A Flexible Approach for the Asymmetric Synthesis of N-Protected (R) -5-Alkyl Tetramates and (R)-5-Alkyl Tetramic Acid Derivatives

Article abstract of DOI:10.1055/s-2003-44968

A flexible two-step asymmetric approach to N-protected (R)-5-alkyl tetramates and (R)-5-alkyl tetramic acid derivatives is described. The method is based on the diastereoselective alkylation of (R)-phenylglycinol derived tetramates 7 and 8, which are the

Full text of DOI:10.1055/s-2003-44968

LETTER
247
A Flexible Approach for the Asymmetric Synthesis of N-Protected (R)-5-Alkyl
Tetramates and (R)-5-Alkyl Tetramic Acid Derivatives
Asymmetric
S
e
ynthesis of
i
N
-Prot
-
ected
(
R
Q
)-5-Alkyl
T
etrama
i
tes ang Huang,* Jun Deng
Department of Chemistry and The Key Laboratory for Chemical Biology of Fujian Province, Xiamen University, Xiamen, Fujian 361005,
P. R. China
Fax +86(592)2186400; E-mail: pqhuang@xmu.edu.cn
Received 27 October 2003
Dedicated to Professor Dr. Khi-Rui TSAI on the occasion of his 90th birthday.
MeO
R
O
Abstract: A flexible two-step asymmetric approach to N-protected
(R)-5-alkyl tetramates and (R)-5-alkyl tetramic acid derivatives is
described. The method is based on the diastereoselective alkylation
of (R)-phenylglycinol derived tetramates 7 and 8, which are the first
synthetic equivalents to chiral nonracemic tetramate 5-carbanionic
synthons A.
O
R
O
N
N
H
H
2
2a R=Me
2b R=Bn, dolapyrrolidone (Dpy)
1
Key words: asymmetric synthesis, alkylations, chiral auxiliaries,
O
tetramates, tetramic acids
H2N
O
O
Cl
Me
Cl
5-Alkyl tetramic acids 1 are sub-structures found in a
number of bioactive natural products.1 Some of them exist
as ‘methyl ester’, namely, methyl 5-alkyl tetramates 2.1
For example, methyl 5-methyl tetramate 2a is found both
in mirabimide E (3, Figure 1),2 a solid tumor selective cy-
totoxin isolated from the terrestrial blue green alga Scy-
tonema mirabile UH strain BY-8-1, and in
dysideapyrrolidone,3 a compound isolated from the tropi-
cal marine sponge Dysidea herbacea (Dictyoceratida) of
specimen D. herbacea from Palau; dolapyrrolidone (Dpy)
2b is a component of antineoplastic dolastatin 15, isolated
in minus amount (1600 kg of mollusk yielded 6.2 mg!)
from the Indian Ocean sea hare Dolabella auricularia.4 In
addition, the multiple functionalities and reactivities pos-
sessed by 1 and 2 render them versatile building blocks for
the synthesis of other classes of bioactive natural prod-
ucts, which include indocarbazole natural product
K252a,5 b-hydroxy-g-amino acids such as statine and
AHPPA,6,7 and pyrrolidine alkaloids.7,8
Me
N
OMe
Cl
Cl
Me
O
mirabimide E (3)
Figure 1
Mitsunobu conditions,14 could not proceed without race-
mization. Thus the development of direct entrances15 to
chiral nonracemic tetramates is highly desirable. Very re-
cently, we have reported a flexible approach to (S)-5-alkyl
tetramic acid derivatives,7 we now wish to report the first
direct asymmetric approach to N-protected alkyl (R)-5-
alkyl tetramates, which is based on the use of (R)-phe-
nylglycinol as a N-chiral auxiliary.
The use of (R)-phenylglycinol as a chiral auxiliary for the
asymmetric synthesis of N-containing compounds has en-
joyed great success, which is due largely to Husson and
co-workers’ efforts in developing CN(R,S) methodolo-
gy.16 The alkylation of the (R)-phenylglycinol derived vi-
nylogous enolates 4 (Figure 2) has also been studied,
which was shown to occur exclusively at the C-3 posi-
tion.17 Only the corresponding silyl dienolethers (2-silyl-
oxypyrrole) 518 and 6,19 or the vinylogous enolates of N-
protected 4-alkyl-3-pyrrolin-2-one20 can react regioselec-
tively with electrophiles at the C-5 position. However,
such methods can neither be used for the synthesis of tet-
ramates, nor allow the introduction of simple alkyl group
at the C-5 position. Gratefully, it was reported that the re-
action of achiral tetramate lithium dienolates with alkylat-
ing agents occurred regioselectively at the C-5 position.21
On the basis of these considerations, we reasoned that the
(R)-phenylglycinol derived chiral tetramates 7/8 would
serve as suitable chiral building blocks for the asymmetric
synthesis of 5-alkyl tetramates 9/10 (Scheme 1).
Since methods have been developed for the introduction
of both C-3 and N-1 substituents starting from 5-alkyl tet-
ramates,9 the key to the asymmetric synthesis of C-3, C-5
and N-1 substituted tetramic acid-related natural products
depends on the synthesis of 5-alkyl tetramic acids 1 or 5-
alkyl tetramates 2. Up to date, a-amino acid-based meth-
6,10
ods
remain the main entrance to optically active 5-
alkyl tetramic acid-related natural products.11 However,
partial racemization has been observed in such ap-
proach.12 Moreover, it was reported that the conversion
of 5-alkyl tetramic acids to methyl 5-alkyl tetramates,
by treatment of 5-alkyl tetramic acids either with di-
methylsulfate or diazomethane2,13 or with methanol under
SYNLETT 2004, No. 2, pp 0247–0250
0
2.
0
2.
2
0
0
4
Advanced online publication: 16.12.2003
DOI: 10.1055/s-2003-44968; Art ID: U22303ST.pdf
© Georg Thieme Verlag Stuttgart · New York

248
P.-Q. Huang, J. Deng
LETTER
whereas those of 7 exhibit as an AB system, which appear
at d = 3.64 and 3.82 ppm (J = 17.6 Hz). Except for the
H-5 signal, both the 1H NMR and 13C NMR spectra of 9a
and 7 are identical, which indicates that the reaction oc-
curred regioselectively at the C-5 position.
R
OTMS
N
OLi
OTBDMS
N
N
MeO
LiO
t-Boc
Ph
Ph
6
4 (R=H, Alkyl)
5
R'O
R'O
Figure 2
Base, THF,
HMPA, –78 °C
O
El
HO
O
N
N
R'O
*
R'O
O
R'O
Electrophiles
HO
Ph
Ph
R
R
N
O
O
O
9 R'=Et
10 R'=Me
7 R'=Et
8 R'=Me
N
N
HO
Ph
HO
Ph
Ph
Scheme 3
9 R'=Et
10 R'=Me
7 R'=Et
8 R'=Me
A
Encouraged by this result, the alkylation of the dianionic
intermediate A (R¢ = Et) was pursued. Thus successive
treatment of 7 with 2.4 equivalents of t-BuLi and methyl
iodide in a mixed solvent system (THF–HMPA 20:1) at
–78 °C for 6 hours afforded readily separable 9b and its
diastereomer in a ratio of 90:10 (combined yield 63%).
Besides, 7% of recovered starting material 7 and 15% of
dimethylated products were also isolated as an un-separa-
ble mixture (Table 1, entry 2).23 Because we were unable
to obtain the dimethylated side products in pure form,
their structure and ratio could not be determined. Methyl-
ation of 8 proceeded similarly, which afforded 10b in
90:10 diastereoselectivity (combined yield 72%, Table 1,
entry 3). Alkylation of the dianion A (R¢ = Et) in situ
generated from 7 with other electrophiles gave the corre-
sponding C-5 alkylated products in similar dia-
stereoselectivities and chemical yields (Table 1, entries
4–6). When benzyl bromide was used as an alkylating
agent, a slightly lower diastereoselectivity was obtained
(Table 1, entries 7, 8). It is important to note that, only in
the case showed in entry 7 (Table 1), did a small amount
(8.6%) of the C-3 alkylated product was obtained.
Scheme 1
Several methods are available for the preparation of 5-un-
substituted tetramates.22 The required chiral tetramate 7
was synthesized by heating a acetonitrile solution of
known ethyl (E)-4-chloro-3-ethoxy-2-butenoate (11)22b,c
and (R)-phenylglycinol in the presence of triethylamine,
which gave the desired N-protected ethyl tetramate 7
(white crystals, mp 114–115 °C) in 50% yield. Attempts
to improve the yield of the condensation reaction by using
pyridine or K2CO3 instead of triethylamine or by using O-
TBDMS protected phenylglycinol were not rewarding.
Although the yield of 7 is only 50%, its ready availability
in just one step makes this approach attractive. Similar
treatment of (R)-phenylglycinol with 12 afforded the
corresponding N-protected methyl tetramate 8 in 55%
yield (Scheme 2).
R'O
OR'
Cl
CH3CN
HO
NH2
Ph
O
N
+
The recovering of a small amount of starting 7 or 8 in all
cases might implicate that the deprotonation of the C-5
proton was uncompleted. It is important to note that the
use of tert-butyllithium as the base is crucial for the reac-
tion. Other bases such as sec-butyllithium or lithium
hexamethyldisilazide gave poorer yields (Table 1, entries
9, 10).
Et3N, 80 °C
R'O
O
HO
Ph
50% for 7
55% for 8
11 R'=Et
12 R'=Me
7 R'=Et
8 R'=Me
Scheme 2
With quantities of synthon equivalents 7 and 8 available,
the generation of the dianions A (R¢ = Et, Me) and subse-
quent reaction with electrophiles (Scheme 3) were inves-
tigated. As a first investigation, compound 7 was
deprotonated with 2.4 molar equivalents of tert-butyllith-
ium in THF at –78 °C, the anion generated in situ was
quenched with deuteriomethanol (MeOD, –78 °C, 20
min), which yielded the C-5 deuterio product 9a and the
recovered starting material 7 in a ratio of 84:16 (yield
To determine the stereochemistry of the alkylated
products 9/10, the transformation of methyl 5-benzyl
tetramate 10f into known 2-benzylpyrrolidin-3-ol (16)24
was undertaken (Scheme 4). Thus treatment of 10f with
concentrated HCl solution for 12 hours gave tetramic
acid 13, which without further purification, was treated
with sodium borohydride in a mixed solvent system
(HOAc:CH2Cl2 = 1:10) at low temperature (–15 to 0
°C),24a to provide cis-14 as the only observable product.
The yield over two steps was 67%. The cis stereochemis-
try of compound 14 was assigned on the basis of observed
vicinal coupling constants (J4,5 = 5.2 Hz).25
1
85%, Table 1, entry 1) as indicated by H NMR spectral
analysis. The deuterio product 9a and the starting material
7 can be easily distinguished by 1H NMR: the C-5 proton
signal for 9a appears at d = 3.65 ppm as a broad singlet,
Synlett 2004, No. 2, 247–250 © Thieme Stuttgart · New York

LETTER
A
sy
Asymmetric Synthesis of N-Protected (R)-5-Alkyl Tetramates
249
Table 1 Results for the Reaction between Lithiated Compounds 7/
8 and Electrophiles
approach to compounds 17–20, which are novel potential
HIV protease inhibitors.24a
1
All the products 9/10 show the same H NMR and 13C
En- Base/starting Electro-
Products
Stereomer- Yield
try tetramates
philes
MeOD
MeI
ic ratioa
(%)
NMR spectral characteristics. This allows us to assume
that they share the same stereochemistry as depicted in
structure 9/10 (Scheme 3). With the stereochemistry as-
certained, it is possible to have an insight into the stereo-
chemical course of the alkylation reaction. It is reasonable
to assume that the reaction passed through the chelating
intermediate B or vinylogous enolate C (Figure 3). The
reaction of B with electrophiles then proceeded with in-
version of configuration26 at the C-5 position, which
would establish the R-configuration at the C-5. On the
other hand, if the intermediate C was involved, then the
electrophiles would approach from the re-face of the viny-
logous enolate to avoid the unfavorable interaction with
the phenyl group, which would establish R-configuration
as well.
1
2
3
4
5
6
7
8
9
t-BuLi/7
t-BuLi/7
t-BuLi/8
t-BuLi/7
t-BuLi/7
t-BuLi/7
t-BuLi/7
t-BuLi/8
s-BuLi/7
9a:7 (86:14)b –c
(85)e
9b
10b
9c
90:10d
63 (68)e
72 (77)e
68 (77)e
61 (67)e
63 (76)e
71 (83)e
58 (73)e
49 (74)e
18 (49)e
MeI
90:10d
87:13d
93:7d
EtI
n-BuI
n-C6H13I
BnBr
9d
9e
92:8d
9f
84:16d
85:15d
93:7d
BnBr
10f
9e
n-C6H11I
n-C6H13I
10 LHMDS/7
9e
87:13d
R'O
H
a The stereochemistry of the major diastereomer was showed in 9/10
(Scheme 3), which was determined by chemical correlation (vide
infra).
R'O
Li
O
O
O
Li
b Ratio determined by 1H NMR.
N
N
H
Li
c Ratio can not be determined by 1H NMR.
d Ratio determined by chromatographical separation.
e Yields based on the recovered starting material.
O
Li
H
C
B
Figure 3
X
HO
In summary, starting from (R)-phenylglycinol, we have
developed a simple, versatile and direct two-step asym-
metric approach to N-protected (R)-5-alkyl-tetramates
and (R)-5-alkyl-tetramic acid derivatives. Among thus
synthesized (R)-5-alkyl tetramate derivatives, methyl (R)-
5-methyl-tetramate and methyl (R)-5-benzyl-tetramate
are antipodes of the tetramate sub-units found respective-
ly in mirabimide E (3), dysideapyrrolidone, and antineo-
plastic dolastatin 15. The transformation of the major
diastereomer 10f into known N-Boc-(2R,3R)-2-ben-
zylpyrrolidin-3-ol (16) both confirms the stereochemistry
of the reaction and provides an approach to some novel
potential HIV protease inhibitors. Work is in progress for
the asymmetric synthesis of naturally occurring tetra-
mates.
NaBH4
Ph
Ph
O
X
HOAc, CH2Cl2
–15 °C–0 °C
N
N
HO
HO
Ph
Ph
14 X=O
10f X=OMe
13 X=OH
H3B·SMe2
THF, 0 °C r.t.
concd HCl
HO
15 X=2H
N
Pd/C, EtOH
(Boc)2O, r.t.
ref. 24a
NH
t-Boc
R
N
X
Ph
Ph
t-Boc
17 R=H, X=β-OH, α-H
18 R=OH, X=β-OH, α-H
19 R=H, X=O
16
20 R=OH, X=O
Scheme 4
Acknowledgment
The reduction of amide 14 with borane dimethylsulfide
complex (H3B·SMe2) gave pyrrolidine 15. Without fur-
ther purification, 15 was subjected to hydrogenolysis con-
ditions (H2, l atm, 10% Pd/C, EtOH) in the presence of
di(tert-butyl) dicarbonate to afford, in one-pot, the N-de-
benzylation–butoxycarbonylation product 16 {mp 78–79
The authors are grateful to the National Science Fund for Distin-
guished Young Investigators, the NNSF of China (29832020;
20072031; 20272048; 203900505), and the Specialized Research
Fund for the Doctoral Program of Higher Education for financial
support.
20
°C; lit.24a mp 71 °C; [a]D +5.8 (c 1.0, CHCl3); lit.24a
References
23
[a]D23 +6.00 (c 1.0, CHCl3) for 2R,3R-enantiomer; [a]D
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configuration. Moreover, the present work provides a new
Synlett 2004, No. 2, 247–250 © Thieme Stuttgart · New York

250
P.-Q. Huang, J. Deng
LETTER
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To a solution of (R)-7 (104 mg, 0.42 mmol) in THF (8.4 mL,
containing 0.37 mL of HMPA as a co-solvent) was added
dropwise t-BuLi (0.67 mL, 1.5 M in pentane) at –78 ºC.
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based on the recovered starting material, 68%). Compound
9b: [a]D20 +19.4 (c 1.1, CHCl3). IR (KBr): 3374, 2981, 2934,
1657, 1625, 1340, 1221, 1029 cm–1. 1H NMR (500 MHz,
CDCl3): d = 7.40–7.20 (m, 5 H, ArH), 5.18 (dd, J = 6.4, 7.7
Hz, 1 H, D2O exchangeable, OH), 5.01 (s, 1 H, O=CCH=),
4.51 (dd, J = 3.1, 7.7 Hz, 1 H, PhCHN), 4.27 (ddd, J = 7.7,
7.7, 12.3 Hz, 1 H, HOCH2), 4.00 (m, 3 H, CH3CH2O and
HOCH2), 3.73 (q, J = 6.8 Hz, 1 H, CH3CHN), 1.37 (t,
J = 7.1 Hz, 3 H, CH3CH2O), 1.28 (d, J = 6.8 Hz, 3 H,
CH3CHN) ppm. 13C NMR (125 MHz, CDCl3): d = 176.42,
173.18, 138.34, 128.80 (2 C), 127.77, 127.05 (2 C), 93.35,
67.10, 64.91, 61.90, 57.17, 15.83, 14.06 ppm. MS (ESI):
m/z (%) = 262 (13) [M + H+], 244 (5) [M + H+ – H2O], 142
(100). HRMS calcd for [C15H19NO3 + H]+: 262.1438. Found:
262.1440. Minor diastereomer of 9b: [a]D20 +17.1 (c 1.1,
CHCl3). IR (KBr): 3367, 2981, 2932, 1659, 1625, 1340,
1222, 1029 cm–1. 1H NMR (500 MHz, CDCl3): d = 7.40–
7.20 (m, 5 H, ArH), 5.02 (s, 1 H, =CHC=O), 4.82 (dd,
J = 3.8, 7.5 Hz, PhCHN), 4.21 (m, 1 H, HOCH2N), 4.09 (m,
2 H, HOCH2 and OH), 3.98 (m, 2 H, CH3CH2O), 3.88 (q,
J = 6.8 Hz, 1 H, CH3CHN), 1.37 (t, J = 7.1 Hz, 3 H,
CH3CH2O), 1.14 (d, J = 6.8 Hz, 3 H, CH3CHN). MS (ESI):
m/z (%) = 262 (100) [M + H+]. HRMS calcd for [C15H19NO3
+ H]+: 262.1438. Found: 262.1432.
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