Synthesis and application of L-N-Boc-N-methyl-β-hydroxyvaline in the preparation of a depsipeptide

Article abstract of DOI:10.1139/v05-033

Enantiopure (>99% ee) L-N-tert-butyloxycarbonyl-N-methyl-β- hydroxyvaline (2) was synthesized in six steps and 43% overall yield from D-serine methyl ester (5). Methyl (4S)-N-(9-phenylfluoren-9-yl)-oxazolidine-4- carboxylate (7) was prepared in two steps

Full text of DOI:10.1139/v05-033

793
Synthesis and application of L-N-Boc-N-methyl-␤-
hydroxyvaline in the preparation of a
depsipeptide1
James E. Dettwiler, Laurent Bélec, and William D. Lubell
Abstract: Enantiopure (>99% ee) L-N-tert-butyloxycarbonyl-N-methyl-β-hydroxyvaline (2) was synthesized in six steps
and 43% overall yield from D-serine methyl ester (5). Methyl (4S)-N-(9-phenylfluoren-9-yl)-oxazolidine-4-carboxylate
(7) was prepared in two steps and 73% yield by N-phenylfluorenation of 5 followed by cyclization of N-(PhF)amino
alcohol 6 with formaldehyde and catalytic p-toluenesulfonic acid (PhF = 9-phenylfluoren-9-yl). The addition of MeLi
to oxazolidine carboxylate 7 produced the tertiary alcohol 8 in 91% yield. Oxazolidines 8 equilibrated with oxazolidine
9 under acidic conditions. Reduction of pure 8 or the mixture of oxazolidines 8 and 9 with NaCNBH3 and hydrochlo-
ric acid in anhydrous dioxane afforded N-methyl amino diol 11 in 86%–92% yields. Attempts to selectively oxidize N-
(PhF)amino diol 11 were unsuccessful; however, hydrogenation of 11 in the presence of di-tert-butyl dicarbonate gave
the corresponding N-(Boc)amino diol 12 in 82% yield. Selective oxidation of diol 12 was performed using a cocktail
containing TEMPO free radical, NaClO2, and NaOCl to give L-N-Boc-N-methyl-β-hydroxyvaline (2) in 87% yield. Cou-
pling of β-hydroxyvaline 2 and (S)-2-hydroxy-3-methylbutanoate (15) was accomplished by using the methodology of
Mitsunobu to provide depsipeptide Boc-(S)-HOMeVal-(R)-Hmb (4) for use as a building block in the synthesis of the
cyclic antifungal depsipeptide aureobasidin B.
Key words: N-methylated amino acid, serine, depsipeptide, aureobasidin.
Résumé : La L-N-tert-butyloxycarbonyl-N-méthyl-β-hydroxyvaline (2) a été synthétisée sous sa forme énantiopure
(>99 % ee) par un protocole comprenant six étapes à partir de l’ester méthylique de la D-sérine (5) avec un rendement
total de 43 %. Le méthyle (4S)-N-(9-phénylfluoren-9-yl)-oxazolidine-4-carboxylate (7) a été préparé en deux étapes
avec un rendement de 73 % par une N-phénylfluorénation de 5, suivie par une cyclisation de l’alcool N-(PhF)amino 6
avec du formaldéhyde en présence catalytique d’acide p-toluène sulfonique (PhF = 9-phenylfluoren-9-yl). L’addition de
MeLi sur l’oxazolidine carboxylate 7 a donné l’alcool tertiaire 8 avec un rendement de 91 %. L’oxazolidine 8 a équili-
bré avec l’oxazolidine 9 dans des conditions acides. La réaction de 8 pur ou du mélange des oxazolidines 8 et 9 avec
du NaCNBH3 et de l’acide chlorhydrique dans du dioxane anhydre a donné l’amino diol N-méthylé 11 avec des rende-
ments de 86 % – 92 %. Les essais d’oxydation du N-(PhF)amino diol 11 ont échoué, cependant, le N-(Boc)amino diol
12 a été obtenu à partir de l’hydrogénation de 11 en présence de di-tert-butyle dicarbonate avec un rendement de
82 %. L’oxydation sélective du diol 12 a été effectuée avec un mélange du radical libre TEMPO, NaClO2 et NaOCl
pour donner la L-N-Boc-N-méthyl-β-hydroxyvaline (2) avec un rendement de 87 %. Le couplage entre la β-hydroxyva-
line 2 et le (S)-2-hydroxy-3-méthylbutanoate (15) a été effectué avec la méthode de Mitsunobu pour donner le depsi-
peptide Boc-(S)-HOMeVal-(R)-Hmb (4) qui est un fragment pour la synthèse de l’auréoabsidine B, un depsipeptide
antifongique et cyclique.
Mots clés : acide aminé N-méthylé, serine, depsipeptide, auréobasidine.
Dettwiler et al. 800
Introduction
well as a quaternary carbon, 1 is a synthetic challenge for
which no efficient route presently exists. Methods to prepare
L-N-methyl-β-hydroxyvaline (1) have usually required multi-
ple steps and have often resulted in low enantiomeric purity
(3–6). First synthesized in racemic form (3) and resolved, L-
N-Boc-N-methyl-β-hydroxyvaline (2) was most recently pre-
pared from D-N-Boc-serine methyl ester in eight steps with
L-N-Methyl-β-hydroxyvaline (1) is an important compo-
nent of biologically active peptides such as the anti-HIV
antibiotic luzopeptin (1) as well as members of the aureo-
basidin (2) family of antifungal peptides (Fig. 1). Featuring
hydroxyl, amino, carboxylate, and N-alkyl substituents as
Received 13 December 2004. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 6 July 2005.
J.E. Dettwiler, L. Bélec, and W.D. Lubell.2 Département de Chimie, Université de Montréal, C.P. 6128, Succursale Centre Ville,
Montréal, QC H3C 3J7, Canada.
1This article is part of a Special Issue dedicated to Professor Howard Alper.
2Corresponding author (e-mail: lubell@chimie.umontreal.ca).
Can. J. Chem. 83: 793–800 (2005)
doi: 10.1139/V05-033
© 2005 NRC Canada

794
Can. J. Chem. Vol. 83, 2005
Scheme 1. Synthesis of L-N-tert-butyloxycarbonyl-N-methyl-β-hydroxyvaline (2).
O
O
O
CH2O, p-TsOH
Ref. 8
78%
OCH3
HO
OCH3
HCl
HO
OCH3
O
NPhF
H2O:THF
93%
.
NH
NHPhF
7
5
6
CDCl3
or
NaCNBH3 , TFA , THF
or
NaCNBH3, HCl , dioxane
OH
SiO2
250 mol% MeLi
THF, 0°C
91%
RO
O
O
NPhF
Ph
FN
74%-92%
8
9: R = H
10: R= Ac
PhF:
Ac2O, DMAP
DMF
OH
O
H2, Pd/C
Boc2O
OH
TEMPO, NaOCl2 , NaClO
CH3CN, Phosphate Buffer
OH
OH
HO
CH3
HO
NBoc
THF
82%
NBoc
87%
NPhF
CH3
CH3
11
12
2
Fig. 1. L-Hydroxymethyl-N-methyl-β-hydroxyvaline (1), L-N-tert-
butyloxycarbonyl-N-methyl-β-hydroxyvaline (2), L-N-tert-butyl-
oxycarbonyl-β-hydroxyvaline (3), depsipeptide Boc-(S)-
HOMeVal-(R)-Hmb (4), and aureobasidin B.
however, unsuccessful using various methods. Protocols
using CH3I and (CH3)2SO4 as methylating agents in the
presence of various bases (NaH, Ag2O, Ag2CO3, or n-BuLi)
were unsuccessful and either resulted in alcohol methylation
or recovered starting material (11). For example, treatment
of 3 with CH3I and NaH in THF gave L-N-Boc-β-methoxy-
valine in 65% yield. Attempts to convert 3 to an oxazolidine
or oxazolidinone were also unsuccessful and resulted pre-
dominantly in recovered starting material. Instead of trying a
selective O-protection, N-alkylation, and O-deprotection
strategy, we decided to develop an alternative route to obtain
L-N-Boc-N-methyl-β-hydroxyvaline (2) in enantiopure form
(Scheme 1) (12). Our protocol involves a six-step synthesis
from D-serine methyl ester (5) in 43% overall yield. This
route employs N-(9-phenylfluoren-9-yl)serine methyl ester
(8) as a chiral educt to deliver enantiopure 2 (>99% ee).
HO
OH
O
O
O
9
OH
N
O
1
NR
8
Me
R'
O
N H
:
1 R = H, R' = Me
N Me
2
O
:
2 R = Boc, R' = Me
7
:
3 R = Boc, R' = H
O
Me N
H N
3
O
6
Ph
N Me
O
N H
OH
O
O
5
4
OH
N
Results and discussion
O
Ph
O
NBoc
O
The starting material D-N-(PhF)serine methyl ester (6) was
synthesized according to the literature method by phenyl-
fluorenation of D-serine methyl ester (Scheme 1) (8). Treat-
ment of amino alcohol 6 with formaldehyde and catalytic
p-toluenesulfonic acid in THF at room temperature afforded
oxazolidine carboxylate ester 7 in 93% yield (13). The qua-
ternary carbon was constructed using our best conditions by
adding 250 mol% of MeLi to a solution of 7 in THF at 0 °C.
Tertiary alcohol 8 could be isolated after filtration through a
plug of silica gel; however, on prolonged exposure to silica
gel, 8 equilibrated with oxazolidine primary alcohol 9.
In chloroform, oxazolidine 8 existed in equilibrium with
oxazolidine primary alcohol 9, the latter of which could be
selectively acetylated. Treatment of a 1:1 mixture of 8 and 9
with acetic anhydride and catalytic 4-dimethylaminopyridine
(DMAP) in chloroform gave 8 and ester 10. Separation of
CH3
Aureobasin B
4
an overall yield of 66%, albeit with an enantiomeric purity
of 90% (4, 5). Racemization during this synthesis was likely
due to the employment of a configurationally labile α-amino
aldehyde intermediate (7). Considering the requirement of
enantiopure building blocks for effective peptide synthesis,
alternative methodology is still necessary for synthesizing
enantiopure 1 in protected form.
We have recently reported a two-step approach for synthe-
sizing L-N-Boc-β-hydroxyvaline (3) from D-N-Boc-serine
methyl ester (8), which involved treatment with MeMgBr
followed by selective catalyzed TEMPO oxidation (9, 10)
Direct N-methylation of L-N-Boc-β-hydroxyvaline (3) was,
© 2005 NRC Canada

Dettwiler et al.
795
Scheme 2. Enantiomeric purity of L-N-tert-butyloxycarbonyl-N-methyl-β-hydroxyvaline (2).
Ph
O
TBTU, HOBt, DIEA, CH3CN
- and
-Phe-OMe . HCl
O
OH
O
OH
D
L
*
OCH3
OH
N
H
NBoc
NR
CH3
CH3
2
13: R = Boc
HCl, CH2Cl2
14: R = H
dr >99:1
the mixture by chromatography on silica gel gave a new
equilibrium and the three compounds (8–10) were isolated
in pure form. In CDCl3, pure 8 was found to equilibrate to a
1:1 mix of 8 and 9 after 12 h. Residual acid in chloroform
may be responsible for this transformation because spectra
of pure 8 and 9 could be obtained in DMSO-d6. The assign-
ment of the structures of oxazolidine alcohols 8 and 9 was
dicarbonate and palladium hydroxide to give the N-
(Boc)amino diol 12 in 82% yield (14). As in our synthesis of
β-hydroxyvaline (10), selective primary alcohol oxidation
was accomplished by treating N-(Boc)amino diol 12 with
TEMPO (2,2,6,6-tetramethyl-1-piperidyloxy) free radical,
sodium chlorite, and sodium hypochlorite in a sodium phos-
phate buffered acetonitrile solution (15). L-N-Boc-N-Methyl-
β-hydroxyvaline (2) was then isolated in pure form after
aqueous extraction in 87% yield.
To ascertain if any racemization had occurred during the
synthesis, the enantiomeric purity of 2 was investigated by
coupling to L- and D-Phe-OMe·HCl using benzotriazol-1-yl-
1,1,3,3-tetramethyluronium tetrafluoroborate (TBTU) (16),
1-hydroxybenzotriazole (HOBt), and diisopropylethylamine
(DIEA) in acetonitrile to produce dipeptides 13, which ex-
isted as mixtures of carbamate isomers (Scheme 2). The N-
Boc protecting group of the crude product was removed by
bubbling a stream of HCl gas into a solution of 13 in
CH2Cl2. The 400 MHz 1H NMR spectrum of crude dipeptide
(R)-14 was examined during incremental additions of (S)-14
in CD3OD. Measurement of the signals for the diastereo-
meric methyl amine singlets at 2.52 and 2.04 ppm demon-
strated dipeptide (S)-14 to be of >99% diastereomeric purity.
Hence, L-N-Boc-N-methyl-β-hydroxyvaline (2) is considered
to be of the same high enantiomeric purity.
1
made based on their H NMR spectra in DMSO-d6 in com-
parison to acetate 10. Of particular note, large differences in
the chemical shifts for the signals of the diastereotopic
methylene and methyl protons were indicative of groups
held in fixed environments by the oxazolidine ring. In the
case of the primary alcohol oxazolidine 9, the difference in
the chemical shifts of the diastereotopic signals for the con-
strained methyl groups were larger (∆δ = 162.4 Hz) than
those observed for tertiary alcohol oxazolidine 8 (∆δ =
94.6 Hz). The chemical shift differences for the diastereo-
topic methylene protons were greater (∆δ = 71.1 Hz) for the
tertiary alcohol oxazolidine 8 relative to those of the primary
alcohol oxazolidine 9 (∆δ = 26.6 Hz). Acetate 10 exhibited a
similar difference in the chemical shifts of its diastereotopic
methyl singlets (∆δ = 143.3 Hz) and methylene protons
(∆δ = 28.2 Hz) as was seen for the primary alcohol
oxazolidine 9. Moreover, relative to the methylene signals of
9, those for 10 were observed to be downfield shifted indica-
tive of acylation of the primary alcohol.
The depsipeptide Boc-(S)-HOMeVal-(R)-Hmb (4) was tar-
geted to serve as a building block for the synthesis of
analogs of the cyclic antifungal depsipeptide aureobasidin B
(17). The preparation of depsipeptides 4, 21, and 22 was
accomplished by the coupling of the corresponding N-Boc
protected amino acid (2, 3, and 17) and (S)-2-hydroxy-3-
methylbutanoic benzyl ester (16) using the Mitsunobu reac-
tion, followed by benzyl ester deprotection (Scheme 3).
Benzyl ester 16 was prepared from (S)-2-hydroxy-3-methyl-
butanoic acid (15), which was synthesized with retention of
configuration via a double inversion from L-valine using the
literature procedure in 52% overall yield (18). Because di-
rect alkylation of 15 resulted in mixtures of ether and ester
products, a four-step, one-pot procedure was developed to
prepare the benzyl ester 16. First, the hydroxyl and car-
boxylate groups were both protected with trimethylsilyl
(TMS) groups. Then, the carboxylic acid was liberated selec-
tively using MeOH and alkylated with benzyl bromide and
Cs2CO3 to provide the silylether benzyl ester, which on
treatment with citric acid gave α-hydroxy ester 16 in 65%
overall yield. The conditions for formation of the depsi
bonds were first optimized with Boc-MeVal (17) and Boc-
The conversion of oxazolidines 8 and 9 to N-methyl diol
11 was accomplished by treatment with NaCNBH3 under
acidic conditions (13). Two conditions were tested. In the
first condition, oxazolidine 8 at 0 °C was treated with
NaCNBH3 in anhydr. THF containing trifluoroacetic acid to
give N-methylamino diol 11 in 74% yield after purification.
In the second condition, oxazolidine 8 in 0.1 mol/L HCl in
dioxane was treated with NaCNBH3 to give N-methylamino
diol 11 in 86% yield after purification. Employing these
later conditions to reduce a mixture of oxazolidines 8 and 9
1
gave amino diol 11 in similar yield. In the H NMR spec-
trum of N-PhF-N-methylamino diol 11 in CDCl3, a doubling
of signals was observed suggesting an equilibrium between
two hydrogen-bonded forms. The doubling of signals could
be avoided by recording the spectrum of 11 in deuterated
methanol containing a trace of trifluoroacetic acid.
Selective oxidation of N-PhF-N-methylamino diol 11 was
unsuccessful using a wide variety of oxidation methods pre-
sumably because of the oxidation of the tertiary amine. On
the other hand, exchanging N-PhF to N-Boc protection could
be effected by hydrogenation in the presence of di-tert-butyl
© 2005 NRC Canada

796
Can. J. Chem. Vol. 83, 2005
Scheme 3. Synthesis of the depsipeptides 4, 21, and 22 (1 atm = 101.325 kPa).
O
O
R1
DIAD, PPh3
+
O R
OH
-20 °C, CH2Cl2,THF
OH
NBoc
R2
1) TMSCl, Et3N
2) MeOH
15: R = H
16: R= Bn
2: R1 = OH, R2 = CH3
17: R1 = H, R2 = CH3
3: R1 = OH, R2 = H
3) Cs2CO3, BnBr
4) Citric acid
65%
H2 (1 atm)
Pd(OH)2/C
THF
R1
O
R1
O
O
OH
O
O
NBoc
O
NBoc
O
R2
R2
18: R1 = OH, R2 = CH3
19: R1 = H, R2 = CH3
20: R1 = OH, R2 = H
4 : Boc-HOMeVal-Hmb : 72%
21 : Boc-MeVal-Hmb
22 : Boc-HOVal-Hmb
: 85%
: 83%
HOVal (3). Optimal conditions were then applied to Boc-
HOMeVal (2). Formation of the depsipeptide bond was per-
formed by an initial reaction of the secondary alcohol of 16
with triphenylphosphine and diisopropyldiazodicarboxylate
(DIAD) in dichloromethane, followed by SN2 displacement
of the phosphine intermediate on addition of the N-Boc pro-
tected amino acid (2, 3, and 17) to form depsipeptides 18–20
and triphenylphosphine oxide (19). Crude depsipeptides 18–
20 were isolated by chromatography on silica gel to remove
the triphenylphosphine oxide. Hydrogenolysis by using
Pd(OH)2 on carbon cleaved the benzyl ester affording acids
4, 21, and 22, in 72%, 85%, and 83% overall yields, respec-
tively (14).3
Triethylamine (Et3N) was distilled from ninhydrin then
CaH2. The 0.2 N HCl – dioxane solution was prepared by
passing a stream of HCl gas bubbles through dry dioxane
and the concentration of the solution was determined by ti-
tration of 1 mL aliquots diluted with 1 mL H2O using a
0.1 N standard NaOH solution and phenolphtalein as the in-
dicator. Chromatography was performed using 230–400
mesh silica gel. Amines were revealed with ninhydrin on
TLC plates; Boc groups were removed by dipping the plates
in TFA before exposure to ninhydrin. Unless otherwise noted,
1H and 13C NMR spectra were recorded in CDCl3 or CD3OD
at 300/75 or 400/100 MHz. Chemical shifts (δ) are reported
in parts per million (ppm) downfield of internal tetramethyl-
silane ((CH3)4Si) or referenced to the residual solvent peak
(CHCl3 δ 7.26, 77.16 ppm; methanol δ 3.31, 49.00 ppm).
Coupling constants are reported in Hz. Chemical shifts of
PhF aromatic carbons are omitted for clarity in the 13C NMR
spectra. HR-MS (EI and FAB) were obtained by the
Université de Montréal Mass Spectrometry facility. Specific
rotations are reported as follows: [α]2D0, concentration (c in
g/100 mL), and solvent. Analytical LC–MS was performed
on an inverse phase column (Anal Waters C18, 250 × 4.6 mm)
with a flow rate of 0.5 mL/min. A volume of 20 µL of sam-
ple with a concentration of 1 mg/mL was injected. Prepara-
tive reversed-phase HPLC was performed on an inverse phase
column (Prep Prevail C18, 250 × 22 mm) with a flow rate of
15 mL/min: elution condition A, gradient 20% CH3CN with
0.1% TFA–H2O with 0.1% TFA to 80% CH3CN with 0.1%
TFA–H2O with 0.1% TFA over 20 min; condition B, 70%
CH3CN with 0.1% TFA–H2O with 0.1% TFA.
Conclusion
Enantiopure (>99%) L-N-tert-butyloxycarbonyl-N-methyl-
β-hydroxyvaline (2) was synthesized in six steps and 43%
overall yield from D-serine methyl ester (5). Application of
2, as well as Boc-MeVal and Boc-HOVal (3), in Mitsunobu
reactions with (S)-benzyl-2-hydroxy-3-methylbutanoate (16)
has provided three depsipeptides: Boc-(S)-HOMeVal-(R)-
Hmb (4), Boc-(S)-MeVal-(R)-Hmb (21), and Boc-(S)-
HOVal-(R)-Hmb (22). These building blocks are presently
being employed in our program on antimicrobial peptide
synthesis (17, 20).
Experimental section
General
For reactions performed under anhydrous conditions,
glassware was either oven- or flame-dried and the reaction
was run under a positive pressure of argon or nitrogen. Tetra-
hydrofuran (THF) was freshly distilled from sodium/benzo-
phenone, CH2Cl2 from P2O5, dioxane from sodium.
Methyl (4S)-N-(9-phenylfluoren-9-yl)-oxazolidine-4-
carboxylate (7)
A solution of N-(PhF)serine methyl ester (6, 11.50 g,
32 mmol, prepared according to ref. 8), p-toluenesulfonic
3 Supplementary data for this article are available on the Web site or may be purchased from the Depository of Unpublished Data, Document
Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0S2, Canada. DUD 3681. For more information on obtaining mate-
rial refer to http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml.
© 2005 NRC Canada

Dettwiler et al.
797
acid (6 mol%, 331 mg, 1.92 mmol), and a 37% aqueous
formaldehyde solution (1500 mol%, 39 mL, 480 mmol) in
THF (320 mL) was stirred for 24 h at room temperature,
washed with a solution of satd. NaHCO3 (2 × 200 mL) and
brine, dried with MgSO4, filtered and evaporated to a solid
residue. Chromatography on silica gel using 10% EtOAc in
hexanes as the eluant gave a white solid (11.05 g, 93%).
TLC Rf = 0.41 (20% EtOAc in hexanes); mp 106 to 107 °C.
(75 MHz, DMSO-d6) δ: 20.5, 27.9, 63.4, 63.9, 76.9, 79.2,
80.9, 81.2, 169.7. HR-MS calcd. for C27H28NO3 ([M + H]+):
414.2064; found: 414.2065. Second to elute was (4R)-4-
hydroxymethyl-5,5-dimethyl-(9-phenylfluoren-9-yl)-oxazoli-
dine (9) as a white foam (12.5 mg, 25%). TLC Rf = 0.53
1
(30% AcOEt in hexanes). [α]2D0 326.0 (c 0.2 DMSO). H
NMR (400 MHz, CDCl3) δ: 0.70 (s, 3H), 1.24 (s, 3H), 2.15
(d, 1H, J = 4.9), 2.80 (dd, 1H, J1 = 6.1, J2 = 11.1), 3.20 (d,
1H, J = 11.0), 4.78 (d, 1H, J = 7.1), 4.96 (d, 1H, J = 7.0),
7.16–7.76 (m, 13H). 1H NMR (300 MHz, DMSO-d6) δ: 0.62
(s, 3H), 1.16 (s, 3H), 1.99 (m, 1H), 3.06 (m, 1H), 3.24 (m,
1H), 4.22 (t, 1H, J = 4.7), 4.59 (d, 1H, J = 7.0), 4.68 (d, 1H,
J = 7.0), 7.14–7.96 (m, 13H). 13C NMR (75 MHz, DMSO-
d6) δ: 20.2, 28.5, 61.6, 66.2, 77.1, 80.8, 81.6. HR-MS calcd.
for C25H26NO2 ([M + H]+): 372.1958; found: 372.1961. Last
to elute was oxazolidine 8 as a white foam (8.9 mg, 18%).
1
[α]2D0 –267.3 (c 1.0 CHCl3). H NMR (400 MHz, CDCl3) δ:
3.30 (t, 1H, J = 6.8), 3.54 (s, 3H), 3.61 (d, 2H), 4.73 (d, 1H,
J = 6.4), 4.93 (d, 1H, J = 6.4), 7.22–7.70 (m, 13H). 13C
NMR (100 MHz, CDCl3) δ: 52.0, 60.8, 69.1, 85.1, 119.8,
173.6. HR-MS calcd. for C24H22NO3 ([M + H]+): 372.1600;
found: 372.1582.
(4S)-4-(1-Hydroxymethylethyl)-3-(9-phenylfluoren-9-yl)-
oxazolidine (8)
(2R)-2-N-(9-Phenylfluoren-9-yl)amino-3-methyl-1,3-
butanediol (11)
To a solution of oxazolidine 7 (3.72 g, 10 mmol) in THF
(182 mL) at 0 °C, a solution of 1.4 mol/L MeLi in Et2O
(250 mol%, 18 mL, 25 mmol) was added dropwise. The re-
action mixture was stirred for 2 h at 0 °C and partitioned be-
tween equal volumes of EtOAc and a satd. NaH2PO4
solution (100 mL). The phases were separated and the aque-
ous phase was extracted with EtOAc (3 × 50 mL). The com-
bined organic phase was washed with brine, dried with
MgSO4, filtered and evaporated to a solid residue. Chroma-
tography on silica gel using 20% EtOAc in hexanes as the
eluant provided 8 as a white solid (3.38 g, 91%). TLC Rf =
0.34 (30% AcOEt in hexanes); mp 156 to 157 °C. [α]D20
Method 1
A solution of oxazolidine 8 (2.22 g, 6 mmol) in 0.2 mol/L
HCl in dioxane (60 mL) was treated with NaCNBH3
(320 mol%, 1.21 g, 19.2 mmol). The reaction mixture was
stirred for 1 h at r.t.
Method 2
A solution of oxazolidine 8 (4.5 g, 14.2 mmol) and
NaCNBH3 (800 mol%, 4.9 g, 114 mmol) in THF (120 mL)
was cooled to 0 °C and treated dropwise with trifluoroacetic
acid (300 mol%, 3.3 mL, 42.6 mmol). The reaction mixture
was stirred for 30 min at 0 °C. After stirring for the desig-
nated time, both reaction mixtures were washed, respec-
tively, with a solution of satd. NaHCO3 and brine, dried with
MgSO4, filtered and evaporated to a solid. Chromatography
on silica gel using 20% EtOAc in hexanes as eluant gave a
white solid (Method 1: 1.93 g, 86%; Method 2: 3.35 g,
74%). TLC Rf = 0.54 (50% AcOEt in hexanes); mp 85 to
1
–519.3 (c 0.3 DMSO). H NMR (400 MHz, CDCl3) δ: 0.91
(s, 3H), 1.05 (s, 3H), 1.95 (s, 1H), 2.49 (dd, 1H, J1 = 3.1,
J2 = 7.5), 2.96 (t, 1H, J = 7.8), 3.49 (dd, 1H, J1 = 3.1, J2 =
8.1), 4.79 (d, 1H, J = 6.7), 5.10 (d, 1H, J = 6.7), 7.15–7.73
1
(m, 13H). H NMR (400 MHz, DMSO-d6) δ: 0.78 (s, 3H),
1.01 (s, 3H), 2.32 (dd, 1H, J1 = 2.4, J2 = 7.2), 2.32 (t, 1H,
J = 7.4), 3.63 (dd, 1H, J1 = 2.5, J2 = 7.2), 4.13 (s, 1H), 4.71
(d, 1H, J = 6.3), 4.89 (d, 1H, J = 6.3), 7.10–7.97 (m, 13H).
13C NMR (100 MHz, DMSO-d6) δ: 26.2, 29.0, 65.2, 66.7,
71.1, 77.6, 85.5. HR-MS calcd. for C25H25NO2 (TOF EI):
371.1885; found: 371.1886.
1
86 °C. [α]2D0 −449.8 (c 1.0 MeOH). H NMR (400 MHz,
CD3OD + TFA) δ: 0.99 (s, 3H), 1.36 (s, 3H), 2.82 (dd, 1H,
J1 = 2.0, J2 = 5.4), 3.12 (dd, 1H, J1 = 6.6, J2 = 5.0), 3.15 (s,
3H), 3.42 (dd, 1H, J1 = 3.5, J2 = 8.1), 7.22–8.10 (m, 13H).
13C NMR (100 MHz, CD3OD) showed a mixture of con-
formers; the chemicals shifts are listed in order of appear-
ance (δ): 26.8, 28.5, 29.6, 33.2, 33.4, 58.2, 60.7, 61.3, 63.0,
75.5, 76.7, 78.5, 79.1, 79.2. HR-MS calcd. for C25H27NO2
(TOF EI): 373.2042; found: 373.2051. Employing a 1:1 mix-
ture of oxazolidines 8 and 9 (155 mg, 0.147 mmol) using
method 2 gave diol 11 in 92% yield.
(4R)-O-Acetoxymethyl-5,5-dimethyl-3-(9-phenylfluoren-
9-yl)-oxazolidine (10)
A solution of oxazolidine 8 (50 mg, 0.135 mmol) in
CDCl3 (1.4 mL) was stirred for 12 h at room temperature,
treated with DMAP (10 mol%, 1.6 mg, 13.5 µmol), and
dropwise with acetic anhydride (240 mol%, 42 µL,
0.324 mmol). The reaction mixture was stirred for 24 h and
concentrated under reduced pressure to give a residue that
was purified by chromatography on silica gel using 15%
EtOAc in hexanes as the eluant. First to elute was the acety-
lated oxazolidine 10 as a white foam (28.0 mg, 50%). TLC
Rf = 0.45 (20% AcOEt in hexanes). [α]2D0 229.3 (c 0.3
(2R)-2-N-(Boc)Amino-3-methyl-1,3-butanediol (12)
A solution of diol 11 (900 mg, 2.41 mmol) and di-tert-
butyl dicarbonate (140 mol%, 737 mg, 3.37 mmol) in THF
(100 mL) was treated with palladium hydroxide on carbon
(270 mg, 20 wt% in Pd) and stirred under 5 atm of hydrogen
(1 atm = 101.325 kPa) for 24 h at room temperature. The
catalyst was removed by filtration on Celite® and washed
with THF (2 × 20 mL) and MeOH (2 × 20 mL). The com-
bined organic phase was evaporated to a residue that was pu-
rified by chromatography on silica gel using 70% EtOAc in
hexanes as the eluant. Evaporation of the ninhydrin positive
fractions (visualized on TLC after treatment with TFA va-
1
DMSO). H NMR (400 MHz, CDCl3) δ: 0.70 (s, 3H), 1.15
(s, 3H), 1.87 (s, 3H), 2.30 (dd, 1H, J1 = 4.5, J2 = 10.3), 3.72
(dd, 1H, J1 = 4.5, J2 = 11.1), 3.90 (t, 1H, J = 10.7), 4.73 (d,
1
1H, J = 7.3), 4.88 (d, 1H, J = 7.3), 7.16–7.60 (m, 13H). H
NMR (300 MHz, DMSO-d6) δ: 0.56 (s, 3H), 1.07 (s, 3H),
1.83 (s, 3H), 2.17 (dd, 1H, J1 = 4.5, J2 = 10.3), 3.57 (dd, 1H,
J1 = 4.5, J2 = 11.1), 3.86 (t, 1H, J = 10.7), 4.65 (d, 1H, J =
7.2), 4.75 (d, 1H, J = 7.1), 7.17–7.94 (m, 13H). 13C NMR
© 2005 NRC Canada

798
Can. J. Chem. Vol. 83, 2005
pors) gave N-(Boc)amino diol 12 as a white solid (461 mg,
82%). TLC Rf = 0.49 (80% AcOEt in hexanes); mp 138
washed with an aq. satd. solution of NaHCO3 (2 × 3 mL),
1 N NaH2PO4 (2 × 3 mL), and brine (3 mL), dried with
MgSO4, filtered and evaporated to a solid residue. The Boc
group was subsequently removed by treating the residue in
dry CH2Cl2 at 0 °C with a stream of HCl gas bubbles, stir-
ring for 15 min, and concentrating the reaction mixture
under reduced pressure to give a solid that was directly ex-
amined by 400 MHz 1H NMR spectroscopy in CD3OD.
Measurement of the signals for the diastereomeric methyl
amine singlets at 2.04 and 2.52 ppm during incremental ad-
ditions of (S)-14 in (R)-14 demonstrated dipeptide (R)-14 to
be of >99% diastereomeric purity. Hence, (S)-β-hydroxy-N-
(Boc)-N-(methyl)valine (2) is considered to be of the same
high enantiomeric purity.
1
to 139 °C. [α]2D0 37.1 (c 1.0 MeOH). H NMR (400 MHz,
CD3OD) showed a 1:1.3 mixture of carbamate isomers (δ):
1.15 (s, 1.6H), [1.16 (s, 1.2H)], [1.25 (s, 1.5H)], 1.26 (s,
1.6H), 1.46 (s, 4.2H), 1.48 (s, 4.5H), [2.89 (s, 1.1H)], 2.93
(s, 1.7H), 3.83–3.92 (m, 2.5H), [4.04 (dd, 0.5H, J1 = 4.0,
J2 = 8.0)]. 13C NMR (100 MHz, CD3OD) showed a mixture
of carbamate isomers; the chemical shifts are listed in order
of appearance (δ): 26.7, 27.8, 28.0, 28.3, 28.9, 57.7, 58.0,
64.8, 72.6, 72.7, 79.9, 80.1, 157.9, 158.0. HR-MS calcd. for
C11H24NO4 ([M + H]+): 234.1705; found: 234.1709. Anal.
calcd. for C11H23NO4: C 56.63, H 9.94, N 6.00; found: C
56.50, H 10.28, N 5.92.
(S,S)-␤-Hydroxy-N-(Boc)-N-(methyl)valinylphenylalanine
methyl ester ((S)-13)
(S)-␤-Hydroxy-N-(Boc)-N-(methyl)valine (2)
A mixture of diol 12 (278 mg, 1.19 mmol), sodium phos-
phate buffer (4.6 mL, 0.67 mol/L, pH 6.7), and TEMPO
(10 mol%, 24 mg, 0.119 mmol) in acetonitrile (6 mL) was
heated to 35 °C and treated dropwise simultaneously over
2 h (Caution! Do not mix bleach and sodium chlorite before
adding to reaction mixture) (15) with a solution of sodium
chlorite (NaClO2, 200 mol%, 2.38 mmol, 1.2 mL of solution :
5.71 g 80% w/w, 50.5 mmol in 25 mL of water) and diluted
bleach (NaOCl, 2 mol%, 0.04 mmol, 1050 µL of solution :
0.66 mL commercial bleach 10.8% v/v 25 mL of water). The
mixture was stirred at 35 °C overnight, cooled to r.t., acidi-
fied with citric acid to pH 3, and extracted with EtOAc (3 ×
10 mL). The organic phases were combined and evaporated.
The residue was dissolved in a solution of satd. Na2CO3
(15 mL) and washed with EtOAc (2 × 10 mL). The aqueous
phase was acidified with 1 mol/L H3PO4 to pH 3, saturated
with NaCl, and extracted with EtOAc (3 × 15 mL). The or-
ganic phases were combined, dried with MgSO4, filtered and
evaporated to give an oil (256 mg, 87%). TLC Rf = 0.43
(CHCl3–MeOH–AcOH, 89:10:1). [α]2D0 –34.4 (c 0.5 MeOH).
1H NMR (400 MHz, CD3OD) showed a 0.7:1 mixture of
carbamate isomers (δ): 1.22 (s, 1.9H), [1.26 (1.2H)], [1.38
(s, 1.2H)], 1.42 (s, 1.9H), [1.46 (s, 3.6H)], 1.47 (s, 5.4H),
[2.95 (s, 1.2H)], 2.96 (s, 1.8H), [4.46 (s, 0.4H)], 4.59 (s,
0.6H). 13C NMR (100 MHz, CD3OD) showed a mixture of
carbamate isomers; the chemicals shifts are listed in order of
appearance (δ): 26.4, 27.6, 27.9, 28.1, 33.2, 33.3, 65.3, 66.2,
72.8, 78.5, 80.5, 81.1, 156.5, 157.4, 172.2, 172.3. HR-MS
calcd. for C11H22NO5 ([M + H]+): 248.1498; found:
248.1500. LC–MS: 98% purity (condition A), RT = 14.5.
TLC Rf = 0.30 (30% AcOEt in hexanes). 1H NMR
(400 MHz, CD3OD) showed a 1:2 mixture of carbamate iso-
mers (δ): 1.13 (s, 3H), 1.20 (s, 3H), 1.47 (s, 9H), 2.68 (s,
2.0H), [2.71 (s, 1.1H)], 2.97 (m, 1H), 3.23 (m, 1H), [3.68 (s,
1.5H)], 3.73 (s, 3.8H), [4.44 (s, 0.28H)], 4.54 (s, 0.65H),
4.76 (dd, 1H, J1 = 4.8, J2 = 9.5), 7.08–7.35 (m, 5H). MS
(TIC) m/z: 309.2 ([M + H]+).
(S,R)-␤-Hydroxy-N-(Boc)-N-(methyl)valinylphenylalanine
methyl ester ((R)-13)
TLC Rf = 0.35 (30% AcOEt in hexanes). 1H NMR
(400 MHz, CD3OD) showed a 1:2 mixture of carbamate iso-
mers (δ): 1.08 (s, 3H), 1.15 (s, 3H), 1.46 (s, 9H), [2.70 (s,
1.1H)], 2.93 (s, 1.9H), 3.00 (m, 1H), 3.24 (m, 1H), [3.66 (s,
1.8H)], 3.72 (s, 3.6H), [4.40 (s, 0.3H)], 4.49 (s, 1H), 4.77
(dd, 0.6H, J1 = 4.50, J2 = 8.9), 7.01–7.34 (m, 5H). MS (TIC)
m/z: 309.2 ([M + H]+).
(S,S)-␤-Hydroxy-N-(methyl)valinylphenylalanine methyl
ester hydrochloride ((S)-14)
1H NMR (400 MHz, CD3OD) δ: 1.07 (s, 3H), 1.29 (s,
3H), 2.04 (s, 3H), 2.84 (dd, 1H, J1 = 10.9, J2 = 14.0), 3.17–
3.23 (m, 2H), 3.58 (s, 3H), 4.82 (m, 1H), 7.14–7.20 (m, 5H).
(S,R)-␤-Hydroxy-N-(methyl)valinylphenylalanine methyl
ester hydrochloride ((R)-14)
1H NMR (400 MHz, CD3OD) δ: 0.84 (s, 3H), 0.87 (s,
3H), 2.52 (s, 3H), 2.83 (dd, 1H, J1 = 11.5, J2 = 14.2), 3.21
(m, 1H), 3.26 (dd, 1H, J1 = 4.5, J2 = 14.2), 3.66 (s, 3H),
4.70 (dd, 1H, J1 = 4.5, J2 = 11.4), 7.14–7.19 (m, 5H).
Enantiomeric purity of (S)-␤-hydroxy-N-(Boc)-N-
(methyl)valine (2)
(S)-N-(Boc)-␤-methoxyvaline
A solution of 2 (30 mg, 0.12 mmol) in acetonitrile (1 mL)
at 0 °C was treated with HOBt (100 mol%, 22 mg, 0.12 mmol)
and TBTU (150 mol%, 58 mg, 0.18 mmol), stirred for
30 min, and treated with a premixed solution of (S)- or (R)-
phenylalanine methyl ester hydrochloride (300 mol%,
78 mg, 0.36 mmol) and diisopropylethylamine (DIEA,
400 mol%, 0.84 µL, 0.48 mmol) in acetonitrile (1 mL) at
0 °C. The reaction mixture was stirred for 48 h at room tem-
perature until TLC showed the complete disappearance of
starting acid 2. TLC Rf = 0.43 (CHCl3–MeOH–AcOH,
89:10:1). The solvent was evaporated and the solid residue
was resuspended in CH2Cl2 (3 mL). The organic phase was
To a solution of (S)-β-hydroxy-N-(Boc)valine (3, 100 mg,
429 µmol, prepared according to ref. 10) and methyl iodide
(800 mol%, 214 µL, 3.43 mmol) in THF (1.5 mL) at 0 °C,
sodium hydride (60% dispersion in mineral oil, 300 mol%,
52 mg, 29 mmol) was added cautiously with gentle stirring.
The suspension was stirred for 24 h, diluted with EtOAc
(2 mL), treated dropwise with water to destroy the excess
sodium hydride, and evaporated to an oily residue that was
partitioned between ether (5 mL) and water (10 mL). The
ether layer was washed with a solution of satd. NaHCO3
(10 mL). The combined aqueous extracts were acidified with
citric acid to pH 3 and extracted with EtOAc (3 × 10 mL).
© 2005 NRC Canada

Dettwiler et al.
799
The organic phases were combined, washed with water (2 ×
10 mL), 5% aqueous sodium thiosulfate (2 × 10 mL), and
water (2 × 10 mL), dried with MgSO4, filtered and evapo-
rated to give an oil that was purified by chromatography on
silica gel using a mixture of CHCl3, methanol, and acetic
acid (94:4:1) to give an oil (69 mg, 65%). TLC Rf = 0.56
(CHCl3–MeOH–AcOH, 89:10:1). [α]2D0 9.5 (c 1.0 MeOH).
1H NMR (400 MHz, CD3OD) δ: 1.26 (s, 3H), 1.28 (s, 3H),
1.45 (s, 9H), 3.23 (s, 3H), 4.2 (s, 1H). 13C NMR (100 MHz,
CD3OD) δ: 21.7, 21.8, 27.7, 49.0, 60.6, 76.1, 79.7, 156.8,
173.0. HR-MS calcd. for C11H21NO5Na ([M + Na]+):
270.1312; found: 270.1319.
step. An analytically pure sample (LC–MS: 96% purity, con-
dition B, RT = 23.7) was obtained by purification on RP-
HPLC using condition A. TLC Rf = 0.28 (20% AcOEt in
1
hexanes). [α]2D0 –22.2 (c 0.5 MeOH). H NMR (300 MHz,
CD3OD) showed a 1:2 mixture of carbamate isomers (δ):
0.92 (d, 3H, J = 6.9), 0.99 (d, 3H, J = 6.9), [1.38 (s, 1.08H)],
1.41 (s, 1.98H), 1.22 (s, 2.01H), [1.25 (s, 1.08H)], 1.47 (s,
9H), 2.16–2.31 (m, 1H), [2.93 (s, 1.16H)], 2.94 (s, 1.96H),
[4.75 (s, 0.33H)], 4.78 (s, 0.62H), 5.15 (d, 1H, J = 12.1),
5.21 (d, 1H, J = 12.1), 7.31–7.38 (m, 5H). 13C NMR
(75 MHz, CD3OD) showed a mixture of carbamate isomers;
the chemical shifts are listed in order of appearance (δ):
16.5, 18.1, 21.3, 26.4, 26.5, 27.6, 28.1, 30.4, 32.3, 33.3,
65.1, 65.6, 67.0, 72.5, 72.6, 77.6, 80.6, 81.0, 128.5, 128.6,
135.9, 156.5, 157.5, 196.6, 170.1. HR-MS calcd. for
C23H36NO7 ([M + H]+): 438.2486; found: 438.2478.
(S)-Benzyl-2-hydroxy-3-methylbutanoate (Hmb-OBn, 16)
To a solution of (S)-2-hydroxy-3-methylbutanoic acid (15,
2 g, 16.9 mmol, prepared according to ref. 18) and TMSCl
(600 mol%, 8.8 mL, 101.4 mmol) in CH2Cl2 (170 mL) at
0 °C, Et3N (600 mol%, 14.2 mL, 101.4 mmol) was added
dropwise. The mixture was heated at reflux for 1 h, cooled
to 0 °C, and treated with dry MeOH (110 mol%, 732 µL,
18.6 mmol). The reaction was stirred for 1 h at room tem-
perature. The volatiles were removed under reduced pressure
on a rotary evaporator. The residue was dissolved in
acetonitrile (170 mL) and treated with anhydrous cesium
carbonate (150 mol%, 8.3 g, 25.4 mmol) and benzyl bro-
mide (500 mol%, 10 mL, 84.5 mmol), heated at reflux and
stirred vigorously for 1 h, cooled to room temperature, treated
with solid citric acid (1000 mol%, 32.5 g, 169 mmol), and
stirred vigorously for 1 h at room temperature. The precipi-
tate was filtered off and the filtrate was concentrated under
reduced pressure. The residue was dissolved in CHCl3
(150 mL), washed with a solution of satd. NaHCO3 (2 ×
50 mL) and brine, dried with MgSO4, filtered and evapo-
rated to a residue. Chromatography on silica gel using a gra-
dient 20%–30% EtOAc in hexanes as the eluant and
evaporation of the collected fractions provided an oil (2.3 g,
65%). TLC Rf = 0.37 (50% EtOAc in hexanes). [α]2D0 –11.3
Boc-(S)-MeVal-(R)-Hmb-OBn (19, 649 mg) was isolated
from the reaction of Hmb-OBn (16, 313 mg, 1.50 mmol)
and Boc-MeVal (17, 521 mg, 2.25 mmol) by chromatogra-
phy on silica gel using 15% EtOAc in hexanes as the eluant
and directly employed in the hydrogenation step. An analyti-
cally pure sample (LC–MS: 99% purity, condition A, RT =
8.0) was obtained by purification on RP-HPLC using condi-
tion A. TLC Rf = 0.35 (10% AcOEt in hexanes). [α]2D0 –46.6
1
(c 0.5 CHCl3). H NMR (400 MHz, CDCl3) showed a 1:1
mixture of carbamate isomers (δ): 0.85–0.95 (m, 6H), 0.95–
1.06 (m, 6H), 1.46 (s, 9H), 2.22 (m, 2H), 2.78 (s, 1.6H),
[2.83 (s, 1.4H)], [4.29 (d, 0.4H, J = 10.3)], 4.59 (d, 0.4H,
J = 10.3), 4.87 (d, 1H, J = 4.1), 7.24–7.46 (m, 5H). 13C
NMR (100 MHz, CDCl3) showed a mixture of carbamate
isomers; the chemical shifts are listed in order of appearance
(δ): 17.2, 19.0, 19.3, 20.0, 27.6, 27.8, 28.5, 30.2, 30.7, 63.1,
64.8, 66.9, 80.0, 80.3, 128.5, 128.7, 135.4, 155.8, 156.4,
169.1, 169.3, 170.8, 171.1. HR-MS calcd. for C23H35NO7
(TOF EI): 421.2464; found: 421.2471.
Boc-(S)-HOVal-(R)-Hmb-OBn (20, 1.3 mg) was isolated
from the reaction of Hmb-OBn (16, 625 mg, 3.0 mmol) and
Boc-HOVal (3, 1050 mg, 4.5 mmol) by chromatography on
silica gel using 15% EtOAc in hexanes as the eluant and di-
rectly employed in the hydrogenation step. An analytically
pure sample (LC–MS: 99% purity, condition B, RT = 16.0)
was obtained by purification on RP-HPLC using condition
A. TLC Rf = 0.38 (25% AcOEt in hexanes). [α]2D0 –2.8 (c 0.5
1
(c 2.0 CHCl3). H NMR (400 MHz, CDCl3) δ: 0.82 (d, 3H,
J = 6.9), 0.98 (d, 3H, J = 6.9), 2.00–2.14 (m, 1H), 3.06–3.21
(m, 1H), 4.01–4.11 (m, 1H), 5.14 (d, 1H, J = 12.1), 5.21 (d,
1H, J = 12.2), 7.25–7.38 (m, 5H). 13C NMR (100 MHz,
CDCl3) δ: 15.9, 18.7, 32.1, 67.0, 75.0, 128.3, 128.4, 128.5,
135.2, 174.7. HR-MS calcd. for C12H16NO3 (TOF EI):
208.1099; found: 208.1098.
1
MeOH). H NMR (400 MHz, CD3OD) δ: 0.93 (d, 3H, J =
6.8), 0.98 (d, 3H, J = 6.8), 1.25 (s, 3H), 1.28 (s, 3H), 1.44 (s,
9H), 2.22 (dsept, 1H, J1 = 4.6, J2 = 6.8), 4.17 (s, 1H), 4.86
(d, 1H, J = 4.4), 5.16 (d, 1H, J = 12.2), 5.21 (d, 1H, J =
12.2), 7.28–7.40 (m, 5H). 13C NMR (100 MHz, CD3OD) δ:
16.6, 18.2, 25.8, 26.0, 27.7, 30.4, 63.1, 67.0, 71.3, 77.7,
79.8, 128.4, 128.5, 128.6, 135.9, 156.8, 169.9, 170.9. HR-
MS calcd. for C22H33NO7 ([M + H]+): 424.2330; found:
424.2315.
General procedure for the preparation of depsipeptide
benzyl esters
A 0.1 mol/L solution of Hmb-OBn (16, 100 mmol%),
Ph3P (250 mol%), and the amino acid (Boc-HOMeVal (2,
100 mol%), Boc-HOVal (3, 150 mol%), or Boc-MeVal (17,
150 mol%)) in CH2Cl2–THF (1:1) was cooled to –20 °C,
treated with diisopropyl azodicarboxylate (DIAD, 255 mol%)
dropwise over 30 min, and stirred at –20 °C for 30 min. The
reaction mixture was removed from the ice bath and allowed
to stir at room temperature overnight. The volatiles were re-
moved under reduced pressure on a rotary evaporator.
Boc-(S)-HOMeVal-(R)-Hmb-OBn (18, 169 mg) was iso-
lated from the reaction of Hmb-OBn (16, 80 mg,
0.384 mmol) and Boc-HOMeVal (2, 91 mg, 0.384 mmol) by
chromatography on silica gel using 18% EtOAc in hexanes
as the eluant and directly employed in the hydrogenation
General procedure for benzyl ester removal from
depsipeptides
A 0.033 mol/L solution containing the depsipeptide
benzyl ester (100 mol%) in THF was treated with palladium
hydroxide on carbon (30 wt% of 20 wt% palladium (wet)),
stirred under 1 atm of hydrogen for 1 h, filtered onto Celite®,
and washed with THF (2×) and MeOH (2×). The filtrate was
concentrated under reduce pressure to a residue that was
© 2005 NRC Canada

800
Can. J. Chem. Vol. 83, 2005
passed through a pad of silica gel, first using 20% EtOAc in
hexanes as the eluant, then followed by 99% EtOAc contain-
ing 1% AcOH.
Acknowledgments
This research was supported in part by the Natural Sci-
ences and Engineering Research Council of Canada
(NSERC), the Ministère de l’Éducation du Québec, and
Valorisation-Recherche Québec. We would like to thank
Mr. Dalbir Sekhon for performing the LC–MS experiments.
Boc-(S)-HOMeVal-(R)-Hmb (4)
The reaction of ester 18 (89 mg) afforded 4 as a white
foam (51 mg, 72%). TLC Rf = 0.40 (CHCl3–MeOH–AcOH,
1
89:10:1). [α]2D0 –28.2 (c 0.5 MeOH). H NMR (300 MHz,
CD3OD) showed a 1:1.5 mixture of carbamate isomers (δ):
0.99 (d, 3H, J = 6.9), 1.03 (d, 3H, J = 6.9), 1.26 (d, 3H, J =
9.5), 1.41 (d, 3H, J = 8.6), 1.48 (s, H), 2.26 (dsept, 1H, J1 =
4.0, J2 = 6.7), [3.01 (s, 1.17H)], 3.03 (s, 1.78H), [4.75 (s,
0.37H)], 4.78 (s, 0.64H), 4.88 (d, 1H, J = 3.9), 4.95 (br s,
1OH). 13C NMR (75 MHz, CD3OD) showed a mixture of
carbamate isomers; the chemical shifts are listed in order of
appearance (δ): 16.4, 18.3, 21.1, 21.3, 26.4, 27.6, 28.1, 30.1,
32.3, 33.3, 65.2, 65.6, 69.4, 72.5, 72.7, 77.5, 78.5, 80.6,
81.0, 156.6, 157.5, 170.1, 171.5. HR-MS calcd. for
C16H30NO7 ([M + H]+): 348.2017; found: 348.2016. LC–
MS: 99% purity (condition A), RT = 21.1.
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Boc-(S)-MeVal-(R)-Hmb (21)
The reaction of ester 19 (250 mg) afforded 21 as an oil
(167 mg, 85%). TLC Rf = 0.58 (CHCl3–MeOH–AcOH,
8. W.D. Lubell and H. Rapoport. J. Org. Chem. 54, 3824 (1989).
9. J.E. Dettwiler and W.D. Lubell. Can. J. Chem. 82, 318 (2004).
10. J.E. Dettwiler and W.D. Lubell. J. Org. Chem. 68, 177 (2003).
11. M. Prashad, D. Har, B. Hu, H.-Y. Kim, O. Repic, and T.J.
Blacklock. Org. Lett. 5, 125 (2003), and refs. 1 and 2 cited
therein.
12. J.E. Dettwiler, L. Bélec, and W.D. Lubell. In The 18th Ameri-
can Peptide Society Symposium Proceedings: Peptide revolu-
tion: Genomics, proteomics & therapeutics. Boston, Mass, 19–
23 July 2003. Edited by M. Chorev and T.S. Sawyer. American
Peptide Society. Cardiff, Calif. 2004. pp. 179–180.
13. W.D. Lubell, T.F. Jamison, and H. Rapoport. J. Org. Chem. 55,
3511 (1990).
1
89:10:1). [α]2D0 –75.6 (c 0.5 MeOH). H NMR (300 MHz,
CD3OD) δ: 0.91 (m, 3H), 0.95–1.07 (m, 6H), 1.25 (d, 3H,
J = 6.2), 1.45 (s, 9H), 2.13–2.33 (m, 2H), 2.84 (s, 3H), [4.27
(d, 0.43H, J = 10.5)], 4.45 (d, 0.45H, J = 10.4), 4.8 (d, 1H,
J = 4.0), 4.84–5.00 (br s, 1OH). 13C NMR (75 MHz,
CD3OD) showed a mixture of carbamate isomers; the chemi-
cal shifts are listed in order of appearance (δ): 16.5, 18.2,
18.4, 18.5, 19.3, 21.3, 27.4, 27.6, 27.8, 29.8, 30.1, 30.3,
63.6, 65.1, 69.4, 77.3, 80.5, 80.8, 156.5, 157.1, 170.9. HR-
MS calcd. for C16H30NO6 ([M + H]+): 332.2073; found
332.2074. LC–MS: 97% purity (condition A), RT = 26.0.
14. R. Sharma and W.D. Lubell. J. Org. Chem. 61, 202 (1996).
15. M. Zhao, J. Li, E. Mano, Z. Song, D.M. Tschaen, E.J.J.
Grabowski, and P.J. Reider. J. Org. Chem. 64, 2564 (1999).
16. R. Knorr, A. Trzeciak, W. Bannwarth, and D. Gillessen. Tetra-
hedron Lett. 30, 1927 (1989).
Boc-(S)-HOVal-(R)-Hmb (22)
The reaction of ester 20 (330 mg) afforded 22 as a white
solid (224 mg, 83%). TLC Rf = 0.38 (CHCl3–MeOH–
AcOH, 89:10:1). [α]2D0 –29.4 (c 0.5 MeOH). 1H NMR
(400 MHz, CD3OD) δ: 1.00 (d, J = 6.9, 3H), 1.03 (d, J =
7.0, 3H), 1.29 (s, 3H), 1.31 (s, 3H), 1.45 (s, 9H), 2.26
(dsept, J1 = 4.1, J2 = 6.7, 1H), 4.14 (s, 1H), 4.87 (d, J = 3.9,
1H). 13C NMR (100 MHz, CD3OD) δ: 14.4, 19.4, 26.9, 27.0,
28.7, 31.2, 64.2, 72.3, 78.5, 80.8, 157.8, 171.7, 173.0. HR-
MS calcd. for C15H28NO7 ([M + H]+): 334.1866; found
334.1876. LC–MS: 98% purity (condition A), RT = 18.5.
17. J.E. Dettwiler and W.D. Lubell. M.Sc. thesis, University of
Montreal. Montréal, Que. 2005.
18. P. Brewster, F. Hiron, E.D. Hughes, C.K. Ingold, and P.A.D.S.
Rao. Nature (London), 166, 179 (1950).
19. O. Mitsunobu. Synthesis, 1 (1981).
20. S. Roy, H.-G. Lombart, R.E.W. Hancock, S.W. Farmer, and
W.D. Lubell. J. Pept. Res. 60, 198 (2002).
© 2005 NRC Canada