Total synthesis of rhodonoids a, b, e, and f, enabled by singlet oxygen ene reactions

Article abstract of DOI:10.1021/acs.joc.9b02968

Singlet oxygen is a versatile reagent for the selective oxidation of organic compounds under mild reaction conditions. It is frequently invoked in biosynthetic pathways, so it is especially suitable for application in the biomimetic synthesis of natural products. Herein, we show that use of the singlet oxygen ene reaction, combined with [2 + 2] cycloadditions, leads to concise, divergent, and redox-economic total syntheses of several polycyclic members of the rhodonoid family of meroterpenoids.

Full text of DOI:10.1021/acs.joc.9b02968

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Article
Total Synthesis of Rhodonoids A, B, E and
F, Enabled by Singlet Oxygen Ene Reactions
Laura Burchill, and Jonathan H. George
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b02968 • Publication Date (Web): 03 Jan 2020
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Total Synthesis of Rhodonoids A, B, E and F, Enabled by Singlet Oxygen Ene Reactions
Laura Burchill and Jonathan H. George*
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Department of Chemistry, The University of Adelaide, Adelaide, South Australia 5005, Australia.
jonathan.george@adelaide.edu.au
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Abstract
HO
OH
H
1O2
O
H
2 steps
O
O
rhodonoid A
HO
HO
HO
HO
HO
H
H
H
H
H
H
1O2
H
O
H
O
H
O
+
1 step
rhodonoid E
rhodonoid F
Singlet oxygen is a versatile reagent for the selective oxidation of organic compounds under mild
reaction conditions. It is frequently invoked in biosynthetic pathways, so it is especially suitable for
application in the biomimetic synthesis of natural products. Herein, we show that use of the singlet
oxygen ene reaction, combined with [2+2] cycloadditions, leads to concise, divergent and redox-
economic total syntheses of several polycyclic members of the rhodonoid family of meroterpenoids.
Introduction
Rhodonoids A-G (1-7, Figure 1) are a family of polycyclic meroterpenoids isolated from
Rhododendron capitatum.1 Despite their sterecochemically rich structures, the rhodonoids were all
found as partial racemates. This implies that key steps in their biosynthesis might not be under
enzymatic control, which makes the rhodonoids attractive targets for biomimetic synthesis.2 We
have previously reported a divergent synthesis of rhodonoids C and D via a biomimetic cascade
reaction that involves ring opening of an epoxide intermediate. 3 A similar strategy was
independently developed by Hsung and Tang,4 who also used epoxide ring opening chemistry in
their total syntheses of rhodonoids A, B, E and F.5 Herein, we propose that singlet oxygen ene
reactions6 are involved in the biosynthesis of rhodonoids A, B, E and F. Application of these
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photooxygenation reactions in a biomimetic strategy has enabled a concise route to rhodonoid
natural products that minimises redox reactions and protecting group operations.
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9
O
O
HO
HO
OH
H
OH
H
H
H
H
O
O
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H
H
O
H
O
O
O
1: rhodonoid A
2: rhodonoid B
3: rhodonoid C
4: rhodonoid D
HO
HO
OH
HO
H
HO
OH
H
H
H
H
H
H
O
H
O
O
5: rhodonoid E
6: rhodonoid F
7: rhodonoid G
Figure 1. Rhodonoids A-G, partially racemic meroterpenoids from Rhododendron capitatum.
Results and Discussion
Chromene 8 has already been shown to the be the likely biosynthetic precursor of rhodonoids C and
D (3 and 4) via biomimetic synthesis,3,4 so we thought it might also be the precursor of rhodonoid A
(1) (Scheme 1). A singlet oxygen ene reaction (often called the Schenck ene reaction) of the prenyl
side chain of chromene 8 could give hydroperoxide 9 as an inconsequential mixture of
diastereomers. A Kornblum-DeLaMare rearrangement7 of 9 would give the enone 10, which could
8
undergo an intramolecular [2+2] photocycloaddition to give rhodonoid A (1). A [2+2]
photocycloaddition of a protected version of 10 was used in Hsung and Tang’s total synthesis of 1.5
OH
O
OH
OH
1O2
Schenck ene
reaction
O
O
8
9
Kornblum-DeLaMare
rearrangement
− H2O
OH
HO
H
[2+2]
photocycloaddition
O
O
H
O
O
1: rhodonoid A
10
Scheme 1. Proposed Biosynthesis of Rhodonoid A
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Given that exposure to light is necessary for both the generation of singlet oxygen (required for the
Schenck ene reaction, 8 → 9) and the intramolecular [2+2] cycloaddition (10 → 1), we hoped to
develop a one-pot, photochemical method for the direct conversion of 8 into 1. Although this goal
has proven elusive, we have developed an efficient two-step sequence to convert 8 into 1 (Scheme
2). Firstly, a one-pot Schenck ene reaction and subsequent Kornblum-DeLaMare rearrangement of
chromene 89 was achieved using visible light and tetraphenylporphyrin (TPP) as the sensitizer to
generate singlet oxygen, with excess pyridine, Ac2O and catalytic DMAP to activate the
hydroperoxide rearrangement. The C-5 phenol was also acetylated during this step to give 13 in
51% yield. These reaction conditions were first reported by Laroche and Nay in their studies on the
biomimetic photooxygenation of resinic diterpenes.10 No [2+2] photocycloaddition of 13 was
observed under these reaction conditions with visible light. Enone 13 was therefore dissolved in
MeOH and exposed to UV light to form 14, which was hydrolysed in the same pot by addition of
K2CO3, giving rhodonoid A (1) in 60% yield.5 This two-step (31% overall yield) sequence
compares favourably to the previous synthesis of rhodonoid A by Hsung and Tang, which required
six steps (11% overall yield) to convert 8 into 1.5
10
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OH
OAc
O2, TPP, Ac2O, DMAP
pyridine, CH2Cl2, visible light
O
O
8
11
synthesised in 1 step
(ref. 9)
Schenck ene
reaction
1O2
OH
O
OAc
OAc
5
Kornblum-DeLaMare
rearrangement
O
51%
O
O
12
13
MeOH
UV light
[2+2] photocycloaddition
AcO
H
HO
H
O
O
K2CO3, MeOH
60%
H
H
O
O
14
1: rhodonoid A
Scheme 2. Biomimetic Total Synthesis of Rhodonoid A
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Singlet oxygen ene reactions were also invoked in the biosynthesis of rhodonoids B, E and F
(Scheme 3). Chromene 15 (as a mixture of E and Z alkene isomers) could undergo a [2+2]
photocycloaddition to give the tetracyclic diastereomers 16 and 17 (epimers at C-12). Singlet
oxygen ene reaction of tetracycle 17 could form the diastereomeric hydroperoxides 18 and 19,
reduction of which would give rhodonoid E (5) and rhodonoid F (6) respectively. Rhodonoid B (2)
could arise from singlet oxygen ene reaction of tetracycle 16 to give peroxy radical 20. This
reactive radical intermediate could undergo a Schenck ene rearrangement11 and H atom abstraction
to give hydroperoxide 21, followed by Kornblum-DeLaMare rearrangement to give 2.
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HOO
HOO
OH
HO
HO
HO
HO
12
12
[2+2]
photocycloaddition
H
H
H
H
H
H
H
H
1O2
H
+
H
H
O
+
H
O
Schenck ene
reaction
O
O
O
15
16
17
18
19
Schenck ene
reaction
reduction
reduction
1O2
O
OH
H
HO
H
HO
H
O
O
O
HO
HO
HO
HO
HO
Kornblum-DeLaMare
rearrangement
Schenck ene
rearrangement
H
H
H
H
H
H
H
+
H
O
H
O
H
O
H
O
H
O
− H2O
+ H
2: rhodonoid B
21
20
5: rhodonoid E
6: rhodonoid F
Scheme 3. Proposed Biosynthesis of Rhodonoids B, E, and F
Divergent synthesis of tetracycles 16 and 17 was achieved using Hsung and Tang’s Fe(OTf)3-
mediated cationic [2+2] cyclization. Firstly, orcinol (22) was condensed with technical grade
farnesal (23, a complex mixture of isomers) in the presence of EDDA to give chromene 15 as a
1:1.4 mixture of cis and trans isomers (Scheme 4).12 These cis and trans alkenes were previously
prepared as by Hsung and Tang as pure stereoisomers, but we carried out a divergent intramolecular
[2+2] cycloaddition on the mixture to give 16 and 17 in 28% and 43% yield respectively.
Tetracycles 16 and 17 were separable by careful flash chromatography on silica gel. Attempted
[2+2] photocycloaddition of 15 using UV light resulted in decomposition.
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O
H
23: farnesal
OH
OH
EDDA, PhMe, 110 °C
5
6
7
84%
O
OH
22: orcinol
15
1:1.4 mixture of cis
and trans alkenes
8
9
Fe(OTf)3
CH2Cl2, rt
[2+2]
10
11
12
13
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15
16
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HO
H
HO
H
H
H
H
+
H
O
O
16
17
43%
28%
Scheme 4. Divergent Synthesis of Tetracycles 16 and 175
Visible light-mediated singlet oxygen ene reaction of tetracycle 17 using TPP as the sensitizer
followed by reduction of the hydroperoxide intermediates with PPh3 gave rhodonoid E (5) in 23%
yield, rhodonoid F (6) in 16% yield, and tertiary alcohol 24 in 40% yield (Scheme 5). These
compounds were separable by careful flash chromatography using silica gel doped with silver
nitrate.13 If our biosynthetic speculation on the origin of rhodonoids E and F is correct, then 24 is a
plausible “undiscovered natural product”. Previously, Hsung and Tang converted 17 into 5 and 6
(28% and 26% overall yield, respectively) via a four-step sequence involving ring opening of an
epoxide intermediate.4
HO
HO
OH
HO
HO
HO
HO
H
H
H
H
H
TPP, CH2Cl2, light
O2, rt, then; PPh3
H
H
H
H
O
H
O
H
O
H
O
+
+
Schenck ene
reaction,
reduction
17
5: rhodonoid E
6: rhodonoid F
24
40%
23%
16%
Scheme 5. Biomimetic Total Synthesis of Rhodonoids E and F
Finally, we used singlet oxygen chemistry to enable a formal synthesis of rhodonoid B (2) via the
generation of tertiary alcohol 27, which has been converted into 2 by Hsung and Tang in two
further steps.5 First, tetracycle 16 was acetylated under standard conditions to give 25. Singlet
oxygen ene reaction of 25 followed by reduction of the hydroperoxide products using PPh3 gave 27
in 45% yield, alongside an inseparable mixture of secondary allylic alcohol diastereomers 26 in
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44% yield (Scheme 6). Hsung and Tang’s previous synthesis of rhodonoid B required seven steps
4
5
(37% yield) to convert 16 into 27, compared to our two-step protocol (36% yield).5
6
OH
7
HO
8
9
HO
AcO
AcO
AcO
Ac2O, DMAP
CH2Cl2, rt
TPP, CH2Cl2, light
O2, rt, then; PPh3
H
H
H
H
H
H
H
H
10
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12
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+
H
O
H
O
H
O
H
O
81%
Schenck ene
reaction,
reduction
16
25
26
44%
1:1 mixture of
diastereomers
27
45%
2 steps to rhodonoid B (2)
(Hsung/Tang, ref. 5)
Scheme 6. Formal Total Synthesis of Rhodonoid B
Conclusion
We have significantly shortened the step count of previous total syntheses of rhodonoids A, B, E
and F through the strategic use of singlet oxygen as a mild and selective oxidant. The 3-step total
synthesis of rhodonoid A features a one-pot singlet ene reaction and Kornblum-DeLaMare
rearrangement, followed by a [2+2] photocycloaddition, to rapidly assemble a complex polycyclic
framework.
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 N2 unless otherwise stated. All
organic extracts were 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 Rf values
were measured 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 (1H at 600 MHz, 13C at 150 MHz) or a 500 MHz
spectrometer (1H at 500 MHz, C at 125 MHz). The solvent used for NMR spectra was CDCl3
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unless otherwise specified. 1H chemical shifts are reported in ppm on the δ-scale relative to TMS (δ
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0.0) and C{1H} NMR are reported in ppm relative to chloroform (δ 77.16). Multiplicities are
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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. Photochemistry with UV light was performed using a generic brand commercial
LED UV light globe; wavelength: 365 nm. Photochemical reactions with visible light were
performed with a conventional commercial LED desk lamp at 240 V with a 4 W 5000 K 32 mA
globe.
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2,7-dimethyl-2-(4-methylpent-3-en-1-yl)-2H-chromen-5-ol (8). To
a
solution of 4-
methylresorcinol (10.0 g, 80.6 mmol, 1.0 equiv) in PhMe (250 mL) at room temperature was added
citral (12.3 mL, 80.6 mmol, 1.0 equiv) and ethylenediamine diacetate (430 mg, 2.42 mmol, 0.03
equiv). The reaction was stirred at reflux for 3 h. The mixture was cooled to room temperature, then
concentrated in vacuo and purified via flash column chromatography on SiO2 (8:1, hexanes:EtOAc)
to afford chromene 8 as an orange oil (17.2 g, 66.6 mmol, 82%). Data for 8 matched that previously
reported in the literature.5 Data for 8: Rf 0.40 (5:1, hexanes:EtOAc); 1H NMR (500 MHz, CDCl3) δ
6.61 (d, J = 10.0 Hz, 1H), 6.24 (s, 1H), 6.11 (s, 1H), 5.49 (d, J = 10.0 Hz, 1H), 5.10 (t, J = 7.1 Hz,
1H), 4.71 (br s, 1H), 2.20 (s, 3H), 2.13 – 2.07 (m, 2H), 1.72 (dd, J = 10.7, 5.9 Hz, 1H), 1.66 (s, 3H),
13
1.66 – 1.63 (m, 1H), 1.58 (s, 3H), 1.37 (s, 3H); C{1H} NMR (125 MHz, CDCl3) δ 154.1, 151.0,
139.5, 131.6, 127.2, 124.2, 117.0, 109.9, 108.3, 106.7, 78.2, 41.1, 26.2, 25.7, 22.7, 21.5, 17.6.
2,7-dimethyl-2-(4-methyl-3-oxopent-4-en-1-yl)-2H-chromen-5-yl acetate (13). Using modified
conditions reported by Nay et al.,10 chromene 8 (450 mg, 1.80 mmol, 1.0 equiv) was dissolved in
CH2Cl2 (30 mL), followed by addition of tetraphenylporphyrin (TPP) (22 mg, 0.036 mmol, 0.02
equiv), pyridine (8.70 mL, 108 mmol, 60 equiv), Ac2O (17.0 mL, 180 mmol, 100 equiv) and
DMAP (4 mg, 0.036 mmol, 0.02 equiv). The reaction was stirred at room temperature in a
borosilicate glass test tube while exposed to visible light at a distance of 10 cm from the irradiation
vessel and with O2 bubbled through the solution for 36 h. The reaction was then quenched by
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addition of distilled H2O (40 mL). The organic phase was then separated, and the aqueous phase
further extracted with CH2Cl2 (2 x 40 mL). The combined organic layers were then washed with 0.5
M CuSO4(aq) (3 x 60 mL), dried over MgSO4, filtered and concentrated in vacuo. The residue was
purified via flash chromatography (neat CH2Cl2) to give the desired enone 13 (283 mg, 1.13 mmol,
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16
17
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51%) as a white solid. Data for 13: Rf 0.30 (9:1, hexanes:EtOAc); H NMR (600 MHz, CDCl3) δ
6.48 (s, 1H), 6.41 (s, 1H), 6.33 (d, J = 10.0 Hz, 1H), 5.92 (s, 1H), 5.71 (d, J = 1.6 Hz, 1H), 5.49 (d,
J = 10.0 Hz, 1H), 2.83 (dddd, J = 69.0, 17.1, 10.1, 5.4 Hz, 2H), 2.29 (s, 3H), 2.25 (s, 3H), 2.08 –
1.93 (m, 2H), 1.90 – 1.79 (m, 3H), 1.38 (s, 3H); 13C{1H} NMR (150 MHz, CDCl3) δ 201.6, 169.3,
153.8, 146.3, 144.3, 139.7, 128.6, 124.9, 117.4, 115.0, 114.7, 111.3, 78.3, 35.8, 32.5, 26.9, 21.6,
20.9, 17.7; IR (neat) 2925, 1769, 1676, 1451, 1190; HRMS (ESI-TOF) m/z: [M+H]+ calculated for
C19H23O4 315.1591; Found 315.1591.
Rhodonoid A (1). A solution of enone 13 (57 mg, 0.181 mmol, 1.0 equiv) in MeOH (3 mL) was
left in a borosilicate sealed vial placed within a water condenser and irradiated from beneath with
UV light at a distance of 5 cm for 24 h at room temperature. K2CO3 (62 mg, 0.450 mmol, 2.5 equiv)
was then added in one portion and the reaction was stirred for 1 h. H2O (5 mL) was added and the
product was extracted with CH2Cl2 (3 x 10 mL). The combined organic layers were then dried
under MgSO4, filtered and concentrated. Purification via flash chromatography (9:1,
hexanes:EtOAc) afforded (±)-rhodonoid A (1) as a white solid (30 mg, 0.110 mmol, 60%). Data for
(±)-rhodonoid A (1) matched that previously reported in literature.5 Data for 1: Rf 0.20 (8:2,
hexanes:EtOAc); mp 179.6 – 180.2 °C (MeOH) (lit. 180 – 181 °C); 1H NMR (600 MHz, CDCl3) δ
6.34 (s, 1H), 6.21 (s, 1H), 4.59 (s, 1H), 3.80 - 3.73 (m, 1H), 2.78 (ddd, J = 18.1, 10.9, 7.1 Hz, 1H),
2.59 (d, J = 9.5 Hz, 1H), 2.42 (ddd, J = 18.3, 6.8, 3.4 Hz, 1H), 2.39 – 2.33 (m, 1H), 2.33 – 2.30 (m,
1H), 2.22 (s, 3H), 2.16 (dd, J = 12.2, 7.1 Hz, 1H), 2.03 (ddd, J = 14.0, 10.9, 6.8 Hz, 1H), 1.44 (s,
3H), 1.16 (s, 3H); C{1H} NMR (150 MHz, CDCl3) δ 216.5, 154.1, 152.8, 137.5, 112.6, 112.0,
13
109.3, 73.5, 51.1, 43.9, 39.0, 34.1, 33.7, 25.4, 24.9, 21.9, 21.3; IR (neat) 2965, 1687, 1625, 1514,
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1420, 1187, 1069; HRMS (ESI-TOF) m/z: [M+H]+ calculated for C17H21O3 273.1485; Found
273.1485.
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6
7
8
9
2-(4,8-dimethylnona-3,7-dien-1-yl)-2,7-dimethyl-2H-chromen-5-ol (15). To a solution of 4-
methylresorcinol (12.9 g, 103.9 mmol, 1.0 equiv) in PhMe (300 mL) at room temperature was
added farnesal (mixture of isomers, 22.9 g, 103.9 mmol, 1.0 equiv) and ethylenediamine diacetate
(190 mg, 10.4 mmol, 0.10 equiv). The reaction was stirred at reflux for 16 h. The mixture was
cooled to room temperature, then concentrated in vacuo. Purification via flash column
chromatography on SiO2 (10:1, hexanes:EtOAc) then afforded chromene 15 as a 1:1.4 mixture of
cis:trans stereoisomers (28.6 g, 87.6 mmol, 84%). Data for 15 matched that previously reported in
the literature.5 Data for 15: Rf 0.50 (8:2, hexanes:EtOAc); 1H NMR (600 MHz, CDCl3) δ 6.60 (d, J
= 9.9 Hz, 1H), 6.24 (d, J = 4.0 Hz, 1H), 6.11 (s, 1H), 5.49 (dd, J = 10.0, 8.4 Hz, 1H), 5.17 – 5.04
(m, 2H), 4.70 (s, 1H), 2.20 (s, 3H), 2.15 – 1.94 (m, 6H), 1.76 – 1.70 (m, 2H), 1.67 (s, 3H), 1.62 –
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1.57 (m, 6H), 1.38 (s, 3H); C{1H} NMR (150 MHz, CDCl3) δ 154.3, 151.2, 139.7, 139.7, 135.5,
135.4, 131.5, 127.4, 127.3, 125.0, 124.5, 124.2, 116.9, 116.8, 110.0, 110.0, 108.4, 106.9, 78.4, 78.3,
41.5, 41.2, 39.8, 32.0, 26.8, 26.7, 26.4, 26.4, 25.9, 25.8, 23.5, 22.8, 22.6, 21.6, 17.8, 17.8, 16.1; IR
(neat) 2967, 1626, 1578, 1448, 1249, 1197, 1091; HRMS (ESI-TOF) m/z: [M+H]+ calculated for
C22H31O2 327.2319; Found 327.2319.
(1R,1aR,1a1S,3aS,8bS)-1,3a,6-trimethyl-1-(4-methylpent-3-en-1-yl)-1a,1a1,2,3,3a,8b-
hexahydro-1H-4-oxabenzo[f]cyclobuta[cd]inden-8-ol (16) and (1S,1aR,1a1,3aS,8bS)-1,3a,6-
trimethyl-1-(4-methylpent-3-en-1-yl)-1a,1a1,2,3,3a,8b-hexahydro-1H-4-
oxabenzo[f]cyclobuta[cd]inden-8-ol (17). Using a modified procedure from Tang et al.,5 Fe(OTf)3
(50 mg, 0.099 mmol, 0.3 equiv) was added in one portion to a solution of a 1:1.4 cis:trans mixture
of chromene 15 (100 mg, 0.306 mmol, 1.0 equiv) in CH2Cl2 (20 mL) under N2 at –78 ºC. The
reaction was warmed to room temperature and left to stir for 12 h, then quenched with sat.
NaHCO3(aq) (30 mL). The product was extracted with CH2Cl2 (3 x 20 mL) and the combined
organic extracts were washed with brine (60 mL). The organic phase was dried over MgSO4,
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filtered and concentrated in vacuo. Careful flash chromatography on silica gel (50:1,
hexanes:EtOAc) then afforded 17 as a white solid (42 mg, 0.131 mmol, 43%) followed by 16 as an
off white solid (28 mg, 0.086 mmol, 28%). Data for 16 and 17 matched that previously reported in
literature.5 Data for 16: Rf 0.20 (20:1, hexanes:EtOAc); 1H NMR (600 MHz, CDCl3) δ 6.32 (s, 1H),
6.18 (s, 1H), 4.94 (t, J = 4.9 Hz, 1H), 4.46 (br s, 1H), 3.12 (d, J = 9.7 Hz, 1H), 2.57 (dd, J = 9.8, 8.2
Hz, 1H), 2.42 (td, J = 8.3, 3.6 Hz, 1H), 2.22 (s, 3H), 2.05 (dt, J = 12.9, 7.5 Hz, 1H), 1.86 – 1.61 (m,
5H), 1.60 (s, 3H), 1.49 (s, 3H), 1.38 (s, 3H), 1.32 (s, 3H), 1.28 – 1.11 (m, 2H); 13C{1H} NMR (150
MHz, CDCl3) δ 154.7, 153.9, 137.5, 130.7, 125.4, 111.6, 108.7, 108.3, 83.7, 46.2, 42.4, 40.2, 38.6,
36.1, 31.2, 30.5, 26.4, 26.1, 25.8, 23.5, 21.4, 17.7; IR (neat) 2950, 1626, 1452, 1375, 1054; HRMS
(ESI-TOF) m/z: [M+H]+ calculated for C22H31O2 327.2319; Found 327.2319. Data for 17: Rf 0.20
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(20:1, hexanes:EtOAc); H NMR (600 MHz, CDCl3) δ 6.32 (s, 1H), 6.17 (s, 1H), 5.17 (t, J = 7.0
Hz, 1H), 4.48 (s, 1H), 3.06 (d, J = 9.6 Hz, 1H), 2.56 (dd, J = 9.6, 7.7 Hz, 1H), 2.47 (t, J = 7.5 Hz,
1H), 2.22 (s, 3H), 2.18 – 2.04 (m, 1H), 2.03 – 1.91 (m, 2H), 1.76 (ddd, J = 13.3, 11.6, 5.1 Hz, 1H),
1.71 (s, 3H), 1.70 – 1.59 (m, 4H), 1.63 (s, 3H), 1.36 (s, 3H), 0.76 (s, 3H); 13C{1H} NMR (150 MHz,
CDCl3) δ 154.6, 154.1, 137.6, 131.4, 125.1, 111.5, 108.5, 108.2, 83.5, 46.8, 44.4, 42.4, 39.1, 38.6,
35.6, 27.4, 25.9, 25.7, 22.9, 21.3, 17.8, 15.1; IR (neat) 2927, 1625, 1584, 1453, 1251, 1052; HRMS
(ESI-TOF) m/z: [M+H]+ calculated for C22H31O2 327.2319; Found 327.2318.
Rhodonoid E (5), rhodonoid F (6) and (1S,1aR,1a1S,3aS,8bS)-1-(((E)-4-hydroxy-4-methylpent-
2-en-1-yl)-1,3a,6-trimethyl)-1a,1a1,2,3,3a,8b-hexahydro-1H-4-oxabenzo[f]-cyclobuta[cd]inden-
8-ol (24). To a borosilicate glass test tube containing 17 (188 mg, 0.661 mmol, 1.0 equiv) in CH2Cl2
(20 mL) was added tetraphenylporphyrin (8 mg, 0.013 mmol, 0.02 equiv). O2 was bubbled through
the solution while it was stirred at room temperature and exposed to visible light at a distance of 10
cm from the irradiation vessel for 6 h. PPh3 (350 mg, 1.32 mmol, 2.0 equiv) was then added and the
reaction was stirred for a further 16 h under N2. The solution was then concentrated in vacuo and
purified via flash chromatography (9:1, CH2Cl2:EtOAc) to give (±)-rhodonoid E (5) as a white solid
(49 mg, 0.150 mmol, 23%) and a mixture of (±)-rhodonoid F (6) and allylic alcohol 24 (133 mg,
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0.407 mmol, 63%). The mixture was then further purified via flash chromatography using 1% w/w
AgNO3 impregnated SiO2 (7:3, hexanes:EtOAc) with early fractions containing allylic alcohol 24
(86 mg, 0.262 mmol, 40%) as a red oil and later fractions containing (±)-rhodonoid F (6) as a white
solid (35 mg, 0.107 mmol, 16%). Data for (±)-rhodonoid E (5) and (±)-rhodonoid F (6) matched
that previously reported.4 Data for (±)-rhodonoid E (5): Rf 0.40 (9:1, CH2Cl2:EtOAc); mp 123.7 –
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125.0 ºC (CHCl3) (lit. 86 – 87 ºC)3; H NMR (600 MHz, CDCl3) δ 6.32 (s, 1H), 6.16 (s, 1H), 4.99
(s, 1H), 4.87 (s, 1H), 4.61 (br s, 1H), 4.10 (t, J = 5.0 Hz, 1H), 3.09 (d, J = 9.6 Hz, 1H), 2.56 (dd, J =
9.6, 7.8 Hz, 1H), 2.43 (td, J = 7.8, 4.1 Hz, 1H), 2.22 (s, 3H), 2.07 – 1.94 (m, 1H), 1.76 (s, 3H), 1.73
– 1.59 (m, 6H), 1.54 – 1.49 (m, 1H), 1.35 (s, 3H), 0.73 (s, 3H); 13C{1H} NMR (150 MHz, CDCl3) δ
154.6, 154.1, 147.7, 137.6, 111.5, 111.3, 108.6, 108.2, 83.6, 76.6, 44.5, 42.3, 42.0, 39.2, 38.8, 35.3,
29.4, 27.3, 25.7, 21.4, 17.8, 15.3; IR (neat) 2925, 1621, 1415, 1259, 1118, 1055; HRMS (ESI-TOF)
m/z: [M+H]+ calculated for C22H31O3 343.2268; Found 343.2280. Data for (±)-rhodonoid F (6): Rf
0.20 (8:2, hexanes:EtOAc); mp 149.0 – 150.6 ºC (lit. 152 – 153 ºC)3; 1H NMR (600 MHz, CDCl3) δ
6.31 (s, 1H), 6.17 (s, 1H), 4.98 (s, 1H), 4.87 (s, 1H), 4.79 (br s, 1H), 4.10 (t, J = 6.2 Hz, 1H), 3.11
(d, J = 9.6 Hz, 1H), 2.55 (dd, J = 9.6, 7.9 Hz, 1H), 2.41 (td, J = 7.4, 3.5, Hz, 1H), 2.21 (s, 3H), 2.02
– 1.95 (m, 1H), 1.87 – 1.80 (m, 1H), 1.76 (s, 3H), 1.70 – 1.50 (m, 6H), 1.34 (s, 3H), 0.73 (s, 3H);
13C{1H} NMR (150 MHz, CDCl3) δ 154.6, 154.2, 147.6, 137.5, 111.5, 111.4, 108.7, 108.3, 83.6,
76.7, 44.7, 42.4, 42.2, 39.2, 38.8, 35.2, 29.5, 27.2, 25.6, 21.4, 17.7, 15.2; IR (neat) 2944, 1624,
1419, 1260, 1137; HRMS (ESI-TOF) m/z: [M+H]+ calculated for C22H31O3 343.2268; Found
1
343.2272. Data for 24: Rf 0.20 (8:2, hexanes:EtOAc); H NMR (600 MHz, CDCl3) δ 6.30 (s, 1H),
6.21 (s, 1H), 5.89 – 5.87 (m, 2H), 5.01 (br s, 1H), 3.05 (d, J = 9.7 Hz, 1H), 2.59 (dd, J = 9.7, 7.7 Hz,
1H), 2.44 – 2.37 (m, 2H), 2.32 (dd, J = 12.5, 3.2 Hz, 1H), 2.21 (s, 3H), 2.0 – 1.96 (m, 1H), 1.75 –
1.56 (m, 3H), 1.36 (s, 9H), 0.79 (s, 3H); C{1H} NMR (150 MHz, CDCl3) δ 154.4, 154.3, 143.3,
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138.0, 123.3, 111.2, 108.4, 107.6, 83.1, 71.0, 48.9, 45.7, 42.4, 38.7, 38.2, 33.2, 29.9, 29.7, 27.4,
25.5, 21.4, 15.3; IR (neat) 2968, 1624, 1585, 1455, 1328, 1137; HRMS (ESI-TOF) m/z: [M+Na]+
calculated for C22H30O3Na 365.2087; Found 365.2085.
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(1R,1aR,1a1S,3aS,8bS)-1,3a,6-trimethyl-1-(4-methylpent-3-en-1-yl)-1a,1a1,2,3,3a,8b-
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hexahydro-1H-4-oxabenzo[f]cyclobuta[cd]inden-8-yl acetate (25). Following a modified
procedure from Tang et al.,5 Ac2O (0.04 mL, 0.367 mmol, 2.0 equiv) was added dropwise to a
solution of 16 (60 mg, 0.184 mmol, 1.0 equiv) and DMAP (33 mg, 0.275 mmol, 1.5 equiv) in
CH2Cl2 (10 mL). The resulting mixture was stirred at room temperature for 30 min, then quenched
by addition of sat. NaHCO3(aq) (20 mL). The resultant mixture was extracted with CH2Cl2 (3 x 20
mL). The combined organic extracts were dried over MgSO4, filtered, and concentrated in vacuo.
Purification via flash chromatography on SiO2 (19:1, hexanes:EtOAc) afforded acetate 25 (55 mg,
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0.149 mmol, 81%) as a colourless oil. Data for 25: Rf 0.30 (19:1, hexanes:EtOAc); H NMR (600
MHz, CDCl3) δ 6.60 (s, 1H), 6.48 (s, 1H), 4.94 (t, J = 7.8 Hz, 1H), 2.99 (d, J = 9.8 Hz, 1H), 2.55 (t,
J = 9.1 Hz, 1H), 2.42 (td, J = 8.5, 5.8 Hz, 1H), 2.29 (s, 3H), 2.27 (s, 3H), 2.04 (dt, J = 13.1, 6.6 Hz,
1H), 1.84 – 1.67 (m, 4H), 1.66 – 1.58 (m, 1H), 1.61 (s, 3H), 1.50 (s, 3H), 1.32 (s, 3H), 1.29 (s, 3H),
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1.21 – 1.12 (m, 1H), 1.01 (ddd, J = 14.0, 12.1, 5.8 Hz, 1H); C{1H} NMR (150 MHz, CDCl3) δ
169.2, 154.8, 149.1, 137.4, 130.7, 125.2, 116.8, 115.2, 114.7, 84.1, 46.1, 42.7, 40.7, 38.9, 36.5,
31.3, 30.6, 26.1, 25.8, 25.7, 23.3, 21.4, 21.3, 17.6; IR (neat) 2949, 1768, 1626, 1451, 1371, 1052;
HRMS (ESI-TOF) m/z: [M+H]+ calculated for C24H33O3 369.2424; Found 369.2428.
(1R,1aR,1a1S,3aS,8bS)-1-(3-hydroxy-4-methylpent-4-en-1-yl)-1,3a,6-trimethyl-
1a,1a1,2,3,3a,8b-hexahydro-1H-4-oxabenzo[f]cyclobuta[cd]inden-8-yl acetate (26)
(1R,1aR,1a1S,3aS,8bS)-1-((E)-4-hydroxy-4-methylpent-2-en-1-yl)-1,3a,6-trimethyl-
and
1a,1a1,2,3,3a8b-hexahydro-1H-4-oxabenzo[f]cyclobuta[cd]inden-8-yl acetate (27). To a solution
of 25 (101 mg, 0.311 mmol, 1.0 equiv) in CH2Cl2 (20 mL) in a 25 mL borosilicate glass test tube
was added tetraphenylporphyrin (TPP) (4 mg, 0.007 mmol, 0.02 equiv) and O2 was sparged through
the solution for 10 min. Visible light was applied to the solution at a distance of 10 cm from the
irradiation vessel which was stirred for 6 h at room temperature. PPh3 (163 mg, 0.621 mmol, 2.0
equiv) was then added to the solution in one portion and the reaction was stirred at room
temperature under N2 for 8 h. The reaction was concentrated in vacuo and purified via flash
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chromatography on SiO2 (9:1, CH2Cl2: EtOAc) to afford 27 (53 mg, 0.139 mmol, 45%) as a yellow
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oil and 26 (42 mg, 0.109 mmol, 44%) as a yellow solid and as a 1:1 mixture of diastereomers. Data
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for 27 matched that previously reported.5 Data for 26: Rf 0.50 (9:1, CH2Cl2:EtOAc); H NMR (500
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MHz, CDCl3) δ 6.61 (s, 1H), 6.47 (s overlapped, J = 6.8 Hz, 1H), 4.80 (d, J = 4.7 Hz, 1H), 4.73 (d,
J = 4.7 Hz, 1H), 3.78 – 3.73 (m, 1H), 3.01 (t, J = 9.3 Hz, 1H), 2.54 (t, J = 9.3 Hz, 1H), 2.44 – 2.39
(m, 1H), 2.27 (s, 6H), 2.08 – 2.01 (m, 1H), 1.90 – 1.82 (m, 1H), 1.74 – 1.67 (m, 2H), 1.63 (s, 3H),
1.59 (s, 3H), 1.59 – 1.55 (m, 1H), 1.30 (s, 3H), 1.28 (s, 3H), 1.10 – 1.09 (m, 1H), 1.00 – 0.92 (m,
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1H); C{1H} NMR (125 MHz, CDCl3) δ 169.3, 169.2, 154.9, 154.9, 149.0, 149.0, 147.8, 147.7,
137.5, 137.5, 116.9, 116.8, 115.3, 115.2, 114.9, 114.8, 110.8, 110.7, 84.3, 84.2, 76.6, 76.5, 46.3,
46.2, 42.5, 42.4 41.0, 40.8, 39.3, 39.2, 36.4, 36.3, 30.7, 30.4, 30.2, 27.7, 27.7, 25.9, 25.7, 25.6, 25.5,
21.4, 21.4, 21.3, 17.8, 17.6; IR (neat) 2970, 1771, 1466, 1306, 1160, 1107; HRMS (ESI-TOF) m/z:
[M+Na]+ calculated for C24H32O4Na 407.2193; Found 407.2199. Data for 27: Rf 0.35 (9:1,
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CH2Cl2:EtOAc); H NMR (500 MHz, CDCl3) δ 6.61 (s, 1H), 6.47 (s, 1H), 5.43 (d, J = 16.0 Hz,
1H), 5.38 – 5.31 (m, 1H), 3.05 (d, J = 9.5 Hz, 1H), 2.54 (t, J = 9.2 Hz, 1H), 2.45 (dt, J = 8.9, 4.2
Hz, 1H), 2.28 (s, 6H), 2.08 – 1.97 (m, 2H), 1.94 – 1.84 (m, 1H), 1.76 – 1.70 (m, 2H), 1.64 – 1.59
(m, 1H), 1.28 (s, 6H), 1.22 (s, 6H); 13C{1H} NMR (125 MHz, CDCl3) δ 169.3, 154.9, 149.0, 140.2,
137.6, 123.7, 117.0, 115.3, 114.8, 84.3, 70.7, 46.4, 42.2, 40.8, 39.4, 36.1, 34.8, 31.1, 30.0, 29.9,
26.0, 25.6, 21.4, 21.3; IR (neat) 2948, 1768, 1750, 1626, 1576, 1371, 1198; HRMS (ESI-TOF) m/z:
[M+NH4]+ calculated for C24H36O4N 402.2639; Found 402.2639.
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at https://
Experimental details including NMR spectra and Tables of 1H and 13C NMR data (PDF)
Acknowledgements
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This work was supported by an Australian Research Council Future Fellowship awarded to J.H.G.
(FT170100437).
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References and Footnotes
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(a) Liao, H.-B.; Lei, C.; Gao, L.-X.; Li, J.-Y.; Li, J.; Hou, A.-J. Two Enantiomeric Pairs of
Meroterpenoids from Rhododendron capitatum. Org. Lett. 2015, 17, 5040. (b) Liao, H.-B.; Huang,
G.-H.; Yu, M.-H.; Lei, C.; Hou, A.-J. Five Pairs of Meroterpenoid Enantiomers from
Rhododendron capitatum. J. Org. Chem. 2017, 82, 1632.
2 (a) Lam, H. C.; Spence, J. T. J.; George, J. H. Biomimetic Total Synthesis of Hyperjapones A-E
and Hyperjaponols A and C. Angew. Chem. Int. Ed. 2016, 55, 10368. (b) Hart, J. D.; Burchill, L.;
Day, A. J.; Newton, C. G.; Sumby, C. J.; Huang, D. M.; George, J. H. Visible Light Photoredox
Catalysis Enables the Biomimetic Synthesis of Nyingchinoids A, B and D, and Rasumatranin D.
Angew. Chem. Int. Ed. 2019, 58, 2791.
3 Day, A. J.; Lam, H. C.; Sumby, C. J.; George, J. H. Biomimetic Total Synthesis of Rhodonoids C
and D, and Murrayakonine D. Org. Lett. 2017, 19, 2463.
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Wu, H.; Hsung, R. P.; Tang, Y. Total Syntheses of (±)-Rhodonoids C, D, E, F, and G and
Ranhuadujuanine B. Org. Lett. 2017, 19, 3505.
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Wu, H.; Hsung, R. P.; Tang, Y. Total Syntheses of (±)-Rhodonoids A and B and C12-epi-
Rhodonoid B. J. Org. Chem. 2017, 82, 1545.
6 For a review of the use of singlet oxygen in total synthesis, see: (a) Ghogare, A. A.; Greer, A.
Using Singlet Oxygen to Synthesize Natural Products and Drugs. Chem. Rev. 2016, 116, 9994. For
a perspective on the use of singlet oxygen in biomimetic synthesis, see: (b) Margaros, I.;
Montagnon, T.; Tofi, M.; Pavlakos, E.; Vassilikogiannakis, G. The power of singlet oxygen
chemistry in biomimetic syntheses. Tetrahedron 2006, 62, 5308.
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7 Kornblum, N.; DeLaMare, H. E. The Base Catalyzed Decomposition of a Di-alkyl Peroxide. J.
Am. Chem. Soc. 1951, 73, 880.
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8 For reviews of [2+2] photocycloadditions in natural product synthesis, see: (a) Bach, T.; Hehn, J.
P. Photochemical Reactions as Key Steps in Natural Product Synthesis. Angew. Chem. Int. Ed. 2011,
50, 1000. (b) Karkas, M. D.; Porco, J. A., Jr.; Stephenson, C. R. J. Photochemical Approaches to
Complex Chemotypes: Applications in Natural Product Synthesis. Chem. Rev. 2016, 116, 9683.
9 Chromene 8 was synthesised according to: Luo, G. Y.; Wu, H.; Tang, Y.; Li, H.; Yeom, H.-S.;
Yang, K.; Hsung, R. P. A Total Synthesis of (±)-Rhododaurichromanic Acid A via an Oxa-[3+3]
Annulation of Resorcinols. Synthesis 2015, 47, 2713.
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10 Laroche, B.; Nay, B. Harnessing the potential diversity of resinic diterpenes through visible light-
induced sensitized oxygenation coupled to Kornblum–DeLaMare and Hock reactions. Org. Chem.
Front. 2017, 4, 2412.
11 Davies, A. G. The Schenck rearrangement of allylic hydroperoxides. J. Chem. Res. 2009, 533.
12 The synthesis of chromenes via this method has been studied extensively by Hsung and co-
workers. For a review, see: Hsung, R. P.; Kurdyumov, A. V.; Sydorenko, N. A Formal [3 + 3]
Cycloaddition Approach to Natural‐Product Synthesis. Eur. J. Org. Chem. 2005, 23.
13 (a) Williams, C. M.; Mander, L. N. Chromatography with silver nitrate. Tetrahedron 2001, 57,
425. (b) Mander, L. M.; Williams, C. M. Chromatography with silver nitrate: part 2. Tetrahedron
2016, 72, 1133.
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