Syntheses of the plant auxin conjugate 2-O-(indole-3-acetyl)-myo-inositol IAInos

Article abstract of DOI:10.1039/C8OB02096E

The plant hormone conjugate 2-O-(indole-3-acetyl)-myo-inositol (IAInos) has been selectively prepared for the first time by two routes from myo-inositol. One of the syntheses depended upon the construction of the 3-indoleacetyl group by a Fischer indole synthesis on an unreactive axial hydroxyl group, while the other via a direct acylation of the equatorially orientated hydroxy group created by conformational constraint of the cyclohexane ring. The latter synthesis produced IAInos in 5 steps and 29% overall yield.

Full text of DOI:10.1039/C8OB02096E

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Syntheses of the plant auxin conjugate 2-O-(indole-3-acetyl)-myo-
inositol IAInos
Received 00th January 20xx,
Accepted 00th January 20xx
Saúl Silvaa, Osvaldo S. Ascensoa, Eva C. Lourençoa, Margarida Archerb, Christopher D. Maycockb,c
M. Rita Venturab,*
,
DOI: 10.1039/x0xx00000x
www.rsc.org/
The plant hormone conjugate 2-O-(indole-3-acetyl)-myo-inositol of the resulting diastereoisomers4a have been reported.
(IAInos) has been selectively prepared for the first time by two
routes from myo-inositol. One of the syntheses depended upon
the the construction of the 3-indoleacetyl group by a Fischer
indole synthesis on an unreactive axial hydroxyl group while the
other, via a direct acylation of the equatorially orientated hydroxy
group created by conformational constraint of the cyclohexane
ring. The latter synthesis produced IAInos in 5 steps and 29%
overall yield.
Indole-3-acetic acid (IAA)
2, the most common naturally
occurring auxin, is a hormone produced by plants, fungi and
bacteria.1 IAA plays a key role in the control of plant growth
and development during its life cycle.2 IAA can form ester
conjugates with sugar and cyclitol moieties, such as in indole-
Figure 1 – IAInos and IAA structure.
Strategies for the preparation of 2-O-myo-inositol esters have
been previously described and rely on the selective protection
of hydroxyl groups 1 and 3-66 or selective acyl group migration
in orthoester derivatives7. Compared to previously reported
examples, indole-3-acetyl is a more functionalized acyl group
and the preparation of its 2-myo-inositol ester imposes
additional challenges. The aim of this work was to develop a
3-acetyl-myo-inositol (IAInos)
1, which is one of the most
important auxin conjugates for the storage and transportation
of auxin.3 Information on its composition, distribution, and
metabolism is important in order to elucidate the related
signal transduction pathways of plant hormones.4 The
conjugated forms of IAA
forms of this hormone for in vitro and in vivo studies.4c,4d Easy
access to pure IAInos would therefore provide a ready source
2 have been used as slow release
new methodology for the selective preparation of IAInos 1.
1
IAInos is a symmetrical inositol derivative and at first
glance appears to be merely a protecting group problem. Thus
an initial strategy consisted of the direct indoleacetylation of
of this compound to be used as a standard for accurate
quantitative biological and analytical tests. Pure isomeric
IAInos 2 can be used for the identification and characterisation
two partially protected myo-inositol derivatives
5 and 6
of specific enzymes involved in the biosynthesis and regulation
of IAA in plants and eventually in other organisms.
(
Scheme 1). These were prepared by selective TBS-protection
of (±)-3,4,5,6-tetrakis-O-benzyl-myo-inositol8
3
or 1,6:3,4-bis-
Several
/EDC;
R1=H, R2=Cl); 1-benzylindole-3-
R1=Bn, R2=Cl)11; succinimido indole-3-
7
R1=H, R2=O-succinimide), (R1=H, R2=OCO2Et), but
[O-(2,3-dimethoxybutane-2,3-diyl)]-myo-inositol9
4.
Despite its biological relevance, a procedure for the
synthesis of IAInos has not been previously described. To our
knowledge, only an enzymatic procedure5 and the unselective
esterification of myo-inositol with subsequent HPLC separation
activated indoleacetic acid reagents
indole-3-acetyl chloride10
acetyl chloride
acetate12
7 were tested: IAA 2
(
7
(
7
(
all failed to introduce the indole-3-acetyl or 1-benzylindole-3-
acetyl groups and starting material was recovered. It appears
that the free hydroxyl groups of compounds
5 and 6 are not
sufficiently reactive probably because they are normally axially
oriented and hindered. Activated indoleacetic acids are
notoriously unstable and polymerize readily, probably due to
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Scheme 1 - Attempts to introduce the indole-3-acetyl group by direct acylation of inositol derivatives 5 and 6.
the formation of ketenes. Thus, they normally acylate only indole dimers. From this result it appears that although indole
reactive alcohols under basic conditions.13
radicals were formed, acetyl free radicals of 5 and 6 were not.
Indolacetylation has previously been recognized as
problematic and an alternative procedure reported by Baran14
for the radical coupling of acetyl esters and indole was
employed (scheme 2). Acetyl esters
very good yield from and , respectively. Unfortunately
submitting and 10 to the oxidative conditions in the presence
9 and 10 were prepared in
5
6
9
of indole resulted in the recovery of starting materials and
Scheme 3 – IAInos 1 preparation attempt via Fischer indole synthesis starting
from 5
2 | J. Name., 2012, 00, 1-3
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Scheme 4 - IAInos 1 preparation via Fischer indole synthesis.
Since these direct attempts to introduce the 3-indoleacetyl attempted using a Fischer indole process (scheme 3). 4-TBSO-
group had failed, the construction of the indolacetyl ester was butanoic acid15 was coupled to inositol derivative
using EDC.
5
The reaction was very slow (4 days) and the use of
stoichiometric DMAP resulted in the highest yield (56%) of
ester 11. Selective deprotection of the primary OTBS group
was affected using Et3N.3HF. Aldehyde 13 was obtained in
excellent yield (96%) by Swern oxidation and subsequent
treatment with phenylhydrazine produced the desired 3-
indoleacetate 14 (89%). Removal of the TBS from 14 using
Et3N.3HF was very slow (7 days), but acyl group migration was
not observed. However, removal of the benzyl groups from 15
proved to be problematic and hydrogenolysis, using palladium
on carbon as catalyst, was not successful.
Scheme 2 – Attempt to introduce indole-3-acetyl group via an acetyl/indole
radical reaction.
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There are no conflicts to declare
In order to eliminate this problem, the same strategy
described in Scheme was used to introduce the 3-
indoleacetyl group into the protected inositol scheme 4) to
3
6
(
Acknowledgements
afford indoleacetate 19. Removal of the TBS group from 19
with TBAF was very slow and some starting material was
recovered (22%) even after 48 h. Hydrolysis of the bis-acetal
M.R.V. acknowledges MostMicro Research Unit (financially
supported by LISBOA-01-0145-FEDER-007660 funded by FEDER
funds through COMPETE2020 (POCI) and by national funds
through FCT), and M.A. acknowledges iNOVA4Health (LISBOA-
01-0145-FEDER-007344, co-funded by FEDER under PT2020)
and financial support from FCT, through Investigador FCT 2014
(IF/00656/2014). The NMR data was acquired at CERMAX,
ITQB-NOVA, Oeiras, Portugal with equipment funded by FCT -
protecting groups on 20 using TFA afforded IAInos
1
1
in
quantitative yield. The H NMR spectrum of
that described in the literature.4a
1 was identical to
Although
1 had been prepared using the route described in
scheme 4, our goal was a more expedient route to this
compound. Therefore, a new synthesis (scheme 5) was
developed using, myo-inositol orthoformate16 21
. Direct
Fundação para
01/SAICT/2016.
a Ciência e Tecnologia, project AAC
acylation of 21 using 3-indoleacetic acid and EDC resulted in a
complex mixture of indoleacetates (mono, bis and tris-acylated
products). However, selective TBS protection of 21 was
possible at an axial hydroxyl and 22 was obtained (65%).17
Direct acylation of 22 occurred selectively at the hydroxyl
group at the 2-position and 23 was obtained in good yield
(83%). In this context, it is notable that direct indolacetylation
Notes and references
1. (a) M. Li, R. Guo, F. Yu, X. Chen, H. Zhao, H. Li and J. Wu, Int J
Mol Sci, 2018, 19, 443; (b) K. Estenson, G. B. Hurst, R. F.
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of myo-inositol derivatives
5 and 6 (scheme 1) was not
possible. Derivative 22 was more reactive probably because
the hydroxyl group at C-2 was fixed in an equatorial
orientation due to the conformational restraints created by
the tricyclic system, while in
5 and 6 it is axial. Removal of the
TBS group from 23 was achieved using TBAF. TFA promoted
hydrolysis of orthoformate 24 was not possible in
dichloromethane or THF due to poor solubility. Using a 1:1
mixture of ethanol and water as solvent, orthoformate 24 was
hydrolyzed but also some hydrolysis of the 3-indoleacetate
ester was observed. Nevertheless, the myo-inositol formed
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1
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Conclusions
In conclusion, the first efficient chemical syntheses of 2-O-
(indole-3-acetyl)-myo-inositol (IAInos) are described in this
1
work. In the first strategy the 3-indoleacetyl group is
introduced via a Fischer indole synthesis and inositol was
converted into IAInos in 8 steps and 16% overall yield, while in
the second it was introduced by direct acylation of
a
conformationally constrained inositol and ester 1 obtained in 5
steps and 29% overall yield from myo-inositol. This latter is an
expedient procedure for the preparation of IAInos, providing
an efficient strategy for the difficult indoleacetylation reaction.
Intermediates 15, 20, and 23 also have the potential to be
used for the preparation of IAInos derivatives. These new
syntheses are efficient methods for the preparation of
sufficient quantities of pure inositol indolylacetates for
biological studies and for analytical standards.
Conflicts of interest
Scheme 5 –Alternative synthesis of IAInos 1.
4 | J. Name., 2012, 00, 1-3
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