Regioselective synthesis of 1,3-thiazines by sequential 4-oxothiazolidine to 1,2-dithiole to 1,3-thiazine transformations: role of intramolecular non-bonded S?O interactions

Article abstract of DOI:10.1016/j.tet.2006.12.075

A new synthetic approach to 2,3-dihydro-4H-1,3-thiazine derivatives based upon reductive rearrangement of 1,2-dithiole-3-ylidene thiones has been developed. In turn, the 1,2-dithiole derivatives were prepared by an efficient ring-opening-closing process of 2-alkylidene-4-oxothiazolidines, induced in the presence of Lawesson's reagent by intramolecular non-bonded 1,5-type S?O interactions in the 4-oxothiazolidine precursors.

Full text of DOI:10.1016/j.tet.2006.12.075

Tetrahedron 63 (2007) 1937–1945
Regioselective synthesis of 1,3-thiazines by sequential
4-oxothiazolidine to 1,2-dithiole to 1,3-thiazine transformations:
role of intramolecular non-bonded S/O interactions
b
c
ꢀ
ꢁ a
ꢁa,d,
*
Aleksandar Rasovic, Peter J. Steel, Erich Kleinpeter and Rade Markovic
aCenter for Chemistry ICTM, PO Box 815, 11000 Belgrade, Serbia
bDepartment of Chemistry, University of Canterbury, PO Box 4800, Christchurch, New Zealand
cUniversita€t Potsdam, Chemisches Institut, PO Box 60 15 53, D-14415 Potsdam, Germany
dFaculty of Chemistry, University of Belgrade, Studentski trg 16, PO Box 158, 11001 Belgrade, Serbia
Received 21 November 2006; revised 20 December 2006; accepted 21 December 2006
Available online 28 December 2006
Abstract—A new synthetic approach to 2,3-dihydro-4H-1,3-thiazine derivatives based upon reductive rearrangement of 1,2-dithiole-3-
ylidene thiones has been developed. In turn, the 1,2-dithiole derivatives were prepared by an efficient ring-opening–closing process of
2-alkylidene-4-oxothiazolidines, induced in the presence of Lawesson’s reagent by intramolecular non-bonded 1,5-type S/O interactions
in the 4-oxothiazolidine precursors.
Ó 2007 Elsevier Ltd. All rights reserved.
Me
N
1. Introduction
O2
S
CO2Me
CO2Me
N
S
Sb
S
Experimental and theoretical studies have shown that intra-
molecular non-bonded S/X (X¼O or S) interactions in a
large number of organosulfur compounds, as exemplified by
acetazolamide 1,1 thiadiazoline 2,2 natural antitumor anti-
biotic leinamycin 3,3 cyclic sulfilimine 44 or 1,3-dithioles
5,5 are important in controlling their structural properties
and chemical reactivity.6 Similarly, among specific exam-
ples, the reactivity enhancement of ethynyl-1,5-azastibocine
67 with respect to diphenyl(phenylethynyl)stibane, in Pd-
catalyzed cross-coupling reactions with organic halides,
has been explained in terms of the central role of an intra-
molecular Sb/N interaction.
Cl
Cl
2.54 Å
S
O
2.68 Å
OMe
S
Ar
2.91 Å
4
5
6
The most important structural feature of the compounds,
exhibiting the 1,5-type S/S, S/O or Sb/N interactions,
is a much shorter distance between the corresponding atoms
˚
˚
than the sum of their van der Waals radii (3.60 A, 3.32 A,
˚
and 3.74 A, respectively). Over the last few years we have
reported that push–pull 2-alkylidene-4-oxothiazolidines of
the general structure 7 (Scheme 1), containing the cis-
configured –S–C]C–C]O moiety, participate in a number
of reactions through activation of (i) the nucleophilic a-
carbon atom of the exocyclic C]C bond,8 or (ii) C(4) and
C(5) positions of the heterocyclic ring via activated vinyl-
ogous N-methyliminium ions.9 In particular, we anticipated
that the 1,5-type S/O close contact, in combination with
the rigid and flat 4-oxothiazolidine ring conjugated with
the C(2) side chain, can be electronically tuned to induce
specific chemical transformation of substrates 7. As reported
in a preliminary communication, this has been confirmed ex-
perimentally by a ring-opening–closing transformation of
selected 4-oxothiazolidine enaminoketones 7 in the presence
of Lawesson’s reagent (LR), to produce functionalized 1,2-
dithiole-3-ylidene thiones 810 (Scheme 1).
O
OH O
2.65 Å
2.76 Å
+ S
S
O
Me
F3C
O
HO
N
–
O
S
S
H2NO2S
NH
N
O
N
H
2.80 Å
N
N
HN N
S
1
2
3
Keywords: Thiazolidines; 1,2-Dithioles; 1,3-Thiazines; Rearrangement;
Non-bonded S/O interaction.
* Corresponding author. Tel.: +381 11 3336 741; fax: +381 11 636 061;
e-mail: markovic@helix.chem.bg.ac.yu
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.12.075

1938
A. Raꢀsovicꢁ et al. / Tetrahedron 63 (2007) 1937–1945
R1
–S–C]C–C]O fragment is also reflected in the n,p-
R2
O
donor/acceptor properties of compounds 7, possessing the
two electron donors (–NH– and –S–), the intervening double
bond and an electron acceptor (C]O). For the reason of
greater reactivity of ketones toward LR, in comparison to that
of amides and esters,13 we chose enaminoketone type thiazo-
lidines 7a–c (Scheme 2) as substrates for thionation. Thus,
colorless 7a–c (R3¼H) react readily with LR in toluene under
reflux to form in one-pot brown-reddish 1,2-dithioles 8a–c
in high yield. When LR was replaced by P4S10 as a thionating
reagent in the reaction with 7a, only 11% of the rearranged
1,2-dithiole 8a was formed after 6 h, confirming LR as being
the superior reagent for this transformation.
O
S
S
S
S
S
R2
R2
R1
N
R1
N
H
S
N
R3
H
7
8
9
Scheme 1.
Herein, we report the full account of the 7/8 conversion
initiated by the directional non-bonded 1,5-type S/O
interaction in the N-unsubstituted 4-oxothiazolidines 7. In
addition, the range of results obtained on the use of 1,2-
dithioles in a subsequent one-pot reductive rearrangement
to new 2,3-dihydro-4H-1,3-thiazine-4-thiones 9, forms the
subject of the second part of our paper, demonstrating the
ease of interconversion of three structurally different hetero-
cyclic classes via two efficient sequential transformations.11
The structures 8a (R2¼H; R1¼Ph) and 9b (R2¼Me; R1¼Ph),
as representatives of the two heterocyclic series, have been
unambiguously confirmed by X-ray analyses.
An alternative method to prepare 1,2-dithioles 8 in good
yields (>55%) involved, in situ base-catalyzed formation
of 2-alkylidene-4-oxothiazolidines 712 from the correspond-
ing b-oxonitriles (1 equiv) and a-mercaptoesters (1.2 equiv)
in chloroform as a solvent, followed by addition of LR. The
formation of a side product, such as oxodithiole 10, in a very
low yield, was also observed in several reactions.14
2. Results and discussion
In contrast to the parent enaminoketones 7a–c, N-methyl
substituted thiazolidine derivative 7d did not give the ex-
pected rearranged product with LR, but instead, thionation
products 11d (48%) and 12d (48%) were formed under
similar reaction conditions. The isolation of the thionation
derivative 11d in 64% yield after short reaction time
(10 min), establishes for the thiazolidinone reactivity the
thionation rates as follows: ketone>lactam\ester. In
practice, the ester functionality at C(5) of 7c and 7d stayed
intact even after prolonged reaction times (5–7 h). Charac-
teristic spectroscopic data of the dithioles 8a–c and oxodi-
thioles 10b,c are compiled in Table 1.
We were attracted to this work by the possibility of inducing
the regioselective reaction of (Z)-2-alkylidene-4-oxothiazo-
lidine involving the push–pull moiety as the reactive site,
due to the proximity of sulfur and oxygen atoms. An X-ray
crystal structure characterization of ethyl (Z)-(5-ethoxycar-
bonylmethyl-4-oxothiazolidin-2-ylidene)ethanoate (7e), in
combination with a NOE experiment,12 gave the evidence
for the Z-configuration and short 1,5-O/S non-bonded
˚
distance (2.87 A), which is less than the sum of the van
˚
der Waals radii (3.22 A). The planarity of the U-shaped
S
N
S
S
O
S
S
toluene
R2
R2
+
R2
R1
N
R1
4-5 h,
N
O
S
8a-c (85-99%)
10a-c (~1-3%)
O
R1
+ LR
N
R2
R3
R2
H
toluene
2 h,
S
S
S
S
7
+
R1
O
R1
S
N
7a (R1 = Ph; R2 = R3 = H)
H
H
Me
Me
12d (48%)
7b (R1 = Ph; R2 = Me; R3 = H)
11d (48%)
7c (R1 = Ph; R2 = CH2CO2Et; R3 = H)
7d (R1 = Ph; R2 = CH2CO2Et; R3 = Me)
7e (R1 = OEt; R2 = CH2CO2Et; R3 = H)
Scheme 2.
Table 1. Selected 13C and 1H NMR (CDCl3) chemical shifts (ppm) of 1,2-dithioles 8a–c and oxodithioles 10b,c
2
S
1
S
2
S
1
S
3
S
3a
S
4
S
O
S
5
R2
5
R2
1
N
R1
other dipolar
resonance forms
R2
R1
N
3
N
3
2'
4
Ph
3'
2'
4
3'
H
H
10a-c
8a-c
8A
Entry
Product
C(20)
C(3)
C(4)
C(5)
C(4)-H
C(30)-H
1
2
3
5
6
8a14
8b
8c
10b
10c
198.6
204.9
201.9
186.4
185.5
187.3
187.8
187.4
185.4
183.7
125.8
126.0
126.4
122.8
123.0
178.3
178.7
177.9
171.0
171.3
8.37
8.42
8.37
7.75
7.75
2.86
3.12
3.40
2.76
3.05

A. Raꢀsovicꢁ et al. / Tetrahedron 63 (2007) 1937–1945
1939
In the 1H NMR spectra the C(4) proton of dithioles 8a–c ab-
sorbs at very low field, viz 8.37–8.42 ppm, which indicate
the aromatic nature of the dithiole ring. In addition, the
13C shifts for C(3), C(4), and C(5) point to highly delocal-
ized derivatives 8a–c, which implicate a significant aromatic
contribution from the 10p electron 3,3al4,4-trithia-1-aza-
pentalene structure 8A15 (vide infra). The values matched
those obtained in similar 1,2-dithiole-3-ylidene thiones.16,17
The lowest field signal at d 198–205 ppm was assigned by an
HMBC experiment to the C atom of the C]S group.
dithiole closing process 14/15, followed by H-transfer
(step 15/8). The lack of rearrangement in the case of the
N-methyl substituted precursor 7d can be attributed to an
unfavorable methyl migration.
In order to prove the 10p electron aromaticity of highly de-
localized 1,2-dithioles 8a–c, the structure of 8a was theoreti-
cally calculated on the B3LYP/6-31G** level of theory.18,19
As expected, a planar [1,2]dithiolo[1,5-b][1,2,4]dithiazole
moiety 8B was obtained; the distances between the three
sulfur atoms are more or less the same (vide infra) and the
phenyl substituent in position 5 is twisted by 30.6ꢀ (Fig. 1).
Thering current effect, which proved to beverycharacteristic
of the aromaticity of the 10p electron system 8B was cal-
culated by the method of Klod and Kleinpeter20 and is also
visualized in Figure 1.
The regiochemical control of the 4-oxothiazolidine/1,2-
dithiole rearrangement can be traced in the mechanistic fea-
tures outlined in Scheme 3. Upon direct replacement of the
carbonyl oxygen in (Z)-7 by sulfur, the non-bonded S/S
distance in the intermediates 12 and 13, should be very sim-
˚
ilar to the original value of 2.87 A, determined for the S/O
distance in enaminoester 7d. The distance is approximately
Iso-chemical-shielding surfaces (ICSS) of ꢁ0.1 ppm21 thus
˚
˚
obtained [above and below the plane ca. 10 A high field shift
0.75 A shorter than the corresponding van der Waals dis-
tance between the two sulfur atoms.12 Subsequently, the
close S/S interaction initiates an intramolecular rearrange-
ment by concerted thioxothiazolidine ring-opening-1,2-
(yellow), in-plane >10 A low field shift (red)] show 8B to
˚
have a stronger ring current effect than benzene20 and,
hereby, can be employed as proof for strong pseudo-aroma-
ticity of the planar [1,2]dithiolo[1,5-b][1,2,4]dithiazole moi-
ety 8B. The chemical shielding in the surrounding of 8B was
calculated based on the idea of NICS by Schleyer et al.21
Therefore, the molecule was placed in the center of a grid
R2
R2
LR
S
S
S
S
S
S
S
S
(Z)-7
R2
˚
of ghost atoms ranging from ꢂ10.0 to +10.0 A in all three
O
Ph
S
Ph
N
H
N
H
N
Ph
˚
dimensions with a step width of 0.5 A. This resulted in
H
13
14
15
a cube of 68,921 ghost atoms. The chemical shielding calcu-
lations were done with the GIAO method22,23 using HF/
6-31G* on the basis of the geometry optimized B3LYP/
6-31G** structure of 8B. Since GIAO is a coupled HF
method that uses gauge independent atom orbital for the cal-
culation of shielding values, it can be applied in the calcula-
tion of NICS. From the GIAO calculations the coordinates
H-transfer
S
S
S
S
S
S
S
S
R2
R2
R2
N
Ph
N
Ph
N
Ph
8B
8A
8
Scheme 3.
Figure 1. Structure and ring current effect of planar [1,2]dithiolo[1,5-b][1,2,4]dithiazole moiety of 8a (ICSSs of different shielding or deshielding: blue stands
for 5 ppm shielding, cyan for 2 ppm shielding, green-blue for 1 ppm shielding, green for 0.5 ppm shielding, yellow for 0.1 ppm shielding and red for ꢂ0.1 ppm
deshielding).

1940
A. Raꢀsovicꢁ et al. / Tetrahedron 63 (2007) 1937–1945
and isotropic shielding values of the ghost atoms were
extracted. After the transformation of the tabulated chemical
shielding into a contour file the ring current effect of 8B can
be visualized as ICSS. In Figure 1 only ICSSs behind the
coordinate system are visualized to show both the different
ICSSs and the structure of the molecule. Thus, it is possible
to map the spatial extension of the ring current effect at a
given chemical shielding value: blue stands for 5 ppm shield-
ing, cyan for 2 ppm shielding, green-blue for 1 ppm shield-
ing, green for 0.5 ppm shielding, yellow for 0.1 ppm
shielding, and red for ꢂ0.1 ppm deshielding.
10p-electron system, represented by trithia-1-azapentalene
structure 8A or 8B.27 It is worth mentioning that numerous
experimental and theoretical investigations presented evi-
dence that the structure of 1,6,6al4,4-trithiapentalene 16
(X¼S) and related compounds is characterized by C2v sym-
metry and no-bond-single-bond-resonance.15b–d
6
X
6a
S
1
X
X
S
X
X
S
X
5
2
3a
4
3
16 (X = S or O)
It should be emphasized that the above calculations were
fully corroborated by a single-crystal X-ray structure deter-
mination of 8a (Fig. 2), which confirms the presence of the
3,3al4,4-trithia-1-azapentalene system as in 8A or 8B,
depicting full or partial bonding between the sulfur atoms,
respectively. It crystallizes as red plates in the monoclinic
space group P21/c. Figure 2 shows a perspective view of
the structure along with selected bond lengths and bond
angles, which have similar values to those in a previously
reported structure containing a heterocyclic ring system of
this type.24
Having studied the synthesis of 3-imino-1,2-dithiole deriva-
tives 8a–c, which is inherently dependent on the spatial ori-
entation of the U-shaped –S–C]C–C]O fragment of the
thiazolidinone precursors 7, we have focused our interest
on their synthetic utility. The aromatic nature of the conju-
gated 1,2-dithiole ring and nucleophilicity of sulfur atoms
reflect the reactivity toward a limited number of electrophilic
reagents.17 On the other hand, reactions are common with a
certain number of oxygen, sulfur, nitrogen or carbon nucleo-
philes, which attack various electrophilic positions of the
dithiole nucleus. In some cases after the initial nucleophilic
attack, a stepwise pathway involving ring cleavage and
subsequent cyclization, sometimes preceded by sulfur extru-
sion, provide an approach for formation of other hetero-
cyclic systems. These facts prompted us to explore the
reductive cleavage of 1,2-dithioles 8 and 1,2-oxodithioles
10 as we assumed that the potential acyclic intermediate
can be formed, en route to another heterocyclic ring system.
The [1,2]dithiolo[1,5-b][1,2,4]dithiazole ring system is pla-
˚
nar [maximum deviation from the mean-plane¼0.022(2) A]
and inclined to the plane of the phenyl ring at an angle of
28.0(1)ꢀ. The molecular packing shows the typical herring-
bone pattern associated with this space group and appears
to be controlled by intermolecular S/S interactions,25 the
˚
shortest being 3.455(1) A. From the data above it is apparent
that the S1–S2 and S2–S3 bonds, being 2.3374(5) and
˚
2.3408(5) A, respectively, are shorter than the sum of the
van der Waals radii, but relatively long in comparison to
Thus, we found that heating 8a–c (X¼S) with 2 equiv of
sodium borohydride in ethanol resulted in ring-opening
followed by cyclization to yellowish 2,3-dihydro-4H-1,3-
thiazine-4-thiones 9a–c in good yields (Scheme 4).18,28
In contrast, attempted reductive rearrangement of oxodi-
thioles 10a,b (X¼O) to analogous 1,3-oxothiazine-4-thiones
failed, and instead (Z)-3-mercapto-3-phenylprop-2-enethio-
amide (16) was isolated, together with the corresponding
a,b-unsaturated aldehyde 17.
˚
a S–S covalent bond (2.08 A) and not exactly of equal
length, as the two fused rings differ. The solid-state structure
8A, having an internal hypervalent sulfur S(2)26 within the
nearly linear S(1)–S(2)–S(3) array (175.8ꢀ) is unequivocal,
but not necessarily identical with that in solution. The struc-
ture of the major 1,2-dithiol-3-imino species 8 in solution
should be considered in conjunction with fused bicyclic
Ph
1. NaBH4, EtOH
S
, 3-3.5 h
R2
N
H
9a-c
S
2. H2O
X = S
X
S
S
R2
1. NaBH4, EtOH
, 3 h
N
Ph
S
SH
Ph
O
8a (R2 = H; X = S)
2
+
R
2. H2O
X = O
8b (R2 = Me; X = S)
8c (R2 = CH2CO2Et; X = S)
10a (R2 = H; X = O)
H2N
H
R2
17
16
10b (R2 = Me; X = O)
Scheme 4.
Figure 2. Perspective view of the crystal structure of 8a. Selected bond
˚
lengths (A): S1–S2 2.3374(5), S1–C1 1.701(2), S2–S3 2.3408(5), S2–C2
The structures of the new products 9a–c are supported by
their spectroscopic data (Table 2) and elemental analysis.
1,3-Thiazines have one strong absorption in their UV–vis
spectra at lmax 269 nm and another, nearly identical maxi-
mum at 370, 369, and 367 nm, respectively. In comparison
to this ring system, 1,2-dithiole precursors 8a–c exhibit
two identical maxima at 333 and 446 nm, indicating that
1.760(2), S3–C4 1.708(2), C1–N1 1.320(2), C1–C11 1.506(2), C2–N1
1.357(2), C2–C3 1.409(2), C3–C4 1.384(2), C4–C41 1.484(2). Selected
bond angles (ꢀ): S(3)–S(2)–S(1) 175.80(2), C(2)–S(2)–S(3) 88.84(5),
C(2)–S(2)–S(1) 86.96(5), C(4)–S(3)–S(2) 94.31(5), N(1)–C(1)–S(1)
121.22(12), C(11)–C(1)–S(1) 119.48(12), N(1)–C(2)–S(2) 122.14(12),
C(3)–C(2)–S(2) 118.88(11), C(4)–C(3)–C(2) 121.29(12), C(3)–C(4)–S(3)
116.66(11).

A. Raꢀsovicꢁ et al. / Tetrahedron 63 (2007) 1937–1945
Table 2. Selected spectroscopic data of 2,3-dihydro-4H-1,3-thiazine-4-thiones 9a–c
1941
Ph
13C and 1H NMR chemical shifts (ppm)
IR (KBr) (n in cmꢂ1
)
UV (lmax in nm)
1
S
2
R2
N
H
S
C(20)H
NH
C(5)H
C]S
C]C
NH
2'
C(4)
C(2)H
9aa
9bb
9cb
190.1
191.6
191.4
4.97–5.06 (m)
4.74–4.82 (m)
4.91–4.99 (m)
1.63 (d, J¼6.6 Hz)
2.00–2.23 (m)
2.28–2.48 (m)
10.34 6.99
1158
1153
1131
1550
1560
1555
3075
3147
3273
269, 370
269, 369
269, 367
8.07 7.11 (d, J¼1.2 Hz)c
8.30 7.12 (d, J¼1.2 Hz)c
a
DMSO-d6.
CDCl3.
NH–C(5)H coupling as determined by HH COSY.
b
c
different substituents R, i.e., hydrogen, methyl or ethoxycar-
bonylmethylene, do not alter the UV–vis absorption pattern
of heterocycles 8a–c and 9a–c. The presence of a strong
band in the IR spectra (KBr) around 1150 cmꢂ1 and a low-
field signal at d¼w191 ppm, unambiguously established
the presence of the thiocarbonyl group.
alternative positions with 85:15 relative occupancies. To
the best of our knowledge, this is the first X-ray crystal struc-
ture of 4H-2,3-dihydro-1,3-thiazine-4-thione.
The rearrangement of 8a–c to 9a–c can be rationalized by
the series of steps, involving an initial hydride ring-opening
of the 1,2-dithiole ring to acyclic intermediate 18, containing
the six-atom skeleton, which subsequently appears in the
final thiazine ring (Scheme 5). Reduction of the thioxo group
(intermediate 19), followed by rotation around the N–C bond
and s-cis/s-trans conformational change 19/20, set the
stage for the cyclization. Thus, the mercapto group attack
on the C]C bond, occurring with the loss of H2S, gives
rise to 1,3-thiazines 9a–c. Attempts to promote the same
type of rearrangement with oxo-1,2-dithioles 10b,c (X¼O)
to obtain analogues 1,3-oxazines, resulted in the formation
of the a,b-unsaturated thioamide 16 (80%) and 17b or 17c
via aldol reaction of enolizable aldehyde, formed by degra-
dation of the common open-chain intermediate 19b or
19c (X¼O). Unsurprisingly, this intermediate, generated by
reductive cleavage from the corresponding 1,2-oxodithiole
10 does not undergo cyclization due to the lower nucleo-
philic ability of the oxygen versus the sulfur atom.
One of the thiazine derivatives, namely 2-ethyl-6-phenyl-
2,3-dihydro-4H-1,3-thiazine-4-thione (9b), has been struc-
turally characterized by X-ray crystallography. This
compound crystallizes as yellow blocks in the triclinic space
group P-1. Figure 3 shows a perspective view of the structure
with selected bond lengths. Not shown in the diagram is the
fact that there is minor disorder in the conformation of the
heterocyclic ring with the sulfur atom occupying two
3. Conclusion
The efficient and general procedure for the synthesis of 2,3-
dihydro-4H-1,3-thiazine-4-thiones via 1,2-dithiole-3-ylidene
thiones, prepared from (Z)-2-alkylidene-4-oxothiazolidines,
has been developed. The overall 4-oxothiazolidine-1,3-
thiazine transformation clearly reveals the importance of
the directional non-bonded 1,5-type S/O interaction in
4-oxothiazolidine precursors to induce further chemical
reactions.
Figure 3. Perspective view of the major contributor in the crystal structure
˚
of 9b. Selected bond lengths (A): S1–C6 1.772(3), S1–C2 1.805(3), C2–N3
1.467(4), C2–C21 1.492(4), N3–C4 1.331(3), C4–S4 1.689(3), C4–C5
1.459(4), C5–C6 1.330(4), C6–C61 1.487(4).
H
H
H
X
S
S
S
SH
Ph
X
S
S
X
S
S
[Red]
X = O
H2N
Ph
R2
R2
R2
N
H
N
H
Ph
N
Ph
8a-c (X = S)
10a,b (X = O)
16
19a-c (X = S or O)
18a-c (X = S or O)
+
α,β-unsaturated
aldehyde 17b,c
X = S
Ph
Ph
HS
-H2S
S
SH
R2
R2
S
N
H
S
N
H
9a-c
20a-c
Scheme 5.

1942
A. Raꢀsovicꢁ et al. / Tetrahedron 63 (2007) 1937–1945
4. Experimental
Melting points were determined on a Micro-Heiztisch Boe-
(Cipso-Ph), 178.5 (C]C–S), 187.3 (N]C–S), 198.6
(C]S). MS (EI): m/z (rel intensity): 251 (M+, 35), 236
(15), 210 (5), 193 (7), 174 (3), 145 (13), 121 (18), 102
(20), 89 (3), 77 (13), 59 (100), 39 (3); UV (DMSO): lmax
(3) 333 nm (15,250) and 446 nm (11,300). Anal. Calcd for
C11H9NS3: C, 52.56; H, 3.61; N, 5.57; S, 38.26. Found: C,
52.68; H, 3.71; N, 5.63; S, 37.88.
€
tius PHMK apparatus or Buchi apparatus and are uncor-
rected. The IR spectra were recorded on a Perkin–Elmer
FTIR 1725X spectrophotometer and are reported as wave
numbers (cmꢂ1). Samples for IR spectral measurements
were prepared as KBr disks. The NMR spectra were
obtained using a Varian Gemini 2000 instrument (1H at
200 MHz, 13C at 50.3 MHz). 13C NMR resonance assign-
ments were aided by the use of the DEPT technique to deter-
mine numbers of attached hydrogens. The 2D ROESY has
been performed on a Bruker Avance 500 NMR spectrometer.
Chemical shifts are reported in parts per million (ppm) on
the d scale from TMS as an internal standard in the solvents
specified. Low-resolution mass spectra were recorded using
a Finnigan MAT 8230 BE spectrometer at 70 eV (EI). Isobu-
tane was used as the ionizing gas for the chemical ionization
(CI) mass spectra. The UV spectra were measured on a
Beckman DU-50 spectrophotometer. Analytical thin layer
chromatography (TLC) was carried out on Kieselgel G
nach Stahl, and the spots were visualized by iodine. Column
4.1.2. N-(5-Phenyl-3H-1,2-dithiol-3-ylidene)propan-
thioamide (8b). From 7b (40 mg, 0.17 mmol) in toluene
(4 mL) and LR (70 mg, 0.17 mmol) after column chromato-
graphy (petroleum ether/EtOAc 10:0 to 10:0.4) the 1,2-
dithiole 8b was isolated; yield 39 mg (85%); Rf (toluene/
ethyl acetate, 9:1)¼0.65; mp 54–56 ꢀC. IR (KBr): nmax
3142, 3013, 2965, 2927, 2891, 1518, 1481, 1448, 1396,
1290, 1227, 1195, 1065, 1000, 944, 841, 761, 693 cmꢂ1
.
1H NMR (CDCl3): d 1.44 (3H, t, J¼7.4 Hz, CH3), 3.12
(2H, q, J¼7.4 Hz, CH2), 7.47–7.50 (3H, m, m- and p-Ph),
7.83–7.88 (2H, m, o-Ph), 8.42 (1H, s, ]CH). 13C NMR
(CDCl3): d 13.8 (CH3), 35.8 (CH2), 126.0 (]CH), 127.5
(o-Ph), 129.1 (m-Ph), 131.0 (p-Ph), 136.7 (Cipso-Ph), 178.7
(C]C–S), 187.8 (N]C–S), 204.9 (C]S); UV (DMSO):
lmax (3) 332 nm (15,250) and 446 nm (11,300). Anal. Calcd
for C12H11NS3: C, 54.32; H, 4.15; N, 5.28. Found: C, 54.14;
H, 4.18; N, 5.26.
˚
chromatography was carried out on SiO2 (silica gel 60 A,
12-26, ICN Biomedicals). Elemental analyses were per-
formed at the microanalysis laboratory at the Department
of Chemistry, University of Belgrade.
4.1.3. Ethyl 3-(5-phenyl-3H-1,2-dithiol-3-ylidenethiocar-
bamoyl)propanoate (8c). From 7c (50 mg, 0.16 mmol) in
toluene (4 mL) and LR (66 mg, 0.16 mmol) after column
chromatography (toluene/EtOAc 3:1 to 1:1) the 1,2-dithiole
8c was isolated; yield 51 mg (92%); Rf (toluene/ethyl ace-
tate, 4:1)¼0.68; mp 65 ꢀC. IR (KBr): nmax 3067, 3006,
2978, 2925, 2855, 1730, 1512, 1483, 1450, 1426, 1401,
1375, 1305, 1177, 1157, 1107, 1049, 1015, 945, 867, 830,
4.1. General procedure for the preparation
of 1,2-dithioles 8a–c
A colorless solution of (Z)-2-alkylidene-4-oxothiazolidines
7a–c (0.164 mmol) and LR (0.164 mmol) in dry toluene
(3 mL) was heated in an oil bath at 90–95 ꢀC (initially a het-
erogeneous solution at room temperature becomes homoge-
nous upon heating at around 75 ꢀC). After a few minutes, the
color of the reaction mixture turned dark reddish brown.
Caution: All reactions involving Lawesson’s reagent, due
to the unpleasant odor, should be carried out in a well-
ventilated hood. The mixture was stirred at this temperature
for additional 4–5 h when TLC indicated the complete con-
sumption of substrates 7a–c. After cooling to room temper-
ature, the solvent was evaporated in vacuo. The residue was
chromatographed (toluene/ethyl acetate, 10:0 to 8:2, v/v)
affording the dark orange crystalline 1,2-dithioles 8a–c in
high yields (85–99%). The structural assignments of all iso-
lated products were made on the basis of spectroscopic data
760, 687 cmꢂ1
.
1H NMR (CDCl3): d 1.26 (3H, t,
J¼7.2 Hz, CH3), 2.92 (2H, t, J¼7.3 Hz, CH2–CO), 3.40
(2H, t, J¼7.3 Hz, CH2–CS), 4.17 (2H, q, J¼7.0 Hz, CH2–
O), 7.41–7.53 (3H, m, m- and p-Ph), 7.78–7.87 (2H, m,
o-Ph), 8.37 (1H, s, ]CH). 13C NMR (CDCl3): d 14.7
(CH3), 32.9 (CH2–CO), 37.5 (CH2–CS), 60.6 (CH2–O),
126.4 (]CH), 127.4 (o-Ph), 129.1 (m-Ph), 131.1 (p-Ph),
136.0 (Cipso-Ph), 172.2 (COester), 177.5 (C]C–S), 187.4
(N]C–S), 201.9 (C]S). MS (EI): m/z (rel intensity): 337
(M+, 38), 304 (65), 277 (7), 264 (8), 236 (100), 211 (15),
194 (10), 178 (20), 145 (38), 117 (73), 102 (33), 71 (45),
55 (65); UV (DMSO): lmax (3) 332.6 nm (15,990) and
446.1 nm (12,720). Anal. Calcd for C15H15NO2S3: C,
53.39; H, 4.48; N, 4.15; S, 28.50. Found: C, 53.15; H,
4.45; N, 4.24; S, 27.97.
1
(IR, H and 13C NMR, MS, UV) and elemental analysis.
Compound 8a was previously described.14 The assignments
are based on the highly delocalized 1,2-dithiole structures 8
with emphasis on an increased contribution of the 3,3al4,4-
trithia-1-azapentalene form 8A.
4.1.4. N-(5-Phenyl-3H-1,2-dithiol-3-ylidene)propan-
amide (10b). A suspension of 1,2-dithiole 8b (71 mg,
0.27 mmol) and Hg(OAc)2 (86 mg, 0.27 mmol) in acetic
acid (6 mL) was kept under reflux for 2.5 h and then cooled
down to room temperature. After addition of water (50 mL),
the suspension was filtered and extracted with CHCl3
(2ꢃ50 mL). The organic layer was dried and concentrated
in vacuo. After column chromatography (toluene/EtOAc
20:0 to 20:5) the title compound 10b was isolated; yield
34.5 mg (52%); Rf (toluene/ethyl acetate, 4:1)¼0.36; mp
109–110 ꢀC. IR (KBr): nmax 3223, 3051, 2968, 2931, 2901,
2360, 1585, 1527, 1488, 1450, 1416, 1360, 1295, 1250,
1217, 1176, 1068, 1025, 1001, 911, 845, 805, 758, 727,
4.1.1. N-(5-Phenyl-3H-1,2-dithiol-3-ylidene)ethanthio-
amide (8a). From 7a (50 mg, 0.228 mmol) in toluene
(4 mL) and LR (87 mg, 0.228 mmol) after column chroma-
tography (toluene/EtOAc 10:0 to 8:2) the 1,2-dithiole 8a was
isolated; yield 57 mg (99%); Rf (toluene/ethyl acetate,
4:1)¼0.76; mp 98 ꢀC. IR (KBr): nmax 3004, 2908, 1638,
1514, 1485, 1447, 1404, 1226, 1205, 989, 846, 757, 690,
1
642 cmꢂ1. H NMR (CDCl3): d 2.86 (3H, s, CH3), 7.42–
7.53 (3H, m, m- and p-Ph), 7.75–7.84 (2H, m, o-Ph), 8.37
(1H, s, ]CH). 13C NMR (CDCl3): d 29.1 (CH3), 125.8
(]CH), 127.4 (o-Ph), 129.0 (m-Ph), 131.0 (p-Ph), 136.2

A. Raꢀsovicꢁ et al. / Tetrahedron 63 (2007) 1937–1945
1943
682, 652 cmꢂ1. 1H NMR (CDCl3): d 1.30 (3H, t, J¼7.4 Hz,
CH3), 2.76 (2H, q, J¼7.4 Hz, CH2), 7.49–7.54 (3H, m,
m- and p-Ph), 7.71–7.73 (o-Ph), 7.74 (1H, s, ]CH). 13C
NMR (CDCl3): d 9.7 (CH3), 31.4 (CH2), 122.8 (]CHCPh),
127.2 (o-Ph), 129.4 (m-Ph), 131.6 (p-Ph), 132.9 (Cipso-Ph),
171.0 (]CSPh), 185.4 (N]CS), 186.4 (CO); CIMS: m/z:
250 (M+1); UV (DMSO): lmax (3) 304.0 nm (10,600) and
381.1 nm (8900). Anal. Calcd for C12H11NOS2: C, 57.81;
H, 4.45; N, 5.62; S, 25.72. Found: C, 57.42; H, 4.46; N,
5.54; S, 25.15.
7.67–7.71 (2H, m, o-Ph), 10.34 (1H, s, NH). 13C NMR
(DMSO-d6): d 19.0 (CH3), 53.7 (CH), 123.2 (]CH),
127.7 (o-Ph), 129.4 (m-Ph), 131.2 (p-Ph), 135.5 (Cipso-Ph),
143.0 (]CHPh), 190.1 (C]S). MS (EI): m/z (rel intensity):
221 (M+, 100), 206 (14), 193 (11), 178 (77), 145 (31), 121
(96), 102 (17), 77 (23); UV (DMSO): lmax (3) 269 nm
(17,000) and 370 nm (23,900). Anal. Calcd for
C11H11NS2: C, 59.69; H, 5.01; N, 6.33. Found: C, 59.42;
H, 5.19; N, 6.10.
4.2.2. 2-Ethyl-6-phenyl-2,3-dihydro-4H-1,3-thiazine-4-
thione (9b). The title compound was obtained as a yellow
solid in 60% yield (101.5 mg) from 191.0 mg (0.7 mmol)
of 1,2-dithiole 8b and 54.5 mg (1.4 mmol) of NaBH4.
Mp 135–136 ꢀC. Rf (toluene/ethyl acetate, 9:1)¼0.55. IR
(KBr): nmax 3147, 3067, 3028, 2966, 2875, 1560, 1501,
4.1.5. Ethyl 4-oxo-(5-phenyl-3H-1,2-dithiol-3-ylidene-
amino)butanoate (10c). Based on the same procedure as
above, from 1,2-dithiole 8c (101.3 mg, 0.3 mmol) and
Hg(OAc)2 (95.7 mg, 0.3 mmol) in acetic acid (6 mL) after
column chromatography (toluene/EtOAc 20:0 to 20:5) the
title compound 10c was isolated; yield 50 mg (52%); Rf
(toluene/ethyl acetate, 4:1)¼0.33; mp 119–121 ꢀC. IR
(KBr): nmax 3434, 3056, 2986, 2927, 2335, 1732, 1577,
1522, 1487, 1450, 1414, 1388, 1364, 1329, 1272, 1229,
1186, 1157, 1076, 1022, 965, 940, 917, 868, 848, 762, 684,
1
1485, 1416, 1368, 1275, 1153, 1099, 764, 702 cmꢂ1. H
NMR (CDCl3): d 1.18 (3H, t, J¼7.6 Hz, CH3), 2.00–2.23
(2H, m, CH2), 4.74–4.82 (1H, m, CH), 7.11 (1H, d,
J¼1.2 Hz, ]CH), 7.39–7.50 (3H, m, m- and p-Ph), 7.67–
7.72 (2H, m, o-Ph), 8.07 (1H, s, NH). 13C NMR (CDCl3):
d 10.2 (CH3), 26.2 (CH2), 59.9 (CH), 123.0 (]CH), 128.2
(o-Ph), 128.9 (m-Ph), 131.1 (p-Ph), 135.6 (Cipso-Ph), 146.0
(]CHPh), 191.6 (C]S). MS (EI): m/z (rel intensity): 235
(M+, 100), 220 (54), 206 (28), 193 (52), 178 (26), 145 (33),
121 (95), 104 (30), 77 (25); UV (DMSO): lmax (3) 269 nm
(29,300) and 369 nm (21,400). Anal. Calcd for C12H13NS2:
C, 61.24; H, 5.57; N, 5.95. Found: C, 61.04; H, 5.55; N, 5.99.
660 cmꢂ1
.
1H NMR (CDCl3): d 1.27 (3H, t, J¼7.2 Hz,
CH3), 2.80 (2H, t, J¼6.9 Hz, CH2CO), 3.05 (2H, t,
J¼6.9 Hz, CH2CON), 4.17 (2H, q, J¼7.2 Hz, CH2), 7.50–
7.55 (3H, m, m- and p-Ph), 7.71–7.76 (o-Ph), 7.75 (1H, s,
]CH). 13C NMR (CDCl3): d 14.2 (CH3), 29.7 (CH2CO),
32.9 (CH2CON), 60.6 (OCH2), 123.0 (]CHCPh), 127.2
(o-Ph), 129.5 (m-Ph), 131.7 (p-Ph), 132.7 (Cipso-Ph), 171.3
(]CSPh), 172.7 (N]CO), 183.8 (N]CS), 185.5 (COester);
CIMS: m/z: 322 (M+1); UV (DMSO): lmax (3) 305.6 nm
(12,600) and 381.1 nm (10,900). Anal. Calcd for
C15H15NO3S2: C, 56.05; H, 4.70; N, 4.36. Found: C, 56.54;
H, 4.97; N, 3.92.
4.2.3. 2-Ethyl-3-(6-phenyl-4-thioxo-3,4-dihydro-4H-1,3-
thiazin-2-yl)propanoate (9c). The title compound was
obtained as a yellow solid in 55% yield (22.0 mg) from
43.8 mg (0.13 mmol) of 1,2-dithiole 8c and 9.8 mg
(0.26 mmol) of NaBH4. Mp 114–116 ꢀC; Rf (toluene/ethyl
acetate, 9:1)¼0.41. IR (KBr): nmax 3273, 3058, 2961, 2927,
4.2. General procedure for the preparation of
1,3-thiazine derivatives 9a–c
1720, 1555, 1487, 1282, 1201, 1131, 1022, 762, 698 cmꢂ1
.
1H NMR (CDCl3): d 1.29 (3H, t, J¼7.0 Hz, CH3), 2.28–
2.48 (2H, m, CH2), 2.59–2.69 (2H, m, COCH2), 4.20 (2H,
q, J¼7.0 Hz, CH2O), 4.91–4.99 (1H, m, CH), 7.12 (1H, d,
J¼1.2 Hz, ]CH), 7.39–7.53 (3H, m, m- and p-Ph), 7.66–
7.70 (2H, m, o-Ph), 8.30 (1H, s, NH). 13C NMR (CDCl3):
d 14.1 (CH3), 27.9 (CH2), 30.1 (COCH2), 57.5 (CH), 61.3
(CH2O), 123.0 (]CH), 128.1 (o-Ph), 129.0 (m-Ph), 131.2
(p-Ph), 135.6 (Cipso-Ph), 144.6 (]CHPh), 172.5 (C]O),
191.4 (C]S). MS (EI): m/z (rel intensity): 307 (M+, 100),
220 (96), 206 (20), 193 (23), 179 (29), 145 (24), 121 (62),
102 (14), 77 (18); UV (DMSO): lmax (3) 269 nm (18,600)
and 367 nm (14,300). Anal. Calcd for C15H17NO2S2: C,
58.60; H, 5.57; N, 4.56. Found: C, 58.46; H, 5.68; N, 4.51.
1,2-Dithiole 8 (1 mmol) and sodium borohydride (2 mmol)
were suspended in ethanol (ca. 30–35 mL) (initially a hetero-
geneous solution at room temperature that becomes homo-
genous upon heating). A dark orange reaction mixture after
stirring at room temperature for 1.5–2 h became less colored
and cloudy. Stirring was continued at 85–90 ꢀC (oil bath)
until the reaction was complete, as monitored by TLC (in
most cases, for about 1.5 h). The reaction mixture was
brought to room temperature and then the solvent was evap-
orated in vacuo. The residue was treated with warm water
(70 mL) and after stirring (10 min), a cold suspension was
extracted with CHCl3 (3ꢃ40 mL), followed by drying and
concentration of the combined organic phases. The residue
was either crystallized or chromatographed on silica gel
(toluene/ethyl acetate gradient 100:0 to 50:50, v/v) to afford
corresponding thiazine 9, as a pale yellow solid.
4.3. X-ray crystallography
Data were collected with a Bruker APEX CCD area detector,
using graphite monochromatized Mo Ka radiation (l¼
˚
4.2.1. 2-Methyl-6-phenyl-2,3-dihydro-4H-1,3-thiazine-4-
thione (9a). The title compound was obtained as a yellow
solid in 62% yield (56 mg) from 101.5 mg (0.4 mmol) of
1,2-dithiole 8a and 30.5 mg (0.8 mmol) of NaBH4. Mp
154–155 ꢀC; Rf (toluene/ethyl acetate, 9:1)¼0.50. IR
(KBr): nmax 3075, 3031, 2920, 2853, 1550, 1489, 1444,
0.71073 A). The structures were solved by direct methods
using SHELXS29 and refined on F2, using all data, by
full-matrix least-squares procedures using SHELXTL.30
Hydrogen atoms were included in calculated positions,
with isotropic displacement parameters 1.2 times the isotro-
pic equivalent of the carrier carbons.
1
1385, 1158, 1084, 766, 738, 701 cmꢂ1. H NMR (DMSO-
d6): d 1.63 (3H, d, J¼6.6 Hz, CH3), 4.97–5.06 (1H, m,
Fulltablesofatomcoordinates, thermalparameters, andbond
lengths and angles have been deposited at the Cambridge
CH), 6.99 (1H, s, ]CH), 7.49–7.55 (3H, m, m- and p-Ph),

1944
A. Raꢀsovicꢁ et al. / Tetrahedron 63 (2007) 1937–1945
(b) Cherkasov, R. A.; Kutyrev, G. A.; Pudovik, A. N.
Tetrahedron 1985, 41, 2567–2624; (c) Jesberger, M.;
Davis, T. P.; Barner, L. Synthesis 2003, 1929–1958.
14. New oxodithioles 10b,c were also synthesized independently
from 1,2-dithioles 8b,c according to procedure reported by
Ammick, G.; Vialle, J. Bull. Soc. Chim. Fr. 1971, 4002–4006.
15. (a) Lozac’h, N. Comprehensive Heterocyclic Chemistry;
Katritzky, A. R., Rees, C. W., Potts, K. T., Eds.; Pergamon:
Oxford, 1984; Vol. 6, pp 1049–1070; (b) Perrin, C. L.; Kim,
Y.-J.; Kuperman, J. J. Phys. Chem. A 2001, 105, 11383–
11387; (c) Spanget-Larsen, J.; Andersen, K. B. Phys. Chem.
Chem. Phys. 2001, 3, 908–916; (d) Bjernemose, J. K.;
Frandsen, E.; Jensen, F.; Pedersen, C. Th. Tetrahedron 2003,
59, 10255–10259; (e) Potts, K. T.; Nye, S. A.; Smith, K. A.
J. Org. Chem. 1992, 57, 3895–3901; (f) Zhang, W.; Henry, Y.
Synlett 2001, 1129–1130.
Crystallographic Data Centre. CCDC 607823–607824 con-
tains the supplementary crystallographic data for this paper.
These data can be obtained free of charge at www.ccdc.
cam.ac.uk/conts/retrieving.html [or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax (internat.): +44-1223/336-033; e-mail:
deposit@ccdc.cam.ac.uk].
4.3.1. Crystal data for 8a. C11H9NS3, MW 251.37, red
plate, 0.50ꢃ0.41ꢃ0.09 mm, monoclinic, P21/c, a¼
ꢀ
˚
13.4197(7), b¼7.2797(4), c¼11.8980(6) A, b¼108.161(2) ,
3
ꢀ
˚
V¼1104.4(1) A , Z¼4, T¼ꢂ180 C, F(000)¼520, m(Mo
Ka)¼0.633 mmꢂ1, Dcalcd¼1.512 g cmꢂ3, 2qmax 55ꢀ (CCD
area detector, 99.6% completeness), wR(F2)¼0.076 (all
2532 data), R¼0.030 (2403 data with I>2sI).
16. Pedersen, B. S.; Lawesson, S.-O. Tetrahedron 1979, 35,
2433–2437.
17. McKinnon, D. M. Comprehensive Heterocyclic Chemistry;
Katritzky, A. R., Rees, C. W., Potts, K. T., Eds.; Pergamon:
Oxford, 1984; Vol. 6, pp 783–811.
4.3.2. Crystal data for 9b. C12H13NS2, MW 235.35, yellow
block, 0.22ꢃ0.09ꢃ0.08 mm, triclinic, P-1, a¼5.6687(8),
b¼11.0149ꢀ(15), c¼11.056ꢀ(2) A, a¼116.540(9)ꢀ, b¼
˚
3
˚
94.285(10) , g¼102.679(7) , V¼590.6(2) A , Z¼2, T¼
ꢂ180 ꢀC,
F(000)¼248,
m(Mo
Ka)¼0.416 mmꢂ1
,
Dcalcd¼1.323 g cmꢂ3, 2qmax 50ꢀ (CCD area detector, 97%
completeness), wR(F2)¼0.092 (all 2033 data), R¼0.044
(1285 data with I>2sI).
18. The ab initio calculations were performed on SGI Octane and
SGI Origin 2000 workstations and a Linux cluster using the
program GAUSSIAN 03: Frisch, M. J.; Trucks, G. W.;
Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman,
J. R.; Montgomery, J. A.; Vreven, T., Jr.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.;
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox,
J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain,
M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.;
Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.;
Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin,
R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
Gaussian 03, Revision C.02; Gaussian: Wallingford, CT, 2004.
19. Geometry optimization was carried out at the B3LYP/6-31G**
level of theory without restrictions; see: Hehre, W. J.; Radom,
L.; Schleyer, P. v. R.; Pople, J. A. Ab initio Molecular Orbital
Theory; Wiley: New York, NY, 1986.
Acknowledgements
Partial financial support by the Ministry of Science,
Technology and Development of the Republic of Serbia,
grant no. 1709 (to R.M.), is acknowledged.
References and notes
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Longo, R. L. Tetrahedron 2003, 59, 10187–10193.
3. Wu, S.; Greer, A. J. Org. Chem. 2000, 65, 4883–4887.
ꢁ
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