Synthesis of (-)-2-fluoroshikimic acid

Full text of DOI:10.1021/jo952132u

3916
J . Org. Chem. 1996, 61, 3916-3919
Sch em e 1
Syn th esis of (-)-2-F lu or osh ik im ic Acid
Robert H. Rich and Paul A. Bartlett*
Department of Chemistry, University of California,
Berkeley, California 94720-1460
Received December 1, 1995
Fluorinated substrate analogs are useful as mechanis-
tic probes and inhibitors of enzymatic reactions. Fluorine
and hydrogen are similar sterically, yet dramatically
different electronically,1 so this substitution can reveal
the nature of charged species in a reaction mechanism
through its influence on the rate.2 Fluorine also serves
as a leaving group in an important class of suicide
inhibitors of amino acid metabolism.3 Depicted in Scheme
1 are the fluorinated derivatives of a number of inter-
mediates along the shikimic acid pathway4,5 that have
been prepared synthetically6-10 and enzymatically.11-14
As anticipated, fluorine has a significant effect on the
reactivity of these analogs. For example, the 6-fluoro
derivatives of 5-enolpyruvylshikimate 3-phosphate (EPSP),
7, have played an important role in deciphering the
mechanism of the 1,4-elimination of phosphate catalyzed
by chorismate synthase.13,15 Bornemann et al. have
recently demonstrated the enzymatic conversion of 6R-
fluoro-EPSP, 7, to 6-fluorochorismic acid, 8, which for the
E. coli enzyme occurs at 0.3% the rate with the normal
substrate.14
Sch em e 2
The 2-fluoro derivative of shikimic acid, 9, has not been
reported previously; we now describe a synthesis of this
material by chemical conversion from shikimic acid itself.
The new derivative provides access to the fluorinated
analogs of shikimate 3-phosphate, EPSP, and, poten-
tially, chorismic acid, which are isomeric to the 6-fluoro
derivatives so far described.
In an earlier report, we outlined the synthesis of
2-chloroshikimic acid, 12, with chlorination of sulfide 10
as the key step in the sequence (Scheme 2).16 That route
did not prove to be adaptable to the synthesis of the
(1) Resnati, G. Tetrahedron 1993, 49, 9385-9445.
(2) Poulter, C. D.; Argyle, J . C.; Marsh, E. A. J . Biol. Chem. 1978,
253, 7227-7233.
(3) Silverman, R. B. Mechanism-Based Enzyme Inactivation: Chem-
istry and Enzymology; CRC Press, Inc.: Boca Raton, FL, 1988; Vols. I
and II.
(4) Haslam, E. Shikimic Acid Metabolism and Metabolites; Wiley-
Interscience: New York, 1993.
(5) Dewick, P. M. Nat. Prod. Rep. 1995, 12, 101-133.
(6) Alberg, D. G.; Lauhon, C. T.; Nyfeler, R.; Fa¨ssler, A.; Bartlett,
P. A. J . Am. Chem. Soc. 1992, 114, 3535-3546.
(7) Lauhon, C. T.; Bartlett, P. A. Biochemistry 1994, 33, 14100-
14108.
(8) Sutherland, J . K.; Watkins, W. J .; Bailey, J . P.; Chapman, A.
K.; Davies, G. M. J . Chem. Soc., Chem. Commun. 1989, 1386-1387.
(9) Bowles, S. A.; Campbell, M. M.; Sainsbury, M.; Davies, G. M.
Tetrahedron 1990, 46, 3981-3992.
(10) Le Mare´chal, P.; Froussios, C.; Azerad, R. Biochimie 1986, 68,
1211-1215.
fluorine analog, since elimination to the unsaturated
sulfide (e.g., 13) occurred in preference to fluorination
under a variety of conditions. We also explored a route
involving the 2-keto analog 14, with the intention of using
(diethylamino)sulfur trifluoride (DAST) to introduce the
unsaturated fluoride directly. However, the desired
transformation was blocked by the highly enolic character
of this intermediate, as a result of the conjugating
carboxyl group at C-1. Therefore, to allow manipulation
at the 2-position without interference from the carboxyl
group, we developed the sequence depicted in Scheme 3,
(11) Pilch, P. F.; Somerville, R. L. Biochemistry 1976, 15, 5315-
5320.
(12) Walker, M. C.; J ones, C. J .; Somerville, R. L.; Sikorski, J . A. J .
Am. Chem. Soc. 1992, 114, 7601-7603.
(13) Balasubramanian, S.; Davies, G. M.; Coggins, J . R.; Abell, C.
J . Am. Chem. Soc. 1991, 113, 8945-8946.
(14) Bornemann, S.; Ramjee, M. K.; Balasubramanian, S.; Abell, C.;
Coggins, J . R.; Lowe, D. J .; Thorneley, R. N. F. J . Biol. Chem. 1995,
270, 22811-22815.
(15) Ramjee, M. N.; Balasubramanian, S.; Abell, C.; Coggins, J . R.;
Davies, G. M.; Hawkes, T. R.; Lowe, D. J .; Thorneley, R. N. F. J . Am.
Chem. Soc. 1992, 114, 3151-3153.
(16) Rich, R. H.; Lawrence, B. M.; Bartlett, P. A. J . Org. Chem. 1994,
59, 693-694.
S0022-3263(95)02132-3 CCC: $12.00 © 1996 American Chemical Society

Notes
J . Org. Chem., Vol. 61, No. 11, 1996 3917
Sch em e 3
in which the carboxyl group is reduced at the outset and
restored at the end.
of 68%. These compounds are separately deprotected at
the primary position by oxidation with DDQ24 to give
alcohols 22 and 23, respectively, which in turn are
oxidized by the Dess-Martin procedure25 or, in better
yield, by PCC,21 to provide the fluoro enal 24 in each case.
It appears that the difluoro derivative undergoes spon-
taneous elimination under the reaction conditions.
Further oxidation of the aldehyde 24 to the carboxylic
acid 25 is carried out with sodium chlorite as oxidant in
DMSO/water in quantitative yield.26 Finally, the robust
nature of the methylene acetal protecting groups was
manifested when it came time to remove them. Cleavage
of the methoxymethyl ether at C-5 is accomplished
readily in aqueous hydrochloric acid, but the cyclic acetal
is more resistant, requiring prolonged reflux in 3 N HCl.
Nonetheless, 2-fluoroshikimic acid, 9, is stable to these
conditions and can be isolated as a hygroscopic powder
in 60% overall yield after purification by reversed-phase
HPLC and lyophilization.
The vigorous conditions anticipated for the DAST
reaction dictated robust protection for the hydroxyl
groups of shikimic acid. Methyl shikimate17 was there-
fore protected as the methylene acetal and methoxy-
methyl ether,18 15, and then reduced to the allylic alcohol
16 with DIBALH.19 A number of protecting groups were
evaluated for the primary hydroxyl group; the 2,5-
dimethoxybenzyl ether proved to be stable to the suc-
ceeding reaction conditions, and, importantly, can be
removed by an oxidative process that preserves the vinyl
fluoride moiety and leaves the acetal protecting groups
intact.20 Hydroboration of the double bond of 17, followed
by oxidation with alkaline hydrogen peroxide, leads to
the saturated alcohol 18, albeit in moderate yield (45%
from 17). The major side reaction appears to be Grob-
like fragmentation of the intermediate organoborane with
opening of the acetal ring (e.g., 26 f 27), since the
byproducts have lost their methylene acetal hydrogens
Preliminary experiments with shikimate kinase sug-
gest that 2-fluoroshikimate is accepted as a substrate and
that the product, 2-fluoroshikimate 3-phosphate, can be
further converted to 2-fluoro-EPSP by EPSP synthase.
The results of these investigations will reported sepa-
rately.
1
by H NMR.
Exp er im en ta l Section 27,28
Meth yl 5â-Meth oxym eth oxy-3r,4r-m eth ylen ed ioxycy-
cloh ex-1-en eca r boxyla te (15). To a stirred solution of methyl
shikimate, 1417 (6.68 g, 35.5 mmol), in chloroform (175 mL, dried
over P2O5) were added dimethoxymethane (175 mL) and P2O5
Oxidation of alcohol 18 with PCC21 to ketone 19 is
straightforward, setting the stage for introduction of
fluorine.22 Reaction with DAST23 produces both geminal
difluoride 20 and vinyl fluoride 21, in a combined yield
(23) Middleton, W. J . J . Org. Chem. 1975, 40, 574-578. Strobach,
D. R.; Boswell, G. A. J . Org. Chem. 1971, 36, 818-820.
(24) Oikawa, Y.; Yoshioka, T.; Yonemitsu, O. Tetrahedron Lett. 1982,
23, 885-888.
(25) Dess, D. B.; Martin, J . C. J . Am. Chem. Soc. 1991, 113, 7277-
7287.
(26) Dalcanale, E.; Montanari, F. J . Org. Chem. 1986, 51, 567-569.
(27) Gen er a l Meth od s. All reactions involving moisture-sensitive
reagents were performed under a dry argon atmosphere. 19F chemical
shifts are reported in ppm from CFCl3 as 0 ppm (downfield positive).
J values are given in Hz. Chromatography refers to the method of Still,
Kahn, and Mitra28 using silica gel 60 (E. Merck, Darmstadt).
(28) Still, W. C.; Kahn, M.; Mitra, A. J . Org. Chem. 1978, 43, 2923-
2925.
(17) Fischer, H. O. L.; Dangschat, G. Helv. Chim. Acta 1934, 17,
1206-1213.
(18) Fuji, K.; Nakaro, S.; Fujita, E. Synthesis 1975, 276-277.
(19) Yoon, N. M.; Gyoung, Y. S. J . Org. Chem. 1985, 50, 2443-2450.
(20) Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic
Synthesis, 2nd ed.; J ohn Wiley & Sons: New York, 1991; p 53.
(21) Hudlicky, M. Oxidations in Organic Chemistry; American
Chemical Society: Washington, D.C., 1990; p 284.
(22) The configurations depicted for the C-1 carbons in 18 and 19
are consistent with coupling constants discerned in selectively de-
coupled 1D 1H NMR spectra and suggest that epimerization to the
more stable C-equatorial isomer occurs after oxidation to the ketone.

3918 J . Org. Chem., Vol. 61, No. 11, 1996
Notes
(90 g), and the mixture was allowed to stir at rt for 20 h. The
reaction mixture was partitioned between ice-cold saturated
NaHCO3 and CH2Cl2 (400 mL each). The reaction flask was
carefully washed with more NaHCO3 and CH2Cl2 (100 mL each),
and the solutions were combined. The organic phase was
washed with NaHCO3 (100 mL) and brine (100 mL), dried,
filtered, and evaporated under reduced pressure. The crude
product was chromatographed (25% ethyl acetate/hexane) to give
fully protected acetal 15 (7.36 g, 85%) as a colorless oil: 1H NMR
δ 6.85 (m, 1), 5.00 (s, 1), 4.95 (s, 1), 4.70 (s, 2), 4.66 (m, 1), 4.13
(t, 1, J ) 6.4), 3.94 (td, 1, J ) 6.6, 4.4), 3.75 (s, 3), 3.36 (s, 3),
2.69 (m, 1), 2.38 (m, 1); 13C NMR δ 166.4, 133.1, 130.8, 95.6,
94.3, 75.4, 72.3, 72.2, 55.4, 52.0, 26.8. FAB-MS (NBA - LiCl)
251.1 (100, MLi+). Anal. Calcd for C11H16O6: C, 54.09; H, 6.60.
Found: C, 54.05; H, 6.66.
5â-(Meth oxym eth oxy)-3r,4r-(m eth ylen ed ioxy)cycloh ex-
1-en em eth a n ol (16). To a stirred solution of ester 15 (7.36 g,
30 mmol) in THF (150 mL) at -78 °C was added a 1.0 M solution
of diisobutylaluminum hydride in hexane (120 mL, 4 equiv). The
solution was stirred for 20 min, allowed to warm to 0 °C, and
stirred for an additional 30 min. Isopropyl alcohol (30 mL) was
added to decompose excess reagent, and the resulting solution
was allowed to stir for 10 min before it was poured into a mixture
of potassium sodium tartrate (170 g, 603 mmol, 20 equiv) in 50%
water/ethyl acetate (800 mL). This mixture was stirred vigor-
ously for 17.5 h, the layers were separated, and the aqueous
phase was extracted with ethyl acetate (two 400-mL portions).
The combined organic layer was dried, filtered, and evaporated
under reduced pressure. Chromatography (ethyl acetate) of the
crude residue gave alcohol 16 (6.47 g) as a colorless oil: 1H NMR
δ 5.75 (br s, 1), 5.01 (s, 1), 4.87 (s, 1), 4.68 (AB-q, 2, J ) 6.8),
4.45 (br s, 1), 4.03 (t, 1, J ) 6.9), 3.98 (br s, 2), 3.75 (m, 1), 3.31
(s, 3), 2.68 (m, 1), 2.31 (m, 1), 2.02 (m, 1); 13C NMR δ 140.8,
117.2, 100.2, 95.8, 94.0, 76.2, 73.2, 65.2, 55.2, 29.1. FAB-MS
(NBA + LiCl) 223.2 (40, MLi+); HRMS calcd for C10H16LiO5
223.1158, found 223.1163. Anal. Calcd for C10H16O5: C, 55.55;
H, 7.46. Found: C, 55.75; H, 7.40.
1-[[(2,5-D i m e t h o x y b e n z y l)o x y ]m e t h y l]-5â-(m e t h -
oxym eth oxy)-3r,4r-(m eth ylen ed ioxy)cycloh exen e (17). To
a stirred solution of allylic alcohol 16 (3.4 g, 16 mmol) in THF
(200 mL) were added consecutively 2,5-dimethoxybenzyl chlo-
ride29 (7.3 g, 39 mmol, 2.5 equiv) and sodium hydride (4.5 g, 94
mmol, 6.0 equiv), and the suspension was heated to reflux for 6
h. The mixture was allowed to cool, NH4OH (50 mL) was added,
and stirring was continued for 5 min. The mixture was diluted
with ether (600 mL), the layers were separated, and the organic
phase was washed with NH4Cl (600 mL), NaHCO3 (600 mL),
and brine (600 mL), dried, filtered, and evaporated under
reduced pressure. The crude oil was chromatographed (20%
ethyl acetate/hexane) to give 2,5-dimethoxybenzyl ether 17 (5.1
g, 88% from 15) as an oil: 1H NMR δ 6.95 (br s, 1), 6.76 (br s,
2), 5.86 (br s, 1), 5.08 (s, 1), 4.94 (s, 1), 4.73 (AB-q, 2, J ) 6.8),
4.51 (m, 1), 4.48 (s, 2), 4.09 (m, 1), 3.99 (s, 2), 3.82 (m, 1), 3.76
(s, 3), 3.75 (s, 3), 3.35 (s, 3), 2.4 (m, 1), 2.11 (m, 1); 13C NMR δ
153.5, 151.1, 138.3, 127.4, 119.4, 114.7, 112.9, 111.2, 95.8, 94.1,
76.2, 73.3, 73.11, 73.07, 66.8, 55.8, 55.6, 55.3, 29.5. Anal. Calcd
for C19H26O7: C, 62.28; H, 7.15. Found: C, 61.89; H, 7.09.
1r-[[(2,5-D im e t h o x y b e n z y l)o x y ]m e t h y l]-5â-(m e t h -
oxym eth oxy)-3r,4r-(m eth ylen edioxy)cycloh exan -2â-ol (18).
To a solution of alkene 17 (5.92 g, 16.2 mmol) in THF (200 mL)
at 0 °C was added a 1.0 M solution of BH3‚THF in THF (113
mL, 7 equiv), and the solution was stirred for 1.7 h. Alkaline
hydrogen peroxide (600 mL, 2:1 30% hydrogen peroxide/3.0 M
NaOH) was then added carefully through a reflux condenser.
The mixture was stirred for 15 min and warmed to room
temperature. After 35 min, the solution was heated to 60 °C
for 2.2 h. The mixture was allowed to cool and diluted with ether
(500 mL), and the organic phase was washed successively with
NaHCO3 (500 mL) and brine (500 mL), dried, filtered, and
evaporated under reduced pressure. The residue was chromato-
graphed (40% ethyl acetate/hexanes) to give the starting olefin
17 (310 mg, 5%) as a colorless oil. Further elution with 60%
ethyl acetate gave the desired alcohol 18 (2.79 g, 45%) as a
colorless oil: 1H NMR δ 6.86 (br s, 1), 6.74 (br s, 2), 5.09 (s, 1),
4.93 (s, 1), 4.63 (s, 2), 4.48 (s, 2), 4.05 (m, 2), 3.93 (m, 1), 3.73 (s,
3), 3.71 (s, 3), 3.60 (m, 1), 3.50 (m, 2), 3.32 (s, 3), 2.04 (m, 1),
1.68 (m, 1), 1.49 (m, 1); 13C NMR δ 153.4, 151.4, 127.0, 115.1,
113.1, 111.3, 95.4, 94.4, 79.6, 77.4, 73.9, 73.2, 71.6, 68.6, 55.9,
55.7, 55.5, 35.1, 27.6. Anal. Calcd for C19H28O8: C, 59.36; H,
7.34. Found: C, 59.37; H, 7.50.
1r-[[(2,5-D im e t h o x y b e n z y l)o x y ]m e t h y l]-5â-(m e t h -
oxym eth oxy)-3r,4r-(m eth ylen edioxy)cycloh exan -2-on e (19).
A solution of alcohol 18 (2.79 g, 7.2 mmol) and pyridinium
chlorochromate (4.69 g, 21.8 mmol, 3.0 equiv) in CH2Cl2 (200
mL) was stirred for 24 h. The crude orange solution was loaded
directly on a silica gel column (equilibrated with 40% ethyl
acetate/hexane) and chromatographed (40% ethyl acetate/hex-
ane) to give ketone 19 (2.18 g, 78%) as a colorless oil: 1H NMR
δ 6.94 (br s, 1), 6.74 (br s, 2), 5.0 (AB-q, 2), 4.74 (AB-q, 2), 4.51
(m, 3), 4.31 (m, 1), 4.21 (m, 1), 3.85 (m, 1), 3.74 (s, 6), 3.50 (m,
1), 3.39 (s, 3), 3.1 (m, 1), 2.35 (m, 1), 1.8 (m, 1); 13C NMR δ 206.3,
153.6, 151.1, 127.8, 114.5, 112.7, 111.2, 95.7, 95.2, 81.0, 77.9,
71.0, 68.3, 67.9, 55.9, 55.8, 55.7, 43.4, 30.4. Anal. Calcd for
C
19H26O8: C, 59.68; H, 6.85. Found: C, 59.50; H, 7.24. Further
elution of the column with 50% ethyl acetate/hexane led to
recovery of starting alcohol 18 (300 mg, 11%).
2,2-Diflu or o-1r-[[(2,5-d im eth oxyben zyl)oxy]m eth yl]-5â-
(m eth oxym eth oxy)-3r,4r-(m eth ylen edioxy)cycloh exan e (20)
a n d 2-F lu or o-1-[[(2,5-d im et h oxyb en zyl)oxy]m et h yl]-5â-
m eth oxym eth oxy-3r,4r-(m eth ylen edioxy)cycloh exen e (21).
To a stirred solution of ketone 19 (2.18 g, 5.7 mmol) in anhydrous
dimethoxyethane (350 mL) was added (diethylamino)sulfur
trifluoride (DAST) (3.77 mL, 28.5 mmol, 10.0 equiv), and the
solution was heated to 80 °C. While heating was continued,
additional portions of DAST (3.77 mL, 28.5 mmol, 10.0 equiv)
were added through the top of the reflux condenser at 2, 8.5,
and 22.5 h total time. The solution was allowed to cool after a
total of 27 h. Excess reagent was carefully decomposed through
the addition of NaHCO3 (500 mL), and the mixture was
extracted with ether (500 mL). The organic phase was washed
with NaHCO3 (500 mL) and brine (500 mL), dried, filtered, and
evaporated under reduced pressure. Chromatography of the
crude product (20% ethyl acetate/hexane) gave difluoride 20 (657
mg, 28%) as a colorless oil: 1H NMR δ 6.95 (br s, 1), 6.76 (br s,
2), 5.23 (s, 1), 4.99 (s, 1), 4.68 (AB-q, 2, J ) 6.9, 15.1), 4.52 (d, 2,
J ) 10.8), 4.24 (br m, 1), 4.17 (br s, 1), 4.08 (br s, 1), 3.88 (dd, 1,
J ) 4.0, 9.6), 3.76 (s, 3), 3.75 (s, 3), 3.51 (d, 1, J ) 9.0), 3.36 (s,
3), 2.45 (br m, 1), 2.17 (m, 1), 1.81 (m, 1); 13C NMR δ 171.0,
153.6, 151.0, 127.7, 114.4, 112.7, 111.2, 96.5, 95.5, 78.1, 74.5,
70.2, 67.8, 67.0, 55.8, 55.7, 55.6, 37.0, 26.4. 19F NMR δ -104.7
(d, J ) 244.4), -120.3 (ddd, J ) 244.5, 27.5, 15.4); FAB-MS
(NBA) 404 (100, M+); HRMS calcd for C19H26O7F2 404.1647,
found 404.1646.
Further elution gave vinyl fluoride 21 (871 mg, 40%) as a
colorless oil: 1H NMR δ 6.93 (br s, 1), 6.77 (br s, 2), 5.10 (s, 1),
5.03 (s, 1), 4.69 (s, 2), 4.67 (m, 1), 4.45 (s, 2), 4.23 (m, 1), 4.16
(m, 2), 3.94 (m, 1), 3.76 (s, 3), 3.75 (s, 3), 3.35 (s, 3), 2.55 (m, 1),
2.34 (m, 1); 19F NMR δ -121.0 (s); 13C NMR δ 153.0, 151.2, 151.0
(d), 127.3, 114.9, 113.6, 113.1, 111.2, 95.6, 94.7, 77.1, 71.7, 71.5,
66.7, 64.8, 55.8, 55.6, 55.4, 27.4. FAB-MS (G) 384 (100, M+);
HRMS calcd for C19H25O7F 384.1584, found 384.1587.
2,2-Diflu or o-1r-(h yd r oxym eth yl)-5â-(m eth oxym eth oxy)-
3r,4r-(m eth ylen ed ioxy)cycloh exa n e (22). A mixture of 2,5-
dimethoxybenzyl ether 20 (657 mg, 1.62 mmol) and 2,3-dichloro-
5,6-dicyanoquinone (DDQ, 443 mg, 1.95 mmol, 1.2 equiv) in 20:1
CH2Cl2/water (210 mL) was stirred for 22 h. The mixture was
diluted with CH2Cl2 (50 mL), washed with brine (three 250-mL
portions), dried, filtered, and evaporated under reduced pressure.
The crude oil was chromatographed (40% ethyl acetate/hexane)
to give the desired difluoro alcohol 22 (290 mg, 70%) as a
colorless oil: 1H NMR δ 5.24 (s, 1), 4.99 (s, 1), 4.69 (m, 2), 4.24
(br m, 1), 4.20 (br s, 1), 4.08 (br s, 1), 3.96 (dd, 1, J ) 11.4, 5.3),
3.74 (dd, 1, J ) 11.3, 5.8), 3.38 (s, 3), 2.33 (br m, 1), 1.97 (m, 1),
1.87 (m, 1); 19F NMR δ -104.0 (d, J ) 244.0), -120.1 (ddd, J )
244.0, 27.5, 15.0); 13C NMR δ 122.0, 96.5, 95.5, 78.0, 74.4, 70.2,
59.7, 55.7, 38.7, 25.9; FAB-MS (NBA) 255.0 (65, MH+). Anal.
Calcd for C10H16F2O5: C, 47.24; H, 6.34. Found: C, 47.40; H,
6.57.
2-Flu or o-1-(h ydr oxym eth yl)-5â-(m eth oxym eth oxy)-3r,4r-
(m et h ylen ed ioxy)cycloh exen e (23). A mixture of 2,5-
dimethoxybenzyl ether 21 (220 mg, 0.57 mmol) together with
2,3-dichloro-5,6-dicyanoquinone (DDQ, 156 mg, 0.69 mmol, 1.2
equiv) in 21:1 CH2Cl2/water (157 mL) was stirred for 22 h. The
(29) O’Shea, K. E.; Fox, M. A. J . Am. Chem. Soc. 1991, 113, 611-
615.

Notes
J . Org. Chem., Vol. 61, No. 11, 1996 3919
mixture was diluted with CH2Cl2 (30 mL) and washed with brine
(three 180-mL portions), and the organic phase was dried,
filtered, and evaporated under reduced pressure. The crude oil
was chromatographed (45% ethyl acetate/hexane) to give the
fluoro alcohol 23 (90 mg, 67%) as a colorless oil: 1H NMR δ 5.06
(s, 1), 4.98 (s, 1), 4.67 (s, 2), 4.58 (m, 1), 4.19 (m, 3), 3.93 (m, 1),
3.33 (s, 3), 2.51 (m, 1), 2.26 (m, 1); 19F NMR δ -20.5 (s); 13C
NMR δ 149.2 (d, J ) 256), 115.5 (d, J ) 9), 95.8, 94.7, 76.9,
71.9, 71.6 (d, J ) 26), 57.6 (d, J ) 7), 55.4, 27.2 (d, J ) 4). Anal.
Calcd for C10H15FO5: C, 51.28; H, 6.45. Found: C, 51.21; H,
6.56.
2-F lu or o-5â-(m eth oxym eth oxy)-3r,4r-(m eth ylen ed ioxy)-
cycloh ex-1-en eca r boxa ld eh yd e (24). A solution of difluoro
alcohol 22 (333 mg, 1.31 mmol), NaHCO3 (220 mg, 2.62 mmol,
2.0 equiv), and pyridinium chlorochromate (1.411 g, 6.54 mmol,
5.0 equiv) in CH2Cl2 (25 mL) was stirred for 2 h. The orange
solution was loaded directly onto a silica gel column (equilibrated
with 20% ethyl acetate/hexane) and eluted (20% ethyl acetate/
hexane) to give aldehyde 24 (184 mg, 60%) as a colorless oil:
1H NMR δ 10.15 (s, 1), 5.09 (s, 1), 5.04 (s, 1), 4.79 (m, 1), 4.65
(AB-q, 2, J ) 6.9), 4.32 (m, 1), 4.14 (m, 1), 3.33 (s, 3), 2.58 (m,
1), 2.47 (m, 1); 19F NMR δ -113.0; 13C NMR δ 187.7 (d, J ) 12),
166.0 (d, J ) 285), 116.1, 95.7, 95.3, 76.8 (d, J ) 7), 71.2 (d, J )
23), 70.7, 55.6, 22.0 (d, J ) 2). Anal. Calcd for C10H13FO5: C,
51.72; H, 5.64. Found: C, 52.09; H, 5.71.
Fluoro aldehyde 24 was also prepared as follows: A solution
of fluoro alcohol 23 (102 mg, 0.44 mmol) and pyridinium
chlorochromate (470 mg, 2.18 mmol, 5.0 equiv) in CH2Cl2 (10
mL) was stirred for 2 h. The orange solution was worked up as
described above to give aldehyde 24 (68 mg, 67%).
2-F lu or o-5â-(m eth oxym eth oxy)-3r,4r-(m eth ylen ed ioxy)-
cycloh ex-1-en eca r boxylic Acid (25). To a stirred solution of
aldehyde 24 (132 mg, 0.57 mmol) in 1:2 DMSO/H2O (6 mL) was
added NaH2PO4 (16 mg, 0.11 mmol, 0.2 equiv) followed by
sodium chlorite (72 mg, 0.80 mmol, 1.4 equiv), and the resulting
solution was allowed to stir for 20 h. The reaction mixture was
neutralized with NaHCO3 (20 mL) and washed with CH2Cl2
(three 50-mL portions). The aqueous phase was acidified to pH
1 with 3 N HCl and extracted with CH2Cl2 (three 50-mL
portions). The organic extract was dried, filtered, and evapo-
rated under reduced pressure to give free acid 25 (141 mg, 99%)
as a colorless oil: 1H NMR δ 5.08 (s, 1), 5.02 (s, 1), 4.67 (m, 3),
4.27 (m, 1), 4.09 (m,1), 3.36 (s, 3), 2.64 (m, 2); 13C NMR δ 165.9,
158.1 (d, J ) 280), 108.2 (d, J ) 3), 95.2, 94.6, 76.4 (d, J ) 7),
71.4 (d, J ) 25), 70.7, 55.2, 26.1; 19F NMR δ -95.2 (s); FAB-MS
(NBA) 247 (M- - H, 100), 249 (M+, 25); HRMS calcd for
C
10H14O6F 249.0774, found 249.0770.
2-F lu or osh ik im ic Acid (9) a n d 2-F lu or o-5â-h yd r oxy-
3r,4r-(m eth ylen edioxy)cycloh ex-1-en ecar boxylic Acid (26).
A stirred solution of carboxylic acid 25 (135 mg, 0.54 mmol) and
concd HCl (2 mL) in water (5 mL) was heated to reflux for 17.5
h. The solution was cooled, diluted with water (15 mL), filtered
through a 0.1 µm filter, and lyophilized. The crude white solid
was purified by preparative reversed-phase HPLC (C-18 Vydac
column, 10 mL/min flow rate, 0.9 mL injection volume, 0.1%
trifluoroacetic acid/water mobile phase). The first product to
elute from the column (7.5 min retention time) was 2-fluoro-
shikimic acid, 9 (42 mg, 40%), which could be isolated as a
hygroscopic white solid after lyophilization: [R]23 ) -137° (c
D
1
) 0.99 in H2O); H NMR (D2O) δ 4.36 (m, 1), 3.82 (m, 1), 3.67
(dd, 1, J ) 9.2, 4.6), 2.67 (m, 1), 2.13 (m, 1); 13C NMR (D2O) δ
167.9, 161.1 (d, J ) 282), 107.9, 71.2 (d, J ) 9), 66.3 (d, J ) 24),
65.5, 29.4; 19F NMR (D2O) δ -100.5 (s); ꢀ187 ) 13 100; IR (KBr)
3481, 3354, 3215, 2900 br, 1692, 1662, 1440, 1262, 1147, 1100,
997, 938, 774, 668 cm-1; FAB-MS (NBA) 191 (M- - H, 100);
HRMS calcd for C7H8O5F 191.0356, found 191.0352.
Continued elution with an increasing concentration of 0.1%
trifluoroacetic acid/acetonitrile in the mobile phase led to the
isolation of methylene acetal 26 (62 mg, 30%) as a hygroscopic
white solid: 1H NMR (D2O) δ 5.01 (s, 1), 4.95 (s, 1), 4.73 (m, 1),
4.22 (m, 1), 3.95 (dd, 1, J ) 11.0, 6.6), 2.57 (m, 1), 2.34 (m, 1);
13C NMR (D2O) δ 167.4, 159.9, 108.7, 94.3, 77.4 (d, J ) 8), 71.2
(d, J ) 25), 65.1, 27.9; 19F NMR (D2O) δ -101.0 (s). This
material was converted to 2-fluoroshikimic acid on further
treatment in aqueous HCl at reflux and purification (35 mg,
62%).
Ack n ow led gm en t. We thank Kimberly Stigers for
her help in characterizing the final product. Support
for this work was provided by the National Institutes
of Health (Grant No. GM-28965).
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