Cyclization for preparing antifolate compounds

This invention provides a process for preparing a compound of formula III ##STR1## wherein R.sup.1 is bromo, iodo or COOR.sup.2 ; PA1 R.sup.2 is H, C.sub.1 -C.sub.4 alkyl, phenyl which may be substituted or benzyl; and PA1 A is a 5- or 6-membered aromatic residue which may contain up to three hetero atoms and which may optionally be substituted with one or two groups selected from the group consisting of halo, hydroxy, C.sub.1 -C.sub.4 alkyl, and C.sub.1 -C.sub.4 alkoxy; or a salt thereof, which comprises PA1 (a) reacting a compound of formula I ##STR2## wherein R is C.sub.1 -C.sub.4 alkyl or phenyl which may be substituted; and PA1 R.sup.1, R.sup.2, and A are as defined above; or a salt thereof, with a sulfurization agent; PA1 (b) cyclizing the reaction product from step (a) with guanidine; and PA1 (c) optionally salifying the reaction product from step (b).

BACKGROUND OF THE INVENTION 
This invention relates to the fields of pharmaceutical and organic 
chemistry, and provides novel intermediates which are useful in the 
synthesis of tetrahydropyrido[2,3-d]pyrimidine (tetrahydrofolic acid) 
antimetabolites of the antifolate type. This invention also relates to 
processes for the preparation of such intermediates. 
Substituted pyrido[2,3-d]pyrimidine-based antifolates have been used for a 
number of years as chemotherapeutic agents in the treatment of cancer. One 
such drug, methotrexate, is now one of the most widely used anticancer 
drugs; and many other compounds in the folic acid family have been 
synthesized, tested and discussed in the chemical and medical literature. 
The compounds have various activities at the enzymatic level; they inhibit 
such enzymes as dihydrofolate reductase, folate polyglutamate synthetase, 
glycinamide ribonucleotide formyltransferase and thymidylate synthase. 
More particularly, a tetrahydrofolic acid antitumor agent, 
5,10-dideaza-5,6,7,8-tetrahydrofolic acid (DDATHF/lometrexol), inhibits 
glycinamide ribonucleotide transformylase (GARFT), an enzyme required in 
the initial stage of de novo purine biosynthesis. See, U.S. Pat. No. 
4,685,653; J. Med. Chem., 28:914 (1985). However, the process for 
preparing lometrexol, and analogs thereof, has not been optimized. 
The present invention provides a novel intermediate, and processes thereto, 
which help to optimize the process for preparing tetrahydrofolic acid 
derivatives. 
SUMMARY OF THE INVENTION 
The present invention relates to compounds having the formula 
##STR3## 
wherein 
R is C.sub.1 -C.sub.4 alkyl or phenyl which may be substituted; 
R.sup.1 is bromo, iodo or COOR.sup.2 ; 
R.sup.2 is H, C.sub.1 -C.sub.4 alkyl, phenyl which may be substituted or 
benzyl; and 
A is an aryl group which may be substituted; or a salt thereof. 
This invention also relates to a process for preparing a 
tetrahydropyrido[2,3-d]pyrimidine of the formula 
##STR4## 
wherein 
R.sup.1 and A are as defined above, which comprises 
(a) reacting a compound of formula I 
##STR5## 
wherein 
R is C.sub.1 -C.sub.4 alkyl or phenyl which may be substituted; and 
R.sup.1 is bromo, iodo or COOR.sup.2 ; 
R.sup.2 is H, C.sub.1 -C.sub.4 alkyl, phenyl which may be substituted or 
benzyl; and 
A is an aryl group which may be substituted; or a salt thereof, with a 
sulfurization agent; 
(b) cyclizing the reaction product from step (a) with guanidine; and 
(c) optionally salifying the reaction product from step (b). 
A further aspect of this invention includes only steps (b) and (c) of the 
process described above. 
Furthermore, this invention relates to another process for preparing a 
compound of formula III 
##STR6## 
wherein 
R.sup.1 is COOR.sup.2 ; 
R.sup.2 is H; and 
A is an aryl group which may be substituted; or a salt thereof, which 
comprises 
(a) reacting a compound of formula I 
##STR7## 
wherein 
R is C.sub.1 -C.sub.4 alkyl or phenyl which may be substituted; 
R.sup.1 is COOR.sup.2 ; 
R.sup.2 is t-butyl; and 
A is an aryl group which may be substituted; or a salt thereof, with a 
sulfurization agent; 
(b) treating the reaction product from step (a) with a strong acid; 
(c) cyclizing the reaction product from step (b) with guanidine; and 
(d) optionally salifying the reaction product from step (c). 
Another aspect of this invention relates only to steps (c) and (d) of the 
process described in the immediately preceding paragraph.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention pertains to novel intermediate compounds which are 
valuable for the preparation of tetrahydropyrido[2,3-d]pyrimidine 
derivatives and processes thereto. 
Tetrahydropyrido[2,3-d]pyrimidines, particularly 
5,10-dideaza-5,6,7,8-tetrahydrofolic acid derivatives such as DDATHF 
(lometrexol, formula IV below), have an inhibitory effect on one or more 
enzymes which utilize folic acid and, in particular, metabolic derivatives 
of folic acid as a substrate. Neoplasms in animals which depend upon such 
enzymes for growth are susceptible to treatment when an effective amount 
of this type of active compound is administered to such an animal. Thus, 
the intermediate compounds of this invention, and processes thereto, are 
useful for the preparation of tetrahydropyrido[2,3-d]pyrimidines which may 
be utilized for the treatment of susceptible neoplasms in animals, 
particularly humans. 
The numbering system used for the pyridopyrimidine moiety of the compound 
of formula IV is shown below. The same numbering system is used for 
compounds of formula III. 
##STR8## 
As shown above in formula IV, the configuration of the L-glutamic acid 
residue is shown unambiguously. The glutamic acid residue for all 
compounds disclosed herein is the L-configuration. In addition, there 
exists an asymmetric center at the 5-position of formula I and II 
compounds and at the 6-position of formula III and IV compounds. If 
desired, the individual enantiomers (formula I-III compounds) or 
diastereomers (formula IV compounds) may be separated by standard methods 
of resolution. Each of the enantiomers/diastereomers which can be 
separated by such a method are included in this invention. 
Furthermore, there exists an asymmetric center at the 3-position of 
compounds of formulae I and II. However, this center is eliminated upon 
cyclization with guanidine, and the configuration does not influence the 
direction or efficiency of the process. 
Compounds of formulae III and IV exist in tautomeric equilibrium with the 
corresponding 4(3H)-oxo compounds. For illustrative purposes, the 
equilibrium for the pyridopyrimidine system are shown below: 
##STR9## 
For convenience, the 4-hydroxy form is depicted for formulae III and IV, 
and the nomenclature is used throughout this specification. However, it is 
understood that such depictions include the corresponding tautomeric 
4(3H)-oxo forms. 
Furthermore, compounds of formula II (including formulae IIa and IIb, see 
Equation 2, infra) exist in tautomeric equilibrium with the 2-mercapto 
compound. For convenience, the 2-thiocarbonyl form is depicted for formula 
II, and the corresponding nomenclature is used throughout this 
specification. However, it is understood that such depictions include the 
corresponding tautomeric 2-mercapto forms. 
The term "C.sub.1 -C.sub.4 alkyl" refers to the straight or branched 
aliphatic chains of 1-4 carbon atoms, including methyl, ethyl, propyl, 
isopropyl, n-butyl, isobutyl, sec-butyl and tert-butyl (t-butyl). 
The term "phenyl which may be substituted" denotes an unsubstituted or 
substituted phenyl residue, optionally having one or two substituents 
selected from halo, nitro, C.sub.1 -C.sub.4 alkyl and C.sub.1 -C.sub.4 
alkoxy. 
The term "C.sub.1 -C.sub.4 alkoxy" represents a C.sub.1 -C.sub.4 alkyl 
group attached through an oxygen bridge, such as, for example, methoxy, 
ethoxy, n-propoxy, isopropoxy and the like. 
The term "an aryl group which may be substituted" as used in describing the 
ring structure identified in A in formulae I, II, III, and IV refers to 5- 
to 6-membered aromatic residues, including heterocyclic groups containing 
up to three heteroatoms (e.g., N, O, and S) therein, such as, for example, 
phenyl, thienyl, pyridyl, furyl, and the like. Of these aromatic residues, 
1,4-phenylene, 2,5-thiophene, and 2,5-furan are preferred. Such aryl 
groups optionally may be substituted, in addition to the R.sup.1 group, 
with one or two substituent groups selected from halo, hydroxy, C.sub.1 
-C.sub.4 alkyl, and C.sub.1 -C.sub.4 alkoxy. 
The carboxyl protecting groups of R and R.sup.2, when R.sup.2 is not H, 
denote groups which generally are not found in final therapeutic 
compounds, but which are intentionally introduced during a portion of the 
synthetic process to protect a group which otherwise might react in the 
course of chemical manipulations, and is then removed at a later stage of 
the synthesis. Since compounds bearing such protecting groups are of 
importance primarily as chemical intermediates (although some derivatives 
also exhibit biological activity), their precise structure is not 
critical. Numerous reactions for the formation and removal of such 
protecting groups are described in a number of standard works including, 
for example, Protective Groups in Oragnic Chemistry, Plenum Press, (London 
and New York, 1973); Greene, Th. W., Protective Groups in Organic 
Synthesis, Wiley, (New York, 1981); and The Peptides, Vol. I, Schrooder 
and Lubke, Academic Press, (London and New York, 1965). 
Representative R carboxyl protecting groups include C.sub.1 -C.sub.4 alkyl 
and phenyl which may be substituted. Representative R.sup.2 groups, when 
R.sup.2 is not H, include C.sub.1 -C.sub.4 alkyl, phenyl which may be 
substituted or benzyl. These groups are selectively removable under 
sufficiently mild conditions so as not to disrupt the desired structure of 
the molecule. 
When R.sup.2 is not H, it is preferred that carboxyl protecting groups R 
and R.sup.2 are not the same group. Thus, preferred R groups are C.sub.1 
-C.sub.4 alkyl, especially ethyl; when R is ethyl, the preferred R.sup.2 
group is t-butyl. 
The process and compounds of this invention also include salts of the 
compounds defined by the above formulas. A particular compound of this 
invention can possess a sufficiently acidic, a sufficiently basic, or both 
functional groups, and accordingly react with any of a number of inorganic 
bases, and inorganic and organic acids, to form a salt. Acids commonly 
employed to form acid addition salts are inorganic acids such as 
hydrochloric acid, hydrobromic acid, hydroiodic acid, sulfuric acid, 
phosphoric acid, and the like, and organic acids such as p-toluenesulfonic 
acid, methanesulfonic acid, oxalic acid, p-bromo-phenyl-sulfonic acid, 
carbonic acid, succinic acid, citric acid, benzoic acid, acetic acid, and 
the like. Examples of such salts thus are the sulfate, pyrosulfate, 
bisulfate, sulfite, bisulfite, phosphate, monohydrogenphosphate, 
dihydrogenphosphate, metaphosphate, pyrophosphate, chloride, bromide, 
iodide, acetate, propionate, decanoate, caprylate, acrylate, formate, 
isobutyrate, caproate, heptanoate, propiolate, oxalate, malonate, 
succinate, suberate, sebacate, fumarate, maleate, butyne-1,4- dioate, 
hexyne-1,6-dioate, benzoate, chlorobenzoate, methylbenzoate, 
dinitrobenzoate, hydroxybenzoate, methoxybenzoate, phthalate, sulfonate, 
xylenesulfonate, phenylacetate, phenylpropionate, phenylbutyrate, citrate, 
lactate, gamma-hydroxybutyrate, glycollate, tartrate, methanesulfonate, 
propanesulfonate, naphthalene-1-sulfonate, naphthalene-2-sulfonate, 
mandelate,and the like. Preferred acid addition salts are those formed 
with mineral acids such as hydrochloric acid and hydrobromic acid, and 
those formed with organic acids such as maleic acid and methanesulfonic 
acid. 
Base addition salts include those derived from inorganic bases, such as 
ammonium or alkali or alkaline earth metal hydroxides, carbonates, 
bicarbonates, and the like. Such bases useful in preparing the salts of 
this invention thus include sodium hydroxide, potassium hydroxide, 
ammonium hydroxide, potassium carbonate. The potassium and sodium salt 
forms are particularly preferred. 
Of course, when the intermediates of this invention are converted to final, 
pharmaceutically active compounds, those compounds may also be in the form 
of a salt, but the salt must be of the pharmaceutically acceptable nature. 
Processes for preparing acid addition, base addition, and pharmaceutically 
acceptable salts (salification) are well known in the art. 
Preferred starting material for preparing compounds of the present 
invention include 2-oxo-3-(C.sub.1 -C.sub.4 alkyl)carboxy-5-[(4-bromo- or 
4-iodophenyl)ethyl] piperidine or 2-oxo-3-(C.sub.1 -C.sub.4 
alkyl)carboxy-5-[(4-protected carboxyphenyl)ethyl]piperidine (both of 
formula I below), depending on the desired R.sup.1 substituent. Other 
preferred starting material include 2-oxo-3-(C.sub.1 -C.sub.4 
alkyl)carboxy-5-[(5-bromo- or 5-iodothienyl or -furanyl)ethyl]piperidine 
or 2-oxo-3-(C.sub.1 -C.sub.4 alkyl)carboxy-5-[(5-protected carboxythienyl 
or carboxyfuranyl)ethyl]piperidine (also of formula I below). The 
desirability and use of different R.sup.1 substituents, bromo, iodo, 
carboxyl or protected carboxyl, are further discussed following Equation 
2. 
Preparation of these starting materials are generally known in the organic 
chemical art (see, e.g. 
Barnett, et al., Tetrahedron Letters, 30:6291-6294 (1989) and Barnett, et 
al., U.S. Pat. No. 5,008,391, respectively). 
Regarding formula I compounds wherein R.sup.1 is bromo or iodo, one will 
note that the desired halogen is present on this starting material prior 
to the first step of the process for preparing compounds of formulae III 
and IV, and can remain as such prior to coupling of compounds of formula 
III with L-glutamic acid to produce therapeutically active tetrahydrofolic 
acid derivatives. 
Likewise, the protected aryl carboxyl group of formula I compounds is also 
present at that position prior to the first step of the process. Preferred 
R.sup.2 carboxyl protecting groups include C.sub.1 -C.sub.4 alkyl, phenyl 
which may be substituted or benzyl, of these, t-butyl is especially 
preferred. As previously mentioned, compounds of formula I possess an 
asymmetric center at the 5-position of the piperidine ring, of which the 
diastereomers thereof may be separated and purified. Under the reactive 
conditions of the processes of the present invention, a selected 
diastereomer will remain as such throughout each step. Thus, if the 6R 
isomer of formula III or formula IV is desired, the 5R isomer of a formula 
I compound should be selected as the starting material. 
In one process, a formula I compound is sulfurized to form a thiopiperidone 
(formula II), which is then cyclized to form a compound of formula III. 
This process is novel and is depicted in Equation 1. 
##STR10## 
wherein R is C.sub.1 -C.sub.4 alkyl or phenyl which may be substituted; 
R.sup.1 is bromo, iodo or COOR.sup.2 ; R.sup.2 is H, C.sub.1 -C.sub.4 
alkyl, phenyl which may be substituted or benzyl; A is an aryl group which 
may be substituted; or a salt thereof. As mentioned above, formula III 
compounds optionally may be salified via known procedures. 
The first step of Equation 1 involves the sulfurization of a formula I 
compound. Although a number of sulfurization agents, such as Lawesson's 
reagent (see, e.g., Organic Synthesis Highlights, Mulzer, et al., 
(Weinhein and New York, 1991), may be used in this step, sulfurization is 
preferably accomplished using phosphorous pentasulfide. 
The amount of sulfurization agent employed is suitably an amount sufficient 
to replace the 2-carbonyl moiety on the piperidone ring by thiocarbonyl. 
Generally, about one equivalent of sulfurization agent per equivalent of 
piperidone is employed. Preferably, an excess of sulfurization agent is 
used. 
The first step shown in Equation 1 is accomplished in the presence of a 
suitable inert, or substantially inert solvent, or mixture of solvents. A 
solvent such as tetrahydrofuran is preferred. 
The temperature employed in this step should be sufficient to effect 
completion of the sulfurization reaction. Typically, a temperature in the 
range from about 50.degree. C. to about 70.degree. C. is sufficient, 
whereas a temperature of about 60.degree. C. is preferred. 
The length of time for this sulfurization step to occur can vary. The 
reaction generally requires from about a few minutes to about a few hours. 
The optimal reaction time can be determined by monitoring the progress of 
the reaction by conventional chromatographic techniques such as thin layer 
chromatography, high performance liquid chromatography or column 
chromatography. 
In the second step of Equation 1, the thiopiperidone of formula II is 
reacted with guanidine, in an appropriate solvent, and forms a compound of 
formula III. For this cyclization to occur, guanidine may be supplied as a 
salt, but it must first be converted to the free base via neutralization 
with a base. Thus, it is preferred to employ guanidine free base in this 
step of the reaction. 
Appropriate solvents include any solvent, or mixture of solvents, which 
will remain inert, or substantially inert, under reaction conditions. 
Typically, appropriate solvents include C.sub.1 -C.sub.4 aliphatic 
alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 
2-butanol and 2 methyl-2-propanol. Of these, ethanol is preferred. 
Suitable amounts of guanidine are those which are sufficient to react with 
all of a formula II compound from step (a) of the reaction shown in 
Equation 1. Generally, from about one equivalent to an excess of guanidine 
per equivalent of a formula II compound is employed. Preferably, an excess 
of guanidine is used. 
Typically, this step proceeds in short periods of time at elevated 
temperatures, but the length of time will vary with the reaction 
conditions employed. This cyclization reaction requires about 10 to about 
30 minutes to proceed when run at the preferred temperature of about 
90.degree. C. However, this reaction may be run in a temperature range 
from about 70.degree. C. to about 100.degree. C. 
Each step of the novel process depicted in Equation 1 may be individually 
run wherein each reaction product is isolated and purified. Not only is 
the entire process shown in Equation 1 novel, but the second step, the 
cyclization of a formula II thiopiperidone with guanidine, [step (b)], 
independently is a novel process. It is preferred that the steps shown in 
Equation 1 are combined into a one-pot process comprising reacting a 
compound of formula I with a sulfurization agent and cyclizing the 
reaction product from the immediately preceding step (a formula II 
compound) by reacting that reaction product with guanidine. The compounds 
of formula II also are novel and are useful as intermediates for the 
preparation of tetrahydrofolic acid derivatives. 
Alternatively, when the processes of this invention are used to prepare an 
intermediate of formula III wherein R.sup.1 is COOR.sup.2 and R.sup.2 is 
H, it is preferred to use the novel process depicted in Equation 2. The 
preferred starting material is a compound of formula I wherein R.sup.1 is 
COOR.sup.2 and R.sup.2 is t-butyl. However, compared to the process shown 
in Equation 1, an additional step is preferred. 
##STR11## 
wherein R is C.sub.1 -C.sub.4 alkyl or phenyl which may be substituted, 
R.sup.1 is COOR.sup.2, R.sup.2 is t-butyl, and A is an aryl group which 
may be substituted; 
##STR12## 
wherein R, R.sup.1, and A are as defined above; 
##STR13## 
wherein R and A are as defined above, or a salt thereof; 
##STR14## 
wherein A is an aryl group which may be substituted, or a salt thereof. 
The reaction product from step (c) also may be salified as previously 
discussed. 
Steps (a), (c), and (d) of the process shown in Equation 2, including all 
general and preferred reagents and reaction conditions, are the same as 
steps (a), (b) and (c) respectively, described above for Equation 1. In 
Equation 2, however, there A is the preferred additional step of 
converting the thiopiperidone of formula IIa to its acid form [step (b)], 
which provides compounds of formula IIb. 
In essence, it is not necessary to convert the protected thiopiperidone of 
formula IIa to the acid form prior to cyclization with guanidine. However, 
it is best to unmask the carboxyl group prior to cyclization because the 
R.sup.2 protected carboxyl group could react with guanidine to produce 
undesirable results. 
Thus, the selection of t-butyl as the R.sup.2 substituent, as shown in 
Equation 2, allows for the conversion of this protecting group to the acid 
form via an acid catalyst [step (b)]. Suitable acid catalysts can be any 
organic or inorganic compound which will facilitate removal of the t-butyl 
protecting group from the benzoic acid moiety of a formula IIa compound. 
Such acid catalysts are well known in the art. See, e.g., Protective 
Groups in Organic Chemistry, J. G. W. McOmie, Ed., Plenum Press (New York, 
1973); and T. W. Greene, Protective Groups in Organic Synthesis, John 
Wiley and Sons (New York, 1981). However, the preferred acid catalyst is 
trifluoroacetic acid, especially when used in substantial excess per 
equivalent of substrate present. 
Generally, this step of the reaction takes place nearly instantaneously, 
but the length of time required depends upon the choice of acid catalyst 
and its effectiveness in the process. 
Although other C.sub.1 -C.sub.4 alkyl substituents could be converted to 
form the acid, the presence of t-butyl as the R.sup.2 substituent permits 
this conversion without hydrolysis or displacement, leaving the R 
substituent undisturbed. By converting the R.sup.2 substituent while 
leaving the R substituent as originally selected, cyclization at the 
desired location is favored and the potential for guanidine to react with 
the R.sup.2 protected carboxyl group is eliminated. 
Each step of the novel process depicted in Equation 2 may be individually 
run wherein each reaction product is isolated and purified. It is 
preferred, however, that the steps shown in Equation 2 are combined in a 
one-pot process comprising reacting a compound of formula I with a 
sulfurization agent, treating the reaction product from the previous step 
with a strong acid, and cyclizing the reaction product from the step (b) 
with guanidine. In addition, the cyclization of a formula IIb compound 
with guanidine is independently a novel process. 
The final reaction product of both Equation 1 and Equation 2, a formula III 
compound, is easily converted to a therapeutically active tetrahydrofolic 
acid derivative by conventional methods. 
Generally, the R.sup.1 substituent on formula III compounds, when R.sup.1 
is bromo or iodo, is replaced with cyano by reaction with a cyano salt 
such as copper cyanide in the presence of N-methylpyrrolidine. The nitrile 
group is then hydrolyzed to obtain a formula III compound with a carboxyl 
or protected carboxyl group (see, e.g., U.S. Pat. No. 5,008,391). 
A formula III compound is then coupled with a protected L-glutamic acid 
derivative in the manner described in U.S. Pat. No. 4,684,653, using 
conventional condensation techniques for forming peptide bonds. The 
protected L-glutamic acid derivative is then subjected to hydrolysis to 
remove the remaining carboxyl protecting groups. 
The following examples further illustrate the novel intermediate compounds, 
and processes thereto, according to the present invention. The examples 
are not intended to be limiting to the scope of the invention in any 
respect, and should not be so construed. 
In the following examples, the terms melting point, nuclear magnetic 
resonance spectra, electron impact mass spectra, field desorption mass 
spectra, infrared spectra, ultraviolet spectra, and elemental analysis, 
are abbreviated mp, NMR, MS(EI), MS(FD), IR, UV, and Anal., respectively. 
In general, the adsorption maxima listed are only those of interest and 
not necessarily all of the maxima observed. 
The NMR spectra were obtained on a General Electric QE-300 300 MHz 
instrument. The chemical shifts are expressed in .delta. values (parts per 
million downfield from tetramethylsilane). The field desorption mass 
spectra were taken on a Varian-MAT 731 Spectrometer using carbon dendrite 
emitters. Electron impact mass spectra were obtained on a CEC 21-110 
instrument from Consolidated Electrodynamics Corporation. Infrared spectra 
were obtained on a Perkin-Elmer 281 instrument. Ultraviolet spectra were 
obtained on Cary 118 instrument. Melting points are uncorrected. 
EXAMPLE 1 
(3RS,5R)-2-Thioxo-3-(ethylcarboxy-5-[2-(4-bromophenyl)ethyl]piperidine 
A mixture of 1.12 g (3.16 mmol) of 
(3RS,5R)-2-oxo-3-ethylcarboxy-5-[2-(4-bromophenyl)ethyl]piperidine and 
0.77 g (1.74 mmol) of phosphorus pentasulfide in 20 mL of tetrahydrofuran 
was heated at 60.degree. C. until complete conversion was obtained (about 
3 hours). The mixture was evaporated to a residue which was purified by 
chromatography on silica gel. The product was eluted with ethyl 
acetate-hexane 1:1, affording 0.729 g (61%) of (3RS,5R) 
-2-thioxo-3-ethylcarboxy-5-[2-(4-bromophenyl)ethyl]piperidine (mixture of 
C-3 epimers)), mp 105.degree.-115.degree. C., .sup.1 H NMR (300 MHz, 
CDCl.sub.3) .delta. 9.47 (s, 1H), 7.41 (d, J=8.7 Hz,); 7.39 (d, J=8.2 
Hz,), total 2H, 7.02 (d, J=8.7 Hz, 2H), 4.20 (m, 2H), 3.94 (m); 3.69 (m), 
total 1H, 3.45 (m, 1H), 2.99 (m, 1H), 2.60 (m, 2H), 2.13 (m, 2H), 1.5-1.87 
(m, 3H), 1.31 (t, J=7.1 Hz); 1.26 (t, J=7.1 Hz), total 3H; IR (CHCl.sub.3) 
2983, 1731, 1540, 1489, 1373, 1073, 1012 cm.sup.-1 ; MS (FD) m/z 371 
(100), 369 (75); UV (EtOH) 219.8 nm (.epsilon. 13 174), 281.4 (13 557); 
Anal. Calcd for C.sub.16 H.sub.20 BrNO.sub.2 S: C, 51.90; H, 5.44; N, 
3.78. Found: C, 51.61; H, 5.38; N, 3.88. 
EXAMPLE 2 
R-(-)-2-Amino-4-hydroxy-6-[(4-bromophenyl)ethyl]-5,6,7,8-tetrahydropyrido[2 
,3-d]pyrimidine 
A suspension of 740 mg (7.77 mmol) of guanidine hydrochloride in 5 mL of 
ethanol was neutralized by addition of 950 mg (7.77 mmol) of potassium 
tert-butoxide. The precipitated potassium chloride was filtered with 
filter aid and washed with an additional 5 mL of ethanol. A solution of 
720 mg (1.94 mmol) of 
(3RS,5R)-2-thioxo-3-ethylcarboxy-5-[2-(4-bromophenyl)ethyl]piperidine in 5 
mL ethanol was added to the filtrate and this solution was reduced to a 
volume of 2-3 mL under vacuum. The solution was heated in an oil bath 
(T=90.degree. C.) under a stream of nitrogen for 2 h. After cooling to 
ambient temperature, the mixture was subjected to further vacuum 
evaporation. The resulting solid was suspended in water, sonicated to form 
a uniform suspension, and neutralized to pH 7 by addition of 1N aq. HCl. 
The resulting suspension was heated to about 90.degree. C. for 15 min, 
then cooled to ambient temp. and filtered. The product was dried in vacuo 
(10 torr), affording 544 mg (80%) of 
(R)-(-)-2-amino-4-hydroxy-6-[2-(4-bromophenyl)ethyl]-5,6,7,8-tetrahydropyr 
ido[2,3-d]pyrimidine mp 275.degree.-283.degree. C., [.alpha.].sub.589 
-30.9.degree. (c 1, DMF), .sup.1 H NMR (DMSO-d.sub.6) .delta. 9.70 (bs, 
1H), 7.40 (d, J=8.3 Hz, 2H), 7.12 (d, J=8.3 Hz, 2H), 6.23 (d, J=8.3 Hz, 
2H), 5.96 (s, 2H), 3.15 (bd, J=11 Hz, 1H), 2.45 (m, 2H), 1.78 (dd, J=8.6, 
15.1, 1H), 1.50 (m, 3H); IR (KBr) 3400, 3343, 1678, 1642 cm.sup.-1 ; MS 
(EI), m/z 350 (78), 348 (72), 166 (100), 165 (95), 164 (31), 151 (68), 139 
(19). The NMR spectrum was identical with that of a sample of (S)-(+)-3 
obtained as described in U.S. Pat. No. 5,008,391. 
EXAMPLE 3 
4-(2-[2-thioxo-3-ethylcarboxypiperidin-5-yl]ethyl)benzoic acid, 
2,2-dimethylethyl ester 
A mixture of 3.75 g (10 mmol) of 
4-(2-[2-oxo-3-ethylcarboxypiperidin-5-yl]ethyl)benzoic acid, 
2,2-dimethylethyl ester and 2.44 g (5.5 mmol) of phosphorus pentasulfide 
in 70 mL of tetrahydrofuran is heated at 60.degree. C. until complete 
conversion is obtained. The mixture is evaporated to a residue which is 
purified by chromatography on silica gel, providing 
4-(2-[2-thioxo-3-ethylcarboxypiperidin-5-yl]ethyl)benzoic acid, 
2,2-dimethylethyl ester. 
EXAMPLE 4 
4-(2-[2-amino-4(1H)-oxo-5,6,7,8-tetrahydropyrido[2,3-d]pyrimidin-6-yl]ethyl 
)benzoic acid, 2,2-dimethylethyl ester 
A suspension of 764 mg (8.0 mmol) of guanidine hydrochloride in 5 mL of 
ethanol is neutralized by addition of 976 mg (8.7 mmol) of potassium 
tert-butoxide. The precipitated potassium chloride is filtered with filter 
aid and washed with an additional 5 mL of ethanol. A solution of 783 mg 
(2.0 mmol) of 4-(2-[2-thioxo-3-ethylcarboxypiperidin-5-yl]ethyl)benzoic 
acid, 2,2-dimethylethyl ester in 5 mL ethanol is added to the filtrate and 
this solution is reduced to a volume of 2-3 mL under vacuum. The solution 
is heated in an oil bath to about 90.degree. C. under a stream of nitrogen 
for about 2 h. After cooling to ambient temperature, the mixture is 
subjected to further vacuum evaporation. The resulting solid is suspended 
in water, sonicated, if necessary, to form a uniform suspension, and 
neutralized to pH 7 by addition of 1N aq. HCl. The resulting suspension is 
filtered and dried, affording 
4-(2-[2-amino-4(1H)-oxo-5,6,7,8-tetrahydropyrido[2,3-d]pyrimidin-6-yl]ethy 
l)benzoic acid, 2,2-dimethylethyl ester.