Biomimetic Synthetic Studies on the Bruceol Family of Meroterpenoid Natural Products

Article abstract of DOI:10.1021/acs.joc.9b02862

A biomimetic approach to total synthesis can offer several benefits, including the development of cascade reactions for the rapid generation of molecular complexity, and guidance in the structure revision of old natural products and the anticipation of ne

Full text of DOI:10.1021/acs.joc.9b02862

Subscriber access provided by BIU Pharmacie | Faculté de Pharmacie, Université Paris V
Article
Biomimetic Synthetic Studies on the Bruceol
Family of Meroterpenoid Natural Products
Aaron J. Day, Christopher J. Sumby, and Jonathan H. George
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b02862 • Publication Date (Web): 18 Dec 2019
Downloaded from pubs.acs.org on December 18, 2019
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the
course of their duties.

Page 1 of 40
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
Biomimetic Synthetic Studies on the Bruceol Family of Meroterpenoid Natural Products
Aaron J. Day, Christopher J. Sumby, and Jonathan H. George*
Department of Chemistry, The University of Adelaide, Adelaide, South Australia 5005, Australia.
jonathan.george@adelaide.edu.au
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Abstract
O
O
O
O
O
O
oxidative
cyclization
sunlight
O
O
HO
O
HO
O
H
H
H
OH
H
H
H
elusive biosynthetic
intermediate
isobruceol
anticipated natural product
eriobrucinol
A biomimetic approach to total synthesis can offer several benefits, including the development of
cascade reactions for the rapid generation of molecular complexity, and guidance in the structure
revision of old natural products and the anticipation of new ones. Herein, we describe how a
biomimetic synthesis of bruceol, a pentacyclic meroterpenoid, led to the anticipation, isolation and
synthesis of isobruceol. The key step in the synthesis of both bruceol and isobruceol was an
intramolecular hetero-Diels-Alder reaction of an o-quinone methide that was formed by
dearomatization of an electron-rich chromene. The synthesis of an elusive biosynthetic intermediate
also allowed a concise synthesis of eriobrucinol via a photochemical [2+2] cycloaddition.
Furthermore, some speculation on the biosynthesis of prenylated bruceol derivatives inspired the
development of a Claisen/Cope/Diels-Alder cascade reaction. We also report the generation of
halogenated bruceol derivatives, and the synthesis of several protobruceol natural products using
singlet oxygen ene reactions.
Introduction
Australian plants of the Philotheca genus are a rich source of complex, polycyclic meroterpenoids
1
(
Figure 1). These natural products generally feature coumarin ring systems (highlighted in red)
derived from polyketide biosynthetic pathways, while the stereochemically rich “citran” and “cyclol”
2
ring systems (highlighted in blue) arise from the cyclization of terpene side chains. The
1
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 2 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
prototypical Philotheca meroterpenoid is bruceol (1), first isolated from Philotheca brucei
(formerly known as Eriostemon brucei) by Jefferies et al. working at the University of Western
3
Australia (UWA) in 1963. The structure of bruceol was determined predominantly by X-ray
crystallography, an early application of this technique towards the elucidation of a complex
Australian natural product that was facilitated by the then recent appointment of Edward Maslen to
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
4
the staff at UWA. NMR spectroscopy was not used in the structure elucidation of bruceol, although
1
a few peaks of a low resolution 60 MHz H NMR spectrum were reported. The absolute
configuration of naturally occurring (–)-bruceol was later determined by X-ray crystallography of
5
chloroacetate and iodoacetate derivatives. Several years later, in 1992, Waterman and co-workers
believed they had re-isolated bruceol (1) from Philotheca brucei, and accordingly they published a
6
full NMR analysis. However, our synthesis of bruceol revealed that Waterman had in fact
7
unwittingly isolated a new isomeric natural product that we termed isobruceol (2). In isobruceol (2),
the orientation of the citran ring system (highlighted in blue) to the coumarin ring system
(highlighted in red) is opposite to that of bruceol (1). In addition to the synthesis of bruceol and
isobruceol, we also isolated isobruceol from Philotheca brucei. Herein we present a full account of
this work, alongside the synthesis of several related natural products. Deoxyisobruceol (4) was
5
isolated from Philotheca brucei in 1981 by Jefferies et al., although a pre-emptive total synthesis
8
was first reported by Crombie and Ponsford in 1968 (vide infra). Initially, Crombie thought he had
synthesised deoxybruceol (3), which has not yet been isolated from a natural source, but later X-ray
9
studies revealed his synthesis of 4. Both structures 3 and 4 are therefore variously referred to as
“deoxybruceol” in the previous literature, despite this name for the natural product 4 being a
misnomer. To avoid further confusion, we strongly recommend use of the trivial names given in
Figure 1. Several cyclobutane-containing pentacyclic natural products have also been isolated from
1
0
11
Philotheca brucei, including eriobrucinol (6) and iso-eriobrucinol A (5). These meroterpenoids
feature a “cyclol” ring system (highlighted in blue).
2
ACS Paragon Plus Environment

Page 3 of 40
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
O
O
O
O
O
O
O
O
HO
citran ring
system
OH
H
H
H
H
H
H
1
: bruceol
2: isobruceol
O
O
O
O
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
O
O
O
H
citran ring
system
H
H
H
H
H
H
H
3
: deoxybruceol
4: deoxyisobruceol
previously known as deoxybruceol
O
O
O
O
O
OH
HO
O
H
H
H
H
H
H
cyclol ring
system
5
: iso-eriobrucinol A
6: eriobrucinol
Figure 1. “Citran” and “cyclol” meroterpenoid natural products related to bruceol.
The biosynthetic pathways to most of the polycyclic meroterpenoids found in Philotheca brucei are
proposed to start from an isomeric pair of tricyclic chromenes: protobruceol-I (7), which was
1
2
isolated by Waterman in 1992, and chromene 8, which has yet to be isolated. The biosynthesis of
bruceol (1) from protobruceol-I (7) has been previously proposed in review articles by Ghisalberti1
1
3
and by Trauner. In detail, the stereoselective epoxidation of 7 could give epoxide 9, which could
ring open to form the reactive o-quinone methide (o-QM) intermediate 10 (Scheme 1).14 An
intramolecular hetero-Diels-Alder reaction of 10 could then generate the polycyclic framework of
15
bruceol (1). In a similar manner, deoxyisobruceol (4) could be formed via tautomerization of
chromene 8, followed by intramolecular hetero-Diels-Alder reaction of the o-quinone methide 11.
Eriobrucinol (6), the prototypical cyclol natural product in the bruceol family, is presumably
biosynthesized via an intramolecular [2+2] cycloaddition of chromene 8.
3
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 4 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
key chromene biosynthetic intermediates
O
O
O
O
O
O
O
O
O
epoxidation
tautomerization
O
OH
O
OH
HO
O
O
O
H
H
H
9
7: protobruceol-I
8
11
o-QM
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ring opening
of epoxide
hetero-Diels-Alder
reaction
[2+2]
O
O
O
H
O
O
O
O
O
hetero-Diels-Alder
reaction
O
HO
O
O
HO
O
HO
O
H
O
O
H
H
H
H
H
H
H
H
H
1
0
1: bruceol
6: eriobrucinol
4: deoxyisobruceol
previously known as deoxybruceol
o-QM
Scheme 1. Proposed Biosynthesis of Bruceol, Deoxyisobruceol and Eriobrucinol From a Pair
of Isomeric Chromenes
Racemic deoxyisobruceol (4) was first synthesised in 1968 by Crombie and Ponsford in a classic
work of biomimetic synthesis – and also natural product prediction, given that its isolation was not
reported until several years later. Firstly, the synthesis of 5,7-dihydroxycoumarin (13) was achieved
by the reaction between phloroglucinol (12) and neat ethyl propiolate in the presence of ZnCl
2
1
6
(
Scheme 2). The second, incredible step of the synthesis involved the condensation of 13 with
citral (14) in pyridine at 110 ºC, which gave ( )-4 in 10% isolated yield. It was initially assumed
±
that Knoevenagel condensation of 13 with 14 followed by oxa-6p-electrocyclization of an
intermediate o-quinone methide gave chromene 8 – a sequence often referred to as a “Crombie
chromenylation”. Chromene 8 could then undergo tautomerization followed by an intramolecular
hetero-Diels-Alder reaction of a second o-quinone methide (11, Scheme 1) to give 4. However,
chromene 8 was never observed as an intermediate in, or a by-product of, this reaction.
4
ACS Paragon Plus Environment

Page 5 of 40
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
O
O
OH
OEt
O
ZnCl2, neat
9
0%
HO
OH
HO
OH
1
2: phloroglucinol
13
O
pyridine
110 °C, 9 h
H
1
4: citral
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
O
O
O
10%
O
O
tautomerization,
hetero-Diels-Alder
reaction
HO
O
H
H
H
H
(
±)-4: deoxyisobruceol
(±)-8
not observed
Scheme 2. Crombie’s Classic Total Synthesis of Racemic Deoxyisobruceol
In fact, when the chromenylation of 13 was conducted at lower temperature (90 ºC), the only
chromene products observed were 15 and protobruceol-I (7), in 13% and 15% yield respectively
(
Scheme 3). Crombie rationalised the regioselective synthesis of chromenes 15 and 7 in preference
to 8 due to their formation via 6p-electrocyclization of stabilised o-quinone methide intermediates
in which the partial aromaticity of the coumarin ring was maintained.
O
O
O
O
O
H
O
O
1
4: citral
+
pyridine, 90 °C, 18 h
HO
OH
HO
O
O
OH
8
not formed!
1
3
"undesired"
(±)-15
3%
(±)-7: protobruceol-I
1
15%
Scheme 3. Crombie’s Synthesis of Protobruceol-I
To explain his synthesis of deoxyisobruceol (4) (Scheme 2), Crombie reasoned that protobruceol-I
(7) must isomerize under the thermal reaction conditions to generate chromene 8 as an
unobservable intermediate. This was proven by heating 7 in pyridine at reflux, which generated 4 as
the major product (Scheme 4). Mechanistically, retro-6p-electrocyclization of 7 could form the
stabilized o-quinone methide 16, which retains the partially aromatic coumarin ring (highlighted in
5
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 6 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
red). Isomerization of 16 could give the less stable o-quinone methide 17, followed by chromene 8
via 6p-electrocyclization. Chromene 8 is more reactive than 7 with respect to tautomerization, as
the resultant o-quinone methide 11 retains the aromatic character of the coumarin ring system (c.f.
the less stable 18). The tautomerization and hetero-Diels-Alder reaction of chromene 8 is therefore
faster than that of 7, and deoxyisobruceol (4) is the major product of the reaction in a good example
of the Curtin-Hammett principle. In 2007, Lee and Kim reported a modified synthesis of 4 in 45%
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
yield by condensation of 13 with 14 using catalytic ethylenediamine diaceatate (EDDA) in DMF,
instead of pyridine.17
O
O
O
O
O
O
O
O
pyridine, 110 °C, 4 days
retro-6π-electrocyclization
6π-electrocyclization
O
OH
O
OH
HO
O
HO
O
isomerization
H
(
±)-7: protobruceol-I
8
1
6
17
o-QM
o-QM
slow tautomerization
tautomerization fast
O
O
O
O
O
O
O
O
hetero-Diels-Alder
reaction
hetero-Diels-Alder
reaction
O
O
O
O
+
O
O
O
O
H
H
H
H
H
H
H
H
H
H
H
H
1
8
(±)-3: deoxybruceol
(±)-4: deoxyisobruceol
11
o-QM
o-QM
2
2% combined yield
(
4 is the major product)
Scheme 4. Crombie’s Thermal Rearrangement of Protobruceol-I
In order to build on Crombie’s seminal work and achieve the first biomimetic synthesis of bruceol
(1), we needed an oxidant capable of epoxidizing the chromene alkene of protobruceol-I (7) in the
presence of the prenyl side chain. Our previous biomimetic synthesis of rhodonoids C and D used
1
8
m-CPBA to epoxidize the prenyl side chain of a similar chromene meroterpenoid, so this reagent
was clearly unsuitable. Examination of the literature revealed that the Jacobsen-Katsuki
19
20
epoxidation performs well in the epoxidation of chromenes. Furthermore, the Jacobsen-type
chiral (salen)manganese(III)-complexes are known to epoxidize (Z)-1,2-disubstituted alkenes in
preference to tri-substituted alkenes, such as the prenyl side chain. Indeed, Jacobsen has exploited
6
ACS Paragon Plus Environment

Page 7 of 40
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
this chemoselectivity in a kinetic resolution of a chromene en route to the asymmetric synthesis of
2
1
the meroterpenoid (+)-teretifolione B (22) (Scheme 5). In this work, the racemic chromene ( )-19
±
was treated with 10 mol % of the chiral (salen)Mn catalyst (R,R)-20 in the presence of m-CPBA and
NMO. To obtain (+)-19 in good enantiopurity, the epoxidation had to be carried out to high
conversion (approx. 80%), as the kinetic resolution occurred with high catalyst-induced selectivity
but low substrate-induced selectivity. The presumed major epoxide product, 21, was undesired and
discarded in this process. However, in the case of our bruceol synthesis, a similar chromene epoxide
would trigger the biomimetic cascade.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
0% (R,R)-20, m-CPBA
O
OAc
O
OAc
NMO, CH2Cl2, -78 ºC
epoxidation
O
kinetic resolution
(±)-19
21
discarded
O
+
O
OH
O
OAc
4
steps
O
(+)-22: teretifolione
(+)-19 (15%, 91% e.e.)
enantioenriched
N
N
Mn
t-Bu
O
O
t-Bu
Cl
t-Bu
t-Bu
(R,R)-20
Scheme 5. Jacobsen’s Kinetic Resolution of a Racemic Chromene Using an Asymmetric
Epoxidation Strategy
Results and Discussion
Before our attempts to synthesise bruceol, racemic protobruceol-I (7) was prepared in two steps and
in multi-gram quantities according to Crombie’s published procedures (Scheme 3). Treatment of
(±)-7 with 10 mol % of Jacobsen’s catalyst (R,R)-20 then gave (–)-1 in a modest but reproducible
yield of 11%, and in high enantiopurity (98:2 e.r.) (Scheme 6). This kinetic resolution presumably
proceeds via a biomimetic cascade reaction involving ring opening of enantiopure epoxide 9,
7
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 8 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
followed by an intramolecular hetero-Diels-Alder reaction of the resultant o-quinone methide 10.
Formation of 2’-epi-bruceol (+)-25 was also observed in low yield during this reaction (3%, 90: 10
e.r.) via ring opening of the diastereomeric epoxide 23 and cyclization of o-quinone methide 24,
indicating the expected low substrate selectivity for the Jacobsen epoxidation. Use of (S,S)-20
resulted in the formation of the unnatural enantiomer of bruceol, (+)-1, alongside (–)-25. However,
careful comparison of the NMR spectra for synthetic bruceol (1) revealed that it did not quite match
the NMR data for “bruceol” isolated by Waterman in 1992. Remarkably, we were able to obtain the
original 1963 isolation sample of bruceol from the University of Western Australia, and its NMR
spectra perfectly matched our synthetic material. We also obtained X-ray structures of both 1 and
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
5. At this point, we thought it was likely that Waterman had isolated isobruceol (2), rather than
bruceol (1).
O
O
O
O
O
O
O
ring opening
of epoxide
hetero-Diels-Alder
reaction
X-ray
O
O
O
O
O
OH
HO
HO
H
O
O
H
H
H
1
0
(−)-1: bruceol
1%, 98:2 e.r.
o-QM
1
10% (R,R)-20, m-CPBA
NMO, CH Cl , -78 ºC
9
(S,S)-20 gave 13% of (+)-1, 96:4 e.r.
2
2
+
O
OH
epoxidation
O
O
O
O
O
O
ring opening
of epoxide
hetero-Diels-Alder
reaction
X-ray
(
±)-7: protobruceol-I
flip structure
O
O
O
OH
O
O
H
H
H
2'
OH
O
OH
H
2
4
(+)-25: 2'-epi-bruceol
o-QM
3%, 90:10 e.r.
2
3
(
S,S)-20 gave 3% of (-)-25, 91:9 e.r.
Scheme 6. Biomimetic Total Synthesis of Bruceol
The low yield for the bruceol cascade could be due to overoxidation of the coumarin ring system,
which gave rise to some highly polar, uncharacterised by-products. This conjecture was supported
by the much higher yield for the epoxidation-cyclization cascades of the simpler chromene (
±
)-27,
which was derived from chromenylation of orcinol (26) (Scheme 7).18
8
ACS Paragon Plus Environment

Page 9 of 40
The Journal of Organic Chemistry
1
2
3
4
14, EDDA, PhMe, 110 ºC
O
OH
HO
OH
6: orcinol
74%
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
(
±)-27
1
0% (R,R)-20, m-CPBA
NMO, CH2Cl2, -78 ºC
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
O
O
O
+
HO
H
H
H
H
H
H
OH
(
−)-28
(−)-29
11%, 94:6 e.r.
2
3%, 92:8 e.r.
(
S,S)-20 gave 29% of (+)-28, 91:9 e.r.
12% of (+)-29, 92:8 e.r.
Scheme 7. A More Efficient Epoxidation-Cyclization Cascade of a Simpler Chromene
Having confirmed that synthetic bruceol (1) matched the original isolation sample in all
characterisation data, we wanted to confirm our hypothesis that Waterman had isolated isobruceol
(
2), rather than bruceol. Initial attempts to synthesize isobruceol met with failure, so we conducted
our own isolation experiments (Figure 2). Several kilograms of Philotheca brucei were collected
from the Koolyanobbing Range (approx. 500 km east of Perth, Western Australia), and the plant
material was extracted via two different methods. Firstly, a simple extraction of the plant material
using Et O and purification by flash chromatography (after removal of volatile eucalyptol) gave an
2
11:1 mixture of bruceol (1) and isobruceol (2). Pure bruceol was easily obtained by crystallization,
whereas purification of isobruceol required HPLC separation. Recrystallisation of isobruceol from
2
2
CHCl
3
/Et O gave crystals suitable for X-ray analysis, and its NMR spectra matched Waterman’s
2
data, thus confirming his isolation of 2 rather than 1. The second extraction involved the pressurized
hot water extraction (PHWE) method developed by Smith and co-workers, which uses an
2
3
unmodified household espresso machine. This method has recently been used to isolate coumarin
2
4
natural products from Australian native plant species. Application of the PHWE approach to
Philotheca brucei gave a mixture of four known coumarin natural products – bruceol (1), isobruceol
(
2), hydroxyeriobrucinol (30) and isoimperatorin (31). (–)-30 was particularly abundant (0.44% dry
9
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 10 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
weight of the plant material). Single crystal X-ray crystallography of (–)-30 allowed its absolute
configuration to be assigned as shown in Figure 1, which agrees with a previous tentative
2
5
assignment. We had hoped to discover chromene 8 to support our biosynthetic speculations, but
ultimately our efforts were unsuccessful.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
O
O
O
O
O
O
O
O
O
O
O
HO
O
O
O
H
H
H
HO
OH
H
H
H
H
H
H
OH
(
−)-1: bruceol
(−)-2: isobruceol (−)-30: hydroxyeriobrucinol 31: isoimperatorin
X-ray X-ray
Figure 2. Coumarin natural products isolated from Philotheca brucei in this work.
Attempts to synthesise isobruceol (2) were thwarted by the impossibility of preparing its likely
biosynthetic precursor 8 via the direct chromenylation of 5,7-dihydroxycoumarin (13) with citral
(14), which gave only the regioisomeric chromenes 15 and 7 (Scheme 3). Indeed, we accumulated
significant quantities of the undesired chromene 15 as we scaled up the synthesis of protobruceol-I
(7). Eventually, we realised that chromene 15 could be isomerized to give 8 via lactone
2
6
methanolysis to give 32, followed by lactonization (Scheme 8). This resulted in an equilibrium
mixture of 15 and 8, which was easily separated by flash chromatography.
1
0
ACS Paragon Plus Environment

Page 11 of 40
The Journal of Organic Chemistry
1
2
3
4
5
O
O
9
OMe
OH
O
NaOH, MeOH, rt
HO
O
HO
9
O
lactone methanolysis
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(
±)-15
32
O
bond rotation
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
9
OH
4
0% (+ 55% r.s.m.)
HO
O
MeO
OHO
9
O
lactonization
(±)-8
32
Scheme 8. Isomerization of 15 to 8 Catalysed by Methoxide
With convenient access to racemic chromene 8, we could investigate its conversion into isobruceol
(
2) under Jacobsen epoxidation conditions (Scheme 9). Reaction of (
alongside m-CPBA and NMO as the stoichiometric oxidants formed (–)-2 in 5% yield, alongside its
C-2’ epimer (+)-33 in 2% yield. Although this kinetic resolution of ( )-8 was low yielding, the
±
)-8 with 10% (S,S)-20
±
products were obtained in good enantiopurity, and use of the opposite catalyst, (R,R)-20, gave
access to (+)-2 in 9% yield. Furthermore, NMR spectra for synthetic 2 perfectly matched our
isolated natural product data.
O
O
O
O
O
O
1
0% (S,S)-20, m-CPBA
NMO, CH2Cl2, −78 °C
O
+
O
O
HO
O
O
OH
H
H
H
H
H
H
OH
(
±)-8
(−)-2: isobruceol (+)-33: 2'-epi-isobruceol
%, 93:7 e.r. 2%, 96:4 e.r.
(R,R)-20 gave 9% of (+)-2, 92:8 e.r.
% of (−)-33, 91:9 e.r.
5
4
Scheme 9. Biomimetic Synthesis of Isobruceol from Chromene 8
As shown in Scheme 1, chromene 8 is also a pivotal intermediate in the biosynthesis of
deoxyisobruceol (4) and eriobrucinol (6), and this elusive intermediate was also proposed to be the
direct precursor of 4 in Crombie’s total synthesis (Scheme 4). In vindication of Crombie’s
mechanistic analysis, we showed that heating chromene 8 at 150 °C in pyridine gave 4 in 77% yield
1
1
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 12 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(Scheme 10). Therefore, the low yield of 4 in Crombie’s cascade is due to the inefficient
interconversion between chromenes 7 and 8, which results in a mixture of deoxybruceol and
deoxyisobruceol products. On the other hand, the final tautomerization of 8 to give o-QM 11
followed by intramolecular hetero-Diels-Alder reaction is clearly an efficient reaction sequence.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
O
O
O
pyridine, 150 °C
µwave
O
O
HO
O
7
7%
H
tautomerization,
hetero-Diels-Alder
reaction
H
H
H
(±)-8
(±)-4: deoxyisobruceol
Scheme 10. Biomimetic Synthesis of Deoxyisobruceol from Chromene 8
Exposure of a solution of chromene 8 in THF to direct sunlight resulted in the gradual formation of
eriobrucinol (6) in good yield via a photochemical [2+2] cycloaddition (Scheme 11). Despite
extensive work in this area, Crombie was unable to investigate the biomimetic conversion of 8 into
eriobrucinol due to his failure to synthesise 8. However, Crombie did achieve biomimetic,
photochemical syntheses of the isomeric natural products, iso-eriobrucinols A and B (vide infra),
and he also confirmed the structure of eriobrucinol (6) via a less direct total synthesis.27
O
O
O
O
sunlight, THF, rt
HO
O
76%
HO
O
H
H
H
[
2+2] cycloaddition
(±)-8
(±)-6: eriobrucinol
Scheme 11. Photochemical Conversion of Chromene 8 into Eriobrucinol
It is worth comparing our four-step total synthesis of eriobrucinol (6) from its direct biosynthetic
precursor 8 with the first total synthesis by Crombie, and also with a more recent synthesis by
Hsung and Tang. As shown in Scheme 12, Crombie’s synthesis of eriobrucinol features an early
stage chromenylation of 2,4-dihydroxy-6-methoxybenzaldehyde (35) to give chromene 36,
followed by acetylation and photochemical [2+2] cycloaddition to give 38. The coumarin ring
1
2
ACS Paragon Plus Environment

Page 13 of 40
1
The Journal of Organic Chemistry
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
system was then installed via a Perkin reaction, and a final deacetylation gave 6 in a total of seven
steps from commercial starting materials.
O
H
OMe
OMe
14: citral
OMe
OMe
HCl, Zn(CN)2, Et2O, rt;
H2O, 100 °C
OHC
HO
pyridine, 90 °C
OHC
HO
Ac O, pyridine, rt
OHC
AcO
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
50%
50%
93%
O
HO
OH
OH
O
3
4
35
36
37
medium-pressure Hg vapour
lamp (100 W), CD3COCD3, 12 h
7
8%
O
H
O
H
O
O
OH
OMe
OHC
AcO
OHC
HCl, MeOH
KOAc, Ac2O, 160 °C
MgI2, Et2O, C6H6, 80 °C
AcO
O
HO
O
AcO
O
O
H
H
H
H
H
H
H
5
8%
30%
62%
H
H
H
(±)-6: eriobrucinol
40
39
38
Scheme 12. Crombie’s Total Synthesis of Eriobrucinol
Hsung et al. recently reported a divergent, three-step total synthesis of eriobrucinol (6) and the
2
8
isomeric natural products iso-eriobrucinol A (5) and iso-eriobrucinol B (43) (Scheme 13). Initial
chromenylation of phloroglucinol (12) gave chromene 41, which underwent acid catalysed [2+2]
cycloaddition to give cyclobutene 42. Late stage coumarin formation using ZnCl in neat ethyl
2
propiolate gave a 21% yield of 6 alongside a 32% yield of an inseparable 3:1 mixture of 5 and 43.
O
H
14: citral
OH
OH
OH
piperidine, Ac2O
TFA:CH2Cl2 (1:5)
EtOAc/toluene, 90 °C
0 °C to rt
HO
O
HO
O
HO
OH
9
3%
80%
H
H
1
2
H
4
1
42
O
O
O
O
OH
O
OEt
ZnCl2, neat
+
+
O
O
O
HO
O
HO
O
H
H
H
H
H
H
H
H
H
(±)-43: iso-eriobrucinol B
(±)-5: iso-eriobrucinol A
(±)-6: eriobrucinol
32%, 3:1 ratio of 5:43
21%
Scheme 13. Hsung and Tang’s Divergent Total Synthesis of Eriobrucinol and Iso-
eriobrucinols A and B
1
3
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 14 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Given the ease of the photochemical [2+2] cycloaddition of 8 on exposure to sunlight,29 we
investigated the solar photocyclizations of protobruceol-I (7) and chromene 15, which resulted in
the formation of iso-eriobrucinol A (5) and iso-eriobrucinol B (43) in 53% and 33% yield
respectively (Scheme 14). Similar (but lower yielding) syntheses of iso-eriobrucinols A and B were
achieved by Crombie, several years before their isolation, via irradiation with a 100 W medium-
pressure mercury lamp.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
O
O
O
sunlight, CHCl3, rt, 5 h (53%)
O
OH
O
OH
Crombie's conditions:
medium-pressure Hg vapour
lamp (100 W), d5-pyridine
24 h (32%)
H
H
H
[
2+2] cycloaddition
(±)-7: protobruceol-I
(±)-5: iso-eriobrucinol A
O
O
O
O
H
H
sunlight, CHCl3, rt, 40 h (33%)
H
O
HO
O
Crombie's conditions:
medium-pressure Hg vapour
HO
lamp (100 W), acetone/t-BuOH,
7 days (18%)
(±)-15
(±)-43: iso-eriobrucinol B
[2+2] cycloaddition
Scheme 14. Biomimetic Synthesis of Iso-eriobrucinols A and B Using Sunlight
In 1994, a family of seven bruceol derivatives (45-51, Figure 3), each with an oxidized prenyl
substituent at C-2’, were isolated by Waterman and co-workers from Philotheca myoporoides
3
0
(
previously known as Eriostemon myoporoides). Philotheca myoporoides is a flowering shrub that
occurs widely along the east coast of Australia. The bruceol derivatives 45-51 are presumably
formed via aerobic oxidation of “prenylbruceol” (44), although this biosynthetic intermediate and
plausible “undiscovered natural product” has not yet been reported.
1
4
ACS Paragon Plus Environment

Page 15 of 40
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
9
O
O
O
O
O
O
O
O
O
2
O
O
O
O
O
O
O
O
HOO
HO
'
H
H
H
H
H
H
H
H
H
H
H
H
4
4: "prenylbruceol"
45
46
47
O
O
O
O
O
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
O
O
O
O
O
O
O
O
O
O
O
HO
O
HO
H
H
H
H
H
H
H
H
H
H
H
H
O
OH
OH
OOH
4
8
49
50
51
Figure 3. C-2’ prenylated bruceol derivatives isolated from Philotheca myoporoides.
The origin of the prenyl side chain of 44 and its oxidized derivatives is biosynthetically intriguing.
One possibility is a direct prenylation at C-2’ of protobruceol-I (7) to initiate the hetero-Diels-Alder
reaction of an intermediate o-QM. However, this would be an unusual site for a prenylation reaction,
and we propose an alternative that invokes a more common aromatic prenyltransferase that could
prenylate the C-7 phenol of protobruceol-I to give 52 (Scheme 15). The aromatic prenyl ether 52
could then undergo an aromatic Claisen rearrangement 31 to give 53, followed by a Cope
rearrangement to give o-QM 54, and a final intramolecular hetero-Diels-Alder reaction to give
“prenylbruceol” (44). Cascade cyclizations initiated by aromatic Claisen rearrangements have been
proposed in the biosynthesis of several caged xanthone natural products.32
O
O
O
O
O
O
O
O
intramolecular
hetero-Diels-Alder
reaction
aromatic Claisen
rearrangement
Cope
rearrangement
7
O
O
O
O
O
O
O
O
O-prenylation
of protobruceol-I
2
'
H
H
H
H
5
2
53
54
o-QM
44: "prenylbruceol"
Scheme 15. Proposed Biosynthesis of “Prenylbruceol”
To investigate a biomimetic synthesis of 44, prenylation of the free phenol of 7 was achieved under
standard conditions to give 52 in good yield (Scheme 16). However, heating 52 in pyridine (150 °C
in a microwave) formed benzofuran 56 (19%) and the bruceol derivative 58 (30%), rather than the
1
5
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 16 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
desired “prenylbruceol” (44). Perhaps unsurprisingly, the initial aromatic Claisen rearrangement
shifted the prenyl group to the unhindered C-8 position to give 55, which cyclized to give 56.
Alternatively, intermediate 55 isomerized to the more reactive chromene 57 via retro-6p-
electrocyclization and 6p-electrocyclization, followed by tautomerization and intramolecular
hetero-Diels-Alder reaction to give 58, in a process that is analogous to the sequence outlined in
Scheme 4.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
O
O
O
O
O
O
O
Br
8
pyridine, 150 °C
µwave
K2CO3, DMF, 0 °C
O-cyclization
O
OH
O
O
O
OH
O
O
7
4%
aromatic Claisen
rearrangement
1:1 d.r.
(
±)-7: protobruceol-I
52
55
56 (19%)
retro-6π, 6π
+
O
O
O
O
tautomerization,
hetero-Diels-Alder
reaction
HO
O
O
O
H
H
H
H
5
7
58 (30%)
Scheme 16. Thermal Rearrangement of Prenylated Protobruceol-I
We also O-prenylated chromene 8 under standard conditions to give prenyl ether 59 (Scheme 17).
In this substrate, there is no unhindered carbon atom adjacent to the C-5 prenyl ether for a facile
aromatic Claisen rearrangement. However, we still did not observe migration of the prenyl
substituent to C-2’ of the chromene. Instead, under thermal conditions in pyridine 59 underwent
3
3
successive Claisen and Cope rearrangements to shift the prenyl group to C-8 of 61, followed by
aromatization and tautomerization to give o-QM 63, and a final intramolecular hetero-Diels-Alder
reaction to give 64. Heating 59 in toluene instead of pyridine initiated a similar Claisen-Cope
cascade, but with a terminating intramolecular [2+2] cycloaddition to give cyclobutane 64 favoured
over the hetero-Diels-Alder reaction. Attempts to catalyse the desired Claisen-Cope cascade using a
europium(III) catalyst were unsuccessful.34
1
6
ACS Paragon Plus Environment

Page 17 of 40
The Journal of Organic Chemistry
1
2
3
4
5
O
5
O
O
O
O
O
O
O
Br
H
Cope
rearrangement
8
pyridine, 150 °C
µwave
K2CO3, DMF, 0 °C
HO
O
O
O
O
O
O
O
aromatic Claisen
rearrangement
8
9%
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
'
8
59
60
61
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
PhMe, 150 °C, µwave
tautomerization
44%
Claisen/Cope/[2+2]
O
H
O
O
O
O
O
O
O
hetero-Diels-Alder
reaction
tautomerization
HO
O
H
H
O
O
O
HO
O
O
69%
H
H
H
H
H
H
6
5
64
63
o-QM
62
Scheme 17. Synthesis of Prenylated Isobruceol Derivative 64 via a Claisen/Cope/Hetero-Diels-
Alder Cascade Reaction and Synthesis of Prenylated Eriobrucinol Derivative 65 via a
Claisen/Cope/[2+2] Cascade Reaction
Bromonium and iodonium-induced cyclizations of protobruceol-I (7) were also investigated, but
these reactions were rather unselective (Scheme 18). Treatment of 7 with NBS in Et
mixture of di- and tri-brominated citran products 68a, 68b, 69a and 69b. The major product 68a
formed in 32% yield) has an equatorial bromine substituent at C-2’ of the citran ring system, and a
bromine substituent at C-8 of the coumarin ring system. Additional bromination at C-3 gave 68b in
2% yield. Lack of diastereoselectivity of the initial bromonium ion formation resulted in
2
O generated a
(
1
significant formation of 69a and 69b (23% and 7% respectively), which both have an axial bromine
substituent at C-2’. The structures of 68a, 68b and 69a were elucidated using single crystal X-ray
crystallography. Iodination of 7 with NIS in Et O gave a similar result, with a mixture of di- and tri-
2
iodinated citran products 72a, 72b, 73a and 73b being formed.
1
7
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 18 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
O
O
O
O
O
X
X
X
3
X
O
O
O
O
O
Br
Br
Br
Br
O
hetero-Diels-Alder
8
NBS, Et2O, rt
bromination
X = H or Br
reaction
O
O
O
2'
O
O
O
OH
O
OH
+
Br
Br
H
Br
H
H
H
H
H
Br
H
66
67
68a (X = H) (38%)
68b (X = Br) (12%)
69a (X = H) (23%)
69b (X = Br) (7%)
o-QM
7: protobruceol-I
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
NIS, Et2O
rt
O
O
O
O
iodination
X
X
X
X
O
O
O
O
Ι
Ι
Ι
Ι
hetero-Diels-Alder
reaction
X = H or Ι
O
O
O
O
O
O
O
OH
+
Ι
Ι
H
H
Ι
Ι
H
H
H
H
H
7
0
71
o-QM
72a (X = H) (47%)
72b (X = Ι) (7%)
73a (X = H) (18%)
73b (X = Ι) (3%)
6
8a
68b
69a
Scheme 18. Bromination and Iodination of Protobruceol-I
Alongside protobruceol-I (7), the biosynthetic precursor to bruceol (1) and isoeriobrucinol A (5),
Waterman discovered five oxidized chromene natural products 74-78 in the aerial parts of
Philotheca brucei (Figure 4).12 Protobruceol-II (74) and protobruceol-III (75) are presumably
derived from reduction of their corresponding hydroperoxides (76) and (77), which in turn are
35
36
probably formed via the addition of singlet oxygen in a Schenck ene reaction. The proposed
structure of protobruceol-IV (78) bears the same regiochemistry as eriobrucinol (6), isobruceol (2),
and their proposed precursor 8.
1
8
ACS Paragon Plus Environment

Page 19 of 40
The Journal of Organic Chemistry
1
2
3
4
5
6
7
8
O
O
O
O
O
O
O
OH
O
OH
O
OH
OH
OH
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
7
: protobruceol-I
74: protobruceol-II
75: protobruceol-III
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
O
O
O
O
O
O
O
OH
O
OH
HO
O
OOH
OOH
HO
7
6
: protobruceol-II
hydroperoxide
77: protobruceol-III
hydroproxide
78: protobruceol-IV
originally proposed structure
Figure 4. Protobruceol natural products.
Protobruceol-II hydroperoxide (76) and protobruceol-III hydroperoxide (77) were synthesised from
7
in 47% and 39% yield respectively by addition of singlet oxygen, generated in situ using
methylene blue as a sensitiser (Scheme 19). Addition of NaBH after the completion of the Schenck
4
ene reaction afforded protobruceol-II (74) and protobruceol-III (75) in 37% and 35% yield
1
resepectively. H NMR spectra for these protobruceol natural products fully matched Waterman’s
isolation data.12
O
O
O
O
O
O
O2, LED light
methylene blue
MeOH, rt
+
O
OH
O
OH
O
OH
Schenck ene reaction
OOH
OOH
(
±)-7: protobruceol-I
(±)-76: protobruceol-II (±)-77: protobruceol-III
hydroperoxide
47%
hydroproxide
39%, 1:1 d.r.
O
O
O
O
O2, LED light
methylene blue
MeOH, rt; then NaBH4
+
O
OH
O
OH
Schenck ene reaction,
reduction
OH
OH
(±)-74: protobruceol-II (±)-75: protobruceol-III
37%
35%, 1:1 d.r.
Scheme 19. Synthesis of Protobruceol-II, Protobruceol-III and their Hydroperoxides
1
9
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 20 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
After the straightforward synthesis of protobruceols 74-77, our attention turned to the synthesis of
protobruceol-IV (78) (Scheme 20). The one-pot procedure used to synthesise alcohols 74 and 75
proved less reliable with chromene 8 due to poor acid/base stability of this system, so instead we
chose a two step method. Oxidation of chromene 8 gave hydroperoxides 79 (32%) and 80 (33%),
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
which were separated by flash chromatography. Careful reduction of 79 using NaBH
4
gave 78 in
6
6% yield, whereas reduction of 80 gave 81 in 77% yield. Unfortunately, there were significant
1
discrepancies between the H NMR data of our synthetic 78 and that of natural protoisobruceol-IV.
1
We therefore believe a structural revision is necessary for protobruceol-IV. However, with only H
NMR data reported by Waterman for protobruceol-IV, we were unable to determine its true
strucure.
O
O
O
O
O , LED light
O
O
2
rose bengal
MeOH, rt
+
HO
O
HO
O
HO
O
Schenck ene reaction
HOO
HOO
(±)-8
(±)-79
2%
(±)-80
33%, 1:1 d.r.
3
NaBH4
MeOH, 0 °C
NaBH4
MeOH, 0 °C
reduction
66%
77%
O
O
O
O
HO
O
HO
O
HO
HO
(±)-78
NMR does not match
protobruceol-IV
(±)-81
:1 d.r.
1
Scheme 20. Synthesis of the Proposed Structure of Protobruceol-IV
Conclusion
We have used biosynthetic speculation to guide the synthesis of several polycyclic meroterpenoids
found in Philotheca brucei from a pair of isomeric chromenes. The synthesis of bruceol featured an
oxidative cascade cyclization triggered by an asymmetric Jacobsen-Katsuki epoxidation of
protobruceol-I, a process that was also a kinetic resolution. Discrepancies between the NMR spectra
for synthetic bruceol and some published NMR data for natural bruceol led us to propose the
2
0
ACS Paragon Plus Environment

Page 21 of 40
1
The Journal of Organic Chemistry
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
existence of isobruceol as a previously unrecognised natural product. This was confirmed by the
isolation of isobruceol from Philotheca brucei, and its synthesis from an isomer of protobruceol-I.
This chromene isomer, an elusive biosynthetic intermediate, was also directly converted into
eriobrucinol via a photochemical [2+2] cycloaddition, and into deoxyisobruceol via a thermal
hetero-Diels-Alder reaction. Further studies on some biosynthetically-intriguing prenylated
derivatives of bruceol remain inconclusive, although we achieved a complex Claisen/Cope/Diels-
Alder reaction sequence. In a general sense, this work shows that biosynthetic speculation can
inspire the development of novel cascade reactions, and also aid the discovery of new natural
products.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Experimental Section
General Information. All chemicals used were purchased from commercial suppliers and used as
received. All reactions were performed under an inert atmosphere of N . All organic extracts were
2
dried over anhydrous magnesium sulfate. Thin layer chromatography was performed using
aluminium sheets coated with silica gel. Visualization was aided by viewing under a UV lamp and
staining with ceric ammonium molybdate stain followed by heating. All R values were measured
f
to the nearest 0.05. Flash chromatography was performed using 40-63 micron grade silica gel.
Melting points were recorded on a digital melting point apparatus and are uncorrected. Infrared
spectra were recorded using an FT-IR spectrometer as the neat compounds. High field NMR was
recorded using a 600 MHz spectrometer ( H at 600 MHz, 13C at 150 MHz) or a 500 MHz
1
1
13
spectrometer ( H at 500 MHz, C at 125 MHz). The solvent used for NMR spectra was CDCl
3
1
unless otherwise specified. H chemical shifts are reported in ppm on the δ-scale relative to TMS (δ
13
0
.0) and C NMR are reported in ppm relative to chloroform (δ 77.0). Multiplicities are reported as
(br) broad, (s) singlet, (d) doublet, (t) triplet, (q) quartet and (m) multiplet. All J-values were
rounded to the nearest 0.1 Hz. ESI high resolution mass spectra were recorded on a Q-TOF mass
spectrometer. Optical rotations were measured on a modular circular polarimeter.
2
1
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 22 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Experimental Procedures. Note: Experimental information for compounds 1, 2, 7, 8, 15, 22, 25,
8, 29, and 33 have been previously reported as part of the bruceol synthesis.7
2
Isolation of Bruceol (1) and Isobruceol (2). Philotheca brucei collected from the Koolyanobbing
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Range (approx. 500 km east of Perth, GPS location E749300 N6581700) (1.0 kg) was cut into a
crude mulch and macerated in Et
filtered and concentrated to ~750 mL, then washed sequentially with 5% HCl (2 × 500 mL), 8%
NaHCO (2 × 500 mL), and 5% NaOH (2 × 500 mL). The organic phase was dried over anhydrous
MgSO , filtered and the solvent was removed in vacuo, giving a green, fragrant oil. The volatiles
(mostly eucalyptol) were removed by vacuum distillation. The residue was purified by sequential
flash column chromatography on SiO (10:1 ® 1:1 petrol/EtOAc, then 2:1 petrol/EtOAc) to give a
2
O (~3 L) for 5 days with intermittent stirring. The extract was
3
4
2
mixture of bruceol 1 and isobruceol 2 (500 mg, 11:1 in favour of 1). The mixture was then
recrystallised from petrol/EtOAc to afford pure 1 as colourless prisms (140 mg), and the residue
was repurified by flash column chromatography to afford an enriched mixture of bruceol 1 and
isobruceol 2 (300 mg, 4.5:1 in favour of 1). Further recrystallization from petrol/EtOAc afforded
more 1 as colourless prisms (36 mg, total of 176 mg, 0.018% w/w). The residue was repurified by
flash column chromatography (2:1 petrol/EtOAc) to afford a further enriched mixture of bruceol 1
and isobruceol 2 (200 mg, 2:1 in favour of 1). No more bruceol 1 in the residue could be removed
by crystallization. The residue was then purified by semi-preparative reverse phase HPLC on an
Ascentis ® C18 column (25 cm × 10 mm, 5 μm) (MeCN/H
2
O, gradient elution) to afford isobruceol
2
as a white solid (11 mg, 0.0011% w/w). A single crystal was obtained by recrystallization from
CHCl /Et O.
Data for 1. R
3
2
1
f
0.20 (1:1 petrol/EtOAc); mp 197 – 209 °C (prisms from petrol/EtOAc); H NMR
(500 MHz, CDCl
3
) δ 7.96 (d, J = 9.6 Hz, 1H), 6.46 (s, 1H), 6.17 (d, J = 9.6 Hz, 1H), 3.85 (dd, J =
8
.4, 2.0 Hz, 1H), 2.93 (d, J = 2.4 Hz, 1H), 2.30 (ddd, J = 11.6, 5.4, 2.6 Hz, 1H), 2.12 (d, J = 8.4 Hz,
1H), 1.91 (dd, J = 15.6, 6.3 Hz, 1H), 1.58 (d, J = 2.5 Hz, 3H), 1.51 (ddd, overlapped, J = 16.2, 14.4,
2
2
ACS Paragon Plus Environment

Page 23 of 40
1
The Journal of Organic Chemistry
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
7.6 Hz, 1H), 1.50 (s, 3H), 1.25 (ddd, J = 13.0, 7.0, 5.3 Hz 1H), 1.07 (s, 3H), 0.59 (tdd, J = 13.6,
13
1
1
1.7, 6.2 Hz, 1H); C{ H} NMR (125 MHz, CDCl
3
) δ 161.6, 160.5, 155.1, 151.4, 138.3, 111.2,
1
09.4, 104.0, 99.2, 85.5, 79.3, 70.9, 47.0, 36.8, 35.9, 29.5, 24.2, 23.9, 21.6; IR (neat) 3529, 3457,
2971, 1702, 1615, 1566, 1447, 1360, 1248, 1196; HRMS (ESI) Calculated for C19
H
19
5
O 327.1238
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
−
[
M−H] , found 327.1235.
1
Data for 2. R
f
0.20 (1:1 petrol/EtOAc); H NMR (500 MHz, CDCl ) δ 7.85 (d, J = 9.6 Hz, 1H), 6.51
3
(s, 1H), 6.14 (d, J = 9.5 Hz, 1H), 3.83 (s, 1H), 2.91 (t, J = 2.4 Hz, 1H), 2.32 (ddd, J = 11.7, 5.6, 2.7
Hz, 1H), 2.14 (s, 1H), 1.92 (dd, J = 15.5, 6.2 Hz, 1H), 1.62 (s, 3H), 1.50 (ddd, J = 15.0, 13.3, 7.2
Hz, 1H) 1.47 (s, 3H), 1.23 (dt, J = 13.2, 6.4 Hz, 1H), 1.07 (s, 3H), 0.56 (tdd, J = 13.6, 11.7, 6.2 Hz,
1
3
1
1
H); C{ H} NMR (125 MHz, CDCl
3
) δ 161.8, 158.3, 154.7, 154.2, 138.1, 111.6, 110.1, 105.9,
97.3, 86.5, 79.3, 71.0, 47.4, 36.7, 36.2, 29.6, 24.4, 24.3, 21.6; IR (neat) 3422, 2925, 1718, 1617,
1
568, 1446, 1387, 1338, 1315, 1258, 1214, 1193; HRMS (ESI) Calculated for C21
H
21
5
O 329.1384
+
[
M+H] , found 329.1386.
Isolation of Hydroxyeriobrucinol (30) and Isoimperatorin (31). Dried and finely powdered
Philotheca brucei (10.0 g) was mixed with sand (3.5 g), placed in the portafilter (sample
compartment) of a conventional, unmodified espresso machine and extracted with 35% EtOH in
water (2 × 100 mL). The process was repeated nine times (100 g total plant material). The
combined extracts were concentrated to ~200 mL on a rotary evaporator (40 °C bath) and the
aqueous phase was extracted with EtOAc (4 × 200 mL). The combined organic extracts were
washed with brine (2 × 200 mL), dried over MgSO
4
, and concentrated in vacuo. The residue was
(10:1 ® 1:1 petrol/Et O, gradient
initially purified by flash column chromatography on SiO
2
2
elution) giving fractions containing a non-polar fragrant oil, shown to be mostly eucalyptol (254
mg), a brown oil containing a complex mixture of compounds (944 mg), and a mixture of (-)-
bruceol (1), (-)-isobruceol (2), and (-)-hydroxyeriobrucinol (30) as well as other minor impurities
(1.1 g). Flash column chromatography on SiO (4:1 petrol/EtOAc) of the brown oil gave crude
2
2
3
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 24 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
isoimperatorin (31) as a brown solid (55 mg), which was recrystallised (petrol/EtOAc) to afford 31
as white needles (18.1 mg, 0.018% w/w). Data for 31. R 0.40 (1:1 petrol/Et O); mp 98.2 – 99.2 °C
) δ 8.16 (dd, J = 9.8, 0.7 Hz, 1H), 7.60 (d, J = 2.3 Hz,
H), 7.16 (t, J = 0.8 Hz, 1H), 6.96 (dd, J = 2.3, 1.0 Hz, 1H), 6.27 (d, J = 9.8 Hz, 1H), 5.54 (tt, J =
f
2
3
7
1
(lit. 110 °C); H NMR (600 MHz, CDCl
3
1
7
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
3
1
.0, 1.5 Hz, 1H), 4.92 (dt, J = 7.1, 0.9 Hz, 2H), 1.80 (s, 3H), 1.70 (s, 3H); C{ H} NMR (150 MHz,
) δ 161.5, 158.3, 152.8, 149.1, 145.0, 140.0, 139.8, 119.2, 114.3, 112.7, 107.7, 105.2, 94.4,
9.9, 26.0, 18.4; IR (neat) 3026, 2997, 2667, 1603, 1507, 1460, 1443, 1286, 1238, 1229, 1179,
CDCl
3
6
1
039; HRMS (ESI) calculated for C16
H
15
O
4
3
271.0968 [M+H]+
,
found: 271.0971. Trituration of the
mixture containing 1, 2, and 30 with CHCl
(20 mL) gave (-)-hydroxyeriobrucinol (30) as a white
powder (438 mg, 0.44% w/w). Data for 30. R
f
0.15 (1:1 petrol/EtOAc); mp 245 – 249 °C
1
(
decomposes) (needles from EtOH); [ꢀ]ꢁꢂ°ꢃ −181 (MeOH, c = 1.0); H NMR (500 MHz, d
5
-
ꢀ
pyridine) δ 8.29 (d, J = 9.6 Hz, 1H), 6.73 (s, 1H), 6.25 (d, J = 9.6 Hz, 1H), 4.51 (d, J = 5.0 Hz, 1H),
3
5
.47 (d, J = 9.7 Hz, 1H), 2.92 (dd, J = 9.7, 7.5 Hz, 1H), 2.75 (d, J = 7.4 Hz, 1H), 2.52 (dd, J = 14.0,
1
3
1
.2 Hz, 1H), 2.25 (d, J = 13.9 Hz, 1H), 1.87 (s, 3H), 1.53 (s, 3H), 0.99 (s, 3H); C{ H} NMR (125
MHz, d
5
-pyridine) δ 161.6, 158.7, 155.6, 154.9, 140.1, 111.1, 111.0, 105.1, 98.7, 85.6, 73.4, 57.2,
4
8.7, 39.0, 38.7, 36.6, 34.5, 29.9, 19.0; IR (neat) 3289, 2972, 2936, 1683, 1605, 1562, 1446, 1405,
+
1
357, 1283, 1253, 1146, 1107; HRMS (ESI) calculated for C19
H
21
O 329.1384 [M+H] , found:
5
329.1384.
Deoxyisobruceol (4). A solution of 8 (43.3 mg, 0.139 mmol) in pyridine (1 mL) was heated to 150
C by microwave for 30 min. The solvent was removed in vacuo and the residue was purified by
°
flash column chromatography on SiO
2
(3:1 petrol/EtOAc) to afford deoxyisobruceol (4) as a
1
colourless oil (33.5 mg, 77%). Data for 4: R
f
0.35 (2:1 petrol/EtOAc); H NMR (500 MHz, CDCl
3
)
δ 7.86 (d, J = 9.5 Hz, 1H), 6.44 (s, 1H), 6.11 (d, J = 9.5 Hz, 1H), 2.90 – 2.84 (m, 1H), 2.24 (ddd, J =
13.4, 4.7, 3.2 Hz, 1H), 2.13 (ddd, J = 11.6, 5.4, 2.8 Hz, 1H), 1.91 (dd, J = 13.4, 1.7 Hz, 1H), 1.81
(ddd, J = 14.9, 5.6, 3.4 Hz, 1H), 1.60 (s, 3H), 1.47 (ddd, J = 15.1, 13.1, 6.8 Hz, 1H), 1.42 (s, 3H),
2
4
ACS Paragon Plus Environment

Page 25 of 40
1
The Journal of Organic Chemistry
1
3
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1.30 (dt, J = 12.8, 6.0 Hz, 1H), 1.07 (s, 3H), 0.64 (tdd, J = 13.4, 11.6, 6.1 Hz, 1H); C{ H} NMR
125 MHz, CDCl ) δ 162.0, 159.8, 154.6, 153.6, 138.3, 112.4, 110.7, 105.1, 96.9, 86.5, 76.4, 46.2,
7.3, 34.8, 29.6, 28.8, 28.0, 24.2, 22.0; IR (neat) 2975, 2933, 1728, 1616, 1568, 1445, 1355, 1258,
(
3
3
+
1138, 1125, 1071; HRMS (ESI) calculated for C19
H
20
4
O Na 335.1254 [M+Na] , found: 335.1259.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Eriobrucinol (6). A solution of 8 (25 mg, 0.80 mmol) in THF (2 mL) was placed in direct sunlight
for 6 h. The solvent was removed and the residue was purified by flash column chromatography on
SiO
.25 (2:1 petrol/EtOAc); mp 163 – 165 °C (from toluene) (lit. = 185 °C); H NMR (500 MHz,
CDCl ) δ 7.98 (d, J = 9.7 Hz, 1H), 6.46 (s, 1H), 6.15 (d, J = 9.6 Hz, 1H), 5.40 (s, 1H), 3.08 (d, J =
.7 Hz, 1H), 2.66 (dd, J = 9.7, 7.6 Hz, 1H), 2.48 (t, J = 7.5 Hz, 1H), 1.96 – 1.87 (m, 1H), 1.75 –
1.67 (m, 1H, overlapped), 1.75 – 1.61 (m, 2H, overlapped), 1.45 (s, 3H), 1.41 (s, 3H), 0.79 (s, 3H);
2
(2:1 petrol/EtOAc) to afford eriobrucinol (6) as a yellow oil (18.9 mg, 76%). Data for 6: R
f
1
0
1
0
3
9
13
1
C{ H} NMR (150 MHz, CDCl
3
) δ 162.1, 157.8, 154.6, 151.3, 138.9, 111.0, 107.1, 103.2, 99.0,
8
4.7, 46.4, 39.1, 38.7, 37.3, 35.6, 34.6, 27.4, 25.7, 18.2; IR (neat) 3225, 2949, 1690, 1615, 1445,
1205, 1355, 1278, 1137, 1109, 1082; HRMS (ESI) calculated for C19
H
21
O
313.1434 [M+H]+,
4
found: 313.1438.
Iso-eriobrucinol A (5). A solution of protobruceol-I (7) (80.0 mg, 0.256 mmol) in CHCl
3
(2 mL)
was placed in direct sunlight for 5 h. The solvent was removed and the residue was purified by flash
column chromatography on SiO
42.5 mg, 53%). Data for 5: R
2
(2:1 petrol/EtOAc) to afford isoeriobrucinol A (5) as a white solid
1
(
f
0.30 (neat Et
2
O); H NMR (500 MHz, d
5
-pyridine) δ 8.09 (d, J = 9.6
Hz, 1H), 6.61 (s, 1H), 6.27 (d, J = 9.6 Hz, 1H), 4.92 (br s, 1H), 3.34 (d, J = 9.6 Hz, 1H), 2.56 (dd, J
= 9.6, 7.5 Hz, 1H), 2.35 (t, J = 7.5 Hz, 1H), 2.04 (td, J = 12.6, 7.5 Hz, 1H), 1.72 – 1.64 (m,
overlapped, 2H), 1.57 (ddd, J = 13.7, 12.3, 7.6 Hz, 1H), 1.49 (s, 3H), 1.47 (s, 3H), 0.98 (s, 3H);
1
3
1
C{ H} NMR (125 MHz, d
5
-pyridine) δ 162.2, 162.1, 156.1, 151.7, 140.0, 110.2, 109.2, 104.2,
95.6, 85.4, 47.1, 39.9, 39.2, 38.5, 37.0, 34.3, 27.9, 26.4, 18.5; IR (neat) 3212, 2959, 1679, 1595,
1
3
572, 1440, 1397, 1366, 1258, 1229, 1137, 1084; HRMS (ESI) calculated for C19
H
20
O Na
4
+
35.1254 [M+Na] , found: 335.1261.
2
5
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 26 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Iso-eriobrucinol B (43). A suspension of 15 (60.0 mg, 0.192 mmol) in CHCl (2 mL) was placed
3
in direct sunlight for a total of 40 h over the course of 5 days. The mixture was diluted with CHCl
3
(3 mL) and filtered to afford isoeriobrucinol B (43) as a pale yellow solid (19.9 mg, 33%). Data for
1
43: R
f
0.30 (neat Et
2
O); H NMR (500 MHz, d -pyridine) δ 8.31 (d, J = 9.5 Hz, 1H), 6.63 (s, 1H),
5
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
6
1
.29 (d, J = 9.5 Hz, 1H), 3.30 (d, J = 9.6 Hz, 1H), 2.50 (dd, J = 9.7, 7.3 Hz, 1H), 2.31 (t, J =7.4 Hz,
H), 2.02 (td, J = 12.8, 7.8 Hz, 1H), 1.64 – 1.57 (m, 1H), 1.56 (s, 3H), 1.53 – 1.45 (m, 1H), 1.37 (s,
3H), 0.87 (s, 3H); 13C{ H} NMR (125 MHz, d
1
-pyridine) δ 161.8, 158.4, 155.9, 155.5, 140.5,
5
1
2
3
10.4, 104.8, 103.7, 100.7, 85.3, 47.2, 39.6, 39.0, 37.9, 36.2, 34.2, 28.0, 26.2, 18.6; IR (neat) 3331,
938, 1686, 1606, 1498, 1455, 1356, 1263, 1135, 1108; HRMS (ESI) calculated for C19
H
21
O
4
+
13.1434 [M+H] , found: 313.1435.
Prenyl ether 52. To a solution of protobruceol (7) (999 mg, 3.20 mmol) in DMF (40 mL) at 0 °C
was added K CO (1.33 g, 9.59 mmol), followed by prenyl bromide (0.41 mL, 3.5 mmol). The
2
3
reaction was stirred at 0 °C for 5 min, then warmed to room temperature and stirred for a further 2 h.
The mixture was diluted with brine (50 mL) and extracted with EtOAc (5 × 50 mL). The combined
organic extracts were washed with brine (5 × 50 mL), dried over MgSO
in vacuo. The residue was purified by flash column chromatography on SiO
afford 52 as a pale yellow oil (900 mg, 74%). Data for 52: R
500 MHz, CDCl
4
, filtered and concentrated
2
(8:1 petrol/EtOAc) to
1
f
0.45 (5:1 petrol/EtOAc); H NMR
(
3
) δ 7.94 (d, J = 9.6 Hz, 1H), 6.67 (d, J = 10.1 Hz, 1H), 6.14 (d, J = 9.6 Hz, 1H),
.48 (d, J = 10.2 Hz, 1H), 5.45 (t, J = 6.6, 1H), 5.08 (t, J = 7.3 Hz, 1H), 4.57 (d, J = 6.6 Hz, 2H),
.15 – 2.07 (m, 2H), 1.80 (s, 3H), 1.80 – 1.67 (m, overlapped, 2H), 1.76 (s, 3H), 1.65 (s, 3H), 1.55
5
2
1
3
1
(s, 3H), 1.42 (s, 3H); C{ H} NMR (125 MHz, CDCl
3
) δ 161.8, 157.7, 155.9, 150.6, 138.83,
1
2
38.75, 132.1, 126.5, 123.9, 119.0, 116.9, 111.1, 106.7, 103.6, 92.6, 80.3, 65.9, 41.3, 26.6, 26.0,
5.8, 22.8, 18.5, 17.8; IR (neat) 2791, 2915, 1733, 1614, 1597, 1437, 1377, 1137, 1107; HRMS
+
(ESI) calculated for C24
H
29
O
4
381.2060 [M+H] , found 381.2062.
Chromene 56 and Citran 58. A solution of 52 (44.6 mg, 0.117 mmol) in pyridine (5 mL) was
heated to 150 °C in a microwave reactor for 1 h. The solvent was removed in vacuo and the residue
2
6
ACS Paragon Plus Environment

Page 27 of 40
1
The Journal of Organic Chemistry
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
was purified by flash column chromatography on SiO
2
(8:1 ® 3:1 petrol/EtOAc, gradient elution)
to afford chromene 56 as a clear and colourless oil (8.3 mg, 19%). Further elution gave citran 58 as
a light yellow solid (13.5 mg, 30%).
1
Data for 56. R
f
0.40 (5:1 petrol/EtOAc); H NMR (600 MHz, CDCl
3
) δ 7.92 (d, J = 9.6 Hz, 1H),
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
6
.48 (d, J = 10.0 Hz, 1H), 6.07 (d, J = 9.7 Hz, 1H), 5.50 (d, J = 10.0 Hz, 1H), 5.08 (t, J = 7.2 Hz,
H), 4.48 (qd, J = 6.6, 1.8 Hz, 1H), 2.10 (tt, J = 11.3, 6.5 Hz, 2H), 1.82 – 1.67 (m, 2H), 1.65 (s, 3H),
1
1.56 (s, 3H), 1.52 (d, J = 1.5 Hz, 3H), 1.42 (s, 3H), 1.39 (d, J = 6.6 Hz, 3H), 1.25 (d, J = 2.6 Hz,
13
1
3
H); C{ H} NMR (150 MHz, CDCl
3
) δ 161.42, 161.41, 158.22, 158.21, 151.2, 150.58, 150.56,
1
39.4, 132.1, 127.0, 126.9, 123.89, 123.88, 116.47, 116.46, 114.24, 114.23, 109.9, 103.60, 103.59,
101.79, 101.77, 91.17, 91.16, 80.5, 44.1, 41.4, 41.3, 26.68, 26.65, 25.8, 25.7, 25.6, 22.81, 22.78,
13
2
1.4, 21.3, 17.8, 14.40, 14.37 (Note: more than 24 C signals are observed as 56 is a 1:1 mixture of
diastereoisomers); IR (neat) 2968, 2926, 1733, 1623, 1603, 1584, 1435, 1380, 1345, 1313, 1136,
+
1
093, 1093; HRMS (ESI) calculated for C24
Data for 58. R
MHz, CDCl ) δ 7.87 (d, J = 9.5 Hz, 1H), 6.31 (dd, J = 17.4, 10.6 Hz, 1H), 6.08 (d, J = 9.5 Hz, 1H),
4.90 (dd, J = 17.4, 1.2 Hz, 1H), 4.86 (dd, J = 10.6, 1.3 Hz, 1H), 2.88 – 2.84 (m, 1H), 2.18 (ddd, J =
3.2, 4.8, 3.1 Hz, 1H), 2.08 (ddd, J = 11.7, 5.4, 2.7 Hz, 1H), 1.85 (dd, J = 13.2, 1.6 Hz, 1H), 1.76
ddd, J = 15.2, 5.1, 2.9 Hz, 1H), 1.66 (s, 3H), 1.65 (s, 4H), 1.58 (s, 3H), 1.47 – 1.41 (m, overlapped,
H), 1.40 (s, 3H), 1.29 (dt, J = 12.8, 5.9 Hz, 1H), 1.06 (s, 3H), 0.64 (tdd, J = 13.5, 11.7, 6.2 Hz,
H
29
O
4
381.2060 [M+H] , found 381.2062.
1
f
0.25 (5:1 petrol/EtOAc); mp 170.9 – 172.3 °C (prisms from EtOAc); H NMR (600
3
1
(
1
1
3
1
1H); C{ H} NMR (150 MHz, CDCl
3
) δ 161.9, 158.9, 152.4, 151.5, 149.7, 138.8, 115.3, 112.2,
1
10.1, 108.3, 105.1, 85.6, 76.2, 46.7, 41.1, 37.4, 34.9, 29.7, 29.6, 29.4, 28.9, 28.5, 24.2, 22.2; IR
(neat) 2975, 2931, 1707, 1638, 1596, 1561, 1458, 1428, 1335, 1310, 1273, 1166, 1136; HRMS
+
(ESI) calculated for C24
H
29
O
4
381.2060 [M+H] , found 381.2063.
Prenyl ether 59. To a solution of 8 (151 mg, 0.484 mmol) in DMF (10 mL) at 0 °C was added
CO (201 mg, 1.45 mmol), followed by prenyl bromide (0.06 mL, 0.53 mmol). The reaction
mixture was stirred at 0 °C for 5 min, then warmed to room temperature and stirred for a further 1 h.
K
2
3
2
7
ACS Paragon Plus Environment

The Journal of Organic Chemistry
Page 28 of 40
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
The reaction was quenched with 1 M HCl (2 mL) and diluted with water (10 mL). The mixture was
extracted with Et O (3 × 15 mL) and the combined organic extracts were washed with brine (5 × 15
mL), dried over MgSO , filtered, and concentrated in vacuo. The residue was purified by flash
column chromatography on SiO
2
4
2
(8:1 petrol/EtOAc) to afford 59 as a pale yellow oil (165 mg,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
8
1
9%). Data for 59: R
f
0.35 (5:1 petrol/EtOAc); H NMR (600 MHz, CDCl
3
) δ 7.84 (d, J = 9.6 Hz,
H), 6.62 (d, J = 10.1 Hz, 1H), 6.53 (s, 1H), 6.17 (d, J = 9.6 Hz, 1H), 5.64 (d, J = 10.1 Hz, 1H),
5.51 (t, J = 7.5 Hz, 1H), 5.07 (t, J = 6.7 Hz, 1H), 4.46 (d, J = 7.4 Hz, 2H), 2.14 – 2.03 (m, 2H), 1.80
–
1
1
1.74 (m, overlapped, 1H), 1.76 (s, 3H), 1.68 – 1.62 (m, overlapped, 1H), 1.65 (s, 3H), 1.60 (s, 3H),
1
3
1
.56 (s, 3H), 1.42 (s, 3H); C{ H} NMR (150 MHz, CDCl
3
) δ 161.4, 158.0, 155.7, 151.9, 140.2,
39.3, 132.2, 129.4, 123.7, 119.2, 116.9, 112.0, 111.9, 108.1, 100.6, 80.0, 73.0, 41.6, 26.9, 26.0,
25.8, 22.8, 18.2, 17.8; IR (neat) 2973, 1736, 1615, 1599, 1564, 1449, 1380, 1367, 1316, 1196, 1136,
+
1
079; HRMS (ESI) calculated for C24
H
29
O
4
381.2060 [M+H] , found 381.2061
Citran 64. A solution of 59 (23.5 mg, 0.0617 mmol) in pyridine (5 mL) was heated to 150 °C in a
microwave reactor for 30 min. The solvent was removed in vacuo, and the residue was purified by
flash column chromatography on SiO
2
(5:1 petrol/EtOAc) to afford 64 as a pale yellow oil (16.3 mg,
1
6
9%). Data for 64: R
f
0.20 (5:1 petrol/EtOAc); H NMR (600 MHz, CDCl
3
) δ 7.86 (d, J = 9.5 Hz,
1H), 6.10 (d, J = 9.4 Hz, 1H), 5.27 (t, J = 7.7 Hz, 1H), 3.43 (d, J = 7.5 Hz, 2H), 2.86 (br s, 1H), 2.24
ddd, J = 13.3, 4.8, 3.2 Hz, 1H), 2.09 (ddd, J = 11.7, 5.5, 2.8 Hz, 1H), 1.88 (dd, J = 13.3, 1.6 Hz,
(
1
H), 1.82 (s, 3H), 1.74 (ddd, J = 14.9, 5.5, 2.7 Hz, 1H), 1.67 (s, 3H), 1.58 (s, 3H), 1.46 (ddd, J =
3.2, 8.7, 6.7 Hz, 1H), 1.43 (s, 3H), 1.29 – 1.24 (m, 1H), 1.03 (s, 3H), 0.59 (tdd, J = 13.5, 11.7, 6.2
1
1
3
1
Hz, 1H); C{ H} NMR (125 MHz, CDCl
3
) δ 162.3, 157.6, 151.7, 151.3, 138.6, 131.8, 122.2, 112.1,
1
10.5, 110.0, 105.1, 86.0, 76.1, 46.6, 37.5, 34.9, 29.7, 29.0, 28.3, 26.0, 24.2, 22.2, 21.4, 18.1; IR
(neat) 2975, 2927, 1724, 1606, 1573, 1442, 1338, 1270, 1239, 1214, 1187, 1166, 1142, 1124;
+
HRMS (ESI) calculated for C24
H
29
O
4
381.2060 [M+H] , found 381.2062.
Cyclol 65. A solution of 59 (28.7 mg, 0.0754 mmol) in toluene (10 mL) was heated to 150 °C in a
microwave reactor for 1.5 h. The solvent was removed in vacuo and the residue was purified by
2
8
ACS Paragon Plus Environment

Page 29 of 40
1
The Journal of Organic Chemistry
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
flash column chromatography on SiO
2
(5:1® 3:1 petrol/EtOAc) to afford 65 as a yellow solid (12.7
1
mg, 44%). Data for 65: R
f
0.10 (5:1 petrol/EtOAc); H NMR (600 MHz, CDCl
3
) δ 7.94 (d, J = 9.6
Hz, 1H), 6.16 (d, J = 9.6 Hz, 1H), 5.22 (t, J = 7.5 Hz, 1H), 4.64 (br s, 1H), 3.45 (d, J = 7.4 Hz, 2H),
.03 (d, J = 9.7 Hz, 1H), 2.67 (t, J = 8.5 Hz, 1H), 2.47 (t, J = 7.2 Hz, 1H), 1.91 – 1.86 (m, 1H), 1.83
s, 3H), 1.73 – 1.64 (m, overlapped, 3H), 1.66 (s, 3H), 1.45 (s, 3H), 1.44 (s, 3H), 0.77 (s, 3H);
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(
1
3
1
C{ H} NMR (150 MHz, CDCl ) δ 162.0, 155.1, 152.0, 148.8, 138.8, 131.7, 122.1, 111.5, 111.1,
3
106.3, 102.8, 84.5, 46.3, 39.0, 38.5, 37.2, 35.9, 34.7, 27.5, 26.0, 25.7, 21.8, 18.3, 18.2; HRMS (ESI)
+
calculated for C24
H
29
O
4
381.2060 [M+H] , found 381.2059.
Bromination of 7. To a solution of 7 (195 mg, 0.624 mmol) in Et
mg, 1.87 mmol) at room temperature for 10 min. The reaction mixture was quenched with Na
filtered through Celite, flushed with Et O (20 mL) and concentrated in vacuo, and the residue was
purified by flash chromatography on SiO (8:1 ® 3:1 petrol/EtOAc, gradient elution) to afford 69b
(23 mg, 7%), 69a (68 mg, 23%), 68b (41 mg, 12%) and 68a (111 mg, 38%).
Data for 69b: R 0.45 (2:1 petrol/EtOAc); mp 206 – 208 °C (white needles from petrol/EtOAc); H
NMR (500 MHz, CDCl ) δ 8.20 (s, 1H), 4.71 (dd, J = 4.4, 2.0 Hz, 1H), 3.11 (dd, J = 4.4, 2.7 Hz,
1H), 2.75 (ddd, J = 11.7, 5.5, 2.7 Hz, 1H), 2.10 (ddd, J = 15.7, 13.0, 7.1 Hz, 1H), 1.74 (dd, J = 15.6,
2
O (10 mL) was added NBS (333
2
S
2
3
O ,
2
2
1
f
3
6
.5 Hz, 1H), 1.63 (s, 3H), 1.61 (s, 3H), 1.35 (dt, J = 13.2, 6.3 Hz, 1H), 1.13 (s, 3H), 0.63 (ddt, 1H);
1
3
1
C{ H} NMR (125 MHz, CDCl
3
) δ 159.4, 159.1, 153.5, 152.5, 141.6, 115.4, 109.5, 108.0, 94.3,
9
1.1, 82.2, 55.5, 44.1, 38.7, 34.4, 31.8, 30.1, 27.1, 23.8; IR (neat) 2979, 2939, 1739, 1611, 1554,
1454, 1433, 1372, 1433, 1372, 1348, 1295, 1240, 1117; HRMS (ESI) calculated for C19
H18Br
3
O
4
+
5
46.8750 [M+H] , found: 546.8746.
1
Data for 69a: R
f
0.35 (2:1 petrol/EtOAc); mp 210 – 212 °C (white needles from petrol/EtOAc); H
NMR (500 MHz, CDCl
Hz, 1H), 3.11 (dd, J = 4.5, 2.7 Hz, 1H), 2.75 (ddd, J = 11.8, 5.5, 2.6 Hz, 1H), 2.10 (ddd, J = 15.6,
3.0, 7.1 Hz, 1H), 1.73 (dd, J = 15.6, 6.5 Hz, 1H), 1.64 (s, 3H), 1.59 (s, 3H), 1.35 (dt, J = 13.3, 6.3
3
) δ 7.87 (d, J = 9.7 Hz, 1H), 6.21 (d, J = 9.6 Hz, 1H), 4.72 (dd, J = 4.5, 2.1
1
1
3
1
Hz, 1H), 1.14 (s, 3H), 0.67 (tdd, J = 14.0, 12.4, 6.4 Hz, 1H); C{ H} NMR (125 MHz, CDCl ) δ
3
2
9
ACS Paragon Plus Environment