Synthesis of conformationally constrained lysine analogues

Article abstract of DOI:10.1021/jo060210f

The synthesis of two conformationally constrained lysine analogues is reported. The synthesis of the novel analogue 1 based on the 3-aza-bicyclo[3.1.0]hexane system is accomplished from the known tricycle 3 in eight steps. The synthesis of the analogue 2 is accomplished in eight steps from 4-hydroxy proline. Both analogues are synthesized appropriately protected for Fmoc/Boc solid-phase peptide synthesis.

Full text of DOI:10.1021/jo060210f

to mimic a gauche(-) ø1 angle, whereas the analogue 2 roughly
mimics a gauche(+) ø1 angle.
Synthesis of Conformationally Constrained
Lysine Analogues
Rakesh Ganorkar, Amarnath Natarajan, Ahmed Mamai, and
Jose´ S. Madalengoitia*
Department of Chemistry, UniVersity of Vermont,
Burlington, Vermont 05405
jmadalen@uVm.edu
ReceiVed January 31, 2006
We envisioned that the synthesis of the lysine analogue 1
could be accomplished from the tricycle 3, available in six steps
from pyroglutamic acid in multigram quantities.5 The ester
functionality is selectively reduced with LiEt3BH at -78 °C
affording the alcohol 4 in 84% yield (Scheme 1). The corre-
sponding aldehyde 5 was also isolated in small amounts (∼5%,
even with 6 equiv of LiEt3BH).6 We next explored the
Mitsunobu reaction of the alcohol 4 with “HCN” as a means to
introduce what would eventually become both the ꢀ-carbon and
the side-chain nitrogen. However, when alcohol 2 was subjected
to Mitsunobu conditions using acetone cyanohydrin as the
cyanide source, the major product isolated was the reduced
DEAD adduct 6.7 Further attempts to modify the conditions were
unsuccessful in furnishing the desired nitrile. Because the direct
conversion of the alcohol to the nitrile proved elusive, we next
explored the two-step process in which the alcohol could be
converted to a suitable leaving group and then displaced with
cyanide. Attempts to generate the tosylate with 1.1 equiv of
TsCl and 1.1 equiv of Et3N in CH2Cl2 at room temperature
resulted in mixtures of the desired tosylate, the chloride 7, and
recovered starting material.8 Not surprisingly, shorter reaction
times resulted in the recovery of large amounts of starting
material, and longer reaction times favored the production of
the chloride. Unfortunately, the chloride could not be displaced
by cyanide under a number of conditions examined. The chloride
to nitrile transformation could be effected by first converting
the chloride to an iodide (NaI, acetone) and then displacing the
iodide with Bu4NCN in an overall yield of 75% (data not
shown). Because this procedure added an extra step, additional
avenues into the nitrile were explored. We found that the
mesylation of alcohol 4 with MsCl proceeds smoothly to yield
the mesylate 8 (70%) as well as the chloride 7 (10-15%,
Scheme 1), which was easily separable by flash chromatography.
The introduction of the nitrile function was next explored.
Although the reaction of mesylates with NaCN in DMSO
represents a standard method for the synthesis of nitriles,9 in
The synthesis of two conformationally constrained lysine
analogues is reported. The synthesis of the novel analogue
1 based on the 3-aza-bicyclo[3.1.0]hexane system is ac-
complished from the known tricycle 3 in eight steps. The
synthesis of the analogue 2 is accomplished in eight steps
from 4-hydroxy proline. Both analogues are synthesized
appropriately protected for Fmoc/Boc solid-phase peptide
synthesis.
The poly-L-proline (PPII) secondary structure has received
much attention in the past decade because of its role in mediating
a number of signal transduction pathways.1-3 Within this
context, our group has been developing a program focused on
the design and synthesis of PPII mimics as inhibitors of these
signaling pathways. Our strategy for the construction of PPII
mimics involves first the synthesis of peptides from what we
call proline-templated amino acids (PTAAs) and then the
synthesis of peptides from PTAAs. OligoPTAAs are designed
to preferentially populate the PPII conformation in solution
because φ is constrained ∼-75° by the pyrrolidine ring and ψ
is constrained ∼145° by the pseudo-A(1,3) strain. The utility
of these PPII mimics is highlighted by a recent report from our
group that uses oligoPTAAs to provide compelling evidence
that cGMP-dependent protein kinase (PKG) binds peptide
substrates in the PPII conformation.4 In this paper, we report
the synthesis of two lysine PTAA analogues (1 and 2), one of
which (1) was critical to probe the active-site occupancy
requirements of PKG. The lysine PTAA analogue 1 is designed
(1) For a PPII review, see: Siligardi, G.; Drake, A. F. Pept. Sci. 1995,
37, 281.
(2) For a survey of PPII helices in globular proteins, see: Adzhubei, A.
A.; Sternberg, M. J. J. Mol. Biol. 1992, 229, 472.
(3) For examples of proteins that bind PPII helices, see: (a) Raj, P. A.;
Marcus, E.; Edgerton, M. Biochemistry 1996, 35, 4314. (b) Lee, C.-H.;
Saksela, K.; Mirza, U. A.; Chait, B. T.; Kuriyan, J. Cell 1996, 85, 931. (c)
Peng, S.; Kasahara, C.; Rickles, R. J.; Schreiber, S. L. Proc. Natl. Acad.
Sci. U.S.A. 1995, 92, 12408. (d) Jardetzky, T. S.; Brown, J. H.; Gorga, J.
C.; Stern, L. J.; Urban, R. G.; Strominger, J. L.; Wiley, D. C. Proc. Natl.
Acad. Sci. U.S.A. 1996, 93, 734. (e) Zeile, W. L.; Purich, D. L.; Southwick,
F. S. J. Cell Biol. 1996, 133, 49.
(5) Zhang, R.; Mamai, A.; Madalengoitia, J. S. J. Org. Chem. 1999, 64,
547.
(6) Pedregal, C.; Ezquerra, J. Tetrahedron Lett. 1994, 35, 2053.
(7) Wilk, B. K. Synth. Commun. 1993, 23, 2481.
(8) Jimenez, J. M.; Rife, J.; Ortun˜o, R. M. Tetrahedron: Asymmetry 1996,
7, 537.
(4) Zhang, R.; Nickl, C. K.; Mamai, A.; Flemer, S.; Natarajan, A.;
Dostmann, W. R.; Madalengoitia, J. S. J. Pept. Res. 2005, 66, 151.
10.1021/jo060210f CCC: $33.50 © 2006 American Chemical Society
Published on Web 06/02/2006
5004
J. Org. Chem. 2006, 71, 5004-5007

SCHEME 1
The incorporation of the final carbon and nitrogen was
accomplished by the use of 0.3 equiv of Bu4NCN in CH2Cl2/
saturated aqueous KCN to give the nitrile 10. Under these
conditions, the dinitrile compounds do not form because the
CN- concentration is lower and the ionic association with the
ammonium cation most likely attenuates the basicity of the
cyanide anion (in contrast to the “naked” cyanide anion in
DMSO). With the final carbon and nitrogen in place, the
structure was elaborated to the desired PTAA. Complete
reduction of the functionality in nitrile, amide, and oxazolidine
functionality was accomplished with 3.5 equiv of BH3 in
refluxing THF to give the amino alcohol 11.10 The crude amine
was then N-Boc protected with Boc2O to give the prolinol 12
in 75% yield for both steps. Interestingly, N-debenzylation failed
to proceed under standard conditions (H2, Pd-C) or under other
conditions (NH4HCO2H, Pd-C, MeOH, reflux) that had previ-
ously worked for us with troublesome N-debenzylations. In this
instance, however, N-debenzylation was effected with H2 and
Pearlman’s catalyst in methanol. The resultant secondary amine
was protected with FmocCl to yield the alcohol 13 in 65% yield
over both steps. Finally, the alcohol was oxidized to the
carboxylic acid with TEMPO/NaOCl/NaOCl2 to give the novel
lysine PTAA 1 in 84% yield suitably protected for Fmoc/Boc
solid-phase peptide synthesis.11
The synthesis of an analogue of 2,4-cis-lysine PTAA 2 (the
N,N′-diBoc analogue) has been previously reported in the
literature in 18 steps,12 and during the preparation of this
manuscript, a synthesis of the unprotected isomer by a route
similar to ours was published.13 We had originally envisioned
that a short synthesis could be adapted starting from 4-hydroxy
proline as the 4-position was appropriately functionalized for
further elaboration into the lysine side-chain functionality.
Protection of the R-nitrogen with CbzCl afforded the carbamate
15 in 99% yield. Protection of the carboxylic acid group was
accomplished with BnBr and NaHCO3 in DMF with a catalytic
amount of NaI to give the benzyl ester 16 in 80% yield.
Oxidation of the hydroxyl group was accomplished with
trifluoroacetic anhydride, DMSO, and Et3N to give the ketone
17 in 82% yield (Caution! Stench). To functionalize the
4-position with the desired two carbons and a nitrogen, we
carried out a stabilized Wittig reaction that afforded the
conjugated nitriles 18a as a mixture of cis/trans-alkene isomers
and the unconjugated nitrile 18b as a 60:40 (18a/18b) mixture
in 89% overall yield. The assignment is based in part on the
nitrile stretching frequencies. The isomers 18a exhibit an IR
band at 2221 cm-1 consistent with a conjugated nitrile, whereas
the isomer 18b exhibits an IR band at 2254 cm-1 consistent
with an unconjugated nitrile. In McCafferty’s synthesis, an
N-Boc-4-keto proline methyl ester is subjected to an analogous
Wittig reaction under conditions that are essentially the same
as ours, but only the presence of cis/trans isomers is reported.
From an examination of their data, we suspect that the
compound they assign as the Z-nitrile might indeed be the
isomers with the exocyclic double bond because the reported
IR band is at 2221 cm-1. It is also possible that the isomer
assigned as the E-nitrile could be the isomer with the endocyclic
double bond because the reported IR frequency is 2251 cm-1
SCHEME 2
our system, the use of a stoichiometric amount of NaCN resulted
in a mixture of products and the use of an excess of NaCN
resulted in the isolation of compounds 9 as a 1:1 mixture of
diastereomers. The formation of diastereomers 9 suggests that
the naked anion is basic enough to deprotonate R to the nitrile
promoting a retro-Michael ring opening to give, after protonation
of the enolate, the R,â-unsaturated nitrile. Under the reaction
conditions, cyanide then adds to the R,â-unsaturated nitrile in
a Michael fashion affording the dinitrile compounds 9 as an
approximate 1:1 mixture of diastereomers.
(10) Brown, H. C.; Choi, Y. M.; Narasimhan, S. Synthesis 1981, 8, 605.
(11) Zhao, M.; Li, J.; Mano, E.; Song, Z.; Tsachen, D. M.; Grabowski,
E. J. J.; Reider, P. J. J. Org. Chem. 1999, 64, 2564.
(12) Wang, Q.; Sasaki, N. A.; Potier, P. Tetrahedron 1998, 54, 15759.
(13) Barkallah, S.; Schneider, S. L.; McCafferty, D. G. Tetrahedron Lett.
2005, 46, 4985.
(9) (a) Lewis, R. N.; Susi, P. V. J. Am. Chem. Soc. 1952, 74, 840. (b)
Moreau, F.; Florentin, D.; Marquet, A. Tetrahedron 2000, 56, 285.
J. Org. Chem, Vol. 71, No. 13, 2006 5005

TABLE 1. Solvent Effects on the Diastereomer Ratio for the
Hydrogenation of Nitrile 18a
SCHEME 3
solvent
EtOAc
19b/19a
1:0.4
1:1
EtOH
AcOH
1:1
i-PrOH
EtOH/H2O (1:1)
H2O
1:1.7
1:2.4
1:2.5
TABLE 2. Isomerization Experiments
entry
base
LDA
LDA
HA
18a/18b/18c conditions % yield (18b)
1
2
3
4
5
AcOH
TFA
1.4:1:0.35
0.8:1:0.23
aa
a
20
25
44
39
56
yield of 18b was highest when quenching with p-TsOH (entry
4). The final optimization of this reaction was accomplished
by transferring the anion to a solution containing 2 equiv of
p-TsOH at -78 °C and then quenching the reaction with water
at -78 °C. Under these conditions, the desired isomer 18b was
obtained in 56% yield from 18a.
LHMDS p-TsOH 0.55:1:0.23
LHMDS TfOH 0.4:1:0.17
LHMDS p-TsOH 0.28:1:0.09
a
a
bb
a Condition a: 1.1 equiv of base is added to a solution of 18a at -78
°C. After 30 min, this solution is transferred via cannula to a THF solution
containing 2 equiv of acid at -78 °C. The reaction mixture is then allowed
to warm to room temperature and quenched with water. b Condition b: 1.1
equiv of base is added to a solution of 18a at -78 °C. After 30 min, this
solution is transferred via cannula to a THF solution containing 2 equiv of
acid at -78 °C. The reaction mixture is quenched with water and then
allowed to warm to room temperature.
After optimization of the isomerization of 18a to 18b, we
explored the completion of the synthesis of PTAA 2 through
the intermediate 19a (Scheme 3). At this point, all that remained
in the synthesis was to protect the R-nitrogen, reduce the nitrile
to a primary amine, and protect this side-chain amine. Protection
of the R-nitrogen was accomplished by subjecting the amine
19a to Fmoc-OSu and Na2CO3 in a 1:1 dioxane/H2O mixture
furnishing the carbamate 20 in 84% yield. Unfortunately, the
carbamate 20 was not obtained in acceptable levels of purity
even after extensive purification efforts. Moreover, alternative
Fmoc-protection conditions fared no better in providing materi-
als of higher purity. We also explored pressing on to the final
PTAA 2 with the hopes of perhaps purifying the material at
this point, but again, the PTAA could not be obtained in
acceptable levels of purity for use in solid-phase peptide
synthesis.
As an alternative strategy, we investigated reduction of the
double bond and nitrile functionalities with PtO2 and H2 in the
presence of Boc2O. This reaction cleanly afforded the N-Boc
carbamate 21 (Scheme 4) in 78% yield. Hydrogenolysis of the
Cbz and benzyl protection groups was then accomplished with
H2 and Pd-C to give the amino acid 22. Finally, protection of
the amine with Fmoc-OSu afforded the PTAA 2 suitably
protected for Fmoc/Boc solid-phase peptide synthesis in 71%
yield from the intermediate 21.
(18b exhibits an IR band at 2254 cm-1). In addition, the alkene
resonance is reported at 5.78-5.82 ppm, and the alkene
resonance of 18b appears at 5.74 ppm. In the McCafferty
synthesis, the alkene isomers are hydrogenated to give a mixture
of the 2,4-cis and 2,4-trans isomers. It has been our experience
that similar compounds are not readily separable by flash
chromatography (data not shown). Thus, to obtain a practical
synthesis of this PTAA, we next explored conditions for the
stereoselective reduction of the double bond with concomitant
hydrogenolysis of the benzyl and Cbz groups in 18a. Table 1
displays some of these results in which a clear solvent preference
was noted. However, under none of the conditions explored did
we obtain acceptable levels of diastereoselectivity. In contrast,
when the nitrile 18b was subjected to hydrogenation conditions,
it cleanly afforded the 2,4-cis diastereomer 19a with no
detectable 2,4-trans diastereomer 19b.
As the quick completion of the synthesis appeared possible
from the nitrile 18b, the isomerization of the R,â-unsaturated
nitrile 18a to the â,γ-unsaturated nitrile 18b was explored (Table
2). Surprisingly, when the nitrile 18a was enolized with LDA
and kinetically protonated with AcOH, it afforded a 1.4:1:0.35
mixture of isomers (18a-c) in which the majority of products
arose from R-protonation.14 To optimize conditions, we inves-
tigated quenching with different acids and noticed that the ratio
of the desired isomer increased with the acidity of the quenching
acid (TfOH > p-TsOH > TFA > AcOH); however, the isolated
In conclusion, we report the synthesis of novel amino acid 1
in 14 steps from pyroglutamic acid and a synthesis of the PTAA
2 in eight steps from 4-hydroxyproline. This amino acid has
proven critical in elucidating the conformation in which peptide
(14) The assignment of isomer 18c is tentative. We were not able to
isolate and definitively characterize this isomer.
5006 J. Org. Chem., Vol. 71, No. 13, 2006

SCHEME 4
(2S,4S)-4-(2-tert-Butoxycarbonylamino-ethyl)-pyrrolidine-1,2-
dicarboxylic Acid Dibenzyl Ester (21). A solution of the uncon-
jugated nitrile 18b (1.0 g, 2.7 mmol) was dissolved in MeOH (40
mL). To this solution was added platinum dioxide (0.121 g, 0.532
mmol), followed by di-tert-butyl dicarbonate (1.16 g, 5.32 mmol).
The reaction mixture was stirred under an H2 atmosphere for 48 h.
The resulting mixture was then filtered through Celite, and the Celite
plug was washed with ethyl acetate (3 × 30 mL). The filtrate was
concentrated to yield the corresponding Boc-protected product 21.
The crude residue was purified by flash column chromatography
(10% EtOAc in CH2Cl2) to yield 1.0 g (78%) of pure product 21:
1
[R]25 ) -48.0° (c 0.1, CHCl3); H NMR (500 MHz, CDCl3) δ
D
7.13-7.18 (10H, m), 5.19-5.09 (2H, m), 4.99-4.97 (2H, m),
4.88-4.84 (1H, m), 4.34-4.29 (1H, dt), 3.84-3.77 (1H, dq), 3.09-
3.03 (3H, m), 2.44-2.42 (1H, m) 2.13 (1H, m), 1.57-1.47 (3H,
m), 1.40-1.39 (9H, s) ppm; 13C NMR (125 MHz, CDCl3) 172.3,
172.0, 155.6, 154.3, 153.7, 136.3, 136.1, 135.4, 135.2, 128.2, 128.1,
128.06, 127.98, 127.9, 127.8, 127.7, 127.6, 127.4, 78.7, 66.74,
66.66, 66.4, 66.3, 59.0, 58.7, 52.1, 51.7, 38.8, 36.6, 36.0, 35.6, 35.3,
substrates bind to the active site of PKG. We expect both PTAAs
to be useful in a number of additional applications.
Experimental Section
32.8, 32.7, 29.3, 28.1; IR film 3363 (br), 2975, 1749, 1706 cm-1
.
(2S)-4-Cyanomethelene-pyrrolidine-1,2-dicarboxylic Acid
Dibenzyl Ester (18a) and (2S)-4-Cyanomethyl-2,5-dihydro-
pyrrole-1,2-dicarboxylic Acid Dibenzyl Ester (18b). Diethyl
cyanomethylphosphonate (19.3 mL, 119 mmol) was added to a
solution of LHMDS (109 mL of 1.0 M solution) in dry THF (60
mL) at -78 °C. After 30 min, a solution of ketone 17 (35.1 g,
99.2 mmol) in dry THF (50 mL) was added dropwise to the solution
of the phosphonate anion. The reaction mixture was allowed to
warm to room temperature, and after 90 min at room temperature,
the reaction mixture was quenched with 10% HCl (50 mL). The
layers were separated, and the aqueous layer was extracted with
ethyl acetate (3 × 100 mL). The combined organic layers were
washed with brine (50 mL), dried over Na2SO4, and concentrated
to give 42.5 g of the crude product. The crude residue was purified
by flash chromatography (2% EtOAc in CH2Cl2) to afford a 3:2
mixture of cis/trans-alkene isomers 18a (20 g) and the unconjugated
nitrile 18b (13.33 g) (89% yield for both) as colorless solids. 18a:
1H NMR (500 MHz, CDCl3) δ 7.25-7.19 (10H, m), 5.15-4.95
(5H, m), 4.6-4.5 (0.5H, 0.5H, t), 4.37 (0.5H, 0.5H, d), 4.19-4.14
(0.5H, 0.5H, d), 3.01-2.87 (1.5H, m), 2.66 (0.5H, m) ppm; 13C
NMR (125 MHz, CDCl3) (cis and trans conformers) 170.28, 170.15,
161.3, 161.0, 160.3, 160.2, 153.4, 153.3, 153.9, 135.5, 135.4, 134.6,
134.5, 127.2, 127.1, 127.0, 114.9, 114.6, 92.5, 92.3, 66.4, 66.1,
58.1, 57.7, 57.4, 50.6, 50.4, 50.0, 49.8, 35.8, 35.2, 35.0, 34.4 ppm;
IR film 2221, 1745, 1710 cm-1; MS (MALDI) 399.0 (M + 23),
377.1 (M + H). Anal. Calcd for C22H20N2O4: C, 70.20; H, 5.36;
N, 7.44. Found: C, 70.03; H, 5.32; N, 7.42. 18b: [R]25D ) -22.2°
MS (MALDI) 505.58 (M + 23), 383.00 (M-Boc). Anal. Calcd for
C27H34N2O6: C, 67.20; H, 7.10; N, 5.81. Found: C, 66.91; H, 7.02;
N, 5.85.
(2S,4S)-4-(2-tert-Butoxycarbonylamino-ethyl)-pyrrolidine-1,2-
dicarboxylic Acid 1-(9H-Fluoren-9-ylmethyl) Ester (2). The
protected amino acid 21, 0.29 g (0.62 mmol), was dissolved in EtOH
(10 mL) and added to a flask containing 10 mol % of Pd-C (0.11
g). An H2 atmosphere was then applied. After 2 h, the catalyst was
removed by filtration through a pad of Celite and the Celite plug
was washed with MeOH (2 × 15 mL). The filtrate was concen-
trated, and 10% Pd-C (0.11 g) was added, followed by MeOH
(10 mL). The mixture was again subjected to an H2 atmosphere
for another 30 min. The reaction mixture was filtered through Celite,
and the Celite plug was washed with MeOH (2 × 15 mL). The
filtrate was concentrated to give 0.16 g of crude product 22 that
was used in the next step without further purification. MS
(MALDI): 259.2 (M + H), 281.2 (M + 23). The amino acid 22
(0.16 g, 0.62 mmol) was dissolved in water (4 mL). To this solution
was added Na2CO3 (0.13 g, 1.2 mmol), followed by a solution of
Fmoc-OSu (0.25 g, 0.74 mmol) in dioxane (10 mL) over a period
of 2 h. After 2 additional hours, the resulting solution was diluted
with water (10 mL) and extracted with ethyl acetate (3 × 25 mL).
The combined organic fractions were washed with brine (10 mL),
dried over Na2SO4, and concentrated. The resulting material was
purified by column chromatography (10% MeOH in CH2Cl2) to
finally obtain 0.21 g (71%) of 2 as a colorless solid: [R]25
)
D
-53.0° (c 0.1, CHCl3); 1H NMR (500 MHz, CD3OD) δ 7.78 (2H,
d), 7.59 (2H, m), 7.39-7.31 (4H, m), 4.39-4.10 (4H, m), 3.71
(0.7H, 0.3H s), 3.34 (0.7H, 0.3H s), 3.08 (3H, m), 2.42 (0.7H, 0.3H
1
(c 0.2, CHCl3); H NMR (500 MHz, CDCl3) δ 7.33-7.20 (10H,
m), 5.75-5.73 (1H, d), 5.13-5.07 (5H, m), 4.23-4.13 (2H, m),
3.00 (2H, s) ppm; 13C NMR (125 MHz, CDCl3) 168.8, 168.5, 153.3,
152.9, 135.8, 135.7, 135.0, 134.8, 132.2, 132.1, 127.99, 127.86,
127.79, 127.7, 127.6, 127.34, 127.28, 127.1, 121.9, 121.8, 115.4,
66.64, 66.57, 66.5, 66.2, 65.8, 54.4, 53.9 ppm; IR film 2254, 1551,
1712 cm-1; MS (MALDI) 399.0 (M + 23), 377.0 (M + H). Anal.
Calcd for C22H20N2O4: C, 70.20; H, 5.36; N, 7.44. Found: C, 66.92;
H, 5.32; N, 7.42.
s), 2.13 (0.7H, 0.3H s), 1.62-1.57 (3H, m), 1.46 (9H, s) ppm; 13
C
NMR (125 MHz, CD3OD) 183.6, 182.4, 160.9, 159.7, 159.1, 148.0,
147.7, 147.6, 145.09, 145.05, 144.9, 131.4, 130.7, 128.8, 128.6,
124.0, 123.4, 82.5, 71.6, 71.5, 65.1, 64.4, 56.3, 56.0, 42.6, 21.1,
39.8, 39.6, 39.4, 36.5, 31.4 ppm; IR film 3355 (br), 1684 cm-1
;
MS (MALDI) 503.71 (M + 23); HRMS calcd for C27H32N2O6 [M
+ H]+ 480.2216, found 480.2218.
Enolization and Kinetic Protonation (18b). A solution of
conjugated nitrile 18a (4.93 g, 13.1 mmol) in dry THF (15 mL)
was added to a cooled solution of LHMDS (14.4 mL of 1 M
solution) in dry THF (20 mL) at -78 °C. After 15 min at -78 °C,
this solution was transferred via a dry ice-cooled cannula into a
flask containing a solution of p-TsOH (4.98 g, 26.2 mmol) in dry
THF (25 mL) at -78 °C. Water (50 mL) was added at -78 °C,
and the mixture was allowed to warm to room temperature. The
layers were separated, and the aqueous layer was extracted with
ethyl acetate (3 × 50 mL). The combined organic fractions were
washed with brine (20 mL), dried over Na2SO4, and concentrated.
The crude residue was purified by flash column chromatography
(2% EtOAc in CH2Cl2) to yield 2.78 g (56%) of pure product 18b.
Acknowledgment. Financial support for this work was
provided by CHE-0411831 from the NSF. HRMS was provided
by the Washington University Mass Spectrometry Resource, an
NIH Research Resource (Grant No. P41RR0954).
Supporting Information Available: Experimental procedures,
characterization data, and copies of 1H NMR spectra for 1, 4-13,
1
and 17; H NMR spectra for 2, 18a, 18b, 19a, 19b, and 21; 13C
NMR spectra for 1, 2, 4, 5, 7, 8, 10-13, 18a, 18b, and 21; and
HMQC for 18b. This material is available free of charge via the
Internet at http://pubs.acs.org.
JO060210F
J. Org. Chem, Vol. 71, No. 13, 2006 5007