New aspects of the formation of 2-substituted thiazolidine-4-carboxylic acids and their thiohydantoin derivatives

Article abstract of DOI:10.24820/ark.5550190.p009.861

Aromatic aldehydes reacted readily with (R)-cysteine in boiling acidified methanol to give diastereomeric mixtures of the corresponding 2-(aryl substituted) thiazolidine-4-carboxylic acids. 4-Nitrobenzaldehyde under similar conditions afforded one isomer of 2-(4-nitrophenyl)-thiazolidine-4-carboxylic acid, which epimerized in the NMR solvents into a diastereomeric mixture. 2-Nitrobenzaldehyde reacted with (R)-cysteine to afford 3,5-bis-(2-nitrophenyl)-tetrahydro-1H-thiazolo[3,4-c]oxazol-1-one as the sole product, which collapsed in the NMR solvent into a diastereomeric mixture of the thiazolidine-4-carboxylic acids. The thiazolidine derivatives reacted smoothly with phenyl isothiocyanate to give single isomers of the corresponding thiohydantoins.

Full text of DOI:10.24820/ark.5550190.p009.861

General Papers
ARKIVOC 2016 (vi) 105-121
New aspects of the formation of 2-substituted
thiazolidine-4-carboxylic acids and their thiohydantoin derivatives
Ahmed R. E. Mahdy, Elghareeb E. Elboray, Ragab F. Fandy,
Hussien H. Abbas-Temirek, and Moustafa F. Aly*
Department of Chemistry, Faculty of Science, South Valley University, Qena 83523, Egypt
E-mail: mosfaly51@yahoo.com
DOI: https://doi.org/10.24820/ark.5550190.p009.861
Abstract
Aromatic aldehydes reacted readily with (R)-cysteine in boiling acidified methanol to give
diastereomeric mixtures of the corresponding 2-(aryl substituted) thiazolidine-4-carboxylic acids.
4-Nitrobenzaldehyde under similar conditions afforded one isomer of 2-(4-nitrophenyl)-
thiazolidine-4-carboxylic acid, which epimerized in the NMR solvents into a diastereomeric
mixture. 2-Nitrobenzaldehyde reacted with (R)-cysteine to afford 3,5-bis-(2-nitrophenyl)-
tetrahydro-1H-thiazolo[3,4-c]oxazol-1-one as the sole product, which collapsed in the NMR
solvent into a diastereomeric mixture of the thiazolidine-4-carboxylic acids. The thiazolidine
derivatives reacted smoothly with phenyl isothiocyanate to give single isomers of the
corresponding thiohydantoins.
Keywords: Thiazolidine-4-carboxylic acids, thiohydantoins, cyclization, epimerization
Introduction
Penicillins are the class of antibiotic drugs contain a β-lactam nucleus fused to thiazolidine-4-
carboxylic acid derivatives (TCDs).1 The thiazolidine-4-carboxylic acid core used in biology as a
pseudo-/thio-proline or bioisostere of proline to improve the desired biological or physical
properties of a compound without making significant changes in the chemical structure.2-7 TCDs
exhibit a broad spectrum of anticancer activities8-12 against, e.g. liver,13,14 breast,13-15 colon,13,16
prostate,17 endometrial13 and melanoma17 cell lines. In the same vein, TCDs are used as, e.g.
antimalarial,18,19 selective Na+/Ca2+ exchange inhibitors,20 immunostimulating agents,21 tyrosinase
inhibitors,22 influenza A neuraminidase inhibitors,23 antitubercular,24 HIV-1 protease inhibitors2,25
and ACE inhibitors.26 Furthermore, TCDs were introduced as versatile scaffolds in the syntheses
of natural products, i.e. (+)-biotin,27,28 (+)-lyngbyabellin M,29 (+)-hyalodendrin,30 prepiscibactin31
and dehydroluciferin.32 Glutathione is a tripeptide works as a cellular protective agent and cannot
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be used as a drug because it does not enter the living cells.33 The vital amino acid building block
in the in vivo synthesis of glutathione is (R)-cysteine, which cannot be supplemented directly due
to toxicity and instability problems.33,34 TCDs work as (R)-cysteine prodrugs33,35 which metabolize
intracellularly to release (R)-cysteine to stimulate glutathione synthesis. Straightforward formation
of TCDs opens the door for this process to be used for detection of cysteine amino acid and N-
terminal cysteines36 and also for estimation/binding of aldehydes.37
The aim of this work is to use acidified methanol as a solvent in the preparation of some TCDs
and to shed some light on the mechanism of their formation, and their conversion into the
corresponding diastereospecific thiohydantoins.
Results and Discussion
The reaction of (R)-cysteine (1), salicylaldehyde (2a) and N-phenylmaleimide (3) (NPM, a
reactive dipolarophile) in acidified methanol under reflux failed to give the expected cycloadduct
6,38 and instead gave a 98% yield of the well known (2RS)-(2-hydroxyphenyl)thiazolidine-(4R)-
carboxylic acids (7a) and (8a) in a 1.5:1 ratio, respectively. It is worth mentioning that using
acidified methanol gave a better yield and an opposite diastereomeric ratio compared to the
previously reported results39 (Table, entry a). This result encouraged us to study the
diastereoselectivity in the formation of 2-(substituted)thiazolidine-4-carboxylic acids by using our
conditions.
Scheme 1. The competition between cycloaddition and cyclization.
It seems that the Schiff’s base 4 undergoes cyclization in such case much faster than the
generation of azomethine ylide 5 and this could be mainly attributed to the bigger size and softer
nature of the sulfur atom. The reaction starts by the formation of imine 4 which suffers an
intramolecular nucleophilic attack by the thiol group from either side to furnish a diastereomeric
mixture of 2-(substituted)thiazolidine-4-carboxylic acids.40,41 The isomerization at C-4 of the
thiazolidine via deprotonation/protonation is ruled out based on spectroscopic data (vide infra). It
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is believed that, under our conditions, the generation of azomethine ylide has not been formed and
hence the chirality at C-4 remained unchanged. Heating under reflux a mixture of (R)-cysteine (1)
and formaldehyde (2b) in acidified aqueous methanol (MeOH/H2O/AcOH 1:1: drops) gave an
84% yield of thiazolidine-(4R)-carboxylic acid (7b) (Entry b).
Table. Formation of 2-(substituted)thiazolidine-4-carboxylic acids
Entry
R
Yield (Lit.)
9838 (78)39
Ratio 7:8i (Lit.)ii
1.5:138 (1:2.3) 39
-
a
b
c
d
e
f
2-HOC6H4
H
84 (82,39 9842)
88 (89,43 7142)
89 (86,42 9139)
75 (65,43 8339)
93 (75,43 7339)
92 (8039)
Ph
2:1 (2.3:1,43 1:1.242)
3:1 (1:1.1,42 1:1.939)
2:1 (1:1.5,43 1:2.739)
1.4:1 (19:1,43 1:1.939)
2:1 (1:139)
4-HOC6H4
2-MeOC6H4
4-ClC6H4
3-NO2C6H4
4-NO2C6H4
g
h
89 (90,42 8043)
100:0iii (1.9:1,42 1:1943)
1H-NMR spectra were recorded in: (i) CDCl3/TFA, (ii) DMSO-d6, (iii) CDCl3/CD3OD
Furthermore, refluxing an equimolar mixture of (R)-cysteine (1) and benzaldehyde (2c) as the
carbonyl component in acidified methanol gave an 88% yield of 2-phenylthiazolidine-4-carboxylic
acid (7c) and (8c) as an isomeric mixture in a 2:1 ratio, respectively (Entry c). The pure
configuration of both diastereomers 7c and 8c was assigned on the basis of the experimental NOE
data which supported the relationship between the substituents at C-2 and C-4 to be cis-7c and
trans-8c. Thus, in the case of 7c, irradiation of 2-H gave 3.8 and 3.7% enhancement for 4-H and
5-Ha protons, respectively. Also, irradiation of 4-H showed 3.1 and 1.4% enhancement of 2-H and
5-Ha protons, respectively. However, in the case of 8c, irradiation of 2-H and 4-H protons gave no
effect across the ring.
Aryl aldehydes 2d-g reacted with (R)-cysteine (1) under the same conditions to produce
diastereomeric mixtures of 2-substituted thiazolidine-4-carboxylic acids 7d-g and 8d-g in 75-93%
yield (Entries d-g). It is worth noting that the diastereomeric ratio depends on the NMR solvent.
Thus, the 1H-NMR spectrum of the isomeric mixture 7f and 8f in CDCl3/CD3OD showed a 2.5:1
ratio, respectively, whereas in CDCl3/TFA, the isomeric ratio of 7f and 8f was 1.4:1, respectively.
This result also confirmed that the epimerization process at C-2 is solvent and pH dependent.44,45
The recorded NOE data of 7f and 8f, yet again, supported the cis-relationship in the case of the
major isomer 7f and trans-relationship of the minor isomer 8f (see experimental).
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Surprisingly, the use of 4-nitrobenzaldehyde (2h) as the carbonyl component lead to the
formation of 2-(4-nitrophenyl)thiazolidine-4-carboxylic acid (7h) as the only product in an 89%
yield (Entry h). The stereochemistry of 7h was established on the basis of its spectral data (see
experimental and chart 1a). Standing compound 7h in the NMR solvent (CDCl3/CD3OD) for
20min, 7h and 24h at room temperature gave an isomeric mixture of 7h and 8h in 21:1, 3:1 and
1.4:1 ratios, respectively (Chart 1b-1d). Extending the time for 48h (Chart 1e) under the same
conditions afforded a 1:1 ratio and no further change was noticed even after one week. It is believed
that the obtained isomer 7h in solution would be in equilibrium with Schiff’s base 4h, which
suffers a nucleophilic attack by the thiol group from the opposite side to give the other cyclization
product 8h (Scheme 2). Repeating the 1H-NMR experiment for compound 7h in a different solvent
(CDCl3/TFA) showed immediately an isomeric mixture of 7h and 8h in a 2:1 ratio, respectively.
It is obvious that the presence of TFA accelerates the epimerization rate, and the unshared pair of
electrons on the thiazolidine nitrogen drives the epimerization process.
Scheme 2. Epimerization of 7h into 8h via ring opening and recyclization.
.
48h
24h
(e)
(d)
7h
(c)
20 min
(b)
(a)
~ 5 min
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
Chemical Shift (ppm)
Chart 1. 1H-NMR (CDCl3/CD3OD) experiments illustrating the epimerization of 7h.
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On the other hand, (R)-cysteine (1) reacted with 2-nitrobenzaldehyde (2i) as the carbonyl
component under the same conditions to give a 46% yield of 3,5-bis(2-nitrophenyl)tetrahydro-1H-
thiazolo[3,4-c]oxazol-1-one (9) as the sole product (Scheme 3). The obtained bicyclic compound
is attributed to the high reactivity of the aldehyde and the steric congestion, which facilitate the
formation of the second oxazolone ring.
The stereochemistry of 9 was established on the basis of its spectral data (see experimental and
Chart 2a). The 1H-NMR (CDCl3/CD3OD) sample of 9 was left at room temperature for 48h, after
which time traces of the trans-thiazolidine-4-carboxylic acid derivative 8i was observed together
1
with 2-nitrobenzaldehyde in 8:1:1 ratio, respectively (Chart 2b). The H-NMR studies of the
obtained compound 9 in a different solvent (CDCl3/TFA) lead to a diastereomeric mixture of 2-
(2-nitrophenyl)thiazolidine-4-carboxylic acids (7i) and (8i) in a 1:1 ratio (Scheme 4). Accordingly,
2-nitrobenzaldehyde was observed and supported the suggested mechanism in scheme 4.
Scheme 3. Synthesis of 3,5-bis(2-nitrophenyl)tetrahydro-1H-thiazolo[3,4-c]oxazol-1-one 9.
Going through the literature revealed that the diastereomeric ratio depends on (i) the conditions
1
at which the reaction was conducted, and (ii) the H-NMR solvent used.39-43 It seems that using
our optimal conditions in the synthesis of 2-(substituted)thiazolidine-4-carboxylic acids affects
both the experimental yield and diastereomeric ratio (Table). Thus, using such conditions afforded
diastereomeric mixtures of 7 and 8 in 75-98% yields, and the cis-diastereomer 7 was obtained as
the major product in all cases. Interestingly, the cis-diastereomer 7h was obtained as the sole
product, and it was also lucky for us to separate thiazolo[3,4-c]oxazol-1-one 9 as the only product
with no trace of the thiazolidine derivatives.
Scheme 4. Collapse of 9 into thiazolidine-4-carboxylic acids 7i and 8i.
Thiohydantoins are common skeletons in organic synthesis and have a broad spectrum of
biological applications.46,47 Annulation of hydantoin ring to thiazolidine derivatives showed
anticancer activity (modulator of P53 activity)11 and assumed to treat Alzheimer via interaction
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with amyloid β peptide (Aβ 25-35).48 It seemed to us that annulation of thiohydantoin moiety to
thiazolidines might have biological applications.
Lalezari49 and Balalaie50 have reported two different methods for obtaining 6-phenyl-5-
thioxotetrahydroimidazo[1,5-c]thiazol-7-one (12b) in 86 and 97% yields, respectively. However,
the same product was formed in 95% yield by using our simple and convenient procedure. Thus,
stirring an equimolar mixture of thiazolidine-4-carboxylic acid (7b) and phenyl isothiocyanate (11)
in acidified methanol at room temperature gave 12b (Scheme 5). The stereochemistry of 12b was
assigned according to its spectral data and by comparison with the previously reported data.50
Scheme 5. Formation of thiohydantoins 12.
.
48 h
(b)
~ 20 min
1
(a)
10
9
8
7
6
5
4
3
2
0
Chemical Shift (ppm)
Chart 2. 1H-NMR spectra (CDCl3/CD3OD) of 9.
Similarly, the reaction of an isomeric mixture of 7c and 8c (R = Ph) with 11 gave 3,6-diphenyl-
5-thioxo-tetrahydro-imidazo[1,5-c]thiazol-7-one 12c as the only product in a 97% yield. However,
the diastereomeric mixtures of 7d-h and 8d-h reacted in the same manner with 11 to afford
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thiohydantoins 12d-h as the only products in excellent yields (91-99%). The stereochemistry of
these thiohydantoins 12 was confirmed based on the NOE data of 12c,f (see experimental).
Thiohydantoins 12 are stable and no sign of isomerization at C-3 was observed and this could be
mainly attributed to the presence of thioxo-group adjacent to N-4. Our results comply with the
literature work, which showed that the N-protection of thiazolidine-4-carboxylic acid prevented
the epimerization at C-2.36
The possible mechanism for thiohydantoins 12 started through the preferential attack of the
thiazolidine nitrogen of 7 on phenyl isothiocyanate (11) from the opposite face to give the
favourable thiourea derivatives 13 (K1>>K2) (Scheme 6).40,51 However, such conformation of 13
prevents the formation of cis-thiohydantoins 14 and alternatively, the intermediate 15 would
consequently lead to the trans-thiohydantoins 12.
Györgydeák et al.,52 reported a different mechanism for the synthesis of thiohydantoin-fused
thiazolidines, assuming a break-down of C-2–N-3 bond in the thiourea intermediate to produce a
ring open intermediate with a positively charged sulfur in order to change the stereochemistry at
C-2. However, the same authors have refused a similar mechanism suggested by referees during
their early work on N-acetylthiazolidines.41 We believe that our proposed mechanism (vide supra)
would serve well to rationalise the stereochemistry at C-3 in the obtained thiohydantoins.
Scheme 6. Plausible mechanism for thiohydantoins 12.
Interestingly, reacting 3,5-bis-(2-nitrophenyl)dihydro-hiazolo[3,4-c]oxazolidine-1-one (9)
with phenyl isothiocyanate (11) under the same conditions gave a 95% yield of 3-(2-nitrophenyl)-
6-phenyl-5-thioxo-tetrahydro-imidazo[1,2-c]thiazol-7-one (12i) as the only product according to
the suggested mechanism (Scheme 7). In the 1H-NMR spectra of thiohydantoins 12, H-3 appeared
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deshielded and this could be attributed to the presence of thioxo-group on N-4 and the almost
perpendicular dihedral angel relationship49 between the thiazolidine ring and the C-3 aryl
substituent. This arrangement positioned the H-3 proton in the deshielding area of the aryl group
at C-3.
Scheme 7. Possible mechanism for thiohydantoin 12i.
Conclusions
2-Substituted thiazolidine-4-carboxylic acids were easily obtained as nonseparable diastereomeric
mixtures via the reaction of aldehydes with (R)-cysteine in acidified methanol. 4-Nitro-
benzaldehyde gave only one isomer which epimerized gradually in the NMR solvent
(CDCl3/CD3OD) to a mixture of diastereomers. 2-Nitrobenzaldehyde reacted with (R)-cysteine to
afford 3,5-bis(2-nitrophenyl)tetrahydro-1H-thiazolo[3,4-c]oxazol-1-one (9) as an exclusive
product which collapsed in the NMR solvents to a diastereomeric mixture of thiazolidines 7i and
8i. The equilibrium between the two diastereomers is time and pH dependent. The obtained
thiazolidine derivatives reacted smoothly with phenyl isothiocyanate to afford the corresponding
thiohydantoins.
Experimental Section
General technical data
Thin layer chromatography (TLC) was carried out on aluminium plates pre-coated with silica gel
60 F254 (Merck), and were visualised using ultraviolet light and/or aqueous KMnO4/I2. Proton
nuclear magnetic resonance spectra were recorded at 300MHz on Bruker DPX300 and Oxford
NMR instruments. Chemical shifts (δ) are reported in parts per million relative to tetramethylsilane
(δ = 0.00) and coupling constants are given in hertz (Hz). The following abbreviations are used: s
= singlet, d = doublet, br = broad, dd = doublet of doublets, dt = doublet of triplets, m = multiplet,
t = triplet, td = triplet of doublets. 13C-NMR spectra were recorded at 75 MHz on a Bruker DPX300
instrument and chemical shifts are reported in parts per million (ppm). 1H-NMR peak assignments
are mainly based on DEPT135, COSY, HMQC and HMBC spectral data. Accurate masses were
obtained using a Bruker Daltonics micrOTOF spectrometer. Mass spectra were recorded at 70ev
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using Shimadzu GCMS-QP1000EX mass spectrometer. The IR spectra were measured on
Shimadzu IR instrument. Melting points (m.p.) were determined on a Kofler hot-stage apparatus
and are uncorrected. All compounds are named according to the IUPAC system using the
ChemBioDraw Ultra 12.0 program.
Method (A): Synthesis of 2-substituted thiazolidine-4-carboxylic acids
Stirring with heating under reflux an equimolar mixture (0.01 mol) of the carbonyl component 2
and (R)-cysteine 1 in acidified methanol (10:0.1 v/v MeOH/AcOH, 10 mL) for an appropriate
time. The corresponding thiazolidine derivatives precipitated out of hot solution during the reflux.
The solvent was cooled, concentrated and filtered to afford the crude product which was
crystallized from aqueous ethanol to give colourless amorphous solid.
Method (B): Synthesis of thiohydantoin 12
Stirring at room temperature an equimolar mixture (0.01 mol) of the thiazolidine derivatives 7 and
8 with phenyl isothiocyanate 11 in acidified methanol (10:0.1 MeOH/AcOH, 10 mL) for two days.
The corresponding thiohydantoin derivative 12 precipitated out of solution then the solvent was
evaporated under reduced pressure, and the resulting residue crystallized from a proper solvent.
(2RS,4R)-2-(2-Hydroxyphenyl)thiazolidine-4-carboxylic acid (7a and 8a). The reaction was
carried out according to general procedure (method A) using salicylaldehyde 1a as a carbonyl
component and heating for 15 minutes. An isomeric mixture of 7a and 8a was obtained in a 1.5:1
ratio, respectively, (0.22 g, 98%)38 (Lit., 78%)39; mp 160–162 ºC. IR (KBr) 3737, 3437 (broad),
3100, 2364, 1623 cm-1: MS (m/z %): 225 (M+, 40), 180 (M-45, 13), 153 (48), 137 (53), 132 (99),
120 (37), 91 (40), 77 (100), 65 (39) and 51 (88); 1H-NMR (CDCl3/TFA) for the major isomer 7a,
δ: 7.69-6.93 (m, 4H, Ar-H), 6.11 (s, 1H, 2-H), 5.1 (dd, 1H, J 4.1 and 6.8, 4-H), 3.81 (dd, 1H, J 8.4
and 8.1, 5-Ha) and 3.67 (dd, 1H, J 5.7, 5-Hb). The minor isomer 8a, δ: 7.69-6.93 (m, 4H, Ar-H),
6.20 (s, 1H, 2-H), 5.15 (t,1H, J 6.6, 4-H), 3.89 (d, 1H, J 7.5, 5-Ha) and 3.85 (d, 1H, J 7.5, 5-Hb).
(R)-Thiazolidine-4-carboxylic acid (7b). The reaction was carried out according to general
procedure (method A) using formaldehyde 2b as a carbonyl component in acidified aqueous
methanol (5:5:0.05 MeOH/H2O/AcOH, 10 mL) and heating for 8h. Crystallization afforded 7b
(1.12 g, 84%) (Lit., 82%,39 98%42); IR (KBr) 3046, 1585 and 1487 cm-1; m/z (%) 133 (M+, 47), 55
1
(100), 57 (85), 60 (48), 69 (83), 73 (57), 87 (58) and 120 (55); H-NMR (CDCl3/TFA) δ: 4.99
(dd,1H, J 4.5 and 6.6, 4-H), 4.70 (d,1H, J 10.7, 2-Ha), 4.52 (d,1H, J 10.7, 2-Hb), 3.62 (dd, 1H, J
4.5 and 12.9, 5-Ha) and 3.57 (dd, 1H, J 6.6 and 12.9, 5-Hb).
(2RS,4R)-2-Phenylthiazolidine-4-carboxylic acid (7c and 8c). The reaction was carried out
according to general procedure (method A) using benzaldehyde 2c as a carbonyl component and
heating for 1h. An isomeric mixture of 7c and 8c was obtained in a 2:1 ratio, respectively, (1.84 g,
88%) (Lit., 71%,42 89%43); IR (KBr) 3421, 2963, 1480, 1137 and 857 cm-1; m/z (ESI+) 210.0603
(100% MH+), 232.0 (3%, MNa+), (Found MNa+ 232.040196 C10H11NNaO2S requires
232.040270); 1H-NMR (CDCl3/CD3OD) for the major isomer 7c, δ: 7.55-7.33 (m, 5H, Ar-H), 5.55
(s, 1H, 2-H), 3.99 (dd, 1H, J 7.3 and 8.6, 4-H), 3.52 (dd, 1H, J 7.3 and 10.4, 5-Ha,) and 3.20 (dd,
1H, J 8.6 and 10.4, 5-Hb). The minor isomer 8c, δ: 7.55-7.33 (m, 5H, Ar-H), 5.77 (s, 1H, 2-H),
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4.27 (dd, 1H, J 5.6 and 7.5, 4-H), 3.45 (dd, 1H, J 7.5 and 10.9, 5-Ha) and 3.31 (dd, 1H, J 5.6 and
10.9, 5-Hb).
NOE data (CDCl3/CD3OD) for 7c:
% Enhancement
Irradiated
protons
2-H
2-H
4-H
5-Ha
3.72
1.44
5-Hb
Ar-H
3.82
-
-
4.53 (δ 7.51)
4-H
3.08
-
NOE data (CDCl3/CD3OD) for 8c:
% Enhancement
Irradiated
protons
2-H
-
4-H
5-Ha
5-Hb
Ar-H
5.04 (δ 7.54)
2.26 (δ 7.38)
4-H
5.44
-
(2RS,4R)-2-(4-Hydroxyphenyl)thiazolidine-4-carboxylic acid (7d and 8d). The reaction was
carried out according to general procedure (method A) using 4-hydroxybenzaldehyde 2d as a
carbonyl component and heating for 2h. An isomeric mixture of 7d and 8d was obtained in a 3:1
ratio, respectively, (2 g, 89%) (Lit., 91%,39 86%42); IR (KBr) 3138, 2918, 2824, 1474, 1203 and
876; cm-1; m/z (ESI+) 226.05 (33% MH+), (Found MH+ 226.053467 C10H12NO3S requires
226.053241);1H-NMR (CDCl3/CD3OD) for the major isomer 7d, δ: 7.42 (d, 2H, J 8.6, Ar-H), 6.86
(d, 2H, J 8.6, Ar-H), 5.50 (s, 1H, 2-H), 4.03 (t, 1H, J 7.5, 4-H), 3.53 (dd, 1H, J 7.5 and 10.8, 5-Ha)
and 3.31 (dd, 1H, J 7.5 and 10.8, 5-Hb). The minor isomer 8d, δ: 7.41 (d, 2H, J 8.7, Ar-H), 6.85
(d, 2H, J 8.7, Ar-H), 5.71 (s, 1H, 2-H), 4.36 (dd, 1H, J 5.6 and 7.4, 4-H), 3.54 (dd, 1H, J 7.4 and
11.1, 5-Ha) and 3.42 (dd, 1H, J 5.6 and 11.1, 5-Hb).
(2RS,4R)-2-(2-Methoxyphenyl)thiazolidine-4-carboxylic acid (7e and 8e). The reaction was
carried out according to general procedure (method A) using 2-methoxy-benzaldehyde 2e as a
carbonyl component and heating for 4h. An isomeric mixture of 7e and 8e was obtained in a 2:1
ratio, respectively, (1.8 g, 75%) (Lit., 83%,39 65%43); IR (KBr) 3420, 2960, 1459, 1269, 1190, 846
and 793 cm-1; m/z (%) 240 (MH+, 7.88), 55 (19), 57 (16), 60 (10), 69 (14), 76 (17), 87 (11), 91
(100), 103 (12), 107 (20), 119 (47), 132 (27), 135 (18), 193 (16), 207 (7); 1H-NMR (CDCl3/TFA)
for the major isomer 7e, δ: 7.54-7.39 (m, 4H, Ar-H), 6.02 (s, 1H, 2-H), 5.04 (dd, 1H, J 3.3 and 7.9,
4-H), 3.95 (s, 3H, O-Me), 3.78 (dd, 1H, J 7.9 and 13.0, 5-Ha) and 3.70 (dd, 1H, J 3.3 and 13.0,
5Hb).
(2RS,4R)-2-(4-Chlorophenyl)thiazolidine-4-carboxylic acid (7f and 8f). The reaction was
carried out according to general procedure (method A) using 4-chlorobenzaldehyde 2f as a
carbonyl component and heating for 2h. An isomeric mixture of 7f and 8f was obtained in a 2.5:1
ratio, respectively, (2.26 g, 93%) (Lit., 73%,39 75%43); IR (KBr) 3427, 2964, 1490, 1436, 1135 and
740; cm-1; m/z (ESI+) 244.0 (65% MH+), (Found MH+ 244.019803 C10H11ClNO2S requires
244.019354); 1H-NMR (CDCl3/CD3OD) for the major isomer 7f, δ: 7.48 (d, 2H, J 8.5, Ar-H), 7.35
(d, 2H, J 8.5, Ar-H), 5.51 (s, 1H, 2-H), 3.98 (dd, 1H, J 7.3 and 8.6, 4-H), 3.51 (dd, 1H, J 7.3 and
10.5, 5-Ha) and 3.19 (dd, 1H, J 8.6 and 10.5, 5-Hb). The minor isomer 8f, δ: 7.44 (d, 2H, J 8.6,
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Ar-H), 7.31 (d, 2H, J 8.6, Ar-H), 5.76 (s, 1H, 2-H), 4.19 (dd, 1H, J 5.9 and 7.2, 4-H), 3.42 (dd, 1H,
J 7.2 and 10.7, 5-Ha) and 3.26 (dd, 1H, J 5.9 and 10.7, 5-Hb).
NOE data (CDCl3/CD3OD) for 7f:
% Enhancement
Irradiated
protons
2-H
2-H
4-H
5-Ha
-
5-Hb
Ar-H
2.45
-
-
1.91 (δ 7.48)
3.19 (δ 7.48)
4-H
3.65
1.74
(2RS,4R)-2-(3-Nitrophenyl)thiazolidine-4-carboxylic acid (7g and 8g). The reaction was
carried out according to general procedure (method A) using 3-nitrobenzaldehyde 2g as a carbonyl
component and heating for 8h. An isomeric mixture of 7g and 8g was obtained in a 2:1 ratio,
respectively, (2.34 g, 92%) (Lit., 80%)39, IR (KBr) 3447, 1548, 1429, 1197, 808 and 697 cm-1; m/z
(%) 255 (MH+ , 12), 55 (17), 57 (20), 63 (67), 66 (14), 68 (17), 70 (12), 79 (100), 86 (10), 181 (9)
1
and 194 (10); H-NMR (CDCl3/TFA) for the major isomer 7g, δ: 8.59-7.69 (m, 4H, Ar-H), 6.09
(s, 1H, 2-H), 5.16 (brt, 1H, J 5.3, 4-H) and 3.88 (brd, 2-H, J 6.12, 5-Ha and 5-Hb). The minor
isomer 8g, δ: 8,95-7.69 (m, 4H, Ar-H), 6.16 (s, 1H, 2-H), 5.22 (t, 1H, J 6.4, 4-H), 3.98 (dd, 1H, J
6.4 and 12.6, 5-Ha) and 3.73 (dd, 1H, J 6.4 and 12.6, 5-Hb).
(2R,4R)-2-(4-Nitrophenyl)thiazolidine-4-carboxylic acid (7h). The reaction was carried out
according to general procedure (method A) using 4-nitrobenzaldehyde 2h as a carbonyl component
and heating for 2h. Compound 7h was obtained as the sole diastereomer, (2.26 g, 89%); IR (KBr)
3029, 1620, 1486, 1381, 1193, 1126, 874 and 778 cm-1; m/z (ESI+) 255.042980 (< 10% MH+),
299.0 (< 10%, (M+ 2Na-H)+), (Found (M+ 2Na-H)+ 299.007241 C10H9N2Na2O4S requires
299.007293); 1H-NMR (CDCl3/CD3OD) δ: 8.24 (d, 2H, J 8.8, Ar-H), 7.72 (d, 2H, J 8.8, Ar-H),
5.61 (s, 1H, 2-H), 4.03 (dd, 1H, J 7.23 and 8.7, 4-H), 3.54 (dd, 1H, J 7.2 and 10.4, 5-Ha) and 3.21
(dd, 1H, J 8.7 and 10.4, 5-Hb). The minor isomer 8h (1H-NMR sample after 24h), δ: 8.19 (d, 2H,
J 8.6, Ar-H), 7.66 (d, 2H, J 8.6, Ar-H), 5.90 (s, 1H, 2-H), 4.10 (t, 1H, J 6.8, 4-H), 3.42 (dd, 1H, J
6.8 and 10.7, 5-Ha) and 3.26 (dd, 1H, J 6.8 and 10.7, 5-Hb).
(3S,5R,7aR)-3,5-Bis-(2-nitrophenyl)tetrahydro-1H-thiazolo[3,4-c]oxazol-1-one (9). The
reaction was carried out according to general procedure (method A) using 2-nitrobenzaldehyde 2i
as a carbonyl component and heating for 8h. Crystallization gave 9, (1.78 g, 46%); IR (KBr), 3034,
1375, 1203, 1018, and 684 cm-1, m/z (%) 387 (M+, 46%), 51 (100), 133 (45), 177 (43), 263 (52),
278 (48); m/z (ESI+) compound 9, 388.0600 (19% MH+, C17H13N3O6S), 797.0940 (< 5%,
(2M+Na)+), compound 7i/8i, 277.0 (< 5%, MNa+), (Found (MNa+ 277.025302 C10H10N2NaO4S
1
requires 277.025349); H-NMR (CDCl3/CD3OD) δ: 8.16 (dd, 1H, J 1.3 and 8.1, Ar-H), 8.02 (d,
1H, J 7.8, Ar-H), 7.75-7.69 (m, 2H, Ar-H), 7.65-7.56 (m, 2H, Ar-H), 7.48 (dt, 1H, J 1.4 and 8.0,
Ar-H), 7.38 (dd, 1H, J 1.1 and 7.7, Ar-H), 7.08 (s, 1H, 3-H), 6.75 (s, 1H, 5-H), 4.21 (dd, 1H, J 0.8
and 6.5, 7a-H), 3.41 (dd, 1H, J 0.8 and 12.5, 7-Ha) and 2.97 (dd, 1H, J 6.5 and 12.5, 7-Hb).
(R)-6-Phenyl-5-thioxotetrahydroimidazo[1,5-c]thiazol-7(3H)-one (12b). The reaction was
carried out according to general procedure (method B) using thiazolidine 7b. Crystallization from
ethanol afforded 12b (2.38 g, 95%) (Lit., 86%,49 97%50); m.p. 158 oC. IR (KBr) 2996, 1758, 1621,
1490, 1460, 1219, 874 and 764, cm-1; m/z (%) 250 (MH+ 251, 10), 55 (55), 59 (54), 77 (100), 86
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(60), 119 (12), 135 (50), and 204 (4); 1H-NMR (CDCl3) δ: 7.53-7.28 (m, 5 H, Ph-H), 5.39 (d, 1H,
J 9.7, 3-Ha), 4.86 (t, 1H, J 8.2, 7a-H), 4.58 (d, 1H, J 9.7, 3-Hb), 3.44 (dd, 1H, J 8.2 and 11.1, 1-
Ha) and 3.23 (dd, 1H, J 8.2 and 11.1, 1-Hb).
(3S,7aR)-3,6-Diphenyl-5-thioxotetrahydroimidazo[1,5-c]thiazol-7(3H)-one
(12c).
The
reaction was carried out according to general procedure (method B) using thiazolidines 7c and 8c.
Crystallization from ethanol afforded 12c (3.16 g, 97%); m.p. 180 oC. IR (KBr) 3023, 1620, 1489,
873, 778 and 674 cm-1; m/z (ESI+) 327.1 (100% MH+), (Found MH+ 327.062930 C17H15N2OS2
requires 327.062032);1H-NMR (CDCl3) δ: 7.59-7.30 (m, 10 H, Ar-H), 6.81 (s, 1H, 3-H), 4.89 (dd,
1H, J 7.1 and 9.2, 7a-H), 3.50 (dd, 1H, J 7.1 and 11.1, 1-Ha) and 3.30 (dd, 1H, J 9.2 and 11.1, 1-
Hb); 13C-NMR (CDCl3), δc: 184.7, 170.4, 138.6, 132.9, 129.4, 129.3, 128.9, 128.8, 128.1, 126.9,
67.0, 67.7 and 32.5.
NOE data (CDCl3) for 12c:
% Enhancement
Irradiated
protons
3-H
7a-H
1-Ha
3-H
7a-H
0.48
1-Ha
-
5.50
1-Hb
0.69
-
Ar-H
3.83 (δ 7.57)
2.82 (δ 7.57)
-
-
0.54
-
1.51
8.38
0.83
26.67
1-Hb
23.92
(3S,7aR)-3-(4-Hydroxyphenyl)-6-phenyl-5-thioxotetrahydroimidazo[1,5-c]thiazol-7(3H)-
one (12d). The reaction was carried out according to general procedure (method B) using
thiazolidines 7d and 8d. Crystallization from ethanol afforded 12d (3.35 g, 98%); m.p. 178 oC; IR
(KBr) 3047, 2882, 1621, 1589, 1488, 855, 731 and 693 cm-1; m/z (%) 342 (MH+, 64), 55 (100),
57 (96), 77 (49), 106 (52), 135 (59), 192 (47), 205 (50), 248 (49), 299 (52), 311 (56) and 326 (60);
1H-NMR (CDCl3), δ: 7.54-7.30 (m, 9H, Ar-H), 6.74 (s, 1H, 3-H), 4.85 (dd, 1H, J 7.1 and 9.2, 7a-
H), 3.48 (dd, 1H, J 7.1 and 11.1, 1-Ha) and 3.30 (dd, 1H, J 9.2 and 11.1, 1-Hb). 13C-NMR (CDCl3)
δc: 184.7, 170.2, 137.0, 134.7, 132.7, 129.5, 129.3, 129.0, 128.4, 128.0, 66.7, 66.5 and 32.5.
(3S,7aR)-3-(2-Methoxyphenyl)-6-phenyl-5-thioxotetrahydroimidazo[1,5-c]thiazol-7-one
(12e). The reaction was carried out according to general procedure (method B) using thiazolidine
o
7e and 8e. Crystallization from chloroform afforded 12e (3.24 g, 91%); m.p. 178 C; IR (KBr)
3001, 1487, 1294, 873and 846 cm-1; m/z (%) 356 C18H16N2O2S2, (MH+, 32), 91 (100), 101 (37),
135 (39), 278 (63), 282 (41), 298 (41) and 327 (34); 1H-NMR (CDCl3), δ: 7.56-7.50 (m, 3H, Ar-
H), 7.39-7.30 (m, 4H , Ar-H), 7.04 (d, 1H, J 7.6, Ar-H), 6.98 (d, 1H, J 8.3, Ar-H), 6.85 (s, 1H, 3-
H), 5.20 (dd, 1H, J 6.9 and 10.2, 7a-H), 3.46 (dd, 1H, J 6.9 and 11.1, 1-Ha) and 3.30 (dd, 1, J 10.2
and 11.1H, 1-Hb).
(3S,7aR)-3-(4-Chlorophenyl)-6-phenyl-5-thioxotetrahydroimidazo[1,5-c]thiazol-7(3H)-one
(12f). The reaction was carried out according to the general procedure (method B) using
thiazolidines 7f and 8f and stirring for 48h. Crystallization from chloroform afforded 12f (3.55 g,
o
98.5%); m.p. 194-196 C; IR (KBr) 1620, 1490, 1338, 872, 778 and 515; cm-1; m/z (%) 342
C17H13ClN2OS2, 77 (100), 86 (17), 116 (40), 135 (40), 205 (19), 116 (12), 295 (14), and 248 (10);
1H-NMR (CDCl3), δ: 7.54-7.30 (m, 9H, Ar-H), 6.74 (s, 1H, 3-H), 4.89 (dd, 1H, J 7.0 and 9.3, 7a-
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H), 3.50 (dd, 1H, J 7.0 and 11.1, 1-Ha) and 3.31 (dd, 1H, J 9.3 and 11.1, 1-Hb); 13C-NMR (CDCl3)
δ: 184.6, 170.4, 137.0, 134.7, 132.7, 129.5, 129.3, 129.0, 128.4, 128.0, 66.8, 66.4 and 32.4.
NOE data (CDCl3) for 12f:
% Enhancement
Irradiated
protons
3-H
7a-H
1-Ha
3-H
7a-H
0.50
1-Ha
1-Hb
0.87
-
Ar-H
-
4.98 (δ 7.53)
2.65 (δ 7.53)
-
-
0.42
-
1.61
5.99
11.01
1.58
31.52
1-Hb
25.29
(3S,7aR)-3-(3-Nitrophenyl)-6-phenyl-5-thioxotetrahydroimidazo[1,5-c]thiazol-7(3H)-one
(12g). The reaction was carried out according to the general procedure (method B) using
thiazolidines 7g and 8g. Crystallization from ethanol afforded 12g (3.57 g, 96.5%); m.p. 200-202
oC; IR (KBr) 3056, 1620, 1489, 1342, 846 and 727 cm-1; m/z (%) 371 C17H13N3O3S2, 77 (100),
118 (10), 135 (35), 175 (23), 219 (12), 347 (15), 293 (12), and 324 (9); 1H-NMR (CDCl3/TFA), δ:
8.48 (s, 1H, Ar-H), 8.27 (td, 1H, J 1.2 and 8.1, Ar-H), 7.96 (dd, 1H, J 0.6 and 8.1, Ar-H), 7.65 (t,
1H, J 8.1, Ar-H), 7.56–7.52 (m, 3H, Ph-H), 7.93–7.27 (m, 2H, Ph-H), 6.80 (s, 1H, 3-H), 5.10 (dd,
1H, J 7.1 and 9.6, 7a-H), 3.58 (dd, 1H, J 7.1 and 11.0, 1-Ha) and 3.41 (dd, 1H, J 9.6 and 11.0, 1-
Hb).
(3S,7aR)-3-(4-Nitrophenyl)-6-phenyl-5-thioxotetrahydroimidazo[1,5-c]thiazol-7(3H)-one
(12h). The reaction was carried out according to general procedure (method B) using thiazolidines
7h. Crystallization from chloroform afforded 12h (3.33 g, 94%); m.p. 160-162 oC; IR (KBr) 3022,
1440, 874, 753 and 629 cm-1; m/z (ESI+) 372.0 (14% MH+), (Found MH+ 372.046864
C17H14N3O3S2 requires 372.047110); 1H-NMR (CDCl3), δ: 8.28 (d, 2H, J 8.7, Ar-H), 7.74 (d, 2H,
J 8.7, Ar-H), 7.57-7.32 (m, 5H, Ph-H), 6.83 (s, 1H, 3-H), 4.92 (dd, 1H, J 7.0 and 9.4, 7a-H), 3.53
(dd, 1H, J 7.0 and 11.2, 1-Ha) and 3.36 (dd, 1H, J 9.4 and 11.2, 1-Hb).
(3S,7aR)-3-(2-Nitrophenyl)-6-phenyl-5-thioxotetrahydroimidazo[1,5-c]thiazol-7(3H)-one
(12i). The reaction was carried out according to general procedure (method B) using 3,5-bis-(2-
nitrophenyl)dihydrothiazolo[3,4-c]oxazol-1-one 9. Crystallization from ethanol afforded 12i (3.51
g, 95 %); m.p. 230-232 oC; IR (KBr) 2887, 1497, 1345, 853, 786 and 689 cm-1; m/z (%) 371, 55
1
(41), 63 (100), 135 (34), 208 (22), 112 (49), 237 (24), 325 (17), and 355 (18); H-NMR
(CDCl3/CD3OD) δ: 8.17 (dd, 1H, J 1.2 and 8.2, Ar-H), 7.74 (dt, 1H, J 1.3 and 8.2, Ar-H), 7.67
(dd, 1H, J 1.5 and 7.9, Ar-H), 7.59-7.48 (m, 4H, Ar-H), 7.37-7.33 (m, 2H, Ar-H), 7.35 (s, 1H, 3-
H), 5.08 (dd, 1H, J 7.4 and 9.3, 7a-H), 3.43 (dd, 1H, J 7.4 and 11.0, 1-Ha) and 3.36 (dd, 1H, J 9.3
and 11.0, 1-Hb).
Acknowledgements
We acknowledge support from South Valley University. M. F. A and E. E. E. thank Leeds
University and Prof Ronald Grigg for undertaking spectroscopic analysis in their labs.
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Supplementary data
The supplementary file is a pdf document showing the relevant HRMS traces that are reported in
this paper.
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