Mimicking dimethylallyltryptophan synthase: Experimental evidence for a biosynthetic cope rearrangement process

Article abstract of DOI:10.1002/anie.201203586

Easy to cope with: Experimental evidence was found to support an enzymatic [3,3]-sigmatropic rearrangement catalyzed by dimethylallyltryptophan (DMAT) synthase (see scheme). A bio-inspired system showed the feasibility of Cope rearrangement to the C-4 pos

Full text of DOI:10.1002/anie.201203586

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DOI: 10.1002/anie.201203586
Biosynthetic Mechanism
Mimicking Dimethylallyltryptophan Synthase: Experimental Evidence
for a Biosynthetic Cope Rearrangement Process**
Darius D. Schwarzer, Philipp J. Gritsch, and Tanja Gaich*
Dedicated to Professor Johann Mulzer on the occasion of his retirement
Prenylation of the indole nucleus is a fundamental process in
the biosynthesis of alkaloids.[1] This step is catalyzed by
various prenyltransferases, which direct the reaction to
different sites of the indole core. Thereby a dimethylallyl
carbocation reacts in an electrophilic aromatic substitution
reaction at the indole nucleus,[2] leading either to a normal
prenylation product (attack at the primary position of the
dimethylallyl cation) or a reverse prenylation product (attack
at the tertiary position).[3] The normal prenylation at C-4 of
tryptophan is catalyzed by dimethylallyltryptophan (DMAT)
synthase, and comprises the first step in the biosynthesis of
ergot alkaloids. This process is especially noteworthy as the
(DMAPP) was postulated to give compound 1.[6] Subsequent
Cope rearrangement shifts the prenyl moiety to the C-4
position and re-aromatization gives dimethylallyltryptophan
2. Evidence favoring this hypothesis was recently provided by
Tanner et al., with a catalytically active mutant DMAT-
synthase (FgaPT2), producing tricyclic compound 3 instead
(Scheme 1B).[7c] Compound 3 is clearly obtained by way of
reverse prenylation, and consecutive aminal formation.
As examples of enzymes catalyzing pericyclic reactions
are rare,[1c,5a,b,6a,8,9] the Arigoni and Wenkert biosynthetic
hypothesis was dismissed because the chemical feasibility of
a sigmatropic prenyl-shift could not be shown in vitro. Model
compounds 4 and 6 (Scheme 2A,B) failed to give C-4
rearrangement products, but provided products 5 and 7
instead.[5b,c]
Herein, we report the first experimental evidence to
support the Cope rearrangement hypothesis as a possible
biosynthetic pathway, and show the synthetic feasibility of this
chemical transformation. The reaction proceeds at room
temperature and is not dependent on the solvent (Scheme 3).
This led us to conclude that the enzyme might catalyze the
reaction by forcing the substrate into the reactive conforma-
tion.
À
enzyme forms a C C bond at the least nucleophilic position
(C-4) of the indole core—instead of the highly nucleophilic
positions C-2 and C-3.[4] One possible explanation for this
observation was given by Arigoni and Wenkert, and is
displayed in Scheme 1A.[5]
In their hypothesis, an initial reverse prenylation of
tryptophan at C-3 with dimethylallyl pyrophosphate
Scheme 1. The Arigoni and Wenkert biosynthetic hypothesis.
Scheme 2. Model systems 4 and 6 that failed to undergo the Cope
rearrangement to the C-4 position.
[*] M. Sc. D. D. Schwarzer, Dipl.-Ing. P. J. Gritsch, Dr. T. Gaich
Leibniz University of Hannover, Institute of Organic Chemistry
Schneiderberg 1, 30167 Hannover (Germany)
[**] We thank Dr. M. Wiebke from the institute of inorganic chemistry for
X-ray analysis, and Dr. J. Fohrer, M. Rettstedt, D. Kçrtje and Prof. E.
Hofer for detailed 2D-NMR analysis. Financial support was
generously granted by the Fonds der Chemischen Industrie (FCI)
through a Liebigstipendium and by the Alexander von Humboldt
Foundation through the Sofja Kovalevskaja prize.
Supporting information for this article (experimental details) is
available on the WWW under http://dx.doi.org/10.1002/anie.
201203586.
Scheme 3. The bio-inspired [3,3]-sigmatropic rearrangment of 8 and
8a. THF=tetrahydrofuran; PhH=benzene; Bn=benzyl.
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Our bio-inspired system 8 mimics the conformational
restrictions the enzyme likely imposes on the substrate
when pre-orienting it in the active site. It contains a spiro-
fused vinyl-cyclopropane ring, responsible for the rigidity
of the system (Scheme 3). Therefore, the stereochemistry
of the vinyl group pointing towards the benzene ring is
crucial for orbital overlap. The methyl group of 8 is
resembles that of 1 and facilitates the Cope rearrange-
ment (divinylcyclopropane rearrangement in this case)
through a gem-dimethyl effect.[10] We think these archi-
tectural features account for the successful rearrangement
of 8 even at room temperature. Compounds 4 and 6, which
failed to undergo rearrangement, are very flexible, and 6
lacks a gem-dimethyl group.[5b,c] Our system 8 deviates
from bio-system 1 with respect to the oxidation state of
the indole nucleus, but at the same time displays identical
geometrical properties namely sp2 hybridization at respec-
tive carbon atoms. To show that the deviations in
electronic nature do not hamper a [3,3]-sigmatropic rear-
rangement, 8a, without a protecting group, was rearranged to
give 9a in good yields (Scheme 4A). Additionally, 8b[11] was
reduced with sodium borohydride in methanol to give
rearranged indol 15 in one pot (Scheme 4B). Indole 15
exhibits the same oxidation state as bio-system 2.
Scheme 5. Synthetic route to diastereomer 14. DIC=N,N-diisopropylcarbo-
diimide; DMAP=4-N,N-dimethylaminopyridine; DBU=1,8-diazabicyclo-
[5.4.0]undec-7-ene; ABSA=p-acetamidobenzenesulfonyl azide; DMSO=di-
methylsulfoxide; DMF=N,N-dimethylformamide; OTf=triflate.
Synthesis of 8 started with a cyclopropanation of 11 with
diazo-isatin 10.[12,13] Thereby, two separable diastereomers
were formed in a 1:1 ratio in 60% combined yield, following
deprotection of the TBS-group and subsequent oxidation of
aldehydes 12 and 13. The Wittig reaction afforded diastereo-
meric olefins 8 and 14, of which 8 was ideally suited for the
rearrangement, whereas diastereomer 14 cannot undergo the
sigmatropic rearrangement (no orbital overlap of the vinyl
group with the aromatic ring). Indeed only one diastereomer
afforded tricycle 9. Therefore we developed a stereoselective
route to 14 to unambiguously assign the structures of 8 and 14.
For this purpose, azido-acid 16 was converted to 18 in
three steps.[14,15] This sequence secured the cis-annelation at
the two vicinal quaternary carbon centers (curved arrows in
Scheme 5). The relative configuration corresponds to diaste-
Figure 1. Reactivity of 8 in contrast to 14 at 608C as monitored by
NMR spectroscopy.
reomer 14 and was confirmed by single-crystal X-ray
analysis of 13a.[16] Reduction of the azide and cycliza-
tion delivered oxindole 19 in 89% yield. Oxidation of
19 followed by protection of the lactam with benzyl
bromide and consecutive Wittig reaction afforded
diastereomer 14.
Spectral correlation revealed, that exclusively
diastereomer 8 had rearranged to tricycle 9 at room
temperature, whereas 14 did not undergo rearrange-
ment. To visualize the reactivity of 8 a mixture of both
diastereomers was subjected to NMR spectroscopy at
608C. The NMR signals of the CH2-group on the
cyclopropane ring were chosen as indicators of the
rearrangement. As seen in Figure 1, 8 reacts whereas
14 remains essentially unchanged.
Scheme 4. Synthesis of the bio-inspired system 8 (A) and 15 (B). TBAF=tetra-
butylammonium fluoride; IBX=2-iodoxybenzoic acid; TBS=tert-butyldimetyl-
silyl; Boc=tert-butyloxycarbonyl.
Apart from the implications of our results on the
mechanism of the enzyme, our reaction provides rapid
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Scheme 6. Natural products exhibiting a tricyclic benzo[cd]indole core.
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À
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Received: May 9, 2012
Revised: August 1, 2012
Published online: October 10, 2012
Keywords: biosynthesis · Cope rearrangement ·
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indole prenylation · oxindole · reaction mechanisms
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