Cycloadditions of 3,4-dihydro-2H-pyrrole N-oxide with thioketones and a selenoketone

Article abstract of DOI:10.1246/bcsj.80.743

Cycloadditions of 2-diphenylphosphinoyl-2-methyl-3,4-dihydro-2H-pyrrole N-oxide (DPhPMPO), 3,4-dihydro-2H-pyrrole N-oxide having a diphenylphosphinoyl group at the C2 position with thioketones afforded the corresponding 1,4,2-oxathiazolidines. Dissociation constants of these 5-membered ring products were determined. The cycloadducts were stabilized by the diphenylphosphinoyl group. The reaction of DPhPMPO with di-tert-butyl selenoketone gave the corresponding selenoamide under microwave irradiation. The formation of the selenoamide indicated that the cycloaddition of DPhPMPO with the selenoketone analogue also proceeded through the formation of the corresponding 5-membered ring product.

Full text of DOI:10.1246/bcsj.80.743

Ó 2007 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 80, No. 4, 743–746 (2007)
743
Cycloadditions of 3,4-Dihydro-2H-pyrrole N-Oxide
with Thioketones and a Selenoketone
ꢀ1
Kosei Shioji, Akio Matsumoto,1 Masahiko Takao,1 Yoshimitsu Kurauchi,1
Toshiyuki Shigetomi,1 Yoshinobu Yokomori,2 and Kentaro Okuma1
1Department of Chemistry, Faculty of Science, Fukuoka University, Jonan-ku, Fukuoka 814-0180
2Department of Chemistry, National Defense Academy, Hashirimizu, Yokosuka 239-8686
Received July 18, 2006; E-mail: shioji@fukuoka-u.ac.jp
Cycloadditions of 2-diphenylphosphinoyl-2-methyl-3,4-dihydro-2H-pyrrole N-oxide (DPhPMPO), 3,4-dihydro-2H-
pyrrole N-oxide having a diphenylphosphinoyl group at the C2 position with thioketones afforded the corresponding
1,4,2-oxathiazolidines. Dissociation constants of these 5-membered ring products were determined. The cycloadducts
were stabilized by the diphenylphosphinoyl group. The reaction of DPhPMPO with di-tert-butyl selenoketone gave the
corresponding selenoamide under microwave irradiation. The formation of the selenoamide indicated that the cycloaddi-
tion of DPhPMPO with the selenoketone analogue also proceeded through the formation of the corresponding 5-mem-
bered ring product.
The pathways of 1,3-dipolar cycloadditions of nitrones to
O
carbon–carbon multiple bonds, such as alkenes, or alkynes to
form isoxazolidines or 2,3-dihydroisoxazoles are involved in
many synthetic strategies.1 Studies on the chemical properties
of N-oxide as a 1,3-dipole are of substantial interest in this re-
gard. It is also well known that compounds having hetero-
atom–carbon multiple bonds react with electron-rich 1,3-di-
poles.2 Black and Watson have described 1,3-cycloaddition
reactions of nitrones including 2,2-dimethyl-3,4-dihydro-2H-
pyrrole N-oxide (DMPO) (1a) with thioketones, such as ada-
mantane-2-thione (2a) and 2,2,4,4-tetramethyl-3-thioxocyclo-
butanone (2b), to afford 1,4,2-oxathiazolidine derivatives.3
Huisgen, Sustmann, et al. have reported kinetic studies on
these cycloadditions and evaluated the high reactivity of the
C=S double bond toward nitrones by using MO calculations.4
Mloston, Heimgartner, and co-worker have reported the cyclo-
addition of azole N-oxide with several thiocarbonyl com-
pounds.5 However, to our knowledge, there are no other reports
on the 1,3-dipolar cycloaddition reactions of thioketones with
nitrones due to the instability and inaccessibility of nitrones.
Furthermore, to our knowledge, cycloaddition of 3,4-dihydro-
2H-pyrrole N-oxide with a C=Se bond has not been reported.
The difference in reactivity between thioketones and selenoke-
tones toward nitrones is also of interest. We have recently
investigated the synthesis of 3,4-dihydro-2H-pyrrole N-oxide
containing a phosphinoyl group as an electron-withdrawing
group at the 2-position.6 In this paper, we report that the cyclo-
addition reactions of 3,4-dihydro-2H-pyrrole N-oxide, 2-di-
phenylphosphinoyl-2-methyl-3,4-dihydro-2H-pyrrole N-oxide
(DPhPMPO) (1b), with several dipolarophiles, such as thioke-
tone and selenoketone, behave in a variety of ways.
O
P
Cl
O
6
Ph
Ph
H2O
NH3
Ph2PCl
Ph2P H
N
CH3CN
EtOH
4
3
H
5
Oxone
O
NaHCO3 aq.
P
Ph
Ph
acetone
N
O
1b
Scheme 1.
available chlorodiphenylphosphine (4) in 97% yield. 2-(Di-
phenylphosphinoyl)-2-methyl-1-pyrrolidine (5) was obtained
by the cyclization of 5-chloro-2-pentanone (6) and ammonia
with 3 (63% yield). Oxidation of 5 was carried out by using
OxoneÔ in acetone to give the desired nitrone 1b in 47% yield
without purification by column chromatography. Purifications
of the overall synthesis were carried out by recrystallization
from water containing acetonitrile. The obtained crystals could
be stored at room temperature.
The structure of 1b was confirmed by H, 13C, 31P NMR,
1
elemental analysis, and X-ray crystallographic analysis. The
X-ray data are reported in Table 1. A single crystal had one
stereoisomer, and the crystal lattice had two water molecules.
No interaction was observed between the two oxygen atoms on
N-oxide and the phosphinoyl moiety. The P–C(1) bond is mod-
erately longer than the other P–C bonds. It appered that the
position of the diphenylphosphinoyl group caused steric hin-
drance at the nitrone face in the crystalline state (Fig. 1).
Thermal Reaction of DPhPMPO (1b) with Thioketones.
We reacted DPhPMPO with adamantane-2-thione (2a). The
reaction proceeded quickly in CHCl3 at room temperature to
Results and Discussion
Synthesis of DPhPMPO (1b). Nitrone 1b was synthesized
by the following three-step reactions described in Scheme 1.
Diphenylphosphine oxide (3) was prepared from commercially
Published on the web April 13, 2007; doi:10.1246/bcsj.80.743

744 Bull. Chem. Soc. Jpn. Vol. 80, No. 4 (2007)
Cycloadditions of 3,4-Dihydro-2H-pyrrole N-Oxide
give the corresponding oxathiazolidine 7a in 90% yield. Al-
though the relative configuration was not elucidated, the ad-
duct was obtained as a single diastereomer. The reaction of
1b with thioxocyclobutanone 2b gave cycloadduct 7b as color-
less crystals in 60% yield (Scheme 2). These structures were
at room temperature. It has been reported that the steric en-
cumbrance of these thioketones 2a–2d increases in the se-
quence of 2a < 2b < 2c < 2d.7 The poor reactivity of 2c
and 2d was caused by steric hindrance of thiocarbonyl com-
pounds.
1
identified by H, 13C NMR and elemental analysis. These re-
The reported cycloadducts of nitrones with thioketones dis-
sociate to a large extent in dilute solution. Huisgen et al. have
reported that the dissociation constant of the oxathiazolidine
9 generated from linear N-(diphenylmethylene)methylamine
N-oxide (1c) with 2b in CDCl3 was 1:2 ꢁ 10ꢂ3 mol Lꢂ1 when
the initial concentrations of the nitrone and 2b were 0.94 and
0.37 mol Lꢂ1, respectively.4 The dissociation constants of ad-
ducts 7a and 7b at 25 ꢃC in CDCl3 were determined by
1H NMR. The dissociation constants of cycloadducts 8 derived
from 1a and 2 were also determined. These results are summa-
rized in Table 2. Cycloadducts 7a and 8a had smaller dissoci-
ation constants than those of cycloadducts 7b and 8b formed
from 2b. This may be because the steric congestion around
O,S-acetal moiety destabilizes the latter cycloadducts. Al-
though the initial concentrations of 1b and 2b for the reaction
of 1b and 2b were lower than those of the linear nitrone 1c and
2b for the corresponding reaction, the dissociation constant of
7b was three times smaller than that of the adduct of 1c with
2b. Compared with these values, our results indicated that the
oxathiazolidines 7 generated from 1b and 2a or 2b are more
stable than 9 generated from the reaction of the linear nitrone
sults were similar to those obtained for the reaction of 1a.
On the other hand, no reaction occurred between 1b and bor-
nane-6-thione (2c) or 1,1,3,3-tetramethylindane-2-thione (2d)
ꢃ
˚
Table 1. Selected Bond Lengths (A) and Angles ( ) of 1b
P(1)–O(1) 1.481(4) O(2)–N(1) 1.298(6) C(2)–C(3) 1.507(9)
P(1)–C(6) 1.800(6) N(1)–C(4) 1.298(6) C(3)–C(4) 1.480(10)
P(1)–C(12) 1.802(6) N(1)–C(1) 1.513(7)
P(1)–C(1) 1.850(6) C(1)–C(2) 1.521(8)
N(1)–C(1)–P(1)
C(2)–C(1)–P(1)
C(5)–C(1)–P(1)
112.7(4)
109.7(4)
109.4(4)
C(4)–N(1)–C(1)
C(3)–C(2)–C(1)
111.0(5)
106.2(6)
Table 2. Dissociation Constants of 1,3-Cycloadducts 7 and 8
1a)
2b)
Adduct
103 Kdiss/mol Lꢂ1 c)
a
a
b
b
c
a
b
a
b
b
8a
8b
7a
7b
9
6:96 ꢄ 0:39
19:5 ꢄ 0:39
0:114 ꢄ 0:025
0:4107 ꢄ 0:041
1.2d)
a) Initial concentration of ½1ꢅ ¼ 0:049 mol Lꢂ1. b) Initial con-
centration of ½2ꢅ ¼ 0:049 mol Lꢂ1. c) Determined by 1H NMR
at 25 ꢃC. d) Ref. 4.
Fig. 1. ORTEP of 1b.
R1
Ph
N
N
O
O
Ph
O
a
R1 =CH3
b
R1 = Ph2P
1c
1
S
O
S
S
S
2c
2d
2a
2b
R1
N
R2 =
Ph
Ph
O
N
O
S
S
R2
O
1 + 2a,b
CHCl3
O
O
7a,b: R1 =Ph2P
8a,b: R1 =CH3
a
b
9
Scheme 2.

K. Shioji et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 4 (2007)
745
O
P
O
P
O
Ph
Ph
P
Ph
But
But
Ph
Ph
M. W.
N
O
Ph
N
+
Se
Se
N
O
Se
But
CHCl3
O
But
But
or
But
1b
10
13
14
O
P
But
But
Ph
Ph
+
O
Se
N
H
11
12
Scheme 3.
plates (Merck, silica gel 60, F254) and flash column chromatogra-
phy was performed with silica (Merck, 70–230 mesh). NMR spec-
tra (1H at 400 MHz; 13C at 100 MHz; 31P at 161 MHz) were re-
corded in CDCl3, and the chemical shifts are expressed in ppm
relative to the internal standard TMS. 31P NMR spectra were ob-
tained in CDCl3 using 85% H3PO4 as an internal standard with
1c with 2b. The dissociation constant of 8b was higher than
that of 7b, because adamantane-2-thione (2a) is less sterically
crowded than thioxocyclobutanone 2b. These observations
suggested that the diphenylphosphinoyl moiety contributes sta-
bility to the adduct 7a and 7b. The bulky diphenylphosphinoyl
moiety did not influence cycloaddition of 1b with 2a and 2b.
The cycloadditions of 1b with 2c or 2d did not proceed at
room temperature. When a microwave reaction between 1b
with 2c was carried out in chloroform, the pale orange solution
originating from the thioketone 2c gradually became colorless.
Using TLC analysis, a product (Rf ¼ 0:65, EtOAc:MeOH =
9:1) was detected along with 1b (Rf ¼ 0:1) and 2c (Rf ¼
0:95). From the reaction of 1b with 2d, a trace amount of a
product (Rf ¼ 0:45) was also detected. However, these prod-
1
broadband H decoupling. The melting points are uncorrected.
Synthesis of 2-Diphenylphosphinoyl-2-methyl-3,4-dihydro-
2H-pyrrole N-Oxide (DPhPMPO) (1b). Water (4.5 mL) was
added to a solution of chlorodiphenylphosphine (4) (4.6 mL, 25
mmol) in acetonitrile (38 mL) and the mixture was refluxed for
6 h. The reaction mixture was then concentrated under reduced
pressure. The residue was extracted with CH2Cl2 (20 mL ꢁ 3)
and dried over MgSO4. The solvent was removed in vacuo to
give diphenylphosphine oxide (3) as a colorless oil (97% yield).
1
ucts were not observed by H NMR of the reaction mixture.
1H NMR (CDCl3) ꢀ 7.98–8.02 (m, 10H), 8.11 (d, 2H, JP{H
¼
468 Hz). 31P NMR (CDCl3) ꢀ 23.1.
Reaction of DPhPMPO with Di-tert-butyl Selenoketone.
Since selenoketones are also useful 1,3-dipolarophiles, several
selenoketones were employed instead of 2.8 The reaction of 1b
with di-tert-butyl selenoketone (10) did not proceed at room
temperature. However, under microwave irradiation, the cy-
cloaddition of 1b with selenoketone 10 afforded the corre-
sponding selenolactam 11 in 92% yield along with the di-
tert-butyl ketone (12) (Scheme 3), suggesting that the initial
adduct of the reaction was oxaselenazolidine 13 or betaine
14. Since the stability of 13 or 14 is lower than that of 7, rear-
rangement took place under these conditions.
5-Chloro-2-pentanone (6) (2.75 mL, 24 mmol) was added to a
solution of diphenylphosphine oxide (4.04 g, 20 mmol) in ethanol
(16 mL), and the mixture was stirred at 0 ꢃC for 1 h. After stirring
at room temperature, the reaction mixture was warmed up to
60 ꢃC, and then gaseous NH3 was bubbled into the solution for
1 h. After stirring for 3 h, the solvent was removed under reduced
pressure. The residue was dissolved in 2 mol Lꢂ1 HCl (50 mL) and
extracted with CH2Cl2 (100 mL ꢁ 3). The organic layer was dried
over MgSO4, and the solvent was removed. The residue was crys-
tallized from ethanol–hexane to give 2-(diphenylphosphinoyl)-2-
methyl-1-pyrrolidine (5) as colorless crystals (63%). 1H NMR
(CDCl3) ꢀ 1.35 (d, 3H, J ¼ 14 Hz), 1.48–1.50 (m, 1H), 1.62–1.65
(m, 1H), 1.76–1.80 (m, 1H), 2.11–2.19 (m, 1H), 2.57–2.59 (m,
1H), 3.07–3.09 (m, 1H), 7.41–7.51 (m, 6H), 8.14–8.20 (m, 4H).
13C NMR (CDCl3) ꢀ 24.8 (JP{C ¼ 9:1 Hz), 25.9 (JP{C ¼ 4:6 Hz),
35.1 (JP{C ¼ 5:1 Hz), 47.8 (JP{C ¼ 6:9 Hz), 62.6 (JP{C ¼ 94:7
Hz), 128.2, 128.3, 128.4, 128.5, 130.8, 131.1, 131.5, 131.6, 131.7,
131.8, 132.1, 132.5, 132.6, 132.7, 132.8, 132.9, 133.1, 133.2.
31P NMR (CDCl3) ꢀ 33.1.
Aqueous NaHCO3 (50 mL, 4.0 g, 48 mmol) and oxone (50 mL,
7.4 g, 12 mmol) were added dropwise to a solution of pyrrolidine 5
in acetone (2.85 g, 10 mmol). After stirring for 1 h at room temper-
ature, the reaction mixture was quenched with 50 mL of 10% so-
dium thiosulfate and acetone was removed under reduced pres-
sure. The residue was extracted with CH2Cl2 (150 mL ꢁ 3) and
dried over MgSO4. The organic layer was concentrated to give
a crude product as a pale yellow oil. Purification was carried
out via recrystallization from water–acetonitrile to give 3,4-dihy-
dro-2H-pyrrole N-oxide 1b as colorless crystals (47%). mp 137–
Conclusion
3,4-Dihydro-2H-pyrrole N-oxide 1b reacted like a 1,3-di-
pole with thiocarbonyl compounds. The dissociation constants
indicated that the formation of 5-membered ring products was
not decelerated by a bulky group attached to the carbon atom
adjacent to the 1,3-dipole. Selenium atom transfer through the
oxaselenazolidine intermediate took place with microwave
irradiation.
Experimental
General Methods. Solvents were distilled under a nitrogen
atmosphere. All chemicals were obtained from a commercial sup-
plier and used without further purification. Adamantane-2-thione
(2a),7 2,2,4,4-tetramethyl-3-thioxocyclobutanone (2b),9 bornane-
6-thione (2c),10 1,1,3,3-tertamethylindane-2-thione (2d),11 and
di-tert-butyl selenoketone (10)8 were prepared according to liter-
ature procedures. Analytical TLC was carried out on precoated

746 Bull. Chem. Soc. Jpn. Vol. 80, No. 4 (2007)
Cycloadditions of 3,4-Dihydro-2H-pyrrole N-Oxide
1
138 ꢃC (decomp.). H NMR (CDCl3), ꢀ 1.72 (d, 3H, J ¼ 6:8 Hz),
thiazolidine 7 in 24 h. The integral was compared with TCE (s,
5.96). 7b: 0.224 mmol by the signal at ꢀ 4.71 (brd, C-2). 1b:
0.022 mmol by ꢀ 6.60 (d, N=CH–), Kdiss ¼ 0:41 ꢄ 0:041 ꢁ 10ꢂ3
mol Lꢂ1 at 25 ꢃC.
General Procedure of Microwave Irradiation Reaction.
The appropriate thioketone 2 or selenoketone 10 was added to a
solution of 1b in CHCl3, and the mixture was stirred for 1 min.
It was then irradiated with a microwave by Electrolux EMC-
M20Y (TOSHIBA). The reaction was monitored by TLC. After
the irradiation, the reaction mixture was dissolved in CDCl3
(0.5 mL) for NMR analysis.
1.86–1.97 (m, 1H), 2.12–2.23 (m, 1H), 2.37–2.46 (m, 1H), 3.18–
3.28 (m, 1H), 6.66 (dd, 1H, J ¼ 5:2, 2.4 Hz), 7.45–7.56 (m, 6H),
8.08–8.12 (m, 2H), 8.39–8.44 (m, 2H). 13C NMR (CDCl3) ꢀ 21.3
(JP{C ¼ 4:6 Hz), 25.8, 30.2, 78.7 (JP{C ¼ 71:0 Hz), 128.4, 128.6,
128.7, 128.8, 128.9, 129.3, 129.9, 132.1, 132.2, 132.4, 132.6,
133.2, 133.3, 136.3. 31P NMR (CDCl3) ꢀ 34.3. Found: C, 64.49;
H, 6.51; N, 4.40%. Calcd for C17H18NO2P H2O: C, 64.35; H,
6.35; N, 4.41%.
ꢆ
X-ray Crystallographic Data of DPhPMPO (1b). X-ray crys-
tallographic data were collected using a DIP Image Plate with
Cu Kꢁ radiation. The reflections were collected at room tempera-
ture (298 K). An orthorhombic lattice was determined from system-
atic absence of lattice (hkl; h þ k ¼ 2n). A P212121 space group
was determined from systematic absence (hol; l ¼ 2n). A good
model was found by direct methods (SIR97) using the P212121
space group.12 Crystal data for 1b: FW 299.31. Orthorhom-
Reaction of 1b with Di-tert-butyl Selenoketone 10. Di-tert-
butyl selenoketone (10) (41.0 mg, 0.2 mmol) was added to a solu-
tion of 1b (38.9 mg, 0.13 mmol) in CHCl3 (2 mL) and irradiated
with a microwave for 5 min. The reaction mixture was concentrat-
ed under reduced pressure to give a white solid, which was recrys-
tallized from benzene to afford colorless crystals of selenoamide
1
bic, space group P212121, a ¼ 11:4480ð5Þ A, b ¼ 10:3370ð4Þ A,
11 (44.0 mg, 0.12 mmol). 11: mp 209–211 ꢃC. H NMR (CDCl3)
˚
˚
3
˚
˚
c ¼ 14:3840ð6Þ A, V ¼ 1702:17ð12Þ A , Z ¼ 4, Dcalcd ¼ 1:168
g cmꢂ3, ꢂ ¼ 1:457 mmꢂ1, R ¼ 0:0778, Rw ¼ 0:2131. Crystallo-
graphic data have been deposited with the Cambridge Crystallo-
graphic Data Centre: Deposit No. CCDC-629371 for Compound
1b. Copies of the data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12, Union Road, Cambridge,
CB2 1EZ, UK: Fax: +44 1223 336033; or e-mail: deposit@ccdc.
cam.ac.uk).
ꢀ 1.60 (d, 3H, J ¼ 15:6 Hz), 1.99–2.13 (m, 2H), 2.59–2.64 (m,
1H), 2.65–2.86 (m, 1H), 7.50–7.67 (m, 6H), 7.83–7.87 (m, 2H),
7.97–8.02 (m, 2H), 8.84 (brs, 1H). 13C NMR (CDCl3) ꢀ 23.0
(JP{C ¼ 7:4 Hz), 33.4 (JP{C ¼ 3:4 Hz), 48.0, 71.4 (JP{C ¼ 75:8
Hz), 127.5, 128.5, 128.6, 129.2, 129.3, 129.4, 129.5, 132.1, 132.3,
132.4, 133.0, 133.1, 207.7 (JP{C ¼ 3:5 Hz). 31P NMR (CDCl3) ꢀ
31.2. Found: C, 56.37; H, 5.04; N, 3.87%. Calcd for C17H18NO-
PSe: C, 56.36; H, 5.01; N, 3.87%.
Reaction of 1b with Adamantane-2-thione (2a). Adaman-
tane-2-thione (2a) (16.6 mg, 0.1 mmol) was added to a solution
of 1b (30 mg, 0.1 mmol) in CHCl3 (1.0 mL), and the mixture
was stirred at room temperature for 15 min. After stirring, the re-
action mixture was concentrated under reduced pressure to give a
white solid, which was recrystallized from hexane–ether to afford
colorless crystals of oxathiazolidine 7a (41.9 mg, 0.09 mmol). 7a:
mp 128–129 ꢃC. 1H NMR (CDCl3) ꢀ 1.63 (d, 3H, JP{H ¼ 15:6 Hz),
1.68–2.19 (m, 18H), 3.96 (dd, 1H, J ¼ 6:0, 8.4 Hz), 7.42–7.56 (m,
6H), 7.98–8.02 (t, 2H, J ¼ 8:4 Hz), 8.26–8.30 (t, 2H, J ¼ 8:4 Hz).
13C NMR (CDCl3) ꢀ 19.2, 25.3, 25.9, 26.4, 30.2, 32.9, 34.0,
34.7, 34.8, 36.2, 37.3, 38.3, 38.7, 69.64 (JP{C ¼ 94:0 Hz), 72.3
(JP{C ¼ 6:3 Hz), 100.9, 127.2, 127.3, 130.7, 130.8, 131.6, 131.7.
31P NMR (CDCl3) ꢀ 31.7. Found: C, 69.43; H, 6.96; N, 2.93%.
Calcd for C27H32NO2PS: C, 69.65; H, 6.93; N, 3.01%.
References
1
For a review see: P. Caramella, P. Grunanger, 1,3-Dipolar
¨
Cycloaddition Chemistry, ed. by A. Padwa, New York, 1984,
Vol. 1, pp. 291–392.
2
a) For a review see: R. Huisgen, 1,3-Dipolar Cycloaddition
Chemistry, ed. by A. Padwa, New York, 1984, Vol. 1, pp. 1–176.
b) L. Fisera, R. Huisgen, I. Kalwinsch, E. Langhals, X. Li, G.
Mloston, K. Polborn, J. Rapp, W. Sicking, R. Sustmann, Pure
Appl. Chem. 1996, 68, 789. c) R. Huisgen, X. Li, Tetrahedron
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3
2491.
4
D. S. C. Black, K. G. Watson, Aust. J. Chem. 1973, 26,
Reaction of 1b with 2,2,4,4-Tetramethyl-3-thioxocyclobuta-
none (2b). Thione 2b (15.6 mg, 0.1 mmol) was added to a solu-
tion of 1b (30 mg, 0.1 mmol) in CHCl3 (1.0 mL), and the mixture
was stirred at room temperature for 15 min. After stirring, the re-
action mixture was concentrated under reduced pressure to give a
white solid, which was recrystallized from hexane–ether to afford
colorless crystals of oxathiazolidine 7b (27.4 mg, 0.06 mmol). 7b:
mp 102–104 ꢃC. 1H NMR (CDCl3) ꢀ 1.1 (s, 3H), 1.29 (s, 3H), 1.31
(s, 3H), 1.38–1.46 (m, 1H), 1.41 (s, 3H), 1.61 (d, 3H, JP{H ¼ 15
Hz), 1.94–1.99 (m, 1H), 2.23–2.36 (m, 1H), 2.54–2.64 (m, 1H),
4.71 (brd, 1H, J ¼ 4:8 Hz), 7.36–7.55 (m, 6H), 8.08 (t, 2H, J ¼
8:4 Hz), 8.21 (t, 2H, J ¼ 8:4 Hz). 13C NMR (CDCl3) ꢀ 18.6, 20.6,
21.0 (JP{C ¼ 4:4 Hz), 22.6, 24.5, 27.0, 33.9 (JP{C ¼ 4:9), 63.4,
68.1, 71.7, 72.7, 75.4 (JP{C ¼ 11 Hz), 103.6, 128.3, 128.4, 128.5,
132.0, 132.1, 132.8, 132.9, 133.0, 219.4. 31P NMR (CDCl3) ꢀ
30.0. Found: C, 65.77; H, 6.65; N, 2.90%. Calcd for C25H30-
NO3PS: C, 65.91; H, 6.64; N, 3.07%.
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Dissociation Constants of Oxathiazolidine 7. DPhPMPO 1b
(0.246 mmol), thioketone (0.246 mmol), and tetrachloroethane
(0.246 mmol) in CDCl3 (5.0 mL) reached equilibrium with oxa-