Synthesis of L-4,4-difluoroglutamic acid via nucleophilic addition to a chiral aldehyde

Article abstract of DOI:10.1021/jo015754q

Fluorine-containing derivatives of amino acids are assuming increasing importance as probes of biological function and enzyme mechanism. We now report a new, flexible route to enantiomerically pure L-4,4-difluoroglutamic acid that exploits the addition of difluorinated nucleophiles to configurationally stable α-aminoaldehydes. Conversion of the difluorinated adducts to L-4,4-difluoroglutamic acid can be accomplished in three steps by Barton - McCombie dehydroxylation and acid hydrolysis.

Full text of DOI:10.1021/jo015754q

J . Org. Chem. 2001, 66, 6381-6388
6381
Syn th esis of L-4,4-Diflu or oglu ta m ic Acid via Nu cleop h ilic Ad d ition
to a Ch ir a l Ald eh yd e
Yun Ding, J ianqiang Wang,† Khalil A. Abboud, Yuelian Xu,‡ William R. Dolbier, J r., and
Nigel G. J . Richards*
Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200
richards@qtp.ufl.edu
Received May 14, 2001
Fluorine-containing derivatives of amino acids are assuming increasing importance as probes of
biological function and enzyme mechanism. We now report a new, flexible route to enantiomerically
pure L-4,4-difluoroglutamic acid that exploits the addition of difluorinated nucleophiles to
configurationally stable R-aminoaldehydes. Conversion of the difluorinated adducts to L-4,4-
difluoroglutamic acid can be accomplished in three steps by Barton-McCombie dehydroxylation
and acid hydrolysis.
In tr od u ction
fluorinated amino acids has been stimulated by our
recent efforts to characterize the structure and mecha-
nism of asparagine synthetase (AS),7 a glutamine-de-
pendent amidotransferase8 that appears to be intimately
linked with progression through the cell cycle9 and
cellular responses to amino acid starvation.10 Specifically,
we required L-4,4-difluoroglutamine 1 to investigate
whether AS could employ this nonnatural amino acid as
a nitrogen source in place of L-glutamine as part of
ongoing efforts to trap and structurally characterize
specific intermediates formed during the kinetic mech-
anism.7c Since DL-4,4-difluoroglutamic acid had been
converted to a racemic mixture of the fluorinated sub-
strate analogue 1 in three steps,11 we wished to prepare
optically pure L-4,4-difluoroglutamate 2 from cheap
starting materials via a route that might be easily
modified to yield functionalized derivatives of 4,4-difluo-
roglutamine. In this regard, the elegant synthesis of
L-4,4-difluoroglutamate 2,12 reported during our studies,
seems to be less suitable for our purposes than the route
Fluorinated derivatives of amino acids are assuming
increasing importance as probes of biological function and
enzyme mechanism,1 particularly because the electron-
withdrawing effects of fluorine substituents2 often have
profound effects on reactivity and conformational prefer-
ences.3,4 For example, fluorinated analogues of glutamic
acid possess antitumor activity5 and are modulators of
folate poly-γ-glutamate biosynthesis.6 Our interest in
* To whom correspondence should be addressed. Phone: (352) 392-
3601. Fax: (352) 846-2095. Portions of the work described in this paper
were presented at the 218th National Meeting of the American
Chemical Society in New Orleans, August 1999 (MEDI-244).
† Present address: Triad Therapeutics, Inc., 5820 Nancy Ridge
Drive, #200, San Diego, CA 92121.
‡ Present address: Clariant LSM (Florida), Gainesville, FL 32609.
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10.1021/jo015754q CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/25/2001

6382 J . Org. Chem., Vol. 66, No. 19, 2001
Ding et al.
Sch em e 1
Sch em e 2a
we describe here. We note that the only other synthesis
of L-4,4-difluoroglutamic acid 2 in enantiomerically pure
form requires the use of an expensive precursor, that is
not readily available,13 and other practical routes for the
large-scale synthesis of 4,4-difluoroglutamic acid14 and
related compounds15,16 either give the target fluorinated
amino acids as racemic mixtures or require tedious
resolution procedures.17 We now report the successful
preparation of 2 by a strategy in which the difluorinated
side chain is constructed by nucleophilic addition to the
configurationally stable aldehyde 3 (Scheme 1).18
a
PhFl ) 9-phenylfluoren-9-yl. Key: (a) 9-bromo-9-phenylfluo-
rene, Et3N; (b) COCl2, Et3N, CH2Cl2; (c) LiAlH4, THF, -78 °C; (d)
4, dry THF, rt.
Although difluoroolefin 4 is a powerful nucleophile that
undergoes facile addition to aromatic aldehydes and
conjugated carbonyl compounds,23 its reaction with al-
dehydes that possess R-hydrogens yields only N,N-
dimethyldifluoroacetamide, presumably due to proton
abstraction by the highly basic double bond.23 Our first
efforts to prepare 2 therefore involved reaction of 4 with
the configurationally stable aldehyde 624 as the electro-
philic component. Molecular modeling studies25 indicated
that the PhFl protecting group shields the C-2 proton
preventing its abstraction by 4, and it is well established
that racemization of the chiral center in 6 is very slow
under a wide variety of conditions.26 Although synthesis
of 2 by this route ultimately requires D-serine as a
starting material, we elected to use 6 in these early
experiments in order to establish the feasibility of this
synthetic approach. Hence, the commercially available
methyl ester of L-serine was converted to the chiral ester
5 following literature procedures.24,26 After some experi-
mentation, we discovered that aldehyde 6 could be
prepared in good yield (and without any significant loss
in enantiopurity) by treatment of 5 with LiAlH4 at -78
°C.27 As anticipated, the difluoroolefin 4 underwent
smooth addition to aldehyde 6 in THF at room temper-
ature, giving an alcohol 7 as a single adduct in a purified
yield of 58% (based on 6) (Scheme 2). We assigned the
Resu lts a n d Discu ssion
The introduction of difluoromethylene groups into
amino acids has been accomplished using a variety of
reagents,19 including DAST,20 N-fluorobenzenesulfon-
imide (NFSi),12,21 and difluorobromoacetate esters.22 In
our initial experiments, we investigated whether 1,1-bis-
(dimethylamino)-2,2-difluoroethene 4, a novel reagent for
building difluoromethylene centers into molecules,23 would
undergo clean addition to a suitably protected chiral
aldehyde (Scheme 2).
(13) Kitagawa, O.; Hashimoto, A.; Kobayashi, Y.; Taguchi, T. Chem.
Lett. 1990, 1307-1310.
(14) Tsukamoto, T.; Kitazume, T.; McGuire, J . J .; Coward, J . K. J .
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Avent, A. G.; Bowler, A. N.; Doyle, P. M.; Marchand, C. M.; Young, D.
W. Tetrahedron Lett. 1992, 33, 1509-1512. (c) Kroger, S.; Haufe, G.
Amino Acids 1997, 12, 363-372. (d) Hudlicky, M.; Merola, J . S.
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Tetrahedron Lett. 1993, 34, 4917-4920. (f) Taguchi, T.; Mitsuhashi,
S.; Yamanouchi, A.; Kobayashi, Y. Tetrahedron Lett. 1984, 25, 4933-
4936.
(16) For general reviews that describe the preparation of fluorinated
amino acids, see: (a) Sutherland, A.; Willis, C. L. Nat. Prod. Rep. 2000,
17, 621-631. (b) Li, K. Q.; Leriche, C.; Liu, H. W. Bioorg. Med. Chem.
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13914.
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14. (b) Tolman, V.; Sedmera, P. J . Fluorine Chem. 2000, 101, 5-10.
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K.; Ohtani, M. Tetrahedron Asymm. 1996, 7, 3545-3551.
(18) (a) Rose, N. G. W.; Blaskovich, M. A.; Wong, A.; Lajoie, G. A.
Tetrahedron 2001, 57, 1497-1507. (b) Rife´, J .; Ortuno, R.; Lajoie, G.
A. J . Org. Chem. 1999, 64, 8958-8961. (c) Blaskovich, M. A.; Lajoie,
G. A. J . Am. Chem. Soc. 1993, 115, 5021-5030. (d) Blaskovich, M. A.;
Lajoie, G. A. Tetrahedron Lett. 1993, 34, 3837-3840.
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M. J .; Herpin, T. F. Tetrahedron 1996, 52, 8619-8683. (c) Fluorine-
Containing Amino Acids, Synthesis and Properties; Kukhar’, V. P.,
Soloshonok, V. A., Eds.; Wiley: Chichester, 1995. (d) Welch, J . T.
Tetrahedron 1987, 43, 3123-3197.
(20) (a) Hart, B. P.; Coward, J . K. Tetrahedron Lett. 1993, 34, 4917-
4920. (b) Taguchi, T.; Mitsuhashi, S.; Yamanouchi, A.; Kobayashi, Y.
Tetrahedron Lett. 1984, 25, 4933-4936.
(21) (a) Vayron, P.; Renard, P. Y.; Valleix, A.; Mioskowski, C. Chem.
Eur. J . 2000, 6, 1050-1063. (b) Kotoris, C. C.; Chen, M. J .; Taylor, S.
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Morizawa, Y.; Matsumura, Y. Angew. Chem., Int. Ed. Engl. 1996, 35,
1019-1021. (d) Davis, F. A.; Han, W.; Murphy, C. K. J . Org. Chem.
1995, 60, 4730-4737. (e) Differding, E.; Ofner, H. Synlett 1991, 187-
189.
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U. M. Synth. Commun. 1996, 26, 4113-4122. (b) Robinson, R. P.;
Donahue, K. M. J . Org. Chem. 1992, 57, 7309-7314. (b) Takahashi,
L. H.; Radhakrishnan, R.; Rosenfeld, R. E.; Meyer, E. F.; Trainor, D.
A. J . Am. Chem. Soc. 1989, 111, 3368-3374. (c) Chou, T. S.; Heath, P.
C.; Paterson, L. E.; Poteet, L. M.; Lakin, R. E.; Hunt, A. H. Synthesis,
1992, 565-570.
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5599-5602. (b) Xu, Y. L.; Dolbier, W. R. J . Org. Chem. 1997, 62, 6503-
6506. (c) Xu, Y. L.; Dolbier, W. R.; Rong, X. X. J . Org. Chem. 1997, 62,
1576-1577.
(24) Lubell, W.; Rapoport, H. J . Org. Chem. 1989, 54, 3824-3831.
(25) The conformational preferences of 6 were analyzed by a Monte
Carlo/energy minimization algorithm in combination with the MM2*
force field, as implemented in MacroModel V6.0. For additional
information on this search procedure, the parameters employed in the
calculation and the software package, see: (a) Guida, W. C.; Bohacek,
R. S.; Erion, M. D. J . Comput. Chem. 1992, 13, 214-228. (b) Allinger,
N. L. J . Am. Chem. Soc. 1977, 99, 8127-8134. (c) Mohamadi, F.;
Richards, N. G. J .; Guida, W. C.; Liskamp, R. M. J .; Lipton, M. A.;
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Chem. 1990, 11, 440-467.
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236-237. (b) Lubell, W. D.; J amison, T. F.; Rapoport, H. J . Org. Chem.
1990, 55, 3511-3522.
(27) Efforts to obtain the aldehyde 6 by direct reduction of 5 were
reported to yield only the alcohol,24 although 6 could be obtained by
treatment of an acylisoxazolide moiety in place of the ethyl ester in
5.24 We note, however, that LiAlH4 reduction was carried out at
temperatures exceeding 0 °C in these studies. The molecular basis for
the clean reduction of ester 5 to aldehyde 6 at very low temperature
under our conditions remains to be established, although a series of
model studies have suggested that the presence of the â-oxygen
substituents may control reactivity by stabilization of the initial
intermediate (Ding, unpublished results). We note that esters are
reduced to aldehydes at RT by LiAlH4 in the presence of diethyl-
amine: Cha, J .; Kwon, S. S. J . Org. Chem. 1987, 52, 5486-5487.

Synthesis of L-4,4-Difluoroglutamic Acid
J . Org. Chem., Vol. 66, No. 19, 2001 6383
Sch em e 3a
F igu r e 1. (A) Felkin-Anh model for the addition reaction
between difluoroolefin 4 and aldehyde 6. (B) Models for the
metal-directed addition of Reformatsky reagent 8 and aldehyde
6 leading to major and minor products 9a and 9b, respectively.
relative and absolute stereochemistry of 7 on the basis
of X-ray crystallography,28 assuming that no racemization
had taken place at C-2 under the conditions.26 The
stereochemical preference for attack on 6 is consistent
with that reported previously and can be rationalized on
the basis of the Felkin-Anh model in which the bulky
protecting group completely hinders the si-face of the
carbonyl group (Figure 1A).29
a
PhFl ) 9-phenylfluoren-9-yl. Key: (a) ZnCF2CO2Et 8, THF,
Although this result confirmed the hypothesis that
reaction of 4 and 6 proceeded with a very high level of
diastereoselection and that the presence of the bulky
N-protecting group suppressed deprotonation at C-2, a
number of technical difficulties precluded the routine use
of difluoroolefin 4 for large-scale preparations of inter-
mediate 7, including the need for its in situ preparation
from 1,1-bis(dimethylamino)-2,2,2-trifluoroethane30 using
n-butyllithium. We therefore investigated whether ad-
dition of the Reformatsky reagent 8 to the chiral aldehyde
6 would proceed with equally high diastereoselectivity.
Dropwise addition of ethyl bromodifluoroacetate to a
solution of zinc and 6 was required for clean reaction and
gave the diastereoisomeric adducts 9a and 9b as a 6:1
mixture in 85% total yield after purification (Scheme 3A).
Stereochemical assignments for these compounds were
based on the observation that treatment of the major
isomer 9a with lithium dimethylamide 10 gave a single
product with physical and spectroscopic properties that
were identical to those of amide 7. The loss in stereo-
chemical control is probably associated with stabilization
of the transition state leading to the minor isomer due
to chelation of zinc by the oxygen in the oxazolidine ring
(Figure 1B).31
rt; (b) LiNMe2 10, THF, rt; (c) Zn, BrCH2CO2Et, THF, rt.
1
assigned on the basis of the H-1H coupling constant (J
) 6.6 Hz) for the protons attached to C-4 and C-3′, which
is similar in magnitude to the dipolar coupling observed
between the cognate protons in 7 (for which the relative
stereochemistry was unequivocally established). The
stereochemical outcome is also consistent with the results
of previous investigations into the reactions of nucleo-
philes with aldehyde 6.24,26
Having devised conditions for the preparation of the
difluorinated secondary alcohols 7 and 9a , we next
investigated conditions for removing the hydroxyl group
so as to complete the synthesis of the side chain.
Methods for accomplishing this synthetic transforma-
tion at carbon atoms adjacent to CF2 groups, especially
in highly functionalized derivatives such as 7 and 9a , do
not seem to have been investigated in detail.32 Given the
inductive effects of the fluorine substituents, we envis-
aged that this reduction might be best accomplished
using radical-based methods, such as the Barton-Mc-
Combie reaction.33 The difluoroamide 7 was therefore
converted to the O-pentafluorophenylthioate 12 by reac-
tion with pentafluorophenylthiochloroformate (Scheme
4). Relatively strong conditions were required for the
Any influence of electronic effects, associated with the
fluorine substituents, on product stereochemistry was
ruled out by the observation that the cognate Refor-
matsky reagent formed from ethyl bromoacetate reacted
with 6 to give a 8:1 ratio of the diastereoisomeric adducts
11a and 11b (Scheme 3B). The major isomer 11a was
(32) Radical-mediated approaches to the dehydroxylation of second-
ary alcohols adjacent to difluoromethyl and trifluoromethyl groups do
not seem to have been the subject of systematic investigation (Ait-
Mohand, S.; Dolbier, W. R. Personal communication). To our knowl-
edge, there is only a single published example for which no experi-
mental details were given: Kim, K. S.; Qian L. Tetrahedron Lett. 1993,
34, 7195-7196. For studies on the reactivity of radicals in fluorinated
hydrocarbons, see: Baguley, P. A.; Walton, J . C. Angew. Chem., Int.
Ed. Engl. 1998, 37, 3073-3082. (b) Dolbier, W. R. Top. Curr. Chem.
1997, 192, 97-163. (c) Siddiqui, M. A.; Driscoll, J . S.; Abushanab, E.;
Kelley, J . A.; Barchi, J . J .; Marquez, V. E. Nucleosides Nucleotides
2000, 19, 1-12.
(33) (a) Barton, D. H. R.; McCombie, S. W. J . Chem. Soc., Perkin
Trans. 1 1975, 1574-1585. (b) Beckwith, A. L. J .; Crich, D.; Duggan,
P. J .; Yao, Q. W. Chem. Rev. 1997, 97, 3273-3312. (c) Barton, D. H.
R.; J ang, D. O.; J aszberenyi, J . C. Tetrahedron 1993, 49, 2793-2804.
(d) Barton, D. H. R.; Dorchak, J .; J aszberenyi, J . C. Tetrahedron 1992,
48, 7435-7446. (e) Barton D. H. R. Tetrahedron 1992, 48, 2529-2544.
(28) Data for 7: C27H26F2N2O5, Mr ) 478.17 g mol-1, orthorhombic,
space group P2(1)2(1)2(1), a ) 8.5056(5) Å, b ) 11.6903(7) Å, c )
23.913(2) Å, R ) â ) γ ) 90°, T ) 173 ( 1 K. Crystallographic data for
7 (excluding structure factors) are included in the Supporting Informa-
tion for this paper.
(29) (a) Anh, N. T. Top. Curr. Chem. 1980, 88, 145-163. (b) Bartlett,
P. A. Tetrahedron 1980, 36, 2-72.
(30) (a) Xu, Y. L.; Dolbier, W. R. J . Org. Chem. 2000, 65, 2134-
2137. (b) Xu, Y. L.; Dolbier, W. R. Tetrahedron Lett. 1998, 39, 9151-
9154.
(31) Blaskovich, M. A.; Lajoie, G. A. Tetrahedron Lett. 1993, 34,
3837-3840.

6384 J . Org. Chem., Vol. 66, No. 19, 2001
Ding et al.
Sch em e 4a
a
PhFl ) 9-phenylfluoren-9-yl. Key: (a) C6F5OC(dO)Cl, DMAP, N-hydroxysuccinimide, toluene, 90 °C; (b) (n-Bu)3SnH, AIBN, toluene,
reflux; (c) KOH, EtOH or 6 N HCl.
Sch em e 5a
a
Z ) PhCH2OCO; Im ) C3H3N2. Key: (a) Cs2CO3, then C5H9BrO, DMF, rt; (b) BF3‚Et2O, CH2Cl2, rt, then Et3N; (c) DMSO, (COCl)2,
-78 °C; (d) ZnCF2CO2Et 8, THF, rt; (e) (C3H3N2)CdS, dry THF, rt; (f) Et3SiH, (PhCO2)2, C6H6, reflux; (g) 6 N HCl, reflux, then anion-
exchange chromatography.
synthesis of the activated O-pentafluorophenylthioate 12,
presumably due to steric hindrance of the secondary
alcohol by the bulky N-protecting group. In addition, the
complete conversion of starting alcohol 7 was difficult to
achieve, even after the addition of additional amounts
of pentafluorophenylthiochloroformate. Radical-mediated
dehydroxylation from 12 also proved to be more difficult
than anticipated. Although the desired difluorinated
amide could be obtained using Bu3SnH, with AIBN as a
radical initiator, a complicated mixture of products was
formed from which 13 was isolated in a disappointing
30% yield. It is possible that the secondary radical
resulting from breakdown of the O-pentafluorophenyl-
thioate moiety undergoes reaction with the electron-rich
phenylfluorenyl-protecting group, although this remains
to be established. Efforts to optimize the yield by chang-
ing the leaving group to a xanthate or thioimidazolide
gave compounds that either did not react with Bu3SnH,
or gave complex product mixtures, under a variety of
deoxygenation conditions (Ding and Richards, unpub-
lished results). Similar results were obtained for the
O-pentafluorophenylthioate 14 that could be prepared
from alcohol 9a . The reasons for these difficulties in using
Bu3SnH for deoxygenation remain unclear, especially in
light of experiments outlined below. Having established
conditions for removal of the hydroxyl group from the
pentafluorophenyl derivatives 12 and 14, albeit in low
yield, we next confronted the problem of hydrolyzing the
oxazolidinone ring. All attempts to prepare the acyclic
material 16 by hydrolysis of either 13 or 15 under acidic
or basic conditions, as previously reported,24 gave only
complex product mixtures. 19F NMR analysis supports
the hypothesis that these technical problems are likely
associated with dehydrofluorination taking place at a rate
similar to heterocyclic ring opening.
Although we had demonstrated that addition of Re-
formatsky reagent 8 to configurationally stable R-amino-
aldehydes proceeds smoothly, these unexpected difficul-
ties in radical-mediated deoxygenation and hydrolysis of
the heterocyclic ring in either 13 or 15 caused us to
reexamine our choice of starting aldehyde. In particular,
we sought to avoid the use of potentially reactive nitrogen
protecting groups that might participate in transforma-
tions leading to the desired difluorinated side chain. We
therefore investigated reaction of the difluorinated re-
agent 8 with the 4-methyl-2,6,7-trioxabicyclo[2.2.2] ortho
ester (OBO) derivative 3 (Scheme 5).
Thus, the N-protected derivative of L-serine 17 was
converted to optically pure aldehyde 3 in three steps,
according to literature procedures.34 After esterification
with 3-methyl-3-(hydroxymethyl)oxetane to give the ester
18, the OBO ester 19 was formed by BF3-catalyzed

Synthesis of L-4,4-Difluoroglutamic Acid
J . Org. Chem., Vol. 66, No. 19, 2001 6385
1
300 spectrometers. For H and 13C, chemical shifts are reported
rearrangement.35 Swern oxidation then gave the target
aldehyde 3 in excellent yield. The optical purity of this
compound was verified by 1H NMR analysis of resonance
associated with the aldehyde proton in the presence of
Eu(hfc)3.18c We note that rearrangement of the cognate
N-phthaloyl derivative 20 under these conditions was
found to proceed in very poor yield.36 Reaction of 3 with
the difluorinated Reformatsky reagent 8 again proceeded
smoothly to give the enantiomerically pure alcohol 21 as
a 7:1 mixture of diastereoisomers that could not be
separated by column chromatography. The relative ster-
eochemistry of the major isomer was assigned as 21a by
analogy to the major product obtained by reaction of 3
and the cognate, nonfluorinated Reformatsky reagent
derived from ethyl bromoacetate.18c As reported previ-
ously, examination of the 1H and 19F NMR spectra
revealed that there was no dipolar coupling between the
protons at C-2 and C-3. Reaction of 8 with 3 therefore
proceeds via a simple Felkin-Anh model in which the
nucleophile attacks the less hindered si-face of the
aldehyde moiety. Again the diastereoselectivity of the
addition of the difluorinated reagent 8 is significantly
reduced relative to that reported in other experiments
employing aldehyde 3 as an electrophilic component. This
may reflect stabilization of the transition state leading
to 21b through zinc chelation or equilibration of the
initial adduct under the reaction conditions. The mixture
of diastereoisomers 21 was then refluxed with thiocar-
bonyl bis-imidazolide in THF overnight to give the
oxythiocarbonylimidazole derivatives 22. In an attempt
to avoid using Bu3SnH to deoxygenate the secondary
alcohol adjacent to the difluoromethylene moiety, we
investigated the use of Et3SiH in the presence of benzoyl
peroxide at 90 °C.32 We note that because BzOOBz also
acts as a trap for triethylsilyl radicals produced during
the reaction,37 relatively high amounts of the peroxide
(1-1.2 equiv) were needed to drive the reaction to
completion. The protected difluoroester 23 was obtained
in 44% overall yield (for three steps), however, from
aldehyde 3. Conversion of 23 to L-4,4-difluoroglutamate
2 could be accomplished by heating in 6 N HCl for 2 h,
with purification using anion-exchange chromatography.
Comparison of the optical and spectroscopic properties
for this material with those reported previously12,13
confirmed our observations that the synthesis had pro-
ceeded with little loss of stereochemical integrity at the
chiral center. We note that this synthesis is amenable to
use on large scale and, therefore, represents a practical
approach to the enantiospecific preparation of this di-
fluorinated amino acid.
in ppm (δ) downfield of tetramethylsilane as an internal
reference (δ 0.0). In the case of 19F, CFCl3 was used as an
internal standard (δ 0.0). Splitting patterns are abbreviated
as follows: s, singlet, d, doublet, t, triplet, q, quartet and m,
multiplet. EI, CI and FAB mass spectra were recorded on a
Finnegan MAT 25Q (high resolution) spectrometer. Methane
was generally employed in obtaining CI mass spectra. Analyti-
cal thin-layer chromatography (TLC) was performed on silica
gel 60F-254 plates. Flash chromatography was performed
using standard procedures38 on Kieselgel (230-400 mesh). All
reagents were purchased from Aldrich or Fisher Scientific, and
were used without further purification except for chromatog-
raphy solvents, which were distilled before use. Moisture-
sensitive reactions were carried out under an argon atmo-
sphere in glassware that was flame-dried with an inert gas
sweep. THF and benzene were freshly distilled before use from
sodium benzophenone ketyl.
(4S)-2-Oxo-3-(9-p h en ylflu or en -9-yl)oxa zolid in e-4-ca r -
boxa ld eh yd e (6). The N-protected methyl ester 5 (2.51 g, 6.5
mmol) was dissolved in dry THF (50 mL) under an argon
atmosphere. After the reaction mixture was cooled to -78 °C,
LiAlH4 (7.8 mL of a 1 M solution in THF, 7.8 mmol) was added
dropwise so that the reaction temperature did not exceed -75
°C. After the mixture was further stirred for 2 h at this
temperature, TLC showed complete consumption of the start-
ing ester and the reaction was quenched by the addition of a
solution of KHSO4 (3 g) in H2O (60 mL). The mixture was then
allowed to warm to rt before being extracted with EtOAc (3 ×
80 mL). The combined organic extracts were washed with brine
(100 mL) and dried (MgSO4) before removal of the solvent
under reduced pressure. The oily residue was purified using
flash chromatography (eluant: 2:1 hexane/EtOAc) to give the
target aldehyde 6 as a white foam: 1.96 g, 85%; [R]20
)
D
-313.3° (c ) 1.00, CHCl3); 1H NMR (CDCl3, 300 MHz) δ 3.95
(1 H, m), 4.12 (1 H, dd, J ) 9.3, 5.5 Hz), 4.34 (1 H, t, J ) 9.3
Hz), 7.27-7.80 (13 H, m), 9.03 (1 H, d, J ) 3.6 Hz); 13C NMR
(CDCl3, 75.4 MHz) δ 61.88 (d), 63.75 (t), 72.71 (s), 120.50 (d),
124.79 (d), 125.34 (d), 126.43 (d), 127.10 (d), 127.68 (d), 128.56
(d), 128.66 (d), 128.86 (d), 129.94 (d), 130.11 (d), 139.61 (s),
139.74 (s), 140.11 (s), 145.09 (s), 146.98 (s), 157.56 (s), 196.02
(d); MS (CI, CH4) 356 (MH+, 4), 355 (M+, 3), 269 (6), 241 (100).
N,N-Dim eth yl (4S,3′S)-2-Oxo-3-(9-p h en ylflu or en -9-yl)-
oxazolidin e-4-(2′,2′-diflu or o-3′-h ydr oxy)pr opion am ide (7).
A solution of 1,1-bis(dimethylamino)-2,2-difluoroethene 4 (11.5
mL of a 0.47 M solution in THF, 5.4 mmol) was added to the
aldehyde 6 (1.3 g, 3.6 mmol) in dry THF (10 mL), and the
reaction mixture was stirred at rt overnight. The reaction was
quenched by the addition of a few drops of H2O, and the
organic solvent was removed under reduced pressure after
drying (MgSO4). The oily residue was then purified using flash
chromatography (eluant:1:1 EtOAc/hexane) to yield the adduct
7 as a white solid. A portion was recrystallized (EtOAc/hexane)
to give 7 as colorless plates: 998 mg, 58%; mp 244.7-245.2
°C; [R]20 ) -464° (c ) 1.22, CHCl3); 1H NMR (CDCl3, 300
D
MHz) δ 2.80 (1 H, d, J ) 3.5 Hz), 2.85 (3 H, s), 2.94 (3 H, dd,
5J HF ) 2.4, 1.5 Hz), 3.46 (1 H, dt, J HF ) 26.1 Hz, J ) 3.4 Hz),
3
4.38 (2 H, m), 4.59 (1 H, dt, J ) 8.3, 1.5 Hz), 7.21-7.54 (10 H,
m), 7.66 (1 H, d, J ) 7.5 Hz), 7.74 (1 H, d, J ) 7.5 Hz), 8.20 (1
H, d, J ) 7.5 Hz); 13C NMR (CDCl3, 75.4 MHz) δ 36.54 (q),
36.68 (q), 55.76 (d), 62.36 (t), 68.56 (ddd, 2J CF ) 28.7, 21.2 Hz),
71.62 (s), 115.56 (dd, 1J CF ) 268.0, 261.0 Hz), 120.09 (d), 120.54
(d), 124.69 (d), 126.05 (d), 127.19 (d), 128.00 (d), 128.12 (d),
128.37 (d), 129.04 (d), 129.51 (d), 139.12 (s), 139.61 (s), 141.65
(s), 145.01 (s), 148.04 (s), 157.92 (s), 161.92 (t, 2J CF ) 27.5 Hz);
19F NMR (CDCl3, 282 MHz) -108.22 (1 F, d, 2J FF ) 290.1 Hz),
-119.56 (1 F, dd, J FF ) 290.1 Hz, J HF ) 25.5 Hz); MS (CI,
CH4) 479 (MH+, 4), 478 (M+, 11), 242 (19), 241 (100); exact
mass calcd for M+ C27H24F2N2O4 requires 478.1704, found
478.1687 (CI).
Exp er im en ta l Section
Melting points were recorded using a Fisher-J ohns melting
point apparatus and are uncorrected. Optical rotations were
measured using a Polyscience model SR-6 polarimeter. 1H, 13C,
and 19F NMR spectra were obtained at 300, 75.4, and 282 MHz,
respectively, using Varian VXR-300, Gemini-300, and Mercury-
2
3
(34) Blaskovich, M. A.; Evindar, G.; Rose, N. G. W.; Wilkinson, S.;
Luo, Y.; Lajoie, G. A. J . Org. Chem. 1998, 63, 3631-3646.
(35) Corey, E. J .; Raju, N. Tetrahedron Lett. 1983, 24, 5571-5574.
(36) The reaction appears to be complicated by the formation of
products in which ring-opening of the N-phthaloyl substituent has
taken place. Complicated mixtures of products were obtained under a
variety of conditions for conversion of the oxetane into the ortho ester
moiety (Ding, Y.; Richards, N. G. J . Unpublished results).
(37) Barton, D. H. R.; J ang, D. O.; J aszberenyi, J . C. Tetrahedron
Lett. 1991, 32, 7187-7190.
Eth yl (4S,3′S)-2-Oxo-3-(9-p h en ylflu or en -9-yl)oxa zoli-
din e-4-(2′,2′-diflu or o-3′-h ydr oxy)pr opan oate (9a) an d Eth -
(38) Still, W. C.; Kahn, M.; Mitra, A. J . Org. Chem. 1978, 43, 2923-
2935.

6386 J . Org. Chem., Vol. 66, No. 19, 2001
Ding et al.
1
yl (4S,3′R)-2-Oxo-3-(9-p h en ylflu or en -9-yl)oxa zolid in e-
(2′,2′-d iflu or o-3′-h yd r oxy)p r op a n oa te (9b). Ethyl bromodi-
fluoroacetate (658 mg, 3.25 mmol) was added slowly to a
stirred mixture of aldehyde 6 (464 mg, 1.3 mmol) dissolved in
dry THF (10 mL) and acid-washed zinc powder (316 mg, 4.87
mmol) at rt under a N2 atmosphere. The reaction mixture was
stirred vigorously for 2 h until TLC analysis showed complete
consumption of the aldehyde. After the addition of NH4Cl (3
mL) and saturated NaCl (3 mL), the mixture was filtered
through Celite. The precipitate was washed well with EtOAc
(3 × 10 mL) and the organic phase separated. After extraction
of the aqueous phase with EtOAc (2 × 10 mL), the combined
organic extracts were dried (MgSO4), and the solvent was
removed under reduced pressure to give an oily residue. This
material was purified by flash chromatography (eluant: 3:1
hexane/EtOAc) to yield the desired adduct as a mixture of
diastereoisomers 9a and 9b (529 mg, 85%), in a 6:1 ratio by
19F NMR. This mixture was sufficiently pure for use in sub-
sequent transformations. Careful column chromatography
yielded a small amount of pure difluoro alcohol 9a , a portion
of which was recrystallized (EtOAc/hexane) to give colorless
11a : 1.92 g, 72%; H NMR (CDCl3, 300 MHz) δ 1.11 (3 H,
t, J ) 7.2 Hz), 1.50 (1 H, d, J ) 4.4 Hz), 1.71 (1 H, dd, J )
15.6, 4.8 Hz), 2.05 (1 H, dd, J ) 15.6, 8.9 Hz), 3.28 (1 H, m),
3.90 (3 H, m), 4.27 (1 H, dd, J ) 8.8, 2.7 Hz), 4.34 (1 H, t, J )
8.6 Hz), 7.18-7.42 (9 H, m), 7.51 (1 H, dt, J ) 7.6, 0.6 Hz),
7.69 (1 H, d, J ) 7.6 Hz), 7.77 (1 H, d, J ) 7.6 Hz), 8.20 (1 H,
d, J ) 7.3 Hz); 13C NMR (CDCl3, 75.4 MHz) δ 13.88 (q), 37.06
(t), 60.06 (d), 60.63 (d), 61.86 (t), 65.88 (t), 71.52 (s), 120.31
(d), 120.52 (d), 124.63 (d), 124.73 (d), 125.98 (d), 127.21 (d),
127.32 (d), 128.03 (d), 128.37 (d), 129.08 (d), 129.68 (d), 139.04
(s), 140.01 (s), 141.46 (s), 145.01 (s), 148.22 (s), 157.73 (s),
169.93 (s); MS (CI, CH4) 443 (M+, 16), 241 (100); exact mass
calcd for MH+ C27H26NO5 requires 444.1811, found 444.1812
(CI).
11b: 512 mg, 23%; 1H NMR (CDCl3, 300 MHz) δ 1.26 (3 H,
t, J ) 7.2 Hz), 1.42 (1 H, d, J ) 4.6 Hz), 1.75 (1 H, dd, J )
15.4, 5.0 Hz), 2.15 (1 H, dd, J ) 15.4, 8.4 Hz), 3.28 (1 H, m),
3.90 (1 H, ddd, J ) 8.4, 2.4, 1.5 Hz), 4.11 (2 H, q, J ) 7.2 Hz),
4.29 (1 H, dd, J ) 8.7, 2.7 Hz), 4.37 (1 H, t, J ) 8.7 Hz), 7.20-
7.42 (9 H, m), 7.52 (1 H, m), 7.70 (1 H, m), 7.79 (1 H, m), 8.21
(1 H, m); 13C NMR (CDCl3, 75.4 MHz) δ 14.15 (q), 36.88 (t),
51.84 (d), 60.08 (d), 61.88 (t), 65.95 (t), 71.66 (s), 120.44 (d),
120.68 (d), 124.71 (d), 124.79 (d), 126.07 (d), 127.35 (d), 127.38
(d), 128.09 (d), 128.49 (d), 129.17 (d), 129.83 (d), 139.20 (s),
140.12 (s), 141.46 (s), 145.14 (s), 148.34 (s), 157.75 (s), 170.39
(s); MS (CI, CH4) 443 (M+, 12), 241 (100).
plates: mp 169-172 °C; [R]20 ) -506° (c ) 1.06, CHCl3); 1H
D
NMR (CDCl3, 300 MHz) δ 1.23 (3 H, t, J ) 7.2 Hz), 1.61 (1 H,
d, J ) 6.6 Hz), 3.41 (1 H, dt, 3J HF ) 20.1 Hz, J ) 6.6 Hz), 4.13
(2 H, m), 4.34 (1 H, dd, J ) 8.4, 1.2 Hz), 4.43 (1 H, t, J ) 8.4
Hz), 4.53 (1 H, ddd, J ) 9.0, 2.4, 1.2 Hz), 7.44 (10 H, m), 7.74
(2 H, m), 8.21 (1 H, m); 13C NMR (CDCl3, 75.4 MHz) δ 13.74
N,N-Dim eth yl (4S,3′S)-2-Oxo-3-(9-p h en ylflu or en -9-yl)-
oxa zolid in e-4-(2′,2′-d iflu or o-3′-oxyth ioca r bon ylp en ta flu -
or op h en yl)p r op ion a m id e (12). The difluorinated alcohol 7
(372 mg, 0.78 mmol) was dissolved in toluene together with
DMAP (220 mg, 1.8 mmol) and N-hydroxysuccinimide (45 mg,
0.39 mmol). After the addition of pentafluorophenylthiochlo-
roformate (408 mg, 1.5 mmol), the reaction was heated at 90
°C overnight. The solution was then filtered through Celite
and dried (MgSO4). After removal of the solvent under reduced
pressure, the crude product was purified using flash chroma-
tography (eluant: 1:2 EtOAc/hexanes) to yield 12 as a yellow-
ish foam: 296 mg, 54% (74% based on recovered 7): 1H NMR
(CDCl3, 300 MHz) δ 2.78 (6 H, s), 4.58 (1 H, m), 4.74 (2 H, m),
4
(q), 56.18 (d), 62.14 (dt, J CF ) 6.9 Hz), 63.19 (t), 68.45 (dd,
2J CF ) 28.0, 21.0 Hz), 71.70 (s), 113.56 (dd, 1J CF ) 261.0, 254.0
Hz), 120.32 (d), 120.68 (d), 124.66 (d), 126.19 (d), 127.47 (d),
127.63 (d), 128.54 (d), 129.27 (d), 129.45 (d), 129.83 (d), 138.82
(s), 140.08 (s), 141.17 (s), 144.77 (s), 148.38 (s), 157.82 (s),
161.09 (t, 2J CF ) 31.0 Hz); 19F NMR (CDCl3, 282 MHz) -112.35
2
3
(1 F, dd, J FF ) 269.0 Hz, J HF ) 6.2 Hz), -124.21 (1 F, dd,
2J FF ) 269.0 Hz, J HF ) 19.2 Hz); MS (CI, CH4) 480 (MH+, 4),
3
479 (M+, 11), 242 (19), 241 (100); exact mass calcd for M+
C27H24F2NO5 requires 480.1622, found 480.1623 (CI).
2
9b: 19F NMR (CDCl3, 282 MHz) -112.98 (1 F, dd, J FF
)
3
2
269.0 Hz, J HF ) 8.5 Hz), -122.96 (1 F, dd, J FF ) 269.0 Hz,
3J HF ) 19.2 Hz).
5.04 (1 H, m), 7.15-7.50 (10 H, m), 7.70 (2 H, m), 8.13 (1 H, d,
4
N,N-Dim eth yl (4S,3′S)-2-Oxo-3-(9-p h en ylflu or en -9-yl)-
oxa zolidin e-4-(2′,2′-d iflu or o-3′-h yd r oxy)p r op ion a m id e (7)
(fr om 9a ). n-BuLi (0.08 mL of a 2.5 M solution in THF, 0.2
mmol) was added to a solution of dimethylamine (0.1 mL of a
2 M solution in THF, 0.2 mmol) at -78 °C. The reaction
mixture was warmed to 0 °C and stirred for 30 min, before a
solution of 9a (76 mg, 0.16 mmol) in THF (0.5 mL) was added.
The mixture was warmed to rt and stirred for a further 1.5 h,
to ensure complete consumption of starting ester, before being
poured into ice (2 g). The aqueous solution was then extracted
with EtOAc (3 × 2 mL). The organic extracts were washed
with saturated brine (2 mL) and dried (MgSO4), and the
solvent was removed under reduced pressure to give an oily
residue. Purification by flash chromatography (eluant: 1:1
EtOAc/hexane) gave a white solid that had spectroscopic
properties identical to those of an authentic sample of 7: 75
mg, 98%.
Eth yl (4S,3′R)-2-Oxo-3-(9-p h en ylflu or en -9-yl)oxa zoli-
d in e-4-(3′-h yd r oxy)p r op a n oa te (11a ) a n d Eth yl (4S,3′S)-
2-Oxo-3-(9-ph en ylflu or en -9-yl)oxazolidin e-4-(3′-h ydr oxy)-
p r op a n oa te (11b). Ethyl bromoacetate (1.14 mL, 10 mmol)
was added slowly to a mixture of aldehyde 6 (2.29 g, 6 mmol)
and acid-washed zinc dust (0.78 g, 12 mmol) in dry THF (40
mL) at rt under a N2 atmosphere. The reaction mixture was
stirred vigorously for 2 h before the addition of aqueous NH4-
Cl (20 mL) and brine (20 mL). After filtration through Celite,
the precipitate was washed well with EtOAc (4 × 10 mL) and
the organic phase separated. After extraction of the aqueous
phase with EtOAc (4 × 10 mL), the combined organic extracts
were dried (MgSO4) and the solvent was removed under
reduced pressure to give an oily residue. This material was
purified by flash chromatography (eluant: 4:1 hexane/EtOAc).
The major diastereoisomer 11a was eluted as the first material
from the column, followed by 11b. Both compounds were
obtained as colorless oils.
J ) 7.6 Hz); 13C NMR (CDCl3, 75.4 MHz) δ 37.19 (qt, J CF
)
3
6.8 Hz), 37.57 (q), 55.41 (dt, J CF ) 5.0 Hz), 63.19 (t), 72.11
2
1
(s), 80.27 (dt, J CF ) 24.1 Hz), 114.01 (dd, J CF ) 268.0, 261.0
Hz), 120.21 (d), 120.46 (d), 124.64 (d), 126.77 (d), 127.47 (d),
128.38 (d), 128.44 (d), 128.59 (d), 129.45 (d), 129.71 (d), 129.83
(d), 137.00 (m), 139.65 (s), 139.83 (s), 140.28 (m), 140.93 (m),
2
141.54 (s), 144.44 (s), 147.33 (s), 157.21 (s), 159.99 (t, J CF
)
26.0 Hz), 189.97 (s); 19F NMR (CDCl3, 282 MHz) -107.37 (2
F, d, 3J HF ) 10.7 Hz), -151.84 (2 F, d, 3J FF ) 17.1 Hz), -156.58
3
(1 F, t, J ) 21.3 Hz), -162.16 (2 F, dt, J FF ) 21.3, 17.1 Hz);
MS (CI, CH4) 705 (MH+, 5), 704 (M+, 14), 242 (20), 241 (100);
exact mass calcd for M+ C34H23F7N2O5S requires 704.1216,
found 704.1224 (CI).
Further elution gave unreacted alcohol 7: 102 mg.
N,N-Dim eth yl (4S)-2-Oxo-3-(9-p h en ylflu or en -9-yl)ox-
a zolid in e-4-(2′,2′-d iflu or o)p r op ion a m id e (13). A solution
of the difluorinated derivative 12 (295 mg, 0.42 mmol) dis-
solved in toluene (8 mL) was degassed for 15 min before being
heated to reflux under an Ar atmosphere. A mixture of Bu3-
SnH (0.45 mL, 1.68 mmol) and AIBN (42 mg, 0.26 mmol)
dissolved in toluene (4 mL) was then added dropwise to the
hot solution. After being heated for an additional 15 min, the
reaction mixture was cooled to rt and quenched by the addition
of H2O (0.1 mL). The solvent was then removed under reduced
pressure, and the residue was purified using flash chroma-
tography (eluant: 5:2 hexane/EtOAc) to yield the deoyxygen-
ated amide 13 as a white foam. A portion was recrystallized
from EtOAc/hexane: 76 mg, 39%; mp 211-211.5 °C; [R]20
-638° (c ) 0.1, CHCl3); H NMR (CDCl3, 300 MHz) δ 2.78 (3
)
D
1
5
3
H, s), 2.89 (3 H, t, J HF ) 1.8 Hz), 3.64 (1 H, ddd, J HF ) 21.3,
12.3 Hz, J ) 8.4 Hz), 4.51 (1 H, dd, J ) 9.0, 8.4 Hz), 4.61 (1 H,
t, J ) 8.6 Hz), 4.85 (1 H, dd, J ) 16.2, 8.1 Hz), 6.58 (2 H, m),
6.70 (1 H, s), 6.84-7.04 (6 H, m), 7.18 (1 H, m), 7.39 (1 H, m),
7.52 (2 H, m), 7.76 (1 H, m); 13C NMR (CDCl3, 75.4 MHz) δ
2
36.90 (q), 37.24 (q), 49.21 (t, J CF ) 22.0 Hz), 58.24 (d), 61.69

Synthesis of L-4,4-Difluoroglutamic Acid
J . Org. Chem., Vol. 66, No. 19, 2001 6387
1
(t), 65.60 (s), 119.15 (t, J CF ) 257.0 Hz), 125.42 (d), 126.01
(MH+, 100), 463 (M+, 40), 254 (22), 241 (21); exact mass calcd
for MH+ C27H24F2NO4 requires 464.1673, found 464.1670 (CI).
Further elution gave the unreacted pentafluorophenyl de-
rivative 14: 116 mg.
(d), 126.08 (d), 127.28 (d), 127.59 (d), 128.15 (d), 128.79 (d),
129.09 (d), 130.39 (d), 130.58 (d), 133.37 (s), 136.42 (s), 138.21
(s), 139.85 (s), 141.43 (s), 158.29 (s), 162.10 (t, 2J CF ) 26.0 Hz);
19F NMR (CDCl3, 282 MHz) -89.86 (1 F, dd, J FF ) 281.4 Hz,
1-[N-Ben zyloxyca r bon yl-(1S,2R)-1-a m in o-3,3-d iflu or o-
3-e t h o x y c a r b o n y l-4-h y d r o x y p r o p y l]-4-m e t h y l-2,6,7-
tr ioxa bicyclo[2.2.2]octa n e (21a ) a n d 1-[N-Ben zyloxyca r -
bon yl-(1S,2S)-1-a m in o-3,3-d iflu or o-3-eth oxyca r bon yl-4-
h y d r o x y p r o p y l]-4-m e t h y l-2,6,7-t r io x a b ic y c lo [2.2.2]-
octa n e (21b). Ethyl bromodifluoroacetate (8 mL, 62.4 mmol)
was slowly added to a well-stirred mixture of acid-washed zinc
dust (6.08 g, 93.6 mmol) and aldehyde 3 (5 g, 15.6 mmol)
dissolved in dry THF (150 mL) at rt, under an argon atmo-
sphere. The reaction mixture was stirred for 2 h, after which
time TLC analysis showed complete consumption of aldehyde.
After the addition of 3% aqueous NH4Cl (80 mL) and brine
(80 mL), the mixture was filtered to remove solid materials.
The solution was then extracted with EtOAc (3 × 150 mL)
and washed with brine (150 mL), and the combined organic
extracts were dried (MgSO4). After removal of the solvent
under reduced pressure and column chromatography (eluant:
2:3 EtOAc/hexane containing 1% Et3N), the desired adduct was
obtained as a 7:1 mixture of diastereoisomers 21a :21b, based
on 19F NMR, as a white foam: 4.85 g, 70%; 13C NMR (CDCl3,
75.4 MHz) δ 14.10 (q), 14.41 (q), 30.95 (s), 52.11 (t), 63.39 (d),
2
3J HF ) 12.7 Hz), -93.43 (1 F, dd, 2J FF ) 281.4 Hz, 3J HF ) 21.4
Hz); MS (CI, CH4) 463 (MH+, 100), 462 (M+, 18), 443 (72), 241
(10); exact mass calcd for MH+ C27H25F2N2O3 requires 463.1833,
found 463.1828 (CI).
Eth yl (4S,3′S)-2-Oxo-3-(9-p h en ylflu or en -9-yl)oxa zoli-
d in e -4-(2′,2′-d iflu o r o -3′-o x y t h io c a r b o n y lp e n t a flu o -
r op h en yl)p r op a n oa te (14a ) a n d Eth yl (4S,3′R)-2-Oxo-3-
(9-p h e n ylflu or e n -9-yl)oxa zolid in e -4-(2′,2′-d iflu or o-3′-
oxyt h ioca r b on ylp en t a flu or op h en yl)p r op a n oa t e (14b ).
The mixture of diastereoisomeric fluorinated alcohols 9 (1.66
g, 3.5 mmol) was dissolved in toluene (50 mL), together with
DMAP (885 mg, 7 mmol) and N-hydroxysuccinimide (403 mg,
3.5 mmol). After the addition of pentafluorophenylchlorothio-
formate (408 mg, 1.5 mmol), the reaction mixture was heated
to 90 °C and stirred overnight. The solution was then filtered
through Celite and dried (MgSO4). After removal of the solvent
under reduced pressure, the crude product was purified using
flash chromatography (eluant: 1:6 EtOAc/hexane) to yield the
desired pentafluorophenyl ester as a mixture of diastereoiso-
mers 14a and 14b: 1.47 g, 60%; 13C NMR (CDCl3, 75.4 MHz)
δ 13.60 (q), 55.09 (d), 63.07 (t), 64.07 (t), 72.21 (s), 79.76 (ddd,
2
67.17 (t), 68.84 (ddd, J CF ) 31.0, 22.9 Hz), 73.08 (t), 108.74
1
2J CF ) 21.7, 30.2 Hz), 111.09 (dd, J CF ) 258.4, 261.7 Hz),
120.39 (d), 120.69 (d), 124.78 (d), 126.42 (d), 127.51 (d), 127.73
(d, J CF ) 225.6 Hz), 128.21 (d), 128.41 (d), 128.57 (d), 129.56
(d), 129.70 (d), 129.74 (d), 137.96 (m), 139.61 (s), 139.87 (s),
140.18 (m), 141.02 (s), 144.32 (s), 146.94 (s), 149.40 (s), 157.12
1
(s), 113.97 (dd, J CF ) 261.9, 250.9 Hz), 128.25 (d), 128.67 (d),
2
136.67 (s), 156.30 (s), 163.47 (dd, J CF ) 32.7, 29.7 Hz); MS
1
(FAB) 446 (MH+, 58), 138 (64), 137 (100), 136 (85); exact mass
calcd for MH+ C20H26F2NO8 requires 446.1626, found 446.1614
(FAB).
2
21a : 1H NMR (CDCl3, 300 MHz) δ 0.80 (3 H, s), 1.32 (3 H,
t, J ) 7.1 Hz), 3.24 (1 H, br. s), 3.92 (6 H, s), 4.26-4.35 (3 H,
m), 4.66 (1 H, dd, 3J HF ) 18.3, J ) 5.4 Hz), 5.13 (2 H, m), 5.43
(1 H, d, J ) 10.3 Hz), 7.32 (5 H, m); 19F NMR (CDCl3, 282
(s), 160.84 (t, J CF ) 29.7 Hz), 189.58 (s).
14a : 1H NMR (CDCl3, 300 MHz) δ 1.25 (3 H, t, J ) 7.2 Hz),
3
4.10 (2 H, m), 4.58 (3 H, m), 5.03 (1 H, dd, J HF ) 15.9 Hz, J
) 8.4 Hz), 7.20-7.48 (10 H, m), 7.71 (2 H, m), 8.07 (1 H, m);
2
3
MHz) -112.69 (1 F, dd, J FF ) 264.6 Hz, J HF ) 6.4 Hz),
19F NMR (CDCl3, 282 MHz) δ -111.76 (1 F, dd, J FF ) 264.7
2
2
3
-125.35 (1 F, dd, J FF ) 264.6 Hz, J HF ) 19.2 Hz).
21b: 19F NMR (CDCl3, 282 MHz) -112.33 (1 F, d, J FF
3
2
3
Hz, J HF ) 8.5 Hz), -116.83 (1 F, dd, J FF ) 264.7 Hz, J HF
)
2
)
16.9 Hz), -151.93 (2 F, d, J ) 19.2 Hz), -156.38 (1 F, t, J )
262.5 Hz), -124.68 (1 F, dd, 2J FF ) 264.6 Hz, 3J HF ) 17.1 Hz).
1-[N-Ben zyloxyca r bon yl-(1S,2R)-1-a m in o-3,3-d iflu or o-
3-eth oxyca r bon yl-4-oxyth ioca r bon ylim id a zolep r op yl]-4-
m eth yl-2,6,7-tr ioxa bicyclo[2.2.2]octa n e (22a ) a n d 1-[N-
B e n zy lo x y c a r b o n y l-(1S ,2S )-1-a m in o -3,3-d iflu o r o -3-
et h oxyca r b on yl-4-oxyt h ioca r b on ylim id a zolep r op yl]-4-
m eth yl-2,6,7-tr ioxabicyclo[2.2.2]octan e (22b). Thiocarbonyl-
diimidazole (349 mg, 1.76 mmol) and the diastereoisomeric
mixture of adducts 21a and 21b (491 mg, 1.1 mmol) were
dissolved in dry THF (15 mL) and stirred at rt, under an argon
atmosphere, for 10 h. The resulting reaction mixture was then
washed with 5% aqueous NH4Cl (15 mL), aqueous NaHCO3
(15 mL), and brine (15 mL). After drying (MgSO4), the solvent
was removed to yield an oily residue that was purified by
column chromatography (eluant: 1:3 EtOAc/hexane containing
1% Et3N). The mixture of thioester derivatives 22a and 22b
was obtained as a white foam: 465 mg, 76%; 13C NMR (CDCl3,
75.4 MHz) δ 13.63 (q), 14.00 (q), 30.72 (s), 52.11 (t), 63.60 (d),
67.34 (t), 72.82 (t), 75.51 (dt, 2J CF ) 27.2 Hz), 107.12 (s), 111.75
3
21.5 Hz), -161.9 (2 F, dt, J FF ) 21.5, 19.2 Hz); MS (CI, CH4)
705 (M+, 7), 704 (16), 241 (100).
14b: 1H NMR (CDCl3, 300 MHz) δ 1.25 (3 H, t, J ) 7.2
Hz), 4.10 (2 H, m), 4.58 (4 H, m), 7.20-7.48 (10 H, m), 7.71 (2
H, m), 8.07 (1 H, m); 19F NMR (CDCl3, 282 MHz) δ -113.29 (1
2
3
2
F, dd, J FF ) 266.7 Hz, J HF ) 8.5 Hz), -118.72 (1 F, dd, J FF
3
) 266.7 Hz, J HF ) 15.0 Hz), -152.57 (2 F, d, J ) 16.9 Hz),
3
-156.75 (1 F, t, J ) 21.5 Hz), -161.87 (2 F, dt, J FF ) 21.5,
16.9 Hz).
Eth yl (4S)-2-Oxo-3-(9-p h en ylflu or en -9-yl)oxa zolid in e-
4-(2′,2′-d iflu or o)p r op a n oa te (15). A solution of the difluori-
nated derivative 14 (490 mg, 0.695 mmol) dissolved in toluene
(15 mL) was degassed for 15 min before being heated to reflux
under an Ar atmosphere. A mixture of Bu 3SnH (0.56 mL, 2.09
mmol) and AIBN (30 mg, 0.15 mmol) dissolved in toluene (1
mL) was then added dropwise to the hot solution. After being
heated for an additional 10 min, the reaction mixture was
cooled to rt and quenched by the addition of H2O (0.1 mL).
The solvent was removed under reduced pressure, and the
residue was purified using flash chromatography (eluant: 5:2
hexane/EtOAc) to yield the deoyxygenated ester 15 as a white
foam. A portion was recrystallized (EtOAc/hexane) to give 15
as white crystals: 119 mg, 37% (49% based on recovered 14);
mp 182.0-183.5 °C; [R]20D ) -41.8° (c ) 1.02, CHCl3); 1H NMR
(CDCl3, 300 MHz) δ 0.84 (3 H, t, J ) 7 Hz), 3.57 (2 H, ddd,
3J HF ) 24.3, 12.9 Hz, J ) 8.4 Hz), 3.87 (2 H, m), 4.69 (3 H, m),
6.54 (2 H, m), 6.71 (1 H, s), 6.87-7.01 (5 H, m), 7.13 (1 H, m),
7.33 (1 H, m), 7.54 (2 H, m), 7.73 (1 H, m); 13C NMR (CDCl3,
1
(t, J CF ) 258.9 Hz), 118.17 (d), 128.19 (d), 128.45 (d), 130.96
2
(d), 135.92 (s), 137.19 (d), 155.80 (s), 161.54 (dd, J CF ) 31.2,
30.7 Hz), 182.63 (s); MS (FAB) 556 (MH+, 33), 154 (70), 91
(100); exact mass calcd for MH+ C24H28F2N3O8S requires
556.1565, found 556.1560 (FAB).
22a : 1H NMR (CDCl3, 300 MHz) δ 0.76 (3 H, s), 1.27 (3 H,
t, J ) 7.1 Hz), 3.77-3.86 (6 H, m), 4.24 (2 H, m), 4.62 (1 H,
dd, J ) 10.8, 1.0 Hz), 5.13-5.19 (3 H, m), 6.74 (1 H, m), 7.04
(1 H, m), 7.36 (5 H, m), 7.62 (1 H, m), 8.34 (1 H, m); 19F NMR
2
3
(CDCl3, 282 MHz) -113.87 (1 F, dd, J FF ) 269.0 Hz, J HF
)
2
2
3
75.4 MHz) δ 13.29 (q), 47.18 (tt, J CF ) 18.0 Hz), 57.57 (d),
61.65 (t), 62.69 (t), 65.51 (s), 116.90 (dd, J CF ) 254.0, 259.0
9.7 Hz), -115.19 (1 F, dd, J FF ) 269.0 Hz, J HF ) 11.7 Hz).
2
1
22b: 19F NMR (CDCl3, 282 MHz) -113.45 (1 F, dd, J FF
)
3 2
Hz), 125.27 (d), 126.11 (d), 127.61 (d), 127.85 (d), 128.40 (d),
128.93 (d), 129.18 (d), 130.01 (d), 130.59 (d), 131.61 (d), 131.75
(d), 133.42 (s), 136.36 (s), 138.11 (s), 139.60 (s), 141.68 (s),
158.13 (s), 161.50 (t, 2J CF ) 32 Hz); 19F NMR (CDCl3, 282 MHz)
270.0 Hz, J HF ) 9.6 Hz), -115.78 (1 F, dd, J FF ) 270.0 Hz,
3J HF ) 11.8 Hz).
1-[N-Ben zyloxyca r b on yl-(1S)-1-a m in o-3,3-d iflu or o-3-
eth oxycar bon ylpr opyl]-4-m eth yl-2,6,7-tr ioxabicyclo[2.2.2]-
octa n e (23). The diastereoisomeric mixtures of thioester
derivatives 22 (430 mg, 0.78 mmol) and Et3SiH (6.2 mL, 39
2
3
δ -89.47 (1 F, dd, J FF ) 262.5 Hz, J HF ) 13.0 Hz), -111.37
(1 F, dd, J FF ) 262.5 Hz, J HF ) 25.7 Hz); MS (CI, CH4) 464
2
3

6388 J . Org. Chem., Vol. 66, No. 19, 2001
Ding et al.
2
mmol) were dissolved in freshly distilled benzene (10 mL).
After the solution was degassed for 15 min using dry argon,
the reaction mixture was heated to reflux prior to the careful
addition of a solution of benzoyl peroxide (37.5 mg, 0.15 mmol)
in dry benzene (0.2 mL). At 30 min intervals, additional
portions of benzoyl peroxide were added to a total amount of
0.8 equiv. The reaction mixture was then refluxed for a further
30 min and then allowed to cool to rt. The solvent was removed
under reduced pressure and the residue was purified by flash
chromatography (eluant: 4:1 hexane/EtOAc containing 1%
Et3N) to give the dehydroxylated OBO ester 23 as a white
foam: 276 mg, 83%. A portion was recrystallized (EtOAc/hex-
(s); 19F NMR (D2O, 282 MHz) -103.36 (1 F, ddd, J FF ) 251.8
Hz, 3J HF ) 21.3, 12.8 Hz), -104.45 (1 F, ddd, 2J FF ) 251.8 Hz,
3J HF ) 19.2, 14.9 Hz); MS (CI, CH4) exact mass calcd for MH+
C5H8F2NO4 requires 184.0421, found 184.0410.
X-r a y Diffr a ction Str u ctu r e Deter m in a tion of N,N-
Dim eth yl (4R,3′S)-2-Oxo-3-(9-p h en ylflu or en yl-9-yl)oxa z-
olidin e-4-(2′,2′-diflu or o-3′-h ydr oxy)pr opion am ide (7). Data
were collected at 173 K on a Siemens SMART PLATFORM
equipped with A CCD area detector and a graphite monochro-
mator utilizing Mo KR radiation (λ ) 0.710 73 Å). Cell
parameters were refined using 5720 reflections. A hemisphere
of data (1381 frames) was collected using the ω-scan method
(0.3° frame width). The first 50 frames were remeasured at
the end of data collection to monitor instrument and crystal
stability (maximum correction on I was <1%). Absorption
corrections by integration were applied based on measured
indexed crystal faces. The structure was solved by direct
methods in SHELXTL539 and refined using full-matrix least-
squares. The non-H atoms were treated anisotropically, whereas
the hydrogen atoms were calculated in ideal positions and were
riding on their respective carbon atoms, except the O3′
hydroxyl and the water molecule protons which were obtained
ane) to give 23 as colorless needles. mp 78.0-78.8 °C; [R]20
)
D
1
-37.6° (c ) 1, EtOAc); H NMR (CDCl3, 300 MHz) δ 0.79 (3
H, s), 1.27 (3 H, t, J ) 7.0 Hz), 2.17-2.36 (1 H, m), 2.58 (1 H,
3
ddd, J HF ) 29.8, 14.9 Hz, J ) 2.3 Hz), 3.87 (6 H, s), 4.13-
4.27 (3 H, m), 4.84 (1 H, d, J ) 10.5 Hz), 5.11 (2 H, m), 7.29-
7.34 (5 H, m); 13C NMR (CDCl3, 75.4 MHz) δ 13.77 (q), 14.14
(q), 30.57 (s), 34.80 (ddt, 2J CF ) 24.4, 23.3 Hz), 49.78 (t), 62.76
1
(d), 66.78 (t), 72.69 (t), 107.73 (s), 115.24 (dd, J CF ) 252.1,
248.9 Hz), 127.99 (d), 128.03 (d), 128.37 (d), 136.34 (s), 155.76
2
(s), 163.90 (dd, J CF ) 32.7, 32.2 Hz); 19F NMR (CDCl3, 282
2
3
MHz) -102.76 (1 F, ddd, J FF ) 266.8 Hz, J HF ) 14.9, 12.8
Hz), -106.29 (1 F, ddd, 2J FF ) 266.7 Hz, 3J HF ) 19.2, 14.9 Hz);
MS (FAB) 430 (MH+, 93), 307 (59), 136 (100), 107 (48); exact
mass calcd for MH+ C20H26F2NO7 requires 430.1677, found
430.1671 (FAB).
from
a difference Fourier map and refined without any
constraints. There is a water molecule of solvation present in
the asymmetric unit. A total of 340 parameters were refined
in the final cycle of refinement using 3880 reflections with I
> 2σ(I) to yield R1 and wR2 of 4.04% and 7.63%, respectively.
Refinement was done using F2.
(2S)-4,4-Diflu or oglu ta m ic Acid (2). The N-protected OBO
ester 23 (190 mg, 0.44 mmol) was dissolved in 6 N aqueous
HCl (20 mL) and the solution heated at 90 °C for 2 h. The
solvent was removed under vacuum to yield a viscous oil that
was redissolved in doubly deionized (dd) H2O. After the
solution pH was adjusted to 7 using aqueous NaHCO3, the
solution was applied to an anion-exchange column (BioRad
AG3-X4 resin) that had been equilibrated with ddH2O. Elution
was then carried out using ddH2O and an increasing gradient
of TFA up to a final concentration of 0.25 M. Ninhydrin
positive fractions were then pooled and lyophilization yielded
the crude product 2 as a solid. L-4,4-Difluoroglutamic acid 2
was then obtained by recrystallization from H2O/iPrOH as
white needles: 35 mg, 37%; mp 174.5-176.5 °C dec (lit.12 mp
Ack n ow led gm en t. We thank the National Insti-
tutes of Health (NIH), National Cancer Institute
(CA28725), and the NSF (W.R.D.) for partial support
of this work. Funding for the X-ray diffraction facilities
was provided by the NSF and the University of Florida
and is gratefully acknowledged (K.A.A.).
Su p p or tin g In for m a tion Ava ila ble: 1H, 13C, and 19F
NMR spectra of 7, 9a , 13, 15, 21, 23, and 2, together with 1H
and 13C spectra for 11a , an ORTEP plot of 7 and full
information concerning the X-ray structure of 7. This material
is available free of charge via the Internet at http://pubs.acs.org.
173-176 °C dec); [R]20 ) 6.5° (c ) 1, ddH2O) (lit.13 [R]20
)
D
D
5.4° (c ) 1.04, H2O)); 1H NMR (D2O, 300 MHz) δ 2.55-2.92 (3
J O015754Q
H, m), 4.32 (1 H, dd, J ) 8.4, 3.8 Hz); 13C NMR (D2O, 75.4
2
1
MHz) δ 34.79 (tt, J CF ) 24.7 Hz), 48.53 (d), 116.42 (dd, J CF
2
) 251.8, 251.3 Hz), 169.00 (dd, J CF ) 28.2, 27.7 Hz), 171.26
(39) Sheldrick, G. M. SHELXTL5; Bruker-AXS: Madison, WI, 1998.