Synthesis and evaluation of C8-substituted 4.5-spiro lactams as Glycogen Phosphorylase a inhibitors

Article abstract of DOI:10.1016/j.tet.2012.12.004

An effective synthesis of 4.5-spiro lactams 28/29, has been completed in nine steps with an overall yield of 5.8%. The 4.5-spiro lactams were made from 2-allyl-Cbz-Pro-OMe 21, which was converted into the corresponding alcohol 22 via a hindered borane reaction with (2-methylbutyl)₂borane. Subsequent Swern oxidation of 22 gave novel aldehyde 23. Aldehyde 23 was treated under Bucherer-Bergs reaction conditions to give hydantoin 26, which was opened to the corresponding amino acid 30 using di-tert-butyl dicarbonate and DMAP followed by hydrolysis. Treatment of amino acid 30 with acidic methanol gave 4.5-spiro lactams 28/29. Only 4.5-spiro lactam 29 displayed moderate activity against GPa with an IC₅₀ of 241 μM.

Full text of DOI:10.1016/j.tet.2012.12.004

Tetrahedron 69 (2013) 1576e1582
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis and evaluation of C8-substituted 4.5-spiro lactams as
Glycogen Phosphorylase a inhibitors
,
b
,
a
a
a
Wendy A. Loughlin a , Stephanie S. Schweiker , Ian D. Jenkins , Luke C. Henderson
*
a Eskitis Institute for Cell and Molecular Therapies, Griffith University, Nathan, Brisbane, QLD 4111, Australia
b Science, Environment, Engineering and Technology Group, Griffith University, Nathan Brisbane, QLD 4111, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 July 2012
Received in revised form 9 November 2012
Accepted 3 December 2012
Available online 7 December 2012
An effective synthesis of 4.5-spiro lactams 28/29, has been completed in nine steps with an overall yield
of 5.8%. The 4.5-spiro lactams were made from 2-allyl-Cbz-Pro-OMe 21, which was converted into the
corresponding alcohol 22 via a hindered borane reaction with (2-methylbutyl)2borane. Subsequent
Swern oxidation of 22 gave novel aldehyde 23. Aldehyde 23 was treated under BucherereBergs reaction
conditions to give hydantoin 26, which was opened to the corresponding amino acid 30 using di-tert-
butyl dicarbonate and DMAP followed by hydrolysis. Treatment of amino acid 30 with acidic methanol
gave 4.5-spiro lactams 28/29. Only 4.5-spiro lactam 29 displayed moderate activity against GPa with an
Keywords:
Diazaspiro[4.5]decane
Hydantoin
IC50 of 241 mM.
Ó 2012 Published by Elsevier Ltd.
Lactam
Mimetic
Glycogen Phosphorylase a
1. Introduction
of Glycogen Phosphorylase a, which could derive from unique se-
quences (such as Pro-Ser) of the glycogen targeting subunit of PP1
known as GL.16 Glycogen Phosphorylase is a molecular therapeutic
target for treating hyperglycemia,17 as it plays a critical in the for-
mation of glucose-1-phosphate. Thus generation of a 4.5-spiro
lactam scaffold with a C8 side chain substituent that would allow
further generation of derivatives from both the N1 (proline) and C8
(ester) termini (Fig. 1), and potential formation of extended mi-
metics, was considered. Previous methods for generating 4.5-spiro
lactams have focused on generation of substituents on the lactam
nitrogen. Reports of other Pro-Ser mimetics have been limited to
sugar derived oligomeric peptides18 and (Z)-alkene dipeptides.19
Herein we report a novel synthesis of a (ꢀ)-4.5-spiro lactam
incorporating a C8 substituent introduced via the BucherereBergs
reaction, and the screening of representative compounds against
Glycogen Phosphorylase a.
Spiro lactams have attracted considerable interest as con-
formationally constrained peptidomimetics1,2 including type II0
b-
turn mimetics.3 Initially, 4.4-spiro lactams containing a pyrrolidone
ring were reported.4 More recently, 4.5-spiro lactams have been
described as nonpeptide ligands of the SH3 domain,5 -turn5 or
b
‘g-turn/distorted type II b
-turn’ mimetics,6 inhibitors of myeloid
differentiation factor 88,7 and thyrotropin-releasing hormone.8 The
size and substitution of the lactam ring has been used to restrict the
torsion fiþ1 and jiþ1 angles in the development of conformation-
ally constrained mimetics containing a lactam.4,5,9
Synthetic approaches to 4.5-spiro lactams have included ther-
mal intramolecular ester aminolysis of proline derivatives,10 cy-
cloaddition reactions of aliphatic aldimines with methyl acrylate or
N-methylmaleimide,11 and hydrolysis of diazabicyclodecenes.12
Synthetic approaches towards more complex polycyclic ring sys-
tems incorporating a 4.5-spiro lactam have included amide bond
formation within an alkaloid natural product,13 internal cyclisation
via a carbanion to generate the natural product brevianamide14 and
generation of 4.5.4-spiro bicyclic ring systems.15
4.5-Spiro lactams can be considered as mimetics of Pro-Ser. In
this context 4.5-spiro lactams were targeted as potential inhibitors
* Corresponding author. E-mail address: w.loughlin@griffith.edu.au (W.A.
Loughlin).
Fig. 1. 4.5-Spiro lactam mimetic of Pro-Ser.
0040-4020/$ e see front matter Ó 2012 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.tet.2012.12.004

W.A. Loughlin et al. / Tetrahedron 69 (2013) 1576e1582
1577
2. Discussion
Our initial approach considered the C2 alkylation of N-Boc-Pro-
OBn 10 with N-Cbz-bromomethyl butyrate 4, to install an amino
side chain at C2 of proline for subsequent synthetic transformation.
Ring opening of amino butyrolactone 1 with hydrobromic acid,
gave butyric acid 2 in excellent yield (88%). Esterification of 2 with
thionyl chloride in methanol gave amino bromomethyl butyrate 3,
which was N-Cbz protected to give methyl butyrate 420 in 69%
overall yield from 1 (Scheme 1). Treatment of N-Cbz-bromomethyl
butyrate 4 with sodium iodide in acetone21 gave complete con-
version to the Finkelstein product 5 (crude yield 99%) (Scheme 1),
which was used directly in the attempted coupling with 10.
Scheme 2. (a) Boc2O, Et3N, N2, 12 h, 99%; (b) Cs2CO3 30 min, N2, BnBr, N2, 12 h, 80%; (c)
NaOH, benzylchloroformate, 0 ꢁC, 30 min; (d) Methanol, H2SO4, rt, 16 h, 81% from 8.
desired product. In our hands poor recovery of starting materials
was observed and was attributed to decomposition of iodide 5.
Lack of reaction between the anion of Boc-Pro-OBn 10 and al-
kylation agents 4 and 5, may be due to quenching of the anion by
the carbamate NH proton. To circumvent this, racemic N-dibenzyl
bromomethyl butyrate 6 was generated from amino bromomethyl
butyrate 3 (in 52% yield) through double reductive amination using
benzaldehyde and NaBH(OAc)3 (Scheme 1). Treatment of the anion
of Boc-Pro-OBn 10 (using LiHDMS) with N-dibenzyl bromomethyl
butyrate 6, gave recovered 6 and 10. An alternative acidic proton
source may be the a-proton of 4, 5, or 6. N-Cbz-bromomethyl bu-
tyrate 4 was treated with 1 equiv of LiHDMS then benzyl bromide to
trap and identify the mono-anion of 4. However, cyclopropane 724
was isolated (76%). X-ray analysis of the formed crystals of 7 con-
firmed the structure of cyclopropane 725 (Scheme 1). Formation of
7 occurs through nucleophilic substitution of the bromine atom of 4
by the a-anion. The simple alkylating agents, halides 4, 5 and 6,
were not suitable for reaction with N-Boc-Pro-OBn and a more
activated alkylating agent, such as allyl bromide, was explored next.
Generation of the anion of 10 using LDA at ꢂ78 ꢁC, followed by
addition of allyl bromide gave racemic 2-allyl-Boc-Pro-OBn 15.5
Attempted boraneemethyl sulfide reduction and direct PCC oxi-
dation of 15 to Boc-Pro-OBn propanal 1715 (Scheme 3) gave very
poor yields (<5% of 17, by mass spectrometry, as part of a complex
mixture). Likewise, hydroboration of 15 with borane$THF followed
by basic hydrogen peroxide workup at 0 ꢁC to obtain alcohol 16
resulted in the formation of a complex mixture (Scheme 3). The
unexpected decomposition of 2-allyl-Boc-Pro-OBn 15 led us to
consider the methyl ester group as the carboxyl protecting group for
N-Boc-proline 9, the use of which had been previously reported.26
Treatment of Pro-OMe 11 with di-tert-butyl dicarbonate gave
N-Boc-Pro-OMe 1226,27 (Scheme 2). Generation of the anion of 12
using LiHDMS at ꢂ78 ꢁC, followed byaddition of allyl bromide for 2 h
gave racemic 2-allyl-Boc-Pro-OMe 18.26 (Scheme 3). Hydroboration
of 18 with borane$THF followed by basic hydrogen peroxide workup
Scheme 1. (a) HBr/AcOH, 100 ꢁC, 6 h, 88%; (b) SOCl2, MeOH, N2, 0/22 ꢁC, 16 h, 94%;
(c) Na2CO3, H2O, CH2Cl2, CbzCl, 0/22 ꢁC, 16 h, 84%; (d) NaI, acetone, N2, 22 ꢁC, 16 h,
crude 99%; (e) PhCH]O, 1 h; NaBH(OAc)3, 22 ꢁC, 16 h, 52%; (f) LiN(SiMe3)2, BnBr,
0/22 ꢁC, 16 h, 76%.
Treatment of proline with di-tert-butyl dicarbonate gave N-Boc-
proline 9,22 which was O-benzylated using benzyl bromide in the
presence of caesium carbonate to obtain Boc-Pro-OBn 1023
(Scheme 2).
With bromide 4, iodide 5 and proline 10 in hand, we attempted
to form the spirocyclic centre using the enolate anion of Boc-Pro-
OBn 10. Treatment with LDA at ꢂ78 ꢁC in THF, followed by addi-
tion of N-Cbz-bromomethyl butyrate 4 gave recovered starting
materials. Disappointingly the use of alternate amide bases
(LiHMDS), higher temperatures (0 ꢁC) and the more reactive iodide
analogue of N-Cbz-bromomethyl butyrate 5 failed to give the
Scheme 3. (a) LDA, ꢂ78 ꢁC, N2, 1 h; allyl bromide, ꢂ78/22 ꢁC, N2, 12 h, 22%; (b) boraneeTHF, N2, 1 h; H2O2, water, NaOH, 0 ꢁC, 1 h, 0%; (c) boraneeDMS, N2, 1 h; PCC, DCM, N2, 4 h,
40 ꢁC, <5%; (d) Et3N, N2, 1 h, Boc2O, 12 h; 18, 97%; 21, 99%; (e) 2-methylbutene, boraneeTHF, ꢂ15 ꢁC, N2, 1 h, 18 or 21, ꢂ78/22 ꢁC, N2, 1 h; H2O2, water, NaOH, 0 ꢁC, 1 h; 19, 79%; 22,
80%; (f) DMSO, oxalyl chloride, ꢂ78 ꢁC, N2, 10 min; 19 or 22, ꢂ78 ꢁC, 1 h; Et3N, ꢂ78/22 ꢁC, N2, 30 h; 20, 80%; 23, 66%.

1578
W.A. Loughlin et al. / Tetrahedron 69 (2013) 1576e1582
at 0 ꢁC gave a complex mixture. In contrast, formation of (2-
methylbutyl)2borane in situ from 2-methylbutene and bor-
aneeTHF28 and reaction with 18, followed by basic hydrogen per-
oxide workup at 0 ꢁC gave alcohol 1929 (79%). Alcohol 19 was cleanly
converted into aldehyde 2028 (80%) by Swern oxidation (Scheme 3).
With the key synthon aldehyde 20 in hand, the generation of
a C-8 substituted 4.5-spiro lactam was explored. Treatment of al-
dehyde 20 with potassium cyanide and ammonium carbonate in
a BucherereBergs reaction gave hydantoin 24 (Scheme 4). In some
cases an intermediate amino nitrile (based on 13C NMR spectro-
30. It was noted that 30 was moderately insoluble in toluene (even at
reflux temperatures) therefore heating 30 in methanol in a pressure
vessel at 100, 120 or 140 ꢁC was used to promote cyclization. Disap-
pointingly, only the reaction carried out at 140 ꢁC gave traces of
a mixture of 4.5-spiro lactams 28 and 29, as determined by mass
spectral analysis. Instead, amino acid 30ꢁwas treated with methanol,
ꢀ
molecular sieves (3 A) and heated at 140 C in a sealed reaction vessel.
4.5-Spiro lactams 28 and 29 were obtained in a combined yield of 50%
from N,N-di-Boc hydantoin 27 (Scheme 4). Analytical samples of 4.5-
spirolactams 28and 29wereobtained usingsilicachromatography in
isolated yields of 2% and 13%, respectively. For synthetic purposes,
spiro lactams 28 and 29 were used after purification by alumina
chromatography, and as a diastereomeric mixture.
4.5-Spiro lactams 28 and 29 were formed in a ratio of 16:83,
respectively, as determined by integration of key peaks in the 1H
NMR spectrum. The structures of 28 and 29 were determined by
COSY, HSQC and HMBC and NOESY NMR spectroscopy. Formation of
the spiro lactam ring was confirmed by a key correlation between
scopic evidence; signal at
d 120 ppm) was observed in the product
mixture. This was attributed to insufficient carbon dioxide con-
centration in situ. In these cases, further treatment of a methanolic
solution of the cyanohydrin intermediate with carbon dioxide gas
led to complete conversion to hydantoin 24. Hydantoin 24 was
reacted with di-tert-butyl dicarbonate in the presence of DMAP,
and completion of the reaction was monitored by mass spectrom-
etry. The di-Boc 25 is necessary for the hydrolysis step to be suc-
cessful as the Boc groups promote hydantoin ring opening.
Subsequent direct hydrolysis of 25 was carried out with potassium
hydroxide using a mild two-step procedure.30 Unfortunately,
a complex mixture of products was obtained in which amino acid
31 (typically <5%) was detectable only by mass spectrometry.
Amino acid 31 was unable to be isolated from this mixture, thus use
of an alternate protecting group, CBz, was considered.
C-6 (
d
172.9 ppm for 29;
d 176.1 ppm for 28) and H-8 (d 4.2 ppm for
29;
d
4.18 ppm for 28) in the HMBC spectrum. In the 1H NMR
spectrum of spiro lactam 28, H8 displayed axialeequatorial and
equatorialeequatorial coupling constants of 5.5 and <1 Hz, re-
spectively, which indicated that H8 was in a pseudo equatorial
orientation for 28. In the spectrum of 28 although one set of signals
for an equatorial H8 was observed, two sets of signals for H10ax/
H10eq and OMe were observed. By comparison in the 1H NMR
spectrum of spiro lactam 29, H8 displayed an axialeaxial and
axialeequatorial coupling constants of 11.7 and 4.7 Hz, respectively,
which indicated that H8 was in a pseudo axial orientation for 29.
Again, only one set of signals for an axial H8 but two sets of signals
for H10ax/H10eq and OMe were observed. The duplication of
signals suggested that both 28 and 29 were epimeric mixtures.
The NOESY spectra of 28 revealed correlations between H4 and
H9 (
3.54e3.60 ppm and
and 2.18e2.25/2.26e2.38 ppm), which suggested that the relative
d
2.09e2.18 ppm and
d
2.18e2.25 ppm); H2 and H3 (
d
d
1.84e2.06 ppm) and H8 and H9/H9 (
d
4.14 ppm
d
orientation of the pyrroline N and the ester groups were cis in 28
(Fig. 2a). The NOESY spectra of 29 revealed correlations between H2
and H10 (
d
3.54e5.61 ppm and
d 1.89e2.02 ppm) and H8 and H10 (d
4.24 ppm and
d
2.14e2.32 ppm) (Fig. 2b), but the relative stereo-
chemistry of 29 could not be unambiguously determined. However,
if the relative orientation of the N-Cbz and ester groups are cis in 28,
it follows that they must be trans in 29. This is consistent with the
relative amounts of 28 and 29 formed. It would be expected that the
trans isomer would be the more stable and therefore the major
isomer formed.
Scheme 4. (a) (NH4)2CO3, KCN, rt, 48 h; 24, 60%; 26, 86%; (b) Boc2O, DMAP, 0 ꢁC, 16 h;
25, 100%; 27, 99%; (c) KOH, rt, 20 h, 31, 0%; 30, 40%; (d) methanol, activated sieves,
140 ꢁC, 16 h, N2, 40%.
Prolinewas treatedwith benzylchloroformatein basetogenerate
Cbz-Pro-OH 13,31 treatment of 13 with catalytic sulfuric acid in
methanol generated Cbz-Pro-OMe 1432 (Scheme 2). Generation
of the anion of 14, followed by addition of allyl bromide gave
racemic 2-allyl-Cbz-Pro-OMe 2133 (Scheme 3). Reaction of (2-
methylbutyl)2borane generated in situ with 21, followed by basic
hydrogen peroxide workup at 0 ꢁC gave alcohol 2229 in excellent
yield (80%). Alcohol 22 was cleanly converted into the previously
unknown aldehyde 23 (66%) by Swern oxidation (Scheme 3).
Treatment of aldehyde 23 under the BucherereBergs reaction con-
ditions gave hydantoin 26. Reaction of 26 with di-tert-butyl dicar-
bonate and DMAP gave N,N-di-Boc hydantoin 27 (Scheme 4).
Hydrolysis of 27 with potassium hydroxide followed by an acidic
workup gave amino acid 30 in a modest but synthetically usable
yield (40%).
Fig. 2. Representation of (a) 4.5-Spiro lactams (ꢀ)-28 and (b) (ꢀ)-29. Arrows indicate
NOESY correlations obtained at 300 MHz in CDCl3.
In order to demonstrate the potential for generation of de-
rivatives via peptide chemistry, both C and N termini must be or-
thogonally protected. Treatment of spiro lactams 28 and 29 with
palladium-on-carbon, in methanol under an atmosphere of
It was envisaged that lactamisation should occur via heating only.
However heating 30 in toluene returned unreacted starting material

W.A. Loughlin et al. / Tetrahedron 69 (2013) 1576e1582
1579
hydrogen gas, gave awhite intractable gum. Variation of the reaction
conditions to include catalytic sulfuric acid resulted in the formation
of spiro lactam ester 32 (85%) (Scheme 5). Spiro lactams 28 and 29
were treated with barium hydroxide and carbon dioxide, upon
workup spiro lactam acid 33 was isolated in 60% yield (Scheme 5).
were discrete compounds and that compound 29 is the active
component bioactivity of the individual compound. The variance in
activity for 28/29 as compared to the individual compounds 28 and
29 was interesting, and suggests that the stereochemical orientation
of key functional groups on the spiro scaffold influenced the ob-
served activity. From the Lipinski calculations it was noted that there
were no correlations between inhibition levels and properties of
TPSA, lipophilicity (LogP) and solubility (LogS) (Table 1). This is il-
lustrated by the comparison of compounds 28 and 29 (Table 1).
4. Conclusion
Spiro lactams 28/29 were readily generated in a nine step pro-
cess from 8 in overall yield of 5.8%. Spiro lactams 28/29 could be
selectively deprotected to allow for further derivatives or in-
corporation into a parent peptide. Spiro lactam 28 displayed
moderate activity against RMGPa. This suggests future exploration
of N- and C8-substituted spiro lactam 28 may be of interest in the
development of potential inhibitors of GPa.
Scheme 5. (a) Methanol, H2SO4, Pd/C, H2, 48 h, 85%; (b) Ba(OH)2eH2O,1 h, CO2,1 h, 60%.
The formation of spiro methyl ester 32 and spiro acid 33 achieved
selectivedeprotection of4.5-spirolactams28and 29, unmaskingthe
carboxylic acid or amine groups. Extension of the spiro lactam
scaffold through either the carboxylic acid group of 33 and amine
group of 32 could lead to the generation of further spiro derivatives,
and is currently being examined as part of further work.
5. Experimental
5.1. General procedure
Starting materials were obtained commercially
a-amino-g-
butyrolactone hydrobromide 1 were purchased from Aldrich. All
solvents were dried by standard methods. 1H NMR (400 MHz) and
13C NMR (100 MHz) spectra were recorded in the deuterated sol-
3. Bioassay
vents indicated. Chemical shifts are reported in parts per million (
units) relative to the reported solvent as an internal standard (e.g.,
CDCl3
13C, 77.0 ppm)). 1H NMR and 13C NMR spectra spectral as-
signments are supported by gCOSY, gHSQC and gHMBC analysis.
Mass spectral data, LRMS were obtained by electrospray ionization
(ESI). For preparative scale chromatography silica gel
(60e200 mesh) was used. Melting points are uncorrected.
d
Spiro lactams 28, 29, 32 and 33 were screened against RMGPa
using an inhibition assay measured in the direction of glycogen
synthesis. Caffeine was used as the internal standard in the assay.
Benchmarking each assay against caffeine allowed some conclu-
sions to be drawn. The GPa assay results (Table 1) showed that the
(
mixture of spiro lactams 28/29 gave an IC50 of 7.43
standard 288 M).
mM (caffeine
m
When the Cbz group (32) or the methyl protecting group (33)
were removed all activity was lost. By comparison isolated spiro
5.2. Experimental for known compounds
lactam 29 gave an IC50 of 241
hibition of GPa (against the comparable caffeine standards). The
amino acid 30 gave an IC50 of 397 M.
mM whereas 28 displayed no in-
See Supplementary data.
m
Analytical HPLC of 30 and 28/29 were carried out in an aqueous
solvent system analogous to the bioassay conditions. Amino acid 30
(tR¼6e7 min) and spiro lactams 28/29 (tR¼5.2, 5.5 min) were run in
a methanol water gradient. Co-spiking of 28/29 and 30 (60/40) gave
an overlay of the two components with no correlation of 30 with 28/
29. To confirm these results the 13C NMR spectra of 30 and 28/29 in
methanol were crosschecked for purityas the 1H NMR spectrum had
strong overlap of key signals. The 13C NMR spectra showed that the
key peaks of C2, C4, and C3 of the amino acid 30 were not present in
the spiro lactams 28/29 spectrum. This confirmed that 30 and 28/29
5.3. Experimental for compounds
5.3.1. (S)-Methyl 4-bromo-2-(dibenzylamino)butanoate (6). Benzal-
dehyde (223 mg, 2.10 mmol, 2.5 equiv) was added to a solution of the
free amine of 3 (166 mg, 0.850 mmol) in CH2Cl2 (6 mL) and the so-
lution stirred for 1 h. Sodium triacetoxy borohydride (445 mg,
2.10 mmol, 2.5 equiv) was added and the suspension stirred at rt for
16 h. Sodium carbonate (saturated aqueous solution, 10 mL) was
added and the biphasic suspension stirred for 1 h. The reaction was
diluted with water (5 mL) then extracted with ethyl acetate
Table 1
GPa (RMGPa) assay results and Lipinski calculations for compounds 28e30, 32 and 33
#
MW
GPa no inhibition
LogP
LogS (gL)c
# ROTB
# ON
# OH NH
TPSA
# Lipinski
violations
(ni)a or IC50 M)b
(m
28
29
30
32
33
28/29
346.38
346.38
364.398
212.24
332.35
346.38
nie
1.51(ꢀ0.39)
ꢂ2.69(ꢀ0.43)
ꢂ2.69(ꢀ0.43)
ꢂ2.63(ꢀ0.19)
ꢂ0.90(ꢀ0.93)
ꢂ2.39(ꢀ0.23)
ꢂ2.69(ꢀ0.43)
5
5
8
2
4
5
7
7
9
5
7
7
1
1
3
2
2
1
84.946
84.946
119.17
67.43
95.94
84.946
0
0
0
0
0
0
241e
397d
nif
1.51(ꢀ0.39)
ꢂ0.41(ꢀ1.05)
ꢂ0.39(ꢀ0.68)
1.00(ꢀ0.65)
1.51(ꢀ0.39)
nif
7.43f
a
Average values from four determinations (duplicates on two separate occasions) with a Hill slope between 0.5 and 3.0.
Estimated IC50 reported with % GPa inhibition observed at maximal concentration.
Calculated from LogS value determined from ALOGPS 2.1.
Caffeine standard 493
Caffeine standard 543
Caffeine standard 288
b
c
d
e
f
m
m
m
M.
M.
M.

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W.A. Loughlin et al. / Tetrahedron 69 (2013) 1576e1582
(3ꢃ10 mL), brine (10 mL), dried (MgSO4, anhydrous). The solvent was
removed in vacuo to give a colourless oil (346 mg). The crude oil was
retreated with sodium triacetoxy borohydride (445 mg, 2.10 mmol)
in CH2Cl2 (6 mL) and stirred at room temperature for 16 h. The so-
lution was worked up as above and the crude product obtained as
a colourless oil (322 mg). Purification by silica gel chromatography
(ethyl acetate/hexane/triethylamine 29:70:1) gave N-dibenzyl bro-
momethyl butyrate 6 (166 mg, 52%) as a colourless oil; [found: C
60.95; H 6.16; N 3.78. C19H22BrNO2 requires C, 60.65; H 5.89; N
3.72%]; Rf (ethyl acetate/hexane/triethylamine 29:70:1) 0.42; nmax
59.5, and 59.3 (C400), 52.9, 52.9, and 52.7 (OCH3), 49.9, 49.8, 49.7, and
49.7 (C5), 38.3, 38.3, 37.3, and 37.1 (C10), 31.0, 30.7, 30.5, and 30.3 (C3),
28.7, 28.6, and 28.5 ((CH3)3), 27.4, 27.2, 27.0, and 26.5 (C20), 24.2, 24.1,
and23.5(C4);LRMS(ESI)m/z378 (100%, MNaþ), HRMS (FTICR): MNaþ,
found 378.1632. C16H26N3NaO6 requires 378.1641.
5.3.4. Di-tert-butyl 4-(20-(100-(tert-butoxycarbonyl)-200-(methoxycarb-
onyl)pyrrolidin-200-yl)ethyl)-20,50-dioxoimidazolidine-10,30-dicarbox-
ylate (25). DMAP (2.5 mg, 0.022 mmol, 0.05 equiv) was added to
a solution of 24 (150 mg, 0.423 mmol, 1.0 equiv) in THF (anhydrous,
5 mL) under a nitrogen atmosphere. The reaction was stirred, cooled
to 0 ꢁC and di-tert-butyl dicarbonate (0.461 g, 2.15 mmol, 5.0 equiv)
was added. After16 h thesolventwasremoved invacuo. The crudeoil
was purified by silica gel column chromatography (DCM/ethyl ace-
tate, 50:50) to give 25 as a gum (234 mg, 60%), which was used di-
rectly in the next step (see Supplementary data); Rf (DCM/ethyl
acetate, 50:50) 0.38; nmax (Nujol) dec; dH (400 MHz, CD3OD) 1.30 (6H,
s, 100-NCO2C(CH3)3), 1.30 (3H, s, 100-NCO2C(CH3)3) 1.50e1.52 (18H, m,
1-NCO2C(CH3)3, 3-NCO2C(CH3)3) 1.72e1.81 (2H, m, H300, H20),
1.81e2.02 (3H, m, H400, H20), 2.02e2.24 (2H, m, H10), 2.24e2.38 (1H,
m, H300), 3.34e3.44 (1H, m, H500), 3.56e3.66 (1H, m, H500), 3.66e3.73
(neat) 3027, 2949 (br), 1733, 1457, 1155 cmꢂ1
; dH (400 MHz, CDCl3)
2.15e2.35 (2H, m, 2ꢃ H3), 3.34e3.50 (2H, m, 2ꢃ H4), 3.56e3.60 (3H,
m, H2, CH2C6H5), 3.79 (3H, s, OCH3), 3.89 (2H, d, J 13.6 Hz, 2ꢃ
CH2C6H5), 7.24e7.29 (2H, m, 2ꢃ C6H5), 7.33e7.37 (8H, m, C6H5); dC
(100 MHz, CDCl3) 172.6 (C]O), 139.2 (2ꢃ ipso C6H5), 129.1 (4ꢃ ortho
C6H5), 128.5 (4ꢃ meta C6H5), 127.4 (2ꢃ para C6H5), 59.5 (C2), 54.9 (2ꢃ
CH2C6H6), 51.5 (OCH3), 33.0 (C3), 30.2 (C4); LRMS (ESI) m/z 376 (100,
MHþ), 378 (97%, MHþ).
5.3.2. 1-tert-Butyl 2-benzyl 2-allylpyrrolidine-1,2-dicarboxylate
(15). Boc-Pro-OBn 10 (100 mg, 0.327 mmol, 1.0 equiv) was added
dropwise to a solution of LDA (360
m
L, 0.721 mmol, 2.2 equiv) in dry
(3H, m, OCH3), 4.55e4.65 (1H, m, H4); dC (100 MHz, CD3OD) d: 176.3,
THF (4 mL) under an atmosphere of nitrogen at ꢂ78 ꢁC. The reaction
and 176.0 (CO2CH3), 172.8, and 171.9 (C2), 155.4, 155.4, 155.3, and
155.2 (100-NC(O)), 152.0, and 151.7 (1-NHC(O)), 151.0, and 150.8 (3-
NHC(O)), 87.8, 87.6, 87.5, and 87.4 (100-NCO2C(CH3)3), 83.7, 83.7,
83.7, and 83.6 (1-NCO2C(CH3)3), 82.3, 82.0, 82.0, and 81.8 (3-
NCO2C(CH3)3), 68.8, 68.6, 68.6, and 68.4 (C200), 61.8, and 61.5 (C4),
52.8, 52.8, 52.8, and 52.7 (OCH3), 49.9, 49.8, and 49.6 (C500), 38.3, and
38.1 (C10), 33.4, and 32.9 (C300), 28.9, 28.7, 28.6, 28.6, 28.4, 28.4, 28.3,
28.3, 28.2, 28.1, and 28.1 (1-NCO2C(CH3)3, 3-NCO2C(CH3)3, 100-
NCO2C(CH3)3), 25.8, and 25.3 (C20), 24.1, 23.7, 23.6, and 23.5 (C400);
LRMS (ESI) m/z 556 (100%, MHþ), HRMS (FTICR): MꢂH, found
554.2796. C26H40N3O10 requires 554.2719.
wasstirredfor1h, warmedto0ꢁC,andstirredfor5min. Allylbromide
(33 mL, 0.393 mmol,1.2 equiv) was added and the reaction stirred at rt
for 12 h. The reaction was diluted with water (5 mL) and extracted
with ethyl acetate (3ꢃ10 mL). The combined organic layers were
washedwithbrine (10 mL), dried (MgSO4, anhydrous)and thesolvent
removed in vacuo to give clear oil. Purification of the oil by silica gel
column chromatography (hexane/ethyl acetate/Et3N, 89:10:1) gave
15 (25 mg, 22%) as a colourless oil. [Found: C 69.40; H 8.09; N 3.80.
C20H27NO4 requires C, 69.54; H 7.88; N 4.05%]; Rf (hexane/ethyl ace-
tate/Et3N, 89:10:1) 0.29; nmax (film) 2976 (br), 1740, 1697, 1391,
1166 cmꢂ1
;
dH (400 MHz, CDCl3):1.36 (6H, s, 2ꢃ CH3),1.42 (3H, s, CH3),
1.76e1.85 (2H, m, H3), 2.02e2.11 (2H, m, H4), 2.54e2.70 (1H, m, H10),
2.95 and 3.15 (1.0 H, dd, J 4.2, 9.6 Hz, H10), 3.24e3.42 (1H, m, H5),
3.52e3.72 (1H, m, H5), 5.05e5.22 (4H, m, CH2C6H5, H30), 5.70e5.80
(1H, m, H20), 7.25e7.37 (5H, m, C6H5); dC (100 MHz, CDCl3) 174.4 (C(O)
OCH2C6H5), 152.0 (C(O)OC(CH3)3), 136.5 (i-C6H5), 133.8 (C20), 129.5
(m-C6H5), 128.3 (p-C6H5), 127.6 (o-C6H5), 119.2 (C30), 80.3 (C(CH3)3),
67.2 (C2), 66.8 (CH2C6H5), 48.8 (C5), 39.5 (C10), 37.4 (C3), 28.5 (3ꢃ
CH3), 22.9 (C4); LRMS (ESI) m/z 346 (100, MHþ), 352 (100%, MLiþ).
5.3.5. 1-Benzyl 2-methyl 2-(2-formylethyl)pyrrolidine-1,2-dicarbo-
xylate (23). A solution of DMSO (4.60 mL, 64.5 mmol, 3 equiv) in
DCM (25 mL, anhydrous) was added to a solution of oxalyl chloride
(2.80 mL, 32.3 mmol, 1.5 equiv) in DCM (anhydrous, 25 mL) at
ꢂ78 ꢁC over a period of 10 min. The reaction was stirred at ꢂ78 ꢁC
for 10 min. Compound 22 (6.90 mL, 21.5 mmol, 1 equiv) in DCM
(100 mL, anhydrous) was added slowly at ꢂ78 ꢁC and the reaction
stirred for 1 h at ꢂ78 ꢁC. Et3N (14.96 mL, 107.5 mmol, 5 equiv) was
added. The reaction was warmed to rt then stirred for 30 h. The
reaction was diluted with water (50 mL), and the aqueous layer
separated. The organic layer was washed with aqueous HCl (20 mL,
10%), brine (50 mL), dried (MgSO4, anhydrous) and the solvent was
removed in vacuo. Purification of the crude oil by silica gel column
chromatography (hexane/ethyl acetate, 50:50) gave 23 (4.543 g,
66%) as a colourless tacky gum; Rf (DCM/ethyl acetate, 50:50) 0.58;
nmax (Nujol) dec; dH (400 MHz, CDCl3) 1.70e2.00 (2H, m, H4),
2.00e2.10 (2H, m, H3), 2.10e2.30 (1H, m, H10), 2.30e2.50 (2H, m,
H20, H10), 2.50e2.60 (1H, m, H20), 3.40e3.45 (2H, m, H5), 3.60 (3H,
s, OCH3), 4.75e5.20 (2H, m, OCH2), 7.23 (5H, s, C6H5), 9.58 (1H, s,
H30); dC (75 MHz, CDCl3) 201.8, and 201.1 (C30), 174.1, and 173.9
(CO2CH3), 154.5, and 154.2 (1-NCO2CH2), 136.5, and 135.9 (i-C6H5),
128.3, 128.2, 128.1, 127.9, 127.7, and 127.4 (o, m, p-C6H5), 67.8, and
67.1 (CH2C6H5), 67.0, and 66.7 (C2), 52.2, and 52.1 (OCH3), 48.9, and
48.1 (C5), 39.0, and 38.7 (C3), 37.3, and 36.2 (C20), 27.5,þand 27.0
(C10), 23.0, and 22.5 (C4); LRMS (ESI) m/z 342 (100%, MNa ), HRMS
(FTICR): MNaþ, found 342.1353. C17H21NNaO5 requires 342.1317.
5.3.3. 1-tert-Butyl 2-methyl 2-(20-(200,500-dioxoimidazolidin-400-yl)
ethyl)pyrrolidine-1,2-dicarboxylate (24). Compound 20 (0.587 g,
2.06 mmol, 1 equiv) was dissolved in methanol (5 mL), and water
(3 mL), then placed under a nitrogen flow. (NH4)2CO3 (99 mg,
1.03 mmol, 5 equiv) was added and the reaction stirred until the car-
bonate dissolved. KCN (aqueous solution, 2.21 M, 1.12 mL, 1.2 equiv)
was added dropwise over 10 min. The reaction was sealed with a glass
stopperandstirred atrt for 48h. The solvent wasconcentratedinvacuo
to give an oil. The oil was washed with water (10 mL), and the water
layer was extracted with DCM(3ꢃ30 mL). The combined organiclayers
were washed with brine (50 mL), dried (MgSO4, anhydrous) and the
solvent was removed in vacuo. Purification of the clear oil by silica gel
column chromatography (DCM/ethyl acetate, 60:40) gave 24 (439 mg,
60%) as a colourless tacky gum and as a diastereomeric rotamer mix-
ture in a 19:16:34:31 ratio. Rf (DCM/ethyl acetate, 60:40) 0.17; nmax
(Nujol) dec; dH (400 MHz, CD3OD) 1.40 (6H, s, (CH3)3), 1.47 (3H, s,
(CH3)3), 1.52e1.81 (2H, m, H3, H20), 1.81e2.02 (3H, m, H4, H20),
2.02e2.24 (2H, m, H10), 2.24e2.38 (1H, m, H3), 3.34e3.44 (1H, m, H5),
3.56e3.66 (1H, m, H5), 3.66e3.73 (3H, m, OCH3), 4.06e4.20 (1H, m,
H400), 7.10 (1H, m, NH300), 9.50 (1H, s, NH100); dC (100 MHz, CD3OD)
177.9, 177.8, 177.7, and 177.6 (500C(O)), 176.4, 176.3, and 176.3 (CO2Me),
160.0, 159.9, and 159.8 (200C(O)), 155.9, 155.7, 155.5, and 155.5 (NCO2),
82.0, 81.9, 81.2, and 81.2 (C(CH3)3), 68.9, 68.7, and 68.6 (C2), 59.8, 59.6,
5.3.6. 1-Benzyl 2-methyl 2-(20-(200,500-dioxoimidazolidin-400-yl)ethyl)
pyrrolidine-1,2-dicarboxylate (26). (NH4)2CO3 (14.3 g, 90.8 mmol,
11 equiv) was added to compound 23 (2.63 g, 8.25 mmol,1 equiv) in
methanol (20 mL) and water (17.5 mL) under a flow of nitrogen gas.
The reaction was stirred until a homogenous solution was obtained.

W.A. Loughlin et al. / Tetrahedron 69 (2013) 1576e1582
1581
Aqueous KCN (2.21 M, 4.50 mL, 1.2 equiv) was added dropwise over
10 min. The reaction was sealed with a glass stopper and stirred at
rt for 84 h. The solvent was concentrated in vacuo to give an oil,
which was washed with water (20 mL). The organic layer was
extracted with DCM (3ꢃ60 mL). The combined organic layers were
washed with brine (50 mL), dried (MgSO4, anhydrous) and the
solvent was removed in vacuo. Purification of the crude oil by silica
gel column chromatography (hexane/ethyl acetate, 27:73) gave 26
(3.2 g, 86%) as a colourless tacky gum; Rf (hexane/ethyl acetate,
27:73) 0.36; nmax (Nujol) dec; Conformer 1 (30%) dH (400 MHz,
CDCl3) 1.60e1.70 (1H, m, H4), 1.70e1.90 (3H, m, H4, H20), 1.90e2.10
(3H, m, H3, H10), 2.22e2.35 (1H, m, H3), 3.40 (2H, m, H5), 3.60 (3H,
s, OCH3), 3.89e4.05 (1H, m, H400), 4.90e5.20 (2H, m, OCH2), 6.69
(1H, d, J 16 Hz, 300-NH), 7.20e7.30 (5H, m, C6H5), 9.35 (1H, s, 100-NH);
Conformer 2 (70%) dH (400 MHz, CDCl3) 1.60e1.70 (1H, m, H4),
1.70e1.90 (3H, m, H4, H20), 1.90e2.10 (3H, m, H3, H10), 2.22e2.35
(1H, m, H3), 3.40e3.45 (2H, m, H5), 3.60 (3H, s, OCH3), 3.89e4.05
(1H, m, H4), 4.90e5.20 (2H, m, OCH2), 6.95 and 7.05 (1H, br s, 300-
NH), 7.20e7.30 (5H, m, C6H5), 9.40 (1H, 1 s, 100-NH); dC (100 MHz,
CDCl3) 175.7, 175.7, and 175.3 (C500), 174.9, 174.7, and 174.7,
(CO2CH3), 158.3, 158.2, and 158.1 (CO2CH2C6H5), 155.0, 154.9, and
154.7 (C200), 136.8, 136.8, and 136.4 (i-C6H5), 128.7, 128.7, 128.7,
128.1, 127.8, and 127.8 (o, m, p-C6H5), 68.3, and 68.3 (C2), 67.4, and
67.4 (CH2C6H5), 59.0, 58.8, 58.7, and 58.6 (OCH3), 52.7, 52.7, and 52.5
(C400), 50.6, 49.5, 49.3, and 48.6 (C5), 37.6, 37.5, 36.3, and 36.0 (C3),
30.7, 30.5, 30.2, and 29.7 (C4), 26.8, 26.3, 26.2, and 25.8 (þC20), 23.3,
23.3, and 22.9 (C10); LRMS (ESI) m/z 390 (100%, MH ), HRMS
(FTICR): MꢂH, found 388.1530. C19H22N3O6 requires 388.1514.
for 20 h, then diluted with water (15 mL) and ethyl acetate
(15 mL). The aqueous layer was washed with ethyl acetate
(3ꢃ30 mL), then neutralized with aqueous citric acid (25%) to pH
3. The aqueous layer was extracted with ethyl acetate (5ꢃ80 mL).
The combined organic layers were washed with brine (50 mL),
dried (MgSO4, anhydrous) and the solvent was removed in vacuo.
Compound 30 (178 mg, 40%) was obtained as a yellow gum and
purified by reverse phase-HPLC (gradient: 10% solution A (0.1% TFA
in MeOH) and 90% solution B (0.1% TFA in millQ water) to 100%
solution B 0% solution A over 6.5 min, tR¼6e7 min); nmax (Nujol)
dec; dH (400 MHz, CDCl3) 1.60e1.79 (1H, m, H40), 1.79e1.96 (2H, m,
H40, H3), 1.96e2.06 (2H, m, H30, H3), 2.06e2.20 (2H, m, H30, H4),
2.20e2.45 (1H, m, H4), 3.40e3.55 (1H, m, H50), 3.62e3.78 (1H, m,
H50), 3.92e4.15 (4H, m, H2, OCH3), 5.00e5.18 (2H, m, OCH2), 5.30
(2H, s, NH2) 7.20e7.40 (5H, m, C6H5), 10.76 (1H, s, CO2H); dC
(100 MHz, CDCl3) 178.2, 178.1, and 178.1 (CO2CH3), 176.0, 176.5, and
175.9 (C1), 157.2, 157.1, 156.8, and 156.7 (10-NCO2CH2), 138.8, 138.8,
and 138.2 (i-C6H5), 130.2, 129.7, 129.6, 129.6, 129.4, 129.4, and
129.3 (o, m, p-C6H5), 70.1, and 69.6 (C20), 69.5, and 69.1 (CH2C6H5),
60.4, 60.3, 60.2, and 60.2 (C2), 51.8, 51.8, and 51.5 (OCH3), 51.1,
50.3, and 50.1 (C50), 39.2, 39.2, 37.9, and 37.8 (C30), 29.4, 28.9, 28.0,
and 27.9 (C3), 24.7, 24.6, and 24.1 (C40), 21.6, and 21.4 (C4); LRMS
(ESI) m/z 351 (100%, MHþ), HRMS (CI): (MꢂCH3þH2)þ, found
351.1556. C17H23N2O6 requires 351.1556.
5.3.9. 1-Benzyl 8-methyl 6-oxo-1,7-diazaspiro[4.5]decane-1,8-
dicarboxylate (28, 29). A solution of 27 (542 mg, 0.920 mmol) in
ꢀ
methanol (2 mL, anhydrous) and a 4 A molecular sieve (1 bead) was
stirred at 140 ꢁC for 16 h under an atmosphere of nitrogen. The re-
action was diluted with methanol (5 mL), filtered, and the solvent
was removed in vacuo to give a yellow oil (214 mg, 40%). Purification
of the yellow gum by silica gel column chromatography (methanol/
ethyl acetate, 20:80) gave analytical samples of compound 28
(70 mg, 13%) as a gum and as two conformers and compound 29
(10 mg, 2%) as a gum and as two conformers. Alternatively, purifi-
cation of the yellow gum by alumina column chromatography gave
a mixture of 28 and 29 (60 mg, 12%) in a 16:83 ratio, respectively.
Reverse phase-HPLC of 28/29 (gradient: 10% solution A (0.1% TFA in
MeOH) and 90% solution B (0.1% TFA in millQ water) to 100% solution
B 0% solution A over 6.5 min, tR¼5.2, 5.5 min).
5.3.7. Di-tert-butyl 4-(20-(100-((benzyloxy)carbonyl)-200-(methoxycarb-
onyl)pyrrolidin-20-yl)ethyl)-2,5-dioxoimidazolidine-1,3-dicarboxylate
(27). Di-tert-butyl dicarbonate (1.40 g, 6.40 mmol, 5 equiv) was added
to a solution of 26 (0.500 g, 1.29 mmol, 1.0 equiv) in THF (anhydrous,
10 mL) at rt. A portion of DMAP (2.80 mg, 0.0230 mmol, 0.018 equiv),
followed by Et3N (0.195 mL, 1.40 mmol, 1.1 equiv) were added. After
stirring at rt for 3 h, a second portion of DMAP (2.80 mg, 0.0230 mmol,
0.018 equiv) was added and the resulting solution was stirred at rt for
16 h. The reaction solvent was removed in vacuo. The residual oil was
dissolved in chloroform (50 mL). The organic layer was washed with
aqueous HCl (1 M, 20 mL), aqueous NaHCO3 (50 mL, saturated), brine
(50 mL), water (3ꢃ30 mL) and dried (MgSO4, anhydrous) then con-
centrated in vacuo. Purification of the yellow oil by silica gel column
chromatography (hexane/ethyl acetate, 70:30 to 50:50) gave 27
(750 mg, 99%) as a yellow gum; Rf (hexane/ethyl acetate, 70:30) 0.38;
nmax (Nujol) dec; dH (300 MHz, CDCl3)1.40e1.50(18H, m,1-NHC(CH3)3,
3-NHC(CH3)3), 1.60e1.70 (1H, m, H400), 1.70e1.90 (3H, m, H400, H10),
1.90e2.10 (3H, m, H300, H20), 2.22e2.35 (1H, m, H300), 3.40e3.45 (2H, m
H500), 3.60 (3H, s, OCH3), 3.89e4.05 (1H, m, H4), 4.90e5.20 (2H, m,
OCH2), 7.20e7.25 (5H, m, C6H5); dC (100 MHz, CDCl3) 173.6, 173.6,
173.5, and 173.5 (CO2CH3), 170.3 (C5), 167.0, 166.8, 166.7, and 166.6
(C2),153.9,153.8, and 153.7 (10-NCO2CH2), 147.6, 147.6, 147.1, and 147.0
(1-NHCO2C(CH3)3), 144.7, and 144.6 (3-NHCO2C(CH3)3), 136.3, 135.8,
and135.6(i-C6H5), 128.0, 127.9, 127.7, 127.5, 127.4, 127.3, and 127.1 (o, m,
p-C6H5), 85.8, 85.7, 84.8, and 84.8 (1-NCO2C(CH3)3, 3-NCO2C(CH3)3),
67.3, and 67.2 (CH2C6H5), 66.6, and 66.5 (C200), 59.7, 58.5, and 58.4 (C4),
51.8, 51.8, and 51.5 (OCH3), 48.7,48.6, 47.9, and47.8(C500), 36.8, and 36.8
(C20), 35.8, and 35.6 (C300), 27.3, 27.3, 27.3, 27.1, 27.1, and 27.0 (1-
NCO2C(CH3)3, 3-NCO2C(CH3)3, 100-NCO2C(CH3)3), 24.2, 24.1, and 24.0
(C10), 22.7, 22.6, 22.2, and 22.1 (C400); LRMS (ESI) m/z 588 (60%, MꢂH),
HRMS (FTICR): MꢂH, found 588.2558. C29H38N3O10 requires
588.2562.
Compound 28 as a 1:2 mixture of conformers a:b; nmax (Nujol) dec;
dH (300 MHz, CD3OD) 1.64e1.72 (0.5H, m, H10a), 1.72e1.80 (0.5H, m,
H10b),1.80e2.06 (2H, m, 2ꢃ H3), 2.06e2.19 (2H, m, 2ꢃ H4), 2.19e2.26
(1H, m, H9b), 2.26e2.42 (1H, m, H9a), 2.42e2.49 (0.5H, m, H10b),
2.49e2.60 (0.5H, m, H10a), 3.50e3.60 (2H, m, 2ꢃ H2), 3.62 (1.3H, s,
OCHb3), 3.80 (0.6H, s, OCHa3), 4.15 (1H, d, J 5.45 Hz, H8), 5.04e5.28 (2H,
m, CH2C6H5), 7.20e7.40 (5H, m, C6H5); dC (75 MHz, CD3OD) 176.0, and
175.9 (C6), 173.2, and 173.1 (COCH3), 156.0, and 155.7 (NCO2), 138.6,
and 138.1 (i-C6H5), 129.5,129.3, 129.5,128.9, 128.8, and 128.7 (o, m, p-
C6H5), 67.8, and 67.7 (CH2C6H5), 66.6 (C5b), 66.3 (C5a), 54.9, and 54.9
(C8), 52.9 (OCHb3), 52.8 (OCH3a), 48.1 (C2), 40.2 (C4a), 39.1 (C4b), 30.8
(C10b), 29.8 (C10a), 24.1 (C3), 24.1 (C9a) 23.9 (C9b); LRMS (ESI) m/z 369
(100%, MNaþ), HRMS (FTICR): MNaþ, found 369.1421. C18H23N2NaO5
requires 369.1426.
Compound 29 as a 5:4 mixture of conformers a:b; nmax (Nujol)
dec; dH (300 MHz, CD3OD) 1.62e1.80 (1H, m, H3a), 1.80e1.90 (1H,
m, H9), 1.90e2.00 (1H, m, 0.5ꢃ H10a, 0.5ꢃ H10b), 2.05e2.22 (3.5H,
m, H3b, 2ꢃ H4, 0.5ꢃ H10b), 2.22e2.4 (1H, m, H9), 2.40e2.55 (0.5H,
m, 0.5ꢃ H10a), 3.55e3.65 (2H, m, 2ꢃ H2), 3.72 (1.1H, s, OCHb3), 3.78
(0.9H, s, OCHa3), 4.25 (1H, dd, J 4.7, 11.7 Hz, H8), 4.90e5.20 (2H, m,
CH2C6H5), 7.20e7.40 (5H, m, C6H5); dC (75 MHz, CD3OD) d: 175.7,
and 175.4 (C6), 172.9, and 172.6 (CO2CH3), 155.8 (NCO2), 138.1, and
137.3 (i-C6H5), 129.6, 129.6, 129.5, 129.5, 129.0, and 128.7 (o, m, p-
C6H5), 68.7, and 67.9 (CH2C6H5), 66.3 (C5b), 66.0 (C5a), 56.0 (C8),
55.8 (C2), 53.0 (OCHb3), 54.0 (OCH3a), 40.1 (C4a), 38.9 (C4b), 33.1
(C10b), 31.9 (C10a), 25.9 (C3), 24.3 (C9a) 23.6 (C9b); LRMS (ESI) m/z
5.3.8. 4-(10-((Benzyloxy)carbonyl)-20-(methoxycarbonyl) pyrrolidin-
20-yl)-2-aminobutanoic acid (30). Aqueous KOH solution (2.00 M,
13.0 mL) was added to a colourless solution of compound 27
(750 mg, 1.27 mmol) in THF (10 mL) at rt. The reaction was stirred

1582
W.A. Loughlin et al. / Tetrahedron 69 (2013) 1576e1582
369 (100%, MNaþ), HRMS (FTICR): MNaþ, found 369.1421.
C18H23N2NaO5 requires 369.1426.
Supplementary data
Supplementary data available: additional experimental de-
scriptions; 1H and 13C NMR spectra of 23e30, 32 and 33; ORTEP Plot
of 7 (CCDC code: 68432). Supplementary data related to this article
can be found at http://dx.doi.org/10.1016/j.tet.2012.12.004.
5.3.10. Methyl 6-oxo-1,7-diazaspiro[4.5]decane-8-carboxylate (32).
Compounds 28/29 (200 mg, 0.543 mmol) in methanol (2 mL) were
added to Pd/C (29 mg, 10%) under an atmosphere of nitrogen. Meth-
anol (20 mL) and H2SO4 (concd 1 drop) was added. The reaction was
placed under an atmosphere of hydrogen and stirred for 48 h. The
reaction was quenched with nitrogen gas and Celite. The mixture was
filtered and the residue washed with methanol. Removal of the solvent
in vacuo gave compound 32 as a gum (100 mg, 85%); nmax (Nujol) dec;
dH (400 MHz, DMSO-d6) 0.90e2.15 (9H, m, 2ꢃ H3, 2ꢃ H9, 2ꢃ H10, 2ꢃ
H4,1-NH), 3.19 (3H, s, OCH3), 3.20e4.25 (3H, m, 2ꢃ H2, H8), 7.30e7.40
(1H, m, 7-NH); dC (100 MHz, DMSO-d6) 172.9 (C6),162.9 (CO2CH3), 60.2
(C5), 55.0 (OCH3), 53.9(C8), 48.6(C2), 35.9, and35.2(C4), 32.3, and29.3
(C10), 28.1 (C3), 23.4, and 22.9 (C9); LRMS (ESI) m/z 221 (100%, MNaþ),
HRMS (FTICR): MHþ, found 213.1238. C10H17N2O3 requires 213.1239.
References and notes
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decane-8-carboxylic acid (33). Compounds 28/29 (46 mg,
0.125 mmol, 1 equiv) in THF (0.5 mL) were added to Ba(OH)2.H2O
(123 mg, 0.718 mmol, 5.7 equiv) in water (3 mL). The reaction
mixture was stirred at rt for 1 h, then carbon dioxide was bubbled
through the reaction mixture for 1 h. A precipitate formed, which
was removed by filtration and washed with water. The water layer
was lypholised to dryness. The residue was dissolved in methanol
(10 mL), filtered and concentrated in vacuo. Compound 33 was
obtained as a white gum (27 mg, 60%). Diastereomer mix (a and b)
in a 6:4 ratio, respectively; nmax (Nujol) dec; dH (300 MHz, CD3OD)
1.10e1.40 (1H, m, H9a), 1.40e1.67 (1H, m, H9b), 1.67e1.80 (1H, m,
H3a), 1.80e2.01 (2H, m, H3b, H10a), 2.01e2.5 (3H, m, H4, H10b),
3.35e3.60 (1H, m, H2a), 3.60e3.80 (1H, m, H2b), 3.90e4.09 (1H, m,
H8), 4.90e5.30 (2H, m, CH2C6H5), 7.30e7.35 (5H, m, C6H5), NH and
COOH not observed; dC (75 MHz, CD3OD) 176.0, and 175.9 (C6),
160.1 (CO2H), 156.2 (NCO2), 138.2, and 137.6 (i-C6H5), 129.6, 129.5,
129.4, 129.3, 129.0, and 128.7 (o, m, p-C6H5), 69.5, 69.0, 68.5, and
68.0 (CH2C6H5), 59.8, and 59.5 (C5), 52.8 (C8), 50.3 (C2), 38.4 (C4a),
37.2, and 37.0 (C4b), 36.4 (C10b), 27.1 (C10a), 26.7 (C3), 24.1 (C9a)
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(1) Financial support for this work was provided by Griffith
University, Natural Product Discovery, Griffith University, and
Eskitis Institute for Cell and Molecular Therapies, Griffith Univer-
sity; and (2) Peter C. Healy for X-ray analysis of 7.
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