Easy access to trans-2,3-disubstituted cyclobutanones, 2,4,5-trisubstituted 3,6-dihydro-2H-pyrans and cis-substituted phenylcyclopropylsulfones by using the highly versatile 1-phenylsulfenyl- or 1-phenylsulfonyl-cyclopropylketones

Article abstract of DOI:10.1039/b906695k

The high versatility of 1-phenylsulfenyl- or 1-phenylsulfonyl- cyclopropylketones has been exploited for the regioselective synthesis of trans-2,3-disubstituted cyclobutanones, 2,4,5-trisubstituted 3,6-dihydro-2H-pyrans and cis-2-alkyl- or cis-2-aryl-cycl

Full text of DOI:10.1039/b906695k

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Easy access to trans-2,3-disubstituted cyclobutanones, 2,4,5-trisubstituted
3,6-dihydro-2H-pyrans and cis-substituted phenylcyclopropylsulfones by using
the highly versatile 1-phenylsulfenyl- or 1-phenylsulfonyl-cyclopropylketones†
Guido Alberti, Angela M. Bernard, Costantino Floris, Angelo Frongia, Pier P. Piras,* Francesco Secci and
Marco Spiga
Received 3rd April 2009, Accepted 11th June 2009
First published as an Advance Article on the web 6th July 2009
DOI: 10.1039/b906695k
The high versatility of 1-phenylsulfenyl- or 1-phenylsulfonyl-cyclopropylketones has been exploited for
the regioselective synthesis of trans-2,3-disubstituted cyclobutanones, 2,4,5-trisubstituted
3,6-dihydro-2H-pyrans and cis-2-alkyl- or cis-2-aryl-cyclopropylphenylsulfones.
Easily obtainable cyclopropyl ketones 1-substituted with an
arylthio group, with the sulfur atoms in different oxidation states,
are versatile reagents that can be easily prepared stereoselectively
by reacting the Corey ylide with the stereochemically defined Z
alkenes 1.1,2 We recently reported that, by a careful choice of the
substituents and by modulation of the oxidation state of the sulfur
atom, the alkenes 1 or the cyclopropanes 2 are useful precursors
of 2,4,5-trisubstituted- or 3,4,5-trisubstituted-2,3-dihydrofurans 4
and 5 (Scheme 1).1
withdrawing effect of the pivaloxymethyl group. It must be
pointed out that, while the synthesis of 1-(phenylthio)cyclopropyl
carbinols and their ring expansion to the corresponding 2-
substituted cyclobutanones is quite successful,6 the reaction
of 2-methyl-1-(phenylthio)cyclopropyllithium with aldehydes or
ketones produced the corresponding alcohols, hypothetical pre-
cursors of 2,3-disubstituted cyclobutanones, in a low yield of
30%.7 In view of these facts, it was of interest to prepare 2-
substituted-1-(phenylthio)cyclopropyl carbinols 6 in high yields
and to search experimental conditions for their ring expansion to
2,3-disubstituted cyclobutanones in order to overcome the above-
mentioned drawbacks (Scheme 2).
Scheme 1
In order to further study the synthetic potential of cyclo-
propylketones 2, we planned the synthesis of 2,3-disubstituted
cyclobutanones 8 or 9, through the intermediacy of the corre-
sponding cyclopropyl carbinols 6 or the oxiranes 7 (Scheme 2).
As a matter of fact only two papers have appeared dealing
with the chiral synthesis of 2,3-disubstituted cyclobutanones by
ring expansion of oxaspiropentanes3 and one by ring expansion
of a-hydroxycyclopropyl carbinols.4 One previous attempt of
preparing these racemic cyclobutanones using 2-pivaloxymethyl-
1-(phenylthio)cyclopropyl carbinols led to the corresponding
2,4-disubstituted cyclobutanones5 by migration of the less sub-
stituted cyclopropyl carbon as a consequence of the electron-
Scheme 2
2,3-Disubstituted cyclobutanones 8 (Method a)
For this part of the study the required cyclopropylcarbinols 6 were
easily prepared from 1-phenylthiocyclopropyl ketones 2, either by
hydride reduction or by reaction with organolithium derivatives.
The reduction of trisubstituted cyclopropyl ketones 2a–g, carried
out with LiAlH4 in THF at -70 ◦C, gave the alcohols 6a,c,e–g
with modest diastereoselectivity while a good diastereoisomeric
excess was obtained when R or R1 was an aromatic group (6b,
6d). Excellent diastereoselectivity was observed for the alcohols
6h,j obtained by reaction of the ketone 2b with methyl- or
n-butyllithium in THF at -78 ◦C (Table 1).
Dipartimento di Scienze Chimiche, Universita` di Cagliari, Complesso
Universitario di Monserrato, S.S 554 Bivio per Sestu, I 09042, Monserrato,
(Cagliari), Italy. E-mail: pppiras@unica.it; Fax: +39 0706754388; Tel: +39
0706754410
† Electronic supplementary information (ESI) available: Analytical data;
NMR spectra. See DOI: 10.1039/b906695k
As the second part of method “a” involves an acidic treat-
ment of 6 to trigger the ring expansion to the corresponding
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Table 1 Synthesis of the alcohols 6a–j
Yield (%) Syn:anti 6a
Scheme 4 Possible mechanism of substituted ketone formation.
Entry
6
R
R1
R2
1
2
3
4
5
6
7
8
9
a
b
c
d
e
f
(CH3)2CH
(CH3)2CH
C6H5CH2CH2
C6H5
C6H5CH2CH2
CH3
CH3
C6H5
CH3
CH3
C4H9
C4H9
CH3
C6H5 CH3
CH3 CH3
C6H5 C4H9 92
H
H
H
H
H
H
H
96
92
85
80
86
70
94
86
66
85:15
94:6
the acid on the carbonyl group and attack of the nucleophile on
the 3-carbon (path b).
70:30
90:10
75:25
60:40
65:35
94:6
Changing the reaction conditions again, we were pleased to
find that the reaction of 6a–c,e,h–j with p-toluenesulfonic acid in
refluxing benzene, saturated with water,6b led to the stereo- and
regio-chemically homogeneous corresponding cyclobutanones 8,
while 6d gave only unidentified decomposition products and 6f,g
led to the ring opened derivatives 11f,g (Table 2). The formation of
only the trans-2,3-disubstituted cyclobutanones 8a–c,e,h–j can be
attributed to the higher migratory aptitude of the most substituted
bond in a cyclopropylcarbinyl-cyclobutyl rearrangement.9 The
assignment of the trans stereochemistry was made on the basis of
the vicinal coupling constants observed in the 1H NMR spectra10,11
and by NOE experiments.
Carrying out the reaction of a 70:30 mixture of syn:anti 6c in
milder conditions in aqueous benzene with PTSA (1 equiv.) at
70 ◦C and monitoring the reaction by GS MS, we discovered that
a kinetic resolution of the syn-anti mixture of 6c and a cis-trans
equilibration of 8c were at work (Table 3).
g
h
i
furyl
(CH3)2CH
C6H5CH2CH2
(CH3)2CH
10
j
>99
a The diastereoselectivity was measured by 1H NMR (300 MHz) and GC
MS on the crude reaction mixture.
cyclobutanones 8, we treated the cyclopropylcarbinols 6a,c with
anhydrous stannic chloride in methylene chloride at room tem-
perature. We did not obtain the expected cyclobutanones 8 but
only the b-chloro-substituted ketones 10a, c, that are extremely
useful intermediates in organic synthesis and act as intermediates
in the synthesis of enones, annulated compounds, heterocyclic
derivatives and dicarbonyl products (Scheme 3).8
As a matter of fact we observed that the cyclopropylcarbinyl-
cyclobutyl rearrangement was considerably slower and that the
major component of the cyclopropylcarbinol mixture reacted
faster than the minor one. We observed also that the cyclobu-
tanones initially formed as a roughly 61:39 cis:trans mixture
Table 2 Synthesis of 2,3-disubstituted cyclobutanones 8a–c,e,h–j
Scheme 3
We then submitted the cyclopropyl carbinols 6b,c,f,g to the Trost
protocol for this ring expansion, that is the use of p-toluenesulfonic
acid (PTSA) in refluxing dry benzene.6b Also in this case, instead
of the expected cyclobutanones 8 we observed the formation of
the acyclic ketones 11c,f,g and only decomposition products in the
case of 6b.
Concerning the reaction mechanism, the results obtained could
suggest two possible pathways as reported in Scheme 4. One
could involve the formation of a carbonium ion (A) and its
rearrangement to the open chain homoallylic carbocation B,
then attack from the nucleophilic species (Cl- or PhS-) and
final hydrolysis of the so obtained enolthioether to give the final
b-substituted ketones 10a,c or 11c,f,g (path a). The other possibil-
ity could involve the intermediate formation of the cyclobutanone
8 and its ring fission caused by the simultaneous coordination of
Entry
6
R
R1
R2
Time/h
8 yield (%)a
1
2
3
4
5
6
7
8
9
a
b
c
d
e
f
(CH3)2CH
(CH3)2CH
C6H5CH2CH2
C6H5
C6H5CH2CH2
CH3
CH3
C6H5
CH3
CH3
C4H9
C4H9
CH3
C6H5
CH3
C6H5
H
H
H
H
H
H
H
CH3
CH3
C4H9
1
2
2
2
2
4
1
5
48
48
65
85
98b
0
84
0c
g
h
i
furyl
0d
(CH3)2CH
C6H5CH2CH2
(CH3)2CH
70
75
75
10
j
a Isolated yield. b 6c was obtained as a 95/5 mixture of trans/cis isomers.
c Only 2-(phenylthio)-nonan-4-one 11f was isolated. d 1-(Furan-2-yl)-1-
(phenylthio)pentan-3-one 11g was isolated.
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Table 3 Variation of the product distribution with reaction time
Table 4 Transformation of the 1-phenylthiocyclopropylketones 2 into
3,6-dihydro-2H-pyrans 12
Reaction time/h
6c (%) (syn:anti)
8c (%) (trans:cis)
0
16
23
40
49
64
80
100 (70:30)
77 (42:58)
68 (34:66)
23 (0:100)
0
0
0
0
23 (39:61)
32 (46:54)
77 (67:33)
100 (80:20)
100 (87:13)
100 (95:5)
Entry
2
R
R1
7 yield (%)
12 yield (%)
1
2
3
4
5
6
7
8
9
a
b
c
d
g
h
i
(CH3)2CH
(CH3)2CH
C6H5CH2CH2
C6H5
CH3
C6H5
CH3
CH3
CH3
CH3
C6H5
CH3
CH3
Tracesa
Tracesa
changed their relative composition to give, after 70 h, the
cyclobutanone trans-8c as the only product with only traces of the
other cis-8c isomer. The 2,4-disubstituted cyclobutanones, that
could have been produced by migration of the less substituted
cyclopropyl carbon, were not observed.
65b
86c
72
2-furyl
p-CH3C6H5
p-CH3C6H5
2-thienyl
76d
82d
68
Concerning the reaction mechanism of this rearrangement, two
possible pathways could be at work as reported in Scheme 5. One
could involve the formation of an intermediate episulfonium ion
A (path a) that, by migration of the more substituted cyclopropyl
carbon, can produce either the cis- or the trans-cyclobutanones, ac-
cording to the geometry of the starting cyclopropyl carbinol, with
final thermodynamic equilibration. Alternatively the formation of
the trans-cyclobutanone could be justified by the intermediacy of
a carbonium ion B, and its subsequent rearrangement to give an
intermediate 1-phenylthiocyclobutene intermediate, that can be
hydrolyzed to 8 (path b). Further studies are in progress to better
understand this mechanistic aspect of the reaction and the results
will be published in due course.
j
k
2-naphthyl
80
a Starting material was recovered together with inseparable traces of the
pyrans 12a,b (1%). b The oxirane 7c was obtained as a 75:25◦mixture
of isomers. c When we carried out the same reaction at 60 C the
oxirane 7d was isolated as a mixture of two inseparable diastereoisomers.
d In a previous work1 we erroneously assigned the structure of 4,5,6-
trisubstituted 3,4-dihydro-2H-pyrans to 12h,i.
cyclopropylketone 2c with the Corey ylide in DMSO at 96–97 ◦C,
clean formation of the oxirane 7c was observed, but with all the
other derivatives 2d,g–k we observed, unexpectedly, formation
of the 3,6-dihydro-2H-pyrans 12d,g–k instead of the expected
cyclopropyl oxiranes 7d,g–k. The important role of the substituent
R is clearly indicated by the fact that the reaction of derivatives
2a,b gave only traces of the corresponding pyrans, with almost
quantitative recovery of the starting material. Only in the case of
cyclopropylketones 2-substituted with an aromatic or heterocyclic
substituent, ring expansion occurred cleanly leading to good yields
of the corresponding pyrans (Table 4).
This unexpected result was furthermore relevant as pyrans are
important synthetic targets due to their frequent occurrence in
biologically active natural products and medicinally important
molecules.12
Our approach for their synthesis constitutes a valid alternative
to the classical methods that include the Prins and related cycliza-
tion reactions,13 the hetero-Diels–Alder cyclization and transition-
metal-catalysed cyclization of hydroxylated derivatives,14 cycliza-
tion to epoxides15 and intramolecular oxy-Michael reactions,16
use of carbohydrate precursors and transition-metal-catalyzed
cyclization of allenes.17
The structures of derivatives 12d,g–k were determined by1H,
13C NMR and GC mass spectroscopies. COSY, GHSQC,
GHMQC and ROESY experiments for compound 12d permit-
ted the unambiguous assignment of all protons and carbon
signals.
Scheme 5 Possible mechanisms of ring expansion of 6 to give 2,3-disub-
stituted cyclobutanones 8.
Failed attempts at synthesizing the 2,3-disubstituted
cyclobutanones 9 (Method b). Formation of 2,4,5-trisubstituted
3,6-dihydro-2H-pyrans
At this point we addressed ourselves to the second goal, that is
the synthesis of cyclobutanones 9 through the acid catalyzed ring
expansion of the oxiranes 7.
An obvious approach for the synthesis of 7 was the reaction of
the cyclopropylketones 2 with the Corey ylide. When we treated
Concerning the reaction mechanism two possible pathways
could be envisioned (Scheme 6). Both paths entail the intermediate
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Table 5 Transformation of the 1-phenylsulfonyl cyclopropylketones 3
into cis-cyclopropylsulfones 13
Entry
2
R
R1
3 yield (%)
13a yield (%)
1
2
3
4
5
a
b
c
d
i
(CH3)2CH
(CH3)2CH
C6H5CH2CH2
C6H5
CH3
C6H5
CH3
CH3
C6H5
80
96
98
89
94
72
82 (13a)
96
93
95
Scheme 6 Mechanism of the formation of the 3,6-dihydro-2H-pyrans.
formation of the corresponding 1-phenylsulfenyl cyclopropyloxi-
rane 7 by addition of the Corey ylide to the carbonyl group.
The intermediate oxirane 7 can undergo (path a) a spontaneous,
thermally induced, ring opening of both the oxirane and the cyclo-
propane rings via a concerted mechanism to give the zwitterionic
intermediate A, which then undergoes ring closure to generate
the pyrans 12. Alternatively the sodium iodide, generated in the
formation of the Corey ylide, can catalyze (path b) the reaction
of 7 by the sequence shown in Scheme 6. The relatively weakly
basic epoxide oxygen could form a complex with the sodium
cation that can undergo fission of the epoxide ring assisted by
the contemporaneous nucleophilic attack of the iodide ion at the
2-position of the cyclopropane ring. The so generated sodium
alcoholate (intermediate B) can finally lead to the pyrans 12 by
intramolecular displacement of the iodide ion.
p-CH3C6H5
a When products 13a,c,d,i were treated with the Corey ylide for 9 h,
cis-trans interconversion was not observed. The assignment of the cis
stereochemistry was determined by comparison with the 1H NMR of the
previously reported trans derivatives.21a,b,d,e
As shown in Scheme 7, it could involve an initial addition of the
ylide to the carbonyl group to give a tetrahedral intermediate
A that could produce, through a cyclic concerted mechanism
(path a), the cis cyclopropylsulfone and a new carbonyl-stabilized
sulfoxonium ylide. Alternatively, the intermediate A could disso-
ciate into a sulfoxonium ion and a sulfonyl stabilized carbanionic
species (path b) that finally could be protonated to give the cis
cyclopropylsulfones 13a,c,d,i.
The most likely mechanism should be the one involved in path
a, as treatment of the isolated oxirane 7d in DMSO in the absence
of sodium iodide at 96 ◦C led cleanly to the expected pyran
12d.
The reaction can also be carried out without isolation of the
intermediate cyclopropanes as a one pot, three step reaction. As
a matter of fact, treatment of the alkene (Z)-3-(phenylthio)-4-p-
tolylbut-3-en-2-one1 (1 equiv.) with the Corey ylide (2.2 equiv) in
DMSO at 96–97 ◦C for 3 h afforded the corresponding pyran 12h
in a reasonably good 70% yield worked out from the GC mass
spectrum of the reaction mixture.
Scheme 7 Proposed mechanism for the cis-2-substituted cyclopropylsul-
fone formation.
cis-Cyclopropylsulfones 13 formation
At this point of the study it was interesting to investigate
also the behaviour of the sulfone derivatives 3a–d,i (Table 5)
obtained by oxidation of the corresponding sulfides 2a–d,i in
CH2Cl2 with MCPBA at room temperature (the oxidation of
the cyclopropane 2h gave 2-tolyl-4-phenylsufonyl-5-methyl-2,3-
dihydrofuran).1 When these derivatives were reacted with the
Corey ylide, we obtained no detectable amounts of the corre-
sponding expected pyrans, but only good yields of the cyclo-
propylsulfones 13a,c,d,i as single cis-isomers, whose structures
were confirmed by comparison with the analogous known trans
derivatives (Table 5).
The concerted mechanism (path a) should the most likely
because it has been previously found that a sulfonyl stabilized
cyclopropyl carbanion should be incapable of retaining its pyra-
midal configuration giving rise to cis-trans interconversion.19a,20–22
(Scheme 7). This hypothesis is further supported by the fact
that, when the sulfones 13a,b were reacted with NaH in DMSO,
under the same reaction conditions, the cis derivative 13a was
formed within 1 h, but after 3 h the exclusive trans-isomer 13a¢
was obtained (Table 5)through a thermodynamically controlled
process involving the isomerization of the cis cyclopropane
intermediate 13a by a deprotonation-protonation sequence. These
experimental results further support that the conjugated base of
We believe that in the cleavage reaction of the compounds 3a–d,i
a mechanism similar to the Haller Bauer reaction18,19 could occur.
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cis-2-substituted cyclopropylsulfones undergoes an inversion of
configuration due to the preference for the trans geometry.21,22
The most important aspect of this reaction was the exclu-
sive formation of the thermodynamically less stable cis-isomer
with no detectable amounts of the corresponding trans-isomer.
As a consequence of the fact that the trans-aryl- and alkyl-
cyclopropylsulfones have been prepared by different synthetic
approaches21 and only few examples are reported21b,22 for the syn-
thesis of the corresponding cis derivatives, our reaction constitutes
a very useful practical access to the latter.
Anyway because of the stereochemical instability of the sulfonyl
stabilized cyclopropyl carbanion, even a deprotonation of a cis-2-
substituted cyclopropylsulfone and subsequent reaction with an
electrophile should give only the corresponding trans derivative
with loss of stereochemical integrity.21e,22b
On the other hand the stereoselective preparation of the alkenes
1 by Knoevenagel reaction and their stereoselective cyclopropana-
tion offers a means to circumvent the above mentioned difficulty.
As a matter of fact, by reducing the derivatives 3a,b with LiAlH4 we
obtained the cis cyclopropyl carbinols 14a,b as a syn:anti mixture
that can be considered as formally derived by reaction of the
anion of the cyclopropyl sulfone 3a (hypothetically capable of
retaining its stereochemistry) with acetaldehyde or benzaldehyde
(Scheme 8).
otherwise stated. Chemical shifts are reported as follows: chemical
shifts, integration, multiplicity and coupling constants.
Infrared spectra were recorded on a Nicolet Nexus 670 FT-IR
spectrophotometer. Microanalyses were carried out on a Carlo
Erba 1106 element analyzer.
Mass spectra analyses were recorded on an Agilent 5973 N
(Cpsil 32 m) and a Nermag R10-10 (quartz-Cpsil 5.25 m)
(E.I. 70eV, C.I. NH3). Analytical thin layer chromatography
was performed using 0.25 mm silica gel 60-F plates. Flash
chromatography was performed using 0.040–0.063 mesh silica gel
(Merk KGaA). Yields refer to chromatography and spectroscopi-
cally pure materials, unless otherwise stated.
THF and diethyl ether were freshly distilled from
˚
sodium/benzophenone. DMSO was dried over 2 A molecular
sieves. Reactions requiring anhydrous conditions were performed
in oven dried glassware under argon atmosphere.
Compounds 1a–i and 2a–k were prepared according to the
previously reported procedure.1
Spectral data of the compounds 1a,d,h,i1,2 and 1g23, 2a,d,h,i1 has
been previously reported.
Analytical data for 1b, 1c, 1e, 1f, 1j, 1k, 2b, 2c, 2e, 2f, 2g, 2j, 2k,
6a–j, 10a,c, 11c,f,g, 3b,c and 14a,b can be found in the Electronic
Supplementary Information (ESI†).
Rearrangement of cyclopropylcarbinols 6a–j to cyclobutanones
8c–j. General procedure
A stirred solution of cyclopropylcarbinol 6a–j (1.2 mmol) and p-
toluenesulfonic acid (20 mg, 1,2 mmol,) in wet benzene (10 mL)
was refluxed for 2 h. The reaction mixture was then washed
with 10% NaHCO3 and brine, dried (Na2SO4) and evaporated
to remove the solvent. The residue was chromatographed on silica
gel with diethyl ether-light petroleum 1:1 as eluent. Spectral data
of the compounds 8b have been previously reported.11
Scheme 8 Synthesis of syn:anti cis-cyclopropyl carbinols.
Use of cyclopropylsulfones like 3 carrying different keto groups
will give ready access to this class of compounds with the above
reported stereochemistry.
(2R,3R)-2-Methyl-3-isopropylcyclobutanone 8a
Characterization of this volatile cyclobutanone was not pursued
and it was identified by GC MS m/z: 126 (M+ (4)), 108 (5), 84
(42), 69 (100), 53 (40). It was isolated as the non-volatile 2,4-
dinitrophenylhydrazone derivative and spectroscopic data were
identical to the previously reported compound.11
Conclusions
In summary we have reported that 1-phenylsulfenyl cyclopropy-
lketones bearing C-2 alkyl substituents, easily reduced to the
corresponding cyclopropylcarbinols, can be the starting materials
for the synthesis of trans-2,3-disubstituted cyclobutanones. On
the other hand by a careful choice of the substituents on the cy-
clopropane ring and a careful control of the reaction temperature,
the synthesis of 2,4,5- trisubstituted 3,6-dihydro-2H-pyrans can be
easily performed by the reaction of the same cyclopropylketones
with the Corey ylide. Furthermore, upon changing the oxidation
state of the sulfur atom of the cyclopropyl ketones, the same
reaction with the Corey ylide gives a useful and remarkably easy
access to the cis-2-alkyl- or 2-aryl-1-phenylsulfonyl cyclopropanes.
(2S*,2R*)-2-Methyl-3-phenylethylcyclobutanone 8c
The assignment of trans stereochemistry for the substituents in the
1
major isomer was determined by comparison with the H NMR
of the previously reported analogous trans and cis compounds.10,11
Mixture 95:5 of two trans:cis isomers. Yield 98%. IR (neat):
1
1781 cm-1. Major isomer: H NMR (CDCl3) d: 1.16 (d, 3H, J =
7.2 Hz), 1.88–2.06 (m, 3H), 2.59–2.70 (m, 3H), 2.86–2.90 (m, 1H),
3.00 (ddd, 1H, J = 17.1 Hz, J = 8.1 Hz, J = 2.4 Hz), 7.17–7.32 (m,
5H). 13C NMR d:7.49, 33.00, 36.34, 36.53, 43.35, 47.73, 125.83,
128.28, 128.86, 132.25, 209.89. MS m/z: 188 (M+ (15)), 146 (51),
131 (15), 115 (14), 91 (100), 77 (14). Anal. Calcd. for C13H16O:
C, 82.94; H, 8.57. Found: C, 82.84; H, 8.52%. Minor cis isomer.
Spectral data worked out by reaction mixture: 1H NMR (CDCl3)
d: 1.7 (d, 3H, J = 7.5 Hz), 1.52–1.64 (m, 3H), 2.38–2.44 (m, 3H),
2.86–2.90 (m, 1H), 3.40 (ddd, 1H, J = 16.8 Hz, J = 9.0 Hz,
Experimental
1H NMR spectra were recorded using 300 and 400 MHz VARIAN
spectrometers at ambient temperature with CDCl3 as solvent and
TMS as internal standard. 13C NMR spectra were recorded at 75
or 100 MHz at ambient temperature with CDCl3 as solvent unless
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J = 3.3 Hz), 7.17–7.32 (m, 5H). MS m/z: 188 (M+ (3)), 146 (14),
under reduced pressure. The residue purified by chromatography
on silica gel (light petroleum-diethyl ether 10:1) gave 7c and 12d,
g–k. When we carried out the same reaction at 60 ◦C with the
cyclopropane 2d we isolated the oxirane 7d as mixture of two
inseparable diastereoisomers.
131 (10), 115 (12), 91 (100), 77 (14).
(2S*,2R*)-2-Butyl-3-phenylethylcyclobutanone 8e
1
Yield 84%. IR (neat): 1760 cm-1. H NMR (CDCl3) d: 0.88 (t,
3H, J = 7.2 Hz), 1.16–1.71 (m, 7H), 1.76–2.10 (m, 2H), 2.52–
2.88 (m, 4H), 2.96–3.05 (m, 1H), 7.14–7.32 (m, 5H).13C NMR d:
13.85, 22.59, 28.87, 29.39, 30.99, 34.67, 38.38, 49.77, 65.62, 125.98,
128.28, 128.44, 132.36, 210.96. MS m/z: 230 (M+ (14)), 188 (62),
146 (15), 117 (24), 105 (94), 91 (100). Anal. Calcd. for C16H22O:
C,83.43; H, 9.63. Found: C, 83.41; H, 9.72%.
2-Methyl-2-(2-phenethyl-1-(phenylthio)cyclopropyl)oxirane 7c
Inseparable 75:25 mixture of two diastereoisomers. Yellow oil.
Yield 65%. Spectral data worked out by reaction mixture. Major
isomer: 1H NMR (CDCl3) d: 0.59 (dd, 1H, J = 6.6 Hz, J = 4.5 Hz),
1.06 (dd, 1H, J = 4.5 Hz, J = 9.0 Hz), 1.19–1.27 (m, 1H), 1.56 (s,
3H), 1.99 (q, 2H, J = 7.5 Hz), 2.26 (d, 1H, J = 4.8 Hz), 2.42 (d,
1H, J = 4.8 Hz), 2.70–2.81 (m, 2H), 7.17–7.47 (m, 10H). MS m/z:
201 (M+-109 (18)), 183 (4), 143 (16), 128 (8), 110 (26), 91 (100).
Minor isomer: H NMR (CDCl3) d: 0.55 (dd, 1H, J = 6.0 Hz, J =
5.1 Hz), 1.06 (dd, 1H, J = 5.1 Hz, J = 9.3 Hz), 1.25–1.31 (m, 1H),
1.54 (s, 3H), 1.95–2.03 (m, 2H), 2.45 (d, 1H, J = 5.7 Hz), 2.50 (d,
1H, J = 4.8 Hz), 2.69 (t, 2H, J = 8.1 Hz), 7.17–7.47 (m, 10H). MS
m/z: 280 (M+ - 30 (10)), 189 (14), 171 (32), 155 (13), 128 (8), 109
(20), 91 (100).
(2S*,3R*)-3-Isopropyl-2-methyl-2-phenylcyclobutanone 8h
Yield 70%. IR (neat): 1781 cm-1. 1H NMR (CDCl3) d: 1.00 (d, 3H,
J = 6.3 Hz), 1.10 (d, 3H, J = 6.3 Hz), 1.52 (s, 3H), 1.87–1.96 (m,
1H), 2.41–2.48 (m, 1H), 2.89 (dd, 1H, J = 17.6 Hz, J = 8.4 Hz),
3.06 (dd, 1H, J = 17.6 Hz, J = 9.6 Hz), 7.18–7.38 (m, 5H). 13C
NMR d: 19.49, 21.36, 22.75, 30.07, 43.04, 47.46, 67.80, 126.08,
126.53, 128.44, 142.94, 211.97. MS m/z: 160 (M+ - 42 (22)), 145
(100), 130 (26), 115 (48), 91 (48), 77 (44). The stereochemistry of
the trans isomer was confirmed by a strong nuclear Overhauser
effect (NOE) between the methyl group (s, 1.52 ppm), and the
proton of the isopropyl group (m, 1.87–1.96 ppm). Anal. Calcd.
for C14H18O: C,83.12; H, 8.97. Found: C, 83.21; H, 9.02%.
2-Methyl-2-(2-phenyl-1-(phenylthio)cyclopropyl)oxirane 7d
Inseparable 70:30 mixture of two diastereoisomers. Yellow oil.
Yield 65%. 1H NMR (CDCl3) d: 1.42–1.53 (m, 4H), 1.70 (s, 3H),
1.71 (s, 3H), 2.39 (dd, 1H, J = 15.6 Hz, J = 5.1 Hz), 2.53 (dd, 1H,
J = 18.3 Hz, J = 5.1 Hz), 2.54–2.62 (m, 2H), 6.97–7.45 (m, 10H).
13C NMR d: 16.74, 17.01, 19.96, 20.05, 26.84, 28.59, 39.14, 39.80,
53.49, 53.84, 57.59, 57.99, 126.30, 126.56, 127.27, 127.40, 127.58,
127.86, 128.22, 128.62, 128.94, 128.02, 132.50, 133.18, 134.14,
137.01, 137.09. Major isomer: MS m/z: 282 (M+ (48)), 267 (2),
191 (4), 173 (80), 145 (37), 105 (100), 77 (51). Minor isomer: MS
m/z: 282 (M+ (15)), 267 (2), 191 (8), 173 (100), 145 (92), 105 (38),
77 (24).
2,2-Dimethyl-3-phenethylcyclobutanone 8i
Yellow oil. Yield 75%. IR (neat): 1781 cm-1. 1H NMR (CDCl3) d:
1.11 (s, 3H), 1.19 (s, 3H), 1.68–1.77 (m, 1H), 1.93–2.10 (m, 2H),
2.61–2.66 (m, 2H), 2.69 (dd, 1H, J = 17.4 Hz, J = 6.9 Hz), 3.12
(dd, 1H, J = 17.4 Hz, J = 9.0 Hz), 7.17–7.32 (m, 5H). 13C NMR
d: 17.11, 23.61, 32.82, 34.77, 35.99, 48.48, 60.76, 125.97, 128.31,
128.43, 141.71, 214.85. MS m/z: 160 (M+- 44 (58)), 145 (2), 129
(6), 115 (12), 91 (100), 77 (20), 69 (82). Anal. Calcd. for C14H18O:
C,83.12; H, 8.97. Found: C, 83.28; H, 9.12%.
(2S*,3R*)-2-Butyl-3-isopropyl-2-phenylcyclobutanone 8j
3,6-Dihydro-5-methyl-2-phenyl-4-(phenylthio)-2H-pyran 12d
Yield 75%. IR (neat): 1780 cm-1. 1H NMR (CDCl3) d: 0.81 (t, 3H,
J = 7.2 Hz), 0.95 (d, 3H, J = 6.6 Hz), 1.16 (d, 3H, J = 6.6 Hz),
1.19–1.48 (m, 7H), 1.92–1.98 (m, 1H), 2.18–2.24 (m, 1H), 2.86 (dd,
1H, J = 17.7 Hz, J = 8.7 Hz), 3.02 (dd, 1H, J = 17.7 Hz, J =
9.3 Hz), 7.08–7.49 (m, 5H). 13C NMR d: 21.44, 22.19, 22.95, 26.92,
32.13, 39.95, 45.64, 47.55, 71.56, 128.26, 128.53, 128.23, 141.76,
211.81. MS m/z: 244 (M+ (1)), 202 (38), 187 (14), 160 (8), 145
(100), 118 (28), 91 (24), 77 (8). Anal. Calcd. for C17H24O: C,83.55;
H, 9.90. Found: C, 83.51; H, 9.82%.
1
Yellow oil. Yield 86%. H NMR (CDCl3) d: 1.93 (s, 3H), 2.29–
2.36(m, 1H), 2.43–2.52 (m, 1H), 4.31–4.42 (m, 2H), 4.59 (dd, 1H,
J = 3.3 Hz, J = 10.5 Hz) 7.15–7.31 (m, 10H). 13C NMR d: 16.21,
38.05, 70.77, 77.00, 121.64, 125.79, 126.12, 127.62, 128.35, 128.96,
129.40, 134.82, 138.48, 141.44. MS m/z: 282 (M+ (60)), 267 (3),
249 (3), 191 (5), 173 (100), 161 (64), 143 (49), 129 (58), 105 (54),
91 (54), 77 (71). Anal. Calcd. for C18H18OS: C, 76.56; H, 6.42; S,
11.35. Found: C, 76.74; H, 6.52; S, 11.56%.
General method for the synthesis of the cyclopropyl oxirane 7c and
the 3,6-dihydro-2H-pyrans 12d,g–k
2-(Furan-2-yl)-3,6-dihydro-5-methyl-4-(phenylthio)-2H-pyran 12g
Yellow oil. Yield 72%. 1H NMR (CDCl3) d: 1.91 (s, 3H), 2.32–2.42
(m, 1H), 2.67–2.78 (m, 1H), 4.24, 4.39 (ABq, 2H, J = 16.5 Hz),),
4.68 (dd, 1H, J = 3.6 Hz, J = 9.9 Hz), 6.23 (d, 1H, J = 3.0 Hz), 6.30
(dd, J = 3.3 Hz, J = 1.8 Hz), 7.14–7.28 (m, 5H), 7.36 (dd, 1H, J =
0.6 Hz, J = 1.8 Hz).13C NMR d: 16.28, 33.89, 70.07, 70.12, 107.12,
110.10, 120.36, 126.14, 128.97, 129.32, 138.56, 139.40, 142.39,
153.39. MS m/z: 272 (M+ (38)), 242 (4), 204 (6), 163 (38), 135
(37), 109 (81), 95 (100), 77 (62). Anal. Calcd. for C16H16O2S: C,
70.56; H, 5.92; S, 11.77. Found: C, 70.52; H, 5.88; S, 11.65%.
Trimethylsulfoxonium iodide (960 mg, 4.36 mmol) was rapidly
added to a stirred suspension of pentane-washed NaH (179 mg,
4.36 mmol, 60% in mineral oil) in DMSO (5 mL) under an argon
atmosphere. After keeping the reaction mixture at room temper-
ature for 2h, a solution of the cyclopropanes 2a–i (2.9 mmol)
◦
was added and the reaction mixture was heated at 96 C for 3h.
The mixture was diluted with diethyl ether, washed with brine
and dried over anhydrous Na2SO4 and the solvent was removed
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3,6-Dihydro-5-methyl-4-(phenylthio)-2-p-tolyl-2H-pyran 12h
1-((1R*,2R*)-2-Isopropylcyclopropylsulfonyl)benzene 13a
1
Orange oil. Yield 76%. 1H NMR (CDCl3) d: 1.91 (s, 3H), 2.27–2.33
(m, 1H), 2.29 (s, 3H), 2.43–2.53 (m, 1H), 4.34–4.35 (m, 2H), 4.53
(dd,1H, J = 3.6 Hz, J = 10.5 Hz), 7.08–7.27 (m, 9H).13C NMR d:
16.21, 21.06, 38.03, 70.74, 77.43, 121.67, 125.73, 126.07, 128.91,
128.97, 129.41, 134.82, 137.21, 138.38, 138.47. MS m/z: 296 (M+
(55)), 281 (12), 207 (45), 187 (71), 176 (100), 161 (96), 143 (75),
129 (54), 105 (50), 91 (61), 77 (24). Anal. Calcd. for C19H20OS: C,
76.99; H, 6.80; S, 10.82. Found: C, 76.74; H, 6.72; S, 11.66%.
Colourless oil. Yield 82%. IR (neat): 1150, 1310 cm-1. H NMR
(CDCl3) d: 1.04 (d, 6H, J = 6.6 Hz), 1.17–1.21 (m, 2H), 1.26–1.31
(m, 1H), 2.13–2.18 (m, 1H), 2.43 (ddd, 1H, J = 5.7 Hz, J = 8.4
H, J = 14.1 Hz), 7.51–7.94 (m, 5H). 13C NMR d: 12.14, 22.99,
26.39, 31.31, 38.78, 127.17, 129.03, 133.06, 141.99. MS m/z: 224
(M+ (2)), 209 (3), 169 (100), 143 (15), 125 (22), 77 (96), 55 (60).
Anal. Calcd. for C12H16O2S: C, 64.25; H, 7.19; S, 14.29. Found: C,
64.32; H, 7.12; S, 14.36%.
1-((1S*,2R*)-2-Isopropylcyclopropylsulfonyl)benzene 13a¢
3,6-Dihydro-5-phenyl-4-(phenylthio)-2-p-tolyl-2H-pyran 12i
Yellow oil. Yield 80%. IR (neat): 1150, 1310 cm-1. 1H NMR
(CDCl3) d: 0.746 (d, 3H, J = 6.3 Hz), 0.89–0.94 (m, 1H), 0.94
(d, 3H, J = 6.3 Hz), 1.03–1.10 (m, 1H), 1.47–1.52 (m, 2H), 2.20
(ddd, 1H, J = 4.5 Hz, J = 9.0 Hz, J = 8.4 Hz), 7.52–7.91 (m,
5H).). 13C NMR d: 12.01, 21.26, 21.36, 27.99, 31.07, 38.57, 127.52,
129.04, 133.20, 139.43. MS m/z: 224 (M+ (1)), 169 (38), 143 (15),
125 (18), 77 (90), 55 (100). Anal. Calcd. for C12H16O2S: C, 64.25;
H, 7.19; S, 14.29. Found: C, 64.28; H, 7.15; S, 14.33%.
Brown crystals, mp 96–98◦C. Yield 82%. H NMR (CDCl3) d: 2.31
(s, 3H), 2.38–2.60 (m, 2H), 4.59–4.62 (m, 2H), 4.68 (dd,1H, J =
3.3 Hz, J = 9.9 Hz), 7.11–7.37 (m, 14H). 13C NMR d: 21.11,
37.42, 71.14, 76.58, 125.07, 125.84, 126.66, 127.86, 128.29, 128.44,
126.91, 129.07, 130.80, 134.04, 137.44, 137.76, 138.33. 140.95. MS
m/z: 358 (M+ (20)), 281 (8), 238 (75), 205 (58), 191 (10), 147 (51),
128 (100), 109 (24), 91 (82),77 (39). Anal. Calcd. for C24H22OS: C,
80.41; H, 6.19; S, 8.94. Found: C, 80.52; H, 6.12; S, 8.79%.
1-((1R*,2S*)-2-Phenethylcyclopropylsulfonyl)benzene 13c
3,6-Dihydro-5-methyl-4-(phenylthio)–2-(thiophen-2-yl)-2H-pyran
12j
Yellow oil. Yield 96%. IR (neat): 1150, 1310 cm-1. 1H NMR
(CDCl3) d: 1.19–1.28 (m, 1H), 1.28–1.39 (m, 2H), 2.11–2-19 (m,
2H), 2.42 (dt, 1H, J = 7.8 Hz, J = 8.4 Hz), 2.76 (t, 2H, J =
7.2 Hz),7.17–7.94 (m, 10H). 13C NMR d: 12.30, 21.72, 28.21, 35.80,
38.12, 125.87, 127.21, 128.30, 128.52, 129.14, 133.17, 141.36,
141.84. MS m/z: 286 (M+ (10)), 195 (3), 169 (10), 144 (54), 129
(50), 109 (12), 91 (100), 77 (54). Anal. Calcd. for C17H18O2S: C,
71.30; H, 6.34; S, 11.19. Found: C, 71.38; H, 6.22; S, 11.12%.
1
Red oil. Yield 68%. H NMR (CDCl3) d: 1.91 (s, 3H), 2.42–2.48
(m, 1H), 2.58–2.69 (m, 1H), 4.30, 4.38 (ABq, 2H, J = 16.5 Hz),
4.86 (dd,1H, J = 3.3 Hz, J = 9.9 Hz), 6.91–6.94 (m, 2H), 7.15–7.28
(m, 6H). 13C NMR d: 16.23, 37.78, 70.38, 72.76, 121.06, 124.00,
124.88, 126.16, 128.98, 129.37, 134.67, 138.51, 144.32. MS m/z:
288 (M+ (88)), 255 (6), 204 (16), 176 (95), 161 (100), 135 (57), 111
(71), 85 (81), 65 (46). Anal. Calcd. for C16H16OS2: C, 66.63; H,
5.59; S, 22.23. Found: C, 66.54; H, 5.37; S, 22.38%.
1-(1R*,2R*)-2-(Phenylcyclopropylsulfonyl)benzene 13d
Colourless oil. Yield 93%. IR (neat): 1150, 1310 cm-1. H NMR
1
3,6-Dihydro-5-methyl-2-(naphthalen-2-yl)-4-(phenylthio)-2H-
pyran 12k
(CDCl3) d: 1.57 (ddd, 1H, J = 5.7 Hz, J = 8.4 Hz, J = 8.7 Hz),
2.17 (ddd, 1H, J = 5.7 Hz, J = 8.4 Hz, J = 5.4 Hz), 2.34 (s,
3H), 2.59 (dt, 1H, J = 5.7 Hz, J = 8.4 Hz), 2.78 (ddd, 1H, J =
5.7 Hz, J = 8.4 Hz, J = 5.4 Hz), 7.03–7.52 (m, 10H). 13C NMR d:
9.16, 24.94, 39.57, 126.99, 127.16, 127.65, 128.51, 129.53, 132.18,
132.81, 140.47. Anal. Calcd. for C15H14O2S: C, 69.74; H, 5.46; S,
12.41. Found: C, 69.52; H, 5.54; S, 12.36%.
Red oil. Yield 80%. H NMR (CDCl3) d: 1.96 (s, 3H), 2.52–2.62 (m,
2H), 4.41 (m, 2H), 4.77 (dd, 1H, J = 3.3 Hz, J = 10.2 Hz), 7.08–
7.80 (m, 12H). 13C NMR d: 16.31, 38.08, 70.78, 77.08 121.66,
123.96, 124.44, 125.82, 126.05, 126.12, 127.60, 127.96, 128.12,
129.01, 129.43, 132.91, 133.20, 134.82, 138.57, 138.87. MS m/z:
332 (M+ (45)), 257 (6), 223 (26), 193 14), 178 (42), 155 (78), 127
(100), 109 (70), 85 (28), 77 (34). Anal. Calcd. for C22H20OS: C,
79.48; H, 6.06; S, 9.64. Found: C, 79.34; H, 6.19; S, 9.52%.
1-((1R*,2R*)-2-p-Tolylcyclopropylsulfonyl)benzene 13i
Yellow oil. Yield 95%. IR (neat): 1140, 1310 cm-1. 1H NMR
(CDCl3) d: 1.57 (ddd, 1H, J = 5.7 Hz, J = 14.1 Hz, J = 8.7 Hz),
2.17 (ddd, 1H, J = 5.7 Hz, J = 8.1 Hz, J = 5.4 Hz), 2.34 (s, 3H),
2.59 (q, 1H, J = 8.4 Hz), 2.78 (ddd, 1H, J = 13.8 Hz, J = 8.4 Hz,
J = 5.4 Hz), 7.03–7.52 (m, 10H). 13C NMR d: 9.33, 21.04, 24.79,
39.62, 127.31, 128.44, 128.56, 129.43, 132.83, 136.70, 140.68. MS
m/z: 272 (M+ (1)), 131 (100), 115 (30), 91 (24), 77 (32). Anal.
Calcd. for C16H16O2S: C, 70.56; H, 5.92; S, 11.77. Found: C, 70.52;
H, 5.89; S, 11.70%.
General method for the synthesis of the cis-cyclopropanes 13a,c,d,i
Trimethylsulfoxonium iodide (960 mg, 4.36 mmol) was rapidly
added to a stirred suspension of pentane-washed NaH (179 mg,
4.36 mmol, 60% in mineral oil) in DMSO (5 mL) under an
argon atmosphere. After keeping the reaction mixture at room
temperature for 2h, a solution of the cyclopropanes 3a–d,i
(2.9 mmol) was added and the reaction mixture was heated at
96 ◦C for 3h. The mixture was diluted with diethyl ether, washed
with brine and dried over anhydrous Na2SO4 and the solvent
was removed under reduced pressure. The residue purified by
chromatography on silica gel (light petroleum-diethyl ether 10:1)
gave 13a,c,d,i.
Acknowledgements
Financial support from the MIUR, Rome, and by the University
of Cagliari (National Project “Stereoselezione in Sintesi Organica
3518 | Org. Biomol. Chem., 2009, 7, 3512–3519
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Metodologie ed applicazioni”) and from CINMPIS is gratefully
acknowledged.
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