“This is the peer reviewed version of the following article: [Organocatalytic Strategies to Stereoselectively Trap
Photochemically Generated Hydroxy-o-quinodimethanes], which has been published in final form at DOI:
10.1002/ejoc.201800081. https://doi.org/10.1002/ejoc.201800081 This article may be used for non-commercial
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Organocatalytic Strategies to Stereoselectively Trap Photochemically
Generated Hydroxy-o-quinodimethanes
Sara Cuadros,[a] and Paolo Melchiorre*[a,b]

Abstract: Light excitation of ortho-alkyl aromatic ketones and
aldehydes enables access to hydroxy-o-quinodimethanes. These
reactive electron-rich intermediates are sufficiently long-lived to
productively engage in chemical processes, mainly acting as dienes
in [4+2]-cycloadditions with electron-poor alkenes. Since the early
discovery of this photoenolization mechanism, which dates back to
1961, a variety of transformations has been developed, providing a
photochemical alternative to classical Diels-Alder chemistry.
However, enantioselective catalytic versions of the photenoliza

1.

tion/Diels-Alder sequence have remained elusive until recently. This
review describes how the field of enantioselective organocatalysis
has provided suitable tools to stereoselectively trap photochemically
generated hydroxy-o-quinodimethanes. Recent studies also
demonstrated that the chemistry is not limited to cycloaddition-type
manifolds, but it can be expanded to develop intermolecular
enantioselective addition processes.

Introduction

In recent decades, synthetic photochemistry has become highly
sophisticated.[1] Many light-driven reactions have been used to
build structurally complex organic molecules,[2] considerably
enriching the synthetic repertoire of modern chemists. One such
reaction is the photoenolization of ortho-alkyl aromatic ketones
and aldehydes 1 to afford transient hydroxy-o-quino-

dimethanes A (Figure 1a),[3] a venerable photochemical process
established in 1961.[4] The unique reactivity of the generated
photoenols A, which can act as dienes in [4+2]-cycloadditions
with electron-poor alkenes 2, provides a photochemical
alternative to classical Diels-Alder chemistry.[5] The resulting
chiral benzannulated carbocyclic products 3 are privileged cores
found in marketed drugs[6] and naturally occurring substances.[7]
Because of the versatility and the synthetic potential of the
photoenolization/Diels-Alder (PEDA) sequence, this light-driven
process has attracted great interest within the chemistry
community over the years.

1.1. Historical Background and Mechanistic Insights

Figure 1. a) The photochemical enolization process of ortho-alkyl aromatic
ketones and aldehydes 1 generating the reactive hydroxy-orthoquinodimethane A. The unique reactivity of photoenols A, which can act as
dienes in [4+2]-cycloadditions with electron-poor alkenes 2, provides access to
chiral cyclic adducts. (b) The pioneering work by Yang and Rivas [Ref. 4].

[a]

ICIQ - Institute of Chemical Research of Catalonia the Barcelona Institute of
Science and Technology,
Avenida Països Catalans 16 – 43007, Tarragona, Spain
E-mail: pmelchiorre@iciq.es
Homepage: http://www.iciq.org/research/research_group/prof-paolo-melchiorre/

[b]

ICREA - Catalan Institution for Research and Advanced Studies,
Passeig Lluís Companys 23 – 08010, Barcelona, Spain

In 1961, Yang and Rivas[4] were the first to recognize the ability
of ortho-alkyl aromatic ketones and aldehydes 1 to form the
corresponding hydroxy-ortho-quinodimethane A upon excitation
by ultraviolet light (Figure 1a). This photochemically generated
transient intermediate is long-lived enough (τ = 1-10-6 s)[8] to
allow chemical trapping with dienophiles 2 in a Diels-Alder
fashion, leading to valuable chiral products 3. In the original
study (Figure 1b), the formation of cycloadduct 6 was secured by
irradiation of an equimolar solution of 2-methylbenzophenone 4
and acetylene-dicarboxylate 5 with a high-intensity mercury
lamp.[4] This process served as the inspiration for developing
other photoenolization/Diels-Alder (PEDA) sequences using a
variety of dienophiles.[3]
The mechanism of formation of the photoenol intermediate A
(Figure 2) has been the subject of intense research.[9] Laser
flash photolysis[10] has allowed the characterization of the
transient species involved in the formation of A. Light absorption
by the carbonyl group in 1 generates a singlet excited state S1-B,
which decays to the triplet state T1-B by intersystem crossing

(ISC).[11] Upon 1,5-hydrogen transfer, which occurs in T1-B, the
1,4-diradical intermediate (Z)-C is formed. The 1,4-diradical
(Z)-C exists in fast equilibrium with its conformer (E)-C.[9f] Both
conformers can undergo an additional ISC process to yield the
corresponding (E)-A and (Z)-A dienols in the ground state. The
short-lived ground state dienol (Z)-A rapidly reverts to the
starting 2-alkylphenylketone 1 via a 1,5-sigmatropic hydrogen
transfer. In contrast, the reketonization of the photoenol (E)-A
requires an intermolecular proton transfer that may occur either
by protonation of the methylene group by an acid, or by proton
transfer from the enol to the solvent or to a base, followed by
carbon protonation of the dienol anion. [3c] Overall, the difficult
reketonization mechanism confers a longer lifetime to the
photoenol (E)-A. Its chemical trapping with a suitable dienophile
feasible. The lifetimes of all the transient species involved in the
photoenol formation are strongly solvent dependent and vary
considerably for different ortho-alkylbenzophenones.[3a,3c]

compounds belonging to the hamigerans family. The key step
here was the Diels-Alder reaction of photoenols, photogenerated
from substituted benzaldehydes.[7a-d]

Figure 3. Examples of naturally occurring substances synthesized through
PEDA sequences.

Figure 2. a) The photoenolization mechanism and the main transient species
involved in the formation of the reactive (E)-photoenol A. S1: singlet excited
state; T1: triplet excited state; ISC: Intersystem crossing.

1.2. Synthetic Applications and Aims of the Microreview
The utility of the PEDA sequence in synthetic endeavours has
been demonstrated in the total synthesis of naturally-occurring
and biologically active substances (Figure 3).[6,7,12] For example,
Kraus and co-workers used this light-driven process to
synthesize biologically active podophyllotoxin[13a] and pleutorin
derivatives.[13b] K. C. Nicolau successfully prepared a variety of

Despite the many applications based on PEDA processes
reported over more than 50 years,[3,7,14] enantioselective catalytic
variants have remained elusive until recently. In the context of a
Diels-Alder-type trapping of the photoenol A, control of the
relative stereochemistry is secured by the innate
stereospecificity of the cycloaddition process. Owing to the welldefined geometry of the reactive (E)-A (Figure 2) and upon
judicious choice of the dienophile geometry, the endo or exo
Diels-Alder products of type 3 (Figure 1a) can be predictably and
selectively formed. However, controlling the absolute
configuration of the products using catalytic methods has proven
challenging.
Here we discuss how the field of enantioselective
organocatalysis[15]
has
provided
suitable
tools
to
stereoselectively intercept photochemically generated hydroxy-oquinodimethanes. In addition, although the inherent reactivity of
A is permeated by its propensity to act as diene in [4+2]cycloaddition pathways, recent studies demonstrated that the
photogenerated enols can also engage in direct intermolecular
enantioselective addition processes, including Michael, [16]
Mannich,[17] and aldol-type[18] reactions (Figure 4). We will
discuss the strategies and the chiral organocatalytic

Sara Cuadros was born in Benidorm (Spain) and she graduated from the University of Alicante with distinction (2014). In 2015, she
completed her M.Sc. degree from the University Rovira i Virgili under the supervision of Prof. Paolo Melchiorre at ICIQ. After, she started her
Ph.D studies with a FPU-fellowship. In November 2017, she moved to Nagoya University (Japan) to carry out a research period under the
supervision of Prof. Takashi Ooi. Her doctoral studies are focused in the development of novel photo-organocatalytic asymmetric
transformations.

Paolo Melchiorre was born in 1973 in Italy. He earned his M.Sc. degree (1999) and then his PhD in Chemistry (2003) from Bologna
University under the supervision of A. Umani-Ronchi and P. G. Cozzi. After a research period with K. A. Jørgensen at Center for Catalysis,
Århus University (Denmark), he joined the research group of G. Bartoli at Bologna University, where he became Assistant Professor in 2007.
In 2009, Paolo moved to Tarragona (Spain) as an ICREA Research Professor and an ICIQ Senior Group Leader. His current scientific
interests lie on the discovery and mechanistic elucidation of new enantioselective organocatalytic and photochemical processes.

intermediates, which have been used to stereoselectively trap
the fleeting photoenol A.

chiral agent 10 and a reaction temperature as low as -60 °C to
afford the cycloaddition products 9 with fairly good yields,
excellent enantioselectivities and good diastereoselectivities.
Attempts were also conducted to use a chiral catalyst to
render the PEDA sequence stereoselective. However, these
attempts were largely unsuccessful. K. C. Nicolau used (R)BINOL-TiCl2 14 as the metal-based catalyst to intercept the
photoenol, generated upon excitation of aldehyde 11, with
methyl vinyl ketone 12 (Figure 6).[7c] The benzannulated product
13 was achieved with a measurable enantioselectivity (25% ee)
only when using a catalyst loading as high as 75 mol%.

Figure 4. Outline: this microreview discusses the stereoselective
organocatalytic trap of the photoenol A through different reactivity pathways: (i)
Diels-Alder-type; (ii) Michael-type; (iii) Mannich-type, and (iv) aldol-type
reaction.

2. Enantioselective Variant: Initial Attempts
Two fundamental issues have historically hampered the
development of a highly enantioselective Diels-Alder trapping of
the reactive photoenols A: (i) their fleeting nature (τ ≈ 1 – 10-6 s),
which complicates the stereoselective trapping event, and (ii) the
difficulty of avoiding racemic background reactions, which occur
by direct interception of the photoenol A from the dienophile
without the assistance of the chiral agent/catalyst.
One example of effective asymmetric method was reported
by Bach in 2003 (Figure 5).[19] In this strategy, a stoichiometric
amount of a complexing chiral agent 10 was used to bind a
purposely designed 2-alkyl carbonyl compound 7, adorned with
a lactam ring serving as a hydrogen-bond (H-bond) binding site.
Such H-bond interaction[20] secured an effective differentiation of
the two enantiotopic faces of the photochemically generated
photoenol 7’, thus channelling the PEDA sequence toward an
enantioselective pattern. The optimal conditions required a
stoichiometric amount (at least 1.2 equivalents) of the

Figure 5. The first highly enantioselective PEDA sequence required
stoichiometric amounts of the chiral complexing agent 10 [Ref. 19].

Figure 6. A low-enantioselective PEDA sequence using metal-based catalysis
[Ref. 7c].

These few precedents testified to the difficulty of developing
enantioselective catalytic variants of the photoenolization/DielsAlder sequence.

3. Organocatalysis for the Enantioselective
Diels-Alder Trapping of Photoenols
Recently, our research laboratories reported a strategy to fill this
gap in catalytic enantioselective methodology, demonstrating
that organocatalysis offers effective tools to stereoselectively
intercept the fleeting photenols. Our studies were motivated by
our interest in using chiral organic catalysts to control the
stereoselective outcome of photochemical reactions, in addition
to their established potential in promoting asymmetric thermal
reactions.[21] In developing a stereoselective catalytic PEDA
reaction, the main idea was to use a chiral organocatalyst that
could rely on multiple, non-covalent weak attractive interactions
to activate the dienophile. An extensive screening of different Hbond donor organocatalysts identified the cinchona-thiourea
17[22] as a suitable catalyst for efficiently mediating the
stereoselective Diels-Alder trapping of photoenols, derived from
o-alkylbenzophenones 15, with maleimides 16 (Figure 7).[23] An
array of functionalized tetrahydronaphthalenols 18 was accessed
in good-to-excellent yields and with high levels of
stereoselectivity. As a limitation of the method, the Nunprotected maleimide afforded the corresponding product 18f
with poor enantiocontrol (50% ee).
Further investigations revealed a more intricate, and perhaps
more interesting, mechanism than expected. During optimization
studies, we observed that the uncatalyzed racemic background
process between 2-methyl benzophenone 15a and maleimide
16a was significantly faster than the stereoselective reaction
catalyzed by the cinchona-thiourea 17 (Figure 8). Since reducing
the rate of the uncatalyzed pathway is generally crucial to

Figure 8. Reactivity of the PEDA reaction of 15a and 16a in the absence of
any catalyst (racemic background process), or in the presence of 20 mol% of
chiral cinchona-thiourea 17, N,N’-dicyclohexylthiourea 19, and quinuclidine 20.

Aside from the mechanistic implications, this study [23]
demonstrated that a readily available chiral organic catalyst
could address a longstanding and elusive problem in the realm
of photo-mediated enantioselective catalysis.

Figure 7. The first example of a highly enantioselective (organo)catalytic
PEDA sequence [Ref. 23].

successfully developing any photochemical catalytic asymmetric
reaction,[24] we tried to rationalize this puzzling observation.
Optical absorption spectroscopic studies excluded the formation
of any photoabsorbing substrate/catalyst 17 aggregation,
confirming that o-alkylbenzophenone 15 was responsible for the
absorption at 365 nm (the operative λ of the system). We then
performed kinetic measurements to investigate how the kinetic of
the PDA process was affected by the individual fragments of the
organocatalyst 17: the quinuclidine and the thiourea moieties.
We found that a catalytic amount of the achiral thiourea 19
accelerated the reaction, in consonance with a selective
activation of the maleimide 16a facilitating the trapping of the
photoenol (Figure 8). In contrast, quinuclidine 20 greatly inhibited
the process. The last observation can be explained with the
established ability of tertiary amines, [25] including quinuclidine,[26]
to quench the triplet state of benzophenones, the key precursor
intermediate in the formation of photoenols (T1-B in Figure 2).
Laser flash photolysis studies confirmed that the quinuclidine
core within catalyst 17 could reduce the formation of the
photoenol.[27]
Overall, these studies suggested that the cinchona-thiourea
17 played two opposite yet cooperative roles when promoting
the PEDA reaction (Figure 8). The quinuclidine moiety interfered
with the photoenolization mechanism, acting as an inhibitor of
the PEDA sequence. This light-wasting process, by lowering the
amount of reactive photoenol available, decreased the possibility
that the background racemic reaction could take place.
Concurrently, the thiourea moiety in 17 acted as a chiral catalyst,
increasing the dienophilic character of the maleimide 16 upon Hbonding activation, while channeling the Diels-Alder process
toward an enantioselective pattern.

4. Organocatalysis for the Enantioselective
Intermolecular Trapping of Photoenols
The demonstration that a chiral organic catalyst could be used to
develop an enantioselective PEDA sequence motivated our
research group to seek other chiral organocatalytic intermediates
that could stereoselectively intercept the fleeting photoenols.
These investigations came about with an unanticipated
reactivity, since the resulting light-triggered processes did not led
to the expected Diels-Alder cyclic adducts. Instead, the linear
addition products were exclusively formed (Figure 4, paths ii-iv).
Overall, these studies demonstrated that the chemistry of
hydroxy-o-quinodimethanes is not limited to cycloaddition-type
manifolds. Rather, it can be expanded to develop intermolecular
addition processes that stereoselectively form one carboncarbon bond.
4.1. Michael-Type Trapping of Photoenols
Iminium ion activation is an established mode of organocatalytic
reactivity that has found many applications in enantioselective
conjugate additions of soft nucleophiles to unsaturated carbonyl
compounds.[28] It relies on the electrophilic character of the
iminium ion intermediate of type 21 (Figure 9), generated upon
condensation of the unsaturated carbonyl substrate with a chiral
amine catalyst. We wondered if intermediate 21 was electrophilic
enough for successfully intercept the photoenol. The feasibility of
this idea was tested by reacting ortho-methylbenzophenone
derivatives 22 and α,β-unsaturated aldehydes 23 in the
presence
of
commercially
available
diphenylprolinol
trimethylsilylether 25[29] as the chiral amine catalyst.[16] The
experiments were conducted under irradiation by an ordinary 15
W black light bulb (BLB, λmax = 365 nm).

Figure 10. Free-energy profile of the Michael-type trapping of the photenol
with iminium ions assisted by water acting as a proton shuttle. The optimized
structure of intermediate 27 (at -29.7 kcal·mol-1) is highlighted. Free energies
in kcal·mol-1; TBS = tert-butyldimethylsilyl.

Figure 9. The photoenolization/β-benzylation sequence following an
unconventional Michael-type addition manifold [Ref. 16]; TBS = tertbutyldimethylsilyl, BLB = black light bulb, DPP = diphenylphosphoric acid, oCl2C6H4 = 1,2-dichlorobenzene.

The light-triggered process led to the exclusive formation of
the conjugate addition product 24. No traces were detected of
the cycloaddition adduct, arising from the expected PEDA
sequence. From a synthetic perspective, the chemistry provided
a straightforward method for the direct β-benzylation of enals 23,
a transformation for which there are few effective catalytic
enantioselective precedents.[30] This Michael-type addition
manifold led to a variety of chiral β-benzylated aldehydes 24 in
good to moderate yields and with high enantioselectivities.
Complete diastereoselectivity was obtained when using obenzylbenzophenone as the photoenolizable substrate (product
24e).
Aside from its synthetic interest, this transformation[16] was
mechanistically intriguing because of the uncommon ability of
the photoenol to exclusively undergo a conjugate addition.
Generally, the formation of conjugate addition-type adducts is
rationalized on the basis of a [4+2] cycloaddition of the photoenol
followed by the opening of the resulting cyclic intermediate.[14a,31]
However, we did not find any experimental evidence supporting
this pathway. For example, when an authentic sample of the
cyclic adduct, synthesized according to a reported procedure, [32]
was subjected to the optimized photochemical organocatalytic
condition, we did not detect the open product of type 24 to any
extent, which excluded a possible ring-opening step. We then
investigated the mechanism using a density functional theory
(DFT) computational study (M06-2X level in a toluene solvent),
which suggested the crucial role of water in dictating the product
distribution. Experimentally, we had notice how the use of
rigorously dry solvents caused an appreciable decrease in
reactivity. The computational studies revealed that the intrinsic
preference for the cycloaddition product was offset by a network
of proton transfer mechanisms, facilitated by the presence of
water (Figure 10).

Specifically, in the computed preferred pathway for the
reaction between the transiently generated photoenol and the
chiral iminium ion 21 (structure in Figure 9), a water molecule
engaged in a hydrogen-bonding network with the photoenol to
form adduct 26 (Figure 10). The approach of 26 to the iminium
ion 21 led to transition state (TS-H2O), which is 2.2 kcal·mol-1
lower than the free energy for the Diels-Alder type manifold (not
shown in Figure 10). This explained why the Michael addition
dominated over the classical cycloaddition mechanism. TS-H2O
then exclusively relaxed to intermediate 27, with the proton being
transferred from the photoenol to the iminium ion nitrogen atom
via a water-assisted proton shuttle mechanism. From the
enammonium ion intermediate 27, an intramolecular proton
transfer readily

Figure 11. Organocatalytic enantioselective photoenolization/Michael addition
sequence using α,β-unsaturated ketones as Michael acceptors [Ref. 33].

formed the intermediate 28, which released the Michael addition
product 24 and the catalyst 21 after hydrolysis.
Shortly thereafter, Ye’s research research group reported a
similar Michael-type addition manifold[33]. The group described
how the chiral iminium ions, generated upon condensation of
cyclic enones 29 with the chiral amino ester catalyst 31, could
stereoselectively capture the photoenols derived from orthoalkylbenzophenones 30 (Figure 11).
A variety of cyclic enones of different size were tolerated,
providing the corresponding β-benzylated ketones 31 in
moderate-to-good yields and with excellent enantioselectivities.
The method could be extended to include acylic α,β-unsaturated
ketones, providing products of type 31c with high stereocontrol.
In addition, quaternary carbon stereocenters could be formed
with high fidelity when using 3-alkyl-2-cyclohexenones (products
31e and 31f), albeit with moderate yields.

emitting diode (black LED, λmax = 365 nm). An extensive
screening of H-bond chiral organic catalysts identified the
dimeric cinchona alkaloid derivative (DHQ) 2PHAL 36, commonly
employed
as
a
ligand
for
Sharpless
asymmetric
dihydroxylation,[36] as the best candidate to promote the
transformation.[17] This process, which has no precedents in the
racemic regime too, led to the exclusive formation of chiral
amines 35 (with moderate stereoselectivity) via a formal
Mannich-type reaction manifold. Here too, we detected no
formation of the [4+2]-cycloaddition adduct.
Regarding the mechanism, flash photolysis studies revealed
that the generation of the transient photoenol was not affected in
the presence of catalyst 36, thus suggesting that 36 controlled
the stereochemical outcome of the reaction by solely interacting
with the imine substrate 34.

4.3. Aldol-Type Trapping of Photoenols
4.2. Mannich-Type Trapping of Photoenols
Given the ability of the photoenol to participate in stereoselective
Michael reactions,[16] we wondered whether these transient
species could also serve as nucleophiles in other classical
addition processes, such as the Mannich reaction. In addition, in
the previously developed stereoselective PEDA sequence
(Figure 7)[23] we demonstrated that the photoenols could be
stereoselectively trapped by activating the dienophiles, by
means of H-bonding interactions with a suitable chiral
organocatalys. Since imines are primed to noncovalent
mechanisms of organocatalytic activations,[34] we considered it
feasible to develop a stereoselective organocatalytic
photoenolization/Mannich-type process (Figure 12). While linear
imines proved unreactive, the highly electrophilic cyclic
benzoxathiazine-2,2-diones 34[35] intercepted the photoenols
generated upon irradiation of 33 by a single black-light-

Figure 12. Organocatalytic enantioselective photoenolization/Mannich addition
sequence using cyclic benzoxathiazine-2,2-diones as electrophiles [Ref. 17].

Our previous studies demonstrated that photoenols could
participate in stereoselective intermolecular addition processes
promoted by chiral organic catalysts. To demonstrate the
generality of this reactivity framework, the next goal was to
exploit the nucleophilicity of hydroxy-o-quinodimethanes to
develop aldol-type addition reactions. Specifically, we recently
developed an organocatalytic strategy for the desymmetrization
of achiral 2-substituted-2-fluorocyclopentane-1,3-diketones 37,
where a prochiral fluorine-containing carbon was preinstalled
(Figure 13).[18] The chemistry capitalized upon the high reactivity
of photenols and the ability of the readily available chiral amidothiourea catalyst 40 to choose between enantiotopic carbonyl
groups within 37 while facilitating a desymmetric intermolecular
aldol process.[37] The resulting symmetry-breaking process
generated two stereocenters simultaneously, and forged a C-F
stereogenic unit within products 39. Since fluorine-containing
functional groups can greatly alter the intrinsic properties of
organic compounds,[38]the catalytic production of fluorinecontaining stereogenicity is a centrally important methodological
goal.

Figure 13. Photochemical organocatalytic intermolecular aldol-based
desymmetrization process [Ref. 18]; o-Cl2C6H4 = 1,2-dichlorobenzene.

The photochemical approach provided straightforward
access to valuable chiral 2-fluoro-3-hydroxycyclopentanones 39
in moderate to high yields and high stereoselectivities.
Interestingly, similar fluorinated products were previously
prepared in racemic fashion, and used for the synthesis of
pyrrolopyridazine JAK3 inhibitors for the treatment of
inflammatory and autoimmune diseases.[39]

[3]

[4]
[5]
[6]
[7]

5. Conclusions
The formation of hydroxy-o-quinodinomethanes upon light
excitation of ortho-alkyl aromatic carbonyl substrates is a
venerable photochemical process established as far as 1961.
When coupled with Diels-Alder chemistry, this photoenolization
mechanism has historically offered the synthetic community with
a straightforward access to synthetically valuable cyclic
compounds. However, developing a catalytic enantioselective
variant of this transformation has remained elusive over the
years. Recently, using the tools of enantioselective
organocatalysis, this longstanding and elusive problem was
addressed. By means of covalent and noncovalent mechanisms
of substrate activation, readily available chiral organic catalysts
could activate electrophilic substrates toward the stereoselective
interception of the fleeting hydroxy-o-quinodimethanes.
Importantly, it was demonstrated that the chemistry is not limited
to cycloaddition-type manifolds. Rather, it can be expanded to
develop intermolecular enantioselective carbon-carbon bond
forming addition processes. We believe that this catalytic
blueprint will find applications in other enantioselective
photoenol-trapping processes. Overall, these examples further
demonstrate how the field of enantioselective organocatalysis
can provide suitable tools for overcoming the challenges of
modern asymmetric synthesis, expanding the way chemists think
about making chiral molecules sustainably.

[8]
[9]

[10]
[11]
[12]

[13]
[14]

[15]
[16]

[17]

Acknowledgements

[18]
[19]

The authors thank the Generalitat de Catalunya (CERCA
Program), Agencia Estatal de Investigación (AEI, CTQ201675520-P), and the European Research Council (ERC 681840CATA-LUX) for financial support. S.C. is grateful to the MECD
for a FPU fellowship (ref. FPU14/06541). P.M. and S.C. thank
Dr. Luca Dell’Amico and all other coworkers who were involved
in developing the organocatalytic strategies to stereoselectively
trap photoenols.

[20]
[21]

[22]

Keywords: Asymmetric catalysis • Organocatalysis •
Photochemistry • Photoenols • Synthetic methods

[23]

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Received: January 17, 2018

Organocatalysis
S. Cuadros, P. Melchiorre* …… 1-9
Organocatalytic
Strategies
to
Stereoselectively Trap Photochemically
Generated
Hydroxy-o-quinodimethanes

Light
excitation
of
2-alkylbenzophen-ones 1 affords transient
hydroxy-o-quinodinomethanes A. For
a long time, because of its high
reactivity and fleeting nature, A could
not be trapped in a stereoselective

catalytic fashion. This minireview
describes
how
the
field
of
enantioselective organocatalysis has
provided suitable tools to effectively
address this long-standing problem.