Reaction of 4-R-2-phenyltetrahydrobenzo[b]pyrylium and 9-R-sym- octahydroxanthylium salts with furfural and related compounds

Article abstract of DOI:10.1007/s10593-008-0133-2

The condensation of furfural, 5-methylfurfural, and 2-thiophenecarbaldehyde with 4-R-2-phenyltetrahydrobenzo[b]pyrylium and 9-R-sym-octahydroxanthylium salts was studied. Condensation conditions under which acidophobic aldehydes of the furan series do not

Full text of DOI:10.1007/s10593-008-0133-2

Chemistry of Heterocyclic Compounds, Vol. 44, No. 8, 2008
REACTION OF 4-R-2-PHENYLTETRA-
HYDROBENZO[b]PYRYLIUM AND
9-R-sym-OCTAHYDROXANTHYLIUM
SALTS WITH FURFURAL AND RELATED
COMPOUNDS
D. R. Sharafutdinov1, E. V. Kapitonova1, and S. K. Klimenko2
The condensation of furfural, 5-methylfurfural, and 2-thiophenecarbaldehyde with 4-R-2-phenyl-
tetrahydrobenzo[b]pyrylium and 9-R-sym-octahydroxanthylium salts was studied. Condensation
conditions under which acidophobic aldehydes of the furan series do not undergo resinification were
found. 8-Furfurylidene and 8-thenylidene derivatives of 4-R-2-phenyltetrahydrobenzo[b]pyrylium and
bisfurfurylidene and bisthenylidene derivatives of 9-R-sym-octahydroxanthylium salts were obtained for
the first time.
Keywords: 5-methylfurfural, 9-R-1,8-bis[thenylidene]-sym-octahydroxanthylium and 9-R-1,8-bis-
[R1-furfurylidene]-sym-tetrahydrobenzo[b]pyrylium, 9-R-sym-octahydroxanthylium and 4-R-2-phenyl-
tetrahydrobenzo[b]pyrylium, 4-R1-2-phenyl-8-thenylidenetetrahydrobenzo[b]pyrylium and 4-R-2-phenyl-
8-(R1-furfurylidene)tetrahydrobenzo[b]pyrylium salts, thiophenecarbaldehyde, furfural, condensation.
It is known [1-3] that derivatives of cycloalka[b]chalcogenopyrylium salts are used as dyes in passive
switches of optical quantum generators OQG [3], are used in photocompositions without silver salts [4], and
exhibit biological activity [2]. Earlier we demonstrated that cycloalka[b]chalcogenopyrylium salts containing a
γ-methyl group in the heterocycle and an α-methylene group in the alicycle react with aromatic aldehydes under
harsh conditions at 100°C in a 1:3 mixture of acetic acid and acetic anhydride and form quantitative yields of the
corresponding arylidene derivatives [1].
R
R
+
+
O
O
Ph
–
–
A
A
1a,b
2a,b
1 A = BF4; 2 A = ClO4; 1,2 a R = H, b R = Ph
__________________________________________________________________________________________
1Saratovorgsintez, Saratov 410059, Russia. 2N. I. Vavilov Saratov State Agrarian University, Institute of
Veterinary Medicine and Biotechnology, Saratov 410001, Russia; e-mail: kvazar19@yandex.ru. Translated from
Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1160-1165, August, 2008. Original article submitted
February 20, 2007.
0009-3122/08/4408-0931©2008 Springer Science+Business Media, Inc.
931

Before the present work furfurylidene derivatives of cycloalka[b]chalcogenopyrylium salts had not been
described. In so far as furfural is a well known pharmacophore it seemed of interest to study the possibility of
synthesizing furfurylidene derivatives of 4-R-2-phenyltetrahydrobenzo[b]pyrylium 1a,b and 9-R-sym-octa-
hydroxanthylium 2a,b salts.
The aim of the present work was to study the condensation of the above-mentioned salts with furfural,
5-methylfurfural, and thiophenecarbaldehyde and also to develop a preparative method for the synthesis of their
furfurylidene derivatives. The conditions in [1] described above are unsuitable for the condensation of
chalcogenopyrylium salts 1 and 2 with acidophobic aldehydes of the furan series. In the present work we
realized this condensation by boiling the reagents in isopropyl alcohol with phosphorus(V) oxide as dehydrating
agent.
Under the developed conditions (method A, see the experimental section) with furfural, 5-methyl-
furfural, and thiophenecarbaldehyde under mild conditions the salts 1,b are converted into the corresponding
products 3a-f with yields of 60-80%.
R
R
+
+
O
O
Ph
–
–
A
A
Y
Y
Y
R1
R1
R1
4a–d
3a–f
3 а–f A = BF4, a–d Y = O; a R = R1= H, b R = H, R1 = Me, c R = Ph, R1= H, d R = Ph, R1 = Me;
e, f Y = S, e R = R1 = H, f R = Ph, R1= H; 4 a–d A = ClO4, a–c Y = O, a R = R1 = H, b R = Ph,
R1 = H, c R = Ph, R1 = Me; d Y = S, R = Ph, R1 = H
TABLE 1. The Characteristics of the Synthesized Salts 3a-f and 4a-d
Found, %
Calculated, %
Yield %,
in method
A (B)
——————
Com-
pound
Empirical
formula
mp, °C
С
Н
S
C20H17BF4O2
C21H19BF4O2
C26H21BF4O2
C27H23BF4O2
C20H17BF4OS
C26H21BF4OS
C23H21ClO7
63.83
63.86
65.37
64.64
69.37
69.05
69.24
69.55
59.67
61.25
67.51
66.68
62.05
62.10
66.53
66.86
4.51
4.56
4.17
4.91
4.81
4.68
4.15
4.97
3.89
4.37
5.03
4.52
4.96
4.76
5.13
4.84
174-176
222-223
246-248
228-229
195-196
264-266
>300
80 (87)
87 (99)
60 (72)
81 (98)
82 (92)
80 (88)
Quant.
Quant.
Quant.
Quant.
3a
3b
3c
3d
3e
3f
8.76
8.17
7.12
6.85
4a
4b
4c
4d
С29Н25СlО7
>300
С31Н29СlО7
67.57
67.82
63.07
62.98
5.67
5.32
4.75
4.56
>300
С29Н25ClO5S2
11.13
11.59
>300
932

933

The yields of the indicated products are increased appreciably if a 1:1 mixture of acetonitrile and
isopropyl alcohol is used as reaction medium (method B) (Table 1).
9-R-sym-Octahydroxanthylium salts 2a,b react smoothly with the aldehydes in isopropyl alcohol at the
two α-methylene groups, forming quantitative yields of compounds 4a-d.
The composition and structure of the obtained products were supported by the results of elemental
1
analysis and also by the data from H NMR and UV spectroscopy. The yields and characteristics of the
synthesized substances are presented in Tables 1-3.
1
The H NMR spectra of compounds 3a-f contain a signal for the H-3 proton of the pyrylium cation in
the region of 8.7-8.5 ppm in the form of a broad doublet for R = H (salts 3a,b,e), indicating that this proton is
coupled with the H-4 proton. If R = Ph the H-3 signal appears in the form of a singlet, while the H-4 signal is
absent. The protons of the other fragments of the compounds resonate in the upfield region. Thus, the multiplets
for the protons of the phenyl substituents at the meta positions are at 8.5-8.3 ppm while those for the ortho and
para protons are at 7.8-7.7 ppm. The signals for the protons of the hetaryl substituent are observed in the region
of 8.4-6.8 ppm, those for the methine proton at 8.5-6.5 ppm, and those for the alicyclic fragment in the region of
3.3-2.0 ppm. The spectra of compounds 4a-d differ from those described above in twice the intensity for the
signals of the protons of the alicyclic and hetaryl fragments and also in the absence of a signal corresponding to
the H-3 proton of the pyrylium derivatives 3. With the exception of the signal for the methine proton the
heteroatom of the Het substituent in compounds 3 and 4 does not have an appreciable effect on the position of
the signals for the protons of the compounds; replacement of the oxygen atom by a sulfur atom leads to a
substantial shift of this signal by 1.64 (cf. compounds 3a and 3e), 1.67 (3c and 3f), and 1.02 ppm (4b and 4d),
which is explained by the descreening effect of the sulfur atom. The introduction of a methyl substituent into the
furan ring leads to a small upfield shift of the methine proton: by 0.28 (cf. compounds 3a and 3b), 0.30
(compounds 3c and 3d), and 0.41 ppm (compounds 4b and 4c), due probably to the induction effect of this
substituent.
The most interesting feature of the electronic spectra of the synthesized compounds 3 and 4 is the
bathochromic shift of the long-wave band (∆λmax, nm) in relation to its position in the spectra of the initial
compounds 1 and 2, which amounts to 144-305 nm (Table 3). During comparison of the spectral characteristics
of compounds 3a and 3b, 3c and 3d, and 4b and 4c it is possible to see a small bathochromic shift (16-36 nm)
for the spectra of compounds 3b,d and 4b having a methyl substituent in Het. The furfurylidene and thenylidene
derivatives 3a and 3e and 3c and 3f do not reveal substantial differences in the position of the long-wave
absorption band.
TABLE 3. Long-Wave Absorption in the Electronic Spectra of the Initial
Salts 1a,b and 2a,b and Condensation Products 3a-f and 4a-d (Solutions in
MeOH)
Initial
compounds
Condensation
product
λ
max, nm (log ε)
λ
max, nm (log ε)
∆λmax, nm
1a
1b
364 (4.12)
3a
3b
3e
513 (4.129)
540 (4.222)
508 (4.417)
149
176
144
368 (4.20)
3c
3d
3f
528 (3.685)
544 (4.469)
528 (3.742)
160
176
160
2a
2b
316 (3.256)
317 (4.026)
4a
584 (3.383)
268
4b
4c
4d
586 (4.104)
622 (3.805)
582 (3.693)
269
305
265
934

Thus, we have obtained previously unknown 8-furfurylidene and 8-thenylidene derivatives of 4-R-2-
phenyltetrahydrobenzo[b]pyrylium salts and also bisfurfurylidene and bisthenylidene derivatives of 9-R-sym-
octahydroxanthylium salts and have proposed conditions for their preparative synthesis.
EXPERIMENTAL
The UV spectra were recorded in methanol on a Specord M-40 spectrophotometer with layer thickness
1
0.1 cm and concentration 10−3 M. The H NMR spectra were obtained in DMSO-d6–CCl4 solution on a Bruker
AC-300 spectrometer (300 MHz).
The initial salts 1a,b and 2a,b were synthesized by known methods [6, 7].
Condensation of Tetrahydrobenzo[b]pyrylium Salts (1a,b) with Furfural, 5-Methylfurfural, or
Thiophene-2-carbaldehyde (General Method). A. To a solution of P2O5 (2.5 mmol) in dry isopropyl alcohol
(15 ml) we added the pyrylium salt 1 (2.5 mmol). The mixture was heated until the salt had completely
dissolved, the aldehyde (2.5 mmol) was added drop by drop, and the mixture was shaken. The crystals of the
product 3 that separated were removed and washed with alcohol and ether.
B. The reaction was conducted as described above with ~10 ml of a 1:1 mixture of isopropyl alcohol and
acetonitrile as solvent.
Condensation of 9-R-sym-octahydroxanthylium Salts 2a,b with Furfural, 5-Methylfurfural, or
Thiophene-2-carbaldehyde. We dissolved the salt 2 (1.3 mmol) in a solution of P2O5 (0.37 g, 2.6 mmol) in dry
isopropyl alcohol (6 ml) and added the aldehyde (2.6 mmol). The reaction mixture was boiled for 1-10 min. The
separated crystals of the product 4 were removed, washed with alcohol and ether, dried, and purified by
recrystallization from a 1:1 mixture of isopropyl alcohol and acetonitrile.
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