Substituent effects in carbon-nitrogen cleavage of thiamin derivatives. Fragmentation pathways and enzymic avoidance of cofactor destruction

Article abstract of DOI:10.1021/ja0173627

The combination of thiamin and benzaldehyde can produce benzoin but also destroys thiamin. The destruction comes from fragmentation of the conjugate of thiamin and benzaldehyde undergoing a process that produces a phenyl thiazole ketone and pyrimidine. The key step in this process is cleavage of the C-N bond between the heterocycles, which occurs by an unknown mechanism. Enzymes that utilize similar intermediates do not fragment the cofactor although fragmentation is inherent to the structure. To analyze the nature of the C-N cleavage step, the rates of fragmentation of a series of phenyl-substituted N1′-methyl-2-(1-hydroxybenzyl)thiamin derivatives were determined under two sets of conditions: (1) where proton transfer in the step prior to C-N bond breaking is rate-determining and (2) where C-N bond breaking is rate-determining. The resulting p values are 1.6 and 1.8, respectively, leading to the conclusion that C-N cleavage is insensitive to substituent effects. On the basis of these results, we conclude that cleavage occurs by a facile process that resembles the outcome of a [1,5] sigmatropic rearrangement. An enzyme may avoid the fragmentation by holding the intermediate in a conformation that prevents such a process, allowing the normal catalytic process to proceed.

Full text of DOI:10.1021/ja0173627

Published on Web 02/01/2002
Substituent Effects in Carbon-Nitrogen Cleavage of Thiamin
Derivatives. Fragmentation Pathways and Enzymic Avoidance
of Cofactor Destruction
Ian F. Moore and Ronald Kluger*
Contribution from the DaVenport Chemical Research Building, Department of Chemistry,
UniVersity of Toronto, Toronto, Ontario, Canada M5S 3H6
Received October 23, 2001
Abstract: The combination of thiamin and benzaldehyde can produce benzoin but also destroys thiamin.
The destruction comes from fragmentation of the conjugate of thiamin and benzaldehyde undergoing a
process that produces a phenyl thiazole ketone and pyrimidine. The key step in this process is cleavage
of the C-N bond between the heterocycles, which occurs by an unknown mechanism. Enzymes that utilize
similar intermediates do not fragment the cofactor although fragmentation is inherent to the structure. To
analyze the nature of the C-N cleavage step, the rates of fragmentation of a series of phenyl-substituted
N1′-methyl-2-(1-hydroxybenzyl)thiamin derivatives were determined under two sets of conditions: (1) where
proton transfer in the step prior to C-N bond breaking is rate-determining and (2) where C-N bond breaking
is rate-determining. The resulting F values are 1.6 and 1.8, respectively, leading to the conclusion that
C-N cleavage is insensitive to substituent effects. On the basis of these results, we conclude that cleavage
occurs by a facile process that resembles the outcome of a [1,5] sigmatropic rearrangement. An enzyme
may avoid the fragmentation by holding the intermediate in a conformation that prevents such a process,
allowing the normal catalytic process to proceed.
The conjugate base of thiamin adds to aldehydes to form
intermediates that can form the equivalent of an acyl carban-
ion.1,2 Enzymes catalyze similar reactions by addition of the
conjugate base of thiamin diphosphate to carbonyl groups. The
formation of benzoin from benzaldehyde that is catalyzed by
thiamin involves a stable intermediate, 2-(1-hydroxybenzyl)-
thiamin, HBzT, which is analogous to the cyanohydrin in the
traditional benzoin condensation (Scheme 1).
The diphosphate of HBzT is an intermediate in the enzyme-
catalyzed decarboxylation of benzoylformate,3-5 so the reaction
patterns of HBzT give important information about the role of
the covalent intermediate in the enzyme. Mieyal and Sable
showed that the C2R proton of HBzT is exchanged for
deuterium in alkaline solution, consistent with its role in the
thiamin-catalyzed formation of benzoin from benzaldehyde.6,7
In a later study, Washabaugh reported that buffers promote the
decomposition of HBzT to thiamin and benzaldehyde, which
competes with the proton exchange reaction.8 However, unlike
the exchange reaction, the role of Brønsted acids and bases in
the elimination reaction is not apparent. Further studies revealed
that while buffers catalyze the decomposition of HBzT, thiamin
and benzaldehyde are not the products.9 These buffer-promoted
products, which form in neutral and acidic solutions, result from
cleavage of the thiazole and pyrimidine portions. They are the
same as those reported by Oka to result from the reaction of
thiamin with benzaldehyde in methanol in the presence of
amines (Scheme 2).10
Kinetic studies revealed that the fragmentation of HBzT is a
consequence of protonation of the pyrimidine, which occurs at
N1′ (HBzTH+). The material is a dication. If HBzT is N1′-
alkylated, the dicationic form (NMHBzT, 1) is present even in
alkaline solution, so that fragmentation is the only observed
process.11 Our recent report showed that the step in the
* To whom correspondence should be addressed. E-mail: rkluger@
chem.utoronto.ca.
(1) Kluger, R. Chem. ReV. 1987, 87, 863-876.
(2) Breslow, R. J. Am. Chem. Soc. 1958, 80, 3719-26.
(3) Dirmaier, L. J.; Garcia, G. A.; Kozarich, J. W.; Kenyon, G. L. J. Am. Chem.
Soc. 1986, 108, 3149-50.
(4) Hasson, M. S.; Muscate, A.; Henehan, G. T. M.; Guidinger, P. F.; Petsko,
G. A.; Ringe, D.; Kenyon, G. L. Protein Sci. 1995, 4, 955-9.
(5) Hasson, M. S.; Muscate, A.; McLeish, M. J.; Polovnikova, L. S.; Gerlt, J.
A.; Kenyon, G. L.; Petsko, G. A.; Ringe, D. Biochemistry 1998, 37, 9918-
9930.
(6) Mieyal, J. J.; Bantle, G.; Votaw, R. G.; Rosner, I. A.; Sable, H. Z. J. Biol.
Chem. 1971, 246, 5213-9.
(7) Mieyal, J. J.; Votaw, R. G.; Krampitz, L. O.; Sable, H. Z. Biochim. Biophys.
Acta 1967, 141, 205-8.
9
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J. AM. CHEM. SOC. VOL. 124, NO. 8, 2002 1669

A R T I C L E S
Moore and Kluger
Scheme 1
Scheme 2
fragmentation in which cleavage of the pyrimidine and thiazole
occurs from the C2R conjugate base (enamine) intermediate is
very fast, comparable in rate to proton transfer.12,13 In contrast,
the enzyme benzoylformate decarboxylase generates the same
enamine intermediate from combining thiamin diphosphate and
benzoylformate and proceeds without any fragmentation. If the
fragmentation process is inherent to the structure, how does the
enzyme avoid it?
The mechanism of the process in the fragmentation that
cleaves the bond joining the heterocycles remains unknown.
The means by which an enzyme would avoid it would be easier
to discern if the mechanism were known. The various possible
stepwise and concerted mechanisms all require highly energetic
species; none of the mechanisms are either ruled out or
supported by the patterns resulting from variation of reaction
conditions.
The effects of systematic variation of substituents in the
reactants on kinetic patterns can provide information beyond
that which comes from a single reactant. We therefore have
performed kinetic measurements on the fragmentation of a series
of phenyl-substituted analogues of HBzT since they can be
analyzed by application of linear free energy relationships.
Experimental Section
Methods. Kinetics of Reactions in Water. Reactions were first
monitored by recording sequential UV-vis spectra to determine the
wavelength where there is a maximum change due to the substituted
thiazole-ketone product. Subsequent kinetic runs were done at this
wavelength. The rate of fragmentation was monitored using buffer
solutions maintained at 40 °C in a jacketed beaker. The pH electrode
was standardized against reference buffers at 40 °C. The ionic strength
of all reaction solutions was maintained at 0.10 or 1.0 by the addition
of potassium chloride. Data were collected with an interfaced computer.
First-order rate constants were calculated from nonlinear regression
fitting of the data to the integrated first-order rate expression over 5
half-lives. Once the kinetic order was determined for a set of conditions,
the data for additional sets of reactions were acquired over a relatively
short interval and analyzed by the method of initial rates.
(8) Crane, E. J., III; Washabaugh, M. W. Bioorg. Chem. 1991, 19, 351-68.
(9) Kluger, R.; Lam, J. F.; Kim, C. S. Bioorg. Chem. 1993, 21, 275-83.
(10) Oka, Y.; Kishimoto, S.; Hirano, H. Chem. Pharm. Bull. 1970, 18, 527-
33.
(11) Kluger, R.; Lam, J. F.; Pezacki, J. P.; Yang, C.-M. J. Am. Chem. Soc.
1995, 117, 11383-9.
(12) Kluger, R.; Moore, I. F. J. Am. Chem. Soc. 2000, 122, 6145-6150.
(13) Moore, I. F.; Kluger, R. Org. Lett. 2000, 2, 2035-2036.
Synthesis. Two general procedures were followed for the synthesis
of thiamin-benzaldehyde conjugates. The procedure of Mieyal and
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1670 J. AM. CHEM. SOC. VOL. 124, NO. 8, 2002

Carbon−Nitrogen Cleavage of Thiamin Derivatives
A R T I C L E S
Sable was followed for the synthesis of HBzT, p-methyl-HBzT, and
p-methoxy-HBzT.14 Doughty’s procedure was followed for the synthesis
of p-chloro, p-cyano, and p-bromo derivatives.15 The thiamin-
benzaldehyde conjugates were methylated at N1′ with dimethyl
sulfate: p-methyl-NMHBzT (2), p-chloro-NMHBzT (3), p-methoxy-
NMHBzT (4), p-cyano-NMHBzT (5), p-bromo-NMHBzT (6). They
were isolated and recrystallized as bisperchlorates.16
Figure 1. Dependence of the observed first-order rate coefficient for
fragmentation of NMHBzT (1) on the concentration of hydrogen phosphate
ion in water at 40 °C, I ) 1.0. The curve is fit to the rate law for a reaction
in which a change in rate-determining step occurs with increasing phosphate
concentration.
Scheme 3
1
2 [X ) p-CH3]. H NMR (300 MHz, DMSO-d6): δ 9.28 (s, 1H),
8.43 (s, 1H), 7.66 (d, 1H, J ) 4.5 Hz), 7.26 (d, 2H, J ) 8.0 Hz), 7.08
(d, 2H, J ) 8.0 Hz), 6.74 (s, 1H), 6.23 (d, 1H, J ) 4.3 Hz), 5.28 (m,
3H), 3.76 (m, 2H), 3.56 (s, 3H), 3.07 (m, 2H), 2.51 (s, 3H), 2.29 (s,
3H), 2.20 (s, 3H). 13C NMR (75 MHz, DMSO-d6): δ 178.07, 161.38,
159.86, 143.13, 142.65, 138.92, 134.93, 134.13, 129.17, 127.88, 107.94,
70.26, 59.47, 46.10, 41.49, 29.56, 21.29, 20.52, 11.27. FABMS:
[C21H28N4O2S]2+, calcd 399.18547 (C21H27N4O2S), found 399.18439.
3 [X ) p-Cl]. 1H NMR (300 MHz, DMSO-d6): δ 9.35 (s, 1H), 8.48
(s, 1H), 7.81 (d, 1H, J ) 4.7 Hz), 7.42 (d, 2H, J ) 8.7 Hz), 7.39 (d,
2H, J ) 8.7 Hz), 6.85 (s, 1H), 6.33 (d, 1H, J ) 4.7 Hz), 5.29 (m, 3H),
3.73 (m, 2H), 3.59 (s, 3H), 3.08 (m, 2H), 2.53 (s, 3H), 2.30 (s, 3H).
13C NMR (75 MHz, DMSO-d6): δ 177.12, 161.59, 159.86, 143.35,
142.76, 136.82, 134.41, 133.99, 129.82, 128.74, 108.11, 69.45, 59.45,
46.13, 41.54, 29.55, 21.41, 11.33. FABMS: [C20H25N4O2SCl]2+, calcd
419.13085 (C20H24N4O2SCl), found 419.13107.
4 [X ) p-OCH3]. 1H NMR (300 MHz, DMSO-d6): δ 9.30 (s, 1H),
8.45 (s, 1H), 7.61 (d, 1H, J ) 4.2 Hz), 7.30 (d, 2H, J ) 8.6 Hz), 6.82
(d, 2H, J ) 8.6 Hz), 6.78 (s, 1H), 6.22 (d, 1H, J ) 3.6 Hz), 5.27 (m,
3H), 3.76 (m, 2H), 3.70 (s, 3H), 3.56, (s, 3H), 3.07 (m, 2H), 2.49 (s,
3H), 2.29 (s, 3H). 13C NMR (50 MHz, DMSO-d6): δ 178.25, 161.29,
159.90, 159.57, 143.07, 142.68, 133.99, 129.74, 129.30, 113.94, 108.03,
69.95, 59.43, 55.22, 46.01, 41.46, 29.51, 21.14, 11.22. FABMS:
[C21H28N4O3S]2+, calcd 415.18039 (C21H27N4O3S), found 415.17907.
46.22, 41.54, 29.59, 21.57, 11.34. FABMS: [C20H25N4O2SBr]2+, calcd
463.08033 (C20H24N4O2SBr), found 463.08136.
Results
The observed first-order rate coefficient for fragmentation of
1 was measured with variation of pH 6.1 hydrogen phosphate
buffer (Figure 1).
The rate coefficients show a saturating dependence on buffer
concentration, as had been seen with the N1′-benzyl derivative,
resulting from a change in rate-determining step from proton
transfer to C-N cleavage. The data are consistent with the steps
in Scheme 3. The scheme is similar to that which was presented
for the fragmentation of N1′-benzyl-HBzT.
1
5 [X ) p-CN]. H NMR (300 MHz, DMSO-d6): δ 9.35 (s, 1H),
8.52 (s, 1H), 7.93 (d, 1H, J ) 4.9 Hz), 7.86 (d, 2H, J ) 8.5 Hz), 7.62
(d, 2H, 8.2 Hz), 6.96 (s, 1H), 6.45 (d, 1H, J ) 4.7 Hz), 5.29 (m, 3H),
3.74 (m, 2H), 3.62 (s, 3H), 3.09 (m, 2H), 2.55 (s, 3H), 2.32 (s, 3H).
13C NMR (75 MHz, DMSO-d6): δ 175.97, 161.78, 159.90, 143.45,
143.23, 142.86, 134.87, 132.78, 128.91, 118.04, 111.86, 108.27, 69.29,
59.44, 46.23, 41.70, 29.60, 21.43, 11.42. FABMS: [C21H25N5O2S]2+
,
calcd 410.16507 (C21H24N5O2S), found 410.16700.
1
6 [X ) p-Br]. H NMR (300 MHz, DMSO-d6): δ 9.35 (s, 1H),
8.52 (s, 1H), 7.86 (d, 1H, J ) 4.9 Hz), 7.52 (d, 2H, J ) 8.5 Hz), 7.37
(d, 2H, J ) 8.5 Hz), 6.87 (s, 1H), 6.33 (d, 1H, J ) 4.7 Hz), 5.36 (m,
3H), 3.73 (m, 2H), 3.60 (s, 3H), 3.08 (m, 2H), 2.55 (s, 3H), 2.30 (s,
3H). 13C NMR (75 MHz, DMSO-d6): δ 177.20, 161.58, 159.87, 143.40,
142.68, 137.25, 134.42, 131.68, 130.15, 122.66, 108.11, 69.57, 59.48,
Application of the steady-state approximation to Scheme 3
gives eq 1, which describes a rectangular hyperbola. Following
kobsd ) (kB[B] + kOH[OH-])kf/(kBH[BH] + kw + kf) (1)
the method outlined by Keefe and Jencks,17 the kinetic
parameters that can be derived from this plot are kB ) 1.5 ×
(14) Mieyal, J. J.; Bantle, G.; Votaw, R. G.; Rosner, I. A.; Sable, H. Z. J. Biol.
Chem. 1971, 246, 5213.
(15) Doughty, M. B.; Risinger, G. E.; Jungk, S. J. Bioorg. Chem. 1987, 15,
15-30.
(16) Zoltewicz, J. A. Synthesis 1980, 218-19.
(17) Keeffe, J. R.; Jencks, W. P. J. Am. Chem. Soc. 1983, 105, 265-279.
9
J. AM. CHEM. SOC. VOL. 124, NO. 8, 2002 1671

A R T I C L E S
Moore and Kluger
Table 1. Pseudo-First-Order Rate Constants for Fragmentation of
Compounds 1-5 at pH 6.9 and 0.003 M Hydrogen Phosphate
compd
k
obsd (s-1
)
compd
kobsd (s-1
)
1
2
3
(2.5 ( 0.1) × 10-4
(1.5 ( 0.1) × 10-4
(4.69 ( 0.05) × 10-4
4
5
(1.58 ( 0.02) × 10-4
(5.21 ( 0.02) × 10-3
Figure 3. A Hammett plot of the observed first-order rate constants for
fragmentation of 1-6 under conditions where C-N bond cleavage is rate-
determining (data from Table 2).
Table 2. Observed First-Order Rate Constants for Fragmentation
of Compounds 1-6 at pH 6.1 and 0.2 M Hydrogen Phosphate
compd
k
obsd (s-1
)
compd
kobsd (s-1
)
1
2
3
(4.9 ( 0.3) × 10-5
(3.36 ( 0.07) × 10-5
(1.11 ( 0.07) × 10-4
4
5
6
(2.8 ( 0.6) × 10-5
(1.56 ( 0.01) × 10-3
(1.15 ( 0.02) × 10-4
Figure 2. A Hammett plot of observed first-order rate constants for
fragmentation of 1-5 (indicated by substituent) with rate-determining
transfer of the proton from C2R (data from Table 1).
of conditions sets are similar and differ only by the fragmenta-
tion step.
10-3 M-1 s-1 and kBH/kf ) 17 M-1. The first rate-determining
step is proton transfer from C2R, which is accelerated by buffer.
The next rate-determining step, fragmentation, competes with
protonation of the conjugate base at C2R but is not facilitated
by Brønsted acids and bases.
Discussion
The rates of fragmentation of 1-6 are proportional to the
concentrations of buffer in dilute solution but become indepen-
dent of buffer concentrations at higher levels. This change in
dependence is consistent with a change in rate-determining step
with respect to the availability of Brønsted acids and bases. The
kinetic expressions for both regions of the plot are given in eqs
2 and 3.
The reaction rates for the set of N1′-methylphenyl-substituted
analogues of HBzT yield linear free energy relationships to
gauge the electronic requirements of the fragmentation step. We
analyzed the substituent effects on both rate-determining steps
by determining the Hammett F value under conditions where
each is rate-determining. Since kf is rate-determining at high
buffer concentrations, the measured value of F is the sum of
the F values for the steps up to and including the fragmentation
step. Because the proton-transfer step precedes the fragmentation
step, the difference in F values obtained under conditions with
the differing rate-determining steps gives the net value for F
for the fragmentation step. The data in Table 1 and Figure 2
were determined in 0.003 M pH 6.9 hydrogen phosphate. Under
these conditions loss of the proton from C2R is rate-determining.
Fitting the data to σp (r ) 0.975) gives F ) 1.6. The relatively
large positive value of the reaction parameter is consistent with
development of negative character in the transition state at C2R
(the net charge on the whole species is positive).
We also measured rates of fragmentation for compounds 1-6
under conditions where the proton-transfer step is faster than
the fragmentation step kf (high buffer concentration; see Figure
1 and Scheme 3). Thus, the rate data in Figure 3 were
determined at pH 6.1 with 0.2 M hydrogen phosphate buffer
(Table 2).
The plot fits the data best using σp parameters (Figure 3).
The plot gives F ) 1.8. Again, the value of F is consistent with
a transition state that has considerably more localized negative
charge than the reactant. The net value of F for the fragmentation
step (kf) is very small since the values for F under the two sets
kobsd ) kB-[B-] + k
(2)
(3)
18
OH
kobsd ) R(kB/kBH)(kf)
The computed value of F for conditions where proton removal
from C2R is rate-limiting is 1.6. This is a reasonable value for
such a process. In the case of ionization of phenols in water, F
) 2.2, where stabilization of the negative charge is provided
solely by the aromatic substituents, while in 1-6 delocalization
into the thiazolium ring is significant in stabilizing the charge.
Using Jordan’s measured pKa ) 15.419 for a related com-
pound as a reasonable estimate for the loss of a proton from
C2R, we calculate the rate coefficient for conversion of the
enamine into the fragments as kf ) 8 × 104 s-1. Therefore, the
initial intermediate formed by loss of a proton from C2R has to
be relatively close in energy to the transition state for cleavage
of the C-N bond.
The difference in the values obtained for F under the two
sets of rate-limiting conditions indicates that the value of F
(18) Shin, W.; Oh, D.; Chae, C.; Yoon, T. J. Am. Chem. Soc. 1993, 115, 12238-
12250.
(19) Bordwell, F. G.; Satish, A. V.; Jordan, F.; Rios, C. B.; Chung, A. C. J.
Am. Chem. Soc. 1990, 112, 792-797.
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1672 J. AM. CHEM. SOC. VOL. 124, NO. 8, 2002

Carbon−Nitrogen Cleavage of Thiamin Derivatives
A R T I C L E S
Scheme 4
associated with kf is approximately 0.2 if we assume that the
structure of the transition state for the endothermic proton
transfer resembles the conjugate base. The scatter in the plot is
sufficient to make this value indistinguishable from zero. Thus,
there is probably little if any increase or decrease of polar
character in the transition state in which the C-N bond is
broken, after the initial intermediate has lost a proton from
carbon. The ketonic product results from the hydroxyl group at
C2R. If the proton were removed prior to the transition state,
negative character would develop and F would increase sig-
nificantly since the substituent is on the portion of the molecule
that becomes part of the ketone. This would occur in a step
that would lead to formation of the conjugate base of the
hydroxyl. If the hydroxyl proton were retained in the transition
state of the rate-determining step, the appearance of the
protonated ketone would have an inverse substituent dependence
and we would expect to see a large decrease in F. Since neither
corresponds to what we observe, an alternative mechanism is
necessary.
The other required process in transforming the intermediate
to the products is breaking the bond connecting the methylene
bridge and the thiazole nitrogen. If this were the initial process,
the intermediate would become locally seriously deficient in
electron density. Therefore, F for that step would be significantly
negative, reducing the net slope. Again, this is not what we
observe, and the scenario is unlikely. Furthermore, any stepwise
process will generate what must be high-energy species, yet
the rate constants associated with each value of kf are large.
Such small barriers do not permit development of even higher
energy species.
the enzymic generation of the C2R conjugate base of thiamin
diphosphate of HBzT in benzoylformate decarboxylase4,5,20 does
not lead to fragmentation of the cofactor, although the inter-
mediate should be prone to this process. The rate constant for
the fragmentation step from the C2R conjugate base of the N1′-
protonated species is as fast as the normal enzymic process.
Suppression of the extent of protonation of the pyrimidine could
slow the rate of the fragmentation, but this dynamic process
cannot be avoided completely. On the other hand, the enzyme
might hold the intermediate in a conformation that would not
allow the proposed sigmatropic-like process. By hydrogen-
bonding of the critical OH in a way that holds it remote from
the methylene bridge, this pathway would be blocked. This is
a form of stereoelectronic control that ensures specificity and
productive catalysis by preventing a competing process, which
can be as important as promoting a desirable process.21,22 If
the fragmentation from the conjugate base were stepwise, such
a restraint would not be effective. In contrast, the catalytic
process, benzaldehyde and thiamin diphosphate separating, is
not subject to this stereoelectronic limitation.
The remaining alternatives to these heterolytic processes are
a homolytic process (via high-energy free radicals that must
recombine) and a concerted process in which C-N cleavage is
accompanied by transfer of the hydroxyl hydrogen to the
bridging methylene group that is exocyclic to the pyrimidine.
We see no route that generates radicals readily that will lead to
the products. A concerted six-electron mechanism that resembles
a suprafacial [1,5] sigmatropic reaction, which is allowed
according to the Woodward-Hoffmann rules, requires front side
substitution at the saturated carbon. The process resembles the
Alder ene reaction in some aspects but differs in the substitution
at the saturated carbon (Scheme 4). The hydroxyl group must
rotate out of the plane to accommodate the necessary overlaps.
Molecular modeling suggests that a chairlike transition-state
structure can be achieved. We do note that we have observed
that the reaction requires that the pyrimidine possess a positive
charge, while the sigmatropic reaction as drawn is not subject
to polar effects. However, weakening of the C-N bond may
well be induced by the strongly electron withdrawing effect of
the positively charged ring. It is well established that polarization
by Lewis acids, which adds charged character, promotes other
electrocyclic processes.
Acknowledgment. The Natural Sciences and Engineering
Research Council of Canada (NSERC) has provided support
through an operating grant. Ian Moore is the recipient of an
NSERC postgraduate scholarship. We thank Soraya Yekta and
J. P. Pezacki for preparing compounds and J. P. Guthrie for
helpful comments.
The structure in Scheme 4 is drawn as the enamine derivative
resonance structure. However, the thiazolium form may be more
significant since it retains aromaticity in the heterocycle. This
permits rotation about the C2 bond, which leads to the chairlike
conformation.
JA0173627
(20) Dirmaier, L. J.; Garcia, G. A.; Kozarich, J. W.; Kenyon, G. L. J. Am. Chem.
Soc. 1986, 108, 3149.
The facility of the fragmentation of these thiamin derivatives
helps to explain why thiamin catalysis of the benzoin condensa-
tion is subject to breakdown after several turnovers. In contrast,
(21) Hammons, G.; Westheimer, F. H.; Nakaoka, K.; Kluger, R. J. Am. Chem.
Soc. 1975, 97, 1568-72.
(22) Kluger, R.; Nakaoka, K. Biochemistry 1975, 13, 910-4.
9
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