Preparation process of .alpha.-aspartyl-L-phenylalanine methyl ester

.alpha.-L-Aspartyl-L-phenylalanine methyl ester (.alpha.-APM) is prepared by catalytic hydrogenation of N-benzyloxycarbonyl-.alpha.-L-aspartyl-L-phenylalanine methyl ester (Z-.alpha.-APM). The catalytic hydrogenation is conducted in an aqueous suspension containing the Z-.alpha.-APM in the form of particles whose average particle size is not greater than 800 .mu.m. The starting Z-.alpha.-APM may contain up to 30 wt. % of its .beta.-isomer provided that the process additionally includes recrystallization of the .alpha.-APM, collection of the .alpha.-APM by filtration and recycling of the filtrate for use in the aqueous suspension of Z-.alpha.-APM.

BACKGROUND OF THE INVENTION 
1) Field of the Invention 
This invention relates to a process for the preparation of 
.alpha.-L-aspartyl-L-phenylalanine methyl ester (hereinafter abbreviated 
as ".alpha.-APM"). 
.alpha.-APM is widely known as a dipeptide sweetener. It has good sweetness 
characteristics and a high degree of sweetness close to 200 times the 
sweetness of sucrose, whereby its demand as a dietetic sweetener is 
increasing considerably. 
2) Description of the Related Art 
.alpha.-APM is a dipeptide compound formed of L-aspartic acid and 
L-phenylalanine methyl ester. Although a number of processes have already 
been known, centering around chemical preparation processes, as to its 
preparation, the common process is to use an N-protected-L-aspartic 
anhydride and L-phenylalanine methyl ester as starting materials. 
For example, a process is known in which .alpha.-APM is obtained by 
reacting N-benzyloxycarbonyl-L-aspartic anhydride and a salt of 
L-phenylalanine methyl ester in an inert solvent containing a base in an 
amount at least equivalent to the salt of L-phenylalanine methyl ester, 
dissolving the thus-formed 
N-benzyloxycarbonyl-.alpha.-L-aspartyl-L-phenylalanine methyl ester 
(hereinafter as abbreviated as "Z-.alpha.-APM") as an alkali salt in 
water, acidifying the solution, extracting the solution with a 
water-immiscible organic solvent and then subjecting the extract to 
catalytic hydrogenation in methanol (U.S. Pat. 3,808,190). In this 
process, however, hydrolysis of Z-.alpha.-APM takes place due to the use 
of the acid and alkali upon the extraction so that 
.alpha.-L-aspartyl-L-phenylalanine (hereinafter abbreviated as 
".alpha.-AP" is byproduced subsequent to the catalytic hydrogenation. 
Further, Japanese Patent Publication No. 40071/1976 discloses a process for 
obtaining .alpha.-APM, in which Z-.alpha.-APM obtained by condensation of 
N-benzyloxycarbonylaspartic anhydride and L-phenylalanine methyl ester in 
an organic solvent is subjected to catalytic hydrogenation in acetic acid 
or a mixed solution of acetic acid and water as a solvent. This process, 
however, requires removal of acetic acid by distillation in order to 
isolate e-APM after the reduction. 
3-Benzyl-6-carboxymethyl-2,5-diketopiperazine (hereinafter abbreviated as 
"DKP") which does not have sweetness is however formed during the 
distillation, leading to a reduction in yield and deterioration in 
quality. 
Japanese Patent Publication No. 25537/1982 discloses a process for the 
preparation of .alpha.-APM, in which Z-.alpha.-APM is reduced using a 
platinum-group catalyst in the presence of an aqueous solution of a 
mineral acid and then neutralizing the aqueous solution of the resultant 
reaction product. This process, however, is accompanied by the 
byproduction of .alpha.-AP due to hydrolysis of the resulting .alpha.-APM 
by the mineral acid during the reduction and requires the step of 
neutralizing the aqueous solution with a base subsequent to the reduction. 
It is also impossible to avoid mixing of salts, which have been formed 
from such mineral acid and base, in .alpha.-APM so isolated, resulting in 
a reduction in the quality of Aspartame. 
Japanese Patent Publication No. 25538/1982 discloses a process for the 
preparation of .alpha.-APM, in which N-benzyloxycarbonylaspartic anhydride 
and L-phenylalanine methyl ester are reacted in an organic aliphatic 
solvent, the resultant Z-.alpha.-APM is, either after isolation or without 
isolation, subjected to catalytic hydrogenation in the presence of at 
least one catalyst selected from the group consisting of iron-group 
catalysts and platinum-group catalysts, resulting .alpha.-APM is dissolved 
in an aqueous solution of a mineral acid, and the solution is then 
neutralized. The Z-.alpha.-APM so obtained is, however, in a solid form 
unsuited for reduction, and it is difficult to grind the same. This 
process is accompanied by the additional drawbacks that, like the 
above-described process due to the use of the aqueous solution of the 
mineral acid, .alpha.-APM is hydrolyzed to byproduce .alpha.-AP having no 
sweetness and inorganic salts are undesirably mixed in the .alpha.-APM so 
purified. 
In any of the conventional art described above, use of 
N-benzyloxycarbonyl-L-aspartic acid as a starting material makes it 
impossible to avoid byproduction of .beta.-APM besides .alpha.-APM as the 
target product. This .beta.-APM does not have sweetening effects but 
conversely gives bitterness so that its inclusion lowers the commercial 
value of .alpha.-APM. 
As a process for isolating .alpha.-APM from such a mixture of .alpha.-APM 
and .beta.-APM, Japanese Patent Publication No. 6305/1974 discloses a 
process in which .alpha.-APM and .beta.-APM are brought into contact with 
.beta.-resorcylic acid in an aqueous medium to convert .alpha.-APM into a 
sparinglysoluble addition product so that .alpha.-APM is separated from 
.beta.-APM as an impurity. Although this process can separate .alpha.-APM 
from the impurity contained in a large amount, it requires cumbersome 
operation due to the use of .beta.-resorcylic acid in the same amount as 
.alpha.-APM and .beta.-APM and the recrystallization of the .alpha.-APM 
addition product from water subsequent to its isolation from a dilute 
aqueous solution thereof and, moreover, it is economically disadvantageous 
because the recovery rate of the expensive .alpha.-APM is low. 
On the other hand, Japanese Patent Publication No. 41425/1974 discloses a 
process in which .alpha.-APM containing .beta.-APM is brought into contact 
with a hydrohalogenic acid in an aqueous medium to form the 
sparingly-soluble hydrohalide of .alpha.-APM, thereby separating 
.beta.-APM copresent as an impurity. This separation process which is 
conducted using an aqueous solution of the hydrohalogenic acid in an 
excess amount is good in separating .beta.-APM from .alpha.-APM in which 
the .beta.-APM is contained. It is, however, accompanied by such drawbacks 
that, because of the dissolution in the aqueous solution of the 
hydrohalogenic acid, hydrolysis of the methyl ester of .alpha.-APM tends 
to proceed, the recovery rate of the hydrohalide of .alpha.-APM is low and 
an expensive acid-resistant material must be used as a material for a 
reactor. 
To obtain .alpha.-APM from a mineral acid salt of .alpha.-APM once isolated 
as an acid addition product as described above, a neutralization step is 
needed. This neutralization is generally conducted by dissolving the 
mineral acid salt of .alpha.-APM in water, adding a base to the solution 
to neutralize the same and then separating .alpha.-APM formed as crystals. 
Since .alpha.-APM is lost in a substantial amount in the aqueous solution, 
the yield becomes low. The filtrate contains a large amount of salts 
formed from the mineral acid and the base, so that it is difficult to use 
it again in the preceding step. As .alpha.-APM isolated in this manner 
contains salts in a large amount, operations such as recrystallization and 
desalting are needed to obtain the final product so that the yield is 
lowered further. 
As has been described above, the previously-known preparation processes of 
.alpha.-APM are accompanied by one. or more drawbacks and are not fully 
satisfactory as industrial preparation processes. To solve the problems in 
the conventional reducing steps of Z-.alpha.-APM, in particular, it is 
desired to conduct a reducing reaction in an aqueous medium. However, no 
process has heretofore been available to efficiently obtain an aqueous 
Z-.alpha.-APM suspension suited for reduction in such an aqueous solvent. 
Moreover, no process has been found for obtaining .alpha.-APM with high 
purity in high yield upon isolation of .alpha.-APM subsequent to catalytic 
reduction of Z-.alpha.-APM containing Z-.beta.-APM. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a process for the 
industrial and efficient preparation of .alpha.-APM having low impurity 
content in high yield. 
The present inventors have proceeded with an extensive investigation in 
order to overcome the problems described above. As a result, it has been 
found that the use of an aqueous suspension of Z-.alpha.-APM in the form 
of fine particles upon preparation of .alpha.-APM by catalytic reduction 
of Z-.alpha.-APM in an aqueous solvent allows the reaction to proceed 
quick and moreover to reduce the formation of byproducts, especially 
L-aspartyl-L-aspartyl-L-phenylalanine methyl ester having 4 isomers 
because of combinations of .alpha. and .beta.. It has also been found 
that, where Z-.alpha.-APM containing 30 wt. % or less of Z-.beta.-APM is 
used as a raw material, the ratio of the .alpha.-isomer to the 
.beta.-isomer (.alpha./.beta. ratio) after catalytic hydrogenation and the 
isolation yield .alpha.-APM can be both improved when an aqueous solution 
of Z-.alpha.-APM is reduced in the presence of a platinum-group catalyst, 
the catalyst is removed at a temperature at which .alpha.-APM so formed is 
completely dissolved, the filtrate is cooled to a temperature at which 
.beta.-APM does not crystallize out, .alpha.-APM so crystallized is 
collected and then recrystallized from an aqueous solution, and the 
aqueous solution separated in the recrystallization step and containing 
.alpha.-APM is recycled for use in the aqueous suspension of 
Z-.alpha.-APM, leading to the completion of the present invention. 
According to the process of this invention, an aqueous solution of 
.alpha.-APM can be obtained in a high yield and a short reaction time by 
the reducing reaction of Z-.alpha.-APM. Further, .alpha.-APM can be 
obtained by simply cooling the reaction mixture subsequent to elimination 
of the catalyst and, if necessary, toluene therefrom. Its 
recrystallization can provide .alpha.-APM with high purity. By reutilizing 
an aqueous solution, which is separated in the recrystallization step and. 
contains .alpha.-APM, in the reducing step of Z-.alpha.-APM, the ratio of 
.alpha.-APM to .beta.-APM after the reduction becomes greater than the 
.alpha./.beta. ratio of the starting Z-APM so that a high yield can be 
achieved upon crystallization and separation of .alpha.-APM under 
conditions not permitting crystallization of .beta.-APM in the 
crystallizing step. In addition, the aqueous solution separated from the 
recrystallization step can be used again without the need for processing 
it through such steps as heating and concentration, so that impurities 
such as DKP and .alpha.-AP are not formed. Moreover, a-APM can be obtained 
from Z-APM without using any mineral acid. No neutralization step is 
therefore needed. .alpha.-APM so prepared, therefore, does not contain any 
salt which would otherwise be formed from the mineral acid and a base. As 
has been described above, the processes of the present invention can be 
employed industrially for the efficient preparation of high-purity 
.alpha.-APM substantially free of impurities.

DETAILED DESCRIPTION OF THE INVENTION 
In a first aspect of this invention, there is thus provided a process for 
the preparation of .alpha.-L-aspartyl-L-phenylalanine methyl ester by 
catalytic hydrogenation of 
N-benzyloxycarbonyl-a-L-aspartyl-L-phenylalanine methyl ester, which 
comprises conducting said catalytic hydrogenation in an aqueous suspension 
containing the N-benzyloxycarbonyl-a-L-aspartyl-L-phenylalanine methyl 
ester in the form of particles whose average particle size is not greater 
than 800 .mu.m. 
In a second aspect of this invention, there is also provided a process for 
the preparation of .alpha.-L-aspartyl-L-phenylalanine methyl ester, which 
comprises reducing an aqueous suspension of 
N-benzyloxycarbonyl-a-L-aspartyl-L-phenylalanine methyl ester, said ester 
containing not more than 30 wt. % of 
N-benzyloxy-.beta.-L-aspartyl-L-phenylalanine methyl ester, with hydrogen 
in the presence of a platinum-group catalyst, filtering off the catalyst, 
cooling the filtrate to a temperature at which 
.beta.-L-aspartyl-L-phenylalanine methyl ester does not crystallize out, 
collecting .alpha.-L-aspartyl-L-phenylalanine methyl ester so collected, 
dissolving the thus-collected filtrate in an aqueous solvent at an 
elevated temperature, cooling the resulting solution, collecting 
crystallized .alpha.-L-aspartyl-L-phenylalanine methyl ester and washing 
the same to obtain purified .alpha.-L-aspartyl-L-phenylalanine methyl 
ester, and recycling the aqueous solution, which has been separated in the 
purification step and contains .alpha.-L-aspartyl-L-phenylalanine methyl 
ester, for use in the aqueous suspension of Z-.alpha.-APM. . 
A primary feature of the first aspect of this invention resides in the use 
of an aqueous suspension of Z-.alpha.-APM in the form of fine particles 
upon catalytic hydrogenation of Z-.alpha.-APM in an aqueous solvent. The 
raw material, Z-.alpha.-APM, is obtained usually by reacting 
N-benzyloxycarbonylaspartic anhydride (hereinafter abbreviated as "Z-Asp 
anhydride") with L-phenylalanine methyl ester (hereinafter abbreviated as 
"L-PM") in an organic solvent and contains a small amount of unreacted 
Z-Asp anhydride. When this Z-.alpha.-APM is subjected to catalytic 
hydrogenation in an organic solvent miscible with water, such as acetic 
acid, the resulting .alpha.-APM reacts with the Z-Asp anhydride, followed 
by catalytic hydrogenation so that two isomers, 
.alpha.-L-aspartyl-.alpha.-L-aspartyl-L-phenylalanine methyl ester and 
.beta.-L-aspartyl-.alpha.-L-aspartyl-L-phenylalanine methyl ester, are 
formed. Where the raw material, Z-.alpha.-APM, contains Z-.beta.-APM, four 
isomers (hereinafter collectively abbreviated as "A.sub.2 PM") are formed 
because of combinations of .alpha. and .beta.. It has, however, been found 
that the production of A.sub.2 PM is reduced when this reducing reaction 
is conducted in an aqueous solvent. If an aqueous suspension of 
Z-.alpha.-APM having a rather large particle size is employed, its effects 
are small. The use of fine Z-.alpha.-APM can significantly lower the 
production of A.sub.2 PM. 
Z-.alpha.-APM employed in the process according to the first aspect of the 
present invention may contain Z-.beta.-APM as an impurity. 
According to the second aspect of the present invention, 
(a) Z-.alpha.-APM containing not more than 30 wt. % of Z-.beta.-APM is 
suspended in an aqueous solvent, followed by catalytic hydrogenation in 
the presence of a platinum-group catalyst, 
(b) the catalyst is removed from the reaction mixture, which has been 
obtained in step (a), at such a temperature that the resulting .alpha.-APM 
is dissolved in toto and, if necessary, byproduced toluene is removed by 
phase separation, 
(c) an aqueous solution obtained in step (b) is cooled to such a 
temperature that .beta.-APM does not crystallize out, and crystallized 
.alpha.-APM is subjected to solid-liquid separation, thereby obtaining 
crude APM, 
(d) the crude APM obtained in step (c) is dissolved at an elevated 
temperature in an aqueous solvent and then cooled to crystallize 
.alpha.-APM, and the .alpha.-APM so crystallized is subjected to 
solid-liquid separation, followed by washing to obtain purified APM, and 
(e) the fraction of an aqueous solution obtained by the solid-liquid 
separation and washing in step (d) and containing .alpha.-APM is 
reutilized as the aqueous solvent in step (a). 
A primary feature of the second aspect of the present invention resides in 
the reutilization of the fraction of the aqueous solution, which has been 
separated in step (d) and contains .alpha.-APM, as the aqueous solvent in 
the catalytic hydrogenation step (a). 
In the fraction of the aqueous solution separated in the recrystallization 
step, .alpha.-APM is contained, for example, in an amount of about 0.6 g 
per 100 g of the filtrate or washing when recrystallized at 5.degree. C. 
although the amount of .alpha.-APM varies depending on the temperature of 
recrystallization and the amount of a washing solvent. The aqueous 
solution fraction also contains .beta.-APM or DKP which is not suitable as 
a sweetener. It is however not desirable to discard .beta.-APM or DKP as 
it is, because its discard results in a reduction in yield. It is 
therefore necessary to recycle it to a preceding step. 
It is technically possible to recirculate the fraction of the aqueous 
solution, which is separated in step (d), as it is within step (d). 
According to this method, however, .beta.-AMP, DKP and the like accumulate 
in the recycled aqueous solution so that the final product is mixed with 
these impurities. 
When the fraction of the aqueous solution containing .alpha.-APM, said 
fraction having been separated in step (d), is recycled to the reducing 
step of Z-APM in step (a) as in the present invention, byproducts such as 
.beta.-APM and DKP are excluded in the solution separated upon isolation 
of the crude APM in step (c) and is separately hydrolyzed to collect 
L-phenylalanine and L-aspartic acid. These byproducts therefore do not 
give any substantial effects to the next step, i.e., step (d), whereby 
they do not accumulate in the solution recycled to and reused in step (a). 
Accordingly, the purified APM obtained from step (d) is isolated from a 
low-impurity crude concentrate under the same conditions every time so 
that the purified APM has stable high-purity quality. Since .alpha.-APM is 
contained in the solution recycled from step (d), the ratio of .alpha.-APM 
to .beta.-APM in the solution after the reducing reaction in step (a) is 
greater than the ratio of Z-.alpha.-APM to Z-.beta.-APM in the raw 
material so that the efficiency of separation between .alpha.-APM and 
.beta.-APM in step (c) becomes high. The overall yield of .alpha.-APM 
based on Z-.alpha.-APM during steps (a) to (d) becomes higher. 
Any Z-.beta.-APM-containing Z-.alpha.-APM can be used in the second aspect 
of this invention irrespective of its synthesis process. Z-.alpha.-APM 
containing Z-.beta.-APM in an amount not greater than 30 wt. % can however 
be employed effectively for the following reasons. Since .alpha.-APM and 
.beta.-APM have substantially the same solubility in water, inclusion of 
Z-.beta.-APM in an amount greater than 30 wt. % results in low yield of 
.alpha.-APM even when the filtrate is cooled, after removal of a reducing 
catalyst by filtration, to such a temperature that .beta.-APM does not 
crystallize out and crystallized .alpha.-APM is isolated. A large amount 
of .alpha.-APM is therefore lost together with .beta.-APM in the filtrate 
so that the efficiency is impaired. 
In both the first and second aspects of the present invention, the starting 
material, i.e., Z-.alpha.-APM may contain an organic solvent in a small 
amount insofar as the organic solvent does not inhibit crystallization of 
.alpha.-APM in steps (c) and (d). Specific examples of the organic solvent 
include lower aliphatic alcohols such as methyl alcohol, ethyl alcohol, 
n-propyl alcohol and isopropyl alcohol; ethers such as tetrahydrofuran and 
dioxane; nitriles such as acetonitrile and propionitrile; organic 
carboxylic acids such as formic acid, acetic acid and propionic acid; 
aromatic hydrocarbons such as benzene, toluene and xylene; and chlorinated 
hydrocarbons such as dichloromethane and 1,2-dichloroethane. 
The Z-.alpha.-APM suspension employed in the process of this invention can 
be prepared by mixing water with a solution of Z-.alpha.-APM in an organic 
solvent. 
(1) If an organic solvent is miscible with water, it is only necessary to 
separate crystallized Z-.alpha.-APM by filtration or the like and then to 
suspend it in water. Examples of such an organic solvent include organic 
carboxylic acids such as acetic acid and propionic acid; amides such as 
dimethylformamide, dimethylacetamide and N,N'-dimethylimidazolidinone; and 
ketones such as acetone, methyl ethyl ketone and methyl isopropyl ketone. 
To obtain fine particulate Z-.alpha.-APM in the process described above, it 
is preferable to add water to an organic solvent containing Z-.alpha.-APM 
upon mixing water with the organic solvent containing Z-.alpha.-APM. Such 
mixing is performed generally in a crystallizer equipped with a stirrer. 
It is preferred to increasing the stirring velocity because the particle 
system of Z-.alpha.-APM becomes smaller as the stirring speed is fast 
during the mixing. The mixing is usually conducted at 0.1 m/sec or faster, 
preferably at 0.2 m/sec or higher in terms of the end linear speed of a 
stirring blade. 
(2) Even if an organic solvent is not miscible with water, it can still be 
used in the process of this invention provided that it forms an azeotrope 
with water or it has a boiling point lower than water. Namely, an aqueous 
suspension can be prepared by distilling off the organic solvent after 
mixing the organic solvent, in which Z-.alpha.-APM is contained, with 
water. Examples of such an organic solvent include hydrocarbons such as 
benzene, toluene and n-hexane; and esters such as ethyl acetate, butyl 
acetate, amyl acetate, methyl propionate and ethyl propionate. 
Although no particular limitation is imposed on the concentration of 
Z-.alpha.-APM in an organic solvent in the process described above, the 
concentration may generally range from 5% to 30%. When the reaction 
between N-benzyloxycarbonyl-L-aspartic anhydride and L-phenylalanine 
methyl ester is conducted in the above-described organic solvent, the 
reaction mixture can be used as it is. 
When an organic solvent immiscible with water is used, it is possible to 
prepare a solution of Z-.alpha.-APM in the organic solvent, to mix the 
solution at once with water and then to distill off the organic solvent. 
According to this method, precipitated Z-.alpha.-APM crystals tend to 
stick together so that the particle size tends to become greater. To 
obtain an aqueous suspension of fine particulate Z-.alpha.-APM, the 
organic solvent is distilled off while adding the organic solvent solution 
dropwise into water. This method makes it possible to obtain a suspension 
containing Z-.alpha.-APM having a small, uniform particle size, whereby 
the reaction velocity in the subsequent reducing step becomes higher and 
the formation of byproducts is substantially reduced. 
Distillation of the organic solvent is conducted at 60.degree. C. or lower. 
If the temperature is higher than 60.degree. C., crystallized 
Z-.alpha.-APM stick together so that the particle size increases. As a 
result, the velocity of the subsequent reducing reaction is extremely 
retarded, resulting in the formation of byproducts. The particle size of 
Z-.alpha.-APM in an aqueous Z-.alpha.-APM suspension in the reducing 
reaction is usually 10-800 .mu.m. The smaller the particle size, the 
faster the reducing reaction. The shorter the reaction time, the smaller 
the production of impurities such as .alpha.-AP and DKP and the higher the 
yield of .alpha.-APM. Preferably, a suspension of Z-.alpha.-APM in an 
average particle size not greater than 200 .mu.m is used for reduction. 
The aqueous solvent in which the catalytic hydrogenation is performed in 
accordance with this invention is either water or a mixture of water and a 
lower alcohol. Illustrative of the lower alcohol include methanol, ethanol 
and propanol, with methanol being particularly preferred. This aqueous 
solvent may contain one or more other organic solvent in a small amount. 
Examples of catalysts usable in the reduction include platinum-group 
catalysts such as palladium, platinum, cobalt, nickel, ruthenium and 
rhodium. Of these, palladium is particularly suited. For example, 
palladium-carbon is preferred. Although no particular limitation is 
imposed on the amount of the catalyst, it is preferable to use the 
catalyst in an amount of 0.5-10 wt. % based on Z-APM. 
The reduction can be carried out either under normal pressure or elevated 
pressure. 
The reducing temperature in the present invention is 80.degree. C. or 
lower, preferably 40.degree.-60.degree. C. As the reducing time, 2-10 
hours are generally sufficient although it varies depending on the 
temperature. 
Although no particular limitation is imposed on 10 the concentration of 
Z-.alpha.-APM in the aqueous suspension in the process of this invention, 
it may generally range from about 3% to about 20%. If the concentration of 
a suspension so prepared exceeds 20%, such a high concentration is not 
preferred because stirring of the suspension becomes difficult and the 
particle size of Z-.alpha.-APM becomes greater. Concentrations lower than 
about 3% are, however, not economical because the volume efficiency is 
low. If the concentration is high, .alpha.-APM formed subsequent to 
reduction is not completely dissolved but takes the form of a slurry so 
that the catalyst cannot be filtered off as it is. The reducing reaction 
can however be brought to completion even in such a state. Even in such a 
case, the catalyst can still be filtered off by adding a solvent or 
raising the temperature of the reaction mixture and dissolving 
.alpha.-APM. 
The temperature at which the catalyst is filtered off subsequent to the 
reduction is not lower than the temperature at which the resulting 
.alpha.-APM and the like are completely dissolved. At 80.degree. C. or 
higher, .alpha.-L-aspartyl-L-phenylalanine and DKP are formed by the 
hydrolysis of .alpha.-APM and the like and intramolecular cyclization, 
respectively, whereby the isolation yield of .alpha.-APM is lowered. The 
catalyst is filtered off preferably at 40.degree.-60.degree. C. The 
concentration of .alpha.-APM upon filtering off the catalyst is preferably 
near that of the saturated solution of .alpha.-APM at the temperature. The 
concentration can be about 2-4 wt. % at the temperature of 
40.degree.-60.degree. C. At concentrations significantly lower than the 
above concentration, less crystals can be crystallized out upon cooling so 
that the yield drops. 
The toluene formed by the removal of the benzyloxycarbonyl group is usually 
removed by phase separation subsequent to removal of the catalyst, 
although the toluene can be eliminated by causing it to evaporate during 
or after the reducing reaction. 
After the catalyst is filtered off and, if necessary, toluene is separated, 
the filtrate can be cooled to collect crystals of crude .alpha.-APM. No 
particular limitation is imposed on the cooling means. When indirect 
cooling is applied, the cooling can be effected either by 
forced-convection heat transfer including mechanical agitation or by 
conduction heat transfer. As a direct cooling method, the solvent can be 
caused to evaporate under reduced pressure conditions so that cooling can 
be effected relying upon its latent heat of evaporation. 
Any crystallizing temperature can be employed as long as it is higher than 
a predetermined temperature at which .beta.-APM becomes saturated. It is 
however desirable to conduct the crystallization at a temperature as low 
as possible so that the yield can be increased. 
As a method for subjecting the precipitated crystals of crude .alpha.-APM 
to solid-liquid separation, a conventional method such as filtration or 
centrifugal separation can be used. 
As a method for purifying the thus-obtained crude .alpha.-APM by 
recrystallizing it from an aqueous solvent, the crude .alpha.-APM is 
dissolved to a concentration of 2-4 wt. % at a temperature not higher than 
80.degree. C., preferably at 40.degree.-60.degree. C. The resultant 
solution is cooled to 5.degree. C. or lower. Crystals of .alpha.-APM so 
precipitated are collected by filtration and then washed, whereby 
.alpha.-APM completely free of .beta.-APM is isolated. An aqueous solution 
of .alpha.-APM separated here is recycled and reused in the reduction of 
Z-APM in step (a). The combined aqueous solution of the filtrate and the 
washing generally has the following composition: about 0.6 wt.% 
.alpha.-APM, about 0.065 wt. % .beta.-APM, about 0.01 wt. % DKP, and about 
0.01 wt. % .alpha.-AP. The reutilization of the aqueous solution in the 
reducing step of Z-APM as described above makes it possible to render the 
ratio of .alpha.-APM to .beta.-APM in the reducing step higher than the 
.alpha./.beta. ratio of the starting Z-APM, so that the crystallization 
and separation of the .alpha. isomer can be facilitated. 
The processes of the present invention will hereinafter be described in 
detail by the following examples. 
REFERENTIAL EXAMPLE 1 
A solution of L-phenylalanine methyl ester (358.4 g) in acetic acid (658.8 
g) and a solution of N-benzyloxycarbonylaspartic anhydride (505.9 g) in 
acetic acid (4382 g) were reacted at 15.degree.-20.degree. C. for 3 hours, 
followed by the concentration of the resultant reaction mixture to 1813 g. 
The concentrate was added dropwise at 25.degree. C. over 30 minutes into 
water (3530 g) under stirring at 300 rpm in a 10l reactor which was 
equipped with a stirring blade of 15 cm in span. A mixture of 
Z-.alpha.-APM and Z-.beta.-APM so crystallized was collected by filtration 
and dried, whereby Z-APM crystals (856.9 g) were obtained. As a result of 
an analysis by high performance liquid chromatography (HLC), the crystals 
were found to contain Z-.alpha.-APM (658.1 g) and Z-.beta.-APM (164.5 g). 
EXAMPLE 1 
A solution of Z-APM (100 g), which had been obtained in Referential Example 
1 and contained Z-.alpha.-APM (76.8 g, 0.179 mol), in acetic acid (137.17 
g) was poured into water (369.5 g) under stirring by a stirring blade 
having a 10 cm span and driven at 400 rpm. Crystallized Z-.alpha.-APM was 
collected by filtration. To the thus-obtained wet cake (201.0 g) which 
contained Z-.alpha.-APM (72.40 g) having an average particle size of 90 
.mu.m, water (1348.4 g) was added. 5% Palladium carbon . (50% wet, 2.93 g) 
was added, followed by catalytic hydrogenation at 60.degree. C. The 
reaction was completed in 3 hours. The solution obtained subsequent to the 
removal of the catalyst by filtration was found to contain .alpha.-APM 
(48.74 g, 0.1656 mol), DKP (0.30 g), .alpha.-AP (0.21 g) and A.sub.2 PM 
(0.0146 g). 
EXAMPLE 2 
A solution of Z-APM (100 g), which had been obtained in Referential Example 
1 and contained Z-.alpha.-APM (76.8 g, 0.179 mol), in acetic acid (137.17 
g) was poured into water (369.53 g) under stirring by a stirring blade 
having a 10 cm span and driven at 200 rpm. Crystallized Z-.alpha.-APM was 
collected by filtration. To the thus-obtained wet cake (201.0 g) which 
contained Z-.alpha.-APM (72.4 g) having an average particle size of 600 
.mu.m, water (1348.4 g) was added. 5% Palladium carbon (50% wet, 2.93 g) 
was added, followed by catalytic hydrogenation at 60.degree. C. The 
reaction was completed in 3 hours. The solution obtained subsequent to the 
removal of the catalyst by filtration was found to contain .alpha.-APM 
(48.74 g, 0.1656 mol), DKP (0.30 g), .alpha.-AP (0.21 g) and A.sub.2 PM 
(0.0054 g). 
COMATIVE EXAMPLE 1 
A solution of Z-APM (100 g), which had been obtained in Referential Example 
1 and contained Z-.alpha.-APM, (76.8 g, 0.179 mol), in acetic acid (137.17 
g) was poured into water (369.53 g) under stirring by a stirring blade 
having a 10 cm span and driven at 400 rpm. Crystallized Z-.alpha.-APM was 
collected by filtration. To the thus-obtained wet cake (200.97 g) which 
contained Z-.alpha.-APM (72.40 g) having an average particle size of 3000 
.mu.m, water (1348.4 g) was added. 5% Palladium carbon (50% wet, 2.93 g) 
was added, followed by catalytic hydrogenation at 60.degree. C. for 5 
hours. The reaction was, however, not brought to completion. The filtrate 
obtained subsequent to the removal of the catalyst by filtration was found 
to contain Z-.alpha.-APM (31.9 g, 0.07453 mol), .alpha.-APM (27.26 g, 
0.09263 mol), DKP (0.98 g), .alpha.-AP (2.08 g) and A.sub.2 PM (0.0054 g). 
EXAMPLE 3 
A solution of L-phenylalanine methyl ester (17.83 g) in 1,2-dichloroethane 
(55.72 g, hereinafter abbreviated as "EDC") and a solution of 
N-benzyloxycarbonylaspartic anhydride (26.0 g) in EDC (370.5 g) were 
condensed at 15.degree.-20.degree. C. for 3 hours. A solution of the 
thus-obtained Z-.alpha.-APM (35.37 g) and Z-.beta.-APM (7.25 g) in EDC 
(426.21 g) was added dropwise to water (749.37 g) at 40.degree. C. under 
reduced pressure over 1 hour while the EDC was distilled off, so that a 
suspension (777.7 g) was obtained. Z-.alpha.-APM and Z-.beta.-APM were 
both found to have an average particle size of 120 .mu.m. 
EXAMPLE 4 
A solution of Z-.alpha.-APM (40.96 g), which had been obtained in 
Referential Example 1, in ethyl acetate (426.21 g) was added dropwise to 
water (749.37 g) at 60.degree. C. under reduced pressure over 1 hour while 
the ethyl acetate was distilled off, so that a suspension (725.2 g) was 
obtained. The average particle size of Z-.alpha.-APM was found to be 170 
.mu.m. 
EXAMPLE 5 
A solution of Z-.alpha.-APM (40.96 g), which had been obtained in 
Referential Example 1, in chloroform (426.21 g) was maintained at 
40.degree. C. under reduced pressure, to which water (749.47 g) was added 
dropwise over 1 hour while the chloroform was distilled off at 60.degree. 
C. A suspension (750 g) was obtained. The average particle size of 
Z-.alpha.-APM was found to be 650 .mu.m. 
EXAMPLE 6 
A solution of Z-.alpha.-APM (48.33 g, 0.1128 mol), which had been obtained 
in Referential Example 1, in butyl acetate (576.0 g) was added to water 
(780 g). The butyl acetate was then distilled off under reduced pressure 
at 45.degree. C. for 1 hour, whereby a suspension (796 g) was obtained. 
The average particle size of Z-.alpha.-APM was found to be 220 .mu.m. 5% 
Palladium carbon (50% wet, 2.87 g) was thereafter added, followed by 
catalytic hydrogenation at 60.degree. C. The reaction was completed in 3 
hours. The solution obtained subsequent to the removal of the catalyst by 
filtration was found to contain .alpha.-APM (31.56 g), DKP (0.51 g), 
.alpha.-AP (0.63 g) and A.sub.2 PM (0.025 g). 
EXAMPLE 7 
A solution of Z-.alpha.-APM (23.33 g, 0.05446 mol), which had been obtained 
in Referential Example 1, in EDC (221.74 g) was added dropwise to water 
(377.8 g) at 40.degree. C. under reduced pressure over 1 hour while the 
EDC was distilled off, so that a suspension (403.5 g) was obtained. The 
average particle size of Z-.alpha.-APM was found to be 110 .mu.m. Water 
(131.8 g) was then added and 5% palladium carbon (50% wet, 1.08 g) was 
also added, followed by catalytic hydrogenation at 60.degree. C. The 
reaction was completed in 3 hours. A solution obtained subsequent to the 
removal of the catalyst by filtration was found to contain .alpha.-APM 
(15.50 g, 0.05267 mol), DKP (0.17 g), .alpha.-AP (0.32 g) and A.sub.2 PM 
(0.005 g). 
COMATIVE EXAMPLE 2 
A solution of Z-.alpha.-APM (40.96 g, 0.09561 mol) in EDC (426.21 g) was 
added dropwise to water (749.37 g) at 80.degree. C. under reduced pressure 
over 1 hour while the EDC was distilled off, so that a suspension (649.7 
g) was obtained. The average particle size of Z-.alpha.-APM was found to 
be 1200 .mu.m. Water (402.3 g) was then added and 5% palladium carbon (50% 
wet, 2.15 g) was added further, followed by catalytic reduction at 
80.degree. C. for 6 hours. The reaction was, however, not brought to 
completion. The solution obtained subsequent to the removal of the 
catalyst by filtration was found to contain Z-.alpha.-APM (7.84 g, 0.01830 
mol), .alpha.-APM (16.47 g, 0.05596 mol), DKP (3.97 g), .alpha.-AP (0.53 
g) and A.sub.2 PM (0.09 g). 
EXAMPLE 8 
A mixture of Z-.beta.-APM (8.6 g) and Z-.alpha.-APM (34.2 g) was suspended 
in water (610 g), to which 5% palladium-carbon (0.9 g) was added. After 
the resultant mixture was subjected to catalytic reduction under normal 
pressure at 60.degree. C. for 2 hours, the catalyst was filtered off at 
the same temperature. After the toluene layer was separated, the water 
layer was gradually cooled and then stirred at 5.degree. C. for 1 hour. At 
the same temperature, precipitated crystals were collected by filtration 
and then washed so that a wet .alpha.-APM cake (64.0 g) was isolated. 
The thus-isolated wet .alpha.-APM cake was added with water (472.7 g) and 
dissolved in the latter at 60.degree. C. The solution so obtained was 
gradually cooled and then stirred at 5.degree. C. for 1 hour. At the same 
temperature, precipitated crystals were collected by filtration, washed 
with water and then dried, whereby .alpha.-APM (15.6 g) was isolated. At 
the same time, a filtrate-washing mixture (537.5 g) containing .alpha.-APM 
(3.7 g) and .beta.-APM (0.4 g) was also obtained. 
The crystals so obtained was analyzed by high performance liquid 
chromatography. As a result, it were found that the content of .alpha.-APM 
was 15.1 g (64.0% based on Z-.alpha.-APM) and .beta.-APM was not contained 
at all. 
A mixture of Z-.beta.-APM (8.6 g) and Z-.alpha.-APM (34.2 g) was suspended 
in a mixture of the recrystallization filtrate-washing mixture (528 g) and 
water (227 g), to which 5% palladium-carbon (0.9 g) was added. After the 
resultant mixture was subjected to catalytic reduction under normal 
pressure at 60.degree. C. for 2 hours, the catalyst was filtered off at 
the same temperature and the toluene layer was then separated. The water 
layer was gradually cooled to 5.degree. C. and, at the same temperature, 
precipitated crystals were collected by filtration and then washed, 
whereby a wet .alpha.-APM cake (75.1 g) was isolated. 
The thus-isolated wet .alpha.-APM cake was added with water (529.4 g) and 
dissolved in the latter at 60.degree. C. The solution so obtained was 
gradually cooled and then stirred at 5.degree. C. for 1 hour. At the same 
temperature, precipitated crystals were collected by filtration, washed 
with water and then dried, whereby .alpha.-APM (18.4 g) was isolated. At 
the same time, a filtrate-washing mixture (623.2 g) containing .alpha.-APM 
(4.1 g) and .beta.-APM (0.4 g) was also obtained. 
The crystals so obtained were analyzed by high performance liquid 
chromatography. As a result, it was found that the content of .alpha.-APM 
was 17.9 g (76.2% based on Z-.alpha.-APM) and .beta.-APM was not contained 
at all. In addition, the contents of Cl, SO.sub.4 and Na ions were all 
found to be 10 ppm or less. 
EXAMPLE 9 
A mixture of Z-.beta.-APM (8.6 g) and Z-.alpha.-APM (34.2 g) was suspended 
in a mixture of the recrystallization filtrate-washing mixture (610.7 g), 
which had been obtained in Example 8, and water (117.1 g), to which 5% 
palladium-carbon (0.9 g) was added. After the resultant mixture was 
subjected to catalytic reduction under normal pressure at 40.degree. C. 
for 3 hours, precipitated crystals were dissolved at 60.degree. C., the 
catalyst was filtered off at the same temperature and the toluene layer 
was then separated. The water layer was gradually cooled to 5.degree. C. 
and stirred for 1 hour and, at the same temperature, precipitated crystals 
were collected by filtration and then washed, whereby a wet .alpha.-APM 
cake (77.1 g) was isolated. 
The thus-isolated wet .alpha.-APM cake was added with water (539.5 g) and 
dissolved in the latter at 60.degree. C. The solution so obtained was 
gradually cooled and then stirred at 5.degree. C. for 1 hour. At the same 
temperature, precipitated crystals were collected by filtration, washed 
with water and then dried, whereby .alpha.-APM (18.5 g) was isolated. At 
the same time, a filtrate-washing mixture (635.8 g) containing .alpha.-APM 
(4.2 g) and .beta.-APM (0.4 g) were also obtained. 
The crystals so obtained was analyzed by high performance liquid 
chromatography. As a result, it was found that the content of .alpha.-APM 
was 17.9 g (76.2% based on Z-.alpha.-APM) and .beta.-APM was not contained 
at all. 
EXAMPLES 10-17 
Reutilizing the recrystallization filtration of Example 9, an operation was 
conducted in a similar manner to Example 9. 
The above operation was repeated 8 times. The isolation yields of 
.alpha.-APM in Examples 8-17 are shown in Table 1. 
TABLE 1 
______________________________________ 
Isolation yield 
Example (based on Z-.alpha.-APM) 
______________________________________ 
8 76.2% 
9 76.2% 
10 76.2% 
11 76.2% 
12 76.1% 
13 76.2% 
14 76.2% 
15 76.0% 
16 76.2% 
17 76.1% 
______________________________________ 
.beta.-APM was not contained at all in any samples obtained in the examples 
up to Example 17. 
EXAMPLE 18 
A mixture of Z-.beta.-APM (12.8 g) and Z-.alpha.-APM (30 g) was suspended 
in water (529.5 g), to which 5% palladium-carbon (0.9 g) was added. After 
the resultant mixture was subjected to catalytic reduction under normal 
pressure at 60.degree. C. for 2 hours, the catalyst was filtered off at 
the same temperature. After the toluene layer was separated, the water 
layer was gradually cooled and then stirred at 5.degree. C. for 1 hour. At 
the same temperature, precipitated crystals were collected by filtration 
and then washed so that a wet .alpha.-APM cake (50.0 g) was isolated. 
The thus-isolated wet .alpha.-APM cake was added with. water (350.0 g) and 
dissolved in the latter at 60.degree. C. The solution so obtained was 
gradually cooled and then stirred at 5.degree. C. for 1 hour. At the same 
temperature, precipitated crystals were collected by filtration, washed 
with water and then dried, whereby .alpha.-APM (11.9 g) was isolated. At 
the same time, a filtrate-washing mixture (420.3 g) containing .alpha.-APM 
(2.9 g) and .beta.-APM (11.9 g) was also obtained. 
The crystals so obtained were analyzed by high performance liquid 
chromatography. As a result, it was found that the content of .alpha.-APM 
was 11.5 g (55.8% based on Z-.alpha.-APM) and .beta.-APM was not contained 
at all. 
EXAMPLE 19 
A mixture of Z-.beta.-APM (12.8 g) and Z-.alpha.-APM (34.2 g) was suspended 
in a mixture of the recrystallization filtrate-washing mixture (411.9 g), 
which had been obtained in Example 18, and water (196.4 g), to which 5% 
palladium-carbon (0.9 g) was added. After the resultant mixture was 
subjected to catalytic reduction under normal pressure at 60.degree. C. 
for 2 hours, the catalyst was filtered off at the same temperature and the 
toluene layer was then separated. The water layer was gradually cooled to 
5.degree. C. and stirred for 1 hour and, at the same temperature, 
precipitated crystals were collected by filtration and then washed, 
whereby a wet .alpha.-APM cake (58.7 g) was isolated. 
The thus-isolated wet .alpha.-APM cake was added with water (396.9 g) and 
dissolved in the latter at 69.degree. C. The solution so obtained was 
gradually cooled and then stirred at 50.degree. C. for 1 hour. At the same 
temperature, precipitated crystals were collected by filtration, washed 
with water and then dried, whereby .alpha.-APM (13.5 g) was isolated. 
The crystals so obtained were analyzed-by high performance liquid 
chromatography. As a result, it was found that the content of .alpha.-APM 
was 13.1 g (63.6% based on Z-.alpha.-APM) and .beta.-APM was not contained 
at all. 
COMATIVE EXAMPLE 3 
A mixture of Z-.beta.-APM (10.7 g) and Z-.alpha.-APM (42.8 g) was suspended 
in methanol (235 ml), to which 1N-hydrochloric acid (210 ml) and 5% 
palladium-carbon (4.7 g) were added. After the resultant mixture was 
subjected to catalytic reduction under normal pressure at room temperature 
for 3 hours, the catalyst was filtered off and the methanol in the 
filtrate was distilled off under reduced pressure. Crystals so 
precipitated were collected by filtration at room temperature and then 
washed, whereby a wet cake (36.9 g) of .alpha.-APM hydrochloride was 
isolated. 
The thus-isolated wet cake of .alpha.-APM hydrochloride was added with 
water (265 ml), followed by neutralization with 10% aqueous ammonia at 
room temperature. The resulting solution was cooled to 5.degree. C., at 
which the solution was stirred for 1 hour. Crystals so precipitated were 
collected by filtration at the same temperature, washed with water and 
then dried, whereby .alpha.-APM (14.9 g) was isolated. 
The crystals so obtained was analyzed by high performance liquid 
chromatography. As a result, it was found that the content of .alpha.-APM 
were 14.4 g (61.2% based on Z-.alpha.-APM) and .beta.-APM was not 
contained at all. However, the content of Cl ions was found to be 300 ppm. 
Comparative Example 4 (Reutilization of filtrate and washing in 
recrystallization step). 
A mixture of Z-.beta.-APM (160.5 g) and Z-.alpha.-APM (642 g) was suspended 
in water (9150 g), to which 5% palladium-carbon (13.5 g) was added. After 
the resulting mixture was subjected to catalytic reduction under normal 
pressure at 60.degree. C. for 3 hours, the catalyst was filtered off at 
the same temperature and the toluene layer was separated. The water layer 
was gradually cooled to 5.degree. C. and stirred for 1 hour and, at the 
same temperature, precipitated crystals were collected by filtration and 
then washed, whereby a wet .alpha.-APM cake (960.7 g) was isolated. 
A portion (64.0 g) of the thus-isolated wet .alpha.-APM cake was added with 
water (472.7 g) and dissolved in the latter at 60.degree. C. The solution 
so obtained was gradually cooled and then stirred at 5.degree. C. for 1 
hour. At the same temperature, precipitated crystals were collected by 
filtration, washed with water and then dried, whereby .alpha.-APM (15.6 g) 
was isolated. At the same time, a filtrate-washing mixture (537.5 g) 
containing .alpha.-APM (3.7 g) and .alpha.-APM (0.4 g) was also obtained. 
The crystals so obtained were analyzed by high performance liquid 
chromatography. As a result, it was found that the content of .alpha.-APM 
was 15.1 g (64.9% based on Z-.alpha.-APM) and .beta.-APM was not contained 
at all. 
A portion (64.0 g) of the wet .alpha.-APM cake was next taken, to which the 
recrystallization filtrate-washing mixture (528 g) was added to dissolve 
the former in the latter at 60.degree. C. The resulting solution was 
gradually cooled and then stirred at 5.degree. C. for 1 hour. At the same 
temperature, precipitated crystals were collected by filtration, washed 
with water and then dried, whereby .alpha.-APM (17.9 g) was isolated. The 
crystals so obtained were analyzed by high performance liquid 
chromatography. As a result, it was found that the content of .alpha.-APM 
was 17.4 g (76.2% based on Z-.alpha.-APM) and .beta.-APM was not contained 
at all. 
The above operation (reutilization of filtrate-washing mixture) was 
repeated 4 times. The isolation yields of .alpha.-APM are shown in Table 
2. 
TABLE 2 
______________________________________ 
Isolation yield 
Operation (based on Z-.alpha.-APM) 
______________________________________ 
0th 64.0% 
1st 76.2% 
2nd 76.0% 
3rd 76.1% 
4th 76.2% 
5th 76.1% 
______________________________________ 
From the third reutilization, .beta.-APM was contained at a concentration 
of 0.1-0.3% in the .alpha.-APM so isolated. It was therefore necessary to 
conduct recrystallization again. 
EXAMPLE 20 
A solution of L-phenylalanine methyl ester (60.6 g) in acetic acid (111.4 
g) and a solution of N-benzyloxycarbonylaspartic anhydride (52.0 g) in 
acetic acid (741.0 g) were reacted at 15.degree.-20.degree. C. for 3 
hours, followed by the concentration of the resultant reaction mixture to 
306.6 g. The concentrate was added dropwise at 25.degree. C. over 30 
minutes into water (597.0 g) under stirring in a 1-l reactor which was 
equipped with a stirring blade having a 10 cm span and driven at 400 rpm. 
A mixture of Z-.alpha.-APM and Z-.beta.-APM so crystallized was collected 
by filtration, whereby a wet cake having an average particle size of 90 
.mu.m was obtained. As a result of an HLC analysis, the crystals were 
found to contain Z-.alpha.-APM (113.8 g) and Z-.beta.-APM (26.7 g). 
A portion (146.8 g) of the wet cake was dissolved in a mixture of the 
recrystallization filtrate-washing mixture (610.7 g), which had been 
obtained in Example 13, and water (117.1 g), followed by the addition of 
5%-palladium-carbon (0.9 g). After the resulting mixture was subjected to 
catalytic reduction under normal pressure at 60.degree. C. for 2 hours, 
the catalyst was filtered off at the same temperature and the toluene 
layer was separated. The water layer was gradually cooled to 5.degree. C. 
and stirred for 1 hour at the same temperature. Precipitated crystals were 
collected by filtration and then washed, whereby a wet cake (87.3 g) 
containing .alpha.-APM (26.2 g) was isolated. As a result of an HLC 
analysis, the content of impurities were found as follows, all based on 
.alpha.-APM: 0.6% DKP, 0.4% .alpha.-AP, and 0.03% A.sub.2 PM. The wet 
.alpha.-APM cake was added with water (522.0 g) and dissolved in the 
latter at 60.degree. C. The solution so obtained was gradually cooled to 
5.degree. C. and, at the same temperature, was stirred for 1 hour. 
Crystals so precipitated were collected by filtration, washed with water 
and then dried, whereby .alpha.-APM (21.6 g) was obtained. As a result of 
an HLC analysis, it was found that the content of .alpha.-APM was 21.0 g 
(71.4% based on Z-.alpha.-APM) while the contents of impurities were 0.2% 
DIP, 0.1% .alpha.-AP and 0.03% A.sub.2 PM, and .beta.-APM was not 
detected.