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International Edition: DOI: 10.1002/anie.201708947
German Edition:
DOI: 10.1002/ange.201708947

Synthetic Methods

Gold(I)-Catalyzed Synthesis of Indenes and Cyclopentadienes: Access
to (Æ)-Laurokamurene B and the Skeletons of the Cycloaurenones and
Dysiherbols
Xiang Yin, Mauro Mato, and Antonio M. Echavarren*
Abstract: The formal (3+2) cycloaddition between terminal
+
allenes and aryl or styryl gold(I) carbenes generated by a retroBuchner reaction of 7-substituted 1,3,5-cycloheptatrienes led to
indenes and cyclopentadienes, respectively. These cycloaddition processes have been applied to the construction of the
carbon skeleton of the cycloaurenones and the dysiherbols as
well as to the total synthesis of (Æ)-laurokamurene B.

We

previously found that cationic gold(I) complexes
promote the retro-Buchner reaction of 7-substituted 1,3,5cycloheptatrienes, such as 1 and 2, via their norcaradiene
tautomers, leading to reactive metal carbenes[1] [LAu=CHR]+
3 and 4 (Scheme 1), which react with alkenes to give

aryl and styryl gold(I) carbenes 3 and 4 react with allenes 5 to
give highly substituted indenes[6, 7] and cyclopentadienes,[8, 9]
respectively, by a formal (3+2) cycloaddition.
+
Indenes are important motifs present in many biologically
relevant natural products,[10] and are building blocks in
organic synthesis, organometallic chemistry, and in the field
of materials science.[11] Similarly, cyclopentadienes are important substrates, mainly as reactive diene components in the
Diels–Alder reaction and as ligands in organometallic
chemistry.[12] To illustrate the application of the new gold(I)catalyzed (3+2) cycloaddition reactions in the context of
+
natural product synthesis, we have developed a route for the
construction of the tetracyclic carbon skeleton common to the
cycloaurenones (8 a–c)[13] and the dysiherbols (9 a–c),[14] and
a total synthesis of (Æ)-laurokamurene B (10)[15] (Figure 1).

Scheme 1. Formal (3+2) cycloaddition between allenes and both aryl
+
and styryl gold(I) carbenes generated by a retro-Buchner reaction.

cyclopropanes[2, 3] or undergo intramolecular Friedel–Craftstype reactions.[4] Cyclopentenes were obtained by the formal
(4+1) cycloaddition of gold(I) carbenes 3 with cyclobutenes
+
or methylenecyclopropanes.[5] We have now discovered that
[*] X. Yin, M. Mato, Prof. A. M. Echavarren
Institute of Chemical Research of Catalonia (ICIQ)
Barcelona Institute of Science and Technology
Av. Països Catalans 16, 43007 Tarragona (Spain)
Prof. A. M. Echavarren
Departament de Química Analítica i Química Orgànica
Universitat Rovira i Virgili
C/ Marcel·li Domingo s/n, 43007 Tarragona (Spain)
E-mail: aechavarren@iciq.es
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201708947.
 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution-NonCommercial License, which permits use,
distribution and reproduction in any medium, provided the original
work is properly cited and is not used for commercial purposes.
Angew. Chem. Int. Ed. 2017, 56, 14591 –14595

Figure 1. Cycloaurenones (8 a–c), dysiherbols (9 a–c), and laurokamurene B (10).

We first studied the reaction of 7-(1-napthyl)-1,3,5-cycloheptatriene (1 a) with allene 5 a with different gold(I) catalysts
(Table 1). The reaction of 1 a with 5 a (2.0 equiv) in the
presence of the gold(I) complex [(JohnPhos)Au(MeCN)]SbF6
(A; 5 mol %) in 1,2-dichloroethane at 120 8C for 8 h gave
indene 6 a in 66 % yield for the isolated product (Table 1,
entry 1). Other gold(I) complexes B–F could also be used in
the reaction (Table 1, entries 2–6), although none of them
outperformed catalyst A. Complex G bearing a phosphite
ligand failed to promote this transformation.
Indenes 6 b–v were obtained under the standard conditions in 41–72 % yield by the reaction of 1,1-disubstituted
allenes 5 a–g (see the Supporting Information for structures)
with cycloheptatrienes with an unsubstituted, ortho- or parasubstituted, or ortho,meta-disubstituted aryl group (substrates
1 b–j), a 1- or 2-naphthyl group (substrates 1 a and 1 k), or a 9phenanthryl group (substrate 1 l; Table 2). This reaction is

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Table 1: Catalyst screening for the reaction of 1 a with allene 5 a.[a]

Entry

Catalyst

6 a Yield [%][b]

1
2
3
4
5
6
7

A
B
C
D
E
F
G

74 (66)[c]
55
61
55
31
57
–[d]

[a] Reaction conditions: 1 a (0.1 m in DCE), 5 a (2.0 equiv), catalyst
(5 mol %), 120 8C, 8 h. [b] The yield was determined by 1H NMR
spectroscopy with 1,3,5-trimethoxybenzene as an internal standard.
[c] The yield of the isolated product is given in parentheses. [d] Not
detected. Cy = cyclohexyl, DCE = 1,2-dichloroethane, Tf = trifluoromethanesulfonyl.

Chemie

perfectly suited for the preparation of spiro compounds, such
as 6 m–p and 6 t. The structure of indenes 6 d, 6 t, and 6 u was
confirmed by single-crystal X-ray diffraction.[16] Although the
resulting indenes have a reactive double bond, 2:1 adducts
were only observed as very minor products in the crude
reaction mixtures. In contrast, a 2:1 adduct was obtained in
the reaction of 1-cyclohexylallene 5 h, a monosubstituted
allene, with 1 a to form 11, whose relative configuration was
determined by X-ray diffraction.[16]
Styryl cycloheptatriene 2 a reacted with allene 5 a to give
cyclopentadiene 7 a in 50 % yield in the presence of gold(I)
complex B (5 mol %) in EtOAc at 100 8C (Table 3). The less
bulky catalyst A performed similarly (Table 3, entry 2),
whereas NHC gold(I) complexes, such as F, were less reactive
(Table 3, entry 4).
Table 3: Optimization of the synthesis of cyclopentadienes.[a]

Entry

Yield [%][b]

1
2
3
4
5
6
7

Table 2: Scope of the gold(I)-catalyzed synthesis of indenes.[a]

Deviation from above[a]
none
catalyst A instead of B
catalyst D instead of B
catalyst F instead of B
DCE instead of EtOAc
90 8C
120 8C

52 (50)[c]
45
–[d]
17
25
33
40

[a] Reaction conditions: 2 a (0.1 m in EtOAc), 5 a (1.5 equiv), catalyst B
(5 mol %), 100 8C, 12 h. [b] The yield was determined by GC–FID with
diphenylmethane as an internal standard. [c] The yield of the isolated
product is given in parentheses. [d] Not detected. FID = flame ionization
detection.

A range of styryl cycloheptatrienes gave rise to 1,1,2,5tetrasubstituted cyclopentadienes bearing both electron-rich
(products 7 b, 7 d) and electron-poor substituents (products
7 c, 7 e, 7 f) in the aryl moiety in 37–56 % yield (Table 4).
A Diels–Alder reaction between 7 g and maleic anhydride
led to the crystalline endo adduct 12 in excellent yield
(Scheme 2). The X-ray diffraction structure of 12[16] allowed
confirmation of the structures assigned to 7 a–k (Table 4).
Table 4: Scope of the synthesis of cyclopentadienes.[a]

[a] Reaction conditions: 1 (0.1 m in 1,2-dichloroethane), allene 5
(2.0 equiv), catalyst A (5 mol %), 120 8C, 8 h. Yields are for the isolated
product. [b] Reaction time: 16 h.

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[a] Reaction conditions: 2 (0.1 m in EtOAc), allene 5 (1.5 equiv), catalyst
B (5 mol %), 100 8C, 12 h. Yields are for the isolated product. [b] The
reaction was carried out with 2.0 equivalents of the allene. [c] The
reaction was carried out with 3.0 equivalents of the allene.

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Scheme 2. Diels–Alder reaction of 7 g with maleic anhydride.

Laurokamurene B (10), isolated from the red algae
Laurencia okamurae, is a member of a small family of natural
compounds displaying antifungal and cytotoxic activity.[15]
(Æ)-Laurokamurene B (10) was readily synthesized in good
yield by simple hydrogenation of cyclopentadiene 7 h in the
presence of the Wilkinson catalyst (Scheme 3). Considering

Scheme 3. Total synthesis of (Æ)-laurokamurene B (10).

that (E)-7-(4-methylstyryl)cyclohepta-1,3,5-triene (2 b), the
cycloheptatriene required for the preparation of 7 h, can be
readily obtained by the treatement of potassium (E)-(4methylstyryl)trifluoroborate with tropylium tetrafluoroborate in almost quantitative yield,[3] this total synthesis requires
just three steps and provides (Æ)-10 in 39 % overall yield from
commercially available starting materials, which compares
favorably with previous syntheses of (Æ)-10.[17]
Cycloaurenones A–C (8 a–c) feature a cis-decalin moiety,
whereas the dysiherbols (9 a–c) show trans fusion of the A/B
rings (Figure 1). They also differ in their absolute configuration. These compounds are biogenetically related to other
natural products isolated from sponges, such as (+)-smenoqu+
alone,[18] ilimaquinone,[19] and smenospongin.[20] Many of
these natural products display antimicrobial, anti-HIV, antiinflamatory, antiproliferative, and antisecretory activities and
have attracted the interest of synthetic chemists.[21] However,
no approach towards the synthesis of the cycloaurenones and
the dysiherbols has been reported. As a first approach to the
synthesis of these natural products, we considered applying
the (3+2) cycloaddition together with an intramolecular
+
radical cyclization to build up the carbocyclic core structure of
8 a–c and 9 a–c (Scheme 4).
The synthesis of allene 5 i was performed on a gram scale
in seven steps from cyclohexanone, starting with the Michael
addition of its cyclohexylimine to acrylonitrile.[22, 23] The
gold(I)-catalyzed reaction between 5 i and cycloheptatriene
1 f was most efficient with [(IPr)Au(PhCN)]SbF6 (F) as the
catalyst and gave spiroindene 13 as a mixture of four
stereoisomers (Scheme 4).[23] Cleavage of the benzoate and
oxidation of the secondary alcohols with Dess–Martin periodinane (DMP) delivered ketones 14 a and 14 b in a 1:1 ratio and
36 % yield over three steps, after only one chromatographic
purification. Ketone 14 a was reduced to the corresponding
Angew. Chem. Int. Ed. 2017, 56, 14591 –14595

Scheme 4. Assembly of the tetracyclic carbon skeleton of the cycloaurenones (structure 17 a) and dysiherbols (structure 16 b). AIBN =
azobisisobutyronitrile, Bz = benzoyl.

alcohol and then treated with triphenylphosphine dibromide
to give alkyl bromide 15 a in 63 % yield over two steps.
Treatment of 15 a with nBu3SnH in the presence of AIBN
triggered a radical cyclization, which delivered the corresponding tetracyclic product 16 a in 33 % yield (4:1 mixture of
epimers at C7). The major isomer displayed the configuration
of cycloaurenones, as shown by X-ray diffraction.[16] Finally,
oxidative deprotection of the methoxy groups with cerium
ammonium nitrate (CAN) and pyridine-2,6-dicarboxylic acid
N-oxide (Py*O)[24] led to quinone 17 a. By the same sequence
of reactions, 16 b, corresponding to the tetracyclic carbon
skeleton of the dysiherbols, was obtained as a 1:1 mixture of
epimers through a remarkably efficient radical cyclization of
15 b (81 % yield).[23] Oxidation of 16 b as before provided 17 b.
Both (3+2) cycloaddition reactions start with the gold(I)+
promoted retro-Buchner reaction of 1 or 2 to release benzene
and generate the gold(I) carbene 3 or 4, respectively, which

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Acknowledgements
We thank the European Research Council (Advanced Grant
No. 321066), MINECO/FEDER, UE (CTQ2016-75960-P),
MINECO-Severo Ochoa Excellence Accreditation 2014–
2018, SEV-2013-0319), and the CERCA Program/Generalitat
de Catalunya for financial support. We also thank the ICIQ
X-ray diffraction unit, CELLEX-ICIQ HTE laboratory, and
Dr. Fedor Miloserdov for helpful discussions.

Conflict of interest
The authors declare no conflict of interest.
Keywords: cycloaddition · cyclopentadienes · gold catalysis ·
indenes · total synthesis
How to cite: Angew. Chem. Int. Ed. 2017, 56, 14591 – 14595
Angew. Chem. 2017, 129, 14783 – 14787

Scheme 5. Proposed mechanisms for the formal (3+2) cycloaddition
+
between allenes and aryl or styryl gold(I) carbenes.

undergoes electrophilic attack at the central carbon atom of
allenes 5 to give an allyl cationic species Ia or Ib (Scheme 5).
In the first case, intramolecular electrophilic aromatic substitution gives intermediate IIa, which can undergo rearomatization and protonolysis of the AuÀC bond to form IIIa,
which can undergo isomerization to give indene 6. Alternatively, protonation at the exocyclic double bond of IIa with
concomitant deauration would directly furnish 6. For the
cyclopentadiene synthesis, the cyclization of Ib would give
IIb, which is converted into 7 by isomerization. Indeed,
monitoring of the reaction by GC–MS allowed observation of
the rapid formation of IIb (as a mixture of anti- and syn-IIb
when R2 ¼ R3), which slowly underwent isomerization to 7.
6
Intermediates IIb were also observed by 1H NMR spectroscopy, and their isomerization to 7 was found to be catalyzed
by gold(I).[23]
In summary, two new gold(I)-catalyzed formal (3+2)
+
cycloaddition reactions have been developed between allenes
and aryl or styryl gold(I) carbenes generated by a retroBuchner reaction of 7-substituted cycloheptatrienes, thus
leading to highly substituted indenes and cyclopentadienes,
respectively. The usefulness of these new methods has been
demonstrated by the shortest total synthesis of laurokamurene B reported to date and by the ready construction of the
carbon skeleton of the cycloaurenones and the dysiherbols.
Efforts directed towards the synthesis of these natural
products are under way.

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Manuscript received: August 30, 2017
Accepted manuscript online: September 20, 2017
Version of record online: October 11, 2017

 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.angewandte.org

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