Enantioselective Prins cyclization: BINOL-derived phosphoric acid and CuCl synergistic catalysis

Article abstract of DOI:10.1039/c4cc02826k

The first catalytic enantioselective Prins cyclization is disclosed. The reaction is catalyzed by the combination of a chiral BINOL-derived bis-phosphoric acid and CuCl. The process consists of a tandem Prins/Friedel-Crafts cyclization that affords the hexahydro-1H-benzo[f] isochromenes products with three new contiguous stereogenic centers in high yields, and good enantio- and excellent diastereoselectivities. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02826k

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Enantioselective Prins cyclization: BINOL-derived
phosphoric acid and CuCl synergistic catalysis†
Cite this: Chem. Commun., 2014,
50, 7495
Claudia Lalli* and Pierre van de Weghe*
Received 16th April 2014,
Accepted 11th May 2014
DOI: 10.1039/c4cc02826k
www.rsc.org/chemcomm
The first catalytic enantioselective Prins cyclization is disclosed. The as p-TSA and MgBr2.10 Inspired by the successful examples on
reaction is catalyzed by the combination of a chiral BINOL-derived the enantioselective addition to oxocarbenium ions and in
bis-phosphoric acid and CuCl. The process consists of a tandem continuation of our interest in the Prins cyclization reaction,11
Prins/Friedel–Crafts cyclization that affords the hexahydro- we reasoned that the combination of the chiral Brønsted acid
1H-benzo[f ]isochromenes products with three new contiguous with an achiral Lewis acid12 could promote the reaction and we
stereogenic centers in high yields, and good enantio- and excellent envisioned that chiral phosphoric acids could activate the
diastereoselectivities.
oxonium ion intermediate and ensure the enantioselectivity.
Herein we report our preliminary results for the first catalytic
The Prins cyclization reaction1 between a homoallylic alcohol and enantioselective Prins cyclization.
an aldehyde in the presence of an acidic promoter or catalyst is a
We chose as model substrates homoallylic alcohols such as
powerful C–C bond forming transformation and one of the most 6-phenylhex-3-en-1-ol 1 containing a tethered nucleophile.13
elegant approach to access highly substituted tetrahydropyrans with The advantage in using those kind of substrates consists in a
excellent diastereoselectivities.2 Despite the recent advances and simplified reaction system to study. Indeed, most of the Prins
the increasing interest in the stereochemical outcome and the cyclization reactions are promoted by stoichiometric amounts of
mechanism of the Prins cyclization,3 to date, to our knowledge, Lewis acid that play a double role, as a promoter and nucleophile
no asymmetric version has been reported.4 The main drawback source to trap the carbocation species which is formed after the
associated with the development of the enantioselective Prins cyclization. The embedded phenyl moiety prevents the use of a
cyclization is the racemization due to the Oxonia Cope rearrangement, stoichiometric external nucleophile which can interfere with the
the allyl transfer or the solvolysis.5 Recently Feng and coworkers catalytic system, and moreover allows a tandem Prins/Friedel–
succeeded in the synthesis of optically pure 4-OH-tetrahydropyrans Crafts process to take place.13e
by a catalytic asymmetric ene reaction followed by a FeCl3 Prins
We began our studies by investigating the reaction between
cyclization.3d However so far direct access to tetrahydropyrans (Z)-6-phenylhex-3-en-1-ol (Z)-1 and p-methoxy-benzaldehyde 2a
by catalytic enantioselective Prins cyclization has not been in DCE (1,2-dichloroethane) at 40 1C, leading to the hexahydro-
achieved and it is therefore highly required.
1H-benzo[ f ]isochromenes product 5a with three new contiguous
Since the seminal independent reports of Akiyama6 and Terada,7 stereogenic centers.
the ability of BINOL-derived chiral phosphoric acids to direct the
A screening of known chiral phosphoric acids 3–4 (Table 1,
enantioselective addition to imines is well established.8 Nevertheless entries 1–4) gave only disappointing results summarized in a lack
the enantioselective addition to oxocarbenium ions is much less of reactivity. We hypothesized that the Brønsted acid alone is not
developed.9 We recently reported the unprecedented synergistic prone to activate the electrophilic carbonyl group. We reasoned
effect in Prins cyclization between non-reactive Brønsted and Lewis that a catalytic system integrating a phosphoric acid and a metal
acids that lack the ability to catalyze the reaction if used alone, such catalyst should synergistically enable our reaction. To confirm our
hypothesis we first examined the combination of phosphoric acids
´
Universite de Rennes 1, UMR 6226 CNRS, Institut des Sciences Chimiques de Rennes,
3 with different achiral Lewis acids. As shown in Table 1, MgBr2 is
completely inactive (entry 5), in contrast first raw transition metal
salts such as Cu(OTf)2 and FeCl3 (entries 6–9) could catalyze the
reaction with up to 45% yield, unfortunately without any enantio-
selection. The association between the bis-phosphoric acid14 4 and
FeCl3 (entry 10) also failed to selectively promote the reaction.
Equipe PNSCM, UFR des Sciences Biologiques et Pharmaceutiques,
´
2 Avenue du Prof. Leon Bernard, 35043 Rennes Cedex, France.
E-mail: claudia.lalli@univ-rennes1.fr, pierre.van-de-weghe@univ-rennes1.fr
† Electronic supplementary information (ESI) available. CCDC 998587. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc02826k
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Table 1 Catalysts identification and reaction optimization of the enantio-
selective Prins cyclizationa
Table 2 Scope of the enantioselective Prins cyclizationa
Entry
1
Product 5
Yieldb (%)
70
e.r.c,d
80 : 20
2
3
4
5
6
7
73
78
76
62
62
63
76
80 : 20
77 : 23
76 : 24
80 : 20
72 : 28
67 : 33
60 : 40
Entry
Catalyst
Lewis acid
Yield 5ab,c (%)
e.r.d,e 5a
1
2
3
4
5
6
7
8
3a
3b
3c
4
3a
3a
3b
3b
3c
4
—
—
—
—
n.r.
n.r.
n.r.
n.r.
n.r.
20
45
28
34
52
24
65
70
29
—
—
—
—
MgBr2
Cu(OTf)2
FeCl3
Cu(OTf)2
Cu(OTf)2
FeCl3
Cu(OTf)2
CuCl2
CuCl
—
53 : 47
50 : 50
50 : 50
50 : 50
50 : 50
80 : 20
79 : 21
80 : 20
67 : 33
9
10
11
12
13
14
4
4
4
4 f
CuCl
a
General conditions: (Z)-1 (0.1 mmol), 2a (0.12 mmol), 3–4 (10 mol%),
Lewis acid (10 mol%) in DCE (c = 0.1 M) at 40 1C for 22 h. b Yields refer to
isolated products. n.r. = no reaction. d Enantiomeric ratio was determined
c
by chiral HPLC analysis. e d.r. 4 95 : 5. The (E)-1 isomer was used.
f
To our delight the combination of 4 and Cu(OTf)2 (entry 11)
afforded the desired product 5a with an exciting 80: 20 enantio-
meric ratio (e.r.) and as a single diastereomer (d.r. 495: 5), the
cis-fused hexahydro-1H-benzo[ f ]isochromene dictated by the Z
geometry of the double bond, even if in moderated yields (24%).
Different trifluoromethanesulfonates such as Zn(OTf)2, Ni(OTf)2,
Bi(OTf)3, AgOTf or LiOTf failed to give better results, in contrast
the variation of the copper source (CuCl2 and CuCl entries 12
and 13 respectively) improved the yield up to 70% (see ESI†).
Finally the effect of the alkene geometry was evaluated and it was
observed that changing the double bond to E resulted in a
dramatic loss in reactivity and selectivity (entry 14), thus the
trans-fused hexahydro-1H-benzo[ f ]isochromene was obtained
8
9
63
72
65 : 35
60 : 40
10
a
General conditions: (Z)-1 (0.1 mmol), 2 (0.12 mmol), 4 (10 mol%),
b
CuCl (10 mol%) in DCE (c = 0.1 M) at 40 1C for 22 h. Yields refer
to isolated products. Enantiomeric ratio was determined by chiral
HPLC analysis. d.r. 4 95 : 5.
c
d
with only 29% yield and 67: 33 e.r. An exploration of different the cases enantioenriched hexahydro-1H-benzo[ f ]isochromenes
reaction conditions was set out to optimize the selectivity. A were obtained with the same range of yields and e.r. values. The
solvent screening revealed chlorinated solvents to be superior p-NO2 benzaldehyde also participated with a slight erosion of
(see ESI†), in particular DCE gave slightly better yields and enantioselectivity (entry 7). The catalytic system proved to be
enantiomeric ratios than CH2Cl2. Decreasing the temperature efficient for aromatic aldehydes substituted with an electron
was detrimental for the reactivity. The increase of the phosphoric donating group in the para position (entry 1) but it is less
acid loading to 20 mol% did not benefit the enantioselectivity.
selective for the o-MeO substitution probably because of the
Encouraged by the promising level of enantiocontrol, our steric hindrance (entry 8). Aliphatic aldehydes undergo Prins
catalytic system was tested on different substituted aromatic cyclization with high yields, excellent diastereoselectivities but
aldehydes (Table 2, entries 1–8). Gratifyingly benzaldehyde 2b moderate enantiocontrol (entries 9 and 10).
(entry 2) smoothly participated in the reaction leading to the
The absolute configuration of 5e was unambiguously deter-
desired product 5b with 73% yield, 80 : 20 e.r. and as a single mined to be 4S,4aR,10bR by single crystal X-ray analysis15
diastereomer. The substitution of the aromatic ring in the ortho (Fig. 1). The three new contiguous stereogenic centers formed
or para position with halogens (entries 3–6) is tolerated as in all are a result of an attack of the alkene on the Si face of the
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Fig. 1 ORTEP representation of the hexahydro-1H-benzo[f]isochromene
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In conclusion, we have disclosed the first enantioselective
Prins cyclization catalyzed by a synergistic combination of
chiral BINOL-derived bis-phosphoric acid and CuCl under mild
conditions. The present method demonstrates the feasibility of
the asymmetric condensation between a homoallylic alcohol and
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´
´
We thank Universite de Rennes 1, CNRS, Rennes Metropole and
´
Region Bretagne for financial support. Thierry Roisnel and the
´
Centre de Diffractometrie X (CDIFX) de l’Institut des Sciences
Chimiques de Rennes are gratefully acknowledged for crystallo-
graphic analysis and the CRMPO for mass measurement. Part of
this work has been performed using the PRISM core facility
´
(Biogenouestr, UMS Biosit, Universite de Rennes 1- Campus de
Villejean- 35043 RENNES Cedex, FRANCE).
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