# Patent

## Title
N-phenylbenzamide derivatives

## Abstract

This application is a divisional of Ser. No. 844,970, filed Mar. 27, 1986, now abandoned.

## Background
The present invention relates to hypoglycemic agents useful as antidiabetic drugs.

Hitherto, as antidiabetic drugs for oral use, there have been widely employed sulfonyl urea which shows hypoglycemic action particularly through a promotion of the secretion of insulin, and a biguanide which shows a hypoglycemic action particularly through the metabolism of sugar. However, they are somewhat unsatisfactory as to their side effects (see Textbook of Endocrinology 4th ed., 1968, p. 719 (Saunders); Diabetes, 19, 785, 1970; Ann. Rev. Pharmacol., 15, 351, 1975).

No report has been found that a D-phenylalanine derivative possesses hypoglycemic action.

According to one aspect of the invention there is provided for pharmaceutical, particularly hypoglycemic, use, a D-phenylalanine derivative represented by the general formula: ##STR3## or a salt thereof, or a precursor which can be converted thereto in the human or animal body. Such compounds can lower the value of blood sugar and thus can be used as an antidiabetic drug for an oral use as well as by injection.

Among the foregoing phenylalanine derivatives, those in the D-form represented by the general formula: ##STR4## and the salts thereof are novel.

In the above general formulae: R.sup.1 is hydrogen, alkyl of 1 to 5 carbon atoms such as methyl, ethyl, n-propyl, isopropyl, n-butyl, and sec-butyl, aryl of 6 to 12 carbon atoms such as phenyl, tolyl, naphthyl, and ##STR5## aralkyl of 6 to 12 carbon atoms such as benzyl, ##STR6## --CH.sub.2 CO.sub.2 R.sup.3, --CH(CH.sub.3)--OCO--R.sup.3, or --CH.sub.2 --OCO--C(CH.sub.3).sub.3, R.sup.2 is a group comprising aryl of 6 to 12 carbon atoms such as phenyl, naphthyl, and indanyl, a hetero six-membered ring such as quinolynyl, pyridyl, a hetero five-membered ring such as 2-benzofuranyl, cycloalkyl such as cyclohexyl and cyclopentyl, bicycloalkyl such as bicycloheptye, and cycloalkenyl such as 1-cyclohexenyl, 2-cyclohexenyl, 3-cyclohexenyl, 1-cyclopentenyl and 2-cycyclopentenyl, any of which groups optionally having one or more substituents; R.sup.3 is hydrogen or lower alkyl such as methyl, ethyl, isopropyl and pentyl; and R.sup.4 stands for those R.sup.2 groups which provide novel compounds hereof, notably phenyl which has one or more alkyl substituents of 2 to 5 carbon atoms, cyclohexyl which has one or more substituents cyclopentyl, bicycloalkyl, cycloalkenyl, indanyl or 2-benzofuranyl, any of which may have one or more substituents.

When an organic group in the above general formulae has a substituent, examples of such substituents include a halogen atom such as fluorine or chlorine, a hydroxyl group, a C.sub.1-5 alkyl group such as methyl, ethyl, trichloromethyl, trifluoromethyl, propyl, isopropyl, n-butyl, sec-butyl, and tert-butyl, a C.sub.1-5 alkenyl group such as ethenyl, propenyl, and butenyl, an alkylidene group such as ##STR7## a C.sub.1-5 alkyloxy such as methoxy and ethoxy, a C.sub.1-5 alkyl group which has been substituted by such C.sub.1-5 alkyloxy group such as methoxymethyl and 1-ethoxyethyl, a C.sub.1-5 alkylene group which has been substituted by such C.sub.1-5 alkyloxy group in the same manner as above such as 1-methoxyethylene. In the case of a substituted bicycloalkyl group as stated above, it can include a bicycloheptyl or a derivative thereof such as bicyclo (2,2,1)heptyl.

In the case of the compound represented by the general formula (I) wherein R.sup.1 stands for a hydrogen atom, it can be formed by conventional methods via the salts thereof with various cations such as an alkali metal, for example, sodium and potassium, an alkali earth metal, for example, calcium, an inorganic base, for example, ammonia, an organic base, for example, cyclohexylamine, N-methyl-D-glucosamine, or a basic amino acid (lysine, arginine and the like).

The D-phenylalanine derivative as shown by the formula (I) mentioned above, can be prepared by using conventional N-acylating reactions as in the Examples given below.

Most of the phenylalanine derivatives supplied by this invention are novel compounds which have not been described yet in the literature.

The D-phenylalanine derivatives used in the present invention are useful as a hypoglycemic agent for treating diabetic mammals including humans. The derivatives can be used for lowering blood sugar by formulating them into a preparation such as tablets, capsules, and elixirs for oral administration and into an aseptic liquid preparation or an aseptic suspension preparation for parenteral administration such as subcutaneous, intramuscular, intavenous injection, and suppositories. The D-phenylalanine derivatives in the present invention can be administered to a subject necessitating such treatment (animals and humans) in a dosage range of 0.1 to 1,000 mg per subject generally several times a day, that is, in a total daily dosage of 0.2 to 2,000 mg. The dosage varies according to the seriousness of disease, the body weight of subjects, and other factors acknowledged by those skilled in the art.

To produce the preparations using the D-phenylalanine derivatives as described above for the present invention, they may be converted to dosage forms such as tablets, granules, powders, capsules, injections and suppositories by conventional methods.

For the production of oral preparations, there may be added to the D-phenylalanine derivative as the principal agent, adjuvants such as fillers, binders, disintegrators, lubricants, colors, and correctives, as necessary, and then formed by conventional methods into tablets, coated tablets, granules, powders, capsules and the like.

Examples of specific materials which can be incorporated into tablets, capsules, and so forth are as follows: fillers such as cornstarch, lactose, white sugar, glucose, sorbitol, and crystalline cellulose; binders such as polyvinyl alcohol, polyvinyl ether, ethyl cellulose, methyl cellulose, gum arabic, tragacanth gelatine, shellac, hydroxypropyl cellulose, hydroxypropyl starch, polyvinyl pyrrolidone; disintegrators such as starch, agar, gelatine powder, cyrstalline cellulose, calcium carbonate, sodium bicarbonate, calcium citrate, dextrin and pectin; lubricants such as magnesium stearate, talc, polyethylene glycol, silica, hardened plant oil; colors such as one which is allowed as an additive for the medicines; correctives such as cocoa powder, mentha herb, aromatic acid, mentha oil, borneol, cinnamon bark powder. These tablets and granules may be coated with sugar, gelatine, or the like, as desired.

For the production of the injectable formulations, there may be added to the phenylalanine derivative as the principal agent, a pH adjusting agent, a buffer agent, a stabilizing agent, preservatives or the like, as necessary to produce a material for subcutaneous, intramuscular or intravenous injection by conventional methods.

The present invention will be further explained in the following examples.

D-Phenylalanine 2 g (12 mmole) was dissolved in 10% aqueous sodium hydroxide solution (10 ml), and acetone (10 ml) was added. An acetone (5 ml) solution of 4-ethyl benzoyl choride (2.5 g, 15 mmole) and a 10% aqueous sodium hydroxide solution were added dropwise to the mixture obtained above while stirring and cooling with ice over 20 minutes, the reaction solution being maintained at pH 10. The reaction solution was returned to the room temperature, stirred for 3 hours, and made an acidic with a dilute hydrochloric acid solution to precipitate crystals. The crystals were filtered, washed with water and recrystallized from ethyl acetate to obtain N-(4-etylbenzoyl)-D-phenylalanine (3.0 g, yield 83%).

m.p. 165.5.degree.-166.degree. C. Specific Rotation [.alpha.].sub.D.sup.23 +4.4.degree. (C=1, methanol).

For Examples 2 to 8, in the same manner as in Example 1, using the following starting materials, each compound of 50 mmole, the following product compounds were produced. The compounds in Examples 9 to 11 were already known, and therefore were produced in accordance with the following literature references: Example Starting          Yield M.P.  Specific No.  Material Product  (%) (.degree.C.) Rotation __________________________________________________________________________ 2    D-phenylalanine N--(4-tolsoyl)- 83  152-155 [.alpha.  .sub.D.sup.28 +46.2.degree. D-phenylalanine    (C = 0.5, methanol) 3    "        N--(2-fluoro- 74  91.5-93.5 [.alpha.] .sub.D.sup.19 -8.8.degree. benzoyl)-D-        (C = 1, methanol) phenylalanine 4    "        N--(3-fluoro- 81  112.5-116 [.alpha.] .sub.D.sup.22 +48.6.degree. benzoyl)-D-        (C = 1, methanol) phenylalanine 5    "        N--(4-fluoro- 80  142-145 [.alpha.] .sub.D.sup.28 +40.4.degree. benzoyl)-D-        (C = 0.5, methanol) phenylalanine 6    "        N--(3-trifluoro- 77  118-119 [.alpha.] .sub.D.sup.28 +40.4.degree. methylbenzoyl)-    (C = 1, methanol) D-phenylalanine 7    "        N--(4-trifluoro- 70    136-137.5 [.alpha.] .sub.D.sup.28 +36.3.degree. methylbenzoyl)-    (C = 1, methanol) D-phenylalanine 8    "        N--(4-anisoyl)-D- 65  85-90 [.alpha.] .sub.D.sup.20 +60.2.degree. phenylalanine      (C = 0.5, methanol) 9    "        N--benzoyl-D- 81 phenylalanine 10   "        N--nicotinoyl- 62 D-phenylalanine 11   "        N--(2-naphthoyl)- 83 D-phenylalanine __________________________________________________________________________

Cyclopentane carboxylic acid (1.5 g, 13 mmole) was dissolved in chloroform (50 ml), and N-hydroxysuccinimide 1.7 g was added. N,N'-Dicyclohexylcarbodiimide (3.0 g) was gradually added to the mixture as obtained above while stirring and cooling with ice, and the mixture was stirredn for 1 hour at the same temperature. The mixture was further stirred for 7 hours at room temperature. Glacial acetic acid (2 ml) was added to the mixture, and stirred for 1 hour. The insoluble matter was removed by filtration. The filtrate was washed with saturated aqueous sodium bicarbonate solution (30 ml), 1N aqueous hydrochloric acid solution (30 ml), and water (30 ml), and dried over magnesium sulfate. The magnesium sulfate was removed by filtration, and the solution thus obtained was concentrated under reduced pressure to dryness. The matter was recrystallized from ethyl acetate to afford cyclopentane carboxylic acid N-hydroxysuccinimide ester (2.5 g, yield 91%).

The ester derivative thus obtained above (2.5 g), was dissolved in chloroform (20 ml). This solution was added to a chloroform solution (40 ml) of D-phenylalanine methyl ester hydrochloride (3.0 g, 14 mmole) and triethylamine (1.4 g), and the mixture thus obtained was stirred for 18 hours at room temperature. The reaction solution was washed with 1N aqueous hydrochloric acid solution (40 ml), saturated aqueous sodium bicarbonate solution (40 ml) and water (40 ml), and dried over magnesium sulfate. The magnesium sulfate was removed by filtration and the filtrate was concentrated under reduced pressure to dryness. The matter was recrystallized from ethyl acetate-n-hexane to afford N-cyclopentylcarbonyl-D-phenylalanine methyl ester (3.0 g, yield 84%).

The methyl ester derivative (3.0 g) thus obtained above, was dissolved in methanol (10 ml), and 1N aqueous sodium hydroxide solution (20 ml) was added. The mixture was stirred for 30 minutes at room temperature and then made acidic with an addition of a dilute hydrochloric acid to precipitate crystals. The crystals were filtered, washed with water, and recrystallized from methanol-water to give the desired product (2.7 g, yield 80%).

m.p. 108.degree.-110.degree. C. Specific Rotation [.alpha.].sub.D.sup.22 -35.2.degree. (C=0.5, methanol).

For Examples 13 to 16, in the same manner as in Example 12, using as the starting material the following compounds, each of 15 mmole, the following products were obtained. The compounds of Examples 17 and 18 stated in the above table were known, and therefore were produced in accordance with the following literature references: Example Starting          Yield M.P. Specific Rotation No.  Material Product   (%) (.degree.C.) [.alpha.] .sub.D.sup.22 (C = 0.5, methanol) __________________________________________________________________________ 13   2-benzofurane N--(2-benzofuran- 59  114-116 +89.6.degree. carboxylic yl-carbonyl)- acid    D-phenylalanine 14   5-indane N--(5-indanyl- 64  160-161 +52.0.degree. carboxylic carbonyl)- acid    D-phenylalanine 15   3-cyclohexene N--(3-cyclo- 62  100-101 -12.6.degree. carboxylic hexenylcarbonyl)- acid    D-phenylalanine 16   bicyclo- N--(bicyclo- 50  179-181 +33.4.degree. [2,2,1]heptan- [2,2,1]heptan-2- 2-ylcarboxylic ylcarbonyl)-D- acid    phenylalanine 17   cyclohexene N--cyclohexyl- 65 carboxylic carbonyl-D- acid    phenylalanine 18   benzoic acid N--benzoyl-D- 65 phenylalanine methyl ester __________________________________________________________________________

Platinum oxide (200 mg) as a catalyst was suspended in acetic acid (20 ml), and then (s)-(-)-perillic acid (2 g, 12 mmole) was added. The mixture thus obtained was stirred for 8 hours at room temperature under a current of hydrogen gas. The catalyst was removed by filtration, and the filtrate was concentrated under reduced pressure to a dryness. The matter was recrystallized from methanol-water to obtain 4-isopropyl cyclohexane carboxylic acid (1.9 g, yield 93%).

After that, in the same manner as the reaction in Example 1, N-(4-isopropylcyclohexylcarbonyl)-D-phenylalanine was produced. It was crystallized from methanol-water to give the desired product (2.5 g, yield 61%).

m.p. 230.degree.-232.degree. C. Specific Rotation [.alpha.].sub.D.sup.22 -28.2.degree. (C=0.5, methanol).

ICR-CDI mice (Male, five weeks old, Body weight: 20 g) which had been bred for one week, were abstained from food for 18 hours, and then used as test subjects.

The phenylalanine derivative of the present invention was suspended in 0.5% CMC-0.05M tris-hydrochloride buffer (pH 7.4). The sample solution thus obtained was administered orally in fixed amounts to the test subjects. A predetermined time later, the percentage decrease in blood glucose with the comparison to the control group was determined. The results are shown in the following table. Decrease in Blood Glucose (%) Example No. Amounts used (mg/kg) 60 Minutes ______________________________________ 1           25             34 2          100             32 3          "               24 4          "               24 5          "               43 6          250             37 7          100             33 8          "               38 9          "               34 10          250             19 11          "               17 12           50             22 13          100             31 14          250             28 15          100             28 16          250             16 17          100             27 18          250             37 19           25             50 ______________________________________

Cumic acid (15.0 g, 91 mmole) was dissolved in chloroform (150 ml), and N-hydroxysuccinimide (11.4 g, 99 mmole) was added thereto. N,N'-Dicyclohexylcarbodiimide (20.4 g, 99 mmole) was added gradually to the mixture obtained above while cooling with ice and stirring, and then the mixture thus obtained was returned to room temperature. The mixture was further stirred for 15 hours at room temperature. Glacial acetic acid (5 ml) was added thereto, and the mixture thus obtained was stirred and the insoluble matter was removed by filtration. The filtrate was washed with saturated aqueous sodium bicarbonate (300 ml) and water (300 ml), and dried over magnesium sulfate. The magnesium sulfate was removed by filtration, and the filtrate thus obtained was concentrated under reduced pressure to dryness. The resultant substance was recrystallized from ethyl acetate to obtain cumic acid N-hydroxysuccinimide ester (18.8 g, yield 72 mmole).

The ester thus obtained above (18.8 g) was added to the chloroform solution (150 ml) of D-phenylalanine methyl ester hydrochloride (23.0 g, 110 mmole) and triethylamine (10.8 g, 110 mmole), and the mixture thus obtained was stirred for 15 hours at room temperature. The reaction solution was washed with 1N aqueous hydrochloric acid solution (300 ml), saturated aqueous sodium bicarbonate (300 ml) and water 300 ml), and dried over magnesium sulfate. The magnesium sulfate thus used was removed by filtration, and the filtrate thus obtained was concentrated under reduced pressure to dryness.

The residue thus obtained was recrystallized from ethyl acetate-n-hexane to obtain N-cumoyl-D-phenylalanine methyl ester (20.5 g, yield 69%).

The methyl ester thus obtained above (20.5 g) was dissolved in methanol (100 ml), and then 1N aqueous sodium hydroxide (100 ml) was added thereto. The mixture thus obtained was stirred for 10 minutes at room temperature, and was made acidic with an addition of diluted aqueous hydrochloric acid solution to precipitate crystals. The crystals were filtered, washed with water and recrystallized from methanol-water to give the desired product (18.1 g, yield 64%).

m.p. 177.degree.-178.degree. C. Specific Rotation [.alpha.].sub.D.sup.20 +25.5.degree. (C=1, methanol).

In the same manner as in Example 21, using as the starting material the following compounds, each at 50 mmole, the following product compounds were produced. Example Starting         Yield M.P. No.  Material Product   (%) (.degree.C.) Specific Rotation __________________________________________________________________________ [.alpha.]  .sub.D .sup.20    (C =  1, methanol) 22   (s)-perillic N--[(s)-  44  109-110 -37.2.degree. acid   perilloyl]-D- phenylalanine 23   trans-4-n- N--(trans-4-n- 48  104-105 -8.8.degree. propylcyclo- propylcyclo- hexane hexylcarbonyl)- carboxylic D-phenylalanine acid 24   trans-4-n- N--(trans-4-n- 50  144-145 -7.5.degree. butylcyclo- butylcyclohexyl- hexane carbonyl)-D- carboxylic phenylalanine acid 25   4-tert-butyl- N--(4-t-butyl- 55  177-178 +51.5.degree. benzoic acid benzoyl)-D- phenylalanine 26   cuminic acid N--cumoyl-L- 63  121-123 [.alpha.] .sub.D.sup.23 -29.3.degree. (C = 1, methanol) phenylalanine 27   cyclopentane N--cyclopentyl- 40  115-117 [.alpha.] .sub.D.sup.23 +30.1.degree. (C = 1, methanol) carboxylic carbonyl-L- acid   phenylalanine 28   trans-4- N--(trans-4- 43  124-125 [.alpha.] .sub.D.sup.23 -11.5.degree. (C = 1, methanol) methyl-cyclo- methylcyclohexyl- hexane carbo- carbonyl)-D-phenyl- xylic acid alanine 29   trans-4-ethyl- N--(trans-4-ethyl- 53  96-97 [.alpha.] .sub.D.sup.23 -11.1.degree. (C = 1, methanol) cyclohexane cyclohexyl-carbo- carboxylic nyl)-D-phenylala- acid   nine 30   trans-4-t- N--(trans-4-t- 49  160-161 [.alpha.] .sub.D.sup.23 -9.0.degree. (C = 1, methanol) butyl-cyclo- butylcyclohexyl- hexane carbo- carbonyl)-D-phenyl- xylic acid alanine __________________________________________________________________________

Platinum oxide (500 mg) as a catalyst was suspended in acetic acid (50 ml) and cumic acid (10 g, 61 mmole) was added thereto. The mixture thus obtained was stirred vigorously for 2 hours at room temperature under a pressure of hydrogen 5 kg/cm.sup.2. The catalyst was removed by filtration, and the filtrate was concentrated under reduced pressure to a solid state. The resultant substance was distilled under reduced pressure of 1 mmHg (1.3.times.10.sup.-3 kg/cm.sup.2), at 113.degree.-116.degree. C. to obtain 4-isopropylcyclohexane carboxylic acid yielding 10 g (96%) ij a ratio of 3 parts of cis-form per 1 part of trans-form by weight.

To methanol (70 ml) at less than -20.degree. C., thionyl chloride (17 ml) was added dropwise, and the carboxylic acid (10 g) as obtained above was added. The mixture thus obtained was stirred for 15 hours at room temperature, and then concentrated under reduced pressure to a solid substance. The substance thus obtained was distilled under reduced pressure of 0.7 mmHg (9.2.times.10.sup.-4 kg/cm.sup.2) at 66.degree. C. to obtain 4-isopropylcyclohexane carboxylic acid methyl ester (9.5 g, yield 88%).

To the methyl ester (9.5 g) thus obtained, sodium hydride (120 mg) was added, and the mixture was heated at 150.degree. C. for 2 hours under a current of nitrogen gas. The reaction solution was cooled and then subjected to a reduced pressure distillation of 0.7 mmHg (9.2.times.10.sup.-4 kg/cm.sup.2) at 66.degree. C. to obtain 4-isopropylcyclohexane carboxylic acid methyl ester in a ratio of 6 parts trans-form per 1 part cis-form.

The methyl ester (9.0 g) thus obtained was dissolved in methanol (50 ml) and 1N aqueous sodium hydroxide solution (50 ml) was added thereto. The mixture thus obtained was stirred for 10 minutes at room temperature and made acidic with an additon of a dilute aqueous hydrochloric acid solution to precipitate crystals. The crystals were filtered, washed with water, and recrystallized from methanol-water to give trans-4-isopropylcyclohexane carboxylic acid (6.8 g, yield 78%).

After that, in the same manner as in Example 21, using as a starting material the carboxylic acid derivative (6.8 g, 40 mmole), N-(trans-4-isopropylcyclohexylcarbonyl)-D-phenylalanine was produced, and recrystallized from methanol-water to give the desired product (8.2 g, yield 65%).

m.p. 129.degree.-130.degree. C. Specific Rotation [.alpha.].sub.D.sup.20 -9.4.degree. (C=1, methanol).

In the same manner as in Example 26, using as the starting material the following compounds, each of 40 mmole, the following product compounds were produced. Example Starting         Yield M.P.  Specific Rotation No.  Material Product   (%) (.degree.C.) [.alpha.] .sub.D.sup.20 (C = 1, __________________________________________________________________________ methanol) 32   trans-4- 4-(trans-4- 52  137-138 +8.8.degree. isopropyl- isopropylcyclo- cyclohexane hexylcarbonyl)-D- carboxylic phenylalanine acid   methyl ester 33   trans-4- N--(trans-4- 56  130-131 +9.5.degree. isopropyl- isopropylcyclo- cyolohexane hexylcarbonyl)- carboxylic L-phenylalanine acid 34   trans-4- N--(trans-4- 66  134-135 -- isopropyl- isopropylcyclo- cyclohexane hexylcarbonyl)- carboxylic 2-phenylethylamine acid 35   trans-4- N--(trans-4- 58    129-130.5 +8.4.degree. isopropyl- isopropylcyclo- cyclohexane hexylcarbonyl)- carboxylic D-phenylalanine acid   benzylester __________________________________________________________________________

4-Isopropylcyclohexane carboxylic acid methyl ester (9.5 g) (cis-form:trans-form=3:1) was obtained in the same manner as in Example 26. Cis-4-isopropylcyclohexane carboxylic acid methyl ester (7 g) was obtained from the product thus obtained by a high performance liquid chromatography of YMC A-043 column using as the solvent a mixture of n-hexane and 1,2-dichloro ethane in a ratio of 75:25.

After that, in the same manner as in Example 21, using as a starting material the cis-form thus obtained (6.5 g, 38 mmole), N-(cis-4-isopropylcyclohexylcarbonyl)-D-phenylalanine was produced, and recrystallized from methanol-water to give the desired product (8 g, yield 66%).

m.p. 111.degree.-112.degree. C. Specific Rotation [.alpha.].sub.D.sup.20 -13.2.degree. (C=1, methanol).

ICR-CDI mice (Male, five weeks old, Body weight: 20 g) were abstained from food for 18 hours, and then used as test subjects.

The phenylalanine derivative of the present invention was suspended in 0.5% CMC-0.14M sodium chloride buffer solution (pH 7.4). The solution thus obtained was administered orally in fixed volume amounts to the test subjects. After a predetermined time, the percentage decrease of the blood glucose against the control group was determined. The results are shown in the following Table. Example Amounts used of sample Decrease in blood glucose No.    mg/kg body weight after 60 minutes (%) ______________________________________ 21      25              26 22     100              43 23     "                35 24     "                30 25     "                32 26     "                 0 27     "                 0 28        6.25          24 29     "                31 30     "                30 31        1.5           30 32        6.25          37 33     100              23 34     "                14 35      25              24 36     100              27 ______________________________________

It is clear from the foregoing that the D-phenylalanine derivatives as described above can be used as an antidiabetic drug for oral administration as well as the more usual parenteral administration.

## Claims

1. A D-phenylalanine derivative of the formula or a salt thereof or a precursor which can be converted into said D-phenylalanine derivative in vivo, wherein: ethyl, ispropyl, tert-butyl, ethene, or isopropene or cyclohexane substituted at the 4- or 5-position by methyl, ethyl, isopropyl, tert-butyl, ethene, or isopropene.

2. The D-phenylalanine derivative of claim 1, wherein R.sup.4 is said substituted cyclohexane.

3. The D-phenylalanine derivative of claim 1, wherein R.sup.4 is said substituted cyclohexane.

4. The D-phenylalanine derivative of claim 1, wherein the said derivative is N-(4-isopropylcyclohexylcarbonyl)-D-phenylalanine.

5. The D-phenylalanine derivative of claim 1, wherein the said derivative is N-(4-isopropylcyclohexylcarbonyl)-D-phenylalanine; N-[(S)-perilloyl]-D-phenylalanine; N-(4-methylcyclohexylcarbonyl)-D-phenylalanine; N-(4-ethylcyclohexylcarbonyl)-D-phhenylalanine; or N-(4-t-butylcyclohexylcarbonyl)-D-phenylalanine.

6. The D-phenylalanine derivataive of claim 1, wherein the said derivative is N-[(s)-perilloyl]-D-phenylalanine; N-(trans-4-methylcyclohexylcarbonyl)-D-phenylalanine; N-(trans-4-ethylcyclohexylcarbonyl)-D-phenylalanine; N-(trans-4-isopropylcyclohexylcarbonyl)-D-phenylalanine; or N-(trans-4-t-butylcyclohexylcarbonyl)-D-phenylalanine.

7. The D-phenylalanine derivative of claim 1, wherein R.sup.1 is hydrogen and R.sup.3 is hydrogen.

8. The D-phenylalanine derivative of claim 1, wherein R.sup.4 is perilloyl.

9. The D-phenylalanine derivative of claim 1, wherein said substituted cyclohexane is substituted at the 4-position.

10. The D-phenylalanine derivative of claim 1, wherein said substituted cyclohexane is substituted at the 5-position.

11. The D-phenylalanine derivative of claim 1, wherein said substituted cyclohexene is substituted at the 4-position.

12. The D-phenylalanine derivative of claim 1, wherein said substituted cyclohexene is substituted at the 5-positon.

13. The D-phenylalanine derivative of claim 1, wherein said substituted cyclohexane or said substituted cyclohexene is substituted with methyl, ethyl, isopropyl or tert-butyl.

14. The D-phenylalanine derivative of claim 1, wherein said substituted cyclohexane or said substituted cyclohexene is substituted by ethene, or isopropene.

15. A pharmaceutical composition, comprising a D-phenylalanine derivative of claim 1 and a pharmaceutical excipient.