Preparation of protected syn-α,β-dialkyl β-amino acids that contain polar side chain functionality

Article abstract of DOI:10.1021/jo034583h

We report the synthesis of syn-α,β-dialkyl β-amino acid derivatives suitably protected for solid-phase synthesis that give rise to residues containing positively charged lysine-like side chains. These amino acids, as well as synα,β-dialkyl β-amino acids that contain diverse hydrophobic side chains, are prepared in good de and ee. The key step in this route involves Davies's protocol for the conjugate addition of a chiral lithium amide to α,β-unsaturated tertbutyl esters (Davies, S. G.; Ichihara, 0.; Walters, I. A. S. J. Chem. Soc., Perkin Trans. 1 1994, 9, 1141). syn-α,β-Dialkyl β-amino acids are interesting building blocks because of their sheet-forming propensity and because of their presence in bioactive compounds.

Full text of DOI:10.1021/jo034583h

The availability of appropriately substituted â-amino
acid monomers4 is one factor that dictates the pace of
â-peptide foldamer research. Convenient synthetic routes
for monomers that are predisposed to form helical
structures5 have accelerated the pace of discoveries
involving â-peptide helices.5k In contrast, many questions
involving â-peptide sheet structure remain unanswered.
For example, what side chains, side-chain orientations,
and side chain-side chain pairings are required for
hairpin structure in parallel and in antiparallel systems?
Is it possible to engineer a â-peptide hairpin that is
structured in water? Progress in â-peptide sheet research
has lagged behind the study of â-peptide helix structure
because a general, scalable synthesis of syn-R,â-dialkyl
â-amino acids with protein-like side chains has not been
described.
P r ep a r a tion of P r otected syn -r,â-Dia lk yl
â-Am in o Acid s Th a t Con ta in P ola r Sid e
Ch a in F u n ction a lity†
J oseph M. Langenhan and Samuel H. Gellman*
Department of Chemistry, University of Wisconsin,
Madison, Wisconsin 53706-1396
gellman@chem.wisc.edu
Received May 5, 2003
Abstr a ct: We report the synthesis of syn-R,â-dialkyl â-ami-
no acid derivatives suitably protected for solid-phase syn-
thesis that give rise to residues containing positively charged
lysine-like side chains. These amino acids, as well as syn-
R,â-dialkyl â-amino acids that contain diverse hydrophobic
side chains, are prepared in good de and ee. The key step in
this route involves Davies’s protocol for the conjugate
addition of a chiral lithium amide to R,â-unsaturated tert-
butyl esters (Davies, S. G.; Ichihara, O.; Walters, I. A. S. J .
Chem. Soc., Perkin Trans. 1 1994, 9, 1141). syn-R,â-Dialkyl
â-amino acids are interesting building blocks because of their
sheet-forming propensity and because of their presence in
bioactive compounds.
A synthesis that fit several criteria was required. (1)
For convenience, the synthesis should afford products in
high diastereomeric and enantiomeric purity in every
case. (2) The route must readily provide diverse hydro-
phobic side chains in both the R and â positions of the
amino acid. (3) The synthetic methodology must allow
the incorporation of polar side chains that confer water
solubility on â-peptide oligomers. These features are
critical for our long-range goal of studying â-peptide sheet
secondary structure in water.
Methodologies that provide a limited set of syn-R,â-
dialkyl â-amino acids are known.6 For instance, L-aspartic
acid has been used to generate syn-R,â-dialkyl â-amino
acids,3a,b,d,7 and ester enolates have been used in conjunc-
tion with tert-butylsulfinyl imines to afford these com-
pounds.8 The alkylation of â3-amino acids, which are
readily obtained from R-amino acids via Arndt-Eistert
homologation,5a was investigated as a means by which
to obtain syn-R,â-dialkyl â-amino acids.9 Unfortunately,
these alkylations are not very diastereoselective, and in
syn-R,â-Dialkyl â-amino acids are relatively rare in
Nature but have been identified in several important
metabolites. For example, dolastatins 11 and 12 contain
(2S,3R)-2-methyl-3-aminopentanoic acid within a cyclic
depsipeptide.1 The dolastatins inhibit the growth of
murine lymphocytic leukemia cells.1b,c Majusculamide C,
a related cyclic depsipeptide, is cytotoxic and exhibits
fungicidal activity against several plant pathogens.2 The
importance of syn-R,â-dialkyl â-amino acids is not limited
to their biological occurrence. syn-R,â-Dialkyl â-amino
acids have proven useful in the development of sheet-
forming â-peptide foldamers.3 The relative configuration
of the CR and Câ alkyl substituents of these amino acids
enforces an anti NCâ-CRC(dO) torsion angle, which
corresponds to an extended conformation of the â-amino
acid backbone atoms.
(4) For reviews, see: (a) Sibi, M. P.; Manyem, S. Tetrahedron 2002,
58, 7991. (b) J uaristi, E., Ed. Enantioselective synthesis of â-amino
acids; 1st ed.; Wiley-VCH: New York, 1997. (c) Cole, D. C. Tetrahedron
1994, 50, 9517.
(5) For example, see: (a) Podlech, J .; Seebach, D. Liebigs Ann. 1995,
1217. (b) Appella, D. H.; LePlae, P. R.; Raguse, T. L.; Gellman, S. H.
J . Org. Chem. 2000, 65, 4766. (c) Wang, X.; Espinosa, J . F.; Gellman,
S. H. J . Am. Chem. Soc. 2000, 122, 4821. (d) LePlae, P. R.; Umezawa,
N.; Lee, H.-S.; Gellman, S. H. J . Org. Chem. 2001, 66, 5629. (e) Porter
E. A.; Wang, X.; Schmitt, M. A.; Gellman, S. H. Org. Lett. 2002, 4,
3317. (f) Woll, M. G.; Fisk, J . D.; LePlae, P. R.; Gellman, S. H. J . Am.
Chem. Soc. 2002, 124, 12447. (g) Lee, H.-S.; Park, J .-S.; Kim, B. M.;
Gellman, S. H. J . Org. Chem. 2003, 68, 1575. (h) Schinnerl, M.; Murray,
J . K.; Langenhan, J . M.; Gellman, S. H. Eur. J . Org. Chem. 2003, 721.
(i) Wipf, P.; Wang, X. Tetrahedron Lett. 2000, 41, 8747. (j) Berkessel,
A.; Glaubitz, K.; Lex, J . Eur. J . Org. Chem. 2002, 2948. (k) Cheng, R.
P.; Gellman, S. H.; DeGrado, W. F. Chem. Rev. 2001, 101, 3219.
(6) For methodologies not mentioned in the text, see: Taggi, A. E.;
Hafez, A. M.; Wack, H.; Young, B.; Ferraris, D.; Lectka, T. J . Am.
Chem. Soc. 2002, 124, 6626. Dudding, T.; Hafez, A. M.; Taggi, A. E.;
Wagerle, T. R.; Lectka, T. Org. Lett. 2002, 4, 387; Hart, D. J .; Lee,
C.-S.; Pirkle, W. H.; Hyon, M. H.; Tsipouras, A. J . Am. Chem. Soc.
1986, 108, 6054. Cimarelli, C.; Palmieri, G. J . Org. Chem. 1996, 61,
5557. J uaristi, E.; Escalante, J . J . Org. Chem. 1993, 58, 2282. J uaristi,
E.; Escalante, J ., Lamatsch, B., Seebach, D. J . Org. Chem. 1992, 57,
2396. Miyabe, H.; Fujii, K.; Naito, T. Org. Lett. 1999, 1, 569. Miyabe,
H.; Fujii, K.; Naito, T. Org. Biomol. Chem. 2003, 1, 381. Minter, A. R.;
Fuller, A. A.; Mapp, A. K. J . Am. Chem. Soc. 2003, 125, 6846.
(7) J efford, C. W.; McNulty, J . Helv. Chim. Acta 1994, 77, 2142.
(8) (a) Tang, T. P.; Ellman, J . A. J . Org. Chem. 1999, 64, 12. (b)
Tang, T. P., Ellman, J . A. J . Org. Chem. 2002, 67, 7819.
† Dedicated to Professor E. J . Walsh (Allegheny College) on the
occasion of his 68th birthday.
(1) (a) Bates, R. B.; Brusoe, K. G.; Burns, J . J .; Caldera, S.; Cui, W.;
Gangwar, S.; Gramme, M. R.; McClure, K. J .; Rouen, G. P.; Schadow,
H.; Stessman, C. C.; Taylor, S. R.; Vu, V. H.; Yarick, G. V.; Zhang, J .;
Pettit, G. R.; Bontems, R. J . Am. Chem. Soc. 1997, 119, 2111. (b) Pettit,
G. R.; Kamano, Y.; Kizu, H.; Dufresne, C.; Herald, C. L.; Bontems, R.
J .; Schmidt, J . M.; Boettner, F. E.; Nieman, R. A. Heterocycles 1989,
28, 553. (c) Bai, R.; Verdier-Pinard, P.; Gangwar, S.; Stessman, C. C.;
McClure, K. J .; Sausville, E. A.; Pettit, G. R.; Bates, R. B.; Hamel, E.
Mol. Pharm. 2001, 59, 462.
(2) Carter, D. C.; Moore, R. E.; Mynderse, J . S.; Niemczura, W. P.;
Todd, J . S. J . Org. Chem. 1984, 49, 236.
(3) (a) Krauthauser, S.; Christianson, L. A.; Powell, D. R.; Gellman,
S. H. J . Am. Chem. Soc. 1997, 119, 11719. (b) Chung, Y. J .; Chris-
tianson, L. A.; Stanger, H. E.; Powell, D. R.; Gellman, S. H. J . Am.
Chem. Soc. 1998, 120, 10555. (c) Seebach, D.; Abele, S.; Gademann,
K.; J aun, B. Angew. Chem., Int. Ed. 1999, 38, 1595. (d) Chung, Y. J .;
Huck, B. R.; Christianson, L. A.; Stanger, H. E.; Krauthauser, S.;
Powell, D. R.; Gellman, S. H. J . Am. Chem. Soc. 2000, 122, 3995. (e)
Daura, X.; Gademann, K.; Schafer, H.; J aun, B.; Seebach, D.; Gun-
steren, W. F. v. J . Am. Chem. Soc. 2001, 123, 2393. (f) Lin, J .-Q.; Luo,
S.-W.; Wu, Y.-D. J . Comput. Chem. 2002, 23, 1551.
10.1021/jo034583h CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/04/2003
6440
J . Org. Chem. 2003, 68, 6440-6443

TABLE 1. Syn th esis of r,â-Un sa tu r a ted â-Am in o Ester s
TABLE 2. Con ju ga te Ad d ition to r,â-Un sa tu r a ted
Ester s
alde-
entry ester hyde
yielda
prod (%) (E/Z)b
dr
R
R1
1
2
1a
1b
2a
2b
CH3
Boc(Bn)N- 3a
ncc
91:9
(CH2)3
yield
product (%) dea
Boc(Bn)N-
(CH2)4
(CH3)2CHCH2
(c-C6H11)CH2
Bn
CH3
CH3
H
3b
ncc
85:15
entry ester
R
R1
3
4
5
6
7
8
1b
1b
1b
1c
1d
1e
2c
2d
2e
2a
2a
2a
H
H
H
Ph
3c
3d
3e
3f
63d
20
95:5
93:7
ncc
87:13
94:6
91:9
1
3a
CH3
Boc(Bn)N-
(CH2)3
5a
12b
90
ncc
82
2
3
4
5
6
7
3b
3c
3d
3f
3g
3h
Boc(Bn)N(CH2)4
(CH3)2CHCH2
(c-C6H11)CH2
CH3
H
H
H
Ph
5b
5c
5d
5f
20b
66
98
98
93
89
86
89
4-(CH3O)Ph 3g
48
54
53
CH3
TMPe
3h
ncc
67
a
b
Yield over three steps. By 1H NMR. c NC ) not calculated
CH3
CH3
4-(CH3O)-Ph 5g
TMPd
5h
ncc
d
because significant impurities were present. ∼2% of a trace
impurity remained present after column chromatography. eTMP
) 3,4,5-trimethoxyphenyl.
By 1H NMR. Yield from ester 1. c NC ) not calculated
a
b
d
because significant impurities were present. TMP ) 3,4,5-
trimethoxyphenyl.
some cases separation of the diastereomers is impossible
via column chromatography. A particularly promising
report by Davies and co-workers described the conjugate
addition of (R)-N-benzyl-N-(R-methylbenzyl)lithium amide
to a small set of R,â-unsaturated esters to afford syn-
R,â-dialkyl â-amino acids in excellent de in each case.10
However, none of these routes has yet provided access
to syn-R,â-dialkyl â-amino acids with polar side chains.11
Here, we report extension of the conjugate addition
protocol developed by Davies et al.10 to prepare protected
syn-R,â-dialkyl â-amino acids that contain lysine-like side
chains. A diverse set of syn-R,â-dialkyl â-amino acids with
hydrophobic side chains is also reported.
R,â-Unsaturated ester (3) starting materials for the
conjugate addition reactions were obtained via an aldol/
elimination strategy as shown in Table 1. tert-Butyl
esters 1 were converted to the corresponding lithium
enolates using LDA, and aldehydes 2 were added. The
crude aldol products were converted to the corresponding
tosyl esters, which were allowed to react with potassium
tert-butoxide to afford R,â-unsaturated esters 3 in modest
overall yields.12 Ester 3e was obtained as a 1:1 mixture
with the undesired phenyl conjugated isomer tert-butyl-
2-methyl-4-phenylbut-3-enoate.
protected nitrogen provided only recovered starting
material in the conjugate addition step10 with (R)-N-
benzyl-N-(R-methylbenzyl)lithium amide (4) (data not
shown), but an N-Boc-N-benzyl-protected nitrogen led to
successful addition reactions with 4 (Table 2, entries 1
and 2). Amino esters 5a and 5b were obtained in modest
yields and in good de as estimated by 1H NMR after
column chromatography. â-Amino esters that contain
hydrophobic side chains were also reliably produced in
good yields via conjugate addition with 4. Amino esters
5g and 5i could not be completely purified by column
chromatography. However, these materials were carried
on to provide pure protected â-amino acids (vide infra).
The ability to carry through materials that are not
rigorously pure bodes well for the scalability of this
methodology.
With â-amino esters 5 in hand, deprotection strategies
to provide Fmoc protection of the â-amino group were
examined. Esters 5a and 5b provided the greatest
challenge. Our initial attempts to obtain 7a and 7b
involved treatment with TFA to remove the tert-butyl
ester group and concomitant removal of the N-Boc group,
which was subsequently reintroduced. Unfortunately,
column chromatography of the Boc-protected tertiary
amino acids was difficult, and yields were consistently
low. The hydrogenolysis of the three benzylic groups was
attempted using a variety of conditions. In all cases, the
side-chain benzyl group was not cleaved. Given these
difficulties, an alternative strategy to obtain 7a and 7b
was pursued.
The results of the conjugate addition reactions are
shown in Table 2. Entries 1 and 2 describe conjugate
additions that lead to â-amino esters that contain side
chains bearing protected amino groups. An N-Boc-
(9) (a) Seebach, D.; Ciceri, P. E.; Overhand, M.; J aun, B.; Rigo, D.;
Oberer, K.; Hommel, U.; Amstutz, R.; Widmer, H. Helv. Chim. Acta
1996, 2043. (b) Seebach, D.; Abele, S.; Gademann, K.; Guichard, G.;
Hintermann, T.; J aun, B.; Matthews, J . L.; Schreiber, J . V.; Oberer,
L.; Hommel, U.; Widmer, H. Helv. Chim. Acta 1998, 81, 932. (c) Abele,
S., Disseration, ETH-Zurich, 1999. (d) Ma, Z. H.; Liu, C.; Zhao, Y. H.;
Li, W.; Wang, J . B. Chinese Chem. Lett. 2002, 13, 721.
(10) Davies, S. G.; Ichihara, O.; Walters, I. A. S. J . Chem. Soc.,
Perkin Trans. 1 1994, 9, 1141. Davies, S. G.; Garrido, N. M.; Ichihara,
O.; Walters, I. A. S. J . Chem. Soc., Chem. Commun. 1993, 1153.
(11) Tang et al. (ref 8b) reported an N-sulfinyl syn-R,â-dialkyl-â-
amino ester that contained an azido group on the R side chain.
However, this material was obtained as a mixture of four diastereomers
that could only be separated by HPLC, and the conversion of this
material to a protected â-amino acid with an amine side chain was
not demonstrated.
The benzylic groups derived from (R)-N-benzyl-N-(R-
methylbenzyl)lithium amide (4) were removed under
transfer hydrogenation conditions (Table 3). The side-
chain benzyl group was not cleaved under these condi-
tions. The free amine was protected as the benzyloxy
carbamate, and the tert-butyl ester was cleaved with TFA
along with concomitant removal of the N-Boc group.
Reintroduction of the N-Boc group afforded protected
amino acids 6 which could be purified via column
chromatography. Dissolving metal reduction to remove
the N-benzyl group, followed by Fmoc-protection, afforded
(12) For details on the major/minor isomer assignments, see the
Supporting Information.
J . Org. Chem, Vol. 68, No. 16, 2003 6441

TABLE 3. P r otectin g Gr ou p Ma n ip u la tion of â-Am in o
side chains that exploits a conjugate addition protocol
developed by Davies and co-workers.10 Access to this class
of â-amino acid monomers will open the door to the
investigation of â-peptide sheet secondary structure in
water. The synthetic route also provides syn-R,â-dialkyl
â-amino acids that contain diverse hydrophobic groups.
Both monomer types can be generated on a multigram
scale. The enantiomers of â-amino acids 7 could be
readily obtained by using commerically available (S)-N-
benzyl-N-(R-methylbenzyl)amine instead of the (R) iso-
mer.
Acid s Th a t Con ta in Lysin e Sid e Ch a in sa
entry ester Z-acid yield (%) product yield (%) eeb (%) deb (%)
1
2
5a
5b
6a
6b
53
56
7a
7b
69
59
91
99
97
94
a
Conditions: (a) (i) 10% Pd-C, HCOONH4, (ii) Cbz-OSu,
Exp er im en ta l Section
NaHCO3, acetone/water, (iii) 80% TFA in CH2Cl2, (iv) Boc2O, Et3N;
(b) (i) Na, NH3, THF, (ii) Fmoc-OSu, NaHCO3, acetone/water.
A representative synthetic procedure for a â-amino acid
bearing a side chain containing an amino group is provided
below. Other procedures can be found in the Supporting Infor-
mation.
b
Determined by examining the corresponding Mosher amides by
19F NMR (see the Supporting Information).
(2S,3R,rR)-ter t-Bu tyl-7-(ben zyl-ter t-bu toxyca r bon yla m i-
n o)-3-(N-b en zyl-N-r-m et h ylb en zyla m in o)-2-m et h ylh ep -
ta n oa te (5b). A flame-dried flask was charged with distilled
THF (150 mL) and cooled to 0 °C. Distilled i-Pr2NH (5.43 mL,
38.8 mmol) was added via syringe, followed by 2.5 M n-BuLi in
hexane (15.9 mL, 37.3 mmol). The solution was allowed to warm
to rt and was then cooled to -78 °C via a dry ice-acetone bath.
tert-Butyl propionate (1b, 5.39 mL, 35.8 mmol) was added to
this solution dropwise via syringe pump over ∼20 min, and the
solution was stirred for 45 min. Aldehyde 2b (10.9 g, 37.3 mmol)
was added, and the solution was stirred 15 min. Saturated aq
NH4Cl was added, and the reaction mixture was allowed to
warm to rt. THF was removed via rotary evaporation, and the
remaining solution was diluted with water. The aqueous layer
was extracted with Et2O three times. The combined organic
extracts were dried over MgSO4 and concentrated. The crude
aldol product was dissolved in CH2Cl2 (186 mL) and cooled to 0
°C. DMAP (5.01 g, 41.0 mmol) was added followed by TsCl (7.82
g, 41.0 mmol) and Et3N (10.4 mL, 74.5 mmol). The solution was
stirred overnight at rt, washed two times with 1 M HCl, dried
over MgSO4, and concentrated. The crude tosylates were purified
by SiO2 column chromatography eluting with hexane/Et2O (5:
1; v/v) affording two inseparable diastereomers (TLC Rf ) 0.20
and 0.14), 16.99 g as a light yellow oil: EI-MS m/z (M + Na)
calcd for C31H45O7NSNa 598.7, obsd 598.2. The tosyl esters were
dissolved in Et2O (200 mL) and cooled to -78 °C. KO-t-Bu (3.64
g, 32.5 mmol) was added, and the reaction mixture was stirred
until the reaction appeared to be complete by TLC (∼2 h). The
reaction mixture was concentrated; the residue was partitioned
between Et2O and 1 M HCl. The layers were separated, and the
aqueous layer was extracted one additional time with Et2O. The
combined organic layers were dried over MgSO4 and concen-
trated. Crude 3b was partially purified by SiO2 column chro-
matography eluting with Et2O/hexanes (1:9; v/v). An 85:15 E/Z
mixture was obtained as a clear oil (TLC Rf ) 0.10), 9.51 g.
Significant inseparable impurity resonances were apparent. Line
broadening in the 1H spectrum and resonance doubling observed
in the 13C spectrum were attributed to tertiary carbamate
rotamers: 1H NMR (CDCl3, 300 MHz) δ 7.26 (m, 5H), 6.63 (tq,
1H, J ) 7.5, 1.3 Hz) [5.75 (t, 0.18H, J ) 7.2 Hz)], 4.41 (s, 2H),
3.18 (m, 2H), 2.11 (m, 2H), 1.76 (s, 3H) [1.83 (s, 0.65H)], 1.48
(m, 22H); 13C NMR (CDCl3, 75.4 MHz) δ 166.9, 166.8, 155.4,
155.1, 140.1, 138.1, 129.4, 128.9, 128.0, 127.2, 126.6, 79.2, 78.9,
50.0, 49.5, 45.8, 28.0, 27.6, 27.3, 25.4, 11.9; EI-MS m/z (M +
Na) calcd for C24H37O4NNa 426.4, obsd 426.2. Conjugate addition
of 4 to 3b (9.51 g, 23.6 mmol) was carried out using the
conditions described by Davies.10 The crude material was
purified by two rounds of SiO2 column chromatography. The first
column was eluted with a gradient of hexane to hexane/Et2O
(5:1; v/v) to remove 2,6-di-tert-butylphenol. The second column
was eluted with 2.5% Et2O in benzene. The product 5b (TLC Rf
) 0.18 in 1:9 Et2O/hexane) was obtained as an oil, 4.64 g (20%
TABLE 4. P r otectin g Gr ou p Ma n ip u la tion of â-Am in o
Acid s Th a t Con ta in Hyd r op h obic Sid e Ch a in s
yield eea dea
product (%) (%) (%)
entry ester
R
R1
1
2
3
4
5
5c
5d
5f
5g
5h
(CH3)2CHCH2
(c-C6H11)CH2
CH3
CH3
CH3
H
H
Ph
7c
7d
7f
7g
7h
63
96 >92
92 >99
99 >99
89 >99
88 >96
42
40b
74
4-(CH3O)Ph
TMPc
14b
a
Determined by examining the corresponding Mosher amides
by 19F NMR (see the Supporting Information). Yield from ester
b
3. c TMP ) 3,4,5-trimethoxyphenyl.
the desired Fmoc-protected â-amino acids 7a and 7b with
Boc protection on the side chain nitrogen. The stereo-
chemical purity of these monomers was assessed by
examining the corresponding Mosher amides by 19F NMR
(see Supporting Information). Over 1 g of protected syn-
R,â-dialkyl â-amino acid 7a or 7b could be obtained in a
time frame of approximately 2 weeks.
The conversion of â-amino esters 5c,d ,f,g,h to the
corresponding Fmoc-acids (7c,d ,f,g,h ) involved transfer
hydrogenation to remove both benzylic groups, removal
of the tert-butyl ester group with TFA and treatment with
Fmoc-OSu (Table 4). The Fmoc-protected â-amino acids
were obtained with good stereochemical purity (as de-
termined by examining the corresponding Mosher amides
by 19F NMR) in all cases and in good yields (except for
7h ). The absolute configuration of these amino acids was
confirmed through an X-ray crystal structure of a parallel
â-peptide hairpin that contained residues 7c and 7f.13
This route provides syn-R,â-dialkyl â-amino acids with
hydrophobic side chains in about 1 week and is quite
scalable. For instance, over 7 g of 7f was obtained in one
round of synthesis.
In summary, we have developed an efficient synthetic
route to syn-R,â-dialkyl â-amino acids containing polar
1
yield over three steps), estimated >98% de by H NMR integra-
(13) Langenhan, J . M.; Guzei, I. A.; Gellman, S. H. Angew. Chem.,
Int. Ed. 2003, 42, 2402.
tion: 1H NMR (CDCl3, 300 MHz) δ 7.38-7.21 (m, 15H), 4.40
6442 J . Org. Chem., Vol. 68, No. 16, 2003

(m, 2H), 3.98 (q, 1H, J ) 6.8), 3.91 (A of AB, 1H, J ) 15.2), 3.77
(B of AB, 1H, J ) 15.2), 3.12 (m, 3H), 2.73 (m, 1H), 2.38 (m,
1H), 1.50-1.38 (m, 23H), 1.26 (d, 3H, J ) 6.9), 0.89 (d, 3H, J )
7.0); 13C NMR (CDCl3, 75.4 MHz) δ 175.5, 155.9, 155.5, 144.6,
142.4, 138.6, 128.3, 128.2, 128.00, 127.96, 127.8, 126.9, 126.7,
126.3, 79.7, 79.3, 60.8, 59.4, 50.9, 50.3, 49.8, 46.5, 43.8, 30.5,
28.3, 27.9, 25.2, 19.7, 16.8; EI-MS m/z (M + H) calcd for
C39H55O4N2 615.9, obsd 615.3.
bottom flask at -78 °C. Sodium (576 mg, 25.1 mmol) was added,
and the resulting blue solution was stirred for 30 min. A solution
of 6b (2.08 g, 4.18 mmol) in THF (46 mL) was added dropwise
to the reaction solution, and the reaction solution was stirred
for 30 min at -78 °C. The reaction was quenched by the addition
of NH4OAc (2.1 g, 27.2 mmol), and the ammonia was allowed to
evaporate. Precautions were taken to ensure that water did not
enter the reaction vessel during evaporation. Once evaporation
was complete, the remaining THF was removed under reduced
pressure. The residue was dissolved in water and lyophilized.
The resulting white crust was dissolved in 2:1 acetone/water (60
mL). The pH was checked to ensure that it was neutral. NaHCO3
(5.10 g, 60.7 mmol) and FmocOSu (6.14 g, 18.2 mmol) were
added, and the mixture was stirred at rt overnight. The acetone
was removed under reduced pressure, and the remaining
mixture was acidified to pH 2 with 1 M HCl and then extracted
three times with CH2Cl2. The combined organic layers were dried
over MgSO4 and concentrated. The crude residue was purified
by SiO2 column chromatography eluting with a gradient of 2:1
hexane/acetone to 1:2 hexane/acetone with 0.5% acetic acid. The
product was recrystallized by dissolving the material in chloro-
form (rt), adding hexane until the solution became turbid, and
refrigerating for 30 min. After filtration, the desired product was
obtained as a white solid, 1.22 g (59% yield) (TLC Rf ) 0.11 in
1:1 acetone/hexanes): mp 146-148 °C; 1H NMR (DMSO-d6, 300
MHz) δ 12.37 (br s, 0.76H), 8.03 (d, 2H, J ) 7.3), 7.86 (m, 2H),
7.58-7.45 (m, 4H), 7.25 (d, 1H, J ) 9.3), 6.90 (t, 1H, J ) 5.2),
4.56-4.35 (m, 3H), 3.79 (m, 1H), 3.05 (m, 2H), 1.57-1.26 (m,
13H), 1.16 (d, 3H, J ) 6.8); 13C NMR (DMSO-d6, 75.4 MHz): δ
176.2, 156.2, 155.6, 144.0, 143.9, 127.6, 127.1, 125.3, 125.2,
120.1,77.3, 65.1, 52.8, 47.0, 44.4, 32.5, 29.3, 28.3, 23.1, 13.9; EI-
(2S,3R)-(7-(Ben zyl-ter t-bu toxyca r bon yla m in o)-3-ben zy-
loxyca r b on yla m in o-2-m et h ylh ep t a n oic Acid (6b ). Amino
ester 5b (4.64 g, 7.54 mmol) was dissolved in t-BuOH (75 mL),
and ∼3 g of 10% Pd-C was added, followed by HCOONH4 (2.38
g, 37.7 mmol). The reaction mixture was heated at a light reflux
overnight, cooled, filtered through Celite, and concentrated. The
residue was dissolved in 2:1 acetone/water (75 mL) and cooled
to 0 °C. NaHCO3 (6.33 g, 75.3 mmol) and Cbz-OSu (2.44 g, 9.80
mmol) were added, and the reaction mixture was stirred at rt
overnight. The acetone was removed under reduced pressure,
and the mixture was acidified to pH 2 with 1 M HCl, extracted
three times with CH2Cl2, dried over MgSO4, and concentrated.
The crude residue was partially purified by SiO2 column
chromatography eluting with 1:1 Et2O/hexane (TLC Rf ) 0.34).
The product was obtained as an oil, 3.37 g. This material was
dissolved in 80% TFA in CH2Cl2 (65 mL) and stirred for 2 h.
The TFA/CH2Cl2 solvent was evaporated using a stream of
nitrogen, and the remaining residue was dissolved in benzene
and evaporated two times to remove residual TFA. The residue
was dissolved in THF (60 mL) and cooled to 0 °C. Et3N (13.5
mL, 97.1 mmol) and Boc2O (1.99 g, 9.11 mmol) were added, and
the reaction mixture was stirred at rt for 2 days. The reaction
mixture was concentrated, and the residue was partitioned
between CH2Cl2 and 1 M HCl. The aqueous layer was extracted
three times with CH2Cl2, and the combined organic layers were
dried over MgSO4 and concentrated. The crude residue was
purified by SiO2 column chromatography eluting with 2:1 Et2O/
hexane with 0.5% acetic acid (TLC Rf ) 0.21). The product acid
was obtained as an oil, 2.05 g (56% yield over two steps). Two
sets of resonances, attributed to tertiary carbamate rotamers,
were observed by NMR: 1H NMR (CDCl3, 300 MHz) δ 11.09 (br
s, 0.87H), 7.33-7.21 (m, 10H), 5.22 and 5.98 (m, 1H, atropiso-
meric), 5.10 (m, 2H), 4.37 (m, 2H), 3.82 and 3.94 (m, 1H,
atropisomeric), 3.13 (m, 2H), 2.64 (m, 1H), 1.51-1.14 (m, 15H),
1.14 (d, 3H, J ) 7.2); 13C NMR (CDCl3, 75.4 MHz) δ 178.7, 169.8,
156.2, 155.7, 138.3, 136.4, 128.3, 128.2, 127.9, 127.5, 127.0, 79.7,
66.6, 53.1, 50.3, 49.7, 46.1, 43.8, 31.0, 28.3, 27.6, 27.3, 23.3, 13.3;
EI-MS m/z (M - H) calcd for C28H37O6N2 497.6, obsd 497.2.
(2S,3R)-7-(Ben zyl-ter t-bu toxyca r bon yla m in o)-3-(9H-flu -
or en -9-ylm eth oxyca r bon yla m in o)-2-m eth ylh ep ta n oic Acid
(7b). Ammonia (200 mL) was condensed in a three-neck round-
MS m/z (M - H) calcd for C28H35O6N2 495.6, obsd 495.2; [R]D
)
-1.20 (c 3.33 × 10-1, CH3OH). The enantiomeric purity was
determined as 99% ee (> 94% de) by integration of the trifluo-
romethyl 19F resonances in the corresponding (R) and (S) Mosher
amides (see the Supporting Information).
Ack n ow led gm en t. This research was supported by
the NIH (GM56414). J .M.L. was supported in part by a
National Science Foundation Pre-Doctoral Fellowship.
Su p p or tin g In for m a tion Ava ila ble: General procedures,
experimental procedures for 2b, 3c,d ,f-h , 5a ,c,d ,g, 6a , and
1
7a ,c,d ,f-h , and H NMR and 13C NMR spectra of compounds
1a , 2b, 3a -d ,f-h , 5a -d ,f-h , 6a ,b, and 7a -d ,f-h . This
material is available free of charge via the Internet at
http://pubs.acs.org.
J O034583H
J . Org. Chem, Vol. 68, No. 16, 2003 6443