Base catalysed synthesis of thiochromans and azo-linked chromenes using allenylphosphonates

Article abstract of DOI:10.1039/c2ob26285a

An efficient base catalysed approach to the synthesis of thiochromans/chromenes from allenylphosphonates (and an allenoate) and substrates having SH/OH and CHO groups at appropriate positions has been developed. Several azo-linked chromenes that are brigh

Full text of DOI:10.1039/c2ob26285a

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PAPER
Base catalysed synthesis of thiochromans and azo-linked chromenes using
allenylphosphonates†
M. Phani Pavan, M. Nagarjuna Reddy, N. N. Bhuvan Kumar and K. C. Kumara Swamy*
Received 5th July 2012, Accepted 10th August 2012
DOI: 10.1039/c2ob26285a
An efficient base catalysed approach to the synthesis of thiochromans/chromenes from
allenylphosphonates (and an allenoate) and substrates having SH/OH and CHO groups at appropriate
positions has been developed. Several azo-linked chromenes that are bright red pigments are also
synthesized. This methodology involves the domino reactions of Michael addition and subsequent
cyclisation by intramolecular aldol reaction.
comparative purposes, the allenoate 2 is also used.
Introduction
Allenes (1,2-dienes) are versatile precursors for diverse syntheti-
cally and biologically useful molecules.1,2 Among these, base/
phosphine catalysed domino cyclisation of allenes with sub-
strates possessing OH and CHO groups at ortho-positions
leading to chromans3 has attracted great attention in recent years
due to diverse biological activities of the products.4,5 This allene
route is one among the more versatile and convenient routes to
chromans.3,6
During our ongoing work on the synthesis of heterocycles
through allenes, we have recently shown that domino cyclisation
of allenylphosphonates with salicylaldehydes and 2-formyl-
indole leads to a variety of phosphono-chromenes7 and phos-
phono-pyrroloindoles.8 We were curious to know whether this
methodology can be adapted efficiently for thiochromans or azo-
substituted chromans via 2-mercaptobenzaldehydes,9 or azo-sub-
stituted salicyclaldehydes.10 We describe the results in this paper
using the allene precursors 1a–g, 2 and 3a–b; the other reactants
are the 2-mercaptobenzaldehydes 4–5 and azo-substituted salicyl-
aldehydes 6–7. It should be noted that synthetic routes to thio-
chromans/chromenes are relatively scanty and azo-substituted
chromenes, that could have interesting photophysical properties,
are also rather rare. The main focus of this work is on the
utility of allenylphosphonates11 because this class of precursors
are important building blocks in organic synthesis.12 For
Results and discussion
First, we treated the allenylphosphonate 1a with 2-mercapto-
benzaldehyde 4 in the presence of K2CO3 in ethanol. This reac-
tion led to mainly three types of products (Scheme 1): An E + Z
isomeric mixture of (β,γ)-cyclised phosphono-thiochroman 8
(E-isomer characterised by X-ray crystallography, Fig. 1, top),
(β,α)-cyclised phosphono-thiochroman 9 (X-ray structure, Fig. 1,
bottom) and the allylphosphonate 10.
We then wanted to see whether the yield of one of the thio-
chromans could be maximized or not using different solvents/
bases by heating at 90 °C for 4 h (oil bath temperature). The
results are summarized in Table 1. Use of PPh3 or DMAP as the
base afforded only low yields of the desired product (Table 1,
entries 9, 10). PEG-400 as a solvent also works (Table 1,
entry 15), but the yields are only moderate. In some cases, both
the regioisomers 8 and 9 (Table 1, entries 1–2, 6–10, 13, 15–16)
are obtained. The conditions previously used in the reaction of
School of Chemistry, University of Hyderabad, Hyderabad 500 046,
A. P., India. E-mail: kckssc@uohyd.ernet.in, kckssc@yahoo.com;
Fax: (+91)-40-23012460; Tel: (+91)-40-231348560
†Electronic supplementary information (ESI) available: Characterization
data, copies of 1H/13
C NMR spectra and CIF data. CCDC
885125–885130. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2ob26285a
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Table 1 Effect of reaction conditions on the yield of product 8 from
the reaction of allene 1a with 2-mercaptobenzaldehyde 4
Entry
Base/solvent
Yield (%)a, (E : Z)
1e
2
3
4
5
6
7
8
9
10
11
12
13e
14e
15e
16f
17
K2CO3/EtOH
Triethanolamine
NaOAc/DMSO
CsF/DMSO
K3PO4/DMSO
DABCO/DMSO
NaHCO3/DMSO
Et3N/DMSO
46 (10 : 3)b,c
57 (2 : 1)b,c
82 (5 : 1)c
69 (10 : 3)c
68 (5 : 2)c
61 (2 : 1)b,c
53 (10 : 3)b,c
59 (5 : 1)b,c
37 (10 : 3)b,c
37 (2 : 1)b,c
71 (5 : 1)
PPh3/DMSO
DMAP/DMSO
DBU/DMSO
Scheme 1
K2CO3/DMF
65 (2 : 1)c
K2CO3/Toluene
K2CO3/CH3CN
K2CO3/PEG-400
K2CO3/Dioxane
K2CO3/DMSO
58 (10 : 1)b,c
57 (5 : 1)c
54 (10 : 3)b,c
26 (10 : 11)b,c
95 (5 : 2)d
a Yields are calculated by using 1H/31P NMR spectroscopy. b In these
cases product 9 (10–38%) is also present. c The product 10 (14–53%) is
present in these cases. d Yield of the isolated product is 80%. e 90 °C
6 h−1. f 110 °C/6 h (temperature mentioned is of the oil bath used).
Fig. 1 ORTEP diagrams of compounds (E)-8 (top) and 9 (bottom).
Selected bond lengths [Å] with esds in parentheses follow. Compound
(E)-8: P–C(6) 1.798(3), C(6)–C(13) 1.343(4), C(13)–C(14) 1.509(4),
S–C(13) 1.758(3), O(4)–C(15) 1.409(4). [Hydrogen bond parameters
(Å, Å, °): O(4)–H(4)⋯O(1) 0.82 1.93 2.743(4) 175.1°; symmetry code:
2 − x, 2 − y, 2 − z]. Compound 9 [hydrogen atoms other than the one
attached to oxygen are not shown]: P–C(6) 1.852(2), C(6)–C(7) 1.529(3),
C(13)–C(14) 1.319(3), C(6)–C(15) 1.574(3), S–C(13) 1.771(2), O(4)–
C(15) 1.406(3) [Hydrogen bond parameters (Å, Å, °): O(4)–H(4)⋯O(1′)
0.84(3) 1.84(3) 2.673(3) 170.1 °; symmetry code: −x, −y, −z].
Fig. 2 31P NMR spectra of the reaction mixture of 1a with 2-mercapto-
benzaldehyde 4 in (a) EtOH at 90 °C/6 h, (b) DMSO at 90 °C/4 h (temp-
erature mentioned is of the oil bath).
salicylaldehyde with allenylphosphonates7 are less effective
(Table 1, entry 11) wherein the yield of 8 is reasonable, but not
the best. It is found that using K2CO3 as the base and dimethyl
sulfoxide (DMSO) as the solvent, phosphono-thiochroman 8
could be obtained as the predominant product [95%, E : Z =
5 : 2; 31P NMR evidence, Fig. 2].
To check the scope and limitations of the above reaction, we
then conducted the reaction of allenylphosphonates 1a–g with
2-mercaptobenzaldehydes 4–5 under the optimized conditions
[K2CO3/DMSO] for the synthesis of (β,γ)-cyclised phosphono-
thiochromans 8 and 11–23 (Scheme 2). The yields of the isolated
products are good (Table 2). We have separated both E and Z
isomers of 8, 11–13, 17–20. For 14 and 21, only E-isomer was
isolated. In the case of 16 and 23, obtained from α-methyl
allenylphosphonate 1g, only E-isomer is formed exclusively. In
Scheme 2 Yields and 31P NMR chemical shifts given in Table 2.
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Table 2 31P NMR data and yields of the compounds 8 and 11–23a
δ(P)
Entry
Compound
(E)
(Z)
9.8b
9.7
9.9
9.8
—
9.2
—
9.7
9.8c
10.3
10.0
—
Yield (%) (E : Z)b
1
2
3
4
5
6
7
8
8
11
12
13
14
15
16
17
18
19
20
21
22
23
9.0
80 (5 : 2)
78 (10 : 3)
79 (5 : 1)
80 (10 : 3)
63 (10 : 3)
68 (5 : 3)
86
79 (10 : 3)
76 (10 : 1)
78 (5 : 1)
81 (5 : 1)
64 (10 : 3)
67 (5 : 2)
85
9.2
9.4
9.2
9.2
9.0
14.9
9.2
9.4
9.4
9.4
9.4
9.2
15.0
9
10
11
12
13
14
Scheme 3
9.4
—
products 9 and 24 are better using this solvent.
a Products 9 and 10 were not detected under these optimized conditions.
b Yield as given here refers to the combined yield of E + Z isomers.
c The isomeric purity is only 95%.
We then performed the reaction of allenoate 2 with 2-mercapto-
benzaldehydes 4–5. The reaction was complete in 10 min and
led to thiochromans 25–26 as exclusive products (Scheme 3).
Longer reaction times led to a mixture of products. Compounds
25 and 26 are (β,α)-cyclisation products; it may be noted that in
the alternative (β,γ)-product I, only one olefinic proton [and an
up-field CH2 multiplet] should have been observed. This reaction
is similar to that reported earlier with normal salicylaldehydes.3a
Since the analogous allenylphosphonate (OCH2CMe2CH2O)P-
(O)CHvCvCH2 underwent isomerisation to the alkyne
(OCH2CMe2CH2O)P(O)CuCMe under these conditions,12a an
exact comparison between the two systems could not be made.
A plausible mechanism for the reaction which proceeds most
likely through a domino oxo-Michael addition followed by aldol
condensation3c,7 is shown in Scheme 4. Here two possibilities
exist: (β,α)-attack and (β,γ)-attack. The first one leads to the
kinetically controlled product (VI) and the second one leads to
thermodynamically controlled product (V). First, the base
(K2CO3) abstracts proton from 2-mercaptobenzaldehyde and
generates thiolate intermediate II which reacts with allenyl-
phosphonate 1a at β-position to give III/III′. Species IV/IV′ are
resonance forms of III/III′.3c Intermediates IV and IV′ undergo
intramolecular aldol reaction followed by protonation to afford
the final products V and V′. Formation of VI occurs via III
without the rearrangement of carbanion, as shown by the last
equation in Scheme 4. In both allenylphosphonates and allenyl-
sulfones, the α-position is sterically hindered with phosphorus
ring/sulfonyl moiety and a substituent. Because of this, (β,α)-
cyclisation is less favoured (minor product) when compared to
(β,γ)-cyclisation product (major product). Cyclisation in the case
of allenoate 2 occurs at the (β,α)-position as the α-position is not
hindered.
Fig. 3 ORTEP diagrams for the compounds (a) (E)-12 and (b) (Z)-12.
Hydrogen atoms except at the OH group are omitted for clarity. Selected
bond lengths [Å] with esds in parentheses: Compound (E)-12: P–C(6)
1.797(3), C(6)–C(14) 1.346(4), C(14)–C(15) 1.503(4), C(15)–C(16)
1.505(5), S–C(14) 1.754(3), O(5)–C(16) 1.414(3). [Hydrogen bond para-
meters: O(5)–H(5)⋯O(1) 0.82 1.90 2.715(4) 171.8°; symmetry code: 1
+ x, y, z]. Compound (Z)-12: P–C(6) 1.803(3), C(6)–C(14) 1.344(4),
C(14)–C(15) 1.505(4), C(15)–C(16) 1.520(5), S–C(14) 1.765(3), O(5)–
C(16) 1.403(4). [Hydrogen bond parameters: O(5)–H(5)⋯O(1) 0.82
2.01 2.809(4) 166.3°; symmetry code: 2 − x, −y, 1 − z].
the case of 15 and 22 the Rf values are very close. The structures
of both E and Z isomers of 12 were confirmed by X-ray crystal-
lography (Fig. 3). The E-stereochemistry for the phosphono-thio-
chroman 16 is also confirmed by X-ray structure determination
(see ESI† for details). Electron withdrawing groups, rather than
electron donating groups on the allene lead to better results. This
is possibly because of the presence of electron withdrawing
groups, the phenolate anion more readily undergoes addition
across allenes and thus can lead to higher yields of the products.
All these are (β,γ)-cyclised products.
We have also isolated the (β,α)-cyclised product 24 [which is
analogous to product 9 (see Fig. 1)] from the reaction of allene
1a with 5-methyl-2-mercaptobenzaldehyde 5 using ethanol as
the solvent medium. This is because the yields of (β,α)-cyclised
After establishing the above reaction, we wanted to synthesize
phosphono-thiochromenes, by dehydrating the hydroxyl substi-
tuted thiochromans prepared by us. This could be readily
accomplished in the reaction using allenylphosphonate 1a with
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Scheme 6
Scheme 7
mercaptobenzaldehydes (4–5) to furnish thiochromenes (28–31)
in decent yield (Scheme 6).
Although the reaction of simple salicylaldehydes with allenyl-
phosphonates had been studied prior to this work,7 it was
thought useful to extend this to those salicylaldehydes with
pendant chromophores such that the products may be useful as
pigments in a later work. With this idea in mind, under the con-
ditions employed for 2-mercaptobenzaldehydes, we first treated
the azo-substituted salicylaldehyde 6 with the allenylphospho-
nates 1a and 1c and obtained the phosphono-chromans 32–33 as
(Z) and (E) isomeric mixtures (Scheme 7). Both the azo-substi-
tuted salicylaldehyde 6 and the corresponding phosphono-chro-
mans 32–33 are red in colour. Both the (Z) and (E) isomers of
32 and 33 have been isolated by using column chromatography.
Under the above conditions, the reaction of allene 1b with the
azo-substituted salicylaldehyde 6 led to a mixture of chromans
and chromenes (like Scheme 5) because one isomer of chroman
underwent faster dehydration. Hence we continued the reaction
for 24 h to obtain the chromene exclusively. Under these con-
ditions, the reaction of azo-substituted salicylaldehydes 6–7 with
allenylphosphonates 1a–d, g afforded phosphono-azo-chro-
menes 34–43 (Scheme 8). We have also separated individual
isomers in all the cases. The overall (combined) isolated yields
of the two isomers are moderate to good (Table 3). Compound
(E)-35 is characterised by X-ray crystallography (Fig. 4).
Scheme 4
Scheme 5
2-mercaptobenzaldehyde 4 by simply prolonging the reaction
time from 4 h to 24 h. Thus we could obtain phosphono-thio-
chromene 27 (Scheme 5) in good yield. In this compound,
appearance of two olefinic protons signals and disappearance of
the signals due to thiochroman ring CH2 protons (present in 8)
in 1H NMR establishes the structure as depicted. Since the
isomer of 8 with (E)-configuration was the major product prior
to dehydration an analogous (E) configuration is assigned to 27.
In an effort to extend this methodology, we have chosen allenyl-
sulfones (3a–b) in place of allenylphosphonates. Compared to
allenylphosphonates, allenylsulfones reacted much faster with
Conclusions
An efficient allene based route to thiochromans was developed.
Under base catalysed reactions, the reactivity of allenylphospho-
nates with 2-mercaptobenzaldehyde leading to thiochromans is
different from that of allenyl sulfones and allenoate (EtO2C)-
CHvCvCH2 (2) under the same conditions. The reaction using
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Experimental section
General information
Chemicals were purified when required according to standard
procedures. All reactions, unless stated otherwise, were per-
formed in a dry nitrogen atmosphere. 1H, 13C and 31P NMR
spectra were recorded using a 400 MHz spectrometer in CDCl3
(unless stated otherwise) with shifts referenced to SiMe4 (δ = 0)
or 85% H3PO4 (δ = 0). Infrared spectra were recorded neat or by
using KBr pellets on an FT-IR spectrometer. Melting points were
determined by using a local hot-stage melting point apparatus
and are uncorrected. Microanalyses were performed using a
CHNS analyzer. For TLC, glass microslides were coated with
silica-gel-GF254 (mesh size 75 μ) and spots were identified using
iodine or UV chamber as appropriate. For column chromato-
graphy, silica gel of 100–200 mesh size was used. LC-MS and
HRMS (ESI-TOF) equipment was used to record mass spectra
for isolated compounds where appropriate. LC-MS data were
obtained using electrospray ionization on a C-18 column at a
flow rate 0.2 mL min−1 using MeOH–water (90 : 10) as eluent.
Characterization data for all the new compounds reported here
are provided in the ESI.†
Scheme 8
Table 3 31P NMR data and isolated yields of the (E) and (Z) isomers
of 34–43
δ(P)
(i) Typical procedure for the synthesis of 9–10 (compound 8
was present but not isolated in this case). To a 25 mL round-bot-
tomed flask containing allene 1a (0.200 g, 0.76 mmol), 2-mercapto-
benzaldehyde 4 (0.136 g, 0.98 mmol) and K2CO3 (0.021 g,
0.15 mmol), was added EtOH (4 mL). The contents were heated
at 90 °C (temperature mentioned is of the oil bath) for 6 h. The
reaction mixture was quenched with water (5 mL) and extracted
with CH2Cl2 (3 × 25 mL). The whole organic layer was washed
with water (3 × 25 mL), dried (Na2SO4), filtered, concentrated
and the products [8–10, Rf values were in the order: 10 > 9 >
(Z)-8 > (E)-8] were isolated by column chromatography
(hexane–EtOAc; 1 : 1) on silica gel. Compounds (Z)-8 and (E)-8
were not isolated in this case.
Entry
Compound
(E)
(Z)
Yield (%)(E : Z)a
1
2
3
4
5
6
7
8
9
10
34
35
36
37
38
39
40
41
42
43
14.9
15.2
15.3
15.0
20.9
14.5
14.7
14.9
14.6
20.5
11.7
11.9
12.0
11.4
17.0
11.2
11.5
11.6
11.0
16.6b
76 (10.9)
72 (4 : 5)
73 (7 : 10)
74 (1 : 1)
80 (5 : 3)
80 (1 : 1)
76 (5 : 3)
76 (1 : 1)
78 (1 : 1)
74 (5 : 2)
a Yields of the isolated pure compounds = combined yield of E + Z
isomers. b The isomeric purity is only 90%.
(ii) General procedure for the synthesis of thiochromans 8,
11–31. To a 25 mL round-bottomed flask containing allene (one
of 1a–g) (0.76 mmol), 2-mercaptobenzaldehyde
4 or 5
(0.98 mmol) and K2CO3 (0.021 g, 0.15 mmol), was added
DMSO (4 mL) under N2 atmosphere. The contents were heated
at 90 °C for 4 h under N2 atmosphere. The reaction mixture was
quenched with water (5 mL) and extracted with CH2Cl2 (3 ×
25 mL). The whole organic layer was washed with water (3 ×
25 mL), dried (Na2SO4), filtered, concentrated and the products
were isolated by column chromatography (hexane–EtOAc; 1 : 1)
on silica gel. Both Z and E isomers are formed in most cases.
Details on the combined yield of Z and E isomers are given in
Table 2. Isolated yield of individual isomer as applicable in each
case is given here.
Fig. 4 An ORTEP diagram of compound (E)-35. Selected bond
lengths [Å] with esds in parentheses: P–C(6) 1.766(2), C(6)–C(7) 1.358(3),
C(7)–C(8) 1.442(3), C(8)–C(16) 1.337(3), O(4)–C(7) 1.381(2).
2 is much faster than that using 1a–g. In the case of α-substi-
tuted allenylphosphonates (1a–g), (β,γ)-attack is favoured
whereas in the case of allenoate 2, (β,α)-attack is favoured. It
appears that subtle electronic/steric factors contribute to these
differences. The reactions of allenes with azo-linked salicyl-
aldehydes afforded bright red coloured pigments that could have
interesting photochemical properties (under investigation).
(iii) General procedure for the synthesis of azo-substituted
phosphono-chromans 32–33 and azo substituted phosphono-
chromenes 34–43. To the allenylphosphonate 1a or 1c
(0.76 mmol), azo-substituted salicylaldehyde 6 (0.221 g,
0.98 mmol) and K2CO3 (0.021 g, 0.15 mmol) in a 25 mL RBF,
was added DMSO (4 mL) and the contents heated at 90 °C for
4 h. The reaction mixture was quenched with water (5 mL) and
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2009, 11, 991; (g) Y.-W. Sun, X.-Y. Guan and M. Shi, Org. Lett., 2010,
12, 5664.
extracted with CH2Cl2 (3 × 25 mL). The whole organic layer
was washed with water (3 × 25 mL), dried (Na2SO4), filtered,
concentrated and the products were isolated by column
chromatography (hexane–EtOAc; 2 : 3) on silica gel. Here, the
E-isomer eluted before the Z-isomer in each case. [i.e. (Rf
(E-isomer) > Rf (Z-isomer)]. Phosphono-chromenes 34–43 were
synthesised using same molar quantities, procedure and the work
up as that for 32 except that the reaction time was 24 h (instead
of 4 h). The products were isolated by column chromatography
(hexane–EtOAc; 1 : 1) on silica gel. Here, the E-isomer eluted
before the Z-isomer in each case. [i.e. (Rf (E-isomer) > Rf
(Z-isomer)].
4 Representative examples of bioactive chromene core structure:
(a) J. P. Cueva, G. Giorgioni, R. A. Grubbs, B. R. Chemel, V. J. Watts
and D. E. Nichols, J. Med. Chem., 2006, 49, 6848; (b) W. Kemnitzer,
J. Drewe, S. Jiang, H. Zhang, C. Crogan-Grundy, D. Labreque,
M. Bubenick, G. Attardo, R. Denis, S. Lamothe, H. Gourdeau, B. Tseng,
S. Kasibhatla and S. X. Cai, J. Med. Chem., 2008, 51, 417; (c) F. Liu,
T. Evans and B. C. Das, Tetrahedron Lett., 2008, 49, 1578; (d) K. Liu,
A. Chougnet and W.-D. Woggon, Angew. Chem., Int. Ed., 2008, 47, 1;
(e) S. G. Das, J. M. Doshi, D. F. Tian, S. N. Addo, B. Srinivasan,
D. L. Hermanson and C.-G. Xing, J. Med. Chem., 2009, 52, 5937;
(f) R. Machauer, K. Laumen, S. Veenstra, J.-M. Rondeau, M. Tintelnot-
Blomley, C. Betschart, A.-L. Jaton, S. Desrayaud, M. Staufenbiel,
S. Rabe, P. Paganetti and U. Neumann, Bioorg. Med. Chem. Lett., 2009,
19, 1366.
5 Representative examples of bioactive thiochroman core structure:
(a) T. Saito, T. Horikoshi, T. Otani, Y. Matsuda and T. Karakasa, Tetra-
hedron Lett., 2003, 44, 6513; (b) S. Bath, N. M. Laso, H. Lopez-Ruiz,
B. Quiclet-Sire and S. Z. Zard, Chem. Commun., 2003, 204;
(c) M. Jafarzadeh, K. Amani and F. Nikpour, Tetrahedron Lett., 2005, 46,
7567; (d) A. Beliaev, D. A. Learmonth and P. Soares-da-Silva, J. Med.
Chem., 2006, 49, 1191; (e) P. Soares-da-Silva, PCT Int. Appl., 2008, WO
2008085074A2 20080717.
6 For recent examples on the synthesis of chromenes, see: (a) H. Zhou,
Y.-H. Xu, W.-J. Chung and T.-P. Loh, Angew. Chem., Int. Ed., 2009, 48,
1; (b) A. Behrenswerth, N. Volz, J. Toräng, S. Hinz, S. Bräse and
C. Müller, Bioorg. Med. Chem., 2009, 17, 2842; (c) I. Torregroza,
T. Evans and B. C. Das, Chem. Biol. Drug Des., 2009, 73, 339;
(d) M. Uemura, I. D. G. Watson, M. Katsukawa and F. D. Toste, J. Am.
Chem. Soc., 2009, 131, 3464; (e) D. B. Ramachary, K. Ramakumar,
A. Bharanishashank and V. V. Narayana, J. Comb. Chem., 2010, 12, 855;
(f) D. B. Ramachary and R. Sakthidevi, Chem.–Eur. J., 2009, 15, 4516.
7 N. N. Bhuvan Kumar, M. Nagarjuna Reddy and K. C. Kumara Swamy,
J. Org. Chem., 2009, 74, 5395.
Acknowledgements
We thank Department of Science
& Technology (DST,
New Delhi) for financial support, and for Single Crystal X-ray
diffractometer. MPP, MNR and NNBK thank Council of
Scientific & Industrial Research (CSIR) for a fellowship.
Notes and references
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