Elimination, ring-contraction, and fragmentation reactions of 1-thioflavanone 1-oxides

Article abstract of DOI:10.1139/v01-096

Epimeric thioflavanone sulfoxides (2b) were selectively transformed into thioflavone (1a), thioaurone (3a), and di(2-cinnamoylphenyl) disulfide (4). Disulfide 4 can be recyclized into thioaurones (3a-c) and thioflavanones (2a, 5) with heterolysis of the S - S bond. The 3-p-anisylidene sulfoxide analog of 2b (6) transforms, with fragmentation, into 4′-methoxythioaurone 3b.

Full text of DOI:10.1139/v01-096

1159
Elimination, ring-contraction, and fragmentation
reactions of 1-thioflavanone 1-oxides
László Somogyi
Abstract: Epimeric thioflavanone sulfoxides (2b) were selectively transformed into thioflavone (1a), thioaurone (3a),
and di(2-cinnamoylphenyl) disulfide (4). Disulfide 4 can be recyclized into thioaurones (3a–c) and thioflavanones (2a,
5) with heterolysis of the S—S bond. The 3-p-anisylidene sulfoxide analog of 2b (6) transforms, with fragmentation,
into 4′-methoxythioaurone 3b.
Key words: chalcone, ring contraction, sulfoxides, thioaurones, thioflavonoids.
Résumé : On a réalisé la transformation sélective des sulfoxydes épimères de la thioflavanone (2b) en thioflavone (1a),
thioaurone (3a) et en disulfure de di(2-cinnamoylphényle) (4). Le disulfure 4 peut être recyclisé en thioaurones (3a–b)
et en thioflavanones (2a, 5) avec hétérolyse de la liaison S—S. Le sulfoxyde de 3-p-anisylidène analogue de 2b (6) su-
bit une fragmentation et se transforme en 4′-méthoxythioaurone (3b).
Mots clés : chalcone, contraction de cycle, sulfoxydes, thioaurones, thioflavonoïdes.
[Traduit par la Rédaction] Somogyi 1165
Introduction
In the course of our efforts for the synthesis of
1-thioflavone derivatives, dehydrogenation of the
dihydrobenzothiopyran (thioflavan) ring became necessary.
Several methods useful in flavonoid chemistry have been
successfully adopted (1, 3c) for thioflavanones, however, in
some cases these methods were found either ineffective or,
just the opposite; destructively drastic. Due to the greater
variability of the electronic states of sulfur as compared to
that of oxygen, additional methods, e.g., treatment (2) with
PCl5, or ceric ammonium nitrate (CAN) oxidation (3a–c) of
thioflavanone (2a), and also alkali-mediated (2, 3b) or
Pummerer-type transformation (3b) of sulfoxides (e.g., 2b,
in ca. 50% yield) have been successfully applied for the
dehydrogenation. The PCl5, CAN, or alkali-mediated trans-
formations may have severe limitations when sensitive func-
tional groups are present. As to the Pummerer-type
transformation, 2-methyl-1-thiochroman 1-oxides (7a, b)
have been transformed (3d) into the corresponding
thiochromenes (8a, b) in nearly quantitative yields by treat-
ment with acetic anhydride. However, due to the different
electronic properties of the Me and Ph groups, the sulfoxides
2b and 6-Me-2b undergo also ring-contraction reaction pho-
tochemically (4b, 4d), thermally (1), or upon treatment with
I2–DMSO (1) to give thioaurones (3a, 5-Me-3a) merely in
14–70% yields. Because of the relatively poor yields and the
competing formation of 1a and 3a, we decided to find a se-
lective conversion of thioflavanone 1-oxide (2b) into 1a or
3a.
Received December 19, 2000. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on August 3, 2001.
L. Somogyi. Department of Organic Chemistry, University of Debrecen, P.O. Box 20, H-4010 Debrecen, Hungary.
(Fax: + 36 (52) 453-836).
Can. J. Chem. 79: 1159–1165 (2001)
DOI: 10.1139/cjc-79-7-1159
© 2001 NRC Canada

1160
Can. J. Chem. Vol. 79, 2001
Fig. 1. ORTEP drawing of compound 4 with 50% of the thermal
ellipsoids.
On varying the acid or base character of the additives,
treatment of sulfoxide 2b with Ac2O–Et3N or
(EtCO)2O–Et3N, and also with 1,3-diisopropylcarbodiimide
(DIC, see Table 1), gave a new high-melting yellow product
sparingly soluble in most of the common organic solvents.
(In the reaction of 2b with DIC a considerable amount of
thioflavanone 1,1-dioxide 2c was also produced, see Ta-
ble 1). At room temperature, the Ac2O–Et3N-mediated trans-
formation of the equatorial (1,2-trans) sulfoxide epimer (5)
2b afforded this product in a yield half times higher than the
axial (1,2-cis) (3c, 5) compound (see Table 1). One the basis
of C, H, and S analyses (see footnotes of Table 1) and the
1
nonanalyzable 200 MHz H NMR spectrum, a C15H11OS
(FW = 239.31) elemental composition was first suggested
for the new compound with some uncertainty, as the hydro-
gen content was concerned. The product was, however, quite
Results and discussion
1
different in all respects (color, mp, TLC, solubility, IR, H
The conversions of thioflavanone 1-oxide (2b) into either
thioflavone (1a) or thioaurone (3a) proceed with loss of the
elements of water, which are consumed by Ac2O. The initial
steps of these elimination reactions are, however, different.
The Pummerer-type reactions of sulfoxides involve (3e, 3f)
transient formation of sulfur and sulfur-stabilized carbonium
cations. Accordingly, reaction conditions favouring the acti-
vation of the acyl moiety of the anhydrides may be advanta-
geous for the formation of thioflavone, which starts with an
electrophilic attack at the sulfoxide group exerted by the
proton of the acid catalyst, or by the acetyl cation generated
from Ac2O by protonolysis or by the electrophilic acetyl
moiety of Ac2O being activated by the strong nucleophile
(15) 4-dimethylaminopyridine (DMAP) (see Scheme 1).
Thus, treatment of sulfoxide 2b with Ac2O–(TsOH),
Ac2O–DMPA, or TFAA–Et3N afforded 1a in excellent
yields (>80% for the pure product, see Table 1).
NMR and EI-mass spectra, etc.) from 1a and 3a, respec-
tively, (C15H10OS, FW = 238.3) or 2a (C15H12OS, FW =
240.3). Eventually, the MALDI-TOF spectrometry (see also
Table 1, footnotes) indicated a C30H22O2S2 (FW = 478.61)
composition. X-Ray diffraction analysis of this new com-
pound revealed the open-chain disulfide structure 4 (see
Fig. 1) with an S—S bond distance of 2.053 Å. (Previously,
photolysis of thiochroman-4-one 1-oxides has been reported
(4d) to give o-hydroxyaralkyl disulfides in 6–8% yields via
thiyl radicals). The trans-chalcone structure of compound 4
was corroborated by the J = 15.77 Hz coupling of the
vinylene hydrogens at
500 MHz H NMR spectrum. Disulfide 4 was presumably
produced from sulfoxide 2b via
δ
= 7.90 and 7.77 ppm in the
1
a
2-cinnamoylbenzenesulfenic acid intermediate (9a). This
step of the ring-opening of 2b seems to be similar to the
well-known
base-mediated
formation
of
The methylene group of 1-(thio)benzopyran-4-ones is ac-
tivated by the neighboring carbonyl and -XCH(Ph)- groups
(X = O, S, SO, SO2), so this is the reactive site of the mole-
cule when treated with bases. Thus, the deprotonated C-3
creates a bond to the electron-deficient sulfur, and then the
transiently generated thiiranium (4) species is stabilized by
ring contraction and loss of proton to give
2-benzylidenebenzo[b]thiophen-3(2H)-one (thioaurone 3a,
see Scheme 1). Accordingly, treatment of sulfoxide 2b with
Ac2O in the presence of NaOAc, which behaved as a strong
base in this medium, afforded thioaurone (3a) almost quanti-
tatively (see Table 1).
2′-hydroxychalcones (e.g., 9b) from flavanones, however,
sulfenic acids (3g) are generally highly unstable. Decompo-
sition of pseudopeptidic sulfoxides via β-elimination to
sulfenic acids (followed by bimolecular recombination to
thiolsulfinates RS(O)SR) and the corresponding alkene has
been reported most recently (16). Upon treatment with
trihaloacetic anhydride [(CX3CO)2O, X = F, Cl] in a
Pummerer-like reaction, thiolsulfinates are known (17) to
transform into disulfides and sulfinyl carboxylates
[RS(O)OCOCX3] which latter gives adducts readily with
olefins.
© 2001 NRC Canada

Somogyi
1161
Scheme 1.
Before performing the MALDI-TOF MS and X-ray mea-
surements discussed above to lead to the identification of the
disulfide-structure for 4, EI-MS (see footnotes of Table 1)
and HR-MS experiments were carried out with the m/z =
240 ion (M = 240.0599 Da). However, neither these mass
idene)thioflavanone (5, see Scheme 2). Cleavage of the
S—S bond, exhibiting acid–base properties can be effected
by both nucleophiles or bases and electrophiles (3g).
The aforementioned anisaldehyde–piperidine-mediated
recyclization of
4
into the thiaindanone and
1
spectral studies nor the 200 and 360 MHz H NMR spectra
thiobenzopyranone derivatives 3b and 5 can be explained by
the heterolysis of the S—S bond followed by cyclization
into 3a and 2a (see Scheme 3), and their subsequent trans-
formation with the aldehyde into 3b and 5, respectively. In-
deed, treatment of thioaurone 3a with 4-anisaldehyde in
piperidine at 150°C for 1.5 h afforded ca. 65% of 3b. Com-
pound 3b was also formed in good yield in a fragmentation
(each containing superimposed signals in the “aromatic” re-
gion; δ = 8.16–7.42 ppm) gave convincing evidence for the
structure of the new compound. The fragment m/z = 237
(base peak in the EI-MS) could be attributed to the forma-
tion of the benzo[b]thieno-[3,2-b]pyrylium ion (A), whose
perchlorate has been previously prepared (7) by the conden-
sation of 1-thia-3-indanone with salicylic aldehyde in
AcOH–HClO4. The same fragment was observed in the
EI-MS of 1,2-cis-2b (3c, 5) and 3a as a base peak, but with
lesser intensity for the 1,2-trans-2b, (5) and not at all in the
spectrum of thioflavone (1a) where m/z = 238 [M+·] was the
base peak (5).
reaction
upon
treatment
of
3-(4-methoxybenzyl-
idene)thioflavanone 1-oxide (6) with Ac2O–TsOH (see
Table 1). Recently, deprotection–cyclization of 4-meth-
oxybenzyl-protected 2′-mercaptochalcones into thio-
flavanones has been reported (14).
Since the reaction of other sulfoxides with active methy-
lene compounds in the presence of dehydrating agents
(Ac2O, P4O10, DCC) has been reported (6) to furnish sulfon-
ium ylides, a possible formation of 1,2- or 1,3-sulfur ylides
similar to the thiiranium intermediate mentioned above was
considered. Therefore, the reactivity of the sparingly soluble
new compound 4 (at that time with uncertain structure) was
also examined (see Table 2). Thus, treatment with
Ac2O–H2SO4 afforded thioflavone (1a) and thioaurone (3a)
in poor yields. Similarly, when reacted with the I2–DMSO
couple or (in contrast to the transformation of 2b in acid me-
dium) with hot AcOH or cold sulfuric acid again 3a was
formed, and the reaction with CAN resulted in thioaurone
1,1-dioxide (3c). Interestingly, upon treatment with
4-methoxybenzaldehyde–piperidine (which is known to
transform 2a into 5), compound 4 underwent transformation
into 4′-methoxythioaurone (3b) and 3-(4-methoxybenzyl-
Experimental
Melting points (uncorrected): Kofler block. Solutions
were concentrated under reduced pressure in a rotary evapo-
rator (<50°C, bath). TLC: Kieselgel 60 F254 (Merck,
Alurolle). IR(KBr discs): PerkinElmer 16 PC-FT spectrome-
1
ter. H NMR: 200 MHz, Bruker WP 200 SY and 500 MHz,
Bruker DRX 500 spectrometers. MS: VG-7035 GC–MS–DS
(EI, 70 eV, direct insertion technique); high-resolution spec-
trometry (EI, 70 eV, resolution 10 000, accuracy ±5 ppm, di-
rect insertion), AEI MS-902; MALDI-TOF measurements,
Bruker BIFLEX IIITM mass spectrometer (19 kV accelera-
tion voltage with pulsed ion extraction (PIETM); detection of
the positive ions, reflection mode (20 kV); laser desorption,
nitrogen laser (337 nm, 1 ns pulse width) operating at 4 Hz,
100–120 shots were summed; external linear calibration,
with poly(ethylene glycol) (Mn = 1450, MWD = 1.02). Solu-
© 2001 NRC Canada

Table 1. Transformation of sulfoxides 2b and 6.a
Substrate
(mmol)
Reaction temperature
(°C)b (time (h))
% Yield
Agents (mmol)
Work-upc
Product
crude (pure)d
Mp (°C) (solvent)
Lit. mp (°C) (solvent)
2b (1)
Ac2O (53)
TsOHe
100 (24)
B, C
B, D
A
E, F
A
A
A
B, G, H
I
B, C, D, F
1a
1a
1a
3a
4s
95.3 (76.9)
(80)i
125 (2-PrOH–hexane)
125 (2-PrOH)
125f (EtOH)
125f (EtOH)
125f (EtOH)
2bg (5.5)
2bg (3)
2bn (2)
2br (6)
2bu (6)
2bu (2)
2br (3)
Ac2O (148)
TFAAj,k (63)
Ac2O (53)
Ac2O (159)
Ac2O (159)
(EtCO)2O (55)
DICw,x (7)
DMAPh (8.25)
Et3Nk,l (14.4)
NaOAco (10)
Et3Nl (29)
Room temp. (72)
Room temp. (2)
105 (4)
Room temp. (22)
Room temp. (19)
Room temp. (24)
Room temp. (24)
85d
124.5m
99.5p (75)
41d (39.4)
65.3d (53)
52.6d (41)
26d
132 (MeOH)
134 to 134.5 (EtOH)
q
229–230m, 238–240t
228–230m, 238v
226–227m, 236–237t
238m
Et3Nl (29)
Et3Nl (7.2)
4s
4s
4s
2cy
3b
(40)
80 (68)
155.5 (2-PrOH)
158 to 159 (2-PrOH)
155 to 156z (2-PrOH)
158 to 159q (EtOH)
6aa (2)
Ac2O (74)
TsOHe
100 (24)
b
c
d
e
f
g
h
aFor general methods see Experimental. Bath. See Experimental. Without work-up of mother liquors. 4-Toluenesulfonic acid, a catalytic amount. Reference 2. 1,2-cis. 4-Dimethylaminopyridine.
iAfter purification by column chromatography (silica gel 60, 0.063–0.2 mm, 110 g, r ~ 4.8 cm, l = 12 cm, CHCl3). Trifluoroacetic anhydride. CAUTION! The agents should be mixed carefully with
j
k
ice–salt cooling. Dried with and distilled from P4O10. mCrude product. Either 1,2-cis or trans. Anhydrous. Contaminated by a small amount of 1a when obtained from 1,2-trans-2b and by a somewhat
l
n
o
p
larger one starting from the 1,2-cis diastereomer. Reference 12. The 1,2-cis isomer. MALDI-TOF-MS (m/z): 584.98 [M + 107Ag+] (calcd. 585.01). EI-MS m/z (%): 240(12), 239(20), 238(63), 237(100),
q
r
s
208(6), 178(3), 165(10), 149(3), 136(23), 134(16). IR (KBr)ν (cm–1): 1650 (s), 1588 (s), 1574 (s), 1496 (w), 1454 (m), 1448 (m), 1338 (s). H NMR ([CD3]2SO, 500.13 MHz) at 700 K δ: 7.90 and
1
7.77 (each d, 1H, J = 15.77 Hz; 2 trans H-vinylene). Anal. calcd. for C3u0H22O2S2 (478.61) (%): C 75.28, H 4.63, S 13.40; found (%): C 75.20, H 4.85, S 13.41. For the stereostructure of the molecule
t
v
x
see Fig. 1. From DMF with addition of some water to the hot solution. The 1,2-trans isomer. Recrystallized from Ac2O. w1,3-Diisopropylcarbodiimide. In the presence of anhydrous benzene (5 mL)
y
z
with or without addition of anhyd Et3N (21 mmol). Identical (TLC, CHCl3–EtOAc (95:5)) with an authentic specimen. Reference 3a: 155°C or 156 to 157°C (from 2-PrOH); ref. 3c: 155 to 156°C
(from 2-PrOH). aaPrepared according to ref. 13.
Table 2. Transformation of disulfide 4 into thioflavone (1a) and thioaurones 3a, b, c.a
Substrate
(mmol)
Reaction temperature
(°C)b (time (h))
% Yield
Agents (mmol)
Ac2O (265)
Work-upc
E, J, F
Product
crude (pure)d
Mp (°C) (solvent)
Lit. mp (°C) (solvent)
125f (EtOH)
e
4 (1.75)
H2SO4 (12.6)
Room temp. (20)
1a
3a
3a
3a
3a
3bk
3cn
(9)
(12)
(50)
49 (43)
(42)
50d (34)
(23)
124 (2-PrOH)
133 to 134 (2-PrOH)
133 to 134 (2-PrOH)
134 (2-PrOH)
132 to 133 (2-PrOH)
159.5 (2-PrOH)
134 to 134.5g (EtOH)
134 to 134.5g (EtOH)
134 to 134.5g (EtOH)
134 to 134.5g (EtOH)
158 to 159g (EtOH)
156–158 (EtOH)o, 164 to
165 (Me2CO–CH2Cl2)p
4 (0.5)
4 (0.5)
4 (0.125)
4 (3)
AcOHh (873)
Bp (21)
Room temp. (0.083)
100 (24)
152 (1.2)
75 (3m + 2.5)
B, D, K
E, J, K
B, L, K
B, C, K
B, D, F
e
H2SO4 (18)
I2 (0.25)
4-MBAi (6.9)
CANl (13.7)
DMSO (35.2)
C5H11Nj (4.6)
4 (1.5)
160 (2-PrOH)
b
c
d
e
f
g
h
i
aFor general methods see Experimental. Bath. See Experimental. Without work-up of mother liquors. Concentrated (96%). Reference 2. Reference 12. 99 to 100%. 4-Methoxybenzaldehyde.
jPiperidine. Besides, 3-(4-methoxybenzylidene)thioflavanone (5, mp 131°C (from 2-PrOH), ref. 11: 132°C (from MeOH)) could be isolated in 3% yield. (NH4)2Ce(NO3)6, in the presence of MeCN (200 mL) and
k
l
H2O (20 mL). mInput. IR (KBr) ν (cm–1): 1708 (C=O), 1296 and 1156 (SO2); ref. 13: 1708, 1294, and 1156 cm–1. Reference 3b. Reference 13.
n
o
p

Somogyi
1163
Scheme 2.
Scheme 3.
© 2001 NRC Canada

1164
Can. J. Chem. Vol. 79, 2001
tions for preparation of samples: 1,8-dihydroxy-9(10H)-
anthracenone matrix–THF (20 mg mL–1), AgTFA–THF
(5 mg mL–1) to enhance the cationization; 0.5 µL of the mix-
ture of solutions analyte (1 mg 4 mL–1 CH2Cl2) – matrix –
AgTFA in 10:50:1 v/v ratio was deposited onto the sample
plate (stainless steel) and allowed to air-dry). X-ray diffrac-
tion analysis (for yellow plate crystals (0.48 × 0.39 ×
0.1 mm) of C30H22O2S2, grown from Ac2O, M = 478.6,
orthorhombic, a = 10.387(1) Å, b = 12.298(1) Å, c =
2 H, 3′′, 5′′-H), 3.94 (s, 2 H, CH2), 3.73 (s, 3 H, OMe). Anal.
calcd. for C23H18O2S (%): C 77.07, H 5.06; found (%): C
77.26, H 5.08.
Acknowledgements
The author is indebted to Prof. L. Szilágyi, D.Sc. (Depart-
ment of Organic Chemistry, University of Debrecen) for the
1
500 MHz H NMR spectrum of 4, to Z. Dinya, Ph.D. (De-
18.471(1) Å, V = 2359.7 Å3, Z = 8, space group: Pbcn, ρcalc
=
partment of Organic Chemistry) for the EI-MS (VG-7035),
and also to Á. Gömöry and K. Vékey, Ph.D. (Chemical Re-
search Center, Hungarian Academy of Sciences, Budapest)
for the high-resolution mass spectra (AEI MS-902), as well
as to S. Kéki, Ph.D. (Department of Applied Chemistry) for
the MALDI-TOF MS. A. Cs. Bényei, Ph.D. (Laboratory for
X-ray Diffraction Analysis, University of Debrecen) is
thanked for the X-ray diffraction analysis supported by a
Tempus JEP (Grant No. 9252–95) and the Hungarian Scien-
tific Research Fund (OTKA Grant No. D25136 and
M28249). The author is greatly obliged to the Hungarian
Scientific Research Fund (for a grant, OTKA T025016).
1.347 g cm–3): Enraf–Nonius MACH3 diffractometer; data
were collected at 293(1) K, Mo Kα radiation λ = 0.71073 Å,
ω-2Θ motion, Θmax = 27.8°, 2222 reflections of which 1612
were unique with I > 2σ(I); decay: 4%. The structure was
solved using the SIR-92 software (8) and refined on F2 us-
ing SHELX-97 program (9); publication material was pre-
pared with the WINGX-97 suite (10); R(F) = 0.039 and
wR(F2) = 0.094 for 2222 reflections, 154 parameters.
General method for the preparation and work-up of
the reaction mixtures
The substrate was added to a mixture of the reagents and
allowed to react, if necessary with stirring, as indicated in
Tables 1 and 2. The reaction mixtures were processed:
(A) The product was filtered off from the cold reaction
mixture; (B) The reaction mixture was concentrated; (C) The
residue was triturated with MeOH and (or) water to give a
crude product; (D) A CHCl3 solution of the residue was
washed with (when DMAP was used at first with aq.
KHSO4) aq. NaHCO3 and water, dried (MgSO4), treated
with fuller’s earth and charcoal and then concentrated; (E)
The cold reaction mixture was poured into ice water; (F)
The crude product was purified by column chromatography
(eluent, CHCl3); (G) The residue was triturated with hexane
to give a crude product; (H) The crude product was
triturated with CHCl3 and the undissolved material was fil-
tered off; (I) The material dissolved in the filtrate was puri-
fied by column chromatography (eluent, CHCl3); (J) The
mixture was extracted with CHCl3 and processed as in (D);
(K) Crystallization from the solvent indicated in Table 1 or
2; (L) A CHCl3 solution of the residue was washed with a
necessary amount of aq. Na2S2O3 and water, dried (MgSO4),
treated with charcoal and then concentrated.
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3-(4-Methoxybenzyl)thioflavone (1b)
According to the known method (11), a mixture of
anisaldehyde (8.753 g, 98%, 63 mmol), thioflavanone (2a,
14.418 g, 60 mmol), and piperidine (2.5 mL, 25.28 mmol)
was heated at 152°C (bath) for 2.5 h (instead of the 1 h re-
ported), cooled and treated with MeOH to give crude
(6.311 g, 29.3%, mp 123–126°C) or recrystallized
3-(4-methoxybenzylidene)thioflavanone (5, 5.572 g, 25.9%),
mp 133–133.5°C (from 2-PrOH); ref. 11: 132°C (from
MeOH), yield 52.2%. The mother liquor of the crude prod-
uct 5 was concentrated and the residue triturated with EtOAc
(10 mL) to give, after successive addition of hexane
(30 mL), crude (2.623 g, 12.2%) or recrystallized colorless
1b (2.245 g, 10.4%), mp 148.5°C (from 2-PrOH). IR (KBr)
(cm–1) ν: 1618 (C=O). 1H NMR (CDCl3, 360 MHz) δ:
8.57–8.55 (m, 1 H, 5-H), 7.60–7.36 (m, 8 H, 6,7,8-H and
Ph), 6.96–6.93 (d-shaped m, 2 H, 2′′, 6′′-H) 6.73–6.69 (m,
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© 2001 NRC Canada