Process for phosphodiesterase IV inhibitors

The invention encompasses a novel process for the formation of enantiomerically enriched mixtures of compounds of Formula I, which are useful precursors in the synthesis of phosphodiestersae IV inhibitors.

BACKGROUND OF THE INVENTION 
This invention relates to a process of making compounds for the treatment 
of diseases by raising the level of cyclic adenosine-3',5'-monophosphate 
(cAMP) through the inhibition of phosphodiesterase IV (PDE IV). 
Many hormones and neurotransmitters modulate tissue function by elevating 
intra-cellular levels of adenosine 3', 5'-cyclic monophosphate (cAMP). The 
cellular levels of cAMP are regulated by mechanisms which control 
synthesis and breakdown. The synthesis of cAMP is controlled by adenyl 
cyclase which may be directly activated by agents such as forskolin or 
indirectly activated by the binding of specific agonists to cell surface 
receptors which are coupled to adenyl cyclase. The breakdown of cAMP is 
controlled by a family of phosphodiesterase (PDE) isoenzymes, which also 
control the breakdown of guanosine 3',5'-cyclic monophosphate (cGMP). To 
date, seven members of the family have been described (PDE I-VII) the 
distribution of which varies from tissue to tissue. This suggests that 
specific inhibitors of PDE isoenzymes could achieve differential elevation 
of cAMP in different tissues, for reviews of PDE distribution, structure, 
function and regulation, see Beavo & Reifsnyder (1990) TIPS, 11: 150-155 
and Nicholson et al (1991) TIPS, 12: 19-27!. 
The availability of PDE isotype selective inhibitors has enabled the role 
of PDEs in a variety of cell types to be investigated. In particular it 
has been established that PDE IV controls the breakdown of cAMP in many 
inflammatory cells, for example, basophils (Peachell P.T. et al., (1992) 
J. Immunol. 148 2503-2510) and eosinophils (Dent G. et al., (1991) Br. J. 
Pharmacol. 103 1339-1346) and that inhibition of this isotype is 
associated with the inhibition of cell activation. Furthermore, elevation 
of cAMP in airway smooth muscle has a spasmolytic effect. Consequently PDE 
IV inhibitors are currently being developed as potential anti-inflammatory 
drugs particularly for the prophylaxis and treatment of asthma, by 
achieving both anti-inflammatory and bronchodilator effects. 
The application of molecular cloning to the study of PDEs has revealed that 
for each isotype there may be one or more isoforms. For PDE IV, it is has 
been shown that there are four isoforms (A, B, C and D) each coded for by 
a separate gene in both rodents (Swinnen J. V. et al., (1989) Proc. Natl. 
Acad. Sci. USA 86 5325-5329) and man (Bolger G. et al., (1993) Mol. Cell 
Biol. 13 6558-6571). 
The existence of multiple PDE IVs raises the prospect of obtaining 
inhibitors that are selective for individual isoforms, thus increasing the 
specificity of action of such inhibitors. This assumes that the different 
PDE IV isoforms are functionally distinct. Indirect evidence in support of 
this comes from the selective distribution of these isoforms in different 
tissues (Swinnen et al., 1989; Bolger et al., 1993; Obernolte R. et al., 
(1993) Gene 129 239-247, ibid) and the high degree of sequence 
conservation amongst isoforms of different species. 
To date full length cDNAs for human PDE IVA, B and D (Bolger et al., 1993 
ibid; Obernolte et al., 1993 ibid; Mclaughlin M. et al., (1993) J. Biol. 
Chem. 268 6470-6476) and rat PDE IVA, B and D (Davis R. et al., (1989) 
Proc. Natl. Acad. Sci. USA 86 3604-3608; Swinnen J. V. et al., (1991) J. 
Biol. Chem. 266 18370-18377), have been reported, enabling functional 
recombinant enzymes to be produced by expression of the cDNAs in an 
appropriate host cell. These cDNAs have been isolated by conventional 
hybridisation methods. However using this approach, only partial cDNAs for 
both human and rat PDE IVC have been obtained. (Bolger et al., ibid. 1993 
and Swinnen et al., ibid. 1989 and International Patent Specification No. 
WO 91/16457.) 
The design of PDE IV inhibitors for the treatment of inflammatory diseases 
such as asthma, has met with limited success to date. Many of the PDE IV 
inhibitors which have been synthesised have lacked potency and/or inhibit 
more than one type of PDE isoenzyme in a non-selective manner. PDE IV 
inhibitors that are relatively potent and selective for PDE IV, are 
reported to be emetic as well. Indeed this side effect has been so 
universal that experts have expressed their belief that the emesis 
experienced upon administration of a PDE IV inhibitor, may be mechanism 
based. 
We had previously reported a novel series of tri-substituted phenyl 
derivatives (Formula III), members of which, when compared to known 
structurally similar compounds, are potent inhibitors of PDE IV and have 
little or no inhibitory action on other PDE isoenzymes. 
##STR1## 
The preparation of these compounds is known in the literature. (See, e.g., 
WO 94/14742, published on Jul. 7, 1994; WO 95/17386, published on Jun. 29, 
1995). However, the known syntheses of the enatiopure compounds involved 
the use of covalently bonded chiral auxilliaries, which lengthened the 
synthetic protocols and incresed the price and difficulty of preparation. 
We have discovered a short and efficient process to the enantiopure 
compounds using a resolution via crystallization. 
SUMMARY OF THE INVENTION 
The invention encompasses a process for the formation of enantiomerically 
enriched mixtures of compounds of Formula I, which are useful in the 
synthesis of phosphodiesterase IV inhibitor compounds, such as that 
represented by Formula III. 
##STR2## 
Also within the scope of the invention are salts of the compound of Formula 
I and (1S)-(+)-3-bromocamphor-10-sulfonic acid or 
(1)-(-)-3-bromocamphor-10-sulfonic acid. DEFINITIONS: 
______________________________________ 
(Br--CSA) = (1S)-(+)-3-bromocamphor-10-sulfonic acid or 
(1R)-(-)-3-bromocamphor-10-sulfonic acid 
cAMP = cyclic adenosine-3',5'-monophosphate 
Cp = cyclopentyl 
DBU = 1,8-diazabicyclo5.4.0!undec-7-ene 
DMF = N,N-dimethylformamide 
ee = enantiomeric excess 
LC = liquid chromatography 
LDA = lithium diisopropylamide 
NSAID = non-steroidal anti-inflammatory drug 
PDE = phosphodiesterase 
Ph = phenyl 
PY = pyridyl 
r.t. = room temperature 
rac. = racemic 
THF = tetrahydrofuran 
______________________________________ 
DETAILED DESCRIPTION OF THE INVENTION 
An embodiment of the present invention is a process for the production of 
an enantiomerically enriched mixture of a compound of Formula I 
##STR3## 
comprising, forming a salt of the compound of Formula I with 
(1S)-(+)-3-bromocamphor-10-sulfonic acid or 
(1R)-(-)-3-bromocamphor-10-sulfonic acid. 
Another embodiment of the present invention is a process for the production 
of an enantiomerically enriched mixture of a compound of Formula I 
##STR4## 
comprising, a) mixing the compound of Formula I with 
(1S)-(+)-3-bromocamphor-10-sulfonic acid or 
(1R)-(-)-3-bromocamphor-10-sulfonic acid in an alcohol solvent system to 
form a mixture; 
b) heating the mixture between about 60.degree. C. and the reflux 
temperature of the alcohol solvent system; 
c) allowing the mixture to cool so that a crystalline, bromocamphorsulfonic 
acid salt of compound I is formed; 
d) filtering of the mixture to separate the crystalline salt from the 
supernatant; and 
e) liberating compound I from the crystalline salt by treating the salt 
with a base. 
A preferred embodiment is the process as recited above, wherein the alcohol 
solvent system is about 0.5% to about 5% water in an alcohol selected from 
the group consisting of: 1-propanol, 2-propanol, and 1-butanol. 
A more preferred embodiment is wherein the bromocamphorsulfonic acid used 
in step (a) is (1S)-(+)-3-bromocamphor-10-sulfonic acid. 
Another more preferred embodiment is wherein the alcohol solvent system 
consists of 1% water in n-propanol. 
A most preferred embodiment wherein the alcohol solvent system in step (a) 
consists of 10% water in n-propanol and is heated to about 70.degree. C. 
in step (b). 
And another embodiment of the present invention is a crystalline salt of 
Formula II 
##STR5## 
or the corresponding enantiomeric salt. 
For purposes of this specification a compound is said to selectively 
inhibit PDE IV in preference to other PDE's if the ratio of the IC50 
concentration for all other PDE inhibition to PDE IV inhibition is 100 or 
greater. 
The term "enantiomerically enriched" as used in the application is intended 
to include compounds that are enantiomerically pure. 
Utility: 
The compounds according to the invention are useful intermediates in the 
preparation of phosphodiesterase inhibitors of Formula III. The PDE IV 
inhibitors of Fromula III are of particular use in the prophylaxis and 
treatment of human diseases where an unwanted inflammatory response or 
muscular spasm (for example bladder or alimentary smooth muscle spasm) is 
present and where the elevation of cAMP levels may be expected to prevent 
or alleviate the inflammation and relax muscle. 
Particular uses for the compounds Formula III include the prophylaxis and 
treatment of asthma, especially inflamed lung associated with asthma, 
cystic fibrosis, or in the treatment of inflammatory airway disease, 
chronic bronchitis, eosinophilic granuloma, psoriasis and other benign and 
malignant proliferative skin diseases, endotoxic shock, septic shock, 
ulcerative colitis, Crohn's disease, reperfusion injury of the myocardium 
and brain, inflammatory arthritis, chronic glomerulonephritis, atopic 
dermatitis, urticaria, adult respiratory distress syndrome, diabetes 
insipidus, allergic rhinitis, allergic conjunctivitis, vernal 
conjunctivitis, arterial restenosis and artherosclerosis. 
Compounds of Formula III also suppress neurogenic inflammation through 
elevation of cAMP in sensory neurones. They are, therefore, analgesic, 
anti-tussive and anti-hyperalgesic in inflammatory diseases associated 
with irritation and pain. Compounds according to the invention may also 
elevate cAMP in lymphocytes and thereby suppress unwanted lymphocyte 
activation in immune-based diseases such as rheumatoid arthritis, 
ankylosing spondylitis, transplant rejection and graft versus host 
disease. 
Compounds of Formula III have also been found to reduce gastric acid 
secretion and therefore can be used to treat conditions associated with 
hypersecretion of gastric acid. 
Compounds of Formula III suppress cytokine synthesis by inflammatory cells 
in response to immune or infectious stimulation. They are, therefore, 
useful in the treatment of bacterial, fungal or viral induced sepsis and 
septic shock in which cytokines such as tumour necrosis factor (TNF) are 
key mediators. They also suppress inflammation and pyrexia due to 
cytokines and are, therefore, useful in the treatment of inflammation and 
cytokine-mediated chronic tissue degeneration which occurs in diseases 
such as rheumatoid or osteoarthritis. 
Over-production of cytokines such as TNF in bacterial, fungal or viral 
infections or in diseases such as cancer, leads to cachexia and muscle 
wasting. Compounds of the invention ameliorate these symptoms with a 
consequent enhancement of quality of life. 
Compounds of Formula III also elevate cAMP in certain areas of the brain 
and thereby counteract depression and memory impairment. 
They also suppress cell proliferation in certain tumour cells and can be 
used, therefore, to prevent tumour growth and invasion of normal tissues. 
Synthesis of Compounds of Formula I 
Racemic phenol 5 may be synthesized by the procedure disclosed in European 
Patent No. 626939, which is hereby incorporated by reference, or by the 
scheme shown below. (1S)-(+)-3-bromocamphor-10-sulfonic acid may be 
purchased as the hydrate from Aldrich (Milwaukee, Wis.). Althernatively, 
either isomer may be synthesized by the method described in Beilstein 11, 
317. 
Scheme 1 
The Grignard reaction of isovanillin (2) with 2.0 equivalent of 
phenylmagnesium bromide at -10.degree. C. produced alcohol 3, which was 
isolated from. Alcohol 3 was then converted to chloride 4 using thionyl 
chloride in toluene at 0.degree. C. The residue after evaporation was 
dissolved in THF and treated with the litho picolate (generated from LDA 
and picoline) at -60.degree. C. After work up and crystallization from 
isopropanol, compound 5 was obtained in racemic form. 
##STR6## 
Scheme 2 
The isolated racemic pyridine phenol 5, a highly crystalline solid, was 
treated with (1S)-(+)-3-bromocamphor-10-sulfonic acid (Br-CSA) (6) in 
aqueous n-propanol to provide the salt 7 which contained an 88% ee of the 
desired enantiomer (R)-7. Recrystallization of the isolated salt 7 from 
aqueous n-propanol afforded optically pure 7 (&gt;99.5% ee). Thus, a highly 
efficient resolution of racemic pyridine phenol 5 was accomplished in good 
overall yield and &gt;99.5% ee. 
To complete the synthesis, the salt 7 was neutralized with 2N NaOH to 
afford the enantiopure phenol (R)-8. Subsequent cyclopentylation of (R)-8 
was carried out with cyclopentyl bromide (CpBr) and cesium carbonate in 
DMF (no racemization was observed). The final product (1) was obtained as 
a bisulfate salt. 
##STR7## 
HPLC ASSAY METHODS 
A. Determination of Purity: 
The purity of the mentioned compounds was determined using an Rx C-8 column 
under the following conditions: 
Flow rate=1.0 mL/min; 
.lambda.=210 nm; 
Mobile phase: A=acetonitrile; B=H.sub.2 O/0.1% H.sub.3 PO.sub.4 ; 
Gradient Utilized: 
______________________________________ 
Time (min.) % A % B 
______________________________________ 
0 30 70 
25 70 30 
26 30 70 
30 30 70 
______________________________________ 
Retention Times: 
Phenol 5: 6.0 minutes 
Compound 1: 12.9 minutes 
B. Determination of Enantiomeric Excess 
Enantiomeric excess was determined using a CHIRALCEL OJ column (available 
from Chiral Technologies, Inc. in Exton, Pa.) under the following 
conditions: 
Flow rate=1.0 mL/min; 
.lambda.=210 nm; 
Mobile phase 30% EtOH/Hexane 
Retention Times: 
(S)-enantiomer 1: 17.4 minutes 
(R)-enantiomer 1: 19.8 minutes

EXAMPLE 1 
##STR8## 
To a 125 ml round bottom flask with magnetic bar and N.sub.2 inlet 
containing 37.5 mL of 1-propanol and 375 .mu.L of water was added 1.73 g 
(5.75 mmol) of racemic phenol 1. 1.87 g (5.69 mmol) of 
(1S)-(+)-3-bromocamphor-10-sulfonic acid was then added and the slurry was 
heated to 70 .degree. C. All solids dissolved. The solution was cooled to 
58 .degree. C. and seeded. The resulting thin slurry was aged at 55 
.degree. C. for one hour and then cooled to room temperature over one 
hour. The white slurry was aged for one hour at room temperature and 
subsequently filtered through a glass funnel. The crystals were washed 
with 2 ml n-propanol and dried at 45 .degree. C. overnight in a vacuum 
oven. The recovered crystalline salt had an enantiomeric excess of 88% as 
determined by chiral LC of the free base under the conditions mentioned 
above. 
EXAMPLE 2 
Recrystallization of the Acid-base Pair 6 
To a 125 ml round bottom flask with magnetic bar and N.sub.2 inlet 
containing 4.0 mL of 1-propanol and 400 .mu.L of water was added 413 mg of 
salt 7. The white slurry was heated to 70 .degree. C. until all the solids 
had dissolved. The reaction mixture was then cooled to 55 .degree. C. and 
seeded. The thin slurry for then aged for one hour at 52 .degree. C., 
cooled to room temp erature over one hour and aged for an additional hour. 
The crystals were then filtered through a glass funnel and washed with 1 
ml n-propanol. The recovered salt 6 had an enantiomeric excess of over 
99.5% as determined by the chiral LC method described above. m.p. 
185-187.degree. C. 
EXAMPLE 3 
Cyclopentylation of the Phenolic Moiety to Yield PDE 1 
To a 125 ml round bottom flask with magnetic stir bar was added 1.02 g 
(1.61 mmol) of salt 6 and 25 mL ethyl acetate. To this slurry was added 15 
mL H.sub.2 O and the pH was adjusted to 4.5 with 2N NaOH (about 1 mL). The 
organic phase was then washed with 20 mL H.sub.2 O and dried with 
magnesium sulfate. The solution was filtered and evaporated to an oil. The 
residue was dissolved in 4.0 mL DMF and placed in a 15 mL round bottom 
flask with a magnetic stir bar and N.sub.2 inlet. 1.07 g (3.3 mmol) of 
cesium carbonate and 354 .mu.L (3.3 mmol) of cyclopentylbromide were then 
added and the thin white slurry was stirred overnight at room temperature. 
The reaction was monitored by LC using the purity assay disclosed above. 
When the starting material was reduced to &gt;0.1% (approximately 16-18 
hours), the reaction was quenched with 10 mL H.sub.2 O. The quenched 
solution was extracted twice with ethyl acetate (about 20 mL total ) and 
the combined organics were wash with 20 mL H.sub.2 O. The organic layer 
was removed under reduced pressure and the crude residue was dissolved in 
6.0 mL of ethyl alcohol. 90.0 mL (1.61 mmol) of sulfuric acid were then 
added and the sulfate salt precipitated from the solution. The slurry was 
aged one hour, filtered and washed with ethyl alcohol. After drying at 
40.degree. C. under vacuum overnight, compound 1 was obtained as a bright 
white crystals.