Quasiracemic synthesis: Concepts and implementation with a fluorous tagging strategy to make both enantiomers of pyridovericin and mappicine

Article abstract of DOI:10.1021/ja025606x

The concept of quasiracemic synthesis is introduced and illustrated with syntheses of both enantiomers of pyridovericin (whose absolute configuration is assigned as R) and mappicine. Like racemic synthesis, quasiracemic synthesis provides both enantiomers in a single synthetic sequence; however, separation tagging is used to ensure that quasiracemic mixtures can be analyzed, separated, and identified on demand. Fluorous tags of differing chain lengths are used to tag two enantiomeric starting materials. The resulting quasienantiomers are mixed to make a quasiracemate, which is then treated like a true racemate in successive steps of the synthesis. Fluorous chromatography is used to separate, or demix, the final quasiracemate into its two components, which are then detagged to provide (true) enantiomeric products. Quasiracemic synthesis is portrayed as the first and simplest of a series of mixture synthesis techniques based on separation tagging, and the prospects for using other types of separation tags are briefly evaluated.

Full text of DOI:10.1021/ja025606x

Published on Web 04/24/2002
Quasiracemic Synthesis: Concepts and Implementation with
a Fluorous Tagging Strategy to Make Both Enantiomers of
Pyridovericin and Mappicine
Qisheng Zhang, Alexey Rivkin, and Dennis P. Curran*
Contribution from the Department of Chemistry, UniVersity of Pittsburgh,
Pittsburgh, PennsylVania, 15260
Received January 15, 2002
Abstract: The concept of quasiracemic synthesis is introduced and illustrated with syntheses of both
enantiomers of pyridovericin (whose absolute configuration is assigned as R) and mappicine. Like racemic
synthesis, quasiracemic synthesis provides both enantiomers in a single synthetic sequence; however,
separation tagging is used to ensure that quasiracemic mixtures can be analyzed, separated, and identified
on demand. Fluorous tags of differing chain lengths are used to tag two enantiomeric starting materials.
The resulting quasienantiomers are mixed to make a quasiracemate, which is then treated like a true
racemate in successive steps of the synthesis. Fluorous chromatography is used to separate, or demix,
the final quasiracemate into its two components, which are then detagged to provide (true) enantiomeric
products. Quasiracemic synthesis is portrayed as the first and simplest of a series of mixture synthesis
techniques based on separation tagging, and the prospects for using other types of separation tags are
briefly evaluated.
Introduction
the other by a mirror, but the molecules differ in one part such
that an atom or group of one quasienantiomer is reflected into
Chiral compounds abound in organic, medicinal, and natural
products chemistry. Because biological activity and other
properties are a function of the absolute configuration, the
synthesis of chiral compounds in enantiopure form is of utmost
importance. Today, there are two ways to make enantiopure
(or enantioenriched) organic molecules: racemic synthesis
followed by resolution, or asymmetric synthesis.1 We introduce
herein a third method, quasiracemic synthesis, which unites
some of the key advantages of racemic and asymmetric
synthesis.
Classical racemic synthesis makes both enantiomers of a
target compound in a single synthesis, but separation (resolution)
and identification of the final enantiomers pose large hurdles.
Asymmetric synthesis employs enantiopure compounds, but two
separate syntheses are needed if both enantiomers are desired.
Like asymmetric synthesis, quasiracemic synthesis starts and
finishes with enantiopure compounds, but like racemic synthe-
sis it provides both enantiomers in a single synthesis. “Quasi-
enantiomers” are used in place of enantiomers, and the separa-
tion and identification of the final quasienantiomers is ensured
by a tagging strategy.2
a similar but not identical atom or group of the other. For
example, (S)-2-chlorobutane and (R)-2-bromobutane are quasi-
enantiomers. “Quasiracemic” follows as a mixture of equal parts
of quasienantiomers.3 The “method of quasiracemates” is a
classical technique used to assign absolute configuration,1 and
innovative new resolution methods based on quasienantiomers
have been introduced by Bergbreiter,4 Vedejs,5 and Zwanen-
burg.6
The tagging strategy of quasiracemic synthesis is shown in a
very general way in Figure 1. Enantiomeric compounds (R)-1
and (S)-1 are tagged with different tags to provide “quasi-
enantiomers” (R)-2a and (S)-2b. The quasienantiomers (R)-2a
and (S)-2b are mixed to make a quasiracemic mixture, which
is then taken through a series of steps to make a final tagged
product mixture (R)-3a/(S)-3b. The mixture is then separated
by a method that is selective based on the tag to provide the
two pure quasienantiomers. The tag is finally removed to
generate the two true enantiomers (R)-4 and (S)-4.
Ideally, tags for quasiracemic synthesis behave as if they are
identical during intermediate steps of the synthesis. This means
that tagged quasienantiomers should behave like true enanti-
omers: they should have the same solubility, reactivity (toward
We use the term quasienantiomers as defined qualitatively
by Eliel.1a Quasienantiomers are not enantiomers because they
are not isomers; “most” of one quasienantiomer is reflected into
(3) The terms “pseudoenantiomers” and “pseudoracemates” are often used with
the same meanings, although pseudoracemate has another meaning (a
crystalline form consisting of a solid solutiuon of two (true) enantiomers).
See ref 1a, p 159.
* Corresponding author. E-mail: curran+@pitt.edu.
(1) (a) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compunds; Wiley-
Interscience: New York, 1994; p 1267. (b) Jacques, J.; Collet, A.; Wilen,
S. H. Enantiomers, Racemates and Resolutions; Wiley: New York, 1981.
(2) The concept of separation tagging is generalized in: Curran, D. P. Angew.
Chem., Int. Ed. Engl. 1998, 37, 1175.
(4) Bergbreiter, D. E.; Zhang, L. J. Chem. Soc., Chem. Commun. 1993, 596.
(5) (a) Vedejs, E.; Chen, X. J. Am. Chem. Soc. 1997, 119, 2584. (b) Vedejs,
E.; Rozners, E. J. Am. Chem. Soc. 2001, 123, 2428.
(6) ten Holte, P.; Thijs, L.; Zwanenburg, B. Org. Lett. 2001, 3, 1093.
9
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Figure 2. Structures of pyridovericin and mappicine.
2) was isolated from the entomopathogenic fungus BeauVeria
bassiana EPF-5 in 1998, and is an inhibitor of the protein
tyrosine kinase.9 This pyridone has one stereocenter of unknown
absolute configuration along with a primary hydroxy group that
serves as a convenient location for the tag. Initial goals were to
synthesize both enantiomers of pyridovericin 5 in enantiopure
form and to assign the absolute configuration of the natural
product. As the work unfolded, only the second goal was met.
To reach the first, we selected the antiviral agent mappicine
6,10 which has been made by a number of groups.11 We have
used an isonitrile cascade annulation12 to make mappicine by
racemic synthesis,13 asymmetric synthesis,14 solid-phase syn-
thesis (with little success15), and solution phase parallel syn-
thesis.12 Herein we use the same strategy for the quasiracemic
synthesis. The quasiracemic synthesis of mappicine along with
the synthesis of analogues by “fluorous mixture synthesis” has
been briefly communicated.16
Figure 1. Quasiracemic synthesis by separation tagging.
achiral species), physical properties and spectral characteristics.
They should also have similar, preferably identical, chromato-
graphic properties on regular and reverse phase silica gel.
However, tags must be chosen such that there is at least one
“orthogonal” reaction or separation method that can be used to
differentiate on demand the quasiracemic mixtures based on the
tag.
While there may be no tag that exhibits “ideal” behavior, we
posit that many tags will be close enough to ideal to be useful.
To demonstrate quasiracemic synthesis, we selected fluorous
tags bearing slightly different perfluorinated fragments. Fluorous
chromatography is the separation method that is complementary
to the tag.7 Fluorous methods have been exploited recently in a
diverse collection of settings with an underlying theme of
separating fluorous-tagged molecules from organic molecules.8
Here we instead capitalize on the ability to separate fluorous-
tagged molecules from each other, or to prevent that separation,
depending on whether fluorous or nonfluorous separation
methods are used.
Results and Discussion
Figure 3 shows the plan for the synthesis of pyridovericin 5.
This plan follows directly from a synthesis of tenellin by Rigby
and Qabar.17 Tenellin (not shown) is an isomer of pyridovericin
with a hydroxyl group on the pyridone nitrogen rather than at
the end of one of the branches of the side chain. The centerpiece
of the synthesis is Rigby’s [4 + 2] annulation18 between
ketoester 7 and vinyl isocyanate 8 to make the pyridone ring.
Working back through two olefinations provides the chiral
monoprotected diols 9. The plan was to prepare both quasi-
(9) (a) Takahashi, S.; Uchida, K.; Kakinuma, N.; Hashimoto, R.; Yanagisawa,
T.; Nakagawa, A. J. Antibiot. 1998, 51, 1051. (b) Takahashi, S.; Kakinuma,
N.; Uchida, K.; Hashimoto, R.; Yanagisawa, T.; Nakagawa, A. J. Antibiot.
1998, 51, 596.
(10) (a) Govindachari, T. R.; Ravindranath, K. R.; Viswanathan, N. J. Chem.
Soc., Perkin Trans. 1 1974, 1215. (b) Pirillo, A.; Verotta, L.; Gariboldi,
P.; Torregiani, E.; Bombardelli, E. J. Chem. Soc., Perkin Trans. 1 1995,
583.
Quasiracemic syntheses of two natural products, pyridovericin
5 and mappicine 6, are described below. Pyridovericin (Figure
(11) (a) Comins, D. L.; Saha, J. K. J. Org. Chem. 1996, 61, 9623. (b) Boger,
D. L.; Hong, J. Y. J. Am. Chem. Soc. 1998, 120, 1218. (c) Yadav, J. S.;
Sarkar, S.; Chandrasekhar, S. Tetrahedron 1999, 55, 5449. (d) Mekouar,
K.; Genisson, Y.; Leue, S.; Greene, A. E. J. Org. Chem. 2000, 65, 5212.
(e) Ishibashi, H.; Kato, I.; Takeda, Y.; Tamura, O. Tetrahedron Lett. 2001,
42, 931. (f) Toyota, M.; Komsii, C.; Ihara, M. J. Org. Chem. 2000, 65,
7110. (g) Das, B.; Madhusudhan, P. Tetrahedron 1999, 55, 7875.
(12) (a) Curran, D. P.; Liu, H. J. Am. Chem. Soc. 1991, 113, 2127. (b) Josien,
H.; Ko, S. B.; Bom, D.; Curran, D. P. Chem. Eur. J. 1998, 4, 67.
(13) Josien, H.; Curran, D. P. Tetrahedron 1997, 53, 8881.
(14) deFrutos, O.; Curran, D. P. J. Comb. Chem. 2000, 2, 639.
(15) Gabarda, A.; Choi, J.-H.; Curran, D. P. Unpublished observations.
(16) Luo, Z. Y.; Zhang, Q. S.; Oderaotoshi, Y.; Curran, D. P. Science 2001,
291, 1766.
(7) Curran, D. P. Synlett 2001, 1488.
(8) (a) Horva´th, I. T. Acc. Chem. Res. 1998, 31, 641. (b) Curran, D. P. The
Cancer J. 1998, 4 (Supp. 1), S73. (c) Barthel-Rosa, L. P.; Gladysz, J. A.
Coord. Chem. ReV. 1999, 192, 587. (d) Curran, D. P.; Hadida, S.; Studer,
A.; He, M.; Kim, S.-Y.; Luo, Z.; Larhed, M.; Hallberg, M.; Linclau, B. In
Combinatorial Chemistry: A Practical Approach; Fenniri, H., Ed.; Oxford
University Press: Oxford, 2001; Vol. 2, p 327. (e) Curran, D. P. In
Stimulating Concepts in Chemistry; Vo¨gtle, F., Stoddart, J. F., Shibasaki,
M., Eds.; Wiley-VCH: New York, 2000; p 25. (f) Curran, D. P. Pure Appl.
Chem. 2000, 72, 1649. (g) Curran, D. P.; Hadida, S.; Studer, A.; He, M.;
Kim, S. Y.; Luo, Z. Y.; Larhed, M.; Hallberg, A.; Linclau, B. In
Combinatorial Chemistry; Fenniri, H., Ed.; Oxford University Press: New
York, 2000; Vol. 233, p 327. (h) Curran, D. P.; Luo, Z. Y. J. Am. Chem.
Soc. 1999, 121, 9069. (i) Luo, Z. Y.; Williams, J.; Read, R. W.; Curran,
D. P. J. Org. Chem. 2001, 66, 4261.
(17) Rigby, J. H.; Qabar, M. J. Org. Chem. 1989, 54, 5852.
(18) Rigby, J. H. Synlett 2000, 1.
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Zhang et al.
perfluorhexylethyl diisopropylsilyl; and “c” series, perfluoro-
octylethyl diisopropylsilyl.
To validate the synthetic plan and confirm the structure, we
first synthesized racemic pyridovericin using a standard (non-
fluorous) protecting group. Monosilylation of 2-ethyl-1,3-
propane diol 10 with triisopropylsilyl chloride gave rac-11a in
98% yield.19 Swern oxidation20 and Wittig olefination provided
unsaturated ester rac-12a in 82% isolated yield (>97% E).
DIBAL reduction21 and Swern oxidation provided aldehyde rac-
13a in 95% yield. This was olefinated with phosphine oxide
1422 to give â-keto ester rac-15a in 72% yield as a mixture of
keto and enol forms (enol form not shown).
Keto ester rac-15a was then condensed with readily available
isocyanate 823 according to the two-step protocol of Rigby and
Qabar.24 Deprotonation of rac-15a with sodium hydride at 0
°C was followed by addition of vinyl isocyanate 7 and warming
to room temperature. After 12 h, workup and flash chromato-
graphic purification provided enamide rac-16a. This existed in
the enol form as indicated, although the geometry of the enol
double bond was not assigned. Heating of rac-16a for 5 min at
250 °C in diphenyl ether provided pyridone rac-17a in 76%
yield. Shorter times or lower temperatures gave incomplete
conversion, while longer times resulted in lower yields due to
product decomposition.
Figure 3. Strategy for quasiracemic synthesis of pyridovericin.
enantiomers of diol 9 protected with different fluorous tags (PG1,
PG2), and then to mix the quasienantiomers for the quasiracemic
synthesis. This plan maximizes the efficiency of the mixture
synthesis by mixing early and demixing late.
We initially planned to remove the MEM group of rac-17a
first followed by the TIPS group, and later to conduct demixing
between these two steps in the quasiracemic synthesis. However,
treatment of rac-17a with TMSCl and sodium iodide at -20
°C for 2 h25 provided the desilylated product with the MEM
group intact (not shown). Prolonged exposure of rac-17a to a
larger excess of reagents then removed the MEM group to
provide rac-pyridoverin 5 in 79% overall yield. This was
identical in all respects (except rotation) with the natural product,
so both the structure and the synthesis were validated. The
Scheme 1 summarizes all four of the syntheses of pyrido-
vericin conducted in this work. Three of these are traditional
racemic syntheses with a nonfluorous and two different fluorous
tags (protecting groups), and the fourth is the quasiracemic
synthesis. Prefixes before numbers have the following mean-
ings: “rac”, true racemate; “R” or “S”, enriched in the R- or
S-enantiomer; and “M”, quasiracemic mixture made by manual
mixing of quasienantiomers. Suffixes designate the alcohol
protecting groups: “a” series, triisopropylsilyl; “b” series,
Scheme 1
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observations in deprotection of rac-17a led us to reformulate
the plan; demixing would be conducted before any deprotection
and then both protecting groups would be removed simulta-
neously by prolonged exposure to TMSCl/NaI.
We chose fluorous diisopropyl silanes bearing -(CH2)2C6F13
and -(CH2)2C8F17 groups as tags for the quasienantiomers. The
requisite known silanes26 were prepared as shown in eq 1.
isocyanate 8 gave rac-16b,c (69%, 58%), and heating of these
enols gave pyridones rac-17b,c (60%, 55%). Removal of both
the silyl group and the MEM group occurred on exposure of
rac-17b to excess TMSCl/NaI, and rac-pyridovericin 5 was
produced in 90% yield. The larger homologue rac-17c was not
deprotected.
Throughout these two separate syntheses, the chromatographic
behavior of the products bearing shorter (b series, C6F13) and
longer (c series, C8F17) tails was compared. In no case could
the pairs of products be differentiated by thin-layer chroma-
tography. This suggests that “accidental” separation of the
quasienantiomers after mixing will not occur. In contrast, the
pairs of products were well differentiated on a fluorous column
(over 6 min difference in retention time with gradient elution
on a Fluofix 120E column), so preparative demixing by fluorous
chromatography was assured.
With the groundwork laid, we then moved ahead to the
quasiracemic synthesis of pyridovericin with the double goals
of producing both enantiomers in enantiopure form and assign-
ing the absolute configuration of the natural product. Enantio-
pure monosilylated propane diol derivatives were prepared as
shown in eq 2. Evans alkylation28 of oxazolidinone (S)-22 with
Addition of diisopropylchlorosilane to the lithium reagents
derived from perfluorohexylethyl iodide 18b and perfluoro-
octylethyl iodide 18c gave the silanes 19b,c. These were then
brominated in situ to give solutions of silyl bromides 20b,c,
which were used immediately for silylations.
To ensure that the fluorous silyl groups behave identically
to each other and to the triisopropylsilyl groups, we then
repeated the synthesis two more times with racemic alcohols
11b,c tagged to the corresponding fluorous silyl tags. The results
of these syntheses are also summarized in Scheme 1. This
proved to be a good decision since one of the steps failed.
Silylation of propane diol 10 with the in situ generated silyl
bromides provided rac-11b and 11c in 75% and 86% yields.
Swern oxidation and olefination then provided rac-12b,c (73%,
79%). Reduction and a second Swern oxidation (72%, 64%)
provided aldehydes rac-13b,c without event. However, ole-
fination of rac-13b with phosphine oxide 14 under the condi-
tions used for the nonfluorous silyl ether rac-13a and a variety
of others failed to provide the keto ester rac-15b. In general,
substantial amounts of the aldehyde were recovered. To solve
this problem, we switched to using a phosphonate27 21 in place
of the phosphine oxide 14. Generation of the dianion of 21 with
2 equiv of LDA followed by addition of rac-13b,c gave keto
esters rac-15b,c in 90% and 78% yields. Keto esters 15b,c again
existed as mixtures of keto/enol tautomers (enol not shown).
With the olefination problem solved, the remaining steps
proceeded without event. Condensation of rac-15b,c with
(benzyloxy)methyl chloride (BOMCl) according to Fukumoto29
followed by removal of the benzyl group provided (S)-23 as a
single diastereomer in 78% yield after chromatography. Silyl-
ation of (S)-23 with the shorter fluorous silyl tag (89%) and
reductive removal of the auxiliary30 gave (S)-11b. In turn, (R)-
11c was prepared by an identical sequence of steps starting from
(R)-22, but silylation was conducted with a longer fluorous silyl
tag.
Equimolar amounts of quasienantiomers (S)-11b and (R)-11c
were then mixed to make the first quasiracemic mixture
M-11b/c, and this was taken forward in the synthesis sum-
marized again in Scheme 1. Swern oxidation and olefination
provided M-12b/c (73%), which was then reduced to give an
alcohol followed by oxidation under Swern conditions to
aldehyde M-13b/c (95%). Olefination with phosphonate 21
provided ketoester M-15b/c (83%, keto/enol mixture). Coupling
(19) McDougal, P. G.; Rico, J. G.; Oh, Y.-I.; Condon, B. D. J. Org. Chem.
1986, 51, 3388.
(20) Gaucher, A.; Ollivier, J.; Marguerite, J.; Paugam, R.; Salau¨n, J. Can. J.
Chem. 1994, 72, 1312.
(21) Diez-Martin, D.; Kotecha, N. R.; Ley, S. V.; Mantegani, S.; Menendez, J.
C.; Organ, H. M.; White, A. D.; Banks, B. J. Tetrahedron 1992, 48, 7899.
(22) van der Gen, A.; van den Goorbergh, J. A. M. Tetrahedron Lett. 1980, 21,
3621.
(23) Rigby, J. H.; Burkhardt, F. J. J. Org. Chem. 1986, 51, 1374.
(24) (a) Rigby, J. H.; Balasubramanian, N. J. Org. Chem. 1984, 49, 4569. (b)
Rigby, J. H.; Balasubramanian, N. J. Org. Chem. 1989, 54, 224. (c) Rigby,
J. H.; Holsworth, D. D.; James, K. J. Org. Chem. 1989, 54, 4019.
(25) Rigby, J. H.; Wilson, J. Z. Tetrahedron Lett. 1984, 25, 1429.
(26) Boutevin, B.; Guida-Pietrasanta, F.; Ratsimihety, A.; Caporiccio, G.;
Gornowicz, G. J. J. Fluorine Chem. 1993, 60, 211. The silanes are
now commercially available from Fluorous Technologies, Inc (www.
fluorous.com).
(28) (a) Gage, J. R.; Evans, D. A. Org. Synth. 1989, 68, 83. (b) Evans, D. A.;
Chapman, K. T.; Hung, D. T.; Kawaguchi, A. T. Angew. Chem., Int. Ed.
Engl. 1987, 26, 1184.
(29) Ihara, M.; Katsumata, A.; Setsu, F.; Tokunaga, Y.; Fukumoto, K. J. Org.
Chem. 1996, 61, 677.
(30) Crimmins, M.; King, B. W. J. Am. Chem. Soc. 1998, 120, 9084.
(27) (a) Svendsen, A.; Boll, P. M. Tetrahedron 1973, 29, 4251. (b) Bodalski,
R.; Pietrusiewicz, K. M.; Monkiewicz, J.; Koszuk, J. Tetrahedron Lett.
1980, 21, 2287.
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Figure 4. 1H NMR spectra of rac-13b, rac-13c, and quasiracemic mixture
M-13b/c.
with isocyanate 8 (58%) followed by heating at 250 °C for 8
min gave protected pyridovericin M-17b/c in 50% purified yield.
The seven mixture steps of the quasiracemic synthesis were
conducted just as in the prior racemic syntheses, and yields were
calculated based on the average molecular weight of the
quasienantiomers. Most products were purified by flash chro-
matography on standard silica gel, and there was no evidence
of separation of the quasienantiomers. We started with 440 mg
of M-11b/c and finished with 230 mg of M-17b/c.
Figure 5. 13C NMR spectra of rac-13b, rac-13c, and quasiracemic mixture
M-13b/c.
Quasiracemic mixtures were characterized in the standard way
by 1H NMR and 13C NMR spectroscopy. Like spectra from true
racemic mixtures, the spectra from the quasiracemic mixtures
appeared to reflect resonances from a single compound. Figure
4 provides as an example the 1H NMR spectra of rac-13b, rac-
13c, and M-13b/c recorded at 300 MHz in CDCl3; these spectra
are indistinguishable. Broad band decoupled 13C NMR spectra
of these same molecules are shown in Figure 5. These spectra
are also identical except in the region of 105-125 ppm, where
the carbons bearing fluorines resonate. This region has a
complicated set of low-intensity peaks (due to C-F coupling)
which differ for the C6F13, C8F17, and mixture samples. With
appropriate experiments and expansions, this complicated region
can actually be fully analyzed to provide information about the
tags. However, this is unnecessary since there are simpler ways
to get this information (see below).
1
Complementary to H and 13C NMR spectroscopy, which
provide information about the substrate but not the tag, is 19F
NMR spectroscopy, which provides information about the tag
and not the substrate. Figure 6 shows the CF2 region of three
19F NMR spectra corresponding to rac-13b, rac-13c, and
M-13b/c. These spectra show characteristic differences. For
example, the peak at -120.5 ppm in rac-13b bearing the C6F13
group integrates to two fluorines (one difluoromethylene group),
while the same peak in rac-13c bearing the C8F17 group
integrates to six fluorines (three overlapping difluoromethylene
groups). In the quasiracemic mixture (M-13b/c), this peak
integrates to the average value of four fluorines. Also, the CF2
group in the C6F13 compound at -122.0 ppm is slightly upfield
from its counterpart in the C8F17 quasienantiomer at -121.8
ppm. There were no detectable variations in the 19F NMR
spectra of any of the intermediates as a function of change of
Figure 6. 19F NMR spectra of rac-13b, rac-13c, and quasiracemic mixture
M-13b/c.
the substrate part of the molecule. Evidently, the fluorines are
too remote to be affected by the structural changes imposed by
the synthesis. Accordingly, 19F NMR spectroscopy can be used
at any time to ensure (within the limits of integration error)
that the quasiracemic mixture does not become deficient in one
of the quasienantiomers due to differential reaction or separation.
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Mass spectroscopy provides information about both the tag
and the substrate, and LCMS with a fluorous hplc column is
especially useful. Quasienantiomers (S)-13b and (R)-13c were
separated by about 7 min under standard conditions (see
Supporting Information), and were present in roughly equi-
molar ratios. Molecular ions corresponding to each quasi-
enantiomer were observed by MS detection. Conveniently,
the quasienantiomers differ in molecular weight by 100 mu
(C2F4).
Prior to detagging, pyridone M-17b/c was demixed into its
individual tagged components by semipreparative fluorous hplc.
Considerably larger columns are now available, but at this time
we were limited to injections of about 7 mg. A 30 min run
included a 10 min linear gradient of 92% MeOH/water up to
100% MeOH followed by 20 min at 100% MeOH. Under these
conditions, (S)-17b and (R)-17c eluted at about 13 and 18 min,
respectively. A series of ten 7 mg injections provided 30 mg of
the lighter 17b and 33 mg of the heavier 17c; the total recovery
was 90%.
Compounds (S)-17b and (R)-17c were then individually
detagged (deprotected) by using TMSCl/NaI in acetonitrile to
give (S)- and (R)-pyridovericin 5, respectively (eq 3). These
Figure 7. Alcohol precursors for Mosher analysis of pyridovericin
intermediates.
sample of about 1 mg of natural pyridovericin, whose rotation
was measured in our lab as -17.8 (c 0.09, MeOH). With these
results, we can confidently assign the absolute configuration of
natural pyridovericin as R. Disappointingly, with estimated ee
values of about 15%, the samples of 5 prepared from quasi-
racemic synthesis are a lot closer to racemic than enantio-
pure.
Unsatisfied with the use of optical rotation to measure
enantiopurity, we experimented with several chiral columns to
resolve pyridovericin 5 or its immediate protected precursor 17,
but without success. We then resorted to Mosher ester methods
(eq 4). The sample of (R)-pyridovericin precursor (R)-17c from
quasiracemic synthesis was degraded by ozonolysis and reduc-
tion31 to regenerate the starting alcohol (R)-11c. This was
converted to its Mosher ester (R)-24c,32 whose de value was
measured by 1H and 19F NMR to be 25%. The 25% ee
determined by derivatization is probably more accurate than the
15% ee from rotation, but the take home message is the same:
substantial racemization occurred during the synthesis.
samples were purified first by flash chromatography and then
by preparative reverse phase hplc (Symmetry C18 column). The
NMR and mass spectra of each enantiomer were identical with
that of natural pyridovericin. The optical rotation of (S)-5 was
+3.0 (c 0.1, MeOH) while that of (R)-5 was -3.3 (c 0.06,
MeOH). These rotations are considerably lower than the reported
value of -20.3 (c 0.1 MeOH) for the natural product. Dr.
Nakagawa of Teikyo University, Japan, kindly provided us a
To provide more information on where racemization was
occurring, we converted three other intermediates to Mosher
(31) Smith, A. B., III; et al. U.S. Patent 5,789,605.
(32) Takeuchi, Y.; Itoh, N.; Kawahara, S.; Koizumi, T. Tetrahedron 1993, 49,
1861.
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A R T I C L E S
Zhang et al.
Scheme 2
esters and measured their de values. Racemic samples of each
Mosher ester were made to facilitate experiments. The results
of these experiments are summarized in Figure 7. Starting
alcohol (R)-11c was derivatized and shown to be enantiopure
within the limits of detection. Two steps later, ester (S)-12b
has decreased to 84% ee. An additional two steps provided
aldehyde (S)-13b of only 64% ee. Clearly, the ee loss comes
not at one but several steps in the sequence. Additional basic
and high-temperature conditions follow in the synthesis, so in
retrospect we were fortunate that our final samples were not
racemic.
In quasiracemic synthesis, racemization can occur by epimer-
ization as in standard asymmetric synthesis. It can also occur
by chemical exchange (scrambling) of the tags from one
enantiomer to the other. Thus, both epimerization and tag
scrambling must be avoided. We felt that tag scrambling was
unlikely and that base-catalyzed (or thermal) epimerization was
the likely culprit. To support this hypothesis, we exposed a THF
solution of (R)-13c to DBU for 48 h at 25 °C (eq 5). As
The first quasiracemic synthesis only met one of the two
initial goals: The absolute configuration of natural pyridovericin
5 was assigned as R by making both enantiomers in a single
synthesis, but the final products were not enantiopure (or even
highly enriched). To demonstrate that enantiopure products
could be made, we turned to our recent asymmetric synthesis
of (S)-mappicine 6,13,14 which was mutated into a quasiracemic
synthesis as shown in Scheme 2. Reduction of readily available
ketone 25 with (+)-B-chlorodiisopinylcampheylborane ((+)-
DIP-chloride)33 afforded (R)-26 in 97% yield with >98% ee,
as reported.14 This was silylated with the smaller silyl bromide
20b to give (R)-27b (82%). Likewise, reduction with (-)-DIP-
chloride provided enantiopure (S)-26 (92%), which was silylated
with the larger silyl bromide 20c to give (S)-27c (90%). These
were then mixed in equimolar amounts for the four-step
quasiracemic synthesis.
Treatment of M-27b/c with ICl followed by demethylation
with BBr3 and purification by flash chromatography over
standard silica gel gave M-28b/c in 48% yield. N-Propargylation
under standard conditions34 provided the radical precursor
M-29b/c in 61% yield. In turn, this was reacted with phenyl
isonitrile under established conditions for the cascade radical
annulation to provide the quasiracemic mixture of protected
mappicines M-30b/c.
This mixture was then separated by fluorous chromatography
employing the same gradient as above. The two quasi-
enantiomers were well separated ((R)-30b, 18 min; (S)-30c, 26
min). Six serial injections of about 10 mg each provided 23
mg of (R)-30b and 25 mg of (S)-30c. Deprotection of the
measured by optical rotation, the ee of the sample dropped from
65% ([R]D -9.4, c 1.0, MeOH) to 45% ([R]D -6.5, c 1.0,
MeOH). Thus, epimerization through enolization of 13 and
related intermediates probably accounts for the loss of enantio-
purity of the tagged quasienantiomers.
(33) Brown, H. C.; Ramachandran, P. V. In AdVances in Asymmetric Synthesis;
Hassner, A., Ed.; Jai Press Inc: Greenwich, CT, 1995; Vol. 1, p 147.
(34) Liu, H.; Ko, S. B.; Josien, H.; Curran, D. P. Tetrahedron Lett. 1995, 36,
8917.
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Quasiracemic Synthesis
A R T I C L E S
resolved quasienantiomers provided the natural (-)-mappicine
(S)-6 (from (S)-30c, 74%) and its enantiomer (+)-mappicine
(R)-6 (from (R)-30b, 68%). The final products (R)/(S)-6 from
the quasiracemic synthesis were analyzed by chiral HPLC on a
ChiralCel OD column and found to be enantiopure within the
limits of detection (>98% ee).11b Unnatural (R)-mappicine has
been isolated previously by resolution,11b,11g and this is its first
asymmetric synthesis.
for strategic separation purposes has become increasingly
common in recent years, and a number of practical strategies
have emerged.3 Extending any one of these strategies for use
in quasiracemic synthesis requires the development of two
different kinds of tags that ideally behave the same under typical
synthetic reaction and separation conditions, but have at least
one “tag specific” reaction or separation that can be used at
will to differentiate tagged molecules.
In the larger picture of solution phase mixture synthesis aided
by separation tags,14 quasiracemic synthesis can be viewed as
the simplest technique: only two compounds are mixed and
prior to tagging the two compounds are enantiomers. Since
enantiomers have identical separation behavior under achiral
conditions, it is not difficult to design tags that will dominate
when paired with a complementary separation technique. Mixing
more compounds involves tagging diastereomers or even
nonisomers, so the tag dominance becomes more challenging.
We have already communicated that fluorous mixture synthesis
can be used to make and separate nonisomeric analogues of
mappicine,16 and we will be reporting in detail on this and other
endeavors soon.
Conclusions
These straightforward syntheses of pyridovericin 5 and
mappicine 6 show that quasiracemic synthesis has value as a
complement to both racemic and asymmetric synthesis. Like
racemic synthesis, both enantiomers are produced in a single
set of steps, yet separation and identification are guaranteed by
the tags. Like asymmetric synthesis, the preparation of enan-
tiopure precursors is required at some stage, and racemization,
either by epimerization or tag exchange, must be avoided. Effort
is economized by tagging early in the synthesis and detagging
late. A fluorous quasiracemic synthesis should be considered
whenever both enantiomers are needed for structure identifica-
tion, biological testing, or any other end. Fluorous quasiracemic
synthesis is readily scaleable to make larger quantities, and the
limitation at demixing is quickly receding as larger fluorous
columns become available.
Conveniently, fluorous quasienantiomers behave like true
enantiomers when subjected to the typical spectroscopic analyses
(1H and 13C NMR) and chromatographic techniques (flash
chromatography) used in modern synthesis. The spectroscopic
similarities were expected since the tag is remote from the
portion of the molecule being analyzed. But the chromatographic
similarities were a pleasant outcome. Adding CF2 groups
decreases the polarity of molecules, so we were concerned at
the outset that quasienantiomers might separate on regular silica
gel. Whether they do or not may be a function of structure,
sorbent, and conditions, so more experience is needed before
the lack of separation can be generalized. At any time, the
quasiracemic mixture can be resolved based on the fluorous
tag by fluorous chromatography. The large retention time
differences in this work show that the incremental change of
tags by two CF2 groups is unneeded; tags differing by one CF2
group should suffice. But even-numbered perfluoroalkyl chains
are currently less expensive, so the incremental change of two
may be preferred.
The classical synthesis of racemic mixtures beautifully
illustrates the problems of mixture synthesis: the individual
enantiomeric components of the mixture are not reliably
analyzed, separated, or identified on demand. The use of
separation tags in quasiracemic synthesis and other more
advanced techniques16 provides a conceptual foundation to
approach problems of mixture synthesis35 when pure final
products are desired. And fluorous tagging provides the first
practical solution to the experimental problems posed by mixture
synthesis.
Acknowledgment. We thank the National Institutes of Health
for funding of this work. We also thank Dr. A. Nakagawa for
providing samples of pyridovericin and Professor James Rigby
for providing experimental details.
Supporting Information Available: Complete experimental
data for all new compounds reported in this paper (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA025606X
Can other tags besides fluorous tags be used for quasiracemic
synthesis? Yes, without doubt. The practice of tagging molecules
(35) Houghten, R. A.; Pinilla, C.; Appel, J. R.; Blondelle, S. E.; Dooley, C. T.;
Eichler, J.; Nefzi, A.; Ostresh, J. M. J. Med. Chem. 1999, 42, 3743.
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