Bone collagen cross-links: A convergent synthesis of (+)-deoxypyrrololine

Article abstract of DOI:10.1021/jo0008901

A convergent total synthesis of (+)-deoxypyrrololine (Dpl, 4), a putative cross-link of bone collagen, is described starting from a commercially available L-glutamic acid derivative, (4S)-5-(tert-butoxy)-4-[(tert-butoxycarbonyl)amino]-5-oxopentanoic acid (16). Condensation of aldehyde (S)-(-)-17 with nitro compound (S)-(-)-27, both of which were prepared from a common precursor (S)-16, gave the α-hydroxynitro compound 28, which upon acetylation afforded α-acetoxynitro compound 14 in good yield. Subsequent condensation and cyclization of α-acetoxynitro compound 14 with benzyl isocyanoacetate (15) in the presence of DBU in THF gave the key pyrrole intermediate (S,S)-(-)-12 in 57% yield. N-Alkylation of pyrrole (S,S)-(-)-12 with iodide (S)-(-)-13 using t-BuOK in THF afforded the 2-benzyloxycarbonyl-1,3,4-substituted pyrrole derivative (-)-29 in 42% yield. Removal of the protective groups in (-)-29 followed by hydrogenolysis and decarboxylation afforded the crosslink (+)-Dpl (4) in good overall yield. The synthesis of an analogue (S)-(+)-24 and formation of a novel tetrahydroindole derivative (-)-31 are also described.

Full text of DOI:10.1021/jo0008901

J . Org. Chem. 2001, 66, 11-19
11
Bon e Colla gen Cr oss-Lin k s: A Con ver gen t Syn th esis of
(+)-Deoxyp yr r ololin e
Maciej Adamczyk,* Donald D. J ohnson, and Rajarathnam E. Reddy
Department of Chemistry (9NM, Bldg AP20), Diagnostics Division, Abbott Laboratories,
100 Abbott Park Road, Abbott Park, Illinois 60064-6016
maciej.adamczyk@abbott.com
Received J une 12, 2000
A convergent total synthesis of (+)-deoxypyrrololine (Dpl, 4), a putative cross-link of bone collagen,
is described starting from a commercially available L-glutamic acid derivative, (4S)-5-(tert-butoxy)-
4-[(tert-butoxycarbonyl)amino]-5-oxopentanoic acid (16). Condensation of aldehyde (S)-(-)-17 with
nitro compound (S)-(-)-27, both of which were prepared from a common precursor (S)-16, gave the
R-hydroxynitro compound 28, which upon acetylation afforded R-acetoxynitro compound 14 in good
yield. Subsequent condensation and cyclization of R-acetoxynitro compound 14 with benzyl
isocyanoacetate (15) in the presence of DBU in THF gave the key pyrrole intermediate (S,S)-(-)-
12 in 57% yield. N-Alkylation of pyrrole (S,S)-(-)-12 with iodide (S)-(-)-13 using t-BuOK in THF
afforded the 2-benzyloxycarbonyl-1,3,4-substituted pyrrole derivative (-)-29 in 42% yield. Removal
of the protective groups in (-)-29 followed by hydrogenolysis and decarboxylation afforded the cross-
link (+)-Dpl (4) in good overall yield. The synthesis of an analogue (S)-(+)-24 and formation of a
novel tetrahydroindole derivative (-)-31 are also described.
In tr od u ction
links (+)-pyridinoline (Pyd, 1)7 and (+)-deoxypyridinoline
(Dpd, 2)8 (Figure 1) have gained much attention because
of their potential clinical utility in the diagnosis of
osteoporosis9,10 and other metabolic bone diseases.11 In
addition to the presence of these pyridinium cross-links
(1 and 2), Scott et al.12 in 1981 postulated the existence
of pyrrole cross-links, pyrrololine (Pyl, 3) and deoxypyr-
rololine (Dpl, 4)13 (Figure 1), in various tissues. Subse-
quent studies by several other groups have provided
Bone is a complex and highly specialized form of
connective tissue, which serves several functions includ-
ing support of the body, protection of internal organs, and
as a reservoir for minerals. In order for bones to respond
and adapt to the mechanical stress as well as to maintain
serum mineral metabolism, they undergo constant re-
modeling. The bone remodeling process, which is also
called bone turnover, begins with resorption of old bone
by osteoclasts, followed by the formation of new bone by
osteoblasts.1 Any alterations or imbalance in the remod-
eling process results in various metabolic bone diseases
such as osteoporosis. Osteoporosis affects more than 25
million of the aged population in the U.S., particularly
postmenopausal women,2 and accounts for nearly 1.3
million fractures annually. The diagnosis of osteoporosis
involves analysis of bone based on histomorphometry and
densitometric measurements.3 Efforts for prevention of
this bone disease as well as to develop an effective
antiresorptive therapy have increased the search for
reliable and noninvasive biochemical markers of bone
resorption.4 The traditional markers of bone resorption,
e.g., urinary calcium5 and hydroxyproline,6 lack the
desired clinical sensitivity and specificity for diagnosis
of osteoporosis. In recent years, the pyridinium cross-
(4) For reviews on the collagen cross-links, see: (a) Watts, N. B.
Clin. Chem. 1999, 45, 1359-1368. (b) Knott, L.; Baily, A. J . Bone 1998,
22, 181-187. (c) J ames, I. T.; Walne, A. J .; Perrett, D. Ann. Clin.
Biochem. 1996, 33, 397-420. (d) Eyre, D. R. In Principles of Bone
Biology; Bilezikan, J . P., Raicz, L. G., Rogan, G., Eds.; Academic
Press: New York, 1996; pp 143-153.
(5) (a) Raisz, L. G. Nature 1963, 197, 1015-1016. (b) Murrils, R. J .
In Principles of Bone Biology; Bilezikan, J . P., Raicz, L. G., Rogan, G.,
Eds.; Academic Press: New York, 1996; pp 1239-1251.
(6) (a) Weiss, P. H.; Klein, L. J . Clin. Invest. 1969, 48, 1-10. (b)
Segrest, J . P. Methods Enzymol. 1982, 82, 398-410. (c) Lippincott, S.;
Chesney, R. W.; Friedman, A.; Pityer, R.; Barden, H.; Mazess, R. B.
Bone 1989, 10, 265-268.
(7) For isolation of Pyd (1), see: Fujimoto, D.; Moriguchi, T.; Ishida,
T.; Hayashi, H. Biochem. Biophys. Res. Commun. 1978, 84, 52-57.
(8) For isolation of Dpd (2), see: Ogawa, T.; Ono, T.; Tsuda, M.;
Kawanishi, Y. Biochem. Biophys. Res. Commun. 1982, 107, 1252-1257.
(9) (a) Eyre, D. R.; Oguchi, H.; Biochem. Biophys. Res. Commun.
1980, 92, 403-410. (b) Gunja-Smith, Z.; Boucek, R. J . Biochem. J . 1981,
197, 759-762. (c) Robins, S. P. Biochem. J . 1983, 215, 167-173.
(10) (a) Gomez, B., J r.; Ardakani, S.; Evans, B. J .; Merrell, L. D.;
J enkins, D. K.; Kung, V. T. Clin. Chem. 1996, 42, 1168-1175. (b)
Rosano, T. G.; Peaston, R. T.; Bone, H. G.; Woitge, H. W.; Francis, R.
M.; Seibel, M. J . Clin. Chem. 1998, 44, 2126-2132. (c) Sarno, M.;
Powell, H.; Tjersland, G.; Schoendorfer, D.; Harris, H.; Adams, K.;
Ogata, P.; Warnick, G. R. Clin. Chem. 1999, 45, 1501-1509.
(11) (a) Luftner, D.; Gunther, S.; Flath, B.; Muller, C.; Echteroff,
K.; Mergenthaler, H.-G.; Wernecke, K.-D.; Possinger, K. Anticancer
Res. 1999, 19, 2537-2544. (b) Delmas, P. D.; Gineyts, E.; Bertholin,
A.; Garnero, P.; Marchand, F. J . Bone Miner. Res. 1993, 8, 643-648.
(c) Hoshi, H.; Kushida, K.; Takahashi, M.; Denda, M.; Yamazaki, K.;
Yamanashi, A.; Inoue, T. Miner. Electrolyte Metab. 1997, 23, 93-99.
(12) (a) Scott, J . E.; Hughes, E. W.; Shuttleworth, A. A. Biosci. Rep.
1981, 1, 611-618. (b) Scott, J . E.; Qian, R.; Henkel, W.; Glanville, R.
W. Biochem. J . 1983, 209, 263-264. (c) Kemp, P. D.; Scott, J . E.
Biochem. J . 1988, 252, 387-393.
* To whom correspondence should be addressed. Phone: 847-937-
0225, Fax: 847-938-8927.
(1) For a comprehensive study on the physiology and pathology of
bones, see: Principles of Bone Biology; Bilezikan, J . P., Raicz, L. G.,
Rogan, G., Eds.; Academic Press: New York, 1996.
(2) (a) Sato, M.; Grese, T. A.; Dodge, J . A.; Bryant, H. U.; Turner,
C. H. J . Med. Chem. 1999, 42, 1-24. (b) Gums, J . G. US Pharmacist
1996 (September), 85-96. (c) Eyre, D. R.; Paz, M. A.; Gallop, P. M.
Annu. Rev. Biochem. 1984, 53, 717-748.
(3) (a) Hagiwara, S.; Yang, S.-O.; Gluer, C. C.; Bendavid, E.; Genant,
H. K. Osteoporosis 1994, 20, 651-669. (b) Genant, H. K.; Lang, T. F.;
Engelke, K.; Fuerst, T.; Gluer, C.-C.; Majumdar, S.; J ergas, M. Calcif.
Tissue Int. 1996, 59 (Suppl 1), S10-S15.
10.1021/jo0008901 CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/04/2000

12 J . Org. Chem., Vol. 66, No. 1, 2001
Adamczyk et al.
respectively, present in the telopeptide domains of the
collagen molecules. Thus, when the aldehyde is hydroxy-
lated (i.e., 6), the pyridinium cross-links 1 and 2 are
generated, and the pyrrole cross-links 3 and 4 are formed
from the unhydroxylated aldehyde (i.e., 9). We have been
interested in measuring the pyridinium cross-links 1 and
2 by immunoassay16 and other techniques17 as markers
for diagnosis of osteoporosis and other metabolic bone
diseases. Because the pyrrole cross-links (3 and 4) are
also excreted in urine during the process of bone degra-
dation along with pyridinium cross-links (1 and 2), it is
essential to consider the presence of cross-links 3 and 4
in the development of any assays for osteoporosis [e.g.,
measurement of Pyd (1) or Dpd (2)]. Additionally, the
pyrrole cross-links 3 and 4 themselves may potentially
be useful as markers in diagnosis and therapy manage-
ment of osteoporosis as well as to study the metabolism
of bones. In this paper, we detail the synthesis of pyrrole
cross-link (+)-deoxypyrrololine (Dpl, 4) starting from
(4S)-5-(tert-butoxy)-4-[(tert-butoxycarbonyl)amino]-5-oxo-
pentanoic acid (16).18
Resu lts a n d Discu ssion
Intrigued by the novel structural features of pyrrole
cross-links of bone collagen (3, 4) and their potential
applications, studies directed toward the total synthesis
of this class of 1,3,4-trisubstituted pyrrole amino acids
seemed to be appropriate. The ultimate goal of our effort
was to devise an efficient total synthesis of (+)-deoxy-
pyrrololine (4), thereby providing material for prelimi-
nary evaluation, and also to prepare analogues useful for
the diagnosis and therapy management of osteoporosis.
The basic synthetic strategy for construction of the cross-
link (+)-Dpl (4) involved (Figure 3) the introduction of a
lysine side chain via N-alkylation of pyrrole derivative
(S,S)-(-)-12 with iodide (S)-(-)-13, followed by hydrolysis
of the protective groups and removal of 2-carboxybenzyl
ester. The Boc and tert-butyl groups were chosen as
protective groups for amine and carboxylic acid function-
alities, respectively, as they are not only stable for the
reaction conditions during the synthetic pathway but also
could be hydrolyzed simultaneously using trifluoroacetic
acid. Additionally, we decided to protect the amino
functionalities as bis-Boc to avoid any interference in the
critical synthetic steps such as N-alkylation. The founda-
tion for the synthesis of key intermediate 2-benzyloxy-
carbonyl-3,4-substituted pyrrole derivative (S,S)-(-)-12
was based on our prior work in the area of 2-carboxy-
3,4-substituted pyrroles and their derivatives.19 Thus, the
key synthon, (S,S)-(-)-12, was envisioned to arise from
F igu r e 1. Structure of bone collagen cross-links (1-4).
further convincing evidence for the existence of pyrrole
cross-links.14,15 Unfortunately, attempts to isolate the
cross-links 3 and 4 have not been successful so far.4a,d,14f
It was proposed that the pyrrole cross-links 3 and 4 are
also formed from the natural (2S,5R)-hydroxylysine and
(S)-lysine residues present in collagen by a lysyl oxidase
mediated enzymatic process, in an analogous fashion to
the pyridinium cross-links 1, 2 as shown in Figure
2.4d,14c,d,f Thus, the reaction of keto-amine (hydroxylysino-
5-oxonorleucine, 5) with aldehyde 6 leads to the formation
of the hydroxylated Schiff base adduct 7, which then
undergoes a series of transformations such as Amadori
rearrangement, tautomerization, and ring closure to
produce the unsaturated ketone 8. Subsequent enoliza-
tion and oxidation of ketone 8 would lead to pyridinium
cross-links 1 and 2. Similarly, the reaction of keto-amine
5 with the aldehyde 9 would produce Schiff base adduct
10, which upon ring closure ultimately leads to the
formation of pyrrole cross-links 3 and 4. The common
basis for formation of cross-links (1, 2 and 3, 4) is the
key reaction of a keto-amine 5 with a free telopeptidyl
aldehyde (6 or 9). Aldehydes 6 and 9 are generated by
lysyl oxidase from the hydroxylysine and lysine residues,
(16) For our studies on pyridinium cross-links (1, 2), see: (a)
Adamczyk, M.; J ohnson, D. D.; Reddy, R. E. Tetrahedron 1999, 55,
63-88. (b) Adamczyk, M.; J ohnson, D. D.; Reddy, R. E. Tetrahedron:
Asymmetry 1999, 10, 775-781. (c) Adamczyk, M.; Akireddy, S. R.;
Reddy, R. E. Tetrahedron: Asymmetry 1999, 10, 3107-3110. (d)
Adamczyk, M.; J ohnson, D. D.; Reddy, R. E. Tetrahedron Lett. 1999,
40, 8993-8994. (e) Adamczyk, M.; J ohnson, D. D.; Reddy, R. E.
Bioconjugate Chem. 2000, 11, 124-130. (f) Adamczyk, M.; Akireddy,
S. R.; Reddy, R. E. Tetrahedron 2000, 56, 2379-2390. (g) Adamczyk,
M.; J ohnson, D. D.; Reddy, R. E. Tetrahedron: Asymmetry 2000, 11,
2289-2298.
(13) The names for pyrrole cross-links pyrrololine (Pyl, 3) and
deoxypyrrololine (Dpl, 4) were assigned in analogy to the pyridinium
cross-links pyridinoline (Pyd, 1) and deoxypyridinoline (Dpd, 2).
(14) (a) Henkel, W.; Glanville, R. W.; Greifendorf, D. Eur. J .
Biochem. 1987, 165, 427-436. (b) Horgan, D. J .; King, N. L.; Kurth,
L. B.; Kuypers, R. Arch. Biochem. Biophys. 1990, 281, 21-26. (c)
Kuypers, R.; Tyler, M.; Kurth, L. B.; J enkins, I. D.; Horgan, D. J .
Biochem. J . 1992, 283, 129-136. (d) Kuypers, R.; Tyler, M.; Kurth, L.
B.; J enkins, I. D.; Horgan, D. J . Meat Sci. 1994, 37, 67-89. (e) Risteli,
J .; Eriksen, H.; Risteli, L.; Mansell, J . P.; Bailey, A. J . Bone. Miner.
Res. 1994, 9, Suppl.1, A186. (f) Hanson, A. A.; Eyre, D.; R. J . Biol.
Chem. 1996, 271, 26508-26516.
(17) Adamczyk, M.; J ohnson, D. D.; Reddy, R. E.; Rege, S. D. J .
Labelled Comp. Radiopharm. 2000, 43, 463-472.
(18) For preliminary results of this work, see: Adamczyk, M.;
J ohnson, D. D.; Reddy, R. E. Angew. Chem. Int. Ed. Engl. 1999, 38,
3537-3539.
(15) Kleter, G. A.; Damen, J . J . M.; Kettenes-van den Bosch, J . J .;
Bank, R. A.; te Koppele, J . M.; Veraart, J . R.; ten Cate, J . M. Biochim.
Biophys. Acta 1998, 1381, 179-190.
(19) (a) Adamczyk, M.; Reddy, R. E. Tetrahedron Lett. 1996, 37,
2325-2326. (b) Adamczyk, M.; Fishpaugh, J . R.; Heuser, K. J .; Ramp,
J . M.; Reddy, R. E.; Wong, M. Tetrahedron 1998, 54, 3093-3112.

Synthesis of (+)-Deoxypyrrololine
J . Org. Chem., Vol. 66, No. 1, 2001 13
F igu r e 2. Formation of pyridinium and pyrrole cross-links (1, 2 and 3, 4) via condensation of keto-amine (5) with aldehydes (6
or 9).
F igu r e 3. Retrosynthetic analysis of (+)-deoxypyrrololine (Dpl, 4).
R-acetoxynitro compound 14 and benzyl isocyanoacetate
(15) via a base-promoted condensation and cyclization
process.20 The R-acetoxynitro compound 14 could be
derived from a common chiral starting material, (4S)-5-
(tert-butoxy)-4-[(tert-butoxycarbonyl)amino]-5-oxopen-
tanoic acid (16), which is a commercially available
L-glutamic acid derivative. To validate this proposed
synthetic strategy, particularly the decarboxylation of
1,3,4-substituted 2-carboxypyrrole derivatives [i.e., (-)-
29], 3,4-diethylpyrrole derivative (S)-(+)-24 (Scheme 1),
a simplified analogue of (+)-Dpl (4), was first targeted.
Analogue (S)-(+)-24 has the desired lysine chain at the
pyrrole nitrogen, which is present in the native (+)-Dpl
(4) and the ethyl groups at 3 and 4 positions, and thus
offers the simplicity for optimization of reaction condi-
tions for crucial steps including the N-alkylation.
(-)-tert-butyl-2-[bis(tert-butoxycarbonyl)amino]-6-iodohex-
anoate (13) from the L-glutamic acid derivative (S)-16
(Scheme 1). Thus, (S)-16 was converted to the known
aldehyde (S)-(-)-17 in three steps on a 20 g scale by
following our previously developed procedure.16b The
aldehyde (S)-(-)-17 was then extended by a methylene
to the corresponding olefin (S)-(-)-18 via a Wittig
reaction,16b which upon hydroboration afforded the alco-
hol (S)-(-)-19 in excellent yield (87%). The hydroxyl
group in (S)-(-)-19 was then converted to the desired
iodide (S)-(-)-13 in THF using I2, PPh3, and imidazole
in 92% yield. The next step in the synthesis of analogue
(S)-(+)-24 was the alkylation of known 2-benzyloxycar-
bonyl-3,4-diethylpyrrole (21)20b with iodide (S)-(-)-13.
Thus, ester 21 was treated with 1.0 equiv of potassium
tert-butoxide in THF, followed by reaction with 2.0 equiv
of iodide (S)-(-)-13 in the presence of 18-crown-6,21 at
room temperature. Purification of the crude compound
by preparative HPLC afforded the N-alkylated pyrrole
derivative (S)-(-)-22 in 66% yield. The synthesis of
analogue (+)-24 required hydrolysis of benzyl ester,
P r ep a r a tion of An a logu e (S)-(+)-24. The synthesis
of analogue (S)-(+)-24 began with the preparation of (S)-
(20) (a) Barton, D. H. R.; Kervagoret, J .; Zard, S. Z. Tetrahedron
1990, 46, 7587-7598. (b) Lash, T. D.; Bellettini, J . R.; Bastian, J . A.;
Couch, K. B. Synthesis 1994, 170-172.

14 J . Org. Chem., Vol. 66, No. 1, 2001
Sch em e 1. P r ep a r a tion of An a log (S)-(+)-24
Adamczyk et al.
Sch em e 2. Syn th esis of Key In ter m ed ia te (S,S)-(-)-12
unmasking the protective groups (i.e., Boc and tert-butyl),
and removal of the carboxyl group at the 2-position of
the pyrrole ring via decarboxylation. We planned to
achieve the last two transformations [i.e., hydrolysis of
protective groups (Boc and tert-butyl) and decarboxyla-
tion] simultaneously using trifluoroacetic acid.22 Accord-
ingly, the benzyl ester in (S)-(-)-22 was first cleaved by
hydrogenolysis in methanol to afford acid (S)-(-)-23 in
almost quantitative yield. Treatment of acid (S)-(-)-23
with trifluoroacetic acid at room temperature and puri-
fication of the crude product by preparative reversed-
phase HPLC afforded the 3,4-dimethylpyrrole derivative
(S)-(+)-24 in 82% yield as its TFA salt.
pleted the preparation of analogue (S)-(+)-24, we then
proceeded to the synthesis of (+)-Dpl (4) (Schemes 2-4).
Our first goal in the synthesis of (+)-Dpl (4) was to
prepare the required R-acetoxynitro compound 14 needed
for construction of the key pyrrole synthon (S,S)-(-)-12.
Accordingly, the aldehyde (S)-(-)-17 was reduced with
NaBH4 in MeOH to the corresponding alcohol (S)-(-)-25
in 95% yield, which was subsequently transformed to the
iodide (S)-(-)-26, using I2, PPh3, and imidazole, in
excellent yield (94%). Treatment of iodide (S)-(-)-26 with
sodium nitrite in DMF23 afforded the corresponding nitro
compound (S)-(-)-27 in 55% yield, a key component
required for condensation with aldehyde (S)-(-)-17.
Treatment of nitro compound (S)-(-)-27 with aldehyde
(S)-(-)-17 in the presence of DMAP in CH2Cl2 allowed
smooth condensation (Henry reaction) to give the R-hy-
droxynitro compound 28 in 91% yield as a mixture of
Syn th esis of Key 2,3,4-Tr isu bstitu ted P yr r ole
In ter m ed ia te (S,S)-(-)-12. Having successfully com-
(21) (a) Tenis, S. P.; Raggon, J . W. J . Org. Chem. 1987, 52, 819-
827. (b) Stefancich, G.; Artico, M.; Silvestri, R. J . Heterocycl. Chem.
1992, 29, 1005-1007.
(22) Wee, A. G. H.; Bunnenberg, E.; Djerassi, C. Synth. Commun.
1984, 14, 383-390.
(23) Blake, A. J .; Boyd, E. C.; Gould, R. O.; Paton, R. M. J . Chem.
Soc., Perkin Trans. 1 1994, 2841-2847.

Synthesis of (+)-Deoxypyrrololine
J . Org. Chem., Vol. 66, No. 1, 2001 15
Sch em e 3. Isola tion of a Novel Tetr a h yd r oin d ole Der iva tive (-)-31
Sch em e 4. Com p letion of th e Syn th esis of (+)-Dp l
pyrrole (S,S)-(-)-12 via alkylation (Scheme 3) by follow-
ing the conditions developed for analogue (+)-24. Thus,
treatment of 2-benzyloxycarbonyl-3,4-substituted pyrrole
derivative (S,S)-(-)-12 with 1.0 equiv of potassium tert-
butoxide in THF and subsequent reaction with 2.0 equiv
of iodide (S)-(-)-13 in the presence of 18-crown-6 afforded
the N-alkylated pyrrole derivative (-)-29 in 42% yield.
The derivative (-)-29 has the required three amino acid
side chains at the 1,3,4-positions on the pyrrole ring.
Synthesis of (+)-Dpl (4) required unmasking the protec-
tive groups and removing the benzyloxycarbonyl group
at the 2-position of the pyrrole ring. Thus, hydrogenolysis
of benzyl ester (-)-29 using 10% Pd/C in ethanol afforded
the acid 30 in 76% yield. The acid 30 was then treated
with trifluoroacetic acid, followed by water, to hydrolyze
the protective groups (i.e., Boc and t-Bu ester) and
simultaneously remove the acid group at the 2-position
via decarboxylation. However, purification of the crude
compound by preparative reversed-phase HPLC afforded
a product which did not correspond to the expected
(4)
1
collagen cross-link, (+)-Dpl (4). In the H NMR, only one
pyrrole proton (δ 7.08) was present, indicating that all
other positions on the ring were substituted. The 13C
NMR spectrum indicated the presence of three carbonyl
groups (δ 182.1, 172.4, and 172.2), one of which had a
distinct chemical shift (δ 182.1). The ESI-mass spectrum
showed (M + H)+ at 381, not the expected (M + H)+ at
399 for (+)-Dpl (4). It became evident that the isolated
product from the TFA-water hydrolysis reaction was a
novel (-)-(2S)-2-amino-6-{(6S)-6-amino-3[(4S)-3-ami-
nocarboxypropyl]-7-oxo-4,5,6,7-tetrahydro-1H-indol-1-yl}-
hexanoic acid (31) but not the desired (+)-Dpl (4).
Tetrahydroindole (-)-31 was isolated as the sole product
from the TFA-water reaction in 74% yield and presumed
to be formed via an acid-catalyzed decarboxylation-
acylation process.24 The formation of (-)-31 led us to
modify our original synthetic plan to achieve the total
synthesis of (+)-Dpl (4). Thus, we thought that hydrolysis
of the protective groups (i.e., Boc and tert-butyl) in acid
30 prior to the decarboxylation process could circumvent
diastereomers. Because the two newly generated chiral
centers at the NO2 and OH groups in R-hydroxynitro
compound 28 would be eventually eliminated during the
pyrrole ring formation, the diastereomeric mixture of 28,
as such, was converted to the corresponding acetate
derivative (14) by treatment with acetic anhydride in
THF in almost quantitative yield (96%). Finally, the
reaction of R-acetoxynitro compound 14 with benzyl
isocyanoacetate (15) in the presence of DBU in THF at 0
°C to room temperature underwent smooth condensation,
followed by cyclization. The crude compound from this
reaction was purified by silica gel column chromatogra-
phy to afford the key intermediate, 2-benzyloxycarbonyl-
3,4-substituted pyrrole derivative (S,S)-(-)-12, in 57%
yield.
Isola tion of a Novel Tetr a h yd r oin d ole Der iva tive
(-)-31 a n d Com p letion of th e Syn th esis of (+)-Dp l
(4). The next key step in the synthesis of (+)-4 was the
installation of a lysine chain at the pyrrole nitrogen in
(24) For the importance of the tetrahydroindole structural unit,
see: (a) J oshi, K. C.; Chand, P. Pharmazie 1982, 37, 1-13. (b) De
Kimpe, N.; Keppens, M. Tetrahedron 1996, 52, 3705-3718 and
references therein.

16 J . Org. Chem., Vol. 66, No. 1, 2001
Adamczyk et al.
resulting orange ylide solution was cooled to 0 °C in an ice
bath, and a solution of (S)-(-)-17 (6.07 g, 15.7 mmol) in THF
(60 mL) was added via a double-ended needle. The mixture
was stirred for 45 min at 0 °C and then quenched with a
saturated aqueous NH4Cl solution (60 mL). The mixture was
diluted with water (300 mL) and extracted with EtOAc (3 ×
300 mL). The combined organic layers were dried (Na2SO4),
and the solvent was removed on a rotary evaporator. The crude
compound was purified by silica gel column chromatography
(10% EtOAc in hexanes) to afford 3.66 g of (S)-(-)-18 in 61%
yield as a colorless viscous oil.16b Rf: 0.62 (15% EtOAc in
hexanes). [R]23D: -16.9 (c 1.48, MeOH). Analytical RP HPLC:
MeCN/0.1% aqueous trifluoroacetic acid 70:30, 2.0 mL/min at
the formation of tetrahydroindole derivative (-)-31.
Accordingly, compound (-)-29 was first treated with
trifluoroacetic acid and water to hydrolyze the Boc and
tert-butyl ester protective groups (Scheme 4). Purification
of the crude compound by preparative reversed-phase
HPLC afforded the benzyl ester (+)-32 in 79% yield. The
benzyl group in (+)-32 was then removed by hydro-
genolysis over 10% Pd/C in MeOH to give the corre-
sponding acid 33, which without purification was treated
with trifluoroacetic acid to initiate the decarboxylation.
Concentration of the crude reaction mixture followed by
preparative reversed-phase HPLC purification and lyo-
philization afforded the cross-link (+)-deoxypyrrololine
(Dpl, 4) in 39% yield as a pale pink powder. Under these
conditions during the transformation of compound (+)-
32 to (+)-Dpl (4), we did not observe tetrahydroindole
31.
1
225 nm, tR 4.90 min, 98%. H NMR (CDCl3): δ 5.88-5.74 (m,
1 H), 5.07-4.95 (m, 2 H), 4.75-4.70 (m, 1 H), 2.22-2.06 (m, 3
H), 2.00-1.89 (m, 1 H), 1.50 (s, 18 H), 1.44 (s, 9 H). 13C NMR
(CDCl3): δ 170.0, 152.6, 137.6, 115.4, 82.7, 81.1, 58.3, 30.4,
28.6, 27.9, 29.8. ESI-MS (m/z): 386 (M + H)+, 788 (2 × M +
NH4)+.
(S)-(-)-ter t-Bu t yl-2-[b is(ter t-b u t oxyca r b on yl)a m in o]-
6-h yd r oxyh exa n oa te (19). (S)-(-)-18 (3.611 g, 9.37 mmol)
in THF (50 mL) was cooled to 0 °C in an ice bath, and a
solution of borane-THF complex (1.0 M solution in THF, 12.2
mL, 12.2 mmol, 1.3 equiv) was added under nitrogen. The
cooling bath was removed, and the mixture was allowed to
warm to room temperature and stirred for 17 h. The reaction
mixture was then cooled to 0 °C, and 1.0 N aqueous NaOH
(14.0 mL, 14.0 mmol, 1.5 equiv) was added followed by 30%
aqueous H2O2 (11.7 mL, 103.0 mmol, 11.0 equiv) and stirred
for 30 min. The mixture was diluted with a 20% aqueous NaCl
solution (100 mL) and extracted with EtOAc (3 × 100 mL).
The combined organic layers were dried (Na2SO4) and con-
centrated on a rotary evaporator. The crude product was
purified by silica gel column chromatography (40% EtOAc in
hexanes) to afford 3.281 g of (S)-(-)-19 in 87% yield as a
colorless viscous oil. Rf: 0.30 (40% EtOAc in hexanes). Analyti-
cal RP HPLC: MeCN/0.05% aqueous trifluoroacetic acid 60:
40, 2.0 mL/min at 215 nm, tR 6.19 min, 99.5%. [a]23D: -16.2 (c
In summary, a general and convergent total synthesis
of a novel pyrrole cross-link of collagen, (+)-deoxypyr-
rololine (Dpl, 4), is described, utilizing a commercially
available L-glutamic acid derivative, (4S)-5-(tert-butoxy)-
4-[(tert-butoxycarbonyl)amino]-5-oxopentanoic acid (16)
as a common source for all three chiral centers.
Exp er im en ta l Section
Gen er a l Meth od s a n d Ma ter ia ls. 1H and 13C NMR
spectra were recorded on a Varian Gemini spectrometer (300
MHz), and the chemical shifts (δ) were reported. Electrospray
ionization mass spectrometry (ESI-MS) was carried on a
Perkin-Elmer (Norwalk, CT) Sciex API 100 benchtop system
employing a Turbo IonSpray ion source, and HRMS were
obtained on a Nermang 3010 MS-50 or a J EOL SX102-A. Thin-
layer chromatography was performed on precoated Whatman
MK6F silica gel 60 Å plates (layer thickness: 250 µm) and
visualized with UV light, KMnO4 solution [KMnO4 (1.0 g) and
NaOH (8.0 g) in water (200 mL)], phosphomolybdic acid
reagent (20 wt % solution in ethanol), or ninhydrin reagent
(0.2% ninhydrin in ethanol). Column chromatography was
performed on silica gel, Merck grade 60 (230-400 mesh).
Anhydrous solvents were freshly distilled (THF from a purple
solution of sodium and benzophenone and CH2Cl2 from CaH2)
under nitrogen. All reagents were purchased from Aldrich
Chemical Co. (Milwaukee, WI) or Sigma Chemical Co. (St.
Louis, MO) and used without purification, except where noted.
All of the solvents employed were of HPLC grade purchased
from EM Science (Gibbstown, NJ ) and used as received.
Analytical reversed-phase (RP) HPLC was performed using a
Waters µBondapak RCM C18 10 µm (8 × 100 mm) column
(solvents ratio v/v reported) unless otherwise stated. Prepara-
tive reversed-phase (RP) HPLC was performed using a Waters
µBondapak RCM C18 10 µm (40 × 100 mm) column (solvents
ratio v/v reported) unless otherwise stated. Optical rotations
were measured on an Autopol III polarimeter from Rudolph
Research (Flanders, NJ ). IUPAC names of all new compounds
were obtained using the ACD/Ilab Web service version 3.5 at
http://www.acdlabs.com/ilab.
(S)-(-)-tert-Butyl-2-[bis-(tert-butoxycarbonyl)amino]-5-oxo-
pentanoate (17) was prepared from a commercially available
(4S)-5-(tert-butoxy)-4-[(tert-butoxycarbonyl)amino]-5-oxopen-
tanoic acid (16) in three steps on a 20 g scale following our
previously developed procedure.16b 2-Benzyl 3,4-diethyl-1H-
pyrrole-2-carboxylate (21) was prepared from 1-ethyl-2-ni-
trobutyl acetate (20) and benzyl isocyanoacetate (15) according
to the known procedure on a 8.6 g scale in 72% yield.20b
(S)-(-)-ter t-Bu t yl-2-[b is(ter t-b u t oxyca r b on yl)a m in o]-
5-h exen oa te (18). n-BuLi (2.5 M solution in hexanes, 12.6
mL, 31.4 mmol, 2.0 equiv) was added dropwise to a suspension
of methyl triphenylphosphonium bromide (11.2 g, 31.4 mmol,
2.0 equiv) in THF (230 mL) at room temperature under
nitrogen. After the mixture was stirred for 30 min, the
1
1.46, MeOH). H NMR (CDCl3): δ 4.70 (dd, 1 H, J ) 9.3, 5.0
Hz), 3.64 (q, 2 H, J ) 12.3, 6.3 Hz), 2.13-2.01 (m, 1 H), 1.93-
1.80 (m, 1 H), 1.66-1.38 (m, 5 H), 1.50 (s, 18 H), 1.44 (s, 9 H).
13C NMR (CDCl3): δ 169.9, 152.5, 82.7, 81.1, 62.5, 58.7, 32.2,
28.8, 28.0, 27.9, 22.5. ESI-MS (m/z): 404 (M + H)+, 442 (M +
K)+. HRMS (FAB, m/z): calcd for C20H38NO7, 404.2648 (M +
H)+; observed, 404.2658.
(S)-(-)-ter t-Bu t yl-2-[b is(ter t-b u t oxyca r b on yl)a m in o]-
6-iod oh exa n oa te (13). Triphenylphosphine (3.12 g, 11.9
mmol, 1.5 equiv), imidazole (0.865 g, 12.7 mmol, 1.6 equiv),
and iodine (3.02 g, 11.9 mmol, 1.5 equiv) were added sequen-
tially to a solution of (S)-(-)-19 (3.197 g, 7.92 mmol) in THF
(80 mL) at room temperature under nitrogen. After the
mixture was stirred for 1 h, it was diluted with EtOAc (80
mL) and water (80 mL). The organic layer was separated, and
the aqueous layer was extracted with EtOAc (2 × 80 mL). The
combined organic layers were dried (Na2SO4), and the solvent
was removed on a rotary evaporator. The crude product was
purified by silica gel column chromatography (10% EtOAc in
hexanes) to afford 3.741 g of (S)-(-)-13 in 92% yield as a pale
yellow viscous oil. Rf: 0.46 (15% EtOAc in hexanes). [R]23
:
D
-22.1 (c 1.73, MeOH). Analytical RP HPLC: MeCN/0.1%
aqueous trifluoroacetic acid 80:20, 2.0 mL/min at 215 nm, tR
1
5.26 min, 99.5%. H NMR (CDCl3): δ 4.72 (dd, 1 H, J ) 9.0,
5.1 Hz), 3.28-3.13 (m, 2 H), 2.22-2.09 (m, 1 H), 2.04-1.81
(m, 3 H), 1.51 (s, 18 H), 1.45 (s, 9 H). 13C NMR (CDCl3): δ
169.4, 152.3, 82.9, 81.4, 57.8, 30.4, 30.2, 28.0, 27.9, 5.8. ESI-
MS (m/z): 500 (M + H)+. HRMS (FAB, m/z): calcd for C20H37
-
NO6I, 514.1666 (M + H)+; observed, 514.1663.
(-)-Ben zyl-1-[(5S)-5-[bis(ter t-bu toxyca r bon yl)a m in o]-
6-(ter t-bu toxy)-6-oxoh exyl]-3,4-d ieth yl-1H-p yr r ole-2-ca r -
boxyla te (22). A solution of 18-crown-6 (0.073 g, 0.276 mmol,
0.1 equiv) in THF (11.0 mL) and a solution of t-BuOK (1.0 M
solution in THF, 2.76 mL, 2.76 mmol, 1.0 equiv) were added
sequentially to a solution of ester 21 (0.709 g, 2.76 mmol) in
THF (11.0 mL) at room temperature under nitrogen. To this
mixture was added a solution of (S)-(-)-iodide 13 (2.832 g, 5.52

Synthesis of (+)-Deoxypyrrololine
J . Org. Chem., Vol. 66, No. 1, 2001 17
mmol, 2.0 equiv) in THF (11.0 mL) via a double-ended needle,
and the reaction mixture was stirred for 7 h. The reaction
mixture was then diluted with 20% aqueous NaCl solution (20
mL) and EtOAc (20 mL). The organic layer was separated, and
the aqueous layer was extracted with EtOAc (2 × 20 mL). The
combined organic layers were dried (Na2SO4), and the solvent
was removed on a rotary evaporator. The crude product was
purified by silica gel column chromatography (20% EtOAc in
hexanes) to give 3.01 g of product, which was further purified
by preparative reversed-phase HPLC (Waters, C18, RCM,
Symmetry, 7.0 µm, 40 × 100 mm; MeCN/0.1 aqueous trifluo-
roacetic acid/93:7, 45 mL/min at 225 nm). Lyophilyzation of
the product afforded 1.14 g of (-)-22 in 66% yield as a pale
nitrogen. The mixture was stirred for 45 min, quenched with
water (200 mL), and extracted with EtOAc (3 × 200 mL). The
combined organic layers were dried (Na2SO4), and the solvent
was removed on a rotary evaporator. The crude compound was
purified by silica gel column chromatography (40% EtOAc in
hexanes) to afford 7.82 g of (S)-(-)-25 in 95% yield as a
colorless viscous oil. Rf: 0.31 (40% EtOAc in hexanes). [R]23
:
D
-14.9 (c 1.28, MeOH). Analytical RP HPLC: MeCN/0.1%
aqueous trifluoroacetic acid 60:40, 2.0 mL/min at 215 nm, tR
1
5.51 min, 99.3%. H NMR (CDCl3): δ 4.74 (dd, 1 H, J ) 9.0,
5.4 Hz), 3.66 (dt, 2 H, J ) 12.3, 6.6 Hz), 2.22-2.11 (m, 1 H),
1.95-1.83 (m, 1 H), 1.67-1.50 (m, 3 H), 1.50 (s, 18 H), 1.44 (s,
9 H). 13C NMR (CDCl3): δ 169.8, 152.5, 82.8, 81.2, 62.2, 58.6,
29.5, 28.0, 27.9, 25.7. ESI-MS (m/z): 390 (M + H)+, 412 (M +
Na)+. HRMS (FAB, m/z): calcd for C19H36NO7, 390.2492 (M +
H)+; observed, 390.2498.
yellow viscous oil. Rf: 0.64 (20% EtOAc in hexanes). [R]23
:
D
-2.3 (c 0.53, MeOH). Analytical RP HPLC (Waters, C18, RCM,
Symmetry, 7.0 µm, 8 × 100 mm): MeCN/0.1% aqueous
trifluoroacetic acid 95:5, 2.0 mL/min at 225 nm, tR 8.14 min,
(S)-(-)-ter t-Bu t yl-2-[b is(ter t-b u t oxyca r b on yl)a m in o]-
5-iod op en ta n oa te (26). Triphenylphosphine (3.78 g, 14.4
mmol, 1.5 equiv), imidazole (1.04 g, 15.3 mmol, 1.6 equiv), and
iodine (3.65 g, 14.4 mmol, 1.5 equiv) were added sequentially
to a solution of (S)-(-)-25 (3.734 g, 9.59 mmol) in THF (100
mL) at room temperature under nitrogen. After the mixture
was stirred for 1.5 h, the solvent was removed on a rotary
evaporator to dryness, and the crude product was purified by
silica gel column chromatography (10% EtOAc in hexanes) to
afford 4.498 g of (S)-(-)-26 in 94% yield as colorless viscous
oil. Rf: 0.51 (20% EtOAc in hexanes). [R]23D: -17.8 (c 1.39,
MeOH). Analytical RP HPLC: MeCN/0.1% aqueous trifluo-
roacetic acid 80:20, 2.0 mL/min at 215 nm, tR 5.26 min, 99.5%.
1H NMR (CDCl3): δ 4.72 (dd, 1 H, J ) 9.0, 5.1 Hz), 3.28-3.13
(m, 2 H), 2.22-2.09 (m, 1 H), 2.04-1.81 (m, 3 H), 1.51 (s, 18
H), 1.45 (s, 9 H). 13C NMR (CDCl3): δ 169.4, 152.3, 82.9, 81.4,
57.8, 30.4, 30.2, 28.0, 27.9, 5.8. ESI-MS (m/z): 500 (M + H)+.
HRMS (FAB, m/z): calcd for C19H35NO6I, 500.1509 (M + H)+;
observed, 500.1511.
1
>99%. H NMR (CDCl3): δ 7.43-7.26 (m, 5 H), 6.57 (s, 1 H),
5.27 (s, 2 H), 4.67 (dd, 1 H, J ) 9.3, 5.1 Hz), 4.22-4.14 (m, 2
H), 2.68 (q, 2 H, J ) 7.5 Hz), 2.38 (q, 2 H, J ) 7.2 Hz), 2.10-
1.24 (m, 6 H), 1.49 (s, 18 H), 1.44 (s, 9 H), 1.15 (t, 3 H, J ) 7.5
Hz), 1.04 (t, 3 H, J ) 7.5 Hz). 13C NMR (CDCl3): δ 169.8, 161.4,
152.5, 136.5, 134.8, 128.5, 128.3, 128.0, 126.0, 124.2, 117.6,
82.7, 81.1, 65.4, 58.8, 49.5, 31.5, 29.0, 28.0, 27.9, 23.9, 19.1,
17.8, 15.7, 14.9. ESI-MS (m/z): 643 (M + H)+, 660 (M + NH4)+.
HRMS (FAB, m/z): calcd for C36H54N2O8, 642.3880 (M)+;
observed, 642.3884.
(-)-1-[(5S)-5-[Bis(ter t-bu toxyca r bon yl)a m in o]-6-(ter t-
bu t oxy)-6-oxoh exyl]-3,4-d iet h yl-1H -p yr r ole-2-ca r boxyl-
ic Acid (23). A solution of benzyl ester (-)-22 (0.268 g, 0.074
mmol) in MeOH (19.0 mL) was treated with 10% Pd/C (0.092
g), and the mixture was stirred under H2 atmosphere, using a
balloon, at room temperature. After the mixture was stirred
for 1 h, it was filtered through a pipet (fitted with cotton and
Celite) and washed with MeOH (2.0 mL). The filtrate was
concentrated on a rotary evaporator to afford 0.221 g of (S)-
(-)-23 in 96% yield as a pale pink gummy material. Rf: 0.29
(20% EtOAc in hexanes). [R]23D: -6.9 (c 0.45, MeOH). Analyti-
cal RP HPLC (Waters, C18, RCM, Symmetry, 7.0 µm, 8 × 100
mm): MeCN/0.1% aqueous trifluoroacetic acid 95:5, 2.0 mL/
(S)-(-)-ter t-Bu t yl-2-[b is(ter t-b u t oxyca r b on yl)a m in o]-
5-n itr op en ta n oa te (27). Sodium nitrite (NaNO2, 1.95 g, 28.3
mmol, 2.0 equiv) was added to a solution of (S)-(-)-26 (7.053
g, 14.1 mmol) in DMF (70 mL) at room temperature under
nitrogen. After the reaction mixture was stirred for 40 min, it
was diluted with 20% aqueous NaCl solution (200 mL) and
EtOAc (200 mL). The organic layer was separated, and the
aqueous layer was extracted with EtOAc (2 × 200 mL). The
combined organic layers were dried (Na2SO4), and the solvent
was removed on a rotary evaporator. The crude product was
purified by silica gel column chromatography (20% EtOAc in
hexanes) to afford 3.263 g of (S)-(-)-27 in 55% yield as a
1
min at 225 nm, tR 3.46 min, 98%. H NMR (CDCl3): δ 6.62 (s,
1 H), 4.69 (dd, 1 H, J ) 9.3, 5.1 Hz), 4.24-4.18 (m, 2 H), 2.75
(q, 2 H, J ) 7.5 Hz), 2.41 (q, 2 H, J ) 7.5 Hz), 2.12-1.26 (m,
6 H), 1.49 (s, 18 H), 1.44 (s, 9 H), 1.23 (t, 3 H, J ) 7.5 Hz),
1.17 (t, 3 H, J ) 7.5 Hz). 13C NMR (CDCl3): δ 169.8, 164.8,
152.5, 136.6, 127.0, 124.5, 116.8, 82.7, 81.2, 58.8, 49.4, 31.3,
29.0, 28.0, 27.9, 23.6, 19.0, 17.7, 15.6, 14.8. ESI-MS (m/z): 575
(M + Na)+. HRMS (FAB, m/z): calcd for C29H48N2O8, 552.3411
(M)+; observed, 552.3404.
colorless viscous oil. Rf: 0.35 (20% EtOAc in hexanes). [R]23
:
D
-21.8 (c 1.97, MeOH). Analytical RP HPLC: MeCN/0.05%
aqueous trifluoroacetic acid 70:30, 2.0 mL/min at 215 nm, tR
5.57 min, 99.7%. 1H NMR (CDCl3): δ 4.75-4.70 (m, 1 H),
4.44-4.38 (m, 2 H), 2.18-1.91 (m, 4 H), 1.51 (s, 18 H), 1.44 (s,
9 H). 13C NMR (CDCl3): δ 169.0, 152.4, 83.2, 81.6, 74.9, 57.9,
28.0, 27.8, 26.0, 24.3. ESI-MS (m/z): 419 (M + H)+, 436 (M +
NH4)+, 854 (2 × M + NH4)+. HRMS (FAB, m/z): calcd for
(+)-6-(3,4-Dieth yl-1H-p yr r ol-1-yl)-L-n or leu cin e (24). Tri-
fluoroacetic acid (10.0 mL) was added to (S)-(-)-23 (0.217 g,
0.393 mmol) at room temperature. After the mixture was
stirred for 2 h, water (0.5 mL) was added, and the stirring
was continued at room temperature for an additional 2 h. The
reaction mixture was then concentrated on a rotary evaporator
(<35 °C bath temperature), and the residue was dissolved in
MeCN/0.1% aqueous trifluoroacetic acid (50:50, 15 mL) and
purified by preparative RP HPLC (Waters, C18, RCM, Sym-
metry, 7.0 µm, 40 × 100 mm; MeCN/0.1% aqueous trifluoro-
acetic acid 35:65, 45 mL/min at 225 nm). The product was
collected and lyophilized to afford 0.117 g of analogue (S)-(+)-
24 as its TFA salt in 82% yield (pale pink gummy material).
[R]23D: +3.96 (c 0.63, MeOH). Analytical RP HPLC (Waters,
C18, RCM, Symmetry, 7.0 µm, 8 × 100 mm): MeCN/0.1%
aqueous trifluoroacetic acid 30:70, 2.0 mL/min at 225 nm, tR
8.54 min, >99%. 1H NMR (CD3OD): δ 6.37 (s, 2 H), 3.82-
3.76 (m, 3 H), 2.36 (q, 4 H, J ) 7.8 Hz), 1.98-1.50 (m, 4 H),
1.48-1.36 (m, 2 H), 1.13 (t, 6 H, J ) 7.2 Hz). 13C NMR (CD3-
OD): δ 172.4, 124.8, 54.4, 32.3, 31.4, 23.4, 19.5, 15.4. ESI-MS
(m/z): 253 (M + H)+. HRMS (FAB, m/z): calcd for C14H25N2O2,
253.1916 (M + H)+; observed, 253.1915.
C
19H35N2O8, 419.2393 (M + H)+; observed, 419.2405.
1,10-Di-ter t-bu tyl-2,9-bis[bis(ter t-bu toxyca r bon yl)a m i-
n o]-2,3,4,5,7,8,9-h eptadeoxy-5-n itr o-D-th r eo-decar ate (28).
A mixture of (S)-(-)-tert-butyl-2-[bis(tert-butoxycarbonyl)-
amino]-5-nitropentanoate (27, 1.907 g, 4.56 mmol) and (S)-
(-)-tert-butyl-2-[bis(tert-butoxycarbonyl)amino]-5-oxopen-
tanoate (17, 1.887 g, 4.87 mmol, 1.1 equiv) were dissolved in
CH2Cl2 (6.0 mL), and 4-(dimethylamino)pyridine (DMAP, 2.23
g, 18.2 mmol, 4.0 equiv) was added at room temperature under
nitrogen. After the reaction mixture was stirred for 2 days,
an additional amount of CH2Cl2 (6.0 mL) was added, and the
mixture was stirred for 6 days. The crude reaction mixture
was then directly purified by silica gel column chromatography
(20-30% EtOAc in hexanes) to afford 2.67 g of R-hydroxynitro
compound 28 in 73% yield as a mixture of diastereomers (on
the basis of the recovered nitro compound (S)-(-)-27, the yield
of 28 was 91%). Rf: 0.48 (30% EtOAc in hexanes). Analytical
RP HPLC: MeCN/0.05% aqueous trifluoroacetic acid 80:20,
(S)-(-)-ter t-Bu tyl-2-[bis-(ter t-bu toxyca r bon yl)a m in o]-
5-h yd r oxyp en t a n oa t e (25). Sodium borohydride (0.798 g,
21.1 mmol, 1.0 equiv) was added to a solution of aldehyde (S)-
(-)-17 (8.177 g, 21.1 mmol) in MeOH (100 mL) at 0 °C under
1
2.0 mL/min at 215 nm, tR 6.87 min, >99%. H NMR (CDCl3):
δ 4.75-4.66 (m, 2 H), 4.56-4.36 (m, 1 H), 4.08-3.84 (m, 1 H),

18 J . Org. Chem., Vol. 66, No. 1, 2001
Adamczyk et al.
2.92-1.80 (m, 9 H), 1.50 (s, 36 H), 1.43 (s, 18 H). ESI-MS (m/
z): 823 (M + NH4)+, 828 (M + Na)+. HRMS (FAB, m/z): calcd
for C38H67N3O15Na, 828.4464 (M + Na)+; observed, 828.4467.
Also, 0.383 g of (S)-(-)-27 (yield: 20%) and 0.333 g of (S)-
(-)-17 in (yield: 18%) of both the starting materials were
recovered, which were recycled.
NaCl solution (20 mL) and EtOAc (20 m). The organic layer
was separated, and the aqueous layer was extracted with
EtOAc (2 × 20 mL). The combined organic layers were dried
(Na2SO4), and the solvent was removed on a rotary evaporator.
The crude product was purified by silica gel column chroma-
tography (20% EtOAc in hexanes) to afford 0.404 g of (-)-29
in 42% yield. Rf: 0.42 (20% EtOAc in hexanes). [R]23D: -6.8
(c 2.0, MeOH). Analytical RP HPLC: MeCN/0.1% aqueous
trifluoroacetic acid 95:5, 2.0 mL/min at 215 nm, tR 8.7 min,
97%. 1H NMR (CDCl3): δ 7.43-7.26 (m, 5 H), 6.58 (s, 1 H),
5.26 (q, 2 H, J ) 48.3, 12.3 Hz), 4.72-4.61 (m, 3 H), 4.11 (t, 2
H, J ) 8.1 Hz), 2.82-2.60 (m, 2 H), 2.44-1.24 (m, 12 H), 1.49
(s, 18 H), 1.48 (s, 18 H), 1.47 (s, 18 H), 1.44 (s, 9 H), 1.43 (s, 9
H), 1.42 (s, 9 H). 13C NMR (CDCl3): δ 169.8, 169.7, 169.6,
161.0, 152.5, 152.4, 136.6, 132.2, 128.5, 127.9, 126.5, 121.4,
118.2, 82.7, 82.5, 82.4, 81.1, 81.0, 77.2, 65.3, 59.3, 58.8, 58.3,
49.7, 31.7, 31.1, 29.2, 29.1, 28.0, 27.9, 23.8, 23.1, 21.3. ESI-
MS (m/z): 1319 (M + NH4)+. HRMS (FAB, m/z): calcd for
1,10-Di-ter t-bu tyl-6-O-a cetyl-2,9-bis[bis(ter t-bu toxyca r -
bon yl)a m in o]-2,3,4,5,7,8,9-h ep ta d eoxy-5-n itr o-D-th r eo-d e-
ca r a te (14). Acetic anhydride (0.463 mL, 4.91 mmol, 1.5 equiv)
and 4-(dimethylamino)pyridine (0.040 g, 0.327 mmol, 0.1
equiv) were added sequentially to a solution of R-hydroxynitro
compound 28 (2.638 g, 3.27 mmol) in THF (25 mL) at room
temperature under nitrogen. After the mixture was stirred for
2 h, it was diluted with water (70 mL) and EtOAc (70 mL).
The organic layer was separated, and the aqueous layer was
extracted with EtOAc (70 mL). The combined organic layers
were dried (Na2SO4), and the solvent was removed on a rotary
evaporator. The residue was purified by silica gel column
chromatography (20% EtOAc in hexanes) to afford 2.67 g of
R-acetoxynitro compound 14 in 96% yield as a mixture of
diastereomers. Rf: 0.23 (20% EtOAc in hexanes). Analytical
RP HPLC: MeCN/0.05% aqueous trifluoroacetic acid 85:15,
C
68H109N4O20Na, 1323.7427 (M + Na)+; observed, 1323.7449.
3,4-Bis[(3S)-3-[bis(ter t-bu toxyca r bon yl)a m in o]-4-(ter t-
bu toxy)-4-oxobu tyl]-1-[(5S)-5-[bis(ter t-bu toxyca r bon yl)-
a m in o]-6-(ter t-bu toxy)-6-oxoh exyl]-1H-p yr r ole-2-ca r box-
ylic Acid (30). 10% Pd/C (0.020 g) was added to a solution of
benzyl ester (-)-29 (0.096 g, 0.074 mmol) in EtOH (4.0 mL),
and the reaction mixture was stirred under H2 atmosphere
using a balloon at room temperature. After the mixture was
stirred for 1 h, it was filtered through a pipet (fitted with cotton
and Celite) and washed with MeOH (2.0 mL). The filtrate was
concentrated on a rotary evaporator, and the crude product
was purified by silica gel column chromatography (30% EtOAc
in hexanes) to give 0.068 g of acid 30 in 76% yield. Analytical
RP HPLC: MeCN/0.1% aqueous trifluoroacetic acid 19:81; 2.0
mL/min at 215 nm, tR 4.89 min, 99%. 1H NMR (CD3OD): δ
6.65 (s, 1 H), 4.80-4.65 (m, 3 H), 4.21-4.16 (m, 2 H), 2.80-
2.72 (m, 2 H), 2.46-2.38 (m, 2 H), 2.36-1.68 (m, 10 H), 1.49
(s, 54 H), 1.43 (s, 27 H). ESI-MS (m/z): 1229 (M + NH4)+.
(-)-(2S)-2-Am in o-6-{(6S)-6-a m in o-3-[(4S))-3-a m in oca r -
b oxyp r op yl]-7-oxo-4,5,6,7-t e t r a h yd r o-1H -in d ol-1-yl}-
h exa n oic Acid (31). Trifluoroacetic acid (2.0 mL) was added
to acid 30 (0.058 g, 0.048 mmol) at room temperature. After
the mixture was stirred for 2 h, water (0.1 mL) was added,
and the stirring was continued at room temperature for an
additional 4 h. The reaction mixture was concentrated on a
rotary evaporator (<35 °C bath temperature). The residue was
dissolved in MeCN/0.05% aqueous trifluoroacetic acid (10:90,
8 mL) and purified by preparative RP HPLC (MeCN/0.05%
aqueous trifluoroacetic acid 5:95, 40 mL/min at 215 nm). The
product was collected and lyophilized to afford 0.025 g of
tetrahydroindole derivative (-)-31 as its TFA salt in 74% yield.
[R]23D: -7.7 (c 0.31, MeOH). Analytical RP HPLC: MeCN/0.1%
aqueous trifluoroacetic acid 5:95, 2.0 mL/min at 215 nm, tR
1
2.0 mL/min at 215 nm, tR 6.02 min, 99%. H NMR (CDCl3): δ
5.28-5.16 (m, 1 H), 4.74-4.60 (m, 3 H), 2.20-1.60 (m, 11 H),
1.50 (s, 36 H), 1.43 (s, 18 H). ESI-MS (m/z): 865 (M + NH4)+,
870 (M + Na)+. HRMS (FAB, m/z): calcd for C40H69N3O16Na,
870.4570 (M + Na)+; observed, 870.4586.
(-)-Ben zyl-3,4-bis[(3S)-3-[bis(ter t-bu toxyca r bon yl)a m i-
n o]-4-(ter t-b u t oxy)-4-oxob u t yl]-1H -p yr r ole-2-ca r b oxy-
la te (12). The diastereomeric mixture of R-acetoxynitro com-
pound 14 (4.457 g, 5.26 mmol) was dissolved in THF (40 mL)
and cooled in an ice bath (0-5 °C), and a solution of benzyl
isocyanoacetate (15, 1.20 g, 6.84 mmol, 1.3 equiv) in THF (10
mL) was added via a double-ended needle under nitrogen. To
this mixture was added via a syringe 1,8-diazabicyclo[5.4.0]-
undec-7-ene (DBU, 2.0 mL, 13.0 mmol, 2.5 equiv), and the
mixture was stirred for 30 min. The cooling bath was then
removed, the reaction mixture was allowed to warm to room
temperature, and the stirring was continued for an additional
5 h. The mixture was partitioned between a 20% aqueous NaCl
solution (130 mL) and EtOAc (130 mL). The organic layer was
separated, and the aqueous layer was extracted with EtOAc
(2 × 130 mL). The combined organic layers were dried (Na2-
SO4), and the solvent was removed on a rotary evaporator. The
residue was purified by silica gel column chromatography (20%
EtOAc in hexanes) to afford 2.75 g of (S,S)-(-)-benzyl ester
(12) in 57% yield. Rf: 0.64 (30% EtOAc in hexanes). ESI-MS
(m/z): 934 (M + NH4)+. A 0.081 g portion of (S,S)-(-)-benzyl
ester (12) was dissolved in MeCN/water (85:15, 10 mL) and
purified by preparative RP HPLC (MeCN/0.05% aqueous
trifluoroacetic acid 85:15; 2.0 mL/min at 215 nm). The com-
pound was collected, and the solvent was removed on a rotary
evaporator to about 100 mL and lyophilized to afford 0.047 g
of analytically pure (S,S)-(-)-benzyl ester (12). Analytical RP
HPLC: MeCN/0.05% aqueous trifluoroacetic acid 85:15, 2.0
mL/min at 215 nm, tR 8.91 min, 97%. [R]23D: -14.9 (c 1.37,
1
3.55 min, 98%. H NMR (CD3OD): δ 7.08 (s, 1 H), 4.36-4.20
(m, 2 H), 4.14-4.06 (m, 1 H), 3.92-3.82 (m, 2 H), 2.98-2.77
(m, 2 H), 2.50-2.42 (m, 3 H), 2.20-2.04, (m, 3 H), 1.96-1.74
(m, 4 H), 1.54-1.34 (m, 2 H). 13C NMR (CD3OD): δ 182.1,
172.4, 172.2, 138.2, 132.7, 125.1, 120.9, 56.4, 54.3, 53.7, 32.3,
31.7, 31.2, 30.8, 22.9, 21.2, 21.1. ESI-MS (m/z): 381 (M + H)+.
HRMS (FAB, m/z): calcd for C18H28N4O5, 381.2138 (M + H)+;
observed, 381.2137.
1
MeOH). H NMR (CDCl3): δ 8.77 (br s, 1 H), 7.42-7.29 (m, 5
H), 6.70 (d, 1 H, J ) 3.0 Hz), 5.28 (q, 2 H, J ) 30.9, 12.0 Hz),
4.74-4.68 (m, 2 H), 2.88-2.68 (m, 2 H), 2.50-2.40 (m, 2 H),
2.38-2.23 (m, 2 H), 2.32-1.92 (m, 2 H), 1.48 (s, 36 H), 1.43 (s,
9 H), 1.42 (s, 9 H). ESI-MS (m/z): 934 (M + NH4)+. HRMS
(FAB, m/z): calcd for C48H73N3O14Na, 938.4978 (M + Na)+;
observed, 938.4985.
(+)-Ben zyl Ester (32). Compound (-)-29 (0.0383 g, 0.294
mmol) was dissolved in a mixture of TFA/water (95:5, 10 mL)
at room temperature and stirred for 2 h. The mixture was
concentrated on a rotary evaporator (<40 °C bath tempera-
ture), and the residual TFA was removed azeotropically using
toluene (2 × 10 mL). The residue was dissolved in MeCN/water
(20:80, 30 mL) and purified by preparative RP HPLC (MeCN/
0.1% aqueous trifluoroacetic acid 19:81, 40 mL/min at 215 nm).
The product was collected and concentrated on a rotary
evaporator (<40 °C bath temperature) to about 75 mL and
lyophilized to afford 0.23 g of (+)-32 in 79% yield as its TFA
(-)-Ben zyl-3,4-bis[(3S)-3-[bis(ter t-bu toxyca r bon yl)a m i-
n o]-4-(ter t-bu toxy)-4-oxobu tyl]-1-[(5S)-5-[bis(ter t-bu toxy-
ca r bon yl)a m in o]-6-(ter t-bu toxy)-6-oxoh exyl]-1H-p yr r ole-
2-ca r boxyla te (29). A solution of 18-crown-6 (0.020 g, 0.074
mmol, 0.1 equiv) in THF (3.0 mL) and a solution of t-BuOK
(1.0 M solution in THF, 0.740 mL, 0.74 mmol, 1.0 equiv) were
added sequentially to a solution of (S,S)-(-)-benzyl ester 12
(0.677 g, 0.79 mmol) in THF (3.0 mL) at room temperature
under nitrogen. To this mixture was added a solution of (S)-
(-)-iodide 13 (0.760 g, 1.48 mmol, 2.0 equiv) in THF (3.0 mL)
via a double-ended needle, and the mixture was stirred for 7
h. The reaction mixture was then diluted with 20% aqueous
salt. [R]23
: +17.9 (c 1.32, MeOH). Analytical RP HPLC:
D
MeCN/0.1% aqueous trifluoroacetic acid 19:81, 2.0 mL/min at
1
215 nm, tR 4.27 min, >99%. H NMR (CD3OD): δ 7.48-7.34
(m, 5 H), 6.86 (s, 1 H), 5.31 (s, 2 H), 4.19 (t, 2 H, J ) 7.5 Hz),
3.92 (t, 1 H, J ) 6.0 Hz), 3.84 (t, 1 H, J ) 6.6 Hz), 3.72 (dist

Synthesis of (+)-Deoxypyrrololine
J . Org. Chem., Vol. 66, No. 1, 2001 19
t, 1 H, J ) 6.6 Hz), 3.98-2.89 (m, 1 H), 2.89-2.76 (m, 1 H),
2.66-2.54 (m, 2 H), 2.20-1.98 (m, 4 H), 1.86-1.74 (m, 2 H),
1.74-1.62 (m, 2 H), 1.40-1.28 (m, 2 H). 13C NMR (CD3OD): δ
172.2, 172.1, 170.0, 162.6, 137.7, 132.3, 129.8, 129.7, 129.5,
128.7, 122.1, 119.7, 67.2, 54.0, 53.9, 53.8, 50.5, 32.9, 32.8, 32.4,
31.2, 23.1, 22.4, 21.1. ESI-MS (m/z): 533 (M + H)+. HRMS
(FAB, m/z): calcd for C26H37N4O8, 533.2611 (M + H)+; ob-
served, 533.2615.
(+)-Deoxyp yr r ololin e (Dp l, 4). 10% Pd/C (0.019 g) was
added to a solution of benzyl ester (+)-32 (0.086 g, 0.087 mmol)
in MeOH (4.0 mL) and stirred under H2 atmosphere using a
balloon at room temperature. After the mixture was stirred
for 1 h, it was filtered through a pipet (fitted with cotton and
Celite powder) and washed with MeOH (2.0 mL). The filtrate
was concentrated on a rotary evaporator (<40 °C bath tem-
perature) and dried on a vacuum pump (0.5 mm/Hg) to give
0.069 g of crude acid 33. Analytical RP HPLC: MeCN/0.1%
aqueous trifluoroacetic acid 6:94, 2.0 mL/min at 215 nm, tR
7.23 min, 70%. ESI-MS (m/z): 443 (M + H)+. A small amount
of the desired decarboxylated compound, (+)-Dpl (4), was also
found to be present in the crude acid 33. Analytical RP
HPLC: MeCN/0.1% aqueous trifluoroacetic acid 6:94, 2.0 mL/
min at 215 nm, tR 4.77 min, 30%. ESI-MS (m/z): 399 (M +
H)+. To this crude mixture of acid 33 and (+)-Dpl (4) (0.055 g)
was added trifluoroacetic acid (2.0 mL) at room temperature,
and the mixture was stirred for 15 min. The reaction mixture
was then concentrated on a rotary evaporator (<35 °C bath
temperature), and the residual trifluorocaetic acid was re-
moved azeotropically using a mixture of toluene and MeOH
(1:1, 2 × 4 mL). The resulting residue was dissolved in MeCN/
water (10:90, 10 mL) and purified by preparative reversed-
phase HPLC (MeCN/0.1% aqueous trifluoroacetic acid 5:95,
40 mL/min at 215 nm). The product was collected and
lyophilized to afford 0.021 g of (+)-Dpl (4) TFA salt in 39%
yield as a pale pink powder. [R]23
: +20.6 (c 0.17, H2O).
D
Analytical RP HPLC: MeCN/0.1% aqueous trifluoroacetic acid
1
4:96, 2.0 mL/min at 215 nm, tR 5.54 min, 96%. H NMR (CD3-
OD): δ 6.51 (s, 2 H), 3.92-3.78 (m, 5 H), 2.64-2.52 (m, 4 H),
2.20-1.98 (m, 4 H), 1.94-1.82 (m, 2 H), 1.82-1.70 (m, 2 H),
1.48-1.38 (m, 2 H). 13C NMR (D2O + 2.0 µL of MeCN): δ 173.7,
120.0, 119.7, 54.1, 54.0, 49.0, 31.6, 30.8, 30.1, 22.1, 20.7, 1.5.
ESI-MS (m/z): 399 (M + H)+. HRMS (FAB, m/z): calcd for
C
18H31N4O6, 399.2244 (M + H)+; observed, 399.2253.
1
Su p p or tin g In for m a tion Ava ila ble: Copies of H NMR
spectra of (S,S)-(-)-12, (S)-(-)-13, 14, (S)-(-)-19, (S)-(-)-22,
(S)-(-)-23, (S)-(-)-25, (S)-(-)-26, (S)-(-)-27, 28, (-)-29, 30, and
1
(+)-32 and the H NMR and 13C NMR spectra of (+)-Dpl (4),
(S)-(+)-24, and (-)-31. This material is published free of charge
via the Internet at http://pubs.acs.org.
J O0008901