2,2-Difluoro-1,3,2-oxazaborolidin-5-ones: Novel approach for selective side-chain protection of serine and threonine

Article abstract of DOI:10.1039/a606295d

2,2-Difluoro-1,3,2-oxazaborolidin-5-ones 1, which are synthesized from BF₃ and salts of amino acids, are highly effective, convenient and, moreover, inexpensive intermediates for the simultaneous protection of both α-amino and α-carboxy groups in α-amino acids. The new method streamlines the hitherto tedious procedures for side-chain protection of Ser and Thr. Ser(Bu′), Thr(Bu′), Ser(Bzl) and Thr(Bzl) are obtained by this procedure in high yields and in pure form using highly reactive reagents.

Full text of DOI:10.1039/a606295d

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2,2-Difluoro-1,3,2-oxazaborolidin-5-ones: novel approach for selective
side-chain protection of serine and threonine
J idong Wang,a Yoshio Okada,b Wei Li,a Toshio Yokoib and J intao Zhua
a
Department of Molecular Biology, Jilin University, Changchun, 130023, China
b Faculty of Pharmaceutical Sciences, Kobe Gakuin University, Nishi-ku, Kobe 651-21, Japan
2,2-D ifluoro-1,3,2-oxazaborolidin-5-ones 1, which are synthesized from BF 3 and salts of amino acids, are
highly effective, convenient and, moreover, inexpensive intermediates for the simultaneous protection of
both á-amino and á-carboxy groups in á-amino acids. The new method streamlines the hitherto tedious
procedures for side-chain protection of Ser and Thr. Ser(But ), Thr(But ), Ser(Bzl) and Thr(Bzl) are
obtained by this procedure in high yields and in pure form using highly reactive reagents.
threonine benzyl ester was prepared, followed by hydrolysis to
Introduction
give the desired compound.8 Although the total yield was rather
low (ca. 16%), the method was thought to be the only practical
approach to Thr(Bzl).
Synthetic peptides are increasingly widely recognized as
potential pharmaceutical agents and are of particular interest
for the design, synthesis and evaluation of peptide libraries.1
Therefore, there are growing demands for amino acid deriv-
atives used as building-blocks in peptide synthesis which are
not only chemically and stereochemically pure, but are also
available at acceptable prices and prepared by convenient
methods for practical large-scale synthesis. In our experi-
ence, the most troublesome preparations have involved the
polyfunctional amino acids, especially in the side-chain protec-
tion of Asp, Glu, Ser, Thr and Tyr, etc.† Although these deriv-
atives are commercially available, they are expensive and often
impure.
Traditionally in peptide synthesis, the side-chain hydroxy
functions in Ser and Thr have been protected as Bzl ethers
based on Nα-Boc protection, and as But ethers in combination
with an Fmoc group as the Nα-protecting group. In order to
prepare Ser(But) and Thr(But), temporary protection of the α-
amino and α-carboxy groups is necessary. In published pro-
cedures, the α-amino group has been blocked by a benzyloxy-
carbonyl group and the α-carboxy group by a p-nitrobenzyl
ester 2 which is removable by hydrogenation after tert-butylation
of the hydroxy groups. Moderate yields were achieved in both
cases.
From the time when the Boc strategy was established, a Bzl
group was selected as the protecting group for side-chain
hydroxy functions in Ser and Thr. More recently, Fmoc-
Ser(Bzl)-OH and Fmoc-Thr(Bzl)-OH have been popularly
employed in convergent solid-phase peptide synthesis.3 How-
ever, methods for preparation of Ser(Bzl) and Thr(Bzl) are even
more complicated. Ser(Bzl)4 and Thr(Bzl)5 were first prepared
by hydrolysis of the ,-N-acetyl derivatives with an acylase,
respectively. Although this method could give optically pure -
form products, it was not a practical method. Boc-Ser(Bzl)-OH
was prepared by benzylation of Boc-Ser-OH under strong basic
conditions.6,7 The yield was moderate, but the procedure was
not suitable for Thr protection; moreover, the Boc protecting
groups had to be removed for preparation of Fmoc-Ser(Bzl)-
OH. With regard to the synthesis of Thr(Bzl), the O-benzyl--
Therefore, the major disadvantages associated with the earl-
ier methods include tedious, multi-step protection and depro-
tection procedures, cumbersome reaction conditions and dif-
ficulty in scale-up. Furthermore, for Bzl side-chain protection,
an effective etherification reagent is definitely necessary. For the
above reasons, our studies have focused on α-amino acid inter-
mediates with simultaneous protection of both α-functional
groups. In this method the protected intermediates are prepared
directly from free-form α-amino acids; free-form side-chain
protected α-amino acids can be obtained after protection of the
side-chain and deprotection of the intermediates. There are sev-
eral methods which lead to simultaneous protection of both α-
amino and α-carboxy groups in α-amino acids. For example, α-
amino acids have been protected by CuII complexes,9 by hexa-
fluoroacetone,10 or by boranes.11,12 Although some amino acid
derivatives have been synthesized through these intermediates,
the applicability of these intermediates is limited due to their
instability to acids or alcohols.
Being aware of the vast variety of boron complexes, which
result from different substituents on the boron atom, we have
prepared 4-substituted 2,2-difluoro-1,3,2-oxazaborolidin-5-
ones 113 (Scheme 1) from aspartic acid and glutamic acid, and
reported the synthesis of Asp(OBut) and Glu(OBut) using 1 as
intermediates which give simultaneous protection of both α-
functional groups and are stable to Lewis acid catalysts. Here
we describe an efficient and practical one-pot synthesis of the
benzyl and tert-butyl-based side-chain protected serine and
threonine derivatives which are useful in Boc and Fmoc chem-
istry, respectively.
Results and discussions
Formation, deprotection and properties of 1
The mono alkali-metal salt of the amino acid was treated with
BF3ؒEt2O (>2 equiv.) in tetrahydrofuran (THF) to give 1 (see
Scheme 1). The selection of salts is dependent on their ability to
form an anhydrous compound. The lithium salts react with BF3
in THF to form a clear solution and are thus preferred. How-
ever, for threonine, the sodium salt was selected because its lith-
ium salt does not easily undergo dehydration. THF is the best
solvent for the formation of 1; other solvents such as ethylene
glycol, dimethyl ether and DMF are also suitable depending on
the solubility of compound 1 in them. The formation of com-
pound 1 proceeds smoothly at room temperature, while at ele-
† All amino acids have the -configuration except where indicated
otherwise. Abbreviations are those recommended by IUPAC-IUB
Commission on Biochemical Nomenclature (Biochem. J., 1967, 102, 23;
1967, 104, 17; 1972, 126, 773): Boc = tert-butyloxycarbonyl, Bzl = ben-
zyl, DMF = N,N-dimethylformamide, DMSO = dimethyl sulfoxide,
Fm = fluoren-9-ylmethyl, Fmoc = fluoren-9-ylmethoxycarbonyl, OSu =
N-hydroxysuccinimidyl and THF = tetrahydrofuran.
J. Chem. Soc., Perkin Trans. 1, 1997
621

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useful in the separation of amino acid derivatives from unsub-
O
O–
C
C
CO2–M+
NH2
BF3
stituted amino acids and inorganic salts in neutral aqueous
solutions and, unlike ion-exchange resins, are able to separate
some compounds sensitive to acids or bases. At pH 6, Ser(But)
or Thr(But) was effectively adsorbed on the resin, while im-
purities arising from neutralization of BF3, H3PO4 and oxaza-
borolidinones, were washed out from the resin by water. After
eluting with aqueous alcohol, pure Ser(But) or Thr(But) can be
isolated in high yields (Table 1). Fmoc-Ser(Bu
F
H
2BF3
C
N:B
H2
F
H
R1CHOR2
F
R1CHOR2
–MBF4
R2O CHR1
1a R1 = H, R2 = H
t)-OH was pre-
H
O
b R1 = H, R2 = But
c R1 = H, R2 = Bn
d R1 = CH3, R2 = H
e R1 = CH3, R2 = But
f R1 = CH3, R2 = Bn
pared from Fmoc-OSu and Ser(But),16 which showed a similar
optical rotation value to that reported.17
C
C
H2N+
O
–
B
F
F
Preparation of Ser(Bzl) and Thr(Bzl)
1
Compound 1a or 1d was allowed to react with benzyl trichloro-
acetimidate18 in dioxane with BF3 as the catalyst, to give 1c or
1f, respectively. The desired compounds can be obtained after
deprotection of the intermediates (Scheme 3). In order to avoid
Scheme 1 Formation of 2,2-difluoro-1,3,2-oxazaborolidin-5-ones
vated temperature the reaction can go to completion, normally
at 40–50 ЊC, in 2–4 h.
The boron complex 1 is sensitive to moisture when the solv-
ent is removed; therefore, the solvents used for the formation
and the reaction of 1 should be anhydrous. On the other hand,
compound 1 in solution is relatively stable. Reactions carried
out under dry N2 give best results. Pure intermediates were
difficult to obtain because of the aforementioned reaction con-
straints; their properties remain, therefore, unknown. As men-
tioned above, deprotection of the compounds proceeds rapidly
in water. In solution, the deprotection is accelerated in the pres-
ence of bases, while even in aqueous acid, compound 1 can be
deprotected at room temperature within hours. These proper-
ties indicate that oxazaborolidinone 1 is a useful intermediate
to effect protection irrespective of the sensitivity of the parent
compound to acidic or basic conditions.
We have reported evidence for the formation of a 5-
membered ring which incorporates both the α-functional
groups of aspartic acid.13 Here we report the 19F NMR spectral
results for compound 1a which showed two multiplets, derived
from the two fluorine atoms which became non-equivalent,
being situated on either side of the molecular plane. The chem-
ical shifts of the two different fluorine atoms in 1a (from tri-
fluoroacetic acid) were δ Ϫ74.92 and Ϫ75.41, respectively.
Compounds 1a and 1d were directly used in the preparation of
side-chain protected Ser and Thr derivatives without further
purification.
BnO CHR
C
HO CHR
H
O
H
Ser(Bn)
(R = H)
or
Thr(Bn)
(R = CH3)
O
C
C
C
Cl3CC(NH)OBn
BF3
1 M NaOH
H2N+
O
H2N+
O
–
–
B
B
F
F
F
F
Scheme 3 Preparation of Ser(Bzl) and Thr(Bzl)
strong basic conditions, benzyl trichloroacetimidate was used
to effect the etherification. As previously reported, benzyl
trichloroacetimidate was commonly used in non-polar solvents,
such as cyclohexane or carbon tetrachloride, in order to avoid
its rearrangement to the corresponding N-benzylamide. In this
study, we found that dioxane was also a suitable solvent for the
reaction. Within 0.5–2 h, the yield was >90% in both cases with
no more than 1.2 equiv. of benzyl trichloroacetimidate; this
indicated that rearrangement was insignificant. We also found
that Amberlite XAD-2 or -4 was a more effective adsorbent for
Ser(Bzl) and Thr(Bzl) than for But-protected derivatives. There-
fore, Ser(Bzl) and Thr(Bzl) can likewise also be purified by
Amberlite XAD-2 or -4 (Table 1). Because Ser(Bzl) and
Thr(Bzl) are not very soluble in cold water, they can be separ-
ated simply by filtration. Fmoc-Thr(Bzl)-OH was prepared
according to the literature.16 Finally, the optical purities of
Ser(Bzl) and Thr(Bzl) were also examined on an HPLC column
with a chiral crown ether as the stationary phase; the results
showed that no detectable racemization occurred in the
preparations.
Preparation of Ser(But ) and Thr(But )
Compound 1a or 1d was allowed to react with isobutylene in
dioxane with BF3–H3PO4 as catalyst to give 1b and 1e, respect-
ively. Hydrolysis with aqueous base then gave Ser(But) and
Thr(But) (Scheme 2). In effecting tert-butylation, two import-
In summary, the present study demonstrates that 2,2-
difluoro-1,3,2-oxazaborolidin-5-ones are highly effective, con-
venient and, moreover, inexpensive intermediates for simul-
taneous protection of both α-amino and α-carboxy groups in α-
amino acids. The new method has streamlined the hitherto
tedious procedures for side-chain protection of Ser and Thr,
and will pave the way for further applications of this technique
to access many variants of side-chain modified amino acids,
especially O-glycosylated amino acids.22
ButO CHR
HO CHR
H
O
Ser(But)
(R = H)
or
Thr(But)
(R = CH3)
H
O
C
C
C
C
isobutylene
BF3/H3PO4
1 M NaOH
H2N+
O
H2N+
O
–
–
B
B
F
F
F
F
Scheme 2 Preparation of Ser(But) and Thr(But)
Experimental
ant factors, the catalyst and solvent, should be considered. It
was reported 14 that a combination of BF3–H3PO4 was a mild
catalyst and most effective in the preparation of tert-butyl
ethers from Ser and Thr derivatives and isobutylene in CH2Cl2.
With dioxane as the solvent, to achieve better results, more
catalyst was needed compared with that for CH2Cl2 as solvent.
At room temperature, the reaction was complete in 2–4 h. A
prolonged reaction time was undesirable because of polymer-
ization of the isobutylene. With THF as the solvent, even more
catalyst was required and significant polymerization of isobutyl-
ene occurred.
General information
Mps were determined on a Yanagimoto micro-melting point
apparatus and are uncorrected. Optical rotations were meas-
ured with an automatic polarimeter, model DIP-360 (Japan
Spectroscopic Co.) and the [α]2D5 values are given in 10Ϫ1 deg cm2
g
Ϫ1. 1H (400, 500 MHz) and 13C (100, 125 MHz) NMR spectra
were recorded either on a Bruker DPX 400 or ARX 500 spec-
trometer. Chemical shift values are expressed as ppm downfield
from tetramethylsilane used as an internal standard (δ values). J
Values are given in Hz. 13C Signal assignments for each signal
were established by measurements of 2D spectra; multiplicities
are indicated by p (primary), s (secondary), t (tertiary) or q
(quaternary). Analytical TLC was carried out on Merck pre-
Amberlite XAD-2 or -4 non-ionic resin is useful in concen-
trating organic compounds in water.15 We found they are also
622
J. Chem. Soc., Perkin Trans. 1, 1997

View Article Online
Table 1 Properties of the Ser and Thr derivatives
Ser(But)
Thr(But)
Ser(Bzl)
Thr(Bzl)
Mp/ЊC
Found
225–226 (decomp.)
22020
234–236 (decomp.)
244–24720
221–222 (decomp.)
219–22121
203–204 (decomp.)
1975
Reported
[α]2D5
(c 1.0 in H2O)
Ϫ17.0
(c 1.0 in MeOH)
Ϫ44.6
(c 1.0 in 3N HCl)
+6.0
(c 1.0 in HOAc)
Ϫ30.8
Found
Reported
Elemental analysis (%) (Calc.)
Ϫ16.5 ± 120
C 51.13 (51.01)
Ϫ43 ± 220
+5.864
Ϫ30.48
C 54.15 (54.14)
C 61.31 (61.53)
C 62.87 (63.14)
H
N
9.58 (9.42)
8.34 (8.50)
H
N
9.64 (9.79)
7.85 (7.89)
H
N
6.60 (6.71)
7.12 (7.18)
H
N
7.16 (7.22)
6.75 (6.70)
Empirical formula
Yield (%)
C7H15NO3ؒ₅¹H2O
C8H17NO3ؒ₈¹H2O
C10H13NO3
94.8
C11H15NO3
90.8


85.6–92.4
73.6–86.7
coated silica gel plates (Kieselgel G 60 F254) employing the sol-
vent system, BuOH–AcOH–H2O (4:1:1, v/v). Non-aqueous
reactions were carried out under a nitrogen atmosphere. THF
and dioxane were dried and redistilled from calcium hydride.
MeOH was distilled from magnesium. Boron trifluoride–diethyl
ether was dried (CaH2) and distilled at reduced pressure. Iso-
butylene was dried (CaSO4) at Ϫ20 ЊC overnight. Salts of
amino acids were lyophilized with FZ-12 Freeze Dry System
(Labconco.) and then warmed to 80 ЊC for 2 h at 3–24 × 10Ϫ3
mmHg. Amberlite XAD-4 adsorbent (Merck) was washed
with water (2 vol. of the resin), MeOH (2 vol.) and water (2 vol.)
before use. Anhydrous phosphoric acid (crystals 98+%) was
purchased from the Aldrich Chemical Co., Inc. Amino acids,
,-Ser(Bzl) and ,-Thr(Bzl) were obtained from Watanabe
Chemical Industries, Ltd. Other laboratory reagents were
used directly without further purification. Benzyl trichloro-
acetimidate was prepared according to the literature18 with
modification: benzyl trichloroacetimidate was purified by distil-
lation in vacuo (0.4 mmHg) to give a colourless liquid and was
used in the preparation of benzyl derivatives immediately after
its purification.
H-Thr(But)-OH
The title compound was prepared from 1d (prepared from 10
mmol of Thr) by following a similar procedure for preparation
of Ser(But) except for the amount of isobutylene (25 cm3) and
the reaction time for tert-butylation (2.5–3.0 h). After isolation
on Amberlite XAD-4 resin, Thr(But) was obtained as a white
solid (1.29–1.52 g).
Fmoc-Ser(But)-OH
The title compound was prepared from Ser(But), which was
obtained as above, and Fmoc-OSu 16; mp 126–128 ЊC; [α]2D5
+25.5 (c 1.0, EtOAc) [lit.,17 mp 126–129 ЊC [α]2D5 +25.4 (c 1.0,
EtOAc)] (Found: C, 68.8; H, 6.53; N, 3.75. Calc. for
C22H25NO5: C, 68.9; H, 6.57; N, 3.65%); δH (500 MHz, CDCl3)
1.18 (9 H, s, But), 3.62 (1 H, dd, J 8.8, 4.0, OCHHaCH), 3.90
(1 H, dd, J 8.8, 2.7, OCHHbCH), 4.24 [1 H, t, J 7.0, 9-H,
fluorenylmethyl (Fm)], 4.37 (1 H, dd, J 10.7, 7.2, CHaH, Fm),
4.42 (1 H, dd, J 10.7, 7.4, CHbH, Fm), 4.51 (1 H, dt, J 8.3, 4.1,
NCHCO), 5.71 (1 H, d, J 8.3, CONH), 7.30 (2 H, t, J 7.4,
2- and 7-H, Fm), 7.39 (2 H, t, J 7.4, 3- and 6-H, Fm), 7.61 (2 H,
t, J 6.9, 1- and 8-H, Fm) and 7.75 (2 H, d, J 7.5, 4- and 5-H,
Fm); δC(125 MHz, CDCl3) 27.3 (p, 3 × C, But), 47.1 (t, 9-C,
Fm), 54.4 (t, 2-C, Ser), 61.8 (s, 3-C, Ser), 67.3 (s, CH2, Fm), 74.1
[q, O-C(CH3)3], 120.0 (t, 2 × C, 4- and 5-C, Fm), 125.1, 125.2 (t,
each 1 × C, 1- and 8-C, Fm), 127.1 (t, 2 × C, 2- and 7-C, Fm),
127.5 (t, 2 × C, 3- and 6-C, Fm), 141.3 (q, 2 × C, 4a- and 4b-C,
Fm), 143.7, 143.9 (q, each 1 × C, 8a- and 9a-C, Fm), 156.3 (q,
CONH) and 175.1 (q, CO2H).
2,2-Difluoro-4-hydroxymethyl-1,3,2-oxazaborolidin-5-one 1a
and 2,2-difluoro-4-(2-hydroxyethyl)-1,3,2-oxazaborolidin-5-one
1d
To the salt of the amino acid (mono lithium salt for Ser, mono
sodium salt for Thr) (10 mmol) suspended in THF (15 cm3),
was added BF3ؒEt2O (6.0 cm3). The mixture was stirred at room
temperature for 6 h and then at 40–45 ЊC for an additional 2 h,
to give 1a or 1d in quantitative yield.
H-Ser(Bzl)-OH
A THF solution of 1a (10 mmol) was evaporated in vacuo at
room temperature to remove the solvent. After most of the
THF had been removed, the residue was kept in vacuo (1.0
mmHg) for an additional 0.5 h. Dioxane (30 cm3) was added to
the residue and the solution was stirred for 10 min. To this
solution, benzyl trichloroacetimidate (2.15 cm3, 11.5 mmol) was
added over 10 min. After being stirred for 2 h, the mixture was
treated with anhydrous methanol (5 cm3), stirred for 10 min
and then heated with 1 mol dmϪ3 aqueous NaOH (30 cm3); it
was then stirred for an additional 30 min. The residue obtained
when the mixture was evaporated in vacuo was dissolved in
water (200 cm3) and the resulting solution was washed with
diethyl ether (3 × 15 cm3). The aqueous phase was adjusted to
pH 6.0 and passed through a column containing Amberlite
XAD-4 resin (л 2.5 × 20 cm). The column was washed with
water and then 50% EtOH. The latter eluent containing the
desired compound was collected and evaporated in vacuo to
give a white solid (1.85 g); δH [500 MHz, (CD3)2SO] 3.47 (1
H, dd, J 7.6, 3.4, NCHCO), 3.67 (1 H, dd, J 10.4, 7.7,
OCHaHCH), 3.79 (1 H, dd, J 10.4, 3.4, OCHbHCH), 4.51 (2
H, s, CH2Ph) and 7.28–7.41 (5 H, m, phenyl); δC[125 MHz,
(CD3)2SO] 54.0 (t, 2-C, Ser), 69.0 (s, CH2Ph), 72.0 (s, 3-C,
Ser), 127.4 (t, 4-C, phenyl), 127.6, 128.1 (t, each 2 × C, 2-, 3-,
5- and, 6-C, phenyl), 137.9 (q, 1-C, phenyl) and 167.6 (q,
CO2H).
H-Ser(But)-OH
A solution of 1a (obtained from 10 mmol of Ser) in THF,
which was prepared as above in a 50 cm3 Pyrex Erlenmeyer
flask, was concentrated to dryness with the aid of a water pump
at room temperature. After removal of the solvent, the evapor-
ation was continued for an additional 0.5 h, after which dioxane
(30 cm3) was added to the residue. The solution was stirred
for 15 min after which H3PO4 (98+%; 0.4 cm3) was added,
and stirring was continued for an additional 15 min. The flask
containing 1a was then cooled to Ϫ20 ЊC and isobutylene (20
cm3) was added at Ϫ20 ЊC. The flask was securely stoppered to
ensure no leakage of isobutylene. The resultant solution was
stirred at room temperature for 2–2.5 h. After discharge of
isobutylene, the solution was poured into a stirred solution of
1 mol dmϪ3 aqueous NaOH (80 cm3) at 0 ЊC and stirring was
continued for 0.5 h. The mixture was then evaporated in
vacuo, the residue was dissolved in water (30 cm3) and the
solution extracted with diethyl ether (3 × 15 cm3). The aque-
ous solution was adjusted to pH 6.0 with 1 mol dmϪ3 HCl,
after which the solution was chromatographed on Amberlite
XAD-4 resin (л 2.5 × 34 cm). The resin was washed with
water and then 80% EtOH. The latter eluent containing
Ser(But) was collected and evaporated to give a white solid
(1.38–1.49 g).
J. Chem. Soc., Perkin Trans. 1, 1997
623

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2 E. Wünsch and J. Jentsch, Chem. Ber., 1964, 97, 2490.
3 P. Lloyd-Williams, F. Albericio and E. Giralt, Tetrahedron, 1993, 49,
1106.
4 K. Okawa, Bull. Chem. Soc. Jpn., 1956, 29, 486.
5 Y. Murase, K. Okawa and S. Adabori, Bull. Chem. Soc. Jpn., 1960,
33, 123.
6 V. T. Hruby and K. W. Ehler, J. Org. Chem., 1970, 35, 1690.
7 H. Sugano and M. Miyoshi, J. Org. Chem., 1976, 41, 2352.
8 T. Mizoguchi, G. Levin, D. W. Woolley and J. M. Stewart, J. Org.
Chem., 1968, 33, 903.
9 A. Neuberger and F. Sanger, Biochem. J., 1944, 38, 125.
10 A. Golubev, N. Sewald and K. Burger, Tetrahedron Lett., 1995, 36,
2037.
11 G. H. L. Nefkens and B. Zwanenburg, Tetrahedron, 1983, 39, 2995.
12 F. Albericio, E. Nicolás, J. Rizo, M. Ruiz-Gayo, E. Pedroso and
E. Giralt, Synthesis, 1990, 119.
H-Thr(Bzl)-OH
The title compound was prepared from 1d (10 mmol) by follow-
ing the procedure as described in the preparation of Ser(Bzl)
above, except for the amount of benzyl trichloroacetimidate
(2.3 cm3, 12.0 mmol); 1.9 g product was obtained; δH [500 MHz,
(CD3)2SO] 1.24 (3 H, d, J 6.5, CH3), 3.15 (1 H, d, J 4.6,
NCHCO2), 4.03 (1 H, dq, J 4.6, 6.4, OCHCH3), 4.47, 4.53 (each
1 H, each d, J 11.9, OCH2Ph), 7.25–7.29 (1 H, m, 4-H, phenyl)
and 7.31–7.37 (4 H, m, 2-, 3-, 5- and 6-H, phenyl); δC[125
MHz, (CD3)2SO] 17.6 (p, CH3), 58.6 (t, 2-C, Thr), 70.3 (s,
CH2ph), 73.4 (t, 3-C, Thr), 127.2 (t, 4-C, phenyl), 127.6, 128.0
(t, each 2 × C, 2-, 3-, 5- and 6-C, phenyl), 138.4 (q, 1-C, phenyl)
and 168.3 (q, CO2H).
13 J. D. Wang, Y. Okada, Z. M. Wang, Y. H. Wang and W. Li, Chem.
Pharm. Bull., 1996, 44, 2189.
14 G.P. 2253924 1973 (Chem. Abstr., 1973, 79, 32301w).
15 S. A. Daignault, D. K. Noot, D. T. Williams and D. M. Huck, Water
Res., 1988, 22, 803.
Fmoc-Thr(Bzl)-OH
The title compound was prepared from Thr(Bzl), obtained as
above, and Fmoc-OSu according to the literature,16 mp 115–
116 ЊC; [α]2D5 +11.5 (c 1.0, MeOH) [lit.,19 mp 114–117 ЊC, [α]2D5
+11.2 (c 1.0, MeOH)] (Found: C, 72.2; H, 5.93; N, 3.23. Calc.
for C26H25NO5: C, 72.4; H, 5.84; N, 3.25%); δH (400 MHz,
CDCl3) 1.25 (3 H, d, J 6.3, CH3), 4.20–4.24 (2 H, 9-H, Fm and
OCHCH3), 4.36–4.47 (4 H, NCHCO2, CH2-Fm and CHaH-
phenyl), 4.58 (1 H, d, J 11.6, CHbH-phenyl), 5.63 (1 H, d, J 9.4,
COHN), 7.22–7.31 (7 H, phenyl and 2- and 7-H, Fm), 7.38 (2
H, t, J 6.8, 3- and 6-H, Fm), 7.60 (2 H, t, J 6.8, 1- and 8-H, Fm),
7.74 (2 H, d, J 7.5, 4- and 5-H, Fm), 9.56 (1 H, br s, CO2H);
δC(100 MHz, CDCl3) 16.2 (p, CH3), 47.1 (t, 9-C, Fm), 58.5 (t,
2-C, Thr), 67.4 (s, CH2-Fm), 71.2 (s, CH2Ph), 74.2 (t, 3-C,
Thr), 120.0 (t, 2 × C, 4- and 5-C, Fm), 125.1, 125.2 (each t,
1- and 8-C, Fm), 127.1 (t, 2 × C, 2- and 7-C, Fm), 127.7 (t,
2 × C, 3- and 6-C, Fm), 127.8, 128.0, 128.4 (t, 5 × C, 2-, 3-, 4-,
5- and 6-C, phenyl), 137.5 (q, 1-C, phenyl), 141.3 (q, 2 × C,
4a- and 4b-C, Fm), 143.7, 143.9 (each q, 8a- and 9a-C, Fm),
156.8 (q, CONH) and 175.6 (q, CO2H).
16 L. Lapatsanis, G. Milias, K. Froussios and M. Kolovos, Synthesis,
1983, 671.
17 C. D. Chang, M. Waki, M. Ahmad, J. Meienhofer, E. Lundell and
J. D. Huag, Int. J. Pept. Protein Res., 1980, 15, 59.
18 (a) F. Cramer, K. Pawelzik and H. J. Baldauf, Chem. Ber., 1958, 91,
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19 G. B. Fields and R. L. Noble, Int. J. Pept. Protein Res., 1990, 35,
161.
20 Catalogue of Fluka Chemika-BioChemika, 1995/96, p. 306 and
p. 307.
21 T. Hayakawa, K. Harada and S. W. Fox, Bull. Chem. Soc. Jpn., 1966,
39, 391.
22 R. S. Richard and M. Josef, Angew. Chem., Int. Ed. Engl., 1980, 19,
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References
Paper 6/06295D
Received 12th September 1996
Accepted 23rd October 1996
1 (a) M. A. Gallop, R. W. Barrett, W. J. Dower, S. P. A. Fodor and
E. M. Gordon, J. Med. Chem., 1994, 37, 1233; (b) G. Lowe, Chem.
Soc. Rev., 1995, 309.
624
J. Chem. Soc., Perkin Trans. 1, 1997