Convenient synthetic route to an enantiomerically pure FMOC α-amino acid

Article abstract of DOI:10.1021/jo801781v

(Chemical Equation Presented) A strategy for the facile α-amination of carboxylic acid menthyl esters is described. The resulting diastereomers, readily separable, can be individually carried on to each enantiomer of the FMOC α-amino acid. A variety of un

Full text of DOI:10.1021/jo801781v

Convenient Synthetic Route to an Enantiomerically Pure FMOC
r-Amino Acid
Douglass F. Taber,* James F. Berry, and Timothy J. Martin†
Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716
taberdf@udel.edu
ReceiVed August 8, 2008
A strategy for the facile R-amination of carboxylic acid menthyl esters is described. The resulting
diastereomers, readily separable, can be individually carried on to each enantiomer of the FMOC R-amino
acid. A variety of unnatural side chains were compatible with this approach. The menthyl ester was
easily removed from the FMOC R-amino acid without racemization.
Introduction
domains4 as well as changes in conformation that take place.5
A plethora of unnatural amino acids have been prepared, with
side chains incorporating, inter alia, perfluorinated tryptophan
derivatives,6 nitrobenzyl groups,7 azides,8 and cyanoanthracene
groups.9
The incorporation of unnatural amino acids into peptides and
proteins has gained importance in recent years.1 Methods such
as nonsense codon suppression have come to the fore as viable
strategies for integrating unnatural amino acids into peptides.2
Unnatural amino acids serve as probes for the identification of
interactions between proteins,3 and for identifying structural
There are two methods for the construction of enantiomeri-
cally pure R-amino acids, enantioselective homologation, and
R-amination of the carboxylic acid. To effect enantioselective
homologation, Belokon has developed achiral nickel(II) com-
plexes used in conjunction with a NOBIN (2-amino-2′-hydroxy-
1,1′-binaphthyl) catalyst.10 Johnson has used Pd-catalyzed
Suzuki cross coupling of halides with enantiopure vinyloxazo-
lidine derivatives.11 Krische has shown rhodium-catalyzed
reductive coupling of alkynes with (N-sulfinyl)iminoacetates to
be a promising method for achieving high regio- and stereo-
† Undergraduate research participant, University of Delaware.
(1) For reviews of the use of unnatural amino acids in the biomedical sciences,
see: (a) Dougherty, D. A. Curr. Opin. Chem. Biol. 2000, 4, 645. (b) England,
P. M. Biochemistry 2004, 43, 11623. (c) Magliery, T. J. Med. Chem. ReV.s
Online 2005, 2, 303. (d) Liao, J. Biotechnol. Prog. 2007, 23, 28. (e) Hausmann,
C. D.; Ling, J.; Ibba, M. Nature Methods 2007, 4, 205. (f) Xie, J.; Schultz, P. G.
Curr. Opin. Chem. Bio. 2005, 9, 548. (g) Xie, J.; Schultz, P. G. Nature ReV.
Mol. Cell Bio. 2006, 7, 775. (h) Pellois, J.-P.; Muir, T. W. Curr. Opin. Chem.
Bio. 2006, 10, 487. (i) Lu, Y. Curr. Opin. Chem. Bio. 2005, 9, 118. (j) Moroder,
L. J. Peptide Sci. 2005, 11, 187.
(2) For more recent references of the incorporation of unnatural amino acids
into peptides, see: (a) Cashin, A. L.; Torrice, M. M.; McMenimen, K. A.; Lester,
H. A.; Dougherty, D. A. Biochemistry 2007, 46, 630. (b) Ryu, Y.; Schultz, P. G.
Nature Methods 2006, 3, 263. (c) Liu, W.; Brock, A.; Chen, S.; Chen, S.; Schultz,
P. G. Nature Methods 2007, 4, 239. (d) Jackson, J. C.; Duffy, S. P.; Hess, K. R.;
Mehl, R. A. J. Am. Chem. Soc. 2006, 128, 11124. (e) Ishida, H.; Kyakuno, M.;
Oishi, S. Peptide Sci. 2004, 76, 69. (f) Caligiuri, A.; D’Arrigo, P.; Rosini, E.;
Tessaro, D.; Molla, G.; Servi, S.; Pollegioni, L. AdV. Synth. Catal. 2006, 348,
2183. (g) Josephson, K.; Hartman, M. C. T.; Szostak, J. W. J. Am. Chem. Soc.
2005, 127, 11727. (h) Gauchet, C.; Labadie, G. R.; Poulter, C. D. J. Am. Chem.
Soc. 2006, 128, 9274. (i) Sando, S.; Kanatani, K.; Sato, N.; Matsumoto, H.;
Hohsaka, T.; Aoyama, Y. J. Am. Chem. Soc. 2005, 127, 7998.
(3) (a) Iida, S.; Asakura, N.; Tabata, K.; Okura, I.; Kamachi, T. ChemBio-
Chem 2006, 7, 1853. (b) Altschuh, D.; Oncul, S.; Demchenko, A. P. J. Mol.
Recognit. 2006, 19, 459. (c) Cashin, A. L.; Torrice, M. M.; McMenimen, K. A.;
Lester, H. A.; Dougherty, D. A. Biochemistry 2007, 46, 630. (d) Mukherji, M.;
Brill, L. M.; Ficarro, S. B.; Hampton, G. M.; Schultz, P. G. Biochemistry 2006,
45, 15529.
(5) (a) Lummis, S. C. R.; Beene, D. L.; Lee, L. W.; Lester, H. A.; Broadhurst,
R. W.; Dougherty, D. A. Nature 2005, 438, 248. (b) Abe, M.; Ohno, S.;
Yokogawa, T.; Nakanishi, T.; Arisaka, F.; Hosoya, T.; Hiramatsu, T.; Suzuki,
M.; Ogasawara, T.; Sawasaki, T.; Nishikawa, K.; Kitamura, M.; Hori, H.; Endo,
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(6) Zhong, W.; Gallivan, J. P.; Zhang, Y.; Li, L.; Lester, H. A.; Dougherty,
D. A. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 12088.
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9334 J. Org. Chem. 2008, 73, 9334–9339
10.1021/jo801781v CCC: $40.75 2008 American Chemical Society
Published on Web 11/06/2008

Synthesis of an Enantiomerically Pure FMOC R-Amino Acid
control in amino acid ester construction.12 Phase transfer
catalysts have also been used for glycine alkylation. O’Donnell
reported the use of a chiral quaternary ammonium salt, derived
from Cinchona alkaloids, to prepare the R-amino acid deriva-
tives in high yield and high ee from the corresponding imine
and alkyl halide.13 Marouka has also developed a series of chiral
quaternary ammonium salts which catalyze the enantioselective
synthesis of R-alkyl-R-amino acid derivatives with very low
catalyst loading.14
An alternative approach is R-amination of the carboxylic acid.
Evans has developed a chiral magnesium bis(sulfonamide)
complex for electrophilic amination with high enantioselectiv-
ity.15 Lectka has employed o-benzoquinone imides with chiral
ketene enolates.16 Zheng has shown the capability of chiral
amide cuprates to be coupled with lithium tert-butyl-N-tosy-
loxycarbamate in high diastereoselectivity and high yield.17 In
the laboratory of Bad´ıa, arylacetamide enolate coupling with
di-tert-butylazidocarboxylate gave the corresponding arylglycine
amino acid precursor with excellent enantioselectivity.18 Zhou19
and Fu20 have both employed chiral copper complexes to effect
NH insertion into the carbene derived from an R-diazoester,
yielding the respective R-amino acid derivative in high enan-
tiomeric excess. Although these last methods of preparing
R-amino acid derivatives via NH insertion are useful, there are
no examples of derivatives containing side chains that include
alkenes or alkynes.
Results and Discussion
Menthol had been used previously for the crystallization,23
separation,24,25 and determination of enantiopurity26 of subse-
quent esters. In this study, we were pleased to observe (Scheme
1) that the 1:1 mixture of diastereomers of the R-aminated
menthyl esters were readily separable by preparatory column
chromatography. Here, the inexpensive L-menthol has three
roles: protection of the sensitive carboxylic acid as an ester, as
a resolving agent for separation of the diastereomers formed
through NH insertion of o-anisidine into the corresponding diazo
ester, and as a reporter of diastereomeric purity. A methyl group
of the L-menthol showed distinct doublets ((R)-2a: δ 0.48; (S)-
2a: δ 0.60) in the 1H NMR spectrum for the two diastereomers.
With use of these doublets, diastereomeric ratios were estab-
lished through integration of the peak areas.
The menthyl esters for this study were prepared by DCC-
mediated coupling27 of menthol with the appropriate acid. Of
these menthyl esters, only 5a and 5e had previously been
prepared.28 It had been reported previously that acylation of an
ester with benzoyl chloride (5a f 6a) proceeded in high yield
by sequential addition of NEt3 and TiCl4, followed by heating
in MeCN for 15 min.22 We found that overnight reaction at
room temperature was more efficient. These milder reaction
conditions should allow for more sensitive substrates to be used
without decomposition.
There are two challenges with each of these approaches. The
first is that the products, although enantiomerically enriched,
are not directly enantiomerically pure. Further, preparation of
the alternate enantiomer of the target amino acid requires
preparation of the enantiomeric chiral auxiliary or catalyst. We
have shown that diazo esters21 couple smoothly with aryl
amines.22 We envisioned (eq 1) the extension of this process to
the production of enantiomerically pure R-amino acids. Several
issues had to be addressed for this approach to be practical.
The first was the discovery of an enantiomerically pure alcohol
such that the diastereomeric esters resulting from amine NH
insertion could be efficiently separated. It was also important
that the protective aromatic group could be removed from the
amine, and that hydrolysis of the ester could be effected without
racemization of the stereogenic center.
We observed (eq 2) that with aromatic side chains (X ) H,
Br, OCH3) one of the diastereomers (the less polar diastereomer,
S configuration at the R carbon as established by X-ray analysis)
of the aromatic ꢀ-keto ester crystallized out. By using this
approach, most of the mixture could be converted to the single
(12) Kong, J.-R.; Cho, C.-W.; Krische, M. J. J. Am. Chem. Soc. 2005, 127,
11269.
(13) (a) O’Donnell, M. J.; Delgado, F.; Hostettler, C.; Schwesinger, R.
Tetrahedron Lett. 1998, 39, 8775. For a review of the use of phase transfer
catalysts with schiff base esters in synthesizing R-amino acids, see: (b) O’Donnell,
M. J. Acc. Chem. Res. 2004, 37, 506.
(14) Kitamura, M.; Shirakawa, S.; Maruoka, K. Angew. Chem., Int. Ed. 2005,
44, 1549.
(15) Evans, D. A.; Nelson, S. G. J. Am. Chem. Soc. 1997, 119, 6452.
(16) (a) Wolfer, J.; Bekele, T.; Abraham, C. J.; Dogo-Isonagie, C.; Lectka,
T. Angew. Chem., Int. Ed. 2006, 45, 7398. (b) Paull, D. H.; Alden-Danforth, E.;
Wolfer, J.; Dogo-Isonagie, C.; Abraham, C. J.; Lectka, T. J. Org. Chem. 2007,
72, 5380.
(17) Zheng, N.; Armstrong, J. D.; McWilliams, J. C.; Volante, R. P.
Tetrahedron Lett. 1997, 38, 2817.
(23) For the initial preparation of R-amino menthyl esters as the hydrochloride
salts, see: (a) Harada, K.; Hayakawa, T. Bull. Chem. Soc. Jpn. 1964, 37, 191.
(b) Hayakawa, T.; Harada, K. Bull. Chem. Soc. Jpn. 1965, 38, 1354. (c) Halpern,
B.; Ricks, J.; Westley, J. W. Aust. J. Chem. 1967, 20, 389.
(24) For the chromatographic separation of racemic amino acids as their
menthyl esters, see: House, D. W. U. S. (1983), 4 pp, CODEN: USXXAM US
4379941 A 19830412.
(25) While preparing this manuscript, another example of employing
L-menthol as a tool for separating diastereomers was published: Sani, M.; Fossati,
G.; Huguenot, F.; Zanda, M. Angew. Chem., Int. Ed. 2007, 46, 3526.
(26) For the use of menthol to establish enantiomeric purity, see: (a)
Kaminsky, Z. J.; Kolesinska, B.; Kolesinska, J.; Sabatino, G.; Chelli, M.; Rovero,
P.; Blaszczyk, M.; Glowka, M. L.; Papini, A. M. J. Am. Chem. Soc. 2005, 127,
16912. (b) Lai, L. M.; Lam, J. W.; Qin, A.; Dong, Y.; Tang, B. Z. J. Phys.
Chem. B 2006, 110, 11128. (c) Lai, L. M.; Lam, J. W.; Tang, B. Z. J. Polym.
Sci., Part A: Polym. Chem. 2006, 44, 2117.
(18) Vicario, J. L.; Bad´ıa, D.; Dom´ınguez, E.; Crespo, A.; Carrillo, L.;
Anakabe, E. Tetrahedron Lett. 1999, 40, 7123.
(19) Liu, B.; Zhu, S.-F.; Zhang, W.; Chen, C.; Zhou, Q.-L. J. Am. Chem.
Soc. 2007, 129, 5834.
(20) Lee, E. C.; Fu, G. C. J. Am. Chem. Soc. 2007, 129, 12066.
(21) For our initial paper showing the preparation of R-diazo esters, see: (a)
Taber, D. F.; You, K.; Song, Y. J. Org. Chem. 1995, 60, 1093. (b) Taber, D. F.;
Gleave, D. M.; Herr, R. J.; Moody, K.; Hennessey, M. J. J. Org. Chem. 1995,
60, 2283.
(27) For details of the esterification step, see: Hu, W.; Timmons, D. J.; Doyle,
M. P. Org. Lett. 2002, 4, 901.
(28) (a) For a previous preparation of 5a, see: Sakakura, A.; Kawajiri, K.;
Ohkubo, T.; Kosugi, Y.; Ishihara, K. J. Am. Chem. Soc. 2007, 129, 14775. (b)
For a previous preparation of 5e, see: Stephen, A.; Hashmi, K.; Sinha, P. AdV.
Synth. Catal. 2004, 346, 432.
(22) Taber, D. F.; Sheth, R. B.; Joshi, P. V. J. Org. Chem. 2005, 70, 2851.
J. Org. Chem. Vol. 73, No. 23, 2008 9335

Taber et al.
SCHEME 1
SCHEME 2
TABLE 1. Yields for the Diazo Transfer and NH Insertion
Reactions
crystalline diastereomer. This may have promise as a practical
method for the preparation of enantiomerically pure ꢀ-keto
esters.
a Based on diazo ester charged. b 35 °C, 3 h. c Room temperature,
overnight.
(Scheme 2), in a 1:1 ratio (1H NMR). With use of the phenyl
diazo ester 1a, the best results (69% yield, based on diazo ester)
were obtained by adding the diazo ester dropwise to a solution
of o-anisidine and catalytic rhodium octanoate in dry Et2O at
reflux. We were pleased (Table 1) to observe that alkenyl (1d)
and alkynyl (1e) functional groups were compatible with these
reaction conditions.
It became apparent that side product formation prevented the
consumption of the entire quantity of o-anisidine, which eluted
with the product diastereomers. Fortunately, we found that
addition of 0.1% triethylamine to the elution solvent assisted
separation. We also discovered that if only 0.8 equiv of
o-anisidine was employed, TLC monitoring of the reaction
mixture showed very little residual o-anisidine, with no decrease
in the yield of the products.
The next step in the sequence was conversion of ꢀ-keto
ester 6 to the corresponding diazo ester 1 (Scheme 2). Our
initial report described the use of p-acetaminobenzenesulfonyl
azide (p-AABS-A) at room temperature for this conversion.22
We found that gentle warming was required to drive the
reaction of the menthyl esters to completion. The yields for
diazo transfer to the several esters are summarized in Ta-
ble 1.
We observed (eq 3) that, after column chromatography,
crystallization of the product diastereomers (dissolving the
compounds in a minimal amount of PE and cooling the solution
to temperatures below 0 °C) afforded the more polar diastere-
The most important step in the synthesis was the rhodium-
catalyzed NH insertion of o-anisidine into the diazo ester 1 to
afford the corresponding amine diastereomers (R)-2 and (S)-2
9336 J. Org. Chem. Vol. 73, No. 23, 2008

Synthesis of an Enantiomerically Pure FMOC R-Amino Acid
Currently most syntheses of R-amino acids employ tert-butyl
esters as protecting groups for the carboxylic acid. Our
methodology described here should be of value since menthyl
esters enable the diastereomeric integrity to be evaluated directly.
Use of this reporter function of the menthyl ester could also
facilitate the preparation of unnatural amino acids from the
corresponding natural amino acids. Esterification with menthol
followed by subsequent transformations could be effected, with
the intermediates easily monitored for diastereomeric purity by
1H NMR.
omer (S)-2 as a crystalline solid (as established by X-ray
analysis), leaving the less polar diastereomer (R)-2 in solution
(eq 3). The more polar diastereomer was recovered in >99%
Conclusion
Adventitious racemization is always a hazard when
manipulating the esters of R-amino acids. We have developed
a simple procedure for the preparation of single enantiomer
FMOC R-amino acids from the corresponding carboxylic
acids. The use of L-menthol as the coupling partner has three
distinct roles: protection of the sensitive carboxylic acid
functionality, separation of the resulting diastereomers formed
through NH insertion, and determination of the diastereomeric
purity through integration of the methyl doublets ((R)-2a: δ
1
diastereomeric purity as established by H NMR.
We also found (eq 4) that a 7:1 mixture of (R)-2a:(S)-2a could
be epimerized using potassium tert-butoxide to afford a 2:1
mixture of diastereomers. Through separation and epimerization,
the two NH insertion products obtained can thus be converted
to a single desired diastereomer.
1
0.48; (S)-2a: δ 0.60) in the H NMR spectrum. The menthyl
ester was easily removed without epimerization. Although
other NH insertion methods19,20 are limited to R-aryl and
R-methyl side chains, our methodology enables the incor-
poration of longer aliphatic side chains. We believe that this
will be a practical method for the laboratory-scale preparation
of a variety of unnatural amino acids.
Experimental Section
(1R,2S,5R)-2-Isopropyl-5-methylcyclohexyl 3-Phenylpropanoate
(5a). L-Menthol (2.01 g, 12.9 mmol), hydrocinnamic acid (1.962
g, 13.1 mmol), and N,N-dimethylaminopyridine (DMAP) (0.39 g,
3.19 mmol) were combined in a round-bottomed flask along with
13 mL of a 1:1 mixture of CH2Cl2 and CH3CN. 1,3-Dicyclohexy-
lcarbodiimide (DCC) (2.92 g, 14.2 mmol) in 2 mL of dry CH2Cl2
was added dropwise over 10 min at 0 °C. The solution was warmed
to rt and stirred overnight. The white precipitate was filtered out
and the filtrate was partitioned between CH2Cl2 and, sequentially,
1 N aqueous HCl, 10% aqueous NaHCO3, and water. The
combined organic extract was dried (Na2SO4) and concentrated
to yield (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl 3-phenyl-
propanoate (5a) (3.69 g, 99% yield) as a clear oil. The 1H NMR
agreed with that of the previously synthesized material.28a 1H
NMR δ 7.21 (m, 2H), 7.11 (m, 3H), 4.59 (td, 1H, J ) 11.2 Hz,
J ) 4.4 Hz), 2.87 (t, 2H, J ) 7.6 Hz), 2.53 (t, 2H, J ) 7.6 Hz),
1.84 (d, 1H, J ) 12.8 Hz), 1.53-1.69 (m, 3H), 1.39 (m, 1H),
1.25 (m, 1H, J ) 11.2 Hz, J ) 2.8 Hz), 0.72-1.01 (m, 9H),
0.62 (d, 3H, J ) 6.8 Hz); 13C NMR32 δ u: 172.5, 140.5, 40.9,
36.2, 34.2, 31.1, 23.4; 13C NMR32 δ d: 128.4, 128.3, 126.2, 74.1,
46.9, 31.4, 26.1, 22.0, 20.8, 16.2.
(1R,2S,5R)-2-Isopropyl-5-methylcyclohexyl 2-Benzoyl-3-oxo-3-
phenylpropanoate (6a). To a solution of the phenyl ester (3.995 g,
13.9 mmol), benzoyl chloride (6.024 g, 43.0 mmol), triethylamine
(8.411 g, 83.3 mmol), and 14 mL of dry CH3CN was added titanium
tetrachloride (1.0 M in toluene; 20.8 mL, 20.8 mmol) over 10 min
at 0 °C under N2 atmosphere. The reaction mixture was warmed to
rt and stirred overnight, then partitioned between water and EtOAc.
The combined organic extract was dried (Na2SO4) and concentrated.
The residue was chromatographed to yield a diastereomeric mixture
of (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl 2-benozyl-3-oxo-3-
After some exploration, we settled (Scheme 1) on the
Hoveyda protocol29 as the best for the removal of the protective
aromatic group. The crude free amine mixture was immediately
exposed to FMOC protection conditions (FMOC-O-Su and
Na2CO3 in a 1:1 H2O:dioxane mixture30 at room temperature
for 24 h). The resulting yield of FMOC amine (R)-7 was 60%
over two steps with little decomposition.
The final step in the sequence was the removal of the menthyl
group to liberate the FMOC protected amino acid. We were
concerned that the use of a strong acid such as TFA could
epimerize the enantiopure center that we had carefully estab-
lished. Use of TFA at 110 °C for 12 min under microwave
conditions did effectively remove the menthyl group. To check
for racemization during the dementhylation step, the resulting
FMOC-D-phenylalanine (R)-8 was subjected to the DCC/DMAP
coupling procedure used previously. To our delight, the 1H NMR
showed the presence of three clear menthyl doublets, indicating
the chiral center had not undergone racemization in the
dementhylation step.31 The use of acetone-d6 as the NMR
solvent is preferred because the three menthyl doublets that are
present are completely distinguishable (RR δ 0.68, 0.77, 0.88),
whereas using CDCl3 causes the chemical shifts to overlap (RR
δ 0.73, 0.83-0.88).
(29) Josephsohn, N. S.; Snapper, M. L.; Hoveyda, A. H. J. Am. Chem. Soc.
2004, 126, 3734.
(30) Carpino, L. A.; Han, G. Y. J. Am. Chem. Soc. 1970, 92, 5748.
(31) A slight impurity in the commercial menthol was present at δ 0.56.
This impurity did not interfere with the analysis.
(32) 13C multiplicities were determined with the aid of a JVERT pulse
sequence, differentiating the signals for methyl and methine carbons as “d” and
for methylene and quaternary carbons as “u”.
J. Org. Chem. Vol. 73, No. 23, 2008 9337

Taber et al.
1
phenylpropanoate (6a) (5.278 g, 97% yield) as an off-white solid.
68-72 °C; H NMR (acetone-d6) δ 7.27 (m, 4H), 7.21 (m, 1H),
6.80 (dd, 1H, J ) 8 Hz, J ) 1.6 Hz), 6.74 (td, 1H, J ) 8 Hz, J )
1.2 Hz), 6.61 (m, 2H), 4.83 (d, 1H, J ) 9.6 Hz), 4.57 (td, 1H, J )
10.8 Hz, J ) 4 Hz), 4.38 (m, 1H), 3.79 (s, 3H), 3.13 (d, 2H, J )
6.8 Hz), 1.84 (m, 1H), 1.62 (m, 3H), 1.26-1.44 (m, 2H), 1.00 (m,
1H), 0.77-0.85 (m, 7H), 0.71 (m, 1H), 0.60 (d, 3H, J ) 6.8 Hz);
13C NMR (acetone-d6) δ u: 173.4, 148.1, 138.0, 137.7, 41.3, 39.2,
34.9, 23.7; 13C NMR (acetone-d6) δ d: 130.3, 129.1, 127.5, 121.8,
118.1, 111.5, 110.9, 75.2, 58.4, 55.9, 47.7, 32.0, 26.6, 22.2, 21.1,
16.3; IR 3401, 1731 cm-1; HRMS calcd for C26H35NO3Na (M +
Na) 432.2515, obsd 432.2514.
The solid was then recrystallized from a 4:1 mixture of MeOH:
PE. TLC Rf (MTBE/PE ) 0.4:9.6) ) 0.29; [R]15 -64.5 (c 1.00,
D
1
CH2Cl2); mp 116-118 °C; H NMR (recrystallized compound) δ
7.99 (dd, 2H, J ) 8.8 Hz, J ) 1.6 Hz), 7.56 (tt, 1H, J ) 7.2 Hz,
J ) 1.6 Hz), 7.45 (t, 2H, J ) 8 Hz), 7.25 (m, 4H), 7.19 (m, 1H),
4.58 (m, 2H), 3.32 (m, 2H), 1.79 (d, 1H, J ) 11.6 Hz), 1.58 (m,
2H), 1.37 (m, 1H), 1.20 (m, 2H), 0.72-0.97 (m, 6H), 0.65 (d, 3H,
J ) 7.2 Hz), 0.41 (d, 3H, J ) 6.4 Hz); 13C NMR δ u: 194.0, 168.8,
138.4, 136.2, 40.3, 34.5, 34.1, 22.9; 13C NMR δ d: 133.4, 129.0,
128.6, 128.6, 128.5, 126.6, 75.5, 56.9, 46.7, 31.3, 25.5, 21.9, 20.6,
15.7; IR 1718, 1686 cm-1; HRMS calcd for C26H33O3 (MH+)
393.2430, obsd 393.2428.
Epimerization. The protected amine (0.023 g, mmol, 7:1 ratio
of (R)-2a to (S)-2a) was dissolved in 3 mL of dry THF. To this
solution was added dropwise 5 mL of a 1 mg/mL solution of
potassium tert-butoxide in dry THF. The reaction mixture was
stirred for 1 h at rt. The solvent was removed under reduced pressure
and an additional 5 mL of potassium tert-butoxide in dry THF was
added. The mixture was then stirred overnight at rt. The residue
was chromatographed to yield a 2:1 mixture ((R)-2a to (S)-2a) of
(1R,2S,5R)-2-Isopropyl-5-methylcyclohexyl 2-Diazo-3-phenyl-
propanoate (1a). Keto ester 6a (1.001 g, 2.6 mmol) was combined
with N-acetoaminobenzenesulfonyl azide (1.228 g, 5.1 mmol) in 1
mL of dry CH3CN. The mixture was stirred at 0 °C under N2
atmosphere for 5 min. DBU (0.583 g, 3.8 mmol) was added
dropwise over 5 min to the solution at 0 °C. The mixture was stirred
for 10 min and additional N-acetoaminobenzenesulfonyl azide
(0.625 g, 2.6 mmol) and DBU (0.193 g, 1.3 mmol) were added.
The solution was heated to 35 °C and stirred for 3 h, then partitioned
between water and CH2Cl2. The combined organic extract was dried
(Na2SO4) and concentrated. The residue was chromatographed to
give (1R,2S,5R)-2-isopropyl-5-methylcyclohexyl 2-diazo-3-phenyl-
propanoate (1a) as a yellow oil (0.592 g, 74% yield). TLC Rf
(MTBE/PE ) 0.4:9.6) 0.46; 1H NMR δ 7.31 (m, 2H, J ) 7.6 Hz),
7.23 (m, 3H), 4.76 (td, 1H, J ) 10.8 Hz, J ) 4.4 Hz), 3.61 (s, 2H),
2.03 (d, 1H, J ) 8.8 Hz), 1.80 (br s, 1H), 1.66 (m, 2H), 1.48 (m,
1H), 1.36 (m, 1H, J ) 12 Hz, J ) 4.8 Hz), 0.93-1.11 (m, 2H),
0.83-0.91 (m, 7H), 0.75 (d, 3H, J ) 6.8 Hz); 13C NMR δ 167.1,
137.4, 128.7, 128.3, 127.0, 77.2, 74.9, 47.1, 41.3, 34.2, 31.4, 29.4,
26.4, 23.6, 22.0, 20.7, 16.5; IR 2082, 1688 cm-1; HRMS calcd for
C19H27N2O2 (MH) 315.2073, obsd 315.2063.
1
diastereomers (0.017 g, 74% yield). The H NMR matched the
aforementioned compounds.
(R)-(1R,2S,5R)-2-Isopropyl-5-methylcyclohexyl) 2-(((9H-Fluo-
ren-9-yl)methoxy)carbonyl)-3-phenylpropanoate ((R)-7). To a solu-
tion of iodobenzene diacetate (0.236 g, 0.73 mmol) and 0.1 mL of
acetic acid in 5 mL of MeOH at 0 °C was added the protected
amine (R)-2a (0.075 g, 0.18 mmol) in 2 mL of MeOH dropwise
over 5 min. The solution was stirred for 10 min at 0 °C and then
warmed to rt and stirred for 30 min. Then, 5 mL of 1 N aqueous
HCl was added and the mixture was stirred for an additional 30
min. Next, 10 mL of 10% aqueous Na2S2O3 was added and the
solution was stirred for 30 min. The solution was brought to pH
∼10-11 with solid Na2CO3 and stirred for 1.5 h. The mixture was
partitioned between CH2Cl2 and H2O and the combined organic
layers were dried (Na2SO4) and concentrated to give the crude free
amine. A mixture of FMOC-O-Su (0.081 g, 0.24 mmol) and
Na2CO3 (0.038 g, 0.36 mmol) in 1.5 mL of a 1:1 dioxane:H2O
mixture was then stirred for 5 min at rt. The crude free amine,
dissolved in 1 mL of dioxane, was added and the solution was
stirred for 24 h at rt. The mixture was partitioned between
CH2Cl2 and H2O and the combined organic layers were dried
(Na2SO4) and concentrated. The residue was chromatographed
to yield (R)-(1R,2S,5R)-2-isopropyl-5-methylcyclohexyl) 2-(((9H-
fluoren-9-yl)methoxy)carbonyl)-3-phenylpropanoate ((R)-7) (0.057
g, 60% yield over two steps) as an off white solid. TLC Rf
(R)-((1R,2S,5R)-2-Isopropyl-5-methylcyclohexyl) 2-(2-Meth-
oxyphenylamino)-3-phenylpropanoate((R)-2a)and(S)-((1R,2S,5R)-
2-Isopropyl-5-methylcyclohexyl) 2-(2-Methoxyphenylamino)-3-
phenylpropanoate ((S)-2a). O-Anisidine (0.147 g, 1.2 mmol) and
rhodium(II) octanoate dimer (0.023 g, 2 mol %) were added to a
round-bottomed flask along with 1.75 mL of dry Et2O. The flask
was equipped with a reflux condenser and the solution was heated
to 45 °C (oil bath temperature) under N2 atmosphere for 10 min.
Diazo ester 1a (0.475 g, 1.5 mmol) in 1.5 mL of dry Et2O was
added dropwise over 10 min. The solution was then allowed to stir
at 45 °C for 3 h. After cooling, the mixture was immediately
concentrated and chromatographed via TLC mesh silica gel to afford
diastereomers (R-((1R,2S,5R)-2-isopropyl-5-methylcyclohexyl) 2-(2-
methoxyphenylamino)-3-phenylpropanoate ((R)-2a) and (S-
((1R,2S,5R)-2-isopropyl-5-methylcyclohexyl) 2-(2-methoxypheny-
lamino)-3-phenylpropanoate ((S)-2a) as clear oils: ((R)-2a (less polar
diastereomer) 0.090 g, (S)-2a (more polar diastereomer) 0.133 g,
mixture of diastereomers 0.203 g, 69% yield overall). The mixture
of diastereomers (0.203 g) was then dissolved in a minimal amount
of PE and placed in the freezer overnight to induce crystallization
of (S)-2a, the more polar diastereomer (0.075 g). (R)-2a (less polar
(MTBE/PE 1.0:9.0) 0.26; [R]15 +28.0 (c 1.00, CH2Cl2); mp
D
1
86-89 °C; H NMR (acetone-d6) δ 7.84 (d, 2H, J ) 7.6 Hz),
7.64 (t, 2H, J ) 6.4 Hz), 7.40 (t, 2H, J ) 7.6 Hz), 7.20-7.33
(m, 7H), 6.86 (d, 1H, exchanges, J ) 8.8 Hz), 4.66 (td, 1H, J )
10.8 Hz, J ) 4.4 Hz), 4.47 (m, 1H), 4.24 (m, 2H), 4.18 (q, 1H,
J ) 6.8 Hz), 3.20 (dd, 1H, J ) 14 Hz, J ) 6 Hz), 3.00 (dd, 1H,
J ) 14 Hz, J ) 9.2 Hz), 1.95 (d, 1H, J ) 12.8 Hz), 1.76 (m,
1H, J ) 6.8 Hz, J ) 2.4 Hz), 1.65 (m, 2H), 1.47 (m, 1H), 1.36
(m, 1H, J ) 12 Hz, J ) 2.8 Hz), 0.93-1.11 (m, 2H), 0.85-0.91
(m, 4H), 0.77 (d, 3H, J ) 6.8 Hz), 0.68 (d, 3H, J ) 7.6 Hz);
13C NMR (acetone-d6) δ u: 172.2, 156.8, 145.0, 142.0, 138.2,
67.2, 41.5, 38.3, 34.9, 23.7; 13C NMR (acetone-d6) δ d: 130.1,
129.2, 128.5, 127.9, 127.5, 126.1, 120.8, 67.2, 56.9, 47.9, 47.7,
diastereomer, clear oil): TLC Rf (MTBE/PE ) 0.4:9.6) 0.45; [R]15
D
1
-23.7 (c 1.00, CH2Cl2); H NMR (acetone-d6) δ 7.30 (m, 4H),
7.23 (m, 1H), 6.82 (d, 1H, J ) 8 Hz), 6.75 (td, 1H, J ) 8 Hz, J )
1.2 Hz), 6.62 (t, 2H, J ) 7.6 Hz), 4.83 (d, 1H, exchanges, J ) 10
Hz), 4.54 (td, 1H, J ) 10.8 Hz, J ) 4.4 Hz), 4.37 (m, 1H), 3.81 (s,
3H), 3.14 (d, 2H, J ) 6.8 Hz), 1.86 (d, 1H, J ) 12 Hz), 1.62 (m,
3H), 1.40 (m, 1H), 1.28 (m, 1H, J ) 11.6 Hz, J ) 3.2 Hz),
0.82-1.03 (m, 6H), 0.70 (d, 3H, J ) 7.2 Hz), 0.48 (d, 3H, J )
6.8); 13C NMR (acetone-d6) δ u: 173.2, 148.1, 138.2, 137.7, 41.6,
39.4, 34.9, 23.5; d: 130.2, 129.1, 127.5, 121.8, 118.2, 111.5, 110.8,
75.2, 59.0, 55.9, 47.7, 32.1, 25.9, 22.3, 21.2, 15.9; IR 3401, 1731
cm-1; HRMS calcd for C26H35NO3Na (M + Na) 432.2513, obsd
432.2511. (S)-2a (more polar diastereomer, crystalline solid): TLC
Rf (MTBE/PE ) 0.4:9.6) 0.41; [R]15D -47.1 (c 1.00, CH2Cl2); mp
32.1, 26.4, 22.3, 21.1, 16.3; IR 3588, 3327, 1721, 1702 cm-1
;
HRMS calcd for C34H39NO4Na (M + Na) 548.2777, obsd
548.2769.
FMOC-D-Phenylalanine ((R)-8). The FMOC-protected amine
(0.075 g, 0.14 mmol) was dissolved in 1.5 mL of TFA and placed in
a microwave reactor for 12 min (5 min warming to 110 °C, followed
by 7 min at 110 °C) at 250 psi. The TFA was then bubbled off under
N2 and the residue was taken up in 3 mL of CH2Cl2 and evaporated
onto flash silica gel. Chromatography of the residue afforded FMOC-
1
D-Phe ((R)-8) (0.047 g, 85% yield) as a white solid. The H NMR
and 13C NMR matched that of the commercial material. Subsequent
9338 J. Org. Chem. Vol. 73, No. 23, 2008

Synthesis of an Enantiomerically Pure FMOC R-Amino Acid
re-esterification with L-menthol was then used to determine the
stereochemical integrity of the amino acid (see below).
Acknowledgment. We thank John Dykins for high-resolution
mass spectra and Glen Yap for X-ray crystallography data. J.F.B.
thanks Paul H. Krolikowski of Merck & Co., Inc. for helpful
discussions. We thank the University of Delaware Honors
Program for partial support of this research.
Re-esterification of FMOC-D-Phenylalanine. L-Menthol (0.020
g, 0.129 mmol), commercial FMOC-D-phenylalanine ((R)-8) (0.050
g, 0.129 mmol), and DMAP (0.016 g, 0.129 mmol) were
combined in a round-bottomed flask along with 1 mL of dry
CH2Cl2. 1,3-Dicyclohexylcarbodiimide (DCC) (0.032 g, 0.154
mmol) in 0.5 mL of dry CH2Cl2 was added dropwise over 5
min at 0 °C. The solution was warmed to rt and stirred overnight.
The mixture was evaporated onto silica gel and chromatographed
to yield (R)-(1R,2S,5R)-2-isopropyl-5-methylcyclohexyl) 2-(((9H-
fluoren-9-yl)methoxy)carbonyl)-3-phenylpropanoate ((R)-7) (0.060
g, 88% yield) as a white solid. The 1H NMR and 13C NMR
matched that of the previously synthesized material.
Supporting Information Available: Experimental data and
1
13C NMR and H NMR spectra for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JO801781V
J. Org. Chem. Vol. 73, No. 23, 2008 9339