Stereocontrolled synthesis of trans-cyclopropyl sulfones from terminal epoxides

Article abstract of DOI:10.1021/jo100844g

(Figure presented) Treatment of a range of (enantiopure) epoxides with the sodium salt of diethyl (phenylsulfonyl)methylphosphonate in DME at 140 °C for 4 h gives a variety of (enantiopure) trans-cyclopropyl sulfones with high diastereoselectivity.

Full text of DOI:10.1021/jo100844g

pubs.acs.org/joc
asymmetric variants of the venerable Simmons-Smith5
Stereocontrolled Synthesis of trans-Cyclopropyl
Sulfones from Terminal Epoxides
and Corey-Chaykovsky reactions,6 though more recently,
there has been an ever-increasing number of organocatalytic
methods developed.7 In spite of these advances, wasteful
resolution processes remain popular in an industrial setting.8
It is evident therefore, that methods to access cyclopropanes
in a stereocontrolled manner, particularly via conceptually
different approaches, are still of significant interest.
Christopher D. Bray* and Giorgio de Faveri
Queen Mary University of London, School of Biological and
Chemical Sciences, Mile End Road, London E1 4NS, U.K.
In 1961, Wadsworth and Emmons reported the direct
conversion of epoxides to cyclopropanes.9 Though poten-
tially very powerful, this method has not found widespread
appeal, and at present, this process is limited to the formation
of cyclopropyl esters using phosphonate esters,10 e.g., 1a, and
to spirocyclic cyclopropyl ketones using keto-stabilized
phosphonates.11-13 In the case of the former examples, the
reaction pathway involves ring-opening of epoxide 2 with
anion 1b (Scheme 1, step 1) followed by transfer of diethyl
phosphite to the newly formed alkoxide of 3 (step 2) and
finally stereospecific intramolecular ring-closure of the sub-
sequently generated R-stabilized anion 414 (step 3) to give
trans-cyclopropyl ester 5. In the synthesis of cyclopropyl
esters, high trans-selectivity is observed. It has been proposed
that this may be the result of (a) ring closure being highly
diastereoselective, which may be due to chelation control,10a,n
or (b) rapid equilibration of a mixture of cyclopropanes to
c.bray@qmul.ac.uk
Received April 29, 2010
Treatment of a range of (enantiopure) epoxides with
the sodium salt of diethyl (phenylsulfonyl)methylphos-
phonate in DME at 140 °C for 4 h gives a variety of
(enantiopure) trans-cyclopropyl sulfones with high dia-
stereoselectivity.
(5) Simmons, H. E.; Smith, R. D. J. Am. Chem. Soc. 1959, 81, 4256.
(6) Corey, E. J.; Chaykovsky, M. J. Am. Chem. Soc. 1965, 87, 1353.
(7) (a) Hansen, H. M.; Longbottom, D. A.; Ley, S. V. Chem. Commun.
2006, 4838. (b) Kunz, R. K.; Macmillan, D. W. C. J. Am. Chem. Soc. 2005,
127, 3240. (c) Papageorgious, C. D.; Cubillo de Dios, M. A.; Ley, S. V.;
Gaunt, M. J. Angew. Chem., Int. Ed. 2004, 43, 4641. (d) Bremeyer, N.; Smith,
S. C.; Ley, S. V.; Gaunt, M. J. Angew. Chem., Int. Ed. 2004, 43, 2681.
(8) For a recent example, see: Beaulieu, P. L.; Gillard, J.; Bailey, M. D.;
Boucher, C.; Duceppe, J.-S.; Simoneau, B.; Wang, X.-J.; Zhang, L.;
Grozinger, K.; Houpis, I.; Farina, V.; Heimroth, H.; Krueger, T.;
Schnaubelt, J. J. Org. Chem. 2005, 70, 5869.
The cyclopropane moiety is ubiquitous in Nature, being
found in numerous amino acids, fatty acids, polyketides, and
terpenes.1 Cyclopropanes are also regularly utilized in medi-
cinal chemistry, including in peptidomimetic approaches,
since they provide the capability to arrange pendant groups
in specific and rigid three-dimensional orientations.2 In
addition, reactions involving metal-catalyzed ring-opening
of cyclopropanes with subsequent cyclopentannulation are
becoming ever more common.3 Methods for the stereo-
controlled synthesis of cyclopropanes are thus of vital
importance, and a great deal of effort has been put into
the development of a multitude of catalytic asymmetric
processes that target them.4 The basis of such reactions lies
predominantly in metal-catalyzed (e.g., Rh or Cu) diazo
decomposition/carbene insertion into alkenes along with
(9) Wadsworth, W. S.; Emmons, W. D. J. Am. Chem. Soc. 1961, 83, 1733.
(10) Examples of Wadsworth-Emmons cyclopropanation in synthesis
^
are as follows: (a) Delhaye, L.; Merschaert, A.; Delbeke, P.; Brione, W. Org.
Process Res. Dev. 2007, 11, 689. (b) Armstrong, A.; Scutt, J. N. Chem.
Commun. 2004, 510. (c) Armstrong, A.; Scutt, J. N. Org. Lett. 2003, 5, 2331.
(d) Singh, A. K.; Rao, M. N.; Simpson, J. H.; Li, W. S.; Thornton, J. E.;
Kuehner, D. E.; Kacsur, D. J. Org. Process Res. Dev. 2002, 6, 618. (e) Petter,
R. C.; Banerjee, S.; Englard, S. J. Org. Chem. 1990, 55, 3088. (f) Petter, R. C.
Tetrahedron Lett. 1989, 30, 399. (g) Fitzsimmons, B. J.; Fraser-Reid, B.
Tetrahedron 1984, 40, 1279. (h) Izydore, R. A.; Ghiradelli, R. G. J. Org.
€
€ €
Chem. 1973, 38, 1790. (i) Tomoskozi, I. Tetrahedron 1966, 22, 179. For the
analogous, but less facile, use of phosphoranylidenes, see: (j) Denney, D. B.;
Vill, J. J.; Boskin, M. J. J. Am. Chem. Soc. 1962, 84, 3944. For related
approaches, see: (k) Clarke, C.; Fox, D. J.; Pedersen, D. S.; Warren, S. Org.
Biomol. Chem. 2009, 7, 1329. (l) Clarke, C.; Foussat, S.; Fox, D. J.; Pedersen,
D. S.; Warren, S. Org. Biomol. Chem. 2009, 7, 1323. (m) Krawczyk, H.;
Wa-sek, K.; Ke-dzia, J. Synthesis 2009, 1473–1476. (n) Krawczyk, H.; Wa-sek,
K.; Ke-dzia, J.; Wojciechowski, J.; Wolf, W. M. Org. Biomol. Chem. 2008, 6,
308–318. (o) Fox, D. J.; Parris, S.; Pedersen, D. J.; Tyzack, C. R.; Warren, S.
Org. Biomol. Chem. 2006, 4, 3108. (p) Boesen, T.; Fox, D. J.; Galloway, W.;
Pedersen, D. S.; Tyzack, C. R.; Warren, S. Org. Biomol. Chem. 2005, 3,
630–637.
(11) Jacks, T. E.; Nibbe, H.; Wiemer, D. F. J. Org. Chem. 1993, 58, 4584.
(12) Wadsworth and Emmons have reported an isolated example of the
use of the diethyl cyanomethylphosphonate for the conversion of styrene
oxide to the corresponding cyanocyclopropane (51% yield); see ref 9.
(13) Katritzky et al. have reported three examples of an analogous
cyclopropanation using 1-(benzotriazol-1-yl)diphenylphosphine oxide; see:
Katrizky, A. R.; Wu, H.; Xie, L.; Jiang, J. J. Heterocycl. Chem. 1995, 32, 595.
(14) An example of the protonated intermediate 4 has been detected by
LC/MS; see ref 10d.
(1) For respective representative examples, see: (a) Parry, R. J.; Mafoti,
R. J. Am. Chem. Soc. 1986, 108, 4681. (b) MacMillan, J. B.; Molinski, T. F.
J. Nat. Prod. 2005, 68, 604. (c) Ringel, S. M.; Greenough, R. C.; Roemer, S.;
Connor, D.; Gutt, A. L.; Blair, B.; Kanter, G.; von Strandtmann, M.
J. Antibiot. 1977, 30, 371. (d) Zheng, G. C.; Ichikawa, A.; Ishitsuka,
M. O.; Kusumi, T.; Yamamoto, H.; Kakisawa, H. J. Org. Chem. 1990, 55,
3677.
(2) Cyclopropane-containing drugs accounted for >$7 billion in sales in
2006. For a review of recent peptidomimetic approaches, see: Youla, S. T.
Acc. Chem. Res. 2008, 41, 1252.
(3) (a) Carson, C. A.; Kerr, M. A. Chem. Soc. Rev. 2009, 38, 3051.
(b) Rubin, M.; Rubina, M.; Gevorgyan, V. Chem. Rev. 2007, 107, 3117.
(4) (a) Goudreau, S. R.; Charette, A. B. Angew. Chem., Int. Ed. 2010, 49,
486. (b) Pellissier, H. Tetrahedron 2008, 64, 7041. (c) McGarrigle, E. M.;
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2007, 107, 5841. (d) Maas, G. Chem. Soc. Rev. 2004, 33, 183. (e) Lebel, H.;
Marcoux, J. F.; Molinaro, C.; Charette, A. B. Chem. Rev. 2003, 103, 977.
(f) Comprehensive Asymmetric Synthesis, 2nd ed.; Ojima, I., Ed.; Wiley-VCH
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4652 J. Org. Chem. 2010, 75, 4652–4655
Published on Web 06/10/2010
DOI: 10.1021/jo100844g
r
2010 American Chemical Society

Bray and de Faveri
JOCNote
SCHEME 1. Reaction Pathway for the Direct Conversion of
Epoxides to trans-Cyclopropyl Esters
TABLE 1. Optimization of the Direct Conversion of Epoxides to
Cyclopropyl Sulfones
entry
solvent
temp/°C
time/h
yielda (%)
1
2
3
4
5
6
7
8
9
PhMe
DME
dioxane
DMF
THF
DME
DME
DME
DME
DME
DME
110
86
100
120
67
16
16
16
16
20
16
16
8
74b
70
67
65
56
86
61c
80
120
140
140
160
140
SCHEME 2. Proposed Reaction Pathway for the Direct
Conversion of Epoxides to Cyclopropyl Sulfones
83d
83d
76
4
2
4
10
11
73e
aIsolated yield following column chromatography (SiO2). bdr 97:3.
cn-BuLi used as base. ddr 98:2. e6b (1.5 equiv).
(generated from 6a using 1.05 equiv of NaH) was heated to
reflux in PhMe with epoxide 2a for 16 h. This gave the desired
trans-cyclopropyl sulfone 9a in a promising 74% yield and
with high diastereocontrol (dr 97:3 (GC)) (Table 1, entry 1).19
In order to optimize this reaction, we examined a number of
solvents (entries 2-5). Yields were generally comparable,
however, from a practical point of view, the insolubility of
6b made the reactions in PhMe and dioxane less straight-
forward due to significant foaming during the deprotonation
of 6a. The promising yield obtained in DME prompted us
to examine this solvent more closely. Attempts to use an
alternate counterion were briefly investigated. Under condi-
tions otherwise comparable to those described for entry 2,
use of n-BuLi as base (entry 6) resulted in precipitation of the
anion and a lower yield of 9a (61%). The use of a potassium
counterion (KH as base) led to an anion which remained in
solution but which led to a far greater product distribution
(as judged by TLC analysis). Next, we began to examine the
effect of temperature. Raising the temperature to 120 °C
instantly led to improvement in yield to 80% (entry 7).
Further increase in temperature to 140 °C (entry 8)20 and
decreasing the reaction time to 8 h increased the yield (83%)
and also the dr (98:2). In fact, reduction of the reaction time
to only 4 h at this temperature led to an identical yield and dr
(entry 9). An attempt to further decrease the reaction time by
increasing the temperature further led to a reduction in yield
(entry 10). Finally, we attempted to reduce the quantity of
anion 6b needed; however, lowering this to 1.5 equiv led to a
noticeable 10% drop in yield (entry 11).
the thermodynamically favored trans-isomers following ring
closure.9
Given the excellent yields and diastereoselectivities often
seen with this reaction,15 coupled with the range of anion-
stabilizing groups one might employ and ready availabilty of
enantiopure epoxides,16 it is highly surprising that the scope
of this reaction has not been examined further. Cyclopropyl
sulfones have found a variety of uses in synthesis,17 and since
sulfones exhibit a similar anion-stabilizing ability to esters,
we investigated whether we could affect the direct conver-
sion of epoxides to cyclopropyl sulfones using sulfone phos-
phonate 6a18 (Scheme 2).
Our work began by examining the reaction between 1,2-
epoxyhex-5-ene 2a and anion 6b. Initially, a slurry of 6b
(15) Reaction between (S)-propylene oxide and 1a has been reported to
give up to 95% yield with dr >98:2; see ref 10a.
(16) Amatore, M.; Beeson, T. D.; Brown, S. P.; MacMillan, D. W. C.
Angew. Chem., Int. Ed. 2009, 48, 5121. (b) Wong, O. A.; Shi, Y. Chem. Rev.
2008, 108, 3958. (c) Schaus, S. E.; Brandes, B. D.; Larrow, J. F.; Tokunaga,
M.; Hansen, K. B.; Gould, A. E.; Furrow, M. E.; Jacobsen, E. N. J. Am.
Chem. Soc. 2002, 124, 1307.
(17) Cyclopropyl sulfones can be subjected to alkylation/acylation; see:
(a) Chang, Y. H.; Pinnick, H. W. J. Org. Chem. 1978, 43, 373. (b) Corey, E. J.;
Weatherhead-Kloster, R. A. Org. Lett. 2006, 8, 171. (c) Tanikaga, R.;
Yamada, S.; Nishikawa, T.; Matsui, A. Tetrahedron 1998, 31, 8933.
(d) Tanaka, K.; Suzuki, H. J. Chem. Soc., Perkin Trans. 1 1992, 2071. They
can be converted to methylene cyclopropanes; see: (e) Baldwin, J. E.;
Adlington, R. M.; Bebbington, D. Tetrahedron 1994, 50, 12015. (f) Lai,
M. T.; Oh, E.; Shih, Y.; Liu, H. W. J. Org. Chem. 1992, 57, 2471. They can
form π-allyl palladium complexes which undergo reactions with electron
deficient alkenes; see: (g) Morizawa, Y.; Oshima, K.; Nozaki, H. Tetra-
hedron Lett. 1982, 23, 2871. They can undergo desulfonylation to give simple
cyclopropanes; see: (h) Kazuta, Y.; Matsuda, A.; Shuto, S. J. Org. Chem.
2002, 67, 1669. They can act as synthons for 1,3-dipoles; see: (i) Trost, B. M.;
Cossy, J.; Burkes, J. J. Am. Chem. Soc. 1983, 105, 1052. They can participate
in Julia-type olefinations; see: (j) Bernard, A. M.; Frongia, A.; Piras, P. P.;
Under these reaction temperatures, it is remarkable that
transfer of the phosphite group to the alkoxide (cf. 7f8) is so
selective over that of the sulfone.21 Considerable synthetic
effort utilizing the endocyclic restriction test has demonstrated
(19) The trans-stereochemistry was assigned by 1H NMR spectroscopy
(NOE, coupling constant analysis and comparison with data in ref 17b) and
ultimately by X-ray crystallographic analysis of 9b,f; see the Supporting
Information.
(20) Temperatures of up to 150 °C have been found to be beneficial in the
reaction between 1b and propylene oxide; see ref 10b.
Secci, F. Synlett 2004, 1064. (k) Aıssa, C. J. Org. Chem. 2006, 71, 360. They
¨
can be regioselectively cleaved to give vinylstannanes; see: (l) Hayashi, N.;
Hirokawa, Y.; Shibata, I.; Yasuda, M.; Baba, A. J. Am. Chem. Soc. 2008,
130, 2912.
(18) Diethyl (phenylsulfonyl)methylphosphonate 6a is readily available
via an Arbusov reaction between triethyl phosphite and chloromethyl phenyl
sulfide and oxidation of the resultant product with Oxone; see the Supporting
Information.
(21) Sulfone anion 6b has previously been used for the synthesis of vinyl
sulfones. While in these cases transfer of diethyl phosphite also occurs in
preference to the sulfone and competitive formation of vinyl phosphonates is
not observed, such reactions are carried out at room temperature or below.
J. Org. Chem. Vol. 75, No. 13, 2010 4653

Bray and de Faveri
JOCNote
SCHEME 3. Reaction Pathway for an Unsuccessful Sulfur(VI)-
Based Analogue of the Wadsworth-Emmons Cyclopropanation
(Homologous-Julia) Reaction
TABLE 2. Examination of Substrate Scope
that the favored pathway for substitution at atoms in the top
right-hand corner of the periodic table including S(VI) is
concerted and involves a transition-state geometry in which
the nucleophile, leaving group, and reacting center are
linearly disposed.22 In contrast, it has been shown that
substitution at phosphorus(V) can proceed via a non-
concerted addition/Berry pseudorotation/elimination pro-
cess, such that a linear arrangement is not always required
for reaction to occur.23 This may explain why the rates for
transfer of sulfur(VI) and phosphorus(V) centered groups in
small rings can be vastly different. To test this hypothesis, we
briefly investigated the reaction between sodium methyl
phenylsulfonylacetate 10a and 1,2-epoxyhex-5-ene 2a. Such
a reaction (homologous Julia) would require transfer of the
sulfone to the alkoxide of 11 in order to form cyclopropyl
ester 5 (Scheme 3); however, despite repeated attempts, we
could find no evidence of cyclopropane formation, even at
elevated temperatures.
aReactions carried out with 6b (2 equiv) at 140 °C in DME for 4 h.
bIsolated yield following column chromotography. cDiethyl (pyridin-
2-yl)sulfonylmethylphosphonate used as reagent.
SCHEME 4. Expedient Synthesis of a Methylecyclopropane
for cyclopropane 9f and so examined the effect of substitu-
tion at the γ-position. Isopropyloxirane 2g gave the expected
cyclopropyl sulfone 9g, but in only 31% yield (entry 6),
whereas reaction with tert-butyloxirane 2h did not give the
expected cyclopropyl sulfone 9h (entry 7). Monitoring of this
latter reaction by in situ 1H NMR analysis revealed the
appearance of signals characteristic of an olefin, indicating
that neopentyl-like rearrangement24 had occurred with loss
of diethyl phosphate; however, it was not possible to isolate
such products. We also examined whether non-terminal
epoxides were substrates for this process; however, cyclo-
hexene oxide 2i gave the desired cyclopropyl sulfone 9i in
only 5% yield. Finally, we briefly examined the use of
heteroaryl sulfones in this process, with a view that the
products could be used in Julia-Kocienski olefination
reactions17k to give methylene cyclopropanes. However,
significant reduction in yield was observed (17%) when a
2-pyridylsulfone was employed (entry 9).
To further demonstrate the utility of our methodology, we
deprotonated cyclopropyl sulfone 9f with n-BuLi and then
alkylated the resultant anion with iodomethyltrimethyl-
silane, which gave the corresponding trisubstituted cyclo-
propane 13 in 65% yield as a single diastereomer (as judged
by 1H NMR) (Scheme 4). Overall, this gave silyl sulfone 13
in only two steps from commercially available materials.
Cyclopropane 13 has been synthesized by Liu and co-workers
in four steps17f en route to [(methylenecyclopropyl)acetyl]-
CoA, the causative agent of Jamaican vomiting sickness,
which results from ingestion of the unusual amino acid
hypoglycin A found in unripe ackee fruit. Silyl sulfone 13
Given the utility of cyclopropyl sulfones,17 we next exam-
ined the scope of this new cyclopropane ring-forming reac-
tion with a variety of enantiopure epoxides (Table 2). Use of
(S)-styrene oxide 2b (>98% ee) under the optimal condi-
tions developed (6b (2 equiv), DME, 140 °C, 4 h) gave the
expected trans-phenyl cyclopropyl sulfone 9b in 86% yield
(entry 1) and with high diastereocontrol (dr >200:1). Ana-
lysis of 9b revealed it to have an ee of >98% (HPLC),
demonstrating that the cyclopropane ring-closing step is
stereospecific. Simple alkyl-substituted enantiopure termi-
nal epoxides (including volatile substrates) also gave good
yields of trans-cyclopropyl sulfones (entry 2 and 3) though
with marginally lower dr (98:2). We next examined sub-
strates whose products would be more amenable to further
synthetic manipulation. (R)-Vinyl oxirane 2e gave good yield
(74%) of the corresponding enantiopure vinyl trans-cyclo-
propyl sulfone 9e (entry 4) as a single diastereomer upon
treatment with 6b. This result is significant since despite the
utility of vinylcyclopropanes in synthesis,3,17g,17i this sub-
strate has not been reported in analogous epoxide to cyclo-
propane conversions.9-13 Though there is the possibility of
π-allyl cation formation following loss of diethyl phosphate
during the reaction pathway, there was no loss of stereo-
integrity (as judged by chiral HPLC) and no evidence of
allylic substitution. Reaction of (S)-benzyl glycidyl ether 2f
with 6b gave the benzyloxymethylene-substituted cyclo-
propyl sulfone 9f in 60% yield (entry 5), again as a single
diastereomer. We were intrigued by the diminished yield
(22) Jarboe, S. G.; Terrazas, M. S.; Beak, P. J. Org. Chem. 2008, 73, 9627.
(23) (a) Tollefson, M. B.; Li, J. J.; Beak, P. J. Am. Chem. Soc. 1996, 118,
9052. (b) Westheimer, F. H. Acc. Che,. Res. 1968, 1, 70.
(24) Streitwieser, A. Chem. Rev. 1956, 56, 571.
4654 J. Org. Chem. Vol. 75, No. 13, 2010

Bray and de Faveri
JOCNote
was readily converted to methylene cyclopropane 1417l on
treatment with TBAF in THF, and hence, the present work
should allow ready access to such compound as single
enantiomers.25
In summary, the present method provides a very efficient
and straightforward elaboration of enantiopure trans-cyclo-
propylsulfones and methylene cyclopropanes from terminal
epoxides which proceeds with high diastereoselectivity. Such
products are highly useful synthetic intermediates in a variety of
processes.17 This methodology should allow for a significant
expansion of the Wadsworth-Emmons cyclopropanation re-
action and considerably increases its synthetic appeal.
4 h. Once cooled, the reaction mixture was dry loaded onto silica
(5 mL) and purified by flash column chromatography (EtOAc/
petrol).
(1S,2R)-2-Vinylcyclopropyl Phenyl Sulfone 9e. According to
the general procedure, (R)-vinyl oxirane (81 μL, 1.00 mmol)
gave the title compound (154 mg, 74%) as a white solid: mp
67-68 °C; [R]D þ6.4 (c 1.0, CHCl3); IR (cm-1) 3066w, 1446 m,
1302s (SO2), 1143s (SO2), 1058 m; 1H NMR (400 MHz, CDCl3)
δ 7.91-7.84 (m, 2H), 7.65-7.58 (m, 2H), 7.57-7.50 (m, 2H),
5.37 (ddd, J = 17.1, 10.2, 7.9 Hz, 1H), 5.15 (d, J = 17.1 Hz, 1H),
5.00 (d, J = 10.2 Hz, 1H), 2.44 (ddd, J = 8.3, 5.3, 4.4 Hz, 1H),
2.42-2.31 (m, 1), 1.62 (dt, J = 9.5 and 5.3 Hz, 1), 1.12 (dt, J =
8.3 and 5.9, 1H); 13C NMR (100 MHz, CDCl3) δ 141.0, 135.6,
133.9, 129.7, 127.8, 117.1, 40.3, 23.2, 13.4; HRMS m/z (M þ
NH4þ, 100) found 226.0892, C11H16O2NS requires 226.0896.
Experimental Section
General Procedure for the Direct Conversion of Epoxides to
trans-Cyclopropylsulfones. To a vigorously stirred suspension of
NaH (49 mg, 2.05 mmol) in anhydrous DME (2 mL) at 25 °C
was added a solution of diethyl (phenylsulfonyl)methylphos-
phonate 6a (584 mg, 2.00 mmol) in anhydrous DME (2 mL)
dropwise over 5 min. The resulting clear solution was stirred for
a further 5 min before the epoxide (1.00 mmol) was added
dropwise over 1 min. The reaction was heated to 140 °C for
Acknowledgment. We thank the Royal Society and the
EPSRC (EP/G041431/1) for project grants, Majid Motevalli
and the EPSRC National Crystallography Service Centre,
Southampton, for obtaining X-ray crystal structures, and the
EPSRC National Mass Spectrometry Service Centre, Swansea.
Supporting Information Available: Spectroscopic data
along with 1H and 13C NMR spectra for compounds 6a, 9a-j,
and 13. This material is available free of charge via the Internet
at http://pubs.acs.org.
(25) Chiral methylene cyclopropanes have been used in tandem rhodium-
catalyzed C-H activation/cycloisomerization reactions; see: Aıssa, C.;
¨
€
Furstner, A. J. Am. Chem. Soc. 2007, 129, 14836.
J. Org. Chem. Vol. 75, No. 13, 2010 4655