkyl group, with an ~-methyl-~-methoxy-hexanoic acid lower alkyl ester (or the corresponding acyl chloride) of formula VII
fC~13 xoc f -CH2-CH2-CH2-CH VII

wherein X may represent -OR3 or -Cl.
Thi.s procedure involves first transforming the methyl phosphonate of formula VI into the corresponding anion by the addition of butyl lithium at -78C in tetrahydrofuran and then contacting it wlth the compound o~
forn1ula VII for about l hour st:ill at the same -tempera-ture. When an optically active form o~ the phosphonate of formula III ls desired, the racemate o~ the ~-methyl ~-methoxy-hexanoic acid is first resolved into the two antipodes by co-nventional procedures7 such as using an optically active base like ephedrlne, atropine or amphetamine, to form the corresponding salts which are separated by fractional crystallization.

9_ ~6361 6 .. --1 o--The separated antipodes are then transformed into the corresponding opticaily active esters or acid chlorides , of formula VII which in their turn are condensed with the meth~lphosphonate of formula VI.
; ~ The compounds of the present invention are potent inhibi-tors o~ the gastric secretion also when they are admini-stered to laboratory animals by the oral route. The extent of this biological activity could not be foreseen at all - by a person skilled in this field, considering that the lO corresponding compounds with a 5-double bond, which are 7 described in Belgian Patent 837.865, when administered per_os are by far less active. In other words it has been ~7! . found that a slight modification in the structure of these ~, compounds has generated remarkable and favorable changes in the biological activity. The gastric antisecretory pro--perties of these compounds when administered per os were evaluated on the basis o~ their e~fectiveness in inhibiting the hyperacidity induced by histamine in dogs. Histamine which is a potent stimulator of the acidic gastric secre tion (see Bertaccini et al., Fur. Journ. Pharmacol., ~, 360, 1974), was administered lntraven~usly b~ con*inuou~
infusion during the experlments.
A group o~ ~ive mongrel dogs was used in the experiment.
The dogs were surgically operated in the stomachs according to the methocl described by Bertaccini et al. (see above) : in order to provide each animal with an innervated main stomach or gastric fistllla (G.F.) and a denervated sto-mach or Haidenhain pouche (H.P.). The main stomach and the Haidenhain pouche were each equipped with a cannula in or-der to allow the qastric juices to drain to the exterior by gravity.

:L:l86~ 6 `\l Said gastric juices were then separately titrated with :d' O.lN NaOH by means of an automatic titrator (Radiometer, ' Copenhagen). The five dogs, after a period of four to five weeks o~ recovery, were first treated with the se-gretagogue alone (doses of histamine were increased pro-gressively every 30 minutes from a minimum of 40 to a maximum of 320 ~g/kg/h) in order to stimulate the acidic ' gastric secretion both in the G.F. and in the H.P.
Every thirty minutes the acidic output both from the G.F.
and the H.P. was collected and titrated as described above.
The values thus obtained (each on~ as a mean of ~ive dogs) were considered as the "control values" o~ "controls".
To evaluate the gastric antisecretory oral activity of the compounds of the invention, they were administered by gavage into the main stomach at a given dosage, expressed as ,ug/kg/h,dissolved in 1 ml of physiological solution, 60 minutes before the secretagogue. The agent stimulating the gastric hypersecretion was administered intravenously by continuous infusion at the dosages indicated be~ore, and every thirty minutes the gastric juia~s ~xom both the !~ G.F. and the H.P. were collected and titrated. So doing it was possible to establish by simple calculations the per-centage inhibition of acidic gastric secretion at given dosages o~ hystamine and active compound.
The results obtained with the compound of example 1 having a/20 = -447, show that very low dosages, from 25 to 100 ~ug/kg/h inhibit the gastric hyperacidity induced by doses varying from 40 to 160 ~g/kg/h of histamine, of from about 95 to about 35~ (calculated versus the controls) in the G.~. and from about 60 to about 30% in the H.P.

;3 ~, In the same experimental conditions, the correspondlng compounds bearing a C-5 double bond are almost inactive.
Another experiment carried ou- in the same conditions as above but administering histamine at a fixed dose of 160 ~g/kg/h instead of at increasina doses, showed that the compound of example 1 with ~/D = ~447 blocks the gastric acid secretion or about 60% while the correspon-ding compounds with a double bond at C-5 are inactive.
The compounds of the present invention are potent inhi-bitors of the gastric hypersecretion also when administeredintravenously. This has been confirmed by evaluating the e~fects, on the gastric hypersecretion induced by hista-mine, produced by single intravenous administrations to anesthetized rats of the compounds of the present inven-: 15 tion. The experiments were carried out essentially according to the methodology described by Ghosh and Schild in Brit. J. Pharm. (1958), 13, 54-61. According to this technique the rat is anesthetized with urethane and its stomach is perfused with a dilute sodium hydroxide solution (N/4000 NaOH at a rate of ~houk 1 ml/min) by way of the oesophagus ancl the pH o~ the ~luid emerging fro~ a cannula in the pylorus is continuously recorded by means of a glass-electrode connected to a direct reading pH-meter and thence ko a recorder. In passing through the stomach, the perfusate collects sufficienk buf~er to act as an approximately linear buffer system over the relevant range, so that the change in p~1 becomes a measure of acid secretion.

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When the N/4000 NaOH solution is collected after having - passed through the unstimulated stomach it gives an ini-tial value of about 6-6.5 (pH
unstimulated stomach A secretory stimulant, histamine in the present case, is then administered intravenously by continuous infusion at a dose of 1.5 mg/kg/h. After a few minutes the pH begins to fall and after about 10 to 2C minutes the secretory effect reaches its maximum and the pH its lowest value (P histamine stimulated stomach) ' g tinuous infusion o histamine, remains constant. The com-pounds to be tested are then administered intravenously and the pH is continuously recorded. By considering the highest pH value recorded, which evidentiates the maximum antisecretor~ effect reached with the test compound (P~l A E ) and using the following equation PHU S PHH. S . S .
the per cent inhibition of gastric acid secxetion may be easily calculated. A 100~ e~ec~ mearls -that the compound tested, at the-dose tested, brought the pH of the hi-stamine stimulated stomach to the lnitial value of the unstimulated stomach, while a 0% effect means that the administration did not af~ect the gastric secretion in-duced by hlstamine.
In representative experirnents with the compound of example 1 with ~ = - 447 we obtained the following results:

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Dose (,ug/kg) ~ Inhi~ition of the acid gastric secretion ,. . .
. 2.5 15 .~ 50 100 " .

Other experiments were carried out which showed that the compounds of the present invention also possess a remarkable cytoprotective activlty which appears at very low oral doses. More particularly, the cytoprotective activity of the compounds of the present invention was evaluated on the basis of their effectiveness in inhibiting the forma-tion and reducing the severity of gastric ulcers in rats caused by the administxation of a high dose of l-(p-chlo-robenzoyl)-S-rnethoxy-2-methyl-3-indolyl- acetic acid (In-domethacin) at: oral doses which are by ~ar lower -t~an those which cause inhibition of th~ gas~rlc aci~ ~cre-tion. In these experiments the rats were deprived ot food the day before the beginning of the experiments but water was given ad libitum. The compounds to be tested were administered by the oral route as suspension in 0.5~
aqueous methocel solu-tion ~5 animals/dose), while indome-thacin was adminis~.ered in-traperitoneall~ at 10.000 mg~kg in the same vehicle. Another group of rats (controls) was administered only with the ulcerogenic agent. The cytopro-tective properties of the compounds of this invention were then e~pressed as "per cent inhibition of the ulcers with .:i ~ Ei3~

respect to controls", which can be easily calculated by - the following ratio:

A U D~controls) ~ A-U-D ~t ~ ~ 100 A U D

(controls) is the average ulceration degree -of the stomachs of the controls and A.U.D.
(treated animals) is the average ulceration degree of the stomachs of the animals which received both indomethacin and the compound s to be tested.
The A.U.D.s are calculated according to the method de-scribed by Thuillier et al. Chim. Ther., 3, 53, 1968, by ~; examining the stomachs of the laboratory animals for pos-sible ulcerations and assigning a score from 0 to 4 which depends on the number and the severity o -the observed ulcerations: 0 means absence o any ulceration on the ga~
stric wall, 4 means perforating ulcerations. A single ul-ceration degree (S.U.D.) is then calculated for each single stomach. The sum of the S.U.D.s dlvlded by -the number o e the animals affords the ~.U.D. of the stomachs o each group o animals.
In these experiments the compound of example 1 having / ~ ~ = - 44.7 proved to be very active even at very low doses. In fact at a ~ose of 3 ~g/kg (the lowest dose te-~ted in these experiments) the per cent inhibition of the ulcers with respect to controls was of about 43, while the ED50 (i.e. the dose which provokes an inhibition of the ulcers with respect to controls of 50~), as calcula-ted by extrapolation, was 6 ~g/kg.

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- It is therefore clear that the major protecti~e action on the gastric mucosa exerted by these compounds is by one or more mechanisms that are independent of inhibition -' OL acid secretion since, as it is apparent from the re-sults of this last experiment, this compound protects in doses which are far too small to inhibit acid secre-;~ tion considerably.
From the above results it derives that the prostaglandin derivatives o~ the present invention are useful in mam-mals to reduce and control excessive gastric acid secre-tion and also to exert, even at very low doses, a protecti-ve action on the gastric mucosa thereby reducing and avoi-ding gastroin~estinal ulcer formation. Thus, according to a further feature of the present invention, there are pro-vided pharmaceutical or veterinary compositions comprising ~' a prostaglandin-like derivative of formula I as the active ingredient.
In the exploitation of the invention the preferred admini-stration rout:e of the new compounds is ~ in the ~orm of capsules, coated tablets or syxups. I~ desired, pax~n-terally adminlstrable dosage ~orms can also be~ prepared as injectable ampoules~ These pharmaceutical dosages are ~ormulated as known in the art (see for instance Rernington's Pharmaceutical Sciences, 13 Ed., Mack Publishing Co., Easton, Pennsylvania) and are prepaxed by common procedu-res. They may contain from about 5 to about 100 ,ug and pre-ferahly from about 10 to abou-t 60 ~g o~ active ingredient.
In addition to the therapeutic principle capsules and coated tablets may contain the usual pharmaceutically acceptable excipients such as inert diluents, luhricating and disinte-~863~@~
~17-.
grating agents. Syrups may contain conventional suspending, weiting, buffering, flavoring agents and preservatives.
The dosage regimen for the prostaglandin-like compounds of the presentinvention in accord with a gastroprotective treatment will depend upon a variety of factors including the type, age, and weight of the mammal. Good results can j be obtained however by administering the prostaglandin compounds of the present invention, at a daily dosage range comprised between about 10 and about 300Jug, preferably in divided doses. It is however clear that a daily dosage beyond the above indicated range may also b~e employed de-pending on the individual conditions of the subject to be treated.
This invention is further illustrated by the following .-~ -15 examples:
! Exam~le 1: lla,l5-dihydroxy-16-methoxy-16-methyl-9-oxo--prosta-13(E)-ene-l-oic acid methyl ester.
(15-R, 16-R) and (15-S, 16-R) or (lS-R, 16-S) and (15-S, 16-S) A) To a mixture of 770 mg of a 81.8~ suspension o~ ~odium hydride in mineral oil (0.026 mol~) and 30 ml o anhydrous ~ dimethoxyethane, a solution of 8 g (0,030 mole) o~ optical-i ly active 3-methoxy-3-methyl-2-oxo-heptylphosphonic acid dimethyl ester having / ~ - +41.2 (C=1% in CHC13) in 40 ml of dimethoxyethane is added dropwise. The resulting mixtu-re is allowed to stand at room temperature for 15 minutes, then a solution of 4.08 g (0.013 mole) of 7-(5~-acetoxy--2~-~ormyl-3a-hydroxy-cyclopent-la-yl)heptanoic acid me-thyl ester in 50 ml of anhydrous dimethoxyethane is ~ra-dually added thereto i3~
~ -18-.
After standing at room temperature for 6 hours, the reaction mixture is then poured into an aqueous sol~tion saturated with NaH2P04 which is subsequently extracted with ethyl ether. The organic extract is dried over MgS04 and concentrated to dryness. The obtained residue is purified ~y silica gel column chromatography by elutin~
with ethyl ether:petroleum ether 1:1 (v/v). Yield :3.9 g Of 9a -acetoxy-llu ~hydroxy-16-methoxy-16-methyl-15-oxo -prosta-13(~)-ene-1-oic acid methyl ester wherein C-16 has absolute coniiguration R or S. /~/D = + 53.8 (C=0.81 in CHC13). NMR absoption peaks in CDC13 (~): 0.89;
1.1-2.1; 1.29; 2.08; 2.30; ~.4-2.6; 3.21, 3.68; ~.11;
5.23; 6.87 ~' ' .
B) A mixture consisting of S g (0.0113 mole) of 9a -ace-toxy~ -hydroxy-16-methoxy-16-methyl-15-oxo-prosta--13(~)-ene-1-oic acid methyl ester (prepared as described under paragraph A)) dissolved in 150 ml of anhydrous ben-zene, 2.6 ml (0.0285 mole) of 2,3-dihydropyran and 70 mg of p-toluenesulfonic acid in 50 ml of anhydrous ben~ene is prepared at a temperatureco~prised be-~ween 5 and 10 C
and is then maintained at room ~emperature ~or about 10 minutes. The reaction mixture is washed ~irst with an aqueous solution saturated with sodium ~icarbona-te and then with water. Evaporation of the solvent affords a re-sidue which is puri~ied by means of silica gel column chromatography, by eluting with petroleum ether: ethyl ether 7:3 (v/v). Yield S g oE 9a-acetoxy-16-methoxy-16--methyl-15-oxo-lla-/(tetrahydro-lH-pyran-2yl)o~y/-prosta--13(E) ene-l-oic acid methyl es-ter wherein the carbon atom at position 16 has absolute con~iguration R or S

NMR absorption peaks in CDC13 (~): 0.88i 1.1-3.0;
1.25; 2.06; 3.23; 3.3-4.~; 3.72; 5.20; 6.8-7.1.

C) To a solution of 1305 g of sodium borohydride in 400 ml of methanol, cooled to -20 C, a solution of S g (0.0093 mole) of 9a-acetoxy-l6-methoxy-l6-methyl-l5-oxo-~-/(tetrahydro-lH-pyran-2-~l)oxy,'-prosta-13(E)-ene-l-oic acid methyl ester (prepared as illustrated under paragraph B)) is added dropwise.
' 10 The resulting solution is kept at -20 C for about 2 hours, ' then it is poured into an aqueous solution saturated with ~' Na~2P0~ which is subsequentl~ extracted with ethyl ether.
The organic extract is dried over Na2504 and evaporated to dryness yielding a residue which is a mixture of the -- 15 two possible isomers at ~-15 of the 9~-acetoxy-15-hydro-xy~l6-metho~y-16-methyl-11~-/(tetrahydro-lH pyran-2-yl)--oxy/-prosta-13(E)-ene-l-oic acid,methyl ester. Said iso-, mers are separated by eluting first with petroleum ether:
ethyl ether 8:2 (v/v) to purify the crude mixture, and then with pet:roleum ether: ethyl e-ther 6:4 (v/v)~
The ~irst eluted procluct (2.05 g of pure produc-t, less po-lar isomer) has the following N~IR spectrunl: main absorption peaks in CDC13 occurring at the following frequencies expressed i.n ~units: 0.90; 1.13; 1.1-2.9; 2.07; 3.28;
:2.5-4.3; 3.73; ~.67; 5.20; 5.7-5.9.
The second eluted product (2.1 g of pure product, more po-lar isomer) has the following N~R spectrum in CDC13 (~
units): 0.93; 1.07; 1.1-2.9; 2.0'7; 3.28; 3.2-4.2; 3.72;
, 4.67; 5.20; 5.7-5.9.

l .

Al ~L8~; 3 ;.
The two products thus obtained have the same absolute configuration at C-16 (R or S) and opposite configurations at C-15. They therefore represent the couple o~ isomers having the following absolute configurations at C-15 and C-16:(15-R, 16-P~) and (15-S, 16-R) or (15-R, 16-S) and ' (15-S, 16-S).
The subsequent chemical modifications which lead to the end products of formula I do not alter the stereochemistry at C-15 and C-16.

D) 2.1 g of the more polar isomer of 9~-acetoxy-15-hydro-xy-16-methoxy-16-methyl-lla-/(tetrahydro-lH-pyran-2-yl)-,..................................................... .
-oxy/-prosta-13(E)-ene-l-oic acid methyl ester, prepared as described under paragraph C), are dissolved in 150 ml of anhydrous benzene. After cooling, 1.3 ml (0.0142 mole) of 2,3-dihydropyran and a solution of 75 mg of p-toluen-sulfonic acid in 40 ml of anhydrous benzene are added and after 15 minutes the solution is poured into a~ueous so-~ dium bicarbonate. The organic layer is separated, dried ,~ 20 over Na2S0~, and taken to dryness. The residuq i5 pu.rified by silica gel column chromatography by eluting with pe troleum ether:ethyl ether - 7.3 (v/v) to yield 2,1 g of 9~ -acetoxy-16-metho~y-16-methyl-11 ~,15-bis-/(tetrahy-dro-lH-pyran-2-yl)ox~ prosta-13(E)-ene-1-oic acid methyl ester wherein the co~bination of the absolute configurations at C-15 and C-16 is one of the four possible ones.
NMR spectrum (CDC13): 0.92; 1.12; 1.15; 1.1-3.0; 2.07;
3.25; 3.32; 3.2-4.2; 3.70; 4.5-5.2; 5.3-5.6 (~ units).
The compound thus obtained is dissolved in 200 ml of anhy-drous methanol and 2.1 g of anhydrous potassium carbonateare added -to the resulting solution.
,:~

63~i ~ -21-.a . ~, After stirring at room tempera-ture for 24 hours the mixtu-re is poured into an aqueous solution saturated with NaH2P04 which is subsequently extracted with ethyl ether.
The organic phase is separated, dried over Na2S04 and 5 concentrated to dryness 1.95 g of 9~-hydroxy-16-metho~y--16-methyl-11~-15-bis-/ (tetrahydro-lH-pyran-2-yl~oxy/-i - -prosta-13(E)-ene-l-oic acid methyl ester (pure product~
A¦
are obtained.
NMR spectrum in CDC13 (~): 0.92; 1.0-2.9; 1.10; 1.13;
10 3.25; 3.32; 3.2-4.4; 4.6-5.0; 5.4-5.9.
7 ~ C ~ ~ i t ~
; ~ E) A mixture consisting of 6.38 g of~ litet 7.25 g of s Collins reagent (Py2.CrO3), and 1.95 y of 9~-hydroxy-' -16-methoxy-16-meth~l-11~-,15-bis-/(tetrahydro-lH-pyran--2 yl)oxy/-prosta-l3(E)-ene-l-oic acid methyl ester in 180 ml of methylene chloride is stirred at room tempera-ture for about 1 hour. The reaction rnixture is poured into 1200 ml of ethyl ether and filtered through celite. The filtrate is decolorized with activated charcoal and con-20 centrated to dr~yness. Yield: 1.77 g of 1~-met~oxy-16-me thyl-9-oxo-llu,15-bis-/(tetrah~dro-lEI pyran-2~yl)ox~
-prosta-13(E)-en~ oic acid me-thyl ~ster.
NMR spectrum in CDC13 (~ units): 0.93; 1.0-2.9; 1.12;
1.15; 3.1-4.g; 3.25; 3.32; 3.70; 5.~-5.8.
F) The `product thus obtained (1.77 g) is suspended into 100 ml of a solution of acetic ~cid:water:tetrahydrofuran 19:11:3(v/v/v). After stirring at 45 C for two hours, the mi:~ture is diluted with water and the pH is brouyht to 7.2 by the addition of sodium bicarbonate.
~ r~ C~

~t363~

The mixture is extracted with ethyl ether which is then boiled off yielding 1.19 g of a raw produc~ which is purified by means of silica gel column chromatography by eluting with ethyl ether.
780 mg of pure 11~, 15-dihydroxy-16-methoxy-16-methyl-9-oxo-prosta-13(E)-ene-l-oic acid methyl ester are obtained wherein the carbon atoms at positions 15 and 16 possess one of the four possible combinations of absolute configurations i.e. (15-R, 16-R) or (15-R, 16-S) or (15-S, 16-R) or (15-S, 16-S). The compound has the following characteristics:
/~20= 44 7o (C=0.98% in CHC13) -~MR in CDC13 (~): 0.92; 1.1-1.7; 1.11; 1.9-2.8; 2.29; 3.1-3.3;
3.24; 3.63; 4.07; 4.12; 5.69.
The compound is a unitary product, as evidentiated by differential scanning calorimetry, which melts at 40-48C.
Example 2 A capsule is prepared containing the followi.ng ingredients:
lla, 15-dihydroxy-16-methoxy-16-methyl-9-oxo-prosta-13(E)-ene-l-oic acid methyl ester prepared by Example 1 40 ~lg Talc 3 mg Lactose 3 mg Sodium-carboxymethylcellulose 3 mg Starch q.s. to 90 mg Exampl _ A coated tablet is prepare~ with:
lla, 15-dihydroxy-16-methoxy-16-methyl-9-oxo-prosta-13(E)-ene-l-oic acid methyl ester prepared by Example 1 60 ~g Sodium-carboxymethylcellulose 4 mg 63~

Magnesium stearate 4 mg Gelatin 7 mg Starch 7 mg Saccarose 20 mg arabic gum, lactose, titanium dioxide, aluminum lac according ~o conventional methods.

Claims (10)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for preparing an optically active 16-methoxy -16-methyl prostaglandin E1 derivative of formula I

I

wherein R stands for a (C1-C4)alkyl group or a non-toxic pharma-ceutically acceptable cation having a chirality at C16 correspond-ing to that at C3 of the stereoisomer of 3-methoxy-3-methyl-2-oxo-heptylphosphonic acid dimethyl ester which has an[.alpha.]? = + 41.2°
(C = 1% in CHCl3) and having a chirality at C15 corresponding to that of the more polar stereoisomer of 9-acetoxy-15-hydroxy-16-methoxy-16-methyl-11.alpha. - [(tetrahydro-1H-pyran-2-yl) oxy] prosta-13 (E)-ene-1-oic acid methyl ester which is the second eluted product in a chromatogrphic separation of stereoisomers on silica gel using sequentially a mixture of petroleum ether/ethyl ether 8:2 (v/v) and petroleum ether/ethyl ether 6:4 (v/v) as the eluent, which process comprises:
(A) splitting off the protective group R4 and, where present, R2 in a compound of the formula VIII

VIII

wherein R stands for a (C1 - C4) alkyl group or a non-toxic pharmaceutically acceptable cation, R2 stands for a hydrogen atom or a protecting group of the hydroxy function, and R4 stands for a protecting group of the hydroxy function, which have a chirality at C-16 and C-15 as defined above, by mild hydrolysis, or (B) separating the compound of formula I from a mixture of the compound and its antipode having the same configuration at C-16 and the opposite configuration at C-15.

2. A process according to claim 1, wherein process (A) is carried out using a compound of formula VIII in which R stands for a methyl group and R2 and R4 stand for a tetrahydro-1 H-pyran-2-yl group, whereby 11 .alpha., 15-dihydroxy-16-methoxy-16-methyl-9-oxo-prosta-13(E)-ene-1-oic acid methyl ester which has the chirality as defined above and melts at about 40 - 48°C. is obtained.

3. An optically active 16-methoxy-16-methyl prostagrandin E1 derivative as defined in claim 1, whenever prepared or produced by the process of claim 1 or an obvious chemical equivalent thereof.

4. A process for preparing an optically active 16-methoxy-16-methyl prostaglandin E1 derivative of formula I

I

wherein R stands for a (C1-C4)alkyl group or a non-toxic pharmaceutically acceptable cation having a chirality at C16 corresponding to that at C3 of the stereoisomer of 3-methoxy-3-methyl-2-oxo-heptylphosponic acid dimethyl ester which has an[.alpha.]? = + 41.2° (c = 1% in CHCl3) and having a chirality at C15 corresponding to that of the more polar stereoisomer of 9-acetoxy-15-hydroxy-16-methoxy-16-methyl-11.alpha.- [(tetrahydro-1H-pyran-2-yl) oxy] prosta-13(E)-ene-1-oic acid methyl ester which is the second eluted product in a chromatographic separation of stereoisomers on silica gel using sequentially a mixture of petroleum ether/ethyl ether 8:2 (v/v) and petroleum ether/ethyl ether 6:4 (v/v) as the eluent, which process comprises:
a) reacting a cyclopentane aldehyde of formula II

II

wherein R is as above and R1 and R2 each independently represent hydrogen or a protecting group of the hydroxy function with an optically active phosphonate of formula III

III

wherein R3 is an alkyl group of 1 to 4 carbon atoms, having [.alpha.] 20 = + 41?2 (c = 1% in CHCl3) when R3 is methyl, whereby a compound of formula IV is obtained IV

wherein R, R1 and R2 are as defined above, b) when R2 is hydrogen and if appropriate, protecting the hydroxy group at C-11 by means of a suitable protecting agent of the hydroxy function, c) reducing the 15-oxo group to 15-hydroxy by treatment of the compound of formula IV with a mixed metal hydride, whereby a compound of formula V is obtained V

wherein R, R1, and R2 are as defined above, d) if appropriate, separately recovering the more polar isomer of the compound V above, e) protecting the hydroxy group at C-15 and, if appropriate when R2 is hydrogen, at C-11, by means of a suitable protecting agent of the hydroxy function, f) when R1 is a protecting group, setting free the hydroxy function at C-9 by removing the protecting group R1, g) oxidizing the hydroxy group at C-9 to oxo, and finally, h) splitting off the protecting groups at C-15 and, where present, at C-11 by mild hydrolysis to restore the hydroxy functions, i) separating the desired optical isomer from a mixture of the isomer and its antipode having the same configuration at C-16 and the opposite configuration at C-15, when the separating step d) is not conducted.

5. A process according to claim 4 for preparing the stereoisomer of the 11.alpha.- 15-dihydroxy-16-methoxy-16-methyl-9-oxo-prosta-13(E)-ene -1-oic acid methyl ester having a chirality at C16 corresponding to that at C3 of the stereoisomer of 3-methoxy-3-methyl-2-oxo-heptyl-phosphonic acid dimethyl ester which has an [.alpha.] 20 of + 41.2° (c = 1%, CHCl3) and having a chirality of C15 corresponding to that of the more polar stereoisomer of 9-acetoxy-15-hydroxy-16-methoxy-16-methyl-11 .alpha.- [(tetrahydro-1H-pyran-2-yl)oxy] prosta-13 (E)-ene-1-oic acid methyl ester, which is the second eluted product a chromatographic separation of stereoisomers on silica gel column using sequentially a mixture petroleum ether/ethyl ether 8:2 (v/v) and then petroleum ether/ethyl ether 6:4 (v/v), wherein in the formula II, R is a methyl group.

6. A process according to claim 4, wherein in the starting materials R1 is an aliphatic acyl group having 2 to 4 carbon atoms and R2 is hydrogen.

7. A process according to claim 6, wherein the step b) is carried out by reacting the compound of the formula IV in which R2 is hydrogen with 2,3-dihydropyran to introduce a tetra-hydro-1H-pyran-2-yl group.

8. A process according to claim 4, 5 or 7, wherein the step e) is carried out by reacting the compound of formula V with 2,3-dihydropyran to introduce a tetrahydro-1H-pyran-2-yl group.

9. A process according to claim 4 or 5, wherein the step d) is carried out using silica gel chromatography or preparative thin layer chromatography on silica gel plates.

10. An optically active 16-methoxy-16-methyl prostagrandin E1 derivative as defined in claim 4, whenever prepared and produced by the process of claim 4 or by an obvious chemical equivalent thereof.