Tuning of the prolyl trans/cis-amide rotamer population by use of C-glucosylproline hybrids

Article abstract of DOI:10.1021/jo0700833

(Chemical Equation Presented) We describe the synthesis of a fused bicyclic C-glucosylproline hybrid (GlcProH) from commercially available 2,3,4,6-tetra-O-benzyl-D-glucopyranose. The GlcProH was incorporated into the model peptides Ac-GlcProH-NHMe and Ac-

Full text of DOI:10.1021/jo0700833

Tuning of the Prolyl trans/cis-Amide Rotamer Population by Use of
C-Glucosylproline Hybrids
Neil W. Owens, Craig Braun, and Frank Schweizer*
Department of Chemistry, UniVersity of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
schweize@ms.umanitoba.ca
ReceiVed January 15, 2007
We describe the synthesis of a fused bicyclic C-glucosylproline hybrid (GlcProH) from commercially
available 2,3,4,6-tetra-O-benzyl-D-glucopyranose. The GlcProH was incorporated into the model peptides
Ac-GlcProH-NHMe and Ac-Gly-GlcProH-NHMe. Postsynthetic modifications can be introduced via
derivatization of the carbohydrate scaffold. Conformational analysis of the GlcProH-modified model
peptides shows that while the conformation of GlcProH remains fixed, the prolyl N-terminal amide
equilibrium (Kt/c) can be varied with different modifications of the carbohydrate scaffold. Simple N-acyl
derivatives studied by NMR spectroscopy showed that in CD3OD there was an increase in the cis-amide
content as the sugar substituents changed from benzyl (10%) to hydroxyl (22%) to acetate (36%). Similar
effects were observed in DMSO-d6. The exact nature of the influence is unclear, but it most likely arises
through intramolecular interactions between sugar groups and the peptidic amide backbone. Overall, our
GlcProH demonstrates variation in Kt/c through tuning of the carbohydrate scaffold: a new concept in
proline peptidomimetics.
Introduction
and thus reverse turn formation. Examples of amide geometry
controlled through steric or stereoelectronic influences includes
Câ-, Cγ-, and Cδ-substituted prolines,3-6 including 4-fluoropro-
line,7 azaprolines,8 and pseudoprolines.9
While these analogues have proved useful for inducing
specific constraints on prolyl N-terminal amide isomerization,
no one analogue can lay claim to the ability to shift the prolyl
amide equilibrium toward both the cis and trans isomers. In
other words, different proline analogues are required to induce
a desired bias in Kt/c. This is in part because none of the present
Proline is uniquely endowed with a side chain that is fused
onto the peptide backbone. This trait restricts the rotation about
its φ dihedral angle, thereby reducing the energy difference
between the prolyl amide cis and trans isomers, making them
nearly isoenergetic. Thus, while most peptide amide bonds exist
almost exclusively in the trans form, proline has a much greater
propensity to form cis-amide bonds; this makes proline crucial
for inducing a reversal in peptide backbone conformation.1 Also,
cis-trans isomerization of proline is of importance because it
becomes the rate-determining step in the folding pathways of
peptides and proteins.2
Variation of the trans/cis ratio is of interest for understanding
the behavior of peptides and proteins. Over the years a number
of proline analogues have been developed to study the structural
and biological properties of proline surrogates in peptides. This
is done by controlling prolyl N-terminal amide isomerization,
(3) Beausoleil, E.; Lubell, W. D. J. Am. Chem. Soc. 1996, 118, 12902-
12908 and references therein.
(4) Delaney, N. G.; Madison, V. J. Am. Chem. Soc. 1982, 104, 6635-
6641.
(5) Samanen, J.; Zuber, G.; Bean, J.; Eggleston, D.; Romoff, T.; Kopple,
K.; Saunders, M.; Regoli, D. Int. J. Pept. Protein Res. 1990, 35, 501-509.
(6) Sharma, R.; Lubell, W. D. J. Org. Chem. 1996, 61, 202-209.
(7) Eberhardt, E. S.; Panasik, N., Jr.; Raines, R. T. J. Am. Chem. Soc.
1996, 118, 12261-12266.
(8) Che, Y.; Marshall, G. R. J. Org. Chem. 2004, 69, 9030-9042 and
references therein.
(9) Tam, J. P.; Miao, Z. J. Am. Chem. Soc. 1999, 121, 9013-9022 and
references therein.
* Corresponding author: fax (+1) 204-474-7608.
(1) Wilmot, C. M.; Thornton, J. M. J. Mol. Biol. 1988, 203, 221-232.
(2) Fischer, G.; Schmid, F. X. Biochemistry 1990, 29, 2205-2212.
10.1021/jo0700833 CCC: $37.00 © 2007 American Chemical Society
Published on Web 05/31/2007
J. Org. Chem. 2007, 72, 4635-4643
4635

Owens et al.
FIGURE 1. General structure of glucose-proline hybrid (GlcProH).
The carbohydrate scaffold is used as a template to constrain Câ, Cγ,
Cδ, and N of the pyrrolidine ring (shown in bold) of L-proline or
(2S,4R)-hydroxyproline; P1, P2, and P3 are hydroxy-derived substituents
used to manipulate the steric and electronic properties of the GlcProH.
FIGURE 2. Conformationally relevant NOE interactions observed
for 8.
for the synthesis of peptidomimetics.19 Because of its stable chair
conformation, glucose provides an ideal scaffold to template
proline since it freezes the orientation of four proline atoms
(Câ, Cγ, Cδ, and N). Furthermore, the sugar hydroxyl groups
amend themselves to derivatization of the building block as
potential sites for influencing the peptide backbone conforma-
tion. It has been shown that hydroxylated amino acids can induce
novel secondary structures in small peptides. For instance,
incorporation of unprotected sugar amino acids into small
peptides (opioid peptides and gramicidin S) prohibited the
formation of the targeted secondary structural motif.20,21 Ap-
pearing instead were unusual turn structures stabilized by
intramolecular hydrogen bonds between sugar hydroxyl groups
and the peptidic amide backbone.22
We envision that similar effects may also be observed with
the GlcProH. For instance, we hope to influence peptide
geometry via intramolecular polar interactions (H-bonding)
between the sugar substituents and the peptide backbone.
Furthermore, through derivatization of the polyhydroxylated
carbohydrate scaffold, we hope to demonstrate the capacity to
tune prolyl N-terminal amide isomerization. To test the effects
of chemically modifying the carbohydrate hydroxyl groups on
prolyl cis-trans isomerization, we will study simple N-acyl and
N-glycyl model compounds. Modifications of the glycine-
proline peptide motif are of particular interest since it is present
in collagen23 as well as in small peptides found to have
neuroprotective properties24 and the ability to inhibit HIV
replication.25 Furthermore, the Gly-Pro-Gly peptide sequence
is known to induce reverse turns.26
building blocks have strategic functional groups positioned for
further derivatization in order to alter the amide equilibrium.
This led us toward the development of a proline analogue
building block in which prolyl cis-trans isomerization could
be tuned through simple chemistry, even after incorporation into
a peptide.
Our concept for a proline analogue was derived from glycosyl
amino acids (GAAs), which are defined by an R-amino acid
group [-CH(NH2)CO2H] either directly attached or carbon-linked
to the anomeric carbon of a carbohydrate scaffold (furan or
pyran).10 The relative rigidity of the furan or pyran ring
combined with the polyfunctional nature of the carbohydrate
scaffold has inspired the design of unusual and conformationally
constrained amino acids and novel peptidomimetics. While there
are many examples of C-glycosylglycine, -alanine, -serine, and
-asparagine,10c few proline-based GAAs exist.11
We report here on the resulting novel fused bicyclic gluco-
proline hybrid (GlcProH) GAA for use as a peptidomimetic to
tune prolyl N-terminal amide isomerization. Our bicyclic
GlcProH rigidly combines the molecular elements of carbohy-
drates (pyran-based polyol) with the unique features of proline
(Figure 1). The resulting GlcProH is a rigid, polyfunctional
building block, which may find use as a proline mimetic,
glycomimetic, or scaffold for combinatorial synthesis. Many
conformationally constrained L-proline analogues have been
developed,12,13 but recently, fused bicyclic prolines have at-
tracted interest due to their increased rigidity, which may permit
better control of the trans/cis ratio.14-16 For instance, bicyclic
proline analogues have been studied as angiotensin17 and
thyroliberin18 analogues, which have served as building blocks
Results
Synthesis of N-Boc-GlcPro-carboxylic Acid 1. The syn-
thesis of GlcProH 1 (Scheme 1) started from known amine 2,27
(10) For recent reviews, see (a) Dondoni, A.; Marra, A. Chem. ReV. 2000,
100, 4395-4421. (b) Schweizer, F. Angew. Chem., Int. Ed. 2002, 41, 230-
264. For recent reviews on the use of glycosyl amino acids as peptidomi-
metics. see (c) Gruner, S. A. W.; Locardi, E.; Lohof, E.; Kessler, H. Chem.
ReV. 2002, 102, 491-514. (d) Chakraborty, T. K.; Ghosh, S.; Jayaprakash,
S. Curr. Med. Chem. 2002, 9, 421-435.
(19) Hanessian, S.; Mcnaughton-Smith, G.; Lombart, H.-G.; Lubell, W.
D. Tetrahedron 1997, 53, 12789-12854 and references 3a-h and 4a-k in
ref 17.
(20) Grotenbreg, G. M.; Timmer, M. S. M.; Llamas-Saiz, A. L.; Verdoes,
M.; van der Marel, G. A.; van Raaij, M. J.; Overkleeft, H. S.; Overhand,
M. J. Am. Chem. Soc. 2004, 126, 3444-3446.
(21) Chakraborty, T. K.; Jayaprakash, S.; Diwan, P. V.; Nagaraj, R.;
Jampani, S. R. B.; Kunwar, A. C. J. Am. Chem. Soc. 1998, 120, 12962-
12963.
(11) Two examples have been reported: (a) Zhang, K.; Schweizer, F.
Synlett 2005, 20, 3111-3115. (b) Cipolla, L.; Redaelli, C.; Nicotra, F. Lett.
Drug Des. DiscoVery 2005, 2, 291-293.
(12) Manzoni, L.; Colombo, M.; May, E.; Scolastico, C. Tetrahedron
2001, 57, 249-255.
(13) Petter, R. C. Tetrahedron Lett. 1989, 30, 399-402.
(14) Jeannotte, G.; Lubell, W. D. J. Org. Chem. 2004, 69, 4656-4662.
(15) (a) Wagaw, S.; Rennels, R. A.; Buchwald, S. L. J. Am. Chem. Soc.
1997, 119, 8451-8458. (b) Kuwano, R.; Sato, K.; Kurokawa, T.; Karube,
D.; Ito, Y. J. Am. Chem. Soc. 2000, 122, 7614-7615. (c) Viswanathan, R.;
Prabhakaran, E. N.; Plotkin, M. A.; Johnston, J. N. J. Am. Chem. Soc. 2003,
125, 163-168.
(22) It has been reported that substitution of D-proline by cis-3-hydroxy-
D-proline (HypC3-OH) in the sequence Boc-Leu-Pro-Gly-NHMe resulted
in a novel pseudo-â-turn-like nine-membered ring structure involving an
intramolecular hydrogen bond between LeuNH: Chakraborty, T. K.;
Srinivasu, P.; Vengal Rao, R.; Kiran Kumar, S.; Kunwar, A. C. J. Org.
Chem. 2004, 69, 7399-7402.
(23) Vitagliano, L.; Beriso, R.; Mazzarella, L.; Zagari, A. Biopolymers
2001, 58, 459-464.
(16) Koep, S.; Gais, H.-J.; Raabe, G. J. Am. Chem. Soc. 2003, 125,
13243-13251 and references therein.
(24) (a) Brimble, M. A.; Trotter, N. S.; Harris, P. W. R.; Sieg, F. Bioorg.
Med. Chem. 2005, 13, 519-532. (b) Alonso, De Diego, S. A.; Munoz, P.;
Gonzalez-Muniz, R.; Herranz, R.; Martin-Martinez, M.; Cenarruzabeitia,
E.; Frechilla, D.; Del, Rio, J.; Jimeno, M. L.; Garcia-Lopez, M. T. Bioorg.
Med. Chem. 2005, 15, 2279-2283. (c) Ioudina, M.; Uemura, E. Exp. Neurol.
2003, 184, 923-929.
(17) Blankley, C. J.; Kaltenbronn, J. S.; DeJohn, D. E.; Werner, A.;
Bennett, L. R.; Bobowski, G.; Krolls, U.; Johnson, D. R.; Pearlman, W.
M.; Hoefle, M. L; Essenberg, A. D.; Cohen, D. M.; Kaplan, H. R. J. Med.
Chem. 1987, 30, 992-998.
(18) Li, W.; Moeller, K. D. J. Am. Chem. Soc. 1996, 118, 10106-10112.
4636 J. Org. Chem., Vol. 72, No. 13, 2007

Tuning of Prolyl trans/cis-Amide Rotamer Population
SCHEME 1. Synthesis of N-Boc-GlcPro-Carboxylic Acid 1
silver acetate in toluene.29 Hydrolysis of 5 by potassium
hydroxide in methanol at elevated temperature provided the
amino alcohol 6 in 78% isolated yield. Protection of the amino
function in 6 was accomplished by use of di-tert-butyl dicar-
bonate to yield the Boc-protected proline analogue 7 in 90%
yield. Subsequently, the alcohol 7 was subjected to Jones
oxidation to afford protected GlcProH 1 in 78% yield. To assign
the stereochemistry of 1, esterfication with benzyl bromide and
cesium carbonate in DMF afforded the protected GlcProH 8 in
40% yield from 7.
SCHEME 2. Synthesis of N-Acetyl-GlcPro N′-Methylamides
11-13
Assignment of Stereochemistry for N-Boc-GlcPro-benzyl
Ester 8. The S-configuration at the 2-position of protected
GlcProH 8 was identified on the basis of observed/unobserved
NOE contacts shown in Figure 2. For instance, subjection of
the H-2 signal in 8 to a one-dimensional GOESY30 experiment
showed interproton effect to H-5 [0.8% NOE31 relative to the
H-2 (dd) signal]. By comparison, no interproton effects were
observed between H-2 and H-9 or H-2 and H-8.
Synthesis of N-Acetyl-GlcPro N′-Methylamides 11-13 and
Determination of Kt/c. With GlcProH 1 in hand, we then
explored the use of the building block in peptide chemistry.
Initially, we focused on analogues for comparison with Ac-
Pro-NHMe 17, which has frequently served as a model to study
the cis-trans isomerization of proline.4,32 The synthesis of
GlcProH-modified amides 11-13 is outlined in Scheme 2. Acid
1 was directly coupled to methylamine by use of O-benzotria-
zolyl-N,N,N′,N′-tetramethyluronium tetrafluoroborate (TBTU)
as coupling reagent in MeCN to produce amide 9 in 95% yield.
Deprotection of the amine (giving 10) followed by acylation
produced diamide 11 in 97% yield over two steps. In addition,
removal of the benzyl ether protecting groups by catalytic
hydrogenolysis in methanol provided the polyhydroxylated
diamide 12, which was further acylated to afford diamide 13 in
quantitative yield.
which was synthesized in seven steps from commercially
available 2,3,4,6 tetra-O-benzyl-D-glucopyranose in an overall
yield of 40%. Protection of the amino function as tert-
butyloxycarbamate (Boc) afforded 3 in quantitative yield.
Installation of the pyrrolidine ring was achieved via ami-
noiodocyclization in 50% CH2Cl2/ether to afford the bicyclic
iodo deriviative 4 in 90% yield as a single stereoisomer together
with 5% unreacted 3.28 Attempts to substitute the iodo function
in 4 by hydroxide ion (KOH) or acetate failed and produced
complex mixtures containing tricyclic carbamate 5. However,
high yields (92%) of 5 could be obtained by exposure of 4 to
For each compound 11-13, the ratio of trans/cis isomers was
calculated by integrating as many well-resolved peaks as
(25) (a) Su, J.; Andersson, E.; Horal, P.; Naghavi, M. H.; Palm, A.; Wu,
Y.-P.; Eriksson, K.; Jansson, M.; Wigzell, H.; Svennerholm, B.; Vahlne,
A. J. Hum. Virol. 2001, 4, 1-7. (b) Laczko, I.; Hollosi, M.; Urge, L.; Ugen,
K. E.; Weiner, D. B.; Mantsch, H. H.; Thurin, J.; Otvos, L., Jr. Biochemistry
1992, 31, 4282-4288.
(26) (a) Hayashi, T.; Asai, T.; Ogoshi, H. Tetrahedron Lett. 1997, 38,
3039-3042. (b) Zerkout, S.; Dupont, V.; Aubry, A.; Vidal, J.; Collet, A.;
Vicherat, A.; Marraud, M. Int. J. Pept. Protein Res. 1994, 44, 378-387.
(27) Cipolla, L.; Lay, L.; Nicotra, F. J. Org. Chem. 1997, 62, 6678-
6681.
(29) Davies, S. G.; Nicholson, R. L.; Price, P. D.; Roberts, P. M.; Smith,
A. D. Synlett 2004, 5, 901-903.
(30) GOESY is a 1D selective NOESY experiment. It provides quantita-
tive information and much higher sensitivity compared to NOESY. First
appeared in Stonehouse, J.; Adell, P.; Keeler, J.; Shaka, A. J. J. Am. Chem.
Soc. 1994, 116, 6037-6038.
(31) A 40-ms Gaussian pulse with a 560-ms mixing time was used.
(32) (a) Taylor, C. M.; Hardre, R.; Edwards, P. J. B.; Park, J. H. Org.
Lett. 2003, 5, 4413-4416. (b) Madison, V.; Kopple, K. D. J. Am. Chem.
Soc. 1980, 102, 4855-4863.
(28) NMR analysis of the crude reaction mixture containing 5 did not
indicate the formation of the other diastereoisomer.
J. Org. Chem, Vol. 72, No. 13, 2007 4637

Owens et al.
SCHEME 3. Synthesis of N-Acetyl-Glycyl-GlcPro
N′-Methylamides 14-16
FIGURE 3. Representative conformationally relevant NOE interactions
observed for GlcProH-derived model compounds. Interproton effects
for the trans-amide rotamer were observed between H-8 and the N-acyl
methyl group (compounds 11-13) or glycinyl protons (compounds 14-
16). By comparison, very small or no interproton effects were observed
between H-2 and the N-acyl methyl group (compounds 11-13) or
glycinyl protons (compounds 14-16).
TABLE 1. Trans/cis Ratioa (Kt/c) and % cis Isomer for 11-13 in
36%). Analysis in DMSO-d6 produced very similar results, with
the N-terminal cis-amide content varying from benzyl (11, 13%)
to hydroxyl (12, 20%) to acetate (13, 33%).
Overall, Kt/c for the unprotected GlcProH 12 (3.5 in CD3-
OD) was very similar to that of the L-proline model compound
17 (Kt/c ) 4),32b leaving the per-O-benzylated 11 favoring the
trans conformer (Kt/c ) 9 in CD3OD) and the per-O-acylated
13 favoring the cis conformer (Kt/c ) 1.8 in CD3OD). Also,
CDCl3 gave higher Kt/c than the more polar solvents (Kt/c ) 30
when R ) Bn, Kt/c ) 6.7 when R ) Ac).
Synthesis of N-Acetylglycyl-GlcPro N′-Methylamides 14-
16 and Determination of Kt/c. GlcProH 10 was coupled to
Fmoc-Gly-OH by use of benzotriazol-1-yloxytris(pyrrolidino)-
phosphonium hexafluorophosphate (PyBOP) as coupling re-
agent, followed by deprotection of the amino function and
acylation providing the triamide 14 in 47% yield over three steps
(Scheme 3). Deblocking of the carbohydrate moiety afforded
polyhydroxylated peptide 15, which was further acylated to
afford triamide 16.
Various Solvents
solvent
compd
CDCl3
D2O
nsb
5.7 (15%)
ns
CD3OD
DMSO-d6
11 (R ) Bn)
12 (R ) H)
13 (R ) Ac)
>30 (<3%)
ns
6.7 (13%)
9 (10%)
3.5 (22%)
1.8 (36%)
6.7 (13%)
4 (20%)
2 (33%)
a Determined by 500 MHz NMR at 25 °C. b Not soluble.
TABLE 2. Trans/cis Ratioa (Kt/c) and % cis Isomer for 14-16 in
Various Solvents
solvent
compd
CDCl3
D2O
nsb
9 (10%)
ns
CD3OD
14 (R ) Bn)
15 (R ) H)
16 (R ) Ac)
19 (5%)
ns
4 (20%)
>30 (<3%)
5.7 (15%)
3 (25%)
a Determined by 500 MHz NMR at 25 °C. b Not soluble.
Again, the average of as many well-resolved peaks as possible
for the major and minor isomers was recorded to find the Kt/c
for respective compounds in each solvent. One-dimensional
GOESY experiments helped to assign the N-terminal amide
geometry. The major isomer was assigned as the trans isomer
on the basis of the interproton effect observed between H-8 and
the glycyl R-protons (Figure 3) (for example, a 6.1% NOE was
observed for 14 in CD3OD). Furthermore, selective inversion
of H-2 of the major isomer showed no interproton effect with
either HR1(Gly) or HR2(Gly) (for example, no such NOE was
observed for 14 in CDCl3). Additionally, selective inversion of
HR2(Gly) of the minor isomer showed no interproton effect with
H-8 of the minor isomer (for example, no such NOE was
observed for 16 in CD3OD).
We found a very similar profile of Kt/c for the Ac-Gly-GlcPro-
NHMe model compounds 14-16 compared to those of the Ac-
GlcPro-NHMe model compounds 11-13 (Table 2). In CD3OD,
there was an increase in the cis content as the sugar substituent
changed from benzyl (14, <3%) to hydroxyl (15, 15%) to
acetate (16, 25%). These results confirm that the substituents
on the sugar are influencing Kt/c.
possible for each isomer and taking the average over all peaks
for respective isomers.32a The assignment of N-terminal amide
geometry for both major and minor isomers of 11-16 was based
on multiple GOESY experiments.30 For example, selective
inversion of the H-8 proton in 11 (CD3OD) showed an
interproton effect to the N-terminal acetate singlet (5.2%) (Figure
3). By comparison, a very small (0.4%) interproton effect was
observed between H-2 and the acetate methyl singlet, and weak
or no interproton effects were observed between H-8 and H-2.
As H-8 and H-2 lie on opposite faces of the pyrrolidine ring,
NOE contact from H-8 to the N-terminal acetate singlet can be
assigned as the trans isomer. Analysis was carried out in
different NMR solvents to understand how the model com-
pounds behaved in different solvent environments. While it
would be most desirable to study the GlcProHs in water, these
results represent a proof of concept, and there is precedence
for studying modifications of prolyl isomerization in nonaqueous
environments.13,33
We found a significant variation of the cis content as the
sugar substituents were varied (Table 1). In CD3OD, there was
an increase in the cis content as the sugar substituents changed
from benzyl (11, 10%) to hydroxyl (12, 22%) to acetate (13,
Analysis of the coupling constants of the pyranose ring
showed it exists in a chair conformation. For example, for 12
in CD3OD, J5,6 was 9.4 Hz, while J6,7 was 9.6 Hz and J7,8 was
9.0 Hz (see Figure 2 for numbering). The average values of
(33) Trabocchi, A.; Potenza, D.; Guarna, A. Eur. J. Org. Chem. 2004,
22, 4621-4627.
4638 J. Org. Chem., Vol. 72, No. 13, 2007

Tuning of Prolyl trans/cis-Amide Rotamer Population
TABLE 3. Comparison of Average Coupling Constants for 11-16 Major and Minor Isomersa with Cγ-endo and Cγ-exo Puckers of
4-Fluoroproline and L-proline
compd
3JR â
1
3JR â
2
3Jγâ
1
3Jγâ
2
3Jδγ
1
11-16 major isomers
9.9 ( 0.3
9.3 ( 0.2
10
1.0 ( 0.1
1.7 ( 0.3
3
10
2-3
7-11
11.8 ( 0.3
7.4 ( 0.2
6.8 ( 0.2
1
7.2 ( 0.1
7.1 ( 0.3
4
11-16 minor isomers
10.9 ( 0.4
4(R)-fluoroproline34 (Cγ-endo)
4(S)-fluoroproline34 (Cγ-exo)
L-proline35 (Cγ-endo)
4
1
5-9
5-9
8
4
3
6-10
7-10
8-12
2-3
6-10
5-9
L-proline35 (Cγ-exo)
a Coupling constants are given in hertz ( standard error.
each coupling constant in comparison to the literature values
match best to a Cγ-endo conformation for the pyrrolidine ring
(Table 3).
acetate (Kt/c of 1.8 in CD3OD for 13) substituents would provide
greater steric bulkiness. However, the trend in Kt/c does not
follow the trend in increase in steric bulk of the sugar
substituents (from 3.5 to 1.8 to 9 for 12 to 13 to 11, respectively).
This led us to consider other factors.
Also, coupling constants indicate that the prolyl 4-position
hydroxyl group (pyrano endocyclic oxygen) is oriented in an
equatorial position relative to the pyrrolidine ring, which is not
its preferred axial orientation.36 Perhaps most importantly, all
coupling constant values changed very little as the sugar
substituents were varied, even as the solvent was varied, and
between the major and minor isomers in each case. Together,
these results indicate that the rigid pyranose ring is restricting
the conformational freedom of the pyrrolidine ring.
A large amount of interest lies in studying the conformation
of proline4,8,23,32,38 and modifications that alter the preferred
conformation of the pyrrolidine ring.3-9,36,37,39 We analyzed the
coupling constants of the sugar and pyrrolidine rings in an
attempt to consider the change in Kt/c as a function of the change
in the pucker of the pyrrolidine ring. Raines and co-workers7,36
proved, through work with 4-fluoroproline model compounds,
that as the stereochemistry of the 4-position EWG is varied, so
is the pucker of the pyrrolidine ring (in an attempt to maximize
the gauche effect). Consequently, as the pucker is changed, the
degree of n f π* donation from the N-terminal CdO to the
C-terminal CdO is also changed, thus affecting Kt/c. In our case,
one could imagine that as the substituents on the sugar are
varied, the pucker of the pyrrolidine ring also changes, influenc-
ing n f π* donation and Kt/c. Through NMR analysis of 11-
16 we found several results; as the sugar substituents are varied,
and even as the solvent is varied, the coupling constants are
held within a very narrow range (Table 3). These results indicate
that the pucker of the pyrrolidine ring does not vary as the
substituents on the sugar scaffold are varied, and the change in
Discussion
After developing a synthetic route to a novel sugar-proline
analogue, we found through NMR experiments that N-terminal
amide isomerization is dependent on the modification of the
gluco scaffold but cannot be correlated with a significant
variation in conformation.
There could be a steric explanation for the trend in Kt/c, as
work by Lubell and co-workers3,37 has shown that the Kt/c for
prolyl N-terminal amide bonds can be influenced through steric
effects. Alkyl-substituted proline derivatives such as 5-tert-
butylproline force a very high N-terminal cis-amide content
(80%+) through steric repulsion of the N-terminal amino acid
side chain and the 5-tert-butyl substituent on proline. Here, we
chose model compounds that provide a minimal steric contribu-
tion to influence the Kt/c. Thus, the N-terminal acetate or glycyl
groups should provide little to no steric bias between the cis
and trans isomers. If there were an explanation for the trend in
K
t/c cannot be explained in terms of a change in the pyrrolidine
pucker.
Most likely, simple intramolecular interactions between the
sugar substituents and the peptide backbone are responsible for
the observed variation in the prolyl N-terminal amide isomer-
ization. For compounds 12 and 15 where R ) H, there is the
potential for a hydrogen bond between the 7-position hydroxyl
group and the prolyl N-terminal amide carbonyl (Figure 4a).
This would stabilize the N-terminal cis-amide isomer to some
extent, and would not be possible for the trans isomer. For
Kt/c based on sterics, then the deprotected sugar-proline hybrid
(Kt/c of 3.5 in CD3OD for 12) should provide the lowest steric
influence, while the benzyl (Kt/c of 9 in CD3OD for 11) and
(34) Gerig, J. T.; McLeod, R. S. J. Am. Chem. Soc. 1973, 95, 5725-
5729.
(35) Cai, M.; Huang, Y.; Liu, J.; Krishnamoorthi, R. J. Biomol. NMR
1995, 6, 123-128.
(38) Madison, V. Biopolymers 1977, 16, 2671-2692.
(39) (a) Montelione, G. T.; Hughes, P.; Clardy, J.; Scheraga, H. A. J.
Am. Chem. Soc. 1986, 108, 6765-6773. (b) Avenoza, A.; Busto, J. H.;
Peregrina, J. M.; Rodriguez, F. J. Org. Chem. 2002, 67, 4241-4249. (c)
Hanessian, S.; Papeo, G.; Angiolini, M.; Fettis, K.; Beretta, M.; Munro, A.
J. Org. Chem. 2003, 68, 7204-7218. (d) Taylor, C. M.; Hardre, R.;
Edwards, P. J. B. J. Org. Chem. 2005, 70, 1306-1315. (e) Koskinen, A.
M. P.; Helaja, J.; Kumpulainen, E. T. T.; Koivisto, J.; Mansikkamaeki, H.;
Rissanen, K. J. Org. Chem. 2005, 70, 6447-6453.
(36) (a) Jenkins, C. L.; Lin, G.; Duo, J.; Rapolu, D.; Guzei, I. A.; Raines,
R. T.; Krow, G. R. J. Org. Chem. 2004, 69, 8565-8573. (b) Bretscher, L.
E.; Jenkins, C. L.; Taylor, K. M.; DeRider, M. L.; Raines, R. T. J. Am.
Chem. Soc. 2001, 123, 777-778.
(37) (a) Halab, L.; Lubell, W. D. J. Org. Chem. 1999, 64, 3312-3321.
(b) Halab, L.; Lubell, W. D. J. Pept. Sci. 2001, 7, 92-104. (c) Halab, L.;
Lubell, W. D. J. Am. Chem. Soc. 2002, 124, 2474-2484.
J. Org. Chem, Vol. 72, No. 13, 2007 4639

Owens et al.
cis-amide rotamer population in various solvents. The cis
rotamer population increased as the substituents on the sugar
scaffold were varied [O-benzyl < OH (unprotected) < O-acetyl]
in CD3OD and DMSO-d6. Until now, the ability to vary the
N-terminal prolyl amide isomer ratio required the synthesis of
proline surrogates by independent synthetic routes. However,
our results demonstrate that one-step modifications of the
hydroxyl groups on the carbohydrate scaffold can be used to
tune the conformational properties of GlcProH-containing model
peptides.
Experimental Section
(2S,3aR,5R,6R,7S,7aS)-1-(tert-Butyloxycarbonyl)-6,7-di-O-
benzyl-5-[(benzyloxy)methyl]octahydropyrano[3,2-b]pyrrole-2-
carboxylic Acid (1). Compound 7 (0.125 g, 0.21 mmol) was
dissolved in 6 mL of acetone. The solution was cooled to 0 °C.
Fresh Jones’ reagent (0.75 mL, 6.3 mmol) was prepared and was
added dropwise. The reaction mixture was stirred for 30 min before
addition of water (5 mL) and then aqueous saturated sodium
bicarbonate (5 mL). The acetone was removed under reduced
pressure. The product was extracted into CH2Cl2 (3 × 15 mL),
then dried (Na2SO4) and concentrated under reduced pressure, and
was normally used directly in the next reaction. Purification by
flash chromatography with 3:1 ethyl acetate/methanol yielded 1 as
a clear oil (0.100 g, 0.17 mmol) (78.4%), [R]D25 ) -12.8° (c 0.4,
FIGURE 4. Proposed intramolecular interactions for 11, 12, and 13:
(a) cis-Amide rotamer isomer can be stabilized through an intramo-
lecular hydrogen bond in 12. (b) cis-Amide rotamer can also be
stabilized through n f π* donation from the prolyl N-terminal amide
to the acetate ester group at the 7-position of GlcProH in 13. (c) The
interactions that stabilize the cis-amide rotamer in 12 and 13 are not
possible for 11. (d) trans-Amide rotamer for 11-13 can be stabilized
through n f π* donation or a hydrogen bond to the C-terminal amide;
analogous interactions can be envisaged for 14-16.
1
CH3OH). H NMR (500 MHz, CD3OD, 298 K) δ ) 7.09-7.37
(m, 15H, aromatic), 4.38-4.76 (m, 7H, -OCH2Ph, H4), 4.25 (br
m, 1H, H2, H2 minor), 4.20 (dd, app t, 0.8H, J7,8 ) 6.0 Hz, J6,7
)
6.3 Hz, H7), 4.10 (dd app, 1H, J8,9 ) 6.6 Hz, H8), [4.06-4.10 (m,
0.2H, H8)], [4.01 (dd app t, 0.2H, H7)], 3.80-3.89 (m, 1H, H5, H5
minor), 3.71 (dd, 0.8H, J10a,10b ) 10.6 Hz, J5,10a ) 6.0 Hz, H10a),
[3.67-3.73 (m, 0.2H, H10a)], 3.59 (dd, 0.8H, J5,10b ) 3.4 Hz, H10b),
[3.57-3.62 (m, 0.2H, H10b)], [3.55 (dd app t, 0.2H, H6)], 3.46 (dd
app t, 1H, J5,6 ) 6.4 Hz, H6), 2.53 (ddd, 1H, J3a,3b ) 12.7 Hz, H3a,
compounds 13 and 16 where R ) Ac, there is the potential for
n f π* donation from the prolyl N-terminal amide carbonyl to
the sugar 7-position acetate ester group (Figure 4b). This would
stabilize the N-terminal cis-amide isomer. The relative decrease
in Kt/c for 13 and 16 (Kt/c of 1.8 and 3, respectively, in CD3-
OD) compared to 12 and 15 (Kt/c of 3.5 and 5.7, respectively,
in CD3OD) may reflect a greater energetic stabilization of the
cis-amide ground state from n f π* donation relative to a
hydrogen bond. Finally, for compounds 11 and 14 where R )
Bn (Figure 4c), the very high trans content is of interest (in
CD3OD, Kt/c is 9 for compound 11 and 30 for compound 14).
The interactions that stabilize the cis-amide geometry for 12
and 13 would not be possible with an ether-linked protecting
group. Most likely there is a steric influence that orients the
N-terminal amide such that it can maximize the n f π* donation
to the C-terminal amide, thereby stabilizing the trans isomer.
This n f π* donation is also probable for the trans isomers of
12 and 13 (Figure 4d). There is also the potential for an
intramolecular hydrogen bond from the N-terminal carbonyl to
the C-terminal amide. Therefore, the overall Kt/c must re-
present the equilibrium established between these competing
interactions.
H3a minor), 1.90-1.97 (m, 1H, H3b, H3b minor), 1.40 (s, 7.2H, tert-
butyl), [1.32 (s, 1.8H, tert-butyl)]. 13C NMR (75 MHz, CD3OD,
298 K) δ ) 168.7, 156.1, 139.8, 139.7, 139.5, (81.9), 81.5, (79.8),
78.4, (76.9), 76.6, 74.9, 74.7, (74.6), (74.5), 74.3, (74.2), 74.1,
(73.9), 72.9, (70.4), 70.2, (59.9), 59.8, 33.9, (28.8), 28.6. HRMS
(ES) calcd for C35H40NO8 (M - H)- 602.2759, found- 602.2755.
1-[2′-(tert-Butyloxycarbonyl)amino-3′,4′,6′-tri-O-benzyl-2′-
deoxy-R-D-glucopyranosyl]-2-propene (3). Compound 2 (0.202
g, 0.43 mmol) was dissolved in 4 mL of methanol. Addition of
triethylamine (0.6 mL, 4.3 mmol) was followed by addition of di-
tert-butyl dicarbonate (0.46 g, 2.1 mmol). The reaction mixture was
stirred for 16 h. All reagents and solvent were removed under
reduced pressure to provide 3 as a white solid (0.244 g, 0.43 mmol)
(quant), [R]D25 ) +11.4° (c 3.7, CHCl3), mp 98-101 °C. 1H NMR
(300 MHz, CDCl3, 298 K) δ ) 7.20-7.40 (m, 15H, aromatic),
5.85 (dddd, 1H, J ) 6.9, 7.0, 10.1, 17.0 Hz, -CHdCH2), 5.61 (d,
1H, J ) 9.8 Hz, NHBoc), 5.02-5.17 (m, 2H, -CHdCH2), 4.46-
4.87 (m, 6H, -OCH2Ph), 4.20 (dd, 1H, J ) 6.1, 6.2 Hz, H5), 3.95
(ddd, 1H, J ) 2.0, 5.6, 7.8 Hz, H1), 3.80-3.89 (m, 2H, H2, H6a),
3.68-3.79 (m, 2H, H3, H6b), 3.55-3.60 (m, 1H, H4), 2.17-2.39
(m, 2H, allylic), 1.45 (s, 9H, tert-butyl). 13C NMR (75 MHz, CDCl3,
298 K) δ ) 155.8, 138.3, 137.8, 137.6, 134.6, 127.4-128.6
(aromatic carbons), 117.1, 79.1, 74.9, 74.9, 73.4, 73.2, 72.1, 71.8,
68.3, 68.0, 48.9, 35.5, 28.4. MS (ES) calcd for C35H43NO6 (M +
Na)+ 596.30, found 596.30. Anal. Calcd for C35H43NO6: C 73.27,
H 7.55, N 2.44. Found: C 73.43, H 7.75, N 2.19.
Conclusions
In order to extend the molecular repertoire of proline
analogues, we became interested in the design and synthesis of
a bicyclic sugar-proline hybrid. The resulting GlcProH is a
rigid, polyfunctional building block, which may find use as a
proline mimetic, glycomimetic, or scaffold for combinatorial
synthesis. GlcProH served as a building block in peptide
synthesis and was incorporated into model peptides. Simple
modifications of the sugar scaffold permitted tuning of the trans/
(2S,3aR,5R,6R,7S,7aS)-1-(tert-Butyloxycarbonyl)-6,7-di-O-
benzyl-2-iodomethyl-5-[(benzyloxy)methyl]octahydropyrano-
[3,2-b]pyrrole (4). Compound 3 (0.29 g, 0.51 mmol) was dissolved
in 10 mL of 1:1 CH2Cl2/diethyl ether. The solution was cooled to
0 °C before addition of iodine (0.39 g, 1.5 mmol). After 1 h, the
reaction mixture was warmed to ambient temperature before being
worked-up by the addition of 20 mL of saturated aqueous sodium
4640 J. Org. Chem., Vol. 72, No. 13, 2007

Tuning of Prolyl trans/cis-Amide Rotamer Population
thiosulfate. With shaking, the solution became colorless. The
product was extracted into CH2Cl2 (3 × 20 mL), dried (Na2SO4),
concentrated, and purified by flash chromatography with 5:1
hexanes/ethyl acetate. The product 4 was isolated as a single
stereoisomer as a pale yellow oil (0.32 g, 0.46 mmol) (89.6%),
[R]D25 ) -23.9° (c 2.5, CHCl3). 1H NMR (300 MHz, CDCl3, 298
K) δ ) 7.13-7.40 (m, 15H, aromatic), 4.32-4.78 (br m, 8H),
3.64-4.08 (broad m’s, 5H), 3.55 (br m, 1H), 3.45 (br t, 2H), 2.40
(br m, 1H, H3a), 1.9 (m, 1H, H3b), 1.5 (s, 9H, tert-butyl). 13C NMR
(75 MHz, CDCl3, 298 K) δ ) 153.8, 138.8, 138.6, 138.4, 127.4-
128.7 (aromatic carbons), 80.7, 77.6, 74.2, 74.0, 73.7, 73.5, 72.5,
70.2, 69.4, 59.5, 56.8, 36.5, 28.8, 14.8. MS (ES) calcd for C35H43-
INO6 (M + H)+ 700.21, found 700.08; calcd for C35H42INNaO6
(M + Na)+ 722.20, found 722.01. Anal. Calcd for C35H42INO6: C
60.09, H 6.05, N 2.00. Found: C 60.02, H 5.74, N 2.39.
Anal. Calcd for C30H35NO5: C 73.59, H 7.21, N 2.86. Found: C
73.23, H 7.52, N 2.86.
(2S,3aR,5R,6R,7S,7aS)-1-(tert-Butyloxycarbonyl)-6,7-di-O-
benzyl-2-hydroxymethyl-5-[(benzyloxy)methyl]octahydropyrano-
[3,2-b]pyrrole (7). Compound 6 (0.048 g, 0.098 mmol) was
dissolved in 4 mL of methanol. Addition of triethylamine (0.68
mL, 4.9 mmol) was followed by addition of di-tert-butyl dicarbonate
(0.11 g, 0.49 mmol). The reaction mixture was stirred for 16 h.
All reagents and solvent were removed under reduced pressure to
25
provide 7 as a colorless oil (0.052 g, 0.088 mmol) (89.7%), [R]D
1
) -10.4° (c 2.0, CHCl3). H NMR (300 MHz, CDCl3, 313 K) δ
) 7.14-7.41 (m, 15H, aromatic), 4.36-4.90 (m, 8H, -OCH2Ph,
H2, H11a), 3.95-4.14 (m, 3H, H11b, H9, H7), 3.85-3.95 (m, 1H,
H8), 3.52-3.88 (m, 4H, H5, H6, H10a, H10b), 2.31 (br s, 1H, H3a),
1.75 (br s, 1H, H3b), 1.45 (s, 9H, tert-butyl). 13C NMR (75 MHz,
CDCl3, 298 K) δ ) 155.8, 138.4, 138.0, 137.9, 126.9-128.7
(aromatic carbons), 80.9, 75.2, 73.4, 73.2, 72.7, 68.9, 66.8, 60.2,
59.8, 32.1, 29.7, 28.4, 22.7, 14.2. MS (ES) calcd for C35H44NO7
(M + H)+ 590.31, found 590.30. Anal. Calcd for C35H43NO7: C
71.28, H 7.35, N 2.38. Found: C 71.03, H 7.59, N 2.33.
(2S,3aR,5R,6R,7S,7aS)-1-(tert-Butyloxycarbonyl)-6,7-di-O-
benzyl-5-[(benzyloxy)methyl]octahydropyrano[3,2-b]pyrrolo-
[1,2-c]oxazol-3-one (5). Compound 4 (0.28 g, 0.41 mmol) was
dissolved in 6 mL of toluene. Addition of silver acetate (0.68 g,
4.1 mmol) made the solution instantly become colorless. The
reaction was stirred for 16 h at ambient temperature. The reaction
mixture was diluted with 10 mL of ethyl acetate and then was
filtered through Celite. The product was concentrated under reduced
pressure and then purified by flash chromatography with 1:1
hexanes/ethyl acetate to yield 5 as a white solid (0.19 g, 0.37 mmol)
(2S,3aR,5R,6R,7S,7aS)-1-(tert-Butyloxycarbonyl)-6,7-di-O-
benzyl-5-[(benzyloxy)methyl]octahydropyrano[3,2-b]pyrrole-2-
carboxylic Acid Benzyl Ester (8). Compound 7 (0.018 g, 0.029
mmol) was dissolved in 3 mL of DMF. Addition of cesium
carbonate (0.015 g, 0.045 mmol) was followed by addition of benzyl
bromide (0.011 mL, 0.089 mmol). The reaction mixture was stirred
for 1 h, and then the solvent was removed under reduced pressure.
The reaction mixture was diluted with 10 mL of water followed
by extraction into CH2Cl2 (3 × 10 mL), dried over anhydrous Na2-
SO4, and concentrated under reduced pressure before being purified
by flash chromatography with 4:1 hexanes/ethyl acetate to yield 8
as a colorless oil (0.11 g, 0.18 mmol) (83.6%), [R]D25 ) -14.2° (c
25
1
(92.1%), [R]D ) +21.9° (c 2.0, CHCl3), mp 106-111 °C. H
NMR (300 MHz, CDCl3, 298 K) δ ) 7.14-7.48 (m, 15H,
aromatic), 4.95 (d, 1H, J ) 11.1 Hz, -OCH2Ph), 4.88 (d, 1H, J )
11.4 Hz, -OCH2Ph), 4.75 (d, 1H, J ) 11.4 Hz, -OCH2Ph), 4.68
(ddd, 1H, J3a,9 ) 1.0 Hz, J3b,9 ) 5.7 Hz, J8,9 ) 5.7 Hz, H9), 4.42-
4.53 (m, 3H, -OCH2Ph, H11a), 4.38 (d, 1H, J ) 12.2 Hz, -OCH2-
Ph), 4.08-4.20 (m, 2H, H11b, H2), 3.95 (dd, 1H, J7,8 ) 6.3 Hz,
H8), 3.72-3.82 (m, 1H, H5), 3.64-3.72 (m, 2H, H6, H7), 3.53-
3.59 (dd, 1H, J10b,10a ) 10.6 Hz, J5,10a ) 4.7 Hz, H10a), 3.47-3.53
(dd, 1H, J5,10b ) 2.8 Hz, H10b), 2.10 (ddd, 1H, J2,3a ) 5.3 Hz, J3a,3b
) 13.2 Hz, H3a), 1.59 (ddd, 1H, J2,3b ) 10.6 Hz, H3b). 13C NMR
(75 MHz, CDCl3, 298 K) δ ) 161.1, 138.4, 138.1, 138.0, 127.6-
128.4 (aromatic carbons), 80.7, 76.9, 75.4, 74.7, 74.4, 73.5, 73.2,
69.5, 66.8, 65.7, 57.5, 38.1. MS (ES) calcd for C31H33NNaO6 (M
+ Na)+ 538.22, found 538.22; Anal. Calcd for C31H33NO6: C 72.21,
H 6.45, N 2.72. Found: C 72.17, H 6.69, N 2.57.
1
0.4, CHCl3). H NMR (300 MHz, CDCl3, 298 K) δ ) 7.05-7.41
(m, 20H, aromatic), 5.19 (d, 1H, J ) 12.1 Hz, -OCH2Ph), 5.11
(d, 1H, J ) 12.3 Hz, -OCH2Ph), 4.35-4.80 (m, 8H, -OCH2Ph,
H2, H9), 4.28 (dd app, 1H, J7,8 ) 5.6 Hz, J6,7 ) 5.7 Hz, H7), 4.17
(dd app t, 1H, J8,9 ) 5.8 Hz, H8), 3.83-3.93 (m, 1H, H5), 3.70
(dd, 1H, J10a,10b ) 10.4 Hz, J5,10a ) 6.3 Hz, H10a), 3.57 (dd, 1H,
J5,10b ) 3.4 Hz, H10b), 3.48 (dd app t, 1H, J5,6 ) 5.8 Hz, H6), 2.40-
2.55 (m, 1H, H3a), 1.80-2.00 (m, 1H, H3b), 1.38 (s, 9H, tert-butyl).
13C NMR (75 MHz, CDCl3, 298 K) δ ) 172.9, 153.6, 138.4, 138.1,
138.1, 135.3, 127.0-128.7 (aromatic carbons), 80.4, 76.0, 74.8,
73.6, 73.5, 73.4, 72.8, 71.2, 69.0, 66.8, 58.8, 58.2, 33.0, 28.1. MS
(ES) calcd for C42H47NNaO8 (M + Na)+ 716.32, found 716.11.
Anal. Calcd for C42H47NNaO8: C 70.37, H 6.61, N 1.95. Found:
C 70.44, H 6.71, N 1.88.
(2S,3aR,5R,6R,7S,7aS)-6,7-di-O-Benzyl-2-hydroxymethyl-5-
[(benzyloxy)methyl]octahydropyrano[3,2-b]pyrrole (6). Com-
pound 5 (0.18 g, 0.35 mmol) was dissolved in 8 mL of methanol.
After addition of potassium hydroxide (1.5 g, 26.2 mmol), the
solution was heated to reflux for 4 h. The reaction mixture was
then cooled to 0 °C, followed by acidification by addition of 5 mL
of 3 M aqueous HCl. The methanol was removed under reduced
pressure. The reaction mixture was brought to pH 9 by addition of
20 mL of aqueous saturated sodium bicarbonate. The product was
extracted into CH2Cl2 (4 × 10 mL) and then dried (Na2SO4),
concentrated under reduced pressure, and purified by flash chro-
matography with first 1:1 hexanes/ethyl acetate and then 5:1 ethyl
acetate/methanol to yield 6 as a pale yellow solid (0.15 g, 0.27
(2S,3aR,5R,6R,7S,7aS)-1-(tert-Butyloxycarbonyl)-6,7-di-O-
benzyl-5-[(benzyloxy)methyl]octahydropyrano[3,2-b]pyrrole-2-
carboxamide N′-Methylamide (9). Compound 1 (0.10 g, 0.16
mmol) was dissolved in 6 mL of acetonitrile. Addition of diiso-
propylethylamine (0.11 mL, 0.64 mmol) was followed by addition
of TBTU (0.10 g, 0.32 mmol) and methylamine hydrochloride (0.02
g, 0.32 mmol). The reaction mixture was stirred at ambient
temperature for 4 h. The reaction mixture was diluted with 15 mL
of water followed by extraction into CH2Cl2 (3 × 15 mL), dried
(Na2SO4), and then concentrated under reduced pressure and
25
mmol) (78.4%), [R]D ) +38.9° (c 2.8, CHCl3), mp 91-94 °C.
1H NMR (300 MHz, CDCl3, 298 K) δ ) 7.15-7.42 (m, 15H,
aromatic), 4.45-4.65 (m, 6H, -OCH2Ph), 4.25-4.35 (m, 1H, H9),
purified by flash chromatography with 3:1 ethyl acetate/hexanes
25
to provide 9 as a clear oil (0.086 g, 0.14 mmol) (84.3%), [R]D
)
1
4.05-4.11 (m, 1H, H5), 3.82 (dd, 1H, J10a,10b ) 10.2 Hz, J5,10a
)
-3.5° (c 0.4, CHCl3); H NMR (300 MHz, acetone-d6, 298 K) δ
) 7.15-7.45 (m, 15H, aromatic), 4.45-4.82 (m, 8H, -OCH2Ph,
H2, H9), 4.39 (dd app t, 1H, J7,8 ) 5.3 Hz, J6,7 ) 5.3 Hz, H7), 4.28
(dd, 1H, J ) 5.9, 7.9 Hz, H2), 4.11 (dd app t, 1H, J8,9 ) 5.5 Hz,
H8), 3.86-3.95 (m, 1H, H5), 3.78 (dd, 1H, J10a,10b ) 10.5 Hz, J5,10a
) 6.1 Hz, H10a), 3.68 (dd, 1H, J5,10b ) 3.8 Hz, H10b), 3.55 (dd app
t, 1H, J5,6 ) 5.7 Hz, H6), 2.80 (d, 3H, -NHCH3), 2.42 (ddd, 1H,
J ) 6.3, 8.1, 13.4 Hz, H3a), 1.89 (ddd, 1H, J ) 5.8, 6.2, 12.0 Hz,
H3b), 1.37 (s, 9H, tert-butyl). 13C NMR (75 MHz, acetone-d6, 298
K) δ ) 173.8, 154.4, 139.7, 139.5 (2), 128.1-129.0 (aromatic
carbons), 79.4, 76.5, 75.9, 74.0, 73.4 (2), 72.9, 71.5, 69.9, 60.8,
6.7 Hz, H10a), 3.77 (dd app t, 1H, J7,8 ) 4.5 Hz, J6,7 ) 4.4 Hz, H7),
3.61 (dd, 1H, J5,10b ) 5.1 Hz, H10b), 3.56 (dd app t, 1H, J5,6 ) 4.2
Hz, H6), 3.41-3.51 (m, 2H, H11a, H2), 3.29 (br s, 0.3H, NH), 3.25
(dd, 1H, J11a,11b ) 12.1 Hz, J11b,2 ) 7.3 Hz, H11b), 3.09 (dd, 1H,
J8,9 ) 3.7 Hz, H8), 2.0 (ddd, 1H, J ) 2.1, 8.0 Hz, J3a,3b ) 14.0 Hz,
H3a), 1.59-1.69 (m, 1H, J ) 6.0, 7.0 Hz, H3b). 13C NMR (75 MHz,
CDCl3, 298 K) δ ) 138.1, 137.8, 137.5, 127.7-128.6 (aromatic
carbons), 74.4, 73.9, 73.4, 73.3, 72.8, 72.7, 71.8, 67.7, 64.1, 60.2,
57.4, 33.9. MS (ES) calcd for C30H36NO5 (M + H)+ 490.26, found
490.39; calcd for C30H35NNaO5 (M + Na)+ 512.24, found 512.36.
J. Org. Chem, Vol. 72, No. 13, 2007 4641

Owens et al.
59.3, 34.6, 28.2, 25.9. MS (ES) calcd for C36H44N2NaO7 (M +
Na)+ 639.30, found 639.27. Anal. Calcd for C36H44N2O7: C 70.11,
H 7.19, N 4.54. Found: C 70.19, H 7.43, N 4.47.
) 13.5 Hz, H3a, H3a minor), 2.10 (s, 2.55H, -NHCH3), [1.93 (ddd,
0.15H, J2,3b ) 1.8 Hz, J3b,9 ) 6.7 Hz, J3a,3b ) 12.9 Hz, H3b)], [1.83
(s, 0.45H, -COCH3)], 1.79 (ddd, 0.85H, J3b,9 ) 7.8 Hz, H3b, H3b
minor). 13C NMR (75 MHz, D2O, 298 K) (major conformer) δ )
174.6, 174.4, 74.5, 73.8, 73.3, 68.6, 61.5, 61.0, 58.1, 28.3, 26.1,
22.3. HRMS (ES) calcd for C12H20N2O6Na (M + Na)+ 311.1212,
found 311.1214.
(2S,3aR,5R,6R,7S,7aS)-6,7-di-O-Benzyl-5-[(benzyloxy)methyl]-
octahydropyrano[3,2-b]pyrrole-2-carboxamide N′-Methylamide
(10). Compound 9 (0.035 g, 0.057 mmol) was dissolved in 1.5 mL
of CH2Cl2. The reaction mixture was then cooled to 0 °C.
Trifluoroacetic acid (0.5 mL, 6.73 mmol) was added slowly. After
1 h the solution was codistilled with toluene (2 × 5 mL), and was
normally used directly in the next reaction. Purification by flash
chromatography with 10:1 ethyl acetate/methanol provided 10 as
a clear oil (0.027 g, 0.14 mmol) (93.1%), [R]D25 ) -11.2° (c 1.0,
(2S,3aR,5R,6R,7S,7aS)-1-Acetyl-6,7-di O-acetyl-5-[(acetyloxy)-
methyl]octahydropyrano[3,2-b]pyrrole-2-carboxamide N′-Me-
thylamide (13). Compound 12 (0.014 g, 0.048 mmol) was dissolved
in 1 mL of pyridine. Acetic anhydride (0.046 mL, 0.48 mmol) was
added and the reaction mixture was stirred at ambient temperature
for 15 h. All solvent and reagent were removed under reduced
pressure, providing compound 13 as a clear oil (0.020 g, 0.048
1
CHCl3). H NMR (500 MHz, CDCl3, 298 K) δ ) 7.58 (br q, 1H,
-NHCH3), 7.20-7.38 (m, 15H, aromatic), 4.47-4.62 (m, 6H,
-OCH2Ph), 4.18-4.27 (m, 1H, H2), 4.04-4.14 (m, 1H, H5), 3.78-
3.91 (m, 2H, H9, H10a), 3.73 (br dd, 1H, J7,8 ) 3.9 Hz, J6,7 ) 3.8
Hz, H7), 3.65 (dd, J10a,10b ) 10.2 Hz, J5,10b ) 5.6 Hz, H10b), 3.59
(br dd, 1H, J5,6 ) 3.8 Hz, H6), 2.89 (br dd, 1H, J ) 3.0, 3.3 Hz,
-NH), 2.78 (d, 3H, J ) 5.0 Hz, -NHCH3), 2.36 (ddd, 1H, J ) 1.96,
25
1
mmol) (quant), [R]D ) +53.3° (c 0.3, CHCl3). H NMR (500
MHz, CDCl3, 298 K, 0.036 M) δ ) 6.37 (br q, 0.87H, -NHCH3),
[5.97 (br q, 0.13H, -NHCH3)], [5.25 (dd, 0.13H, H7)], 5.22 (dd
app t, 0.87H, J7,8 ) 9.8 Hz, J6,7 ) 10.0 Hz, H7), 5.10 (ddd, 0.87H,
J3a,9 ) 11.8 Hz, J3b,9 ) 7.1 Hz, J8,9 ) 7.3 Hz, H9), [5.03 (dd, 0.13H,
J6,7 ) 9.5 Hz, J5,6 ) 9.2 Hz, H6)], 4.99 (dd app t, 0.87H, J5,6 ) 9.9
9.6, 14.4 Hz, H3a), 1.95 (ddd, 1H, J ) 5.0, 7.0, 14.4 Hz, H3b). 13
C
NMR (75 MHz, CDCl3, 298 K) δ ) 175.3, 138.1, 137.8, 137.6,
127.6-128.6 (aromatic carbons), 74.2, 73.5, 73.3, 72.8, 72.6, 72.4,
71.6, 67.6, 60.9, 58.8, 36.7, 25.7. MS (ES) calcd for C31H37N2O5
(M + H)+ 517.27, found 517.30. Anal. Calcd for C31H36N2O5: C
72.07, H 7.02, N 5.42. Found: C 72.11, H 7.13, N 5.36.
Hz, H6), [4.70 (ddd, 0.13H, J3a,9 ) 11.6 Hz, J3b,9 ) 6.8 Hz, J8,9
)
7.0 Hz, H9)], [4.54 (dd, 0.13H, J7,8 ) 7.8 Hz, H8)], [4.41 (dd,
0.13H, J2,3a ) 9.4 Hz, J2,3b ) 1.5 Hz, H2)], 4.27-4.35 (m, 1.87H,
H2, H10a, H10a minor), 4.19 (dd, 0.87H, H8), 4.08 (dd, 1H,
J5,10b ) 2.4 Hz, J10a,10b ) 12.5 Hz, H10b, H10b minor), 3.98
(ddd, 0.87H, J5,10a ) 4.3 Hz, H5), [3.89 (ddd, 0.13H, J5,10b ) 2.6
Hz, J5,10a ) 4.9 Hz, H5)], [2.84 (d, 0.39H, J ) 4.8 Hz, -NHCH3)],
(2S,3aR,5R,6R,7S,7aS)-1-Acetyl-6,7-di-O-benzyl-5-[(benzyloxy)-
methyl]octahydropyrano[3,2-b]pyrrole-2-carboxamide N′-Me-
thylamide (11). Compound 10 (0.029 g, 0.056 mmol) was dissolved
in 2 mL of pyridine followed by addition of acetic anhydride (0.053
mL, 0.56 mmol). The reaction mixture was stirred for 15 h, and
then the solvent and reagents were removed under reduced pressure
and the product was purified by flash chromatography with 10:1
ethyl acetate/methanol to provide 11 as a white solid (0.030 g, 0.054
2.81 (d, 2.61H, J ) 4.8 Hz, -NHCH3), [2.73 (ddd, 0.13H, J3a,3b
)
12.8 Hz, H3a)], 2.48 (ddd, 0.87H, J2,3a ) 9.8 Hz, J3a,3b ) 12.3 Hz,
H3a), 2.13 (s, 2.61H, -COCH3), 2.09 (s, 3H, -COCH3, -COCH3
minor), 2.06 (s, 2.61H, -COCH3), [2.04 (s, 0.39H, -COCH3)],
[2.03 (s, 0.39H, -COCH3)], [2.02 (s, 0.39H, -COCH3)], 2.00-
2.05 (m, 0.87H, H3b), 2.01 (s, 2.61H, -COCH3), [1.86 (s, 0.39H,
-COCH3)]. 13C NMR (75 MHz, CDCl3, 298 K) (major conformer)
δ ) 171.8, 170.7, 170.3, 169.9, 169.5, 73.9, 73.8, 69.7, 68.0, 62.2,
58.6, 58.1, 28.2, 26.4, 22.0, 20.9, 20.7, 20.6. HRMS (ES) calcd
for C18H27N2O9 (M + H)+ 415.1711, found 415.1711.
25
mmol) (97% over two steps), [R]D ) +49.6°(c 1.0, CHCl3),
decomposed at 142-147 °C. 1H NMR (500 MHz, CDCl3, 298 K,
0.036 M) δ ) 7.23-7.41 (m, 13H, aromatic), 7.11-7.20 (m, 2H,
aromatic), 5.97 (br q, 1H, -NHCH3), 4.98 (ddd, 1H, J3a,9 ) 11.7
Hz, J3b,9 ) 7.3 Hz, J8,9 ) 7.4 Hz, H9), 4.93 (d, 1H, J ) 11.2 Hz,
-OCH2Ph), 4.80 (d, 1H, J ) 10.8 Hz, -OCH2Ph), 4.52-4.64 (m,
4H, -OCH2Ph), 4.25 (dd app d, 1H, J2,3a ) 9.5 Hz, J2,3b ) 0.8 Hz,
H2), 4.03 (dd, 1H, J7,8 ) 9.2 Hz, H8), 3.63-3.78 (m, 4H, H5, H6,
N-Acetylglycyl-(2S,3aR,5R,6R,7S,7aS)-6,7-di-O-benzyl-5-[(ben-
zyloxy)methyl]octahydropyrano[3,2-b]pyrrole-2-carboxamide N′-
Methylamide (14). Compound 10 (0.105 g, 0.203 mmol) was
dissolved in 6 mL of N,N-dimethylformamide and cooled to 0 °C.
The reaction was stirred under inert atmosphere. Diisopropylethy-
lamine (0.212 mL, 1.218 mmol) and PyBOP (0.317 g, 0.609 mmol)
were added and the solution was stirred for 10 min. Fmoc-Gly-
OH (0.181 g, 0.609 mmol) was added and the reaction mixture
was stirred for a further 5 min before being allowed to warm to
ambient temperature, where it was stirred for 18 h. The red solution
was diluted with ethyl acetate, washed with 1 M HCl (10 mL) and
then brine (10 mL), dried, and evaporated, giving a red oil. The
product was purified by flash chromatography with ethyl acetate,
giving a white solid product (0.085 g) (53%), along with unreacted
starting material (0.025 g) (24%). The coupled product was
dissolved in 4 mL of dichloromethane, cooled to 0 °C, and treated
with piperidine (1 mL). The reaction mixture was stirred for 1 h
before the solvent and reagents were removed under reduced
pressure, leaving a white solid. The intermediate was dissolved in
4 mL of pyridine followed by addition of acetic anhydride (0.5
mL). The reaction mixture was stirred for 15 h, and then the solvent
and reagents were removed under reduced pressure and the product
was purified by flash chromatography with 10:1 ethyl acetate/
methanol to provide 14 as a clear oil (0.059 g, 0.096 mmol) (47.1%
H10a, H10b), 3.57 (dd app t, 1H, J6,7 ) 9.0 Hz, H7), 2.82 (d, 3H, J
) 4.2 Hz, -NHCH3), 2.35 (ddd, 1H, J3a,3b ) 12.3 Hz, H3a), 2.15 (s,
3H, -COCH3), 2.05 (ddd, 1H, H3b). 13C NMR (75 MHz, CDCl3,
298 K) (major conformer) δ ) 172.1, 171.6, 137.8, 137.7, 137.5,
127.8-128.5 (aromatic carbons), 83.0, 78.0, 75.9, 74.9, 73.7, 73.6,
73.3, 68.8, 60.1, 57.8, 28.4, 26.4, 22.9. HRMS (ES) calcd for
C33H39N2O6 (M + H)+ 559.2802, gounf 559.2801.
(2S,3aR,5R,6R,7S,7aS)-1-Acetyl-6,7-dihydroxy-5-(hydroxym-
ethyl)octahydropyrano[3,2-b]pyrrole-2-carboxamide N′-Methy-
lamide (12). Compound 11 (0.027 g, 0.048 mmol) was dissolved
in 10 mL of methanol. Addition of Pearlman’s catalyst (20%
palladium hydroxide on carbon) (0.030 g, approximately 0.028
mmol) was followed by addition of 1 M aqueous HCl (0.072 mL,
0.072 mmol). The reaction mixture was stirred vigorously under
hydrogen atmosphere (10 psi) for 4.5 h, after which it was flushed
with nitrogen and filtered. The product was then concentrated under
reduced pressure to provide 12 as a clear oil (0.014 g, 0.048 mmol)
(quant), [R]D25 ) +20.5° (c 1.0, CHCl3); 1H NMR (500 MHz, D2O,
298 K, 0.035 M) δ ) 4.55-4.64 (m, 0.85H, H9), 4.32 (dd app d,
0.85H, J2,3a ) 10.2 Hz, J2,3b ) 1.3 Hz, H2), [4.11 (dd, 0.15H, J8,9
) 7.7 Hz, J7,8 ) 8.0 Hz, H8)], 3.99 (dd, 0.85H, J8,9 ) 7.2 Hz, J7,8
) 9.2 Hz, H8), 3.70 (dd, 0.85H, J5,10a ) 2.2 Hz, J10a,10b ) 12.4 Hz,
25
1
over three steps), [R]D ) +23.5°(c 1.0, CHCl3). H NMR (500
MHz, CDCl3, 298 K, 0.033 M) δ ) 7.10-7.38 (m, 15H, aromatic),
6.30 (t, 1H, -NH(Gly), -NH(Gly) minor), 6.22 (q, 1H, -NHCH3,
-NHCH3 minor), 4.95 (d, 0.95H, J ) 11.3 Hz, -OCH2Ph), 4.83
(ddd, 0.95H, J3b,9 ) 7.6 Hz, J3a,9 ) 12.0 Hz, J8,9 ) 7.2 Hz, H9),
4.75 (d, 0.95H, J ) 10.8 Hz, -OCH2Ph), 4.51-4.64 (m, 4.15H,
H10a), [3.67-3.72 (m, 0.15H, H10a)], 3.62 (dd, 0.85H, J5,10b ) 5.0
Hz, H10b), [3.60 (dd, 0.15H, J ) 1.7, 12.5 Hz, H10b)], 3.51-3.56
(m, 0.85H, H5), 3.51 (dd app t, 0.85H, J6,7 ) 9.9 Hz, H7, H7 minor),
[3.45-3.50 (m, H5)], [3.35 (dd, 0.15H, J5,6 ) 9.9 Hz, H6)], 3.32
(dd app t, 0.85H, J5,6 ) 9.7 Hz, H6), [2.63 (s, 0.45H, -COCH3)],
2.58 (s, 2.55H, -COCH3), 2.55 (ddd, 0.85H, J3a,9 ) 11.7 Hz, J3b,3a
4642 J. Org. Chem., Vol. 72, No. 13, 2007

Tuning of Prolyl trans/cis-Amide Rotamer Population
-OCH2Ph, -OCH2Ph minor, H9 minor), [4.22 (dd, 0.05H, J8,9
)
Methylamide (16). Compound 15 (0.012 g, 0.035 mmol) was
dissolved in 1 mL of pyridine. Acetic anhydride (0.034 mL, 0.35
mmol) was added and the reaction mixture was stirred at ambient
temperature for 15 h. All solvent and reagent were removed under
reduced pressure and the product was purified by flash chroma-
tography with 10:1 ethyl acetate/methanol to provide 16 as a clear
oil (0.014 g, 0.030 mmol) (74%), [R]D25 ) +37.3°(c 0.3, CHCl3).
1H NMR (500 MHz, CDCl3, 298 K, 0.03 M) δ ) 6.56 (br q, 0.2H,
J ) 4.6 Hz, -NHCH3), [6.39 (br dd, 0.2H, -NH(Gly))], 6.31 (dd,
0.8H, -NH(Gly)), 6.16 (q, 0.8H, J ) 4.6 Hz, -NHCH3), [5.27 (dd,
0.2H, J7,8 ) 8.3 Hz, J6,7 ) 9.1 Hz, H7)], 5.19 (dd app t, 0.8H, J7,8
) 9.4 Hz, J6,7 ) 9.9 Hz, H7), 5.03 (ddd, 0.8H, J3a,9 ) 11.8 Hz,
J3b,9 ) 7.5 Hz, J8,9 ) 7.2 Hz, H9), [5.00-5.05 (m, 0.2H, H6)], 4.99
(dd app t, 0.8H, J5,6 ) 9.8 Hz, H6), [4.84 (ddd, 0.2H, J3a,9 ) 10.9
Hz, J3b,9 ) 6.7 Hz, J8,9 ) 7.3 Hz, H9)], [4.52 (dd app t, 0.2H, J )
6.5 Hz, J7,8 ) 8.2 Hz, H8)], 4.17 (dd app d, 0.95H, J2,3b ) 1.0 Hz,
J2,3a ) 9.7 Hz, H2), 4.05-4.13 (m, 1.9H, H8, HR1(Gly), H2 minor),
4.02 (dd, 0.95H, JHR2(Gly),NH ) 4.1 Hz, JHR1(Gly),HR2(Gly) ) 17.1 Hz,
HR2(Gly)), 3.68-3.76 (m, 2H, H7, H10a, H7 minor, H10a minor), 3.61-
3.67 (m, 2.1H, H5, H10b, H5 minor, H10b minor, HR2(Gly) minor,
HR1(Gly) minor), 3.53 (dd app t, 0.95H, J6,7 ) 9.7 Hz, J5,6 ) 9.4 Hz,
H6), [3.52-3.58 (m, 0.05H, H6)], [2.80 (d, 0.15H, J ) 5.0 Hz,
-NHCH3)], 2.74 (d, 2.85H, J ) 5.0 Hz, -NHCH3), 2.36 (ddd, 1H,
J3a,3b ) 12.5 Hz, H3a, H3a minor), 1.99 (ddd, 1H, H3b, H3b minor),
1.92 (s, 3H, -COCH3, -COCH3 minor). 13C NMR (75 MHz,
CDCl3, 298 K) δ ) 171.5, 170.5, 169.5, 137.8, 137.6, 137.3,
127.7-128.8 (aromatic carbons), 81.8, 78.2, 75.7, 74.8, 73.8, 73.7,
73.4, 68.7, 59.0, 58.4, 42.9, 28.6, 26.4, 22.7. HRMS (ES) calcd
for C35H41N3O7Na (M + Na)+ 638.2837, found 638.2841.
N-Acetylglycyl-(2S,3aR,5R,6R,7S,7aS)-6,7-dihydroxy-5-(hy-
droxymethyl)octahydropyrano[3,2-b]pyrrole-2-carboxamide N′-
Methylamide (15). Compound 14 (0.020 g, 0.032 mmol) was
dissolved in 5 mL of methanol. Addition of Pearlman’s catalyst
(20% palladium hydroxide on carbon) (0.020 g, approximately
0.019 mmol) was followed by addition of 1 M aqueous HCl (0.010
mL, 0.010 mmol). The reaction mixture was stirred vigorously under
hydrogen atmosphere (10 psi) for 4.5 h, after which it was flushed
with nitrogen and filtered. The product was then concentrated under
reduced pressure to provide 15 as a yellow oil (0.012 g, 0.035
mmol) (quant), [R]D25 ) -2.5° (c 0.6, CHCl3). 1H NMR (500 MHz,
D2O, 298 K, 0.035 M) δ ) 4.57-4.66 (m, 0.9H, H9), 4.47 (d, 0.9H,
7.7 Hz, H8)], 4.44 (dd, 1H, JHR1(Gly),NH ) 6.3 Hz, JHR1(Gly),HR2(Gly) )
17.4 Hz, HR1(Gly), H2 minor), 4.26-4.35 (m, 1.8H, H2, H10a, H10a
minor), 4.23 (dd, 0.8H, H8), 4.08 (dd, 1H, J5,10b ) 2.3 Hz, J10a,10b
) 12.2 Hz, H10b, H10b minor), 3.97 (ddd, 0.8H, J5,10a ) 4.2 Hz,
H5), [3.86-3.92 (m, 0.4H, H5, HR1(Gly))], 3.86 (dd, 0.8H, JHR2(Gly),NH
) 3.1 Hz, HR2(Gly)), [3.56 (dd, 0.2H, JHR2(Gly),NH ) 3.5 Hz,
JHR1(Gly),HR2(Gly) ) 16.9 Hz, HR2(Gly))], [2.83 (d, 0.6H, J ) 4.6 Hz,
-NHCH3)], 2.80 (d, 2.4H, J ) 4.6 Hz, -NHCH3), [2.68 (m, 0.2H,
J2,3a ) 9.5 Hz, J3a,3b ) 12.6 Hz, H3a)], 2.55 (ddd, 0.8H, J2,3a
)
10.0 Hz, J3a,3b ) 12.5 Hz, H3a), 2.13 (s, 2.4H, -COCH3), 2.09 (s,
3H, -COCH3), 1.98-2.04 (m, 1H, H3b, H3b minor), [2.02 (s, 0.6H,
-COCH3)], 2.01 (s, 3H, -COCH3, -COCH3 minor), 1.99 (s, 3H,
-COCH3, -COCH3 minor). 13C NMR (75 MHz, CDCl3, 298 K)
δ ) (major conformer) 171.4, 170.7, 170.4, 170.4, 169.4, 168.8,
73.6, 73.4, 69.8, 68.1, 62.1, 58.6, 57.1, 41.9, 28.1, 26.5, 22.9, 20.9,
20.7, 20.5. HRMS (ES) calcd for C20H29N3O10Na (M + Na)+
494.1745, found 494.1743.
JHR1(Gly),HR2(Gly) ) 17.2 Hz, HR1(Gly)), 4.35 (dd app d, 1H, J2,3a
)
10.3 Hz, J2,3b ) 1.0 Hz, H2, H2 minor), [4.14 (dd app t, 0.1H, J8,9
) 7.2 Hz, J7,8 ) 7.7 Hz, H8)], 4.01 (dd, 0.9H, J8,9 ) 7.2 Hz, J7,8
) 9.0 Hz, H8), 3.97 (d, 0.9H, HR2(Gly)), [3.87 (d, 0.1H, JHR1(Gly),HR2(Gly)
) 17.1 Hz, HR1(Gly))], 3.70 (dd, 1H, J5,10a ) 2.3 Hz, J10a,10b ) 12.2
Hz, H10a, H10a minor), 3.62 (dd, 1H, J5,10b ) 5.0 Hz, H10b, H10b
minor), [3.45-3.51 (m, 0.1H, H5)], 3.50-3.58 (m, 1.9H, H5, H7,
H7 minor), [3.36 (dd, 0.1H, H6)], 3.32 (dd app t, 0.9H, J6,7 ) 9.9
Hz, J5,6 ) 9.5 Hz, H6), [2.64 (s, 0.3H, -NHCH3)], 2.58 (s, 2.7H,
-NHCH3), 2.54 (ddd, 1H, J3a,9 ) 11.2 Hz, J3a,3b ) 12.7 Hz, H3a,
H3a minor), 1.90 (s, 2.7H, -COCH3), 1.79 (ddd, 1H, J3b,9 ) 7.7
Hz, H3b, H3a minor). 13C NMR (75 MHz, D2O, 298 K) δ ) 175.0,
174.4, 171.3, 74.5, 73.8, 73.5, 68.8, 61.1, 60.4, 58.6, 42.4, 27.9,
26.2, 22.0. HRMS (ES) calcd for C14H23N3O7Na (M + Na)+
368.1428, found 368.1427.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada (NSERC) and the
University of Manitoba (UMGF) for financial support.
Supporting Information Available: 1H and 13C NMR spectra
of new compounds, tables of coupling constants for 11-16 (Tables
4 and 5), and tables of observed NOE interproton effects for 11-
16 (Tables 6 and 7). This material is available free of charge via
the Internet at http://pubs.acs.org.
N-Acetylglycyl-(2S,3aR,5R,6R,7S,7aS)-6,7-di-O-acetyl-5-[(acetyl-
oxy)methyl]octahydropyrano[3,2-b]pyrrole-2-carboxamide N′-
JO0700833
J. Org. Chem, Vol. 72, No. 13, 2007 4643