Stereoselective cyclopropanation of serine- and threonine-derived oxazines to access new morpholine-based scaffolds

Article abstract of DOI:10.1039/b808895k

A general strategy for the synthesis of novel, orthogonally protected scaffolds based on the unique 2-oxa-5-azabicyclo[4.1.0]heptane structure is presented. The described reaction sequence takes advantage of easily available starting materials such as serine and threonine and leads to stereochemically dense structures in few, high-yielding synthetic steps. We show how the stereochemistry can be easily tuned by starting from different β-hydroxy-α-amino acids and also by means of a transition metal-catalyzed cyclopropanation step. The compounds find application as constrained templates for the construction of geometrically diversified libraries of compounds.

Full text of DOI:10.1039/b808895k

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Stereoselective cyclopropanation of serine- and threonine-derived oxazines to
access new morpholine-based scaffolds†
Filippo Sladojevich, Andrea Trabocchi* and Antonio Guarna
Received 28th May 2008, Accepted 7th July 2008
First published as an Advance Article on the web 29th July 2008
DOI: 10.1039/b808895k
A general strategy for the synthesis of novel, orthogonally protected scaffolds based on the unique
2-oxa-5-azabicyclo[4.1.0]heptane structure is presented. The described reaction sequence takes
advantage of easily available starting materials such as serine and threonine and leads to
stereochemically dense structures in few, high-yielding synthetic steps. We show how the
stereochemistry can be easily tuned by starting from different b-hydroxy-a-amino acids and also by
means of a transition metal-catalyzed cyclopropanation step. The compounds find application as
constrained templates for the construction of geometrically diversified libraries of compounds.
Introduction
One of the initial steps in the drug discovery process is the
identification of leads which bind to receptors or other tar-
gets of interest. To address this, a common and established
approach is the screening of libraries of compounds. While
combinatorial chemistry initially tended towards the synthesis
of very large libraries of structurally similar products, nowadays
this initial emphasis on creating mixtures of very large num-
bers of structures is giving way to a more measured approach
based on arrays of fewer, well-characterized compounds.1 This
is particularly noticeable in the move towards the synthesis
Fig. 1 Biologically relevant morpholine-based compounds.
of complex and highly diversified mixtures of molecules that
bear a structural resemblance to approved natural-product-based
chemistry,12 we developed the idea of rigidifying a chiral morpho-
line structure through fusion to a functionalized cyclopropane
ring. This approach provides access to a stereochemically rich
and rigid backbone, allowing us to generate different scaffolds by
means of geometric variation of the scaffold itself.13
Herein we present the new template 2-oxa-5-azabicyclo-
[4.1.0]heptane heterocycle (Scheme 1) and we demonstrate a
general synthetic strategy which can guarantee the introduction
of diversification positions with a high degree of diastereocontrol.
The presence of oxygen and nitrogen atoms in the morpholine ring
can be exploited in a wide range of retrosynthetic analyses9 that
allow easy control of the stereochemistry of the carbon atoms,
drugs2 or to “privileged medicinal scaffolds”.3 There is a strong
drive towards the generation of new chemotypes, displaying
increasing complexity and possessing features that can be related
to pharmacologically relevant structures.4 Among the possible
alternatives, nitrogen-containing heterocycles with a saturated
backbone have attracted considerable attention in the design of
biologically active products.5,6 Most marketed compounds and
several promising leads fall into this category, and the discovery of
small-molecular-weight scaffolds with a high degree of diversity
belonging to this family is a tool of primary importance in the
drug discovery process.7 Among the various structures proposed
by medicinal chemists, the morpholine ring represents a common
motif.8 Many carbon-substituted morpholines display biological
activity and have found application as antidepressants,9 appetite
suppressants,10 and antioxidants.11 Some selected examples are
shown in Fig. 1.
During our ongoing research program toward the develop-
ment of constrained, morpholine-based platforms for medicinal
Department of Organic Chemistry “Ugo Schiff”, University of Florence,
Polo Scientifico e Tecnologico, Via della Lastruccia 13, I-50019, Sesto
Fiorentino (FI), Italy. E-mail: andrea.trabocchi@unifi.it; Fax: +39 055
4573531; Tel: +39 055 4573507
† Electronic supplementary information (ESI) available: 1H and 13C
NMR spectra of compounds 4, 5, Cbz-8, 10–13, 14a, 14b, 15a, 15b
and 18–20; molecular modeling methods; total energies and cartesian
coordinates of compound 20 (axial and equatorial conformers). See DOI:
10.1039/b808895k
Scheme 1 Retrosynthetic analysis of 2-oxa-5-azabicyclo[4.1.0]heptane
scaffolds.
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especially starting from readily available amino acid derivatives.
In order to obtain the 2-oxa-5-azabicyclo[4.1.0]heptane skeleton
we planned to use amino acid-derived dihydrooxazine structures14
(Scheme 1) as substrates for a diastereoselective, transition metal-
catalyzed cyclopropanation using diazo-acetates. The cyclopropa-
nation creates three new adjacent stereocenters and at the same
time introduces a strong conformational constraint. This approach
allows the synthesis of structures bearing functionalizable groups
in different, reciprocal stereochemical relationships. Chirality is
first introduced using readily available, enantiomerically pure b-
hydroxy-a-amino acids as starting materials and is followed by a
cyclopropanation step, whose stereochemical outcome is generally
governed by the stereochemistry of the ligand used.
Scheme 3 Attempted synthesis of compound 7.
Results and discussion
The strategy was developed using serine and threonine as starting
materials and the Cbz as nitrogen protecting group. The serine-
derived dihydroxazine 5 was prepared starting from amine 3,14
which was protected using Cbz-Cl and then cyclized in refluxing
toluene upon treatment with p-TsOH and in the presence of
˚
4 A molecular sieves, in order to promote acid-catalyzed trans-
acetalyzation and subsequent elimination of MeOH (Scheme 2).
Scheme 4 Synthesis of threonine-derived dihydroxazine 13.
upon treatment with Cbz-Cl under various reaction conditions.
We reasoned that this poor reactivity was caused by high steric
hindrance imposed by the TBDMS group, and thus amine 10
was subjected to harsher acid conditions, in order to deprotect
the hydroxyl group and simultaneously obtain trans-acetalization.
The amino group of the resulting cyclic acetal 11 proved to be
much more reactive than 10, and the crude product was easily
protected upon treatment with Cbz-Cl/NaHCO3 (aq). Subsequent
˚
treatment of cyclic acetal 12 with p-TsOH and 4 A molecular
sieves in refluxing toluene furnished the dihydroxazine scaffold 13
in good overall yield, requiring only one purification step by flash
chromatography (Scheme 4).
We then focused our attention on the study of the cyclopropana-
tion of the obtained dihydroxazine scaffolds using diazoacetates.
Preliminary reactions using ethyl diazoacetate were carried out in
order to establish the most effective metal catalyst and the best
reaction conditions (Scheme 5 andTable 1, entries 1–4). We found
that CuOTf (obtained in situ from Cu(OTf)2 and phenylhydrazine)
and Rh2(OAc)4 were both effective and gave comparable yields.
Slow addition of ethyl diazoacetate was necessary to maintain
a low “active carbene” concentration in solution, therefore
preventing dimerization of the carbene itself.16
Scheme 2 Synthesis of serine-derived dihydroxazine (S)-5.
When this synthetic strategy was extended to threonine,
problems initially arose in the reductive amination step with
dimethoxyacetaldehyde. In fact, upon treatment of L-threonine
methyl ester hydrochloride (6) with 2 and Pd/C in the presence of
triethylamine and under a hydrogen atmosphere, the only observed
product was not the expected 7, but the oxazolidine 8 (Scheme 3).
Protection of the hydroxyl group of threonine with TBDMSCl15
before performing the reductive amination was essential, and
allowed the synthesis of 10 in almost quantitative yield (Scheme 4).
Unfortunately, amine 10 was found to be completely unreactive
The best experimental conditions identified using ethyl diazoac-
etate were then extended to the cyclopropanation using t-butyl
Scheme 5 Cyclopropanation of dihydroxazine (S)-5.
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Table 1 Experimental conditions for cyclopropanation of dihydroxazine (S)-5
Time for diazoacetate
addition
Combined yield
14 + 15
Entry
R
Catalyst
Ligand
EDA eq.
Ratioa 14:15
1
2
3
4
5
6
7
8
Et
Et
Et
Et
Rh2(OAc)4
Rh2(OAc)4
—
—
—
—
2
3
3
4.5
4.5
4.5
4.5
4.5
10 min
5 h
5 h
6 h
6 h
6 h
5 h
6 h
41%
72%
75%
86%
73%
72%
67%
80%
1.6 : 1
1.6 : 1
1.5 : 1
1.5 : 1
1 : 6
1.4 : 1
1 : 5
Cu(OTf)2/PhNHNH2
Cu(OTf)2/PhNHNH2
Cu(OTf)2/PhNHNH2
Cu(OTf)2/PhNHNH2
Cu(OTf)2/PhNHNH2
Cu(OTf)2/PhNHNH2
Et
(S,S)-t-BuBOX
—
(S,S)-t-BuBOX
(S,S)-t-BuBOX
t-Bu
t-Bu
t-Bu
1 : 6
a Estimated by 1H-NMR.
diazoacetate, so as to obtain orthogonality with the methyl ester
from the dihydroxazine. The introduction of a more hindered
ester did not affect yield or diastereoselectivity (Table 1, entry
6), and cyclopropanation products could be isolated in good
yield. Low diastereoselectivity between the two diastereomers 14
and 15 was observed, and only traces of the other two possible
diastereomers were detected. Although diastereomers 14 and
15 could be separated by standard chromatography, the use of
suitable chiral ligands was taken into account to enhance the
diastereocontrol.
whereas the same chiral ligand in combination with (S)-5 is the
mismatched pair.
Cyclopropanation of dihydroxazine 13, derived from L-
threonine methyl ester 6, with (S,S)-t-BuBOX and t-butyl dia-
zoacetate resulted in compound 18 as the major stereoisomer and
only traces of a second stereoisomer (Scheme 7), indicating an
additional effect of the methyl group at C-2 of dihydroxazine 13
on the stereoselectivity.
The bisoxazoline ligand (S,S)-2,2ꢀ-methylenebis(4-t-butyl-2-
oxazoline), (S,S)-t-BuBOX),17 proved to be effective when used
in combination with copper(I) triflate. We observed that the stere-
ochemistry of the cyclopropanes formed was mainly controlled
by the ligand chirality, regardless of the stereochemistry of the
dihydroxazine scaffold, suggesting in each case that the chiral
ligand orients the attacking carbene to the same alkene face.
In fact, when dihydroxazine (S)-5 (derived from L-serine) was
treated with t-butyl diazoacetate and (S,S)-t-BuBOX, the ratio
between the diastereomers 14b and 15b was 1 : 6, whereas when
the enantiomeric dihydroxazine (derived from D-serine) was used,
the ratio of the two diastereomers 16:17 (enantiomeric to 14b and
15b, respectively) was reversed, giving a 9 : 1 mixture in favour
of compound 16 (Scheme 6). Comparison of these data with
the diastereomeric ratios obtained in the absence of the chiral
ligand (Table 1, entries 3, 4 and 6) suggested that the combination
of (S,S)-t-BuBOX with dihydroxazine (R)-5 is the matched pair,
Scheme 7 (S,S)-t-BuBOX–Cu(OTf)-catalyzed cyclopropanation of dihy-
droxazine 13.
The structural assignment of the diastereomers 14a/b, 15a/b
and 18 was accomplished by analyzing the values of the coupling
constants between the hydrogen atoms in the cyclopropane ring
and by means of NOE experiments. We used the relationship that
coupling constants less than 7 Hz are associated with a trans
relationship between two protons in a cyclopropane ring.18 In
all the diastereomeric bicyclic compounds isolated, J couplings
of H-7 with the other two protons of the cyclopropane ring (H-1
and H-6) were 2.4–3.6 Hz, indicating a trans relationship between
Scheme 6 (S,S)-t-BuBOX–CuOTf-catalyzed cyclopropanation of dihydroxazines (S)- and (R)-5.
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semiempirical method21 was used to optimize the global minimum
conformer. The geometries of the most abundant minimum energy
conformers were successively subjected to ab initio single-point
energy calculation at the 3-21G*/HF level22 of quantum chemical
theory. The conformation having axial C-2 and C-3 substituents
resulted in a twisted half-chair structure for the morpholine
moiety, whereas a twisted half-boat structure was obtained for the
conformation having the same substitutents in equatorial position.
Also, the conformation with axial C-2 and C-3 substituents proved
to be more stable by about 2.3 kcal mol−1 (Eax = −232.14
kcal mol−1; Eeq = −229.86 kcal mol−1). Computation of the
dihedral angle formed by H-2, C-2, C-3, H-3 atoms for the axial
and equatorial conformations resulted in −73.6◦ and −144.6◦,
respectively, which, in conjunction with 1H-NMR data indicating
absence of coupling between H-2 and H-3, suggested the axial
conformer as the more favourable in chloroform solution (Fig. 4).
Table 2 Vicinal coupling constants for the hydrogens of the cyclopropane
ring in the scaffolds 14 and 15
Compd
J
6,7/Hz
J
1,7/Hz
J1,6/Hz
14a
14b
15a
15b
18
3.2
3.6
3.6
3.2
3.2
3.2
2.4
2.4
3.2
3.2
7.2
7.2
7.2
7.2
7.2
the H-7 and H-1/H-6 protons, and consequently a cis relationship
between H-1 and H-6. (Fig. 2 and Table 2).
Fig. 2 Scaffolds 14, 15 and 18.
A definitive structure elucidation was based on NOE spectra
for the bicyclic morpholine-based scaffolds deriving from cyclo-
propanation with t-butyl diazoacetate. In particular, a diagnostic
NOE effect between H-4 and H-7 was observed for compound 14b
(Fig. 3). For compound 15b, H-7 provided only a weak NOE effect
with the methyl ester protons at C-4, and a strong NOE effect was
observed between H-7 and one of the two methylenic protons at
C-3 of the morpholine ring (Fig. 3). NOE spectra of compound 18
deriving from L-threonine resulted in NOE interactions between
the protons of the methyl group at C-3 and the two protons H-1
and H-6. This strongly supported the structure in Fig. 3, with the
two esters in a trans relationship.
Fig. 4 Energy-minimized conformations of 18 bearing axial (left) and
equatorial (right) substituents at C-2 and C-3.
Further corroboration of the preferential axial conformation for
18 was given by NOE experiments, which showed NOE correlation
of H-7 with methyl ester protons (and not H-2), and of H-6 with
the methyl group at C-2.
In order to extend the versatility of the bicyclic structures
reported herein, the selective transformation of the t-butyl ester
into a primary alcohol using compound 15b as substrate was
carried out. Specifically, the orthogonality of the two esters was
used for selective deprotection of the t-butyl ester under standard
acid conditions, followed by reduction of the resulting acid with
isobutyl chloroformate/NaBH4 (Scheme 8). Both transformations
proved to be completely stereoselective, and no epimerization at
C-7 was observed by 1H-NMR, giving the corresponding alcohol
20 in overall 69% yield from 18, and demonstrating the feasibility
of such scaffold as a template for subsequent appendage diversity.
Fig. 3 Most significant NOEs observed for structures 14b, 15b and 18.
Conclusions
Molecular modeling calculations were carried out on com-
pound 18 so as to assess the most stable conformation
and to gain insight into the detailed structure of the bi-
cyclic scaffold. Energy-minimized conformations of the 2-oxa-5-
azabicyclo[4.1.0]heptane-based scaffold 18 were achieved using
SPARTAN Version 5.119 running on a SGI IRIX 6.5 workstation.
Conformational searches of 18 were carried out using a Monte
Carlo method within the MMFF94 force field,20 and the AM1
In summary we have developed an efficient strategy which gives
access to a new series of scaffolds based on the unique heterocyclic
structure of the 2-oxa-5-azabicyclo[4.1.0]heptane. The strategy
allows the generation of compounds with up to five stereogenic
centers in enantiopure form starting from readily available b-
hydroxy-a-amino acids and by means of a diastereoselective cy-
clopropanation achieved using diazoacetates in conjunction with
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B, J = 6.0 Hz, 0.5 H, CH2Ph), 4.70 (dd, J = 7.2, 4.0 Hz, 0.5 H),
4.65–4.58 (m, 1 H), 4.49 (dd, J = 7.2, 4.0 Hz, 0.5 H), 4.05–3.77 (m,
2 H), 3.71 (s, 1.5 H, CO2CH3), 3.58 (s, 1.5 H, CO2CH3), 3.68–3.54
(m, 2 H) 3.45 (s, 1.5 H, OCH3), 3.43 (s, 1.5 H, OCH3), 3.31 (s, 1.5
H, OCH3), 3.28 (s, 1.5 H, OCH3), 3.26–3.21 (m, 1 H); dC (50 MHz;
CDCl3) mixture of rotamers 170.0 (s, CO2CH3), 156.3 and 156.1
(s, NCO2), 135.7 (s, i-Ph), 128.2–127.6 (d, 5 C, Ph), 103.5 and
102.9 [d, CH(OCH3)2], 67.7 and 67.5 (t, PhCH2), 62.8 and 62.3
(d, NCH), 60.7 and 60.4 (t, CH2OH), 55.1 and 54.8 (q, CO2CH3),
54.3 and 52.0 (q, 2 C, OCH3), 49.2 (t, NCH2); MS m/z 309 (M+ −
CH3OH, 1.9), 277 (0.7), 264 (0.7), 250 (0.6), 234 (0.5), 220 (0.1),
91 (100).
Scheme 8 Synthesis of compound 20.
(R)-2-[Benzyloxycarbonyl-(2,2-dimethoxyethyl)amino]-3-
hydroxypropionic acid methyl ester [(R)-4]
Cu(I)OTf and a chiral t-BuBOX ligand. The cyclopropanation
outcome proved to be mainly controlled by the stereochemistry
of the bis-isooxazolidine ligand. A variety of scaffolds can be
obtained using this strategy, each of them differing for the spatial
orientation of the orthogonally protected diversification sites.
Some final manipulations of the synthesized structures have been
presented, in order to prove the versatility and the orthogonal
relationship between the various protecting groups introduced.
Compound (R)-4 was prepared as for (S)-4 starting from D-serine
methyl ester hydrochloride and (2), with identical NMR data to
the enantiomeric compound (S)-4. (Found: C, 56.20; H, 6.84; N
4.02. C16H23NO7 requires C, 56.30; H, 6.79; N, 4.10%); [a]2D5 +41.7
(c 1.0, CHCl3).
(S)-2,3-Dihydro-[1,4]oxazine-3,4-dicarboxylic acid 4-benzyl ester
3-methyl ester [(S)-5]
Experimental
A solution of compound (S)-4 (1.12 g, 3.28 mmol) in toluene
(45 mL) containing a catalytic amount of p-toluenesulfonic acid
monohydrate (63 mg, 0.33 mmol) was placed in a single-necked
round-bottomed flask equipped with a reflux condenser and a
Chromatographic separations were performed on silica gel using
flash-column techniques. Rf values refer to TLC carried out on
25 mm silica gel plates (Merck F254) with the same eluant as for
column chromatography. 1H and 13C NMR spectra were recorded
with NMR instruments operating at 200 MHz and 400 MHz for
proton and at 50 MHz for carbon, and using CDCl3 solutions
unless otherwise stated. EI mass spectra were carried out at 70 eV
ionizing voltage.
˚
dropping funnel containing approximately 16 g of 4 A molecular
sieves. The mixture was refluxed for 2.5 h, then cooled to room
temperature and filtered through a thin layer of NaHCO3. Toluene
was removed under reduced pressure, and the crude product was
purified by flash column chromatography (hexanes–EtOAc 3 : 1)
to yield compound (S)-5 as a colourless oil (729 mg, 78%). (Found:
C, 60.81; H, 5.55; N, 5.01. C14H15NO5 requires C, 60.64; H, 5.45;
N, 5.05%); [a]2D5 +8.6 (c 1.0, CHCl3); dH (400 MHz, CDCl3) 3 : 2
mixture of rotamers a and b 7.39–7.30 (m, 5 H, Ph), 6.45 (d, J =
2.4 Hz, 0.4 H, H-6 b), 6.33 (d, J = 2.6 Hz, 0.6 H, H-6 a), 6.03
(d, J = 2.4 Hz, 0.4 H, H-5 b), 5.90 (d, J = 2.6 Hz, 0.6 H, H-5
a), 5.29–5.15 (m, 2 H, CH2Ph), 4.95 (s, 0.6 H, H-2 a), 4.83 (s, 0.4
H, H-2 b), 4.65 (dd, J = 10.8, 0.8 Hz, 0.6 H, H-2 a), 4.57 (d, J =
10.8, 0.8 Hz, 0.4 H, H-2 b), 3.97–3.92 (m, 1 H, H-3), 3.78 (s, 1.8 H,
OCH3 a), 3.71 (s, 1.2 H, OCH3 b); dC (50 MHz, CDCl3) mixture of
rotamers 168.2 and 168.0 (s, CO2CH3), 151.7 and 151.0 (s, NCO2),
135.4 (s, i-Ph), 129.4 and 128.2 (d, C-6), 128.1–127.6 (d, 5 C, Ph),
105.8 and 105.3 (d, C-5), 67.7 and 67.5 (t, CH2Ph), 65.1 and 64.7
(t, C-2), 54.4 (q, CO2CH3), 53.7 and 52.5 (d, C-3); MS m/z 277
(M+, 4), 249 (11), 91 (100).
(S)-2-[Benzyloxycarbonyl-(2,2-dimethoxyethyl)amino]-3-
hydroxypropionic acid methyl ester [(S)-4]
L-Serine methyl ester hydrochloride (1) (5.34 g, 34.3 mmol) was
dissolved in MeOH (110 mL), then triethylamine (4.79 mL,
34.3 mmol), 60% aqueous solution of dimethoxyacetaldehyde (2)
(5.95 g, 34.3 mmol) and 10% Pd/C (477 mg) were successively
added, and the resulting mixture was stirred overnight at room
temperature under a hydrogen atmosphere. Then, the suspension
was filtered over Celite and the organic solvent was removed under
reduced pressure. The crude reaction mixture was dissolved in H2O
(60 mL), and NaHCO3 (5.76 g, 68.6 mmol) was added. EtOAc
(75 mL) was added and the mixture was cooled at 0 ◦C with an ice
bath. Cbz-Cl (4.80 mL, 33.61 mmol) was added dropwise, then,
after 1 h stirring, the ice bath was removed and the mixture was
stirred overnight. The reaction was diluted with EtOAc (200 mL)
and the aqueous layer was discarded. The organic phase was
washed with 1 M HCl, brine, dried over Na2SO4, concentrated
and purified by flash chromatography (EtOAc–hexanes 3 : 2) to
provide compound (S)-4 as a colourless oil (10.01 g, 85%). (Found:
C, 56.40; H, 6.95; N 4.01. C16H23NO7 requires C, 56.30; H, 6.79;
N, 4.10%); [a]2D6 −42.9 (c 1.0, CHCl3); dH (400 MHz; CDCl3) 1 :
1 mixture of rotamers 7.36–7.29 (m, 5 H, Ph), 5.21 (AB, part A,
J = 6.0 Hz, 0.5 H, CH2Ph), 5.13 (s, 1 H, CH2Ph), 5.13 (AB, part
(R)-2,3-Dihydro-[1,4]oxazine-3,4-dicarboxylic acid 4-benzyl ester
3-methyl ester [(R)-5]
Compound (R)-5 was prepared as for (S)-5 starting from (R)-4,
with identical NMR data to the enantiomeric compound (S)-5.
(Found: C, 60.78; H, 5.51; N, 5.09. C14H15NO5 requires C, 60.64;
H, 5.45; N, 5.05%); [a]2D5 −7.2 (c 2.5, CHCl3).
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(2R/S,4S,5R)-2-Dimethoxymethyl-5-methyloxazolidine-4-
carboxylic acid methyl ester (8): synthesis of the Cbz-protected
derivative of 8
protection step. An analytically pure sample was obtained after
purification by flash column chromatography (EtOAc). (Found:
C, 50.86; H, 8.08; N, 7.22. C8H15NO4 requires C, 50.78; H, 7.99;
N, 7.40%); dH (400 MHz, CDCl3) 3 : 2 mixture of diastereomers
a and b 4.47 (s, 0.4 H, H-6 b), 4.40 (dd, J = 8.8, 2.4 Hz, 0.6 H,
H-6 a), 3.88 (qd, J = 5.0, 1.8 Hz, 0.4 H, H-2 b), 3.74 and 3.73 (s,
3 H, CO2CH3), 3.65 (qd, J = 4.2, 1.2 Hz, 0.6 H, H-2 a), 3.50 (s,
1.8 H, OCH3 a), 3.39 (s, 1.2 H, OCH3 b), 3.27 (d, J = 9.4 Hz, 0.4
H, H-3 b), 3.18 (d, J = 9.4 Hz, 0.6 H, H-3 a), 3.04 (dd, J = 12.4,
2.4 Hz, 0.6 H, H-5 a), 2.92–2.90 (m, 0.8 H, H-5 b), 2.59 (dd, J =
12.4, 8.8 Hz, 0.6 H, H-5 a), 1.75–1.95 (br, 1 H, NH), 1.25 (d, J =
6.4 Hz, 1.8 H, CHCH3 a), 1.15 (d, J = 6.0 Hz, 1.2 H, CHCH3 b);
dC (50 MHz, CDCl3) mixture of diastereomers 171.1 (s, CO2CH3),
100.6 and 95.6 (d, C-6), 73.7 and 65.4 (d, C-2), 63.6 and 62.8 (d,
C-3), 56.1 and 54.5 (q, OCH3), 52.1 (q, CO2CH3), 47.9 and 47.2
(t, C-5), 18.2 (q, CHCH3).
Compound 8 was obtained as a by-product starting from L-
threonine methyl ester (6) (2.42 g, 14.3 mmol) according to the
reported procedure for the preparation of 4 (see ref. 14). The
crude product 8 was then characterized as the corresponding
Cbz-protected derivative, after treatment of crude 8 with Cbz-Cl
according to procedure as for 4. Pure Cbz-protected compound
(2.76 g, 12.4 mmol) was obtained after chromatographic purifica-
tion (hexanes–EtOAc 1 : 3) in 87% yield over two steps. (Found: C,
57.96; H, 6.61; N, 3.74. C17H23NO7 requires C, 57.78; H, 6.56; N,
3.96%); dH (200 MHz, CDCl3) mixture of diastereomers 7.31 (m,
5 H, Ph), 5.31 [s, 1 H, CH(OCH3)2], 5.15 (s, 2 H, CH2Ph), 4.58 (m,
1 H, OCHN), 4.46 (m, 1 H, CHCH3), 4.07 (m, 1 H, NCHCO2),
3.69 (s, 3 H, CO2CH3), 3.46 (s, 6 H, OCH3), 1.39 (d, J = 5.6 Hz,
3 H, CHCH3); dC (50 MHz, CDCl3) mixture of diastereomers
169.6 (s, CO2CH3), 135.6 (s, i-Ph), 128.2 (d, 2 C, Ph), 127.9 (d,
2 C, Ph), 127.5 (d, Ph), 104.2 [d, CH(OCH3)2], 88.6 (d, OCHN),
67.5 (t, CH2Ph), 64.4 (d, CHCH3), 55.7 (d, NCHCO2), 52.3
(q, CO2CH3), 19.5 (q, CHCH3).
(2R,3S,6R/S)-6-Methoxy-2-methylmorpholine-3,4-dicarboxylic
acid 4-benzyl ester 3-methyl ester (12)
Crude cyclic acetal 11 was dissolved in H2O (5 mL) and NaHCO3
(297 mg, 3.54 mmol) was added. The mixture was stirred until
complete dissolution of the salt, then dioxane (8 mL) was added.
The flask was cooled at 0 ◦C with an ice bath and Cbz-Cl (253 mg,
1.77 mmol) was added dropwise. After 10 minutes the ice bath was
removed and the reaction mixture was stirred for 1 day at room
temperature. Afterwards, EtOAc (25 mL) and water (10 mL) were
added. The aqueous layer was discarded and the organic phase was
washed with 1 M HCl, brine, and dried over Na2SO4. The solvent
were removed under reduced pressure, and the crude material
was used without purification for the elimination reaction. An
analytically pure sample was obtained after purification by flash
column chromatography (hexanes–EtOAc 3 : 1). (Found: C, 59.77;
H, 6.78; N, 4.24. C16H21NO6 requires C, 59.43; H, 6.55; N,
4.33%); dH (400 MHz, CDCl3) mixture of diastereomers, mixture
of rotamers 7.34–7.25 (m, 5 H, Ph), 5.23–5.01 (m, 2 H, CH2Ph),
4.82–4.78 (m, 1 H, H-6), 4.70–4.63 (m, 1 H, H-2), 4.32–4.10 (m,
2 H, H-3 and H-5), 3.98–3.42 (m, 3 H, CO2CH3), 3.42–3.38 (m,
3 H, OCH3), 1.43–1.35 (m, 3 H, CHCH3); dC (50 MHz, CDCl3)
mixture of diastereomers, mixture of rotamers 170.2 (s, CO2CH3),
135.9 (s, i-Ph), 128.3–127.6 (d, 5 C, Ph), 97.0 (d, C-6), 69.1 (d, C-2),
67.6 (t, CH2Ph), 59.5 (d, C-3), 55.3 (q, OCH3), 52.3 (q, CO2CH3),
44.3 (t, C-5), 20.0 and 18.9 (q, CHCH3).
(2S,3R)-3-(t-Butyldimethylsilanyloxy)-2-(2,2-
(dimethoxy)ethylamino)butyric acid methyl ester (10)
Compound 9 (3.70 g, 14.9 mmol) was dissolved in MeOH
(45 mL), then 60% aqueous solution of dimethoxyacetaldehyde
(2) (2.59 g, 14.9 mmol) and 10% Pd/C (329 mg) were successively
added, and the resulting mixture was stirred overnight at room
temperature under a hydrogen atmosphere. Then, the suspension
was filtered on Celite and MeOH was removed under reduced
pressure. The resulting mixture was partitioned between water
and Et2O. The combined organic layers were washed with brine,
dried over Na2SO4 and concentrated under reduced pressure to
yield compound 10 as a colourless oil (4.95 g, 99%). (Found: C,
53.86; H, 10.00; N, 4.22. C15H33NO5Si requires C, 53.70; H, 9.91;
N, 4.17%); [a]2D5 −11.4 (c 1.1, CH2Cl2); dH (400 MHz, CDCl3) 4.53
[t, J = 5.2 Hz, 1 H, CH(OCH3)2], 4.18 (qui, J = 5.3 Hz, 1 H,
OCHCH3), 3.73 (s, 3 H, CO2CH3), 3.37 (s, 6 H, OCH3), 3.37 (m,
1 H, NCHCO2), 2.94 (dd, J = 12.2, 5.8 Hz, 1 H, CH2CH), 2.73
(dd, J = 12.2, 5.0 Hz, 1 H, CH2CH), 1.25 (d, J = 6.4 Hz, 3 H,
CHCH3), 0.85 [s, 9 H, (CH3)3CSi], 0.44 (s, 3 H, CH3Si), 0.14 (s,
3 H, CH3Si); dC (50 MHz, CDCl3) 171.9 (s, CO2CH3), 102.9 [d,
CH(OCH3)2], 69.1 (d, OCHCH3), 66.8 (d, NCHCO2), 54.4 (q,
OCH3), 53.6 (q, OCH3), 52.0 (q, CO2CH3), 48.9 (t, CH2CH), 25.7
[q, 3 C, (CH3)3CSi], 20.8 (q, CHCH3), 17.9 [s, (CH3)3CSi], −4.2
(q, CH3Si), −5.1 (q, CH3Si); MS m/z 304 (M+ − CH3O, 8), 291
(15), 278 (6), 246 (13), 159 (38), 73 (100).
(2R,3S)-2-Methyl-2,3-dihydro-[1,4]oxazine-3,4-dicarboxylic acid
4-benzyl ester 3-methyl ester (13)
Crude protected acetal 12 was dissolved in toluene (10 mL) con-
taining a catalytic amount of p-toluenesulfonic acid monohydrate
(34 mg, 0.18 mmol) and placed in a single-necked round-bottomed
flask equipped with a reflux condenser and a dropping funnel
(2R,3S,6R/S)-6-Methoxy-2-methylmorpholine-3-carboxylic acid
methyl ester (11)
SOCl2 (511 lL, 7 mmol) was added dropwise, at 0 ◦C, to 7 mL of
MeOH. The resulting solution was used to dissolve compound 10
(600 mg, 1.79 mmol). The resulting mixture was refluxed for 4 h,
and then concentrated under reduced pressure. The crude material
was dissolved again in MeOH, neutralized with Amberlyst A-
21, and the solvent was evaporated to dryness. The product
was directly used without further purification for the subsequent
containing approximately 10g of 4 A molecular sieves. Themixture
˚
was refluxed for 2 h, then cooled to room temperature and filtered
through a thin layer of NaHCO3. Toluene was removed under
reduced pressure, and the crude product was purified by flash
column chromatography (hexanes–EtOAc 7:2) to yield compound
13 as colourless oil (406 mg, 78% over 3 steps from compound 10).
(Found: C, 61.80; H, 6.01; N, 4.91. C15H17NO5 requires C, 61.85;
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H, 5.88; N, 4.81%); [a]D26 −7.2 (c 0.2, CHCl3). dH (400 MHz, CDCl3)
3 : 2 mixture of rotamers a and b 7.39–7.30 (m, 5 H, Ph), 6.39 (d,
J = 4.8 Hz, 0.4 H, H-6 b) 6.27 (d, J = 4.8 Hz, 0.6 H, H-6 a), 5.88
(d, J = 4.8 Hz, 0.4 H, H-5 b) 5.75 (d, J = 4.8 Hz, 0.6 H, H-5
a), 5.25 (AB, part A, J = 12 Hz, 0.4 H, CH2Ph b), 5.24 (s, 1.2 H,
CH2Ph a), 5.16 (AB, part B, J = 12 Hz, 0.4 H, CH2Ph b), 4.83
(qd, J = 6.4, 1.2 Hz, 0.6 H, H-2 a), 4.72 (qd, J = 6.4, 1.2 Hz,
0.4 H, H-2 b), 4.68 (s, 0.6 H, H-3 a), 4.55 (s, 0.4 H, H-3 b), 3.77 (s,
1.8 H, CO2CH3 a), 3.69 (s, 1.2 H, CO2CH3 b), 1.31 (d, J = 6.4 Hz,
1.8 H, CHCH3 a) 1.30 (d, J = 6.4 Hz, 1.2 H, CHCH3 b); dC
(50 MHz, CDCl3) mixture of rotamers 168.4 (s, CO2CH3), 152.8
(s, NCO2), 135.6 (s, i-Ph), 128.4–127.7 (d, 5 C, Ph), 126.8 and 125.7
(d, C-6), 104.7 and 104.3 (d, C-5), 69.7 and 69.1 (d, C-2), 68.0 and
67.7 (t, CH2Ph), 58.1 and 57.3 (d, C-3), 52.6 (q, CO2CH3), 17.2
(q, CHCH3); MS m/z 291 (M+, 4.6), 188 (15.9), 91 (100).
stereoisomer, or in higher amounts according to general procedure
B. (Found: C, 60.02; H, 5.71; N, 3.70. C18H21NO7 requires C, 59.50;
H, 5.83; N, 3.85%); [a]D25 −48.1 (c 1.2, CHCl3); dH (400 MHz,
CDCl3) 2 : 1 mixture of rotamers a and b 7.35–7.29 (m, 5 H, Ph),
5.29 (AB, part A, J = 12.5 Hz, 0.66 H, CH2Ph a), 5.17 (AB, part B,
J = 12.5 Hz, 0.66 H, CH2Ph a), 5.24–5.11 (m, 0.66 H, CH2Ph b),
4.47 (t, J = 4.8 Hz, 0.66 H, H-4 a), 4.19 (t, J = 4.8 Hz, 0.33 H, H-4
b), 4.20–4.03 (m, 3 H, CO2CH2CH3 and H-1), 4.03 (dd, J = 11.6,
5.2 Hz, 0.66 H, H-3 a), 3.94 (dd, J = 11.6, 5.2 Hz, 0.33 H, H-3 b),
3.84 (dd, J = 11.6, 4.8 Hz, 1 H, H-3), 3.78 (s, 2 H, CO2CH3 a), 3.67
(s, 1 H, CO2CH3 b), 3.54–3.49 (m, 1 H, H-6), 2.00 (t, J = 3.2 Hz,
1 H, H-7), 1.98 (t, J = 7.2 Hz, 2 H, CO2CH2CH3); d (50 MHz,
C
CDCl3) mixture of rotamers 169.4 (s, CO2), 169.2 (s, CO2), 155.9
(s, NCO2), 135.7 (s, i-Ph), 128.2–127.1 (d, 5 C, Ph), 67.8 and 67.7
(t, CH2Ph), 64.6 and 64.1 (t, C-3), 60.7 (t, CO2CH2CH3), 59.1 (d,
C-1) 53.1 and 52.6 (d, C-4) 52.0 (q, CO2CH3), 33.9 (d, C-6), 26.4
and 26.0 (d, C-7), 14.2 (q, CO2CH2CH3); MS m/z 363 (M+, 0.8),
318 (0.2), 290 (2), 228 (15), 182 (18), 91 (100).
Cyclopropanation with Cu(OTf)2 and (S,S)-2,2ꢀ-isopropylidene-
bis(4-t-butyl-2-oxazoline): general procedure A
To a solution of dihydroxazine 5 (626 mg, 2.24 mmol) in dry
CH2Cl2 (4 mL) cooled in an ice–salt bath were added Cu(OTf)2
(16 mg, 0.045 mmol), (S,S)-2,2ꢀ-isopropylidene-bis(4-t-butyl-2-
oxazoline) (16 mg, 0.056 mmol) and phenylhydrazine (4.4 lL,
0.045 mmol). After 30 min, a 1.2 M solution of diazoacetate in dry
CH2Cl2 was added (quantity and time according to Table 1). The
reaction was then gently warmed to room temperature and stirred
for 16 h. Then, the mixture was concentrated under reduced pres-
sure and the residue was purified by flash column chromatography
(hexanes–EtOAc 3 : 1) to yield the cyclopropanated products.
(1S,4S,6R,7R)-2-Oxa-5-azabicyclo[4.1.0]heptane-4,5,7-
tricarboxylic acid 5-benzyl ester 7-t-butyl ester 4-methyl ester
(14b)
Compound 14b was obtained from (S)-5 as the minor diastere-
omer according to general procedure A, or in higher amounts
according to general procedure B. (Found: C, 61.06; H, 6.71; N,
3.55. C20H25NO7 requires C, 61.37; H, 6.44; N, 3.58%); [a]2D5 −61.3
(c 1.1, CHCl3); dH (400 MHz, CDCl3) 2 : 1 mixture of rotamers a
and b 7.38–7.28 (m, 5 H, Ph), 5.28 (AB, part A, J = 12.4 Hz, 0.66
H, CH2Ph a), 5.24 (AB, part A, J = 7.9 Hz, 0.33 H, CH2Ph b),
5.14 (AB, part B, J = 12.4 Hz, 0.66 H, CH2Ph a), 5.10 (AB, part B,
J = 8.0 Hz, 0.33 H, CH2Ph b), 4.45 (t, J = 4.8 Hz, 0.66 H, H-4 a),
4.38 (t, J = 4.8 Hz, 0.33 H, H-4 b), 4.10 (m, 0.33 H, H-1 b), 4.09
(dd, J = 7.2, 3.2 Hz, 0.66 H, H-1 a), 3.94 (dd, J = 11.6, 3.2 Hz,
0.66 H, H-3 a), 3.90 (dd, J = 11.6, 4.8 Hz, 0.33 H, H-3 b), 3.82
(dd, J = 11.6, 4.8 Hz, 1 H, H-3), 3.77 (s, 2 H, CO2CH3 a), 3.65 (s,
1 H, CO2CH3 b), 3.47 (dd, J = 7.2, 3.2 Hz, 0.33 H, H-6 b), 3.42
(dd, J = 7.2, 3.2 Hz, 0.66 H, H-6 a), 1.92 and 1.89 (t, J = 3.2 Hz,
1 H, H-7), 1.43 [s, 3 H, CO2C(CH3)3 b], 1.37 [s, 6 H, CO2C(CH3)3
a]; dC (50 MHz, CDCl3) mixture of rotamers 169.4 (s, CO2), 168.1
(s, CO2), 156.0 (s, NCO2), 135.6 (s, i-Ph), 128.3–127.2 (d, 5 C, Ph),
81.0 [s, C(CH3)3], 67.8 (t, CH2Ph), 64.5 and 64.1 (t, C-3), 58.8 (d,
C-1), 53.1 (d, C-4), 52.6 (d, C-6) 52.1 (q, CO2CH3), 33.5 (d, C-7),
28.0 and 27.4 [q, 3 C, C(CH3)3]; MS m/z 335 (M+ − t-Bu, 2), 318
(0.4), 291 (3), 200 (15), 91 (100).
Cyclopropanation with Cu(OTf)2, without chiral ligand: general
procedure B
To a solution of dihydroxazine scaffold (417 mg, 1.49 mmol) in dry
CH2Cl2 (3 mL) cooled in an ice–salt bath were added Cu(OTf)2
(11 mg, 0.030 mmol) and phenylhydrazine (2.9 lL, 0.030 mmol).
After 30 min, a 1.2 M solution of diazoacetate in dry CH2Cl2 was
added (quantity and time according to Table 1). The reaction
was then gently warmed to room temperature and stirred for
16 h. Then, the mixture was concentrated under reduced pressure
and the residue was purified by flash column chromatography
(hexanes–EtOAc 3 : 1) to yield the cyclopropanated products.
Cyclopropanation with Rh2(OAc)4: general procedure C
To a solution of dihydroxazine scaffold (522 mg, 1.87 mmol) in dry
CH2Cl2 (4 mL) cooled in an ice–salt bath Rh2(OAc)4 (2.5 mol%),
and a 1.2 M solution of diazoacetate in dry CH2Cl2 was added
(quantity and time according to Table 1). The reaction was then
gently warmed to room temperature and stirred for 16 h. The
mixture was concentrated under reduced pressure and the residue
was purified by flash column chromatography (hexanes–EtOAc 3 :
1) to yield the cyclopropanated products.
(1R,4S,6S,7S)-2-Oxa-5-azabicyclo[4.1.0]heptane-4,5,7-tricarbo-
xylic acid 5-benzyl ester 7-ethyl ester 4-methyl ester (15a)
626 mg (2.24 mmol) of compound (S)-5 were treated according
to general procedure A using 4.5 equivalents of ethyl diazoacetate
(6 h time of addition) to yield 512 mg (63%) of 15a. (Found: C,
60.1; H, 5.11; N, 3.62. C18H21NO7 requires C, 59.50; H, 5.83; N,
3.85%); [a]2D5 +5.5 (c 1.2, CHCl3); dH (400 MHz, CDCl3) 3 : 2
mixture of rotamers a and b 7.35–7.28 (m, 5 H, Ph), 5.27 (AB,
part A, J = 12.5 Hz, 0.6 H, CH2Ph a), 5.22 (AB, part B, J =
12.5 Hz, 0.6 H, CH2Ph a), 5.21 (AB, part A, J = 13.1 Hz, 0.4 H,
CH2Ph b), 5.13 (AB, part B, J = 13.1 Hz, 0.4 H, CH2Ph b), 4.27
(d, J = 3.2 Hz, 0.6 H, H-4 a), 4.24 (d, J = 3.2 Hz, 0.4 H, H-4 b),
(1S,4S,6R,7R)-2-Oxa-5-azabicyclo[4.1.0]heptane-4,5,7-
tricarboxylic acid 5-benzyl ester 7-ethyl ester 4-methyl ester (14a)
626 mg (2.24 mmol) of compound (S)-5 were treated according
to general procedure A using 4.5 equivalents of ethyl diazoacetate
(6 h time of addition) to yield 81 mg (10%) of 14a as the minor
3334 | Org. Biomol. Chem., 2008, 6, 3328–3336
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4.20–4.05 (m, 4 H, CO2CH2CH3, H-3, and H-1), 3.84 (dd, J =
11.6, 3.6 Hz, 0.6 H, H-3 a), 3.80 (dd, J = 11.6, 3.6 Hz, 0.4 H, H-3
b), 3.74 (s, 1.8 H, CO2CH3 a), 3.63 (s, 1.2 H, CO2CH3 b), 3.53 (dd,
J = 7.2, 3.6 Hz, 0.4 H, H-6 b), 3.49 (dd, J = 7.2, 3.6 Hz, 0.6 H,
H-6 a), 2.38 (dd, J = 3.6, 2.4 Hz, 0.6 H, H-7 a), 2.29 (dd, J = 3.6,
2.4 Hz, 0.4 H, H-7 b), 1.26 (t, J = 7.2 Hz, 1 H, CO2CH3 b), 1.21
(t, J = 7.2 Hz, 2 H, CO2CH3 a); dC (50 MHz, CDCl3) mixture of
rotamers 170.5 and 170.0 (s, CO2), 169.9 and 169.8 (s, CO2), 156.1
and 155.4 (s, NCO2), 135.8 and 135.5 (s, i-Ph), 128.2–127.2 (d, 5
C, Ph), 67.7 and 67.6 (t, CH2Ph), 65.9 and 65.5 (t, C-3), 60.6 (t,
CO2CH2CH3), 58.1 and 57.8 (d, C-1), 55.4 and 54.9 (d, C-4), 52.6
(q, CO2CH3), 35.3 and 35.2 (d, C-6), 27.5 and 27.3 (d, C-7), 14.2
(q, CO2CH2CH3); MS m/z 363 (M+, 1.2), 246 (15.2), 228 (17.4),
91 (100).
(Found: C, 61.31; H, 6.34; N, 3.49. C20H25NO7 requires C, 61.37;
H, 6.44; N, 3.58%); [a]2D5 −15.8 (c 1.0, CHCl3).
(1R,3R,4S,6S,7S)-3-Methyl-2-oxa-5-azabicyclo[4.1.0]heptane-
4,5,7-tricarboxylic acid 5-benzyl ester 7-t-butyl ester 4-methyl
ester (18)
To a solution of dihydroxazine 13 (450 mg, 1.54 mmol) in
dry CH2Cl2 (4 mL) cooled in an ice–salt bath were added
Cu(OTf)2 (14 mg, 0.038 mmol), (S,S)-2,2ꢀ-isopropylidene-bis(4-
t-butyl-2-oxazoline) (11 mg, 0.038 mmol) and phenylhydrazine
(3.0 lL, 0.031 mmol). After 30 min, a 1.2 M solution of t-butyl-
diazoacetate (5 eq.) in dry CH2Cl2 was added over 6 h. During the
addition the volume was maintained constant, expelling CH2Cl2
by passing nitrogen through the flask. The reaction was then gently
warmed to room temperature and stirred 16 h. The mixture was
concentrated under reduced pressure and the residue was purified
by flash column chromatography (hexanes–EtOAc 3 : 1) to yield
405 mg (65%) of 18. (Found: C, 62.42; H, 6.93; N, 3.52. C21H27NO7
requires C, 62.21; H, 6.71; N, 3.45%); [a]2D4 +34.6 (c 1.9, CHCl3);
dH (400 MHz, CDCl3) 3 : 1 mixture of rotamers a and b 7.34–7.24
(m, 5 H, Ph), 5.29 (AB, part A, J = 12.3 Hz, 0.75 H, CH2Ph a),
5.21 (AB, part A, J = 12.2 Hz, 0.25 H, CH2Ph b), 5.13 (AB, part
B, J = 12.6 Hz, 0.75 H, CH2Ph a), 5.06 (AB, part B, J = 12.3 Hz,
0.25 H, CH2Ph b), 4.34 (q, J = 6.8 Hz, 0.75 H, H-2 a), 4.27 (q,
J = 6.8 Hz, 0.25 H, H-2 b), 4.08 (s, 0.75 H, H-4 a), 4.03 (s, 0.25
H, H-4 a), 3.79 (dd, J = 7.2, 3.2 Hz, 0.25 H, H-1 b), 3.75 (dd,
J = 7.2, 3.2 Hz, 0.75 H, H-1 a), 3.68 (s, 2.25 H, CO2CH3 a), 3.58
(s, 0.75 H, CO2CH3 b), 3.54 (dd, J = 7.2, 3.2 Hz, 0.25 H, H-6 b),
3.48 (dd, J = 7.2, 3.2 Hz, 0.75 H, H-6 a), 2.27 (t, J = 3.2 Hz, 0.75
H, H-7 a), 2.17 (t, J = 3.2 Hz, 0.25 H, H-7 b), 1.44 (d, J= 6.4 Hz,
2.25 H, CHCH3 a), 1.44–1.41 (m, 0.75 H, CHCH3 b), 1.41 [s, 2.25
H, CO2C(CH3)3 b], 1.35 [s, 6.75 H, CO2C(CH3)3 a]; dC (50 MHz,
CDCl3) mixture of rotamers 170.5 (s, CO2), 169.4 (s, CO2), 156.8 (s,
NCO2), 135.7 (s, i-Ph), 128.3–127.2 (d, 5 C, Ph), 80.7 [s, C(CH3)3],
70.1 and 69.6 (d, C-3), 67.6 (t, CH2Ph), 58.8 and 58.4 (d, C-1),
52.7 (q, CO2CH3), 52.3 (d, C-4), 34.9 and 34.7 (d, C-6), 27.9 and
27.6 [q, 3 C, C(CH3)3], 27.5 (d, C-7), 17.8 (q, CHCH3); MS m/z
405 (M+, 0.1), 305 (2), 260 (9), 214 (51), 91 (100).
(1R,4S,6S,7S)-2-Oxa-5-azabicyclo[4.1.0]heptane-4,5,7-
tricarboxylic acid 5-benzyl ester 7-t-butyl ester 4-methyl ester
(15b)
625 mg (2.24 mmol) of compound (S)-5 were treated according to
general procedure A using 4.5 equivalents of t-butyl diazoacetate
(6 h time of addition) to yield 447 mg (51%) of 15b. (Found: C,
61.36; H, 6.31; N, 3.46. C20H25NO7 requires C, 61.37; H, 6.44; N,
3.58%); [a]2D6 +14.1 (c 0.6, CHCl3); dH (400 MHz, CDCl3) 2 : 1
mixture of rotamers a and b 7.37–7.27 (m, 5 H, Ph), 5.30 (AB,
part A, J = 13.1 Hz, 0.66 H, CH2Ph a), 5.17 (AB, part B, J =
13.1 Hz, 0.66 H, CH2Ph a), 5.22–5.12 (m, 0.66 H, CH2Ph b), 4.25
(d, J = 2.8 Hz, 0.66 H, H-4 a), 4.23 (d, J = 2.8 Hz, 0.33 H, H-4 b),
4.13–4.06 (m, 0.66 H, H-1 b and H-3 b), 4.02 (dd, J = 7.2, 2.4 Hz,
0.66 H, H-1 a), 4.01 (d, J = 12.0 Hz, 0.66 H, H-3 a), 3.83 (dd,
J = 12.0, 3.6 Hz, 1 H, H-3), 3.74 (s, 2 H, CO2CH3 a), 3.64 (s, 1 H,
CO2CH3 b), 3.45 (dd, J = 7.2, 3.6 Hz, 0.33 H, H-6 b), 3.41 (dd,
J = 7.2, 3.6 Hz, 0.66 H, H-6 a), 2.27 (dd, J = 3.6, 2.4 Hz, 0.66
H, H-7 a), 2.19 (dd, J = 3.6, 2.4 Hz, 0.33 H, H-7 b), 1.44 [s, 3 H,
CO2C(CH3)3 b], 1.37 [s, 6 H, CO2C(CH3)3 a]; dC (50 MHz, CDCl3)
mixture of rotamers 170.1 (s, CO2), 169.1 (s, CO2), 156.2 (s, CO2),
135.7 (s, i-Ph), 128.2–127.2 (d, 5 C, Ph), 80.9 [s, C(CH3)3], 67.7 (t,
CH2Ph), 65.9 and 65.4 (t, C-3), 57.6 (d, C-1), 54.9 (d, 2 C, C-4
and C-6) 52.5 (q, CO2CH3), 35.0 (d, C-7), 28.4 and 28.1 [q, 3 C,
C(CH3)3]; MS (m/z) 335 (M+ − t-Bu, 2), 318 (0.3), 291 (1), 200
(16), 91 (100).
(1R,4S,6S,7S)-3-Methyl-2-oxa-5-azabicyclo[4.1.0]heptane-4,5,7-
tricarboxylic acid 5-benzyl ester 4-methyl ester (19)
(1R,4R,6S,7S)-2-Oxa-5-azabicyclo[4.1.0]heptane-4,5,7-
tricarboxylic acid 5-benzyl ester 7-t-butyl ester 4-methyl ester
(16)
Compound 15b (240 mg, 0.61 mmol) was dissolved in CH2Cl2
(2.8 mL) and TIS (125 lL, 0.61 mmol) and TFA (1.2 mL) were
added sequentially. The mixture was stirred for 50 minutes at
room temperature and then the solvents were removed under
reduced pressure. The crude product obtained was redissolved in
5% Na2CO3 (20 mL) and the solution was extracted with Et2O. The
acqueous phase was acidified at pH 1–2 with concentrated HCl
and extracted with CH2Cl2 (4 × 10 mL). The dichloromethane
exctracts were combined, dried over Na2SO4 and concentrated
under reduced pressure to obtain compound 19 (174 mg, 85%).
(Found: C, 57.36; H, 5.21; H, 4.19. C16H17NO7 requires C, 57.31;
H, 5.11; N, 4.18%); [a]D26 +5.8 (c 1.0, CHCl3); dH (400 MHz, CDCl3)
3 : 2 mixture of rotamers aand b 7.36–7.28 (m, 5 H, Ph), 5.27–5.10
(m, 2 H, CH2Ph), 4.25 (d, J = 2.8 Hz, 0.6 H, H-4 a), 4.21 (d, J =
2.8 Hz, 0.4 H, H-4 b), 4.17–4.05 (m, 2 H, H-3), 3.85–3.78 (m,
1 H, H-1), 3.74 (s, 1.8 H, CO2CH3 a), 3.61 (s, 1.2 H, CO2CH3 b),
390 mg (1.41 mmol) of compound (R)-5 were treated according to
general procedure A using 4.5 equivalents of t-butyl diazoacetate
(6 h time of addition) to yield 336 mg (61%) of 16, with identical
NMR data as for 14b. (Found: C, 61.11; H, 6.62; N, 3.54.
C20H25NO7 requires C, 61.37; H, 6.44; N, 3.58%); [a]2D5 +58.6 (c
1.0, CHCl3).
(1S,4R,6R,7R)-2-Oxa-5-azabicyclo[4.1.0]heptane-4,5,7-
tricarboxylic acid 5-benzyl ester 7-t-butyl ester 4-methyl ester
(17)
Compound 17 was obtained from (R)-5 as the minor diastereomer
according to general procedure A, or in higher amounts according
to general procedure B, with identical NMR data as for 15b.
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3.59–3.55 (m, 1 H, H-6), 2.39 (s, 0.6 H, H-7 a), 2.29 (s, 0.4 H,
H-7 b); dC (50 MHz, CDCl3) mixture of rotamers 175.6 and 175.0
(s, CO2), 170.5 and 170.2 (s, CO2), 156.3 and 155.8 (s, NCO2),
135.8 and 135.4 (s, i-Ph), 128.4–127.3 (d, 5 C, Ph), 68.0 and 67.8
(t, CH2Ph), 65.9 and 65.5 (t, C-3), 58.4 and 58.2 (d, C-4), 55.4 and
54.9 (d, C-1), 52.6 (q, CO2CH3), 36.0 and 35.8 (d, C-6), 27.4 (d,
C-7); MS m/z 335 (M+, 0.2), 290 (0.3), 246 (6), 232 (3), 200 (11),
91 (100).
R. S. L. Chang, V. J. Lotti, D. J. Cerino, T. B. Chen, P. J. Kling, K. A.
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(1R,4S,6S,7R)-7-Hydroxymethyl-3-methyl-2-oxa-5-
azabicyclo[4.1.0]heptane-4,5-dicarboxylic acid 5-benzyl ester
4-methyl ester (20)
N-Methylmorpholine (52 lL, 0.47 mmol) and isobutyl
chloroformiate (61 lL, 0.45 mmol) were added, at 0 ◦C, to
a solution of compound 19 (144 mg, 0.43 mmol) in dry THF
(4 mL). After 25 minutes, the white suspension was added
dropwise at −78 ◦C to a suspension of NaBH4 (32 mg, 0.86 mmol)
in THF–MeOH 3 : 1 (4 mL). After 30 minutes at −78 ◦C a
second portion of NaBH4 (32 mg, 0.86 mmol) was added and
the mixture was stirred for another 30 minutes at −78 ◦C and
then was gently warmed to −40 ◦C, until all the mixed anhydride
was consumed (TLC monitoring). The reaction was quenched
with 10% AcOH–H2O (2 mL), diluted with H2O (8 mL), and
extracted with EtOAc. The combined organic extracts were
washed with brine, dried over Na2SO4 and concentrated under
reduced pressure to a residue which was purified by flash
column chromatography (EtOAc–hexanes 3 : 1, then EtOAc)
to yield alcohol 20 (112 mg, 81%). (Found: C, 59.98; H, 5.71;
N, 5.02. C16H19NO6 requires C, 59.81; H, 5.96; N, 4.36%); [a]D25
+74.6 (c 1.1, CHCl3); dH (400 MHz, CDCl3) 1 : 1 mixture of
rotamers a and b 7.40–7.29 (m, 5 H, Ph), 5.29–5.10 (m, 2 H,
CH2Ph), 4.16 (dd, J = 12.0, 3.2 Hz, 1 H, H-3), 4.03 (d, J =
12.0 Hz, 1 H, H-3), 3.83–3.72 (m, 2 H, CH2OH), 3.74 (s, 1.5 H,
CO2CH3 a), 3.64–3.59 (m, 1 H, H-4), 3.58 (s, 1.5 H, CO2CH3 b),
3.26–3.19 (m, 1 H, H-1), 2.76–2.70 (m, 1 H, H-6), 1.82–1.80 (m,
1 H, H-7); dC (50 MHz, CDCl3) mixture of rotamers 170.8 and
170.5 (s, CO2), 156.6 (s, NCO2), 135.8 and 135.6 (s, i-Ph), 128.4–
127.8 (d, 5 C, Ph), 67.8 (t, CH2OH), 66.1 and 65.8 (t, CH2Ph), 61.1
and 60.8 (t, C-3), 56.2 and 55.7 (d, C-1), 55.4 and 54.9 (d, C-4),
52.5 (q, CO2CH3), 30.3 and 30.0 (d, C-6), 28.6 and 28.2 (d, C-7);
MS m/z 303 (M+ − OH, 1), 290 (4), 218 (2), 200 (2), 91 (100).
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Acknowledgements
The authors thank Universita` degli Studi di Firenze, CINMPIS,
Istituto Superiore di Sanita`, and MUR for financial support.
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3336 | Org. Biomol. Chem., 2008, 6, 3328–3336
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The Royal Society of Chemistry 2008
©