From Acenaphthenes to (+)-Delavatine A: Visible-Light-Induced Ring Closure of Methyl (α-Naphthyl) Acrylates

Article abstract of DOI:10.1002/chem.201804735

Disclosed herein is a visible light mediated cyclization of methyl (α-naphthyl) acrylates and heteroaromatic analogues yielding substituted acenaphthenes and azaacenaphthenes. This highly functional-group-tolerant transformation was put to the test in an enantioselective formal synthesis of delavatine A. Mechanistic details were elucidated by DFT-calculations revealing an unusual intramolecular H-transfer mediated by a primary amine. The generality of this transformation enables a novel synthetic strategy of five membered ring annulation at an advanced stage, allowing reliance upon naphthalene chemistry up to the point of acenaphthene construction.

Full text of DOI:10.1002/chem.201804735

A Journal of
Accepted Article
Title: From Acenaphthenes to (+)-Delavatine A: Visible-Light-Induced
Ring Closure of Methyl (α-Naphthyl) Acrylates
Authors: Theodor Peez, Jan-Niclas Luy, Klaus Harms, Ralf Tonner,
and Ulrich Koert
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To be cited as: Chem. Eur. J. 10.1002/chem.201804735
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From Acenaphthenes to (+)-Delavatine A: Visible-Light-Induced
Ring Closure of Methyl (α-Naphthyl) Acrylates
Theodor Peez, Jan-Niclas Luy, Klaus Harms, Ralf Tonner and Ulrich Koert*
Abstract: Disclosed herein is a visible light mediated cyclization of
methyl (α-naphthyl) acrylates and heteroaromatic analogues yielding
substituted acenaphthenes and azaacenaphthenes. This highly
functional-group-tolerant transformation was put to the test in an
enantioselective formal synthesis of delavatine A. Mechanistic details
were elucidated by DFT-calculations revealing an unusual
processes such as the burning of fossil fuels, making
acenaphthene and acenaphthylene two of the 16 PAHs monitored
by the EPA. As CP-PAHs represent some of the most stable
forms of carbon, they are believed to be present in interstellar
medium, detectable in diffuse interstellar absorption bands.[6]
Acenaphthylenes
are
synthetically
accessible
by
intramolecular H-transfer mediated by
a primary amine. The
dehydrogenation of acenaphthenes.[7]
generality of this transformation enables a novel synthetic strategy of
five membered ring annulation at an advanced stage, allowing
reliance upon naphthalene chemistry up to the point of acenaphthene
construction.
Substituted (aza)acenaphthenes 9 were previously obtained by
annulation and aromatization of a six-membered ring to an indane
8, 18]
7 or 8 (Scheme 1).[5c,
Naphthalenes, quinolines and
isoquinolines are easily accessible building blocks, which render
them good precursors for acenaphthenes. This makes annulation
of the five-membered ring attractive from a retrosynthetical point
of view, as it allows splitting of the acenaphthene into a
naphthalene “core” and a C2-“bridge”. The most widely-used
methods for this annulation are harsh, involving flash vacuum
pyrolysis,[9] carbocations 10[10] and related Friedel-Crafts
acylations.[11] An annulation of the five-membered ring could also
start from a vinyl naphthalene 11. Here we show that a visible light
mediated ring closure of substituted vinyl naphthalenes 11 is an
efficient and functional-group-tolerant synthetic route to
substituted acenaphthenes.
Acenaphthene 1 and acenaphthylene 2 occur as substructures in
different chemical areas. Acenaphthene has been described as a
“Cinderella” among coal-tar hydrocarbons due to its unique
structure, photophysical properties and intriguing chemistry.[1]
Because of these features, acenaphth(yl)ene derivatives have
found application in different fields of chemistry including
medicinal 3,[2] environmental 4[3] and materials[4] chemistry 6. The
motif is also present in the natural product delavatine A (5),[5]
(Figure 1).
Figure 1. Acenaphthene 1 and acenaphthylene 2 as subunits in various
structures 3-6 from different areas of chemistry.
Cyclopenta-fused polyaromatic hydrocarbons (CP-PAHs) are
ubiquitous environmental carcinogens, emitted by combustion
Scheme 1. Different synthetic routes to substituted acenaphthenes.
[*]
T. Peez, J.-N. Luy, PD Dr. Ralf Tonner, Dr. Klaus Harms,
Prof. Dr. U. Koert
Fachbereich Chemie
Philipps-Universität Marburg
Hans-Meerwein Straße 4
35032 Marburg
Given our group’s past work in the synthesis of highly
functionalized naphthalenes and natural products,[14] we sought
to merge the field of naphthalene chemistry with the field of
acenaphth(yl)enes by developing a general five-membered ring
annulation and putting it to the test in a synthesis of delavatine A.
We turned to photochemistry, as pioneering work of Lapouyade
showed formation of a five-membered ring on similar under
UV-irradiation in the presence of an excess of different amines in
high dilution.[12] Acrylate 12a was selected as a substrate for
optimization of the photocyclization to 13a (Table 1) as it carries
E-mail: koert@chemie.uni-marburg.de
Supporting information for this article is given via a link at the end of
the document.
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an ester as a synthetically versatile functional group on the double
bond and its derivatives are readily available from
1-bromonaphthalenes by Stille cross-coupling with the
appropriate acrylstannane (see Supporting Information). Usage of
visible light by implementation of triplet-energy transfer was
envisioned to provide a broad functional-group-tolerance and
applicability in complex molecule synthesis. Calculation of the
singlet-triplet excitation energy (TSE) of 12a (53.0 kcal mol 1,
BP86/def2TZVPP) showed Ir(dFppy)3 (14) to be an appropriate
sensitizer excitable by visible light.
which are of potential interest to materials chemistry.[17]
Heteroaromatic systems (13p - s) showed lower conversion
under standard conditions and required more forcing conditions,
possibly due to photophysical loss of energy. Substitution in
2-position is not tolerated, as it most likely inhibits adoption of the
reactive conformation (12v, 12w).
Table 1. Course of optimization
14
[mol%]
equiv.
amine
Yield [%]
13a
#
amine
solvent c [mM] T [°C] t [h]
1
2
3
4
5
5
5
3
3
3
Et3N
5
5
5
5
1
MeCN
MeCN
5
5
r.t.
r.t.
r.t.
40
40
48
48
72
48
36
<20
27[b]
75
tBuNH2
tBuNH2
tBuNH2
tBuNH2
toluene
toluene
toluene
5
5
82
70
99
[a] Kessil A160WE (420-480 nm, see Figures S9 and S10 for emission spectra)
[b] 77 % brsm
Addition of a tertiary amine, e.g. Et3N caused formation of
byproducts, most likely due to photoredox chemistry of Ir(dFppy)3
(entry 1). CV-measurements revealed tBuNH2 as one of the most
abundant primary amines unable to engage in redox chemistry
with Ir(dFppy)3 (Figures S1, S2). Substitution for Et3N afforded
only the desired product, albeit with low conversion (entry 2).
Running the reaction in toluene (entry 3) and raising the
temperature (entry 4) accelerated conversion, allowed loading of
amine to be decreased to 1.0 equiv. and substrate concentration
to be raised to 70 mM, resulting in formation of 13a in quantitative
yield (entry 5), see Supporting Information for optimization details
(Table S1) and effects of deviation from standard conditions
(Table S2). Formation of the five-membered ring was confirmed
by single crystal X-ray diffraction.[16] After having identified the
optimum conditions, the substrate scope of the photocyclization
12 → 13 was investigated (Scheme 2). As envisioned,
implementation of visible light resulted in
a variety of
acenaphthenes bearing electron-donating as well as -withdrawing
functional groups at different positions being formed in excellent
yields. Synthetically valuable functionalities such as Bpin (13j)
and bromide (13h), as well as different protecting groups, were
also tolerated in the cyclization. Ring closure also occurred readily
on polyaromatic compounds, furnishing CP-PAHs 13c and 13e,
Scheme 2. Investigation of the substrate scope on the naphthyl ring. Isolated
yields, 0.2 mmol scale, 2.85 mL toluene (70 mM). [a] Kessil A160WE (420-480
nm). [b] Reaction conditions: 15 equiv. tamylamine, 5 mol% Ir(dFppy)3, blue
LED,[a] 6.40 mL toluene (30 mM), 80 °C, 36 h.
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Scheme 3. a) aminoacetaldehyde dimethyl acetal (16, 1.0 equiv.), molecular sieves (3 Å), toluene, r.t., 4 h, then 1 mol% PtO2, H2 (1 bar), EtOAc,, r.t., 48 h, then
glyoxylic acid (50% in water), 37% HCl/EtOH (1:1), reflux, 2 h, then H2SO4 (2.0 equiv.), MeOH, reflux, 18 h b) (CH2O)n (3.2 equiv.), Bu4NI (13 mol%), K2CO3
(3.1 equiv.), toluene, 80 °C, 18 h c) tamylNH2 (15.0 equiv.), Ir(dFppy)3 (5 mol%), toluene, Kessil A160WE (420-480 nm), 80 °C, 48 h d) MnO2 (5.0 equiv.), 1,2-
dichloroethane, 80 °C, 2 h e) DIBAL (2.2 equiv.), Me3Al (1.1 equiv), toluene, -95 to -40 °C, 1 h f) [Ru(p-cymene)Cl2]2 (5 mol%), (R)-MeO-BIPHEP (25) (11 mol%),
toluene, H2 (100 bar), 60 °C, 18 h, g) NBSH (23, 3 equiv.), PPh3 (2 equiv.), DIAD (2 equiv.), N-methylmorpholine, -30 °C (2 h) to r.t. (2 h).
With the substrate scope established, we turned our attention to
the synthesis of delavatine A (5). Delavatine A was isolated in
2016 from the mountain flowering plant Incarvillea delavayi and
showed considerable cytotoxic properties.[5a] As delavatine A had
recently been synthesized by Li,[18] we chose intermediate 23 as
a target to display the applicability of our novel retrosynthetic
strategy, constructing the isoquinoline core first and building the
bridge after, as shown in Scheme 3. Starting off with a Bobbitt-
type isoquinoline synthesis[19] (15 → 17 → 18) and subsequent
esterification, 3-methoxybenzaldehyde (15) was converted into
ester 19. Aldol condensation with formaldehyde under phase
transfer conditions yielded acrylate 12p. Under the developed
forcing conditions, visible-light mediated cyclization of 12p
afforded azaacenaphthene 13p in good yield. Dehydrogenation to
azaacenaphthylene 21 was accomplished using MnO2. The
presence of a Lewis acid (AlMe3) was found to suppress side
reactions on the isoquinoline system in the reduction of 21. As no
precedence of enantioselective acenaphthylene hydrogenation
was found, this transformation was developed for the synthesis of
chiral alcohol 22. Rhodium- and iridium-based catalysts allowed
for mild conditions yet afforded low enantioselectivities. Synthesis
of a ruthenium complex from ligand 25 and its usage in a
hydrogenation under high pressure and temperature afforded an
ee of 91%, after commercially available catalysts delivered
unsatisfactory results (Table S4). The absolute configuration of
resulting alcohol 22 was established as (R) by single crystal X-ray
diffraction.[16] Deoxygenation[20] yielded azaacenaphthene 24, the
intermediate of Li’s synthesis.[18] This protecting-group-free formal
synthesis of delavatine A demonstrates the utility of visible-light
photocatalysis in natural product synthesis[21] and represents a
rare case of using acenapthylenes as intermediates. Having
accomplished the formal synthesis, we became interested in the
mechanism of the photocyclization. After confirming quenching of
the triplet state of sensitizer 14 by acrylate 12a through a Stern-
Volmer quenching experiment (Figures S6, S7), running the
reaction with standard triplet sensitizers instead of potentially
redox-active Ir(dFppy)3 also afforded product (Table S3). In
conjunction with the previously performed CV-measurements
(Figures S1, S2), a triplet energy transfer instead of a redox-
based mechanism thus seems likely. To study the course of the
reaction after sensitization, the mechanism (12p → 13p) was
studied using DFT-calculations (Figure 2). As indicated by
aforementioned experiments, compound 12p is most likely
converted by sensitizer 14 from the singlet (S0) ground state into
the first excited triplet state (T1). Ring closure to the 5-membered
ring can occur on the triplet potential energy surface (PES) via
transition state TS leading to intermediate IM. The rather
delocalized nature of the unpaired electrons in IM is shown by the
spin density (Figure S14). At this point, the necessary H-shift from
IM to 13p requires the amine. On the triplet PES a barrier of 130
kJ mol -1 (Figure S12) for the H-transfer was found, leading to an
energetically less stable product 13p (T1). Instead, we found the
amine to catalyze the reaction on the singlet PES resulting in a
barrierless step from IM to 13p (S0). The amine thus acts as an
intramolecular H-shuttle in the reaction (see the animated GIF in
the Supporting Information).
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Figure 2. Mechanism of the photocyclization from 12p to 13p as elucidated by DFT-calculations (BP86, def2-TZVPP). Singlet (S0) and triplet (T1) potential energy
surfaces are shown, arrows indicate the proposed reaction pathway. A structure (non-stationary point) along the amine-assisted H-transfer (circled atom) pathway
is shown. Bold numbers designate C-C bond lengths.
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intramolecular
amine-assisted
H-transfer
reaction
by
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Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft
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Marburg, CSC Frankfurt and HLR Stuttgart for providing
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Keywords: photochemistry • synthetic methods • natural
products • acenaphthenes • delavatine A
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Title
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Theodor Peez, Jan-Niclas Luy, Klaus
Harms, Ralf Tonner and Ulrich Koert*
Page No. – Page No.
From Acenaphthenes to (+)-
Delavatine A: Visible-Light-Induced
Ring Closure of Methyl (α-Naphthyl)
Acrylates
Hampered by harsh reaction conditions, the synthesis of functionalized acenaphthenes
remains the key challenge in the exploration of their chemistry. Addressing this challenge,
a visible light and amine-catalyzed photocyclization is reported. Elucidation of the
mechanism using DFT-calculations and application in
(+)-delavatine A pave the way for further application.
a formal synthesis of
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