Chemistry of vinyl sulfones. Approach to novel conformationally restricted analogues of glutamic acid

Article abstract of DOI:10.1016/j.tet.2004.10.072

A totally different approach to conformationally restricted glutamic acid analogues is described, in which one of the acid functions is replaced by a cyclopropanol. The reactivity of cyclopropanol vinyl sulfones toward addition of lithiated Sch?llkopf bislactim ether provides a facile synthesis of α-amino acid diastereoisomers. Conformational analysis of these analogues, incorporating solvation effects, and docking to a glutamate receptor model, are used to show the relevance of the conformational restrictions employed. Novel conformationally restricted analogues of glutamic acid have been synthetised.

Full text of DOI:10.1016/j.tet.2004.10.072

Tetrahedron 61 (2005) 699–707
Chemistry of vinyl sulfones. Approach to novel conformationally
restricted analogues of glutamic acid
a,
David Dıez, P. Garcıa, Isidro S. Marcos, N. M. Garrido, P. Basabe, Howard B. Broughton
a
a
a
a
b
*
´
´
and Julio G. Uronesa
a
´ ´ ´
Departamento de Quımica Organica, Facultad de Ciencias Quımicas, Universidad de Salamanca, Plaza de los Caidos 1-5,
37008 Salamanca, Spain
bLilly S.A., Avda. de la Industria, 30, 28108 Alcobendas, Madrid, Spain
Received 29 July 2004; revised 21 October 2004; accepted 25 October 2004
Available online 21 November 2004
Abstract—A totally different approach to conformationally restricted glutamic acid analogues is described, in which one of the acid
¨
functions is replaced by a cyclopropanol. The reactivity of cyclopropanol vinyl sulfones toward addition of lithiated Schollkopf bislactim
ether provides a facile synthesis of a-amino acid diastereoisomers. Conformational analysis of these analogues, incorporating solvation
effects, and docking to a glutamate receptor model, are used to show the relevance of the conformational restrictions employed.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
conformational rigidity via inclusion of a methyl group,
suggesting that amino acids such as 1 or 2 could be of
interest (Fig. 1).
S-Glutamic acid (Glu) is the major excitatory neuro-
transmitter in the central nervous system of vertebrates.1
Many strategies have been devised to reduce or eliminate
the conformational flexibility of the natural ligand. Among
these, the most widely used technique is ring insertion.
There has been a vast body of work in this area, with the
synthesis of three to six-membered carbocyclic analogues,
bicyclo[1.1.1]pentane,2 spiro[2.2]pentane,3 bicyclo[2.1.1]-
hexane,4 and compounds built upon the bicyclo[2.2.1]hep-
tane,5 7-azanorbornane6 and 2-azanorbornane7 skeletons.
Among all these analogues, the carboxycyclopropylglycines
(CCGs) represent the most important source of active
analogues for the Glu receptors.8 The vast majority of
analogues described up to the present time have two
carboxylic acids, so in order to obtain a totally new
approach, we decided to replace one of the carboxylic acids
with a cyclopropanol group. This change may also be
justified on the basis that the delta carboxylate of glutamate
is known to interact with at least some of its ionotropic
receptors via a hydrogen bond to an uncharged alcohol
sidechain of serine, in contrast to the alpha carboxylate
which interacts via a salt bridge with an arginine residue and
would probably be more difficult to replace with an
uncharged group. We also planned to provide greater
Figure 1. Novel conformationally restricted analogues of glutamic acid.
2. Results and discussion
Recently, we have described a new methodology for the
synthesis of cyclopropanols9 that leads, by choice of the
appropriate protecting group and double bond stereo-
chemistry of the allyl sulfone, to trans or cis 2-substituted
cyclopropanols (Scheme 1).10,11
The synthesis of amino acid 1 has been described recently10
12
following Schollkopf’s methodology, starting from the
¨
vinyl sulfone 5, which in turn is obtained stereoselectively
from allyl sulfones 3 or 4. In this paper we communicate the
study of the addition of the lithiated bislactim ether to
the vinyl sulfone 6 which presents the cis stereochemistry in
the cyclopropane ring, and describe how following the same
methodology leads to various potential analogues of
glutamate.
Keywords: Cyclopropanes; Allyl and vinyl sulfones; Aminoacids.
* Corresponding author. Tel.: C34 923294474; fax: C34 923294574;
e-mail: ddm@usal.es
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.10.072

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D. Dıez et al. / Tetrahedron 61 (2005) 699–707
700
Scheme 1.
¨
The initial coupling of lithiated Schollkopf’s bislactim ether
with the vinyl sulfone 6 proceeds in the same way as for the
corresponding trans diastereoisomer 5, to give (in this case
with complete diastereoselectivity) only one diastereo-
isomer 8. This stereoselectivity can be understood by
reference to transition states similar to those proposed by
The hydrolysis of the auxiliary was again found to be
difficult. Problems with the deprotection of this auxiliary
have already been reported by Undheim et al.13 When
compound 14 was deprotected with TBAF the cyclo-
propanol 15 was obtained, although in low yield, but if the
crude mixture was treated with MOM chloride the yield of
protected material 16 rose to 88%. Deprotection of the
auxiliary in this case takes place smoothly to give the amino
ester 17, but this could not be deprotected under any
conditions.
¨
Schollkopf for the addition of the lithiated bislactim ether to
a-b unsaturated ester or nitro compounds.12 The stereo-
chemistry of the addition product 8 was established by
examination of the NMR spectra and by means of NOE
experiments (Scheme 2).
In order to circumvent this problem, the strategy for
deprotection was changed. We decided to replace the TBS
group by something that could be hydrolyzed in the
organism but which would be stable to deprotection under
mild acidic conditions, and selected the acetoxy group as an
appropriate example. Compound 14 was deprotected and
acetylated to give acetate 18 in one step. This compound
was submitted to the usual hydrolysis conditions giving the
corresponding amino ester 19 in excellent yield. Trans-
formation to compound 20 is observed in the NMR tube
where the acetoxy group migrates to the neighbouring
amino group. Column chromatography of the mixture gives
a small amount of urethane 21 (Scheme 4).
Deprotection of the auxiliary was more problematic than
expected. Hydrolysis of compound 8 under the usual
conditions gives the required compound 9 in low yield.
When compound 8 was treated with TBAF in THF the
unstable cyclopropanol 10 was obtained. If, after deprotec-
tion of the TBS group, the resulting crude mixture is treated
in situ under hydrolytic conditions, compound 11 was
obtained as the major component in 84% yield. When
shorter reaction times were used, 11 was obtained in 30%
yield along with 12 (60% yield). In order to purify 11, it was
submitted to acetylation. Acetate 13 was isolated
(Scheme 3). The structures of both compounds (12 and
13) were determined by extensive NMR studies.
We have already described our work10 on a comparative
conformational analysis of trans-1 with the conformation of
glutamic acid proposed to be required for activity at group II
metabotropic glutamate receptors (based upon the activities
Due to the problems encountered in the hydrolysis of
compound 8 we decided to desulfonylate first. Desulfonyl-
ation of 8 under the usual conditions gave 14 (Scheme 4).
Scheme 2.

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D. Dıez et al. / Tetrahedron 61 (2005) 699–707
701
Scheme 3.
of conformationally restricted analogues as discussed
above), showing that the cyclopropanol OH group is almost
perfectly superimposed on the d acid oxygen of glutamate.
previously to confirm that the amino acid cis-2 would
superimpose well on the conformation of glutamic acid
found in the crystal structures of model proteins for the
ionotropic glutamate receptors, such as the 1LBC structure
from the protein databank.14 The conformational preference
We have carried out a similar study to that performed
Scheme 4.

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D. Dıez et al. / Tetrahedron 61 (2005) 699–707
702
of both trans-1 and cis-2 was determined using macro-
model15 with the MMFFs forcefield,16 TNCG minimization
with up to 5000 interactions and default convergence
criteria and torsion driving about the C1–CA–CB–CG and
CA–CB–CG–HG torsions (torsion1 and torsion2, respect-
ively) through a full circle with a 158 increment. The GB/SA
water solvation model17 was used and the zwitterionic form
of the amino acid was modeled. The solvation model was
essential, since otherwise the conformational energy land-
scape was dominated by intramolecularly hydrogen-bonded
species, in agreement with literature precedent on
Kainate.18 Using this method, a clear energy minimum
was located at the (torsion1, torsion2) value of (1658, 758)
for both compounds. While this means that the alpha
and beta carbons occupy the same locations relative to the
a-aminoacid function, the alcohol on the cyclopropane
occupies markedly different space (Fig. 2).
and the cyclopropanol cis-2 merged into the structure in its
place. Hydrogen atoms were added to the entire system in
idealized positions and a short molecular dynamics
simulation in which only the hydrogen atoms were allowed
˚
to move, and only the zone of 8 A around the cyclopropanol
cis-2 was considered of interest, was used to remove strain
in the active site. The simulation ran for 500 fs at a
temperature of 10 K with 10 fs coupling, momentum
removal and nonbonded reset times, with velocity scaling.
A further 500 fs of simulation was then done with all atoms
˚
within 5 A of the ligand permitted to move, but the
remainder held rigid. Energy minimization to default
convergence, using MMFF94S as the forcefield, was then
used to produce a model in which the ligand was docked
into the glutamate site. This docking is shown in Figure 3 as
a relaxed-eye stereoview. The pocket is shown as a
separated surface calculated with Molcad in Sybyl, based
˚
upon the ligand atoms and non-water atoms within 5 A of
the ligand. Glutamic acid as observed in the 1LBC crystal
structure is shown with orange carbon atoms, while cis-2 is
shown with green carbon atoms. All the interactions of the
amino acid portion of glutamate with the protein appear to
be reproduced by this ligand, along with a hydrogen bond
from the cyclopropanol OH group to a serine sidechain that
forms an interaction with the delta carboxylate of glutamic
acid in the crystal structure 1LBC.
The structures shown below each graph represent (with
black carbons) the lowest-energy conformation found for
the cyclopropane in question and (with green carbons) a
reference compound with the a-amino acid in the same
orientation. For the trans cyclopropane the conformation-
ally rigid group II metabotropic glutamate receptor ligand
LY 354740 is shown. For the cis cyclopropane glutamic
acid as observed in the crystal structure 1LBC19 from the
protein databank is shown.
Based on these observations, we believe that it is likely that
compounds such as cis-2 may have activity against certain
glutamate receptors, and that they may show selectivity as
between different classes of receptor.
Given the clear similarity between the cis cyclopropanol
and glutamate in the conformation found in the 1LBC
structure (a model for the ionotropic glutamate receptor type
2, iGluR2), we also examined how well the ligand would fit
to the glutamate site of the protein. Thus, the cyclopropanol
in its lowest-energy conformation was superimposed using
the carboxylate and alpha carbons and the amino nitrogen
on the Glu326 glutamate ligand in 1LBC, using Sybyl
version 6.9.20 Glu326 was then excised from the structure
3. Conclusions
In this manner we have opened a new way for the synthesis
of amino acid analogues of glutamic acid using a totally
Figure 2. Conformational preferences of trans-1 and cis-2 energies in kcal/mol.

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D. Dıez et al. / Tetrahedron 61 (2005) 699–707
703
Figure 3. Docking of cyclopropanol cis-2 into the glutamate site of the protein.
different approach, and have devised methods for over-
coming the more difficult deprotection of the cis as
compared to the trans cyclopropane intermediates. While
the molecular modeling studies do not show conclusively
that the ligands we have designed will be ligands for the
glutamate receptors, they do suggest that it may be possible
to develop such ligands using the synthetic technology that
has been discussed here. Both docking of the ligand to the
active site of a relevant protein model and conformational
search support the view that these ligands may be able to act
selectively on subgroups of the glutamate receptors, and the
method used for the conformational analysis demonstrates
the importance of incorporating solvent effects in the study
of such polar systems.
H2O and saturated brine, then dried over anhydrous
Na2SO4, filtered and the solvent removed. The crude was
purified by flash silica column chromatography (hexane–
EtOAc 90:10) to yield 293 mg (0.56 mmol, 95%) of 8 as a
white solid. [a]2D2ZC8.8 (c 1.04, CHCl3); TLC (7:3
hexane/EtOAc) RfZ0.45; IR: 2955, 2859, 1692, 1464,
1445, 1310, 1235, 150, 1040, 1011, 839; 1H NMR
(400 MHz, CDCl3) d 0.07 and 0.08 (2s, 3H each, CH3–
Si), 0.52 (dt, JZ3.2, 6.2, 6.2 Hz, 1H, Hb-300), 0.68 and 0.99
(2d, JZ6.8 Hz, 3H each, (CH3)2CH–), 0.75 (dt, JZ9.2, 6.2,
6.2 Hz, 1H, Ha-300), 0.84 (s, 9H, tBu–Si), 1.16 (m, 1H,
H-100), 2.19 (dhep, JZ3.6, 6.8 Hz, 1H, (CH3)2CH–), 2.60
(m, 1H, H-10), 2.89 (dd, JZ3.2, 14.6 Hz, 1H, HA-20), 3.15
(dd, JZ9.2, 14.6 Hz, 1H, HB-20), 3.44 (dt, JZ3.2, 6.2,
6.2 Hz, 1H, H-200), 3.48 and 3.67 (2s, 3H each, MeO–), 3.93
(t, JZ3.6 Hz, 1H, H-5), 4.27 (t, JZ3.6 Hz, 1H, H-2), 7.53
(m, 2H, Hmeta –SO2Ph), 7.62 (m, 1H, Hpara –SO2Ph), 7.87
(m, 2H, Hortho –SO2Ph); 13C NMR (100 MHz, CDCl3) d
K5.4 and K4.9 (CH3–Si), 14.5 (C-300), 16.8 and 18.8
[(CH3)2CH–], 17.7 [(CH3)3CSi], 18.2 (C-100), 25.4
[(CH3)3CSi], 32.4 [(CH3)2CH–], 36.2 (C-10), 50.0 (C-200),
52.2 and 52.3 (MeO–), 57.6 (C-20), 58.9 (C-2), 61.0 (C-5),
127.9 (Cortho, –SO2Ph), 129.0 (Cmeta, –SO2Ph), 133.1
(Cpara, –SO2Ph), 140.2 (Cipso, –SO2Ph), 162.4 (C-6),
164.5 (C-3); MS: m/z (%) 523 (25) [MC1]C, 141 (70), 73
(100); HRMS: calcd for C26H43N2O5SSi [MC1]C:
523.2662, found 523.2622.
4. Experimental
4.1. General
NMR spectra were recorded in CDCl3 using Varian 200 VX
and Bruker DRX 400 instruments. IR spectra were
registered on a BOMEM 100 FT IR spectrophotometer.
Optical rotations were determined in a Perkin-Elmer 241
polarimeter in 1 dm cells. The electron impact (EI) mass
spectra were run on a VG-TS 250 spectrometer at 70 eV
ionising voltage. HRMS were recorded in a VG Platform
(Fisons) spectrometer using Chemical Ionisation (ammonia
as gas) or Fast Atom Bombardment (FAB) technique. THF
was distilled over Na/benzophenone.
4.1.2. (2S,3R,10R,20R)-2-Amino-4-benzenesulfonyl-3-[20-
(tert-butyl-dimethyl-silanyloxy)-cyclopropyl]-butyric
acid methyl ester (9). To a solution of 8 (63.4 mg,
0.121 mmol) in THF (1 mL) was added HCl 1 M (0.3 mL,
0.30 mmol). The mixture was left to stir for 15 h at room
temperature. Concentrated ammonia was then added to the
stirred mixture until a pH of 9 was reached. The product was
extracted into CH2Cl2, dried over anhydrous Na2SO4,
filtered and the solvent removed. The crude product was
purified by flash silica column chromatography (hexane–
EtOAc 50:50) to yield 16 mg (0.037 mmol, 31%) of 9 as a
colourless oil. TLC (EtOAc) RfZ0.43; 1H NMR (200 MHz,
CDCl3) d 0.06 and 0.09 (2s, 3H each, CH3–Si), 0.12 (dt, JZ
3.2, 6.2, 6.2 Hz, 1H, Hb-30), 0.61 (m, 1H, Ha-30), 0.88 (s,
4.1.1. (2S,5R,10R,100R,200R)-2-{2-Benzenesulfonyl-1-[2-
(tert-butyl-dimethyl-silanyloxy)-cyclopropyl]-ethyl}-5-
isopropyl-3,6-dimethoxy-2,5-dihydro-pyrazine (8). BuLi
1.6 M in hexane (0.78 mL, 1.25 mmol) was added to a
K78 8C solution of R-2,5-dihydro-3,6-dimethoxy-2-iso-
propylpyrazine 7 (220 mg, 1.19 mmol) in dry THF
(4 mL). After 20 min, a solution of 6 (200 mg, 0.59 mmol)
in dry THF (3 mL) was added. After 30 min at K78 8C the
reaction mixture was quenched with saturated NH4Cl
solution (2 mL). The product was then extracted into
EtOAc. The organic extracts were combined, washed with

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D. Dıez et al. / Tetrahedron 61 (2005) 699–707
704
9H, tBu–Si), 1.17 (m, 1H, H-10), 2.23 (m, 1H, H-3), 3.22
(dd, JZ5.2, 14.8 Hz, 1H, HA-4), 3.36 (dt, JZ3.2, 6.2,
6.2 Hz, 1H, H-20), 3.46 (dd, JZ6.2, 14.8 Hz, 1H, HB-4),
3.69 (s, 3H, –COOMe), 3.84 (m, 1H, H-2), 7.50 (m, 3H,
stir for 1 h at room temperature. Concentrated ammonia was
then added to the stirred mixture until a pH of 9 was
reached. The product was extracted into CH2Cl2, dried over
anhydrous Na2SO4, filtered and the solvent removed.
Compounds were separated by flash column chromato-
graphy on silica (hexane–EtOAc 50:50) to yield 25.6 mg
(0.065 mmol, 60%) of 12 and 15.5 mg (0.038 mmol, 35%)
of 11. Compound 12: [a]2D2ZK8.0 (c 1.00, CHCl3); TLC
(EtOAc) RfZ0.40; IR: 3600–3100, 2959, 2930, 1672, 1466,
1447, 1306, 1231, 1148; 1H NMR (400 MHz, CDCl3) d 0.11
(dt, JZ3.2, 6.0, 6.0 Hz, 1H, Hb-300), 0.61 (dt, JZ9.2, 6.0,
6.0 Hz, 1H, Ha-300), 0.93 and 1.00 (2d, JZ6.9 Hz, 3H each,
(CH3)2CH–), 0.96 (m, 1H, H-100), 2.16 (m, 1H, (CH3)2CH–),
2.60 (tt, JZ2.8, 2.8, 10.4, 10.4 Hz, 1H, H-10), 3.30 (dt, JZ
3.2, 6.2, 6.2 Hz, 1H, H-200), 3.36 (dd, JZ2.8, 14.6 Hz, 1H,
HA-20), 3.76 (m, 1H, H-5), 3.78 (s, 3H, MeO–), 4.51 (dd,
JZ10.0, 14.6 Hz, 1H, HB-20), 4.56 (t, JZ2.8 Hz, 1H, H-2),
4.66 (br s, 1H, –OH), 6.45 (d, JZ2.6 Hz, 1H, –NH), 7.56
(m, 2H, Hmeta –SO2Ph), 7.64 (m, 1H, Hpara –SO2Ph), 7.94
(m, 2H, Hortho –SO2Ph); 13C NMR (100 MHz, CDCl3) d
13.7 (C-300), 17.6 and 18.8 [(CH3)2CH–], 19.4 (C-100), 32.4
[(CH3)2CH–], 37.2 (C-10), 49.4 (C-200), 53.9 (MeO–), 57.1
(C-2), 58.4 (C-20), 59.6 (C-5), 127.9 (Cortho, –SO2Ph), 129.2
Hmeta and Hpara –SO2Ph), 7.84 (m, 2H, Hortho –SO2Ph); 13
C
NMR (50 MHz, CDCl3) d K5.0 and K4.7 (CH3–Si), 13.7
(C-30), 18.1 [(CH3)3CSi], 19.4 (C-10), 25.8 [(CH3)3CSi],
37.1 (C-3), 51.2 (C-20), 52.3 (–COOMe), 56.7 (C-2), 57.6
(C-4), 128.3 (Cortho, –SO2Ph), 129.4 (Cmeta, –SO2Ph), 133.7
(Cpara, –SO2Ph), 140.1 (Cipso, –SO2Ph), 176.0 (–COOMe).
4.1.3. (2S,5R,10R,100R,200R)-2-{20-Benzenesulfonyl-10-[200-
hydroxy-cyclopropyl]-ethyl}-5-isopropyl-3,6-dimethoxy-
2,5-dihydro-pyrazine (10). To a solution of 8 (44.8 mg,
0.068 mmol) in THF (0.75 mL) was added tetrabutyl-
ammonium fluoride 1.0 M solution in THF (130 mL,
0.13 mmol). The mixture was left to stir for 2 h. It was
then diluted with EtOAc, washed with H2O and saturated
brine. The organic layer was dried over anhydrous Na2SO4,
filtered and concentrated. Attempts were made to purify the
crude product by flash column chromatography on silica
gel, but cyclopropanol 10 proved unstable and could not be
isolated.
(Cmeta, –SO2Ph), 133.5 (Cpara –SO2Ph), 139.9 (Cipso
,
4.1.4. (2S,3R,20R,100R,200R)-20-[2-Amino-4-benzenesul-
fonyl-3-(200-hydroxy-cyclopropyl)-butyryl amino]-30-
methyl-butyric acid methyl ester (11). To a solution of 8
(23.6 mg, 0.045 mmol) in THF (0.75 mL) was added
tetrabutylammonium fluoride 1.0 M solution in THF
(60 mL, 0.60 mmol). The mixture was left to stir for 1 h.
The reaction was quenched with H2O (0.2 mL) and HCl 1 M
solution (0.13 mL, 0.13 mmol) was then added. It was left to
stir for 3 h at room temperature. Concentrated ammonia was
then added to the stirred mixture until a pH of 9 was
reached. The product was extracted into CH2Cl2, dried over
anhydrous Na2SO4, filtered and the solvent removed. The
crude product was purified by flash column chromatography
on silica (hexane–EtOAc 30:70) to yield 15.6 mg
(0.038 mmol, 84%) of slightly impure compound 11. TLC
(EtOAc) RfZ0.14; IR: 3600–3100, 2965, 1742, 1669, 1516,
–SO2Ph), 163.9 (C-6), 171.0 (C-3); MS: m/z (%) 395 (10)
[MC1]C, 186 (5); HRMS: calcd for C19H27N2O5S [MC
1]C: 395.1641, found 395.1632.
4.1.6. (2S,3R,20R,100R,200R)-20-[3-(200-Acetoxy-cyclopro-
pyl)-2-acetylamino-4-benzenesulfonyl-butyryl amino]-
30-methyl-butyric acid methyl ester (13). To a solution
of 11 (30 mg, 0.073 mmol) in pyridine (1 mL) was added
Ac2O (0.5 mL). The solution was allowed to stir for 21 h.
Ice was then added and the mixture was poured into a
separating funnel and extracted with EtOAc. The organic
extracts were combined, washed with a solution of 2 N HCl,
a solution of NaHCO3 (5%), H2O, and brine, and then dried
over anhydrous Na2SO4, filtered and the solvent removed.
The product was purified by column chromatography
(hexane–EtOAc 50:50) to yield 25.3 mg of 13
(0.057 mmol, 78%). [a]2D2ZC6.2 (c 1.17, CHCl3); TLC
(EtOAc) RfZ0.20; IR: 3500–3200, 2967, 1744, 1663, 1632,
1447, 1375, 1306, 1236, 1148; 1H NMR (400 MHz, CDCl3)
d 0.26 (dt, JZ3.5, 7.1, 7.1 Hz, 1H, Hb-300), 0.95 (m, 1H, Ha-
300), 0.97 and 1.00 (2d, JZ6.8 Hz, 3H each, (CH3)2CH–),
1.30 (m, 1H, H-100), 2.01 and 2.02 (2s, 3H each, CH3CO–),
2.23 (m, 2H, H-3 and (CH3)2CH–), 3.30 (dd, JZ3.6,
14.7 Hz, 1H, HA-4), 3.59 (dd, JZ9.2, 14.7 Hz, 1H, HB-4),
3.73 (s, 3H, –COOMe), 4.24 (dt, JZ3.04, 6.6, 6.6 Hz, 1H,
H-200), 4.44 (dd, JZ4.8, 8.3 Hz, 1H, H-2 ), 5.08 (dd, JZ4.5,
7.7 Hz, 1H, H-2), 6.71 (d, JZ7.6 Hz, NHAc), 7.12 (d, JZ
8.2 Hz, 1H, NHCO), 7.59 (m, 2H, Hmeta –SO2Ph), 7.67 (m,
1H, Hpara –SO2Ph), 7.98 (m, 2H, Hortho –SO2Ph); 13C NMR
(100 MHz, CDCl3) d 10.8 (C-300), 17.2 (C-100), 17.8 and 19.1
[(CH3)2CH–], 20.7 and 23.1 (CH3CO–), 30.7 [(CH3)2CH–],
34.70(C-3), 52.1 (–COOMe), 53.4 (C-200), 54.0 (C-2), 57.6
1
1447, 1306, 1148; H NMR (200 MHz, CDCl3) d 0.10 (m,
1H, Hb-300), 0.67 (m, 1H, Ha-300), 0.94 (m, 1H, H-100), 1.02
(d, JZ6.9 Hz, 6H, (CH3)2CH–), 2.26 (m, 1H, (CH3)2CH–),
3.23 (m, 1H, H-3), 3.36 (dd, JZ3.2, 14.6 Hz, 1H, HA-4),
3.43 (m, 1H, H-20), 3.59 (dd, JZ11.0, 14.6 Hz, 1H, HB-4),
3.76 (s, 3H, –COOMe), 3.75 (m, 1H, H-2), 4.14 (d, JZ
4.8 Hz, 1H, –NH), 4.53 (dd, JZ4.8, 8.0 Hz, 1H, H-20), 7.60
(m, 3H, Hmeta and Hpara –SO2Ph), 7.93 (m, 2H, Hortho
13
–SO2Ph); C NMR (50 MHz, CDCl3) d 14.4 (C-300), 18.2
and 19.6 [(CH3)2CH–], 18.8 (C-100), 31.0 [(CH3)2CH–], 39.0
(C-3), 49.6 (C-200), 52.5 (–COOMe), 54.6 (C-20), 58.0 (C-2),
58.5 (C-4), 127.9 (Cortho, –SO2Ph), 129.8 (Cmeta, –SO2Ph),
134.3 (Cpara, –SO2Ph), 139.4 (Cipso, –SO2Ph), 172.2 and
173.0 (CO).
4.1.5. (2S,5R,10R,100R,200R)-2-[20-Benzenesulfonyl-10-(200-
hydroxy-cyclopropyl)-ethyl]-5-isopropyl-6-methoxy-2,5-
dihydro-1H-pyrazin-3-one (12). To a solution of 8
(56.3 mg, 0.108 mmol) in THF (1 mL) was added tetra-
butylammonium fluoride 1.0 M solution in THF (140 mL,
0.14 mmol). The mixture was left to stir for 1 h. The
reaction was quenched with H2O (0.2 mL) and HCl 1 M
solution (0.35 mL, 0.35 mmol) was then added. It was left to
(C-2 ), 58.3 (C-4), 128.1 (Cortho, –SO2Ph), 129.4 (Cmeta
,
–SO2Ph), 134.0 (Cpara, –SO2Ph), 138.8 (Cipso, –SO2Ph),
169.6 (CONH), 170.2 (NHCOCH3), 171.2 (OCOCH3),
171.6 (–COOMe); MS: m/z (%) 497 (10) [MC1]C, 366
(10), 196 (10); HRMS: calcd for C23H33N2O8S [MC1]C:
497.1958, found 497.1954.

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D. Dıez et al. / Tetrahedron 61 (2005) 699–707
705
4.1.7.
(2S,5R,10R,100R,200R)-2-{10-[200-(tert-Butyl-di-
filtered and concentrated. The crude product was submitted
for MOM protection. It was dissolved in dry CH2Cl2
(1 mL). To the ice-cooled solution, diisopropylethyl amine
(20 mL, 0.109 mmol) and MOMCl (30 mL, 0.40 mmol) were
added. After 1 h 30 min the reaction mixture was quenched
with H2O (0.5 mL). The product was extracted into Et2O.
The organic extracts were combined, washed with H2O and
saturated brine, then dried over anhydrous Na2SO4, filtered
and the solvent removed. The crude was purified by flash
column chromatography on silica (hexane–EtOAc 90:10) to
yield 24.8 mg (0.079 mmol, 88%) of 16 as a colourless oil.
[a]2D2ZK49.0 (c 1.20, CHCl3); TLC (7:3 hexane–EtOAc)
RfZ0.49; IR: 2961, 1697, 1458, 1236, 1047; 1H NMR
(200 MHz, CDCl3) d 0.26 (dt, JZ3.2, 6.0, 6.0 Hz, 1H, Hb-
300), 0.70 and 1.04 (2d, JZ7.0, 6.600Hz, respectively, 3H
each, (CH3)2CH–), 0.75 (m, 1H, Ha-3 ), 0.77 (d, JZ7.2 Hz,
3H, H-20), 1.22 (m, 1H, H-100), 2.02 (m, 1H, (CH3)2CH–),
2.25 (m, 1H, H-10), 3.42 (s, 3H, –CH2OCH3), 3.57 (dt, JZ
3.2, 6.6, 6.6 Hz, 1H, H-200), 3.67 and 3.70 (2s, 3H each,
MeO–), 3.94 (t, JZ3.2 Hz, 1H, H-5), 4.17 (t, JZ3.2 Hz,
1H, H-2), 4.70 (m, 2H, –CH2OCH3); 13C NMR (50 MHz,
CDCl3) d 10.6 (C-300), 14.4 (C-20), 16.9 and 19.3
[(CH3)2CH–], 20.7 (C-100), 32.1 [(CH3)2CH–], 35.5 (C-10),
52.4 and 52.5 (MeO–), 55.4 (C-200), 55.9 (–CH2OCH3), 59.4
(C-2), 60.9 (C-5), 97.1 (–CH2OCH3), 163.7 (C-6 and C-3);
MS: m/z (%) 313 (75) [MC1]C, 281 (30), 183 (25), 141
(100); HRMS: calcd for C16H29N2O4 [MC1]C: 313.2127,
found 313.2143.
methyl-silanyloxy)-cyclopropyl]-ethyl}-5-isopropyl-3,6-
dimethoxy-2,5-dihydropyrazine (14). A solution of 8
(163 mg, 0.312 mmol) in dry MeOH (4 mL) was added to
Na–Hg 5% amalgam (1 g) via cannula. The reaction mixture
was left to stir for 1 h 30 min at room temperature under Ar.
The residue was filtered and diluted with EtOAc. The
mixture was washed with H2O and brine, dried over
anhydrous Na2SO4, filtered and the solvent removed. The
crude product was purified by flash column chromatography
on silica (hexane–EtOAc 95:5) to yield 98 mg (0.256 mmol,
82%) of 14 as a white solid. [a]2D2ZK21.8 (c 0.93, CHCl3);
TLC (7:3 hexane–EtOAc) RfZ0.63; IR: 2959, 2859, 1699,
1
1462, 1366, 1235, 1152, 1045, 1015, 839, 775; H NMR
(200 MHz, CDCl3) d 0.09 (s, 6H, CH3–Si), 0.12 (m, 1H, Hb-
300), 0.62 (dt, JZ9.6, 5.8, 5.8 Hz, 1H, Ha-300), 0.70 and 1.04
(2d, JZ7.0 Hz, 3H each, (CH3)2CH–), 0.74 (d, JZ6.8 Hz,
3H, H-20), 0.89 (s, 9H, tBu–Si), 1.050(m, 1H, H-100), 2.06 (m,
1H, (CH3)2CH–), 2.25 (m, 1H, H-1 ), 3.49 (dt, JZ3.2, 6.2,
6.2 Hz, 1H, H-200), 3.64 and 3.70 (2s, 3H each, MeO–), 3.93
(t, JZ3.2 Hz, 1H, H-5), 4.21 (t, JZ3.2 Hz, 1H, H-2); 13C
NMR (50 MHz, CDCl3) d K5.0 and K4.7 (CH3–Si), 12.9
(C-300), 14.0 (C-20), 16.8 and 19.2 [(CH3)2CH–], 18.1
[(CH3)3CSi–], 19.8 (C-100), 25.8 [(CH3)3CSi–], 32.1
[(CH3)2CH–], 35.6 (C-10), 50.9 (C-200), 52.3 and 52.4
(MeO–), 59.4 (C-2), 60.8 (C-5), 163.6 (C-6), 164.2 (C-3);
MS: m/z (%) 383 (10) [MC1]C, 199 (15), 141 (35), 73
(100); HRMS: calcd for C20H39N2O3Si [MC1]C:
383.2730, found 383.2701.
4.1.10. (2S,3R,10R,20R)-2-Amino-3-(20-methoxymethoxy-
cyclopropyl)-butyric acid methyl ester (17). To a solution
of 16 (24.8 mg, 0.079 mmol) in THF (1.5 mL) was added
HCl 0.5 M (0.5 mL, 0.25 mmol). The mixture was left to stir
for 14 h at room temperature. Concentrated ammonia was
then added to the stirred mixture until a pH of 9 was
reached. The product was extracted into CH2Cl2, dried over
anhydrous Na2SO4, filtered and the solvent removed. The
crude product was purified by flash column chromatography
on silica (CHCl3–MeOH 98:2) to yield 13 mg (0.059 mmol,
75%) of 17. [a]2D2ZK52.3 (c 0.73, CHCl3); TLC (9:1
CHCl3–MeOH) RfZ0.45; IR: 3385, 3320, 2959, 1746,
1441, 1225, 1159, 1045; 1H NMR (200 MHz, CDCl3) d 0.26
(dt, JZ3.2, 6.2, 6.2 Hz, 1H, Hb-30), 0.72 (m, 1H, Ha-30),
0.93 (m, 1H, H-10), 1.03 (d, JZ7.4 Hz, 3H, H-4), 1.82 (m,
1H, H-3), 3.30 (br s, 2H, –NH2), 3.40 (–CH2OCH3), 3.54
(dt, JZ3.4, 6.2, 6.2 Hz, 1H, H-20), 3.72 (m, 1H, H-2), 3.74
(s, 3H, –COOMe), 4.68 (s, 2H, –CH2OCH3); 13C NMR
(50 MHz, CDCl3) d 10.8 (C-30), 15.5 (C-4), 20.7 (C-10),
36.2 (C-3), 52.1 (–COOMe), 54.9 (C-20), 56.0
(–CH2OCH3), 58.5 (C-2), 97.0 (–CH2OCH3), 176
(–COOMe); MS: m/z (%) 218 (60) [MC1]C, 186 (15);
HRMS: calcd for C10H20NO4 [MC1]C: 218.1392, found
218.1378.
4.1.8. (2S,5R,10R,100R,200R)-2-[10-(200-Hydroxy-cyclo-
propyl)-ethyl]-5-isopropyl-3,6-dimethoxy-2,5-dihydro-
pyrazine (15). To a solution of 14 (56.2 mg, 0.147 mmol) in
THF (1 mL) was added tetrabutylammonium fluoride 1.0 M
solution in THF (190 mL, 0.19 mmol). The mixture was left
to stir for 30 min. It was then diluted with EtOAc, washed
with H2O and saturated brine. The organic layer was dried
over anhydrous Na2SO4, filtered and concentrated. The
crude product was purified by flash column chromatography
on silica (hexane–EtOAc 80:20) to yield 15.3 mg
(0.057 mmol, 39%) of 15 as a colourless oil. [a]2D2ZC
14.6 (c 1.12, CHCl3); TLC (7:3 hexane–EtOAc) RfZ0.38;
IR: 3600–3100, 2961, 1694, 1462, 1308, 1236, 1015, 774;
1H NMR (200 MHz, CDCl3) d 0.24 (dt, JZ3.2, 6.2, 6.2 Hz,
1H, Hb-300), 0.70 (m, 1H, Ha-300), 0.72 and 1.05 (2d, JZ6.6,
7.2 Hz, respectively, 3H each, (CH3)2CH–), 0.84 (m, 1H,
H-100), 1.04 (d, JZ7.0 Hz, 3H, H-20), 2.12 (m, 1H,
(CH3)2CH–), 2.24 (m, 1H, H-10), 3.43 (dt, JZ3.0, 6.6,
6.6 Hz, 1H, H-200), 3.71 (s, 6H, MeO–), 3.97 (t, JZ3.6 Hz,
1H, H-5), 4.02 (t, JZ3.6 Hz, 1H, H-2); 013C NMR (50 MHz,
CDCl3) d 12.8 (C-300), 16.6 (C-2 ), 17.1 and 19.3
[(CH3)2CH–], 20.9 (C-100), 32.1 [(CH3)2CH–], 36.4 (C-10),
50.0 (C-200), 52.6 and 52.8 (MeO–), 59.5 (C-5), 61.3 (C-2),
163.8 (C-6), and 165 (C-3).
4.1.11. (2S,5R,10R,100R,200R)-2-[10-(200-Acetoxy-cyclo-
propyl)-ethyl]-5-isopropyl-3,6-dimethoxy-2,5-dihydro-
pyrazine (18). To a solution of 14 (27.3 mg, 0.071 mmol) in
THF (0.75 mL) was added tetrabutylammonium fluoride
1.0 M solution in THF (80 mL, 0.08 mmol). The mixture
was left to stir for 1 h. It was then diluted with EtOAc, and
washed with H2O and saturated brine. The organic layer was
dried over anhydrous Na2SO4, filtered and concentrated.
The crude product was submitted for acetylation. It was
4.1.9. (2S,5R,10R,100R,200R)-5-Isopropyl-3,6-dimethoxy-2-
[10-(200-methoxymethoxy-cyclopropyl)-ethyl]-2,5-di-
hydro-pyrazine (16). To a solution of 14 (34.7 mg,
0.091 mmol) in THF (1 mL) was added tetrabutyl-
ammonium fluoride 1.0 M solution in THF (100 mL,
0.10 mmol). The mixture was left to stir for 45 min. Then,
it was diluted with EtOAc, washed with H2O and saturated
brine. The organic layer was dried over anhydrous Na2SO4,

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D. Dıez et al. / Tetrahedron 61 (2005) 699–707
706
dissolved in pyridine (1 mL) and Ac2O (0.5 mL) was added.
The solution was allowed to stir for 6 h. Ice was then added
and the mixture was poured into a separating funnel and
extracted with EtOAc. The organic extracts were combined,
washed with a solution of 2 N HCl, a solution of NaHCO3
(5%), H2O, and brine, and then dried over anhydrous
Na2SO4, filtered and the solvent removed. The product was
purified by column chromatography (hexane–EtOAc 90:10)
to yield 15.4 mg (0.050 mmol, 70%) of 18. [a]2D2ZC27.3 (c
1.24, CHCl3); TLC (7:3 hexane–EtOAc) RfZ0.55; IR:
2965, 1748, 1697, 1236; 1H NMR (200 MHz, CDCl3) d 0.42
(dt, JZ3.4, 6.2, 6.2 Hz, 1H, Hb-300), 0.68 and 1.04 (2d, JZ
7.0 Hz, 3H each, (CH3)2CH–), 0.76 (d, JZ7.0 Hz, 3H,
H-20), 0.95 (dt, JZ9.6, 6.6, 6.6 Hz, 1H, Ha-300), 1.42 (m, 1H,
H-100), 1.89 (m, 1H, (CH3)2CH–), 2.08 (CH3CO), 2.26 (m,
1H, H-10), 3.67 and 3.69 (s, 3H each, MeO–), 3.94 (t, JZ
3.6 Hz, H-5), 4.05 (t, JZ3.8 Hz, H-2), 4.24 (dt, JZ3.4, 6.6,
6.6 Hz, 1H, H-200)0; 13C NMR (50 MHz, CDCl3) d 10.7
(C-300), 14.1 (C-2 ), 16.8 and 19.3 [(CH3)2CH–], 20.0
(C-100), 21.1 (CH3CO–), 32.0 [(CH3)2CH–], 35.5 (C-10),
52.4 and 52.5 (MeO–), 53.5 (C-200), 59.2 (C-2), 60.8 (C-5),
163.4 (C-6), 163.6 (C-3), 172.5 (CH3CO–); MS: m/z (%)
311 (100) [MC1]C, 267 (25), 183 (25), 141 (80); HRMS:
calcd for C16H27N2O4 [MC1]C: 311.1971, found
311.1962.
4.1.14. (2S,3R,10R,20R)-3-(20-Acetoxy-cyclopropyl)-2-[3-
(20-acetoxy-cyclopropyl)-2-methoxycarbonyl-propyl]-
ureido}-butyric acid methyl ester (21). Compound 21 was
isolated during attempts to purify compound 19. TLC (9:1
1
CHCl3–MeOH) RfZ0.35; H NMR (400 MHz, CDCl3) d
0.38 (m, 2!1H, Hb-30), 0.96 (m, 2!2H, Ha-30 and H-10),
1.06 (d, JZ7.1 Hz, 2!3H, H-4), 1.73 (m, 2!1H, H-3),
2.09 (s, 2!3H, CH3CO), 3.69 (s, 2!3H, –COOMe), 4.25
(dt, JZ3.5, 6.7, 6.7 Hz, 2!1H, H-20), 4.50 (dd, JZ4.4,
9.2 Hz, 2!1H, H-2), 5.31 (d, JZ9.2 Hz, 2!1H, NH); 13C
NMR (100 MHz, CDCl3) d 9.9 (C-30), 16.3 (C-4), 19.0
(C-10), 20.9 (CH3CO), 34.5 (C-3), 51.9 (–COOMe), 53.2
(C-20), 56.5 (C-2), 157.1 (–NHCONH–), 172.7 (CH3CO),
173.1 (–COOMe); MS: m/z (%) 479 (30) [MCNa]C, 457
(15) [MC1]C, 216 (15); HRMS: calcd for C21H33N2O9
[MC1]C: 457.2186, found 457.2203.
Acknowledgements
The authors thank the CICYT (BQU 2001-1034), Junta de
´
Castilla y Leon, for financial support (SA 13-009) and
Universidad de Salamanca for a doctoral fellowship to P. G.
4.1.12. (2S,3R,10R, 20R)-3-(20-Acetoxy-cyclopropyl)-2-
amino-butyric acid methyl ester (19). To a solution of
18 (13 mg, 0.042 mmol) in THF (1 mL) was added HCl
0.5 M (0.2 mL, 0.10 mmol). The mixture was left to stir for
7 h at room temperature. Concentrated ammonia was then
added to the stirred mixture until a pH of 9 was reached. The
product was extracted into CH2Cl2, dried over anhydrous
Na2SO4, filtered and the solvent removed. Compound 19
[8.1 mg (0.038 mmol, 90%)] was obtained. TLC (9:1
CHCl31–MeOH) RfZ0.35; IR: 3300, 2961, 1744, 1373,
1236; H NMR (200 MHz, CDCl3) d 0.44 (dt, JZ3.4, 6.6,
6.6 Hz, 1H, Hb-30), 0.90 (m, 1H, Ha-30), 1.05 (d, JZ7.0 Hz,
3H, H-4), 1.13 (m, 2H, H-10), 1.13 (m, 2H, H-10 and H-3),
2.07 (s, 3H, CH3CO), 3.36 (br s, 2H, –NH2), 3.70 (m, 1H,
H-2), 3.73 (s, 3H, –COOMe), 4.22 (dt, JZ3.4, 6.6, 6.6 Hz,
1H, H-20); 13C NMR (50 MHz, CDCl3) d 10.97 (C-30), 14.8
(C-4), 20.1 (C-10), 21.0 (CH3CO), 35.3 (C-3), 52.5
(–COOMe), 53.1 (C-20), 56.8 (C-2), 172.3 (–COOMe and
CH3CO); MS: m/z (%) 216 (10) [MC1]C, 91 (35); HRMS:
calcd for C10H18NO4 [MC1]C: 216.1236, found 216.1262.
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4.1.13. (2S,3R,10R, 20R)-2-Acetylamino-3-(20-hydroxy-
cyclopropyl)-butyric acid methyl ester (20). Compound
20 was isolated during attempts to purify compound 19.
[a]2D2ZC4.3 (c 0.14, CHCl3); TLC (9:1 CHCl3–MeOH)
RfZ0.23; IR: 3600–3100, 2969, 1744, 1659, 1377; 1H NMR
(200 MHz, CDCl3) d 0.26 (dt, JZ3.4, 6.2, 6.2 Hz, 1H, Hb-
30), 0.51 (dt, JZ9.6, 6.6, 6.6 Hz, 1H, Ha-30), 0.79 (m, 1H,
H-10), 1.13 (d, JZ7.4 Hz, 3H, H-4), 1.64 (br s, 1H, –OH),
1.84 (m, 1H, H-3), 2.02 (s, 3H, CH3CO), 3.52 (dt, JZ3.4,
6.6, 6.6 Hz, 1H, H-20), 3.75 (s, 3H, –COOMe), 4.58 (dd, JZ
5.2, 7.8 Hz, 1H, H-2), 6.60 (br s, 1H, NH); 13C NMR
(50 MHz, CDCl3) d 13.3 (C-30), 17.6 (C-4), 21.0 (C-10),
23.3 (CH3CO), 35.4 (C-3), 50.7 (C-20), 52.4 (–COOMe),
56.8 (C-2), 170.4 (CH3CO), 173.1 (–COOMe); MS: m/z (%)
216 (10) [MC1]C, 178 (10), 95 (40), 69 (80); HRMS: calcd
for C10H18NO4 [MC1]C: 216.1236, found 216.1232.
´
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