Ring expansion vs. cleavage of the exocyclic double bond of 2- cycloalkylidenedihydrobenzothiazoles and -benzimidazoles by methanesulfonyl azide

Article abstract of DOI:10.1002/1099-0690(200004)2000:7<1229::AID-EJOC1229>3.0.CO;2-N

Deprotonation of the 2-cycloalkylbenzothiazolium perchlorates 1f-h with sodium hydride in the presence of methanesulfonyl azide (3) affords the spirocyclic dihydro-1,4-benzothiazines 9f-h together with small amounts of the 2-iminobenzothiazole 5. The 2-cycloalkylidenedihydrobenzimidazoles 11e-h are generated by deprotonation with potassium hydride from the corresponding 2-cycloalkylbenzimidazolium salts 10e-h, and trapped with 3 to yield the zwitterions 12e-h. Whereas 12h is thermally unstable, 12e-g are isolated and thermolysed at 20-80 °C. The cyclopropyl zwitterion 12e decomposes in an ill-defined way, only at temperatures above 80 °C. In contrast, 12f,g, and, in particular, 12h, decompose more readily to furnish the products of ring expansion (13f-h) and those of a [3 + 2] cycloreversion (6 and 14) of intermediate spirocyclic triazolines. Products that might be indicative of the intervention of hypothetical zwitterions of type 8 cannot be detected.

Full text of DOI:10.1002/1099-0690(200004)2000:7<1229::AID-EJOC1229>3.0.CO;2-N

FULL PAPER
Ring Expansion vs. Cleavage of the Exocyclic Double Bond of
2
-Cycloalkylidenedihydrobenzothiazoles and -benzimidazoles by
Methanesulfonyl Azide
[a]
[a]
Helmut Quast* and Svetlana Ivanova
Keywords: Azides / Cleavage reactions / Cycloadditions / Nitrogen heterocycles / Ring expansion / Sulfur heterocycles /
Synthetic methods
Deprotonation of the 2-cycloalkylbenzothiazolium perchlor- isolated and thermolysed at 20–80 °C. The cyclopropyl zwit-
ates 1f–h with sodium hydride in the presence of meth-
anesulfonyl azide (3) affords the spirocyclic dihydro-1,4-
benzothiazines 9f–h together with small amounts of the 2-
terion 12e decomposes in an ill-defined way, only at temper-
atures above 80 °C. In contrast, 12f,g, and, in particular, 12h,
decompose more readily to furnish the products of ring ex-
iminobenzothiazole 5. The 2-cycloalkylidenedihydrobenzim- pansion (13f–h) and those of a [3 + 2] cycloreversion (6 and
idazoles 11e–h are generated by deprotonation with potas- 14) of intermediate spirocyclic triazolines. Products that
sium hydride from the corresponding 2-cycloalkylbenzimida- might be indicative of the intervention of hypothetical zwit-
zolium salts 10e–h, and trapped with 3 to yield the zwitter-
ions 12e–h. Whereas 12h is thermally unstable, 12e–g are
terions of type 8 cannot be detected.
Introduction
A number of heterocyclic ketene N,X-acetals, e.g. of type
2
a–d and 11b–d, readily add electrophilic azides such as
methanesulfonyl azide (3, Schemes 1 and 2). While 2-alkyl-
idenedihydrobenzimidazoles 11b–d yield zwitterions 12
Scheme 2), which can be isolated in most cases,[ the ad-
2]
(
ducts of the benzothiazole derivatives 2a–d (and other het-
erocycles) with 3 immediately loose molecular nitrogen and
undergo ring expansion, or are cleaved into N-sulfonylimine
5
and a diazo compound 6. The zwitterions 12b–d behave
likewise, but in general only at higher temperatures. Previ-
ous studies have uncovered structural features that favour
the useful ring expansion reaction over the undesired diazo-
[
3,4]
alkane cleavage:
Soft heteroatoms like sulfur and selen-
ium, whose tendency to form bridged cations is well-
[
5]
known, and one alkyl group or, better, two methyl groups
at the exocyclic carbon atom of the heterocyclic N,X-acetals
as in 2b–d and 11b–d favour ring expansion. We have now
used two representative 2-alkylidene heterocycles, whose
exocyclic carbon atom is a member of a cycloalkane ring,
to study the effect of the size of that ring. The cyclopro-
pylidene compounds 2e and 11e were particularly intriguing
because of the notorious reluctance of the cyclopropane
ring to undergo nucleophilic substitution reactions,[ and
the possibility of detecting products that would be indicat-
ive of ring opening of hypothetical cyclopropyl cations,[
e.g. 8e.
6]
7]
Scheme 1
Cycloalkylidenedihydrobenzothiazoles 2e–h were gener-
ated by deprotonation of the benzothiazolium perchlorates flash chromatography. The structures of new compounds
1
e–h with sodium hydride, and trapped in situ by meth- are based on elemental analyses (Table 4) and spectroscopic
anesulfonyl azide. Products were separated and purified by evidence (Table 2 and Table 3). Except for the cyclopro-
pylidene compound 2e, satisfactory material balances were
[
a]
Institut für Organische Chemie der Universität Würzburg,
Am Hubland, D-97074 Würzburg, Germany
Fax: (internat.) ϩ 49-(0)541-8141935
obtained in all cases. Again with exception of 2e, the spiro-
cyclic dihydrobenzothiazines 9f–h were the major products.
Only about 10% of the N-sulfonyliminodihydrobenzothia-
E-mail: ivanova_svetlana@hotmail.com
Eur. J. Org. Chem. 2000, 1229Ϫ1233

WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1434Ϫ193X/00/0407Ϫ1229 $ 17.50ϩ.50/0
1229

H. Quast, S. Ivanova
FULL PAPER
Table 1. Products, yields obtained in preparative experiments, melt-
ing points after recrystallisation from ethanol, and ratios of prod-
ucts (route A:route B) as calculated from integration of the methyl
signals in proton spectra recorded for solutions of the crude prod-
ucts; ranges of decomposition for the crude zwitterions are given
in brackets
Scheme 2
zole 5 was formed by cleavage of the carbon–carbon double
bond in 2f–h. In contrast, no trace of a ring expansion
product was obtained from the cyclopropylidene compound
2
e. Instead, only a moderate yield of 5 was obtained. The
results are summarised in Table 1.
Solutions of the cycloalkylidenedihydrobenzimidazoles
1
1e–h in tetrahydrofuran were prepared by deprotonation
of the benzimidazolium salts 10e–h with potassium hydride,
which were subsequently treated with methanesulfonyl az-
ide. Like the isopropylidene compound 11d,[ the cyclohex-
ylidene derivative 11h formed a highly unstable zwitterion
3]
[
a]
Solvents: AN ϭ acetonitrile or [D
3
]acetonitrile, T ϭ trichlorome-
[b] [3]
thane or [D]trichloromethane, THF ϭ tetrahydrofuran. – Ref.:
m.p. 164–165 °C. –
1
2h, which could be characterised only at low temperatures
[c]
Equal amounts of the cation 10e and the
1
(
Table 2 and Table 3) because of rapid decomposition at imine 14 were observed besides unidentified products ( H NMR). –
[d]
[3]
[e]
Ref.: m.p. 206–208 °C. – Small-scale experiment, carried out
temperatures above 0 °C. The other zwitterions 12e–g were
prepared at room temperature and isolated as almost col-
ourless solids in good yields (Table 1). These compounds
could be kept at 0 °C or even at room temperature (12e, f)
without decomposition.
in an NMR sample tube.
The thermal stability of the isolated zwitterions 12e–g in-
The present results extend the range of structural para-
creased with decreasing size of the cycloalkyl ring. While meters that allow ring expansion of heterocyclic N,X-acetals
thermal decomposition of the cyclopropyl compound 12e with electrophilic azides. With the exception of the first
required heating in boiling acetonitrile for several hours, the members of the series, viz. the 2-cyclopropylidene hetero-
cyclobutyl zwitterion 12f decomposed at 60 °C, and a solu- cycles 2e and 11e, the larger cycloalkylidene compounds 2f–
tion of the cyclopentyl zwitterion 12g began to evolve nitro- h and 11f–h afford reasonable to good yields of spirocyclic
gen already at temperatures as low as 0 °C.
products that are otherwise difficult to come by. Due to the
Except for the cyclopropyl zwitterion 12e, which yielded, complexity of the various conceivable mechanistic schemes,
besides numerous unidentified compounds, small amounts all of which involve several intermediates and elementary
1
of the cleavage products 10e and 14 (1:1, H NMR), ther- steps, a rationalisation of the influence of substituents dis-
mal decomposition of 12f–h resulted in a reasonable mat- closed in the present and the foregoing studies[
2–44]
must
erial balance. Ring expansion to the spirocyclic tetrahy- remain speculative. We note that a 1,2 shift to a cyclopropyl
droquinoxalines 13f–h occurred to the same or greater ex- carbon atom bearing a very good leaving group is disfa-
[6]
tent as cleavage to the N-methylsulfonylimine 14, rendering voured, as are S 2 reactions at the cyclopropane ring.
N
this sequence as a useful ring expansion method (Table 1). These features may be invoked to rationalise the chemical
The occurrence of benzimidazolium ions, which would in- behaviour of the cyclopropyl compounds 2e and 12e. The
dicate reversibility of the zwitterion formation,[ was not relatively stable zwitterions 12b, c, and 12e prefer the two
observed when the thermolysis of 12f–h was monitored by modes of decomposition over ring expansion, viz. reversion
proton spectroscopy. This is a further difference between of their formation, thus affording eventually benzimidazol-
2]
these zwitterions and the cyclopropyl zwitterion 12e.
ium ions, and cyclisation to the spirocyclic intermediates
1230
Eur. J. Org. Chem. 2000, 1229Ϫ1233

Ring Expansion vs. Cleavage in 2-Cycloalkylidenedihydrobenzothiazoles and -benzimidazoles
FULL PAPER
1
Table 3. Chemical shifts (δ values) in 13C NMR spectra
Table 2. Chemical shifts (δ values) in H NMR spectra
[
a]
6 3
Solvents: A ϭ [D ]acetone, AN ϭ [D ]acetonitrile, T ϭ [D]tri-
b] [c]
[
chloromethane. –
tem. – The spectrum was recorded at a temperature of –50 °C.
AAЈBBЈ spin system. – AAЈXXЈ spin sys-
[
d]
followed by their cleavage into N-methylsulfonylimine 14
and diazo compounds 6.
Experimental Section
General: Yields, melting points, and ratios of products: Table 1. –
1
13
H NMR: Table 2. – C NMR: Table 3. – Molecular formulae and
masses, and elemental analyses: Table 4. – Melting points: Kofler
1
13
apparatus from Reichert, Vienna. – H and C NMR: Bruker AC
00 and AC 250. – Flash chromatography: (40 ϫ 3)cm glass col-
umn packed with silica gel 32–63 µm (ICN Biomedicals), UV de-
2
2
tector Knauer 87.00 (λ ϭ 254 nm), 1.5 bar N . – MS (70 eV): Finni-
gan MAT 8200.
Tetrahydrofuran was dried with powdered KOH and distilled from
[
a]
Solvents: see Table 2. – [b] The spectrum was recorded at a tem-
perature of –50 °C.
NaH. Petroleum ether (PE), boiling range 50–70 °C, and ethyl acet-
ate (EA) were distilled from P
2
O
5
. Acetonitrile was distilled from
. – NaH and KH, sus-
CaH . [D ]Acetonitrile was dried with CaH
2
3
2
gave colourless needles (12.0 g, 52%), m.p. 232–234 °C (m.p. 234–
pended in paraffin oil, were washed three times with pentane and
dried in a stream of Ar. – Experiments involving ketene N,X-acetals
were carried out in dry solvents under Ar (99.998%). – The follow-
[9]
2
36 °C ).
2-Cyclohexyl-1H-benzimidazole: Prepared from cyclohexane carb-
ing compounds were available from previous work: 1e–h (X ϭ oxylic acid, as described in the preceding experiment for 2-cyclo-
[
8]
[3]
[8]
ClO
4
), 3, 10f–h (X ϭ ClO
4
).
propyl-1H-benzimidazole, and recrystallised from ethanol/water
2:1). Colourless needles (23.5 g, 81%), m.p. 281–282 °C (m.p. 281–
(
2
(
-Cyclopropyl-1H-benzimidazole: A mixture of o-phenylenediamine
[10]
2
82 °C ).
15.7 g, 0.145 mol), cyclopropane carboxylic acid (25.0 g, 0.29 mol)
and 3 drops of conc. sulfuric acid was heated for 8 h. The temper-
ature of the mixture was gradually raised from 120 °C to 220 °C, X ؍ BF
and water was allowed to distil. After cooling, the solid product to a stirred suspension of NaHCO
was washed with a sat. aq. solution of KHCO and water until the propyl-1H-benzimidazole (6.1 g, 39 mmol) in water (40 mL).
2-Cyclopropyl-1,3-dimethylbenzimidazolium Tetrafluoroborate (10e,
4
): Dimethyl sulfate (12.3 g, 96 mmol) was added dropwise
(9.7 g, 116 mmol) and 2-cyclo-
3
[
11]
3
washings were neutral. Recrystallisation from ethanol/water (1:1)
The mixture was stirred for 14 h, and the solid removed by filtra-
1231
Eur. J. Org. Chem. 2000, 1229Ϫ1233

H. Quast, S. Ivanova
FULL PAPER
ϩ
Table 4. Molecular formulae, masses, and elemental analyses
245 (100) [M – SO
17), 136 (32).
2
Me], 230 (48), 204 (16), 150 (10), 138 (18), 137
(
Synthesis of the Zwitterions 12. – General Procedures: a) Preparative
Experiments: A suspension of a powdered 2-cycloalkyl-1,3-di-
methylbenzimidazolium salt (10e–g, 3 mmol) and KH (0.24 g,
6
mmol) in tetrahydrofuran (30 mL) was stirred in an 80-mL centri-
fuge tube, equipped with a septum, until the gas evolution had
ceased (1–2 days). The solid material was removed with the help of
a centrifuge and washed with tetrahydrofuran (5 mL). The com-
bined solutions were transferred via syringe into a 100-mL flask,
and a solution of 3 (0.40 g, 3.3 mmol) in tetrahydrofuran (2 mL)
was added dropwise with stirring. After stirring at room temper-
ature for 1 h, the precipitate was collected by filtration. Yields and
temperature ranges of decomposition are listed in Table 1.
b) Small-Scale Experiments at Low Temperatures: A suspension of
a powdered 2-cycloalkyl-1,3-dimethylbenzimidazolium salt (10g, h,
0
8
.2 mmol) and KH (40 mg, 1 mmol) in [D ]tetrahydrofuran
(
0.7 mL) was stirred for 1 day in a 10-mL centrifuge tube, equipped
with a septum. The solid material was removed with the help of a
centrifuge. The solution was transferred into an NMR sample tube
and frozen by cooling in liquid nitrogen. Careful attention was paid
to avoid condensation of Ar. Azide 3 (24 mg, 0.2 mmol) was placed
on top of the frozen solution. The mixture was placed into a bath
of –40 °C, briefly shaken and kept therein for 3 h. The pale brown
precipitate was collected with the help of a centrifuge at –30 °C
and dissolved in cold [D]trichloromethane (0.7 mL, –50 °C).
tion. A sat. aq. solution of NaBF
4
(20 mL), acidified with 3 drops
of aq. HBF (50%), was added dropwise to the filtrate to afford
4
colourless crystals, 9.8 g (93%). Recrystallisation from ethanol/
water (10:1) furnished colourless needles (6.0 g, 57%), m.p. 188–
190 °C.
1
-[1-(1,3-Dimethylbenzimidazol-2-ylio)cyclopropyl]-3-methyl-
In situ Trapping of the Ketene N,S-Acetals 2e–h with Methanesul-
fonyl Azide (3). – General Procedure: A suspension of powdered 1
sulfonyltriazenide (12e): a) Pale brown powder (0.58 g) from 10e.
(
3
X ϭ ClO
4
, 3.3 mmol), NaH (0.12 g, 5 mmol), and 3 (0.42 g,
1
-[1-(1,3-Dimethylbenzimidazol-2-ylio)cyclobutyl]-3-methyl-
.5 mmol) in tetrahydrofuran (30 mL) was stirred in an 80-mL
sulfonyltriazenide (12f): a) Pale yellow powder (0.93 g) from 10f.
centrifuge tube, equipped with a septum, until the gas evolution
had ceased (1 d). The solid material was removed with the help of
a centrifuge and washed with tetrahydrofuran (5 mL). The solvent
was distilled in vacuo to afford a solid residue which was purified
and/or separated by flash chromatography (PE/EA, 1:1). The crude
products obtained from the different fractions by distillation of the
solvent in vacuo were recrystallised from ethanol to yield colour-
less crystals.
1
-[1-(1,3-Dimethylbenzimidazol-2-ylio)cyclopentyl]-3-methyl-
sulfonyltriazenide (12g): a) Pale yellow powder (0.72 g) from 10g.
b) Proton and 13C NMR spectra were recorded at –50 °C. Gas
evolved at room temperature. Subsequently, the proton spectrum
indicated the presence of 13g and 14 (7:3). The solvent was evapor-
ated in a stream of N
acetonitrile. The proton spectrum showed no signals of 10g.
2 3
, and the solid residue was dissolved in [D ]-
2
,3-Dihydro-3-methyl-2-[N-(methylsulfonyl)imino]benzothiazole (5):
1
-[1-(1,3-Dimethylbenzimidazol-2-ylio)cyclohexyl]-3-methyl-
From 1e. Flash chromatography gave colourless crystals (0.46 g,
1
3
sulfonyltriazenide (12h): b) Proton and C NMR spectra were re-
corded at –50 °C. Gas evolved at room temperature. Subsequently,
the proton spectrum indicated the presence of 13h and 14 (7:3).
The solvent was evaporated in a stream of N
was dissolved in [D
57%), m.p. 158–165 °C.
3
,4-Dihydro-4-methyl-3-[N-(methylsulfonyl)imino]-2H-1,4-benzo-
thiazine-2-spiro-1Ј-cyclobutane (9f) and 5: From 1f. Flash chromato-
graphy gave a colourless solid (9f, 0.74 g, 76%, m. range 90–130
2
, and the solid residue
3
]acetonitrile. The proton spectrum showed no
°
C). The second fraction yielded colourless crystals (5, 80 mg, 10%,
signals of 10h.
ϩ
m. range 100–140 °C). – 9f: MS; m/z (%): 296 (84) [M ], 268 (51),
2
Thermolysis of the Zwitterions 12. – General Procedures: a) Prepar-
ative Experiments: A suspension of 12e–g (2.5 mmol) in acetonitrile
or trichloromethane was heated under reflux until the evolution of
gas had subsided. The solution was immediately placed on top of
a silica gel column followed by flash chromatography with ethyl
acetate. Distillation of the solvent in vacuo from the various frac-
tions and recrystallisation of the residues from ethanol yielded col-
ourless crystals. Yields and melting points are listed in Table 1.
ϩ
53 (72), 237 (27), 217 (86) [M – SO
2
Me], 216 (27), 202 (18), 200
(36), 189 (100), 162 (21), 155 (22), 149 (34), 137 (39), 136 (85).
3
,4-Dihydro-4-methyl-3-[N-(methylsulfonyl)imino]-2H-1,4-benzo-
thiazine-2-spiro-1Ј-cyclopentane (9g) and 5: From 1g. Flash chroma-
tography gave colourless prisms (9g, 0.80 g, 82%, m.p. 130–139 °C).
The second fraction yielded colourless crystals (5, 70 mg, 9%, m.
ϩ
range 130–160 °C). – 9g: MS; m/z (%): 310 (26) [M ], 231 (21)
ϩ
2
[M – SO Me], 217 (16), 216 (100), 138 (19), 137 (20), 136 (29).
b) Small-Scale Experiments. Solutions of 12e–g (0.1 mmol) in
3
3
,4-Dihydro-4-methyl-3-[N-(methylsulfonyl)imino]-2H-1,4- [D ]acetonitrile or [D]trichloromethane (0.7 mL) contained in
benzothiazine-2-spiro-1Ј-cyclohexane (9h) and 5: From 1h. Flash
chromatography gave a colourless solid (9h, 0.78 g, 80%, m.p. 130–
NMR sample tubes were heated while the conversion was moni-
tored by proton spectroscopy until 12 had disappeared. The ratios
of 13:14 (route A:route B) were calculated from integration of the
methyl signals. Temperatures and results are listed in Table 1.
1
33 °C). The second fraction yielded colourless crystals (5, 80 mg,
ϩ
10%, m.p. 164–166 °C). – 9h: MS; m/z (%): 324 (42) [M ], 246(18),
1232
Eur. J. Org. Chem. 2000, 1229Ϫ1233

Ring Expansion vs. Cleavage in 2-Cycloalkylidenedihydrobenzothiazoles and -benzimidazoles
FULL PAPER
Thermolysis of 12e: a) Acetonitrile (5 mL), 5 h. Flash chromato-
graphy gave colourless crystals (14, 42 mg, 7%, m.p. 200–204 °C).
chromatography of the resulting solution with EA gave, as first
fraction, a yellow oil (13h, 0.61 g), and colourless crystals as second
fraction (14, 0.19 g, m.p. 203–205 °C). Recrystallisation from eth-
anol afforded colourless crystals (13h, 0.36 g). – 13h: MS; m/z (%):
3
b) [D ]Acetonitrile, 5 h, 80 °C. The proton spectrum indicated the
presence of 14 and the cation 10e (1:1) as well as several unidenti-
fied compounds.
ϩ
ϩ
3
21 (11) [M ], 234 (18), 242 (100) [M – SO Me], 184 (6), 160 (12),
2
1
59 (12).
Thermolysis of 12f: a) Trichloromethane (25 mL), 2 h. Flash chro-
matography gave yellow crystals (13f, 0.36 g, 49%, m.p. 124–126
Acknowledgments
°
C), and colourless crystals as second fraction (14, 0.26 g, 44%,
ϩ
We thank Mrs. E. Ruckdeschel and Dr. M. Heubes for recording
NMR spectra, and Dr. G. Lange and Mr. F. Dadrich for measuring
the mass spectra. Financial support by the Deutsche Forschungsge-
meinschaft and the Fonds der Chemischen Industrie, Frankfurt am
Main, is gratefully acknowledged. S. I. thanks the Deutscher
Akademischer Austauschdienst (DAAD) for a stipend.
m.p. 205–206 °C). – 13f: MS; m/z (%): 293 (2) [M ], 265 (20), 250
ϩ
(10), 215 (129), 214 (81) [M – SO
2
Me], 187 (15), 186 (100), 146
(16), 154 (13).
b) [D
presence of 13f and 14 (7:3) as well as a small amount of 10f. –
D]Trichloromethane, 60 °C, 1.5 h. The proton spectrum indicated
the presence of 13f and 14 (1:1). The solvent was evaporated in a
stream of N . The proton spectrum of the solid residue, dissolved
in [D ]acetonitrile, showed the presence of 13f and 14 (1:1), but no
signals of 10f. The same result was obtained when the solution of
2f in [D]trichloromethane was kept for 2 weeks at room temper-
3
]Acetonitrile, 60 °C, 2 h. The proton spectrum indicated the
[
[1]
The results are taken from the dissertation by S. Ivanova, Uni-
2
versity of Würzburg, 1998.
[
[
[
[
2]
3]
4]
3
H. Quast, M. Ach, S. Ivanova, E.-M. Peters, K. Peters, H. G.
von Schnering, Liebigs Ann. 1996, 1551–1558.
H. Quast, M. Ach, E.-M. Peters, K. Peters, H. G. von Schner-
ing, Liebigs Ann. 1992, 1259–1269.
1
ature.
H. Quast, S. Ivanova, E.-M. Peters, K. Peters, H. G. von
Schnering, Liebigs Ann. 1996, 1541–1549.
Thermolysis of 12g: a) Trichloromethane (20 mL), 2 h. Flash chro-
matography gave colourless crystals (13g, 0.42 g, 56%, m.p. 150–
5] [5a]
E. Block, Reactions of Organosulfur Compounds, 1st ed.,
[5b]
Academic Press, New York 1978, chapter 4. –
N. S. Isaacs,
1
57 °C), and colourless crystals as second fraction (14, 0.15 g, 25%,
Physical Organic Chemistry, 2nd ed., Longman, Harlow, Essex,
England, 1995, chapter 13.
6]
ϩ
m.p. 200–204 °C). – 13g: MS; m/z (%): 307 (12) [M ], 229 (16), 228
[
[
ϩ
K. Banert, Chem. Ber. 1985, 118, 1564–1574.
(100) [M – SO
2
Me], 187 (15), 160 (9), 159 (19).
7] [7a]
E. C. Friedrich, in The Chemistry of the Cyclopropyl Group.
(
1
Ed.: Z. Rappoport), 1st ed., Wiley, New York, 1987, chapter
1
,2,3,4-Tetrahydro-1,4-dimethyl-2-[N-(methylsulfonyl)imino]-
[7b]
1. –
G. Boche, H. M. Walborsky, Cyclopropane Derived
quinoxalin-3-spiro-1Ј-cyclohexane (13h): A suspension of powdered
0h (X ϭ BF , 0.95 g, 3 mmol), KH (0.24 g, 6 mmol) in tetrahy-
Reactive Intermediates (Eds.: S. Patai, Z. Rappoport), 1st ed.,
Wiley, New York, 1990, chapter 3.
1
4
[
[
8]
9]
drofuran (30 mL) was stirred in an 80 mL centrifuge tube, equipped
with a septum, until the gas evolution had ceased (1 day). The solid
material was removed with the help of a centrifuge and washed
with tetrahydrofuran (5 mL). The combined solutions were trans-
ferred via syringe into a 100 mL flask, and a solution of 3 (0.40 g,
H. Quast, E. Schmitt, Chem. Ber. 1969, 102, 568–574.
R. Pellicciari, R. Fringuelli, B. Natalini, L. Brucato, A. R.
Contessa, Arch. Pharm. 1985, 318, 393–399.
[10]
D. Jerchel, H. Fischer, M. Kracht, Liebigs Ann. 1952, 575,
1
62–173.
[11]
H. Quast, E. Schmitt, Chem. Ber. 1968, 101, 4012–4014.
Received July 6, 1999
[O99399]
3.3 mmol) in tetrahydrofuran (2 mL) was added dropwise with stir-
ring. The mixture was stirred at room temperature for 1 h. Flash
Eur. J. Org. Chem. 2000, 1229Ϫ1233
1233