Divergent gold-catalysed reactions of cyclopropenylmethyl sulfonamides with tethered heteroaromatics

Article abstract of DOI:10.1039/c9cc06241f

Cyclopropenylmethyl sulfonamides with tethered heteroaromatics have been demonstrated to undergo divergent gold-catalysed cyclisation reactions. A formal dearomative (4+3) cycloaddition takes place with furan-tethered substrates, yielding densely functionalised 5,7-fused heterocycles related to the bioactive curcusone natural products. Indole-tethered substrates display divergent reactivity giving biologically important tetrahydro-β-carbolines via a Friedel-Crafts mechanism.

Full text of DOI:10.1039/c9cc06241f

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Divergent gold-catalysed reactions of
cyclopropenylmethyl sulfonamides with tethered
heteroaromatics†
Cite this: Chem. Commun., 2019,
55, 13971
Received 14th August 2019,
Accepted 11th October 2019
a
a
Melanie A. Drew,
Matthias Rudolph,b A. Stephen K. Hashmi *b and Christopher J. T. Hyland
Sebastian Arndt,b Christopher Richardson,
a
*
DOI: 10.1039/c9cc06241f
rsc.li/chemcomm
Cyclopropenylmethyl sulfonamides with tethered heteroaromatics intramolecular reactions of cyclopropenes bearing heteroatom
have been demonstrated to undergo divergent gold-catalysed cyclisa- tethers at the sp3 C3 position have been far less investigated than
tion reactions. A formal dearomative (4+3) cycloaddition takes the sp2 C2-tethered systems.10 Furthermore, to the best of our
place with furan-tethered substrates, yielding densely functionalised knowledge there have been no studies on trapping cyclopropene-
5,7-fused heterocycles related to the bioactive curcusone natural derived vinyl gold-carbenes with pendant heteroaromatic
products. Indole-tethered substrates display divergent reactivity systems.11,12 Lee and co-workers reported elegant studies on
giving biologically important tetrahydro-b-carbolines via a Friedel– the intermolecular reactions of furans and indoles (3) under-
Crafts mechanism.
going direct attack to vinyl gold carbenes 2 derived from the ring-
opening of simple cyclopropenes 1 (Scheme 1).5 We reasoned
Gold-catalysed reactions of cyclopropenes are emerging as that employing tethered systems of type 5 should encourage the
powerful tools in organic synthesis.1 For example, their facile vinylogous attack of the pendant heteroaromatic based on
ring opening enables them to act as useful stable precursors to formation of the most favoured ring-size – ultimately leading to
highly versatile and reactive vinyl gold carbene intermediates.1–5 complex heterocycles. Herein, we report the divergent reactivity of
Cyclopropenes have also been used to study the nature of gold(I) disubstituted alkenylgold carbenes I derived from cyclopropene
carbene bonding.6 The accessibility of cyclopropenes with derivatives with heteroaromatic groups tethered to a C3 amino-
pendant heteroatoms has enabled new emergent methods for methyl linker (Scheme 1). Our studies reveal the first examples of
intramolecular trapping of these vinyl gold carbenes. Notably, vinyl gold carbenes undergoing a formal (4+3) cycloaddition with
Cossy, Meyer and co-workers demonstrated intramolecular a furan tether to yield 5,7-fused heterocycles 6. This reaction adds
cyclopropanation of mono-substituted vinyl gold carbenes to the existing suite of coinage metal-catalysed (4+3) cycloaddition
generated from O- and N-tethered cyclopropenes to form processes, including those using allenes13 and propargyl esters.14
stereodefined bicycles.3 In contrast, we have shown that Crucially, when indole/pyrrole tethers are used, divergent
C1–C2-disubstituted cyclopropenylmethyl acetates undergo
alternative gold-catalysed regioselective ring-opening to form
a disubstituted vinyl gold-carbene, which participates in a
formal 1,2-acetoxy transfer to form Z-acetoxydienes.4,7 As such,
cyclopropenes provide an important, complementary, route to
vinyl gold-carbenes along with other important methods from
propargyl esters,8 vinyldiazoesters9a–h and via the retro-Buchner
reaction of alkenyl- or aryl-cycloheptatrienes.9i–k Despite straight-
forward synthesis via cyclopropenation of alkynes, gold-catalysed
a School of Chemistry and Molecular Bioscience, and Molecular Horizons Research
Institute, University of Wollongong, Northfields Avenue, Wollongong, 2522,
Australia. E-mail: chris_hyland@uow.edu.au
b Organisch-Chemisches Institut, Heidelberg University, Im Neuenheimer Feld 270,
69120, Heidelberg, Germany
† Electronic supplementary information (ESI) available. CCDC 1909426 and
1909427. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c9cc06241f
Scheme 1 Au-Catalysed reactions of cyclopropenes with a pendant
heteroatom.
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Table 1 Substrate scope for the (4+3) dearomative cycloaddition of
cyclopropenyl furyl sulfonamides 5
reactivity emerges via a Friedel–Crafts process to give tetra-
hydro-b-carbolines 7. Heterocycles 6 are important as they are
related structurally to the daphnane/tigliane/rhamnofolane
diterpenes (e.g. curcusones I and J) that have numerous biological
activities including anti-HIV, antimicrobial, antitumour, anti-
malarial and neurotropic activity (Scheme 1).15,16 The tetrahydro-
b-carboline skeleton obtained from the indole-tethered system is
also found in an array of bioactive molecules, including the
antihypertensive ajmalicine.17
To investigate the proposed reactivity of C3-heteroaromatic
tethered cyclopropenes, a 5-methylfuran-tethered (5aa), an
N-methylindole-tethered (5ba) and a 5-methylthiophene-tethered
system 5c were synthesised via a short sequence utilising an
alkyne cyclopropenation reaction as the key step. Treatment of
the furan-tethered cyclopropene (5aa) with the NHC gold catalyst
system IPrAuCl/AgBF4 provided the oxa-bridged 5,7-fused cyclo-
adduct 6a as a single diastereoisomer in good yield (Scheme 2a –
for optimisation see Table S1, ESI†). This product arises from an
apparent formal (4+3) cycloaddition between an in situ generated
vinyl gold carbene and the furan tether. Importantly, we showed
An X-ray crystal structure was obtained for heterocycle 6b (CCDC
1909426, contains the supplementary crystallographic data, ESI).
that gold–phosphine catalysts and the Brønsted acid HNTf2 were which was likely due to the presence of an electron-withdrawing
incapable of effecting this transformation (see ESI†). The N-Me bromo-substituent in the R3 position. Unsymmetrical, mono-
indole tethered cyclopropene 5ba diverged in reactivity when substituted cyclopropenyl furyl sulfonamides 5ag–5ai were inves-
treated under the same conditions to give tetrahydro-b-carboline tigated, although yields for the corresponding heterocycles 6g–6i
7a in excellent yield (Scheme 2b). This latter reaction is consis- dropped substantially which, in our previous experience, is due
tent with a Friedel–Crafts process involving alkylation with the to the less-substituted cyclopropenes being unstable towards
vinylgold carbene. Use of the more aromatic and less nucleo- decomposition. The unsymmetrical, disubstituted substrates 5aj
philic thiophene tether of derivative 5c resulted in diene 8 (not (R1 = ÀC6H13, R2 = ÀCH2OTBS) and 5ak (R1 = Ph, R2 = Me – not
shown, see ESI†) forming – likely via a 1,2-H shift and subse- shown, see ESI†) failed to produce significant amounts of the
quent deauration. With the reactivity pattern of the tethered corresponding heterocycles 6j and 6k. In the case of 5aj the diene
cyclopropenylmethyl sulfonamides established, the substrate was the major product, likely a result of the OTBS facilitating a 1,2-H
scope for the (4+3) cycloaddition was explored (Table 1). Short- shift. The presence of two aryl substituents on the cyclopropene
ening the alkyl chains R1 and R2 by one carbon produced (5al – not shown, see ESI†) also prevented the cycloaddition
heterocycle 6b in good yield and the reaction conditions also from occurring, presumably because of the additional stabilisa-
tolerated a change in protecting group to a brosyl and provided tion of the gold carbene by the aromatic substituent resulting
6c in good yield. The importance of a substituent in the furan in a poor reactivity.
5-position was probed by preparing cyclopropenyl furyl sulfon-
The substrate scope with respect to indole and pyrrole-
amide 5ad (R3 = H), which provided the oxa-bridged 5,7-fused tethered cyclopropenes was next explored (Table 2). The indole
heterocycle 6d in good yield. Cycloadduct 6d also provided X-ray could also be exchanged with a pyrrole ring to produce hetero-
quality crystals that gave definitive structural assignment and cycle 7b in good yield. To our delight, when the monosubsti-
relative configuration. Changing R3 to a ÀCH2OEt substituent tuted cyclopropene example 5bc was reacted, heterocycle 7c was
gave heterocycle 6e. Heterocycle 6f was isolated in low yield, obtained in good yield, which contrasts with the low yield
obtained for the (4+3) cycloaddition reaction with the analo-
gous furan-tethered substrate (6h). Pleasingly, we found that
inductively electron-withdrawing halogens (Br and Cl) in the
5-position of the indole ring were well-tolerated, giving hetero-
cycles 7d and 7e in excellent yields. Such products are (vide infra)
amenable to further functionalisation through cross-coupling
reactions. A benzyl protecting group for the indole was well-
tolerated giving heterocycle 7f in excellent yield, however, as
expected, the electron-withdrawing tosyl group shut down the
reaction pathway, which we attribute to the reduced nucleophilicity
of the pyrrole, resulting in diene formation. A proposed mechanism
for the formation of 5,7-fused heterocycles 6 from cyclo-
propenyl furyl sulfonamides 5a involves gold-activation of the
Scheme 2 Initial gold-catalysed reaction of tethered cyclopropenes.
13972 | Chem. Commun., 2019, 55, 13971--13974
electron-rich cyclopropene double bond that leads to rapid
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gold allylic cations stabilised by gold at the central carbon.11
The regioselectivity of the cycloaddition with monosubstituted
cyclopropene substrates can be explained based on the prefer-
ential ring-opening to place the gold on the less substituted
carbon atom. This observation matches that reported pre-
viously and our own computational studies.3,4,7 Two plausible
Friedel–Crafts-type mechanisms for cyclopropenyl indolyl (and
pyrrole) sulfonamides 5b are postulated (Scheme 3).18 Each
pathway invokes the alkenylgold(I) carbene intermediate I as a
crucial initial species by the same cyclopropene ring opening
chemistry. The first proposed pathway involves attack from the
nucleophilic 3-position of the indole in intermediate I (blue
arrows) to generate intermediate III, which could subsequently
form heterocycle 7 following re-aromatisation and protodeaura-
tion. The proposed pathway for the pyrrole-tethered system
involves attack from the nucleophilic 2-position of the pyrrole
ring to generate intermediate II (red arrows). A Wagner–
Meerwein shift (green arrows) then provides intermediate III
which, following re-aromatisation and protodeauration, delivers
the observed heterocycle 7.19 In both reactions, conjugate attack
on the alkenylgold carbene may be facilitated by the NHC-ligand
reducing carbocation character at C8, combined with the for-
mation of a more favourable 5-membered vs. 7-membered ring.
Table 2 Substrate scope for the Friedel–Crafts cyclisation of cyclopro-
penyl pyrrolyl- and indolyl-sulfonamides 5
The synthesis of 7c was conducted at À10 1C. An X-ray crystal structure
was obtained for heterocycle 7a (CCDC 1909427 contains the supple-
mentary crystallographic data, ESI).
ring-opening furnishing the alkenylgold–carbene intermediate Chemical transformations were performed on heterocycles 6 and
I (Scheme 3). 7 to demonstrate their versatility. Desilylation of heterocycle 6h
Conjugate nucleophilic attack from the furan 2-position to using TBAF in THF at rt gave the corresponding alcohol 9 in 87%
the vinylgold carbene then forms the spirocyclic intermediate yield (Scheme 4a). Tosyl deprotection with Mg in MeOH/THF was
II. Nucleophilic attack of the vinyl gold onto the oxonium ion successfully performed with heterocycle 7a to obtain the free
provides the observed heterocycle 6 (purple arrows) and regen- amine 10 in 76% yield (Scheme 4b). A Suzuki cross-coupling was
erates the gold(I) catalyst. The major diastereoisomer obtained carried out on 7d and heterocycle 7g was obtained in 80% yield
in the cycloaddition reaction matches that of the curcusone (Scheme 4c). This result proved important as the range of
I and J natural products and also that obtained from the accessible products could be further expanded for heterocycles
previously reported Au-catalysed allene (4+3) cycloaddition to 7 given the vast range of commercially available boronic acids.
furans.13b If a stepwise mechanism is operating, it is suggested
In conclusion, the gold(I)-catalysed rearrangements of cyclo-
that initial nucleophilic attack of the furan might be reversible, propenyl sulfonamides 5 (Scheme 1) can be controlled via the
and that only one diastereoisomer of II undergoes favourable identity of the tethered heterocycle. When an indole tether is
cyclisation to the observed diastereoisomer of the product. present, a Friedel–Crafts reaction provides tetrahydro-b-carbolines
While there have been no reports of vinyl gold carbenes of type
II undergoing concerted cycloaddition reactions, such a process
cannot be ruled out. If a concerted pathway is occurring from I
then exo TS1 would explain the relative stereochemistry of the
cycloaddition product. Similar transition states have been
proposed for (4+3) cycloaddition reactions with allene-derived
Scheme 4 Conditions: (i) TBAF (2 eq.), THF, rt; (ii) Mg (50 eq.), MeOH/THF
(3 : 1), sonication; (iii) Pd(dppf)Cl2ÁCH2Cl2 (12 mol%), K2CO3 (2.3 eq.),
Scheme 3 Proposed mechanism for the formation of 6 and 7.
dioxane : H2O (4 : 1), 85–95 1C.
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Council Discovery project: grant number DP180100904, the
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