Probing structural effects on replication efficiency through comparative analyses of families of potential self-replicators

Article abstract of DOI:10.1002/chem.200600460

A formidable synthetic apparatus for the creation of nanoscale molecular structures and supramolecular assemblies through molecular structures can potentially be created from systems that are capable of parallel automultiplication (self-replication). In order to achieve this goal, a detailed understanding of the relationship be tween molecular structure and replication efficiency is necessary. Diastereoisomeric templates that are capable of specific and simultaneous autocatalysis have been synthesised. A systematic experimental and theoretical evaluation of their behaviour and that of structurally-related systems reveals the key determinants that dictate the emergence of self-replicative function and defines the structural space within which this behaviour is observed.

Full text of DOI:10.1002/chem.200600460

DOI: 10.1002/chem.200600460
Probing Structural Effects on Replication Efficiency through Comparative
Analyses of Families of Potential Self-Replicators
Eleftherios Kassianidis, Russell J. Pearson, and Douglas Philp*[a]
Abstract: A formidable synthetic appa-
ratus for the creation of nanoscale mo-
lecular structures and supramolecular
assemblies through molecular struc-
tures can potentially be created from
systems that are capable of parallel au-
tomultiplication (self-replication). In
order to achieve this goal, a detailed
understanding of the relationship be-
tween molecular structure and replica-
tion efficiency is necessary. Diastereo-
specific and simultaneous autocatalysis
have been synthesised. A systematic
experimental and theoretical evalua-
tion of their behaviour and that of
structurally-related systems reveals the
key determinants that dictate the emer-
gence of self-replicative function and
defines the structural space within
which this behaviour is observed.
A
Keywords: kinetics · molecular rec-
ognition · recognition-mediated re-
actions · self-replication · supramo-
lecular chemistry
Introduction
wish to exploit replicating systems in the construction[6] and
amplification[7] of large molecular and supramolecular as-
semblies.
The formulation of a generally-applicable conceptual frame-
work that would facilitate the rapid development and de-
ployment of self-replicating[1] molecular architectures[2]
could potentially revolutionise materials fabrication at the
nanometer scale. One can envision that the emergence of
such a self-replication protocol would deliver a formidable,
self-instructed synthetic apparatus[3] functioning in an eco-
nomical and environmentally benign fashion. In order to
achieve this ambitious goal, a fundamental understanding of
recognition-mediated processes that allow molecules to tem-
plate their own formation is an important requirement.
Within this broad objective, the development of efficient
protocols which allow self-replication and genotypic (Dar-
winian) evolution[4] within supramolecular assemblies is es-
sential. A clear understanding of the nature of the recogni-
tion-mediated processes which allow molecules to template
their own formation is therefore an important requirement.
We have become interested in the factors that govern the
choice of reaction pathway adopted by systems whose reac-
tivity is mediated[5] by recognition processes. Ultimately, we
In the last 15 years, several examples[8] of self-replicating
systems capable of templating and catalysing their own syn-
thesis have appeared[9] in the chemical literature. Almost all
of the examples of synthetic self-replicators reported in this
period are based on the minimal model[10] shown in
Figure 1. This minimal model of self-replication contains
three distinct reaction channels. The first is the uncatalysed
bimolecular reaction between reagents A and B to afford
the template T. A key requirement of the model is that A
and B bear complementary recognition sites such that A
and B can associate with each other to form a binary com-
plex, [A·B]. The presence of this complex opens a second
reaction channel—the binary complex channel—in which A
and B are preorganised[11] with respect to each other and
the reaction between them is pseudo-intramolecular. The
product of this reaction channel forms a closed template
Tinactive in which the recognition used to assemble the binary
complex lives on in the template, thus, although rate acceler-
ation is achieved[5] by this mechanism, this template is inert
catalytically.
The third reaction channel available to the system is the
[a] Dr. E. Kassianidis, Dr. R. J. Pearson, Dr. D. Philp
Centre for Biomolecular Sciences, School of Chemistry
University of St Andrews, North Haugh, St Andrews KY16 9ST
(UK)
autocatalytic[12] cycle. In this channel, A and B bind revers-
ibly to the open template T to form a catalytic ternary com-
plex [A·B·T]. In a manner similar to the [A·B] complex, the
reaction between A and B is also rendered pseudo-intramo-
lecular. Bond formation occurs between A and B to give the
product duplex [T·T], which then dissociates to return two
Fax : (+44)1334-463-808
E-mail: d.philp@st-andrews.ac.uk
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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FULL PAPER
tion, ii) the efficiency of self-replication, iii) the suppression
(or ideally eradication) of the binary reactive complex route
in favour of the self-replicative route, iv) the enhancement
of diastereoselectivity (translation of stereochemical infor-
mation from parent template to offspring), and v) the social
behaviour of self-replicating species (i.e., emergence of syn-
ergistic or cross-catalytic behaviour, autocratic or independ-
ent/non-cooperative networks). Having established these
factors, we must incorporate our conclusions to current
methodologies, thus expanding the scope and applicability
of the self-replication technology.
In this work, we describe two systems which each contain
a pair of diastereoisomeric replicators which are capable of
specific autocatalysis without concomitant cross-catalysis.
This behaviour is rationalised and explained using experi-
mental and computational methods in terms of the different
recognition-mediated reaction pathways—facilitated by the
amidopicoline–carboxylic acid[16] recognition motif—which
predominate in these templates. Additionally, we demon-
strate, through programmed variation of the constitution of
the components of the recognition-mediated reactions, that
the structural window within which this behaviour is ob-
served is rather narrow.
Figure 1. Minimal model of self-replication. Reagents A and B can react
through three pathways. Channel 1: uncatalysed bimolecular reaction.
Channel 2: Recognition-mediated pseudounimolecular pathway mediated
by a binary complex [A·B]. Channel 3: Recognition-mediated pseudouni-
molecular autocatalytic cycle mediated by a ternary complex [A·B·T].
Results and Discussion
molecules of T to the start of the autocatalytic cycle. Thus,
assuming the open template T presents its recognition sites
in the correct orientation, it can act as a template for its
own formation, transmitting molecular information through
the formation of identical copies of itself. Our initial
forays[13] in the field of molecular self-replication focused on
the utilisation of bifunctional precursor subunits which in-
corporate mutually complementary recognition elements
(essential for reversible, intermolecular assembly) and reac-
tive functionalities (essential for the irreversible construc-
tion of scaffolds). These features are combined to engineer
linear self-complementary platforms capable of autocatalytic
self-propagation through the mechanism illustrated in
Figure 1.
In our overall strategy, the development of replicating sys-
tems that can operate independently or in concert within
the same reaction mixture is of great significance. We identi-
fied the Diels–Alder reaction between alkylfurans and male-
imides as a particularly suitable platform[14] upon which to
conduct the search for systems that display this important
property. This reaction[15] affords two products—the endo
cycloadduct and the exo cycloadduct—which differ in the
relative stereochemistry at the 6–5 ring junction and these
two cycloadducts have markedly different geometries. The
reaction proceeds under mild conditions without the need
for external reagents, making it ideal for study by NMR
methods. Our approach to studying molecular replication in-
volves rigorous experimental and theoretical analyses of sys-
tems that are intimately related in a constitutional sense.
From these studies we wish to pinpoint the key determi-
nants that enable: i) the emergence of self-replicative func-
In order to investigate the relationship between structure
and function in minimal self-replicating systems, it is neces-
sary to examine the role that systematic structural variation
plays in determining the recognition-mediated reactivity dis-
played by a particular system. To this end, we designed and
synthesised (Figure 2) furans 1 through 4 and maleimides 5a
and 6a. All dienes contain a carboxylic acid recognition site
which can associate in non-polar solvents—through two hy-
drogen bonds (Figure 2)—with the amidopyridine recogni-
tion site located on the dienophiles. Reaction of the diene
with the dienophile gives rise to two diastereoisomeric cy-
cloadducts, endo and exo. The structural variation inherent
in this system affords three sites of variability—the number
of methylene spacers between the recognition sites and the
reactive sites (m and n in Figure 2), and the location of the
spacer on the diene (furan substitution in Figure 2).
Therefore, pairwise combination of each of the dienes
with each of the dienophiles gives rise to eight distinct sys-
tems—labelled system I through VIII in Figure 2—with the
capacity for recognition-mediated reactivity and therefore,
potentially, replication. It is noteworthy that the diastereo-
isomeric pairs of endo and exo cycloadducts (produced from
common precursors, for example, 1 and 5a) possess dissimi-
lar configurational and geometrical features. Consequently,
although identical, peripherally extending recognition ele-
ments are affixed to identical positions on the tricyclic core
of respective endo and exo cycloadducts through identical
alkyl spacer units, these recognition sites explore disparate
conformational landscapes. This difference is significant,
since the three-dimensional structure and folding of each di-
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D. Philp et al.
a function of time. Product
concentrations were calculated
1
from the H NMR spectroscop-
ic data by deconvolution meth-
ods. All kinetics experiments
were performed under thermo-
statically controlled conditions
(Æ0.18C).
We did not observe decom-
position or the formation of
unexpected side products in
any of the kinetics experiments
performed. Retro Diels–Alder
reactions[17] involving furans
and maleimides[18] are well
documented and known to
occur rapidly at elevated tem-
peratures (T > 1008C). In the
temperature range and time-
scale of our investigations,
complications resulting from
extensive interconversion of
endo and exo cycloadducts did
not arise. However, minor ef-
fects from this phenomenon
Figure 2. Design of self-replicating templates. The cartoons relate the chemical structures to the schematic
mechanism shown in Figure 1.
astereoisomer determines its self-templating capacity. Addi-
tionally, we introduced variation in the position of substitu-
tion on the furan ring since we expected there to be behav-
ioural and mechanistic discrepancies between constitutional
isomers such as exo-8a and exo-12a.
In any assessment of reactivity, suitable control com-
pounds are required. We therefore designed and synthesised
maleimides 5b and 6b as suitable controls for systems I
through VIII since they lack the appropriate amidopyridine
recognition site whilst retaining the other structural and
electronic features of 5a and 6a. In the discussion which fol-
lows, any compound with the designator b refers to a control
compound and any compound with the designator a refers
to a compound with the capacity for recognition. All com-
pounds were synthesised by standard methods—details are
given in the Experimental Section and the Supporting Infor-
mation.
were taken into account in our kinetic modelling studies
(see Supporting Information). In summary, experimental
time courses for the formation of cycloadducts endo-7a
through 14a and exo-7a through 14a were obtained using
reaction mixtures containing the appropriate dienes and di-
enophiles ([diene]=[dienophile]=25.0 mm) in CDCl3.
Measures of the kinetic parameters and diastereoselectivi-
ties of the non-recognition-mediated mode of reactivity be-
tween dienes and dienophiles were obtained through kinetic
experiments in which the original dienophiles 5a or 6a were
replaced by non-associating maleimides (NAMs) 5b or 6b,
respectively. Starting concentrations of [diene]=[NAM]=
25.0 mm were employed in all cases. These model reactions
served as excellent reference points, enabling direct compar-
isons of catalysed (recognition-mediated/pseudounimolecu-
lar) and uncatalysed (non recognition-mediated/bimolecu-
lar) pathways and, ultimately, permitting quantification of
self-replicative proficiency.
Kinetic investigations: Our standard kinetic analysis[12] of
each of the systems I through VIII, employed four diagnos-
tic experiments—detailed below—performed on each
system at a range of temperatures. Computational kinetic
simulations and electronic structure calculations of reaction
transition states complemented these experiments. All kinet-
ic experiments reported in detail here were undertaken in
CDCl3 at 358C (systems I–VI) or 458C (systems VII and
VIII).
Reaction mixtures were prepared and assayed at fixed in-
tervals by 500 MHz 1H NMR spectroscopy, and reaction
progress was assessed by monitoring the disappearance or
appearance of the appropriate resonances in the region d
6.80 to 6.99 (maleimide) or d 5.20 to 5.60 (cycloadducts) as
Competitive inhibition experiments involved the addition
of 4.0 equivalents of benzoic acid (BA) to reaction mixtures
consisting of diene and dienophile building blocks ([diene]=
[dienophile]=25.0 mm, [BA]=100.0 mm). These experi-
ments assisted qualitative evaluation of the significance of
hydrogen bonding to the observed rate accelerations. Benzo-
ic acid obstructs the assembly of the catalytically crucial en-
sembles [diene·dienophile·adduct], by competitively occupy-
ing amidopicoline recognition sites of dienophiles and cyclo-
adducts. Hence, reactions that are genuinely orchestrated by
hydrogen bonding should display reductions in reaction rate
in the presence of benzoic acid.
Lastly, the self-replicative/autocatalytic capacity of the
Diels–Alder cycloadducts was examined by incorporating
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Self-Replicators
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catalytic amounts of the prefabricated cycloadducts
A
ing stoichiometric concentrations of the constituent frag-
ments ([diene]=[dienophile]=25.0 mm): It was envisaged
that substantial increases in reaction velocity—proportional
to the increase of concentration of catalytically active spe-
cies (in comparison to reactions where [adduct]initial =0)—
should be observed in cases where adducts exhibited self-
replicative function. On the contrary, the injection of sub-
stoichiometric amounts of self-complementary scaffolds
bearing recognition sites associated in an intramolecular
fashion (Tinactive in Figure 1), should not result in increases in
reaction velocity. In effect, presynthesised templates acted
as mechanistic probes, invaluable for mapping out predomi-
nant reactive pathways: self-replication (or ternary complex)
pathway in the first instance and binary complex pathway in
the second instance. Nevertheless, in the majority of situa-
tions, mathematical treatment of experimental data pointed
to some concurrent operation of all three reaction channels.
The combination of these four experiments—performed
on each system in turn—allows an initial assessment of the
presence (or absence) of a dominant recognition-mediated
reaction channel in a given situation. Finer-grained analysis
of the behaviour of the systems becomes possible after the
simulation and fitting of the kinetic data to the appropriate
reaction model and the careful analysis of the best-fit kinetic
parameters thus obtained for each system.
As a result of the sheer volume of data generated within
this study, we will focus in depth only on systems displaying
unusually interesting characteristics.
Figure 3. Rate profiles at 358Cin CDCl3, for the formation of a) endo-8a
or endo-8b and b) exo-8a or exo-8b from building blocks 1 and 6a and 1
and 6b, respectively. Starting concentrations of diene and dienophile
*
were 25 mm. : Reaction between 1 and 6a (Recognition-mediated reac-
~
System II: In this system, the diene has a single methylene
spacer between the recognition site and the furan ring and
there are two methylenes connecting the dienophile to the
amidopyridine recognition site. The control reaction be-
tween 1 and 6b reveals that, after 16 h at 358C in CDCl3
where [1]=[6b]=25.0 mm, the total conversion was 19%
and the endo-8b:exo-8b ratio was 1.6:1. Analysis of this
data by kinetic simulation and fitting afforded the initial
*
tion). : Reaction between 1 and 6b (Bimolecular reaction). : Reaction
between 1 and 6a in the presence of 4 equiv PhCO2H (competitive inhib-
itor). : Reaction between 1 and 6a in the presence of preformed endo-
8a (10 mol%) or exo-8a (8 mol%). Solid lines represent the best fit of
the experimental data to the appropriate kinetic model using the
SimFit[19] package. See Supporting Information for further details of the
kinetic models used.
~
rates of formation of endo-8b and exo-8b as r
A
of the initial rates of cycloadducts formation back to levels
almost identical to those observed in the control (bimolecu-
4.14”10À5 mmsÀ1 and r
ACHTREUNG
tively (Figure 3).
lar) reaction: r
A
G
When the control dienophile 6b was replaced by the rec-
ognition-capable dienophile 6a, the initial rates of formation
of cycloadducts endo-8a and exo-8a increased significantly.
In the reaction between 1 and 6a ([1]=[6a]=25.0 mm), the
3.85”10À5 mmsÀ1. This observation demonstrates that recog-
nition is pivotal for the emergence of rate acceleration
within this system, whilst simultaneously ruling out adventi-
tious general Lewis acid catalysis of the cycloaddition reac-
tion arising from the presence of carboxylic acid moieties
that are located on precursor 1 and products endo-8a and
exo-8a. Although this experiment demonstrates the recogni-
tion-mediated nature of the processes forming endo-8a and
exo-8a, two mechanistic possibilities still exist. Each cyclo-
adduct, endo-8a and exo-8a, could function as an isolated
self-replicator, autonomously self-propagating in the ab-
sence of synergism or inhibition with its diastereoisomer.
Alternatively, each diastereoisomer could, in addition to
self-replication, participate in the operation of parallel, in-
terdependent subnetworks (hypercycles), sustained by col-
initial rates of reaction increased to r
ACHTREUNG
10À5 mmsÀ1 (2.4-fold increase) and
r
ACHTREUNG
10À5 mmsÀ1 (2.9-fold increase). After 16 h, in the presence of
recognition the total conversion was now 41%. A minor
change in the diastereoselectivity was also observed (endo-
8b:exo-8b=1.1:1). These observations when combined, in-
sinuate the presence of catalytic forces, possibly emanating
from the assembly of ternary aggregates [1·6a·endo-8a],
[1·6a·exo-8a] or the binary aggregate [1·6a].
The introduction of benzoic acid as competitive inhibitor
([1]=[6a]=25.0 mm, [BA]=100.0 mm) resulted in reduction
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D. Philp et al.
laborative
catalysis—where
proliferation of endo-8a is cat-
alysed by itself and by exo-8a
and vice versa.
According to the general
protocol described above, the
ability of the reaction products
(templates) endo-8a and exo-
8a to synchronously bind their
constituent fragments (impos-
ing the propinquity of reactive
functionalities and, hence,
bringing about increases in re-
action velocity) can be evaluat-
ed by computing initial rates
of generation of endo-8a and
exo-8a in reactions between 1
and 6a ([1]=[6a]=25.0 mm)
in the presence of endo-8a
(2.5 mm) and exo-8a (2.3m m).
Figure 4. a) Ball-and-stick model of a representative low energy conformation of endo-8a. b) Ball-and-stick
model of a representative low energy conformation of exo-8a. c) Ball-and-stick model of a representative low
energy conformation of [endo-8a·endo-8a]. d) Ball-and-stick model of a representative low energy conforma-
tion of [exo-8a·exo-8a]. All structures were calculated using AMBER* force field. Carbon atoms are coloured
white, oxygen atoms are coloured light grey, nitrogen atoms are coloured dark grey. Some hydrogen atoms
have been omitted for clarity. Hydrogen bonds are represented by dashed lines.
When such template-directed reactions were performed, the
expected rate increases were observed—the formation of
the absence of any cross-catalytic capacity in the templates.
In order to probe this hypothesis, we calculated the struc-
tures of the exo transition state docked on the exo-8a tem-
plate and the endo transition state docked on the endo-8a
template. Additionally, we attempted to dock the diastereo-
merically mismatched transition states and templates. The
results of these calculations are revealing. The diastereomer-
ically-matched transition states are supported elegantly
(Figure 5) by the corresponding templates suggesting that
these reactions can proceed readily on the matched tem-
plate. However, it proved impossible to locate plausible
transition state structures for the mismatched pairs suggest-
ing that it is not possible for a ternary complex to enter the
reaction cross-section leading to a cross-catalytic transition
state.
both endo-8a and exo-8a was accelerated, with r
C
AHCTREUNG
sponding to accelerations of 3.8- and 4.6-fold over the bimo-
lecular control. It was therefore concluded that the produc-
tion of endo-8a and exo-8a was mediated by the ternary
complexes [1·6a·endo-8a] and [1·6a·exo-8a]. In these ex-
periments, marginal increases (in comparison with the reac-
tion between 1 and 6a) in the fraction of exo-8a produced
(endo-8a:exo-8a=1:1) and total conversion (54%) were
also observed. These results suggest that endo-8a and exo-
8a replicate independently. In other words, endo-8a is a cat-
alyst for its own formation, but not for the formation of exo-
8a and vice versa. In other words, neither template engages
in cross-catalytic behaviour.
The source of this dramatic catalytic selectivity must arise
from the location of the recognition elements in space and
the congruency of the transition state with the supporting
template structure.
In order to probe these effects more deeply, we per-
formed a series of calculations using both molecular me-
chanics (AMBER* force field) and electronic structure
methods (at the B3LYP/6-31G level of theory). We calculat-
ed the minimum energy conformations of the isolated tem-
plates endo-8a and exo-8a, the corresponding homodimeric
duplexes [endo-8a·endo-8a] and [exo-8a·exo-8a] and the
heterodimeric duplex [endo-8a·exo-8a].
It is evident from an examination of Figure 4 that endo-
8a and exo-8a are geometrically incongruent. The homo-
dimers [endo-8a·endo-8a] and [exo-8a·exo-8a] are capable
of supporting four hydrogen bonds. However, the hetero-
dimer [endo-8a·exo-8a] cannot support more than a single
association between an amidopyridine of one template and
the carboxylic acid of its diastereoisomer. Therefore, one
might expect that the transition states leading to endo-8a
and exo-8a might also be basically incongruent resulting in
Figure 5. a) Ball-and-stick representation of the calculated (B3LYP/6-
31G(d)) transition state for the formation endo-8a within the [1·6a·endo-
8a] complex. b) Ball-and-stick representation of the calculated (B3LYP/
6-31G(d)) transition state for the formation exo-8a within the [1·6a·exo-
8a] complex. Carbon atoms are coloured white, oxygen atoms are col-
oured light grey, nitrogen atoms are coloured dark grey. Some hydrogen
atoms have been omitted for clarity. Hydrogen bonds are represented by
dashed lines.
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Self-Replicators
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Table 1. Calculated kinetic and thermodynamic effective molarity values
associated with formation of endo-8a and exo-8a through the operation
of the self-replicative and the binary reactive complex pathways at 358C
in CDCl3.
Experimental support for these computational results
comes from extensive gradient NOE difference analyses
(Figure 6). Equimolar mixtures of endo-8a and exo-8a
([endo-8a]=[exo-8a]=10.0 mm) were subjected to analysis
Reaction path-
kEM
(endo-
tEM
U
kEM
(exo-
tEM(exo-
N
1
by 500 MHz H NMR spectroscopy in CDCl3 at 258C. These
way (358C)
8a) [mm]
8a) [mm]
8a) [mm]
8a) [mm]
studies were able to establish a number of NOE enhance-
ments which could only arise from the homodimeric struc-
tures [endo-8a·endo-8a] and [exo-8a·exo-8a]. For example,
irradiation (Figure 6) at the resonance frequency of He, situ-
self-replication
binary reactive
complex
663289
70
110067
31
3
96
37
the two diastereoisomeric templates indicate that the level
of transition state stabilisation present in the ternary com-
plexes is relatively low—of a similar magnitude to the free
energy of binding of the components of the replicator. This
result is not unsurprising given the relative flexibility of the
templates and the resistance of cycloaddition reaction to ac-
celeration by intramolecularisation. The kEM for exo-8a is
significantly higher than that for endo-8a and this observa-
tion probably reflects a better fit of the transition state to
the template (Figure 5). The most interesting aspect of this
system is the presence of two independent autocatalytic
channels that operate independently in such a structurally
simple system.
Figure 6. Inter- and intramolecular NOE within the homodimeric aggre-
gate [endo-8a·endo-8a] observed in CDCl3 at 358C using 500 MHz
1H NMR spectroscopy.
ated upon the endo-8a scaffold, positive intramolecular
NOE values were observed for adjacent protons Hf and pos-
itive intermolecular NOE values for Hg, signifying the pres-
ence of the homodimer [endo-8a·endo-8a]. It is noteworthy
that this observed NOE signal is consistent with the struc-
ture of the [endo-8a·endo-8a] duplex shown in Figure 4. No
NOE evidence emerged which supported the presence of
significant quantities of the heterodimeric structure [endo-
8a·exo-8a] in solution under these conditions.
Elucidation of the mechanistic subtleties of recognition-
mediated processes is predicated upon the construction of
reaction models that can faithfully reproduce experimental
time courses by numerical simulation. In this way, we can
maximise the amount of information we can extract from
our kinetic experiments by determining more accurately the
relative contributions of the various reaction pathways. In
the case of this system, we postulated that species endo-8a
and exo-8a are formed through the concurrent operation of
the three channels shown in Figure 1 and that the replica-
tion channel for each diastereoisomer (exo-8a and endo-8a)
operate in a mutually-exclusive manner. Details of the reac-
tion model used are given in the Supporting Information.
Fitting of this reaction model to the concentration versus
time data obtained for this system (solid lines in Figure 3)
enabled the calculation of rate constants and, hence, kinet-
ic[20] and thermodynamic[21] effective molarity[22] values (de-
noted as kEM and tEM, respectively), characteristic of the
binary reactive complex channel and the templated (ternary
complex) channel (Table 1).
System III: The diastereoisomeric pair of templates endo-
9a/exo-9a are constitutional isomers of endo-8a/exo-8a. The
length of the spacers that connect the recognition sites to
the reactive sites differentiates the two pairs of cycloadducts.
In system II, the carboxylic acid is connected to the diene
by a single CH2 and the amidopyridine is connected to the
maleimide by a CH2CH2 group. In system III, the placement
of these spacer groups is reversed—the carboxylic acid is
connected to the diene by a CH2CH2 group and the amido-
pyridine is connected to the maleimide by a single CH2. By
examining the behaviour of system III, we hoped to gain in-
sight into the resilience of the replicating behaviour of
system II to small structural changes.
Once again, kinetic experiments (Figure 7) were per-
formed at 358C in CDCl3. In the control reaction, the
Diels–Alder cycloaddition between 2 and 5b ([2]=[5b]=
25.0 mm), afforded endo-9b and exo-9b with respective ini-
tial velocities of r
U
G
Table 2. Measured initial rates of formation of endo-9 and exo-9 at 358C
in CDCl3. Relative rates are calculated assuming the reaction between 2
and 5b has a rate of 1 mmsÀ1
.
Reaction
(358C/
CDCl3)
r
G
Relative
rate
r
U
Relative Total con-
)
]
)
]
rate
version %
(in 16 h)
(endo-9) [mmsÀ1
(exo-9)
2 + 5b
2 + 5a
2 + 5a
+ BA
2 + 5a
+ tem-
plate
6.731.0 .85
3
1.0
25
11.50
10.57
1.7
8.27
5.24
2.2
1.4
38
30
1.5
2.4
The results clearly demonstrate that, in this system, the ef-
ficiency of the binary complex channel is rather low—uni-
formly an order of magnitude less than the autocatalytic
channel. The kinetic effective molarities of close to 1m for
15.80
11.40
3.0
44
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8803

D. Philp et al.
ed reaction between carboxylic acid 2 and 5a ([2]=[5a]=
25.0 mm) rate enhancements of 1.7-fold for endo-9a and 2.2-
fold for exo-9a are observed.
stereoisomeric replicators is precluded by the geometric in-
congruence of the endo and exo cycloadducts.
As before we attempted to reproduce the experimental
rate profiles through kinetic simulation and fitting. Fitting of
the appropriate reaction model to the concentration/time
data obtained for this system (solid lines in Figure 7) ena-
bled the calculation of rate constants and, hence, kinetic and
thermodynamic effective molarities (denoted as kEM and tEM
,
respectively), characteristic of the binary reactive complex
and the autocatalytic reaction channels (Table 3).
Table 3. Calculated kinetic and thermodynamic effective molarity values
associated with formation of endo-9a and exo-9a through the operation
of the self-replicative and the binary reactive complex pathways at 358C
in CDCl3.
Reaction path-
kEM
(endo-
tEM
U
kEM
(exo-
tEM(exo-
N
way (358C)
9a) [mm]
9a) [mm]
9a) [mm]
9a) [mm]
self-replication
binary reactive
complex
475
46
799
216
719
65
78
16
Once again, the binary complex pathway is suppressed ef-
fectively. Templates endo-9a and exo-9a participate in inde-
pendent self-replicating circuits, the maintenance of which is
reliant upon the capacity of endo-9a and exo-9a—exhibiting
dissimilar geometrical features—to independently intercept
their precursor fragments 2 and 5a on their respective rec-
ognition surfaces. As before, the kinetic effective molarities
of below 1m for the two diastereoisomeric templates indi-
cate that the level of transition state stabilisation present in
the ternary complexes is relatively low—slightly less than
the stabilisation of the system through formation of the ap-
propriate ternary complex. Given that the total number of
rotors is identical to system II, this result is not unsurprising.
The kEM for exo-9a is significantly higher than that for endo-
9a and this observation once again probably reflects a
better fit of the transition state to the template. It is pleasing
to note that, in contrast to other replicating systems stud-
ied[23] in our laboratory, the systems reported here are resist-
ant to disruption caused by the reversal of spacer location.
Despite the broad similarity in behaviour between sys-
tems II and III more detailed comparisons of the results
reveal subtle, but important differences. The effective molar-
ities determined for the endo templates at 358C suggest that
the role of recognition is somewhat different in endo-8a and
Figure 7. Rate profiles, at 358C in CDCl3, for the formation of a) endo-
9a or endo-9b and b) exo-9a or exo-9b from building blocks 2 and 5a
and 2 and 5b, respectively. Starting concentrations of diene and dieno-
*
phile were 25 mm. : Reaction between 2 and 5a (recognition-mediated
~
*
reaction). : Reaction between 2 and 5b (bimolecular reaction). : Reac-
tion between 2 and 5a in the presence of 4 equiv PhCO2H (competitive
~
inhibitor). : Reaction between 2 and 5a in the presence of preformed
endo-9a (10 mol%) or exo-9a (7 mol%). Solid lines represent the best
fit of the experimental data to the appropriate kinetic model using the
SimFit[19] package. See Supporting Information for further details of the
kinetic models used.
The addition of the competitive inhibitor benzoic acid
([2]=[5a]=25.0 mm, [BA]=100.0 mm) induced a reduction
of the initial velocity for the formation of endo-9a and exo-
9a (Table 2), thus providing firm evidence of the connection
between hydrogen bonding and rate acceleration.
endo-9a. Whilst kEM
that the major role of recognition is transition state stabilisa-
tion, kEM(endo-9a) tEM(endo-9a), suggesting that the
A
N
A
<
ACHTREUNG
Finally, the addition of 2.5 mm of endo-9a and 1.7 mm of
exo-9a into reaction mixtures containing equimolar concen-
trations of 2 and 5a ([2]=[5a]=25.0 mm), resulted in meas-
major role of recognition is product ground state stabilisa-
tion. By contrast, both exo-8a and exo-9a appear to operate
through transition-state stabilisation with exo-8a achieving a
slightly higher effective molarity. These distinctions between
the two systems are clearly a result of minor structural dif-
ferences in the templates and coconformational divergences
in the ternary complexes. However, the exact origins of
these differences have not proven amenable to elucidation
by computational methods.
ured initial rates of r
C
A
A
2.4- and 3-fold in comparison to the bimolecular reaction
(Table 4). The latter observation demonstrates the ability of
both endo-9a and exo-9a to template their own formation.
As in the case of system II, cross-catalysis between the dia-
8804
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Self-Replicators
FULL PAPER
Table 4. Measured initial rates of formation of endo-10 and exo-10 at
358C in CDCl3. Relative rates are calculated assuming the reaction be-
tween 2 and 6b has a rate of 1 mmsÀ1
System IV: The diastereoisomeric pair endo-10a and exo-
10a has an additional methylene rotor when compared to
either system II or III. In our previous studies, the addition
of a methylene rotor to a system that operated predomi-
nantly through a replicating pathway was to redirect the re-
action through the binary complex. In order to test this hy-
pothesis, we performed the appropriate kinetic experiments
at 358C in CDCl3. The results of these experiments, which
were conducted in a manner analogous to that described
above, are given in Figure 8 and Table 4.
Reaction
(358C/
CDCl3)
r
N
r
G
Relative Total con-
rate version %
(exo-10) (in 16 h)
[mmsÀ1
]
2 + 6b
2 + 6a
2 + 6a
+ BA
6.04
8.32
4.21
1.0
1.4
0.7
5.09
10.68
5.15
1.0
2.1
1.0
22
41
25
2 + 6a
+ tem-
plate
13.49
2.3
13.90
2.7
63
Table 5. Calculated kinetic and thermodynamic effective molarity values
associated with formation of endo-10a and exo-10a through the opera-
tion of the self-replicative and the binary reactive complex pathways at
358C in CDCl3.
Reaction path-
way (358C)
kEM(endo-
10a) [mm]
tEM(endo-
10a) [mm]
kEM
10a) [mm]
G
tEM(exo-
10a) [mm]
G
self-replication
binary reactive
complex
536
36
588
43
448
65
75
63
enhancements in endo-10a and exo-10a are, once again,
similar to those observed in systems II and III. In this case,
kEM(endo-10a) ꢀtEM(endo-10a) suggesting that both transi-
tion state stabilisation and product ground state stabilisation
play some role. By contrast, kEM
suggesting that transition state stabilisation is dominant.
However, since kEM(exo-10a) < 1m, it would appear that
A
N
AHCTREUNG
the transition state stabilisation is less than the ground state
stabilisation of the system caused by assembly of 2 and 6a
on the exo-10a template.
System I: In previous work, we noted[23] that the system
with the shortest spacer combination is usually the most effi-
cient replicator. In kinetic experiments performed on system
I at 15, 25 and 358C in CDCl3, only marginal differences
were noted between initial rates of the bimolecular control
reactions and recognition-mediated processes. These results
suggest that the limited conformational freedom present in
this system precludes reaction through either the binary
complex route or the self-replication cycle.
Figure 8. Rate profiles, at 358C in CDCl3, for the formation of a) endo-
10a or endo-10b and b) exo-10a or exo-10b from building blocks 2 and
6a and 2 and 6b, respectively. Starting concentrations of diene and dieno-
*
phile were 25 mm. : Reaction between 2 and 6a (recognition-mediated
~
*
reaction). : Reaction between 2 and 6b (bimolecular reaction). : Reac-
tion between 2 and 6a in the presence of 4 equiv PhCO2H (competitive
~
inhibitor). : Reaction between 2 and 6a in the presence of preformed
Given the apparent resilience of replication within sys-
tems I through IV, we decided to introduce a larger structur-
al and electronic perturbation into the systems under inves-
tigation. This perturbation was the exchange of the 3-substi-
tuted furan ring present in systems I through IV for a 2-sub-
stituted furan ring that characterises systems V through
VIII. Overall, 2-alkyl furans are less reactive than their 3-
substituted analogues and the change in substitution pattern
introduces significant geometrical change in the structure of
the templates.
endo-6a (10 mol%) or exo-6a (8 mol%). Solid lines represent the best
fit of the experimental data to the appropriate kinetic model using the
SimFit[19] package. See Supporting Information for further details of the
kinetic models used.
Further rationalisation of the experimental results was as-
sisted by calculation of effective molarities for the pathways
operating in system IV (Table 5).
Surprisingly, interposing an additional rotor into the
system does not have the predicted effect. Although the ef-
fective molarity values for system IV are lower than the
other two systems discussed so far, the predominant path-
way is still self-replication. The origins of the observed rate
Systems V and VI: Meaningful analyses of systems V and
VI were problematic as a consequence of the low reactivity
of diene 3 and dienophiles 5a and 6a at 358C. Increasing
Chem. Eur. J. 2006, 12, 8798 – 8812
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8805

D. Philp et al.
the reaction temperature to 458C did not circumvent the
low reactivity, since total conversion to products in uncata-
lysed bimolecular fusions between 3 and 5b or 6b remained
in the region of 3%, whilst in the recognition-mediated re-
actions between 3 and 5a or 6a fashioned products, total
conversion was only around 5%.
System VIII: System VIII is the 2-alkyl furan analogue of
system IV. We were interested to discover the effect that the
change in furan substitution pattern would have on the reac-
tion pathways adopted by this system. Molecular mechanics
calculations (Figure 9) suggested that the structure of exo-
14a is such that reaction through a binary complex is by far
Figure 10. Rate profiles, at 358C in CDCl3, for the formation of a) endo-
14a or endo-14b and b) exo-14a or exo-14b from building blocks 4 and
6a and 4 and 6b, respectively. Starting concentrations of diene and dieno-
Figure 9. a) Ball-and-stick model of a representative low energy confor-
mation of endo-14a. b) Ball-and-stick model of a representative low
energy conformation of exo-14a. All structures were calculated using
AMBER* force field. Carbon atoms are coloured white, oxygen atoms
are coloured light grey, nitrogen atoms are coloured dark grey. Some hy-
drogen atoms have been omitted for clarity. Hydrogen bonds are repre-
sented by dashed lines.
*
phile were 25 mm. : Reaction between 4 and 6a (Recognition-mediated
~
*
reaction). : Reaction between 4 and 6b (Bimolecular reaction). : Re-
action between 4 and 6a in the presence of 4 equiv PhCO2H (competitive
~
inhibitor). : Reaction between 4 and 6a in the presence of preformed
endo-14a (10 mol%) or exo-14a (8 mol%). Solid lines represent the best
fit of the experimental data to the appropriate kinetic model using the
SimFit[19] package. See Supporting Information for further details of the
kinetic models used.
the most likely recognition-mediated process for this system.
Given the low reactivity of the 2-alkyl furan diene, the
four kinetic experiments were performed in CDCl3 at 458C.
Once again, the results are summarised as concentration/
time profiles (Figure 10) and in tabular form (Table 6).
It is immediately clear from these results that system VIII
does indeed operate predominantly through the binary reac-
tive complex channel. The key evidence is the complete ab-
sence of any increase in reaction rate on addition of pre-
formed template. In light of this observation, kinetic simula-
tion and fitting omitted the autocatalytic cycle from the re-
action model. Excellent fits of the experimental data to the
reaction model were obtained (solid lines in Figure 10). The
effective molarity values derived from the simulation and
fitting for this system are given in Table 7.
Table 6. Measured initial rates of formation of endo-14 and exo-14 at
458C in CDCl3. Relative rates are calculated assuming the reaction be-
tween 4 and 6b has a rate of 1 mmsÀ1
.
Reaction
(458C/
CDCl3)
r
N
r
G
Relative Total con-
rate version %
(exo-14) (in 16 h)
[mmsÀ1
]
4 + 6b
4 + 6a
4 + 6a
+ BA
4 + 6a
+ tem-
plate
7.56
9.531.2 .10
6.37
1.0
23 6.7
0.9
3.43
9.15
1.0
2.7
6.2
19
16
45
48
8.22
1.1
21.37
Table 7. Calculated kinetic and thermodynamic effective molarity values
associated with formation of endo-14a and exo-14a through the binary
reactive complex pathways at 458C in CDCl3.
Despite the apparently substantial rate acceleration ob-
served for the formation of exo-14b, the calculated kEM
values are rather low suggesting that the preorganisation of
the reactive groups within the binary complex [4·6a] is poor.
Other similar systems described previously by our laborato-
ry have kEM values significantly in excess of 1m. The unex-
Reaction path-
way (458C)
kEM(endo-
14a) [mm]
tEM(endo-
14a) [mm]
kEM
14a) [mm]
G
tEM(exo-
14a) [mm]
G
binary reactive
complex
52
639
275
128
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Chem. Eur. J. 2006, 12, 8798 – 8812

Self-Replicators
FULL PAPER
Table 8. Measured initial rates of formation of endo-13 and exo-13 at
458C in CDCl3. Relative rates are calculated assuming the reaction be-
pectedly high value of the tEM for endo-14a (639 mm) indi-
cates that there is a significant level of product ground state
stabilisation that is consistent with the result of the molecu-
lar mechanics calculations performed on this system. Al-
though an investigation of the effect of temperature on the
efficiency of this system would be instructive, investigations
at temperatures T < 458C were hampered by the low in-
trinsic reactivities of 4 and 6b at the lower temperatures.
tween 4 and 5b has a rate of 1 mmsÀ1
.
Reaction
(458C/
CDCl3)
r
N
r
G
Relative Total con-
rate version %
(exo-13) (in 16 h)
[mmsÀ1
]
4 + 5b
4 + 5a
4 + 5a
+ BA
7.84
9.04
6.530.9
1.0
1.1
3.64
9.09
1.4
1.0
2.5
18
19
31
5.11
4 + 5a
+ tem-
plate
8.32
1.0
10.34
2.7
27
System VII: Kinetic examination of system VII (Figure 11
and Table 8) reveals striking behavioural similarities with
system VIII.
in the transformation of system VIII into system VII might
make the reaction more favourable entropically[24] by be-
tween 13and 21 eu. This favourable entropy change seems
to be offset, in this case, by problems in accessing the cor-
rect coconformation in the [4·5a] complex to permit entry
into the exo transition state. As expected, calculation of the
effective molarity values for this system (Table 9) reveals
uniformly low values consistent with the poor performance
of this system.
Table 9. Calculated kinetic and thermodynamic effective molarity values
associated with formation of endo-14a and exo-14a through the binary
reactive complex pathways at 458C in CDCl3.
Pathway
(458C)
kEM(endo-
13a) [mm]
tEM(endo-
13a) [mm]
kEM
13a) [mm]
G
tEM(exo-
13a) [mm]
G
binary reactive 42
complex
316
140
29
It is worth noting in passing that system III and VII have
identical spacers between the recognition sites and the reac-
tive sites. Despite this apparent similarity, the change in sub-
stitution pattern in the diene (3-substituted in system III
and 2-substituted in system VII) is enough to make their ki-
netic behaviours entirely different.
Figure 11. Rate profiles, at 358C in CDCl3, for the formation of a) endo-
13a or endo-13b and b) exo-13a or exo-13b from building blocks 4 and
5a and 4 and 5b, respectively. Starting concentrations of diene and dieno-
Conclusion
*
phile were 25 mm. : Reaction between 4 and 5a (recognition-mediated
By examining the relative rate accelerations achieved
(Figure 12) by the eight systems studied here we can extract
some important general observations concerning the rela-
tionship between structure and function.
~
*
reaction). : Reaction between 4 and 5b (bimolecular reaction). : Reac-
tion between 4 and 5a in the presence of 4 equiv PhCO2H (competitive
inhibitor). : Reaction between 4 and 5a in the presence of preformed
endo-13a (10 mol%) or exo-13a (8 mol%). Solid lines represent the best
fit of the experimental data to the appropriate kinetic model using the
SimFit[19] package. See Supporting Information for further details of the
kinetic models used.
~
It is clear from the data presented in Figure 12 that in
three of the four systems based on 3-substituted furans (sys-
tems II, III and IV), both diastereoisomers are capable of
synthesising themselves through the autocatalytic pathway.
In general, the endo cycloadducts are slightly more efficient
templates than the exo cycloadducts since the addition of
template (white bars, Figure 12) has a greater effect on the
rate enhancement observed. In the case of system I, the
system displays no recognition-mediated reactivity at all. A
dramatic change in behaviour is brought about by the re-
The formation of the endo cycloadduct is essentially un-
changed by the introduction of the recognition sites. By con-
trast, the formation of the exo cycloadduct is enhanced by
the introduction of recognition. Surprisingly, the magnitude
of this rate enhancement is smaller than that observed for
system VIII. One could expect that the removal of a rotor
Chem. Eur. J. 2006, 12, 8798 – 8812
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8807

D. Philp et al.
recognition site on the diene. The kinetic behaviours of the
systems described here and previously are summarised in
Figure 13.
An examination of Figure 13reveals that the self-replicat-
ing behaviour observed in these structurally related systems
is clustered in one region only—namely system II, III and
IV in the current work. Almost all of the other recognition-
mediated reactivity observed occurs through a binary com-
plex reaction channel. An analysis of the number of each
type of behaviour observed reveals that, out of 32 potential
templates, nine are capable of templating their own forma-
tion (of which six are relatively efficient), nine are capable
of recognition-mediated reaction through a binary complex
channel and 14 are inactive. It would therefore appear that
our chances of creating an efficient replicating system suc-
cessfully even from a rationally designed starting point is
only around 1 in 5.
An interesting comparison can also be made between the
systems shown in Figure 13b and the work published by von
Figure 12. Comparison of the relative rate enhancements observed in a)
the endo diastereoisomers and b) the exo diastereoisomers of the eight
systems studied. Rate enhancements are expressed as the log10 of the
ratio of the recognition-mediated rate to the bimolecular rate (grey bars)
and the log10 of the ratio of the recognition-mediated rate including
added template to the bimolecular rate (white bars). No entry for a
system signifies that the system does not exhibit measurable recognition-
mediated reactivity.
placement of the 3-alkyl furan diene with a 2-alkyl furan
diene to create systems V through VIII. In this second
group, significant recognition-mediated reactivity is limited
to the exo cycloadducts of systems VII and VIII and occurs
exclusively through the binary reactive complex. The con-
trast between system II and VI is striking. These two sys-
tems are constitutional isomers differing only in the position
of substitution on the furan ring. However, whilst in system
II both diastereoisomers are rather efficient replicators,
system VI shows no recognition-mediated reactivity at all.
This observation serves to emphasise the point that the
structural window in which replication is observed is rather
narrow.
It is instructive to broaden our comparison to include sys-
tems described previously[23] by our laboratory. The relation-
ship between the two series of replicators is simple. In the
work reported here, the acid recognition site is located on
the diene and the amidopyridine recognition site on the di-
enophile. In the cases reported previously, the acid recogni-
tion site is located on the dienophile and the amidopyridine
Figure 13. a) Kinetic behaviours of the system reported in this work. b)
Kinetic behaviours of structurally related systems reported in ref. [23].
The kinetic behaviour of each diastereoisomer is labelled as follows: – =
no recognition-mediated reactivity observed. AB = Reaction occurs pre-
dominantly through a binary reactive complex. SR = Reaction occurs
predominantly through an autocatalytic cycle.
Kiedrowski[8a] on a structurally related system in which the
furan component is replaced by a cyclohexadiene. In terms
of length of spacer, the most appropriate comparison
(Figure 14) is between our system where m=1 and n=1
(Figure 13b). The cyclohexadiene-based system exhibits
nearly exponential replication, whereas the furan-based
system shows only weak replication signatures for the exo
and endo diastereoisomers. This striking difference in the
observed kinetics of the two systems serves to highlight the
fact that the design of predetermined dynamic behaviour is
extremely difficult. Presumably, the slight differences in the
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Self-Replicators
FULL PAPER
Melting points were determined using an Electrothermal 9200 melting
point apparatus and are uncorrected. Microanalyses (CHN) were carried
out at the University of St Andrews. Infra-red Spectra (IR) were record-
ed as KBr discs or thin films (PTFE plates) using a Perkin–Elmer Para-
gon 1000 spectrometer.
1H Nuclear magnetic resonance (NMR) spectra were recorded on a
Bruker Avance 300 (300.1 MHz), a Varian Gemini 2000 (300.0 MHz), a
Bruker Avance 500 (500.0 MHz) or
a Varian UNITYplus 500
(500.1 MHz) spectrometer using the deuterated solvent as the lock and
the residual solvent as the internal reference in all cases. 13C NMR spec-
tra using the PENDANT sequence were recorded on a Bruker Avance
300 (75.5 MHz) spectrometer. All other 13C spectra were recorded on a
Varian Gemini 2000 (75.5 MHz) spectrometer using composite pulse 1H
decoupling. 19F NMR spectra were recorded on a Bruker Avance 300
(282.3MHz) spectrometer using the deuterated solvent as the lock and
the residual solvent as the internal reference. All spectra were recorded
at 298 K unless otherwise stated. All coupling constants are quoted to the
nearest 0.1 Hz. In the assignment of 1H NMR spectra the symbols br, s,
d, t, q, and m denote broad, singlet, doublet, triplet, quartet and multip-
let, respectively.
Figure 14. Two structurally related replicating systems have markedly dif-
ferent kinetic behaviour.
location of the diene termini (arrows, Figure 14) and the dif-
ferent inherent reactivity of the furan diene and the cyclo-
hexadiene are responsible for the observed effects. Develop-
ing a computational framework that is capable of predicting
these subtle differences is a significant challenge.
Electron impact mass spectrometry (EIMS) and high-resolution mass
spectrometry (HRMS) were carried out on a VG AUTOSPEC mass
spectrometer or on a Micromass GCT orthogonal acceleration time of
flight mass spectrometer. Chemical Ionisation Mass Spectrometry
(CIMS) was carried out on a VG AUTOSPEC instrument or on a Micro-
mass GCT orthogonal acceleration time of flight mass spectrometer.
Electrospray mass spectrometry (ESMS) and high-resolution mass spec-
trometry (HRMS) was carried out on a Micromass LCT orthogonal time
of flight mass spectrometer.
Moving forward, it is clear that our ultimate goal of devel-
oping a robust supramolecular assembly strategy based on
replication requires a number of additional developments.
Firstly, we must improve the magnitude of the rate accelera-
tions achieved in our systems—the best system described
here manages an acceleration of around half an order of
magnitude. Secondly, we must use computational methods
more in a design role to create systems with a high chance
of successfully replicating rather than in a post-rationalisa-
tion role. The comparisons drawn in Figures 13and 14 illus-
trate that the structural window for replication is narrow
and we must exploit modern computational methods to
design templates that can be shown computationally to be
capable of supporting the transition state leading to them-
selves. Finally, control and sequencing must be included
within our designs in order that replication can be incorpo-
rated within more complex kinetic schemes. All of these
issues are currently being addressed in our laboratory.
General procedure for kinetic experiments: Stock solutions of precursory
subunits and/or additives (templates, inhibitors) were prepared by dis-
solving the appropriate amount of reagent in CDCl3 using Volac 5 mLÆ
0.025 mL or 2 mLÆ0.015 mL volumetric flasks. Masses of reagents were
measured using a Sartorius BP 211D balance (Æ0.01 mg). Stock solutions
were pre-equilibrated to the appropriate temperature in a water bath for
a minimum 30 minutes. Miscellaneous reaction mixtures were prepared
by sequential injection of appropriate volumes of reagent solutions (pre-
cursory subunits, templates, inhibitors) into
a
Wilmad 507 or
528PP NMR tube, which was subsequently fitted with a polyethylene
pressure cap to minimise solvent evaporation. Volumes of reagent solu-
tions were measured accurately via utilisation of a 1.0 or 0.5 mL Hamil-
ton gas tight syringe. Reaction mixtures were monitored systematically
(spectra recorded every 790, 1890 or 1900 s) by 500 MHz 1H NMR over
16 h. The extent of reaction for kinetic experiments was determined
using the deconvolution tool available in 1D WINNMR (Version 6.2.0.0,
Bruker Daltonik GmbH, Germany, 2000). Kinetic simulation and fitting
of the resultant data to the appropriate kinetic models was achieved
using the fitting mode of SimFit.[19]
Computational methods: All molecular mechanics calculations were per-
formed on a Linux workstation the AMBER* force field together with
the GB/SA solvation model for chloroform as implemented in Macromo-
del (Version 7.1, Schrodinger Inc., 2000). Ab initio electronic structure
calculations were carried out using GAMESS[25] running on a Linux clus-
ter. The version dated 22 November 2004 was used in all calculations.
The transition state for the reaction between furan and maleimide was lo-
cated by generation of an initial guess using the linear synchronous trans-
it (LST) method and then refinement at the HF/6-31G(d) level of theory
within GAMESS. This model transition state was then used to construct
an initial guess for the transition state leading to the appropriate cycload-
duct. This guess was refined at the B3LYP/6-31G(d) level of theory to a
transition state structure possessing single imaginary vibration that corre-
sponded to the reaction coordinate.
Experimental Section
General procedures: Chemicals and solvents were purchased from Acros
Organics, Avocado, Bamford Laboratories, Fisher Scientific, Fluka, Lan-
caster or Sigma-Aldrich and were used as received unless otherwise
stated. Diethyl ether (Et2O) and tetrahydrofuran (THF) were dried by
heating to reflux in the presence of sodium/benzophenone under an N2
atmosphere and were collected by distillation. Acetonitrile (CH3CN),
N,N’-dimethylformamide (DMF) and CH2Cl2 were dried by heating
under reflux over calcium hydride and distilled under N2.
Thin-layer chromatography (TLC) was performed on aluminium plates
coated with Merck Kieselgel 60 F254. Developed plates were air-dried and
scrutinised under a UV lamp (366 nm), and where necessary, stained with
iodine, phosphomolybdic acid (PMA) or ninhydrin or potassium perman-
ganate to aid visualisation. Column chromatography was performed
using Kieselgel 60 (0.040–0.063mm mesh, Merck 9385) or MP Silica
(silica gel, 0.032–0.063 mesh).
Synthetic procedures
Preparative methods and characterisation data for compounds 1, 2, 3, 4,
5a, 5b, 6a, 6b are provided in the Supporting Information.
{4-[(6-Methyl-pyridin-2-ylcarbamoyl)-methyl]-3,5-dioxo-10-oxa-4-aza-tri-
cyclo[5.2.1.02,6]dec-8-en-8-yl} acetic acid (endo-7a and exo-7a): A 1:1
mixture of 1 (0.5 mL of a 50 mm solution in CDCl3) and 5a (0.5 mL of a
Chem. Eur. J. 2006, 12, 8798 – 8812
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8809

D. Philp et al.
50 mm solution in CDCl3) in an NMR tube was heated at 508C for 2 d.
Adducts endo-7a and exo-7a (5.4 mg, 58%, inseparable mixture of dia-
stereoisomers, endo-7a/exo-7a 1:1) precipitated from the reaction mix-
ture as a colourless crystalline solid and were subsequently filtered off
and washed with copious amounts of CHCl3. M.p. 205.8–208.18C;
1H NMR (500 MHz, [D6]DMSO, 258C, TMS): d=10.65 (brs, 1Hexo),
with copious amounts of CHCl3. M.p. 211.8–212.98C; 1H NMR
(500 MHz, [D6]DMSO, 258C, TMS): d=10.39 (brs, 1H), 7.84 (d, 1H, J=
8.0 Hz), 7.64 (t, 1H, J=7.8 Hz), 6.95 (d, 1H, J=7.4 Hz), 6.10 (brs, 1H),
5.06 (br, 1H), 4.98 (s, 1H), 3.65 (t, 2H, J=7.4 Hz), 3.01 (d, 1H, J=
6.6 Hz), 2.94 (d, 1H, J=6.6 Hz), 2.62 (t, 2H, J=7.4 Hz), 2.47–2.43(m,
4H), 2.40 (s, 3H); 13C NMR (125 MHz, [D6]DMSO, 258C, TMS): d=
176.1, 176.0, 173.5, 169.1, 156.3, 151.1, 150.1, 138.2, 128.7, 118.4, 110.5,
82.3, 81.0, 48.7, 46.6, 34.2, 33.8, 31.4, 23.4, 22.0; IR (KBr): n˜ = 3464,
3079, 2995, 1712, 1616, 1582, 1457, 1409, 1327, 1265, 1226, 1169 cmÀ1; MS
(ESÀ): m/z (%): 398 (70) [MÀH]+; HRMS (ESÀ): m/z: calcd for
C20H20N3O6: 398.1352, found 398.1356 [MÀH]+.
exo-{4-[(6-Methyl-pyridin-2-ylcarbamoyl)-methyl]-3,5-dioxo-10-oxa-4-aza-
tricyclo[5.2.1.02,6]dec-8-en-1-yl} acetic acid (exo-11a): A 1:1 mixture of 3
(0.5 mL of a 50 mm solution in CDCl3) and 5a (0.5 mL of a 50 mm solu-
tion in CDCl3) in an NMR tube was heated at 458C for 4 d. exo-11a
(3.1 mg, 33%) precipitated from the reaction mixture as a colourless
crystalline solid and was subsequently filtered off and washed with copi-
ous amounts of CHCl3. M.p. 151.4–152.58C; 1H NMR (500 MHz,
[D6]DMSO, 258C, TMS): d=10.59 (brs, 1H), 7.74 (d, 1H, J=7.7 Hz),
7.63(t, 1H, J=7.7 Hz), 6.96 (d, 1H, J=7.3Hz), 6.36 (brs, 2H), 5.26 (d,
1H, J=5.5 Hz), 4.08 (s, 2H), 3.77–3.74 (dd, 1H, J=5.5, 7.5 Hz), 3.61 (d,
1H, J=7.7 Hz), 3.05 (d, H, J=15.0 Hz), 2.97 (d, 1H, J=15.0 Hz), 2.39 (s,
3H); 13C NMR (125 MHz, [D6]DMSO, 258C, TMS): d=174.1, 174.0,
170.4, 164.4, 156.4, 150.6, 138.4, 134.8, 133.6, 118.7, 110.2, 87.2, 78.4, 48.7,
47.8, 40.7, 33.5, 23.4; IR (KBr): n˜ =3444, 2358, 1770, 1698, 1457, 1399,
1323, 1159 cmÀ1; MS (ESÀ): m/z (%): 370 (100) [MÀH]+; HRMS (ESÀ):
m/z: calcd for C18H16N3O6: 370.1039, found 370.1043 [MÀH]+.
{4-[2-(6-Methyl-pyridin-2-ylcarbamoyl)-ethyl]-3,5-dioxo-10-oxa-4-azatri-
cyclo[5.2.1.02,6]dec-8-en-1-yl} acetic acid (endo-12a and exo-12a): A 1:1
mixture of 3 (0.5 mL of a 50 mm solution in CDCl3) and 6a (0.5 mL of a
50 mm solution in CDCl3) in an NMR tube was heated at 458C for 4 d.
Adducts endo-12a and exo-12a (2.4 mg, 25%, inseparable mixture of dia-
stereoisomers: endo-12a/exo-12a 1:3) precipitated from the reaction mix-
ture as a colourless crystalline solid and were subsequently filtered off
and washed with copious amounts of CHCl3. M.p. 180.4–180.98C;
1H NMR (500 MHz, [D6]DMSO, 258C, TMS): d=10.39 (br, 1Hexo), 10.34
(br, 1Hendo), 7.84 (d, 1Hexo, J=7.6 Hz), 7.84 (d, 1Hendo, J=7.6 Hz), 7.65 (t,
1Hendo, J=7.6 Hz), 7.63(t, 1H exo, J=7.7 Hz), 6.95 (d, 1Hexo, J=7.5 Hz),
6.95 (d, 1Hendo, J=7.5 Hz), 6.56 (d, 1Hexo, J=1.3Hz), 6.55 (d, 1H exo, J=
5.7 Hz), 6.36 (d, 1Hendo, J=5.7 Hz), 6.31 (d, 1Hendo, J=5.6 Hz), 5.24 (dd,
1Hendo, J=1.2, 5.5 Hz), 5.07 (d, 1Hexo, J=1.4 Hz), 3.65 (dt, 2Hexo, J=3.4,
9.0 Hz), 3.53–3.48 (m, 4Hendo), 3.08 (d, 1Hexo, J=6.5 Hz), 3.08–3.05 (d,
1Hendo, J=15.5 Hz), 3.02 (d, 1Hexo, J=6.5 Hz), 2.98–2.95 (d, 1Hendo, J=
15.2 Hz), 2.95 (d, 1Hexo, J=16.9 Hz), 2.91 (d, 1Hexo, J=16.9 Hz), 2.64–
2.60 (dt, 2Hexo, J=2.6, 7.2 Hz), 2.54 (t, 2Hendo, J=7.3Hz), 2.40 (brs,
3Hexo, 3 Hendo); 13C NMR (125 MHz, [D6]DMSO, 258C, TMS, endo-12a in
parentheses): d=175.8, 174.6, (174.6), (174.4), 170.6, (170.0), (169.1),
169.1, (156.3), 156.3, 151.2, 139.0, (138.3), 138.2, (136.4), 136.3, (134.5),
(133.5), 118.4, 110.5, (110.3), (87.6), (87.0), 79.7, (78.5), 49.8, 48.4, 47.4,
(39.8), (37.0), 34.4, 34.2, (33.9), 33.8, (33.5), 23.4; IR (KBr): n˜ =3459,
3276, 3122, 3084, 2949, 2372, 1772, 1700, 1618, 1575, 1455, 1397, 1320,
1157 cmÀ1; MS (ESÀ): m/z (%): 384 (100) [MÀH]+; HRMS (ESÀ): m/z:
calcd for C19H18N3O6: 384.1196, found 384.1186 [MÀH]+.
exo-{4-[(6-methylpyridin-2-ylcarbamoyl)-methyl]-3,5-dioxo-10-oxa-4-aza-
tricyclo[5.2.1.02,6]dec-8-en-1-yl} propionic acid (exo-13a): A 1:1 mixture
of 4 (0.5 mL of a 50 mm solution in CDCl3) and 5a (0.5 mL of a 50 mm
solution in CDCl3) in an NMR tube was heated at 508C for 2 d. exo-13a
(4.4 mg, 46%) precipitated from the reaction mixture as a colourless
crystalline solid and was subsequently filtered off and washed with copi-
ous amounts of CHCl3. M.p. 191.0–191.58C; 1H NMR (500 MHz,
[D6]DMSO, 258C, TMS): d=12.10 (brs, 1H), 10.68 (brs, 1H), 7.78 (d,
1H, J=7.0 Hz), 7.66 (t, 1H, J=7.7 Hz), 6.99 (d, 1H, J=7.5 Hz), 6.59 (d,
1H, J=5.5 Hz), 6.51 (d, 1H, J=5.6 Hz), 5.12 (d, 1H, J=1.3Hz), 4.27 (d,
2H, J=4.0 Hz), 3.20 (d, 1H, J=6.6 Hz), 3.01 (d, 1H, J=6.5 Hz), 2.57–
2.48 (m, 1H), 2.44–2.37 (m, 1H), 2.43 (s, 3H), 2.13–2.07 (m, 1H);
13C NMR (125 MHz, [D6]DMSO, 258C, TMS): d=175.6, 174.3, 173.7,
164.5, 156.5, 150.7, 138.4, 138.2, 137.2, 118.7, 110.2, 90.4, 79.8, 50.2, 48.7,
41.0, 29.5, 24.6, 23.4; IR (KBr): n˜ = 3448, 3209, 3064, 2362, 1777, 1710,
10.61 (brs, 1Hendo), 7.75 (d, 1Hendo, 1Hexo, J=7.0 Hz), 7.63(t, 1H
,
endo
1Hexo, J=7.7 Hz), 6.96 (d, 1Hendo, 1Hexo, J=7.6 Hz), 6.30 (d, 1Hexo, J=
1.5 Hz), 6.11 (br, 1Hendo), 5.27 (d, 1Hendo, J=4.6 Hz), 5.23(d, 1H endo, J=
4.7 Hz), 5.10 (br, 1Hexo), 5.06 (s, 1Hexo), 4.23(s, 2H exo), 4.09 (s, 2Hendo),
3.67 (m, 2Hendo), 3.33 (s, 2Hexo), 3.26 (d, 1Hexo, J=6.6 Hz), 3.18 (d,
1Hendo, J=17.3Hz), 3.09 (d, 1H exo, J=6.6 Hz), 2.94–2.90 (dd, 1Hendo, J=
1.8, 17.9 Hz), 2.40 (s, 3Hexo), 2.39 (s, 3Hendo); 13C NMR (125 MHz,
[D6]DMSO, 258C, TMS, endo-7a in parentheses): d=175.9, 175.7,
(174.3), (174.0), 171.1, (170.8), 164.6, 156.5, 150.7, 144.1, (142.7), (142.4),
(140.2), 138.4, (134.8), 132.3, 129.5, 118.7, (111.7), (110.3), 110.2, 82.3,
80.9, (80.3), (79.3), 48.4, (47.0), 46.8, (45.9), 41.0, (40.7), (33.7), 32.8,
(30.2), (23.4); IR (KBr): n˜ =3464, 3079, 2995, 1712, 1616, 1582, 1457,
1409, 1327, 1265, 1226, 1169 cmÀ1; MS (ESÀ): m/z (%): 370 (5) [MÀH]+;
HRMS (ESÀ): m/z: calcd for C18H16N3O6: 370.1039, found 370.1033
[MÀH]+.
{4-[2-(6-Methylpyridin-2-ylcarbamoyl)-ethyl]-3,5-dioxo-10-oxa-4-aza-tri-
cyclo[5.2.1.02,6]dec-8-en-8-yl} acetic acid (endo-8a and exo-8a): A 1:1
mixture of 1 (0.5 mL of a 50 mm solution in CDCl3) and 6a (0.5 mL of a
50 mm solution in CDCl3) in an NMR tube was heated at 508C for 2 d.
Adducts endo-8a and exo-8a (6.1 mg, 63%, inseparable mixture of dia-
stereoisomers, endo-8a/exo-8a 1:1) precipitated from the reaction mix-
ture as a colourless crystalline solid and were subsequently filtered off
and washed with copious amounts of CHCl3. M.p. 172.0–173.18C;
1H NMR (500 MHz, [D6]DMSO, 258C, TMS): d=12.43(br, 1H
,
endo
1Hexo), 10.39 (br, 1Hendo, 1Hexo), 7.84 (d, 1Hendo, 1Hexo, J=8.4 Hz), 7.65
(t, 1Hendo, J=8.0 Hz), 7.64 (t, 1Hexo, J=8.1 Hz), 6.96 (d, 1Hendo, 1Hexo, J=
7.3Hz), 6.30 (d, 1H exo, J=1.5 Hz), 6.10 (br, 1Hendo), 5.27 (d, 1Hendo, J=
5.1 Hz), 5.22 (d, 1Hendo, J=5.3Hz), 5.07 (s, 1H exo), 5.03(br, 1H exo), 3.6 (t,
2Hexo, J=7.6 Hz), 3.59 (q, 2Hendo, J=5.0 Hz), 3.50 (dd, 2Hendo, J=7.9,
15.2 Hz), 3.34 (s, 2Hexo), 3.18 (d, 1Hendo, J=2.6 Hz), 3.17 (d, 1Hexo, J=
6.3Hz), 2.99 (d, 1H exo, J=6.3Hz), 2.93–2.89 (dd, 1H endo, J=1.9, 17.5 Hz),
2.61 (t, 2Hexo, J=7.6 Hz), 2.53–2.50 (m, 2Hendo), 2.41 (s, 3Hendo, 3 H );
exo
13C NMR (125 MHz, [D6]DMSO, 258C, TMS, endo-8a in parentheses):
d=176.2, (176.0), (174.6), 174.3, (171.1), 170.6, (169.3), (169.0), 169.0,
(161.8), 156.2, 151.1, 144.0, 142.7, (140.2), (138.1), (134.4), 132.2, 129.1,
118.4, (111.7), 110.4, (110.3), 82.3, 80.9, (80.3), (79.3), (54.4), (48.2), 46.6,
45.5, (34.2), 32.8, 32.4, (26.7), 23.4; IR (KBr): n˜ =3449, 3276, 3074, 2353,
1700, 1616, 1577, 1452, 1399, 1323, 1260, 1193, 1159 cmÀ1; MS (ESÀ): m/z
(%): 384 (100) [MÀH]+; HRMS (ESÀ): m/z: calcd for C19H18N3O6:
384.1196, found 384.1202 [MÀH]+.
exo-3-{4-[(6-Methyl-pyridin-2-ylcarbamoyl)-methyl]-3,5-dioxo-10-oxa-4-
aza-tricyclo[5.2.1.02,6]dec-8-en-8-yl} propionic acid (exo-9a): A 1:1 mix-
ture of 2 (0.5 mL of a 50 mm solution in CDCl3) and 5a (0.5 mL of a
50 mm solution in CDCl3) in an NMR tube was heated at 358C for 3d.
exo-9a (3.9 mg, 40%) precipitated from the reaction mixture as a colour-
less crystalline solid and was subsequently filtered off and washed with
copious amounts of CHCl3. M.p. 179.5–180.48C; 1H NMR (500 MHz,
[D6]DMSO, 258C, TMS): d=10.64 (br s, 1H), 7.75 (d, 1H, J=7.5 Hz),
7.62 (t, 1H, J=7.7 Hz), 6.95 (d, 1H, J=7.5 Hz), 6.09 (brs, 1H), 5.07 (br,
1H), 5.00 (br s, 1H), 4.23(brs, 2H), 3.10 (d, 1H, J=6.4 Hz), 3.02 (d, 1H,
J=6.5 Hz), 2.47–2.41 (m, 4H), 2.40 (s, 3H); 13C NMR (125 MHz,
[D6]DMSO, 258C, TMS): d=175.8, 175.6, 173.4, 164.5, 156.4, 150.7, 150.1,
138.4, 128.7, 118.7, 110.2, 82.2, 81.0, 48.8, 46.7, 41.0, 31.4, 23.4, 21.9; IR
(KBr): n˜ =3440, 3238, 3074, 2372, 1700, 1585, 1460, 1421, 1325, 1267,
1209, 1161 cmÀ1; MS (ESÀ): m/z (%): 384 (100) [MÀH]+; HRMS (ESÀ):
m/z: calcd for C19H18N3O6: 384.1196, found 384.1193 [MÀH]+.
exo-3-{4-[2-(6-methyl-pyridin-2-ylcarbamoyl)-ethyl]-3,5-dioxo-10-oxa-4-
azatricyclo[5.2.1.02,6]dec-8-en-8-yl} propionic acid (exo-10a): A 1:1 mix-
ture of 2 (0.5 mL of a 50 mm solution in CDCl3) and 6a (0.5 mL of a
50 mm solution in CDCl3) in an NMR tube was heated at 458C for 3d.
exo-10a (4.0 mg, 40%) precipitated from the reaction mixture as a col-
ourless crystalline solid and was subsequently filtered off and washed
8810
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 8798 – 8812

Self-Replicators
FULL PAPER
1585, 1455, 1416, 1335, 1262, 1209, 1171 cmÀ1; MS (ESÀ): m/z (%): 384
(100) [MÀH]+; HRMS (ESÀ): m/z: calcd for C19H18N3O6: 384.1196,
found 384.1188 [MÀH]+.
Acknowledgements
BBSRC (Grant 6/11855), EPSRC (DTA award to E.K.) and the Univer-
sity of St Andrews supported this work financially. We are grateful to
Caroline Horsburgh (Mass spectrometry) and Melanja Smith (NMR) for
technical assistance. We thank Prof G. von Kiedrowski for providing us
with a copy of his SimFit program.
3-{4-[2-(6-Methylpyridin-2-ylcarbamoyl)-ethyl]-3,5-dioxo-10-oxa-4-azatri-
cyclo[5.2.1.02,6]dec-8-en-1-yl} propionic acid (endo-14a and exo-14a): A
1:1 mixture of 4 (0.5 mL of a 50 mm solution in CDCl3) and 6a (0.5 mL
of a 50 mm solution in CDCl3) in an NMR tube was heated at 508C for
2 d. Adducts endo-14a and exo-14a (3.5 mg, 35%, mixture of diaster-
eoisomers: endo-14a/exo-14a, 1:3) precipitated from the reaction mixture
as a colourless crystalline solid and were subsequently filtered off and
washed with copious amounts of CHCl3. M.p. 149.4–150.98C; 1H NMR
(500 MHz, [D6]DMSO, 258C, TMS): d=10.37 (brs, 1Hexo), 10.33 (br,
1Hendo), 7.81 (d, 1Hexo, J=8.2 Hz), 7.81 (d, 1Hendo, J=8.2 Hz), 7.61 (t,
1Hendo, J=7.7 Hz), 7.59 (t, 1Hexo, J=7.7 Hz), 6.91 (d, 1Hendo, J=7.3Hz),
[1] For examples of complex replicable entities see: a) L.-H. Eckardt,
K. Naumann, W. M. Pankau, M. Rein, M. Schweitzer, N. Windhab,
G. von Kiedrowski, Nature 2002, 420, 286; b) J. W. Szostak, D. P.
Bartel, P. L. Luisi, Nature 2001, 409, 387–390; c) N. C. Seeman, P. S.
Lukeman, Rep. Prog. Phys. 2005, 68, 237–270; d) R. Wick, P. Walde,
P. L. Luisi, J. Am. Chem. Soc. 1995, 117, 1435–1436; e) A. Veronese,
P. L. Luisi, J. Am. Chem. Soc. 1998, 120, 2662–2663.
6.90 (d, 1Hexo, J=7.3Hz), 6.52 (dd, 1H exo, J=1.3, 5.7 Hz), 6.43 (d, 1Hexo
,
J=5.7 Hz), 6.32 (d, 1Hendo, J=5.7 Hz), 6.27 (d, 1Hendo, J=5.6 Hz), 5.19
(dd, 1Hendo, J=1.3, 5.5 Hz), 5.03 (d, 1Hexo, J=1.5 Hz), 3.65–3.58 (m,
2Hexo), 3.47 (t, 2Hendo, J=7.4 Hz), 3.24 (d, 1Hendo, J=7.4 Hz), 3.02 (d,
1Hexo, J=6.5 Hz), 2.83(d, 1H exo, J=6.5 Hz), 2.83–2.80 (dd, 1Hendo, J=
8.0, 9.1 Hz), 2.64–2.60 (dt, 2Hexo, J=2.2, 7.9 Hz), 2.50 (t, 2Hendo, J=
7.3Hz), 2.45–2.02 (m, 7H exo, 7Hendo); 13C NMR (125 MHz, [D6]DMSO,
258C, TMS, endo-12a in parentheses): d=176.1, (174.9), 174.7, 174.0,
(173.9), (173.5), (171.8), (169.3), 169.2, (156.4), 156.4, 151.3, (138.5),
138.4, 138.3, 137.3, (136.1), (135.2), 118.6, 110.6, (110.4), (90.5), (90.4),
80.0, (78.3), 50.2, (49.2), 48.7, (47.9), 34.7, (34.3), (34.0), 33.9, (33.6), 32.0,
29.7, (29.2), (27.2), 24.7, 23.6; IR (KBr): n˜ = 3444, 3213, 3079, 2934,
2357, 1770, 1702, 1611, 1577, 1452, 1395, 1327, 1159 cmÀ1; MS (ESÀ): m/z
(%): 398 (100) [MÀH]+; HRMS (ESÀ): m/z: calcd for C20H20N3O6:
398.1352, found 398.1351 [MÀH]+.
endo-3-{4-[(Methylphenylcarbamoyl)-ethyl]-3,5-dioxo-10-oxa-4-azatricy-
clo[5.2.1.02,6]dec-8-en-8-yl} propionic acid (endo-10b): A 1:1 mixture of 2
(0.5 mL of a 50 mm solution in CDCl3) and 6b (0.5 mL of a 50 mm solu-
tion in CDCl3) in an NMR tube was heated at 508C for 5 d. Subsequent-
ly, the solvent was removed under reduced pressure and the crude prod-
uct was purified via column chromatography (silica gel, hexane/EtOAc
2:1), affording endo-10b (3.8 mg, 38%) as a colourless oil. 1H NMR
(500 MHz, CDCl3, 258C, TMS): d=7.45–7.16 (m, 5H), 5.82 (br, 1H),
5.18 (d, 1H, J=4.2 Hz), 5.03(d, 1H, J=4.2 Hz), 3.56 (t, 2H, J=7.6 Hz),
3.47 (m, 2H), 3.22 (s, 3H), 2.44–2.13 (m, 6H); 13C NMR (125 MHz,
CDCl3, 258C, TMS): d=176.1, 174.6, 174.1, 169.8, 148.8, 143.2, 129.8,
128.1, 127.1, 126.4, 81.3, 80.1, 47.1, 46.0, 37.2, 34.7, 31.5, 31.2, 23.2; IR
(KBr): n˜ =3444, 2969, 2365, 1962, 1646, 1582, 1453, 1400, 1327, 1265,
1246, 1120 cmÀ1; MS (ESÀ): m/z (%): 397 (100) [MÀH]+, 353 (25);
HRMS (ESÀ): m/z: calcd for C21H21N2O6: 397.1400, found 397.1403
[MÀH]+.
[2] a) A. Mulder, J. Huskens and D. N. Reinhoudt, Org. Biomol. Chem.
2004, 2, 3409–3424; b) G. M. Whitesides, M. Boncheva, Proc. Natl.
Acad. Sci. USA 2002, 99, 4769–4774; c) G. M. Whitesides, B. Grzy-
bowski, Science 2002, 295, 2418–2421; d) H. Cçlfen, S. Mann,
Angew. Chem. 2003, 115, 2452–2468; Angew. Chem. Int. Ed. 2003,
42, 2350–2365; e) I. W. Hamley, Angew. Chem. 2003, 115, 1730–
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Chem. Eur. J. 2006, 12, 8798 – 8812
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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8811

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Received: April 3, 2006
Published online: September 1, 2006
8812
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Chem. Eur. J. 2006, 12, 8798 – 8812