(R,R,R)-2,5-diaminocylohexanecarboxylic acid, a building block for water-soluble, helix-forming β-peptides

Full text of DOI:10.1021/jo000277h

4766
J . Org. Chem. 2000, 65, 4766-4769
derivatives. We now report a synthesis of a protected
(R,R,R)-2,5-Dia m in ocyloh exa n eca r boxylic
form of (R,R,R)-2,5-diaminocyclohexanecarboxylic acid
(DCHC), a monomer that has allowed us to study 14-
helix formation by short â-peptides in aqueous solution.3b
We also report an improved synthesis of protected
versions of ACHC itself.
Acid , a Bu ild in g Block for Wa ter -Solu ble,
Helix-F or m in g â-P ep tid es
Daniel H. Appella, Paul R. LePlae, Tami L. Raguse, and
Samuel H. Gellman*
Our approach relies on the method of Kobayashi et al.
for enzymatic desymmetrization (via selective ester
hydrolysis) of diester 1.7 For ACHC, the resulting chiral
monoacid was subjected to a Curtius rearrangement, as
previously reported7 (Scheme 1). The resulting isocyanate
can be converted to a Cbz-protected amino group via
reaction with benzyl alcohol;7 in our experience, however,
2 could not be easily purified from the excess benzyl
alcohol needed for efficient trapping of the isocyanate.
We therefore carried the impure material forward.
(Attempts to trap the isocyanate with tert-butyl alcohol
were low yielding.) After hydrogenation of 2, to remove
the alkene and the Cbz protecting group, benzyl alcohol
was easily separated from the amine product by acid-
base extraction. The amino group was then reprotected
to yield 3. Epimerization of 3 yielded 4, which was
contaminated with approximately 8% of 3. One recrys-
tallization provided pure 4. Hydrolysis of the methyl ester
provided Boc-ACHC that was comparable in terms of
optical rotation to Boc-ACHC prepared by an alternative
enantioselective route.3c Standard protecting group ma-
nipulations allowed conversion of 4 to 5a , which could
be hydrolyzed to provide Cbz-ACHC, 5b.
Department of Chemistry, University of Wisconsin,
Madison, Wisconsin 53706-1396
gellman@chem.wisc.edu
Received February 29, 2000
Unnatural oligomers with discrete and predictable
folding propensities (“foldamers”) are subjects of consid-
erable current interest.1 Attention in our laboratory and
several others has focused on oligomers of â-amino acids
(“â-peptides”).1-3 We have shown, for example, that
oligomers of trans-2-aminocyclohexanecarboxylic acid
(ACHC) adopt a helical conformation that has approxi-
mately three residues per turn and contains a network
of 14-membered ring hydrogen bonds between backbone
CdO and N-H groups (“14-helix”).3 Independent studies
by Seebach et al. demonstrate that â-peptides constructed
from acyclic residues bearing a single R- or â-substitutent
also adopt the 14-helical conformation.2 Recent results
from our laboratory show that acyclic residues have a
lower intrinsic propensity for helical folding than do
cyclohexane-based residues.3b,4
To synthesize DCHC, which bears an “extra” amino
group at the 5-position of the cyclohexyl ring, we first
synthesized 6 from 2 using published procedures (Scheme
2).8 The first step in the conversion involves hydrolysis
of the methyl ester of 2, and excess benzyl alcohol from
the Curtius rearrangement was removed by an acid-base
extraction after this hydrolysis. A one-pot procedure was
developed to introduce a protected amino group ste-
reospecifically at the 5-position. First, methanesulfonyl
chloride (MsCl) was added to a solution of 6 at 0 °C, and
then tetra-n-butylammonium azide was added at 0 °C.
We believe that this reaction generates an allylic azide.
All attempts to isolate this putative intermediate, how-
ever, were complicated by the presence of an additional
isomer, presumably from sigmatropic rearrangement of
the allylic azide moiety.9 When the intermediate azide
was reduced to the amine at 0 °C (with tri-n-butylphos-
phine and water), followed by treatment with di-tert-
To pursue potential applications of 14-helical â-pep-
tides, we must develop efficient syntheses of protected
forms of ACHC and derivatives that bear substituents
on the cyclohexyl ring. The “extra” substituents on ACHC
derivatives should occupy equatorial positions on the
cyclohexane ring in order to avoid steric repulsions with
residues at positions i + 3 and/or i - 3 in the 14-helical
conformation. The extensive literature on asymmetric
synthesis of â-amino acids5 suggests a few routes for
preparation of enantiomerically pure ACHC.6,7 Our pre-
vious efforts relied on a crystallization-based resolution
of N-benzoyl-ACHC, which proved to be capricious.3c
Excellent chemistry developed by Kobayashi et al.7,8
seemed likely to provide a more secure route to protected
versions of ACHC and to appropriately substituted
(1) Reviews on foldamers: (a) Gellman, S. H. Acc. Chem. Res. 1998,
31, 173. (b) Kirshenbaum, K.; Zuckermann, R. N.; Dill, K. A. Curr.
Opin. Struct. Biol. 1999, 9, 530. (c) Stigers, K. D.; Soth, M. J .; Nowick,
J . S. Curr. Opin. Chem. Biol. 1999, 3, 714. (d) Barron, A. E.;
Zuckermann, R. N. Curr. Opin. Chem. Biol. 1999, 3, 681.
(2) Reviews on â-peptides: (a) Seebach, D.; Matthews, J . L. J . Chem.
Soc., Chem. Commun. 1997, 2015. (b) DeGrado, W. F.; Schneider, J .
P.; Hamuro, Y. J . Peptide Res. 1999, 54, 206. (c) Gademann, K.;
Hintermann, T.; Schreiber, J . V. Curr. Med. Chem. 1999, 6, 905.
(3) (a) Appella, D. H.; Christianson, L. A.; Karle, I. L.; Powell, D.
R.; Gellman, S. H. J . Am. Chem. Soc. 1996, 118, 13071. (b) Appella,
D. H.; Barchi, J . J .; Durell, S.; Gellman, S. H. J . Am. Chem. Soc. 1999,
121, 2309. (c) Appella, D. H.; Christianson, L. A.; Karle, I. L.; Powell,
D. R.; Gellman, S. H. J . Am. Chem. Soc. 1999, 121, 6206. (d) Barchi,
J . J .; Huang, X.; Appella, D. H.; Christianson, L. A.; Durell, S. R.;
Gellman, S. H. J . Am. Chem. Soc. 2000, 122, 2711.
(4) Conformational analysis of â-peptides containing exclusively
acyclic residues in aqueous solution: (a) Abele, S.; Guichard, G.;
Seebach, D. Helv. Chim. Acta 1998, 81, 2141. (b) ref 3b. (c) Gung, B.
W.; Zou, D.; Stalcup, A. M.; Cottrell, C. E. J . Org. Chem. 1999, 64,
2176. (d) â-Peptide nucleic acid analogues: Diederichsen, U.; Schmitt,
H. W. Eur. J . Org. Chem. 1998, 63, 827. (e) Hamuro, Y.; Schneider, J .
P.; DeGrado, W. F. J . Am. Chem. Soc. 1999, 121, 12200.
(5) For general references on â-amino acid synthesis, see: (a) Cole,
D. C. Tetrahedron 1994, 50, 9517. (b) J uaristi, E. Enantioselective
Synthesis of â-Amino Acids; Wiley-VCH: New York, 1997. For recent
developments, see: (c) Davis, F. A.; Reddy, G. V.; Liang, C.-H.
Tetrahedron Lett. 1997, 38, 5139. (d) Ishitani, H.; Ueno, M.; Kobayashi,
S. J . Am. Chem. Soc. 1997, 119, 7153. (e) Sibi, M. P.; Shay, J . J .; Liu,
M.; J asperse, C. P. J . Am. Chem. Soc. 1998, 120, 6615. (f) Tang, T. P.;
Ellman, J . A. J . Org. Chem. 1999, 64, 12. (g) Zhu, G.; Chen, Z.; Zhang,
X. J . Org. Chem. 1999, 64, 6907. (h) Dexter, C. S.; J ackson, R. F. W.
J . Org. Chem. 1999, 64, 7579.
(6) (a) Schultz, A. G. Acc. Chem. Res. 1990, 23, 207. (b) Xu, D.;
Prasad, K.; Repic, O.; Blacklock, T. J . Tetrahedron: Asymmetry 1997,
8, 1445. (c) Enders, D.; Wiedemann, J . Liebigs. Ann./ Recueil 1997,
699.
(7) Kobayashi, S.; Kamiyama, K.; Ohno, M. Chem. Pharm. Bull.
1990, 38, 350-354.
(8) Kobayashi, S.; Kamiyama, K.; Ohno, M. J . Org. Chem. 1990, 55,
1169-1177.
(9) Trost, B. M.; Pulley, S. R. Tetrahedron Lett. 1995, 36, 8737.
10.1021/jo000277h CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/23/2000

Notes
J . Org. Chem., Vol. 65, No. 15, 2000 4767
Sch em e 1
stain (0.5 g of ninhydrin, 150 mL of n-butanol, 5 mL of glacial
acetic acid). Chromatography was performed on EM Science
silica gel 60 (230-400 mesh). Solvent mixtures used for TLC
and column chromatography are reported in v/v ratios.
Com p ou n d 2 was prepared as described in ref 7; however, 2
could not be efficiently separated from residual benzyl alcohol
used in the reported protocol. Therefore, impure 2 was carried
on to the next step.
Sch em e 2
Com p ou n d 3. To a solution of 2 (ca. 18 mmol) in methanol
(25 mL) was added 10% Pd-C (0.26 g). The mixture was shaken
under H2 (50 psi) for 18 h. The mixture was then filtered through
a plug of glass wool, concentrated, and dried under vacuum to
obtain a yellow liquid. Water was added to the liquid, and then
3 N HCl was added to the solution until a pH of 2 was obtained.
The resulting solution was extracted twice with diethyl ether
to remove benzyl alcohol. To the aqueous layer was added
K2CO3 (in small amounts to avoid vigorous evolution of gas) until
a pH of 9 was obtained. Dioxane was added, followed by Boc2O
(5.3 g, 22 mmol). The resulting solution was stirred for 20 h.
The solution was then mixed with water and extracted three
times with ethyl acetate. The combined organic extracts were
dried over MgSO4, concentrated, and dried under vacuum to
obtain a viscous yellow liquid. The product was purified by SiO2
column chromatography, eluting with 6:1 hexane/ethyl acetate.
The fractions containing the desired product were combined and
concentrated to afford 1.73 g of 3 as a clear oil (31% yield from
butyl-dicarbonate, 7 was isolated in pure form. Hydro-
genation of 7 followed by reprotection with Cbz-Cl pro-
vided 8, and epimerization yielded 9 (8 was not detected
after epimerization). The relative and absolute stereo-
chemistry of 9 was verified by conversion to 10, for which
a crystal structure was obtained.
1): [R]23 ) 77.8 (c 0.55, CHCl3); IR (neat) 3445 (NH), 1716
D
(CdO) cm-1
;
1H NMR (CDCl3, 300 MHz) δ 5.32 (br, 1H), 3.85
(m, 1H), 3.69 (s, 3H), 2.79 (bq q, J ) 4 Hz, 1H), 2.07-1.93 (m,
1H), 1.84-1.55 (m, 4H), 1.51-1.25 (m, 12H; includes large
singlet at 1.43); 13C NMR (CDCl3, 75.4 MHz) δ 174.3 (C), 155.2
(C), 79.0 (C), 51.4 (CH3), 49.0 (CH), 44.8 (CH), 29.7 (CH2), 28.3
(CH3), 26.8 (CH2), 23.7 (CH2), 22.5 (CH2); EI-MS m/z (M+) calcd
for C13H23NO4 257.1627, obsd 257.1628.
Short â-peptides containing DCHC and/or ACHC,
which were constructed from building blocks that in-
cluded 5b and 9, have been shown to display high
population of the 14-helical conformation in aqueous
solution.3b The water-solubility of these â-peptides de-
pended upon protonation of the 5-amino group on the
DCHC residue. This amino group can also serve as a
point of attachment for a wide variety of “sidechains,”
which should allow us to use 14-helical â-peptides as
scaffolds to display functional groups in specific and
predictable arrangements.
Com p ou n d 4. Sodium (0.17 g, 7.4 mmol) was placed into a
flame-dried Schlenk flask outfitted with a condenser. The flask
was cooled to 0 °C, and dry methanol (10 mL) was added. The
mixture was kept under N2 and vented to remove evolved gases
until all of the sodium dissolved. In a separate flask, 3 (2.3 g,
8.9 mmol; dried under vacuum) was dissolved in dry methanol
(10 mL) and transferred to the NaOMe solution via cannula.
The resulting solution was refluxed for 5 h. After the solution
was cooled to room temperature, 0.5 M aqueous NH4Cl was
added. The solvent was mostly removed on a rotary evaporator,
and the precipitate was collected by suction filtration to obtain
2.0 g of white solid that contained 4 and about 8% 3. The solid
was recrystallized from n-heptane to afford 1.36 g (70% yield)
of 4 as colorless crystals: mp 90 °C, [R]23D ) -14 (c 1.0, CHCl3);
Exp er im en ta l Section
Gen er a l P r oced u r es. Melting points are uncorrected. Opti-
cal rotations were measured using sodium light (D line, 589.3
nm). Carbon NMR data were assigned using distortionless
enhancement by polarization transfer (DEPT) spectra obtained
with a phase angle of 135°: (C) not observed; (CH) positive; (CH2)
negative; (CH3) positive. Dry methanol was prepared by distil-
lation from Mg(OMe)2 (Mg turnings (5 g), I2 (0.5 g), methanol
(100 mL)). Triethylamine (Et3N) was distilled from CaH2 and
stored under N2. Methanesulfonyl chloride (MsCl) was distilled
from P2O5 and stored under N2. Unless otherwise noted, all other
commercially available reagents and solvents were purchased
from Aldrich and used without further purification, except for
4 N HCl in dioxane, which was purchased from Pierce, and tetra-
n-butylammonium azide, which was purchased from TCI America.
Analytical thin-layer chromatography (TLC) was carried out on
Whatman TLC plates precoated with silica gel 60 (250 µm layer
thickness). Visualization was accomplished using either a UV
lamp, potassium permanganate stain (2 g of KMnO4, 13.3 g of
K2CO3, 3.3 mL of 5% (w/w) NaOH, 200 mL of H2O), or ninhydrin
IR (KBr) 3361 (NH), 1727 (CdO) cm-1 1H NMR (CDCl3, 300
;
MHz) δ 4.51 (br, 1H), 3.67 (s and m, 4H), 2.24 (dt, J ) 12, 3.5
Hz, 1H), 2.09-1.99 (m, 1H), 1.96-1.85 (m, 1H), 1.81-1.52 (m,
2H), 1.51-1.29 (m, 11H; includes large singlet at 1.42), 1.27-
1.09 (m, 2H); 13C NMR (CDCl3, 75.4 MHz) δ 174.4 (C), 154.9
(C), 79.2 (C), 51.7 (CH), 51.2 (CH3), 50.1 (CH), 33.9 (CH2), 28.4
(CH2), 28.3 (CH3), 24.7 (CH2), 24.4 (CH2); EI-MS m/z (M+) calcd
for C13H23NO4 257.1627, obsd 257.1628.
Com p ou n d 5a . To 4 (0.26 g, 1.0 mmol) was added 4 N HCl
in dioxane (3 mL), and the resulting solution was stirred for 1
h (white solid precipitated during this time). The solvent was
removed under a stream of N2, and the residue was dried under
vacuum. The residue was then suspended in CH2Cl2 (10 mL),
and DIEA (0.55 mL, 3.1 mmol) was added, which caused the
residue to dissolve. Benzylchloroformate (0.23 mL, 1.6 mmol)

4768 J . Org. Chem., Vol. 65, No. 15, 2000
Notes
was added, followed by DMAP (10 mg, 0.08 mmol). The resulting
solution was stirred for 26 h; during this time the solution
became yellow. The solution was then washed with 1 N HCl and
with water, and the organic layer was dried over MgSO4 and
concentrated. The product was purified by SiO2 column chro-
matography, eluting with 5:1 hexane/ethyl acetate. Fractions
containing the product were combined and concentrated to afford
0.22 g (70% yield) of 5 as a white solid, which appeared to be
pure by NMR. This material could be recrystallized from
n-heptane without obvious improvement in purity: mp 79 °C;
1.83 (ddd, J ) 14, 7, 3 Hz, 1H), 1.44 (s, 9H); 13C NMR (CDCl3,
75.4 MHz) δ 172.9 (C), 155.8 (C), 155.0 (C), 136.3 (C), 130.2 (CH),
129.7 (CH), 128.5 (CH), 128.0 (CH), 79.7 (C), 66.8 (CH2), 52.0
(CH3), 46.8 (CH), 44.0 (CH), 41.8 (CH), 29.9 (CH2), 28.3 (CH3);
EI-MS m/z (M+ - C4H9) calcd for C17H20N2O6 348.1321, obsd
348.1329; FAB-MS m/z 405.2 (M + H+), 349.1 (M + H+ - C4H9),
305.1 (M + H+ - Boc).
Com p ou n d 8. To a solution of 7 (1.1 g, 2.6 mmol) in methanol
(25 mL) was added 10% Pd-C (0.24 g). The resulting mixture
was shaken under H2 (40 psi) for 12 h. The reaction was
monitored by TLC (1:1 hexane/ethyl acetate). The reaction was
judged to be complete when a single spot was observed at the
origin of the TLC plate. If the reaction was not complete after
12 h, additional 10% Pd-C (0.05 to 0.1 g) was added, and the
mixture was shaken for an additional 12 h. After the reaction
was complete, the mixture was filtered through a plug of glass
wool, and the filtrate was concentrated to obtain a thick oil. The
oil was dissolved in CH2Cl2 (30 mL), and DIEA (1.9 mL, 6.2
mmol) was added, followed by benzylchloroformate (0.6 mL, 4.2
mmol) and DMAP (10 mg, 0.08 mmol). The resulting solution
was stirred for 27 h; during this time the solution became orange.
The solvent was removed on a rotary evaporator, methanol was
added, and a white solid precipitated. The precipitate was
collected by suction filtration to afford 0.78 g (74% yield) of 8 as
a white solid that appeared to be pure by NMR. This material
could be recrystallized from methanol without obvious improve-
[R]23 ) -18 (c 0.9, CHCl3); IR (KBr) 3331 (NH), 1733 (CdO)
D
1
cm-1; H NMR (CDCl3, 300 MHz) δ 7.38-7.27 (m, 5H), 5.07 (s,
2H), 4.08 (br, 1H), 3.73 (tdd, J ) 11, 9, 4 Hz, 1H), 3.61 (s, 3H),
2.27 (td, J ) 11, 3 Hz, 1H), 2.06 (m, 1H), 1.92 (m, 1H), 1.81-
1.49 (m, 3H), 1.47-1.09 (m, 3H); 13C NMR (CDCl3, 75.4 MHz) δ
174.3 (C), 155.4 (C), 136.6 (C), 128.4 (CH), 128.0 (CH), 66.5
(CH2), 51.7 (CH and CH3), 49.7 (CH), 32.8 (CH2), 28.6 (CH2),
24.6 (CH2), 24.3 (CH2); EI-MS m/z (M+) calcd for C16H21NO4
291.1471, obsd 291.1474.
Com p ou n d 5b. Methanol (50 mL) and water (17 mL) were
added to 5a (1.04 g, 3.57 mmol), followed by LiOH‚H2O (1.80 g,
42.9 mmol) and 26% aqueous H2O2 (2.3 mL, 18 mmol). The
mixture was stirred at room temperature for 24 h. A solution of
Na2SO3 (6.76 g, 53.7 mmol) in water (40 mL) was then added at
0 °C, and the mixture was stirred for 10 min. The methanol was
removed via rotary evaporation, which caused formation of a
precipitate. Aqueous sodium hydroxide solution was added with
heating until the precipitate redissolved. The aqueous layer was
washed twice with ethyl acetate. The organic layers were
discarded, and the aqueous layer was cooled to 0 °C and acidified
with 3 M HCl until white solid precipitated. This mixture was
treated with ethyl acetate, which caused the precipitate to
dissolve. The aqueous layer was acidified to pH 2 with 3 M HCl
and re-extracted with the same ethyl acetate and then with four
additional aliquots ethyl acetate. The organic extracts were
combined, dried over MgSO4, and concentrated. Toluene was
added and removed via rotary evaporation three times to remove
any acetic acid produced during the workup. The residue was
dried under vacuum to afford 0.862 g (87% yield) of a white solid
that appeared to be pure by NMR. This material could be
recrystallized from ethyl acetate/hexane without obvious im-
ment in purity: mp 184-185 °C, [R]23 ) 5.0 (c 1, CHCl3); IR
D
(KBr) 3357 (NH), 1730 (CdO), 1720 (CdO) cm-1
;
1H NMR
(CDCl3, 300 MHz) δ 7.40-7.29 (m, 5H), 5.71 (br d, J ) 9 Hz,
1H), 5.08 (s, 2H), 4.33 (br, 1H), 3.80 (tt, J ) 10, 4.5 Hz, 1H),
3.72 (s, 3H), 3.46 (br, 1H), 2.95 (q, J ) 4.5 Hz, 1H), 2.41 (br d,
J ) 13 Hz, 1H), 2.04-1.93 (m, 3H), 1.43 (s and m, 10H), 1.25
(qd, J ) 12, 5 Hz, 1H); 13C NMR (CDCl3, 75.4 MHz) δ 173.6 (C),
155.7 (C), 155.0 (C), 136.5 (C), 128.5 (CH), 128.1 (CH), 79.4 (C),
66.7 (CH2), 52.0 (CH3), 49.7 (CH), 45.6 (CH), 43.5 (CH), 34.1
(CH2), 31.7 (CH2), 28.4 (CH3), 27.8 (CH2); EI-MS m/z (M+ + H+)
calcd for C21H30N2O6 406.2104, obsd 406.2105.
Com p ou n d 9. In a Schlenk flask outfitted with a condenser
was placed 8 (0.69 g, 1.7 mmol), and this material was dried
under vacuum overnight. Dry methanol (35 mL) was added to
obtain a suspension. Sodium (0.2 g, 8.5 mmol) was placed into
a separate flame-dried flask. The flask was cooled to 0 °C, and
dry methanol (10 mL) was added. After the sodium dissolved,
the NaOMe solution was added via cannula to the suspension
of 8 in methanol. The resulting mixture was refluxed under N2
for 5 h; the solution became clear soon after heating began. After
the solution cooled to room temperature, 0.5 M aqueous NH4Cl
(45 mL) was added, and a white precipitate formed. The solid
was collected by suction filtration, washed with additional water,
and dried under vacuum. This material was recrystallized from
methanol to afford 0.4 g (60% yield) of 9 as a white solid: mp
provement in purity: mp 147-148.5 °C; [R]23 ) -27.1 (c 1,
D
CHCl3); IR (thin film) 3328 (NH), 1706 (CdO) cm-1
;
1H NMR
(CDCl3, 300 MHz) δ 10.43 (br, 1H), 7.38-7.25 (m, 5H), 6.12,
5.25-4.85 (m, 3H), 3.85-3.63 (m, 1H), 2.38-2.19 (m, 1H), 2.13-
1.92 (m, 2H), 1.79-1.48 (m, 3H), 1.46-1.10 (m, 3H); 13C NMR
(CD3CN, 75.4 MHz) δ 175.9 (C), 156.5 (C), 138.4 (C), 129.4 (CH),
128.8 (CH), 128.6 (CH), 66.6 (CH2), 52.4 (CH), 49.6 (CH), 33.4
(CH2), 29.9 (CH2), 25.5 (CH2), 25.2 (CH2); FAB-MS m/z calcd for
C
15H19NO4Na (M + Na+) 300.1212, obsd 300.1221.
Com p ou n d 7. Dry CH2Cl2 (50 mL) was added to 6 (prepared
according to ref 8; 2.2 g, 6.7 mmol), and the resulting solution
was cooled to 0 °C. To this solution was added Et3N (2.2 mL, 16
mmol) and then MsCl (1.3 mL, 16 mmol), and the resulting
solution was stirred for 15 min at 0 °C, under N2. In a separate
flask, tetrabutylammonium azide (2.9 g, 10 mmol) was dissolved
in CH2Cl2 (10 mL); this solution was added via cannula to the
solution of mesylate. The resulting solution was stirred at 0 °C
under N2 for 1.5 h. To the solution was added P(n-Bu)3 (5 mL,
20 mmol), followed by water (0.4 mL). The resulting solution
warmed to room temperature and stirred under N2 for 10 h. To
this solution was added Boc2O (4.4 g, 20 mmol), and the resulting
solution was stirred open to air for 24 h. The solvent was
removed on a rotary evaporator, 2:1 hexane/ethyl acetate (200
mL) was added, and the white solid that precipitated was
collected by suction filtration. The filtrate was concentrated to
obtain a yellow liquid that contained the product. The product
was purified by SiO2 column chromatography, eluting with 2:1
hexane/ethyl acetate. The fractions containing the product were
collected and concentrated to afford 1.1 g of white solid. This
material was recrystallized from n-heptane/benzene to afford 1.0
192-193 °C; [R]23 ) 7.1 (c 0.7, CHCl3); IR (KBr) 3362 (NH),
D
3315 (NH), 1734 (CdO) cm-1; 1H NMR (CDCl3, 300 MHz) δ 7.39-
7.29 (m, 5H), 5.07 (s, 2H), 4.70 (br, 1H), 4.41 (br, 1H), 3.71 (m,
1H), 3.61 (s, 3H), 3.47 (br, 1H), 2.39 (td, J ) 12.5, 4 Hz, 1H),
2.27-2.08 (m, 2H), 2.05-2.00 (m, 1H), 1.53-1.21 (m, 12H;
includes large singlet at 1.43); 13C NMR (CDCl3, 75.4 MHz) δ
173.1 (C), 155.5 (C), 155.1 (C), 136.5 (C), 128.5 (CH), 128.0 (CH),
79.5 (C), 66.7 (CH2), 52.0 (CH3), 51.2 (CH), 48.3 (CH), 47.9 (CH),
34.9 (CH2), 31.5 (CH2), 28.4 (CH3); FAB-MS m/z 429.4 (M + Na+),
373.3 (M + Na+ - C4H9).
Com p ou n d 10. TFA (1.5 mL) was added to 9 (0.05 g, 0.13
mmol), and the resulting solution was stirred for 30 min. The
solvent was removed under a stream of N2, and the residue was
dried under vacuum. The residue was dissolved in CH2Cl2 (10
mL), and DIEA (0.06 mL, 0.35 mmol) was added, followed by
adamantylfluoroformate (0.03 g, 0.15 mmol) and DMAP (2 mg,
0.02 mmol). The resulting solution was stirred for 24 h. The
solvent was removed on a rotary evaporator, and the product
was purified by SiO2 column chromatography eluting with 8:1
CH2Cl2/CH3CN. The fractions containing the product were
combined and concentrated to afford 0.05 g (84% yield) of white
solid. The solid was dissolved in ethanol (4 mL), 10% Pd-C (0.01
g) was added, and the mixture was shaken under H2 (40 psi)
for 11 h. The mixture was the filtered through a plug of glass
wool, and the filtrate was concentrated to obtain a white solid.
The solid was dissolved in CH2Cl2 (5 mL), and DIEA (0.03 mL,
g (88% yield) of 7 as a white solid: mp 97-100 °C; [R]23
)
D
-140.4 (c 1.2, CHCl3); IR (KBr) 3355 (NH), 1734 (CdO) cm-1
;
1H NMR (CDCl3, 300 MHz) δ 7.39-7.28 (m, 5H), 5.76 (AB
quartet, J ) 10.5 Hz, 1H), 5.72 (AB quartet, J ) 10.5 Hz, 1H),
5.47 (br d, J ) 10 Hz, 1H), 5.08 (s, 2H), 4.59 (m, 2H), 4.22 (m,
1H), 3.65 (s, 3H), 3.00 (m, 1H), 2.28 (ddd, J ) 14, 8.5, 6 Hz, 1H),

Notes
J . Org. Chem., Vol. 65, No. 15, 2000 4769
0.20 mmol) was added, followed by TsCl (0.04 g, 0.20 mmol) and
DMAP (2 mg, 0.02 mmol). The resulting solution was stirred
for 14 h. The solution was diluted with CH2Cl2 (30 mL) and
washed with 1 N HCl. The organic layer was dried over MgSO4,
concentrated and dried under vacuum to obtain a yellow oil. This
oil was purified by SiO2 column chromatography, eluting with
6:1 CH2Cl2/ethyl acetate. The fractions containing the product
were combined and concentrated to afford 0.02 g (61% yield from
9) of clear oil. X-ray quality crystals of 10 were grown by vapor
diffusion of 1:1 n-heptane/1,2-dichloroethane into a solution of
10 in 1,2-dichloroethane: mp 171-172 °C; IR (KBr) 3408 (NH),
3147 (NH), 1718 (CdO) cm-1; 1H NMR (CDCl3, 300 MHz) δ 7.72
(d, J ) 8 Hz, 2H), 7.29 (d, J ) 8 Hz, 2H), 4.67 (d, J ) 7.5 Hz,
1H), 4.36 (br, 1H), 3.49-3.24 (m and s, 5H), 3.42 (s, 3H), 2.42
(s, 3H), 2.37 (td, J ) 12, 3 Hz, 1H), 2.23-2.01 (m, 11H), 1.93
(m, 1H), 1.64 (m, 7H), 1.36 (q, J ) 12 Hz, 2H), 1.18 (qd, J ) 12,
3 Hz, 1H); FAB-MS m/z 527.2 (M + Na+).
Ack n ow led gm en t. We thank Dr. R. Hayashi for
determining the crystal structure of 10. This research
was supported by the National Institutes of Health
(GM56414). D.H.A. was supported in part by a Chem-
istry-Biology Interface Training Grant from NIGMS
and by a fellowship from Procter & Gamble, P.R.L. in
part by a fellowship from Kodak, and T.L.R. in part by
a fellowship from the National Science Foundation.
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
data (including DEPT-135) for compounds 3-10 and solid-
state structure of 10. This material is available free of charge
via the Internet at http://pubs.acs.org.
J O000277H