A Diels-Alder approach to anthrapyran antibiotics

Article abstract of DOI:10.1055/s-0031-1290529

A new entry to the synthesis of anthrapyran antibiotics has been accomplished through the synthesis of a 4H-anthra[1,2-b]pyran-4,7,12-trione model. The key step features a Diels-Alder reaction between a substituted 5-vinyl-3,4-dihydro-2H-pyran and naphthoquinone as the dienophile. The resulting tetracyclic adduct is then processed towards the targeted trione in a few steps. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1290529

768
LETTER
A Diels–Alder Approach to Anthrapyran Antibiotics
D
iels–Alder
Ap
a
proac
h
to
A
u
nthrapyran
r
ntibioti
a
cs Foulgoc,a Drissa Sissouma,a,b Michel Evain,c Sylvain Collet,*a André Guingant*a
a
Université de Nantes, CNRS, Chimie et Interdisciplinarité: Synthèse, Analyse, Modélisation (CEISAM), UMR CNRS 6230,
2 Rue de la Houssinière, BP 92208, 44322 Nantes Cedex 03, France
Fax +33(2)51125402; E-mail: andre.guingant@univ-nantes.fr; E-mail: sylvain.collet@univ-nantes.fr
Laboratoire de Chimie Organique Structurale, UFR SSMT, Université de Cocody-Abidjan, Abidjan, Ivory Coast
Institut des Matériaux Jean Rouxel, 2 Rue de la Houssinière, BP 92208, 44322 Nantes Cedex 03, France
b
c
Received 24 November 2011
bers of the family have spurred recently some synthetic
interest. While no synthesis of a complete skeleton of a
pluramycin has been achieved to date,2 several syntheses
of 4H-anthra[1,2-b]pyran antibiotics including pluramy-
Abstract: A new entry to the synthesis of anthrapyran antibiotics
has been accomplished through the synthesis of a 4H-anthra[1,2-
b]pyran-4,7,12-trione model. The key step features a Diels–Alder
reaction between a substituted 5-vinyl-3,4-dihydro-2H-pyran and
naphthoquinone as the dienophile. The resulting tetracyclic adduct cin aglycones (pluramycinones) have appeared in the
literature.3 In these reported syntheses, anionic condensa-
is then processed towards the targeted trione in a few steps.
tions and Diels–Alder reaction were mainly used to fash-
ion the DCB framework of the molecules whereas ring A
was elaborated by creation of the O1–C2 or the C4–C4a
bonds from a suitable precursor. Our approach to this
class of compounds is different and inspired from our pre-
vious work accomplished in the field of angucycline syn-
thesis.4 We thus planned to construct, in one single
operation, a tetracyclic precursor to the 4H-anthra[1,2-
b]pyran-4,7,12-trione framework with creation of ring B
by means of a hetero Diels–Alder reaction [ABCD → AB
+ CD strategy]. In addition to its convergency this strategy
would allow the preparation of several analogues by
changes in the structure of the diene and (or) of the dieno-
phile. To test the feasibility of such an approach, the 4H-
anthra[1,2-b]pyran-4,7,12-trione (1) was chosen as a
model compound. A retrosynthetic overview of our key
strategic bond disconnections is depicted in Scheme 1.
Key words: anthrapyran antibiotics, Diels–Alder reaction, quin-
ones, Stille reaction, 4H-anthra[1,2-b]pyran-4,7,12-trione
The pluramycins1 are a group of naturally occurring an-
thrapyran antibiotics with antitumor activity. They dis-
play a 4H-anthra[1,2-b]pyran-4,7,12-trione structure with
C-glycoside moieties typically attached at C8 and C10 (or
C5 and C10 for the subgroup of altromycins) and a lateral
chain branched at C2 bearing one or two epoxide func-
tionalities. These compounds intercalate in DNA with se-
lective positioning of the chain epoxide in the major
groove where it can form an adduct with the N7 of a gua-
nine residue. In addition to the C-glycosylated pluramy-
cins, the family of 4H-anthra[1,2-b]pyran antibiotics also
contains bioactive compounds having no epoxide func-
tionality and compounds bearing no carbohydrate moi-
eties. For the purpose of illustration, Figure 1 gives an
overview of their structural diversity.
We thus envisioned that trione 1 could be obtained by ar-
omatisation of the A ring of 2a or 2b, which could in turn
be prepared by aromatisation of the B ring of 3a or 3b, re-
spectively. These latter could be the result of a Diels–
The original structure and mode of action of pluramycins
added to the promising antitumoral activity of some mem-
O
H
H
O
HO
O
H
OH
D
O
C
O
B
OH
OH
O
O
O
OH
O
O
O
NMe2
O
A
HO
O
O
O
CO2H
O
NMe2
H
O
sapurimycin
γ-indomycinone
H
H
HO
hedamycin (pluramycin group)
Figure 1 Selected natural anthrapyran antibiotics
SYNLETT 2012, 23, 768–772
x
x.
x
x.
2
0
1
2
Advanced online publication: 24.02.2012
DOI: 10.1055/s-0031-1290529; Art ID: D71111ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Diels–Alder Approach to Anthrapyran Antibiotics
769
OBn
OBn
OBn
O
O
O
O
C
O
B
O
O
H
X
H
H
H
H
X
A
O
Y
Y
D
O
1
O
Diels–Alder
2a (X = Y = O)
2b (X = OTBDMS, Y = H)
3a (X = Y = O)
3b (X = OTBDMS, Y = H)
OBn
OBn
O
O
O
or
+
H
OTBDMS
H
O
H
O
Stille coupling
4
5
6
OBn
O
H
O
OBn
I
7
OSiMe3
H
O
5
6
O
OBn
+
H
O
OMe
9
10
11
OBn
O
H
OTBDMS
H
I
8
Scheme 1 Retrosynthetic analysis
Alder reaction between naphthoquinone 4 and dienes 5 and 8 could be envisaged from a same precursor 9, itself
(→ 3a) or 6 (→ 3b). Each of these dienes could be access- being the result of a heterocyclic Diels–Alder reaction be-
ed from an iodoether precursor (7 or 8) through an orga- tween aldehyde 10 and Danishefsky’s diene 11.
nometallic coupling reaction. Finally, the preparation of 7
1) ZnCl2 (1 equiv/10)
THF, 40 °C
2) TFA, CCl4
15 min, r.t.
OBn
OBn
I2
CCl4⋅Pyr (1:1)
H
0 °C to r.t., 2 h
O
O
+
H
H
OBn
86%
87%
H3CO
OSiMe3
O
O
O
10
11
9
I
7
OBn
OBn
CeCl3⋅H2O (1.1 equiv)
NaBH4, THF–MeOH (1:1)
–50 °C to r.t., 1 h
TBDMSCl (2.2 equiv)
imidazole (3 equiv)
DMF, r.t., 12 h
O
O
H
I
H
60–66% (2 steps)
OH
OTBDMS
I
8
12
SnBu3
SnBu3
OBn
OBn
Pd2(dba)3 (20 mol%)
P(o-tol)3 (40 mol%)
DMF, 90 °C, 12 h
Pd2(dba)3 (20 mol%)
P(o-tol)3 (40 mol%)
DMF, 90 °C, 12 h
O
O
H
H
OTBDMS
8
7
66%
75%
O
H
5
6
Scheme 2
© Thieme Stuttgart · New York
Synlett 2012, 23, 768–772

770
L. Foulgoc et al.
LETTER
The synthesis of dienes 5 and 6 commenced with the het- were isolated as single diastereomers. The structure of ad-
ero Diels–Alder condensation of 3-benzyloxyacetalde- duct 17 could be fully ascertained by single-crystal X-ray
hyde (10) with the Danishefsky’s diene (11) to afford the diffraction12 (Figure 2) and structures of adducts 3b, 14,
dihydropyran-4-one 9.5 The latter was next treated with an and 15 were attributed by analogy. The complete stereo-
excess of iodine (ca. 2 equiv) in a 1:1 mixture of CCl4 and selectivity of the cycloaddition process can be accounted
pyridine to give the iododihydropyran-4-one 7 in good for by an endo transition state with minimisation of steric
yield.6 Reduction of 7 under Luche conditions7 provided interactions (i.e., attack of the dienophile on the face of
the sole iodo alcohol 12 which was subsequently the diene opposite the OTBDMS and CH2OBn groups) as
OTBDMS-protected to give the iodo ether 8. Dienes 5 and pictured in Scheme 4 for cycloaddition of juglone (16)
6 could then be reached by palladium-catalysed Stille cou- with diene 6.
pling reaction between tributylvinylstannane8 and iodo
ethers 7 and 8, respectively (Scheme 2).
With access to dienes 5 and 6, we were poised to study the
key cycloaddition reaction to form 3a and 3b (Scheme 3).
Initial reaction conducted with naphthoquinone 4 showed
a clear difference of reactivity between dienes 5 and 6. If
diene 6 smoothly condensed with 4 to give adduct 3b in
good yield,9,10 diene 5 failed to react in the same condi-
tions. Diene 5, however, reacted smoothly with the bro-
mo-activated naphthoquinone 13 to give adduct 14 in
good yield.11 Diene 6 reacted similarly to give adduct 15.
Additionally, diene 6 was also reacted with juglone 16 to
furnish adduct 1710 (Scheme 4).
Although our synthetic strategy implies subsequent arom-
atisation of the B-ring of Diels–Alder adducts, it is not
without interest to point out that all the above adducts
Figure 2 X-ray structure of 17
OBn
OBn
O
O
O
H
H
H
H
MeCN, r.t., 24 h
77%
OTBDMS
O
+
H
OTBDMS
O
O
4
6
3b
OBn
O
OBn
O
O
H
O
O
O
H
Br
H
diene 6
H
Br
H
diene 5
H
Br
MeCN, r.t., 24 h
MeCN, r..t, 4 d
OTBDMS
76%
59%
O
O
O
14
13
15
Scheme 3
H
O
H
OBn
O
H
OH
O
OH
O
O
H
H
H
diene 6
O
CH3CN, r.t., 24 h
OTBDMS
H
H
OBn
80%
O
O
16
O
17
H
H
TBDMSO
Scheme 4
Synlett 2012, 23, 768–772
© Thieme Stuttgart · New York

LETTER
Diels–Alder Approach to Anthrapyran Antibiotics
771
Processing further on to reach trione 1, we next consid- at 80 °C.15 Under these conditions, the targeted trione 116
ered the possibility of carrying out the aromatisation of could be isolated in 93% yield (Scheme 7).
ring B in adducts 14, 15, and 3b. Treatment of adduct 14
with triethylamine resulted in the formation of the B-ring-
opened product 18 (Scheme 5) and we also failed to iso-
late compound 2a in several other conditions. Adduct 15
In summary, we have achieved the synthesis of the 4H-an-
thra[1,2-b]pyran-4,7,12-trione (1) embodying the tetra-
cyclic framework displayed by pluramycins and related
natural products. The key step is a Diels–Alder cycloaddi-
behaved similarly in similar conditions.
tion, which allows, in one single operation, the regioselec-
After several unfruitful research efforts we finally discov- tive assemblage of a CD unit to a A-ring precursor with
ered that compound 2b could be derived from 3b through formation of an advanced tetracyclic intermediate. Inves-
a two-step process featuring epoxidation13 of 3b to give tigations directed towards the implementation of this
epoxide 19 (one single diastereomer) and treatment of the strategy for the synthesis of more elaborated targets in-
latter with an excess of triethylamine. Moreover, we cluding natural products are ongoing.
found that a substantial improvement of the yield (49% to
67%) could be achieved when a one-pot procedure was
applied (Scheme 6).
Acknowledgment
We thank Aurélien Planchat for X-ray crystallographic structure de-
termination.
At this point, completion of the synthesis of trione 1 re-
quired deprotection and oxidation of the C4 alcohol and
introduction of the C2–C3 double bond. Treatment of 2b
References and Notes
with the Jones reagent effected both deprotection of the
OTBDMS ether and oxidation of the resulting alcohol to
give ketone 20 (Scheme 7). The same transformation was
also achieved in two steps, that is, OTBDMS deprotection
(CuCl2·7H2O, acetone–H2O, reflux, 48h; 70%)14 and
PCC oxidation (3 equiv of reagent, CH2Cl2, reflux, 3 d;
80%), without significant yield improvement. Finally, in-
troduction of the C2–C3 double bond to reach trione 1 was
best accomplished by exposure of 20 to iodine in DMSO
(1) Hansen, M. R.; Hurley, L. H. Acc. Chem. Res. 1996, 29, 249.
(2) (a) Fei, Z. B.; McDonald, F. E. Org. Lett. 2007, 9, 3547.
(b) O’Keefe, B. M.; Mans, D. M.; Kaelin, D. E. Jr.; Martin,
S. F. J. Am. Chem. Soc. 2010, 132, 15528.
(3) (a) Hauser, F. M.; Rhee, R. P. J. Am. Chem. Soc. 1979, 101,
1628. (b) Hauser, F. M.; Rhee, R. P. J. Org. Chem. 1980, 45,
3061. (c) Uno, H.; Sakamoto, K.; Honda, E.; Fukuhara, K.;
Ono, N.; Tanaka, J.; Sakanaka, M. J. Chem. Soc., Perkin
Trans. 1 2001, 229. (d) Krohn, K.; Vitz, J. Eur. J. Org.
OBn
OBn
OBn
O
O
H
O
O
O
H
O
HO
Br
H
H
H
H
Et3N (1–3 equiv)
CH2Cl2, r.t., 1 h
O
O
O
H
O
O
14
18
Scheme 5
OBn
OBn
OBn
O
O
H
O
O
O
O
H
H
H
H
H
H
H
MCPBA (1.1 equiv)
CH2Cl2, 20 °C, 24 h
Et3N
1 h
OTBDMS
OTBDMS
OTBDMS
H
O
67%
H
O
O
O
3b
19
2b
Scheme 6
OBn
OBn
OBn
O
O
O
O
O
O
Jones reagent
I2 (20 mol%)
H
H
acetone
DMSO
0 °C, 15 min
OTBDMS
O
80 °C, 4 h
O
52%
93%
O
O
O
2b
20
1
Scheme 7
© Thieme Stuttgart · New York
Synlett 2012, 23, 768–772

772
L. Foulgoc et al.
LETTER
part of an ABXY system, JAB = 18 Hz, J6–7 = 6 Hz, J6–
Chem. 2004, 209. (e) Fei, Z. B.; McDonald, F. E. Org. Lett.
2005, 7, 3617. (f) Krohn, K.; Vidal, A.; Vitz, J.;
6a = 1.2 Hz, J6¢–6a = 6.0 Hz, J6¢–5 = 1.6 Hz, J6¢–12b = 1.2 Hz, 2
H, H6 and H6¢), 1.92 and 1.66 (AB part of ABXY system,
JAB = 14.1 Hz, J3–2 = 6.0 Hz, J3–4 = 4.2 Hz, J3¢–4 = 3.7 Hz,
J3¢–2 = 3.7 Hz, 2 H, H3 and H3¢), 0.84 (s, 9 H, H22), 0.02 (s, 3
H, H20), –0.01 (s, 3 H, H20¢). 13C NMR (75 MHz, CDCl3):
d = 197.5 and 196.44 (2 Cq, C7 and C12), 138.7 (Cq, C16),
137.2 and 136.4 (2 Cq, C7a and C11a), 136.8 (Cq, C4a), 133.5
and 133.1 (2 CH, C9 and C10), 128.2 (2 CH, C17 and C17¢),
127.5 (2 CH, C18 and C18¢), 127.3 (CH, C19), 126.2 and 125.7
(2 CH, C8 and C11), 121.6 (CH, C5), 72.8 (CH, C2), 72.5
(CH2, C15), 70.8 (CH, C4), 70.7 (CH2, C13), 63.1 (CH, C12b),
51.8 (CH, C12a), 43.65 (CH, C6a), 37.4 (CH2, C3), 25.7 (3
CH3, C22), 22.31 (CH2, C6), 17.9 (Cq, C21), –4.6 (CH3, C20),
–5.1 (CH3, C20’). IR (KBr): 2930, 1699, 1254, 1095, 1052
cm–1. MS (EI): m/z (%) = 386 (16), 370 (8), 353 (14), 327
(7), 295 (7), 265 (13), 261 (7), 133 (22) 91 (100), 73 (25).
MS (CI, NH3): m/z = 536 [(M + NH4)+]. HRMS (Maldi
DHB/MeCN + PEG600): m/z calcd for C31H38O5SiNa
[MNa]+: 541.2381; found: 541.2397, D = 2.3 ppm.
(11) For an analysis of substituent effects on regioselectivities of
cycloadditions to naphthoquinones, see: Rozeboom, M. D.;
Tegmo-Larsson, I.-M.; Houk, K. N. J. Org. Chem. 1981, 46,
2338.
Westermann, B.; Abbas, M.; Green, I. Tetrahedron:
Asymmetry 2006, 17, 3051. (g) Tietze, L. F.; Gericke, K.
M.; Singidi, R. R. Angew. Chem. Int. Ed. 2006, 45, 6990.
(h) Tietze, L. F.; Singidi, R. R.; Gericke, K. M. Org. Lett.
2006, 8, 5873. (i) Krohn, K.; Tran-Thien, H. T.; Vitz, J.;
Vidal, A. Eur. J. Org. Chem. 2007, 1905. (j) Tietze, L. F.;
Singidi, R. R.; Gericke, K. M. Chem. Eur. J. 2007, 13, 9939.
(k) Gattinoni, S.; Merlini, L.; Dallavalle, S. Tetrahedron
Lett. 2007, 48, 1049. (l) Wright, B. J. D.; Hartung, J.; Peng,
F.; Van de Water, R.; Liu, H.; Tan, Q.-H.; Danishefsky, S. J.
J. Am. Chem. Soc. 2008, 130, 16786. (m) Dallavalle, S.;
Gattinoni, S.; Mazzini, S.; Scaglioni, L.; Merlini, L.; Tinelli,
S.; Beretta, G. L.; Zunino, F. Bioorg. Med. Chem. Lett. 2008,
18, 1484. (n) Elban, M. A.; Hecht, S. M. J. Org. Chem.
2008, 73, 785.
(4) (a) Collet, S.; Remi, J.-F.; Cariou, C.; Laib, S.; Guingant, A.;
Vu, N.-Q.; Dujardin, G. Tetrahedron Lett. 2004, 45, 4911.
(b) Vu, N.-Q.; Dujardin, G.; Collet, S.; Raiber, E.-A.;
Guingant, A.; Evain, M. Tetrahedron Lett. 2005, 46, 7669.
(c) Sissouma, D.; Maingot, L.; Collet, S.; Guingant, A.
J. Org. Chem. 2006, 71, 8384. (d) Maingot, L.; Thuaud, F.;
Sissouma, D.; Collet, S.; Guingant, A.; Evain, M. Synlett
2008, 263.
(12) Crystal Structure Data for 17
(5) (a) Danishefsky, S. J.; Larson, E.; Askin, D.; Kato, N. J. Am.
Chem. Soc. 1985, 107, 1246. (b) Danishefsky, S. J.;
Pearson, W. H.; Harvey, D. F.; Maring, C. J.; Springer, J. P.
J. Am. Chem. Soc. 1985, 107, 1256.
(6) Examples of dihydropyran-4-one iodination: (a) Chemler,
S. R.; Iserloh, U.; Danishefsky, S. J. Org. Lett. 2001, 3,
2949. (b) Gardiner, J. M.; Mills, R.; Fessard, T. Tetrahedron
Lett. 2004, 45, 1215. (c) Leonelli, F.; Capuzzi, M.;
Calcagno, V.; Passacantilli, P.; Piancatelli, G. Eur. J. Org.
Chem. 2005, 2671.
(7) (a) Luche, J.-L.; Gemal, A. L. J. Am. Chem. Soc. 1979, 101,
5848. (b) Danishefsky, S. J.; Bednarski, M. Tetrahedron
Lett. 1985, 26, 3411.
(8) Seyferth, D.; Stone, F. G. A. J. Am. Chem. Soc. 1957, 79,
515.
C31H38O6Si, Mr = 534.7, monoclinic, P21/c, a = 11.1097
(11), b = 18.9124 (12), c = 13.8124 (15) Å, b = 106.699 (9),
V = 2779.7 (5) Å3, Z = 4, rcalcd = 1.2773 g cm–3, m = 1.27
mm–1, F(000) = 1144, colourless block, 0.19 × 0.18 × 0.17
mm3, 2qmax = 56°, T = 120 K, 48226 reflections, 6629
unique (98% completeness), Rint = 0.096, 346 parameters,
GOF = 1.54, wR2 = 0.1231, R = 0.0563 for 4795 reflections
with I > 2s(I). CCDC 855339 contains the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
(13) For an example of B-ring aromatisation via an epoxide in the
field of angucyclines, see: Krohn, K.; Micheel, J.
Tetrahedron 1998, 54, 4827; aromatisation of 14 could be
explained by aerial oxidation of ring C followed by
abstraction of a proton at C6 (or C12c) of ring B with
subsequent opening of the epoxide and dehydration of the
resulting alcohol.
(9) For a Lewis acid catalysed cycloaddition with a related
diene, see: Huang, H.-L.; Liu, R.-S. J. Org. Chem. 2003, 68,
805.
(10) Procedure for the Preparation of 3b
To a solution of (2-benzyloxymethyl-5-vinyl-3,4-dihydro-
2H-pyran-4-yloxy)-tert-butyl(dimethyl)silane (diene 6, 890
mg, 2.45 mmol) in MeCN (13 mL) was added 1,4-naphtho-
quinone (4, 465 mg, 2.94 mmol). The reaction was stirred for
24 h at r.t. H2O (30 mL) was then added, and the aqueous
layer was extracted with CH2Cl2 (2 × 50 mL). The combined
organic phases were dried (MgSO4), filtered, and
concentrated. The residue was purified by silica gel column
chromatography (Et2O–PE = 1:4) to yield Diels–Alder
adduct 3b (1.18 g, 77%) as a white solid (mp 97 °C). 1H
NMR (400 MHz, CDCl3): d = 8.07–8.02 (m, 1 H, H11), 7.95–
7.89 (m, 1 H, H8), 7.65–7.59 (m, 2 H, H9 and H10), 7.34–7.18
(m, 5 H, H-Ph), 5.81 (dd, J5–6 = 5.9 Hz, J5–6¢ = 1.6 Hz, 1 H,
H5), 4.79 (dd, J12b–12a = 5.2 Hz, J12b–6b = 1.2 Hz, 1 H, H12b),
4.30 (X part of an ABX system, JXA = 4.2 Hz, JXB = 3.7 Hz,
1 H, H4), 4.31 and 4.26 (AB system, JAB = 12.0 Hz, 2 H,
CH2Ph), 3.76 and 3.32 (AB part of an ABX system,
JAB = 11.0 Hz, JAX = 7.6 Hz, JBX = 4.5 Hz, 2 H, CH2O),
3.64–3.56 (m, X part of 2 ABX systems, 1 H, H2), 3.44 (M
part of an AMX system, J12a–12b = 5.2 Hz, J12a–6a = 5.8 Hz, 1
H, H12a), 3.37 (M part of an ABMX system, J12a–6a = 5.8 Hz,
J6a–6 = 6.0 Hz, J6a–6¢ = 1.2 Hz, 1 H, H6a), 3.0 and 2.13 (AB
(14) Tan, Z.; Wang, L.; Wang, J. Chin. Chem. Lett. 2000, 11, 753.
(15) Patonay, T.; Cavaleiro, J.; Lévai, A.; Silva, A. Heterocycl.
Commun. 1997, 3, 223.
(16) Spectroscopic Data for Trione 1
Tan solid (mp 165 °C). 1H NMR (300 MHz, CDCl3):
d = 8.63 (d, J6–5 = 8.3 Hz, 1 H, H6), 8.37 (d, J5–6 = 8.3 Hz, 1
H, H5), 8.33–8.27 (m, 2 H, H8 and H11), 7.90–7.79 (m, 2 H,
H9 and H10), 7.49–7.30 (m, 5 H, H-Ph), 6.68 (s, 1 H, H3),
4.80 (s, 2 H, CH2Ph), 4.57 (d, J = 0.8 Hz, 2 H, CH2O). 13
C
NMR (100 MHz, CDCl3): d = 182.4 and 181.3 (2 Cq, C7 and
C12), 176.6 (Cq, C4), 167,6 (Cq, C12b), 154.7 (Cq, C2), 137.9
(Cq, C6a), 137.1 (Cq, C16), 134.9 and 134.2 (2 CH, C9 and
C10), 134.3 (Cq, C11a), 132.3 (Cq, C7a), 132.1 (CH, C6), 128.8
(2 CH, C17, C17¢), 128.5 (Cq, C4a), 128.3 (CH, C19), 128.0 (2
CH, C18 and C18¢), 127.3 and 127.2 (2 CH, C8 and C11), 123.3
(CH, C5), 122.7 (Cq, C12a), 110.1 (CH, C3), 73.8 (CH2, C15),
67.9 (CH2, C13). IR (KBr): 1674, 1657, 1589, 1418, 1324,
1283, 1122 cm–1. MS (EI): m/z (%) = 280 (20), 125 (27), 111
(30), 97 (47), 83 (45), 71 (59), 57 (100), 43 (69). MS (CI,
NH3): m/z = 397 [(M + H)+]. ESI-HRMS: m/z calcd for
C25H17O5 [MH]+: 397.1071; found: 397.1056, D = 3.6 ppm.
Synlett 2012, 23, 768–772
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