A straightforward synthesis of alkenyl nonaflates from carbonyl compounds using nonafluorobutane-1-sulfonyl fluoride in combination with phosphazene bases

Article abstract of DOI:10.1055/s-2007-991084

An α-deprotonation of carbonyl compounds with phosphazene bases in the presence of the internal quenching reagent, nonafluorobutane-1-sulfonyl fluoride furnishes the corresponding alkenyl nonaflates. The new general method provides high yields of alkenyl nonaflates from aldehydes and cyclic ketones. However, it is not applicable to acyclic ketones whose nonaflate derivatives undergo fast E2 elimination to give alkynes. Successful synthesis of nonaflates from aldehydes requires carefully controlled reaction conditions to avoid the subsequent elimination to alkynes. A kinetic control enables high regioselectivities in favor of least substituted nonaflate regioisomers derived from cyclic ketones and modest Z-selectivities of alkenyl nonaflates derived from aldehydes. A new efficient protocol for highly selective removal of perfluorosulfolane admixture from technical nonafluorobutane-1-sulfonyl fluoride by basic hydrolysis is described. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2007-991084

LETTER
2907
A Straightforward Synthesis of Alkenyl Nonaflates from Carbonyl
Compounds Using Nonafluorobutane-1-sulfonyl Fluoride in Combination
with Phosphazene Bases
Alkenyl
N
onaflates fro
i
m
K
eto
c
nes and
A
l
h
dehydes: Sco
a
pe and Lim
e
itations l A. K. Vogel,a,b Christian B. W. Stark,b Ilya M. Lyapkalo*c
a
b
c
Institute of Chemical & Engineering Sciences Ltd., 1 Pesek Road, Jurong Island, 627833 Singapore, Singapore
Freie Universität Berlin, Institut für Chemie, Organische Chemie, Takustr. 3, 14195 Berlin, Germany
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo n. 2, 16610 Prague 6,
Czech Republic
Fax +420(2)20183578; E-mail: ilya.lyapkalo@uochb.cas.cz
Received 8 August 2007
This paper is dedicated to Prof. D. Seebach on the occasion of his 70th birthday.
formation of side products7b or deteriorate analytical char-
acteristics of isolated nonaflates.
Abstract: An a-deprotonation of carbonyl compounds with phos-
phazene bases in the presence of the internal quenching reagent,
nonafluorobutane-1-sulfonyl fluoride furnishes the corresponding
alkenyl nonaflates. The new general method provides high yields of
alkenyl nonaflates from aldehydes and cyclic ketones. However, it
is not applicable to acyclic ketones whose nonaflate derivatives un-
dergo fast E2 elimination to give alkynes. Successful synthesis of
nonaflates from aldehydes requires carefully controlled reaction
conditions to avoid the subsequent elimination to alkynes. A kinetic
control enables high regioselectivities in favor of least substituted
nonaflate regioisomers derived from cyclic ketones and modest Z-
selectivities of alkenyl nonaflates derived from aldehydes. A new
efficient protocol for highly selective removal of perfluorosulfolane
admixture from technical nonafluorobutane-1-sulfonyl fluoride by
basic hydrolysis is described.
Herein, we would like to report on efficient purification of
the technical quality nonafluorobutane-1-sulfonyl fluo-
ride (1) and its application in combination with phos-
phazene bases for a new high-yielding synthesis of
alkenyl nonaflates from enolizable carbonyl compounds.
We have found that a vigorous stirring of the technical
product consisting of the perfluorinated compounds 1 and
2 with the concentrated aqueous buffer solution of
K2HPO4 and K3PO4 (pH 12–13) for 96 hours at r.t. led to
highly selective nucleophilic ring opening of the per-
fluorosulfolane (2) to give potassium 1,1,2,2,3,3,4,4-
octafluorobutanesulfonate10 with NfF (1) remaining
essentially intact (Scheme 1). Phase separation followed
by distillation over P2O5 furnished NfF (1; 92% yield) of
>99% purity according to 19F NMR.
Key words: chemoselectivity, nonafluorobutane-1-sulfonyl fluo-
ride, ketones, regioselectivity, alkenyl nonaflates
Sulfonic acid enol esters (alkenyl sulfonates) constitute a
synthetically important link that enables the extension of
the transition-metal-catalyzed cross-coupling methodolo-
gy to enolizable carbonyl compounds, one of the most
abundant and ubiquitous pools of organic substrates. So
far, alkenyl triflates proved to be the enol derivatives most
frequently used in Pd(0)-catalyzed cross-coupling reac-
tions.1 However, alkenyl nonaflates (nonafluorobutane-
sulfonates) represent a useful alternative2–6 to the triflates
not least owing to the advantageous properties of the
nonafluorobutane-1-sulfonyl fluoride (1, NfF),7 a sulfo-
nylating reagent routinely used for the preparation of the
nonaflates from carbonyl derivatives.7b,8,9 The compound
1 is a technical product obtained in 90–94% purity by the
electrochemical fluorination of inexpensive 2,5-dihy-
drothiophene 1,1-dioxide, with perfluorosulfolane (2) be-
ing the admixture (6–10 mol%, see Scheme 1, left).
Although the presence of the sulfolane 2 apparently does
not affect the performance of alkenyl nonaflates in one-
pot cross-coupling protocols,2a,d,f,6c it may lead to the
CF3(CF2)3SO2F
K2HPO4, K3PO4
H2O, r.t., 96 h
1 (90–94 mol%)
CF3(CF2)3SO2F
+
1 (>99 mol%)
F
F
F
F
F
F
S
F
F
H(CF2)4SO3K
O
O
2
(6–10 mol%)
Scheme 1 Purification of NfF by basic treatment with aqueous
phosphate buffer solution
High stability of NfF (1) towards nucleophilic (basic)
treatment (Scheme 1) prompted us to investigate a possi-
bility of applying this pure reagent in combination with
metal-free nitrogen bases11 to obtain alkenyl nonaflates
from ketones or aldehydes. After the extensive experi-
mentation,12 we were rewarded by finding out that (tert-
butylimino)tris(1-pyrrolidinyl)phosphorane13 (hereinafter
called P1-base) and 1-(tert-butylimino)-1,1,3,3,3-penta-
kis(dimethylamino)-1l5,3l5-diphosphazene (hereinafter
called P2-base14), commercially available representatives
of the family of phosphazene bases (Figure 1), advanta-
SYNLETT 2007, No. 18, pp 2907–2911
x
x
.x
x
.2
0
0
7
Advanced online publication: 12.10.2007
DOI: 10.1055/s-2007-991084; Art ID: G26907ST
© Georg Thieme Verlag Stuttgart · New York

2908
M. A. K. Vogel et al.
LETTER
Me
Me
of the P1-base provided perfect regioselectivity control in
favor of the deprotonation of the ketone 3h at the position
most remote to the ring nitrogen to give nonaflate 4h as a
single isomer.6c,17 However, the P1-base was found to be
nonregioselective in a-methine vs. a-methylene deproto-
nation of 2-methyl cyclopentanone (3f) and 2-methyl
cyclohexanone (3g). Fortunately, the regioselectivity was
crucially improved when the much stronger P2-base was
employed under kinetically controlled conditions.18 In
both cases, the nonaflates 4f,g bearing the less substituted
double bonds were formed in high yields and regioselec-
tivities.
Me
Me
Me
N
P
N
Me
N
N
P
N
(Me2N)3P
NMe2
NMe2
N
P1-base
P2-base
Figure 1 Phosphazene bases employed for the synthesis of alkenyl
nonaflates from carbonyl compounds
R2
H
R2
R1
O
ONf
R1
NfF, P-base
Nonaflation of the aldehydes 3i–k proceeded appreciably
faster than that of the cyclic ketones apparently owing to
higher acidity of the a-hydrogens of the aldehydes. How-
ever, in the case of 3j,k, the room-temperature reactions
were deteriorated by subsequent base-induced elimination
of NfOH to give terminal alkynes as side products.6c
Gratifyingly, carrying out the reactions at lower tempera-
ture (less than –30 °C) enabled a perfect kinetic discrimi-
nation between nonaflation and elimination steps in favor
of the former resulting in good yields of the desired
nonaflates 4j,k. We believe that the observed moderate Z-
selectivities of 4j,k are owing to the stabilizing anti-
periplanar overlap of the sC–H and the incipient s*C–ONf in
the open-chain transition state. It is noteworthy that the
low-temperature transformation of 6-oxo-heptanal 3k into
the enol nonaflate 4k was successfully accomplished,
with unprotected ketone functionality remaining intact.
– [P-base]H+F–
R
R
3
4
Scheme 2 Synthesis of nonaflates 4 using P-bases in combination
with NfF
geously introduced and developed by Schwesinger et al.,
are fully compatible with NfF.
This enabled us to develop a novel synthesis of alkenyl
nonaflates achieved in a single operation step by having
the electrophilic component NfF present during the depro-
tonation of the carbonyl compound15 by the phosphazene
base (Scheme 2).
The reactions were found to proceed smoothly in common
nonprotogenic solvents, THF or DMF.16
P1-base induced smooth and high-yielding conversion of
cyclic plane-symmetric ketones 3a–e to the desired
nonaflates 4a–e.17 The metal-free noncoordinating nature
Table 1 Synthesis of Alkenyl Nonaflates 4a
Starting material 3
Reaction conditions
Product 4
Isolated yield (%) (isomer ratio)
Structure
P1-base, DMF, 16 h
DBU, DMF, 24 h
96
94
ONf
O
4a
Me
4b
Ph
4c
3a
Me
3b
Ph
3c
O
ONf
ONf
P1-base, THF, 16 h
P1-base, THF, 20 h
96
95
O
O
ONf
P1-base, DMF, 18 h
77
3d
3e
4d
4e
P1-base, THF, 16 h
DBU, THF, 24 h
Et3N,b DMF, 22 h
95
94
72
O
ONf
Synlett 2007, No. 18, 2907–2911 © Thieme Stuttgart · New York

LETTER
Alkenyl Nonaflates from Ketones and Aldehydes: Scope and Limitations
2909
Table 1 Synthesis of Alkenyl Nonaflates 4a (continued)
Starting material 3
Reaction conditions
Product 4
Isolated yield (%) (isomer ratio)
Structure
ONf
O
P2-base, DMF, –30 °C, 17 h
P1-base, DMF, 21 h
84 (ca. 24:1)c
89 (1.3:1)c
Me
Me
4f
3f
O
ONf
ONf
P2-base, DMF, –20 °C, 65 hd
Me
Me
Me
93e (99:1)c
84 (1:2)c
75
4g
3g
O
Me
P1-base, DMF, 111 h
P1-base, DMF, 18 h
3g
4g¢
ONf
O
N
Me
N
Me
3h
4h
Me
Me
O
P1-base, DMF, 4 h
89
ONf
Me
3i
Me
4i
O
Me
( )4
O
P1-base, DMF, –30 °C, 19 h
P1-base, DMF, –30 °C, 21 h
84 (Z/E = 4.3:1)
93 (Z/E = 5:1)
( )4
Me
4j
O
ONf
3j
( )3
ONf
O
( )3
Me
Me
3k
4k
a With P-bases (1.15 equiv) and NfF (1.15 equiv) at r.t. unless otherwise stated.17,18
b Amount of Et3N: 4 equiv.
c In favor of the regioisomer shown.
d An 88% conversion was observed after 24 h.
e P-base (2.0 equiv) and NfF (2.0 equiv) were required in order to achieve a complete conversion of the starting ketone.
Acyclic ketones failed to produce alkenyl nonaflates un- In conclusion, we have developed a general chemo- and
der our reaction conditions. In a typical example, a treat- regioselective synthesis of alkenyl nonaflates20 from
ment of 3-methylbutan-2-one with equimolar amounts of cyclic ketones and aldehydes using phosphazene bases21
NfF and P2-base at –78 °C to room temperature produced combined with nonafluorobutane-1-sulfonyl fluoride. The
a ca. 1:1 mixture of the starting material and isopropyl latter reagent was obtained in >99% purity by treatment of
acetylene (Scheme 3) suggesting that the reaction rate of the industrial product of technical grade with the basic
the desired nonaflate formation is far lower than the aqueous buffer. As compared to previously reported
following base-induced elimination of NfOH leading to methods, our synthesis provides broader scope of applica-
the alkyne.19
tion and higher yields of the requisite nonaflates from car-
bonyl compounds,9 and does not require the intermediacy
of trimethylsilyl enol ethers.7b The efficiency of the syn-
thesis described here along with the earlier observations
of higher reactivity of nonaflates as compared to the cor-
responding triflates in solvolysis22 and Pd-catalyzed
cross-coupling reactions4a,5b,23 should pave the way to
much broader application of the nonaflates24 in organic
synthesis.
NfF (1 equiv)
P2-base (1equiv)
ONf
O
Me
Me
P2-base
Me
Me
slow
fast
Me
Me
+
Me
O
ca. 50%
unconsumed
Me
Me
Me
Scheme 3
Synlett 2007, No. 18, 2907–2911 © Thieme Stuttgart · New York

2910
M. A. K. Vogel et al.
LETTER
Willaredt, J.; Dambacher, T.; Breuer, T.; Ottaway, C.;
Fletschinger, M.; Boele, J.; Fritz, H.; Putzas, D.; Rotter, H.
W.; Bordwell, F. G.; Satish, A. V.; Ji, G. Z.; Peters, E. M.;
Peters, K.; von Schnering, H. G.; Walz, L. Liebigs Ann.
1996, 7, 1055; and references cited therein.
Acknowledgment
Financial support by the Agency for Science Technology and Re-
search (A*STAR Singapore) is gratefully acknowledged. We are
indebted to Lanxess Deutschland GmbH for the generous donation
of nonafluorobutane-1-sulfonyl fluoride.
(15) This mode of reactivity termed as internal quenching was
originally described by Corey and Gross for highly regio-
and stereoselective synthesis of silyl enolates by
References and Notes
deprotonation of carbonyl compounds with lithium
dialkylamide bases in the presence of trialkylsilyl chlorides.
See: Corey, E. J.; Gross, A. W. Tetrahedron Lett. 1984, 25,
495.
(1) (a) Scott, W. J.; McMurry, J. E. Acc. Chem. Res. 1988, 21,
47. (b) Ritter, K. Synthesis 1993, 735. (c) Cacchi, S. Pure
Appl. Chem. 1996, 68, 45. (d) Brückner, R.; Suffert, J.
Synlett 1999, 657.
(2) For Heck reaction, see: (a) Webel, M.; Reissig, H.-U.
Synlett 1997, 1141. (b) Bräse, S. Synlett 1999, 1654.
(c) Lyapkalo, I. M.; Webel, M.; Reissig, H.-U. Eur. J. Org.
Chem. 2001, 4189. (d) Lyapkalo, I. M.; Webel, M.; Reissig,
H.-U. Eur. J. Org. Chem. 2002, 3646. (e) Högermeier, J.;
Reissig, H.-U.; Brüdgam, I.; Hartl, H. Adv. Synth. Catal.
2004, 346, 1868. (f) Lyapkalo, I. M.; Högermeier, J.;
Reissig, H.-U. Tetrahedron 2004, 60, 7721. (g) Vogel, M.
A. K.; Stark, C. B. W.; Lyapkalo, I. M. Adv. Synth. Catal.
2007, 349, 1019.
(16) While THF is more convenient and environmentally benign
solvent, DMF is found to be advantageous for the Pd-
catalyzed cross-couplings of alkenyl nonaflates.2g,5b,8 Hence,
should one choose to carry out a subsequent coupling
reaction without isolation of the nonaflate 4, DMF is a
preferable solvent for the one-pot nonaflation–coupling
sequence.
(17) General Procedure: A one-necked round-bottomed
reaction flask equipped with a three-way tap and a teflon-
coated magnetic stirring bar was heated with a heat-gun
under vacuum for a few minutes and then cooled under an
atmosphere of dry argon. A solvent (1 mL), a carbonyl
compound 3 (1.00 mmol) and NfF (1.15 mmol) were
successively added via syringe into the reaction flask. The
mixture was cooled to 0 °C under vigorous stirring before
P-base (1.15 mmol) was added dropwise. The three-way tap
was quickly replaced with a glass stopper, and the reaction
mixture was stirred at r.t. unless stated otherwise for
a-methylcycloalkanones and aldehydes 3j,k (see Table 1).
After the carbonyl compound 3 had been fully consumed (1H
NMR control), the resulting mixture was quenched with H2O
(5 mL) and extracted with pentane (4 × 25 mL). The
combined organic phase was washed with H2O (20 mL) and
dried (MgSO4). After the volatiles were removed carefully
under reduced pressure on a rotary evaporator (≥ 100 mbar
for 4a; £20 °C water-bath temperature for all the
(3) For Negishi reaction, see: Bellina, F.; Ciucci, D.; Rossi, R.;
Vergamini, P. Tetrahedron 1999, 55, 2103.
(4) For Stille reaction, see: (a) Wada, A.; Ieki, Y.; Ito, M.
Synlett 2002, 1061. (b) Wada, A.; Ieki, Y.; Nakamura, S.;
Ito, M. Synthesis 2005, 1581.
(5) For Suzuki reaction, see: (a) Högermeier, J.; Reissig, H.-U.
Synlett 2006, 2759. (b) Högermeier, J.; Reissig, H.-U.
Chem. Eur. J. 2007, 13, 2410.
(6) For Sonagashira reaction, see ref. 4 and: (a) Suffert, J.;
Eggers, A.; Scheuplein, S. W.; Brückner, R. Tetrahedron
Lett. 1993, 34, 4177. (b) Okauchi, T.; Yano, T.; Fukamachi,
T.; Ichikawa, J.; Minami, T. Tetrahedron Lett. 1999, 40,
5337. (c) Lyapkalo, I. M.; Vogel, M. A. K. Angew. Chem.
Int. Ed. 2006, 45, 4019; Angew. Chem. 2006, 118, 4124.
(7) (a) Zimmer, R.; Webel, M.; Reissig, H.-U. J. Prakt. Chem.
1998, 340, 274; and references cited therein. (b) Lyapkalo,
I. M.; Webel, M.; Reissig, H.-U. Eur. J. Org. Chem. 2002,
1015. (c) Aulenta, F.; Reissig, H.-U. Synlett 2006, 2993.
(8) Lyapkalo, I. M.; Webel, M.; Reissig, H.-U. Synlett 2001,
1293.
compounds), the residue was subjected to flash
chromatography [silica gel, pentane for 4a–j, hexane–
EtOAc (1:1) for 4k] to give pure enol nonaflates 4 as
colorless or yellowish liquids.
(18) Kinetically controlled nonaflation of a-methylcyclo-
alkanones 3f,g and aldehydes 3j,k was carried out according
to the above procedure except that the temperature was kept
at –30 °C in the case of 3f,j,k, and at –20 °C in the case of
3g. For the conversion of aldehydes 3j,k, 1.08 equivalents of
P1-base was used.
(9) Subramanian, L. R.; Bentz, H.; Hanack, M. Synthesis 1973,
293.
(10) Bürger, H.; Heyder, F.; Pawelke, G.; Niederprüm, H. J.
Fluorine Chem. 1979, 13, 25l.
(11) (a) Lithium amide bases readily react with NfF even at low
temperature to give the anticipated nonafluorobutane-1-
sulfonamides: Lyapkalo, I. M.; Reissig, H.-U.; Schäfer, A.;
Wagner, A. Helv. Chim. Acta 2002, 85, 4206. (b) For this
reason, only the stepwise procedure is possible. See refs. 5b
and 8.
(12) Trialkylamines are not basic enough to promote the
nonaflation whereas DBU gives low conversions due to the
side reaction with NfF. The results will be reported in a
subsequent full account.
(13) It has the highest basicity (pKa = 28.35 in MeCN) among
commercially available P1-bases: Schwesinger, R.;
Willaredt, J.; Schlemper, H.; Keller, M.; Schmitt, D.; Fritz,
H. Chem. Ber. 1994, 127, 2435.
(14) (a) Commercially available as a 2 M solution in THF (pKa =
33.49 in MeCN). (b) According to the primary
classification given by Schwesinger, the subscript
designates the number of P-atoms in the molecule. For the
synthesis and properties of P2- or higher-order phosphazene
bases, see: Schwesinger, R.; Schlemper, H.; Hasenfratz, C.;
(19) A general synthesis of alkynes or allenes from acyclic
ketones and NfF depending on the base employed and the
structural features of the ketones will be described by us
elsewhere: Vogel, M. A. K.; Stark, C. B. W.; Lyapkalo, I.
M.; manuscript in preparation.
(20) Spectroscopic Data: 1H NMR (400.23 MHz) and 13C NMR
(100.65 MHz) data in CDCl3 (d in ppm from internal SiMe4)
of the selected products 4 are given below.
4b: 1H NMR (400.23 MHz, CDCl3): d = 0.99 (d, 3J = 6.4 Hz,
3 H, Me), 1.39–1.49 (1 H), 1.68–1.87 (3 H), 2.20–2.34 (2 H),
2.35–2.45 (1 H) (all m, 3 × CH2, CHMe), 5.73 (m, 1 H,
CH=). 13C NMR (100.65 MHz, CDCl3): d = 20.6 (Me), 27.3
(CHMe), 27.4, 30.6, 32.0 (all CH2), 118.0 (CH=C), 149.3
(CH=C). 4e: 1H NMR (400.23 MHz, C6D6): d = 2.99 (s, 2 H,
CH2), 6.17 (s, 1 H, CH=), 6.86–6.89 (1 H), 6.94–7.08 (all m,
3 H, CHAr). 13C NMR (100.65 MHz, C6D6): d = 37.5 (CH2),
119.8 (CH=C), 122.4, 123.9, 126.3, 127.3 (all CHAr), 137.7,
140.3 (CAr), 153.7 (CH=C). 4h: 1H NMR (400.23 MHz,
CDCl3): d = 1.13 (t, 3J = 7.2 Hz, 3 H, Me), 2.30–2.36 (m, 2
Synlett 2007, No. 18, 2907–2911 © Thieme Stuttgart · New York

LETTER
Alkenyl Nonaflates from Ketones and Aldehydes: Scope and Limitations
2911
H, CH2), 2.56 (q, 3J = 7.2 Hz, 2 H, NCH2Me), 2.58 (t, 3J =
(21) Although we did not aim at recovering phosphazene bases
from side products, phosphazenium fluorides, the bases
could be recovered via crystallization of their salts followed
by treatment with a sufficiently strong inorganic base.13,14
We are currently working on the procedure employing
catalytic quantities of the phosphazene bases in combination
with heterogeneous strong inorganic base NaH or KH.
(22) Subramanian, L. R.; Hanack, M. Chem. Ber. 1972, 105,
1465.
(23) Rottländer, M.; Knochel, P. J. Org. Chem. 1998, 63, 203.
(24) Nonaflate anion can easily be precipitated from aqueous
solutions in form of a stable, non-hygroscopic potassium salt
which is described as an efficient flame retardant additive in
transparent polycarbonate compositions. See: (a) Singh, R.
K.; Lin, Y.-G. U.S. Patent Appl., US 2005009968, 2005.
(b) Singh, R. K.; Rosenquist, N. R.; Fischer, J. M. U.S.
Patent Appl., US 6462111, 2002.
5.8 Hz, 2 H, NCH2CH2), 3.14 (m, 2 H, NCH2C=), 5.85 (m, 1
H, CH=C). 13C NMR (100.65 MHz, CDCl3): d = 12.3 (Me),
24.3 (CH2), 48.5, 51.4, 52.6 (all NCH2), 116.6 (CH=C),
146.2 (CH=C). (Z)-4k: 1H NMR (400.23 MHz, CDCl3): d =
1.70 (m, 2 H, CH2CH2CH2), 2.14 (s, 3 H, MeCO), 2.22 (q,
3J = 7.6 Hz, 2 H, CH2CH=), 2.46 (t, 3J = 7.3 Hz, 2 H,
COCH2), 5.23 (td, 3J = 5.6, 7.6 Hz, 1 H, CH=CHONf), 6.61
(br d, 3J = 5.6 Hz, 1 H, CH=CHONf). 13C NMR (100.65
MHz, CDCl3): d = 22.4, 23.6, 42.6 (all CH2), 29.9 (Me),
119.4 (CH=CHONf), 136.2 (CH=CHONf), 208.0 (C=O).
(E)-4k: 1H NMR (400.23 MHz, CDCl3, non-overlapping
signals only): d = 2.08 (q, 3J = 7.7 Hz, 2 H, CH2CH=), 5.74
(dt, 3J = 7.7, 11.8 Hz, 1 H, CH=CHONf), 6.56 (br d, 3J = 11.8
Hz, 1 H, CH=CHONf). 13C NMR (100.65 MHz, CDCl3): d
= 22.6, 26.0, 42.3 (all CH2), 30.0 (Me), 121.7 (CH=CHONf),
136.8 (CH=CHONf), 207.9 (C=O).
Synlett 2007, No. 18, 2907–2911 © Thieme Stuttgart · New York

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