Synthesis of 3,3-dimethoxy-2-aryl-2,3-dihydro-1-oxa-cyclopenta[l] phenanthren-2-ols and their conversion to 2(3H)- and 3(2H)-furanones

Article abstract of DOI:10.1016/j.tet.2005.03.011

A number of new 3,3-dimethoxy-2-aryl-2,3-dihydro-1-oxacyclopenta[l] phenanthren-2-ols were synthesized by the base-catalyzed reaction between phenanthrenequinone and 4-substituted acetophenones in methanol. These furanols are unstable and are easily converted to stable 3-methoxy-3-aryl-3H-1- oxacyclopenta[l]phenathren-2-ones in excellent yields. The furanols are converted to 2-aryl-1-oxacyclopenta[l]phenanthren-3-ones on treatment with acids. Plausible mechanisms for some of these new transformations are proposed.

Full text of DOI:10.1016/j.tet.2005.03.011

Tetrahedron 61 (2005) 4601–4607
Synthesis of 3,3-dimethoxy-2-aryl-2,3-dihydro-1-
oxa-cyclopenta[l]phenanthren-2-ols and their conversion
to 2(3H)- and 3(2H)-furanones
*
Ambily M. Jacob, Roshini K. Thumpakkara, Sreedharan Prathapan and Binoy Jose
Department of Applied Chemistry, Cochin University of Science and Technology, Kochi 682 022, India
Received 30 August 2004; revised 16 February 2005; accepted 3 March 2005
Available online 31 March 2005
Abstract—A number of new 3,3-dimethoxy-2-aryl-2,3-dihydro-1-oxacyclopenta[l]phenanthren-2-ols were synthesized by the base-
catalyzed reaction between phenanthrenequinone and 4-substituted acetophenones in methanol. These furanols are unstable and are easily
converted to stable 3-methoxy-3-aryl-3H-1-oxacyclopenta[l]phenathren-2-ones in excellent yields. The furanols are converted to 2-aryl-1-
oxacyclopenta[l]phenanthren-3-ones on treatment with acids. Plausible mechanisms for some of these new transformations are proposed.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
a,b-Unsaturated ketones are conveniently prepared by
Aldol condensation.1–4 Dibenzoylstyrene (3), for example,
is synthesized by the base-catalyzed condensation between
benzil (1) and acetophenone (2). Thermolysis of 3 and
related systems leads to 2(3H)-furanones 4, which exhibit
rich photochemistry (Scheme 1).5,6 Furthermore, due to
their common occurrence in nature, oxygen containing
heterocycles are frequent and important targets for synthesis
either as final products or as useful synthetic intermediates.
The synthesis of lactones can be achieved by the lactoniza-
tion of hydroxy acids, Baeyer–Villiger oxidation, the
insertion of a carbonyl group by transition metals,
intramolecular cyclization of 1,4-diones, etc.7 We reasoned
Scheme 2.
that the condensation between phenanthrenequinone (5) and
should undergo thermal rearrangement to give 2(3H)-
furanone derivatives 7 and/or 8 (Scheme 2).
acetophenone (2) should lead to phenanthrenone-9-ylidene
ketones 6, which may be regarded as novel analogues of
dibenzoylalkenes. Accordingly, these ketones, in principle,
Close examination of the structural features of 6 reveals that
it can be considered as a quinonemethide as well. Hence, 6
and its derivatives are expected to undergo transformations
typical of other quinonemethides including Diels–Alder
reactions, nucleophilic additions, etc. The reported pro-
cedure for the preparation of 6 involves the Wittig reaction
between phenanthrenequinone and the triphenylphospho-
nium ylide prepared from phenacyl bromide.8 Dimerization
of 6 through a hetero Diels–Alder pathway is a major side
reaction here making this procedure synthetically unviable.
So, we explored the possibility of synthesizing 6 through the
base-catalyzed reaction between phenanthrenequinone and
acetophenones. In this article, we report our findings on the
Scheme 1.
Keywords: Furanol; Furanone; Phenanthrenequinone; Rearrangement.
*
Corresponding author. Tel.: C91 484 257 5804;
e-mail: binoy@cusat.ac.in
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.03.011

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A. M. Jacob et al. / Tetrahedron 61 (2005) 4601–4607
base-catalyzed reaction between phenanthrenequinone and
acetophenones.
undergoes oxidation by single electron transfer to either
oxygen10 or phenanthrenequinone11 followed by further
transformations to give the hydroperoxide intermediate
11.12–14 Under the conditions of work up, 11 is transformed
to the dihydrofuranol 12.15–18 The effect of oxygen on
this reaction was studied by bubbling oxygen through the
reaction mixture. No increase in the yield of 12 was
observed by this method. When the reaction was repeated in
degassed methanol, no noticeable change in yield or product
distribution was observed. However, when the reaction was
repeated with phenanthrenequinone and acetophenone
taken in a 2:1 ratio, the yield of 12 improved moderately
(35% vs 30%) suggesting a possible additional role for
phenanthrenequinone on the course of the reaction. The
poor solubility of phenanthrenequinone in other solvents
such as ethanol, and t-butanol prevented us from carrying
out the reaction in these solvents.
2. Results and discussion
We proposed to synthesize phenanthrenone-9-ylidene
ketones 6a–f by the condensation of phenanthrenequinone
(5) with selected acetophenones 2a–f in methanol in the
presence of potassium hydroxide as catalyst (Scheme 2).
Aryl ketones of our choice were acetophenone (2a),
4-methylacetophenone (2b), 4-methoxyacetophenone (2c),
4-bromoacetophenone (2d), 4-chloroacetophenone (2e),
and 4-phenylacetophenone (2f).
The condensation of phenanthrenequinone with aceto-
phenones in the presence of potassium hydroxide in
methanol gave a product in 20–50% yield. These com-
pounds were unstable and underwent slow degradation in
solution. However, we obtained reliable elemental analysis
and mass spectral data on freshly prepared samples. The
UV spectra of all these compounds were dominated by
absorption due to the phenanthrene residue present in them.
Surprisingly, the IR spectra of the products did not indicate
the presence of carbonyl groups, but showed absorption at
3400 cmK1 indicating the presence of a hydroxy group in
the molecule. 1H NMR spectra of 12a–f showed the
presence of two methoxy groups.9 13C NMR spectra
(noise decoupled) of two representative examples such as
12b,d also indicated the absence of carbonyl groups in the
molecule, but indicated the presence of two signals
attributable to methoxy groups and another signal at wd
108 attributable to a ketal group. Based on spectral and
analytical data, the structure of the products was assigned as
dihydrofuranols 12a–f.
The dihydrofuranol derivatives 12a–f underwent facile
thermal rearrangement when heated up to their respective
melting points. Neat thermolysis of 12a–f in sealed tubes
gave 14a–f as colorless crystalline solids in high yield
(O80%). The structures of 14a–f were arrived at on the
basis of spectral and analytical data. All the thermolysis
products 14a–f showed strong IR absorptions at
w1813 cmK1 indicating the presence of a g-lactone
component in the molecule. UV absorption spectra of
these compounds are similar to that of phenanthrene
indicating the presence of phenanthrene components.
1H NMR spectra of 14a–f showed a singlet at wd 3.4
(3H) indicating the presence of a single methoxy group. In
the 13C NMR spectra, signals were observed at wd 54
(OCH3), 85, 114–150 (aromatic) and 175 (C]O). Mass
spectral data suggested the loss of elements of methanol
from the furanol precursors. Based on these data, the
compounds were assigned the phenanthro-2(3H)-furanone
structures 14a–f. The structures of these compounds were
unequivocally determined by single crystal X-ray diffrac-
tion analysis of a representative example such as 14a
(Fig. 1).19 The mechanism for the formation of phenanthro-
2(3H)-furanones is given in Scheme 4. Upon heating, loss
of a molecule of methanol from 12 and consequent bond
A possible mechanism for the formation of dihydro-2-
furanol 12 involving the intermediacy of phenanthrenone-9-
ylidene ketones 6 is presented in Scheme 3. Nucleophilic
addition of methanol to 6 leads to the formation of 9.
The carbanion intermediate 10 generated by abstraction
of the moderately acidic proton at the 3-position of 9
Scheme 3. XZ (a) H, (b) CH3, (c) OCH3, (d) Br, (e) Cl, (f) Ph.

A. M. Jacob et al. / Tetrahedron 61 (2005) 4601–4607
4603
1
further illustrated by a simple NMR experiment: H NMR
spectra of 12a–f were collected over a period of 0–120 h.
Freshly prepared samples of 12a–f in CDCl3 gave
1
1
acceptable H NMR spectra. However, H NMR spectra
of these samples recorded after storing at room temperature
for 48–120 h indicated complete transformation of 12a–f to
14a–f. While this experiment gave no evidence in support of
the formation of the dibenzoylalkene-type intermediate 13,
it indicated that the nature of the substituents on the 2-aryl
group has a remarkable effect on the relative stability of
these furanols. Bromo (12d), chloro (12e), and phenyl (12f)
substituents at the fourth position of the 2-aryl moiety seem
to impart higher stability to these furanols. On the other
hand, compounds 12a,b were particularly unstable and
underwent fast conversion to 14a,b.
Figure 1. ORTEP diagram of molecular structure of 14a in the crystal.
Furanol 12 may be considered as the dimethylketal of
3(2H)-furanone 15. 3(2H)-Furanone moiety is identified as
a central structural unit in a growing number of natural
products including simple compounds such as bullatenone21
and geipavarin22 and more complex compounds such as
jatrophone, eremantholide,23 and lychnophorolide.24 Many
of these natural products possess significant tumor-
inhibiting properties. Therefore, the synthesis of 3(2H)-
furanones has attracted considerable attention.25–28 So, it
was of interest to us to study the conversion of 12 to 15.
Treatment of 12 with base did not lead to any reaction.
However, treatment of 12 with acid led to the transformation
to 3(2H)-furanone 15. Further, we acetylated a representa-
tive furanol derivatives, 12f using acetic anhydride in
reorganizations lead to the formation of methoxydibenzoyl-
alkene 13. Subsequently 13 undergoes thermal transform-
ation analogous to that reported for other dibenzoylalkenes
to yield the corresponding 2(3H)-furanone 14.5,6 Our
attempts to isolate 13 were not successful.
The thermolysis of 12 in refluxing o-dichlorobenzene also
yielded 14 in comparable yields. In continuation, we carried
out the neat thermolysis of 12 in an open vessel and obtained
the furanones 14 in near quantitative amounts. Thermo-
gravimetric analysis of 12a indicated that its decomposition
is completed in the narrow temperature range between 130
and 140 8C. The observed transformation appears to be a
three-stage process.20 The unstable nature of 12a–f was
Scheme 4. XZ (a) H, (b) CH3, (c) OCH3, (d) Br, (e) Cl, (f) Ph.
Scheme 5.

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A. M. Jacob et al. / Tetrahedron 61 (2005) 4601–4607
pyridine. Under these conditions, the acetylated phenanthro-
3(2H)-furanone 16 was formed in 62% yield (Scheme 5).
Anal. Calcd for C24H20O4: C, 77.40; H, 5.41. Found: C,
77.10; H, 5.04.
4.3.2. 3,3-Dimethoxy-2-p-tolyl-2,3-dihydro-1-oxacyclo-
penta[l]phenanthren-2-ol (12b). Light yellow solid
(20%); mp 118–120 8C (dec.); IR nmax (KBr) 3406 (OH),
and 2941, 1633, 1500 cmK1; UV lmax (CH3CN) 215
(3 14,000), 251 (3 5800), 341 nm (3 1500); 1H NMR
(CDCl3) d 2.3 (s, 3H, CH3); 3.2 (s, 3H, OCH3), 3.5 (s, 3H,
OCH3), 5.6 (s,1H, OH), 7.0–8.6 (m, 12H, aromatic); 13C
NMR (CDCl3) d 21.1, 51.2, 52.2, 108.7, 110.5, 121.6, 122.8,
123, 123.1, 123.5, 123.9, 124.1, 125.7, 126.6, 126.9, 127.1,
128.3, 128.4, 129.3, 130.1, 132.7, 134.6, 138.6; MS (m/z)
354 {(MKCH3OH)C}, 326, 221, 119, 91, and other peaks;
ESI-MS (m/z): calcd for C25H22NaO4 [MCNa]C: 409.14.
Found 409.2; HRMS (MKCH3OHCNa)C found 377.1156.
C24H18NaO3 requires 377.1154; Anal. Calcd for C25H22O4:
C, 77.70; H, 5.74. Found: C, 77.98; H, 5.63.
3. Conclusions
In summary, we have synthesized several phenanthrofur-
anols by the base-catalyzed reaction between phenanthre-
nequinone and acetophenones. These phenanthrofuranols
are converted to 2(3H)- or 3(2H)-furanones in near-
quantitative yields under suitable reaction conditions. The
furanone derivatives thus obtained have immense potential
for further investigations.
4. Experimental section
4.1. General procedures
All melting points are uncorrected and were determined on a
Neolab melting point apparatus. All reactions and chroma-
tographic separations were monitored by thin layer
chromatography (TLC). Glass plates coated with dried
and activated silica gel or aluminum sheets coated with
silica gel (Merck) were used for thin layer chromatography.
Visualization was achieved by exposure to iodine vapor or
UV radiation. Column chromatography was carried out with
slurry-packed silica gel (Qualigens 60–120 mesh). Absorp-
tion spectra were recorded using Shimadzu 160A spectro-
meter and infrared spectra were recorded using Shimadzu-
DR-8001 series FTIR spectrophotometer, respectively. The
1H and 13C NMR spectra were recorded at 300 and 75 MHz,
respectively, on a Brucker 300 FT-NMR spectrometer with
tetramethylsilane as internal standard.
4.3.3. 3,3-Dimethoxy-2-(4-methoxyphenyl)-2,3-dihydro-
1-oxacyclopenta[l]phenanthren-2-ol (12c). Light yellow
solid (50%); mp 136–138 8C (dec.); IR nmax (KBr) 3408,
2992, and 1635 cmK1; UV lmax (CH3CN) 248 (3 42,000),
1
302 (3 8300), 359 nm (3 2600); H NMR (CDCl3) d 3.2
(s, 3H, OCH3), 3.5 (s, 3H, OCH3), 3.6 (s, 3H, OCH3), 5.6
(s, 1H, OH), 6.9–8.7 (m, 12H, aromatic); MS (m/z) 370
{(MKCH3OH)C}, 342, 107, and other peaks; ESI-MS
(m/z): calcd for C25H22NaO5 [MCNa]C: 425.14. Found
425.1; Anal. Calcd for C25H22O5: C, 74.61; H, 5.51. Found:
C, 74.43; H, 5.28.
4.3.4. 2-(4-Bromophenyl)-3,3-dimethoxy-2,3-dihydro-1-
oxacyclopenta[l]phenanthren-2-ol (12d). Light brown
crystals (25%); mp 140–141 8C (dec.); IR nmax (KBr)
3400, 2943, 1614, 1589, and 1520 cmK1; UV lmax (CH3CN)
224 (3 32,200), 248 (3 37,000), 308 (3 7100) 341 nm (3
4.2. Starting materials
1
2000); H NMR (CDCl3) d 3.2 (s, 3H, OCH3), 3.5 (s, 3H,
OCH3), 5.6 (s, 1H, OH), 7.2–8.8 (m, 12H, aromatic); 13C
NMR d 51.3, 52.2, 108.7, 109.9, 111.0, 122.9, 124.1, 124.2,
126.8, 127.2, 128.9, 130.8, 132.8, 136.9, 154.2; MS (m/z)
420 {(MKCH3OH)C}, 390, 221, 185, and other peaks;
FABMS, m/z 453 {(MCH)C}. ESI-MS (m/z): calcd for
C24H20BrO4 [MC1]C: 451.9. Found: 451.1; Anal. Calcd
for C24H19O4Br: C, 63.87; H, 4.24. Found: C, 63.82; H,
4.17.
Phenanthrenequinone (1) and acetophenone (2) were
purchased from E. Merck and were used as obtained.
Ketones 2b–f were prepared using a known procedure.29
4.3. Preparation of 12a–f
12a–f were prepared by the condensation of phenanthrene-
quinone with appropriate acetophenone derivatives in the
presence of potassium hydroxide in methanol. In a typical
experiment, a mixture of phenanthrenequinone (0.025 mol),
acetophenone (0.027 mol), and powdered potassium
hydroxide (1 g) in methanol (30 mL) was stirred at 60 8C
for 1 h and then kept in a refrigerator for 48 h. The solid
product separated out was filtered and purified by
recrystallization from a mixture (2:1) of methanol and
dichloromethane.
4.3.5. 2-(4-Chlorophenyl)-3,3-dimethoxy-2,3-dihydro-1-
oxacyclopenta[l]phenanthren-2-ol (12e). Light yellow
crystals (20%); mp 142–143 8C (dec.); IR nmax (KBr)
3419 (OH), 2949, 1637, 1603, 1506, 756 cmK1; UV lmax
(CH3CN) 209 (3 55,000), 263 (3 56,000), 320 (3 8200),
410 nm (3 3300); 1H NMR (CDCl3) d 3.2 (s, 3H, OCH3), 3.5
(s, 3H, OCH3), 5.6 (s, 1H, OH), 6.5–8.5 (m, 13H, aromatic);
MS (m/z) 374 {(MKCH3OH)C}, 346, 315, 139, 111, and
other peaks; Anal. Calcd for C24H20O4: C, 70.85; H, 4.71.
Found: C, 71.03; H, 4.53.
4.3.1. 3,3-Dimethoxy-2-phenyl-2,3-dihydro-1-oxacyclo-
penta[l]phenanthren-2-ol (12a). Light yellow solid
(30%); mp 116–118 8C (dec.); IR nmax (KBr) 3408 cmK1
(OH); UV lmax (CH3CN) 224 (3 20,000), 248 (3 30,000),
302 nm (3 5100), 359 nm (3 2400); 1H NMR (CDCl3) d 3.3
(s, 3H, OCH3), 3.5 (s, 3H, OCH3), 5.6 (s, 1H, OH), 7.0–8.8
(m, 12H, aromatic); ESI-MS (m/z): calcd for C24H20NaO4
[MCNa]C 395.1. Found 394.9; HRMS (MKCH3OHC
Na)C found 363.0995. C23H16NaO3 requires 363.0997;
4.3.6. 3-Biphenyl-4-yl-3,3-dimethoxy-2,3-dihydro-1-oxa-
cyclopenta[l]phenanthren-2-ol (12f). Light yellow solid
(30%); mp 132–134 8C (dec.); IR nmax (KBr), 3404 (OH),
1635, 1610 and 1506 cmK1; UV lmax (CH3CN) 203
(3 60,000), 257 (3 59,000), 308 (3 9200), 341 nm (3 2400);
1H NMR (CDCl3) d 3.3 (s, 3H, OCH3), 3.5 (s, 3H, OCH3),

A. M. Jacob et al. / Tetrahedron 61 (2005) 4601–4607
4605
˚
5.6 (s, 1H, OH), 7.4–8.8 (m, 17H, aromatic); MS (m/z) 416
{(MKCH3OH)C}, 416, 388, 357, 181, and other peaks;
ESI-MS (m/z) calcd for C30H25O4 [MC1]C: 449.1. Found:
448.0; HRMS (MKCH3OHCNa)C Found: 439.1313.
C29H20NaO3 requires 439.1310; Anal. Calcd for
C30H24O4: C, 80.34; H, 5.39. Found: C, 80.30; H, 5.14.
10.9698(5) A, aZ100.368(5)8, bZ110.282(4)8, gZ
3
K3
˚
102.529(7)8, UZ836.51(11) A , ZZ4, DCZ1.351 g cm
,
mZ0.715 mmK1, (Cu Ka, lZ1.54180 A), TZ293 K. Of
3402 reflections measured on an Enraf-Nonius CAD4
diffractometer and corrected by Psi scan for absorption,
3172 were unique (RintZ0.0280). The structure was solved
by direct methods and refined on F2 values. Hydrogen atoms
were refined isotropically. RZ0.0537 [F values, IO2s(I)].
wR2Z0.1521 (F2 values, all data), goodness-of-fitZ1.141,
˚
4.4. Reaction of phenanthrenequinone and acetophenone
under oxygen bubbling
final difference map extremes C0.316 and K0.225 eAK3
.
Software: SHELXS-97, SHELXL-97.
A mixture of phenanthrenequinone (0.025 mol), aceto-
phenone (0.027 mol), and powdered potassium hydroxide
(1 g) in methanol (30 mL) was stirred at 60 8C, under
oxygen bubbling for 1 h and later kept in a refrigerator for
48 h. The solid product separated out was filtered and
purified by recrystallisation from a mixture (2:1) of
methanol and dichloromethane to give 12a (28%); mp
116–118 8C.
4.7.3. 3-Methoxy-3-p-tolyl-3H-1-oxacyclopenta[l]phen-
anthren-2-one (14b). White solid (82%); mp 160–162 8C;
IR nmax (KBr) 1813 cmK1 (lactone C]O); UV lmax
(CH3CN) 208 (3 19,600), 223 (3 14,000), 245 (3 19,300),
256 (3 15,900), 276 (3 7100), 308 (3 3400), 337 (3 800),
1
355 nm (3 700); H NMR (CDCl3) d 2.3 (3H, s, methyl); d
3.4 (3H, s, OCH3); 7.0–8.9 (12H, m, aromatic); 13C NMR
(CDCl3) d 21.5, 54.3, 85.3, 114.0, 120.1, 122.6, 122.8,
123.4, 123.5, 123.7, 124.0, 124.2, 126.0, 127.5, 127.5,
127.7, 128.0, 128.3, 128.8, 128.9, 129.1, 132.6, 149.6,
175.3.; MS (m/z) 354 (MC), 326, 91, and other peaks; Anal.
Calcd for C24H18O3: C, 81.34; H, 5.12. Found: C, 81.62; H,
5.34.
4.5. Reaction of phenanthrenequinone and 4-phenyl-
acetophenone in degassed methanol
A mixture of phenanthrenequinone (0.025 mol), 4-phenyl-
acetophenone (0.027 mol), and powdered potassium
hydroxide (1 g) in degassed methanol (30 mL) was stirred
at 60 8C, for 4 h and later kept in a refrigerator for 48 h.
The solid product separated out was filtered and purified by
recrystallisation from a mixture (2:1) of methanol and
dichloromethane to give 12f (28%); mp 132–134 8C.
4.7.4. 3-Methoxy-3-(4-methoxyphenyl)-3H-1-oxacyclo-
penta[l]phenanthren-2-one (14c). White solid (86%); mp
135–138 8C; IR nmax (KBr) 1815 cmK1 (lactone C]O); UV
lmax (CH3CN) 208 (3 23,300), 225 (3 20,000), 246
(3 29,900), 257 (3 22,000), 275 (3 10,500), 304 (3 5000),
339 (3 2400), 355 nm (3 2000); 1H NMR (CDCl3) d 3.3 (3H,
s, OCH3), 3.8 (3H, s, OCH3), 6.8–8.9 (12H, m, aromatic);
13C NMR (CDCl3) d 54.2, 55.3, 85.1, 114.1, 120.0, 122.6,
122.9, 123.4, 123.5, 123.7, 124.0, 124.2, 126.0, 127.5,
127.5, 127.7, 128.0, 128.3, 128.8, 128.9, 129.2, 132.5,
149.6, 175.3.; MS (m/z) 371 (MC), 341, 204, 135, and other
peaks; Anal. Calcd for C24H18O4: C, 77.82; H, 4.9. Found:
C, 78.1; H, 4.94.
4.6. Reaction of 2 equiv of phenanthrenequinone with
1 equiv of acetophenone
A mixture of phenanthrenequinone (0.05 mol), aceto-
phenone (0.025 mol), and powdered potassium hydroxide
(1 g) in methanol (30 mL) was stirred at 60 8C for 1 h and
later kept in a refrigerator for 48 h. The solid product
separated out was filtered and purified by recrystallisation
from a mixture (2:1) of methanol and dichloromethane to
give 12a (35%); mp 116–118 8C.
4.7.5. 3-(4-Bromophenyl)-3-methoxy-3H-1-oxacyclo-
penta[l]phenanthren-2-one (14d). White solid (82%);
mp 191–193 8C; IR nmax (KBr) 1811 cmK1 (lactone
C]O); UV lmax (CH3CN) 205 (3 23,000), 226 (3 19,500),
244 (3 29,200), 257 (3 21,000), 274 (3 10,000), 304 (3 4000),
339 (3 2400), 355 nm (3 2000); 1H NMR (CDCl3) d 3.3 (3H,
s, OCH3), 7.2–8.9 (12H, m, aromatic); MS (m/z) 420 (MC),
394, 361, 155, and other peaks; Anal. Calcd for
C23H15O3Br: C, 65.9; H, 3.61. Found: C, 65.56; H, 3.76.
4.7. Thermolysis experiments
4.7.1. Neat thermolysis of 12a. The samples 12a–f were
heated in sealed tubes to give phenanthro-2(3H)-furanones
14a–f. In a typical experiment, a sample of 12a (100 mg,
0.27 mmol) was heated in a sealed tube at 150 8C for 4 h.
The solid residue was chromatographed over silica gel.
Elution with a mixture (4:1) of hexane and dichloromethane
gave 14a (79 mg, 0.232 mmol).
4.7.6. 3-(4-Chlorophenyl)-3-methoxy-3H-1-oxacyclo-
penta[l]phenanthren-2-one (14e). White solid (81%); mp
193–194 8C; IR nmax (KBr) 1813 cmK1 (lactone C]O););
UV lmax (CH3CN) 208 (3 21,400), 245 (3 21,500), 258
(3 17,800), 276 (3 7900), 308 (3 24,100), 338 (3 1000),
4.7.2. -Methoxy-3-phenyl-3H-1-oxacyclopenta[l]phen-
anthren-2-one (14a). White solid (86%); mp 214–216 8C;
IR nmax (KBr) 1813 cmK1 (lactone C]O); UV lmax
(CH3CN) 208 (3 17,200), 222 (3 13,000), 245 (3 17,600),
257 (3 14,100), 276 (3 6300), 308 (3 3000), 337 (3 900),
1
354 nm (3 900); H NMR (CDCl3) d 3.3 (3H, s, OCH3),
7.2–8.8 (12H, m, aromatic); MS (m/z) 374 (MC), 346, 239,
163,111, and other peaks; Anal. Calcd for C23H15O3Cl: C,
73.85; H, 4.03. Found: C, 74.02; H, 4.05.
1
355 nm (3 800); H NMR (CDCl3) d 3.4 (3H, s, OCH3),
7.2–8.9 (13H, m, aromatic); MS (m/z) 340 (MC), 312, 281,
239, 125, and other peaks; Anal. Calcd for C23H16O3: C,
81.16; H, 4.74. Found: C, 81.22; H, 4.74.
4.7.7. 3-Biphenyl-4yl-3-methoxy-3H-1-oxacyclopenta-
[l]phenanthren-2-one (14f). White solid (82%); mp 152–
154 8C; IR nmax (KBr), 1813 cmK1 (lactone C]O); UV
Crystal data of (14a). C23H16O3. MZ340.36, triclinic,
space group P1, aZ8.4695(7) A, bZ10.2384(9) A, cZ
˚
˚

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A. M. Jacob et al. / Tetrahedron 61 (2005) 4601–4607
lmax (CH3CN) 207 (3 41,000), 262 (3 39,000), 306 (3 8000),
342 nm (3 4600) 338 (3 2800), 356 nm (3 1800); H NMR
30 min and then refluxed for 2 h. The mixture was cooled
and diluted with dichloromethane. Then it was washed
with dil. H2SO4, sodium bicarbonate, and with water. The
organic layer was separated, dried over anhydrous sodium
sulfate, and solvent was evaporated to give 16f. It was then
recrystallised from a mixture (2:1) of dichloromethane and
hexane.
1
(CDCl3) d 3.4 (3H, s, OCH3), 7.3–8.8 (17H, m, aromatic);
13C NMR (CDCl3) d 54.3, 85.4, 114.1, 120.1, 122.7, 123.5,
123.6, 124.2, 126.2, 126.7, 127.5, 127.6, 128.8, 128.9,
132.7, 136.2, 140.5, 142.0, 149.8, 175.1.; MS (m/z) 416
(MC), 388, 357, 281, and other peaks; Anal. Calcd for
C29H20O3: C, 83.64; H, 4.84. Found: C, 83.48; H, 4.86.
4.12.1. 2-Biphenyl-4-yl-3-oxo-2,3-dihydro-1-oxacyclo-
penta-[l]phenanthren-2-yl acetate (16f). White solid
(62%); mp 182–185 8C; IR nmax (KBr), 1774, 1713, 1620,
1600 cmK1; UV lmax (CH3CN) 202 (3 45,000), 255
(3 31,000), 306 (3 8200), 340 nm (3 2100); 1H NMR
(CDCl3) d 2.3 (3H, s, acetoxy), 7.2–8.8 (17H, m, aromatic);
13C NMR (CDCl3) d 20.7, 102.0, 108.0, 109.0, 120.8, 122.9,
123.1, 123.7, 124.0, 126.1, 126.9, 127.1, 127.3, 127.5,
127.7, 128.8, 131.8, 132.0, 136.0, 140.3, 143.0, 168.3,
171.2, 194.2; MS (m/z) 444 (MC), 386, 263, 181, and other
peaks; Anal. Calcd for C30H20O4: C, 81.07; H, 4.54. Found:
C, 81.30; H, 4.14.
4.8. Thermolysis in open vessel
The samples 12a–f were thermolyzed in open vessels to
give phenanthro-2(3H)-furanones. In a typical experiment,
a sample of 12a (100 mg, 0.27 mmol) was heated in conical
flask at 150 8C for 4 h. The solid was extracted with
dichloromethane. Column chromatography using a mixture
(4:1) of hexane and dichloromethane gave 14a as a white
solid (75 mg, 83%).
4.9. Thermolysis in o-dichlorobenzene
The samples 12a–f were refluxed in o-dichlorobenzene to
give the corresponding phenanthro-2(3H)-furanones. In a
typical experiment, a sample of 12a (100 mg, 0.27 mmol)
was dissolved in o-dichlorobenzene and refluxed for 4 h.
Solvent was removed under reduced pressure. The solid was
extracted with dichloromethane. Column chromatography
by using a mixture of hexane and dichloromethane (4:1)
gave 14a as a white solid (71 mg, 79%).
Acknowledgements
We thank Professor T. Pradeep for X-ray structure
determination. We also thank CDRI, Lucknow, STIC,
Kochi for elemental analysis and Professor A. Srikrishna
for HRMS data. Financial support from Department of
Science and Technology, India (SP/S1/G-23/2000) is
gratefully acknowledged. AMJ and RKT gratefully
acknowledge DST and CSIR for financial support in the
form of Junior Research Fellowships.
4.10. Reaction of 12a with base
A sample of 12a (100 mg, 0.27 mmol) was dissolved in
dichloromethane (10 mL) and potassium hydroxide (0.5 g)
in methanol (2 mL) was added and stirred for 12 h. The
progress of the reaction was monitored by TLC. No change
was observed. The solution was then refluxed for 6 h. and
was monitored by TLC. The unchanged 12a was recovered
almost quantitatively (90 mg).
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