A highly stereospecific and efficient synthesis of homopentafluorophenylalanine

Article abstract of DOI:10.1021/jo049206z

A short and efficient synthesis of homopentafluorophenylalanine (6) from oxazolidine aldehyde 1 in 57% overall yield and in >98% ee is described. The enantiomeric excess of the product was determined by ¹⁹F NMR analysis of the coupling product derived from 5 and L-Ser(O-t-Bu)-OCH₃, by comparison to a dipeptide obtained from racemic 5.

Full text of DOI:10.1021/jo049206z

A High ly Ster eosp ecific a n d Efficien t
Syn th esis of Hom op en ta flu or o-
p h en yla la n in e
I. Ramesh Babu, Edward K. Hamill, and
Krishna Kumar*
Department of Chemistry, Tufts University,
Medford, Massachusetts 02155
F IGURE 1. Electrostatic potential surfaces for toluene (left)
and pentafluorotoluene (right): red codes for negative elec-
trostatic potential and blue codes for positive potential.
Calculations were carried out on fully geometry-optimized
structures using the 6-31G** basis set as described by Mecozzi
et al.10
krishna.kumar@tufts.edu
Received May 11, 2004
Abstr a ct: A short and efficient synthesis of homopentafluo-
rophenylalanine (6) from oxazolidine aldehyde 1 in 57%
overall yield and in >98% ee is described. The enantiomeric
excess of the product was determined by 19F NMR analysis
of the coupling product derived from 5 and L-Ser(O-t-Bu)-
OCH3, by comparison to a dipeptide obtained from racemic
5.
solvent water and are typically hydrophobic.7 Residues
at sites flanking the hydrophobic core, namely the e and
g positions, are frequently seen to form interhelical
electrostatic contacts, providing a secondary specificity
motif. Analysis of the distances between CR carbons of
the i and (i′ + 5) residues in coiled coil structures suggests
a range from 9.5 to 11 Å.8,9 This distance is readily
spanned by lysine and homophenylalanine (homoPhe)
residues to provide the appropriate geometry for inter-
helical π-cation interactions between the alkylammonium
group (Lys) and the center of the aromatic ring (ho-
moPhe). Substitution of the aromatic ring hydrogens with
fluorine atoms provides a nearly isosteric compound
differing only its ability to participiate in π-cation
interactions (Figure 1). Our design, therefore, required
large quantities of homopentafluorophenylalanine (homo-
Pf-Phe) in chiral form for solid-phase peptide synthesis.
Our synthesis of homo-Pf-Phe commenced from the
configurationally stable11 Garner aldehyde 1 which was
synthesized according to literature procedures (Scheme
1).12 The pentafluorobenzyl moiety was introduced by
Wittig olefination of aldehyde 1 with pentafluorobenzyl
bromide and potassium tert-butoxide as the base to yield
oxazolidine-olefin 2. The reaction produced exclusively
the trans-olefin 2 in 92% isolated yield.13 The open-chain
Fluorinated amino acids have recently emerged as an
important class of structural and functional building
blocks.1,2 A number of research groups3 including ours4
have recently described the successful incorporation of
fluorinated amino acids into peptides and proteins,
including the concomitant effects on structure and func-
tion of the resulting ensembles.
Our laboratory has reengineered model coiled coil
proteins to assess the effect of π-cation interactions in
determining the stability of these ubiquitous folds.5
Extensive statistical analysis of available protein struc-
tures reveals that π-cation interactions are manifested
in the pairing of the cationic side chains of lysine or
arginine with the aromatic amino acids phenylalanine,
tyrosine, or tryptophan.6
Coiled coils are superhelical ensembles consisting of
two or more R-helices. Their assembly is made possible
by a heptad repeat in the primary sequence (abcdefg)n,
where the a and d residues are largely shielded from
(1) (a) Yoder, N. C.; Kumar, K. Chem. Soc. Rev. 2002, 31, 335-341.
(b) Bilgic¸er, B.; Kumar, K. J . Chem. Educ. 2003, 80, 1275. (c) Marsh,
E. N. G. Chem. Biol. 2000, 7, R153-R157.
(7) (a) Lupas, A. Trends Biochem. Sci. 1996, 21, 375-382. (b) Crick,
F. H. C. Acta Cyrstallogr. 1953, 6, 689. (c) Harbury, P. B.; Kim, P. S.;
Alber, T. Nature 1994, 371, 80-83. (d) Harbury, P. B.; Zhang, T.; Kim,
P. S.; Alber, T. Science 1993, 262, 1401-1407. (e) O’Shea, E. K.; Lumb,
K. J .; Kim, P. S. Curr. Biol. 1993, 3, 658-667. (f) Burkhard, P.;
Strelkov, S. V.; Stetefeld, J . Trends Cell Biol. 2001, 11, 82-88. (g)
Kohn, W. D.; Hodges, R. S. Trends Biotechnol. 1998, 16, 379-389.
(8) Crystal structures of the GCN4 leucine zipper (PDB code: 2zta)
contain interhelical salt bridges between Glu20 and Lys15′ (d ) 9.63
Å) and between Glu22 and Lys27′ (d ) 10.4 Å).
(2) Xing, X.; Fichera, A.; Kumar, K. J . Org. Chem. 2002, 67, 1722-
25.
(3) (a) Tang, Y.; Ghirlanda, G.; Vaidehi, N.; Kua, J .; Mainz, D. T.;
Goddard, W. A.; DeGrado, W. F.; Tirrell, D. A. Biochemistry 2001, 40,
2790-2796. (b) Tang, Y.; Ghirlanda, G.; Petka, W. A.; Nakajima, T.;
DeGrado, W. F.; Tirrell, D. A. Angew. Chem., Int. Ed. 2001, 40, 1494-
+. (c) Tang, Y.; Tirrell, D. A. J . Am. Chem. Soc. 2001, 123, 11089-
11090. (d) Holmgren, S. K.; Taylor, K. M.; Bretscher, L. E.; Raines, R.
T. Nature 1998, 392, 666-667. (e) Bretscher, L. E.; J enkins, C. L.;
Taylor, K. M.; DeRider, M. L.; Raines, R. T. J . Am. Chem. Soc. 2001,
123, 777-778. (f) Renner, C.; Alefelder, S.; Bae, J . H.; Budisa, N.;
Huber, R.; Moroder, L. Angew. Chem., Int. Ed. 2001, 40, 923-925.
(4) (a) Bilgic¸er, B.; Fichera, A.; Kumar, K. J . Am. Chem. Soc. 2001,
123, 4393-4399. (b) Bilgic¸er, B.; Xing, X.; Kumar, K. J . Am. Chem.
Soc. 2001, 123, 11815-11816. (c) Bilgic¸er, B.; Kumar, K. Tetrahedron
2002, 58, 4105-4112.
(9) Lumb, K. J .; Kim, P. S. Science 1995, 268, 436-438.
(10) Mecozzi, S.; West, A. P., J r.; Dougherty, D. A. Proc. Natl. Acad.
Sci. U.S.A. 1996, 93, 10566.
(11) Campbell, A. D.; Raynham, T. M.; Taylor, R. J . K. Synthesis
1998, 1707-1709.
(12) (a) Garner, P.; Park, J . M. J . Org. Chem. 1987, 52, 2361-2364.
(b) Garner, P.; Park, J . M. J . Org. Chem. 1988, 53, 2979-2984. (c)
Garner, P.; Park, J . M.; Malecki, E. J . Org. Chem. 1988, 53, 4395-
4398. (d) Angrick, M. Montash. Chem. 1985, 116, 645-649.
(13) Configuration of the double bond was assigned on the basis of
the 1H-1H coupling constants of the vinylic protons in 2 (J ) 16.44
Hz) and 3 (J ) 16.50 Hz).
(5) Babu, I. R.; Hamill, E. K.; Kumar, K. Unpublished results.
(6) Gallivan, J . P.; Dougherty, D. A. Proc. Natl. Acad. Sci. U.S.A.
1999, 96, 9459-9464.
10.1021/jo049206z CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/13/2004
5468
J . Org. Chem. 2004, 69, 5468-5470

dipeptide 8 was analyzed using 19F NMR spectroscopy.
In the case of the dipeptide obtained from racemic 5,16
two different sets of 19F signals were observed, whereas
5 from the present synthesis yielded a dipeptide with
signals in the ratio 117:1 indicating an enantiomeric
excess of 98.3%.17
SCHEME 1a
In summary, we have developed a short and efficient
route for conversion of Garner aldehyde 1 to 2S-homo-
Pf-Phe in 57% overall yield. The construction of 2R-homo-
Pf-Phe is similarly achieved from the corresponding
oxazolidine aldehyde derived from L-serine. Studies
detailing the incorporation of 5 into peptides and its effect
on secondary and quaternary structure will be reported
shortly.
a
Reagents and conditions: (a) C6F5CHdPPh3, THF, -78 °C,
14 h, 92%; (b) (i) CH3COCl, CH3OH, 0 °C, 0.5 h, (ii) (t-Boc)2O, TEA,
CH2Cl2, 4 h, 98%; (c) Pd-black, EtOH, H2, 98%; (d) PDC, DMF, rt,
24 h, 64%; (e) HCl, CH3OH, rt, quant.
saturated alcohol 4 can, in principle, be obtained by
reduction of the oxazolidine-olefin 2, followed by ring
opening. However, attempts to hydrogenate the double
bond in the cyclic compound 2 using several hydrogena-
tion catalysts proved only marginally successful. We
suspected that the double bond might be inaccessible to
the catalyst surface because of geometric constraints
imposed by the oxazolidine ring. Thus, an alternative
strategy with ring opening followed by hydrogenation was
employed to obtain compound 4. Several literature
reports that describe the removal of acetonide protection
in oxazolidine compounds similar to 2 utilize methanol
and p-toluenesulfonic acid14 to accomplish solvolysis.
While this method generally yields the intact N-t-Boc
compound, with 2 the reaction resulted in incomplete
conversion with significant N-t-Boc deprotection, neces-
sitating chromatographic purification. Thus, the ac-
etonide in the oxazolidine-olefin was cleaved by HCl
generated in situ by using an acetyl chloride/methanol
couple, which also resulted in the removal of the t-Boc
group.15 The amino alcohol so obtained was t-Boc pro-
tected in a subsequent step in the same pot. This method
gave near-quantitative conversion of oxazolidine-olefin
2 to the t-Boc protected olefin-alcohol 3, which could be
used without further purification. Reduction of the double
bond in the olefin-alcohol 3 was achieved by catalytic
hydrogenation using Pd-black (10% by wt) in ethanol at
room temperature in 12 h (98%). The resulting alcohol 4
was subjected to oxidation using pyridinium dichromate
in DMF to deliver t-Boc-protected 2S-homo-Pf-Phe 5 in
64% yield, which was further processed to yield the
hydrochloride salt of homo-Pf-Phe (6).
Exp er im en ta l Section
(S,E)-ter t-Bu tyl 2,2-Dim eth yl-4-(p er flu or ostyr yl)oxa zo-
lid in e-3-ca r boxyla te (2). To a solution of PPh3 (3.15 g, 12
mmol) in dry THF (20 mL) pentafluorobenzyl bromide (3.13 g,
12 mmol) was added with a syringe in one portion. The mixture
was stirred for 15 min, the resulting solution of phosphonium
salt was taken into a syringe and added dropwise over a period
of 15 min to a flask containing potassium tert-butoxide (1.32 g,
11.7 mmol) in dry THF (15 mL) at 0 °C. The mixture was stirred
for 1.5 h at 0 °C following which the flask was cooled to -78 °C
in a dry ice-acetone bath. To this a solution of Garner aldehyde
1 (458 mg) in dry THF (10 mL) was added over a period of 15
min. The resulting pale yellow suspension was stirred for 6 h
at -78 °C followed by 8 h stirring at room temperature. THF
was removed under vacuum and the oily suspension obtained
was dissolved in Et2O and salts were removed by filtration. The
filtrate was concentrated to 30 mL to which 150 mL of hexane
was added to precipitate triphenylphosphine oxide that was
removed by filtration. The filtrate was concentrated and the
residue was subjected to flash chromatography with 5% EtOAc
in hexane as eluant to give oxazolidine-pentafluorophenyl-olefin
2 as a white solid (721 mg, 92%): 1H NMR (300 MHz, CDCl3) δ
6.50-6.37 (m 2H), 4.59-4.44 (2m 1H), 4.14 (dd, 1H, J ) 6.2 Hz,
9.0 Hz), 3.86 (dd, 1H, J ) 1.7 Hz, 9.1 Hz), 1.75-1.35 (m, 15H);
13C NMR (75.5 MHz, CDCl3) δ 151.9, 146.5, 143.1, 139.4, 138.3,
137.7, 136.1, 115.6, 112.0, 94.5, 80.7, 80.1, 74.5, 68.0, 60.0, 52.0,
28.5, 27.4, 26.7, 24.7, 23.7; 19F NMR (282.6 MHz, CDCl3) δ
-145.69 and -146.03 (2m, 2F), -159.17 and -159.63 (2m, 1F),
-165.87 and -166.38 (2m, 2F); mp 74 °C; [R]25 ) +66.6 (c 1,
D
CHCl3); ES HRMS m/z for C18H20NO3F5Na+ calcd 416.1261, obsd
416.1265. Anal. Calcd for C18H20F5NO3: C, 54.96; H, 5.12; N,
3.56; F, 24.15. Found: C, 55.02; H, 5.10; N, 3.44; F, 24.28.
(S,E)-ter t-Bu tyl 1-Hyd r oxy-4-(p er flu or op h en yl)bu t-3-en -
2-ylca r ba m a te (3). To methanol (6 mL) at 0 °C was added
acetyl chloride (0.6 mL, 8.4 mmol) with stirring. After 10 min,
olefin 2 (315 mg, 0.8 mmol) was added in one portion to this
solution. The reaction was stirred for 0.5 h. Removal of methanol
yielded a white solid. To this were added t-Boc2O (262 mg, 1.2
mmol), triethylamine (0.42 mL, 3 mmol), and methylene chloride
(5 mL), and the reaction was stirred at rt for 3 h. Methylene
chloride was removed under vacuum, and to the residue were
The optical purity of 6 was verified by coupling 5 to a
protected methyl ester of L-serine (7) and the resulting
(14) (a) Hu, X. E.; Kim, N. K.; Gray, J . L.; Almstead, J . I. K.; Seibel,
W. L.; Ledoussal, B. J . Med. Chem. 2003, 46, 3655-3661. (b) Ghosh,
A. K.; Bischoff, A.; Cappiello, J . Eur. J . Org. Chem. 2003, 5, 821-832.
(15) (a) Felpin, F. X.; Lebreton, J . Tetrahedron Lett. 2003, 44, 527-
530. (b) Muir, J . C.; Pattenden, G.; Ye, T. J . Chem. Soc., Perkin. Trans.
1 2002, 20, 2243-2250.
(16) The racemic sample was generated using the procedure de-
scribed in: Yamada, S.; Hongo, C.; Yoshioka, R.; Chibata, I. J . Org.
Chem. 1983, 48, 843-846.
(17) Xing, X.; Fichera, A.; Kumar, K. Org. Lett. 2001, 3, 1285-1286.
J . Org. Chem, Vol. 69, No. 16, 2004 5469

added ethyl acetate (60 mL) and brine (20 mL). The layers were
separated, and the organic layer was washed sequentially with
brine (20 mL × 2), 5% aq KHSO4 (20 mL × 2), and brine (20
mL × 2). The organic layer was dried over anhydrous Na2SO4
and the solvents removed to yield olefinol 3 (276 mg, 97.6%):
1H NMR (300 MHz, CDCl3) δ 6.60-6.54 (dd, 1H, J ) 0.9 Hz,
16.5 Hz), 6.53-6.48 (d, 1H, J ) 16.2 Hz), 5.03 (bm, 1H), 4.44
(bm, 1H), 3.80 (bm, 2H), 2.03 (bs, 1H), 1.47 (s, 9H); 13C NMR
(75.5 MHz, CDCl3) δ 155.9, 146.6, 143.4, 136.3, 116.1, 80.4, 65.1,
28.5, 27.9; 19F NMR (282.6 MHz, CDCl3) δ -145.83 (m, 2F),
-146.88 to -147.54 (m, 2F), -160.30 to -160.63 (m, 1F),
-165.64 to -166.01 (m, 2F); [R]25 ) +22.9 (c 1.87, CHCl3); ES
D
HRMS m/z calcd for C15H16NO4F5Na+ 392.0897, obsd 392.0893.
Anal. Calcd for C15H16F5NO4: C, 48.79; H, 4.37; N, 3.79, F, 25.72.
Found: C, 47.06; H, 4.26; N, 3.55; F, 24.55.
(S)-1-Ca r b oxy-3-(p er flu or op h en yl)p r op a n -1-a m in iu m
ch lor id e (6): 1H NMR (300 MHz, CD3OH) δ 4.05 (t, 1H, J )
6.19 Hz), 3.27 (m, 2H), 2.24-2.04 (m, 2H); 13C NMR (75.5 MHz,
CD3OH) δ 171.3, 148.3, 145.0, 143.1, 140.7, 137.4, 114.9, 53.5,
30.9, 19.4; 19F NMR (282.6 MHz, CD3OH) δ -147.35 (dd, 2F, J
-159.06 (t, 1F, J ) 20.6 Hz), -166.00 (m, 2F); mp 105 °C; [R]25
D
) 8 Hz, 22.2 Hz), -160.61 (t, 1F, J ) 20.3 Hz), -165.67 (m, 2F);
) +20.85 (c 0.84, CHCl3); ES HRMS m/z for C15H16NO3F5Na+
calcd 376.0948, obsd 376.0944. Anal. Calcd for C15H16F5NO3: C,
51.00; H, 4.56; N, 3.96; F, 26.89. Found: C, 50.97; H, 4.63; N,
3.82; F, 26.62.
+
[R]25 ) +31.3 (c 0.5, CH3OH); ESI-MS calcd for C10H9NO2F5
D
270.05, obsd 270.08. Anal. Calcd for C10H9ClF5NO2: C, 39.30;
H, 2.97; N, 4.58; F, 31.08. Found: C, 38.01; H, 3.05; N, 4.32; F,
29.26.
(S)-ter t-Bu tyl 1-Hyd r oxy-4-(p er flu or op h en yl)bu ta n -2-yl-
ca r ba m a te (4). To a solution of olefin 3 (247 mg, 0.7 mmol) in
5 mL of ethanol was added Pd-black (30 mg) and the suspension
stirred under H2 atmosphere (using a balloon) for 12 h. Upon
removal of Pd-black by filtration and concentration of the filtrate
under reduced pressure, compound 4 was obtained in pure form
(246 mg, 98%): 1H NMR (300 MHz, CDCl3) δ 4.74 (d, 1H, J )
5.1 Hz), 3.70-3.59 (m, 3H), 2.79 (t, 2H, J ) 8.2 Hz), 2.31 (bs,
1H), 1.82 (m, 1H), 1.67 (m, 1H), 1.52 (2s, 9H); 13C NMR (75.5
MHz, CDCl3) δ 156.5, 146.8, 143.6, 139.5, 136.0, 114.7, 80.1, 65.5,
52.4, 31.1, 28.5, 19.4; 19F NMR (282.6 MHz, CDCl3) δ -147.35
(dd, 2F, J ) 8 Hz, 22.2 Hz), -160.61 (t, 1F, J ) 20.3 Hz), -165.67
(m, 2F); mp 107 °C; ES HRMS calcd for C15H18NO3F5Na+
378.1105, obsd 378.1110; [R]25D ) +1.5 (c 1, CHCl3). Anal. Calcd
for C15H18F5NO3: C, 50.71; H, 5.11;, N, 3.94; F, 26.74. Found:
C, 50.55; H, 5.06; N, 3.89; F, 27.02.
(S)-2-(ter t-Bu toxyca r bon yl)-4-(p er flu or op h en yl)bu ta n o-
ic Acid (5). To a mixture of alcohol 4 (142 mg, 0.4 mmol) and
pyridinium dichromate (602 mg, 1.6 mmol) under argon was
added dry DMF (3 mL), and the mixture was stirred overnight
at rt. To the reaction mixture was added 50 mL of water, the
aqueous mixture was extracted with Et2O (50 mL × 3), and the
organic layers were dried over anhydrous Na2SO4. Ether was
removed, to the resulting residue was added 20 mL of 2 N NaOH,
and the mixture was then stirred for 15 min after which it was
washed with Et2O (20 mL × 3). The aqueous layer was acidified
to pH 2 with solid KHSO4 and was extracted with Et2O (20 mL
× 3). The crude product was purified by flash chromatography
(1:9 CH3OH/CH2Cl2 with 0.1% AcOH) and 5 recovered as a white
solid (95 mg, 64%): 1H NMR (300 MHz, CDCl3) δ 10.10 (bs, 1H),
7.04 (d, 0.5H, J ) 7 Hz), 5.17 (d, 0.5H, J ) 8.1 Hz), 4.40-4.22
(2m, 1H), 2.83-2.65 (m, 2H), 2.20 (m, 1H), 1.98 (m, 1H), 1.46
and 1.36 (2s, 9H); 13C NMR (75.5 MHz, CDCl3) δ 175.2, 174.3,
155.8, 154.8, 154.5, 145.7, 142.4, 138.1, 134.8, 112.6, 81.2, 79.6,
53.0, 51.9, 30.3, 27.2, 17.4; 19F NMR (282.6 MHz, CDCl3) δ
(S)-Meth yl 3-ter t-Bu toxy-2-((S)-2-(ter t-bu toxyca r bon yl)-
4-(p er flu or op h en yl)bu ta n a m id o)p r op a n oa te (8). To N-t-
Boc-homopentafluorophenylalanine (18.5 mg, 0.05 mmol), (O-
tert-butyl)-2S-serinemethylester hydrochloride (13 mg, 0.06
mmol), and HBTU (23 mg, 0.06 mmol) in dry DMF was added
0.031 mL of diisopropylethylamine, and the mixture was stirred
for 20 min. DMF was removed, and to the residue was added
10 mL of water followed by extraction with CH2Cl2. The organic
layers were pooled and washed sequentially with brine (10 mL),
5% aq KHSO4 (10 mL × 3), brine (10 mL), 5% aq Na2CO3 (10
mL × 3) ,and brine (10 mL). The organic layer was dried on
anhydrous Na2SO4, and the solvent was removed to yield the
dipeptide 8, which was directly used for further analysis: 1H
NMR (300 MHz) δ 6.89 (d, 1H, J ) 8.4 Hz), 5.41 (d, 1H, J )
9.45 Hz), 4.83 (m, 1H), 4.35 (m, 1H), 3.96 (dd, 1H, J ) 2.8 Hz,
9.12 Hz), 3.74 (s, 3H), 3.57 (dd, 1H, 3.0 Hz, 9 Hz), 2.80 (s, 2H),
2.22 (m, 1H), 1.87 (m, 1H), 1.58 and 1.56 (2s, 9H), 1.26 and 1.25
(2s, 9H); 13C NMR (75.5 MHz, CDCl3) δ 170.0, 169.5, 154.4,
145.7, 142.4, 140.4, 138.1, 137.1, 113.1, 79.1, 61.5, 60.6, 52.7,
51.9, 51.4, 38.8, 33.8, 27.2, 26.2, 17.5; 19F NMR (282.6 MHz,
CDCl3) δ -144.24 (dd, 2F, J ) 8.3 Hz, 22.4 Hz), -157.73 (t, 1F,
J ) 20.8 Hz), -162.95 (m, 2F).
Ack n ow led gm en t. We thank Sandro Mecozzi (Uni-
versity of Wisconsin, Madison) for calculation of the
electrostatic potential surfaces of toluene and pentafluo-
rotoluene. This work was supported in part by the
National Institutes of Health (GM65500) and the Na-
tional Science Foundation (CHE-0236846). K.K. is a
DuPont Young Investigator. E.K.H. was supported by
the 2003 Tufts Summer Scholars Program.
J O049206Z
5470 J . Org. Chem., Vol. 69, No. 16, 2004