A novel diastereoselective synthesis of β-phosphonato unsaturated thioimidates

Article abstract of DOI:10.1055/s-0029-1216877

A novel, convenient, and efficient approach to the synthesis of α,β-unsaturated thioimidates with a phosphonate functional group in b-position has been reported based on a multicomponent reaction. The generated zwitterions from the reaction of trialkyl ph

Full text of DOI:10.1055/s-0029-1216877

PAPER
2737
A Novel Diastereoselective Synthesis of b-Phosphonato Unsaturated
Thioimidates
Synthesis of
b
-Pho
b
sphonato U
n
d
saturated T
h
o
ioimidate
s
lali Alizadeh,* Nasrin Zohreh
Department of Chemistry, Tarbiat Modares University, P. O. Box 14115-175, Tehran 18716, Iran
Fax +98(21)88006544; E-mail: abdol_alizad@yahoo.com; E-mail: aalizadeh@modares.ac.ir
Received 14 April 2009; revised 23 April 2009
via an alkylating agent.7,11 Palladium-catalyzed coupling
reaction between bromobenzene, tert-butyl isocyanide
and anionic S-nucleophiles is also reported for their syn-
thesis.12 A more recent method is based on the use of im-
Abstract: A novel, convenient, and efficient approach to the syn-
thesis of a,b-unsaturated thioimidates with a phosphonate function-
al group in b-position has been reported based on a multicomponent
reaction. The generated zwitterions from the reaction of trialkyl
phosphites and dialkyl acetylenedicarboxylates are trapped with idoylbenzotriazoles as synthetic equivalents of the very
hydrolysis sensitive imidoyl chlorides.13 Synthesis of a,b-
unsaturated thioimidates from imines and alkynyl sulfides
benzenecarbonyl isothiocyanate to produce the title compounds in
good yield.
Key words: thioimidate, trialkyl phosphite, dialkyl acetylenedicar-
boxylate, benzenecarbonyl isothiocyanate, multicomponent reac-
tion
in the presence of Sc(OTf)3 was reported by Kobayashi
and co-workers.14
In 1962, Johnson and Tebby established the reaction of
P(III) with dimethyl acetylenedicarboxylate (DMAD).
This early finding forms the basis for a large class of new
backbones thus accessible.15
Thioimidates,1 especially unsaturated thioimidates2 are
important building blocks in organic synthesis for the syn-
thesis of heterocyclic compounds. Stereoselective synthe-
sis of g-lactams by iodine-induced lactamization of g,d-
unsaturated thioimidates has been reported by Takahata
and co-workers.3 Coupling reaction of a cyclic thioimi-
date with oxazolones is a key step in the synthesis of car-
zinophilin, which is an antitumor and natural antibiotic.4
Rearrangement of thiomethylimidate cyclopropane to a
pyrrolothiomethylimidate intermediate was reported in
the synthesis of dihydropyrroles.5 Synthesis of amidra-
zones using an engineered papain nitrile hydratase and
cysteine protease-like structures by trapping thioimidate
also shows their importance.6 Macrocyclic ligands con-
taining the thioimidate group also exhibited good affinity
towards metal ions, the complexes of which are known to
act as a catalyst for RNA cleavage.7 Recently, glycosyl
thioimidates are being used in glycol conjugates and oli-
gosaccharide synthesis.8
Some of our previous findings included the trapping of
1:1 intermediate of phosphorus nucleophiles and acety-
lenes (TPP/DMAD) with various electrophiles such as
isocyanates (Scheme 1).16a,b,d
Along the same line, we became interested in the applica-
tion of benzenecarbonyl isothiocyanate in these types of
reactions for the synthesis of title compounds. Our new
synthetic method, which has not been previously reported
to our knowledge, is illustrated in Table 1. Reaction be-
tween trialkyl phosphites 1, dialkyl acetylenedicarboxy-
lates 2, and benzenecarbonyl isothiocyanate proceeds in
toluene at room temperature to produce b-phosphonato
unsaturated thioimidates 3 in 70–75% yields (Table 1).
1
The data obtained from elemental analyses, IR, H, 13C
and 31P NMR, and mass spectra confirmed the molecular
structure of products. The mass spectrum of 3a displayed
molecular ion peak at m/z 429. The most important ab-
sorption band in IR spectrum is due to the P=O stretching
frequency of phosphonate group, which appeared at 1249
cm–1. Absorption bands at 1731 and 1690 cm–1 are due to
The most important method for their preparation includes
the reaction of imidoyl halides9 or nitriles6a,10 with S-nu-
cleophiles and conversion of thioamides to thioimidates
O
PT3
O
R
R
O
R
R
N
N
+
N
N
Et2O, r.t.
E
POT2
OH
Ph
or
or
E
E
E
O
PT3
E
Ph
Ph
+
O
O
PT3
T = OR
E = CO2R
R = ArSO2
R
N
•
O
T = Ph
E = COPh
R = ArSO2 or Ph
T = OR
E = COPh
R = ArSO2
Scheme 1 Three-component reaction between phosphorus nucleophiles, acetylenes, and isocyanates studied earlier
SYNTHESIS 2009, No. 16, pp 2737–2740
x
x.
x
x
.
2
0
0
9
Advanced online publication: 26.06.2009
DOI: 10.1055/s-0029-1216877; Art ID: Z07209SS
© Georg Thieme Verlag Stuttgart · New York

2738
A. Alizadeh, N. Zohreh
PAPER
action of a trialkyl phosphite 1 and a dialkyl acetylenedi-
carboxylate 2. Zwitterion 4 is trapped by benzenecarbonyl
isothiocyanate to generate intermediate 5. In the presence
of another molecule of intermediate 5 and probably be-
cause of polarizability and nucleophilicity of sulfur atom,
S-alkylation takes place to form the products 3 in 70–75%
yield.
Table 1 b-Phosphonato Unsaturated Thioimidates 3 Prepared
O
P
E
E
O
toluene
S
N
Ph
R1O
R1O
(R1O) P
+
+
•
3
Ph
N
r. t., 12 h
E
SR1
O
E = CO2R2
E
1
Product
3a
3
2
R1
R2
Me
Et
Yield (%)
Overall, we have succeeded in reporting a novel synthetic
method for b-phosphonato unsaturated thioimidates via
benzenecarbonyl isothiocyanates in a multicomponent re-
action. More importantly, the reaction is stereoselective
and only one of the two isomers is obtained. In compari-
son with other reported synthetic methods, the reaction is
performed in neutral conditions and substrates are mixed
with no bases or catalysts. The simplicity of the present
procedure makes it an interesting alternative to the com-
plex multistep approaches.
Me
Me
Et
75
72
75
70
3b
3c
Me
Et
3d
Et
the two C=O of esters and one carbonyl group. In the 1H
NMR spectrum of 3a, characteristic singlet signal at
d = 2.67 showed that S-alkylation had taken place, and
this signal is due to the thiomethoxy group while the hy-
drogen chemical shift of NR group appears at about 4–5
ppm.17 The spectrum exhibited also two sharp singlets
recognized as arising from methoxy groups (d = 3.76 and
Trialky phosphites, dialkyl acetylenedicarboxylates, and benzene-
carbonyl isothiocyanate, were obtained from Merck (Germany) and
Fluka (Switzerland) and were used without further purification. El-
emental analyses for C, H and N were performed using a Heraeus
3.85). A doublet at 3.62 ppm (3JP,H = 11.6 Hz) is due to the CHN–O–Rapid analyzer. Mass spectra were recorded on a Finnigan
Matt 8430 mass spectrometer operating at an ionization potential of
70 eV. 1H, 13C, and 31P NMR spectra were recorded (CDCl3 solu-
teristic signals in the aromatic region of the spectrum. The
PO(OCH3)2 group. The phenyl moiety gave rise to charac-
tion) on a Bruker DRX-500 Avance spectrometer at 500.1 and
1H decoupled 13C NMR spectrum of 3a showed 19 distinct
resonances compatible with the proposed structure that 12
125.7 MHz, respectively. IR spectra were recorded on a Shimadzu
IR-460 spectrometer. Chromatography columns were prepared
peaks of them are due to the six carbons, which split with
from Merck silica gel (230–240 mesh).
the P atom. The signal at d = 15.20, which is due to the
Dimethyl 2-[(Benzoylimino)(methylsulfanyl)methyl]-3-
(dimethoxyphosphoryl)but-2-enedioate (3a); Typical Proce-
dure
thiomethoxy group also confirmed S versus N-alkylation.
The presence of 31P in 3a–d helps in the assignment of the
1
signals by long range coupling with H and 13C nuclei.
To a magnetically stirred mixture of DMAD (0.14 g, 1 mmol) and
benzenecarbonyl isothiocyanate (0.16 g, 1 mmol) in anhyd toluene
useful tool for assigning the Z/E-structure. In general, (3 mL) in a 5 mL flat-bottomed flask was added trimethyl phosphite
Vicinal 31P-13C coupling constant through a p-bond is a
3JC,P-trans coupling constant is much larger than 3JC,P-cis.18
(0.12 g, 1 mmol) and allowed to stir for 12 h. The solvent was re-
moved under reduced pressure and product was separated by col-
umn chromatography over silica gel (Merck 230–240 mesh) using
a hexane–EtOAc (7:1) mixture as eluent; yield: 0.32 g (75%); yel-
The comparison of observed 3JC,P for the C=N group with
the desired values indicates the product geometries. In the
3
13C NMR spectrum of 3a, the JC,P coupling constant of
low oil.
24.1 Hz (d = 161.93, C=N) showed that there is E rela-
IR (KBr): 1728 (C=O of ester), 1654 (PhC=O), 1611 (C=C), 1565
tionship between PO(OMe)2 and C=N.18a,19 Finally, it is
and 1433 (Ar), 1249 (P=O), 1188 and 1155 (C–O of ester), 1050
found that, there is only one of the two probable diastereo-
isomers (E or Z), in the products. Thus, the reaction is dia-
stereoselective.
and 1022 cm–1 (P–OMe).
1H NMR (500.1 MHz, CDCl3): d = 2.67 (s, 3 H, SCH3), 3.62 [d,
3JP,H = 11.6 Hz, 6 H, PO(OCH3)2], 3.76 (s, 3 H, OCH3), 3.85 (s, 3 H,
OCH3), 7.44 (t, 3JH,H = 7.4 Hz, 2 H, 2 CH of Ar), 7.54 (t, 3JH,H = 7.7
Hz, 1 H, CH of Ar), 8.16 (d, 3JH,H = 7.9 Hz, 2 H, 2 CH of Ar).
Although we have not established the mechanism of this
reaction in an experimental manner, a possible explana-
tion is proposed in Scheme 2. Based on the established 13C NMR (125.7 MHz, CDCl3): d = 15.20 (SCH3), 53.24 (OCH3),
chemistry of phosphorus nucleophiles,16 it is reasonable to
assume that the zwitterion 4 is readily formed from the re-
53.43 (OCH3), 53.61 [d, 2JP,C = 5.2 Hz, PO(OCH3)2], 128.28 (2 CH
of Ar), 130.41 (2 CH of Ar), 133.16 (CH of Ar), 133.98 (Cipso of Ar),
1
134.71 (d, JP,C = 169.82 Hz, POC=C), 144.03 (d, JP,C = 6.4 Hz,
3
POC=C), 161.93 (d, JP,C = 24.1 Hz, C=NCO), 164.88 (d,
2JP,C = 9.4 Hz, CO2Me), 171.88 (d, 3JP,C = 7.0 Hz, CO2Me), 175.88
(PhC=O).
2
O
S
E
COPh
N
E
•
N
Ph
(RO)2P
(RO)3P
+
3
1
2
31P NMR (202.4 MHz, CDCl3): d = 9.49.
O
R
E
S
MS: m/z (%) = 429 (5, [M+]), 382 (15), 370 (10), 320 (10), 105
(100), 77 (30), 51 (10).
E
5
4
Anal. Calcd for C17H20NO8PS (429.38): C, 47.55; H, 4.69; N, 3.26.
Found: C, 47.58; H, 4.71; N, 3.28.
Scheme 2 Plausible mechanism for the formation of phosphonato
unsaturated thioimidates 3
Synthesis 2009, No. 16, 2737–2740 © Thieme Stuttgart · New York

PAPER
Synthesis of b-Phosphonato Unsaturated Thioimidates
2739
Diethyl 2-[(Benzoylimino)(methylsulfanyl)methyl]-3-
(dimethoxyphosphoryl)but-2-enedioate (3b)
Yield: 0.33 g (72%); yellow oil.
3JH,H = 7.2 Hz, 2 H, OCH2CH3), 4.16–4.25 [m, 4 H, PO(CH2CH3)2],
4.31 (q, 3JH,H = 7.2 Hz, 2 H, OCH2CH3), 7.44 (t, 3JH,H = 7.3 Hz, 2 H,
3
2 CH of Ar), 7.53 (t, JH,H = 7.4 Hz, 1 H, CH of Ar), 8.17 (d,
3JH,H = 7.0 Hz, 2 H, 2 CH of Ar).
IR (KBr): 1724 (C=O of ester), 1660 (PhC=O), 1611 (C=C), 1568
and 1443 (Ar), 1241 (P=O), 1176 and 1151 (C–O of ester), 1089
and 1021 cm–1 (P–OMe).
13C NMR (125.7 MHz, CDCl3): d = 13.40 (SCH2CH3), 13.69
3
(OCH2CH3), 13.85 (OCH2CH3), 15.99 [d, JP,C = 6.5 Hz,
PO(OCH2CH3)2], 26.58 (SCH2CH3), 62.19 (OCH2CH3), 62.65
3
1H NMR (500.1 MHz, CDCl3): d = 1.17 (t, JH,H = 7.4 Hz, 3
2
(OCH2CH3), 63.42 [d, JP,C = 4.7 Hz, PO(OCH2CH3)2], 128.22 (2
H,OCH2CH3), 1.32 (t, 3JH,H = 7.3 Hz, 3 H,OCH2CH3), 2.64 (s, 3 H,
CH of Ar), 130.45 (2 CH of Ar), 132.98 (CH of Ar), 134.23 (Cipso
of Ar), 135.28 (d, 1JP,C = 167.1 Hz, POC=C), 144.30 (d, 2JP,C = 6.2
3
SCH3), 3.65 [d, JP,H = 11.5 Hz, 6 H, PO(OCH3)2], 4.21 (q,
3
3JH,H = 7.0 Hz, 2 H, OCH2CH3), 4.31 (q, JH,H = 7.1 Hz, 2 H,
3
Hz, POC=C), 162.31 (d, JP,C = 24.0 Hz, C=NCO), 164.71 (d,
3
OCH2CH3), 7.43 (t, JH,H = 7.1 Hz, 2 H, 2 CH of Ar), 7.53 (t,
3
2JP,C = 9.5 Hz, CO2Et), 171.62 (d, JP,C = 7.2 Hz, CO2Et), 175.91
(PhC=O).
3JH,H = 7.1 Hz, 1 H, CH of Ar), 8.15 (d, 3JH,H = 7.9 Hz, 2 H, 2 CH of
Ar).
31P NMR (202.4 MHz, CDCl3): d = 5.67.
13C NMR (125.7 MHz, CDCl3): d = 13.67 (OCH2CH3), 13.81
2
(OCH2CH3), 15.21 (SCH3), 53.63 [d, JP,C = 5.3 Hz, PO(OCH3)2],
MS: m/z (%) = 448 (8), 410 (21), 384 (31), 334 (41), 277 (17), 192
(45), 105 (100), 77 (79), 43 (55).
62.38 (OCH2CH3), 62.81 (OCH2CH3), 128.35 (2 CH of Ar), 130.36
(2 CH of Ar), 133.10 (CH of Ar), 134.05 (Cipso of Ar), 134.69 (d,
Anal. Calcd for C22H30NO8PS (499.51): C, 52.90; H, 6.05; N, 2.80.
Found: C, 52.95; H, 6.08; N, 2.79.
2
1JP,C = 169.0 Hz, POC=C), 144.14 (d, JP,C = 6.3 Hz, POC=C),
3
2
161.40 (d, JP,C = 24.2 Hz, C=NCO), 164.31 (d, JP,C = 9.6 Hz,
CO2Et), 171.50 (d, 3JP,C = 7.1 Hz, CO2Et), 175.87 (PhC=O).
31P NMR (202.4 MHz, CDCl3): d = 8.61.
References
MS: m/z (%) = 457 (7, [M+]), 410 (21), 384 (19), 339 (27), 279 (58),
105 (100), 77 (42), 51 (12).
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Anal. Calcd for C19H24NO8PS (457.43): C, 49.89; H, 5.29; N, 3.06.
Found: C, 49.92; H, 5.27; N, 3.08.
Dimethyl 2-[(Benzoylimino)(ethylsulfanyl)methyl]-3-(diethoxy-
phosphoryl)but-2-enedioate (3c)
Yield: 0.35 g (75%); yellow oil.
IR (KBr): 1729 (C=O of ester), 1657 (PhC=O), 1612 (C=C), 1565
and 1430 (Ar), 1250 (P=O), 1178 and 1155 (C–O of ester), 1041
and 1013 cm–1 (P–OEt).
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3
1H NMR (500.1 MHz, CDCl3): d = 1.17 [t, JH,H = 7.0 Hz, 6 H,
PO(OCH2CH3)2], 1.47 (t, 3JH,H = 7.4 Hz, 3 H, SCH2CH3), 3.29 (q,
3JH,H = 7.4 Hz, 2 H, SCH2CH3), 3.72 (s, 3 H, OCH3), 3.87 (s, 3 H,
OCH3), 3.95–4.02 [m, 4 H, PO(CH2CH3)2], 7.44 (t, 3JH,H = 7.7 Hz,
2 H, 2 CH of Ar), 7.52 (t, 3JH,H = 7.4 Hz, 1 H, CH of Ar), 8.17 (d,
3JH,H = 7.1 Hz, 2 H, 2 CH of Ar).
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13C NMR (125.7 MHz, CDCl3): d = 13.29 (SCH2CH3), 15.90 [d,
3JP,C = 6.6 Hz, PO(OCH2CH3)2], 26.66 (SCH2CH3), 53.06 (OCH3),
53.35 (OCH3), 63.53 [d, 2JP,C = 4.9 Hz, PO(OCH2CH3)2], 128.25 (2
CH of Ar), 130.50 (2 CH of Ar), 133.07 (CH of Ar), 134.16 (Cipso
of Ar), 135.37 (d, 1JP,C = 167.5 Hz, POC=C), 143.52 (d, 2JP,C = 6.3
Hz, POC=C), 162.19 (d, JP,C = 24.1 Hz, C=NCO), 165.08 (d,
2JP,C = 9.2 Hz, CO2Me), 171.89 (d, 3JP,C = 7.1 Hz, CO2Me), 175.98
(PhC=O).
3
31P NMR (202.4 MHz, CDCl3): d = 5.34.
MS: m/z (%) = 442 (12), 410 (35), 334 (23), 306 (17), 262 (33), 105
(100), 77 (65), 51 (15).
Anal. Calcd for C20H26NO8PS (471.46): C, 50.95; H, 5.56; N, 2.97.
Found: C, 50.99; H, 5.57; N, 2.98.
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Diethyl 2-[(Benzoylimino)(ethylsulfanyl)methyl]-3-(diethoxy-
phosphoryl)but-2-enedioate (3d)
Yield: 0.35 g (70%); yellow oil.
IR (KBr): 1725 (C=O of ester), 1655 (PhC=O), 1616 (C=C), 1565
and 1459 (Ar), 1244 (P=O), 1170 and 1161 (C–O of ester), 1090
and 1014 cm–1 (P–OEt).
3
1H NMR (500.1 MHz, CDCl3): d = 1.18 (t, JH,H = 7.0 Hz, 3 H,
3
OCH2CH3), 1.22 (t, JH,H = 7.1 Hz, 3 H, OCH2CH3), 1.32 [t,
3
3JH,H = 7.1 Hz, 3 H, PO(OCH2CH3)2], 1.46 (t, JH,H = 7.3 Hz, 3 H,
3
SCH2CH3), 3.28 (q, JH,H = 7.3 Hz, 2 H, SCH2CH3), 3.98 (q,
Synthesis 2009, No. 16, 2737–2740 © Thieme Stuttgart · New York

2740
A. Alizadeh, N. Zohreh
PAPER
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Synthesis 2009, No. 16, 2737–2740 © Thieme Stuttgart · New York