Asymmetric epoxidation of chromenes mediated by iminium salts: Synthesis of mollugin and (3S,4R)-trans-3,4-dihydroxy-3,4-dihydromollugin

Article abstract of DOI:10.1016/j.tet.2016.10.070

Organocatalytic asymmetric epoxidation of chromenes mediated by iminium salt catalysts under non-aqueous conditions provided ees as high as 99%. Contrastingly, reaction under aqueous conditions can form the corresponding diol products with ees as high as 71%. The process has been used for the synthesis of the East African medicinal plant metabolite (3S,4R)-trans-3,4-dihydroxy-3,4-dihydromollugin.

Full text of DOI:10.1016/j.tet.2016.10.070

Accepted Manuscript
Asymmetric epoxidation of chromenes mediated by iminium salts: Synthesis of
mollugin and (3S,4R)-trans-3,4-dihydroxy-3,4-dihydromollugin
Philip C. Bulman Page, Yohan Chan, Abu Hassan Noor Armylisas, Mohammed
Alahmdi
PII:
S0040-4020(16)31129-2
10.1016/j.tet.2016.10.070
TET 28211
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 30 July 2016
Revised Date: 22 October 2016
Accepted Date: 31 October 2016
Please cite this article as: Page PCB, Chan Y, Noor Armylisas AH, Alahmdi M, Asymmetric epoxidation
of chromenes mediated by iminium salts: Synthesis of mollugin and (3S,4R)-trans-3,4-dihydroxy-3,4-
dihydromollugin, Tetrahedron (2016), doi: 10.1016/j.tet.2016.10.070.
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ACCEPTED MANUSCRIPT
Graphical Abstract

Asymmetric Epoxidation of Chromenes Mediated by Iminium Salts: Synthesis of Mollugin and (3S,4R)-
ACCEPTED MANUSCRIPT
trans-3,4-Dihydroxy-3,4-Dihydromollugin
a*
a
b
c
Philip C. Bulman Page, Yohan Chan, Abu Hassan Noor Armylisas, Mohammed Alahmdi
a
School of Chemistry, University of East Anglia, Norwich Research Park, Norwich, Norfolk NR4 7TJ, U.K.
Synthesis and Product Development Unit, Advanced Oleochemical Technology Division, Malaysian Palm
b
Oil Board, 6 Persiaran Institusi, Bandar Baru Bangi, 43000 Kajang, Selangor, Malaysia.
c
Department of Chemistry, Tabuk University, P O Box 741, 74191, Kingdom of Saudi Arabia.
ABSTRACT: Organocatalytic asymmetric epoxidation of chromenes mediated by iminium salt catalysts
under non-aqueous conditions provided ees as high as 99%. Contrastingly, reaction under aqueous
conditions can form the corresponding diol products with ees as high as 71%. The process has been used for
the synthesis of the East African medicinal plant metabolite (3S,4R)-trans-3,4-dihydroxy-3,4-
dihydromollugin.
Introduction
The use of organocatalysts in the preparation of optically active compounds has attracted much attention
over the last few years, although organocatalytic processes have been known for many decades. For
example, one of the very early organocatalytic transformations was reported in 1860 by von Liebig, to form
1
oxamide from cyanogen, catalysed by acetaldehyde; the reaction was used on an industrial scale by
Degussa. Many transformations have since been shown to be amenable to organocatalysis, including a
2
number of enantioselective processes, including asymmetric epoxidation. Two of the most successful
organocatalyst types for asymmetric epoxidation are dioxiranes and oxaziridinium salts. Dioxiranes, derived
from chiral ketones, such as those of Yang, Denmark, Armstrong, and Shi, have been reported to give
3
enantioselectivities as high as 97% ee. Oxaziridinium salts were first reported as reactive electrophilic
4
5
oxygen transfer reagents by Lusinchi in 1976, and may be derived in situ from iminium salts. We have
extensively investigated and developed iminium salt asymmetric epoxidation catalysts (Figure 1) based on
6
,12
7
dihydroisoquinolinium (e.g., 1, 2),
biphenylazepinium (e.g., 3, 4 and 5), and binaphthylazepinium
8
9
species (e.g., 6, 7 and 8); these catalysts have shown excellent enantioselectivities of up to 99% ee.
2

ACCEPTED MANUSCRIPT
Figure 1: Enantioselective iminium salt catalysts
We have generally used two sets of conditions, aqueous and non-aqueous, for iminium salt-catalysed
asymmetric epoxidation processes. Aqueous conditions employ the triple salt oxone® as a stoicheiometric
oxidant in the presence of Na CO or NaHCO as the base, and acetonitrile:water as the typical solvent
2
3
3
1
0
mixture. However, the use of aqueous media restricts the range of temperatures at which the epoxidation
can be performed to above about –10 °C. An upper temperature limit is established by the stability of oxone,
which decomposes relatively quickly under basic conditions at room temperature. Our non-aqueous system
11
uses tetraphenylphosphonium monoperoxysulfate (TPPP) as the oxidant. The use of TPPP eliminates the
need for water as a co-solvent; in an organic solvent, typically chloroform, the reaction can be carried out at
much lower temperatures, –40 °C or lower. We have found that the seven-membered ring catalysts are
dramatically more reactive than the dihydroisoquinolinium species; further, the binap-derived catalysts are
usually poorly effective under non-aqueous conditions. We were interested to examine the relative values of
the two sets of conditions, and chose chromenes for the investigation as we have previously shown these to
be excellent substrates for our systems; we have synthesized several biologically active chromene
1
2
13
14
derivatives including levcromakalim 9, scuteflorin A 10, lomatin 11 and trans-khellactone 12 (Figure
) through highly enantioselective epoxidation reactions.
2
1
5
Chromene structural motifs are found in many natural products. These compounds exhibit a wide range of
1
6
17
18
19
biological properties such as anti-tumour, anti-fungal, anti-juvenile hormone in insects, oestrogenic,
2
0
and anti-bacterial activities.
Chromenes extracted from rubiaceous plants across the world have similarly shown a wide range of
21
biological activities. Examples of chromenes isolated from this family of plants include mollugin 13 and
its trans-diol 14 and epoxide 15 derivatives. Mollugin is one of the major chromenes isolated from
Rubiaceae species such as Putoria calabrica and Rubia cordifolia, a medicinal plant used in China and India
for the treatment of a range of conditions including arthritis, rheumatism, menstrual pain, and haemorrhage;
2
2
anti-mutagenic and anti-viral activities have been reported.
3

Both cis- and trans- 3,4-dihydroxy-3,4-dihydromollugin 14 have been isolated from the East African
ACCEPTED MANUSCRIPT
medicinal plant Pentas longiflora, used to treat tapeworm infections, rashes, malaria, and other
22b
conditions.
Synthetic approaches to chromenes including structures related to mollugin include high temperature
2
3
24
cyclization of phenol propargyl ethers, metal-catalysed cycloisomerization, phenylboronic acid-mediated
2
5
electrocyclization reaction of phenol with unsaturated aldehydes, base-catalysed condensation of phenols
2
6
with α
,β−unsaturated aldehydes or their acetal derivatives, Lewis acid-catalysed condensation of phenols
2
7
28
with epoxides, α
,
β-unsaturated aldehydes or allylic alcohols, and, recently, metalloradical activation.
Mollugin and derivatives have been prepared using several of the methodologies described above, including
2
9
Lewis acid-catalysed condensation of phenols with allylic alcohols, Lewis acid-catalysed condensation of
3
0
resorcinols with unsaturated aldehydes, phenylboronic acid-mediated electrocyclization reaction of
2
2,31
32
resorcinols with unsaturated aldehydes,
and electrocyclization of prenylated quinones.
Figure 2: Biologically active compounds containing a chromene core
Results and Discussion
6-Cyano-2,2-dimethylchromene 16 is an intermediate in the synthesis of the anti-hypertensive agent,
1
2,33
levcromakalim 9.
We prepared the precursor 6-cyano-2,2-dimethylchromene 16 using two approaches:
3
4
base-catalysed condensation of 4-cyanophenol and diethyl acetal 17, and cyclization of propargyl ether 18
3
5
(
Scheme 1). The synthesis of chromene 16 using the first method began with the preparation of the diethyl
3
6
acetal 17 using North’s procedure in 86% yield. Acetal 17 was subsequently heated under reflux with 4-
cyanophenol in p-xylene in the presence of 3-picoline as the base to give chromene 16 in 87% yield.
4

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Reagents and conditions: (i) NH NO , EtOH, r.t., 24 h, 86%; (ii) 4-cyanophenol, 3-picoline, p-xylene, reflux, 24 h, 87%; (iii)
4
3
CaCl , CuCl, Cu, cold aqueous HCl, 0 °C, 5 h; (iv) 4-cyanophenol, K CO , KI, acetone, reflux, 99%; (v) ethylene glycol, 210-215
2
2
3
°C, 24 h, quant.
Scheme 1: Syntheses of chromene 16
The rearrangement/cyclization of a propargyl ether is a general method for the synthesis of chromenes
2
3c
developed by Harfenist and Thom. 2-Methyl-3-butyn-2-ol was converted into propargyl chloride 19 using
calcium chloride, cuprous chloride, a catalytic amount of copper powder and cold concentrated hydrochloric
3
7
acid; the resulting chloride was used without purification due to its stability. The O-alkylation of 4-
cyanophenol was achieved by heating the phenol and 3-chloro-3,3-dimethylbutyne 19 with potassium
carbonate and potassium iodide in acetone, giving propargyl ether 18 in up to 99% yield. Cyclization of
ether 18 in ethylene glycol led to the formation of chromene 16 in quantitative yield (Scheme 1).
Asymmetric epoxidation reactions under both aqueous and non-aqueous conditions were then carried out on
6-cyano-2,2-dimethylchromene 16 using the selected iminium salts 1-4, 6, 7. Under non-aqueous conditions,
excellent enantioselectivity was observed for catalysts 2, 3 and 4 for the asymmetric epoxidation reactions
of chromene 16, affording the corresponding epoxy-chromane (−)-(3S,4S)-20 (Table 1). The highest ee was
observed using biphenyl-derived iminium salt catalyst 3, affording the corresponding epoxide in >99% ee
under non-aqueous conditions. When catalysts 1, 6 and 7 were used under non-aqueous conditions, no
conversion to the epoxide was detected using 1H NMR spectroscopic analysis of the unpurified reaction
mixtures. The reduced reactivity for these catalysts has been reported previously in our group for
3
8
asymmetric epoxidation of commercially available alkenes.
a
Table 1. Asymmetric epoxidation of 6-cyano-2,2-dimethylchromene 16 under non-aqueous conditions.
5

b
Catalyst
Yield/% ee/%
ACCEPTED MANUSCRIPT
2
3
4
59
20
20
97
99
97
a
b
Reagents and conditions: Iminium salt (10 mol%), TPPP (6 equiv.), CHCl , 0 °C, 1 d; Enantiomeric excesses were determined
3
c
by chiral HPLC on Chiralcel OD column; The absolute configuration of the major enantiomers were determined by comparison
12,33
of optical rotations with those reported in the literature
The corresponding epoxide product was indeed generated in situ, but the expected epoxy-chromane (3S,4S)-
0 was unstable under the aqueous reaction conditions, fully converting 16 into the corresponding diol (+)-
3S,4R)-21 in up to 71% ee, presumably through trans-diaxial ring-opening at the benzylic position (Table
). The data collected in Table 2 show that the biphenyl-derived catalysts 3 and 4 are the most reactive and
2
(
2
selective, giving the best ees of 68% and 71%, respectively. We were expecting that the reactions using
binaphthyl-derived catalysts 6 and 7 under aqueous conditions would be more successful, as these catalysts
are generally more reactive under aqueous conditions. The binaphthyl-derived catalysts 6 and 7, however,
gave zero and low enantioselectivity, respectively. This is a very surprising finding as we have previously
found catalyst 6 to be one of the most enantioselective iminium catalysts that we have developed, giving as
8
b
high as 95% ee for epoxidation of 1-phenyl-3,4-dihydronaphthalene.
a
Table 2. Formation of diol (+)-(3S,4R)-21 under aqueous conditions.
b
Catalyst
Conversion/% ee/%
1
2
3
4
6
7
65
44
60
57
68
71
0
100
100
25
52
20
a
b
Reagents and conditions: Na CO (4 equiv.), Oxone® (6 equiv.), iminium salt catalyst (10 mol%), MeCN/H O (1:1), 0 °C, 1 d;
2
3
2
c
Enantiomeric excesses were determined by chiral HPLC on Chiralcel OD column; The absolute configuration of the major
enantiomers were attributed by comparison of optical rotations with values reported in the literature 39
We next turned our attention to the synthesis of mollugin 13. Methylation of 1,4-dihydroxynaphthalene-2-
carboxylic acid using sulfuric acid in methanol led to isolation of the desired 1,4-dihydroxynaphthalene-2-
6

4
0
carboxylic ester 22 in a very low yield (5%) alongside dimethylated product 23, formed in 90% yield.
ACCEPTED MANUSCRIPT
Compound 23 could, however, be converted into 22 in 91% yield by treatment of a solution of 23 in
2
2a
anhydrous dichloromethane with aluminum chloride at 0 ºC (Scheme 2). Compound 22 was also formed
in 96% yield when a solution of 1,4-dihydroxynaphthalene-2-carboxylic acid and methyl iodide in
dimethylformamide was treated with anhydrous sodium hydrogen carbonate overnight at room
3
1,41
temperature.
Reagents and conditions: (i) CH OH, H SO , 8 h, 0 ºC-r.t., [22, 5%; 23, 90%]; (ii) AlCl (2 equiv.), 0 ºC, 2 h, 91%; (iii) MeI (2
3
2
4
3
equiv.), NaHCO (2 equiv.), DMF, 0 ºC-r.t., overnight, 96%.
3
Scheme 2: Synthesis of 22
Jahng has described a synthesis of mollugin 13 from 22 using an ortho-directing intermolecular
2
2a
phenylboronic acid-mediated annelation with 3-methyl-2-butenal. In our hands, this annelation of 3-
methyl-2-butenal with 22 was achieved in a reasonable 53% yield (Scheme 3). Using North’s general
3
4
procedure, compound 22 was heated under reflux in p-xylene with α,β-unsaturated acetal 17 in the
presence of 3-picoline to give mollugin in 64% yield. Alternatively, using Harfenist and Thom’s general
2
3c
methodology, ester 22 was heated under reflux for 24 h with propargyl chloride 19 in the presence of
potassium carbonate to yield mollugin 13 in 19% yield alongside recovered starting material 22 (79%
recovery). Modification of the reaction conditions, replacing acetone by toluene and increasing the
temperature from 50 °C to reflux, provided an experimentally convenient and high-yielding process,
allowing isolation of mollugin 13 in 81% yield (Scheme 3).
OH
O
OH
O
O
(i) or (ii) or (iii)
O
O
OH
22
13
Reagents and conditions: (i) 3-Methyl-2-butenal (2.2 equiv.), PhB(OH) (1.1 equiv.), toluene/AcOH, reflux, 17 h, 53%; (ii) acetal
2
1
7 (1 equiv.), 3-picoline (0.25 equiv.), p-xylene, reflux, 24 h, 64%; (iii) 3-Chloro-3-methyl-1-butyne 19 (3.1 equiv.), CuCl (1
equiv.), Cu (5 mol%), K CO (2 equiv.), KI (2 equiv.), toluene, reflux, 24 h, 81%.
2
3
Scheme 3: Synthesis of mollugin 13
7

ACCEPTED MANUSCRIPT
Synthesis of racemic mollugin oxide 15 was first attempted using standard epoxidation conditions.
Unfortunately, m-CPBA oxidation of mollugin 13 in anhydrous dichloromethane was unsuccessful: epoxide
24 was not isolated and decomposition of the starting material was observed. Suspecting that the free
phenolic hydroxyl may interfere with epoxidation or destabilize the epoxide moiety, we prepared the
4
2
benzylated product 25 in 86% yield. Attempts to oxidize 25 using m-CPBA and sodium hydrogen
carbonate in anhydrous dichloromethane, however, again led only to the decomposition of the starting
material (Scheme 4).
Reagents and conditions: (i) m-CPBA (1 equiv.), NaHCO (2 equiv.), 0 °С, 50 min; (ii) BnBr (1.5 equiv.), K CO (1.5 equiv.),
3
2
3
DMF, 70 °C, 4h, 86%.
Scheme 4: Attempted oxidation of 13 and 25
We conjectured that strongly electron-donating groups may destabilize the epoxide moiety under standard
epoxidation conditions, and we therefore prepared the triflate derivative 27 in 98% yield by treatment of
mollugin 13 with triethylamine and trifluoromethanesulfonic anhydride in anhydrous dichloromethane.
Racemic epoxide 28 was prepared from 27 in 81% yield using m-CPBA (Scheme 5).
O
S
O
S
OH
O
F C
O
O
O
F C
O
O
3
3
O
O
O
(i)
(ii)
O
O
O
O
O
13
27
(±)-28
Reagents and conditions: (i) TEA (2 equiv.), (CF SO )O (3 equiv.), DCM, 0 °C-r.t., 24 h, 91 %; (ii) m-CPBA (1 equiv.), NaHCO
3
2
3
(
2 equiv.), 0 °С, 2 h, 81%.
Scheme 5: Synthesis of 27 & (±)-28
Biphenyl catalyst 3 was tested under aqueous conditions and non-aqueous conditions in the asymmetric
epoxidation of 27. Surprisingly, epoxidation of 27 under non-aqueous conditions was unsuccessful at −30,
−10 and 0 °C even over 3 days, and the starting material was recovered in each case. Under aqueous
conditions, however, the reactivity of catalyst 3 proved excellent, and full conversion to (−)-(3S,4S)-epoxide
8

2
8 was observed after 15 min (Scheme 6).
ACCEPTED MANUSCRIPT
O
S
O
S
O
S
F C
O
O
O
F C
O
O
O
F C
O
O
O
3
3
3
O
O
O
(
i)
(ii)
O
O
O
O
OH
OH
2
7
(−)-(3S,4S)-28
(+)-(3S,4R)-29
Reagents and conditions: (i) catalyst 3 (10 mol%), Na CO (5 equiv.), Oxone® (2 equiv.), MeCN:H O (1:1), 15 min, 91%; (ii)
2
3
2
acetone/1M H SO (2:1), r.t., 1 h, 74%.
2
4
Scheme 6: Synthesis of diol (+)-(3S,4R)-29
The reduced stability of developing positive charge at the benzylic position in 28 coupled with a reduced
quantity of oxone® and an increased quantity of base allowed the epoxide product to be isolated in 91%
yield and 71% ee, conversion into the diol under the reaction conditions not being observed under these
conditions. Acid-catalysed ring-opening of the epoxide moiety of 28 with dilute sulfuric acid in aqueous
acetone gave the corresponding (+)-(3S,4R)-trans-diol 29 in 74% yield.
Removal of the trifluoromethane sulfonate group was initially attempted with potassium hydroxide in a
mixture of water and ethanol (1:1), the reaction was heated under reflux for 30 min until complete
disappearance of 29 was observed using TLC. Preferential hydrolysis of the methyl ester had, however,
occurred, leading to the formation of carboxylic acid 30. Indeed the presence of the triflate group was
1
9
43
confirmed using F NMR spectroscopy. A selective triflate removal procedure was then attempted: 29
was dissolved in dimethoxyethane in the presence of caesium carbonate and the reaction mixture heated to
80 °C. However, the saponification product 30 was the only product observed (62% yield) alongside
recovered starting material (28%) (Scheme 7).
Reagents and conditions: i) KOH (1 equiv), H O/EtOH (1:1), reflux, 30 min; ii) Cs CO (1.2 equiv.), MeOCH CH OMe, 80 °C, 1
2
2
3
2
2
h.
Scheme 7: Attempted removal of the trifluoromethane sulfonate group
9

Finally, compound 29 was dissolved in methanol in the presence of caesium carbonate, leading to the
ACCEPTED MANUSCRIPT
formation of (3S,4R)-trans-3,4-dihydroxy-3,4-dihydromollugin 31 in quantitative yield. When triflate 30
was submitted to the same reaction conditions, 31 was also obtained in quantitative yield (Scheme 7).
Reagents and conditions: i) Cs CO (2.5 equiv.), MeOH, 2 h.
2
3
Scheme 8: Triflate group removal
Conclusion
Aqueous and non-aqueous systems have been compared for the organocatalytic asymmetric epoxidation of
chromenes mediated by several iminium salt catalysts. Under non-aqueous conditions, the epoxides were
isolated with ees as high as 99%. Contrastingly, reaction under aqueous conditions could induce in situ
hydrolysis of the epoxides, giving the corresponding diol products with ees of up to 71%. The reaction has
been used for the synthesis of (3S,4R)-trans-3,4-dihydroxy-3,4-dihydromollugin 31 from mollugin.
Acknowledgments
This research has enjoyed the support of University of East Anglia, the Kingdom of Saudi Arabia, and the
Malaysian Palm Oil Board. We are also indebted to the EPSRC Mass Spectrometry Unit, Swansea.
EXPERIMENTAL SECTION
General Experimental Methods
Commercially available reagents and solvents were used as supplied, without further purification, unless
otherwise stated. Light petroleum (bp 40-60 °C), was distilled from calcium chloride prior to use. Ethyl
acetate was distilled over calcium sulfate or chloride. Dichloromethane was distilled over phosphorus
pentoxide or calcium hydride. Tetrahydrofuran was distilled under a nitrogen atmosphere from sodium-
benzophenone ketyl radical. Triethylamine was stored over sodium hydroxide pellets. Air- and moisture-
sensitive reactions were carried out using glassware that had been dried overnight in an oven at 240 °C, and
allowed to cool in a desiccator over self indicating silica gel pellets, under a nitrogen atmosphere; the
reactions were carried out under a slight positive pressure of nitrogen. Flash chromatography was carried out
using Merck 9385 Kieselgel 60-45 (230-400 mesh), typically using ethyl acetate-petroleum ether mixtures,
with the solvent polarity adjusted to provide an Rf of about 0.3. Thin layer chromatography (TLC) was
1
0

carried out on aluminium plates coated with a silica gel layer 0.25 mm thickness. Compounds were
ACCEPTED MANUSCRIPT
visualized by UV irradiation at a wavelength of 254 nm, or stained by exposure to an ethanolic solution of
phosphomolybdic acid (acidified with concentrated sulfuric acid), followed by charring where appropriate.
Infra-red absorption spectra were recorded on a Perkin-Elmer Paragon 2001 FT-IR spectrometer instrument
-1
1
13
in the range of 4000-600cm . H and C NMR spectra were recorded on Bruker DPX 400 and Avance
DPX 500 instruments.
3
6
1,1-Diethoxy-3-methyl-2-butene 17
3-Methylbut-2-enal (1.00 g, 11.90 mmol) and triethyl orthoformate (2.0 mL, 11.90 mmol) were dissolved in
EtOH (20 mL) at room temperature. The mixture was stirred for 5 min and NH NO (0.24 g, 2.97 mmol, 25
4
3
mol%) added. The mixture was stirred for 24 h. Saturated aqueous NaHCO (20 mL) and brine (10 mL)
3
were added. The mixture was extracted using Et O (3 x 20 mL), the organic layers were combined, dried
2
and filtered, and the solvents were removed under reduced pressure to give the title compound 17 as a
-1
brown liquid, which was used without further purification. νmax (film)/cm 2975, 2930, 2914, 2879, 1682,
1
1
6
1
1
447, 1377, 1358, 1348, 1206, 1142, 1115, 1083, 1053, 1017, 991. H-NMR (500 MHz, CDCl ): δ 1.23 (m,
3
H), 1.72 (d, 3H, J = 1.0 Hz), 1.75 (d, 3H, J = 1.0 Hz), 3.42 – 3.58 (m, 2H), 3.60 – 3.63 (m, 2H), 5.14 (d,
1
3
H, J = 6.5 Hz), 5.30 (d, 1H, J = 6.5Hz); C-NMR (125 MHz, CDCl ): δ 15.5, 18.4, 25.6, 60.4, 98.6, 125.1,
3
37.6.
4
4
3-Chloro-3-methyl-1-butyne 19
CaCl (56 g, 0.5 mol), CuCl (40 g, 0.3 mol), and Cu bronze powder (400 mg, 6.3 mmol) were suspended in
2
cold concentrated aqueous HCl (37%, 430 mL). The mixture was flushed with argon several times and
cooled in an ice bath with stirring. 2-Methylbut-3-yn-2-ol (96 mL, 1 mol) was added over 30 min. Stirring
was continued for 1 h at 0 – 5 °C. The upper organic layer was separated and washed immediately with cold
HCl (0-5 °C, 37%, 3 x 100 mL), water (2 x 100 mL), dried over anhydrous K CO , filtered, and the solvents
2
3
were removed under reduced pressure to give propargyl chloride 19, which was used without further
-1
1
purification. νmax (film)/cm 2983, 2927, 1739, 1666, 1616, 1447, 1370, 1111, 938, 836, 604; H-NMR (500
1
3
MHz, CDCl ): δ 1.87 (s, 6H), 2.62 (s, 1H); C-NMR (125 MHz, CDCl ): δ 34.6, 56.9, 71.9, 86.5.
3
3
35
3-(3-Cyanophenoxy)-3-methylbut-1-yne 18
4
-Cyanophenol (0.5 g, 4.2 mmol), anhydrous K CO (0.58 g, 4.2 mmol) and KI (0.07 g, 0.42 mmol, 0.1
2 3
equiv.) were stirred in acetone (25 mL per gram of phenol) under a nitrogen atmosphere. 3-Chloro-3-methyl-
-butyne 19 (1.3 g, 8.8 mmol, 2.1 equiv) was added and the mixture heated under reflux for 18 h. The
1
reaction was allowed to cool, and water (50 mL) added. The mixture was extracted with diethyl ether (3 x 25
mL), and the organic layers were combined, washed with NaOH (2N, 2 x 50 mL), HCl (2N, 50 mL), and
1
1

water (50 mL), dried over anhydrous Na SO , filtered, and the solvent removed under reduced pressure to
2
4
ACCEPTED MANUSCRIPT
-1
yield the title compound 18 as light yellow oil (0.77 g, 99%). ν (film)/cm 3292, 3103, 3078, 3048, 2991,
max
2939, 2226, 2112, 1602, 1572, 1504, 1464, 1452, 1420, 1384, 1366, 1254, 1227, 1172, 1137, 956, 911, 839,
1
7
36, 696, 671, 651. H-NMR (500 MHz, CDCl ): δ 1.70 (s, 6H), 2.65 (s, 1H,), 7.28 (d, 2H, J = 8.5 Hz), 7.57
3
1
3
(
d, 2H, J = 8.5 Hz); C-NMR (125 MHz, CDCl ): δ 29.5, 72.7, 75.3, 84.7, 105.1, 119.1, 120.0, 133.4,
3
159.4.
6-Cyano-2,2-dimethylchromene 16
Method A: 1,1-Diethoxy-3-methylbut-2-ene 17 (1.74 g, 11.0 mmol), 4-cyanophenol (2.65 g, 22.0 mmol)
and 3-picoline (0.27 mL, 2.75 mmol) were dissolved in p-xylene (20 mL/g of phenol). The mixture was
heated under reflux for 24 h, and the reaction mixture allowed to cool to ambient temperature. The clear,
golden mixture was diluted with EtOAc (50 mL) and washed with HCl (1N, 2 x 25 mL). The aqueous layers
were combined and washed with EtOAc (2 x 25 mL). The combined organic layers were washed with
NaOH (1N, 25 mL), brine (25 mL), dried over anhydrous MgSO and filtered. The solvents were removed
4
under reduced pressure to give a bright yellow solid. The yellow solid was recrystallized from light
petroleum to give the title compound 16 as a yellow powder (1.78 g, 87%).
Method B: Phenyl propargyl ether 18 (1.0 g, 5.40 mmol) was dissolved in ethylene glycol (5 mL/g of the
propargyl ether) and the reaction mixture heated to 210-215 °C for 24 h. The mixture was allowed to cool to
room temperature and water added. The mixture was extracted with Et O (2 x 25 mL), the organic layers
2
were combined, dried over anhydrous MgSO and filtered. The solvents were removed under reduced
4
pressure to give a bright yellow solid, recrystallized from light petroleum to give the title compound 16 as a
36
-1
yellow powder (1.0 g, quant.), mp 47-48 °C [Lit. mp 47 °C]. νmax (film)/cm 3054, 2985, 2226, 1605,
1
1
1
4
1
487, 1421, 1369, 1265, 1212, 1148, 1128, 1107, 961, 896, 828, 739, 705. H-NMR (400 MHz, CDCl ): δ
3
.45 (s, 6H), 5.70 (d, 1H, J = 11.6 Hz), 6.28 (d, 1H, J = 11.6 Hz), 6.78 (d, 1H, J = 8.4 Hz), 7.24 (d, 1H, J =
13
.0 Hz), 7.37 (dd, 1H, J = 8.4 Hz, 4.0 Hz); C-NMR (100 MHz, CDCl ): δ 28.4, 77.9, 103.8, 117.2, 119.3,
3
20.6, 121.7, 130.1, 132.2, 133.3, 156.8.
4
5
Preparation of Dimethyldioxirane (DMDO)
Distilled H O (20 mL), acetone (30 mL), and NaHCO (24 g, 0.285 mol) were combined and chilled in an
2
3
ice/water bath with magnetic stirring for 20 min. Stirring was halted and oxone® (25 g, 0.164 mol) added in
a single portion. The flask was loosely covered and the slurry was stirred vigorously for 15 min while still
submerged in the ice bath. After 15 min, the stirrer bar was removed and rinsed with a small portion of
distilled water. The flask containing the reaction slurry was attached to a rotary evaporator with the bath at
room temperature. The rotary evaporator splash trap (250 mL) was chilled in a dry ice/acetone bath and a
vacuum of 155 mmHg was applied via a benchtop diaphragm pump and an accompanying in-line vacuum
1
2

regulator. During this process, the flask was rotated vigorously (210 rpm) to prevent bumping of the slurry
ACCEPTED MANUSCRIPT
into the bump trap. After 15 min, the bath temperature was raised to 40 °C over the course of 10 min. When
the bath reached 40 °C, the distillation was halted immediately by releasing the vacuum and raising the flask
from the heated water bath. The pale yellow solution of DMDO was decanted from the rotary evaporator
splash trap directly into a graduated cylinder to measure the total volume of the solution (an average of 25
mL) and the solution was dried over Na SO . The solution was filtered and rinsed with acetone (10 mL).
2
4
The concentration of DMDO was determined by iodometric titration.
General Procedure for the Formation of Racemic Epoxides using DMDO
The alkene was dissolved in CHCl (2 mL per 0.1 g alkene) and the solution was cooled to 0 °C. The
3
DMDO solution in acetone (0.03 M, 1.5 equiv) was gradually added. After the reaction had reached
completion, the solvent was removed under reduced pressure at room temperature. The residue was purified
by column chromatography.
General Procedure for the Formation of Racemic Diols
The racemic epoxide was dissolved in acetone (50 mL/g of epoxide) and stirred at room temperature for 5
min. Aqueous sulfuric acid (1 M, 5.5 equiv) was added to the solution, and the mixture stirred for 1 h at
room temperature. After the reaction had reached completion, the reaction mixture was neutralized to pH 7
using sodium hydrogen carbonate. Dichloromethane (150 mL/g of epoxide) was added to the reaction
mixture and the organic phase separated. The aqueous layer was extracted with dichloromethane (2 x 150
mL/g of epoxide), and the organic layers were combined and dried (Na SO ). The solvents were removed
2
4
under reduced pressure, and the residue purified by column chromatography.
General Procedure for the Catalytic Asymmetric Epoxidation of Alkenes Mediated by Iminium Salts
using Oxone® under Aqueous Conditions
Oxone® (2 equiv.) was added with stirring to an iced-cooled solution of sodium carbonate (4 equiv.) in
water (12 mL per 1.50 g of Na CO ), and the resulting foaming solution was stirred for 5-10 min, until most
2
3
of the effervescence had subsided. A solution of the iminium salt (10 mol%) in CH CN (6 mL per 1.50 g of
3
Na CO ), was added, followed by a solution of the alkene substrate (approx. 100 mg) in CH CN (6 mL per
2
3
3
1
.50 g of Na CO ). The suspension was stirred with ice-bath cooling until the substrate was completely
2 3
consumed according to TLC. The mixture was diluted with ice-cooled diethyl ether (20 mL per 100 mg
substrate), and the same volume of water added immediately. The aqueous phase was washed four times
with diethyl ether, and the combined organic solutions were washed with brine and dried (MgSO4).
Filtration and removal of solvents gave a yellow or light brown residue, which was purified by column
chromatography, typically using ethyl acetate/light petroleum (1:99), to provide the pure epoxide.
1
3

ACCEPTED MANUSCRIPT
4
6
Tetraphenylphosphonium Monoperoxysulfate (TPPP)
Tetraphenylphosphonium chloride (15.0 g, 40 mmol) was dissolved in CH Cl (200 mL) and cooled in ice-
2
2
water bath. A solution of oxone® (15.0 g, 48 mmol) in deionised water (300 mL) at 0 °C was added to the
solution of tetraphenylphosphonium chloride over a period of 5 min. The resulting biphasic mixture was
stirred vigorously for 1 h in the ice-water bath, the organic layer separated, and the solvents were removed
under reduced pressure at room temperature. The white solid residue was transferred to a sintered glass
funnel and washed with deionized water (3 x 80 mL). The solid was re-dissolved in CH Cl (150 mL) and
2
2
dried over MgSO ; hexane was added to this solution until a solid precipitate just started to form, and the
4
flask was placed in the freezer overnight, producing a colourless crystalline solid with 94% purity in
1
peroxide. H-NMR (400 MHz, CDCl ): δ 7.63 – 7.67 (m, 8H), 7.76 – 7.80 (m, 8H), 7.88 – 7.92 (m, 4H),
3
9.34 (s, 1H). The oxygen content was measured by comparing the integration of the aromatic signals with
the hydroxyl proton.
General Procedure for Catalytic Asymmetric Epoxidation of Alkenes Mediated by Iminium Salts
using Tetraphenylphosphonium Monoperoxysulfate (TPPP)
Tetraphenylphosphonium monoperoxysulfate (2 equiv.) was dissolved in the desired solvent (2 mL per 0.1 g
oxidant) and the solution cooled to the required temperature. The iminium salt (10 mol%) was added
dropwise over 15-20 min to the reaction mixture as a solution in the desired solvent (0.5 mL per 0.1 g
oxidant) at the same temperature as the solution containing the oxidant. A solution of the alkene (approx.
100 mg) in the reaction solvent (0.5 mL per 0.1 g oxidant) was added dropwise. The mixture was stirred at
the reaction temperature until the alkene was completely consumed according to TLC. Et O (pre-cooled to
2
the reaction temperature, 20 mL per 0.1 g oxidant) was added to induce precipitation of the remaining
oxidant, and the mixture filtered through Celite®. The solvents were removed, Et O (40 mL) was added to
2
the residue, and the solution passed through a short pad of silica gel to remove catalyst residues. The
solvents were removed under reduced pressure to give the epoxide. If the reaction did not reach completion,
the epoxide was separated from the alkene by column chromatography, eluting with ethyl acetate/light
petroleum 1:99.
1
2
6-Cyano-3,4-epoxy-2,2-dimethylchromane 20
Prepared according to the general procedure for the catalytic asymmetric epoxidation of alkenes mediated
by iminium salts using TPPP under non-aqueous conditions from 6-cyano-2,2-dimethylchromene 16. 6-
Cyano-3,4-epoxy-2,2-dimethylchromane 20 was isolated as a colourless solid after purification by column
-1
chromatography eluting with EtOAc/light petroleum (5:95) plus 2% Et N. ν (film)/cm 3055, 2987, 2305,
3
max
2228, 1616, 1580, 1494, 1466, 1421, 1385, 1369, 1344, 1265, 1236, 1207, 1162, 1133, 1100, 1040, 958,
1
4

1
9
1
1
1
35, 920, 896, 868, 828, 816, 737, 705. H-NMR (500 MHz, CDCl ): δ 1.30 (s, 3H), 1.60 (s, 3H), 3.54 (d,
3
ACCEPTED MANUSCRIPT
H, J = 4.5 Hz), 3.91 (d, 1H, J = 4.5 Hz), 6.87 (d, 1H, J = 8.5 Hz), 7.53 (dd, 1H, J = 8.5, 2.0 Hz), 7.65 (d,
1
3
H, J = 2.0 Hz); C-NMR (125 MHz, CDCl ): δ 23.0, 25.5, 49.9, 62.3, 74.7, 104.3, 118.8, 119.1, 121.1,
3
33.8, 134.4, 156.5.
3
9
6-Cyano-3,4-dihydroxy-2,2-dimethylchromane 21
Prepared according to the general procedure for the catalytic asymmetric epoxidation of alkenes mediated
by iminium salts using oxone under aqueous conditions from 6-cyano-2,2-dimethylchromene 16. The
product 21 was isolated as a yellow solid after purification by column chromatography eluting with
EtOAc/light petroleum (50:50) plus 2% Et N, followed by drying at 60 °C under reduced pressure, m.p.
3
-1
1
47-149 °C; νmax (film)/cm 3397, 2984, 2919, 2851, 2229, 1611, 1580, 1489, 1372, 1311, 1273, 1195,
1
1
147, 1126, 1072, 1037, 1000, 952, 921, 836. H-NMR (500 MHz, CDCl ): δ 1.25 (s, 3H), 1.52 (s, 3H), 3.62
3
(d, 1H, J = 9.0 Hz), 4.59 (d, 1H, J = 9.0 Hz), 6.85 (d, 1H, J = 8.5 Hz), 7.46 (ddd, 1H, J = 8.5, 2.5, 1.0 Hz),
13
7
.81 (dd, 1H, J = 2.5, 1.0 Hz); C-NMR (125 MHz, CDCl ): δ 19.3, 26.6, 68.6, 75.8, 79.8, 104.0, 118.0,
3
119.2, 124.4, 132.4, 133.3, 156.0.
22a
Methyl 1-hydroxy-4-methoxy-2-naphthoate 23
,4-Dihydroxy-2-naphthoic acid (1.1 g, 5.38 mmol) was dissolved in methanol (12 mL), and dilute aqueous
H SO (1M, 5 mL) was added slowly at 0 °C. The resulting brown solution was heated under reflux for 2 h,
1
2
4
allowed to cool, and water (6 mL) added slowly. The solvents were removed under reduced pressure and the
residue dissolved in ethyl acetate (20 mL). The solution was transferred to a separating funnel and washed
with brine (10 mL). The organic layer was isolated, dried over anhydrous MgSO , and filtered. The solvents
4
were removed under reduced pressure and the residue was purified using silica gel column chromatography
(petroleum ether / ethyl acetate, 5:1) to give 23 as a colourless solid (1.12 g, 90%) and 22 as a yellow solid
2
2a
-1
(
0.05 g, 5%). For 23: mp 130-132 °С [Lit. mp 136-137 °C]; νmax (neat)/cm 3384, 2924, 1713, 1461, 846;
1
H-NMR (500 MHz, CDCl ): δ 3.97 (s, 3 H), 4.00 (s, 3 H), 7.03 (s, 1 H), 7.57 (ddd, 1 H, J = 8.2, 6.9, 1.3
3
Hz), 7.64 (ddd, 1 H, J = 8.3, 6.9, 1.3 Hz), 8.20 (ddd, 1 H, J = 8.4, 1.3, 0.7 Hz), 8.39 (ddd, 1 H, J = 8.3, 1.3,
1
3
0
1
.7 Hz), 11.62 (s, 1 H); C-NMR (125 MHz, CDCl ): δ 52.4, 55.8, 100.6, 104.4, 122.0, 123.9, 125.7, 126.6,
3
29.2, 130.0, 147.8, 155.8, 171.5.
22a
Methyl 1,4-dihydroxy-2-naphthoate 22
Method A: Compound 23 (0.43 g, 1.85 mmol) was suspended in anhydrous dichloromethane under a
nitrogen atmosphere. AlCl (0.49 g, 3.70 mmol) was added at 0 °C, and the mixture stirred for 2 h at 0 °C.
3
The reaction was quenched by slow addition of water (2 mL), followed by dilute aqueous HCl (1M, 1 mL).
The title compound was extracted from the aqueous layer with dichloromethane (3×10 ml). The combined
1
5

organic layers were washed with brine (2 × 10 mL), dried over anhydrous Na SO and filtered. The solvents
2
4
ACCEPTED MANUSCRIPT
were removed under reduced pressure and the residue purified using silica gel column chromatography
petroleum ether / ethyl acetate, 3:1) to give 22 as a yellow solid (0.36 g, 91%).
(
Method B: 4-Dihydroxy-2-naphthoic acid (0.20 g, 0.9 mmol) and anhydrous NaHCO (0.10 g, 1.1 mmol)
3
were suspended in anhydrous dimethylformamide and the suspension was stirred for 30 minutes under a
nitrogen atmosphere at room temperature. Methyl iodide (0.20 g, 0.09 ml, 1.4 mmol) was added using a
syringe. The mixture was stirred for 22 h, until all the starting material was consumed. Saturated brine (10
mL) was added, followed by dilute aqueous HCl (1M, 6 mL). The mixture was extracted with diethyl ether
(
3 x 30 mL), the organic phases were dried over anhydrous MgSO , and the solvents evaporated under
4
reduced pressure. The residue was purified using flash column chromatography (petroleum ether / ethyl
9
acetate, 4:1), to give 22 as a yellow solid (0.21 g, 96%), mp 191-193 °С [Lit. mp 192-193 °C]; ν
max
-1
1
(
neat)/cm 3434, 2981, 1635, 1376, 929; H-NMR (500 MHz, D -DMSO): δ 3.95 (s, 3 H), 7.10 (s, 1 H),
6
7
1
1
.61 (ddd, 1 H, J = 8.2, 6.9, 1.3 Hz), 7.67 (ddd, 1 H, J = 8.3, 6.9, 1.3 Hz), 8.10-8.13 (m, 1 H), 8.24-8.27 (m,
1
3
H), 9.85 (s, 1 H), 11.32 (s, 1 H); C-NMR (125 MHz, D -DMSO): δ 52.6, 103.8, 104.7, 122.2, 123.2,
6
24.8, 126.4, 128.7, 129.0, 145.1, 152.6, 170.5.
3
0a
Mollugin 13
Method A: Phenylboronic acid (0.25 g, 2.00 mmol) was added to a solution of methyl 1,4-dihydroxy-2-
naphthoate (0.45 g, 2.00 mmol) in toluene, followed by glacial acetic acid (3 mL). 3-Methylbut-2-enal (0.30
ml, 3.00 mmol) was added dropwise. The reaction was heated under reflux using a Dean-Stark trap under a
nitrogen atmosphere for 18 h. After cooling, aqueous sodium bicarbonate (30 mL) was added. The aqueous
layers were extracted with ethyl acetate (3 x 30 mL) and washed with brine (10 mL); the combined organic
extracts were dried over anhydrous MgSO . Removal of the solvents under reduced pressure afforded a
4
brown solid. The residue was purified using silica gel column chromatography (petroleum ether / ethyl
acetate, 11:1), to give 13 as a yellow solid (0.31 g, 56%).
Method B: Diethyl acetal 17 (0.55 mL, 6.6 mmol) and methyl 1,4-dihydroxy-2-naphthoate 22 (1.43 g, 6.6
mmol) were dissolved in p-xylene (10 mL). The mixture was stirred under a nitrogen atmosphere for 20
min. 3-Picoline (0.16 mL, 1.65 mmol) was added, and the reaction heated under reflux for 24 h. During the
reflux, the solution turned black. The mixture was allowed to cool to room temperature, dissolved in diethyl
ether, and washed with water. The organic layer was separated and charcoal added. The mixture was filtered
through a pad of celite to afford an oily residue. The residue was purified using silica gel column
chromatography (petroleum ether / ethyl acetate, 13:1), to give 13 as a yellow solid (1.2 g, 56%).
Method C: Methyl 1,4-dihydroxy-2-naphthoate 22 (0.53 g, 2.43 mmol), cuprous chloride (0.24 g, 2.43
mmol) and fine copper powder (7.7 mg, 0.12 mmol) were combined under a nitrogen atmosphere.
Anhydrous toluene and K CO (0.67 g, 4.86 mmol) were added. The mixture was stirred vigorously, and 3-
2
3
1
6

chloro-3-methyl-but-1-yne (0.7 g, 7.38 mmol) added. The mixture was heated under reflux until the starting
ACCEPTED MANUSCRIPT
material was consumed by TLC (24 h), allowed to cool to room temperature, and water (10 ml) added. The
mixture was extracted with diethyl ether (3 × 15 mL). The combined organic layers were washed with
saturated aqueous NaHCO (10 mL) and brine (5 mL), and dried over anhydrous MgSO . The solvents were
3
4
removed removed under reduced pressure to yield a yellow oil, purified by column chromatography on
silica gel (petroleum ether / ethyl acetate, 9:1) to furnish 13 as a yellow powder (0.56 g, 81%), mp 128-130
1
0
-1
1
°
С [Lit. mp 131-133 °C]; νmax (neat)/cm 3435, 2986, 2305, 1712, 1264, 896; H-NMR (500 MHz,
CDCl ): δ 1.45 (s, 6 H), 3.91 (s, 3 H), 5.59 (dd, 1 H, J = 10.0 Hz), 7.03 (dd, 1 H, J = 10.0 Hz), 7.41-7,45 (m,
3
1
3
1
H), 7.51-7,54 (m, 1 H), 8.09 (d, 1 H, J = 8.3 Hz), 8.29 (d, 1 H, J = 8.3 Hz), 12.10 (s, 1 H); C-NMR (125
MHz, CDCl ): δ 26.9, 52.3, 74.5, 102.2, 112.5, 121.9, 122.3, 124.0, 125.0, 126.3, 128.8, 129.0, 129.3, 141.5,
3
156.5, 172.5.
Methyl-2,2-dimethyl-6-(((trifluoromethyl)sulfonyl)oxy)-2H-benzo[h]chromene-5-carboxylate 27
Mollugin 13 (0.45 g, 1.58 mmol) was dissolved in anhydrous dichloromethane (20 mL) at 0 °C under an
argon atmosphere. Triethylamine (0.44 ml, 3.16 mmol) was added. The resulting yellow solution was stirred
for 20 min. Trifluoromethanesulfonic anhydride (0.8 ml, 4.74 mmol) was added, causing the mixture to turn
brown. After the addition was completed, the mixture was stirred at room temperature for 24 h. Water (5 ml)
was added slowly, and the organic layer was separated, washed with brine (7 ml), dried over anhydrous
Na SO and filtered. The solvents were removed under reduced pressure, and the residue purified by column
2
4,
chromatography on silica gel using petroleum ether / ethyl acetate (7:1) to give the title compound 27 as a
-1
1
colourless oil (0.59 g, 91%), νmax (neat)/cm 2976, 1724, 1149, 819, 751; H-NMR (500 MHz, CDCl ): δ
3
1
1
7
1
.55 (s, 6H), 3.99 (s, 3H), 5.75 (d, 1H, J = 10 Hz), 6.62 (d, 1H, J = 10 Hz), 7.57-7.64 (m, 2H), 8.01-8.04 (m,
19
13
H), 8.23-8.27 (m, 1H); F-NMR (471 MHz, CDCl ): δ −73.34; C-NMR (125 MHz, CDCl ): δ 28.0, 53.0,
3
3
7.5, 113.1, 118.7 (q, J= 318 Hz), 119.6, 121.7, 122.0, 122.6, 126.71, 126.74, 128.0, 128.7, 130.8, 135.8,
+
+
48.8, 165.1; m/z found for [M+NH ] : 434.0872; [C H F O S+NH ] requires 434.0872.
4
18 15
3
6
4
(±)-Methyl-2,2-dimethyl-6-(((trifluoromethyl)sulfonyl)oxy)-2H-benzo[h]chromene-5-carboxylate oxide
28
Na CO (0.13 g, 1.24 mmol) was added to a solution of 27 (0.26 g, 0.62 mmol) in anhydrous
2
3
dichloromethane at 0 °C. A solution of m-CPBA (0.1 g, 0.62 mmol) was added slowly at 0 °C. Complete
conversion of alkene was observed by TLC after 2 h. The residue was purified using column
chromatography on silica gel, eluting with petroleum ether / ethyl acetate (4:1), buffered with 3% Et N, to
3
-1
furnish the racemic epoxide 28 as a colourless solid (0.21 g, 81%), mp 175-176 ˚C; νmax (neat)/cm 2976,
1
1
724, 1149, 819, 751; H-NMR (500 MHz, CDCl ): δ 1.38 (s, 3H), 1.73 (s, 3H), 3.62 (d, 1H, J = 4.5 Hz),
3
4.02 (s, 3H), 4.44 (d, 1H, J = 4.5 Hz), 7.61 (ddd, 1 H, J = 8.1, 6.9, 1.1 Hz), 7.67 (ddd, 1 H, J = 8.4, 6.9, 1.2
1
7

1
9
13
Hz), 8.03-8.07 (m, 1 H), 8.23-8.27 (m, 1 H); F-NMR (471 MHz, CDCl ): δ −73.18; C-NMR (125 MHz,
3
ACCEPTED MANUSCRIPT
CDCl ): δ 23.2, 25.7, 47.8, 53.3, 63.0, 74.8, 111.3, 118.7 (q, J= 318 Hz), 121.9, 122.8, 124.7, 127.3, 127.4,
3
+
+
1
28.3, 129.3, 135.9, 149.3, 164.6; m/z found for [M+H] : 433.0558; [C H F O S+H] requires 433.0563.
18 15 3 7
(3S,4S)-Methyl-2,2-dimethyl-6-(((trifluoromethyl)sulfonyl)oxy)-2H-benzo[h]chromene-5-carboxylate
oxide 28
Water (0.2 mL), acetonitrile (2 mL) and Na CO (0.19 g, 1.80 mmol) were combined and cooled in an ice-
2
3
bath at 0 °C. The mixture was stirred for 2 min, and oxone® (0.44 g, 0.72 mmol) and catalyst 3 (0.25g, 10
mol%) were added. After 5 min, a solution of 27 (0.15 g, 0.36 mmol) in acetonitrile was added dropwise.
The mixture was stirred at 0 °C until complete conversion of the alkene was observed by TLC (15 min). The
reaction mixture was diluted with diethylether (5 mL), and the resulting suspension filtered through a mixed
pad of celite® and Na SO . The solvents were removed under reduced pressure. Epoxide (−)-(3S,4S)-28 was
2
3
obtained by flash column chromatography on silica gel eluting with petroleum ether / ethyl acetate (7:1),
containing 3% TEA. (0.14 g, 91%). Chiral HPLC trace (99.5:0.5, hexane:iso-propanol, flow rate; 0.5
mL/min); 22.22 min (14.75 %), 32.60 min (85.25 %).
(+)-(3S,4R)-Methyl-3,4-dihydroxy-2,2-dimethyl-6-(((trifluoromethyl)sulfonyl)oxy)-3,4-dihydro-2H-
benzo[h]chromene-5-carboxylate 29
Compound (−)-(3S,4S)-28 (83 mg, 0.91 mmol) was dissolved in acetone (1 mL) at room temperature. Dilute
sulfuric acid (1M, 0.5 mL) was added dropwise, and the mixture stirred for 1 h. Progress of the reaction was
monitored by TLC until the starting material had been completely consumed. The mixture was neutralized
to pH 7 using NaHCO . Dichloromethane (10 mL) and water (5 mL) were added, the layers separated, and
3
the aqueous layer was extracted with dichloromethane (10 mL). The combined organic extracts were washed
with brine (20 mL), and dried over anhydrous Na SO . The solvents were removed under reduced pressure,
2
4
and the residue was purified using column chromatography on silica gel using petroleum ether / ethyl
-1
acetate (1:1), to afford (+)-(3S,4R)-29 as a colourless solid (68 mg, 74%). mp 186-188 ˚C; νmax (neat)/cm
1
3
283, 2850, 1765, 1230, 893; H-NMR (400 MHz, CDCl ): δ 1.42 (s, 3H), 1.60 (s, 3H), 3.79 (d, 1H, J = 6.8
3
1
9
Hz), 3.98 (s, 3H), 4.89 (d, 1H, J = 6.8 Hz), 7.59-7.70 (m, 2 H), 8.01-8.05 (m, 1 H), 8.24-8.28 (m, 1 H); F-
13
NMR (471 MHz, CDCl ): δ −73.22; C-NMR (100 MHz, CDCl ): δ 20.4, 25.7, 53.4, 68.8, 75.4, 79.3,
3
3
114.4, 115.7 (q, J = 318 Hz), 122.0, 123.2, 123.9, 127.0, 128.4, 129.1, 136.4, 148.3, 166.9; m/z found for
+
+
[
M+NH ] : 468.0925; [C H F O S+NH ] requires 468.0934.
4 18 17 3 8 4
(+)-(3S,4R)-3,4-Dihydroxy-2,2-dimethyl-6-(((trifluoromethyl)sulfonyl)oxy)-3,4-dihydro-2H-
benzo[h]chromene-5-carboxylic acid 30
1
8

(
+)-(3S,4R)-29 (50 mg, 0.11 mmol) was dissolved in dimethoxyethane (5 mL). Cs CO (43 mg, 0.13 mmol)
2 3
ACCEPTED MANUSCRIPT
was added and the mixture heated to 80 °C for 1 h. The mixture was allowed to cool, and water (10 mL)
added. The mixture was extracted using ethyl acetate (3 x 5 mL), and the combined organic layers were
dried over anhydrous Na SO . The solvents were removed under reduced pressure, and the residue purified
2
4
using column chromatography on silica gel using petroleum ether / ethyl acetate (1:1) to afford (+)-(3S,4R)-
-1
1
3
0 as a yellow foam (40 mg, 84%). νmax (neat)/cm 3280, 1763, 1230, 1141, 1042, 942, 896; H-NMR (400
MHz, CDCl ): δ 1.59 (s, 3H), 1.71 (s, 3H), 3.76 (d, 1H, J = 9.2 Hz), 5.45 (d, 1H, J = 9.2 Hz), 7.43 (s, 1H),
3
1
3
7
7
1
.71-7.76 (m, 2 H), 8.12-8.17 (m, 1 H), 8.19-8.24 (m, 1 H); C-NMR (100 MHz, CDCl ): δ 21.0, 27.3,
3
4.1, 77.7, 84.0, 116.1, 118.9 (q, J = 319 Hz), 119.1, 122.6, 123.1, 127.7, 128.7, 129.1, 129.2, 135.0, 146.6,
+
+
65.7; m/z found for [M+NH ] : 454.0773; [C H F O S+NH ] requires 454.0778.
4
17 15
3
8
4
(+)-(3S,4R)-3,4-Dihydroxy-2,2-dimethyl-6-(((trifluoromethyl)sulfonyl)oxy)-3,4-dihydro-2H-
benzo[h]chromene-5-carboxylic acid 31
Method A: (+)-(3S,4R)-30 (40 mg, 0.09 mmol) was dissolved in methanol (5 mL). Cs CO (74 mg, 0.23
2
3
mmol) was added, and the mixture stirred for 2 h. The solvents were removed under reduced pressure, and
water (10 mL) and ethyl acetate (5 mL) added to the residue. The layers were separated, and the aqueous
layer was extracted with ethyl acetate (5 mL). The combined organic layers were dried over anhydrous
Na SO . The solvent was removed under reduced pressure to afford (+)-(3S,4R)-31 as a yellow foam (28
2
4
mg, quant.).
Method B: (+)-(3S,4R)-29 (65 mg, 0.14 mmol) was dissolved in methanol (10 mL). Cs CO (117 mg, 0.36
2
3
mmol) was added, and the mixture stirred for 2 h. The solvents were removed under reduced pressure, and
water (10 mL) and ethyl acetate (5 mL) added to the residue. The layers were separated, and the aqueous
layer was extracted with ethyl acetate (5 mL). The combined organic layers were dried over anhydrous
Na SO . The solvent was removed under reduced pressure to afford (+)-(3S,4R)-31 as a yellow foam (43
2
4
-1
1
mg, quant.). νmax (neat)/cm 3280, 1760, 1213, 1195, 1158, 1042, 948; H-NMR (400 MHz, CDCl ): δ 1.54
3
(s, 3H), 1.64 (s, 3H), 3.72 (d, 1H, J = 9.2 Hz), 5.42 (d, 1H, J = 9.2 Hz), 7.43 (s, 1H), 7.56-7.61 (m, 1 H),
13
7
.63-7.70 (m, 1 H), 8.08 (d, 1 H, J = 8.4 Hz), 8.33 (d, 1 H, J = 8.4 Hz); C-NMR (100 MHz, CDCl ): δ
3
20.8, 27.4, 74.7, 79.3, 82.3, 104.0, 116.4, 122.0, 124.4, 126.6, 126.7, 128.6, 129.2, 139.9, 148.2, 171.3; m/z
+
+
found for [M+NH ] : 322.1281; [C H O +NH ] requires 322.1285.
4
16 16
6
4
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