Influence of Side Chain Conformation and Configuration on Glycosyl\nDonor Reactivity and Selectivity as Illustrated by Sialic Acid Donors Epimeric\nat the 7-Position
Two N-acetyl 4O,5N-oxazolidine-protected sialyl thioglycosides epimeric at the 7-position have been synthesized and their reactivity and stereoselectivity in glycosylation reactions compared. It is demonstrated that the natural 7S-donor is both more reactive and more \xce\xb1-selective than the unnatural 7R-isomer. The difference in reactivity is attributed to the side chain conformation and specifically to the proximity of O7 to the anomeric center. In the natural 7S-isomer O7 is closer to the anomeric center than in its unnatural 7R-epimer and therefore better able to support incipient positive charge at the locus of reaction. The difference in selectivity is also attributed to the side conformation, which in the unnatural 7R-series is placed perpendicularly above the \xce\xb1-face of the donor and so shields it to a greater extent than in the 7S-series. These observations are consistent with earlier conclusions on the influence of the side chain conformation on reactivity and selectivity derived from conformationally locked models in the glucose and galactose series and corroborate the suggestion that those effects are predominantly stereoelectronic rather than torsional. The possible relevance of side chain conformation as a factor in the influence of glycosylation stereoselectivity by remote protecting groups and as a control element in enzymic processes for glycosidic bond formation and hydrolysis are discussed. Methods for assignment of the anomeric configuration in the sialic acid glycosides are critically surveyed.
Introduction<!>Results<!>Discussion<!>Conclusion
<p>The development of efficient, stereocontrolled methods for glycosidic bond
synthesis is a key element in the continued evolution of the field of chemical
glycobiology.1-2 The rational development of such
methods necessarily depends on an understanding of the mechanisms of glycosylation
and on the factors that influence them.3-9 In this respect the
influence of side chain protecting groups and conformation on the reactivity and
selectivity of glycosyl donors is an area of considerable current
interest.10 Interest in
the role of side chain conformation stems from the discovery of Fraser-Reid and
co-workers of the retarding effect of cyclic protecting groups on the reactivity of
glycosyl donors, which they termed the torsionally disarming effect,11-12 and from the critical role played by
4,6-O-benzylidene acetals in
β-mannopyranosylation.3,13-15 Bols and co-workers dissected the benzylidene
effect into torsional and electronic components based on the relative rates of
hydrolysis of a series of cyclic 6-methoxy-7-carba-glucopyranoside probes (Fig 1a),16 reaching the conclusion that the major part of the effect
is electronic and arises from the locking of the C5-C6 bond in the
tg conformation17-18 (Fig 2a). Subsequently, however, the Bols
laboratory working in the mannopyranosyl series with related cyclic 7-carba analogs
lacking the additional methoxy group suggested that the torsional component plays a
larger role in the benzylidene effect than was originally thought.19 In our laboratory we prepared a
series of cis-fused 6-methoxy-7-carba-galactopyranoside probes, and
determined their relative rates of hydrolysis by the Bols protocol. We found the
conformer in which the 6-methoxy group is antiperiplanar to the C5-O5 to be the
least reactive (Fig 1b) consistent with the
earlier work in the trans-fused gluco-system.20 Taking into account the additional
torsional interactions introduced by the replacement of an ether oxygen by a
methoxymethyl group (Fig 1a,b) or by a simple
methylene group,19 we concluded
that the Bols-type probes over-estimate the torsional component of the disarming
effect of the fused ring, and therefore that maximization of the
electron-withdrawing effect of the C6-O6 bond is of the greatest
importance.20 This
conclusion is re-enforced by our earlier studies with a series of 6-fluoro-,
6,6-difluoro-, and 6,6,6-trifluororhamnopyranosyl donors in which it was found that
the decomposition temperature and β-selectivity of the intermediate rhamnosyl
triflates increased with increasing fluorine content, ie, that donor reactivity is
related inversely to the electron-withdrawing ability of substituents at the
6-position.21</p><p>Continuing our studies on the effect of side chain conformation on glycosyl
donor reactivity, we have now turned our attention to a comparison of the reactivity
and selectivity of sialic acid donors and their 7-epi-isomers (Figure 3). This study is based on the prediction that the two
isomers will exhibit different predominant side chain conformations and so different
reactivities and stereoselectivities. In so far as this study does not make use of
substrates with artificially restricted side chain conformations, it obviates the
problem of additional torsional that characterizes the Bols-type system. To exclude
any possibility that donors of different side chain configuration might react via
different conformations of the pyranose ring, we elected to carry out this study
with the rigid
N-acetyl-4O,5N-oxazolidinone-protected
donors22-23 which retain the
2C5 conformation in the crystal and in
solution22,24 and which, along with the
N-desacetyl analogs,25-26 have proven to be
some of the most selective sialyl donors available for the synthesis of
O-,27-37C-,38-39 and S-sialosides.40 The sensing of the side chain
conformation and configuration in the sialic acid series by substituents at the
anomeric position follows from the anomeric configurational dependence in
1H NMR spectra of i) the
3JH7,H8 coupling constant,41-42 and ii) the H9a,H9b chemical shift
difference,42 both of
which have been applied as methods for the determination of anomeric configuration.
Knowledge of the influence of side chain conformation and configuration on anomeric
reactivity will ultimately contribute to our mechanistic understanding of
glycosylation and so to the design of improved systems.</p><!><p>We begin with a definition of the staggered conformations about the exocyclic
side bonds employed in this Article. With respect to the hydroxymethyl groups of the
hexopyranoses the conformation is described by the stereochemical relationship
(gauche, or trans) of O6 to i) the ring
oxygen, and ii) to C4 (Fig 2a).17-18 Thus, for example, the methylidene protected glucose
derivative 4 is described as having the tg conformer
of its "hydroxymethyl" group (Fig
2a). In the case of the higher carbon sugars, as noted by Grindley for
the sialic acids,18 formally the
definition of the conformation of the exocyclic bond is based on the stereochemical
relationship of the next carbon along the chain to i) the ring oxygen, and ii) to C4
(C5 in the sialic acids). According to this definition compounds 2 and
3 (Fig 2a) are formally both
tg conformers. For consistency, and to highlight the importance
of the C-O bond at the exocyclic position, all conformations in this paper are
defined by the relationship of the C-O bond at the exocyclic position to i) the ring
oxygen, and ii) to the ring carbon (C4 in the hexoses and C5 in the neuraminic
acids) as was done in the earlier papers by Bols and ourselves.16,19-20 To indicate
structures in which a side chain oxygen atom has been used as reference atom rather
than a formally more correct side chain carbon the conformational descriptors have
been prefaced by the term [O].</p><p>Synthesis of a 7-episialyl donor began with the known adamantanyl
thiosialoside 923 and
proceeded through Zemplen deacetylation standard installation of an
8,9-O-acetonide giving 10.43 Adapting literature
methods,43-45 reaction of diol 10 with acetyl
chloride at −20 °C enabled the selective esterification of the less
hindered 4-OH giving 11, of which Parikh-Doering oxidation46 then afforded the 7-keto sialyl
donor 12. Selective reduction of 12 with Luche's
reagent47 provided the
7-epi sialyl donor 13 in 77% yield and 10:1 selectivity and this was
followed by treatment with Boc2O in the presence of DMAP in hot THF to
give the N,O-di-Boc-derivative 14
(Scheme 1).</p><p>Zemplen deacetylation of 14 followed by treatment with 3M HCl in
methanol afforded the 7-epi neuraminate 15, which was immediately
converted into the 4O,5N-oxazolidinone
16 by treatment with triphosgene in the presence of
Hünig's base at −50 °C. We note in passing that the more
conventional use of trifluoroactic acid/water,23 or 2 M HCl in ether,48 or more dilute solutions of HCl in methanol for the removal
of the Boc groups from 14 were complicated by formation of the
oxazinone 18 alongside the desired 15, which we interpret
as being due to the rapid cleavage of the N-Boc group with
subsequent cyclization onto the remaining O-Boc moiety. Acetylation
of 16 with acetic anhydride in pyridine and then with acetyl chloride
and Hünig's base finally gave the requisite donor 17
(Scheme 2). When the reaction of
15 with 4-nitrophenyl chloroformate and sodium bicarbonate in
aqueous acetonitrile the regioisomeric
5N,7O-oxazinone 18 was obtained in
67% yield (Scheme 2); subsequent acetylation
afforded the donor 19. The formation of the oxazinone 18
provides a first indication of the influence of stereochemistry at C7 on general
reactivity in this series, as application of the same protocol to the analogue
20 with the natural stereochemistry, reliably affords the
4O,5N-oxazolidinone 21, two stage
peracetylation of which gives the donor 22 as described previously
(Scheme 2).23</p><p>With the stereoisomeric donors 17 and 22 in hand,
we began by comparing their reactivity toward activation with the
N-iodosuccinimide/triflic anhydride combination. To this end a 1:1
mixture of 17 and 22 were activated with NIS/TfOH at
−78 °C in a 2:1 mixture of dichloromethane and acetonitrile and
3Å acid-washed molecular sieves in the presence of the acceptor
23 with quenching at −78 °C after 1 h by the addition
of triethylamine (Scheme 3). Examination of
the crude reaction mixture by HPLC and 1H NMR spectroscopy revealed clean
preferential activation of the natural 7S-isomer 22
over the inverted 7R-epimer 17. Consistent with this
observation unreacted 17 was recovered from the reaction mixture in 96%
yield, while the glycoside 24 was isolated in 92% yield as a single
α-anomer in agreement with the original report on the coupling of
17 and 23 under comparable conditions.23 This experiment clearly
demonstrates the influence of side chain configuration (and conformation) on
anomeric reactivity.</p><p>Subsequent experimentation revealed a temperature of around −60
°C to be the lowest at which donor 17 was activated in a
reasonable time frame by the NIS/TfOH combination in the 2:1
CH2Cl2/acetonitrile solvent mixture in the presence of
acid washed molecular sieves. Accordingly, with a view to comparing the
stereoselectivity of the epimers 17 and 22, the two donors
were coupled separately with acceptors 23, 25,
26, and 27 under a standard set of conditions giving
rise to the products 24 and 28-34 presented in Table 1.</p><p>With donor 22, the natural 7S-configuration,
all couplings were α-selective (Table
1) albeit and expectedly less so than for comparable reactions23 previously conducted at −78
°C. With the epimeric 7R-isomer 17, coupling to
the relatively unhindered primary carbohydrate acceptor 23 and to
1-octanol 27 (Table 1, entries 1
and 4) was α-selective albeit to smaller extent than with the natural
7S-configured donor 22. With the two secondary
alcohol acceptors studied, couplings to the unnatural 7R-configured
donor were moderately β-selective (Table
1, entries 2 and 3). Overall, therefore, the
7R-configured donor 17 is less α-selective than
its 7S-isomer 22, and with the less reactive alcohols
this selectivity is even inverted to a modest preference for β-selectivity.
When the galactose-3,4-diol 25 was employed as acceptor both couplings
took place through the less sterically hindered 3-OH, as is well-known in the
field,49-51 giving rise to the
α,β-(2→3)-configured disaccharides 28 and
32.</p><p>Returning to the question of relative reactivity we prepared and isolated
the anomeric sialyl phosphates 35 and 36 from
17 and 22, respectively, following the method of Wong
and coworkers (Scheme 4).28 Again, a difference in selectivity
was observed with the unnatural 7R-isomer 22 affording
a 3:1 α:β-mixture of phosphates 35, while the
7S-epimer gave a single β-product 36.</p><p>Phosphates 35 and 36 were then examined by ESI
mass spectrometry when, using the standard cone voltage of 40 V, they both showed
clean sodiated molecular ions and the absence of fragmentation. The ESI mass spectra
were then recorded at increasing cone voltages until the onset of fragmentation as
determined by the observation of a daughter ion resulting from the loss of
dibutylphosphoric acid. With the natural isomer 36 fragmentation was
detectable at a cone voltage of 85 V, consistent with the literature,52 whereas the unnatural required
isomer 36 required a cone voltage of 98 V before fragmentation was
observed. While it is the case that the energies required for mass spectral
fragmentation of glycosides are dependent on anomeric configuration,53 this experiment clearly
demonstrates that both anomers of 35 are considerably less susceptible
to decomposition in this manner than the β-isomer of 36, which
is considered to be the most stable in view of previous observations on the
equilibration of sialyl glycosides.</p><p>With the 7R-oxazinone 19 in hand, we briefly
examined conditions for its activation and its selectivity in a glycosylation
reaction. Consistent with earlier work from our laboratory with the analogous
7S-configured oxazinone 37,54 this donor was substantially less
reactive than the oxazolidinones. For this reason, as in the previous work with
37, we employed the potent diphenyl sulfoxide/triflic anhydride
combination55 for
activation. Using 1-adamantanol as acceptor the glycoside 38 was
obtained in 27% yield as a single α-anomer along with 70% of the recovered
donor (Scheme 5). This selectivity of this
coupling is the opposite of that observed previously with 37 when the
β-glycoside 39 was the sole product (Scheme 5), and indeed with that observed by Tanaka and
co-workers with the closer 7S-analog 40.56</p><p>The side chain conformation of thioglycosides 17 and
22, as well as of a number of O-glycosides in both
the natural and 7-epi series was examined by NMR spectroscopy. The NOE contacts and
the 3J6,7 coupling employed to this end along
with the predominant conformation about the exocyclic C6-C7 bond for each substance
studied are listed in Table 2. In the
unnatural 7R-configured oxazolidinone-protected series strong nOe
interactions between the axial ring hydrogen H5 and the side chain hydrogen H8,
together with 3J6,7 coupling constants
between 2.4 and 3.6 Hz, consistent with earlier observations by Bandgar and Zbiral
on simple peracetylated sialosides,44 indicate that, with two exceptions, an approximate
[O]gt conformation about the
C6-C7 bond predominates in the unnatural series for both the α- and
β-glycosides (Table 2). Furthermore,
the assignment of the [O]gt side
chain conformation to the protected 7-episialosides is consistent with the
observations of Zbiral and coworkers on the side chain conformation of
7-epi-N-acetyl neuraminic acid itself.57 Accordingly, we represent the side
of these substances in the [O]gt
conformation in this Article. The two exceptions to this rule are the
α-glycosides 32 and 33 of secondary alcohols, when
the H8-H5 nOe interaction, while still present, is noticeably weaker and the
3J6,7 coupling constants much larger at
7.2 Hz, suggesting an important contribution from the
[O]tg conformation. In the
natural 7S-series all oxazolidinone protected glycosides examined
(Table 2), including the
α-glycoside of a secondary alcohol 29α showed a strong
nOe correlation between the axial ring hydrogen H6 and the side chain hydrogen H8.
Together with their observed 3J6,7 coupling
constants of 1.5-2.4 Hz this suggests a predominant
[O]gg conformation as is
typically found in the N-acetyl neuraminic acid series.57-62</p><!><p>It is clear from the comparison of the reactivity of conformationally locked
donors 17 and 22 under standard glycosylation conditions,
as well as from the differing minimum cone voltages required to promote
fragmentation of the isomeric glycosyl phosphates 35 and
36, that the stereochemical configuration at C7 in the
N-acetylneuraminic acid series affects anomeric reactivity. As
the glycosylation reactions, whether they proceed through associative
SN2-like or dissociative SN1-like pathways, and the mass
spectral fragmentations, whether stepwise or concerted,52 proceed with an initial build-up of positive
charge at the anomeric position, this influence on reactivity must correlate with
the ability of the donor to support nascent positive charge at the anomeric carbon.
In the more reactive natural series, with the 7S-configuration the
[O]gg conformation
predominates in all systems studied (Table 2)
whereas in the less reactive unnatural 7R-configured systems (Table 2) the
[O]gt conformation predominates
with the exception of the two hindered α-glycosides 32α
and 33α to which we return later. The greater reactivity of the
system predominantly adopting the
[O]gg over the isomeric system
with its predominant [O]gt
conformation is fully consistent with the findings of Bols and workers16 and of our laboratory20 on the relative reactivities of
conformationally-locked gluco- and galactopyranosyl donors (Figure 1). Making the reasonable assumption that the side chains
predominantly adopt closely related conformations in any
4H5 half chair-like oxocarbenium ions as
they do in both the α- and β-glycosides, the greater reactivity of the
natural system with its [O]gg
conformation is best accounted for by the ability of O7 to stabilize positive charge
on the anomeric carbon through space by virtue of the periplanar alignment of the
C7-O7 bond and the vacant p-orbital on C2 (Figure
4a). Conversely, in the unnatural 7R system with its
predominant [O]gt conformation the
C7-O7 bond is inappropriately located to stabilize the π-system of any
anomeric oxocarbenium ion (Figure 4b). Parallel
arguments apply to reactions proceeding through SN2-like mechanisms with
the [O]gg conformation better placed
for stabilization of the partial bonds at the transition state by the C7-O7 bond.
Alternatively, O7 is spatially closer to the point charge (or partial point charge)
of the anomeric carbenium ion in the
[O]gg conformer than in the
[O]gt conformer and so better
able to provide electrostatic stabilization following the models of
Deslongchamps,63
Woods,64
Woerpel,65 and
Bols.66-67 The overall situation is consistent with the
result of DFT computations by Yang and Woerpel (Figure
5) according to which the gg conformation of the
1-methoxy-5-methoxymethyl) tetrahydropyranosyl cation is some 0.4
kcal.mol−1 lower in energy than the corresponding
gt conformation.68 Self-evidently, this rationalization only applies to the
4H5 or closely related conformation which
is imposed on the system by the trans-fused oxazolidinone ring in
the systems under study.</p><p>While we have not conducted a thorough study of the oxazinone-protected
donors, we previously noted54 the
lack of reactivity of the 7S-configured 37 under the
NIS/TfOH conditions (Scheme 5) which forced us
to employ the more potent diphenyl sulfoxide/Tf2O activating system.
Parallel observations were made by Tanaka and coworkers with the
7S-configured donor 38 (Scheme 5).56 As
reported here the unnatural 7R-configured donor 19 is
relatively unreactive even with activation by diphenyl sulfoxide/Tf2O
(Scheme 5). The main difference between
the oxazinone-protected systems 19, 37, and
38, and the oxazolidinone-protected ones 17 and
22 is the imposition of the
[O]tg conformer in the oxazinones
which leads to a reduction in activity consistent with the earlier studies on
conformationally locked gluco and galactopyranosyl donors (Figure 1).16,20 We refrain from interpreting
differences in reactivity between the oxazinones 19, 37,
and 38 because of the inconsistencies in the thioglycosides employed,
in their anomeric stereochemistry, and in the side chain protecting groups all of
which influence anomeric reactivity.3-9</p><p>Turning to the influence of side chain configuration on anomeric
stereoselectivity, all glycosylation reactions with the natural
7S-configured donor 22 were α-selective (Table 1) consistent with the broad use of this
class of oxazolidinone-protected donor in the literature.22-36 When
hindered secondary acceptors were employed the selectivity was lower than previously
reported,23 a fact that we
attribute to the higher reaction temperatures necessitated by the comparative
studies. As we have discussed previously,52 the high levels of α-selectivity observed with donor
22 are best accommodated by associative transition states
proceeding with inversion of configuration at the anomeric center, even if the exact
nature of the leaving group has yet to be pinpointed. Associative transition states
are favoured by the oxazolidinone-protected class of sialyl donors because of the
strong electron-withdrawing nature of the oxazolidinone whose dipole is in the mean
plane of the pyranose ring and strongly destabilizes positive charge at the anomeric
center.52 As noted above,
the unnatural 7R-configured donor 17 is less reactive
than 22, which is a consequence of the conformation of its side-chain
and the reduced stabilization afforded to positive charge at the anomeric position
by O7. Donor 17 is, however, less selective than donor 22
even if its relative reactivity suggests a higher degree of association in its
anomeric substitution reactions. This reduced selectivity in the unnatural series is
apparently a function of the additional shielding of the α-face by the
predominant [O]gt conformation of the
side chain, which retards approach of the acceptor from that face and so promotes
either a competing β-selective associative mechanisms or a more dissociative
process. The additional shielding of the α-face in the unnatural
7R-series is illustrated in (Figure 6) for an associative displacement of an unspecified leaving
group; similar interactions can be expected to be present in the product forming
step of a dissociative substitution.</p><p>The steric origin of the reduced selectivity of donor 22 is
most apparent with the secondary alcohol acceptors when the β-anomeric
products 32β and 33β are favoured (Table 1, entries 2 and 3). It is pertinent in
this regard that the minor anomers 32α and
33α are the only compounds in the unnatural
7R-series to display a different side chain conformation to the
otherwise predominant [O]gt
arrangement. This change in side chain conformation presumably arises because of a
more highly developed form of the steric clash between C8 in the
[O]gt conformation and the
aglycone that is the cause of the reduced selectivity in this series. Conversely, in
the oxazinone series the unnatural 7S-configured donor
19 is more α-selective than comparable
7R-configured systems 37 and 40 (Scheme 5). Consistent with the above rationale
for the greater α-selectivity of 22 over 17, this
is clearly the result of the axial side chain in 37 and 40
being located directly above the α-face of the system.</p><p>As the side chain conformations of both the natural 7S and
unnatural 7RN-acetyloxazolidinone protected
sialosides found in this work correspond to those found57-62 in
simpler systems lacking the cyclic protecting group, we discount the possibility
that the oxazolidinone N-acetyl group interferes with the side
chain conformation of either epimer to any significant extent. As illustrated in
Figure 6 both systems adopt conformations
in which the C7-H7 bond is oriented toward the oxazolidinone
N-acetyl group, thereby minimizing steric interactions with it.</p><p>The anomeric configuration of all compounds prepared in this study was
determined primarily by the measurement of the
3JC,H coupling constant between the
anomeric carboxylate carbon and the axial H3 (Tables 3 and 4, supporting information),
which provided the 2C5 chair conformation
pertains is diagnostic,69-72 and are supported by consistent
nOe patterns such as those laid out in Table
2. In the natural 7S-series,
3JC1,H3ax for the α-glycosides was
consistently found in the range 5.0-6.3 Hz, while the corresponding
β-glycosides showed 3JC1,H3ax = 0, in
full agreement with the literature.69-72 On the other
hand, in the unnatural 7R-series of compounds, while the
α-glycosides displayed a similar range of
3JC1,H3ax (4.7-6.5 Hz) to the natural
series, the β-isomers deviated significantly from the norm and were found in
the range 0-3.8 Hz (Tables 3 and 4, supporting information). This deviation from the 0 Hz for the
3JC1,H3ax heteronuclear coupling constant
in the 7-epi-β-sialosides is not a consequence of a significant distortion of
the MeO2C-C2-C3-H3ax dihedral angle from the ideal 60° (i.e., of a
change in ring conformation) as the
3JH3ax,H4,
3JH4,H5, and
3JH5,H6 homonuclear coupling constants
indicate the antiperiplanar nature of those hydrogens in all cases. Rather we
consider this deviation from the standard69-72 pattern in the
case of the 7-epi-β-glycosides to be a consequence of the influence of the
[O]gt side chain conformation
side chain either directly on the electron density of the
MeO2C-C2-C3-H3ax spin system or indirectly by its influence on the
glycosidic torsion angle, which in turn modulates electron density on the
MeO2C-C2-C3-H3ax spin system. Generally we are of the opinion that
other systems73 for the assignment
of configuration in the sialic acid glycosides based on chemical shifts of specific
resonances and/or chemicals shift differences of pairs of resonances
(Î´H3eq,74
Î´H4,41-42 Î´H7,41,75 and Î´H8,41,75) are unreliable
even in the natural series, particularly when only one anomer is available, as they
are somewhat dependent on the nature of the aglycone (Tables 3 and 4, supporting information). For
example, compound 28α and 28β (in the
natural series for which the rules are formulated) displays Î´H4 of 3.95 and
4.55, respectively, while glycosides 29α and
29Î² (also in the natural series) have Î´H4 of 4.11 and
3.45, respectively, that is, with the inverted order. As the only difference between
glycosides 28α,β and 29α,β is
the presence (28) or absence (29) of a benzyl ether on O4
of the acceptor this reversal of chemical shift order underlines the influence of
the aglycone and even its protecting groups on chemical shift patterns within the
glycoside. Methods for the assignment of anomeric configuration based on side chain
coupling constants (3J7,8 coupling
constant41-42) and chemical shift differences between pairs of
side chain resonances (Δδ H9a and H9b.42,75) cannot
be transposed from the natural (7S) to the unnatural
(7R) series (Tables 3 and 4, supporting information)
because of the change in side chain conformation.</p><!><p>The stereochemical configuration at C7 in the oxazolidinone-protected sialyl
donors influences both reactivity and stereoselectivity, as a consequence of the
differing conformations of the side chains. In the natural
7S-series the C6-C7 bond predominantly adopts the
[O]gg conformation, which
confers a greater reactivity on the donor than the predominant
[O]gt conformation found in
the unnatural 7R-series as a consequence of the increased through
space stabilization of positive charge at the anomeric center in the
[O]gg conformation. The
[O]gt side chain conformation
found in the unnatural series affords greater steric shielding to the α-face
of the donor resulting in lower stereoselectivity in its coupling reactions. These
observations are consistent with earlier conclusions on the influence of the side
chain conformation of reactivity and selectivity derived from conformationally
locked models in the glucose16 and
galactose20 series and
corroborate the suggestion that those effects are predominantly stereoelectronic
rather than torsional.16,20 It follows from these arguments
that the use of atypical protecting group systems in sialyl donors, such as the
recent use of silyl ethers at O4 and/or O7,76 may influence anomeric selectivity through the modulation
of side chain conformation as well as more directly through their arming or
disarming nature. It also follows, however, that care must be taken in the
assignment of anomeric configuration in such systems because standard empirical
rules do not necessarily transpose to the new system; 41-42,73-75 the only reliable method in our opinion being the magnitude
of the 3JC,H heteronuclear coupling constant
between the anomeric carboxylate carbon and the axial H3 for compounds with
established chair conformations.</p><p>Although the results presented in this Article have been developed through
the use of N-acetyl neuraminic acid derivatives and their
7-epimers, the conclusions on the influence of the side chain configuration and
conformation on glycosyl donor reactivity and selectivity are likely to have broader
implications, not the least in the formation of the legionaminic and pseudaminic
acid (Figure 7) glycosides found in various
pathogenic bacteria.77-81 Furthermore, in so far as the side
chain conformation of glycosyl donors might be influenced by protecting groups at
other positions around the pyranose ring, for steric and or stereoelectronic
reasons, it appears likely that side conformation has a role to play in the
continuing debate on the influence of remote protecting groups on glycosylation
reactions.10,82-85 Finally, it is interesting to speculate that Nature may
modulate the activity of enzymes involving in glycosidic bond formation and/or
hydrolysis through the evolution of binding sites tailored to lock the side chain in
suitable conformations.</p>