Synthesis and analysis of the sterically constrained L-glutamine analogues (3S,4R)-3,4-dimethyl-L-glutamine and (3S,4R)-3,4-dimethyl-L-pyroglutamic acid

Article abstract of DOI:10.1016/S0040-4020(01)00501-4

The nonproteinogenic amino acids (3S,4R)-3,4-dimethyl-L-pyroglutamic acid and (3S,4R)-3,4-dimethyl-L-glutamine - found in the cyclic depsipeptides callipeltin B, callipeltin A, and papuamide A - were synthesized from a common intermediate derived from L-pyroglutamic acid. The diastereoselective introduction of the methyl groups was accomplished by cuprate addition and enolate alkylation, followed by a kinetic epimerization of the C-4 methyl substituent. (3S,4R)-3,4-Dimethyl-L-glutamine shows a conformational restriction of its side chain which may be related to its biological function in the natural products where it is found.

Full text of DOI:10.1016/S0040-4020(01)00501-4

TETRAHEDRON
Pergamon
Tetrahedron 57 (2001) 6353±6359
Synthesis and analysis of the sterically constrained l-glutamine
analogues ꢀ3S,4R)-3,4-dimethyl-l-glutamine and
ꢀ3S,4R)-3,4-dimethyl-l-pyroglutamic acid
²
Cristina M. Acevedo, Eugene F. Kogut and Mark A. Liptonp
Department of Chemistry, Purdue University, West Lafayette, IN 47907-1393, USA
Received 23 January 2001; accepted 17 April 2001
AbstractÐThe nonproteinogenic amino acids (3S,4R)-3,4-dimethyl-l-pyroglutamic acid and (3S,4R)-3,4-dimethyl-l-glutamineÐfound in
the cyclic depsipeptides callipeltin B, callipeltin A, and papuamide AÐwere synthesized from a common intermediate derived from
l-pyroglutamic acid. The diastereoselective introduction of the methyl groups was accomplished by cuprate addition and enolate alkylation,
followed by a kinetic epimerization of the C-4 methyl substituent. (3S,4R)-3,4-Dimethyl-l-glutamine shows a conformational restriction of
its side chain which may be related to its biological function in the natural products where it is found. q 2001Elsevier Science Ltd. All rights
reserved.
1. Introduction
and 5a. Additionally, the side chain methylation of 1 might
be assumed to result in a conformational restriction of its
side chain in comparison to that of l-glutamine.
The nonproteinogenic a-amino acid (3S,4R)-3,4-dimethyl-
l-glutamine (1) was ®rst encountered by Minale and
co-workers during their characterization of callipeltin A
(2), a cyclic depsipeptide with potent antiviral and anti-
fungal properties isolated from the shallow water Lithistida
sponge Callipelta sp.1 In a companion publication, Minale
and coworkers also isolated and characterized a related
cyclic depsipeptide, callipeltin B (3), in which 1 had been
cyclized to form (3S,4R)-3,4-dimethyl-l-pyroglutamic acid
(4).2 Both callipeltins A and B were shown to possess anti-
tumor activity against several different tumor cell lines.2
More recently, 1 has also been found in a family of related
natural products, papuamides A±D (5a±d), isolated from
Theonella mirabilis and Theonella swinhoe.3 Papuamides
A (5a) and B (5b) were shown, like callipeltin A, to inhibit
the replication of HIV-1in vitro; papuamide A was also
found to be cytotoxic to a panel of tumor cell lines with a
As part of our efforts directed toward the synthesis and
mean IC50 of 75 ng mL21
.
It is noteworthy that in all of these cyclic depsipeptide
natural products, each of which possesses antitumor, anti-
viral or antifungal activity, some form of 1 is present. While
the modes of action of these molecules are not known, the
conservation of 1 throughout the family suggests that it may
play a role in one or more of the biological functions of 2, 3
Keywords: asymmetric synthesis; amino acids; conformational analysis;
callipeltin A.
p
Corresponding author. Tel.: 1765-494-0132; fax: 1765-494-0239;
e-mail: lipton@purdue.edu
Present address: Eli Lilly and Co., Indianapolis, IN 46285, USA.
²
0040±4020/01/$ - see front matter q 2001Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(01)00501-4

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C. M. Acevedo et al. / Tetrahedron 57 .2001) 6353±6359
the lactam nitrogen to facilitate nucleophilic ring opening of
the lactam by a nitrogen nucleophile, avoiding epimeriza-
tion of Ca by suitable protection of the carboxylate and
producing the desired cis relationship of the two methyls
in 4. This latter concern was addressed through diastereo-
selective epimerization of the C-4 methyl.
2. Results and discussion
Reduction of l-pyroglutamic acid 6 to (5S)-5-hydroxy-
methylpyrrolidin-2-one 7 was required to reduce the acidity
of Ca and thereby prevent racemization during later
synthetic steps. Reduction of 6 to 7 was accomplished in
91% yield in one pot by conversion of the acid to the methyl
ester followed by reduction with NaBH4 to the alcohol
(Scheme 1). Alcohol 7 was protected as the silyl ether 8
with tert-butyldimethylsilyl chloride and imidazole in
DMF, followed by acylation using (Boc)2O and DMAP in
acetonitrile to yield 9 in 95% overall yield. The a,b-
unsaturation of intermediate 10 was installed in 72% yield
from 9 by a-selenation of the lactam5 and oxidative
elimination in the presence of pyridine to avoid racemiza-
tion of 10. It was found in accord with literature precedent6
that successful cuprate addition to 10 could be achieved
using an excess of cuprate reagent. The resulting enolate
was found to react with methyl iodide only when the
reaction was allowed to warm to room temperature. Methyl-
ation of the enolate resulted in nearly exclusive formation of
the trans, trans diastereomer 11. The cis relationship of the
C-4 methyl to the silyl ether and its trans relationship to the
C-3 methyl were established by NOE spectroscopy.7
structure elucidation of callipeltins A and B, we wish to
report herein the syntheses of (3S,4R)-3,4-dimethyl-l-gluta-
mine (1) and (3S,4R)-3,4-dimethyl-l-pyroglutamic acid (4)
from a common precursor derived initially from l-pyro-
glutamic acid.4 We also have now investigated the confor-
mational consequences of the side chain methylation of 1 by
comparing the conformation preferences of a protected
derivative of 1 to those of an analogously protected version
of l-glutamine. The major concerns attending the syntheses
of 1 and 4 from l-pyroglutamic acid were the acylation of
Enolization of compound 11 with LHMDS (Scheme 2) and
quenching with acetic acid at 2788C gave a mixture of 11
and its C-4 epimer 12 in a 1to 4 ratio. Epimers 11 and 12
were separated using ¯ash chromatography and recovered
11 was resubmitted to the epimerization conditions. From
Scheme 1.

C. M. Acevedo et al. / Tetrahedron 57 .2001) 6353±6359
6355
Scheme 2.
this intermediate (12), deprotection of the silyl ether and
oxidation of the alcohol provides a Boc-protected version
of 4. In the event, both operations were accomplished in one
step, in 77% yield, using a Jones oxidation. The product thus
obtained (13) could be cleanly deprotected with TFA in
methylene chloride to afford (3S,4R)-3,4-dimethyl-l-pyro-
glutamic acid 4.
followed by treatment with several different oxidizing
agents led to no decrease in the formation of 15.
A second strategy was therefore explored in which N-(tert-
butoxycarbonyl)-(3S,4R)-3,4-dimethyl-l-pyroglutamic acid
(13) was used as the precursor of 1. This strategy began with
conversion of 13 to the t-butyl ester 17 (Scheme 4) using
N,N0-diisopropyl-O-tert-butylisourea.9 Ring opening of
lactam 17 using ammonia in the presence of catalytic
KCN resulted in the formation of N-(tert-butoxycarbonyl)-
(3S,4R)-3,4-dimethyl-l-glutamine tert-butyl ester 18.10
Deprotected 1 can be obtained from 18 by treatment with
TFA in methylene chloride.
The advanced intermediate 12 was also employed as a
synthetic precursor to (3S,4R)-3,4-dimethyl-l-glutamine.
In our early efforts, the acylated lactam 12 was treated
with ammonia in the presence of AlMe3,8 opening the ring
to form the acyclic amide 14 (Scheme 3). However, when 14
was submitted to Jones oxidation, the desired carboxylic
acid was produced in low (,40%) yield, and a relatively
nonpolar compound (15) was obtained as the major product.
Isolation and characterization of 15 showed it to be a cyclic
imide, presumably formed by cyclization of the intermedi-
ate a-amino aldehyde 16 and oxidation of the resultant
cyclic hemiaminal. Deprotection of the silyl group of 14
Upon completion of the syntheses of protected forms of 1
and 4, 1H-NMR and computational studies were undertaken
to elucidate the conformational consequences of the 3,4-
dimethyl substituents on the l-glutamine side chain. To
this end, vicinal coupling constants for the side chain
protons of 18 and N-(tert-butoxycarbonyl)-l-glutamine
Scheme 3.

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C. M. Acevedo et al. / Tetrahedron 57 .2001) 6353±6359
Scheme 4.
19 using the AMBERp force ®eld,11 the GB/SA solvation
model12 for water and the torsional Monte Carlo conforma-
tional search algorithm13 as implemented in the Macro-
Model molecular modeling package.14 Vicinal coupling
constants of individual conformations and Boltzmann-
weighted averages of all conformations were computed
using the Altona equation.15
Table 1. Experimental and computed coupling constants found in 18 and 19
a
Jbc (Hz)
a
Jb c (Hz)
0
Jab (Hz)
0
Compound
Jab (Hz)
A comparison of the experimentally derived and calculated
coupling constants shows that the observed coupling
constants of 19 are more consistent with those of an average
of conformations than with any single conformation
examined. This was in accord with the observation that
the diastereotopic Cg protons of 19 appear in the 300 MHz
NMR spectra as a single resonance and behave as magneti-
cally shift equivalent protons. In contrast, the observed
coupling constants of 18 are marginally more consistent
with those computed for the lowest energy conformer
obtained from molecular mechanics calculations than with
those of an ensemble average. Additionally, it was found
that the number of available conformations of 19 (41) within
3 kcal mol21 of the ground state was substantially greater
than the comparable number of conformations of 18 (30).
Even more striking was a comparison of the number of
conformers within 1kcal mol 21 of the ground state: 19
had seven minima in that range, whereas 18 had only two.
18 (experimental)
18 (calculated)b
19 (experimental)
19 (calculated)b
4.8
±
8.4
±
4.3 (4.6)
3.8
4.3 (3.5)
6.4 (5.2)
7.4
9.0 (7.1)
7.6
11.6 (11.1)
7.8
3.3 (5.0)
a
0
Coupling constants to Hc and Hc are reported as a single value due to
magnetic shift equivalence.
b
The value outside of parentheses is the coupling constant calculated for
the lowest energy conformer. The value inside the parentheses is a
Boltzmann-weighted average obtained from all conformers within
3 kcal mol21 of the global minimum.
tert-butyl ester (19) were obtained from homonuclear
decoupling experiments performed in methanol-d4. The
results are shown in Table 1, along with vicinal coupling
constants computed from molecular mechanics calculations.
Further information was obtained from an examination of
the global minima in the two conformational searches. The
molecular mechanics global minimum of 19 (Fig. 1) has
side chain torsion angles x1 ˆ 21708 and x2 ˆ 2538;
whereas that of 18 (Fig. 2) has x1 ˆ 2488 and x2 ˆ
2418: This difference represents a fundamental change in
The calculated vicinal coupling constants shown in Table 1
were obtained from conformational searches of both 18 and
Figure 1. Stereo pair view of the lowest energy conformation of 19 obtained from molecular mechanics calculations.

C. M. Acevedo et al. / Tetrahedron 57 .2001) 6353±6359
6357
Figure 2. Stereo pair view of the lowest energy conformation of 18 obtained from molecular mechanics calculations.
side chain conformation, as that of 19 adopts an anti,
-gauche conformation where that of 18 exists in a slightly
distorted -gauche, -gauche arrangement. Examination of the
other low energy conformer of 18 reveals that it too exists in
a -gauche, -gauche conformation that differs only subtly
from the global minimum. Thus, it appears that the methyl
substituents on the side chain of 18 may bias its conforma-
tion to -gauche, -gauche. The only analogous conformer
found for the parent system 19 was the highest energy
conformer in the search, nearly 3 kcal mol21 above the
ground state. Therefore, the methyl substituents appear not
only to restrict the rotation of the side chain, but also to
selectively stabilize a high energy conformer of the
glutamine side chain.
evolve gas. The reaction was then allowed to warm to room
temperature overnight producing a milky white solution.
The reaction was quenched by addition of a 5% solution
of citric acid (100 mL) until the solution became clear
with a solid gray precipitate. The solution was then decanted
from the precipitate and the solvent removed under reduced
pressure leaving a thick oil. To the oil was added a 25%
methanol/ethyl acetate solution (200 mL) and the resulting
solution was ®ltered. The solvent was removed under
reduced pressure from the supernatant and the solid was
dissolved in CH2Cl2 (200 mL) and ®ltered. The CH2Cl2
was removed under reduced pressure to yield 7 as a white
solid (1.63 g, 14.2 mmol, 91%): 1H NMR (CDCl3) d 7.54 (s,
1H), 4.89 (br s, 1H), 3.73 (m, 1H), 3.64 (dd, Jˆ11.1, 3.6 Hz,
1H), 3.47 (m, 1H), 2.29 (m, 2H), 2.10 (m, 1H), 1.73 (m, 1H);
13C NMR (CDCl3) d 179.7, 65.7, 56.7, 30.4, 22.7;
FABHRMS calculated: 116.0712, found: 116.0714.
3. Conclusion
4.1.2. ꢀ5S)-5-{[ꢀtert-Butyl)dimethylsilyloxy]methyl}pyrro-
lidin-2-one ꢀ8). To a solution of 7 (1.63 g, 14.2 mmol) in
dimethylformamide (DMF) (100 mL) was added imidazole
(2.41g, 35.4 mmol) and tert-butyldimethylsilyl chloride
(TBSCl) (2.57 g, 17.0 mmol). After 24 h the solution was
diluted with Et2O (300 mL) and washed with water
(3£100 mL) and brine (3£100 mL) and dried over
Na2SO4. The solvent was removed to give 8 as a colorless
oil (3.14 g, 13.6 mmol, 96%): 1H NMR (CDCl3) d 5.99 (br
s, 1H), 3.74 (m, 1H), 3.61 (dd, Jˆ10.1, 4.1 Hz, 1H), 3.44
(dd, Jˆ10.0, 7.6 Hz, 1H), 2.33 (m, 2H), 2.16 (m, 1H), 1.73
(m, 1H), 0.87 (s, 9H), 0.05 (s, 6H); 13C NMR (CDCl3) d
178.6, 66.6, 55.9, 30.0, 25.9, 25.8, 22.9, 11.2, 23.5, 25.4,
25.5.
Protected derivatives of the nonproteinogenic amino acids
(3S,4R)-3,4-dimethyl-l-pyroglutamic acid (4) and (3S,4R)-
3,4-dimethyl-l-glutamine (1) were synthesized in good
yields from readily available l-pyroglutamic acid in 8 and
10 steps, respectively. It was also shown that (3S,4R)-3,4-
dimethyl-l-glutamine adopts a different conformation than
l-glutamine in methanol and that it appears to rigidify the
side chain into a conformation unavailable to that of l-gluta-
mine. This observation may provide an insight into the role
of the methyl groups of (3S,4R)-3,4-dimethyl-l-glutamine
in the biological activity exhibited by the natural products
that contain it.
4. Experimental
4.1.3.
ꢀ5S)-N-ꢀtert-Butoxycarbonyl)-5-{[ꢀtert-butyl)di-
methylsilyloxy]methyl}-pyrrolidin-2-one ꢀ9). To a solu-
tion of 8 (3.14 g, 13.61 mmol) in acetonitrile (CH3CN)
(100 mL) at 08C was added DMAP (0.17 g, 1.361 mmol)
and di-tert-butyl dicarbonate (5.94 g, 27.22 mmol) with stir-
ring. The reaction was allowed to slowly warm to room
temperature overnight. The solvent was removed under
reduced pressure to yield an dark red oil which was puri®ed
by ¯ash chromatography using 10% ethyl acetate/CH2Cl2 to
yield 9 as an amber oil (4.49 g, 13.59 mmol, 99%):
[a]24 ˆ259.78 (c 0.10, CHCl3) {lit. [a]23 ˆ262.28 (c
4.1. Data for compounds
4.1.1. ꢀS)-5-Hydroxymethyl-2-pyrrolidinone ꢀ7). To a
solution of (S)-pyroglutamic acid (6) (2.00 g, 15.5 mmol)
in methanol (50 mL) at 2158C was added thionyl chloride
(SOCl2) (2.04 g, 17.1 mmol) dropwise with stirring. After
stirring for 30 min at 2158C the reaction was allowed to
warm to room temperature over an hour and stirred for an
additional hour at room temperature. The solvent was then
removed under reduced pressure producing a thick, clear oil.
This oil was then redissolved in ethanol (50 mL) and cooled
to 08C. To the solution was slowly added sodium boro-
hydride (NaBH4) (1.18 g, 31.0 mmol) which would rapidly
D
D
1
1.23, CHCl3)}; H NMR (CDCl3) d 4.41(m, 1H), 3.89
(dd, Jˆ10.4, 4.1 Hz, 1H), 3.66 (dd, Jˆ10.4, 2.3 Hz, 1H),
2.67 (m, 1H), 2.34 (m, 1H), 2.04 (m, 1H), 1.50 (s, 9H), 0.85

6358
C. M. Acevedo et al. / Tetrahedron 57 .2001) 6353±6359
(s, 9H), 0.02 (s, 3H), 0.00 (s, 3H); 13C NMR (CDCl3) d
175.1, 150.2, 82.8, 64.5, 59.0, 32.5, 28.2, 26.0, 21.3, 18.3,
25.4, 25.5; FABHRMS calculated: 330.2102, found:
330.2100.
3H), 0.86 (t Jˆ2.9 Hz, 9H), 0.03 (d, Jˆ4.6 Hz, 6H); 13C
NMR d 176.6, 150.5, 82.5, 64.7, 63.1, 40.6, 32.9, 28.0,
25.8, 18.1, 15.8, 10.1, 25.5, 25.6; IR (neat) 2957.4,
2861.4, 1787.6, 1754.1, 1711.7, 1466.2 cm21; FABHRMS
calculated: 358.2414, found: 358.2412; Anal. Calcd for
C18H35NO4Si: C, 60.46; H, 9.87; N, 3.92. Found: C,
60.56; H, 9.97; N, 3.95.
4.1.4.
ꢀ5S)-N-ꢀtert-Butoxycarbonyl)-5-{[ꢀtert-butyl)di-
methylsiloxy]methyl}-3-pyrrolin-2-one ꢀ10). To a solu-
tion of 9 (7.93 g, 24.0 mmol) in THF (200 mL) at 2788C
was slowly added LiHMDS in hexanes (26.4 mL,
26.4 mmol) and left stirring for 15 min. A solution of
phenylselenenyl bromide (9.89 g, 41.9 mmol) in THF
(50 mL) was transferred via cannula to the reaction and
allowed to stir for 1h at 2788C. The reaction was quenched
by addition of saturated NH4Cl (50 mL) and Et2O (100 mL)
and allowed to warm to room temperature. The solution was
washed with sat. NH4Cl (3£100 mL) followed by brine
(3£100 mL) and the organic layer dried over Na2SO4. The
solvent was removed under reduced pressure leaving behind
a red oil that was then dissolved in CH2Cl2 (200 mL) and
cooled to 2788C. Pyridine (5.81mL) was added, followed
by slow addition of 30% H2O2 (8.13 mL) and the tempera-
ture was allowed to warm to 08C over 1h. The solution was
diluted with Et2O (500 mL) and washed with H2O
(3£100 mL) followed by brine (3£100 mL). The organic
phase was dried over Na2SO4 and concentrated under
reduced pressure. The resulting oil was puri®ed using
¯ash column chromatography with 10% EtOAc/petroleum
ether giving the desired product as a colorless solid (6.64 g,
4.1.6. ꢀ3R,4R,5S)-N-ꢀtert-Butoxycarbonyl)-5-{[ꢀtert-butyl)-
dimethylsiloxy]methyl}-3,4-dimethylpyrrolidin-2-one ꢀ12).
To a solution of 11 (0.20 g, 0.57 mmol) in THF (4.5 mL) at
2788C was added LiHMDS in hexanes (0.85 mL, 1M,
0.85 mmol). The reaction was allowed to stir at 2788C for
1h, the temperature was lowered to 2988C and glacial
HOAc (49 mL, 0.85 mmol) was added quickly. The reaction
was quenched after 5 min by the addition of sat. NH4Cl
(2 mL) and sat. NaHCO3 (2 mL) and diluted with Et2O
(5 mL) and H2O (5 mL). The mixture was allowed to
warm to room temperature and the layers separated. The
organic layer was washed with H2O (3£10 mL) and brine
(3£10 mL). After drying the organic layer over Na2SO4 and
removing the solvent under reduced pressure, the isomers
were separated via ¯ash column chromatography with 10%
EtOAc/petroleum ether giving the desired product 12 in a
4:1ratio (0.20 g, 97%) with 11: [a]24 ˆ243.98 (c 1.1,
D
1
CDCl3); H NMR (CDCl3) d 4.03 (dd, Jˆ10.3, 4.3 Hz,
1H), 3.69 (dd, Jˆ10.3, 2.2 Hz, 1H), 3.53 (m, 1H), 2.09
(m, 1H), 1.96 (m, 1H), 1.53 (s, 9H), 1.22 (d, Jˆ7.1Hz,
3H), 1.15 (d, Jˆ6.6 Hz, 3H), 0.87 (s, 9H), 0.03 (d,
Jˆ3.2 Hz, 6H); 13C NMR (CDCl3) d 176.8, 150.7, 82.9,
64.6, 61.5, 45.6, 35.2, 28.3, 26.0, 18.8, 18.5, 15.0, 25.2,
25.3; mp 57±598C; IR (KBr) 1781.5, 1701.3,
1460.8 cm21; Anal. Calcd. for C18H35NO4Si: C, 60.46; H,
9.87; N, 3.92. Found C, 60.58; H, 9.86; N, 3.85.
19.4 mmol, 81% yield): [a]24 ˆ21698 (c 1.0, CDCl3){lit.
D
[a]23 ˆ2175.68 (c 0.9, CHCl3)}; 1H NMR (CDCl3) d 7.26
D
(m, 1H), 6.12 (dd, Jˆ6.1, 1.7 Hz, 1H), 4.60 (m, 1H), 4.15
(dd, Jˆ9.6, 3.7 Hz, 1H), 3.71 (dd, Jˆ9.6, 6.8 Hz, 1H), 1.55
(s, 9H), 0.86 (s, 9H), 0.04 (d, Jˆ3.9 Hz, 6H); 13C NMR
(CDCl3) d 169.7, 150.0, 149.7, 127.3, 83.2, 63.8, 62.7,
28.4, 25.9, 18.4, 25.2, 25.3; mp 57±598C; IR (KBr)
1781.5, 1701.3, 1460.8 cm21
;
Anal. Calcd for
4.1.7. ꢀ2S)-N-ꢀtert-Butoxycarbonyl)-ꢀ3S,4R)-3,4-dimethyl-
pyroglutamic acid ꢀ13). To a solution of 12 (0.41g,
1.2 mmol) in acetone at 08C was added Jones reagent
(0.86 mL, 2.32 mmol), which had been cooled to 08C. The
solution instantly became bright orange and after 10 min a
precipitate had formed and the solution had turned amber in
color. The reaction stirred for 30 min at 08C and 1h at room
temperature. The reaction was quenched by addition of
isopropanol (25 mL) and slow addition of sat. NaHCO3
(10 mL) at 08C. The volatiles were removed under reduced
pressure and the resultant mixture was diluted with H2O
(15 mL) and ethyl acetate (25 mL). The solution was then
extracted with Et2O (3£20 mL) and the aqueous layers were
combined and acidi®ed with 0.1N HCl to pH 3 at 08C. The
acidi®ed solution was extracted with ethyl acetate
(3£20 mL). The combined organic fractions were dried
over Na2SO4. Removal of the solvent gave 13 as a glassy
C18H35NO4Si: C, 60.46; H, 9.87; N, 3.92. Found: C,
60.58; H, 9.86; N, 3.85.
4.1.5. ꢀ3S,4R,5S)-N-ꢀtert-Butoxycarbonyl)-5-{[ꢀtert-butyl)-
dimethylsiloxy]methyl}-3,4-dimethylpyrrolidin-2-one ꢀ11).
To a yellow±orange mixture of copper iodide (6.04 g,
31.8 mmol) in Et2O at 08C methyl lithium in Et2O
(46.38 mL, 63.5 mmol) was added turning it into a clear
solution. The lithium dimethyl cuprate solution was cooled
to 2788C and a solution of 10 (1.49 g, 4.54 mmol) in Et2O
was added via cannula forming a cloudy yellow solution.
The reaction was allowed to stir at 2788C for 1h and then
methyl iodide (12.90 g, 90.8 mmol) was added. Formation
of a white precipitate is observed at this point. The reaction
was left stirring for 30 min and then the temperature was
allowed to slowly rise to room temperature over a period of
3 h. The reaction was quenched by addition of sat. NH4Cl
(3 mL) and Et2O (10 mL). The solution was washed with
NH4Cl (3£30 mL), H2O (3£30 mL), and brine (3£30 mL).
The organic phase was dried over Na2SO4 and concentrated
under reduced pressure. Column chromatography puri®ca-
tion eluted with 10% EtOAc/petroleum ether gave
solid (0.23 g, 77% yield): [a]24 ˆ210.38 (c 0.8, CDCl3);
D
1H NMR (CDCl3) d 10.66 (br s, 1H), 4.20 (d, Jˆ1.7 Hz,
1H), 2.79 (m, 1H), 2.54 (m, 1H), 1.46 (s, 9H), 1.08 (m, 6H);
13C NMR (CDCl3) d 176.0, 175.6, 150.0, 84.0, 64.4, 40.8,
34.4, 28.0, 25.8, 15.4, 10.0; mp 107±110 C; IR (KBr)
3190.1, 2986.9, 1788.4, 1741.6 cm21
.
compound 11 (1.19 g, 3.33 mmol, 73% yield): [a]24
ˆ
D
2558 (c 0.5, CDCl3); 1H NMR (CDCl3) d 3.84 (dd,
Jˆ10.3, 5.2 Hz, 1H), 3.74 (dd, Jˆ10.3, 2.7 Hz, 1H), 3.66
(m, 1H), 2.96 (p, Jˆ7.5 Hz, 1H), 2.43 (p, Jˆ7.5 Hz, 1H),
1.53 (s, 9H), 1.07 (d, Jˆ7.3 Hz, 3H), 0.98 (d, Jˆ7.1Hz,
4.1.8.
ꢀ4S)-N-ꢀtert-Butoxycarbonyl)-4-{[ꢀtert-butyl)di-
methylsiloxy]methyl}-ꢀ2R,3S)-dimethylglutamine ꢀ14).
To a solution of 12 (75 mg, 0.21mmol) in CH 2Cl2
(4.2 mL) at room temperature was bubbled NH3(g) for

C. M. Acevedo et al. / Tetrahedron 57 .2001) 6353±6359
6359
3 min. Trimethylaluminum (0.16 mL, 0.32 mmol) was
added rapidly dropwise, and left stirring for 6 h. The reac-
tion was quenched by addition of 0.1N HCl (3 mL) and the
resulting suspension was stirred for 5 min and ®ltered. The
solution was extracted with Et2O (3£10 mL), the organic
layer dried over Na2SO4 and concentrated under reduced
pressure. Puri®cation using column chromatography with
30% EtOAc/petroleum ether gave the desired product 14
0.96 mmol) in THF (0.38 mL) at 2358C catalytic amounts
of KCN were added. Ammonia (,1mL) was condensed
and the reaction was left to reach room temperature in a
sealed, high pressure vessel. After 24 h of stirring, crude
18 was submitted to ¯ash column chromatography with
20% EtOAc/petroleum ether, yielding an oil (27 mg,
1
86%): [a]24 ˆ29.48 (c 0.5, CDCl3); H NMR (CDCl3) d
D
7.07 (bs,1H), 5.53 (bs, 1H), 5.31 (d, Jˆ9 Hz, 1H), 4.15 (dd,
Jˆ9, 9 Hz,1H), 2.47 (m, 2H), 1.47 (s, 9H) 1.45 (s, 9H), 1.18
(d, Jˆ7 Hz, 3H), 0.92 (d, Jˆ7 Hz, 3H); 13C NMR (CDCl3) d
175.8, 171.1, 156.3, 82.6, 80.5, 57.2, 42.1, 39.9, 28.3, 28.0,
16.2, 12.2; HRMS calculated: 331.2233, found 331.2221;
IR (®lm) 3348.6, 1703.5, 1680.5, 1650.2, 1368.0,
as
a white solid (57 mg, 0.16 mmol, 72% yield):
1
[a]24 ˆ254.88 (c 1.01, CDCl3); H NMR (CDCl3) d 7.65
D
(br s, 1H), 5.44 (br s, 1H), 5.08 (d, Jˆ9.8 Hz, 1H), 3.76 (dd,
Jˆ10.1, 2.5 Hz, 1H), 3.60 (m, 2H), 2.53 (m, 1H), 1.60 (m,
1H), 1.45 (s, 9H), 1.09(d, Jˆ7.08 Hz, 3H), 0.88 (s, 9H), 0.86
(s, 6H), 0.11 (s, 3H), 0.10 (s, 3H); 13C NMR (CDCl3) d
176.0, 157.4, 80.1, 63.2, 54.6, 39.5, 38.8, 28.4, 25.9, 18.3,
16.6, 11.7, 25.5; mp 105±1068C; FABHRMS calculated:
375.2679, found 375.2662; IR (KBr) 3438.0, 3206.1,
1155.2 cm21
.
References
2957.6, 1669.0, 1626.3 cm21
.
1. Zampella, A.; D'Auria, M. V.; Paloma, L. G.; Casapullo, A.;
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4.1.9. [ꢀ4S,5R)-Dimethyl-2,6-dioxo-piperidin-3S-yl]-car-
bamic acid tert-butyl ester ꢀ15). To a solution of 14
(20 mg, 0.077 mmol) in acetone at 08C was added Jones
reagent (0.17 mL, 1.67 mmol) which had been cooled to
08C. The solution instantly became bright orange and after
10 min a precipitate had formed and the solution had turned
amber in color. The reaction stirred for 30 min at 08C and
1h at room temperature. The reaction was quenched by
addition of isopropanol (1mL) and slow addition of sat.
NaHCO3 (0.5 mL) at 08C. The volatiles were removed
under reduced pressure and the resultant mixture was
diluted with H2O (5 mL) and diethyl ether (2 mL). The
mixture was extracted with Et2O (3£5mL) and the
combined organic fractions were dried over Na2SO4.
Removal of the solvent yielded a white solid (13 mg, 62%
yield): 1H NMR (CDCl3) d 7.83 (br s, 1H), 5.44 (br s, 1H),
4.54 (m, 1H), 2.87 (m, 1H), 2.69 (m, 1H), 1.47 (s, 9H), 1.28
(d, Jˆ7.2 Hz, 3H), 0.85 (d, Jˆ7.2 Hz, 3H); 13C NMR
(CDCl3) d 173.7, 171.1, 80.5, 57.6, 41.2, 35.7, 29.7, 28.3,
28.2, 12.4, 8.0, 1.0; HRMS calculated: 257.1501, found
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Â
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5. (a) Ohfune, Y.; Tomita, M. J. Am. Chem. Soc. 1982, 104,
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(d) Ikota, N. Chem. Pharm. Bull. 1992, 40, 1925.
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257.1499; IR (®lm) 2979.7, 2931.8, 1712.7, 1494.9 cm21
.
7. An NOE in the cis epimer 12 was observed between the two
methyl groups, whereas there was no corresponding NOE seen
in the trans epimer 11.
4.1.10. tert-Butyl ꢀ2S)-N-ꢀtert-Butoxycarbonyl)-ꢀ3S, 4R)-
3,4-dimethylpyroglutamate ꢀ17). To a solution of 13
(60 mg, 0.23 mmol) in CH2Cl2 (2 mL) freshly distilled
N,N0-diisopropyl-O-tert-butylisourea (47 mg, 2.3 mmol)
was added and allowed to stir at room temperature for
16 h. The solvent was removed under reduced pressure
and the resultant solid was puri®ed using ¯ash column chro-
matography with 30% Et2O/petroleum ether yielding a
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white solid (60 mg, 83%): [a]24 ˆ212.98 (c 1.2, CDCl3);
12. Still, W. C.; Tempczyk, A.; Hawley, R. C.; Hendrickson, T.
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D
1H NMR (CDCl3) d 4.10 (d, Jˆ5 Hz,1H), 2.65 (m, 1H),
2.30 (m, 1H), 1.44 (s, 9H), 1.42 (s, 9H), 1.10 (m, 6H); 13C
NMR (CDCl3) d 175.3, 169.8, 149.7, 83.0, 82.1, 65.0, 40.4,
34.2, 27.9, 15.4, 9.8; mp 62±648C; IR (®lm) 2979.2, 2928.3,
2880.5, 1792.7, 1735.0, 1717.2, 1457.7, 1370.1 cm21; Anal
Calcd for C16H27NO5: C, 61.32; H, 8.68; N, 4.47. Found: C,
61.59; H, 8.59; N, 4.44.
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glutamine-tert-butyl ester ꢀ18). To a solution of 17 (30 mg,