The organocatalytic asymmetric prins cyclization

Article abstract of DOI:10.1002/anie.201500219

We describe here the design and development of an organocatalytic Prins cyclization. In the presence of a confined chiral imidodiphosphoric acid catalyst, salicylaldehydes react with 3-methyl-3-buten-1-ol to afford highly functionalized 4-methylenetetrahydropyrans in excellent regio- and enantioselectivity. The extreme steric demand of the acid catalyst is key for the success of this transformation. A bulky cat.: In an organocatalytic asymmetric Prins cyclization, salicylaldehydes react with 3-methyl-3-buten-1-ol in the presence of a chiral imidodiphosphoric acid catalyst to afford highly functionalized 4-methylenetetrahydropyrans in excellent regio- and enantioselectivities (see scheme).

Full text of DOI:10.1002/anie.201500219

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Chemie
International Edition: DOI: 10.1002/anie.201500219
German Edition: DOI: 10.1002/ange.201500219
Organocatalysis
The Organocatalytic Asymmetric Prins Cyclization**
Gavin Chit Tsui, Luping Liu, and Benjamin List*
Abstract: We describe here the design and development of an
organocatalytic Prins cyclization. In the presence of a confined
chiral imidodiphosphoric acid catalyst, salicylaldehydes react
with 3-methyl-3-buten-1-ol to afford highly functionalized 4-
methylenetetrahydropyrans in excellent regio- and enantiose-
lectivity. The extreme steric demand of the acid catalyst is key
for the success of this transformation.
T
he acid-catalyzed condensation between homoallylic alco-
hols and aldehydes, known as the Prins cyclization, is a direct
and efficient method to construct functionalized tetrahydro-
pyran (THP) rings.[1] Due to the prevalence of THPs in
natural products and pharmaceuticals, the Prins cyclization
has become an important strategy in novel and elegant total
syntheses of THP-containing targets.[2] Since the first report
by Hanschke in 1955 on forming the THP rings by combining
3-buten-1-ol with aldehydes or ketones in the presence of an
acid,[3] great strides have been made towards understanding
the mechanism,[4] scope,[5] and selectivity[6] of the Prins
cyclization. New catalytic systems[7] and cascade reactions[8]
have also been developed to improve efficiency and broaden
reactivity. Despite this tremendous progress, surprisingly only
one example of an enantioselective Prins cyclization using
a dual phosphoric acid and CuCl catalytic system has been
recently reported affording up to 80:20 enantiomeric ratio.[9]
A purely organocatalytic variant, until today, has remained
entirely unknown. Here we report a highly enantioselective
Prins cyclization that is catalyzed by our recently introduced
confined chiral imidodiphosphate catalysts.
Scheme 1. Chiral Brønsted acid catalyzed asymmetric Prins cyclization.
reactions.[10] The role of the chiral counteranion (X*À) is
twofold: 1) enantio-induction by asymmetric counteranion-
directed catalysis (ACDC)[11] and 2) deprotonation to gen-
erate the alkene product.
We initially investigated the reaction between benzalde-
hyde (1a) and 3-methyl-3-buten-1-ol (2) [Eq. (1)]. We tested
A key intermediate in the Prins cyclization pathway is the
oxocarbenium ion I formed in the condensation between the
olefinic alcohol and the aldehyde (Scheme 1).[1b] This highly
reactive intermediate is subsequently attacked by the pendant
alkene creating a stereogenic center in cation II. We
À
envisioned that this key enantiodiscriminating C C bond-
forming step can be governed by our newly developed C2-
symmetric imidodiphosphoric acid catalysts 5, which have
demonstrated excellent enantiocontrol over the nucleophilic
attack to oxocarbenium ions in asymmetric acetalization
different chiral Brønsted acids, including phosphoric acids,[12]
disulfonimides,[13] and imidodiphosphates,[10] which all showed
either poor yield and/or low enantioselectivity (see the
Supporting Information). The highest e.r. (95:5) was obtained
with catalyst 5a but the yield was not synthetically useful
(11%) and significant side-product formation was observed.
[*] Dr. G. C. Tsui, L. Liu, Prof. Dr. B. List
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany)
E-mail: list@kofo.mpg.de
Remarkably, salicylaldehyde (1b) gave
a significantly
[**] Generous support from the Max-Planck-Society and the European
Research Council (Advanced grant “High Performance Lewis Acid
Organocatalysis, HIPOCAT” to B.L.) is gratefully acknowledged.
G.C.T. thanks the Alexander von Humboldt Foundation and Bayer
Science & Education Foundation for a postdoctoral fellowship. We
also thank Sylvia Ruthe and the members of our GC, HPLC, and
crystallography departments for their excellent support.
improved reaction profile with 62% yield and an excellent
e.r. of 97:3. After extensive optimization of the reaction
conditions we found that using cyclohexane as the solvent
with 5 Š molecular sieves at room temperature gave the
optimal combination of yield and enantioselectivity. Catalyst
5a exhibited superior control in both enantioselectivity and
regioselectivity. The exocyclic alkene product 3 was isolated
as the sole product.[14]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500219.
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Table 1: Asymmetric Prins cyclization of salicylaldehyde derivatives.[a]
and required a mixed solvent system, cyclohexane/toluene
(9:1 v/v). Substituents at the 4- and 3-positions were also
tolerated (3j–l). However, the latter required heating to 458C
to reach comparable yields.
Dihalogenated products 3m–o could also be obtained in
high enantioselectivities. A substrate substituted at the 6-
position did not provide the corresponding product (3p). A
control experiment using 2-methoxybenzaldehyde gave prod-
uct 6 in only low yield and poor enantioselectivity. The
absolute configuration of product 3b was unambiguously
determined by single-crystal X-ray analysis of a derivative
(see the Supporting Information).
We also explored homoallylic alcohols with additional
substituents. For example, racemic alcohol 7 gave the
corresponding product as an inseparable mixture of cis/trans
isomers [Eq. (2)]. Both isomers were formed with excellent
enantioselectivity in a stereodivergent, parallel kinetic reso-
lution. When the reaction was stopped after 56% conversion,
no significant enantiomer differentiation was observed (cis/
trans = 1:1.3, e.r.cis = 97.5:2.5, e.r.trans = 97.5:2.5). We also stud-
ied alcohol 9, which could potentially lead to different exo-
and endocyclic alkene regioisomers [Eq. (3)]. Consistent with
[a] Unless specified otherwise, reactions were performed using catalyst
5a (5 mol%), 1.0 equivalent each of 1 and 2 and 5 Š molecular sieves
(0.3 gmmolÀ1) in cyclohexane (0.125m), at room temperature for 5 days.
[b] The enantiomeric ratios were determined by GC and HPLC on a chiral
stationary phase, regioselectivities all >20:1. [c] Solvent=cyclohexane/
toluene (9:1 v/v). [d] At 458C. [e] Catalyst=5b.
the results in Table 1, the exocyclic alkene product 10 was
obtained exclusively. The E isomer was favored and obtained
in excellent enantioselectivity; the minor Z isomer gave poor
enantioselectivity. We have also tested several other unsatu-
rated alcohols as substrates but found them to be less
reactive.[15]
The hydroxy group in products 3 can serve as a useful
synthetic handle for further derivatization (Scheme 2). For
example, triflation of product 3b took place smoothly to
generate product 11. Suzuki coupling between aryl triflate 11
and an arylboronic acid gave biaryl derivative 12. Alterna-
tively, a reductive detriflation protocol allowed the removal of
the hydroxy group and afforded the previously challenging
Next, we studied the scope of the asymmetric Prins
cyclization (Table 1). A variety of commercially available
salicylaldehyde derivatives bearing different substituents
were subjected to the reaction conditions using 5 mol% of
catalyst 5a. Functional groups including electron-rich (3c),
electron-poor (3i), sterically demanding (3e), and halogens
(3 f–h) were tolerated at the 5-position of the aromatic ring.
Some substrates were not completely soluble in cyclohexane
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oxocarbenium ion in A which also contains the chiral
imidodiphosphate counteranion. Ion pair A can also be
described as a protonated ortho-quinone methide, resonance
structure B.[18] The key enantiodifferentiating C C bond
À
formation furnishes THP C bearing a new chiral center and
a tertiary carbocation. In the final deprotonation step, the
sterically encumbering imidodiphosphate anion kinetically
prefers removing a proton at the primary position to
selectively afford the exocyclic alkene product 3b.
In summary, we have developed an organocatalytic
asymmetric Prins cyclization. The reaction possibly proceeds
by means of an unusual mode of enantiocontrol involving
challenging o-quinone methide intermediates.[19] We have
introduced the new sterically confined imidodiphosphoric
acid 5a, a powerful Brønsted acid catalyst controlling
reactivity and enantio- and regioselectivity. Our method can
be applied to the enantioselective synthesis of diverse
functionalized tetrahydropyrans with an exocyclic methylene
moiety. Further exploration of the asymmetric Prins cycliza-
tion, particularly towards expanding the substrate scope, is in
progress in our laboratories.
Scheme 2. Derivatization of product 3b.
phenyl-substituted THP product 3a in high enantiopurity [cf.
Eq. (1)]. Note that the enantiomeric ratios remained
unchanged in the products of these transformations.
Keywords: asymmetric catalysis · Brønsted acids ·
organocatalysis · Prins cyclization · tetrahydropyrans
How to cite: Angew. Chem. Int. Ed. 2015, 54, 7703–7706
Angew. Chem. 2015, 127, 7814–7818
The results using a methyl ether instead of the salicylal-
dehyde giving product 6 clearly demonstrated that the
hydroxy group ortho to the aldehyde is crucial for reactivity
and enantioselectivity. The increased reactivity can be
rationalized by invoking an internal activation through intra-
molecular hydrogen bonding between the hydroxy and
carbonyl groups.[16] It is also known that the bifunctional
catalyst can form intermolecular hydrogen bonds with both
the hydroxy and carbonyl/imino groups in transition states,
thus activating the aldehyde.[17] A plausible mechanism is
suggested in Scheme 3. The initial condensation between
aldehyde 1b and alcohol 2 leads to the formation of the
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of salicylaldehyde 1b and alcohol 2 catalyzed by a chiral imidodiphos-
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Received: January 9, 2015
Revised: March 25, 2015
Published online: May 26, 2015
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