Preparation of L-N(α)-Fmoc-4-[di-(tert- butyl)phosphonomethyl]phenylalanine from L-tyrosine

Article abstract of DOI:10.1016/S0957-4166(99)00385-7

The unnatural amino acid analogue, 4-(phosphonomethyl)phenylalanine (Pmp, 2), has proven to be a valuable tool for studying protein-tyrosine kinase dependent signal transduction, where it is most often incorporated into peptides or peptide mimetics as a phosphatase-stable phosphotyrosyl mimetic. Although Pmp has been prepared previously bearing a number of protection strategies, the N(α)-Fmoc 4-[di-(tert-butyl)phosphonomethyl] phenylalanine form [(N(α)-Fmoc-L-Pmp((t)Bu₂)-OH, 3] is particularly attractive since it can be cleanly introduced into peptides using standard Fmoc protocols. Synthesis of 3 was first reported as its (D/L)-racemate, and subsequently as its L-3 enantiomer, with the latter synthesis having relied on induction of chirality using a camphor sultam auxillary. Reported herein is an alternate enantioselective synthesis of L-3 in high enantiomeric purity by procedures which derive the stereochemistry of the final product directly from the starting amino acid, without the need for chiral induction. A key feature of the route is the racemization-free nucleophilic substitution of lithium di-tert-butyl phosphite onto protected 4-bromomethylphenylalanine (17).

Full text of DOI:10.1016/S0957-4166(99)00385-7

Pergamon
Tetrahedron: Asymmetry 10 (1999) 3727–3734
Preparation of L-Nα-Fmoc-4-[di-(tert-butyl)-
phosphonomethyl]phenylalanine from L-tyrosine
Zhu-Jun Yao, Yang Gao and Terrence R. Burke Jr. ∗
Laboratory of Medicinal Chemistry, Division of Basic Sciences, National Cancer Institute, National Institutes of Health,
Bldg 37, Rm 5C06, Bethesda, MD 20892-4255, USA
Received 14 July 1999; accepted 25 August 1999
Abstract
The unnatural amino acid analogue, 4-(phosphonomethyl)phenylalanine (Pmp, 2), has proven to be a valuable
tool for studying protein-tyrosine kinase dependent signal transduction, where it is most often incorporated
into peptides or peptide mimetics as a phosphatase-stable phosphotyrosyl mimetic. Although Pmp has been
prepared previously bearing a number of protection strategies, the Nα-Fmoc 4-[di-(tert-butyl)phosphonomethyl]
phenylalanine form [(Nα-Fmoc-L-Pmp(tBu2)-OH, 3] is particularly attractive since it can be cleanly introduced into
peptides using standard Fmoc protocols. Synthesis of 3 was first reported as its (D/L)-racemate, and subsequently as
its L-3 enantiomer, with the latter synthesis having relied on induction of chirality using a camphor sultam auxillary.
Reported herein is an alternate enantioselective synthesis of L-3 in high enantiomeric purity by procedures which
derive the stereochemistry of the final product directly from the starting amino acid, without the need for chiral
induction. A key feature of the route is the racemization-free nucleophilic substitution of lithium di-tert-butyl
phosphite onto protected 4-bromomethylphenylalanine (17). © 1999 Elsevier Science Ltd. All rights reserved.
1. Introduction
The unique functional role of phosphotyrosyl residues (pTyr, 1) in protein-tyrosine kinase (PTK) de-
pendent signal transduction has made ‘pTyr motifs’ valuable thematic starting points for the development
of signal transduction modulators.1 However, a potential limitation of pTyr residues in vivo is hydrolytic
lability of the phosphoryl-ester linkage in the presence of protein-tyrosine phosphatases (PTPs). In order
to overcome such enzymatic lability, a number of pTyr mimetics have been reported, including L-(4-
phosphonomethyl)phenylalanine (L-Pmp, 2), which can exhibit biological properties similar to the parent
pTyr residue, yet is stable to phosphatases.2 For this reason, Pmp has proven to be a valuable analogue
in a variety of signal transduction systems.1 Recent reports of highly potent Pmp-containing Grb2 SH2
∗
Corresponding author. Tel: 301-402-1717; fax: 301-402-2275; e-mail: tburke@helix.nih.gov
0957-4166/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(99)00385-7

3728
Z.-J. Yao et al. / Tetrahedron: Asymmetry 10 (1999) 3727–3734
domain inhibitors such as 43 and 54 have renewed interest in the development of suitably protected
analogues of Pmp for the preparation of inhibitors directed against this important signal transduction
protein.
Since its original preparation in the racemic, free phosphonic acid form,5 Pmp has been synthesized
bearing a number of phosphonate protection strategies, most notably ethyl or methyl.6–8 Alternatively,
preparation of Pmp in its Nα-Fmoc 4-[di-(tert-butyl)phosphonomethyl]phenylalanine form [(Nα-Fmoc-
Pmp(tBu2)-OH, 3] is particularly attractive, since this possesses orthogonal protection compatible with
standard Fmoc peptide synthesis protocols. Synthesis of 3 was first reported as its (D/L)-racemate,9 and
subsequently as its L-3 enantiomer, with the latter synthesis having relied on induction of chirality using
the camphor sultam auxillary.10,11 In light of the above mentioned utility of Nα-Fmoc-L-Pmp(tBu2)-OH
for the synthesis of extremely potent Grb2 SH2 domain inhibitors,3 there is a renewed interest in the
further development of new preparations of L-3 from readily available starting materials. Accordingly,
reported herein is an alternate enantioselective synthesis of L-3 in which chirality is derived directly from
readily available L-tyrosine without the need for chiral induction.
2. Synthesis
The 4-formylphenylalanine analogue 10 was envisioned to serve as a key intermediate for the intro-
duction of di-tert-butyl phosphonic acid functionality via nucleophilic addition at the aldehyde site. Our
initial approach toward 10 utilized palladium-catalyzed carbonylation12 of either 4-iodophenylalanine
analogue 7 or a tyrosine triflate analogue 6. However, these reactions either failed to provide the desired
product, or did so in extremely low yield. Alternatively, Yokomatsu et al. have shown that ozonolysis of a
4-vinylphenylalanine yields the corresponding 4-formyl derivative in high yield.13 Accordingly, 4-vinyl-
substituted 8 appeared to be an ideal precursor which could provide a clean, high yield route to desired
10 (Scheme 1). In practice, readily available tyrosine triflate analogue 6 proved to be superior over the
more expensive 4-iodophenylalanine analogue 7, because of the greater ease of removal of tin species by
silica gel chromatography, although both 6 and 7 gave similar yields of 8. In a departure from Yokomatsu
et al.,13 the vinyl group of 8 was oxidized to diol 9 by 4-methylmorpholine N-oxide in the presence of 1
mol% OsO4 (85% yield), then oxidatively cleaved (NaIO4 in ether:H2O) to give a quantitative yield of
key intermediate 10. The entire sequence of steps was amenable to large scale up, and allowed the ready
preparation of gram quantities of 10.
With 10 in hand, two approaches toward L-Pmp(tBu2)-OH (11) were examined. In the first route,

Z.-J. Yao et al. / Tetrahedron: Asymmetry 10 (1999) 3727–3734
3729
Scheme 1.
which appeared deceptively simple and direct, reaction of aldehyde 10 with the lithium salt of di-tert-
butyl phosphite at −78°C in THF gave hydroxyphosphonate 12 (74% yield). Unfortunately, contrary to
expectations, susequent hydrogenolysis cleaved the benzyl ester and CBz groups, but did not remove
the phosphonate benzylic hydroxyl. Even under pressure in the presence of palladium black, reduction
would not proceed beyond intermediate 13, which was obtained in quantitative yield. Therefore, several
alternate methods were examined in attempts to remove the benzylic hydroxyl of 12. In one of these, it
was found that thiocarbonyl diimidazole-mediated conversion to thio-ester 14 afforded deoxygenated 15
in 96% yield following reduction with tributyltin hydride and AIBN (2 steps).14 However, the presence
of residual sulfur species in 15 rendered the approach impractical due to resistance of benzyl protecting
groups to subsequent catalytic hydrogenolysis.
Based on the reported ready displacement of benzylic bromides using di-tert-butyl phosphite,15
a
new strategy centred around 4-(bromomethyl)phenylalanine analogue 17 was developed (Scheme 2).
Reduction of aldehyde 10 (NaBH4) afforded intermediate benzyl alcohol 16 (quantitative yield), which
upon treatment with CBr4 and Ph3P in acetonitrile, provided desired bromide 17 (79%). Reaction of
17 with one equivalent of the lithium or sodium salt of di-tert-butyl phosphite in THF:HMPA (−78°C
to 0°C) gave the fully protected L-Pmp-derivative 15 (57%). When the reaction was repeated at room
temperature overnight with 1.2 equivalents of di-tert-butyl phosphite lithium salt in an attempt to drive
the reaction to completion, partial racemization (44% ee) was observed. Under even more rigorous
conditions (reflux), total racemization occurred. Hydrogenolysis of 15 gave a quantitative yield of free

3730
Z.-J. Yao et al. / Tetrahedron: Asymmetry 10 (1999) 3727–3734
amino acid 11, which was finally protected as the title Fmoc derivative 3 (71%) by standard procedures.
The enantiomeric purity of 3 as measured by HPLC analysis of the Pmp-leucine dipeptide indicated
>97% single enantiomer (see below).
Scheme 2.
3. Determination of enantiomeric purity
In order to determine the enantiomeric purity of final product 3, leucine amide dipeptides were pre-
pared by solid-phase techniques, with HPLC separation performed on resulting diastereomers
(Scheme 3). Racemic D,L-leucine-containing dipeptides (18), having free terminal amino groups, first
served as standards which showed good separation of diastereomers (diastereomeric retention time
difference of 4.9 min). Next, dipeptide 19 was prepared using enantiomerically pure L-leucine, and
shown to have less than 3% diastereomeric contamination resulting from D-Pmp.
Scheme 3.

Z.-J. Yao et al. / Tetrahedron: Asymmetry 10 (1999) 3727–3734
3731
4. Conclusions
Starting from readily available L-tyrosine, a practical synthesis of L-Nα-Fmoc-Pmp(tBu)-OH (3) in
high enantiomeric purity is described. The route offers the conceptual advantage over the previously
reported enantioselective synthesis11 of deriving final stereochemistry from natural L-tyrosine itself
without reliance on chiral induction, although this comes at a cost of increased length and lowered overall
yield. The ready availability of 3 by this alternate route should facilitate biochemical studies in the signal
transduction field. Exemplary of this is the recent utilization of 3 for the clean preparation of highly
potent Grb2 SH2 domain inhibitor 4 on a large scale for animal studies in breast cancer model systems.16
5. Experimental
5.1. General
Melting points were determined on a MeI Temp II melting apparatus and are uncorrected. Elemental
analysis were obtained from Atlantic Microlab Inc., Norcross, GA. Fast atom bombardment mass spectra
(FABMS) were acquired with a VG Analytical 7070E mass spectrometer under the control of a VG
2035 data system using 4-nitrobenzyl alcohol (NBA) matrices. 1H NMR data were obtained on a Bruker
AC250 (250 MHz) instrument. Removal of solvents was performed by rotary evaporation under reduced
pressure.
5.2. L-Nα-Benzyloxycarbonyl-4-[1-(2-ethenyl)]phenylalanine benzyl ester (8)
To a solution of L-Nα-Cbz-4-(O-trifluoromethanesulfonate)tyrosine benzyl ester (6) (9.81 g, 18.26
mmol), PdCl2(PPh3)2 (256 mg, 0.365 mmol) and LiCl (5.42 g, 128 mmol) in DMF (100 mL) was
added tributylvinyltin (5.31 mL, 18.3 mmol) at room temperature under argon. The mixture was warmed
to 90°C and stirred (24 h), then cooled and poured onto ice-water (250 mL) and extracted with
EtOAc (2×100 mL). Combined organic extracts were washed with brine (2×100 mL), dried (Na2SO4),
concentrated and purified by silica gel flash chromatography (EtOAc:hexanes, from 1:20 to 1:10) to
provide 8 as a white solid (5.52 g, 73%): mp 77–79°C; [α]23 +4.2 (CHCl3, c 0.12); 1H NMR (CDCl3) δ
D
7.34 (10H, m), 7.24 (2H, d, J=8.3 Hz), 6.97 (2H, d, J=8.1 Hz), 6.67 (1H, dd, J=10.7, 17.6 Hz), 5.71 (1H,
dd, J=1.0, 17.6 Hz), 5.23 (1H, dd, J=0.8, 10.5 Hz), 5.20 (1H, m), 5.14 (2H, d, J=3.9 Hz), 5.10 (2H, s),
4.69 (1H, m), 3.10 (2H, d, J=5.6 Hz). FABMS (+VE, NBA) m/z 416 [MH+]. Anal. calcd for C26H25NO4:
C, 75.2; H, 6.1; N, 3.4. Found: C, 75.0; H, 6.1; N, 3.3.
5.3. L-Nα-Benzyloxycarbonyl-4-(1,2-dihydroxyethyl)phenylalanine benzyl ester (9)
To a solution of 8 (1.78 g, 4.29 mmol) in acetone:THF:H2O (6:2:1; 27 mL) was added 4-
methylmorpholine N-oxide (560 mg, 4.72 mmol) followed by 0.05 M OsO4 in tert-butanol (0.9 mL,
0.045 mmol). The mixture was stirred at room temperature overnight, then treated with 10% Na2SO3
(10 mL) for 30 min, diluted with H2O (50 mL) and extracted with EtOAc (3×50 mL). The combined
organic extracts were washed with brine (50 mL), dried (Na2SO4), concentrated and purified by silica
gel flash chromatography (CHCl3:MeOH, from 40:1 to 20:1) to provide 9 as a white foam (1.81 g, 85%):
1H NMR (CDCl3) δ 7.30–7.41 (10H, m), 7.21 (2H, d, J=8.1 Hz), 6.99 (2H, d, J=7.6 Hz), 5.21 (1H, m),
5.15 (2H, d, J=4.1 Hz), 5.10 (2H, s), 4.77 (1H, dd, J=3.4 Hz, 7.8 Hz), 4.70 (1H, m), 3.73 (1H, dd, J=3.4

3732
Z.-J. Yao et al. / Tetrahedron: Asymmetry 10 (1999) 3727–3734
Hz, 11.0 Hz), 3.62 (1H, dd, J=8.0 Hz, 11.2 Hz), 3.11 (2H, d, J=5.6 Hz), 2.44 (1H, brs), 2.05 (1H, brs).
FABMS (+VE, NBA) m/z 450 [MH+].
5.4. L-Nα-Benzyloxycarbonyl-4-(formyl)phenylalanine benzyl ester (10)
To a solution of diol 9 (1.70 g, 3.79 mmol) in ether (36 mL) was added a solution of NaIO4 (890 mg,
4.14 mmol) in H2O (7.2 mL) and the mixture was stirred at room temperature (3 h). The mixture was
diluted with ether (50 mL) then washed with water (50 mL) and brine (2×50 mL), dried (Na2SO4) and
concentrated under vacuum to provide 9 as a white solid sufficiently pure for further use (1.489 g, 94%):
mp 65–67°C; [α]23 +4.9 (CHCl3, c 0.32); 1H NMR (CDCl3) δ 9.95 (1H, s), 7.69 (2H, d, J=8.3 Hz), 7.35
D
(10H, m), 7.14 (2H, d, J=8.0 Hz), 5.05–5.29 (5H, m), 4.76 (1H, m), 3.15 (2H, m). FABMS (+VE, NBA)
m/z 418 [MH+]. Anal. calcd for C25H23NO5·1/4H2O: C, 71.1; H, 5.6; N, 3.3. Found: C, 71.1; H, 5.6; N,
3.3.
5.5. L-Nα-Benzyloxycarbonyl-4-(hydroxymethyl)phenylalanine benzyl ester (16)
To a solution of aldehyde 9 (2.47 g, 5.92 mmol) in THF (10 mL) with EtOH (50 mL) was added
NaBH4 (225 mg, 5.92 mmol) at 0°C, and the mixture was stirred (20 min) then diluted with H2O (100
mL) and stirring continued. The mixture was extracted with EtOAc (2×50 mL) and dried (Na2SO4)
then concentrated and purified by silica gel flash chromatography (CHCl3:MeOH, from 100:1 to 50:1) to
provide 16 as a white solid (2.47 g, 99%): mp 97–99°C; [α]23 +5.3 (CHCl3, c 0.25); 1H NMR (DMSO-
D
d6) δ 7.86 (1H, d, J=8.0 Hz), 7.17–7.40 (14H, m), 5.11 (2H, s), 4.98 (2H, dd, J=12.6, 15.9 Hz), 4.46 (2H,
s), 4.27 (1H, m), 3.04 (1H, dd, J=5.4, 13.6 Hz), 2.87 (1H, dd, J=10.0, 13.7 Hz). FABMS (+VE, NH3) m/z
437 [(M+NH4)+]. Anal. calcd for C25H25NO5: C, 71.6; H, 6.0; N, 3.3. Found: C, 71.3; H, 6.0; N, 3.3.
5.6. L-Nα-Benzyloxycarbonyl-4-(bromomethyl)phenylalanine benzyl ester (17)
To a solution of alcohol 16 (2.35 g, 5.60 mmol) in anhydrous MeCN (50 mL) was added CBr4 (2.23 g,
6.72 mmol) followed by Ph3P (1.91 g, 7.28 mmol) at 0°C under argon. After stirring at room temperature
(30 min), additional CBr4 (930 mg, 2.8 mmol) and Ph3P (734 mg, 2.8 mmol) were added and stirring
was continued (30 min). The mixture was concentrated and residue was purified by silica gel flash
chromatography (hexane:EtOAc, from 7:1 to 4:1) to provide 17 as a white solid (2.12 g, 79%): mp
90–92°C; [α]23 +4.4 (CHCl3, c 0.05); 1H NMR (CDCl3) δ 7.37 (10H, m), 7.23 (2H, d, J=8.3 Hz), 6.97
D
(2H, d, J=8.1 Hz), 5.22 (1H, d, J=8.8 Hz), 5.14 (2H, d, J=4.9 Hz), 5.10 (2H, s), 4.71 (1H, m), 4.45 (2H,
s), 3.10 (2H, d, J=5.7 Hz) [Note: -OH did not show]. FABMS (+VE, NH3) m/z 499 and 501 [(M+NH4)+].
Anal. calcd for C25H24BrNO4: C, 62.2; H, 5.0; N, 2.9. Found: C, 62.5; H, 5.1; N, 2.9.
5.7. L-Nα-Benzyloxycarbonyl-4-[(di-tert-butyl)phosphonomethyl]phenylalanine benzyl ester [L-N-Cbz-
Pmp(tBu2)-OBn] (15)
To a solution of di-tert-butyl phosphite (305 mg, 1.575 mmol) in THF (10 mL) was added 1.0 M
lithium bis(trimethylsilyl)amide (LHMDS) in THF (1.6 mL, 1.6 mmol) at −78°C under argon. After 30
min, hexamethylphosphoramide (HMPA) (1.3 mL) was added and the mixture was stirred (30 min), then
bromide 17 (760 mg, 1.575 mmol) in THF (3 mL) was added slowly and stirring was continued first
at −78°C (2 h) then at 0°C (2 h). The reaction mixture was quenched by addition of aqueous NH4Cl
(20 mL), diluted with EtOAc (50 mL), and washed sequentially with H2O (50 mL), aqueous NH4Cl (50

Z.-J. Yao et al. / Tetrahedron: Asymmetry 10 (1999) 3727–3734
3733
mL) and brine (50 mL), then dried (Na2SO4) and concentrated. Crude product was purified by silica gel
flash chromatography (hexane:EtOAc, from 3:1 to 2:1) to provide 15 as a clear oil which crystallized on
standing (537 mg, 57%): mp 132–134°C; 1H NMR (CDCl3) δ 7.34 (10H, m), 7.12 (2H, dd, J=8.1, 2.2
Hz), 6.93 (2H, d, J=7.9 Hz), 5.05–5.26 (5H, m), 4.67 (1H, m), 3.09 (2H, m), 3.00 (2H, d, J=21.5 Hz),
1.40 (18H, s). FABMS (+VE, NH3) m/z 613 [(M+NH4)+] and 596 [MH+]. Anal. calcd for C33H42NO7P:
C, 66.5; H, 7.1; N, 2.4. Found: C, 66.6; H, 7.2; N, 2.3.
5.8. L-4-((Di-tert-butyl)phosphonomethyl)phenylalanine [L-Pmp(tBu2)-OH] (11)
A solution of L-N-Cbz-Pmp(tBu2)-OBn (15) (1.286 g, 2.16 mmol) in MeOH (20 mL) and THF (10
mL) was hydrogenolyzed (H2-filled balloon) in the presence of palladium black (100 mg) at room
temperature (overnight). Catalyst was removed by filtration and the filtrate was taken to dryness to
provide 11 as a white foam (795 mg, 99%): 1H NMR (DMSO-d6) δ 7.17 (4H, m), 3.37 (1H, dd, J=3.4,
8.1 Hz), 3.30–3.70 (2H, brm), 3.10 (1H, dd, J=4.7, 14.2 Hz), 2.96 (2H, d, J=21.2 Hz), 2.82 (1H, dd, J=8.1
Hz, 14.6 Hz), 1.37 (18H, s) [Note: COOH did not show]. FABMS (+VE, NH3) m/z 389 [(M+NH4)+].
Anal. calcd for C18H30NO5P·1/4H2O: C, 57.5; H, 8.2; N, 3.7. Found: C, 57.5; H, 8.2; N, 3.7.
5.9. L-Nα-(9-Fluorenylmethoxycarbonyl)-4-[(di-tert-butyl)phosphonomethyl]phenylalanine [L-N-
Fmoc-Pmp(tBu2)-OH] (3)
A mixture of L-Pmp(tBu2)-OH (15) (621 mg, 1.675 mmol), (9-fluorenylmethyloxy)succinimidyl
carbonate Fmoc-OSu (572 mg, 1.7 mmol) and NaHCO3 (422 mg, 5 mmol) in dioxane:water (1:1; 24
mL) was stirred at room temperature (overnight). The mixture was cooled to 0°C, acidified to pH 3 with
1N HCl, diluted with H2O (50 mL), extracted with EtOAc (3×25 mL), and the combined organic phases
were dried (Na2SO4) and concentrated. Crude product was purified by silica gel flash chromatography
(CHCl3:MeOH, 20:1) to provide 3 with ee≥94% (see below) as a white solid (710 mg, 71%): mp
78–80°C: [α]23 +22 (CHCl3, c 0.20) [lit.11 [α]23 +31 (CHCl3, c 0.20)]; 1H NMR (DMSO-d6) δ 12.60
D
D
(1H, brs), 7.88 (2H, d, J=7.1 Hz), 7.68 (3H, m), 7.41 (2H, dd, J=6.8, 7.6 Hz), 7.33 (2H, m), 7.17 (4H,
m), 4.18 (4H, m), 3.05 (1H, m), 2.94 (2H, d, J=21.2 Hz), 2.83 (1H, dd, J=10.8, 13.4 Hz), 1.32 (18H, s).
5.10. Determination of enantiomeric purity
Dipeptides 18 and 19 were prepared from L-N-Fmoc-Pmp(tBu2)-OH (3) using Rink amide resin17 (0.4
meq/g, purchased from Bachem Corp., Torrance, CA) with Fmoc-protocols similar to those described
previously.18 Fmoc-D,L-Leu and Fmoc-L-Leu-Rink amide resins were prepared by coupling the appro-
priate Fmoc-protected amino acids to Rink resin, with the resulting Fmoc-protected resins (12.5 mg)
then being washed well with several 1 mL portions of N-methyl-2-pyrolidoinone (NMP). Fmoc amino
protection was removed by treatment with 20% piperidine in NMP (0.5 mL, 1 min followed by 0.5 mL,
20 min). Resins were washed well with NMP (10×1 mL) then coupled overnight with a solution of active
ester formed by reacting 12.5 µmol each of L-N-Fmoc-Pmp(tBu2)-OH (3), 1-hydroxybenzotriazole
(HOBt) and 1,3-diisopropylcarbodiimide (DIPCDI) in NMP (1.0 mL, 10 min). Resins were washed with
NMP (10×1 mL), and N-terminal Fmoc-protection was removed by treatment with 20% piperidine in
NMP (0.5 ml, 5 min). Deblocked resins were first washed with NMP (10×1 mL) and dichloromethane
(10×2 mL), then dipeptides were cleaved from the resin using a mixture of trifluoroacetic acid (TFA,
1.85 mL), H2O (100 µl) and triethylsilane (50 µl) (1 h), taken to dryness and analyzed by HPLC.
Retention times of diastereomeric peaks, as determined using dipeptide 18 prepared from racemic D,L-

3734
Z.-J. Yao et al. / Tetrahedron: Asymmetry 10 (1999) 3727–3734
leucine, indicated diastereomers eluting at 18.8 min and 23.8 min. Enantiomeric contamination of L-N-
Fmoc-Pmp(tBu2)-OH (3) was then determined by similar analysis of dipeptide 19, where diastereomeric
contamination accounted for an area less than 3% of that observed for the major diastereomer. These
results indicated greater than 94% enantiomeric purity.
Acknowledgements
Appreciation is expressed to Dr. James Kelley of the LMCH for mass spectral analysis.
References
1. Burke Jr., T. R.; Yao, Z.-J.; Smyth, M. S.; Ye, B. Curr. Pharm. Des. 1997, 3, 291–304.
2. Domchek, S. M.; Auger, K. R.; Chatterjee, S.; Burke, T. R.; Shoelson, S. E. Biochemistry 1992, 31, 9865–9870.
3. Yao, Z.-J.; King, C. R.; Cao, T.; Kelley, J.; Milne, G. W. A.; Voigt, J. H.; Burke Jr., T. R. J. Med. Chem. 1999, 42, 25–35.
4. Schoepfer, J.; Fretz, H.; Gay, B.; Furet, P.; GarciaEcheverria, C.; End, N.; Caravatti, G. Bioorg. Med. Chem. Lett. 1999, 9,
221–226.
5. Marseigne, I.; Roques, B. P. J. Org. Chem. 1988, 53, 3621–3624.
6. Garbay-Jaureguiberry, C.; McCort-Tranchepain, I.; Barbe, B.; Ficheux, D.; Roques, B. P. Tetrahedron: Asymmetry 1992,
3, 637–650.
7. Cushman, M.; Lee, E. S. Tetrahedron Lett. 1992, 33, 1193–1196.
8. Dow, R. L.; Bechle, B. M. Synlett. 1994, 293–294.
9. Burke Jr., T. R.; Russ, P.; Lim, B. Synthesis 1991, 11, 1019–1020.
10. Josien, H.; Matin, A.; Chassaing, G. Tetrahedron Lett. 1991, 32, 6547–6550.
11. Liu, W. Q.; Roques, B. P.; Garbay-Jaureguiberry, C. Tetrahedron: Asymmetry 1995, 6, 647–650.
12. Hartman, G. D.; Halczenko, H. Synth. Commun. 1991, 21, 2103–2107.
13. Yokomatsu, T.; Yamagishi, T.; Matsumoto, K.; Shibuya, S. Tetrahedron 1996, 52, 11725–11738.
14. Rasmussen, J. R.; Slinger, C. J.; Kordish, R. J.; Newman-Evans, D. D. J. Org. Chem. 1981, 46, 4843–4846.
15. Baczko, K.; Liu, W. Q.; Roques, B. P.; GarbayJaureguiberry, C. Tetrahedron 1996, 52, 2021–2030.
16. Whole animal studies with Pmp-containing Grb2 SH2 domain inhibitor 4 are currently in progress.
17. Rink, H. Tetrahedron Lett. 1987, 28, 3787–3790.
18. Burke, T. R.; Yao, Z. J.; Zhao, H.; Milne, G. W. A.; Wu, L.; Zhang, Z. Y.; Voigt, J. H. Tetrahedron 1998, 54, 9981–9994.