CF3-Containing para-Quinone Methides for Organic Synthesis

Article abstract of DOI:10.1002/ejoc.202000295

A new family of CF₃-containing para-quinone methides (CF₃-QMs) was systematically investigated for its suitability in organic synthesis. Addition of different nucleophiles gives access to target molecules with a benzylic CF₃-containing stereogenic center straightforwardly. The electrophilicity parameter E of the prototypical CF₃-QM 2,6-di-tert-butyl-4-(2,2,2-trifluoroethylidene)cyclohexa-2,5-dien-1-one was determined to be –11.68 according to the Mayr scale, making it one of the most reactive quinone methides known so far.

Full text of DOI:10.1002/ejoc.202000295

A Journal of
Accepted Article
Title: CF3-Containing para-Quinone Methides for Organic Syntheses
Authors: Michael Winter, Roman Schütz, Andreas Eitzinger, Armin R.
Ofial, and Mario Waser
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10.1002/ejoc.202000295
European Journal of Organic Chemistry
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CF3-Containing para-Quinone Methides for Organic Syntheses
Michael Winter,[a] Roman Schütz,[a] Andreas Eitzinger,[a] Armin R. Ofial,[b] and Mario Waser*[a]
Abstract: A new family of CF3-containing para-quinone methides
(CF3-QMs) was systematically investigated for its suitability in organic
synthesis. Addition of different nucleophiles gives access to target
apart from these two reports, such potentially useful building
blocks have not been reported and utilized for further
transformations until very recently, when we described the first
synthesis of the CF3-QM 1a.[13,14] Starting from the bulk chemical
2, p-QM 1a could be obtained in two steps via a benzylic
trifluoromethylation first,[15] followed by oxidation to the QM
(Scheme 1C).[13,16] This compound was then successfully used in
the asymmetric phase-transfer catalyst (PTC) controlled reaction
with pronucleophile 3 to access the masked b2,2-amino acid
derivative 4 with very high levels of stereoselectivity.[13]
molecules with
a benzylic CF3-containing stereogenic center
straightforwardly. The electrophilicity parameter E of the prototypical
CF3-QM 2,6-di-tert-butyl-4-(2,2,2-trifluoroethylidene)cyclohexa-2,5-
dien-1-one was determined to be -11.68 according to the Mayr scale,
making it one of the most reactive quinone methides known so far.
Considering this very promising initial application of the novel
building block 1a, we now became interested in investigating this
novel quinone methide-platform more systematically (Scheme
1B). Besides utilizing 1a for a variety of different transformations,
we also wanted to install alternative groups R at the phenol part
and we became interested in determining the electrophilicity
parameter E for at least one (stable) derivative.
Introduction
para-Quinone methides (p-QMs) have emerged as versatile
reagents for a variety of different (asymmetric) transformations
over the last years.[1] Most commonly, arylidene-based p-QMs
(Ar-QMs) have been utilized as highly electrophilic acceptor
molecules,[2] which easily undergo 1,6-addition reactions with a
multitude of C- or hetero-atom nucleophiles under a variety of
(catalytic) conditions (Scheme 1A).[1,3-6] Surprisingly however,
alternative ylidene groups have been much less explored so far.[1]
Given the broad interest in CF3-containing (chiral) organic
molecules,[7,8] we reasoned that the introduction and systematic
investigation of CF3-based para-quinone methides (CF3-QMs)
may be a worthwhile task to establish a new platform of versatile
prochiral starting materials (Scheme 1B).[8] While racemic
trifluoromethylation reactions of arylidene-based Ar-QMs have
been reported before,[9] reactions of the CF3-containing acceptor
compounds CF3-QMs with different nucleophiles would result in
an unprecedented and complementary synthesis strategy to
access
a
broad variety of (chiral) target molecules
straightforwardly. Furthermore, to get a more comprehensive
understanding about the reactivity of such novel electrophiles and
to predict new reactions thereof as well as to obtain a direct
comparison with the well-established Ar-QMs,[2] it would be
beneficial to determine the electrophilicity parameter E (according
to Mayr’s free energy relationship equation 1[10]) of at least one
new CF3-QM derivative.
Scheme 1. A) General reactivity mode of p-QMs; B) The herein targeted
investigations concerning novel CF3-QMs; C) Our previous report describing the
synthesis of CF3-QM 1a and its use in a single asymmetric transformation[13]
.
Interestingly, in 1985 already Umemoto and co-workers described
the formation of a CF3-containing p-QM[11] and the intermediate
formation of CF3-containg p-QMs was also observed in reactions
of trifluoroethanol-containing phenols some years ago,[12] but
Results and Discussion
E-Parameter.
Arylidene-based p-quinone methides Ar-QMs have been studied
by Mayr’s group several years ago. They analysed the second-
order rate constants of the reactions of Ar-QMs with several
nucleophiles[2] according to the following linear free energy
relationship equation (Eqn. 1).[10]
[a]
[b]
DI M. Winter, R. Schütz, DI A. Eitzinger, Prof. M. Waser
Johannes Kepler University Linz, Institute of Organic Chemistry
Altenbergerstraße 69, 4040 Linz, Austria
Email: mario.waser@jku.at
www.jku.at/orc/waser
Dr. A. R. Ofial
Equation 1: lg k(20 °C) = sN (N + E)
Department Chemie, Ludwig-Maximilians-Universität München,
Butenandtstraße 5–13, 81377 München, Germany
The availability of electrophilicity parameters E for certain
acceptor molecules, such as Ar-QMs or CF3-QMs, not only allows
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/ejoc.202000295
European Journal of Organic Chemistry
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for
a reactivity comparison between different electrophilic
compounds, but also provides a tool for the reliable prediction of
novel reactions upon considering the E-parameter of the
electrophile and the N-parameter of the nucleophile.[10] In general,
if (E + N) > -5 is fulfilled, a certain electrophile/nucleophile-
combination can be expected to react at room temperature.
Thus, we started our investigations by determining the E-
parameter of the stable CF3-QM 1a by studying the kinetics of its
reactions with the carbanions 5a–d[2a,17] as reference nucleophiles
in DMSO at 20 °C (Scheme 2).
Figure 1. Electrophilicity E of 1a in comparison with established arylidene-
based p-QMs[2]
.
Syntheses of CF3-QMs.
For the synthesis of the tBu-containing 1a our recently developed
Scheme 2. Kinetic measurements of the reactions of 1a with the reference
nucleophiles 5a (Solvent: DMSO; counteranion for 5a-d; K+).
strategy starting from phenol
2 turned out to be highly
reproducible and was easily carried out on several gram scale
(Scheme 3A).[13] Hereby, the CF3-group was first installed by
means of a benzylic trifluoromethylation of 2 using Togni’s second
generation reagent 7[18] in analogy to a recently published
procedure.[16] Conversion of 8 to the quinone methide 1a was then
achieved by employing 2,3-dichloro-5,6-dicyano-benzoquinone
(DDQ) as an oxidant.
The CF3-QM 1a has a lmax of 292 nm (in DMSO) and a molar
absorption coefficient of 2.5 × 104 M–1 cm–1, which enabled us to
follow its reactions with the colourless nucleophiles 5a–d to the
hypsochromically shifted phenol derivatives 6a-d photometrically
(5a-d were generated by deprotonation of the pronucleophiles 5-
H with 0.5 equiv. of KOtBu).[16] As well established for other
nucleophile/electrophile combinations before,[2,10,17] the reaction
kinetics were determined by employing stopped-flow UV/Vis
photometry to follow the fading of the coloured 1a upon reaction
with a large excess of the carbanions 5 (resulting in absorbance
decays that follow first-order kinetics). Based on these
measurements, it was then possible to calculate the
corresponding experimental second-order rate constants for
these four reactions.[2,10,16] By using these rate constants together
with the known parameters (N, sN) of our reference
nucleophiles,[2,17] it was finally possible to determine the
electrophilicity parameter E for CF3-QM 1a being -11.68.[16] By
comparing this value with E-parameters for well-known tert-butyl-
substituted Ar-QMs[2] (Fig. 1), it becomes obvious that the
presence of the CF3-group significantly boosts the electrophilicity
of 1a, which makes it a very promising building block for further
transformations: The CF3-QM 1a should be capable of reacting
with various types of nucleophiles provided that their nucleophilic
reactivity parameter N exceeds the value of +7.
Unfortunately, this route was found to be not suited for other
phenol derivatives (e.g. when using mesitylene derivatives to
access QM 1b, the radical trifluoromethylation was not site-
selective and the subsequent purification and oxidation turned out
to be rather problematic). Therefore, we opted for a different
strategy to access the alternatively substituted QMs 1b-e next. It
was shown before,[12,19] that phenol 9b can be converted into the
CF3-containing benzylic chloride 12b in a two-step procedure by
first carrying out an SEAr-reaction with trifluoroacetaldehyde
hemiacetale 10 (giving alcohol 11b), followed by chlorination
using SOCl2 (Scheme 3B). Interestingly, compound 12b already
contains traces of the corresponding QM 1b and we found that
treatment with Et3N allows for the direct formation of 1b with a
reasonable isolated yield of 54%. It should however be noted that
the dimethyl-QM 1b was found to be significantly less stable than
the di-t-butyl-QM 1a, making its purification and isolation a difficult
task. The benzylic chlorides 12c-e could be accessed
analogously from the corresponding phenols 9c-e, but in neither
of these cases was it possible to isolate the final QMs 1c-e upon
treatment with base. Interestingly however, as compounds 12c-e
degraded quickly under basic conditions, and given the fact that
we were able to detect traces of QMs 1 in crude samples of
precursors 12 already, it seems very likely that the less-stable
QMs 1c-e are actually formed from 12c-e under basic conditions
and that it may be possible to generate and utilize them for further
transformations in situ (vide infra).
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10.1002/ejoc.202000295
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access to a variety of trifluoromethylated functionalized phenol
derivatives. The moderate yields obtained for products 6 can to
some extent be rationalized by a certain sensitivity of these
products under basic reaction conditions in the presence of an
excess of nucleophile. The synthesis of indole derivative 13a
performed well under established Lewis acid (BF3.OEt2)-mediated
conditions.[20] In sharp contrast, when reacting 1a with indole in
the absence of any Lewis acid, no reaction occurs. This result is
in perfect accordance with the hypothesis that
a given
electrophile/nucleophile-combination can be expected to react at
room temperature if (E + N) > -5,[10] as the experimentally
determined nucleophilicity of indole (N < 5.6)[21] is too low to fulfil
this requirement in combination with 1a (E = -11.68) in the
absence of any Lewis acid. It should be noted that we also tested
the addition of NaBH4 and NaBD4 to QM 1a, which resulted in
quantitative formation of precursor 8 and the monodeuterated 8-
D respectively.[16]
Having established that the most stable QM 1a reacts well with a
variety of different nucleophiles, we next focused on the use of
QM 1b (preformed and/or in situ formed from 12b) and on QM-
precursors 12c-e (Scheme 5).[16] First experiments carried out
with 1b allowed for the straightforward synthesis of the glycine
Schiff base-containing 17b and the indole-based 13b. The later
reaction again required the use of BF3.OEt2 as a Lewis acid,
indicating that the dimethyl QM 1b is not significantly more
electrophilic than the di-t-butyl QM 1a (vide supra). We next tested
if it may be possible to use QM precursors like 12d directly for the
reaction with indole. When 12d was reacted with indole and
BF3.OEt2 in the absence of any base, no product 13d was formed.
However, when 12d was first stirred with one equivalent of Et3N,
before adding indole and BF3.OEt2, the target 13d was obtained
straightforwardly. This strongly supports our hypothesis that the
in situ formation and utilization of QMs 1 from precursors 12 is
indeed possible. Thus, we next reacted 12b-e with diethyl
malonate and two different amines in the presence of Cs2CO3,
which allowed for the direct formation of the products 6e-h and
14d-k in reasonable yields as well.
Scheme 3. Syntheses strategies to access CF3-QMs 1a-e.
We also tested if the SEAr strategy outlined in Scheme 3B may be
applicable to the corresponding di-tBu-phenol 9a, but surprisingly
in this case the yields were rather low and unpractical (compared
to the route outlined in Scheme 3A).
Application Scope.
As the tBu-QM 1a was found to be the most stable of the so far
accessed CF3-QM derivatives 1, we started our investigations of
the application scope by reacting 1a with a variety of C- and
heteroatom-nucleophiles (Scheme 4).
Scheme 4. Racemic addition of different nucleophiles to QM 1a.[16]
All the racemic products shown in Scheme 4 were obtained
straightforwardly under operationally simple conditions,[16] giving
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10.1002/ejoc.202000295
European Journal of Organic Chemistry
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Scheme 5. Reactions of QM 1b and QM-precursors 12b-e.[16]
Scheme 6. Asymmetric transformations of 1a and debutylation/deprotection
strategy for the synthesis of the free amino acid 19.[16]
Finally, as QM 1a could be successfully employed for the highly
asymmetric phase transfer-catalysed synthesis of the masked
b
2,2-amino acid derivative 4 (Scheme 6A),[13] we also became
We have recently shown that Ar-QMs can also undergo highly
enantioselective spirocyclopropanation reactions with chiral
ammonium ylides (formed in situ upon deprotonation of
ammonium salts 20).[6e] While the addition of achiral ammonium
salts 20 (with R3N = Me3N) to CF3-QM 1a yielded the racemic
trans-spirocyclopropans 21a and 21b straightforwardly (Scheme
6C), the asymmetric synthesis turned out to be more difficult (i.e.
compared to the highly asymmetric spirocyclopropanation of Ar-
QMs[6e]). When testing different chiral ammonium ylide-precursors
20, we found that Cinchona alkaloids allow for some moderate
enantioselectivity in the synthesis of 21a only, but unfortunately
the synthesis of 21b turned out to be rather sluggish and
unselective. While this example demonstrates the sometimes
significant reactivity difference of achiral and chiral ammonium
ylides in asymmetric cyclization reactions,[24,25] it still serves as
another illustrative example about the general use of CF3-QMs 1
for asymmetric transformations.
interested in testing the two other asymmetric transformations
depicted in Scheme 6. First, 1a was reacted with the glycine Schiff
base 18[22] under phase-transfer conditions to access the
protected a-amino acid derivative 17a. The asymmetric addition
of nucleophiles 18 to Ar-QMs was recently reported by the groups
of Fan and Deng,[23] and in analogy to Fan’s work, the use of
Cinchona alkaloid-based chiral PTCs, i.e. ammonium salt B,
allowed for reasonable selectivities for the addition of 18 to 1a
(Scheme 6B). Unfortunately, it was not possible to assign the
relative and absolute configuration of enantioenriched 17a as we
were not able to obtain single crystals of sufficient quality for a
detailed X-ray analysis. However, it was easily possible to fully
deprotect and debutylate 17a under acidic conditions, which gave
a direct entry to b-CF3-a-tyrosine 19 (Scheme 6B).
CF2H-Quinone Methide.
Difluoromethyl (CF2H)-containing (chiral) molecules have
attracted more and more attention recently,[26] especially because
CF2H-containing compounds show unique properties like e.g. H-
bonding capability, in comparison to analogous CF3-
derivatives.[27] This results in a powerful tool to alter the properties
of a given structural motif by subtle structural changes only, and
we were therefore interested to obtain a first proof-of-concept if
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10.1002/ejoc.202000295
European Journal of Organic Chemistry
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our herein developed strategy to access and utilize the CF3-QMs
1b-e in situ, starting from the benzylic chlorides 12b-e, may also
be useful to access analogous and so far unprecedented CF2H-
QMs.
Experimental Section
General details can be found in the online supporting information. This
document also contains detailed synthesis procedures and analytical data
of all novel compounds and reaction products, details about the kinetic
investigations to determine the E parameter, as well as copies of NMR
spectra and HPLC traces.
Synthesis of QM 1a:
Step 1: CuI (190 mg; 1 mmol) and Togni’s reagent 7 (4.74 g; 15 mmol)
were dissolved in 50 mL DMF. Then the phenol 2 (2.2 g; 10 mmol) was
added and the mixture was stirred at 40 °C. After 1 h the reaction mixture
was diluted with EtOAc and washed with NaHCO3. The organic layer was
dried over Na2SO4, filtered, and concentrated in vacuo. The crude product
was purified by column chromatography on silica gel (Heptanes/EtOAc =
20:1) to yield the product 8 in 91% (2.62 g; 9.1 mmol).
Step 2: The trifluoroethylated phenol 8 (2.88 g; 10 mmol) was dissolved in
200 mL MeOH and DDQ (5.67 g; 25 mmol) was added. The reaction
mixture was stirred at room temperature for 1 h. After completion of the
reaction, the solvent was evaporated and the crude reaction mixture was
purified by column chromatography (heptanes/CH2Cl2 = 10:1) to afford
product 1a in 84% yield (2.4 g; 8.4 mmol).
Scheme 7. A) Synthesis route to access the QM-precursor 12b-CF2H and B)
applications thereof.[16]
Analytic details of 1a: m.p. = 34.5 – 35.0 °C; 1H NMR (700 MHz, CDCl3,
298 K): δ = 7.33 (s, 1H), 6.78 (s, 1H), 6.03 (q, J = 8.9 Hz, 1H), 1.29 (s, 9H),
1.28 (s, 9H) ppm; 19F-NMR (282 MHz, CDCl3, 298 K): δ = -55.40 (d, J =
8.9 Hz, 3F) ppm; 13C-NMR (125 MHz, CDCl3, 298 K): δ = 186.2, 151.7,
151.4, 138.4 (q, J = 5.5 Hz), 132.4, 125.3, 124.1 (q, J = 34.7 Hz), 123.0 (q,
J = 271.0 Hz), 35.8, 35.4, 29.5 ppm. HRMS (ESI): m/z calculated for
C16H21F3O: 317.1734 [M-H+MeOH]-; found: 317.1730.
The difluoromethyl-based hemiacetale 22 is commercially
available and we were glad to see that the synthesis of the QM-
precursor 12b-CF2H could be carried in the same manner
(Scheme 7A) as for the CF3-analogs (compare with Scheme 3B).
NMR of purified 12b-CF2H showed around 10-15% of the target
quinone methide 1b-CF2H, but unfortunately we were not able to
isolate this QM after treatment with base (decomposition).
Nevertheless, it was again possible to use the precursor 12b-
CF2H directly for further manipulations, as demonstrated for the
synthesis of the two CF2H-phenol derivatives 6e-CF2H and 14db-
CF2H (Scheme 7B).
General Synthesis of QM-precursors 12 and Synthesis of QM 1b:
The corresponding phenol
9 (30 mmol) and trifluoroacetaldehyde
hemiacetale 10 (30 mmol) were mixed and K2CO3 (1.5 mmol) was added.
The mixture was heated to 60 °C and stirred for 16 h. After cooling to room
temperature, the reaction mixture was dissolved with ethyl acetate and
washed with ammonium chloride, H2O and brine. The organic layer was
dried over Na2SO4 and evaporated to dryness. The crude products 11
were directly used for the next step.
Conclusions
Step 2: A mixture of compound 11 (10 mmol) and SOCl2 (14 mmol) in 15
mL dry toluene was cooled to 0-5 °C and pyridine (10 mmol) was added.
After 1 h the mixture was heated to 70 °C and stirred for another 2 h. After
cooling to room temperature, the mixture was poured on 20 g ice and
stirred for another 30 min. Then the organic layer was separated and the
aqueous layer was extracted with ethyl acetate twice. The combined
organic layers were dried over Na2SO4 and evaporated to dryness. The
crude reaction mixture was purified by column chromatography
(CH2Cl2/heptanes = 2:1) giving the products 12 in the yields reported in
Scheme 3.
A new family of either preformed, or in situ generated, CF3-
containing para-quinone methides was developed and
systematically investigated for their use as starting materials to
access phenol derivatives with
a CF3-substituted benzylic
stereogenic center. In addition, we determined the electrophilicity
parameter E (according to Mayr’s linear free energy relationship
equation) for one derivative being -11.68. This is one of the
highest E parameters for a quinone methide so far, making these
quinone methides very useful and highly reactive building blocks
that undergo 1,6-addition reactions with a variety of C- and
heteroatom nucleophiles straightforwardly. In addition, it was also
shown that an analogous novel CF2H-QM precursor can be
accessed in a similar manner and utilized directly for further
manipulations.
Step 3 (Synthesis of 1b): Precursor 12b (1.19 g; 5 mmol) was dissolved in
20 mL CH2Cl2 and triethylamine (0.76 mL; 5.5 mmol) was added. The
reaction mixture was stirred at room temperature overnight (completion
shown by TLC) and H2O was added. The organic layer was washed with
1 N HCl followed by NaHCO3 (sat.) and brine. After drying the organic layer
over Na2SO4 it was evaporated to dryness and the quinone methide 1b
was isolated by column chromatography (heptanes/EtOAc = 20:1 - 1:1) in
54% yield (0.55 g; 2.7 mmol).
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Analytic details of 1b: 1H NMR (300 MHz, CDCl3, 298 K): δ = 7.34 (s, 1H),
6.84 (s, 1H), 6.00 (q, J = 9.0 Hz, 1H), 2.05 (s, 3H), 2.02 (s, 3H) ppm; 19F-
NMR (282 MHz, CDCl3, 298 K): δ = -55.40 (d, J = 9 Hz, 3F) ppm; 13C-NMR
(125 MHz, CDCl3, 298 K): δ = 186.9, 140.0, 139.4, 137.8 (q, J = 5.5 Hz),
135.8, 128.7, 124.0 (q, J = 34.8 Hz), 122.7 (q, J = 271.7 Hz), 16.8, 16.2
ppm. HRMS (ESI): m/z calculated for C10H9F3O: 201.0533 [M-H]-; found:
201.0535.
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3
Analytic details of 12c: 1H NMR (300 MHz, CDCl3, 298 K): δ = 7.60 (s, 2H),
6.07 (s, 1H), 5.00 (q, J = 6.7 Hz, 1H) ppm; 19F-NMR (282 MHz, CDCl3, 298
K): δ = -73.45 (d, J = 6.7 Hz, 3F) ppm; 13C-NMR (125 MHz, CDCl3, 298
K): δ = 151.1, 132.5, 128.7, 123.2 (q, J = 278.8 Hz), 110.2, 50.0 (q, J =
34.4 Hz) ppm. HRMS (ESI): m/z calculated for C8H4Br2ClF3O: 366.8342
[M+H]+; found: 366.8348..
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Analytic details of 12d: 1H NMR (300 MHz, CDCl3, 298 K): δ = 7.22 (s, 1H),
7.17 (s, 1H), 5.03 (q, J = 7.0 Hz, 1H), 2.27 (s, 3H), 1.42 (s, 9H) ppm; 19F-
NMR (282 MHz, CDCl3, 298 K): δ = -73.21 (d, J = 7.0 Hz, 3F) ppm; 13C-
NMR (125 MHz, CDCl3, 298 K): δ = 154.2, 138.0, 136.1, 129.2, 128.8,
128.4, 126.1, 125.4, 123.7, 123.7 (q, J = 279.1 Hz), 123.5, 59.2 (q, J =
34.1 Hz), 34.7, 29.7, 16.1 ppm. HRMS (ESI): m/z calculated for
C13H16ClF3O: 281.0915 [M+H]+; found: 281.0915.
4
Analytic details of 12e: 1H NMR (300 MHz, CDCl3, 298 K): δ = 6.71 (s, 2H),
5.65 (s, 1H), 5.03 (q, J = 6.7 Hz, 1H), 3.92 (s, 6H) ppm; 19F-NMR (282
MHz, CDCl3, 298 K): δ = -73.20 (d, J = 6.7 Hz, 3F) ppm; 13C-NMR (125
MHz, CDCl3, 298 K): δ = 147.1, 136.4, 129.1, 128.4, 125.4, 123.8 (q, J =
282.8 Hz), 123.1, 105.8, 59.3 (q, J = 35.0 Hz), 56.6 ppm. HRMS (ESI): m/z
calculated for C10H10ClF3O3: 271.0343 [M+H]+; found: 271.0339.
5
6
Hydride addition: T. Pan, P. Shi, B. Chen, D.-G. Zhou, Y.-L. Zeng, W.-D. Chu, L.
He, Q.-Z. Liu, C.-A. Fan, Org. Lett. 2019, 21, 6397-6402.
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Acknowledgements
This work was generously supported by the Austrian Science
Funds (FWF): Project No. P30237 (financed by the Austrian
National Foundation for Research, Technology and Development
and Research Department of the State of Upper Austria). The
used NMR spectrometers were acquired in collaboration with the
University of South Bohemia (CZ) with financial support from the
European Union through the EFRE INTERREG IV ETC-AT-CZ
program (project M00146, "RERI-uasb")."). We thank Robert J.
Mayer (LMU München) for helpful discussions.
7
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8
9
Keywords: Quinone Methides
•
Trifluoromethylation
•
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12 The intermediate formation of a similar CF3-QM was observed in reactions of
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Novel CF3-containing para-quinone methides were
systematically investigated for their use in (asymmetric)
transformations.
Quinone Methides
Michael Winter, Roman
Schütz, Andreas Eitzinger,
Armin R. Ofial and Mario
Waser*
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CF3-Containing para-
Quinone Methides for
Organic Syntheses
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