Visible-Light Photoredox Catalysis Enables the Biomimetic Synthesis of Nyingchinoids A, B, and D, and Rasumatranin D

Article abstract of DOI:10.1002/anie.201814089

The total synthesis of nyingchinoids A and B has been achieved through successive rearrangements of a 1,2-dioxane intermediate that was assembled using a visible-light photoredox-catalysed aerobic [2+2+2] cycloaddition. Nyingchinoid D was synthesised with a competing [2+2] cycloaddition. Based on NMR data and biosynthetic speculation, we proposed a structure revision of the related natural product rasumatranin D, which was confirmed through total synthesis. Under photoredox conditions, we observed the conversion of a cyclobutane into a 1,2-dioxane through retro-[2+2] cycloaddition followed by aerobic [2+2+2] cycloaddition.

Full text of DOI:10.1002/anie.201814089

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Accepted Article
Title: Visible Light Photoredox Catalysis Enables the Biomimetic
Synthesis of Nyingchinoids A, B and D, and Rasumatranin D
Authors: Jacob Hart, Laura Burchill, Aaron Day, Christopher Newton,
Christopher Sumby, David Huang, and Jonathan Harry
George
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201814089
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10.1002/anie.201814089
Angewandte Chemie International Edition
COMMUNICATION
Visible Light Photoredox Catalysis Enables the Biomimetic
Synthesis of Nyingchinoids A, B and D, and Rasumatranin D
Jacob D. Hart, Laura Burchill, Aaron J. Day, Christopher G. Newton, Christopher J. Sumby, David M.
Huang and Jonathan H. George*
Abstract: The total synthesis of nyingchinoids A and B has been
achieved via successive rearrangements of 1,2-dioxane
intermediate that was assembled using a visible light photoredox
catalysed aerobic [2+2+2] cycloaddition. Nyingchinoid was
workers, with each natural product identified as a scalemic
mixture by chiral HPLC.3 We were particularly interested in the
biosynthetic relationships between nyingchinoids A (1), B (2)
and D (3) and a possible biosynthetic precursor, chromene 4
(Scheme 1). While nyingchinoid D (3) is most likely derived from
an intramolecular, photochemical [2+2] cycloaddition of 4, the
biosynthetic origin of nyingchinoids A (1) and B (2) is less
obvious. We propose that 1,2-dioxane 5 is an undiscovered
natural product or biosynthetic intermediate that links chromene
4 to 1 and 2. The 1,2-dioxane 5 could arise via a [2+2+2]
cycloaddition between oxygen and the chromene and prenyl
alkenes of 4. Intramolecular dearomatization of 5 driven by
cleavage of the weak O-O bond could then form 2.5 Nucleophilic
attack at the epoxide of 2 by the newly formed tertiary alcohol
could then rearomatize the system via an unusual C-C bond
scission to give 1.
a
D
synthesised by a competing [2+2] cycloaddition. Based on NMR
data and biosynthetic speculation, we proposed a structure revision
of the related natural product rasumatranin D, which was confirmed
through total synthesis. Under photoredox conditions, we observed
the conversion of a cyclobutane into a 1,2-dioxane via retro-[2+2]
cycloaddition followed by aerobic [2+2+2] cycloaddition.
The renaissance of visible light photoredox catalysis during
the past decade has significantly enhanced the toolkit of organic
synthesis through the improved ability to access radicals and
radical ions under mild conditions.1 However, the application of
photoredox catalysis to the total synthesis of natural products
has been relatively slow, with most examples to date involving
the reductive generation of alkyl radicals or photocatalysed [4+2]
and [2+2] cycloadditions.2 Herein, we report the first total
In biosynthesis, the aerobic [2+2+2] cycloaddition has only
been proposed to occur in the formation of gracilioethers A and
H,6 although this reaction has not yet been successfully applied
in a total synthesis of these,7 or any other, 1,2-dioxane natural
products. The majority of 1,2-dioxane natural products are
biosynthesised via [4+2] cycloadditions between 1,3-dienes (of
either terpene or polyketide origin) and singlet oxygen, a
reaction that is amenable to biomimetic synthesis.8 The
biomimetic synthesis of 1,2-dioxane natural products has also
been achieved using cascade radical cyclizations that
incorporate triplet oxygen.9
synthesis application of
cycloaddition as the key step of
nyingchinoid3 and rasumatranin4 meroterpenoid natural products.
a
photocatalytic aerobic [2+2+2]
a
biomimetic route to
H
H
[2+2]
H
O
HO
O
HO
3: nyingchinoid D
4
As a synthetic method, the photocatalytic aerobic [2+2+2]
cycloaddition of electron-rich 1,1-disubstituted styrenes to give
1,2-dioxanes using 9,10-dicyanoathracene (DCA) as the
photocatalyst was first reported by Gollnick.10 Synthesis of
bicyclic endoperoxides via aerobic [2+2+2] cycloaddition of
electron-rich bis(styrene) substrates was later disclosed by
Miyashi, again using DCA as the photocatalyst.11 More recently,
the scope of the aerobic [2+2+2] cycloaddition was significantly
extended by Yoon to include less electron-rich bis(styrene)
O
O
O
O
H
H
O2
H
O
HO
O
HO
[2+2+2]
4
5
dearomatization and
O-O cleavage
HO
H
O
C-C cleavage
and aromatization
2+
substrates using Ru(bpz)3 as an efficient photocatalyst.12 Most
O
O
H
H
recently, and most relevantly to this work, Nicewicz showed that
the use of triarylpyrylium salts as organic photocatalysts allowed
the aerobic [2+2+2] cycloaddition of dienes bearing one styrene
and one aliphatic alkene (Scheme 2).13
H
O
HO
O
H
O
1: nyingchinoid A
2: nyingchinoid B
Scheme 1. Proposed biosynthesis of nyingchinoids A, B and D.
4-MeO-TPT, CH2Cl2, -41 ºC
1 atm O2, 470 nm LED
MeO
Nyingchinoids A-H are a family of polycyclic meroterpenoids
isolated from Rhododendron nyingchiense by Hou and co-
O
O
Ar
70%
BF4
MeO
Ar
O
Ar = 4-MeOC6H4
Ar
4-MeO-TPT
[*]
J. D. Hart, L. Burchill, A. J. Day, Dr. C. G. Newton, Prof. C. J.
Sumby, Dr. D. M. Huang, Dr. J. H. George
Department of Chemistry, The University of Adelaide
Adelaide, SA 5005, Australia
Scheme 2. Nicewicz’s photocatalysed cyclization-endoperoxidation cascade
of a tethered diene using a triarylpyrylium catalyst. 4-MeO-TPT = 2,4,6-tris(4-
methoxyphenyl)pyrylium tetrafluoroborate.
E-mail: jonathan.george@adelaide.edu.au
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201814089
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COMMUNICATION
O
2 , 5 h
71%
O
O
H
4-MeO-TPT, DCE, 0 ºC
H
H
H
O2
TBSCl, imidazole
DMF, RT
=
1 atm O2, 470 nm LED
11
11
H
O
H
O
or
TBSO
TBSO
HO
O
TBSO
O
after 20 min: 8 (87%) sole product
after 7 h: 7 (60%) sole product
75%
4
6
7
8
O
2 then TBAF
then TFA
O2
53%
then TBAF
92%
TBAF, THF, RT
86%
TBAF, THF, RT
61%
HO
H
H
O
H
O
H
O
TFA, CH2Cl2, RT
58%
H
H
H
HO
HO
O
O
O
O
2 (X-ray)
1 (X-ray)
1: nyingchinoid A
2: nyingchinoid B
3: nyingchinoid D
Scheme 3. Biomimetic synthesis of nyingchinoids A, B and D. TBSCl = tert-butyldimethylsilyl chloride, DMF = N,N-dimethylformamide, DCE = 1,2-dichloroethane,
TBAF = tetra-n-butylammonium fluoride, THF = tetrahydrofuran.
Our biomimetic synthesis of nyingchinoids A, B and D is
outlined in Scheme 3. Racemic chromene was first
Our experimental results clearly demonstrate that the
photoredox catalyzed [2+2] cycloaddition of 6 is reversible using
the 4-MeO-TPT photocatalyst. Indeed, re-subjecting cyclobutane
8 to this catalyst under aerobic conditions formed 1,2-dioxane 7
in 71% yield, with inversion of the relative configuration at C-11
indicating that full cycloreversion had taken place before the
[2+2+2] cycloaddition. In previous studies of photocatalytic,
intermolecular [2+2] cycloadditions of electron-rich styrenes,
Yoon has observed significant cycloreversion of the cyclobutane
4
synthesised in one step by condensation of orcinol with citral
according to a known procedure.14 We then attempted to convert
chromene 4 into 1,2-dioxane 5 using various oxidation protocols.
Firstly, exposure of 4 to conditions known to generate singlet
oxygen (e.g. Rose Bengal sensitiser, O2, MeOH, visible light)
gave products exclusively formed by oxidation of the prenyl side
chain, rather than the electron-rich chromene.15 Secondly,
attempts to oxidise 4 to a phenoxy radical in the presence of O2
resulted in degradation of starting material.16 We therefore
protected 4 as TBS-ether 6, which we thought could be oxidised
to a radical cation under photoredox conditions. Treatment of 6
with a slightly modified version of Nicewicz’s conditions for
pyrylium photoredox catalysed aerobic [2+2+2] cycloaddition (2
mol% 4-MeO-TPT photocatalyst, DCE, 0 °C, 1 atm O2, 470 nm
LED) gave, after 20 min, cyclobutane 8 as the only product
observed in the 1H NMR spectrum of the reaction mixture.17
After purification, 8 was obtained in 87% yield. Deprotection of 8
using TBAF in THF gave nyingchinoid D (3) in 86% yield.
However, on repeating this [2+2] cycloaddition we noticed the
gradual formation of the desired 1,2-dioxane 7 (as a single
diastereoisomer) when the reaction was run for extended time
periods. After 7 h, 7 was obtained in 60% yield. No reaction was
products when using the highly oxidizing photocatalyst
2+ 19
Ru(bpz)3
.
A mechanistic cycle for the aerobic [2+2+2]
that incorporates reversible [2+2]
cycloaddition of
6
a
cycloaddition is outlined in Scheme 4. The 4-MeO-TPT
photocatalyst is first activated by blue LED light to give an
excited state capable of oxidizing the electron-rich chromene 6
to give radical cation 9. 5-exo-trig cyclization of 9 generates the
diasteromeric, distonic radical cations 10 and 12. Diastereomer
10 has the correct relative configuration to undergo further
cyclization to give the cyclobutane radical cation 11, followed by
reduction to give cyclobutane 8 as the overall kinetic product of
the reaction. Diastereomer 12 is unable to undergo reductive
3
cyclization in this manner, so it is trapped by O2 to give 14 via
the peroxy radical cation 13. Single electron reduction of 13 then
gives 1,2-dioxane 7 as the thermodynamic product. Re-oxidation
observed in either the absence of photocatalyst or blue LED light. of 8 by the excited photocatalyst could regenerate radical cation
Treatment of 7 with TBAF formed nyingchinoid B (2) in 92%
yield via intramolecular dearomatization of the intermediate
phenoxide anion.18 Acid catalysed rearrangement of 2 using
TFA in CH2Cl2 then gave nyingchinoid A (1) in 58% yield. Next,
we investigated some telescoped reactions of chromene 6 to
form 1 and 2 in one-pot procedures. Pleasingly, photoredox
catalysed aerobic [2+2+2] cycloaddition of 6 followed by addition
of TBAF gave 2 in 61% yield, while addition of TBAF followed by
TFA gave 1 in 53% yield. The latter transformation of 6 into 1
involves the construction of 2 rings, 3 stereocentres, 4 C-O
bonds and 1 C-C bond, alongside the scission of 1 O-O bond
and 1 C-C bond.
9 via retro-[2+2] cycloaddition of 11, thus allowing the kinetic
product 8 to be “recycled” to give 7 via radical cation 12. The
high oxidation potential of the excited state of 4-MeO-TPT+
(+1.74 V) is presumably required to oxidize
8 prior to
cycloreversion. This stepwise mechanistic pathway is supported
by computational modeling using density functional theory,
which shows that both 5-exo-trig cyclizations of radical cation 9
are reversible and confirms 7 as the thermodynamic product, but
indicates that the system becomes kinetically trapped as 11
instead of forming 12 (see the Supporting Information for full
details).
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O
O
H
H
*
Ar
blue LED
H
Ar
BF4
Ar
TBSO
O
TBSO
O
Ar
O
Ar
Ar
O
7
6
4-MeO-TPT*
+e−
4-MeO-TPT
Ar
SET
Ar = 4-MeO-C6H4
+e−
O
O
Ar
O
Ar
4-MeO-TPT
H
H
H
H
H
O
TBSO
H
O
14
TBSO
TBSO
O
8
9
+e−
−e−
O
O
5-exo-trig
5-exo-trig
H
H
H
H
TBSO
O
H
O
13
H
H
TBSO
H
O
H
11
TBSO
TBSO
O
3O2
12
10
Scheme 4. Proposed mechanistic cycle for the photocatalysed cyclization of chromene 6 to give 1,2-dioxane 7 and cyclobutane 8.
Finally, we compared the NMR spectra of nyingchinoid A (1),
whose structure has been unequivocably proven by X-ray
crystallography, with that of the related natural product
rasumatranin D, which suggested that the previously proposed
structure 15 was incorrect (Figure 1).4 We believed that the
substitution pattern of the aromatic ring and the relative
configuration at C-11 of rasumatranin D should be reassigned as
structure 16, in line with the structure of nyingchinoid A. The
coupling constant of 14 Hz between H-3 and H-11, and the
absence of an nOe interaction, indicates a trans relationship at
this ring junction of 16. Furthermore, the co-isolation of the
natural rasumatranin D, thus confirming the structure revision.
The synthesis of rasumatranin D was significantly streamlined
by conducting a one-pot conversion of 18 into 16 in 49% yield.
Ph
Ph
O
O
H
4-MeO-TPT, DCE, 0 ºC
1 atm O2, 470 nm LED
H
O2
=
H
O
TBSO
TBSO
O
69%
18
19
O2 then TBAF, then TFA
49%
Ph
TBAF, THF, RT
Ph
93%
cyclobutane natural product 17 alongside rasumatranin
D
suggested a common chromene biosynthetic precursor with the
same aromatic substitution pattern as the nyingchinoids.20
HO
H
H
O
O
O
TFA, CH2Cl2, RT
62%
Ph
Ph
H
H
OH
HO
H
H
O
O
O
O
O
3
H
H
O
O
3
16: rasumatranin D
20
H
H
H
H
H
O
11
11
Ph
HO
O
O
HO
Scheme 5. Biomimetic synthesis of rasumatranin D.
15: rasumatranin D
originally proposed structure
16: rasumatranin D
revised structure
17
In conclusion, we have achieved the biomimetic synthesis of
nyingchinoids A, and D, and the revised structure of
Figure 1. Proposed structure revision of rasumatranin D.
B
rasumatranin D, using [2+2] and [2+2+2] cycloadditions that
were initiated by oxidation of electron-rich chromenes. The use
of visible light mediated photoredox catalysis to oxidize these
substrates enabled the cycloadditions to be telescoped with
further rearrangements in multi-step, one-pot procedures. The
predisposed, highly diastereoselective, biomimetic synthesis of
nyingchinoids A and B from a common chromene intermediate
gives good insight into the biosynthesis of these natural products,
which is supported by the structure revision of rasumatranin D.
Furthermore, we observed for the first time the conversion of a
Our synthesis of the revised rasumatranin D structure 16 is
outlined in Scheme 5. Firstly, 18 was synthesised by TBS-
protection of a known chromene.21 Visible light photoredox
catalysed aerobic [2+2+2] cycloaddition of 18 then gave 1,2-
dioxane 19. Treatment of 19 with TBAF formed the nyingchinoid
B analogue 20 (presumably an undiscovered natural product),
which gave rasumatranin D (16) on exposure to TFA. NMR data
for 16 showed excellent agreement with the published data for
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cycloaddition / aerobic [2+2+2] cycloaddition cascade reaction.
Acknowledgements
This work was supported by an Australian Research Council
Future Fellowship awarded to J.H.G. (FT170100437).
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COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Jacb D. Hart, Laura Burchill, Aaron J.
Day, Christopher G. Newton,
Christopher J. Sumby, David M. Huang,
and Jonathan H. George*
Page No. – Page No.
Visible Light Photoredox Catalysis
Enables the Biomimetic Synthesis of
Nyingchinoids A, B and D, and
Rasumatranin D
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