Enhanced site-selectivity in acylation reactions with substrate-optimized catalysts on solid supports

Article abstract of DOI:10.1039/c7cc00655a

A concept for site selective acylation of poly-hydroxylated substrates is presented where polymer-supported catalysts are employed: catalytically active DMAP units were combined with a library of small molecule peptides attached to the solid phase with the goal to identify substrate-optimized catalysts through library screening. For selected examples, we demonstrate how the optimized catalysts can convert “their” substrate with a markedly enhanced site-selectivity, compared to only DMAP. Due to the solid support, product purification is significantly simplified, and the peptidic catalysts can be easily reused in multiple cycles while conserving its efficiency.

Full text of DOI:10.1039/c7cc00655a

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Enhanced site-selectivity in acylation reactions
with substrate-optimized catalysts on solid
supports†
Cite this:DOI: 10.1039/c7cc00655a
Received 23rd January 2017,
Accepted 20th February 2017
My Linh Tong, Florian Huber, Estelle S. Taghuo Kaptouom, Torsten Cellnik and
Stefan F. Kirsch*
DOI: 10.1039/c7cc00655a
rsc.li/chemcomm
A concept for site selective acylation of poly-hydroxylated substrates is through the catalyst recognition of a particular functional unit
presented where polymer-supported catalysts are employed: catalyti- within the molecule.10 In contrast, catalytic methods that exhibit
cally active DMAP units were combined with a library of small molecule selectivity for one substrate, rather than one functional unit,
peptides attached to the solid phase with the goal to identify substrate- remained underdeveloped, even though this strategy is the
optimized catalysts through library screening. For selected examples, closest to how enzymes work. The most commonly used concept
we demonstrate how the optimized catalysts can convert ‘‘their’’ to identify substrate-optimized catalysts is by mimicking natural
substrate with a markedly enhanced site-selectivity, compared to only evolution, i.e. the selection from diversity, in the laboratory
DMAP. Due to the solid support, product purification is significantly through directed evolution of enzymes.11a Nevertheless, the
simplified, and the peptidic catalysts can be easily reused in multiple fine-tuning of enzyme activity remains a great challenge, in
cycles while conserving its efficiency.
particular with specific non-natural substrates.11
Motivated by the prospect of a synthetic access to substrate-
Enzymes are virtually perfect catalysts: they have a narrow scope specific catalysts that are, like enzymes, also site-selective within
that is paired with an outstanding specificity for the substrate the substrate, we became inspired by the fascinating studies
and the chemical task they have been designed for following by Miller and co-workers demonstrating that low-molecular
ages of evolution. Enzymes, therefore, are the major tools used weight peptides are highly selective and readily tuned catalysts
by Nature to make site-selective reactions with complex multi- for a number of transformations.12,13 In particular, the site-
functionalized biomolecules possible.1 It is remarkable that selectivities obtained with polyfunctional targets underline the
the natural strategy for the synthesis of densely functionalized potential of short peptidic sequences to create high degrees of
biomolecules never relies on less economic pathways such as both selectivity and substrate specificity, even in comparison
protecting group manipulations, in stark contrast to the de novo- to full grown proteins.14 We started our own activities in the
synthesis methods developed by chemists where protecting field with the idea to not use the peptide itself as the catalyst
group manipulations are still a necessary evil due to the lack but to attach a catalytically active unit to the peptide core.
of reactions with competitive selectivity.2
It was expected that the catalytically active unit of the new
Gaining precise reactivity with predictable selectivity is a key system should still be accountable for catalysis while the
challenge in contemporary synthetic chemistry, in particular, when peptidic chain may influence the selectivity, ultimately affording
the controlled modification of multiple-functionalized molecules is substrate-optimized catalysts. Following the lead that 4-(dimethyl-
tackled. In the context of pursuing new methods for the site-selective amino)pyridine (DMAP) is a widely used nucleophilic acylation
modification of complex molecules,3 chemists have compiled catalyst,15 the concept was tested first with the goal to discover
a reaction compendium that includes catalytic methods, for DMAP-based acylation catalysts that could convert specific
example, C–H bond oxidations4 and alkylations,5 the carbo- substrates with a significantly higher site-selectivity than one
hydrate oxidation,6 the organoboron-catalyzed functionalization would achieve by using mere DMAP as the acylation catalyst. To
of diols and carbohydrates,7 and the base-catalyzed modification this end, a library of small peptides containing the artificial
of cis-diols.8,9 In many ways, these methods reflect the idea of amino acid 1 was screened for selected substrates, and it was
achieving preferential reactivity of one group over another group demonstrated that the identification of optimized catalysts was
indeed possible.16 As exemplified by the benzoylation of polyol
2 providing 3b (Scheme 1b), the substrates of interest were
¨
Organic Chemistry, Bergische Universitat Wuppertal, Gaußstr. 20,
typically converted with a markedly enhanced site-selectivity,
42119 Wuppertal, Germany. E-mail: sfkirsch@uni-wuppertal.de
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7cc00655a compared to the parent DMAP.
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Scheme 2 The acylation of glucose-derived diol 5: (a) substrate-
optimized catalyst vs. DMAP,16 (b) substrate-optimized on solid support.
also formed. We then found that a markedly higher selectivity
(6a: 6b = 28 : 1) can be obtained when employing 10 mol% of
Ac-Val-Pro-Phe-1-Leu-Asp-NH2, as an substrate-optimized but still
homogeneous catalyst (Scheme 2a). The same peptidic sequence
was a powerful catalyst when remaining attached at the solid
phase: Ac-Val-Pro-Phe-1-Leu-Asp-resin (10 mol%) provided the
monobenzoylated product 6a in an excellent 96% isolated yield.
A range of other acylations were also possible, and the esters 6d–h
were isolated in yields between 66 and 90%. Of primary impor-
tance, one catalyst was, upon filtration and washing, reused for all
Scheme 1 General strategy for substrate-optimized catalysts: (a) artificial
amino acid 1, (b) substrate-optimized catalyst for the formation of 3b
from 2,16 (c) creation of a library of DMAP–peptide conjugates through
Fmoc-SPPS on Boc-Gly Merrifield resin.
The strength of the concept was the ease of constructing the reactions summarized in Scheme 2b.
the catalyst library through an automated solid-phase peptide
We then questioned whether the site-selectivity with the solid
synthesis using Fmoc-chemistry,17 and one may attach any supported catalyst might be simply due to the involvement of the
other catalytically active unit than DMAP to the peptide chain resin, fully independent from the peptidic sequence bound to
via alkyne–azide cycloaddition.18 However, the use of relatively resin. However, screening of a small library of 21 catalysts on
cost-intensive peptide catalysts with high molecular masses solid support unequivocally demonstrated that subtle changes
(in comparison to the substrates) was a constant issue, and in the catalyst structure influence the eventual selectivity: for
product purification was problematic and time-consuming in example, the use of Ac-1-Pro-Lys-Thr-Ala-Ser-resin resulted only
most cases, in particular, when polar substrates were employed.
The efficient reisolation of the peptides for further use was only
possible with preparative HPLC; yet numerous careful attempts
to reuse the catalysts failed.
We now report the advantages of polymer-supported DMAP–
peptide conjugates 4 (Scheme 1c):19,20 automated solid-phase
synthesis on a Boc-glycine Merrifield resin provides a random
library of catalytically active peptides still attached to the solid
support where each member of the library owns the artificial
amino acid 1 containing the DMAP unit (see the ESI† for further
details). Of note, the peptide structures we generated possessed an
acetyl group at the N-terminus, and the C-terminus was connected
to the glycine-modified Merrifield resin. Activity screening with the
solid-supported catalysts resulted in the direct identification of
hits, and the substrate-optimized catalysts were easily separated
from the reaction mixtures through simple washing and filtration,
allowing for multiple reuse.
We began our studies with the optimization of the DMAP-
catalyzed acylation of dihydroxylated substrate 5 with regard to
the site-selectivity (Scheme 2). The benzoylation of glucose-derived
diol 5 resulted in a poor selectivity with DMAP (6a :6b = 2.8 :1),
and substantial amounts of the dibenzoylated product were
Scheme 3 Optimization of the benzoylation of glucose-derived diols 5
and 7.
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Scheme 4 Optimization of the benzoylation of ouabagenin derivative 9.
in the formation of the monobenzoylated sugar 6a while mixtures these exact reaction conditions with DMAP, instead of the
with 6b and 6c were obtained through the use of other catalysts. substrate-optimized catalyst, the dibenzoylated product 10c
When using DMAP under solution conditions for the benzoylation was found to be the sole product of the reaction.
of diol 7, a compound closely similar to 5 but with an additional
In summary, we have presented how acylation reactions
ether group, we only obtained the dibenzoylated product 8c, catalyzed by solid-supported derivatives of DMAP can be opti-
presumably due to the low solubility of the starting diol 7 under mized with the goal to achieve increased site-selectivity in the
the reaction conditions, compared to an enhanced solubility of reactions. The catalysts consist of a catalytically active DMAP
the mono- and dibenzoylated compounds 8 (Scheme 3b). On the unit and vary in a peptide chain. The catalysts that were found to
other hand, the use of the supported catalysts allowed for the be optimal for a particular substrate can be recovered and reused
straightforward formation of the monobenzoylated compounds while maintaining both their activity and selectivity; a library
and underlines the hypothesis that the solid support is not once created can be used for screening purposes in a continuous
an innocent bystander; instead, the resin may influence the way. As a result, the use of the solid-supported catalysts is
substrate-specificity through, for example, polarity and diffusion significantly more cost-effective than using the related DMAP–
effects. We optimized the benzoylation of diol 7 with the solid- peptide conjugates not attached to the solid support.
supported catalysts and found that Ac-Phe-Asp-Gln-Thr-Val-1-
Ala-His-resin and Ac-1-Gln-Ala-Phe-Val-Leu-Lys-Ser-resin performed
equally well: the monobenzoylated compound 8a was obtained
as the exclusive product. This result highlights how fully unequal
peptide sequences can afford the same selectivity effect while
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