Asymmetric Catalysis via Cyclic, Aliphatic Oxocarbenium Ions

Article abstract of DOI:10.1021/jacs.6b11993

A direct enantioselective synthesis of substituted oxygen heterocycles from lactol acetates and enolsilanes has been realized using a highly reactive and confined imidodiphosphorimidate (IDPi) catalyst. Various chiral oxygen heterocycles, including tetrahydrofurans, tetrahydropyrans, oxepanes, chromans, and dihydrobenzofurans, were obtained in excellent enantioselectivities by reacting the corresponding lactol acetates with diverse enol silanes. Mechanistic studies suggest the reaction to proceed via a nonstabilized, aliphatic, cyclic oxocarbenium ion intermediate paired with the confined chiral counteranion.

Full text of DOI:10.1021/jacs.6b11993

Communication
pubs.acs.org/JACS
Asymmetric Catalysis via Cyclic, Aliphatic Oxocarbenium Ions
Sunggi Lee, Philip S. J. Kaib, and Benjamin List*
Max-Planck-Institut fur Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mulheim an der Ruhr, Germany
̈
̈
S
* Supporting Information
All of these examples are limited to the use of resonance-
stabilized oxocarbenium ions within six-membered ring
systems. In contrast, the enantioselective formation of
tetrahydrofurans or tetrahydropyrans via nucleophilic addition
reactions to aliphatic, cyclic oxocarbenium ions remained as a
long-standing challenge. Their high electrophilicity often leads
to catalyst degradation, and their small size and the absence of
further functionalization makes enantiodifferentiation difficult
to accomplish. We have recently introduced confined
imidodiphosphoric acid catalysts (IDPs) that catalyze cycliza-
tion reactions via aliphatic oxocarbenium ions.12 However,
intermolecular reactions proceeding via unmodified, aliphatic,
cyclic oxocarbenium ions could not be accomplished with these
catalysts. We envisioned that our newly developed imidodi-
phosphorimidates (IDPi) could perhaps provide a solution to
this significant problem.13 Their acidity is higher than that of
IDPs and even of disulfonimides (DSIs), enabling silylium-
based asymmetric counteranion-directed catalysis (Si-
ACDC).14,15 In addition, their highly stabilized counteranions
should support the generation of ion pairs with strongly
electrophilic cations. Finally, the confined active site should
allow efficient enantiodiscrimination of sterically and electroni-
cally unbiased cationic intermediates. On the basis of these
considerations, a plausible catalytic cycle can be envisioned (eq
1), which is initiated with the in situ generation of the silylium
ABSTRACT: A direct enantioselective synthesis of
substituted oxygen heterocycles from lactol acetates and
enolsilanes has been realized using a highly reactive and
confined imidodiphosphorimidate (IDPi) catalyst. Various
chiral oxygen heterocycles, including tetrahydrofurans,
tetrahydropyrans, oxepanes, chromans, and dihydrobenzo-
furans, were obtained in excellent enantioselectivities by
reacting the corresponding lactol acetates with diverse enol
silanes. Mechanistic studies suggest the reaction to
proceed via a nonstabilized, aliphatic, cyclic oxocarbenium
ion intermediate paired with the confined chiral counter-
anion.
xocarbenium ions are key intermediates in enzymatic
O
glycosidations but also in a variety of synthetic
transformations.1 The stereocontrol of such reactions is
challenging, and catalytic enantioselective approaches have
only recently been described.2 Asymmetric nucleophilic
substitutions via non-aromatic, cyclic oxocarbenium ions are
particularly difficult to control, as these cations display neither
Lewis basic sites nor large π-surfaces for substrate−catalyst
interactions. We show here that silylium-based asymmetric
counteranion-directed Lewis acid catalysis provides a general
solution to this challenge; various lactol acetates react with enol
silanes in the presence of a highly active and confined pre-Lewis
acid organocatalyst to furnish substituted oxygen heterocycles
with high enantioselectivity.
Despite the frequent appearance of saturated oxygen
heterocycles in drugs and natural products,3 general methods
for their enantioselective synthesis are relatively rare.4−11 One
straightforward approach is the direct addition of nucleophiles
to in situ-generated cyclic oxocarbenium ions. In pioneering
studies, Braun and Kotter2a showed that a chiral titanium
complex enables the dynamic kinetic asymmetric nucleophilic
substitution of allyltrimethylsilane with silyl ethers and acetals.
However, high enantioselectivity was achieved only with
aromatic substrates. Recently the Jacobsen group reported
enantioselective substitution reactions of silyl ketene acetals
with aromatic glycosyl chlorides using anion binding
catalysis.2b,r−t Chiral 1-substituted isochromanes and related
products were also obtained via oxidative enamine catalysis, via
alkynylation using a chiral copper catalyst combined with
stoichiometric Lewis acids, via chiral silver-salt-catalyzed
reductions, and via nucleophilic addition reactions to pyrylium
salts using different catalysts.2c−o Most recently, the Mattson
group developed a silandiol-catalyzed chromenone functiona-
lization,2p and the Toste group utilized anionic phase transfer
catalysis for the formation of 2,4-diarylbenzopyrans.2q
ion pair catalyst from the strongly Brønsted acidic precatalyst
HX* and a silylated nucleophile (TBS-Nu). The cyclic
oxocarbenium ion is subsequently generated via acetate
abstraction from a lactol acetate by the silicon Lewis acid.16
In the following nucleophilic addition of the silylated
nucleophile to the oxocarbenium ion, the counteranion directs
the enantiodifferentiation, leading to formation of the product
and regeneration of the silylium Lewis acid catalyst.
Received: November 21, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b11993
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
To test this design, we explored the reaction of different
oxocarbenium ion precursors 1 with ketene acetal 2a in the
presence of a variety of acidic catalysts previously developed in
our group (Table 1). Chiral disulfonimide catalyst 4 enabled
−78 °C (entry 10). Furthermore, the amount of nucleophile
could be reduced to 1.2 equiv without affecting the
enantioselectivity or conversion (entry 11). Under the optimal
conditions, as little as 0.1 mol% catalyst 6d was sufficient to
furnish tetrahydrofuran 3a in 88% yield after 16 h (entry 12). It
is noteworthy that product 3a is the starting material for several
syntheses of biologically active molecules but there are only a
few procedures for its synthesis, using enzymatic kinetic
resolutions.17
a
Table 1. Reaction Development
With the optimal conditions in hand, we next investigated
the reaction scope (Table 2). Various nucleophiles and
electrophiles can be utilized in our reaction. The size of the
nucleophile does not seem to influence the selectivity and
reactivity much (entries 1−3). In addition to silyl ketene
acetals, the silyl enol ether derived from acetophenone (2d)
was also applicable, although with slightly lower enantiose-
lectivity (entry 4). A conjugated silyl enol ether gave product 3e
with high enantioselectivity (entry 5). Extended nucleophiles
such as vinylogous ketene acetal 2f and dioxinone-derived
silyloxydiene 2g reacted at the γ-position, furnishing the
corresponding heterocycles with >95:5 er in moderate to
excellent yields (entries 6 and 7). Tetrahydropyran and
oxepane products 3h and 3i could be obtained under identical
conditions with high enantiomeric ratios from the correspond-
ing six- and seven-membered lactol acetates (entries 8 and 9).
In addition to simple aliphatic substrates, aromatic substrates
possessing chroman and benzodihydrofuran moieties could also
be converted under our reaction conditions but required longer
reaction times and higher concentration. Full conversion to
products 3j and 3k with excellent and good enantioselectivity
was observed after 7 and 4 days, respectively (entries 10 and
11). In contrast, the reaction with 1-acetoxyisochroman 1h was
complete within 10 min and gave product 3l in high yield but
with only moderate enantioselectivity (entry 12).
b
entry
substrate catalyst
temp
conv.
er
cd
,
1
2
1a
1a
1a
1a
1a
1a
1a
1b
1c
1a
1a
1a
4
r.t.
>95
<10
>95
>95
>95
>95
>95
>95
50:50
ND
cd
,
5
r.t.
c
3
6a
6a
6b
6c
6d
6d
6d
6d
6d
6d
r.t.
59.5:40.5
61.5:18.5
60:40
84:16
94:6
4
r.t.
5
−20 °C
−20 °C
−20 °C
−20 °C
−20 °C
−78 °C
−78 °C
−78 °C
6
7
8
93:7
e
9
>95
>95
>95
87:13
97:3
Remarkably, when 2,4-bis(acetoxy)tetrahydrofuran (1i) was
subjected to the reaction conditions using 2.4 equiv of
nucleophile 2b, the entire mixture of stereoisomers (meso-1i,
(S,S)-1i, and (R,R)-1i) was converted into a single major
product stereoisomer (eq 2). The initial 1.5:1 racemic trans:cis
10
11
f
98:2
fg
12 ,
h
>95 (88 )
98:2
a
Unless otherwise indicated, reactions were conducted on a 0.05
mmol scale with 3.0 equiv of 2a and 1.0 mol% catalyst in 0.5 mL of
toluene. The er was determined by GC. ND = not determined.
b
c
Determined by NMR analyses with an internal standard. The
d
e
reaction was carried out in CH2Cl2. 5 mol% catalyst. A 19% yield of
product was observed by crude NMR analysis. 1.2 equiv of
nucleophile. 1.0 mmol scale, 0.1 mol% catalyst. Isolated yield.
f
g
h
full conversion to the desired product 3a within 4 h but without
any enantioselectivity (entry 1). In contrast, imidodiphosphate
catalyst 5 gave no conversion, likely because of its insufficient
acidity. Remarkably, however, IDPi catalyst 6a efficiently
promoted the reaction with promising enantioselectivity
(entries 3 and 4). The corresponding 2-naphthyl-substituted
catalyst 6b, previously used in the asymmetric Hosomi−Sakurai
allylation of aldehydes, showed no improvement in the
enantioselectivity (entry 5). We found that para substituents
on the 3,3′-phenyl groups of the BINOL backbone strongly
affect the enantioselectivity. A stepwise increase in enantiose-
lectivity was observed by varying from small methyl to bulky
tert-butyl groups, affording up to 94:6 er at −20 °C in toluene
(entries 6 and 7). Switching from acetate 1a to benzoate 1b did
not influence reactivity or selectivity (entry 8), and 2-
chlorotetrahydrofuran (1c) gave only a 19% yield of the
desired product with reduced enantioselectivity. The selectivity
was further increased to 97:3 er by lowering the temperature to
ratio of substrate 1i gave a 15:1 trans:cis ratio in product 3m,
which was obtained with superb enantiomeric ratio (>99:1) in a
diasteroconvergent and enantioconvergent kinetic resolution.
We were also able to utilize α-branched nuclophiles. For
example, 2-(trimethylsiloxy)furan (2h) reacted with lactol
acetate 1a to give lactone product syn-3n with excellent er
and moderate dr (eq 3).18
Toward elucidating the mechanism of our reaction, we
monitored the enantiomeric excesses of substrate 1a and
product 3a during the course of the reaction (Figure 1). While
the enantiomeric excess of the product was constantly high
B
DOI: 10.1021/jacs.6b11993
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
Table 2. Substrate Scope
Figure 1. Proposed mechanism and enantiomeric excess vs
conversion. s = selectivity factor, c = conversion, ee = enantiomeric
excess, sm = starting material.
selectivity factor (s) of ca. 4. Both the decrement of the
reaction rate by electron deficiency on oxygen (Table 2, entries
10 and 11) and the fact that both enantiomers are converted
into the same enantiopure product are consistent with an SN1
mechanism involving a cyclic oxocarbenium ion intermediate
paired with the chiral counteranion.
In conclusion, we have developed a catalytic enantioselective
synthesis of oxygen-containing heterocycles from readily
accessible lactol acetate precursors and silylated nucleophiles.
We suggest the reaction to proceed via an ACDC-type SN1
mechanism involving a nonstabilized, aliphatic, cyclic oxocar-
benium ion paired with the confined IDPi counteranion. The
scope of our reaction is broad, and five-, six-, and seven-
membered-ring lactol acetates react with various nucleophiles.
Moreover, our catalyst system also tolerates aromatic
substrates, furnishing chroman and dihydrobenzofuran prod-
ucts. Finally, a highly diastereo- and enantioconvergent catalytic
asymmetric transformation has been realized with substrate 1h,
providing a C2-symmetric tetrahydrofuran product.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b11993.
■
S
Experimental procedures, characterizations and analytical
data of products, and NMR and HPLC spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
*list@kofo.mpg.de
■
a
Unless otherwise indicated, reactions were conducted with 1.0 equiv
of substrate, 1.2 equiv of nucleophile, and 1.0 mol% catalyst in toluene
ORCID
b
(0.1 M). The er was determined by HPLC or GC. 1.0 mmol scale,
Benjamin List: 0000-0002-9804-599X
Notes
c
d
e
f
g
0.1 mol% catalyst. 0.3 mol% catalyst. 0.5 M. 7 days. 4 days. 10
min.
The authors declare no competing financial interest.
from the beginning of the reaction, not unexpectedly, one
enantiomer of substrate 1a reacted faster than the other,
affording a gradual enrichment of the enantiomeric excess of
the substrate. This phenomenon corresponds well to a
theoretical graph using the equation of Kagan19 with a
ACKNOWLEDGMENTS
■
Generous support by the Max Planck Society and the European
Research Council (Advanced Grant “CHAOS”) is gratefully
acknowledged. We thank Dr. Roberta Properzi and Lucas
C
DOI: 10.1021/jacs.6b11993
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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D
DOI: 10.1021/jacs.6b11993
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX