Total synthesis of (+)-deoxypyrrololine: A potential biochemical marker for diagnosis of osteoporosis

Article abstract of DOI:10.1002/(SICI)1521-3773(19991203)38:23<3537::AID-ANIE3537>3.0.CO;2-R

The collagen cross-link (+)-deoxypyrrololine (Dpl, 1), a potential biochemical marker for diagnosis of osteoporosis, has been obtained by a general and convergent total synthesis. The key synthetic features involve utilization of a L-glutamic acid derivat

Full text of DOI:10.1002/(SICI)1521-3773(19991203)38:23<3537::AID-ANIE3537>3.0.CO;2-R

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this dimer, complex 5 was prepared in a zinc reduction using an
analogous procedure as described for 2. Experimental details will be
reported in a separate manuscript.
[27] A more electron-deficient rhodium center might also accelerate
insertion reactions at RhIII intermediates, which would result in
increased activity.
[28] The formation of an intermediate with the same characteristics as 7
was also observed in a reaction of 1-phenylpropanal with 6. At this
point an alternative structure for 7 can not be excluded (for example,
the branched isomer as in 3').
[29] C. P. Lenges, P. S. White, M. Brookhart, J. Am. Chem. Soc. 1999, 121,
4385.
Total Synthesis of (‡)-Deoxypyrrololine:
A Potential Biochemical Marker
for Diagnosis of Osteoporosis
Maciej Adamczyk,* Donald D. Johnson, and
Rajarathnam E. Reddy
Osteoporosis is a crippling degenerative bone disease that
affects the aged population, particularly postmenopausal
women.[1] This metabolic disease is a consequence of an
inbalance in the bone renewal process, which occurs when
bone resorption exceeds bone formation. The current meth-
ods for diagnosis of osteoporosis involve analysis of bone
based on histomorphometry and densitometric measure-
ments.[2] The 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.[1c, 3] The traditional markers of
bone resorption, for example urinary calcium[4] and hydrox-
yproline,[5] lack clinical sensitivity and specificity for diagnosis
of osteoporosis.
that 2 and 4 are formed from natural (2S,5R)-hydroxylysine
and (S)-lysine present in collagen by a lysyl oxidase mediated
enzymatic process, in an analogous fashion to the pyridinium
cross-links 1 and 3.[3d, 13c,d,f] Here we report the first synthesis
of the pyrrole cross-link (‡)-deoxypyrrololine (Dpl, 4) from
(4S)-5-(tert-butoxy)-4-[(tert-butoxycarbonyl)amino]-5-oxo-p-
entanoic acid (7).
The synthesis of 4 involves alkylation of the pyrrole
derivative (S,S)-( )-5 with iodide (S)-( )-6, followed by
hydrolysis of the protecting groups and removal of the
2-carboxybenzyl ester moiety (Scheme 1). It was envisioned
that the key intermediate (S,S)-( )-5 could be prepared from
the a-acetoxynitro compound 13 (see Scheme 2) and benzyl
In recent years, the pyridinium cross-links (‡)-pyridinoline
(Pyd, 1)[6] and (‡)-deoxypyridinoline (Dpd, 3)[7] have gained
much attention owing to their poten-
tial clinical utility in the diagnosis of
osteoporosis and other bone diseas-
es.[8±10] In 1981 Scott et al.[11] postu-
lated the existence of pyrrole cross-
links pyrrololine (Pyl, 2) and deoxy-
pyrrololine (Dpl, 4)[12] in various
tissues. Subsequent studies by sever-
al other groups have provided fur-
ther convincing evidence for the
existence of pyrrole cross-links.[13, 14]
Unfortunately, the attempts to iso-
late cross-links 2 and 4 have not been
successful so far.[3a,d] It was proposed
Scheme 1. Retrosynthesis of (‡)-deoxypyrrololine (Dpl, 4). Bn ˆ benyzl, Boc ˆ tert-butyloxycarbonyl.
isocyanoacetate (14)[15, 16] by a base-promoted condensation
and cyclization process. Compound 13 could be obtained
from 7.
Accordingly, the commercially available (S)-7 was convert-
ed into the aldehyde (S)-( )-8,[17] which was then reduced
[*] Dr. M. Adamczyk, D. D. Johnson, Dr. R. E. Reddy
Division Organic Chemistry Research (9NM, AP20)
Diagnostics Division, Abbott Laboratories
100 Abbott Park Road, Abbott Park, IL 60064-6016 (USA)
Fax: (‡1)847-938-8927
E-mail: maciej.adamczyk@abbott.com
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yield using I2, PPh3, and imidazole in THF.
Treatment of (S,S)-( )-5 with 1.0 equiva-
lent of tBuOK in THF and subsequent
reaction with 2.0 equivalents of (S)-( )-6
in the presence of [18]crown-6[21] at room
temperature afforded the N-alkylated pyr-
role derivative ( )-17 in 42% yield after
purification by column chromatography on
silica gel.[22] The derivative ( )-17 has the
required amino acid chains at the 1-,3-, and
4-positions of the pyrrole ring. The syn-
thesis of (‡)-Dpl (4) would only require
unmasking the protecting groups and re-
moval of the benzyloxycarbonyl group at
the 2-position of pyrrole ring. Thus, ( )-17
was first treated with trifluoroacetic acid
and water,[9a] and the mixture obtained was
purified by preparative reverse-phase
HPLC to afford (‡)-18 in 79% yield. The
Scheme 2. Synthesis of (S,S)-( )-5. Reagents and conditions: a) See ref. [17]; b) NaBH4
(1.0 equiv), MeOH, 08C, 45 min, 95%; c) I2 (1.5 equiv), PPh3 (1.6 equiv), imidazole (2.0 equiv),
THF, 238C, 1.5 h, 94%; d) NaNO2 (2.0 equiv), DMF, 238C, 30 min, 55%; e) (S)-( )-8 (1.1 equiv),
DMAP, (4.0 equiv), CH2Cl2, 238C, 8 d, 91%; f) Ac2O (1.5 equiv), DMAP (0.1 equiv), THF, 238C,
2 h, 96%; g) BnO2CCH2NC (14, 1.3 equiv), DBU (2.5 equiv), THF, 0 ± 238C, 5 h, 57%. DBU ˆ 1,8-
diazabicyclo[5.4.0]undec-7-ene, DMAP ˆ 4-dimethylaminopyridine.
with NaBH4 in MeOH to the alcohol
(S)-( )-9 in 95% yield (Scheme 2).[18]
The hydroxyl group in (S)-( )-9 was
converted into the iodide to provide
(S)-( )-10, which was subsequently
treated with sodium nitrite in DMF[19]
to afford the corresponding nitro com-
pound (S)-( )-11 in 55% yield. Con-
densation of (S)-( )-11 with aldehyde
(S)-( )-8 (Henry reaction)[20] in the
presence of 4-dimethylaminopyridine
in CH2Cl2 gave a-hydroxynitro com-
pound 12 in 91% yield as a mixture of
diastereomers (1:1 ratio). Since the two
new chiral centers (the locations of the
NO2 and OH groups) in 12 are even-
tually eliminated during the pyrrole ring
formation, the diastereomeric mixture
of 12 was converted into the corre-
sponding acetate derivative 13 by treat-
Scheme 3. Synthesis of (‡)-4. Reagents and conditions: MePPh3Br (2.0 equiv), nBuLi (2.0 equiv),
THF 08C, 45 min, 61%; b) B2H6 ´ THF (1.3 equiv), THF, 0 !238C, 17 h, 87%; c) I2 (1.5 equiv), PPh3
(1.6 equiv), imidazole (2.0 equiv), THF, 238C, 1 h, 92%; d) (S,S)-( )-5, tBuOK (1.0 equiv), [18]crown-
6 (0.1 equiv), THF, 238C, 7 h, 42%; e) TFA/water (95/5), 238C, 2 h, 79%; f) 10% Pd/C, H2, MeOH,
238C, 1 h, then TFA, 238C, 15 min, 39%. TFA ˆ trifluoroacetic acid.
ment with Ac2O in THF in almost
quantitative yield (96%). Finally, con-
densation of the diastereomeric mixture
of a-acetoxynitro compound 13 with
benzyl isocyanoacetate (14)[15, 16] in the
presence of DBU in THF at 08C to
room temperature, followed by column chromatography on
benzyl group in (‡)-18 was removed by hydrogenation over
10% Pd/C in MeOH to give the acid 19, which without
purification was decarboxylated with trifluoroacetic acid.[23]
Concentration of the crude reaction mixture followed by
purification by preparative reverse-phase HPLC and lyophi-
lization afforded 4[24] in 39% yield as a pale pink powder
([a]2D3 ˆ ‡20.6 (c ˆ 0.17, H2O)).
silica gel, gave the key intermediate (S,S)-( )-5 in 57% yield
([a]2D3
ˆ
14.9 (c ˆ 1.37, MeOH)).
The next step in the synthesis of (‡)-4 was installation of
lysine chain at the pyrrole nitrogen atom in (S,S)-( )-5 by
alkylation with 6. The iodide (S)-( )-6 was prepared from
aldehyde (S)-( )-8 in three steps (Scheme 3). First, the (S)-
( )-8 was extended by a methylene group to the correspond-
ing olefin (S)-( )-15 by Wittig reaction,[17] which upon
hydroboration afforded the alcohol (S)-( )-16 in excellent
yield (87%). The hydroxyl group in (S)-( )-16 was then
converted to provide the desired iodide (S)-( )-6 in 92%
In summary a general and convergent total synthesis of (‡)-
deoxypyrrololine (Dpl, 4), a pyrrole cross-link of collagen, has
been achieved utilizing a commercially available chiral
starting material (7). The method is currently being applied
to the synthesis of pyrrololine (2) and for the preparation of
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Angew. Chem. Int. Ed. 1999, 38, No. 23

COMMUNICATIONS
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Keywords: collagen
´ natural products ´ osteoporosis ´
pyrrololines ´ total synthesis
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[22] ( )-17: Rf ˆ 0.42 (silica gel, EtOAc/hexanes 2/8); [a]D28
ˆ
6.8 (c ˆ 2.0,
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C18 column, 10 mm, 8 Â 10 mm; MeCN/0.1% aq. trifluoroacetic acid
95/5, 2.0 mLmin 1, detection at 215 nm): Rt ˆ 8.7 min (97%); 1H NMR
(300 MHz, CDCl3, 238C, TMS): d ˆ 7.43 ± 7.26 (m, 5H, Ar-H), 6.58 (s,
1H, pyrrole-H), 5.26 (q, 2H, J ˆ 48.3, 12.3 Hz, Ar-CH2), 4.72 ± 4.61 (m,
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2.82 ± 2.60 (m, 2H), 2.44 ± 1.24 (m, 12H), 1.49 (s, 18H), 1.48 (s, 18H),
1.47 (s, 18H), 1.44 (s, 9H), 1.43 (s, 9H), 1.42 (s, 9H); 13C NMR
(75 MHz, CDCl3, 238C, TMS): d ˆ 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,
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[24] (‡)-Dpl (4): [a]2D0 ˆ ‡20.6 (c ˆ 0.17, H2O); analytical reverse-phase
HPLC (Waters, Rcm, mBondapak C18 column, 10 mm, 8 Â 10 mm;
MeCN/0.1% aq. trifluoroacetic acid 4/96, 2.0 mLmin 1, detection at
215 nm): Rt ˆ 5.54 min (96%); 1H NMR (300 MHz, CD3OD, 238C,
TMS): d ˆ 6.51 (s, 2H, pyrrole-H), 3.92 ± 3.78 (m, 5H, 3  N-CH-
CO2H and pyrrole-N-CH2), 2.64 ± 2.52 (m, 4H), 2.20 ± 1.98 (m, 4H),
1.94 ± 1.82 (m, 2H), 1.82 ± 1.70 (m, 2H), 1.48 ± 1.38 (m, 2H); 13C NMR
(75 MHz, D2O (0.5 mL ‡ MeCN (2.0 mL), 238C): d ˆ 173.7, 120.0,
119.7 (CN), 54.1, 54.0, 49.0, 31.6, 30.8, 30.1, 22.1, 20.7, 1.5 (CH3); ESI-
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‡
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‡
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Angew. Chem. Int. Ed. 1999, 38, No. 23
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