An efficient strategy to orthogonally protected (R)- and (S)-α-methyl(alkyl)serine-containing peptides via a novel azlactone/oxazoline interconversion reaction

Article abstract of DOI:10.1021/jo952068g

A novel strategy for the synthesis of (R)- and (S)-α-methyl(alkyl)serine-containing peptides is presented. Using (S)-phenylalanine cyclohexylamide 6 as chiral auxiliary, the optically pure azlactones (R)- and (S)-2 were synthesized via a novel azlactone/oxazoline interconversion reaction (Figures 3 and 6). These azlactones constitute fully protected and activated synthetic equivalents of (R)- and (S)-α-methylserine and can be directly incorporated into peptides without further protective group manipulations. Like other α,α-dialkylated glycines, optically pure α-alkylserines can be used to stabilize β-turn and α-helical conformations in short peptides.

Full text of DOI:10.1021/jo952068g

4080
J . Org. Chem. 1996, 61, 4080-4086
An Efficien t Str a tegy to Or th ogon a lly P r otected (R)- a n d
(S)-r-Meth yl(a lk yl)ser in e-Con ta in in g P ep tid es via a Novel
Azla cton e/Oxa zolin e In ter con ver sion Rea ction †
Daniel Obrecht,* Michael Altorfer,‡ Christian Lehmann, Peter Scho¨nholzer, and Klaus Mu¨ller
Pharma Research, F. Hoffmann-La Roche AG, CH-4070 Basel, Switzerland
Received November 22, 1995X
A novel strategy for the synthesis of (R)- and (S)-R-methyl(alkyl)serine-containing peptides is
presented. Using (S)-phenylalanine cyclohexylamide 6 as chiral auxiliary, the optically pure
azlactones (R)- and (S)-2 were synthesized via a novel azlactone/oxazoline interconversion reaction
(Figures 3 and 6). These azlactones constitute fully protected and activated synthetic equivalents
of (R)- and (S)-R-methylserine and can be directly incorporated into peptides without further
protective group manipulations. Like other R,R-dialkylated glycines, optically pure R-alkylserines
can be used to stabilize â-turn and R-helical conformations in short peptides.
1. In tr od u ction
investigating the conformational properties of (R)- and
(S)-R-methylserine 1 (Figure 1) and analogues since the
conformational properties of R,R-disubstituted glycines
bearing functionalized and polar side chains have been
much less investigated.
Interestingly, calculated Ramachandran plots20,21 of
(R)- and (S)-N-acetyl-R-methylserine N-methylamide (Fig-
ure 2) revealed a strong preference for R-helical confor-
mations, similarly to R-aminoisobutyric acid (AIB).22
In addition, it was known from screening the Protein
Crystal Structure Data Base,23 that (S)-serine is rela-
tively often found in the last C-terminal turn of R-helices.
In this position, the side chain hydroxyl group of serine
can be engaged in a H-bond with an amide carbonyl
group in the preceding turn.24
In recent years, non-proteinogenic amino acids have
been the focus of many investigations.1-3 Among the
nonstandard amino acids, the R,R-dialkylated glycines
are especially interesting, because they can stabilize
short peptides in rather well defined conformations, such
as â-turns,4-9 310- and R-helical10-14 and extended9,15-18
conformations, depending on the nature of the R-substit-
uents.
We have developed a general method for the prepara-
tion of a wide range of cyclic and open-chain R,R-
disubstituted glycines, and we have studied the confor-
mational properties of such building blocks when in-
corporated into small peptides.6-10 These building blocks,
together with other templates, were used in a program
with the aim to mimic exposed epitopes of structurally
known proteins with conformationally constrained pep-
tides.19 As part of this program we were interested in
For these structural aspects and for reasons of in-
creased chemical stability we were interested in studying
the conformational properties of (R)- and (S)-R-meth-
ylserine 1 as a potential C-terminal R-helix stabilizing
building block.
* To whom inquiries should be addressed.
‡ Part of the Ph.D. thesis of M. Altorfer, University of Zurich, 1996.
† Presented in part at the annual meeting of the New Swiss
Chemical Society, Abstract published in Chimia 1994, 48, 276.
X Abstract published in Advance ACS Abstracts, May 15, 1996.
(1) Williams, R. M. Synthesis of optically active R-amino acids;
Organic Chemistry Series; Pergamon: New York, 1989.
(2) Duthaler, R. O. Tetrahedron 1994, 50, 1540-1650.
(3) Ottenheijm, H. C. J . Chimia 1985, 39, 89.
In this paper we present a novel synthetic strategy
featuring the (R)- and (S)-4-(bromomethyl)-4-methyl-2-
phenyloxazol-5(4H)-ones (“azlactones”) 2 (Figure 3) as
fully protected and activated analogues of (R)- and (S)-
1, ready for incorporation into peptides.
(4) Wipf, P.; Kunz, R. W.; Prewo, R.; Heimgartner, H. Helv. Chim.
Acta 1988, 71, 268-273.
(5) Wipf, P.; Heimgartner, H. Helv. Chim. Acta 1988, 71, 258-267.
(6) Obrecht, D.; Bohdal, U.; Broger, C.; Bur, D.; Lehmann, C.;
Ruffieux, R.; Scho¨nholzer, P.; Spiegler, C.; Mu¨ller, K. Helv. Chim. Acta
1995, 78, 563-580.
2. Syn th esis of th e Op tica lly P u r e Azla cton es (R)-
a n d (S)-2
Several publications dealing with the asymmetric
synthesis of fully orthogonally protected R-methylserine
derivatives appeared in the literature.25-35 Much less is
(7) Obrecht, D.; Karajiannis, H.; Lehmann, C.; Scho¨nholzer, P.;
Spiegler, C.; Mu¨ller, K. Helv. Chim. Acta 1995, 78, 703-714.
(8) Obrecht, D.; Bohdal, U,; Lehmann, C.; Scho¨nholzer, P.; Mu¨ller,
K. Tetrahedron 1995, 51, 10883-10900.
(19) Mu¨ller, K.; Obrecht, D.; Knierzinger, A.; Stankovic, C.; Spiegler,
C.; Bannwarth, W.; Trzeciak, A.; Englert, G.; Labhardt, A. M.;
Scho¨nholzer, P. Perspectives in Medicinal Chemistry; Testa, B., Kyburz,
E., Fuhrer, W., Giger, R., Eds.; Helvetica Chimica Acta: Basel, 1993;
Chapter 33, pp 513-531.
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K. Helv. Chim. Acta 1995, 78, 1567-1587.
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Blasio, B.; Grimaldi, P.; Lelj, F.; Pavone, V.; Pedone, C.; Bonora, G.
M.; Lingham, I. J . Am. Chem. Soc. 1984, 106, 8146-8152.
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Pavone, V.; Pedone, C. Biopolymers 1985, 24, 1759.
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Barone, V.; Fraternali, F.; Hardy, P. M.; Maia, H. L. S. Biopolymers
1991, 31, 637.
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(23) Cambridge Structural Database, Version 5.06, October 1993.
(24) Richardson, J . S. Biophysik. J . 1992, 63, 1186-1209.
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yashi, T. Tetrahedron Lett. 1988, 29, 235-238.
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S0022-3263(95)02068-8 CCC: $12.00 © 1996 American Chemical Society

(R)- and (S)-R-Methyl(alkyl)serine-Containing Peptides
J . Org. Chem., Vol. 61, No. 12, 1996 4081
Sch em e 1
F igu r e 1.
F igu r e 2. Ramachandran plot of N-Ac-(S)-R-Me-Ser(OH)-
NHMe. Contour spacing: 0.25 kcal/mol. Contour lines (Φ/Ψ):
min 7.57 kcal/mol (Φ ) +61°/Ψ ) +40°), max 9.82 kcal/mol
(Φ ) 0°/Ψ ) 0°).
iodomethylated with CH2I2, NaH as base in DMF/THF
(1:1), giving rise to the key precursor (rac)-5a -d in
medium to good overall yields (40-75%).38
The racemic 4-(iodomethyl)-4-methyl-2-phenyloxazol-
5(4H)-one 5a was further treated in analogy to our
previously reported method10 with the chiral auxiliary
(S)-Phe-cyclohexylamide 6 in N-methylpyrrolidone (NMP)
and diisopropylethylamine (DIPEA) at 55 °C to form the
easily separable diastereomeric oxazolines 7 and 8 in 46%
and 48% isolated yield, respectively, after separation by
flash chromatography (FC).39 The absolute configura-
tions of dipeptides 7 and 8 were unambiguously deter-
mined by a crystal structure of 8 (Figure 4).51
F igu r e 3. Fully protected and activated analogues of (R)- and
(S)-R-methylserine.
Treatment of 7 and 8 with 33% HBr in acetic acid and
acetic anhydride, to assure anhydrous conditions, gave
after recovery of 6‚HBr (85% yield, >99% optical purity)
the two optically pure azlactones (R)- and (S)-2 in 70-
80% isolated yield after FC.
known about the incorporation of these building blocks
into peptides and the conformational behavior of (R)- and
(S)-R-methylserine-containing peptides.36,37
Our synthetic strategy starts from the corresponding
commercially available racemic N-benzoylated amino
acids 3a -d as outlined in Scheme 1.
After cyclization of rac-3a -d , using N,N-dicyclohexyl-
carbodiimide (DCC) in CH2Cl2, the intermediate racemic
4-monosubstituted-2-phenyl-oxazol-5(4H)-ones 4a-d were
In summary these azlactones (R)- and (S)-2, which
constitute fully orthogonally protected and activated
analogues of (R)- and (S)-R-methylserine 1, were obtained
in four steps (including the separation of the diastereo-
mers 7 and 8) in an overall yield of roughly 30% (Scheme
1).
(29) Fukuyama, T.; Xu, L. J . Am. Chem. Soc. 1993, 115, 8449-8450.
(30) Seebach, D.; Aebi, J . D.; Gander-Goquoz, M.; Neaf, R. Helv.
Chim. Acta 1987, 70, 1192-1216.
3. Hyd r olytic Oxa zolin e Rin g Op en in gs
(31) Zembower, D. E.; Gilbert, J . A.; Ames, M. M. J . Med. Chem.
1993, 36, 305-313.
In order to test the selective hydrolytic cleavage of the
oxazoline moiety,40 we treated the diastereomeric oxazo-
lines 7 and 8 separately with 2 N aqueous HCl in dioxane
at room temperature giving cleanly the intermediate free
amines (R/ S)-9 and (S/ S)-10, after neutralization of the
(32) Scho¨llkopf, U.; Hartwig, W.; Groth, U.; Westphalen, K. O.
Liebigs Ann. Chem. 1981, 696-708.
(33) Scho¨llkopf, U.; Groth, U.; Hartwig, W. Liebigs Ann. Chem. 1981,
2407-2418.
(34) Coussedie`re, D.; Gigliotti, G.; Moguilewski, M. Appl. EP 305,
270, 1989.
(35) Wipf, P.; Venkatraman, S.; Miller, C. P. Tetrahedron Lett. 1995,
36, 3639-3642.
(36) Altmann, E.; Altmann, K. H.; Nebel, K.; Mutter, M. Int. J .
Peptide Protein Res. 1988, 32, 344-351.
(38) Obrecht, D.; Bohdal, U.; Ruffieux, R.; Mu¨ller, K. Helv. Chim.
Acta 1994, 77, 1423.
(39) Still, W. C.; Kahn, M.; Mitra, A. J . Org. Chem. 1978, 43, 2923.
(40) Meyers, A. I.; Schmidt, W.; McKennon, M. J . Synthesis 1993,
250-262.
(37) Pavone, V.; Di Blasio, B.; Lombardi, A.; Maglio, O.; Isernia, C.;
Pedone, C.; Benedetti, E.; Altmann, E.; Mutter, M. Int. J . Pept. Protein
Res. 1993, 41, 15-20.

4082 J . Org. Chem., Vol. 61, No. 12, 1996
Obrecht et al.
F igu r e 4. ORTEP diagram of 8.
F igu r e 5. ORTEP diagram of 11.
Sch em e 2
Sch em e 3
acidic reaction mixture with aqueous NaHCO3 and
extraction with CHCl3 or EtOAc (Scheme 2).
The amines (R/ S)-9 and (S/ S)-10 were immediately
N-benzoylated with benzoyl chloride and DIPEA in CH2-
Cl2 at room temperature affording the N,O-dibenzoylated
dipeptides (R/ S)-11 and (S/ S)-12 in high yield. The
absolute configurations of 11 and 12 were reconfirmed
by a crystal structure of 11 (Figure 5).51
The interesting â-turn I conformation observed in the
X-ray structure of 11 will be discussed in more detail in
section 6. It is important to note that under the reaction
conditions we did not observe an O f N-benzoyl-shift in
the intermediate amines 9 and 10, which is an important
prerequisite for our strategy to incorporate the key
azlactones (R)- and (S)-2 into peptides without further
protective group manipulations.
ramethyluronium tetrafluoroborate (TATU)41 and 1-hy-
droxy-7-azabenzotriazole (HOAT) as coupling reagents
to give the fully protected tripeptides 17 and 18 in 81%
and 74% yield, respectively. The highly efficient coupling
reagent TATU/HOAT developed recently by Carpino
works especially well for the coupling of sterically hin-
dered amines as noted also by Rich.42
In peptides 17 and 18 the O-benzoyl group is stable
towards the hydrogenation conditions (Pd black, EtOH,
H2) to liberate the free C-terminus or to the acidic
conditions to remove the Boc-group (CF3COOH, CH2Cl2)
to liberate the N-terminus. In addition, treatment of 17
and 18 with NaCN in BnOH at 50 °C cleanly liberates
4. In cor p or a tion of Azla cton es (R)- a n d (S)-2 in to
Tr ip ep tid es
As outlined in Scheme 3 we have efficiently coupled
the azlactones (R)- and (S)-2 with (S)-alanine benzyl ester
hydrochloride in the presence of DIPEA in NMP at 50
°C to yield the corresponding oxazoline derivatives (R/
S)-13 and (S/ S)-14 in 85% and 92% isolated yield,
respectively.
After hydrolysis of 13 and 14 using 2 N aqueous HCl
in dioxane at room temperature, neutralization of the
reaction mixture with NaHCO3 and extraction with
CHCl3 or EtOAc, the free amines (R/ S)-15 and (S/ S)-
16 were immediately coupled to Boc-(S)-Ala-OH in DMF
and DIPEA using O-(7-azabenzotriazol-1-yl)-1,1,3,3-tet-
(41) Carpino, L. A. J . Am. Chem. Soc. 1993, 115, 4397-4398.
(42) Angell, Y. M.; Garcia-Echeverria, C.; Rich, D. H. Tetrahedron
Lett. 1994, 35, 5981.

(R)- and (S)-R-Methyl(alkyl)serine-Containing Peptides
J . Org. Chem., Vol. 61, No. 12, 1996 4083
Sch em e 6. P u ta tive Mech a n ism for th e Oxa zolin e/
Azla cton e In ter con ver sion
F igu r e 6. Azlactone/oxazoline interconversion.
Sch em e 4
Sch em e 5. P u ta tive Mech a n ism for th e Azla cton e/
Oxa zolin e In ter con ver sion
of N(3). After ring opening and formation of III, the
N-benzoyl group displaces, in an intramolecular base-
assisted SN2-reaction, the side chain bromo group to form
the stable oxazoline ring moiety, similar to the formation
of oxazolines starting from â-halo serine amides.44
The oxazoline/azlactone interconversion (II f I, Figure
6) is depicted in Scheme 6 for the transformation of 7
into (S)-2.
Under the strongly acidic reaction conditions the
oxazoline ring is protonated at N(3) and presumably
opened by bromine attack at C(5) forming the linear
dipeptide intermediate IV (in analogy to reference45 ). As
previously shown,6,10 dipeptides of type IV are selectively
cleaved forming 6‚HBr, which can be recovered, and (S)-
2. The driving force for this selective amide cleavage and
for the preferred formation of the 4,4-disubstituted
azlactone (S)-2, can be attributed to a strong “gem-dialkyl
effect” as shown earlier.46
the free serine hydroxyl group by transesterification
(compare reference43 ) without modification of the C- and
N-terminal protective groups. These experiments dem-
onstrate the orthogonality of the three protective groups
in the peptides 17 and 18 and confirm our strategy to
use building blocks (R)- and (S)-2 as fully protected and
activated R-methylserine analogues.
These building blocks were also converted successfully
into the oxazoline methyl esters (R)- and (S)-19 as shown
in Scheme 4, which after reduction with LiAlH4 in THF
gave the corresponding oxazoline alcohols (S)- and (R)-
20 in high yields.
6. Discu ssion of th e Cr ysta l Str u ctu r es
The peptide backbone conformations of the compounds
8 and 11 in the crystalline state (cf. stereo plots in
Figures 4 and 5) exhibit some interesting aspects of
peptide folding.
Whereas the oxazoline derivative 8, due to the blocked
N-terminal amide group, is not capable of forming a
â-turn, the open chain derivative 11 exhibits a nearly
perfect â-turn type I47,48 conformation as outlined in
Figure 7. The former extended structure 8 is found in a
conformational space region populated by 1% of turn
types in the aforementioned representative protein da-
tabase extract. Furthermore both structures have their
phenylalanine side chain in a (-) synclinal orientation,
allowing the central amide group to be engaged in
intermolecular hydrogen bonding as observed in the
crystal packing of the structures. The torsionally unre-
stricted O-benzoyl hydroxymethyl group in 11 is oriented
in a (+) synclinal conformation. This is a nice example
These alcohols constitute versatile optically pure tri-
functional building blocks for synthesis of a range of (R)-
and (S)-R-methylserine analogues.
5. Mech a n istic Con sid er a tion s
The synthesis of the key building blocks (R)- and (S)-2
and their integration into the fully protected tripeptides
17 and 18 (Scheme 3) take advantage of a novel direct
azlactone/oxazoline interconversion (I T II, Figure 6).
In Scheme 5 we describe a putative mechanism for the
azlactone/oxazoline interconversion (I f II, Figure 6) as
shown for the conversion of (S)-2 f (R)-19. In a first
step we anticipate an attack of the nucleophile to the
activated azlactone carboxyl group C(5) by protonation
(44) Heine, H. W. J . Am. Chem. Soc. 1956, 78, 3708.
(45) Stammer, C. H.; Wilson, A. N.; Spencer, C. F.; Bachelor, F. W.;
Holly, F. W.; Folkers, K. J . Am. Chem. Soc. 1957, 79, 3236.
(46) Stierli, F.; Obrecht, D.; Heimgartner, H. Chimia 1984, 38, 432.
(47) Wilmot, C. M.; Thornton, J . M. J . Mol. Biol. 1988, 203, 221.
(48) Wilmot, C. M.; Thornton, J . M. Protein Eng. 1990, 3, 479.
(43) Herzig, J .; Nudelman, A.; Gottlieb, H. E.; Fischer, B. J . Org.
Chem. 1986, 51, 727-730.

4084 J . Org. Chem., Vol. 61, No. 12, 1996
Obrecht et al.
F igu r e 7.
of the gauche effect49 showing a preferentially staggered
orientation of the more electronegative carbon hetero-
atom bonds.
r a c-4-Meth yl-2-ph en yl-4H-oxazol-5-on e (4a). To a stirred
suspension of rac-N-benzoyl-Ala-OH 3 (10.0 g, 51.7 mmol) in
CH2Cl2 (150 mL) at 0 °C and under Ar was added DCC (11.1
g, 53.8 mmol) in small portions, so that the reaction temper-
ature did not exceed 5 °C. The reaction mixture was stirred
at 0 °C for 30 min and at rt for 40 min and filtered, and the
solvent was evaporated. The residue was triturated in Et2O
(200 mL) and filtered. Evaporation of the solvent yielded 9.1
g (quantitative) of 4a as a colorless oil that was used in the
next step without further purification.
7. Su m m a r y a n d Ou tlook
Starting from commercial rac-N-benzoyl-Ala-OH the
key (R)- and (S)-4-(bromomethyl)-4-methyl-2-phenylox-
azol-5(4H)-ones 2 were efficiently prepared in optically
pure form (Scheme 1).
Using a series of azlactone/oxazoline interconversions
(Figure 6), these building blocks were incorporated
without additional protective group manipulations into
the tripeptides 17 and 18. It is noteworthy, that after
the hydrolytic cleavage of the oxazolines 7 and 8 (Scheme
2), 13 and 14 (Scheme 3) no O f N-benzoyl-shift was
observed under the reaction conditions employed for the
subsequent acylation of the intermediate amines 9, 10,
15, and 16. This prerequisite in combination with the
novel direct azlactone/oxazoline interconversion forms the
cornerstones of our strategy to synthesize and incorporate
the fully protected and activated R-methylserine ana-
logues (R)- and (S)-2 into tripeptides 17 and 18 (Scheme
3).
As indicated in Scheme 2, our strategy should not be
limited to the (R)- and (S)-R-methylserine 1, since the
R-iodomethylation of the 4-monosubstituted-2-phenylox-
azol-5(4H)-ones 4b-d 38 proceeded smoothly to give the
racemic 4-iodomethylated azlactones 5b-d in high yields.
These azlactones 5b-d correspond to the precursors of
the “R-chimeras”6 (R,R-disubstituted glycines combining
two side chains of proteinogenic amino acids at the
R-center) of Ser-Phe 5b, Ser-Val 5c, and Ser-Leu 5d .
Work to synthesize the corresponding optically pure
azlactones (R)- and (S)-2b-d is in progress and will be
reported in due course.
r a c-4-(Iod om e t h yl)-4-m e t h yl-2-p h e n yl-4H -oxa zol-5-
on e (5a ). To a stirred solution of 4a (9.1 g, 52 mmol) and
CH2I2 (20.9 mL, 0.26 mol) in dry THF/DMF (1:1, 300 mL) at
0 °C and under Ar, was added NaH dispersion (60%, 2.48 g,
62 mmol) in small portions so that the reaction temperature
did not exceed 15 °C. The reaction mixture was stirred at 0
°C for 30 min and poured onto ice, 5% aqueous NaH2PO4 (200
mL), and CH2Cl2 (200 mL). The organic layer was extracted
with H2O (200 mL) and saturated brine (200 mL) and dried
(Na2SO4), and the solvents were evaporated. The residue was
chromatographed on SiO2 (500 g) with hexane/EtOAc (10:1-
7:1) to yield 5.5 g (34%) of 5a as white crystals. Mp 98-100
°C. IR (KBr): 3435w, 3064w, 3028w, 2969w 1814s, 1649s,
1577w, 1450m, 1317m, 1293m, 1229m, 1006s, 929m, 884m,
694s. 1H-NMR (CDCl3, 250 MHz): 8.1-8.0 (m, 2H); 7.65-
7.45 (m, 3H); 3.57, 3.50 (2d, J ) 10, 2H); 1.70 (s, 3H). MS
(EI): 315 (3, (M)), 188 (35), 174 (12), 105 (100), 77 (44), 51
(16).
(R)-4-Met h yl-2-p h en yl-4,5-d ih yd r ooxa zole-4-ca r b ox-
ylic Acid (S)-[1-(cycloh exylca r ba m oyl)-2-p h en yleth yl]-
a m id e (7) a n d (S)-4-Meth yl-2-p h en yl-4,5-d ih yd r ooxa zole-
4-ca r boxylic Acid (S)-[1-(cycloh exylca r ba m oyl)-2-p h en -
yleth yl]a m id e (8). A mixture of azlactone 5a (1.65 g, 5.24
mmol), 6 (1.99 g, 8.1 mmol) and DIPEA (1.85 mL, 10.48 mmol)
in NMP (15 mL) was stirred for 18 h at 55 °C, cooled to rt,
and poured onto H2O (50 mL) and EtOAc (80 mL). The organic
fraction was extracted with H2O (2 × 30 mL) and dried
(MgSO4), and the solvent was evaporated. The residue was
chromatographed on SiO2 (300 g) with EtOAc/hexane (1:2-2:
3) to give first, after crystallisation from Et2O/hexane, 1.05 g
(46%) of 7 as a white solid. Mp 137-138 °C. [R]D ) +47 (c )
0.2, CHCl3). IR (KBr): 3376w, 3302m, 3062w, 3029w, 2930m,
2853w, 1645s, 1521s, 1495m, 1451m, 1352m, 1301w, 1063m,
696s. 1H-NMR (250 MHz, CDCl3): 8.0-7.9 (m, 2H); 7.6-7.35
(m, 4H); 7.35-7.2 (m, 5H); 5.38 (d (br), 1H); 4.61, 4.23 (2d, J
) 8.5, 2H); 4.55-4.45 (m, 1H); 3.7-3.55 (m, 1H); 3.2-3.0 (m,
2H); 1.75-1.4 (m, 5H); 1.49 (s, 3H); 1.35-0.75 (m, 5H). MS
(EI): 433 ((M),1), 307 (16), 273 (8), 160 (100), 132 (32), 105
(35), 104 (44), 91 (18), 77 (32), 55(28), 41 (28).
Exp er im en ta l Section
Gen er a l. See ref 6. Optical rotation measurements were
made at 20 °C, and the values are expressed as specific rotation
[R]D in angular degrees. Melting points are given in °C.
(49) Wolfe, S. Acc. Chem. Res. 1972, 5, 102.
(50) IUPAC-IUB Commission on Biochemical Nomenclature. Bio-
chemistry 1970, 9, 3471.
(51) The author has deposited atomic coordinates for this structure
with the Cambridge Crystallographic Data Centre. The coordinates
can be obtained, on request, from the Director, Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK.
Further elution yielded, after recrystallisation from Et2O/
hexane, 1.10 g (48.%) of 8 as a white solid. Mp 121.5-123.0

(R)- and (S)-R-Methyl(alkyl)serine-Containing Peptides
J . Org. Chem., Vol. 61, No. 12, 1996 4085
°C. [R]D ) -55 (c ) 0.2, CHCl3). IR (KBr): 3381w, 3292m,
3060w, 2932m, 2855w, 1676s, 1644s, 1533s, 1450w, 1353w,
1301w, 1062m, 697s. 1H-NMR (250 MHz, CDCl3): 8.0-7.9 (m,
2H); 7.6-7.45 (m, 3H); 7.25 (d (br), 1H); 7.09 (s, 5H); 5.69 (d
(br), 1H); 4.6-4.45 (m, 1H); 4.48, 4.19 (2d, J ) 8.5, 2H); 3.8-
3.65 (m, 1H); 3.15-2.95 (m, 2H); 1.95-1.5 (m, 5H); 1.55 (s, 3H);
1.45-0.9 (m, 5H). MS (EI): 433 ((M),1), 307 (17), 273 (7), 160
(100), 132 (27), 105 (34), 104 (41), 91 (16), 77 (39), 55(26), 41
(23).
(S)-4-(Br om om et h yl)-4-m et h yl-2-p h en yl-4H -oxa zol-5-
on e [(S)-2]. A solution of 7 (400 mg, 0.92 mmol) in HBr/AcOH
(33%, 2 mL) and Ac2O (0.8 mL) was stirred in a sealed tube at
70 °C for 4.5 h. The solvents were removed under high
vacuum, and the residue was chromatographed on SiO2 (15
g) with hexane/EtOAc (3:1) to yield 175 mg (71%) of (S)-2 as
colorless oil. [R]D ) -16 (CHCl3, c ) 0.1). IR (film) 3065w,
3035w, 2984w, 2934w, 1824s, 1655s, 1580w, 1493w, 1451m,
1321m, 1293s, 1269w, 1244w, 1131w, 1094m, 1007s, 934m,
887m, 781m, 696s. 1H-NMR (CDCl3, 250 MHz): 8.1-8.0 (m,
2H); 7.65-7.45 (m, 3H); 3.75, 3.69 (2d, J ) 10, 2H); 1.66 (s,
3H). MS (EI): 269, 267 (8, (M)), 174 (25), 144 (10), 105 (100),
104 (30), 77 (35), 51 (10).
(R)-4-(Br om om et h yl)-4-m et h yl-2-p h en yl-4H-oxa zol-5-
on e [(R)-2]. To a stirred mixture of 8 (500 mg, 1.15 mmol) in
33% HBr/AcOH (2 mL) in a pyrolysis tube at 0 °C under Ar
was added acetic anhydride (0.8 mL). The reaction mixture
was stirred for 5 h at 80 °C and cooled to rt, and the solvents
were removed under reduced pressure. The residue was dried
over K2CO3 in a desiccator and chromatographed on SiO2 (40
g) with EtOAc/hexane (1:4) to yield, after drying under reduced
pressure, 231 mg (75%) of (R)-2 as a colorless oil. [R]D ) +17
(c ) 0.15, CHCl3). IR, MS, and NMR spectra in close
agreement to (S)-2.
(R)-2-(Ben zoyla m in o)-2-[[(S)-1-(cycloh exylca r ba m oyl)-
2-p h en yleth yl]ca r ba m oyl]p r op yl Ben zoa te (11). To a
stirred solution of 7 (910 mg, 2.10 mmol) in dioxane (3 mL) at
0 °C was added 2 N aqueous HCl (3 mL). The reaction mixture
was stirred for 2h at rt and poured onto ice, saturated aqueous
NaHCO3 (15 mL), and CHCl3 (15 mL). The organic layer was
dried (MgSO4) and the solvent evaporated. After drying under
high vacuum overnight, the residue was dissolved in NMP (4
mL), and DIPEA (0.72 mL, 4.2 mmol) and benzoyl chloride
(0.37 mL, 3.15 mmol) were added at 0 °C. The reaction
mixture was stirred at rt for 4 h and poured onto ice, 2 N
aqueous HCl (10 mL), and EtOAc (20 mL). The aqueous layer
was extracted with EtOAc (2 × 10 mL), the combined organic
fractions were dried (MgSO4), and the solvents were evapo-
rated. The residue was recrystallized from EtOAc/hexane to
yield 1.06 g (91%) of 11 as a white solid. Mp 202.5-204.0 °C.
[R]D ) +10 (c ) 0.2, CHCl3). IR (KBr): 3359m, 3324m, 3065w,
2935w, 2854w, 1704s, 1659s, 1641s, 1581w, 1539m, 1366w,
1280s, 1258m, 1119w, 722m, 695w. 1H-NMR (250 MHz,
CDCl3): 8.0-7.9 (m, 2H); 7.75-7.65 (m, 2H); 7.6-7.5 (m, 2H);
7.5-7.35 (m, 4H); 7.25-7.1 (m, 5H); 7.10 (s (br), 1H); 6.64 (d
(br), J ) 8, 1H); 6.25 (d (br), J ) 8, 1H); 4.91, 4.67 (2d, J ) 12,
2H); 4.65-4.55 (m, 1H); 3.8-3.6 (m, 1H); 3.3-3.0 (m, 2H);
1.85-1.5 (m, 5H); 1.62 (s, 3H); 1.4-1.0 (m, 5H). MS (ISP):
578.6 ((M + Na)+, 50), 556.6 ((M + H)+, 100).
°C under Ar was added DIPEA (0.56 mL, 3.25 mmol). The
reaction mixture was stirred at 50 °C overnight and poured
onto ice, H2O (30 mL), and EtOAc (30 mL). The organic layer
was extracted with saturated brine (30 mL) and dried (MgSO4),
and the solvent was evaporated. The residue was chromato-
graphed on SiO2 (15 g) with hexane/EtOAc (2:1) to yield 203
mg (85%) of 13 as a colorless oil. [R]D ) +65 (CHCl3, c ) 0.08).
IR (KBr): 3385w, 2979w, 2930w, 1742s, 1677s, 1644m, 1512m,
1451m, 1354w, 1298w, 1258w, 1199m, 1171m, 1139w, 1064m,
1026w, 967w, 750w, 697s. 1H-NMR (CDCl3, 250 MHz): 8.0-
7.9 (m, 2H); 7.55-7.35 (m, 3H); 7.3-7.2 (m, 6H); 5.17, 5.09
(2d, J ) 12, 2H); 4.61, 4.25 (2d, J ) 9, 2H); 4.7-4.55 (m, 1H);
1.58 (s, CH3); 1.47 (d, J ) 7, 3H). MS (ISP): 367 (100, [M +
H]+).
Ben zyl (S)-2-[(S)-4-Meth yl-2-p h en yl-4,5-d ih yd r ooxa zol-
4-yl]ca r bon yl]a m in o]p r op ion a te (14). To a stirred solution
of (R)-2 (220 mg, 0.82 mmol) and H-(S)-Ala-OBn‚HCl (354 mg,
1.64 mmol) in DMA (3 mL) was added under Ar at 0 °C DIPEA
(0.70 mL, 4.1 mmol). The reaction mixture was stirred for 17
h at 50 °C, cooled to rt, and poured onto ice, H2O (40 mL), and
EtOAc (40 mL). The organic layer was extracted with satu-
rated brine (40 mL) and dried (MgSO4), and the solvents were
evaporated. The residue was chromatographed on SiO2 (25g)
with EtOAc/hexane (1:2) to yield 275 mg (92%) of 14 as a white
foam. [R]D ) -26.0 (c ) 0.2, CHCl3). IR (KBr): 3380w ,
3060w, 3025w, 2975w, 1742s, 1676s, 1644m, 1513m, 1353w,
1298w, 1200w, 1171w, 1065m, 697s. 1H-NMR (250 MHz,
CDCl3): 8.05-7.95 (m, 2H); 7.6-7.3 (m, 8H); 7.23 (br d, J )
7.5, 1H); 5.23, 5.16 (2d, J ) 12.0, 2H); 4.65, 4.25 (2d, J ) 8.0,
2H); 4.7-4.55 (m, 1H); 1.56 (s, 3H); 1.41 (d, J ) 7.5, 3H). MS
(ISP): 367.4 ((M + H)+, 100).
(R)-2-[[(S)-1-(Ben zyloxyca r bon yl)eth yl]ca r ba m oyl]-2-
[[(S)-2-(ter t-b u t oxyca r b on yla m in o)p r op ion yl]a m in o]-
p r op yl Ben zoa te (17). To a stirred solution of 13 (200 mg,
0.55 mmol) in dioxane (3 mL) at 5 °C under Ar was added 0.5
N aqueous HCl (2.2 mL). The reaction mixture was stirred
at rt for 2 h and poured onto ice, saturated aqueous NaHCO3
(40 mL), and CH2Cl2 (40 mL). The organic layer was dried
(MgSO4), and the solvent was evaporated to yield crude 15.
To a stirred solution of Boc-(S)-Ala-OH (155 mg, 0.82 mmol),
TATU (263 mg, 0.82 mmol), and HOAT (149 mg, 1.09 mmol)
in DMF (5 mL) at 0 °C under Ar were added DIPEA (0.19 mL,
1.10 mmol) and crude 15. The reaction mixture was stirred
at rt for 1 h and poured onto H2O (40 mL) and EtOAc (40 mL).
The organic layer was extracted with saturated aqueous
NaHCO3 (40 mL) and 1 N aqueous HCl (40 mL) and dried
(MgSO4), and the solvents were evaporated. The residue was
chromatographed on SiO2 (15 g) with hexane/EtOAc (1:1) to
give 246 mg (81%) of 17 as a white powder.
Mp 167.5-168.5 °C. [R]D ) -2 (MeOH, c ) 0.1). IR (KBr):
3377w, 3300m, 3068w, 2982w, 2938w, 1741m, 1726s, 1683s,
1654s, 1532m, 1454w, 1389w, 1365w, 1271s, 1174m, 1109m,
1071w, 1024w, 715w. 1H-NMR (CDCl3, 250 MHz): 8.1-8.0
(m, 2H); 7.6-7.35 (m, 3H); 7.35-7.25 (m, 5H); 7.16 (m, 2H);
5.18, 5.11 (2d, J ) 12, 2H); 4.95 (s (br), 1H); 4.77, 4.65 (2d, J
) 12, 2H); 4.7-4.5 (m, 1H); 4.15-3.95 (m, 1H); 1.68 (s, 3H);
1.39 (s, 9H); 1.39 (d, J ) 7, 3H); 1.34 (d, J ) 7, 3H). MS
(ISP): 578 (100, (M+Na)+), 556 (92, (M + H)+), 500 (25), 328
(20), 327 (22). Anal. Calcd for C29H37N3O8 (555.63): C 62.69,
H 6.71, N 7.56. Found: C 62.38, H 6.82, N 7.39.
(S)-2-(Ben zoyla m in o)-2-[(S)-1-(cycloh exylca r ba m oyl)-
2-p h en yleth yl]ca r ba m oyl]p r op yl Ben zoa te (12). A mix-
ture of oxazoline 8 (910 mg, 2.10 mmol) in dioxane (3 mL) was
treated according to oxazoline 7 to yield, after recrystallization
from EtOAc/hexane, 1.05 g (90%) of 12 as a white solid. Mp
166-167 °C. [R]D ) +15.0 (c ) 0.2, CHCl3). IR (KBr): 3418w
(br), 3328w (br), 3063w, 2932w, 2854w, 1726m, 1645s, 1601w,
1534m (br), 1451m, 1315w, 1272s, 1112w, 713m, 695w. 1H-
NMR (250 MHz, CDCl3): 8.2-8.1 (m, 2H); 7.93 (s (br), 1H);
7.75-7.45 (m, 8H); 6.9-6.7 (m, 5H); 6.32 (d (br), 1H); 4.74,
4.49 (2d, J ) 12, 2H); 4.75-4.6 (m, 1H); 3.85-3.65 (m, 1H);
3.4-2.65 (m, 2H); 1.95-1.55 (m, 5H); 1.71 (s, 3H); 1.45-1.05
(m, 5H). MS (ISP): 578.65 ((M + Na)+, 40), 556.5 ((M + H)+,
100).
(S)-2-[[(S)-1-(Ben zyloxyca r bon yl)eth yl]ca r ba m oyl]-2-
[[(S)-2-(ter t-b u t oxyca r b on yla m in o)p r op ion yl]a m in o]-
p r op yl Ben zoa te (18). To a stirred solution of 14 (250 mg,
0.68 mmol) in dioxane (3 mL) at 0 °C was added 0.5 N aqueous
HCl (1 mL). The reaction mixture was stirred for 3 h at rt
and poured onto ice, saturated aqueous NaHCO3, and CH2-
Cl2. The organic layer was dried (MgSO4), the solvents were
evaporated, and the residue was dried under reduced pressure
to give crude 16, which was not further purified. The residue
was dissolved in CH2Cl2 (1 mL), followed by addition of a
prestirred mixture of Boc-(S)-Ala-OH (257 mg, 1.36 mmol),
1-hydroxybenzotriazole (HOBT) (230 mg, 1.70 mmol), N-[3-
(dimethylamino)propyl]-N-ethylcarbodiimide hydrochloride
(EDCI) (274 mg, 1.43 mmol) and DIPEA (0.23 mL) in DMF (5
mL) at 0 °C. The reaction mixture was stirred for 18 h at rt
and poured onto H2O and EtOAc, the organic layer was
Ben zyl (S)-2-[(R)-4-Meth yl-2-p h en yl-4,5-dih yd r ooxa zol-
4-yl]ca r bon yl]a m in o]p r op ion a te (13). To a stirred solution
of (S)-2 (175 mg, 0.65 mmol) and H-(S)-Ala-OBn‚HCl (280 mg,
1.30 mmol) in N,N-dimethylacetamide (DMA) (2.5 mL) at 0

4086 J . Org. Chem., Vol. 61, No. 12, 1996
Obrecht et al.
extracted with H2O, saturated aqueous NaHCO3 solution, and
1 N aqueous HCl and dried (MgSO4), and the solvents were
evaporated. The residue was crystallized from EtOAc/hexane
to yield, after drying under reduced pressure, 280 mg (74%)
of 18 as a white amorphous solid.
Mp 169.5-171 °C. [R]D ) -69 (c ) 0.15, EtOH). IR (KBr):
3390w , 3301m, 3060w, 2875w, 1731s, 1681s, 1651s, 1532 m,
1454m, 1365w, 1272s, 1164s, 1110w, 713w. 1H-NMR (250
MHz, DMSO-d6): 8.19 (s (br), 1H); 8.05-7.95 (m, 2H); 7.81 (d
(br), J ) 7, 1H); 7.7-7.45 (m, 3H); 7.40-7.25 (m, 5H); 7.05 (br
d, J ) 6, 1H); 5.08 (s, 2H); 4.84, 4.51 (2d, J ) 10, 2H); 4.45-
4.3 (m, 1H); 3.95-3.8 (m, 1H); 1.43 (s, 3H); 1.34 (s, 9H); 1.28
(d, J ) 7.5, 3H); 1.05 (d, J ) 7.0, 3H). MS (FAB): 578.2 ((M
+ Na)+, 10), 556.2 ((M + H)+,70), 217 (80), 149.1 (25), 109.1
(30), 91.2 (100).
Met h yl (S)-4-Met h yl-2-p h en yl-4,5-d ih yd r ooxa zole-4-
ca r boxyla te [(S)-19]. A solution of 8 (5.0 g, 11.5 mmol) in
HBr/AcOH (33%, 25 mL) and Ac2O (10 mL) was stirred in a
sealed tube at 70 °C for 4.5 h. The solvents were removed
under high vacuum, the residue was dissolved in MeOH (10
mL), DIPEA (10 mL) was added, and the reaction mixture was
stirred at 50 °C overnight and poured onto ice, H2O (100 mL),
and EtOAc (100 mL). The organic layer was dried (Na2SO4),
and the solvents were evaporated. The residue was chromato-
graphed on SiO2 (100 g) with hexane/EtOAc (9:1-4:1) to give
1.61 g (63%) of (S)-19 as white crystals.
(3.30 mL of a 1 M solution in THF, 3.30 mmol). The reaction
mixture was stirred at rt for 30 min and cooled to 0 °C. 2 N
NaOH (5 mL), saturated aqueous NaHCO3 (20 mL), and Et2O
(100 mL) were added, and the mixture was stirred for 10 min
at rt. The organic layer was decanted, the aqueous layer was
extracted with Et2O (3 × 30 mL), the combined organic phases
were dried (Na2SO4), and the solvents were evaporated. The
residue was chromatographed on SiO2 (50 g) with hexane/
EtOAc (4:1-1:2) to give 590 mg (93%) of (R)-20 as white
crystals.
Mp 120.5-122 °C. [R]D ) +17 (CHCl3, c ) 0.1). IR (KBr):
3173s (br), 2973m, 2894m, 2862m, 1641s, 1602w, 1578m,
1487m, 1451m, 1424w, 1379w, 1355s, 1326s, 1304m, 1263m,
1204m, 1149w, 1075, 1061s, 1038m, 973s, 929w, 870w, 844w,
794m, 742w, 697s, 638w. 1H-NMR (CDCl3, 250 MHz): 7.95-
7.85 (m, 2H); 7.5-7.3 (m, 3H); 4.47, 4.09 (2d, J ) 8, 2H); 3.85-
3.75 (m, 1H); 3.55-3.45 (m, 1H); 2.6-2.5 (m, 1H); 1.33 (s, 3H).
MS (EI): 161 (12), 160 (100, [M - CH2OH]), 132 (24), 105 (17),
104 (50), 77 (19).
(S)-(4-Meth yl-2-p h en yl-4,5-d ih yd r ooxa zol-4-yl)m eth a -
n ol [(S)-20]. A solution of (R)-19 (120 mg, 0.55 mmol) in THF
(2 mL) was treated according to (R)-20 to yield 89 mg (85%) of
(S)-20 as white crystals. Mp 119-120.5 °C. [R]D ) -18
(CHCl3, c ) 0.1). IR, MS, and NMR spectra are in close
agreement to (R)-20.
Mp 73-74 °C. [R]D ) +60 (CHCl3, c ) 0.1). IR (KBr): 3433s
(br), 2991w, 2952m, 2899w, 1743s, 1635s, 1601w, 1578w,
1407m, 1449m, 1379m, 1363s, 1322m, 1304s, 1283s, 1215s,
1185m, 1155s, 1132m, 1083m, 1065s, 1020w, 1000w, 982w,
965s, 935w, 881w, 832w, 791m, 769w, 726m, 708s, 702s, 686w,
572w. 1H-NMR (CDCl3, 250 MHz): 8.05-7.95 (m, 2H); 7.55-
7.35 (m, 3H); 4.83, 4.19 (2d, J ) 9, 2H); 3.80 (s, 3H); 1.63 (s,
3H). MS (EI): 220 (0.5, (M + H)+), 161 (12), 160 (100, [M -
COOCH3]), 132 (20), 105 (17), 104 (24), 77 (13).
Ack n ow led gm en t. We wish to thank our colleagues
from Pharma Research New Technologies, F. Hoffmann-
LaRoche AG for IR (Mr. A. Bubendorf), NMR (Dr. W.
Arnold), mass spectra (Dr. W. Vetter and Mr. W.
Meister) and elemental analyses (Dr. S. Mu¨ller). We
thank Profs. J . Baldwin (Oxford) and H.-J . Hansen
(Zu¨rich) for their valuable advice and fruitful discussion.
Met h yl (R)-4-Met h yl-2-p h en yl-4,5-d ih yd r ooxa zole-4-
ca r boxyla te [(R)-19]. A solution of 7 (100 mg, 0.23 mmol)
in HBr/AcOH (33%, 0.5 mL) and Ac2O (0.2 mL) was treated
according to (S)-19 to yield 32 mg (63%) of (R)-19 as white
crystals. Mp 72.5-74 °C. [R]D ) -58 (CHCl3, c ) 0.1). IR,
MS, and NMR spectra are in close agreement to (S)-19.
(R)-(4-Meth yl-2-p h en yl-4,5-d ih yd r ooxa zol-4-yl)m eth a -
n ol [(R)-20]. To a stirred solution of (S)-19 (720 mg, 3.30
mmol) in THF (12 mL) at 0 °C under Ar was added LiAlH4
1
Su p p or tin g In for m a tion Ava ila ble: Copies of H-NMR
spectra for compounds (S)-2, (R)-2, 5, 7, 8, 11-14, 17, 18, (S)-
19, (R)-19, (S)-20, and (R)-20 (15 pages). This material is
contained in libraries on microfiche, immediately follows this
article in the microfilm version of the journal, and can be
ordered from the ACS; see any current masthead page for
ordering information.
J O952068G