An efficient and large-scale enantioselective synthesis of PNP405: A purine nucleoside phosphorylase inhibitor

Article abstract of DOI:10.1021/jo020256i

An efficient and large-scale enantioselective synthesis of PNP405 (1), a purine nucleoside phosphorylase inhibitor, is described. This synthesis of 1 involved eight steps starting from o-fluorophenylacetic acid with a 21.6% overall yield and >99.5% enantiopurity. The key stereogenic center with (R)-configuration was created using Evans' asymmetric alkylation methodology. This synthesis also features the racemization-free reductive removal of the chiral auxiliary in 5 using sodium borohydride, protection of the γ-cyano alcohol 6 as the trityl ether by a new water-assisted tritylation with trityl chloride and triethylamine or with trityl alcohol and catalytic trifluoroacetic acid, and an efficient one-pot cyclo-guanidinylation of 10 using cyanamide as the guanidinylating agent.

Full text of DOI:10.1021/jo020256i

An Efficien t a n d La r ge-Sca le En a n tioselective Syn th esis of
P NP 405: A P u r in e Nu cleosid e P h osp h or yla se In h ibitor
Mahavir Prashad,* Denis Har, Lijian Chen,† Hong-Yong Kim, Oljan Repicˇ, and
Thomas J . Blacklock
Process Research and Development, Chemical and Analytical Development, Novartis Institute for
Biomedical Research, One Health Plaza, East Hanover, New J ersey 07936
mahavir.prashad@pharma.novartis.com
Received April 11, 2002
An efficient and large-scale enantioselective synthesis of PNP405 (1), a purine nucleoside
phosphorylase inhibitor, is described. This synthesis of 1 involved eight steps starting from
o-fluorophenylacetic acid with a 21.6% overall yield and >99.5% enantiopurity. The key stereogenic
center with (R)-configuration was created using Evans’ asymmetric alkylation methodology. This
synthesis also features the racemization-free reductive removal of the chiral auxiliary in 5 using
sodium borohydride, protection of the γ-cyano alcohol 6 as the trityl ether by a new water-assisted
tritylation with trityl chloride and triethylamine or with trityl alcohol and catalytic trifluoroacetic
acid, and an efficient one-pot cyclo-guanidinylation of 10 using cyanamide as the guanidinylating
agent.
during quenching of the reaction mixture. Additionally,
low-temperature (-78 °C) conditions also presented
equipment limitations. We found that the use of 0.97
equiv of commercial lithium bis(trimethylsilyl)amide in
THF at -15 to -20 °C followed by a reverse quench of
the reaction mixture yielded a highly reproducible dia-
stereomeric ratio of 7:1, and the desired diastereoisomer
5 was obtained in 80% yield with >99% de by a recrys-
tallization from tert-butyl methyl ether. Thus, an increase
in the temperature of the recation from -78 to -20 °C
led to only a slight loss (6%) in the yield of 5 but
eliminated equipment limitations. The effect of the
substituents in the chiral auxiliary on the diastereo-
selectivity during the asymmetric alkylation was studied
using several auxiliaries. The (R)-4-benzyl- and (4R,5S)-
4-methyl-5-phenyl-2-oxazolidinones gave poorer diaste-
reoselectivity (5.3:1 and 3.7:1, respectively) and yield (56
and 52%, respectively, with >99% de). Such a low
diastereoselectivity using (4S,5R)-4-methyl-5-phenyl-2-
oxazolidinone was also recently reported4 during the
asymmetric alkylation of (4S,5R)-N-phenylacetyl-4-meth-
yl-5-phenyl-2-oxazolidinone with bromoacetonitrile, af-
fording a 3.9:1 diastereomeric mixture from which the
required diastereomer was obtained in 60% yield. The
(R)-4-isopropyl-2-oxazolidinone afforded the highest di-
astereoselectivity (13.4:1), but the product purification
required a silica gel chromatography and afforded the
desired diastereomer in the same yield (81% with >99%
de) as with the 4-phenyl substituent. (R)-4-Phenyl-2-
oxazolidinone (3) was used for the large scale preparation
because it afforded crystalline intermediate 5 as well as
because it is a cheap and commercially readily available
chiral auxiliary.
2-Amino-7-[1-(2-fluorophenyl)-2-(R)-hydroxyethyl]-3,5-
dihydro-4H-pyrrolo[3,2-d]pyrimidin-4-one (1; PNP405) is
a purine nucleoside phosphorylase (PNP) inhibitor from
a 9-deazaguanine structural class. As a potent, orally
active PNP inhibitor, it is targeted as a therapy for
transplant rejection and potentially for rheumatoid ar-
thritis, autoimmune diseases, and T-cell-mediated in-
flammatory diseases.1 Our goal was to develop an eco-
nomical, efficient, and enantioselective synthesis of
PNP405 (1) that was suitable for large scale prepara-
tion.
Asymmetric alkylation approach1 using Evans’ chiral
auxiliary was considered suitable for large scale prepara-
tion to create the key stereogenic center with (R)-
configuration that would also afford the primary alcohol
functionality in 1 after reductive removal of the auxiliary.
Recently, we developed a one-pot, convenient, and practi-
cal method for the N-acylation of 2-oxazolidinones di-
rectly with arylacetic acids in the presence of pivaloyl
chloride and triethylamine at 80-85 °C in toluene.2 This
new method avoided the necessity of preparing and
isolating the acid chloride. Thus, the N-acylation of (R)-
4-phenyl-2-oxazolidinone (3) with o-fluorophenylacetic
acid (2) under these conditions afforded 4 in >85% crude
yield, which was recrystallized from tert-butyl methyl
ether to obtain a 73% yield (Scheme 1). The asymmetric
alkylation3 of 4 with bromoacetonitrile in the presence
of sodium bis(trimethylsilyl)amide at -78 °C initially
proved problematic with variable diastereoselectivity,
perhaps, due to the varying degree of epimerization
† Present Address: Pfizer, 10777 Science Center Drive, San Diego,
CA 92121.
(1) McQuire, L. W.; Mugrage, B. B.; Prashad, M.; van Duzer, J . H.
U.S. Patent 6,043,365, Mar 28, 2000.
(2) Prashad, M.; Kim, H. Y.; Har, D.; Repicˇ, O.; Blacklock, T. J .
Tetrahedron Lett. 1998, 39, 9369- 9372.
Having developed the highly reproducible asymmetric
alkylation conditions, we were ready to carry out the
(3) Evans, D. A.; Ennis, M. D.; Mathre, D. J . J . Am. Chem. Soc.
1982, 104, 1737-1739.
(4) Azam, S.; D’Souza, A. A.; Wyatt, P. B. J . Chem. Soc., Perkin
Trans. 1 1996, 621-627.
10.1021/jo020256i CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/27/2002
6612
J . Org. Chem. 2002, 67, 6612-6617

Enantioselective Synthesis of PNP405
SCHEME 1a
SCHEME 2
covered by a simple filtration, and it was also recycled
satisfactorily. Although alcohol 6 could be purified by
silica gel chromatography, we found that crude 6 could
be used in the next step without purification.
The next step involved the protection of the primary
alcohol 6. Of a few protecting groups investigated, the
trityl group was our choice for further development
because the trityl ether 7 was a solid that could be
recrystallized from either tert-butyl methyl ether or
methanol and it was stable in subsequent steps utilizing
basic conditions. These recrysallization conditions also
efficiently removed any residual chiral auxiliary and the
undesired (S)-enantiomer, produced by racemization
(∼10%) when LiBH4 was used in the preceding step in
the pilot plant, affording 7 with >99% ee. Surprisingly,
the tritylation of 6 with trityl chloride in the presence of
Et3N in toluene was slow and could not be driven to
completion, even upon addition of an excess of reagents,
and the results varied from batch to batch. A Karl Fischer
analysis of three different crude batches of 6 in toluene
suggested that more water gave a better conversion. On
the basis of these results, we postulated that tritylation
of 6 was assisted by water. A highly reproducible process
for the tritylation of 6 with trityl chloride (1.52 equiv) in
the presence of Et3N (2.01 equiv) in toluene at 35 °C for
70 h using water (0.55 equiv) as the catalyst was
developed to afford 7 in 66% yield that also scaled-up well
in the pilot plant. We hypothesized that, due to the
presence of the nitrile group, under the basic conditions
6 may exist in equilibrium with the cyclic iminoether I
(Scheme 2), which may be the reason for the inhibition
of the tritylation. Water, perhaps, drives the equilibrium
toward 6 by opening I back to γ-cyano alcohol 6 via the
transition state II. Indirect evidence for the suppression
of the tritylation by the participation of the nitrile group
was obtained by the satisfactory tritylation of (()-2-
phenyl-1-propanol (lacking the nitrile group) without
water. Water-assisted reactions have been reviewed
recently,7 and such a tritylation reaction is not reported
therein. This new water-assisted tritylation of a γ-cyano
alcohol is an interesting addition to the list of water-
assisted reactions. Having recognized that the nitrile
group may be suppressing the tritylation of 6 under basic
conditions, we next studied the tritylation of 6 with trityl
a
Reagents: (a) 3, Me3CCOCl, Et3N, toluene, 80-85 °C (75%);
(b) LiHMDS, BrCH2CN, THF, -20 °C (80%); (c) NaBH4, THF-
H2O, 23 °C (100%); (d) Ph3CCl, Et3N, toluene, water, 35 °C (66%)
or Ph3COH, TFA, toluene, reflux (73%); (e) LDA, HCO2C2H5, THF,
-10 °C (100%); (f) diethylaminomalonate hydrochloride, NaOAc,
C2H5OH, 23 °C (100%); (g) C2H5ONa, C2H5OH (93%); (h) HCl,
C2H5OH, 50% aq NH2CN, aq NaOH (53%).
reductive removal of the chiral auxiliary in 5 to the
γ-cyano alcohol 6 with LiBH4. The reported conditions5
using 1.1 equiv of LiBH4 and 1.1 equiv of water at 0 °C
in THF afforded 6 as an oil, but with significant racem-
ization (6%). Further optimization of these conditions
revealed that an increase in the rate of reaction by
increasing the amount of LiBH4 to 1.5 equiv led to a
decrease in the racemization (2%). However, a scale-up
of these conditions in the pilot plant led to 10% racem-
ization. The racemization problem as well as the fact that
LiBH4 is an expensive and commercially nonreadily
available reagent led us to develop an alternative method
for this transformation. LiAlH4 has been used in the
literature3 for similar reductions; however, it gave a
complex mixture in the case of 5, perhaps, due to the
presence of the nitrile group. We then developed a new,
economical, practical, and racemization-free method for
the reductive removal of 2-oxazolidinones with inexpen-
sive NaBH4 in a mixture of THF and water at room
temperature.6 This new method afforded the desired
γ-cyano alcohol 6 in quantitative yield with undetectable
racemization (Scheme 1). The chiral auxiliary was re-
(5) Penning, T. D.; Djuric, S. W.; Haack, R. A.; Kalish, V. J .;
Miyashiro, J . M.; Rowell, B. W.; Yu, S. S. Synth. Commun. 1990, 20,
307-312.
(6) Prashad, M.; Har, D.; Kim, H. Y.; Repicˇ, O. Tetrahedron Lett.
1998, 39, 7067-7070.
(7) Ribe, S.; Wipf, P. Chem. Commun. 2001, 299-307.
J . Org. Chem, Vol. 67, No. 19, 2002 6613

Prashad et al.
SCHEME 3a
realized that 13 would be more basic than 1 and should
be removed easily as the water-soluble salt at a pH at
which 1 does not form the salt. Consequently, we
developed conditions for the purification of 1 that in-
volved the treatment of a solution of crude 1 in aqueous
sodium hydroxide with acetic acid to adjust the pH to
∼4.5. These conditions yielded pure 1 as a crystalline
solid in 53% yield and >99% purity. Thus, an efficient
one-pot process for the construction of the six-mem-
bered 2-aminopyrimidone ring by cyclo-guanidinylation
of 10 with cyanamide was developed that also scaled-
up well in the pilot plant. To ascertain the enantio-
meric purity of 1, an authentic sample of the (S)-
enantiomer (S)-1 was also synthesized. A chiral HPLC
analysis indicated that the enantiomeric purity of 1 was
>99.5%. Thus, an efficient and large-scale enantioselec-
tive synthesis of PNP405 (1) was developed that was a
linear 8-step synthesis with an overall yield of 21.6%
(Scheme 1).
a
Reagents: (a) HCl, C2H5OH; (b) concd HCl, 50% aq H2HCN,
C2H5OH; (c) aq NaOH.
In summary, an efficient and large-scale enantioselec-
tive synthesis of a purine nucleoside phosphorylase
inhibitor, 2-amino-7-[1-(2-fluorophenyl)-2(R)-hydroxyethyl]-
3,5-dihydro-4H-pyrrolo[3,2-d]pyrimidin-4-one (1; PNP405)
is described. This synthesis of 1 involved eight steps
starting from o-fluorophenylacetic acid with a 21.6% over-
all yield and >99.5% enantiopurity. The key stereogenic
center with (R)-configuration was created using Evans’
asymmetric alkylation methodology. This synthesis also
features the racemization-free reductive removal of the
chiral auxiliary in 5 using sodium borohydride, protection
of the γ-cyano alcohol 6 as the trityl ether by a new water-
assisted tritylation with trityl chloride and triethylamine
or with trityl alcohol and catalytic trifluoroacetic acid,
and an efficient one-pot cyclo-guanidinylation of 10 using
cyanamide as the guanidinylating agent.
alcohol under acidic conditions. Thus, treatment of 6 with
trityl alcohol (1.1 equiv) in the presence of catalytic
amounts of trifluoroacetic acid in refluxing toluene for
only 4 h afforded the desired intermdiate 7 in 73% yield.
Thus, this method was more efficient than the water-
assisted conditions.
Formylation of 7 with ethyl formate using LDA as a
base in THF yielded 8 cleanly and in quantitative yield.
The reaction of 8 with diethyl aminomalonate hydrochlo-
ride in ethanol in the presence of sodium acetate yielded
the enamine 9 as a mixture of stereoisomers.8-10 Some
of the pyrrole 10 was also formed under these conditions
as observed by HPLC. Treatment of crude 9 with sodium
ethoxide in ethanol yielded the pyrrole 10 in high yield
(93% overall from 7). With pyrrole 10 in hand, we next
turned our attention toward constructing the six-mem-
bered 2-aminopyrimidinone ring. We reasoned that a
direct cyclo-guanidinylation approach using cheap and
commercially readily available cyanamide11 as the guanid-
inylating agent would provide an efficient method for this
purpose. Thus, reaction of 10 with 50% aqueous cyana-
mide in ethanol in the presence of HCl followed by
basification with NaOH yielded PNP405 (1) efficiently.
Initially, the reaction of 10 with HCl led to the depro-
tection of the trityl group to give alcohol 11 (Scheme 3).
The alcohol 11 then reacted with 50% aqueous cyanamide
to afford acyclic guanidine 12, which underwent cycliza-
tion with aqueous sodium hydroxide to afford crude 1 in
73% yield. An adjustment of the pH to 12 with aqueous
NaOH was critical for the cyclization of 12. The acyclic
guanidine 12 can also be isolated in 85% yield. Attempts
to purify the crude 1 by recrystallization from a mixture
of methanol and water or ethanol and water not only gave
poor recovery of 1 but also did not improve the purity.
The major impurity, which was isolated by repetitive
HPLC, was characterized as 13 (Scheme 3) on the basis
of spectroscopic data. Having identified this impurity, we
Exp er im en ta l Section
The enantiopurity of 1 was determined using a Chiralcel
OD-R column (4.6 × 250 mm) and a mixture of CH3CN/0.05
M NH4OAc buffer (20:80 v/v) as the mobile phase (isocratic at
a flow rate of 0.5 mL/min and UV detector at 235 nm) at a
column temperature at 40 °C. The retention times of PNP405
(1) and its enantiomer (S)-1 were 18 and 16 min, respectively.
The diastereomeric purity of 5 was determined using a Waters
µ-Bondapak, C-18 column (4.5 × 250 mm) and a mixture of
CH3OH/H2O (45:55 v/v) as the mobile phase (isocratic at a flow
rate of 1.0 mL/min and a UV detector at 210 nm) at a column
temperature at 40 °C. The retention times of 5 and its
diastereomer (5a ) were 11 and 14 min, respectively. The
enantiopurity of 6 was determined using a Chiralcel OJ
column (4.6 × 250 mm) and a mixture of hexane/C2H5OH/TFA
(93:7:1 v/v/v) as the mobile phase (isocratic at a flow rate of 1
mL/min and UV detector at 260 nm). The retention times of 6
and its enantiomer (S)-6 were 23 and 31 min, respectively.
The enantiopurity of 7 was determined using a Chiralcel OD
column (4.6 × 250 mm) and a mixture of hexane/2-propanol
(95:5 v/v) as the mobile phase (isocratic at a flow rate of 1.0
mL/min and UV detector at 230 nm). The retention times of 7
and its enantiomer (S)-7 were 9.5 and 18 min, respectively.
(8) Elliott, A. J .; Montgomery, J . A.; Walsh, D. A. Tetrahedron Lett.
1996, 37, 4339-4340.
(9) Elliott, A. J .; Morris, P. E.; Petty, S. L.; Williams, C. H. J . Org.
Chem. 1997, 62, 8071-8075.
(10) Ferreira, V. F.; de Souza, M. C. B. V.; Cunha, A. C.; Pereira, L.
O. R.; Ferreira, M. L. G. Org. Prepr. Proced. Int. 2001, 33, 411-454.
(11) Theiling, L. F.; McKee, R. L. J . Am. Chem. Soc. 1952, 74, 1834-
1836.
3-[2-(2-F lu or op h en yl)-1-oxoet h yl]-4-(R)-p h en yl-2-ox-
a zolid in on e (4). A mixture of o-fluorophenylacetic acid (2,
107.7 g, 0.7 mol), (R)-4-phenyl-2-oxazolidinone (3, 100.0 g,
0.613 mol), Et3N (186.0 g, 1.84 mol), and toluene (1.0 L) was
heated to an internal temperature of 80 °C to afford a solution.
To this solution was added pivaloyl chloride (88.5 g, 0.734 mol)
6614 J . Org. Chem., Vol. 67, No. 19, 2002

Enantioselective Synthesis of PNP405
over a period of 30 min with efficient stirring while an internal
temperature 80-85 °C was maintained, and the mixture was
stirred at this temperature for an additional 2 h. Additional
pivaloyl chloride (44.1 g, 0.365 mol) was added over a period
of 15 min while an internal temperature 80-85 °C was
maintained, and the mixture was stirred at the same temper-
ature for an additional 3 h. The mixture was cooled to an
internal temperature of 40 °C, and prewarmed (40 °C) water
(300 mL) was added in 2 min while an internal temperature
of 40-45 °C was maintained (slightly exothermic reaction).
The organic layer was separated and washed with a pre-
warmed (40 °C) 2.2 N solution of HCl (358 mL) and prewarmed
(40 °C) water (100 mL) sequentially while an internal tem-
perature of 40-45 °C was maintained. The organic layer was
concentrated under reduced pressure to collect 940 mL of
solvent to obtain 3-[2-(2-fluorophenyl)-1-oxoethyl]-4-(R)-phen-
yl-2-oxazolidinone (4) as a semisolid. The semisolid was
suspended in tert-butyl methyl ether (400 mL), and the
mixture was heated to reflux to obtain a yellow solution. The
solution was cooled to 20-25 °C over a period of 20 min and
stirred at this temperture for 4 h. The mixture was further
cooled to 5-9 °C and stirred at this temperature for an
additional 2 h. The solids were filtered and washed with
precooled (5-9 °C) tert-butyl methyl ether (3 × 9 mL). The
product was dried at 45-50 °C (86-99 mbar) for 12 h to obtain
pure 3-[2-(2-fluorophenyl)-1-oxoethyl]-4-(R)-phenyl-2-oxazoli-
dinone (4, 137.0 g, 75%) as a white solid: mp 110-111 °C;
[R]D -154.8 (c ) 1, CH3OH); 1H NMR (300 MHz, CDCl3) δ 4.15
(dd, 1H, J ) 8.9, 3.8 Hz), 4.28 (dd, 2H, J ) 9.3 Hz), 4.54 (t,
1H, J ) 8.9 Hz), 5.34 (dd, 1H, J ) 8.9, 3.8 Hz), 6.94-7.34 (m,
9H). 13C NMR (300 MHz, DMSO-d6) δ 169.3, 162.8, 159.5,
153.9, 138.9, 131.8, 129.4, 128.6, 126.2, 124.0, 121.0, 120.8,
115.3, 115.2, 72.3, 57.6, 36.1. Anal. Calcd for C17H14FNO3: C,
68.2; H, 4.68; N, 4.68; F, 6.35. Found: C, 68.33; H, 4.61; N,
4.70; F, 6.55.
Chromatography of the mother liquor furnished the dias-
tereomer â-(S)-(2-fluorophenyl)-γ,2-dioxo-4-(R)-phenyl-3-oxazo-
lidinebutyronitrile (5a ): mp 138-140 °C; [R]D +118.85 (c ) 1,
1
DMSO); H NMR (300 MHz, DMSO-d6) δ 2.96-3.04 (m, 2H),
4.13 (dd, 1H, J ) 8.8, 4.6 Hz), 4.76 (t, 1H, J ) 8.8 Hz), 5.42 (t,
1H, J ) 6.6 Hz), 5.56 (dd, 1H, J ) 8.8, 4.6 Hz), 6.74 (t, 1H, J
) 6.2 Hz), 7.07-7.40 (m, 8H); 13C NMR (300 MHz, DMSO-d6)
δ 169.4, 161.8, 158.6, 152.8, 138.9, 130.2, 128.7, 128.2, 126.2,
125.7, 124.4, 123.3, 123.1, 118.5, 115.9, 115.6, 70.1, 57.2, 20.1.
Anal. Calcd for C19H15FN2O3: C, 67.35; H, 4.43; F, 5.61; N,
8.27. Found: C, 67.51, H, 4.34; F, 5.76; N, 8.24.
2-F lu o r o -(â-(R )-h yd r ox ym e t h yl)b e n ze n e p r o p io n i-
tr ile (6). To a slurry of â-(R)-(2-fluorophenyl)-γ,2-dioxo-4-(R)-
phenyl-3-oxazolidinebutyronitrile (5, 60.0 g, 0.177 mol) in THF
(540 mL) was added a freshly prepared solution of NaBH4 (27.0
g, 0.714 mol) in water (170 mL) over a period of 45 min while
an internal temperature 20-25 °C was maintained. The
reaction mixture was stirred at 20-25 °C for 1 h. A solution
of NaCl (40.0 g) in 2 N HCl (400 mL) was added over a period
of 45 min while an internal temperature of 20-25 °C was
maintained. The top organic layer was separated, and the
aqueous layer was extracted with toluene (175 mL). The
combined organic layers were washed with brine (175 mL),
filtered, and concentrated (to collect 580 mL of solvent) under
reduced pressure. To the residue was added toluene (100 mL),
and the mixture was concentrated under reduced pressure to
a volume of about 110 mL. The mixture was cooled to 0-5 °C
and stirred for 1-2 h. The solid (chiral auxiliary 3) was filtered
and washed with cold (∼5 °C) toluene (3 × 25 mL). The filtrate
was concentrated to dryness under reduced pressure to obtain
crude 2-fluoro-(â-(R)-hydroxymethyl)benzenepropionitrile (6,
31.8 g, 100%), which was stored below 0 °C and used for the
1
next step as is: oil; ee >99%; [R]D -37.6 (c ) 1, CH3OH); H
NMR (300 MHz, CDCl3) δ 2.66-2.86 (m, 2H), 3.08-3.15 (m,
1H), 3.45-3.58 (m, 1H), 3.70-3.78 (m, 2H), 7.01-7.26 (m, 4H);
13C NMR (300 MHz, CDCl3) δ 162.3, 159.1, 129.4, 128.7, 124.6,
118.6, 115.9, 63.5, 37.9, 19.3. Anal. Calcd for C10H10FNO: C,
66.9; H, 5.5; N, 7.8. Found: C, 66.8; H, 5.56; N, 7.72.
â-(R)-(2-F lu or op h en yl)-γ,2-d ioxo-4-(R)-p h en yl-3-oxa zo-
lid in ebu tyr on itr ile (5). A solution of 3-[2-(2-fluorophenyl)-
1-oxoethyl]-4-(R)-phenyl-2-oxazolidinone (4, 546.0 g, 1.824 mol)
in dry THF (2.18 L) was cooled to an internal temperature of
-5 to -8 °C. A solution of lithium bis(trimethylsilyl)amide (1
M in THF, 1.773 L, 1.773 mol) was added over a period of 55
min while an internal temperature of 0 to -5 °C was
maintained. The solution was stirred at an internal temper-
ature of 0 to -5 °C for 1 h and then cooled to an internal
temperature of -20 °C. To the resulting mixture was added
bromoacetonitrile (258.3 g, 2.153 mol) over a period of 40 min
while an internal temperature of -15 to -20 °C was main-
tained. The reaction mixture was stirred at this temperature
for 4.5 h and was added to a precooled (0-5 °C) solution of
NaCl (478.0 g) in water (2.164 L) and concentrated HCl (214.0
g) over a period of 20 min while the internal temperature was
maintained below 9 °C. After addition of THF (500 mL) as a
rinse, the slurry was stirred for 15 min at an internal
temperature of 5-8 °C and then warmed to room temperature
over a period of 30 min. The organic layer was separated,
washed with a solution of NaCl (632.0 g) in water (2.180 L),
and concentrated under reduced pressure to a final volume of
1.4-1.6 L. To the mixture was added tert-butyl methyl ether
(2.5 L), and the slurry was vigorously stirred for 1 h at room
temperature. The solid was filtered, washed with tert-butyl
methyl ether (2 × 150 mL), and dried at 45-50 °C (86-95
mbar) for 20 h to obtain â-(R)-(2-fluorophenyl)-γ,2-dioxo-4-(R)-
phenyl-3-oxazolidinebutyronitrile (5, 480.0 g, 80%) as an off-
white crystalline solid: mp 182-3 °C; de >99.0%; [R]D -234.13
(c ) 1, DMSO); 1H NMR (300 MHz, DMSO-d6) δ 3.0-3.04 (m,
2H), 4.17 (dd, 1H, J ) 8.5, 3.3 Hz), 4.69 (t, 1H, J ) 8.5 Hz),
5.50 (t, 1H, J ) 6.6 Hz), 5.57 (dd, 1H, J ) 8.5, 3.3 Hz), 7.20-
7.39 (m, 9H); 13C NMR (300 MHz, DMSO-d6) δ 169.5, 161.7,
158.5, 152.8, 139.3, 130.2, 129.1, 128.8, 128.1, 125.8, 124.7,
123.4, 123.2, 118.3, 115.8, 115.5, 70.5, 57.5, 20.5. Anal. Calcd
for C19H15FN2O3: C, 67.35; H, 4.43; F, 5.61; N, 8.27. Found:
C, 67.23, H, 4.31; F, 5.56; N, 8.23.
2-F lu or o-â-(R )-[(t r ip h e n ylm e t h yl)oxy]b e n ze n e p r o-
p ion itr ile (7). Meth od A. To a solution of 2-fluoro-(â-(R)-
hydroxymethyl)benzenepropionitrile (6, 59.5 g, 0.332 mol) in
toluene (500 mL) was added triphenylmethyl chloride (117.65
g, 0.442 mol), and the mixture was stirred at 20-22 °C for 10
min to afford a brown solution. To this solution was added
Et3N (60.0 g, 0.593 mol) rapidly over a period of 10 min while
an internal temperature of <30 °C was maintained. The
reaction mixture was stirred at an internal temperature of 35
°C for 4 h to obtain a light-brown suspension. Water (1.1 g,
0.061 mol) was added, and the stirring was continued at the
same temperature for an additional 20 h. Additional triph-
enylmethyl chloride (19.0 g, 0.068 mol), Et3N (7.7 g, 0.076 mol),
and water (1.1 g, 0.061 mol) were added. The resulting
suspension was stirred for an additional 24 h, additional water
(1.1 g, 0.061 mol) was added, and the stirring was continued
at 35 °C for an additional 22 h (or until starting material is
<10%). The mixture was cooled to room temperature, and
methanol (64 mL) was added dropwise over 10 min while an
internal temperature of 30-35 °C was maintained. The
reaction mixture was stirred for 40 min at an internal
temperature of 20-25 °C. The mixture was concentrated under
reduced pressure to obtain a red semisolid. The semisolid was
suspended in methanol (880 mL). The slurry was stirred at
an internal temperature of 50 °C for 1 h and then at 20-22
°C for 30 min. The solid was filtered, washed with methanol
(3 × 40 mL), and dried at 45-50 °C (30 mbar) for 4 h or to a
constant weight to afford 2-fluoro-â-(R)-[(triphenylmethyl)oxy]-
benzenepropionitrile (7, 93.0 g, 66%): mp 145-8 °C; ee >99%;
[R]D -21.93 (c ) 1, CHCl3); 1H NMR (300 MHz, CDCl3) δ 2.65-
2.68 (m, 2H), 3.35-3.52 (m, 3H), 6.69-7.37 (m, 19H). 13C NMR
(300 MHz, CDCl3) δ 162.3, 159.1, 143.8, 129.3, 129.1, 128.8,
128.7, 128.5, 127.9, 127.6, 127.3, 127.1, 126.0, 125.9, 125.7,
124.4, 124.3, 118.2, 115.8, 115.5, 87.0, 64.7, 36.4, 19.7. Anal.
J . Org. Chem, Vol. 67, No. 19, 2002 6615

Prashad et al.
Calcd for C29H24FNO: C, 82.6; H, 5.6; N, 3.3; F, 4.5. Found:
C, 82.4; H, 5.7; N, 3.32; F, 4.55.
m et h yliden yl]-â-(R)-[(t r iph en ylm et h yoxy)m et h yl]ben -
zenepropionitrile (9, 0.142 mol) and anhydrous 2B ethanol (240
mL) was stirred for 20 min and cooled to 10 ( 5 °C. A 21%
sodium ethoxide solution (80 mL, 0.214 mol) in ethanol was
added over a period of 20 min while an internal temperature
at 10-20 °C was maintained. The mixture was stirred at 20-
25 °C overnight (14 h). The reaction mixture was cooled to -5
to 0 °C, and acetic acid (20 mL) was added at -5 to 0 °C over
a period of 15 min. After the mixture was stirred for 20 min,
ethyl acetate (300 mL) and 20% NaCl (400 mL) were added
(pH of the aqueous layer should be 4-6), and the mixture was
stirred for 20 min at the same temperature. The organic layer
was separated, and the aqueous layer was extracted with ethyl
acetate (120 mL). The combined organic layers were washed
with saturated NaHCO3 (300 mL) and then with brine (300
mL). The organic layer was filtered and concentrated to
dryness under reduced pressure. Anhydrous 3A ethanol (150
mL) was added, and the mixture was stirred for 20 min. The
mixture was concentrated to dryness under reduced pressure.
This treatment was repeated two additional times with 150
mL of anhydrous 3A ethanol each time. The residue was then
dissolved in anhydrous 3A ethanol (180 mL). The resulting
solution was added to precooled (0-5 °C) water (600 mL) over
a period of 30 min while an internal temperature at 0-5 °C
was maintained. The mixture was stirred at 5 ( 5 °C for 1 h,
and brine (200 mL) was added. Stirring was continued for an
additional 1 h at 5 ( 5 °C. The solid was filtered, washed with
water (2 × 100 mL), and dried at room temperature (22-25
°C) under reduced pressure (30 mbar) for 48 h to obtain ethyl
3-amino-4-[1-(2-fluorophenyl)-2-(R)-[(triphenylmethoxy)ethyl-
2-1H-pyrrole-2-carboxylate (10, 70.49 g, 93%) as a brown
powder: mp 59-62 °C; [R]D -10.73 (c ) 1, CH3OH); 1H NMR
(300 MHz, DMSOd6) δ 1.22 (t, 3H, J ) 7.2 Hz), 3.44 (d, 2H, J
) 5.8 Hz), 4.18 (q, 2H, J ) 7.0 Hz), 4.55 (t, 1H, J ) 6.0 Hz),
4.77 (bs, 2H), 6.56 (s, 1H), 7.20-7.29 (m, 19H), 10.66 (s, 1H).
13C NMR (300 MHz, CDCl3) δ 162.7, 159.5, 144.0, 130.1, 128.7,
128.5, 128.4, 128.3, 127.9, 127.1, 124.3, 124.2, 115.5, 115.2,
86.8, 66.1, 59.6, 34.8, 14.8. Anal. Calcd for C34H31FN2O3: C,
76.3; H, 5.7; N, 5.2; F, 3.6. Found: C, 76.23; H, 5.96; N, 5.01;
F, 3.6.
Meth od B. A solution of 2-fluoro-(â-(R)-hydroxymethyl)-
benzenepropionitrile (6, 21.2 g, 118.24 mmol) and triphenyl-
methanol (33.9 g, 130.2 mmol) in toluene (206 mL) was stirred
at 20-22 °C for 10 min to obtain a hazy yellow solution. To
the resulting solution was added trifluoroacetic acid (0.674 g,
5.91 mmol) dropwise in 5 min. The mixture was heated to an
internal temperature of 111-113 °C to achieve a gentle reflux,
and refluxing was continued for 4 h. The reaction mixture was
cooled to room temperature and concentrated under reduced
pressure to obtain a semisolid. The semisolid was suspended
in methanol (300 mL) and heated to an internal temperature
of 60-65 °C. The slurry was stirred at 60-65 °C for 1 h, cooled
to 20-22 °C, and stirred at this temperature for an additional
1 h. The solid was filtered, washed with methanol (3 × 30 mL),
and dried at 45-50 °C (30 mbar) for 4 h or to a constant weight
to afford 2-fluoro-â-(R)-[(triphenylmethyl)oxy]benzenepro-
pionitrile (7, 36.0 g, 73%).
2-F lu or o-r-(h yd r oxym et h ylid en yl)-â-(R)-[(t r ip h en yl-
m eth oxy)m eth yl]ben zen ep r op ion itr ile (8). A mixture of
2-fluoro-â-(R)-[(triphenylmethyl)oxy]benzenepropionitrile (7,
59.81 g, 0.142 mol) and anhydrous THF (300 mL) was stirred
at 20-25 °C under nitrogen for 15 min. The yellowish solution
was cooled to -10 °C (internal temperature), and a 2.0 M
solution of LDA (78.0 mL, 0.156 mol) in heptane/THF was
added over a period of 20 min while an internal temperature
-10 to -5 °C was maintained. The mixture was stirred at -10
to -5 °C for 15 min, and ethyl formate (26.6 g, 0.359 mol) was
added over a period of 20 min while an internal temperature
-10 to -5 °C was maintained. The reaction mixture was
stirred at -10 to -5 °C for 30 min, and to it was added to a
solution of glacial acetic acid (15 mL) and 20% NaCl solution
(300 mL) over a period of 20 min while an internal temperature
of 0-5 °C was maintained. After the mixture was stirred at
0-5 °C for 15 min, the organic layer was separated, and the
aqueous layer was extracted with ethyl acetate (150 mL). The
combined organic layers were washed with saturated NaHCO3
(300 mL, the pH of the aqueous layer after the wash should
be 8-10) and then with brine (300 mL). The organic solution
was dried and concentrated to dryness under reduced pressure
toobtain crude2-fluoro-R-(hydroxymethylidenyl)-â-(R)-[(triphenyl-
methoxy)methyl]benzenepropionitrile (8, 0.142 mole) as an oil,
which was used in the next step without purification (the
product was unstable and should be stored at 0-4 °C).
2-F lu o r o -r-[[[d i(e t h o x y c a r b o n y l)m e t h y l]a m in o ]-
m et h ylid en yl]-â-(R)-[(t r ip h en ylm et h yoxy)m et h yl]b en -
zen ep r op ion itr ile (9). A mixture of sodium acetate (34.92
g, 0.426 mol), diethyl aminomalonate hydrochloride (60.06 g,
0.284 mol), and anhydrous 2B ethanol (150 mL) was stirred
at 20 to 25 °C for 20 min. A solution of crude 2-fluoro-
R-(hydroxymethylidenyl)-â-(R)-[(triphenylmethoxy)methyl]-
benzenepropioni-trile (8, 0.142 mol) in anhydrous 2B ethanol
(150 mL) was added at 20-25 °C over a period of 20 min. The
mixture was stirred at 20-25 °C for 72 h. Ethyl acetate (240
mL) and water (300 mL) were then added, and the mixture
was stirred at 20-25 °C for 20 min. The organic layer was
separated, and the aqueous layer was extracted with ethyl
acetate (120 mL). The combined organic layers were washed
with saturated NaHCO3 (240 mL) and brine (240 mL) and
concentrated to dryness under reduced pressure. To the
residue was added toluene (150 mL), and the mixture was
stirred at 20-25 °C for 20 min and concentrated to dryness
under reduced pressure. This treatment was repeated two
additional times with 150 mL of toluene each time to obtain
2-Am in o-7-[1-(2-flu or op h en yl)-2-(R)-h yd r oxyeth yl]-3,5-
d ih yd r o-4H-p yr r olo[3,2-d ]p yr im id in -4-on e (1). A mixture
of ethyl 3-amino-4-[1-(2-fluorophenyl)-2-(R)-[(triphenylmethoxy)-
ethyl-2-1H-pyrrole-2-carboxylate (10, 69.8 g, 0.131 mol) and
anhydrous 2B ethanol (300 mL) was stirred at 20-25 °C for
20 min. Concentrated HCl (54 mL) was added at 20-40 °C
over a period of 15 min. The mixture was heated to an internal
temperature of 40-45 °C and stirred at 40-45 °C for 16 h.
The mixture was cooled to 0-5 °C and filtered. The filter cake
was washed with 2B ethanol (2 × 50 mL). The filtrate was
heated to an internal temperature of 40-45 °C, and a total of
100 mL of 50% (w/w) aqueous solution of cyanamide (1.31 mol)
was added in five 20 mL portions at 2 h intervals. The pH
was checked after each addition; if the pH was above 2,
concentrated HCl (5 mL) was added. The reaction mixture was
cooled to 0-5 °C 2 h after the last addition of cyanamide, and
50% (w/w) aqueous NaOH (45 mL) was then added (to adjust
the pH to 12) at a rate to maintain the internal temperature
below 15 °C. The mixture was cooled to 0-5 °C and stirred at
0-5 °C for 2 h. Concentrated HCl (45 mL) was added (to adjust
pH to 2) at a rate to maintain the internal temperature at
5-10 °C. The mixture was concentrated under reduced pres-
sure until the batch volume was about 350 mL. To the residue
was added water (200 mL), and the mixture was stirred at
20-25 °C for 20 min. Ethyl acetate (150 mL) was added, and
the mixture was again stirred for 20 min. The organic layer
was separated, and the aqueous layer was extracted with ethyl
acetate (100 mL). The aqueous layer was saved. The combined
organic layers were washed with 0.1 N HCl (2 × 90 mL). All
the aqueous layers were combined and cooled to an internal
temperature of 0-5 °C, and 50% (w/w) aqueous NaOH (15 mL)
was added (to adjust the pH to 8-9) at a rate to maintain the
cr u de
2-flu or o-R-[[[di(et h oxyca r bon yl)m et h yl]a m in o]-
m et h yliden yl]-â-(R)-[(t r iph en ylm et h yoxy)m et h yl]ben -
zenepropionitrile (9, 0.142 mol) as an oil, which was used in
the next step without purification.
3-Am in o-4-[1-(2-flu or op h en yl)-2-(R)-[(tr ip h en ylm eth -
oxy)eth yl-2-1H-p yr r ole-2-ca r boxyla te (10). A mixture of
cr u de
2-flu or o-R-[[[di(et h oxyca r bon yl)m et h yl]a m in o]-
6616 J . Org. Chem., Vol. 67, No. 19, 2002

Enantioselective Synthesis of PNP405
internal temperature at 0-15 °C with vigorous agitation. The
mixture was stirred at 0-5 °C for 2 h and then at room
temperature for 16 h. The solid was filtered, washed with
water (2 × 100 mL), and dried at 50 °C (20-45 mbar) for 16
h to afford crude 2-amino-7-[1-(2-fluorophenyl)-2-(R)-hydroxy-
ethyl]-3,5-dihydro-4H-pyrrolo[3,2-d]pyrimidin-4-one (1, 25.7 g,
73%).
DMSO-d6) δ 3.99 (m, 2H), 4.56 (m, 1H), 5.32 (bs, 1H), 6.10
(bs, 2H), 7.01-7.36 (m, 5H), 10.82 (bs, 1H), 11.47 (bs, 1H); 13
NMR (300 MHz, DMSO-d6) δ 162.1, 158.9, 154.7, 150.7, 145.1,
130.4, 127.9, 126.2, 124.2, 115.3, 115.0, 112.7, 65.6, 37.4. Anal.
Calcd for C14H13FN4O2: C, 58.25; H, 4.5; F, 6.5; N, 19.4.
Found: C, 58.36, H, 4.39; F, 6.63; N, 19.45.
C
2-Am in o-7-[1-(2-flu or op h en yl)-2-(S)-h yd r oxyeth yl]-3,5-
d ih yd r o-4H-p yr r olo[3,2-d ]p yr im id in -4-on e ((S)-1). Pre-
pared from 5a using the same procedures as described above
for 1. (S)-1: mp 265-268 °C; ee >99.5%; [R]D -35.0 (c ) 1,
A mixture of crude 2-amino-7-[1-(2-fluorophenyl)-2-(R)-
hydroxyethyl]-3,5-dihydro-4H-pyrrolo[3,2-d]pyrimidin-4-one (1,
25.7 g) and 1.0 N NaOH (98 mL) was heated to an internal
temperature of 40-45 °C and stirred for 30 min to obtain a
clear brown solution. Charcoal (1.29 g) was added, and the
mixture was stirred at 40-45 °C for 20 min. The mixture was
filtered through a pad of Celite (3.0 g). The filter cake was
washed with water (20 mL). To the filtrate was added 2B
ethanol (120 mL), and the resulting solution was heated to
an internal temperature of 35-40 °C. Glacial acetic acid (10.2
mL) was added dropwise at 35-45 °C over 20 min with
moderate agitation. The mixture was stirred at 40-45 °C for
2 h, cooled to 20-25 °C over a period of 30 min, and stirred at
the same temperature for 4 h. The solid was filtered and
washed with 50% aqueous 2B ethanol (2 × 40 mL). The wet
cake was suspended in water (150 mL) and stirred for 4 h.
The solid was filtered, washed with water (2 × 30 mL), and
dried at 50 °C (25-40 mbar) for 16 h to obtain pure 2-amino-
7-[1-(2-fluorophenyl)-2-(R)-hydroxyethyl]-3,5-dihydro-4H-pyr-
rolo[3,2-d]pyrimidin-4-one (1, 20.15 g, 53.5%): mp 267-269
°C; ee >99.5%; [R]D +34.74 (c ) 1, DMSO); 1H NMR (300 MHz,
1
DMSO); H NMR (300 MHz, DMSO-d6) δ 3.99 (m, 2H), 4.56
(m, 1H), 5.32 (bs, 1H), 6.10 (bs, 2H), 7.01-7.36 (m, 5H), 10.82
(bs, 1H), 11.47 (bs, 1H); 13C NMR (300 MHz, DMSO-d6) δ
162.1, 158.9, 154.7, 150.7, 145.1, 130.4, 127.9, 126.2, 124.2,
115.3, 115.0, 112.7, 65.6, 37.4. Anal. Calcd for C14H13FN4O2:
C, 58.25; H, 4.5; F, 6.5; N, 19.4. Found: C, 58.42, H, 4.42; F,
6.91; N, 19.11.
Ack n ow led gm en t. We thank Dr. Wen Shieh, Dr.
Liladhar Waykole, Dr. Leslie McQuire, Dr. Yugang Liu,
and Ms. J . Xu for helpful discussions.
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra for compounds 1, (S)-1a , 4-7, 5a , and 10. This
material is available free of charge via the Internet at
http://pubs.acs.org.
J O020256I
J . Org. Chem, Vol. 67, No. 19, 2002 6617