Conformationally Constrained Dipeptide Surrogates with Aromatic Side-Chains: Synthesis of 4-Aryl Indolizidin-9-one Amino Acids by Conjugate Addition to a Common α,ω-Diaminoazelate Enone Intermediate

Article abstract of DOI:10.1021/jo0355855

Four methyl 9-oxo-8-(N-(Boc)-amino)-4-phenyl-1-azabicyclo[4.3.0]nonane carboxylates (11, 4-Ph-I⁹aa-OMe) were synthesized from (2S,8S,5E)-di-tert-butyl-4-oxo-5-ene-2,8-bis[N-(PhF)amino]azelate [(5E)-7, PhF = 9-(9-phenylfluorenyl)] via a seven-step process featuring a conjugate addition/reductive amination/lactam cyclization sequence. Various nucleophiles were used in the conjugate addition reactions on enone (5E)-7 as a general route for making α,ω-diaminoazelates possessing different substituents in good yield albeit low diastereoselectivity except in the case of aryl Grignard reagents (9/1 to 15/1 drs). 6-Phenylazelates (6S)-8d and (6R)-8d were separated by chromatography and diastereoselective precipitation and independently transformed into 4-Ph-I⁹aa-OMe. From (6S)-8d, (2S,4R,6R,8S)-4-Ph-I⁹aa-OMe 11 was prepared selectively in 51% yield. Reductive amination of (6R)-8d provided the desired pipecolates 9 along with desamino compound 10, which was minimized by performing the hydrogenation in the presence of ammonium acetate. Subsequent ester exchange, lactam cyclization, and amine protection provided three products (2R,4S,6S,8R)-,(2R,4S,6S,8S)-, and (2S,4S,6R,8S)-4-Ph-I⁹aa-OMe 11 in 10, 6, and 6% yields, respectively, from (6R)-8d. Ester hydrolysis of (2S,4R,6R,8S)-11 furnished 4-phenyl indolizidin-9-one N-(Boc)amino acid 3 as a novel constrained Ala-Phe dipeptide surrogate for studying conformation-activity relationships of biologically active peptides.

Full text of DOI:10.1021/jo0355855

Con for m a tion a lly Con str a in ed Dip ep tid e Su r r oga tes w ith
Ar om a tic Sid e-Ch a in s: Syn th esis of 4-Ar yl In d olizid in -9-on e
Am in o Acid s by Con ju ga te Ad d ition to a Com m on
r,ω-Dia m in oa zela te En on e In ter m ed ia te
J e´roˆme Cluzeau and William D. Lubell*
De´partement de Chimie, Universite´ de Montre´al, C. P. 6128, Succursale Centre Ville,
Montre´al, Que´bec, Canada H3C 3J 7
lubell@chimie.umontreal.ca
Received October 28, 2003
Four methyl 9-oxo-8-(N-(Boc)-amino)-4-phenyl-1-azabicyclo[4.3.0]nonane carboxylates (11, 4-Ph-
I9aa-OMe) were synthesized from (2S,8S,5E)-di-tert-butyl-4-oxo-5-ene-2,8-bis[N-(PhF)amino]azelate
[(5E)-7, PhF ) 9-(9-phenylfluorenyl)] via a seven-step process featuring a conjugate addition/
reductive amination/lactam cyclization sequence. Various nucleophiles were used in the conjugate
addition reactions on enone (5E)-7 as a general route for making R,ω-diaminoazelates possessing
different substituents in good yield albeit low diastereoselectivity except in the case of aryl Grignard
reagents (9/1 to 15/1 drs). 6-Phenylazelates (6S)-8d and (6R)-8d were separated by chromatography
and diastereoselective precipitation and independently transformed into 4-Ph-I9aa-OMe. From (6S)-
8d , (2S,4R,6R,8S)-4-Ph-I9aa-OMe 11 was prepared selectively in 51% yield. Reductive amination
of (6R)-8d provided the desired pipecolates 9 along with desamino compound 10, which was
minimized by performing the hydrogenation in the presence of ammonium acetate. Subsequent
ester exchange, lactam cyclization, and amine protection provided three products (2R,4S,6S,8R)-,
(2R,4S,6S,8S)-, and (2S,4S,6R,8S)-4-Ph-I9aa-OMe 11 in 10, 6, and 6% yields, respectively, from
(6R)-8d . Ester hydrolysis of (2S,4R,6R,8S)-11 furnished 4-phenyl indolizidin-9-one N-(Boc)amino
acid 3 as a novel constrained Ala-Phe dipeptide surrogate for studying conformation-activity
relationships of biologically active peptides.
In tr od u ction
of the indolizidin-9-one Leu-enkephalin analogue also
exhibited a curve shape characteristic of a turn confor-
mation in support of the importance of a turn about the
glycine residues for activity of the native peptide.
Turn motifs play important roles in the recognition and
activity of biologically relevant peptides and proteins.1
Conformationally constrained mimics of peptide turns
have thus become important synthesis targets because
of their utility for studying the relationship between
structure and function as well as their potential to
improve the pharmacological properties of the native
peptide.2 Among turn mimics,3 azabicyclo[X.Y.0]alkane
amino acids have proven to be useful for constraining
peptide backbone and side-chain geometry for studying
structure-activity relationships.4-9
In the context of our research on peptide mimicry, we
have employed different azabicyclo[X.Y.0]alkane amino
acids in structure-activity studies of various peptides.10-12
For example, introduction of indolizidin-9-one amino acid
1 (I9aa) into a Leu-enkephalin (YGGFL) analogue as a
rigid surrogate of its Gly-Gly residue produced an active
mimic that exhibited greater duration of action relative
to the parent peptide.10 The circular dichroism spectrum
Approaches for installing side-chain functionality at
the different ring-positions on the indolizidin-9-one amino
acid may furnish improved dipeptide surrogates for
peptide mimicry. Although methodology is abundant for
adding side-chain functionality to the 3-,5e,i,x 4-,5a,b,d,l,s,v
5-,5c,6a,b 6-,5o 7-,5o,p,q,u,y,6a,b,c and 8-positions5u,v of the
related fused 6,5-ring system, indolizidin-2-one amino
acid 2 (I2aa), few methods have been reported for adding
side-chains onto its fused 5,6-ring counterpart, indolizi-
din-9-one 1 (Figure 1).7b,c For example, we have previ-
ously reported on three effective routes for appending
a series of aliphatic as well as a set of heteroatomic
side chains onto the 5- and 7-positions of indolizidin-2-
one amino acid, using a common â-keto ester starting
material obtained from the Claisen condensation of
R-tert-butyl γ-methyl N-(PhF)glutamate (Scheme 1).6
On the other hand, Boc-IBTM 4 (Figure 1), a condensa-
tion product of the Asp-Trp dipeptide represents a rare
example of an indolizidin-9-one amino acid possessing
side-chain functionality.7b Replacement of the D-Phe-Pro
residue of gramicidin S (c-[D-Phe-Pro-Val-Orn-Leu]2) with
IBTM has produced analogues with potency similar to
the parent antibiotic,13 contingent on the indolizidin-9-
(1) Rose, G. D.; Gierasch, L. M.; Smith, J . A. Adv. Protein Chem.
1985, 37, 1-109.
(2) Ball, J . B.; Alewood, P. F. J . Mol. Recogn. 1990, 3, 55-64.
(3) For review see: Hanessian, S.; McNaughton-Smith, G.; Lombart,
H.-G.; Lubell, W. D. Tetrahedron 1997, 53, 12789-12854.
(4) Pyrrolizidinone amino acids: Dietrich, E.; Lubell, W. D. J . Org.
Chem. 2003, 68, 6988-6996 and references therein.
10.1021/jo0355855 CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/11/2004
1504
J . Org. Chem. 2004, 69, 1504-1512

Synthesis of 4-Aryl Indolizidin-9-one Amino Acids
SCHEME 1. Gen er a l Str a tegy for Aza bicyclo[X.Y.0]a lk a n e Am in o Acid Syn th esis
one ring-fusion stereochemistry. The promising biological
activity of the enkephalin analogue (Tyr-I9aa-Phe-Leu)
and antibiotic peptide analogue (c-[IBTM-Val-Orn-Leu-
D-Phe-Pro-Val-Orn-Leu]), possessing indolizidin-9-one
amino acids 1 and 4, illustrates their effectiveness as
mimics of secondary structure and suggests similar
potential for peptide mimicry with related analogues
possessing side-chain ring-substituents.
In our general strategy for constructing indolizidin-9-
one amino acid systems, linear R,ω-diaminoazelate enone
(5E)-7 was converted to the heterocycle via a reductive
amination/lactam cyclization sequence (Scheme 1).7a
Conjugate addition (1,4-addition) reactions on enone
(5) Indolizidin-2-one amino acids: (a) Hanessian, S.; Ronan, B.
Bioorg. Med. Chem. Lett. 1994, 4, 1397-1400. (b) Li, W.; Hanau, C.
E.; d’Avignon, A.; Moeller, K. D. J . Org. Chem. 1995, 60, 8155-8170.
(c) Hanessian, S.; McNaughton-Smith, G. Bioorg. Med. Chem. Lett.
1996, 6, 1567-1572. (d) Li, W.; Moeller, K. D. J . Am. Chem. Soc. 1996,
118, 10106-10112. (e) Salimbeni, A.; Paleari, F.; Canevotti, R.;
Criscuoli, M.; Lippi, A.; Angiolini, M.; Belvisi, L.; Scolastico, C.;
Colombo, L. Bioorg. Med. Chem. Lett. 1997, 7, 2205-2210. (f) Gennari,
C.; Mielgo, A.; Potenza, D.; Scolastico, C.; Piarulli, U.; Manzoni, L. Eur.
J . Org. Chem. 1999, 379-388. (g) Belvisi, L.; Gennari, C.; Mielgo, A.;
Potenza, D.; Scolastico, C. Eur. J . Org. Chem. 1999, 389-400. (h)
Wessig, P. Tetrahedron Lett. 1999, 40, 5987-5988. (i) Boatman, P. D.;
Ogbu, C. O.; Eguchi, M.; Kim, H.-O.; Nakanishi, H.; Cao, B.; Shea, J .
P.; Kahn, M. J . Med. Chem. 1999, 42, 1367-1375. (j) Angiolini, M.;
Araneo, S.; Belvisi, L.; Cesarotti, E.; Checchia, A.; Crippa, L.; Manzoni,
L.; Scolastico, C. Eur. J . Org. Chem. 2000, 2571-2581. (k) Estiarte,
M. A.; Rubiralta, M.; Diez, A.; Thormann, M.; Giralt, E. J . Org. Chem.
2000, 65, 6992-6999. (l) Mulzer, J .; Schulzchen, F.; Bats, J .-W.
Tetrahedron 2000, 56, 4289-4298. (m) Beal, L. M.; Liu, B.; Chu, W.;
Moeller, K. D. Tetrahedron 2000, 56, 10113-10125. (n) Wang, W.;
Xiong, C.; Hruby, V. J . Tetrahedron Lett. 2001, 42, 3159-3161. (o)
Belvisi, L.; Colombo, L.; Colombo, M.; Di Giacomo, M.; Manzoni, L.;
Vodopivec, B.; Scolastico, C. Tetrahedron 2001, 57, 6463-6473. (p)
Manzoni, L.; Colombo, M.; May, E.; Scolastico, C. Tetrahedron 2001,
57, 249-255. (q) Zhang, X.; J iang, W.; Schmitt, A. C. Tetrahedron Lett.
2001, 42, 4943-4945. (r) Millet, R.; Domarkas, J .; Rombaux, P.; Rigo,
B.; Houssin, R.; He´nichart, J .-P. Tetrahedron Lett. 2002, 43, 5087-
5088. (s) Zhang, J .; Xiong, C.; Wang, W.; Ying, J .; Hruby, V. J . Org.
Lett. 2002, 4, 4029-4032. (t) Sun, H.; Moeller, K. D. Org. Lett. 2002,
4, 1547-1550. (u) Wang, W.; Yang, J .; Ying, J .; Xiong, C.; Zhang, J .;
Cai, C.; Hruby, V. J . J . Org. Chem. 2002, 67, 6353-6360. (v) Zhang,
J .; Xiong, C.; Ying, J .; Wang, W.; Hruby, V. J . Org. Lett. 2003, 5, 3115-
3118. (w) Hanessian, S.; Sailes, H.; Munro, A.; Therrien, E. J . Org.
Chem. 2003, 68, 7219-7233. (x) Gardiner, J .; Abell, A. D. Tetrahedron
Lett. 2003, 44, 4227-4230. (y) Artale, E.; Banfi, G.; Belvisi, L.;
Colombo, L.; Colombo, M.; Manzoni, L.; Scolastico, C. Tetrahedron
2003, 59, 6241-6250.
(6) (a) Polyak, F.; Lubell, W. D. J . Org. Chem. 1998, 63, 5937-5949.
(b) Polyak, F.; Lubell, W. D. J . Org. Chem. 2001, 66, 1171-1180. (c)
Feng, Z.; Lubell, W. D. J . Org. Chem. 2001, 66, 1181-1185.
(7) Indolizidin-9-one amino acids: (a) Gosselin, F.; Lubell, W. D. J .
Org. Chem. 1998, 63, 7463-7471. (b) De La Figuera, N.; Rosas, I.;
Garcia-Lopez, M. T.; Gonzalez-Muniz, R. J . Chem. Soc., Chem. Comm.
1994, 613. (c) Lamazzi, C.; Carbonnel, S.; Calinaud, P.; Troin Y.
Heterocycles 2003, 60, 1447-1456. (d) Shimizu, M.; Nemoto, H.;
Kakuda, H.; Takahata, H. Heterocycles, 2003, 59, 245-255.
(8) Pyrroloazepinone amino acids: (a) Tremmel, P.; Geyer, A. J . Am.
Chem. Soc. 2002, 124, 8548-8549. (b) Gosselin, F.; Lubell, W. D. J .
Org. Chem. 2000, 65, 2163-2171. (c) Geyer, A.; Moser, F. Eur. J . Org.
Chem. 2000, 1113-1120. (d) Bracci, A.; Manzoni, L.; Scolastico, C.
Synthesis 2003, 15, 2363-2367.
F IGURE 1. Representative azabicylo[X.Y.0]alkanone amino
acids.4,7a-c,9
(5E)-7 have now provided a series of R,ω-diaminoazelate
ketones 8 for conversion to a variety of substituted
indolizidin-9-ones.14 Conditions for the conversion of
(11) Roy, S.; Lombart, H.-G.; Hancock, R. E. W.; Farmer, S. W.;
Lubell, W. D. J . Pept. Res. 2002, 60, 198-214.
(12) Halab, L.; Becker, J . A. J .; Darula, Z.; Tourwe, D.; Kieffer, B.
L.; Simonin, F.; Lubell, W. D. J . Med. Chem. 2002, 45, 5353-5357.
(13) Andreu, D.; Ruiz, S.; Carreno, C.; Alsina, J .; Albericio, F.;
J ime´nez, M. A.; De La Figuera, N.; Herranz, R.; Garcia-Lopez M. T.;
Gonzalez-Muniz, R. J . Am. Chem. Soc. 1997, 119, 10579-10586.
(9) Lombart, H.-G.; Lubell, W. D. J . Org. Chem. 1994, 59, 6147-
6149. (b) Lombart, H.-G.; Lubell, W. D. J . Org. Chem. 1996, 61, 9437-
9446.
(10) Gosselin, F.; Tourwe, D.; Ceusters, M.; Meert, T.; Heylen, L.;
J urzak, M.; Lubell, W. D. J . Pept. Res. 2001, 57, 337-344.
J . Org. Chem, Vol. 69, No. 5, 2004 1505

Cluzeau and Lubell
SCHEME 2. Syn th esis of 6-Su bstitu ted
Dia m in oa zela tes
tion with the Grignard reagent increased the diastereo-
selectivity of the phenyl and iso-propyl products (6R)-8d
and (6R)-8f up to ratios of 15:1 and 6.5:1, respectively
(entries 10 and 13).
Diastereomeric mixtures from the conjugate addition
were generally difficult to separate using chromatogra-
phy. However, in the case of the phenyl analogue, a
ternary eluant (45/45/10, iso-octane/toluene/iso-propyl
ether) was developed that provided baseline separation
of the diastereomers on 100 mg scale. Later, we found
that 1.8 g of (6S)-8d could be precipitated diastereose-
lectively from 3.1 g of a 2/1 mixture of (6S)- and (6R)-8d
in a 0.035 M solution of i-PrOH containing 1% H2O. This
method provided each isomer in a diastereomeric ratio
of 13 to 1. Diastereomerically enriched samples (13:1 and
1:13) of (6S)- and (6R)-8d were used in subsequent
reductive amination/lactam cyclization sequences de-
scribed below. Stereochemical assignment at the 6-posi-
tion of compound 8d was made on the basis of assign-
ments for (2S,4R,6R,8S)-4-phenylindolizidin-9-one 11,
from cyclization of (6S)-8d , as characterized below.
TABLE 1. Con ju ga te Ad d ition on En on e 7
entry
nucleophile
product [% yield, dr]
1
2
MeO2CCH2CO2Me/NaH
KCN/18Crown6
CH3NO2/DBU
Ph2CuCN(MgBr)2
PhCu.S(CH3)2
8a [77, 1:1]
8b [68, 1:1]
8c [94, 2:1]
8d [98, 1:2]
8d [74, 1:3]
8e [98, 1:2]
8f [80, 5:1]
8d [83, 5:1]
8d [86, 9:1]
8d [100a, 15:1]
8e [82, 10:1]
8f [71b, 3:1]
8f [80a, 6:1]
3
4
5
6
(p(CH3O)-C6H4)2CuCN(MgBr)2
i-Pr2CuCN(MgBr)2
i-PrMgBr/CuI (cat)
PhMgBr
7
8
9
10
11
12
13
PhMgBr/MgBr2
(2S,6R,8S)-Di-tert-butyl-4-oxo-6-phenyl-2,8-bis[N-(Ph-
F)amino]azelate [(6S)-8d (13:1 dr)] was converted to
(2S,4R,6R,8S)-methyl-9-oxo-8-(N-(Boc)-amino)-4-phenyl-
1-azabicyclo[4.3.0]nonane carboxylate [(2S,4R,6R,8S)-4-
Ph-I9aa-OMe, 11] in 51% overall yield by our reductive
amination/lactam cyclization protocol (Scheme 3).7a Hy-
drogenation using 10% palladium-on-carbon in a mixture
of EtOH and AcOH (9/1) provided the trisubstituted
pipecolate (2S′,2S,4R,6R)-9 as a single diastereoisomer
as determined by 1H and 13C NMR spectroscopy. The tert-
butyl esters were then removed using concentrated HCl
and replaced by methyl esters using HCl gas in MeOH,
p(CH3O)-C6H4-MgBr
i-PrMgBr
i-PrMgBr/MgBr2
a
b
% conversion. 1,2-Adduct (23%) was also isolated.
ketones 8 into 4-aryl indolizidin-9-one amino acids are
now presented by the syntheses of constrained Ala-Phe
dipeptide surrogate 3 (N-Boc-4-Ph-I9aa). In particular,
we have developed an effective six-step synthesis of
(2S,4R,6R,8S)-9-oxo-8-(N-(Boc)-amino)-4-phenyl-1-azabi-
cyclo[4.3.0]nonane carboxylic acid 3 from (5E)-7 with 42%
overall yield.
1
as followed by H NMR spectroscopy by monitoring the
methyl ester singlets at 3.90 and 3.84 ppm, as well as by
TLC. Lactam cyclization was performed by treating the
diester in MeOH with Et3N at reflux until NMR spec-
troscopy showed replacement of the two methyl ester
singlets by a new singlet at 3.86 ppm (approximately
24 h). The amine was then protected with Boc2O and
Et3N in CH2Cl2. Chromatography of the final residue
provided (2S,4R,6R,8S)-4-Ph-I9aa-OMe 11 in 51% overall
yield from ketone (6S)-8d .15 Ester (2S,4R,6R,8S)-11 was
then converted to the corresponding N-(Boc)amino acid
(2S,4R,6R,8S)-3, suitable for application in peptide syn-
thesis, using potassium trimethylsilanolate in Et2O for
1 h at room temperature (Scheme 3).
(2S,6R,8S)-Di-tert-butyl-4-oxo-6-phenyl-2,8-bis[N-(Ph-
F)amino]azelate [(6R)-8d (13:1 dr)] was initially hydro-
genated using the same conditions as in the synthesis of
pipecolate (2S′,2S,4R,6R)-9 from ketone (6S)-8d . The
desired pipecolates 9 and their desamino counterparts
10 were isolated as mixtures of diastereoisomers in 27
and 56% respective yields (Scheme 4). Loss of the amine
function was presumed to arrive from imine to enamine
tautomerization followed by â-elimination of ammonium
ion to form the R,â-unsaturated imine, which underwent
subsequent reduction.16 In related hydrogenation
processes, we have reported â-elimination of nitrogen
from a â-amino imine intermediate4 and of oxygen
Resu lts a n d Discu ssion
Conjugate addition reactions on enone (5E)-7 were
studied using various nucleophiles as a general route for
making R,ω-diaminoazelates possessing different sub-
stituents (Scheme 2). Various nucleophiles reacted ef-
fectively on enone (5E)-7 in good yield albeit with low
diastereoselectivity. For example, methylmalonate, cya-
nide, and nitromethane all were introduced onto ketone
(5E)-7 in good yield and from 1:1 to 2:1 diastereoselec-
tivity (entries 1-3). Different alkyl and aryl substituents
were introduced as organocopper species, namely, higher
order cyano cuprates, aryl copper reagents, and Grignard
reagents in the presence of catalytic amounts of copper.
These various conditions employing copper reagents gave
generally good yield and low diastereoselectivity (2/1 to
5/1, entries 4-8), with limited influence of the copper
species on the diastereomeric ratio observed for the final
ketone.14
Remarkable regio- and diastereoselectivity was ob-
served when ordinary aryl Grignard reagents were added
to ketone 7.14 For example, phenylmagnesium bromide
added selectively in a 1,4 manner to provide a 9:1 ratio
in favor of (6R)-8d (entry 9). Furthermore, the addition
of metal salts to the reaction mixture was shown to give
a significant improvement in the diastereoselectivity. For
example, addition of magnesium bromide prior to reac-
(15) Using the 13/1 diastereomeric mixture, (2R,4S,6R,8S)-11 was
isolated in 3% yield from the residual (6R)-8d .
(14) Cluzeau, J .; Lubell, W. D. Israel J . Chem. 2002, 41, 271-281.
1506 J . Org. Chem., Vol. 69, No. 5, 2004
(16) Swarbrick, M. E.; Lubell, W. D. Chirality 2000, 12, 366-373.

Synthesis of 4-Aryl Indolizidin-9-one Amino Acids
SCHEME 3. Syn th esis of 4-P h -I9a a -OMe (2S,4R,6R,8S)-11 via Red u ctive Am in a tion of (6S)-8d
SCHEME 4. Hyd r ogen a tion of (6R)-8d P r oceed ed
w ith â-Elim in a tion
Practical conditions for obtaining the diamino com-
pound 9 were found when the reaction mixture was
saturated with ammonium acetate (entry 4), which
according to Le Chatelier’s law was expected to suppress
the â-elimination product. In the absence of added AcOH,
this saturation technique was not always reproducible
(entries 4 and 5), such that a protocol was developed in
which the ammonium acetate was first suspended in and
dried by evaporation from toluene and then left to sit
under high vacuum overnight. The resulting white
crystalline solid was then employed in the hydrogenation
of (6R)-8d using Pd/C in a solvent system of EtOH/THF/
AcOH (7/3/0.1) for 48 h, which provided selectively the
pipecolates 9. Desamino compound 10 was not detectable
1
by H and 13C NMR spectroscopy of the crude product,
yet was visible when concentrated samples were spotted
on TLC.
As described above for the synthesis of (2S,4R,6R,8S)-
11, the tert-butyl esters of pipecolates 9 were removed
using concentrated HCl and replaced with methyl esters
employing HCl gas in MeOH (Scheme 5). Because the
1H NMR spectrum was too complex, formation of methyl
esters was best monitored by TLC. The lactam cycliza-
tion was then performed in MeOH at reflux with Et3N
for 48 h followed by amine protection using Et3N and
(Boc)2O in CH2Cl2 for 12 h. After two chromatographies,
eluting first with 60/40 hexane/EtOAc, followed by 85/
10/5 hexane/i-Pr2O/i-PrOH, three diastereomeric prod-
ucts, (2R,4S,6S,8S)-, (2R,4S,6R,8S)-, and (2R,4S,6S,8R)-
4-Ph-I9aa 11 were isolated in 6, 6, and 10% overall yields,
respectively, from (6R)-8d .
TABLE 2. In flu en ce of P d Ca ta lyst on Hyd r ogen a tion of
(6R)-8d
dr of 9
(6S:6R) ratio of 9:10
entry
catalysta
dr of 10
1
Pd/C 10% wet
Pd/C 10%
Pd(OH)2/C 20%
Pd/C 20% eggshell
1/3
1/1
1/1
3/1
1/1
1/1
1/2c
1/4
1/3
1/2
1/2d
3/1
2
3b
4
a
Hydrogenation was performed in EtOH/AcOH (9/1) under 9
b
atm of H2. When MeOH/EtOAC (7/3) was employed, the ratio
for 9 was 1:4 and that for 10 was 1:4. c At 0 °C, a 1:4 ratio was
d
obtained. At 50 °C, a 9:1 ratio was obtained.
from â-hydroxy, â-silyloxy, and â-acetoxy imine inter-
mediates.6b,16 To prevent â-elimination, different proton
sources, catalysts, and reaction temperatures were stud-
ied. For example, four forms of Pd catalyst were exam-
ined at 9 atm of H2 in 9/1 EtOH/AcOH (Table 2). Their
effect on the elimination process under these conditions
was significant, and the ratio of pipecolate 9 to â-elimi-
nation product 10 changed from 1:1 with wet Pd/C (10%
w/w) up to 1:4 using “eggshell” Pd/C (20 wt %).
In our synthesis of pyrrolizidinone amino acid 6, a
similar loss of amine due to a â-elimination process was
inhibited significantly by diminishing the quantity of acid
employed in the hydrogenation with Pd/C as catalyst.4
In the case of (6R)-8d , no reaction was observed when
using only 100 mol % AcOH, and a 1:1 ratio of 9 and 10
was obtained when using 100 mol % of pyridinium para-
toluenesulfonate (Table 3, entries 2 and 3).
En a n tiom er ic P u r ity a n d Ster eoch em ica l Assign -
m en t of In d olizid in es 11. The configurational integrity
of the 4-phenyl-indolizidin-9-one amino acid was evalu-
ated by the preparation and spectral analysis of diaster-
eomeric N-(p-toluenesulfonyl)prolinamides 12 using
(2S,4R,6R,8S)-4-Ph-I9aa-OMe 11, which was obtained
from the hydrogenation/lactam cyclization sequence us-
ing (6S)-8d . After cleavage of the Boc protecting group
using HCl gas, the resulting hydrochloride salt was
treated with Et3N and coupled respectively to L- and DL-
N-(p-tolylsulfonyl)prolyl chloride in CH2Cl2 (Scheme 6).
Observation of the diastereomeric methyl ester singlets
1
at 3.75 and 3.73 ppm by 400 MHz H NMR spectroscopy
in C6D6 during incremental additions of the diastereo-
meric mixture demonstrated prolylamide (2′S)-12 to be
of >98% diastereomeric excess. Hence, (2S,4R,6R,8S)-4-
J . Org. Chem, Vol. 69, No. 5, 2004 1507

Cluzeau and Lubell
TABLE 3. In flu en ce of th e P r oton Sou r ce on Hyd r ogen a tion of (6R)-8d
entry
catalyst
solvent
proton source
dr of 9 of (6S/6R)
ratio of 9:10
dr of 10
1
2
3
4
5
6
Pd/C 10%
Pd/C 10%
Pd/C 10%
Pd/C 10%
Pd/C 10%
Pd/C 10%
EtOH
AcOH (excess)
AcOH (1 equiv)
PPTS (1 equiv)
NH4OAc (200 equiv)
NH4OAc (dry, 200 equiv)a
NH4OAc (dry, 200 equiv)a
AcOH (1%/solv)
1/1
1/2
nr
1/1
6/1
nr
>100/1
2/1
EtOH/THF (8/2)
EtOH/THF (8/2)
EtOH/THF (7/3)
EtOH/THF (7/3)
EtOH/THF (7/3)
1/4
4/1
1/4
1/1
3/1
3/1
7
Pd(OH)2/C 20%
MeOH/EtOAc (7/3)
NH4OAc (200 equiv)
4/1
1/1
a
Ammonium acetate was dried as discussed in the Experimental Section.
SCHEME 5. Syn th esis of (4S)-4-P h -I9a a -)OMe 11 via Red u ctive An im a tion of (6R)-8d
SCHEME 6. En a n tiom er ic P u r ity of
(2S,4R,6R,8S)-11
of the NMR signals were well resolved at distinct
chemical shifts (Table 4). All protons were assigned using
COSY and HMQC experiments, which established their
linear sequence around the bicycle from the carbamate
NH to the C-2 proton. Diastereomeric protons cis and
trans to the carboxylate at C-2 are designated â and R,
respectively. The assignments of the R and â protons at
C-3, C-5, and C-7 were made using their signal multiplic-
ity and coupling constants and supported by NOE
observed in the NOESY experiments.
1
In the H NMR spectrum of (2S,4R,6R,8S)-4-Ph-I9aa-
OMe 11 in C6D6, the protons at C-2, C-3â, C-4, C-5â, and
C-6 all exhibited large coupling constants (10.4-12.5 Hz)
indicative of their axial position on the pipecolate, which
adopted a chair conformation (Figure 2). For example,
the C-2 proton was observed as a doublet of doublets and
shared, respectively, vicinal coupling constants of 3.4 and
11.2 Hz with the C-3R and C-3â protons, corresponding
to dihedral angles of (60° and (160°.17 The pseudoaxial
conformation of the C-7â proton in the five-member
lactam was assigned on the basis of observation of an
Ph-I9aa-OMe 11 as well as its corresponding acid
(2S,4R,6R,8S)-3 are presumed to be of similarly high
enantiomeric purity.
The relative stereochemistry of the different diastere-
oisomers of 11 was established using one- and two-
dimensional NMR experiments (Figure 2). The majority
1508 J . Org. Chem., Vol. 69, No. 5, 2004

Synthesis of 4-Aryl Indolizidin-9-one Amino Acids
TABLE 4. 1H NMR Da ta for N-(Boc)-4-P h en yl-in d olizid in -9-on e Am in o Ester s 11
2
3R
3â
1.80 q 2.08 t
12.3 11.9
1.89 q 2.80 tt
12.6
4
5R
1.31 d 1.00 q
9.6 11.7
2.06 d 1.52 q
12.3
5â
6
7R
7â
8
NH
Me t-Bu
(2S,4R,6R,8S)-11 3.48 dd 1.70 d
(C6D6, 400 MHz) 3.4, 11.2 12.5
(2R,4S,6S,8S)-11 3.88 dd 2.10 d
(CDCl3,600 MHz) 3.5, 12.0 13.4
2.34 t 2.25 br s 1.11 q 4.16 m
10.4
3.74 m 2.24 t
6.9
5.00 br d 3.62 1.42
5.2
11.3
4.17 m 5.01 br s 3.80
1.44
3.3, 12.4 12.9
(2S,4S,6R,8S)-11 4.26 t
(C6D6, 400 MHz) 5.0
1.93 ddd 1.56 m 2.77 br t 1.56 m 1.82 ddd 2.91 m 2.42 m
1.16 q 4.35 m
11.0
5.12 d
6.0
3.37 1.43
3.7, 5.9,
9.6
9.7, 12.3,
13.9
between the C-6 and C-8 protons on one face as well as
between the axial C-3â and C-5â protons on the other face.
1
In the H NMR spectrum of (2R,4S,6S,8S)-4-Ph-I9aa-
OMe 11, the first of the two minor isomers from the
cyclization of (6R)-8d , the C-2, C-3â, C-4, and C-5â protons
all exhibited large coupling constants in CDCl3 (11.9 to
13.4 Hz), indicative of their axial position on the pipe-
colate, which adopted a chair conformation. For example,
the C-4 proton was observed as a triplet of triplets with
coupling constants of 3.3 and 12.4 Hz corresponding to
dihedral angles close to (61° with both the C-3R and
C-5R protons and close to (175° with both of the C-3â
and C-5â protons.16 With the (4S)-phenyl substituent
sitting equatorial, the data suggested epimerization at
C-2 and 2R,4S,6S stereochemistry for the pipecolate ring.
The 2R,4S,6S,8S configuration was confirmed using
NOESY experiments in which NOEs were observed
between the axial C-2, C-4, and C-6 protons, confirming
complete equatorial substitution about the piperidine
cycle. Additional NOE between the C-5â and C-8 protons
on the opposite face established the 8S stereochemistry.
1
In the H NMR spectrum of the second minor isomer
from cyclization of (6R)-8d , (2S,4S,6R,8S)-4-Ph-I9aa-OMe
11, in C6D6, the coupling constant pattern for protons in
the piperidine cycle differed from that of a chair confor-
mation. For example, the C-2 proton exhibited a small 5
Hz coupling constant and appeared as a triplet, suggest-
ing an axial position for the ester function. The C-5â
proton was observed as a double doublet of doublets (ddd)
with three large coupling constants (9.7, 12.3, 13.9 Hz),
indicative of a pseudoequatorial position for the phenyl
ring. The C-7â proton exhibited an 11.0 Hz coupling
constant and quadruplet multiplicity indicative of a trans
relationship with the C-8 and C-6 protons. The C-6
stereocenter was assigned the 6R relative configuration
because, in the NOESY spectrum, correlations were
observed between the C-8 and C-6 protons and between
the C-6 and C-2 protons, in agreement with the 2S,6R,8S
relative stereochemistry. Additional NOEs were observed
between the C-7â proton and both the carbamate NH and
the C-5â protons, as well as between ortho aromatic
protons and both C-2 and C-6 protons, which helped to
confirm the stereochemistry of (2S,4S,6R,8S)-11.
The diverse NMR data confirmed the presence of
(2S,4R,6R,8S)-4-Ph-I9aa-OMe 11, which might come from
residual (6S)-8d in the hydrogenation of (6R)-8d ; how-
ever, the mass of isolated product exceeded expectations.
The presence of its enantiomer, (2R,4S,6S,8R)-11, coming
from epimerization at both the C-2 and C-8 stereocenters
during transformation of (6R)-8d was validated by cou-
pling L-N-(p-tolylsulfonyl)-prolyl chloride to the isolated
sample of (2S,4R,6R,8S)-4-Ph-I9aa-OMe 11; the amide
products 12 were shown to be of only 33% diastereomeric
excess (66:33 dr).
F IGURE 2. NOE correlations observed in the NOESY NMR
spectra of 11.
apparent quadruplet, due to similar vicinal couplings
with the pseudoaxial C-8 proton and the axial C-6 proton
as well as a geminal coupling with the C-7R proton. The
concave shape of the bicyclic system and the (4R)-
stereochemistry, which positions the C-2, C-4, C-6, and
C-8 protons all on the same face, were supported by
NOESY experiments. Characteristic NOE correlations
were observed between the three piperidine axial protons
at C-2, C-4, and C-6. In addition, NOE was measured
(17) Calculated using the equation 3J ) Kcos2φ; where Ka-a ) 12.5
Hz, Ka-e ) Ke-a ) 14.3 Hz, and Ke-e ) 12.9 Hz. φ is the dihedral angle,
and K values are calculated from 1,3,5-trimethyl-cyclohexane. (a) Xie,
X.-Q.; Melvin, L. S.; Makriyannis, A. J . Biol. Chem. 1996, 271, 10640-
10647. (b) Kriwacki, R. W.; Makriyannis, A. Mol. Pharmacol. 1989,
35, 495-503. (c) Booth, H. Prog. NMR Spectrosc. 1969, 5, 149-381.
J . Org. Chem, Vol. 69, No. 5, 2004 1509

Cluzeau and Lubell
SCHEME 7. Hyd r ogen a tion of (6R)-8d Ma y P r oceed w ith â-Elim in a tion a s w ell a s C-2 a n d C-8
Ep im er iza tion
Ster eoselection a n d Ep im er iza tion . In the synthe-
sis of the parent indolizidin-9-one system, I9aa 1, the six-
membered ring iminium ion was preferentially reduced
on the face opposite the ester, leading predominantly to
the concave diastereomer (9:1).7a In the synthesis of (4R)-
4-Ph-I9aa, the phenyl substituent works in harmony with
the ester function to favor their pseudoequatorial orien-
tation in the iminium ion such that attack occurs
exclusively to give the concave 6R diastereoisomer (Scheme
3). In the case of the (4R)-phenyl isomer, the phenyl and
carboxylate substituents battle to avoid pseudoaxial
orientations in the iminium ion, such that two conformers
exist in equilibrium, with a likely preference for the
conformer having the phenyl group positioned equatori-
ally (Scheme 5). Hydrogenation of the more stable
iminium ion favored formation of the piperidine in a chair
conformer having the C-4 and C-6 substituents positioned
equatorially, which was transformed into (2R,4S,6S,8R)-
and (2R,4S,6S,8S)-4-Ph-I9aa-OMe 11.18 Reduction of the
less stable iminium ion having the phenyl group posi-
tioned pseudoaxially favored a piperidine in a chair
conformer with the C-2 and C-6 substituents positioned
equatorially, which furnished (2S,4S,6R,8S)-4-Ph-I9aa-
OMe 11.
contingent on the nitrogen protecting group. Imine
derivatives of R-amino acids are particularly sensitive to
R-epimerization even in the absence of base21 and as
iminium ions in acidic conditions.19 To the best of our
knowledge, no R-epimerization of iminium ion intermedi-
ates has been previously reported in applications of
reductive aminations to form substituted prolines and
pipecolates using catalytic hydrogenation under mildly
acidic conditions. Although a detailed assessment of the
source of C-2 epimerization was not possible because the
pipecolate diastereomers were inseparable, a driving
force for epimerization during hydrogenation of the
iminium ion would be the formation of the more stable
intermediate having both substituents positioned pseu-
doequatorially (Scheme 5). A less likely alternative would
involve C-2 epimerization by selective enolization of an
R-ammonium carboxylate after hydrogenation during
cleavage of the tert-butyl esters and formation of the
methyl esters under acidic conditions. Epimerization at
C-8 was only observed in the product from hydrogenation
of (6R)-8d in the presence of excess NH4OAc. Application
of NH4OAc to prevent formation of desamino pipecolate
10 apparently increased the potential for conjugate
addition of ammonia to the unsaturated iminium ion,
product from â-elimination, prior to reduction of the
iminium ion intermediate (Scheme 7). The preferred
formation of (2R,4S,6S,8R)-4-Ph-I9aa-OMe 11 instead of
product epimerized only at C-8 remains inexplicable with
our present data.
Epimerization of one and both R-amino carboxylate
centers of azelate (6R)-8d occurred during the synthesis
of (2R,4S,6S,8S)- and (2R,4S,6S,8R)-4-Ph-I9aa-OMe 11,
respectively. Epimerization of different stereocenters has
been sometimes reported in the syntheses of mono- and
bicyclic amino acids. In particular, the R-proton of cyclic
amino acids has been removed under basic conditions7c,19,20
(19) Subasinghe, N. L.; Khalil, E. M.; J ohnson, R. L. Tetrahedron
Lett. 1997, 38, 1317-1320.
(20) Beausoleil, E.; L’Archeveˆque, B.; Be´lec, L.; Atfani, M.; Lubell,
W. D. J . Org. Chem. 1996, 61, 9447-9454.
(18) Deslongchamps, P. Stereoelectronic Effect in Organic Chemistry;
Pergamon Press: Oxford, 1983.
(21) Mkairi, A.; Hamelin, J . Tetrahedron Lett. 1987, 28, 1397-1400.
1510 J . Org. Chem., Vol. 69, No. 5, 2004

Synthesis of 4-Aryl Indolizidin-9-one Amino Acids
solution of THF (36 mL), EtOH 95% (126 mL), and AcOH (18
mL) and treated with palladium-on-carbon 10% (175 mg, 10
wt %). The vessel containing the suspension was filled, vented,
and refilled three times with hydrogen. After stirring for 24 h
under 9 atm of H2, the suspension was filtered through Celite
and concentrated on a rotary evaporator. The residue was
partitioned between saturated NaHCO3 (50 mL) and CHCl3/
i-PrOH (4/1, 50 mL). The aqueous phase was extracted (3 ×
20 mL) with CHCl3/i-PrOH (4/1). The organic phases were
combined, dried with MgSO4, and concentrated under vacuum
to give the free amine as a colorless oil: TLC Rf 0.1 (90/9/1
Con clu sion
Toward a general approach for constructing 4-substi-
tuted indolizidin-9-one amino acids, conjugate addition
reactions on enone 7 were studied using various nucleo-
philes, and a versatile route has been developed for
making 6-substituted R,ω-diaminoazelates in good yield
and diastereomeric ratios from 1:1 to 15:1. 6-Phenyl-
substituted diaminoazelates, (6S)- and (6R)-8d , were
obtained in high diastereoselectivity (13:1 dr) by selec-
tive precipitation from a 0.035 M solution of i-PrOH/
H2O (99/1). Enantiopure (2S,4R,6R,8S)-9-oxo-8-(N-(Boc)-
amino)-4-phenyl-1-azabicyclo[4.3.0]nonane carboxylic acid
((2S,4R,6R,8S)-3) was synthesized from (6S)-8d in six
steps and 42% overall yield via a reductive amination/
lactam cyclization sequence. On the other hand, reductive
amination of (6R)-8d was accompanied by â-elimination
leading to desamino analogue 10. Although â-elimination
could be avoided by performing the hydrogenation in the
presence of excess ammonium acetate, epimerization
during the conversion of (6R)-8d to 9 was detected at both
the C-2 and C-8 stereocenters and three products,
(2R,4S,6S,8R)-, (2R,4S,6S,8S)-, and (2S,4S,6R,8S)-11,
were finally isolated from the lactam cyclization protocol.
A novel synthesis methodology for preparing 4-substi-
tuted indolizidin-9-one amino acids has thus been dem-
onstrated by the synthesis of 4-Ph-I9aa 3, a new con-
strained Ala-Phe dipeptide surrogate for examining
conformation-activity relationships of biologically active
peptides.
1
hexane/i-PrOH/Et3N); H NMR (CDCl3) δ 7.33-7.28 (m, 2H),
7.22 (m, 3H), 3.58 (br s, 1H), 3.42 (d, 1H, J ) 11.0 Hz), 2.95
(br s, 1H), 2.73 (t, 1H, J ) 12.2 Hz), 2.19 (d, 1H, J ) 12.7 Hz),
2.07 (br s, 2H), 1.90 (m, 1H), 1.82 (d, 1H, J ) 12.7 Hz), 1.61
(m, 1H), 1.52 (q, 1H, J ) 12.0 Hz), 1.46 and 1.45 (2s, 18H),
1.43-1.33 (m, 2H); 13C NMR δ 175.5, 172.2, 145.4, 128.4, 126.7,
126.3, 81.0 (2C), 59.3, 52.5, 51.9, 42.8, 40.8, 39.9, 36.7, 27.9
(2C). The oil was dissolved in 12 N HCl (30 mL) and stirred
for 15 min, at which point 1H NMR spectroscopy in CD3OD
showed the disappearance of the tert-butyl singlets at 1.55 and
1.51 ppm. Evaporation of the volatiles under vacuum gave the
free diamino acid as the HCl salt: TLC Rf 0.05 (4/1/1 n-BuOH/
H2O/AcOH); 1H NMR (CD3OD) δ 7.36-7.23 (m, 5H), 4.26-
4.22 (m, 2H), 3.88 (m, 1H), 3.10 (tt, 1H, J ) 12.7, 2.9 Hz), 2.48
(m, 2H), 2.31 (d, 1H, J ) 13.7 Hz), 2.21-2.15 (m, 2H), 1.90 (q,
1H, J ) 13.7 Hz), 1.77 (q, 1H, J ) 13.8 Hz); 13C NMR (CD3-
OD) δ 171.0, 170.8, 144.4, 130.0, 128.3, 128.0, 59.0, 55.4, 50.7,
41.2, 36.0, 35.0, 33.9. The crude white solid was dissolved in
methanol (65 mL) and treated with bubbles of HCl gas until
complete disappearance of starting material was observed by
TLC (4/1/1 n-BuOH/H2O/AcOH), approximately 1 h. The
volatiles were removed under vacuum to give the diester as a
white solid. TLC Rf 0.35 (4/1/1 n-BuOH/H2O/AcOH); 1H NMR
(CD3OD) δ 7.32-7.23 (m, 5H), 4.47-4.28 (m, 2H), 3.90 (s, 3H),
3.85 (m, 4H), 3.16 (m, 1H), 2.60 (m, 1H), 2.42 (m, 1H), 2.30
(m, 2H), 2.05-1.85 (m, 2H); 13C NMR δ 170.8, 170.0, 144.4,
130.0, 128.4, 128.0, 58.9, 55.3, 54.5, 54.1, 50.7, 40.9, 35.8, 35.0,
33.8. The solid was dissolved in MeOH (45 mL), treated with
Et3N (0.54 mL, 200 mol %), and heated at reflux for 48 h at
which point 1H NMR spectroscopy in CD3OD of an aliquot
showed complete disappearance of the methyl singlets at 3.90
and 3.84 ppm and appearance of a new methyl singlet at 3.71
ppm. After removal of the volatiles under vaccum, the crude
oil was dissolved in CH2Cl2 (45 mL), treated with Et3N (2.7
mL, 1000 mol %) and Boc2O (0.846 g, 200 mol %), stirred for
15 h, and concentrated on a rotary evaporator. Purification
by column chromatography was performed using an eluant of
hexane/EtOAc (70/30) as an eluant. First to elute was
(2R,4S,6S,8S)-methyl-9-oxo-8-(N-(Boc)-amino)-4-phenyl-1-
azabicyclo[4,3,0]nonane carboxylate [(2R,4S,6S,8S)-11, 22 mg,
3%]: TLC Rf 0.1 (85/10/5 hexanes/i-Pr2O/i-PrOH). Next to elute
was (2S,4R,6R,8S)-methyl-9-oxo-8-(N-(Boc)-amino)-4-phenyl-
1-azabicyclo[4.3.0]nonane carboxylate [(2S,4R,6R,8S)-11, 340
mg, 51%]: TLC Rf 0.13 (70/30 Hex/EtOAc); [R]20D -36.1; 13C
NMR (C6D6) δ 171.7 (2 C), 170.8 (C), 144.6 (C), 129.0 (2 CH),
128.5 (CH), 127.1 (2 CH), 79.4 (C), 56.4 (CH), 53.9 (CH), 52.6
(CH), 52.3 (CH), 40.6 (CH2), 38.5 (2CH2), 36.2 (2 CH3), 28.6;
MS (FAB+) m/z 389.1 (M + H+); HRMS calcd for C21H28N2O5
(MH+) 389.2077, found 389.2098.
Exp er im en ta l Section
(2S,6S,8S)- a n d (2S,6R,8S)-Di-ter t-bu tyl-4-oxo-6-p h en -
yl-2,8-bis[N-(P h F )a m in o]-a zela te [(6S)- a n d (6R)-8d ]. Un-
der an argon flow, CuCN (1.25 g, 240 mol %) was gently flame
dried, allowed to cool to room temperature, cooled further to
-48 °C, treated with a 0.5 M solution of PhMgBr in THF (48.5
mL, 400 mol %), stirred for 30 min, treated with a solution of
enone (5E)-77a (4 g, 4.8 mmol) in THF (50 mL), stirred for 2 h,
and quenched with saturated NH4Cl (20 mL). The resulting
suspension was extracted with ether (3 × 20 mL), and the
combined organic layers were washed with brine (30 mL),
dried with Na2SO4, and concentrated on a rotary evaporator.
The residue obtained was purified by column chromatography
using hexane/EtOAc (90/10) as an eluant to give 8d (3.2 g,
72%) as a 2/1 mixture of (6S)-/(6R)-8d as measured by the tert-
butyl singlets at 1.16-1.12 and 1.16 ppm in the 1H NMR
spectrum. The mixture was dissolved in i-PrOH (100 mL, 0.035
M), treated with 1 mL of water, and stirred for 18 h. The
mother liquor was decanted and evaporated to give a 1/13
mixture of [(6S)-/(6R)-8d ,1.1 g, 25%]: Rf 0.12 (45/45/10 toluene/
1
iso-octane/i-Pr2O); Rf 0.31 (85/15, hexane/EtOAc); H NMR δ
(CDCl3) 7.68-7.52 (m, 4 H), 7.44-7.05 (m. 27 H), 3.46 (t, 1H,
J ) 7.4 Hz), 3.15 (br s, 2 H), 2.73 (t, 1 H, J ) 5.1 Hz), 2.47-
2.35 (m, 3 H), 2.25-2.14 (m, 2 H), 1.79-1.64 (m, 2 H), 1.19 (s,
18 H); 13C NMR δ (CDCl3) 206.1, 175.0, 173.2, 80.9, 80.7, 72.9,
72.8, 54.2, 52.7, 49.4, 48.7, 42.7, 36.7, 27.9, 27.7; MS (FAB+)
m/z 901.3 (M + H+). The dry white precipitate was a 13/1
mixture of [(6S)-/(6R)-8d ,1.8 g, 42%]: Rf 0.17 (45/45/10 toluene/
iso-octane/i-Pr2O); 1H NMR δ (CDCl3) 7.69-7.63 (m, 4 H),
7.37-7.12 (m. 27 H), 3.26 (m, 1 H), 2.95 (br s, 1 H), 2.79 (br t,
1 H), 2.47 (m, 3 H), 2.35 (dd, 1 H), 2.21 (dd, 1 H), 1.73 (t, 2 H),
1.20 (s, 9 H), 1.16 (s, 9 H); 13C NMR δ (CDCl3) 206.1, 175.0,
173.2, 80.9, 80.7, 72.9, 72.8, 54.8, 52.9, 50.5, 48.2, 42.4, 37.5,
(4S )-Me t h y l-9-o x o -8-(N -(B o c )-a m in o )-4-p h e n y l-1-
a za bicyclo[4.3.0]n on a n e ca r boxyla te [(4S)-11]. Ketone
(6R)-8d (900 mg, 1 mmol) was dissolved in THF (15 mL) and
treated with a solution of NH4OAc (suspended in and dried
by evaporation from toluene, and then left to sit under high
vacuum overnight, 4.27 g, 5500 mol %) in 95% EtOH (35 mL)
followed by AcOH (0.5 mL) and Pd/C 10% (90 mg, 10 wt %).
The vessel containing the suspension was filled, vented, and
refilled three times with hydrogen. After stirring for 24 h under
9 atm of H2, the suspension was then filtered through Celite
and concentrated on a rotary evaporator to give 9: TLC Rf
0.1 (90/10 hexane/i-PrOH). Distinct signals for the major
27.8, 27.6; MS (FAB+) m/z 901.2 (M + H+); [R]D -136.1 (c
0.018, CHCl3).
(2S,4R,6R,8S)-Meth yl-9-oxo-8-(N-(Boc)-a m in o)-4-p h en -
yl-1-a za bicyclo[4,3,0]n on a n e Ca r boxyla te [(2S,4R,6R,8S)-
11]. Ketone (6S)-8d (1.75 g, 1.94 mmol) was dissolved in a
20
J . Org. Chem, Vol. 69, No. 5, 2004 1511

Cluzeau and Lubell
isomer of (4S,6S)-9: 1H NMR (CDCl3) δ 7.34 (m, 3H), 7.20 (m,
2H), 3.40 (d, 2H, J ) 10.3 Hz), 2.91 (m, 1H), 2.71 (t, 1H, J )
11.9 Hz), 2.20 (d, 1H, J ) 12.8 Hz), 1.87 (m, 3H), 1.70-1.50-
(m, 2H), 1.45 (s, 18H), 1.36 (q, 1H, J ) 12.0 Hz); 13C NMR
(CDCl3) δ 175.4, 172.1, 145.7, 128.5 (2C), 126.9 (2C), 125.8,
81.1 (2C), 59.5, 55.1, 54.2, 42.8, 41.1, 39.6, 36.4, 28.0 (6 CH3);
MS (FAB+) m/z 405.2 (M + H+). As described above for the
synthesis of (2S,4R,6R,8S)-11, ester exchange, lactam forma-
tion, and N-protection were performed to provide a residue
that was purified by column chromatography using hexane/
EtOAc (70/30) as an eluant, which gave (2R,4S,6S,8S)-methyl-
9-oxo-8-(N-(Boc)-amino)-4-phenyl-1-azabicyclo[4.3.0]nonane car-
boxylate [(2R,4S,6S,8S)-11, 18 mg, 6%]: TLC Rf 0.25 (85/10/5
hexane/i-Pr2O/i-PrOH); 13C NMR (CDCl3) δ 170.3, 155.9, 129.0,
128.0, 127.1, 126.9, 80.2, 56.8, 56.5, 52.7, 51.5, 41.4, 38.9, 35.1,
33.6, 28.5; MS (FAB+) m/z 389.2 (M + H+). HRMS calcd for
C21H28N2O5 (MH+) 389.2077, found 389.2085. A second fraction
was collected containing a mixture of two compounds that were
separated by column chromatography using hexane/i-Pr2O/i-
PrOH (85/10/5) as an eluant. First to elute was (2S,4S,6R,8S)-
methyl-9-oxo-8-(N-(Boc)-amino)-4-phenyl-1-azabicyclo[4.3.0]-
nonane carboxylate [(2S,4S,6R,8S)-11, 17 mg, 6%]: TLC Rf
0.22 (85/10/5 hexane/i-Pr2O/i-PrOH); 13C NMR (C6D6) δ 171.8,
171.7, 156.3, 146.3, 79.3, 63.9, 52.4, 52.1, 48.4, 38.2, 36.9, 34.7,
31.6, 28.6; MS (FAB+) m/z 389.1 (M + H+); HRMS calcd for
) 8.2 Hz), 7.57 (d, 1H, J ) 6.5 Hz), 7.13-7.04 (m, 3H), 6.86
(d, 2H, J ) 8.4 Hz), 6.80 (d, 2H, J ) 8.0 Hz), 4.36 (m, 1H),
4.19 (dd, 1H, J ) 3.0, 8.6 Hz), 3.66 (s, 3H), 3.53 (dd, 1H, J )
4.0, 11.0 Hz), 3.27 (m, 1H), 2.90 (m, 1H), 2.48 (m, 1H), 2.34
(m, 1H), 2.18-2.07 (m, 2H), 1.89 (s, 3H), 1.85 (m, 1H), 1.70
(dd, 1H, J ) 1.5, 13.2 Hz), 1.67-1.52 (m, 2H), 1.33 (d, 1H, J
) 11.1 Hz), 1.20-1.11 (m, 2H), 1.02 (m, 1H). The limits of
detection were determined by observation of the diastereomeric
singlets at 3.75 and 3.73 ppm in a 400 MHz 1H NMR spectrum
of (2′S)-12 in C6D6 during incremental additions of the dia-
stereomeric mixture (2′RS)-12, which demonstrated (2′S)-12
to be of >98% diastereomeric purity. Distinct signals for (2′R)-
12 include: 1H NMR (C6D6) δ 7.63 (d, 2H, J ) 8.2 Hz), 7.47
(d, 1H, J ) 8.2 Hz), 6.88 (d, 2H, J ) 7.5 Hz)), 6.74 (d, 2H, J )
8.0 Hz), 4.78 (m, 1H), 3.64 (s, 3H), 1.85 (s, 3H).
(2S ,4R ,6R ,8S )-9-Oxo-8-(N -(Boc)-a m in o)-4-p h e n yl-1-
a za bicyclo[4.3.0]n on a n e Ca r boxylic Acid [(2S,4R,6R,8S)-
3]. Ester (2S,4R,6R,8S)-11 (519 mg, 1.34 mmol) in Et2O (40
mL) was treated with potassium trimethylsilanolate (257 mg,
150 mol %), stirred for 2 h, quenched with water (1 mL), and
evaporated to a residue that was partitioned between CHCl3
(50 mL) and 10% citric acid solution (50 mL). The phases were
separated, and brine (25 mL) was added to the aqueous phase,
which was extracted with CHCl3 (4 × 25 mL). The combined
organic layer was dried over MgSO4, filtered, and concentrated
to provide (2S,4R,6R,8S)-9-oxo-8-(N-(Boc)-amino)-4-phenyl-1-
azabicyclo[4.3.0]nonane carboxylic acid [(2S,4R,6R,8S)-3, 410
mg, 82%]: 1H NMR (C6D6) δ 9.84 (br s, 1H), 7.15 (m, 2H), 7.07
(m, 1H), 6.94 (m, 2H), 5.81 (d, 1H, J ) 5.6 Hz), 5.10 (d, 1H, J
) 5.6 Hz), 4.40 (br s, 1H), 3.58 (m, 1H), 2.66 (br t, 1H, J )
11.6 Hz), 2.43 (m, 2H), 1.57-1.48 (m, 3H), 1.45 (s, 9H), 1.08
(q, 1H, J ) 11.8 Hz); 13C (C6D6) δ 174.0, 173.2, 156.7, 145.1,
129.1, 127.5, 127.1, 80.0, 53.3, 52.5, 52.4, 39.3, 38.8, 35.2, 33.7,
28.8; MS (FAB+) m/z 375.1 (M + H+); HRMS calcd for
C
21H28N2O5 (MH+) 389.2077, found 389.2093. Second to elute
was (2R,4S,6S,8R)-methyl-9-oxo-8-(N-(Boc)-amino)-4-phenyl-
1-azabicyclo[4.3.0]nonane carboxylate [(2R,4S,6S,8R)-11, 28
20
mg, 10%]: TLC Rf 0.17 (85/10/5 hexane/i-Pr2O/i-PrOH); [R]D
+0.8. The NMR spectral data were identical with those of
(2S,4R,6R,8S)-11. The enantiomeric purity of (2R,4S,6S,8R)-
11 was assessed by the preparation of diastereomeric prolyl
amides (2′S,2R,4S,6S,8R)-12 as described below and measured
at 33% ee.
C
20H26N2O5 (MH+) 375.1920, found 375.1915.
E n a n t iom er ic P u r it y of (2S,4R,6R,8S)-Met h yl-9-oxo-
8-(N-(Boc)-a m in o)-4-p h en yl-1-a za b icyclo[4.3.0]n on a n e
Ca r b oxyla t e 11. Compound (2S,4R,6R,8S)-11 (28 mg,
72 µmol) was dissolved in CH2Cl2 (5 mL) and treated with
HCl gas for 1 h. The volatiles were removed under reduced
pressure, and the product was dissolved in CH2Cl2 (3 mL),
treated with DIEA (9 µL, 140 mol %) and either L- or
DL-N-(p-toluenesulfonyl)prolyl chloride (10 mg, 100 mol %),
stirred for 6 h at rt, diluted with CH2Cl2 (3 mL), and
washed sequentially with 10% citric acid (3 mL), 1 N NaOH
(3 mL), and brine (3 mL), dried (MgSO4), filtered, and
concentrated. In the case of the DL-diastereomer, 18 mg (93%)
of (2′RS,2S,4R,6R,8S)-methyl-9-oxo-8-(N-(p-toluenesulfonyl)-
prolinamido)-4-phenyl-1-azabicyclo[4,3,0]nonane carboxylate
12 was obtained as a 1:1 mixture of diastereoisomers as
determined by measuring the signals of the methyl ester
singlets at 3.75 and 3.73 ppm. For the (2′S)-diastereoisomer,
12 mg (62%) of (2′S,2S,4R,6R,8S)-methyl-9-oxo-8-(N-(p-tolu-
enesulfonyl)prolinamido)-4-phenyl-1-azabicyclo[4.3.0]nonane
carboxylate 12 was obtained: 1H NMR (C6D6) δ 7.69 (d, 2H, J
Ack n ow led gm en t. This research was supported in
part by the Natural Sciences and Engineering Research
Council (NSERC) of Canada and the Ministe`re de
l’EÄ ducation du Que´bec. We are grateful to Sylvie Bilo-
deau, Robert Mayer, and Dr. M. T. Phan Viet of the
Regional High-Field NMR Laboratory for their as-
sistance. Benoit J olicoeur is thanked for preparing
advanced intermediates.
Su p p or tin g In for m a tion Ava ila ble: 1H, 13C, COSY, and
NOESY NMR spectra of (2S,4R,6R,8S)-11, (2R,4S,6S,8S)-11,
1
and (2S,4S,6R,8S)-11; H and 13C NMR spectra of 3, 8c, 8b,
8a , 9, 10, and 12; and Experimental for 8c, 8b, 8a , (4S,6R)-9,
(4S,6S)-10, and (4S,6R)-10. This material is available free of
charge via the Internet at http://pubs.acs.org.
J O0355855
1512 J . Org. Chem., Vol. 69, No. 5, 2004