Synthesis of 4-cis-Phenyl-l-proline via Hydrogenolysis

Full text of DOI:10.1021/jo005625u

J . Org. Chem. 2001, 66, 3593-3596
3593
Ch a r t 1. Exa m p les of P r olin e-Rela ted ACE
Syn th esis of 4-cis-P h en yl-L-p r olin e via
In h ibitor s
Hyd r ogen olysis
Makoto Tamaki, Guoxia Han, and Victor J . Hruby*
Department of Chemistry, University of Arizona,
Tucson, Arizona 85721
hruby@u.arizona.edu
Received August 27, 2000
In tr od u ction
Proline is an important amino acid in many naturally
occurring bioactive peptides such as gramicidin1,2 and
R-melanotropin,3,4 both of which have important biologi-
cal activities. In addition, proline and its 4-substituted
derivatives have been extensively used in the pharma-
ceutical industry, such as in angiotensin-converting
enzyme (ACE) inhibitors, including Captopril,5 Enala-
pril,6 Fosinopril,7 and Lisinopril6 (Chart 1), which are
widely introduced to treat hypertension and congestive
heart failure. Furthermore, among the 20 naturally
occurring amino acids proline is the only residue found
in proteins that is cyclic and a secondary amine, provid-
ing novel constraints. Hence, its unique structural prop-
erties and medicinal potentials have directed our re-
search to the design and synthesis of novel proline
derivatives. It has been found that appropriate confor-
mational constraints provide a powerful method for the
design of peptide ligands with potent bioactivities and
selectivities.8-17 In this regard, proline provides a special
structure for stabilizing turns and other secondary
structures in compounds that incorporate it.18-22 In
searching for novel constrained amino acids for our
design of selective and potent ligands, we have found a
simple method for substitution at the 4-position of proline
starting from 4-trans-hydroxy-proline (2), a naturally
abundant amino acid.
Resu lts
More than a decade ago, it was reported that 4-cis-
phenylproline (1) can be made by hydrogenation (Scheme
1, Path A) of the double bond in compound 3.7 However,
attempts to repeat the literature methods were not
successful in our hands. To make this compound readily
available for our ongoing investigation of its application
in biological research, we have investigated a new and
practical route to this compound, which has been suc-
cessfully synthesized by hydrogenolysis (Scheme 1, Path
B) of the 4-cis-hydroxy-4-trans-phenylproline derivative
(7). Initially, hydroxy proline (2) was esterified in dry
methanol saturated with dry hydrogen chloride gas to
give 5 (Scheme 2) in a quantitative yield. The NR-amino
group was then Boc protected, and the hydroxyl group
was oxidized by CrO3 in a rather low yield (ca. 40%).23
Attempts to optimize this procedure by slow addition of
the oxidant did not improve the yield significantly, and
lowering the temperature did not either. By applying
PDC as the oxidant, a slightly improved yield (50%) for
6b was achieved.
* Phone: 520-621-6332. Fax: 520-621-8407.
(1) Tamaki, M.; Okitsu, T.; Araki, M.; Sakamoto, H.; Takimoto, M.;
Muramatsu, I. Bull. Chem. Soc. J pn. 1985, 58, 531-535.
(2) Prenner, E. J .; Lewis, R.; McElhaney, R. N. Biochim. Biophys.
Acta-Biomembr. 1999, 1462, 201-221.
(3) Cody, W. L.; Wilkes, B. C.; Muska, B. J .; Hruby, V. J .; Castrucci,
A. M. D.; Hadley, M. E. J . Med. Chem. 1984, 27, 1186-1190.
(4) Hruby, V. J .; Wilkes, B. C.; Cody, W. L.; Sawyer, T. K.; Hadley,
M. E. Pept. Protein Rev. 1984, 3, 1-64.
(5) Ondetti, M. A.; Cushman, D. W. Science 1977, 196, 441-444.
(6) Patchett, A. A.; Harris, E.; Tristram, E. W.; Wyvratt, M. J .; Wu,
M. T.; Taub, D.; Peterson, E. R.; Ikeler, T. J .; Tenbroeke, J .; Payne, L.
G.; Ondeyka, D. L.; Thorsett, E. D.; Greenlee, W. J .; Lohr, N. S.;
Hoffsommer, R. D.; J oshua, H.; Ruyle, W. V.; Rothrock, J . W.; Aster,
S. D.; Maycock, A. L.; Robinson, F. M.; Hirschmann, R.; Sweet, C. S.;
Ulm, E. H.; Gross, D. M.; Vassil, T. C.; Stone, C. A. Nature 1980, 288,
280-283.
(7) Krapcho, J .; Turk, C.; Cushman, D. W.; Powell, J . R.; Deforrest,
J . M.; Spitzmiller, E. R.; Karanewsky, D. S.; Duggan, M.; Rovnyak,
G.; Schwartz, J .; Natarajan, S.; Godfrey, J . D.; Ryono, D. E.; Neubeck,
R.; Atwal, K. S.; Petrillo, E. W. J . Med. Chem. 1988, 31, 1148-1160.
(8) Narasimhan, L. S.; Rubin, J . R.; Holland, D. R.; Plummer, J . S.;
Rapundalo, S. T.; Edmunds, J . E.; St-Denis, Y.; Siddiqui, M. A.;
Humblet, C. J . Med. Chem. 2000, 43, 361-368.
(15) Boteju, L. W.; Nikiforovich, G. V.; HaskellLuevano, C.; Fang,
S. N.; Zalewska, T.; Stropova, D.; Yamamura, H. I.; Hruby, V. J . J .
Med. Chem. 1996, 39, 4120-4124.
(16) Toth, G.; Russell, K. C.; Landis, G.; Kramer, T. H.; Fang, L.;
Knapp, R.; Davis, P.; Burks, T. F.; Yamamura, H. I.; Hruby, V. J . J .
Med. Chem. 1992, 35, 2384-2391.
(9) Ranganathan, D.; Haridas, V.; Kurur, S.; Nagaraj, R.; Bikshapa-
thy, E.; Kunwar, A. C.; Sarma, A. V. S.; Vairamani, M. J . Org. Chem.
2000, 65, 365-374.
(17) Hruby, V. J .; Sharma, S. D. Curr. Opin. Biotechnol. 1991, 2,
599-605.
(10) Costantino, G.; Macchiarulo, A.; Pellicciari, R. J . Med. Chem.
1999, 42, 2816-2827.
(18) Saviano, M.; Isernia, C.; Rossi, F.; Di Blasio, B.; Iacovino, R.;
Mazzeo, M.; Pedone, C.; Benedetti, E. Biopolymers 2000, 53, 189-199.
(19) Suh, J . Y.; Lee, Y. T.; Park, C. B.; Lee, K. H.; Kim, S. C.; Choi,
B. S. Eur. J . Biochem. 1999, 266, 665-674.
(11) Oosterom, J .; Nijenhuis, W. A. J .; Schaaper, W. M. M.; Slootstra,
J .; Meloen, R. H.; Gispen, W. H. H.; Burbach, J . P. H.; Adan, R. A. H.
J . Biol. Chem. 1999, 274, 16853-16860.
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10.1021/jo005625u CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/25/2001

3594 J . Org. Chem., Vol. 66, No. 10, 2001
Sch em e 1. P r oced u r es to Syn th esize 4-cis-P h -P r olin e
Notes
Sch em e 2. Syn th esis of 7b
Sch em e 3. Syn th esis of 7d
Ta ble 1. Gr ign a r d Ad d ition of 6 w ith P h MgBr
carbonyl much less electrophilic and resulted in a greatly
improved yield (ca. 80%, Table 1) for the Grignard
reaction. Only a single diastereoisomer, 4-cis-hydroxyl-
4-phenyl-L-proline derivative (7d ), was obtained. In ad-
dition, the tert-butyl ester protecting group made the final
deprotecting step easier, since both NR-Boc and tert-butyl
ester groups could be removed under mild condition by
trifluoroacetic acid (TFA) in one step.
product
R
yield (%)a
7a
7b
7c
7d
H
CH3
Bn
40-45b
20-30
36c
t-Bu
78-80
Isolated yields. THF was the reaction media. c When THF
was used as the solvent, no desired product was obtained.
a
b
The diastereoselectivity of the Grignard addition of
phenylmagnesium bromide to 6 was assessed. For com-
pound 6b (R ) Me), only one diastereoisomer, methyl
4-cis-hydroxyl-4-phenyl-L-prolinate (7b), was obtained.
Neither high-resolution NMR nor HPLC with a diode
array detector were able to detect any of the other
diastereoisomer. An X-ray structure of compound 7b
demonstrated that the hydroxyl group is cis to the acid
group (see Supplementary Information). Hence, the
phenyl group attacked the carbonyl group only trans to
the acid group. Similarly, for other analogues, 7a , 7c, and
7d , only one diastereoisomer, which has a hydroxyl group
cis to the acid group, was obtained.
Low yields (ca. 20-30%) were obtained for the Grig-
nard addition with phenylmagnesium bromide to 6b, and
quite a few side products were detected by TLC. We
postulated that the problem might arise from a competi-
tive reaction with the carbonyl groups in 6b. Hence, the
carboxyl group was protected in the form of several esters
(Table 1). With a benzyl group as the protecting group,
the yield improved to 36%. Though the phenyl group can
provide steric protection, its electron-withdrawing prop-
erty may increase the electrophilicity of the carbonyl
group and the combined effect may lead to only a small
improvement in the yield. Surprisingly, when no protec-
tion was used for the acid group, the Grignard addition
gave a modestly improved isolated yield of 40-45%,
presumably because the negative ion reduced electrophi-
licity of the acid carbonyl group. When the bulky tert-
butyl group was used for ester protection (Scheme 3), its
steric effect and electron-donating character made the
Hydrogenolysis of 7d by hydrogen with Pd(OH)2 in
methanol led to almost pure dehydroxyl product 9d
(Scheme 4) with a de > 95%. In addition, the chiral center
at the 4-position was inverted, as can be seen from the
X-ray structure of 9d (see Supplementary Information).

Notes
J . Org. Chem., Vol. 66, No. 10, 2001 3595
dropwise to a solution of NR-Boc-4-trans-hydroxy-L-proline (8,
3.00 g, 13 mmol) in THF (30 mL) over a period of 10 min at
room temperature, and then the mixture was heated to 55-60
°C for 4 h. Additional O-tert-butyl N,N′-diisopropylisourea (2.98
g, 13 mmol) was added to the mixture, and stirring was
continued overnight. After the mixture cooled, the precipitated
urea was filtered off and the filtrate was evaporated in vacuo to
give a solid residue, which was purified by flash column
chromatography (hexanes/ethyl acetate ) 1:1, v/v) to give the
corresponding tert-butyl ester. Yield, 3.2 g (86%), mp 62-64 °C.
Sch em e 4. Syn th esis of 1
[R]25 ) -64.8 (c ) 1.3, CHCl3). HR-MS calcd for C14H26NO5:
D
[M + H]+ 288.1857, found 288.1821. Rf ) 0.58 (CHCl3/MeOH )
9:1). 1H NMR (CDCl3): 4.44 (hump, 1H), 4.30-4.23 (m, 1H),
3.62-3.49 (m, 2H), 2.33-2.20 (m, 1H), 2.07-1.99 (m, 1H), 1.45-
1.43 (3s, 18H). 13C NMR (CDCl3): 172.14, 154.21, 81.07, 80.12,
69.06, 58.52, 54.53, 39.10, 38.34, 28.28, 27.94, 27.87.
ter t-Bu tyl Nr-Boc-4-k eto-L-p r olin a te (6d ). Tetrapropylam-
monium perruthenate (TPAP, 63 mg) was added in one portion
to a stirred mixture of NR-Boc-4-trans-hydroxy-L-proline (4d , 1
g, 3.47 mmol), N-methylmorpholine N-oxide (0.62 g, 15.6 mmol),
and powdered molecular sieves (4 Å, 1.78 g) in dichloromethane
(7 mL) at room temperature under Ar. The mixture was stirred
for 3 h and then filtered and evaporated in vacuo to give a black
residue. The product was purified by flash column (CH2Cl2/
EtOAc ) 1:1, v/v) and recrystallized from ether and hexane to
give the corresponding ketone 6d . Yield, 0.9 g (90%), mp 66-67
We were not able to separate any intermediate. In fact,
when TLC was used to monitor the reaction, only the
starting material 7d and the product 9d were detected.
Further investigation is needed for the identification of
the reaction mechanism for this step.
Attempts to deprotect both amino and acidic groups
of 9d were carried on in hydrogen chloride gas in ethyl
ether or hydrochloric acid (6 M in water) and gave
mixtures of products. However, deprotection in almost
pure TFA (with 0.2% methionine, Scheme 4) provided
essentially pure amino acid 1 as an oil. Upon treatment
with hydrogen chloride gas in deionized water, 4-cis-
phenyl-L-proline hydrogen chloride (1) was obtained, after
extraction with diethyl ether and subsequent evaporation
of the aqueous solution, as a white powder that was pure
on the basis of NMR, TLC and HPLC.
°C (lit. 67-68 °C).23 [R]25 ) +11.2 (c 1.2, CHCl3), lit. [R]25
)
D
D
10.65 (c 1.23, CHCl3).23 HR-MS calcd for C14H24NO5 [M + H]+
286.1703, found 286.1665. Rf ) 0.55 (hexane/EtOAc ) 7:3). 1H
NMR (CDCl3): 4.70-4.58 (q, J ) 10.4 Hz, 1H), 3.81-3.88 (m,
2H), 3.06-2.85 (m, 1H), 2.58-2.48 (d, J ) 18.8 Hz, 1H). 13C NMR
(CDCl3): 208.88, 208.07, 170.82, 154.31, 153.62, 82.31, 80.97,
57.02, 56.56, 52.92, 52.48, 41.37, 40.84, 28.21, 27.86.
ter t-Bu tyl Nr-Boc-cis-4-h ydr oxy-4-ph en yl-L-pr olin ate (7d).
Phenylmagnesium bromide (3 M in Et2O, 4.5 mL, 13.5 mmol)
was added over a period of 15 min to a magnetically stirred
solution of Boc-4-keto-L-proline-tert-butyl ester (6d , 1.5 g, 5.25
mmol) in dry diethyl ether (75 mL) at -60 °C. After 5 more
minutes of stirring after the completion of the addition of
PhMgBr, a saturated solution of ammonium chloride (15 mL)
was added slowly to quench the reaction. Then the solution was
stirred at room temperature for 1 h, and diethyl ether (50 mL)
was added. The organic layer was successively washed with
water (15 mL × 2) and brine (15 mL) and dried overnight over
anhydrous MgSO4. The solution was concentrated in vacuo. The
resulting residue was recrystallized from hexanes. Yield of 7d ,
Con clu sion
A practical new pathway to the synthesis of 4-cis-
phenyl-L-proline (1) has been developed. A key step
involving a regio- and diastereoselective Grignard reac-
tion has been thoroughly investigated. All of the steps
are easily scalable. Currently, we are investigating other
aryl-substituted prolines using the procedures reported
here.
1.5 g (80%), mp 105-107 °C. [R]25 ) -11.8 (c 1.3, CHCl3). HR-
D
MS calcd for C20H30NO5 [M + H]+ 348.2175, found 348.2177. Rf
1
) 0.55 (hexane/EtOAc ) 7:3). H NMR (CDCl3): 7.54-7.28 (m,
5H), 4.44-4.36 (q, J ) 10.1 Hz, 1H), 3.99-3.90 (m, 1H), 3.79-
3.68 (q, J ) 11.5 Hz, 1H), 2.74-2.67 (m, 1H), 2.32-2.26 (t, J )
14.2 Hz, 1H), 1.55-1.49 (4s, 18H). 13C NMR (CDCl3): 174.20,
173.99, 154.27, 153.80, 141.47, 141.36, 128.38, 127.60, 125.26,
82.82, 82.62, 80.46, 80.30, 80.06, 79.19, 61.42, 60.70, 59.45, 59.38,
44.39, 43.31, 28.35, 28.26, 27.95, 27.88.
ter t-Bu tyl Nr-Boc-cis-4-p h en yl-L-p r olin a te (9d ). tert-Butyl
NR-Boc-cis-4-hydroxy-trans-4-phenyl-L-prolinate (7d , 3.06 g, 8.40
mmol) was hydrogenated over Pd(OH)2 (0.7 g) in methanol (70
mL) for 4 days at room temperature under a H2 atmosphere (3
atm). The catalyst was filtered off, and the filtrate was concen-
trated in vacuo to give a residue, which was purified by
recrystallization from hexane and ethyl acetate. Mp 92-94 °C,
Exp er im en ta l Section
Unless otherwise noted, all materials were obtained from
commercial suppliers and used without further purification.
Analytical thin-layer chromatography was carried out on pre-
coated silica gel plates. Flash column chromatography was
performed using E. Merck Silica gel (230 mesh). Independent
analysis of purity for synthetic products was achieved using
analytical HPLC system (column, Vydac C18; detector, diode
array; flow rate, 1.0 mL/min; gradient, 10-90% acetonitrile, 90-
10% water (0.1% TFA) over 40 min. High-resolution mass
spectra (HR-MS) were obtained at the University of Arizona
Mass Spectroscopy Facility in the Department of Chemistry. All
NMR spectra were obtained on a 500 MHz spectrometer. Optical
rotations were measured on a polarimeter using a 0.5-dm cell.
X-ray crystallography was performed at the University of
Arizona Crystallography Facility.
[R]25 ) -45.4 (c 1.0, CHCl3). HR-MS calcd for C20H28NO4, [M
D
+ H]+ 348.2175, found 348.2177. Rf ) 0.68 (hexane/EtOAc )
7:3). 1H NMR (CDCl3): 7.38-7.27 (m, 5H), 4.33-4.25 (m, 1H),
4.14-3.96 (m, 1H), 3.48-3.43 (m, 1H), 3.42-3.34 (m, 1H), 2.77-
2.68 (m, 1H), 2.07-2.00 (m, 1H), 1.52-1.50 (3s, 18H).
cis-4-P h en ylp r olin e HCl Sa lt (1). NR-Boc-trans-4-phenyl-
L-proline-tert-butyl ester (9d , 100 mg, 0.29 mmol) was dissolved
in TFA (0.2% methionine, 4 mL), and the mixture was stirred
for 2 h at room temperature. The solution was then concentrated.
To the residue was added diethyl ether, and the resulting
precipitate was filtered and dried in vacuo. The solid was then
dissolved in water (20 mL). Pure HCl gas was bubbled into the
solution. The solution was extracted with diethyl ether (20 mL),
and the aqueous layer was then evaporated in vacuo to yield a
ter t-Bu tyl Nr-Boc-4-tr a n s-h yd r oxy-L-p r olin a te (4d ). O-
tert-Butyl N,N′-diisopropylisourea24 (2.98 g, 13 mmol) was added
(24) Bergmeier, S. C.; Cobas, A. A.; Rapoport, H. J . Org. Chem. 1993,
58, 2369-2376.
white solid. Yield of 1, 60 mg (90%), mp 175-177 °C, [R]25
)
D

3596 J . Org. Chem., Vol. 66, No. 10, 2001
Notes
+4.27 (c 0.5, MeOH). Rf ) 0.42 (n-BuOH/AcOH/H2O ) 4:1:1).
HR-MS calcd for C11H13NO2 [M + H]+ 192.1052, found 192.1018.
1H NMR (D2O): 7.36-7.26 (m, 5H), 4.50-4.47 (t, J ) 9.0 Hz,
1H), 3.77-3.73 (t, J ) 9.6 Hz, 1H), 3.62-3.58 (m, 1H), 3.36-
3.31 (t, J ) 11.3 Hz, 1H), 2.82-2.77 (m, 1H), 2.19-2.12 (m, 1H).
13C NMR (D2O): 171.72, 137.59, 129.01, 127.75, 127.13, 59.92,
50.92, 43.00, 35.58.
(m, 2H), 3.77 (s, 3H), 2.99-2.89 (m, 1H), 2.60-2.56 (d, J ) 18.7
Hz, 1H), 1.48-1.45 (3s, 9H). 13C NMR (CDCl3): 208.34, 207.54,
172.21, 154.26, 153.47, 81.26, 56.22, 55.50, 52.78, 52.45, 41.15,
40.71, 28.19, 27.88.
Meth yl Nr-Boc-cis-4-h yd r oxy-4-tr a n s-p h en yl-L-p r olin a te
(7b). The reaction between NR-Boc-4-keto-proline methyl ester
(6b, 490 mg, 2.0 mmol) and PhMgBr (2.0 M in Et2O, 1.0 mL)
was carried out by the same method outlined for the preparation
of tert-butyl NR-Boc-cis-4-hydroxy-4-phenyl-L-prolinate. Yield,
Ben zyl Nr-Boc-4-k eto-p r olin a te (6c). To a solution of NR-
Boc-4-trans-hydroxy-proline (8, 10 g, 43.2 mmol) and benzyl
bromide (9.59 g, 56.2 mmol) in THF (40 mL) was added
triethylamine (7.86 mL, 56.2 mmol) at 0 °C, and then the
mixture was stirred overnight. After filtration, the filtrate was
evaporated in vacuo to give a residue, which was subsequently
purified by flash chromatography (hexane/ethyl acetate ) 4:6,
v/v) to give benzyl NR-Boc-4-trans-hydroxy-L-proline ester as a
colorless oil. Yield, 12.6 g (91%). Benzyl NR-Boc-4-trans-hydroxy-
proline ester (12.6 g, 39 mmol) was added to a solution of PDC
(45 g, 120 mmol) in DMF (100 mL) at 0 °C, and the mixture
was stirred for 8 h at room temperature. To this mixture was
added water (50 mL), and the solution was extracted by ether
(150 mL × 3). The solution was evaporated in vacuo to give a
residue, which was purified by flash chromatography (hexanes/
ethyl acetate ) 7:3, v/v) to yield a colorless oil. Yield of 6c, 8.0
g (64%), [R]25 ) +0.73 (c 1.5, CHCl3), lit. [R]20 ) -5.4 (c 1,
200 mg (31%), mp 85-86 °C. [R]25 ) -20.1 (c 0.8, CHCl3). HR-
D
MS calcd for C17H24NO5: [M + H]+ 322.1656, found 322.1655.
Rf ) 0.47 (hexane/EtOAc ) 7:3). 1H NMR (CDCl3): 7.53-7.29
(m, 5H), 4.59-4.48 (dd, J ) 9.4, 9.0 Hz, 1H), 4.00-3.71 (m, 5H),
2.73-2.67 (m, 1H), 2.39-2.33 (t, J ) 15.6 Hz, 1H), 1.49-1.47
(3s, 9H). 13C NMR (CDCl3): 175.42, 175.17, 154.37, 153.58,
141.30, 141.23, 128.44, 128.42, 127.67, 125.22, 125.17, 80.63,
80.58, 79.21, 61.10, 60.20, 58.51, 58.31, 52.90, 52.59, 44.23, 43.26,
28.33, 28.24. A small portion of the product was recrystallized
in a mixture of hexanes and ethyl acetate (95:5, v/v) and was
used for the X-ray structural determination.
Nr-Boc-4-k eto-p r olin e (6a ). A mixture of pyridine (44 mL)
in dry CH2Cl2 (100 mL) was cooled at 0 °C. To this mixture was
added CrO3 (26.4 g). Stirring was continued at 0 °C for 30 min,
and then the mixture was allowed to warm to room temperature
over a period of 30 min. A solution of NR-Boc-4-hydroxy-proline
(8, 10.4 g, mmol) in CH2Cl2 (160 mL) was added over 5 min,
and stirring was continued for 1 h. The reaction mixture was
filtered and evaporated in vacuo. The residue was extracted with
diethyl ether (250 mL × 3). The combined ether exacts were
washed successively with HCl (5%, 2 × 100 mL) and NaCl (1
M, 3 × 50 mL) and then dried over MgSO4 overnight. The
organic solution was then decolorized with active charcoal,
filtered, and evaporated to give a yellow oil. The residue was
then recrystallized from a mixture of ethyl acetate and hexane
to yield a colorless solid 6a . Yield, 5.05 g (49%), mp 155-157 °C
D
D
CHCl3).23 HR-MS calcd for C17H22NO5: [M + H]+ 320.1489,
found 320.1496. Rf ) 0.57 (hexane/EtOAc ) 7:3). 1H NMR
(CDCl3): 7.35 (s, 5H), 5.30-5.12 (m, 2H), 4.90-4.72 (2d, J )
9.8, 8.8 Hz, 1H), 3.95-3.87 (m, 2H), 3.03-2.91 (m, 1H), 2.65-
2.56 (d, J ) 8.8 Hz, 1H), 1.51 & 1.41 (2s, 9H). 13C NMR
(CDCl3): 208.34, 207.54, 171.55, 171.51, 154.21, 153.39, 13.504,
134.92, 128.64, 128.57, 128.42, 128.09, 81.24, 67.47, 67.32, 67.31,
67.30, 67.28, 67.26, 67.25, 67.22, 56.26, 55.64, 52.80, 52.39, 41.12,
40.62, 40.54, 28.19, 28.01.
Ben zyl Nr-Boc-cis-4-h yd r oxy-4-p h en yl-L-p r olin a te (7c).
The coupling between benzyl NR-Boc-4-keto-L-prolinate (6c, 2.0
g, 6.23 mmol) and phenylmagnesium bromide was carried out
by the same method (using dry diethyl ether as the solvent)
outlined for the synthesis of tert-butyl NR-Boc-cis-4-hydroxy-4-
phenyl-L-prolinate. (For this reaction, when THF was used as a
solvent, we did not obtain any of the desired compound). After
purification by flash chromatography, the titled compound was
obtained as a colorless oil which crystallized as a white solid 7c
(decomp), lit. 160-162 °C (decomp.).23 [R]25 ) -16.5 (c 0.9,
D
MeOH), lit. [R]24 ) +20.9 (c 0.49, CH3COCH3).23 HR-MS calcd
D
for C10H16NO5: [M + H]+ 230.0990, found 230.1021. Rf ) 0.28
(CHCl3/MeOH ) 7:3). 1H NMR (CD3OD): 4.74-4.69 (m, 1H),
3.92-3.76 (m, 2H), 3.15-3.06 (m, 1H), 2.60-2.50 (m, 1H), 1.50-
1.45 (3s, 9H).
Nr-Boc-cis-4-h yd r oxy-4-p h en yl-L-p r olin e (7a ). PhMgBr (3
M in THF, 1.33 mL, 4 mmol) was added over 15 min to a stirred
solution of NR-Boc-4-keto-proline (6a , 230 mg, 1 mmol) in THF
(15 mL) at -50 °C. The mixture was stirred at -50 °C for 1 h.
Ammonium chloride solution (10%, 10 mL) was added slowly at
-50 °C, acidified to pH 2 by HCl (6 M), extracted by ethyl acetate
(30 mL × 3), and concentrated to give a brown residue. The
residue was dissolved in NaOH (2%), and the solution was
washed with ether (20 mL × 2). The aqueous solution was
acidified by HCl (6 M) and workup (EtOAc) to give a residue,
which was recrystallized from diethyl ether to afford a white
solid. Yield, 145 mg (45%), mp 177-78 °C. [R]25D ) -43.9 (c 0.5,
MeOH). HR-MS calcd for C16H22NO5: [M + H]+ 308.1460, found
308.1489. Rf ) 0.40 (CHCl3/MeOH ) 7:3). 1H NMR (CD3OD):
7.52-7.28 (m, 5H), 4.53-4.45 (m, 1H), 3.80-3.69 (m, 2H), 2.79-
2.73 (m, 1H), 2.48-2.45 (d, J ) 13.3 Hz, 1H), 1.50-1.48 (2s,
9H).
after long standing. Yield, 0.9 g (36%), mp 92-94 °C. [R]25
)
D
-25.6 (c 0.36, CHCl3), lit. [R]25 ) -63.34 (c 1, CHCl3).23 HR-
D
MS calcd for C23H28NO5: [M + H]+ 398.1967, found 398.1967.
Rf ) 0.50 (hexane/EtOAc ) 7:3). 1H NMR (CDCl3): 7.48-7.28
(m, 10H), 5.36-5.15 (m, 2H), 4.61-4.48 (2d, J ) 9.8 Hz, 1H),
3.96-3.68 (m, 2H), 2.70-2.65 (t, J ) 11.8 Hz, 1H), 2.37-2.30
(q, J ) 13.8 Hz, 1H), 1.46, 1.36 (2s, 18H).
Met h yl Nr-Boc-4-tr a n s-h yd r oxy-p r olin a t e (4b ). Methyl
4-trans-hydroxy-prolinate hydrochloride (5, 7.25 g, 50 mmol) was
dissolved in a mixture of dioxane and water (2:1, v/v, 100 mL)
at 0 °C. To the solution was added triethylamine (10.56 mL, 75
mmol) and Boc anhydride (14.53 g, 55 mmol). The reaction
mixture was then stirred for 2 h at 20 °C and evaporated in
vacuo. The resulting residue was washed with ethyl acetate (100
mL) and water (50 mL). The aqueous layer was successively
washed with ethyl acetate (100 mL × 2). The combined organic
layer was washed with HCl (0.5 M, 25 mL), water (25 mL × 2),
sodium carbonate (5%, 25 mL), water (25 mL × 2), and brine
(25 mL) and finally was dried overnight by anhydrous MgSO4.
The product was obtained as a colorless oil 4b after the solvent
was evaporated in vacuo and was used directly in the next step.
Yield, 9.0 g (89%).
Ack n ow led gm en t. We thank Dr. M. Doyle for use
of his polarimeter. X-ray structural analysis for com-
pounds 7b and 9d were made by Dr. Michael Carducci.
This research was supported by grants from the U.S.
Public Health Service DK17420 and DA06284. The
opinions expressed herein are all those of the authors
and do not necessary reflect those of the U.S. Public
Health Service.
Meth yl Nr-Boc-4-k eto-p r olin a te (6b). The synthesis (crude
4b was used directly) was carried out by the same procedures
used for the preparation of benzyl NR-Boc-4-keto-prolinate (6c).
Methyl NR-Boc-4-keto-prolinate (6b) was isolated as a colorless
oil. Yield, 5.4 g (50%). [R]25 ) +9.82 (c 2.6, CHCl3), lit. [R]26
)
D
D
-21.7 (c 1.24, CH3OH).25 HR-MS calcd for C11H18NO5: [M + H]+
244.1216, found 244.1191. Rf ) 0.52 (hexane/EtOAc ) 7:3). 1H
NMR (CDCl3): 4.82-4.71 (dd, J ) 9.3, 2.7 Hz, 1H), 3.91-3.88
Su ppor tin g In for m ation Available: Detailed X-ray struc-
tural reports on compounds 7b and 9d . This material is
available free of charge via the Internet at http://pubs.acs.org.
(25) Dormoy, J . R.; Castro, B. Synthesis 1986, 81-82.
J O005625U