Synthesis of chiral pilocarpine analogues via a C-8 ketone intermediate.

Article abstract of DOI:10.1021/jo0111210

The synthesis of a chiral pilocarpine analogue 3 in which the lactone ring is replaced by an oxazolidinone and the bridging methylene group is in the ketone oxidation state has been accomplished. The utility of this compound as a key intermediate for the preparation of more complex structures was demonstrated by its reduction to two alcohol epimers and its reaction with a methylene ylide.

Full text of DOI:10.1021/jo0111210

Syn th esis of Ch ir a l P iloca r p in e An a logu es via a C-8 Keton e
In ter m ed ia te
Kenneth G. Holden,*,† Matthew N. Mattson,‡ Kyung Hoi Cha,‡ and Henry Rapoport‡
Holden Laboratories, 179 Mal Paso Road, Carmel, California 93923, and Department of Chemistry,
University of California, Berkeley, California 94720
KenGHolden@aol.com
Received December 3, 2001
The synthesis of a chiral pilocarpine analogue 3 in which the lactone ring is replaced by an
oxazolidinone and the bridging methylene group is in the ketone oxidation state has been
accomplished. The utility of this compound as a key intermediate for the preparation of more complex
structures was demonstrated by its reduction to two alcohol epimers and its reaction with a
methylene ylide.
Pilocarpine (1a , Figure 1), an alkaloid first isolated
over 100 years ago, is currently used for the treatment
of glaucoma and xerostomia (salivary gland dysfunction).
Recently there has been renewed interest in the thera-
peutic utility of this type of muscarinic agonist, particu-
larly those active in the central nervous system.1 For
example, because of its potential for the treatment of
F IGURE 1.
SCHEME 1
Alzheimer’s disease, pilocarpine thiolactone (1b) was
developed as a clinical candidate.2 A cyclic carbamate
analogue (2) of pilocarpine has been prepared using
histidine as the chiral educt.3 Biological evaluation of 2
showed it to be equivalent with pilocarpine using a
standard in vitro assay.3a However, 2 is expected to have
a longer duration of biological action, because two sites
for inactivation of pilocarpine (epimerization and lactone
hydrolysis) have been eliminated. In addition to its
equivalent biological potency, carbamate analogues are
more easily synthesized since, in addition to having
greater stability, one of the two stereogenic centers has
been eliminated. On the basis of these advantages, we
sought to prepare a chiral intermediate that could be
utilized for the preparation of additional pilocarpine
analogues, particularly those with constrained conforma-
tions. For this purpose, ketone 3 was selected as a
desirable target that might be derived from D-serine (4a )
and 1-methylimidazole (6) as shown in Scheme 1.
hyde and sodium borohydride as described for the
correponding L-methyl ester.5 Initial studies on the
formation of oxazolidinone 5a using the literature pro-
cedure (carbonyldiimidazole, CDI)5 were complicated by
partial epimerization.6 However, substituting 1,1′-car-
bonylbis(3-methylimidazolium)triflate (CBMIT)7 for CDI
resulted in oxazolidinone formation without significant
epimerization. Thus, acid 5b was obtained by hydro-
genolysis with an er > 98/2.
The initial approach (Scheme 2) began with the es-
terification of D-serine (4a ) to give the benzyl ester as its
tosylate salt 4b,4 which after conversion to the hydro-
chloride 4c, was N-alkylated to give 4d using acetalde-
The regioselective functionalization of 1-methylimida-
zole was achieved using a route outlined by El Borai et
al.,8 which was presented without experimental details
(Scheme 3). Iodination of the dilithium salt of 1-meth-
† Holden Laboratories.
‡ University of California.
(1) Felder, C. C.; Bymaster, F. P.; Ward, J .; DeLapp, N. J . Med.
Chem. 2000, 43, 4333.
(2) (a) Shapiro, G.; Floershime, J .; Boelsterli, J .; Amstutz, R.;
Bolliger, G.; Gammenthaler, H.; Gmelin, G.; Supavilai, P.; Walkinshaw,
M. J . Med. Chem. 1992, 35, 15. (b) Enz, A.; Boddeke, H.; Sauter, A.;
Rudin, M.; Shapiro, G. Life Sci. 1993, 52, 513.
(3) (a) Sauerberg, P.; Chen, J .; WoldeMussie, E.; Rapoport, H. J .
Med. Chem. 1989, 32, 1322. (b) Rapoport, H.; Sauerberg, P. U.S. Patent
5 025 027, J une 18, 1991. (c) Gonzalez, F.; Baz, J . P.; Espina, M. I. R.
Tetrahedron Lett. 1989, 30, 2145.
(5) Groneberg, R. D.; Miyazaki, T.; Stylianides, N. A.; Schulze, T.
J .; Stahl, W.; Schreiner, E. P.; Suzuki, T.; Iwabuchi, Y., Smith, A. L.;
Nicolaou, K. C. J . Am. Chem. Soc. 1993, 115, 7593.
(6) Determined by HPLC after conversion to the acid and coupling
with (R)-(+)-R-methylbenzylamine.
(7) Saha, A. K.; Schultz, P.; Rapoport, H. J . Am. Chem. Soc. 1989,
111, 4856.
(4) Folsh, G. Acta Chem, Scand. 1959, 13, 1407
10.1021/jo0111210 CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/26/2002
J . Org. Chem. 2002, 67, 5913-5918
5913

Holden et al.
SCHEME 2a
SCHEME 4a
a
(a) H2O, MeCHO then NaBH4, <15 °C; (b) i-BuOCOCl, NaOH;
(c) Na2CO3, ∆; (d) MeSO3H, EtOH, ∆.
product. Only small amounts of starting material and the
corresponding N,N-diethyl derivative were present. This
result suggests that the five-membered N,O-acetal is the
dominant species present during the reduction11. Acyl-
ation with isobutyl chloroformate gave the carbamate,
which could be isolated as a glass following extraction
with ethyl acetate (73%). However, it was found prefer-
able to cyclize this intermediate directly by heating in
the presence of sodium carbonate to give 5b (60% from
D-serine) as a crystalline solid, identical with that
produced by the previous route (Scheme 2). Although this
method for preparing 5b is clearly superior, there are two
negative factors that must be taken into account: (1)
complete hydrolysis back to N-ethyl-D-serine is a compet-
ing reaction and (2) the high water solubility of 5b makes
extraction (EtOAc/tert.-BuOH, 2/1) difficult even after
NaCl saturation of the aqueous.
Activation of 5b as its THP ester 5c proved problematic
because the ester was unstable at room temperature,
could not be readily assayed due to decomposition on
silica, and often failed to form because of precipitation
of the starting material. This problem was circumvented
by the use of ethyl ester 5d , which was stable and could
be formed under standard Fischer conditions.
Regioselective activation of the imidazole fragment was
achived by direct bromination of 1-methylimidazole with
NBS.8 Although the yield was relatively low, the desired
5-bromo-1-methylimidazole (9) could be easily separated
from the other major product, 4,5-dibromo-1-methylimi-
dazole, by simple distillation. In addition, the intermedi-
ate Grignard product 10 was completely soluble, while
that from the iodide formed a suspension and eventually
a gum if stirring was interrupted. The solubility of 10
proved to be a considerable advantage, since the reaction
sequence required transfer of this material by syringe.
Using these modifications, as shown in Scheme 5,
ketone 3 could be prepared in gram quantities. In
addition, the product was obtained in high enantiomeric
purity (>99/1 as determined by chiral HPLC). Even so,
the yield of 3 was modest (49% after chromatography)
and could not be significantly improved by using a variety
of reaction conditions or substituting a more hindered
Girgnard reagent (isopropylmagnesium chloride). In ad-
dition to 3, a small amount (7%) of a more polar product
was formed. On the basis of analogy with the stereo-
chemical outcome of hydride reductions of 3 (see below),
this product is assigned structure and stereochemistry
11.
a
(a) PhCH2OH, p-TSA, CCl4, ∆, 9 h, 95%; (b) NaHCO3, then
HCl gas, 0 °C, 88%; (c) MeOH, Et3N, MeCHO, 0 °C, then NaBH4,
58%; (d) CBMIT, 0 °C, 92%; (e) Pd/C, H2, EtOAc, quantitative; (f)
DHP, 10-fold excess, CH3SO3H, 0 °C, 88%.
SCHEME 3a
a
(a) n-BuLi, 2.4 equiv, TMEDA, hexanes -20 °C to 22 °C, then
I2, 2.45 equiv in THF, -65°, 89%; (b) n-BuLi, THF, 0°, 71%; (c)
DCM, EtMgBr in Et2O; (d) DMF, 0 °C.
ylimidazole (6) gave the 2,5-diiodo derivative 7, which
could be selectively deiodinated to the desired 5-iodo-1-
methylimidazole (8a ). After some development, it was
possible to carry out the conversion of 6 to 8a (65%) in a
one-flask reaction. Treatment of 8a with EtMgBr gave
the intermediate heterocyclic Grignard reagent 8b, which
produced the known 1-methylimidazole-5-caboxaldehyde
(8c)9 on reaction with DMF, thus confirming the regio-
selectivity of this reaction sequence.
To produce target ketone 3, acid 5b was first converted
to its THP ester 5c10 (Scheme 2), followed by treatment
of this material (generated in situ) with Grignard reagent
8b, produced from 8a . The product of this reaction, 3,
isolated by chromatography as a crystalline solid in
variable yield (54% maximum) depending on reaction
conditions, was determined to have an enantiomeric
purity of 93-96% using chiral HPLC.
Although this synthetic route provided 3 in milligram
amounts, improvements were needed to produce gram
quantities required for analogue synthesis. In particular,
the rather lengthy route to 5b, which presented ester
interchange and epimerization problems, suggested that
a more direct route via the free acid might have advan-
tages. Also, the preparation of 8a required 300 mol % of
n-BuLi and failed to give an acceptable yield on at-
tempted scale-up. The route devised for the preparation
of 5b (Scheme 4) was carried out entirely in aqueous
solution. Thus, a solution of molar equivalents of D-serine
and acetaldehyde in water was first treated with solid
sodium borohydride to give N-ethyl-D-serine as the major
(10) Mattson, M. N.; Rapoport, H. J . Org. Chem., 1996, 61, 6071.
(11) Saavedra, J . E. J . Org. Chem., 1985, 50, 2271; Reddy, G. V.;
Rao, G. V.; Streevani, V.; Iyengar, D. S., Tertahedron Lett. 2000, 41,
949.
(8) El Borai, M.; Moustafa, A. H.; Anwar, A. M.; Abdel Hay, F. I.
Pol. J . Chem. 1981, 55, 1659.
(9) Matthews, H. R.; Rapoport, H., J . Am. Chem. Soc. 1973, 95, 2297.
5914 J . Org. Chem., Vol. 67, No. 17, 2002

Synthesis of Chiral Pilocarpine Analogues
SCHEME 5
Exp er im en ta l Section
Gen er a l. TLC analyses were performed on silica gel plates,
and the products were visualized with UV light followed by
iodine staining. Column chromatography was carried out on
silica gel (28-200 or 70-230 mesh) or alumina. 1H and 13C
NMR spectra are reported in ppm (δ) downfield from tetram-
ethylsilane. Melting points were determined on an open
capillary apparatus and are uncorrected. Elemental analyses
were conducted by the Analytical Laboratory at the University
of California at Berkeley.
D-Ser in e Ben zyl E st er , p -Tolu en esu lfon ic Acid Sa lt
(4b). A mixture of D-serine (21 g, 0.2 mol), p-TsOH (41.8 g,
0.22 mol), and benzyl alcohol (100 mL) in carbon tetrachloride
(100 mL) was heated at reflux until the formation of water
ceased (9 h). The mixture was concentrated in vacuo, and the
residue was treated with diethyl ether (300 mL). After being
seeded with a small sample of 4b, the mixture was placed in
the refrigerator for 1-2 days. Additional diethyl ether (900
mL) was added, and the mixture was stirred at 0 °C for 30
min, filtered, and washed with diethyl ether to give 70.0 g
(95%) of 4b as a white solid: mp 80 °C (lit. 94-95°);4 it has
been reported that the reaction product is frequently difficult
to crystallize due to byproducts resulting from self-esterifica-
tion and O-benzylation); [R]20D +2.3° (c 4.55, MeOH); 1H NMR
(400 MHz, DMSO) δ 2.29 (s, 3H), 3.78-3.91 (m, 2H), 4.23 (s,
1H). 5.24 (ABq, J ) 12.55 Hz, 2H), 7.12, 7.49 (AA′BB′, J )
7.77 Hz, 4H), 7.31-7.41 (m, 5H), 8.40 (s, 2H).
SCHEME 6
D-Ser in e Ben zyl Ester , Hyd r och lor id e Sa lt (4c). D-
Serine benzyl ester p-toluenesulfonic acid salt (4b, 70 g, 0.19
mol) was carefully dissolved in 5% sodium bicarbonate solution
(350 mL) at 0 °C. The resulting solution was extracted with
CHCl3/i-PrOH (4:1, 3 × 300 mL), dried over sodium sulfate,
and evaporated to a residue. The residue was dissolved in
dichloromethane (500 mL), and HCl gas was bubbled into the
solution at 0 °C for 10-15 min followed by stirring for 1 h.
The solid formed was filtered, washed with dichloromethane
and diethyl ether, and dried to yield 38.68 g (87.6%) of 4c as
a white solid. A sample of this material was further purified
by recrystallization from i-PrOH/MeOH (13:1) in 78% yield,
As shown in Scheme 6, Wittig reaction of 3 gave the
expected methylene analogue 12 along with a more polar
compound, which appeared to contain elements of the
intermediate ylide. When the reaction was conducted in
DMSO, the latter was the major product. Low-temper-
ature reduction of 3 with L-Selectride gave predomi-
nantly (30/1) alcohol 13 assigned the syn (R) configura-
tion. This is the expected product resulting from approach
of a bulky hydride reagent from the less hindered â face
of the molecule (away from the N-ethyl group). This
assignment is also consistent with its 1H NMR vicinal
coupling constant (J 4-8 ) 8.2 Hz) compared to the anti
(â) epimer 14 with a value of 2.5 Hz, both of which are
in agreement with those predicted from molecular model-
ing. For preparative purposes, it was found that reduction
with sodium borohydride was superior even though both
epimers were produced (R/â, 1/1.2), since they were easily
separated by fractional crystallization and thus readily
available for biological evaluation.
mp 172-174 °C; [R]20 + 4.03° (c 4.63, MeOH); 1H NMR (400
D
MHz, DMSO) δ 3.80-3.90 (m, 2H), 4.19 (brs, 1H), 5.24 (ABq,
J ) 12.67 Hz, 2H), 5.69 (brs, 1H), 7.30-7.50 (m, 5H); 13C NMR
(100 MHz, DMSO) δ 54.9, 60.0, 67.3, 128.4, 128.9, 135.8, 168.4.
D-N-Eth ylser in e Ben zyl Ester (4d ). D-Serine benzyl ester
hydrochloride salt (4c, 78.8 g, 0.34 mol) was treated with dry
methanol (300 mL) followed by dry triethylamine (47.5 mL,
0.34 mol). The reaction was carried out under nitrogen at 0
°C with stirring. Cold acetaldehyde (20 mL, 0.36 mol) was
added via syringe and stirring at 0 °C was continued for 2 h.
Sodium borohydride (25.8 g, 0.68 mol) was then slowly added
over 3 h, keeping the internal temperature at -7 to -13 °C.
After being stirred for an additional 20 min, the reaction
mixture was slowly added to a stirred solution of 3 M HCl (340
mL) at 0 °C; CHCl3/i-PrOH (3:1, 800 mL) was then added
followed by 5 M NaOH to pH 10.5. After saturation with solid
NaCl, the organic phase was separated, and the aqueous was
extracted with CHCl3/i-PrOH (3:1, 2 × 600 mL) while main-
taining the pH of the aqueous at 10.5. The combined organic
phases were dried over sodium sulfate and evaporated below
35 °C until precipitation occurred. Filtration gave 44.3 g
(58.2%) of 4d as a white solid. Evaporation of the mother
liquor, followed by crystallization from dichloromethane, gave
an additional 3.8 g (5.0%) of 4d , mp 100-101 °C; [R]20D +17.7°
(c 1.17, CHCl3), 1H NMR (400 MHz, CDCl3) δ 1.11 (t, J ) 7.13
Hz, 3H), 1.64 (brs, 1H), 2.57 (dq, J ) 11.21, 7.18 Hz, 1H), 2.74
(dq, J ) 11.21, 7.06 Hz, 1H), 3.43 (dd, J ) 6.60, 4.57 Hz, 1H),
3.59 (dd, J ) 10.60, 6.63 Hz, 1H), 3.79 (dd, J ) 10.60, 4.52
Hz, 1H), 5.19 (s, 2H), 7.33-7.40 (m, 5H); 13C NMR (100 MHz,
CDCl3) δ 15.4, 42.6, 62.6, 66.9, 128.2, 128.7, 135.7, 173.6. Anal.
Calcd for C12H17NO3: C, 64.55; H, 7.67; N, 6.27. Found: C,
64.65; H, 7.80; N, 6.27.
It will be of interest to determine the relative musca-
rinic potency of these pilocarpine analogues and also to
investigate the possibility that structures 3, 13, and 14
may represent metabolites of 2. The availability of
relatively large amounts of ketone 3 by the synthetic
route described makes the synthesis of additional ana-
logues a reasonable goal. This is particularly important
because, although well-known and medically useful,
pilocarpine has received relatively little attention in
terms of structure-activity relationships.
J . Org. Chem, Vol. 67, No. 17, 2002 5915

Holden et al.
(R)-3-Eth yl-2-oxo-4-oxa zolid in eca r boxylic Acid Ben zyl
Ester (5a ). D-N-Ethylserine benzyl ester (4d , 45 g, 0.2 mol)
was suspended in dry nitromethane12 (262 mL), cooled to 0
°C, and treated with 1-methylimidazole (2.1 mL, 0.026 mol).
To the stirred suspension at 0 °C was added a CBMIT solution
[prepared by adding methyl triflate13 (86 g, 0.52 mol) to a
suspension of CDI (42.5 g, 0.26 mol) in nitromethane (205 mL)
at 0 °C and stirring for 30 min] over 2-3 h. The reaction
mixture was concentrated in vacuo, diluted with chloroform
(3 L), washed with 1 M HCl, water, 5% NaHCO3, water, and
brine in succession, dried over sodium sulfate, and evaporated
to a residue. Chromatography (EtOAc/hexane, 1:1) gave 46.11
g (92%) of 5a : [R]20D +40.5° (c 1.0, CHCl3); 1H NMR (400 MHz,
CDCl3) δ 1.14 (t, J ) 7.28 Hz, 3H), 3.19 (dq, J ) 14.20, 7.12
Hz, 1H), 3.61 (dq, J ) 14.27, 7.30 Hz, 1H), 4.31 (dd, J ) 8.02,
4.11 Hz, 1H), 4.37-4.47 (m, 2H), 5.23 (ABq, J ) 12.04, Hz,
2H), 7.34-7.42 (m, 5H); 13C NMR (100 MHz, CDCl3) δ 12.5,
38.3, 56.6, 64.2, 67.8, 128.5, 128.8, 128.9, 134.6, 157.4, 169.6.
Anal. Calcd for C13H15NO4: C, 62.63; H, 6.07; N, 5.62. Found:
C, 62.81; H, 6.21; N, 5.79.
3H, diastereomeric mixture), 1.61-1.88 (m, 6H), 3.24 (approxi-
mately sextet, 1H), 3.62-3.87 (m, 3H), 4.35-4.40 (m, 2H),
4.41-4.51 (m, 1H), 6.11, 6.12 (two s, 1H, diastereomeric
mixture).
2,5-Diiodo-1-m eth ylim idazole (7). 1-Methylimidazole (4.45
mL, 52.8 mmol) was added over 20 min to a well-stirred
solution of TMEDA (20.0 mL, 135 mmol, 240 mol %) and
n-BuLi (52 mL, 2.54 M in hexanes, 132 mmol, 237 mol %) in
pentane (55 mL) under nitrogen at -20 °C. The stirred, cloudy,
yellow, heterogeneous mixture was warmed to 22 °C for 1 h
and then diluted with anhydrous THF (50 mL) and cooled to
-65 °C. A solution of iodine (34.7 g, 137 mmol, 245 mol %) in
anhydrous THF (180 mL) was added over 90 min. During the
addition, the internal temperature of the thick, light-brown
reaction mixture rose to -39 °C. The stirred reaction mixture
was allowed to warm to 22 °C over 9 h. During this process,
the reaction mixture suddenly became a dark, homogeneous
solution at -19 °C. The reaction was quenched by the addition
of ethyl acetate (10 mL) followed by water (10 mL), dichloro-
methane (250 mL), and saturated Na2SO3 solution (125 mL).
The organic layer was separated, washed with saturated Na2-
SO3 (3 × 50 mL) and brine (3 × 25 mL), dried over MgSO4,
filtered, and evaporated to yield 7 (16.7 g, 89%) as a golden-
brown solid which could be recrystallized from chloroform: mp
150-152 °C (lit. 154 °C):8 Rf 0.62 (CH2Cl2/MeOH/NH4OH, 88:
(R)-3-Eth yl-2-oxo-4-oxa zolid in eca r boxylic Acid (5b).
Benzyl ester 5a (23.1 g, 93 mmol) in EtOAc (125 mL) was
hydrogenated in the presence of 5% Pd/C (3.5 g) at 60 psi and
room temperature. Hydrogen was consumed rapidly, but the
reaction was allowed to continue for 1.5 h. Filtration through
Celite followed by evaporation of the solvent gave 14.4 g
(quantitative) of 5b as a white solid: mp 103-109 °C (crude),
1
10:2); H NMR (CDCl3) δ 7.21 (s, 1H, CH, C-4), 3.65 (s, 3H,
CH3).
110-111.5 °C after recrystallization (EtOAc/diethyl ether);
5-Iod o-1-m eth yim id a zole (8a ). To a stirred solution of 7
(2.80 g, 8.39 mmol) in anhydrous THF (90 mL) at -78 °C under
nitrogen was slowly added n-BuLi (3.45 mL, 2.55 M in
hexanes, 105 mol %) during 18 min. After being stirred for an
additional 40 min, the reaction was quenched by the addition
of methanol (3 mL) and diluted with dichloromethane (150
mL). The organic layer was washed with brine (2 × 20 mL),
dried (MgS04), filtered, and evaporated to give a yellow-brown
solid (1.92 g, >100%), which was recrystallized from chloroform
to yield 8a (1.23 g, 71%): mp 104-105 °C (lit. 107 °C):8 Rf 0.4
1
[R]21 +47.8° (c 1.025, CHCl3); H NMR (400 MHz, CDCl3) δ
D
1.20 (t, J ) 7.31 Hz, 3H), 3.27 (dd, J ) 14.22, 7.14 Hz, 1H),
3.65 (dd, J ) 14.28, 7.37 Hz, 1H), 4.41-4.45 (m, 2H), 4.54 (like
t, J ) 10.02 Hz, 1H), 8.94 (brs, 1H); 13C NMR (100 MHz,
CDCl3) δ 12.4, 38.3, 56.3, 64.9, 158.5, 172.6.
Deter m in a tion of En a tiom er ic P u r ity of 5b. Carboxylic
acid 5b (1 mmol) was dissolved in dry THF (12 mL), cooled to
-20 °C, and treated with isobutylchloroformate (1.2 mmol)
with stirring, under a nitrogen atmosphere for 40-60 min. To
the reaction mixture was added (R)-(+)-R-methylbenzylamine
(1.1 mmol), and the reaction was then allowed to warm to room
temperature. The solvent was removed in vacuo, and the
residue was dissolved in EtOAc, washed successively with 1
M HCl, water, 5% NaHCO3, water and brine, dried over
sodium sulfate, and evaporated. The crude amide residue was
evaluated for enatiomeric purity using HPLC (Microsorb Si,
mobile phase EtOAc/n-hexane, 3:1); the er was found be be
approximately 98/2. When CDI was used in place of CBMIT
for formation of the oxazolidinone, the enantiomeric purity was
determined to be in the range of 80-88% and could not be
substantially improved by recrystallization.
(R)-3-Eth yl-2-oxo-4-oxa zolid in eca r boxylic Acid , Tet-
r a h yd r op yr a n yl Ester (5c). To a stirred solution of 5b (159
mg, 1.0 mmol) in dichloromethane (5 mL) at 0 °C were added
dihydropyran (0.91 mL, 10 mmol) and methanesulfonic acid
(0.65 µL, 0.01 mmol). After being stirred for 1 h at 0 °C, the
reaction was quenched by addition of 1,2-ethylenediamine
(2.67 µL, 0.04 mmol) followed by chilled NaHCO3 solution
(H2O/brine/sat. NaHCO3, 1:1:1, 0.5 mL). After dilution with
dichloromethane (30 mL), the organic phase was washed with
NaHCO3 and brine and dried over Na2SO4/Na2CO3. Evapora-
tion of the solvent gave 215 mg (88%) of 5c as a colorless oil:
1H NMR (400 MHz, CDCl3) δ 1.20, 1.21 (two t, J ) 7.2 Hz,
1
(EtOAc/MeOH/NH4OH, 88:10:2); H NMR (CDCl3) δ 7.71 (s,
1H), 7.08 (s, 1H), 3.61 (s, 3H, CH3).
5-Iod o-1-m eth ylim id a zole (8a ), Dir ectly fr om 1-Meth -
ylim id a zole. To a stirred solution of TMEDA (32.3 mL, 214
mmol, 237 mol %) in dry pentane (75 mL) under nitrogen at
-20 °C was added n-BuLi (94.4 mL, 2.3 M in hexanes, 217
mmol, 240 mol %). 1-Methylimidazole (7.21 mL, 90.4 mmol,
100 mol %) was added dropwise over 25 min, and the cooling
bath (-25 °C) was removed approximately halfway through
the addition. The stirred cloudy, yellow mixture was warmed
to 22 °C for 1 h, whereupon a yellow/white precipitate formed.
The suspension was cooled to -65 °C and anhydrous THF (220
mL) was added at such a rate that the internal temperature
remained below -20 °C. A solution of iodine (33.3 g, 131 mmol,
145 mol %) in anhydrous THF (140 mL) was added to the
vigorously stirred suspension at such a rate that the internal
temperature (initially at -65 °C) did not rise above -20 °C.
Stirring was continued as the reaction was gradually warmed
to 0 °C over 2 h while the solids dissolved. TLC indicated
virtually no diiodoimidazole remaining. After 0.5 h at 0 °C,
the reaction mixture was quenched by adding methanol (15
mL) followed by brine (70 mL). After addition of dichlo-
romethane (350 mL), the organic phase was separated, washed
with saturated Na2SO3 (2 × 50 mL) and brine (3 × 25 mL),
dried (Na2SO4), filtered, and evaporated to yield 8a (12.3 g,
65%) as a yellow-brown solid (mp 104 °C), which was recrys-
tallized from chloroform to give 7.93 g: mp 104-5 °C, identical
with the product derived from 7.
(12) Dry nitromethane was prepared by treating with CaCl2 or
CaSO4 for one day followed by molecular sieves (3 Å, freshly activated
at 400˚ C) twice for two days and stored in the dark. It has been
reported that nitromethane may explode when heated with basic
impurities.
(13) This was prepared according to the following procedure: tri-
fluoromethanesulfonic acid (1 mol) was added via a dropping funnel
into stirred dimethyl sulfate (1.2 mol, previously distilled from P2O5).
The reaction mixture was then distilled to give colorless methyl triflate
(bp 92-100˚ C/760 mm Hg). Methyl triflate is a powerful methylating
agent and is thus a suspected carcinogen that should be handled with
appropriate care.
1-Meth ylim id a zole-5-ca r boxa ld eh yd e (8c). To a stirred
solution of 8a (104 mg, 0.50 mmol, 100 mol %) in dichloro-
methane (2.2 mL) at 20 °C under nitrogen was added ethyl-
magnesium bromide (203 µL, 2.7 M in ether, 110 mol %) over
2 min. After stirring for 1 h, the heterogeneous reaction
mixture was cooled to 0 °C, and dimethylformamide (77 µL,
1.0 mmol, 200 mol %) was added. After being warmed to 22
5916 J . Org. Chem., Vol. 67, No. 17, 2002

Synthesis of Chiral Pilocarpine Analogues
°C and stirring for 2 h, the reaction was quenched with
saturated Na2HPO4 (4 mL) and extracted with dichlormethane/
2-propanol (DCM/IPA, 3:1,1 × 35 mL, then 4 × 10 mL). The
combined organic extracts were dried (MgSO4), filtered, and
evaporated to give 8c (55 mg, ∼100%, containing a small
amount of DMF): 1H NMR (400 MHz, CD3OD) δ 9.07 (d, 1H,
CHO), 7.72 (s, 1H), 7.58 (s, 1H), 3.88 (s, 1H, CH3). This
material was identical to an authentic sample of 1-methylimi-
dazole-5-carboxaldehyde previously prepared by an unambigu-
was purified by column chromatography (CHCl3/methanolic
NH3, 14:1) to give 13 (30 mg, ∼quantitative). This material
appeared to be a diastereomeric mixture by NMR (13/14, 30/
1). The oily product crystallized on standing, and on trituration
with diethyl ether produced pure 13 as a white solid: mp 128-
129 °C; 1H NMR (400 MHz, CDCl3) δ 1.22 (t, J ) 7.1 Hz, 3H),
3.54-3.65 (m, 2H), 3.74 (s, 1H), 3.78-3.80 (m, 1H), 4.20-4.80
(m, 2H), 4.70 (d, J ) 8.2 Hz, 1H), 6.81 (s, 1H), 7.40 (s, 1H); 13
C
NMR (100 MHz, CDCl3) δ 12.7, 32.3, 39.3, 57.5, 64.8, 68.5,
126.6, 130.7, 139.2, 158.4.
ous route14
.
(R)-3-E t h yl-4-(1′-m et h yl-5′-im id a zoyl)-2-oxa zolid in o-
n e (3). To a stirred solution of 5b (139 mg, 1.0 mmol) in
dichlomethane (8 mL) at -20 °C under nitrogen were added
dropwise dihydropyran (0.11 mL, 1.2 mmol) and methane-
sulfonic acid (0.65 µL, 0.01 mmol). The reaction was allowed
to warm to 0 °C during 1.5 h and stirred for 2 h. Separately,
a Grignard reagent was prepared by adding ethylmagnesium
bromide solution (2.0 mL, 2.3 mmol) to a stirred solution of
8a (437 mg, 2.1 mmol) in dichloromethane (9 mL) at 20 °C
under nitrogen over 1 h. After stirring for an additional 20
min, the resulting suspension was transferred via cannula to
the previously prepared solution of 5c. The addition was
carried out at -20 °C over 10-15 min, and the reaction was
allowed to warm to 0 °C over 1-1.5 h. After stirring over.night
at 4 °C (cold room), the reaction was stirred at room temper-
ature for 1 h and then poured into a stirred mixture of 1 M
KH2PO4/dichoromethane (1:1, 30 mL) at 5 °C. The pH was
adjusted to 5.5, the organic phase was separated, and the
aqueous phase was extracted with dichloromethane (3 × 30
mL). The combined organic phases were washed with satu-
rated NaHCO3/water (1:1, 10 mL), dried over sodium sulfate,
and evaporated to give 3 (216 mg, 97%). The crude product
was purified by column chromatography (chloroform/methanol,
15:1) to afford 3 (120 mg, 54%) as a white solid: mp 93-98 °C
(R,R)-1-(3-E t h yl-2-oxo-4-oxa zolid in yl)-1-(1′-m et h yl-5′-
im idazolyl)m eth an ol (14), by Redu ction of 3 with Lith iu m
Bor oh yd r id e. A 2.0 M solution of LiBH4 (135 µL, 0.27 mmol)
was added dropwise to a stirred solution of 3 (40 mg, 0.18
mmol) in THF (7 mL) at -78 °C. After being stirred at -78
°C for 2 h, the reaction mixture was warmed to -10 °C and
quenched with phosphate buffer (pH 6, 2 mL). Extraction with
CHCl3/i-PrOH (3:1, 3 × 20 mL) followed by washing of the
combined organic phase with phosphate buffer (pH 6.0, 10 mL),
drying (Na2SO4), and evaporation gave the crude product,
which was purified by column chromatography (CHCl3/metha-
1
nolic NH3, 13:1). NMR analysis of the separated components
indicated a mixture of 13 and 14 (21 mg, 51%, 13/14, 1/1.3)
and a borane complex (14 mg, 13/14, 1/1.7). The borane
complex was dissolved in CHCl3/i-PrOH (3:1, 15 mL) and
stirred with 1 M phosphoric acid. The pH was adjusted to 6.0
with NaHCO3 and the aqueous phase was extracted with
CHCl3/i-PrOH (3:1, 2 × 10 mL). The combined organic extracts
were dried over sodium sulfate and evaporated to give ad-
ditional 13 and 14 (6 mg, 15%, 13/14, 1/1.3): total yield 66%.
A 12 mg sample of the mixture was recrystallized from EtOAc/
MeOH to give 5 mg of 14 as a white solid: mp 203-204 °C,
1H NMR (400 MHz, CD3OD) δ 1.18 (t, J ) 7.2 Hz, 3H), 3.16
(approximately sextet, 1H), 3.55 (approximately sextet, 1H),
3.75 (s, 3H), 4.33-4.35 (m, 1H), 4.45 (approximately d, J )
7.1 Hz, 2H), 5.01 (like d, 1H), 6.18 (s, 1H), 7.63 (s, 1H).
(before recrystallization), mp 102.5-103 °C (after recrystal-
1
liztion from EtOAc/hexane); [R]20 +82.0° (c 1.11, CHCl3); H
D
NMR (400 MHz, CDCl3) δ 1.16 (t, J ) 7.3 Hz, 3H), 3.12-
(approximately sextet, 1H), 3.63 (approximately sextet, 1H).
3.98 (s, 3H), 4.22 (dd, J ) 8.8, 5.5 Hz, 1H), 4.57 (approximately
t, J ) 8.90 Hz, 1H), 4.99 (dd, J ) 9.8, 5.4 Hz, 1H), 7.70 (s,
1H), 7.82 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 12.7, 35.0,
38.2, 59.4, 64.9, 128.5, 139.2, 145.1, 157.8, 185.8. Anal. Calcd
for C10H13N3O3: C, 53.81; H, 5.87; N, 18.82. Found: C, 53.72;
H, 5.91; N, 18.78.
Deter m in a tion of Sta bility a n d En a n tiom er ic P u r ity
of 3. The purity of 3 was evaluated using a CHIRACEL OD-R
reverse phase chiral column, 250 × 4.6 mm. The HPLC
conditions were as follows: flow rate, 0.5 mL/min; mobile
phase, CH3CN/H2O (1:9); injection volume, 20 µL; detector, 260
nm; retention time, 28 min for minor and 32 min for major
enantiomer. The sample of ketone 3 used in this analysis was
found to have an enantiomeric ratio of 96:4.
Three samples of 3 were treated under various conditions
to investigate the conformational stability of the ketone. The
first sample was kept neat at 50 °C for 2 h, the second sample
was subjected to column chromatography using CHCl3/i-PrOH
(7:1) as eluant (column residence time 2 h), and the third
sample was treated with diisopropylethylamine (100 mol %)
in THF at room temperature for 3 h. As shown by chiral HPLC,
all samples were stable under the described conditions.
(R,S)-1-(3-Eth yl-2-oxo-4-oxa zlid in yl-1-(1′-m eth yl-5′-im i-
d a zolyl)m eth a n ol (13) by Red u ction of 3 w ith L-Selec-
tr id e. To a stirred solution of 3 (30 mg, 0.13 mmol) in THF (5
mL) under nitrogen at -78 °C was slowly added a solution of
L-Selectride (0.72 mL, 1.0 M, 0.72 mmol). After 2 h at -78
°C, the reaction was quenched by the addition of phosphate
buffer (pH 6.0, 2 mL) and extracted with CHCl3/i-PrOH (3:1,
3 × 20 mL). The combined organic phases were dried over
sodium sulfate and evaporated to yield a crude product, which
(R)-3-Eth yl-2-oxo-4-oxa zolid in eca r boxylic Acid (5b),
Dir ectly fr om D-Ser in e. A solution of D-serine (52.5 g, 0.50
mol) and acetaldehyde (24.2 g, 0.55 mol) in water (500 mL)
was stirred at 0 °C while solid sodium borohydride (19.0 g,
0.50 mol) was added at such a rate that the internal temper-
ature did not rise above 15 °C. After 2 h, stirring was stopped,
and the reaction mixture (heavy white precipitate) was allowed
to stand overnight at room temperature. To destroy excess
borohydride, the cooled reaction mixture was carefully treated
with 20% HCl until strongly acidic (pH 1-2). Following
adjustment of the pH to 6.5 with 50% NaOH and then to 8.5
with 4 M NaOH, the stirred reaction mixture was treated
alternately with isobutyl chloroformate and 4 M NaOH,
keeping the pH between 7.5 and 9.0 (about 7 h). After standing
overnight, the reaction mixture was filtered, and the filtrate
was treated with saturated sodium carbonate solution (450
mL) and distilled during 2 h to give approximately 500 mL of
a mixture of isobutanol and water. The cooled pot residue was
acidified to pH 1-2, treated with sodium chloride (250 g), and
extracted with ethyl acetate (9 × 100 mL). Evaporation of the
combined and dried (sodium sulfate) organic phases gave 39.51
g of crude product. Re-extraction of the aqueous with ethyl
acetate/tert-butyl alcohol (2:1, 6 × 100 mL) gave, after drying
and evaporation, an additional 8.48 g for a total of 47.99 g
(0.302 mol, 60%). The crude product could be recrystallized
from ethyl acetate/cyclohexane or methylene chloride (chilled
in freezer at - 20 °C) to give 5b identical with that prepared
by the benzyl ester route.
(R)-3-Eth yl-2-oxo-4-oxa zolid in eca r boxylic Acid Eth yl
Ester (5d ). A solution of acid 5b (15.90 g, 0.10 mol) in 99.5%
ethanol (100 mL) containing methanesulfonic acid (0.48 g, 0.05
mol) was slowly distilled over 2 h, collecting 45 mL of distillate.
More 99.5% ethanol (50 mL) was added to the pot residue,
and distillation was continued for another 2 h. The cooled
reaction mixture was treated with sodium bicarbonate and
evaporated to near dryness. The residue was taken up in
(14) Dener, J . M.; Zang, L. -H.; Rapoport, H. J . Org Chem., 1993,
58, 1159.
J . Org. Chem, Vol. 67, No. 17, 2002 5917

Holden et al.
ethyl acetate (30 mL) and water (15 mL). After the organic
phase was separated, the aqueous was extracted with ethyl
acetate (2 × 20 mL), and the combined organic phases were
dried (sodium sulfate) and evaporated to give 17.83 g of 5d
(0.0956 mol, 95%). A sample of this material was distilled (bp
125-129°, 2 mm) to give a clear oil: Rf ) 0.74 (methylene
chloride/ethyl acetate, 7:3). Anal. Calcd for C8H13NO4: C, 51.3;
H, 7.0; N, 7.5. Found: C, 51.22; H, 6.99; N, 7.50.
ester route. Further elution of the column with 5% ethanol in
methylene chloride gave 11 (0.22 g, 0.87 mmol, 7%), which
was purified by recrystallization from ethyl acetate/ether, mp
161-162 °C, (Rf ) 0.28, methylene chloride/ethyl acetate/
ethanol, 5:3:2). Anal. Calcd for C12H19N3O3: C, 56.9; H, 7.56;
N, 16.59. Found: C, 56.87; H, 7.65; N, 16.45.
(R)-1-(Eth yl-2-oxo-4-oxa zolid in yl)-1-(1′-m eth yl-5′-im i-
d a zolyl)eth en e (12). To a stirred suspension of methyltri-
phenylphosphonium bromide (0.90 g, 2.4 mmol) in anhydrous
THF (5 mL) under an argon atmosphere at 0 °C was added 2
M n-butyllithium in cyclohexane (1.15 mL, 2.3 mmol). After
stirring for 30 min, a solution of 3 in THF (3 mL) was added
gradually over a few minutes. The reaction mixture was
allowed to stir for 1 h while it warmed to room temperature.
After being stirred at reflux for 3 h, the cooled reaction mixture
was stirred with 1 mL of 50% sodium hydroxide for 15 min
and then neutralized with 20% hydrochloric acid. Evaporation
of most of the solvent gave a residue, which was distributed
between water (10 mL) and methylene chloride (10 mL).
Additional acid was added to bring the aqueous pH to 1-2.
This was extracted with methylene chloride (2 × 10 mL) to
remove neutrals (triphenylphosphine and triphenylphosphine
oxide). The aqueous was made basic (pH 9) with 4 M sodium
hydroxide and extracted with methylene chloride/2-propanol,
3:1 (4 × 10 mL), dried (sodium sulfate), and evaporated to give
0.44 g of crude product which contained 12 (Rf ) 0.53,
methylene chloride/ethyl acetate/ethanol, 5:3:2) and an uni-
dentified product (Rf ) 0.46). Despite their similar Rfs on silica,
these materials were easily separated by chromatography on
activity III, neutral alumina (20 g). Elution with methylene
chloride and 1% ethanol in methylene chloride gave 12 (0.27
g, 61%), which was purified by recrystallization from ethyl
acetate/hexanes, mp 98-98 °C. Anal. Calcd for C11H15O2N3:
C, 59.71; H, 6.83; N, 18.99. Found: C, 59.68; H, 7.05; N, 19.00.
(R,S)-1-(3-E t h yl-2-oxo-4-oxa zolid in yl)-1-(1′-m et h yl-5′-
im id a zolyl)m eth a n ol (13) a n d (R,R)-1-(3-Eth yl-2-oxo-4-
oxa zold in yl)-1-(1′-m eth yl-5′-im id a zolyl)m eth a n ol (14) by
Red u ction of 3 w ith Sod iu m Bor oh yd r id e. To a stirred
solution of sodium borohydride (0.084 g, 2.2 mmol) in water
(3 mL) at 0 °C was added a solution of 3 (0.45 g, 2.0 mmol) in
2-propanol (2 mL) during 3 min. After being stirred for 1 h
while the reaction mixture warmed to room temperature, it
was treated dropwise with 20% hydrochloric acid until the pH
reached 1-2. After 10 min, the pH was adjusted to 8-9 with
4 M NaOH and extracted with methylene chloride (6 mL)
followed by methylene chloride/2-propanol (3:1, 2 × 6 mL). The
combined organic phases were dried (sodium sulfate) and
evaporated (0.48 g, >100%). Chromatography on 30 g of
neutral, activity III alumina gave only partial separation of
the epimeric alcohols, both eluting with methylene chloride
containing increasing amounts of ethanol (1.5-15%). However,
recrystallization from ethanol/ethyl acetate gave pure 13 (Rf
) 0.25, methylene chloride/ethyl acetate/ethanol, 5:3:2), mp
135-136 °C. Anal. Calcd for C10H15O3N3: C, 53.32; H, 6.71;
N, 18.66. Found: C, 53.26; H, 6.74; N, 18.61. Pure 14 could
be obtained by recrystallization of the mother liquor residue
from ethanol/ethyl acetate (Rf ) 0.18), mp 207-208 °C. Anal.
Calcd for C10H15O3N3: C, 53.32; H, 6.71; N, 18.66. Found: C,
53.06; H, 6.92; N, 18.31. Except for small differences in melting
points, these samples of 13 and 14 were identical with those
produced with other reducing agents.
5-Br om o-1-m eth ylim id a zole (9). This was prepared es-
sentially as described in the reference8 except that purification
did not involve chromatography, and the yield of product was
consistently lower than reported. To a stirred solution of
1-methylimidazole (76.8 g, 0.80 mol) in chloroform (500 mL)
was added N-bromosuccinimide (171 g, 0.96 mol) in portions
such that the reaction temperature remained just below reflux.
The reaction mixture was then heated at reflux for 2 h. The
cooled reaction mixture was stirred with saturated sodium
carbonate (200 mL) and filtered. The organic phase was
separated and washed with 0.25 M trisodium phosphate (2 ×
175 mL), and the aqueous phases were extracted, with
methylene chloride (2 × 50 mL). The combined organic phases
were dried (sodium sulfate) and evaporated to give 80.66 g of
crude 9. Distillation (89-121 °C at 2.5 mm) gave material
(48.71 g, 0.30 mol, 38%) that was predominantly 9 (Rf ) 0.31,
methylene chloride/ethyl acetate, 7:3) but was contaminated
with some starting material; the pot residue was mainly 4,5-
dibromo-1-methylimidazole (Rf ) 0.61). Attempted removal of
1-methylimidazole by extraction with aqueous base was only
partially successful; however, redistillation at 68-71 °C/ 2 mm
gave a low melting solid. Crystals from this source were
washed with cold 1,1,1-trichloroethane and distilled via a
short-path apparatus to give an analytical sample, mp 47-49
°C (reported8 as an oil). Anal. Calcd for C4H5N2Br: C, 29.84;
H, 3.13; N, 17.40. Found: C, 29.82; H, 3.08; N, 17.36.
(R)-3-E t h yl-4-(1′-m et h yl-5′-im id a zoyl)-2-oxa zolid in o-
n e (3) a n d (R,S)-1-(3-Eth yl-2-oxo-4-oxa zolid in yl)-1-(1′-
m eth yl-5′-im id a zolyl)p r op a n ol (11). A stirred solution of
5-bromo-1-methylimidazole (9, 1.93 g, 12 mmol) in anhydrous
methylene chloride (10 mL), under an argon atmosphere, was
treated with a 3 M solution of ethylmagnesium bromide (4.0
mL, 12 mmol) at such a rate that the reaction mixture was
kept just below reflux. A precipitate began to form during the
addition but redissolves at completion to give a clear golden
solution. After an additional 15 min, the solution of 10 was
added in portions, via syringe, to a stirred solution of oxazol-
dinone ester 5d (1.87 g, 10 mmol) in methylene chloride (10
mL), which was maintained under an argon atmosphere at 0
°C. The addition took place over 1 h, during which time a dense
precipitate formed and eventually turned into sticky clumps
that inhibited stirring. After stirring for an additional 1 h at
0 °C, the reaction was quenched by the addition of 1 M
hydrochloric acid (15 mL) to give an aqueous solution with a
pH of 6.0-6.5. The organic phase was separated, dried (sodium
sulfate), and evaporated to give 2.11 g. Extraction of the
aqueous with ethyl acetate/tert-butyl alcohol (2 × 20 mL) gave
0.52 g. The aqueous was treated with 3.0 g of ammonium
chloride, adjusted to pH 8.0-8.5, and extracted with ethyl
acetate/tert-butyl alcohol (3 × 20 mL) to give an additional
0.19 g. Since all three factions contained 3 (Rf ) 0.50,
methylene chloride/ethyl acetate/ethanol, 5:3:2, strong UV),
they were combined and chromatographed on neutral, activity
III alumina (50 g). Early fractions eluted with hexanes/
methylene chloride 1:1 and methylene chloride contained
mostly starting materials (9 and 5d along with 1-methylimi-
dazole and products resulting from the addition of 1 or 2 mol
of ethylmagneium bromide to ester 5d ) totaling 1.52 g.
Fractions eluting later, with methylene chloride and 2%
ethanol in methylene chloride, contained mostly 3 (1.10 g, 4.93
mmol, 49%). Recrystallization from ethyl acetate/cyclohexane
gave pure 3, mp 117-119 °C, which, except for its higher
melting point, appeared identical with 3 prepared via the THP
Ack n ow led gm en t. K. H. Cha thanks the Korea
Science and Engineering Foundation for partial support.
Su p p or tin g In for m a tion Ava ila ble: Chiral HPLC data
on samples of ketone 3 prepared under different reaction
conditions. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O0111210
5918 J . Org. Chem., Vol. 67, No. 17, 2002