PROSTAGLANDINS
SYNTHESIS AND BIOLOGICAL ACTIVITY OF 2-DECARBOXY-2-(TETRAZOL-5-YL)PROSTAGLANDINS Norman A. Nelson, Robert W. Jackson and Andrew T. Au Research Laboratories, The Upjohn Company, Kalamazoo, Michigan 49001 ABSTRACT The synthesis of 2-decarboxy-2-(tetrazol-5-yl)prostaglandins of the E,, Ez, F,a and F,a series is described together with their biological effects on gerbil colon smooth muscle and rat blood pressure. ACKNOWLEDGEMENT The authors gratefully acknowledge the Prostaglandin Screening Laboratory of The Upjohn Company under the direction of Charles F. Lawson for the data in Table II. INTRODUCTION Chemists have long recognized the tetrazole ring system to have aromatic character and to be resistant to acids and bases in addition to oxidizing and reducing reagents (1) - characteristics desirable for a group to be carried in a multi-step organic synthesis. Perhaps most remarkable, is the fact that 5-methyltetrazole (and presumably other 5-alkyltetrazoles such as V-VIII) has a pKa of about 5.6 in water, a value close to that of alkyl carboxylic acids acetic acid, pKa 4.8) (1). Consequently, one might predict that, (e.g., in those situations where the steric bulk of two additional nitrogen atoms can be tolerated, tetrazoles might mimic a carboxylic acid group. Indeed, a number of compounds in which the carboxylic acid group of an active compound has been replaced with a 5-tetrazoyl group, display biological activity, albeit, of an unpredictable nature (increased, decreased or antagonistic activity (1, 2). In our search for prostaglandin analogs which would be resistant to $-oxidative breakdown of the acidic side chain, and, because of considerations described above, we replaced the carboxy group of four prostaglandins with a tetrazol5-yl group to determine the resultant effect on prostaglandin-like activities. MATERIALS AND METHODS Synthesis of 2-Decarboxy-2-(tetrazol-5-yl)prostaglandins. A mixture of 5-bnomovaleronitrile and triphenylphosphine in toluene was refluxed overnight to give 4-cyanobutyltriphenylphosphonium bromide (I), m.p. 230". Treatment of I with sodium azide and ammonium chloride in
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303
PROSTAGLANDINS
dimethylformamide at 115" for 72 hours gave crude 4-(tetrazol-5-y& butyltriphenylphosphonium halide(s) which was dissolved in water and treated with sodium bicarbonate to precipitate the zwitterion product II, m.p. 274-277" (methanol-acetone). Conversion of II to the ylid with dimsyl sodium in dimethyl sulfoxide and addition of III (3, 4) gave IV which, on removal of protective groups yielded 2-decarboxy-2-(tetrazol-5-yl)-PGF,a (V) (non-crystalline), or, after oxidation with Jones reagent and removal of protective groups gave 2decarboxy-2-(tetrazol-5-yl)-PGEz (VI)(nOn-Crystalline). Selective hydrogenation of IV with 5% palladium-on-charcoal in ethyl acetate at room temperature for about 3 hours at atmospheric pressure yielded the 5,6-dihydro compound which was transformed to the corresponding 2-decarboxy-2-(tetrazol-5-yl)-PG, products VII iwaxy solid, mp 81-83") and VIII (waxy solid, mp 8286"). Intermediates and final products were established as pure by thin layer chromatography on silica gel plates with several solvent systems and by spectral analyses. Identification of the various compounds was based on the chemistry involved and spectral analyses (NMR and especially high resolution mass spectrometry). Crystalline compounds had acceptable elemental analyses. Rf data are given in Table I. 8 N-N &Hs),-&CH&CN I@ --+ (C&)s-&H&-C; 11 N-N I II
d
-_ 0‘
OH
-9 wDTHP
THPO=
=
:
:
,z
O=THP
THPi
III
304
V, R = a-OH
VII, R = a-OH
VI, R=
VIII, R = 0
0
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1975
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PROSTAGLANDINS
Gerbil Colon Smooth Muscle Stimulation and Rat Blood Pressure Assays. These assays were conducted essentially as described previously (5) and the results are described in Table II. RESULTS Gerbil Colon Smooth Muscle Stimulation. The PGF analogs V and VII showed diminished gerbil colon activity while the PGE analogs VI and VIII were similar to the reference compound PGEl (Table II). Rat Blood Pressure. The PGF. analog V gave atypical results in that there was an initial depressor response followed by a slight pressor response in some rats, but not others (Table II). It is not possible, therefore, to express a meaningful result relative to the standards. The PGF, analog VII showed a slight depressor response rather than the expected pressor response for a PGF compound in this assay. Both PGE analogs VI and VIII showed depressor responses roughly comparable to PGE1. The biological results illustrate that replacement of the carboxyl group of a prostaglandin with a tetrazol-5-yl group will maintain biological activity, but to an unpredictable extent. TABLE I Mobilities of Compounds on Silica Gel Thin Layer Chromatography Plates
Compound and Comparison Substance
Rf AIX System*
Rf MI1 System**
Compound V
0.05
0.17
Prostaglandin F,a
0.21
0.27
Compound VI
0.18
0.30
Prostaglandin Ea
0.35
0.33
Compound VII
0.05
0.19
Prostaglandin F,a
0.19
0.29
Compound VIII
0.13
0.36
Prostaglandin E1
0.38
0.43
*AIX System:
**MI1 System:
AUGUST
1975
organic phase from ethyl acetate--acetic acid--2,2,4trimethylpentane-water 90:20:50:100 (6). organic phase from ethyl acetate--methanol--water 8:2:5 (7).
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PROSTAGLANDINS
TABLE II Activities of P-Decarboxy-2-(tetrazol-5-yl)prostaglandins in Gerbil Colon Smooth Muscle and Rat Blood Pressure Assays*
Compound
V VI VII VIII
Gerbil Colon Smooth Muscle Stimulation (PGE~ = 100)
Rat Blood Pressure (PGE, = 1004) (PGF;~ = loo+)
< 30.3 > 6.9 - ca. 13
Atypical
< 224 > 68.8 - ca. 119
< 78.2 > 39.2 - ca. 53)
< 2.5 > 1.3 - ca. 2
< 0.4 > 0.2 - ca. 0.2 $
< 226 > 90.2 - ca. 136
< 66.9 > 34.1 - ca. 47i
*4-Point assays (5). ADDENDUM In the course of this work, and later, patents have appeared which describe the independent synthesis of the compounds described in this communication (8, 9). REFERENCES 1.
2.
3.
4. 5.
6. 7. 8.
9.
306
Benson, F. R. "The Tetrazoles," in R. C. Elderfield's "Heterocyclic Compounds," John Wiley and Sons Inc., New York, N.Y. 8:1, 1967. Drain, D. J., B. Davy, M. Horlington, J.G.B. Howes, J. M. Scruton and R. A. Selway. The Effects of Substituting Tetrazole for Carboxyl in Two Series of Anti-inflammatory Phenoxyacetic Acids, J. Pharm. and Pharmacol. 23:857, 1971. Corey, E. J., T. K. Schaaf, W. Huber, U. Koelliker and N. M. Weinshenker. Total Synthesis of Prostaglandins Fla and Ez as the Naturally Occurring Forms, J. Am. Chem. Sot. 92:397, 1970. Yankee, E. W., U. Axen and G. L. Bundy. Total Synthesis of 15-Methylprostaglandins, J. Am. Chem. Sot. 96:5865, 1974. Weeks, J. R., D. W. DuCharme, W. E. Magee and W. L. Miller. The Biological Activity of the (15S)-15-Methyl Analogs of Prostaglandins Ea and F2a, J. Pharm. Exp. Ther. 186:67, 1973. Hamberg, M. and B. Samuelsson. Prostaglandins in Human Seminal Plasma, J. Biol. Chem. 241:257, 1966. Ramwell, P. W. and E. G. Daniels. Chromatography of the Prostaglandins, Lipid Chrom. Anal. 2:313, 1969. Pfizer Inc. 2-Decarboxy-2-(tetrazol-5_yl)prostaglandins, Netherlands Patent Application No. 72.11860, September 1, 1971. Sandoz AG. 2-Tetrazol Analogs of Prostaglandins, German Offenlegungsschrift 2405-255, August 8, 1974.
AUGUST
1975
VOL. 10 NO. 2
SYNTHESIS AND BIOLOGICAL ACTIVITY OF 2-DECARBOXY-2-(TETRAZOL-5-YL)PROSTAGLANDINS Norman A. Nelson, Robert W. Jackson and Andrew T. Au Research Laboratories, The Upjohn Company, Kalamazoo, Michigan 49001 ABSTRACT The synthesis of 2-decarboxy-2-(tetrazol-5-yl)prostaglandins of the E,, Ez, F,a and F,a series is described together with their biological effects on gerbil colon smooth muscle and rat blood pressure. ACKNOWLEDGEMENT The authors gratefully acknowledge the Prostaglandin Screening Laboratory of The Upjohn Company under the direction of Charles F. Lawson for the data in Table II. INTRODUCTION Chemists have long recognized the tetrazole ring system to have aromatic character and to be resistant to acids and bases in addition to oxidizing and reducing reagents (1) - characteristics desirable for a group to be carried in a multi-step organic synthesis. Perhaps most remarkable, is the fact that 5-methyltetrazole (and presumably other 5-alkyltetrazoles such as V-VIII) has a pKa of about 5.6 in water, a value close to that of alkyl carboxylic acids acetic acid, pKa 4.8) (1). Consequently, one might predict that, (e.g., in those situations where the steric bulk of two additional nitrogen atoms can be tolerated, tetrazoles might mimic a carboxylic acid group. Indeed, a number of compounds in which the carboxylic acid group of an active compound has been replaced with a 5-tetrazoyl group, display biological activity, albeit, of an unpredictable nature (increased, decreased or antagonistic activity (1, 2). In our search for prostaglandin analogs which would be resistant to $-oxidative breakdown of the acidic side chain, and, because of considerations described above, we replaced the carboxy group of four prostaglandins with a tetrazol5-yl group to determine the resultant effect on prostaglandin-like activities. MATERIALS AND METHODS Synthesis of 2-Decarboxy-2-(tetrazol-5-yl)prostaglandins. A mixture of 5-bnomovaleronitrile and triphenylphosphine in toluene was refluxed overnight to give 4-cyanobutyltriphenylphosphonium bromide (I), m.p. 230". Treatment of I with sodium azide and ammonium chloride in
AUGUST
1975
VOL. 10 NO. 2
303
PROSTAGLANDINS
dimethylformamide at 115" for 72 hours gave crude 4-(tetrazol-5-y& butyltriphenylphosphonium halide(s) which was dissolved in water and treated with sodium bicarbonate to precipitate the zwitterion product II, m.p. 274-277" (methanol-acetone). Conversion of II to the ylid with dimsyl sodium in dimethyl sulfoxide and addition of III (3, 4) gave IV which, on removal of protective groups yielded 2-decarboxy-2-(tetrazol-5-yl)-PGF,a (V) (non-crystalline), or, after oxidation with Jones reagent and removal of protective groups gave 2decarboxy-2-(tetrazol-5-yl)-PGEz (VI)(nOn-Crystalline). Selective hydrogenation of IV with 5% palladium-on-charcoal in ethyl acetate at room temperature for about 3 hours at atmospheric pressure yielded the 5,6-dihydro compound which was transformed to the corresponding 2-decarboxy-2-(tetrazol-5-yl)-PG, products VII iwaxy solid, mp 81-83") and VIII (waxy solid, mp 8286"). Intermediates and final products were established as pure by thin layer chromatography on silica gel plates with several solvent systems and by spectral analyses. Identification of the various compounds was based on the chemistry involved and spectral analyses (NMR and especially high resolution mass spectrometry). Crystalline compounds had acceptable elemental analyses. Rf data are given in Table I. 8 N-N &Hs),-&CH&CN I@ --+ (C&)s-&H&-C; 11 N-N I II
d
-_ 0‘
OH
-9 wDTHP
THPO=
=
:
:
,z
O=THP
THPi
III
304
V, R = a-OH
VII, R = a-OH
VI, R=
VIII, R = 0
0
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1975
VOL. 10 NO. 2
PROSTAGLANDINS
Gerbil Colon Smooth Muscle Stimulation and Rat Blood Pressure Assays. These assays were conducted essentially as described previously (5) and the results are described in Table II. RESULTS Gerbil Colon Smooth Muscle Stimulation. The PGF analogs V and VII showed diminished gerbil colon activity while the PGE analogs VI and VIII were similar to the reference compound PGEl (Table II). Rat Blood Pressure. The PGF. analog V gave atypical results in that there was an initial depressor response followed by a slight pressor response in some rats, but not others (Table II). It is not possible, therefore, to express a meaningful result relative to the standards. The PGF, analog VII showed a slight depressor response rather than the expected pressor response for a PGF compound in this assay. Both PGE analogs VI and VIII showed depressor responses roughly comparable to PGE1. The biological results illustrate that replacement of the carboxyl group of a prostaglandin with a tetrazol-5-yl group will maintain biological activity, but to an unpredictable extent. TABLE I Mobilities of Compounds on Silica Gel Thin Layer Chromatography Plates
Compound and Comparison Substance
Rf AIX System*
Rf MI1 System**
Compound V
0.05
0.17
Prostaglandin F,a
0.21
0.27
Compound VI
0.18
0.30
Prostaglandin Ea
0.35
0.33
Compound VII
0.05
0.19
Prostaglandin F,a
0.19
0.29
Compound VIII
0.13
0.36
Prostaglandin E1
0.38
0.43
*AIX System:
**MI1 System:
AUGUST
1975
organic phase from ethyl acetate--acetic acid--2,2,4trimethylpentane-water 90:20:50:100 (6). organic phase from ethyl acetate--methanol--water 8:2:5 (7).
VOL. 10 NO. 2
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PROSTAGLANDINS
TABLE II Activities of P-Decarboxy-2-(tetrazol-5-yl)prostaglandins in Gerbil Colon Smooth Muscle and Rat Blood Pressure Assays*
Compound
V VI VII VIII
Gerbil Colon Smooth Muscle Stimulation (PGE~ = 100)
Rat Blood Pressure (PGE, = 1004) (PGF;~ = loo+)
< 30.3 > 6.9 - ca. 13
Atypical
< 224 > 68.8 - ca. 119
< 78.2 > 39.2 - ca. 53)
< 2.5 > 1.3 - ca. 2
< 0.4 > 0.2 - ca. 0.2 $
< 226 > 90.2 - ca. 136
< 66.9 > 34.1 - ca. 47i
*4-Point assays (5). ADDENDUM In the course of this work, and later, patents have appeared which describe the independent synthesis of the compounds described in this communication (8, 9). REFERENCES 1.
2.
3.
4. 5.
6. 7. 8.
9.
306
Benson, F. R. "The Tetrazoles," in R. C. Elderfield's "Heterocyclic Compounds," John Wiley and Sons Inc., New York, N.Y. 8:1, 1967. Drain, D. J., B. Davy, M. Horlington, J.G.B. Howes, J. M. Scruton and R. A. Selway. The Effects of Substituting Tetrazole for Carboxyl in Two Series of Anti-inflammatory Phenoxyacetic Acids, J. Pharm. and Pharmacol. 23:857, 1971. Corey, E. J., T. K. Schaaf, W. Huber, U. Koelliker and N. M. Weinshenker. Total Synthesis of Prostaglandins Fla and Ez as the Naturally Occurring Forms, J. Am. Chem. Sot. 92:397, 1970. Yankee, E. W., U. Axen and G. L. Bundy. Total Synthesis of 15-Methylprostaglandins, J. Am. Chem. Sot. 96:5865, 1974. Weeks, J. R., D. W. DuCharme, W. E. Magee and W. L. Miller. The Biological Activity of the (15S)-15-Methyl Analogs of Prostaglandins Ea and F2a, J. Pharm. Exp. Ther. 186:67, 1973. Hamberg, M. and B. Samuelsson. Prostaglandins in Human Seminal Plasma, J. Biol. Chem. 241:257, 1966. Ramwell, P. W. and E. G. Daniels. Chromatography of the Prostaglandins, Lipid Chrom. Anal. 2:313, 1969. Pfizer Inc. 2-Decarboxy-2-(tetrazol-5_yl)prostaglandins, Netherlands Patent Application No. 72.11860, September 1, 1971. Sandoz AG. 2-Tetrazol Analogs of Prostaglandins, German Offenlegungsschrift 2405-255, August 8, 1974.
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1975
VOL. 10 NO. 2
