Directing group assisted nucleophilic substitution of propargylic alcohols via o -quinone methide intermediates: Br??nsted acid catalyzed, highly enantio- and diastereoselective synthesis of 7-alkynyl-12a-acetamido-substituted benzoxanthenes

Article abstract of DOI:10.1021/ol503662g

BINOL-based, chiral phosphoric acids catalyze the substitution of 1-(o-hydroxyphenyl)propargylic alcohols with enamides to furnish 7-alkynyl-12a-acetamido-substituted benzo[c]xanthenes and related heterocycles in a one-pot operation with excellent diastereo- and enantioselectivity. Ambient reaction temperature, operationally simple reaction conditions, low catalyst loading, high yields, and excellent stereocontrol are attractive features of this process and make it a highly practical and versatile transformation.

Full text of DOI:10.1021/ol503662g

Letter
pubs.acs.org/OrgLett
Directing Group Assisted Nucleophilic Substitution of Propargylic
Alcohols via o‑Quinone Methide Intermediates: Brønsted Acid
Catalyzed, Highly Enantio- and Diastereoselective Synthesis of
7‑Alkynyl-12a-acetamido-Substituted Benzoxanthenes
Satyajit Saha and Christoph Schneider*
Institute of Organic Chemistry, University of Leipzig, Johannisallee 29, D-04103 Leipzig, Germany
S
* Supporting Information
ABSTRACT: BINOL-based, chiral phosphoric acids catalyze the substitution of 1-(o-hydroxyphenyl)propargylic alcohols with
enamides to furnish 7-alkynyl-12a-acetamido-substituted benzo[c]xanthenes and related heterocycles in a one-pot operation with
excellent diastereo- and enantioselectivity. Ambient reaction temperature, operationally simple reaction conditions, low catalyst
loading, high yields, and excellent stereocontrol are attractive features of this process and make it a highly practical and versatile
transformation.
tudies on the direct nucleophilic displacement of the
philes.13 Pursuing this strategy, we have successfully established
Shydroxyl group in propargylic alcohols are currently among one-pot and high-yielding syntheses of various chromene- and
the prime objectives in organic synthesis.1 The highly versatile
xanthene-based oxygen heterocycles as well as triarylmethanes
with excellent stereocontrol. These transformations proceeded
through in situ generated o-quinone methides (o-QM)
activated through the hydrogen-bonded chiral phosphoric
acid catalyst.14 o-QMs are highly reactive 1-oxabutadienes
which have frequently been employed in the synthesis of
natural products and bioactive molecules.15 However, the
transient nature of o-QM principally poses challenges to
synthetic applications, and it was only recently that a range of
catalytic, enantioselective processes have been successfully
developed for o-QM chemistry including palladium, cinchona
alkaloid, BINOL, Brønsted acid, and NHC catalyzed
reactions.16
We report herein the Brønsted acid catalyzed, highly
enantioselective substitution of 1-(o-hydroxyphenyl)propargylic
alcohols 1 with enamides 2 which upon subsequent N,O-
acetalization generate highly functionalized 7-alkynyl-12a-
acetamido-substituted benzo[c]xanthenes 4 and related hetero-
cycles carrying three contiguous chiral centers. The products
were obtained in good yield and excellent diastereo- and
enantioselectivity in a one-pot operation (Scheme 1b). The o-
hydroxy group within 1 helps to form the highly reactive o-QM,
which readily adds the enamide nucleophile in a conjugate
nature of the alkyne moiety makes such reactions highly
attractive in organic synthesis. Fully or semisaturated products
are easily obtained by catalytic hydrogenation, and various
other functional groups are readily accessible by a range of
known transformations of the alkynyl group.2
Accordingly, transition-metal-catalyzed propargylic substitu-
tion reactions have attracted a great deal of attention recently.3
Gold-, ruthenium-, and copper-catalyzed processes have been
disclosed by the groups of Campagne,4 Toste,5 Sheppard,6
Nishibayashi,7 Alexakis, and others, respectively.8 In addition,
Nishibayashi et al. have disclosed powerful ruthenium- and
copper-catalyzed enantioselective alkylation and amination
reactions of propargylic alcohols and derivatives thereof.9
They have also developed cooperative ruthenium−prolinol
ether- and ruthenium−copper-catalyzed asymmetric reactions
of propargylic alcohols and esters.10 Recently, Hu11 and van
Maarseeven12 and co-workers have reported various copper-
catalyzed, enantioselective substitution reactions of propargylic
acetates as well. Nevertheless, there is a great demand for novel,
enantioselective propargylic substitution reactions that are
applicable to a broad substrate range.
Recently, our group has developed a series of highly efficient
Brønsted acid catalyzed, enantioselective substitution reactions
of o-hydroxybenzhydryl alcohols employing 1,3-dicarbonyl
compounds, enamides, indoles, and naphthols as nucleo-
Received: December 20, 2014
© XXXX American Chemical Society
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DOI: 10.1021/ol503662g
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Letter
(entry 3). Neither yield nor stereoselectivity of the reaction
were affected through the use of molecular sieves.
To assess the applicability of this process, various 1-(o-
hydroxyphenyl)propargylic alcohols 1a−l were subsequently
reacted with enamide 2a under the above-optimized reaction
conditions, and the results are summarized in Table 2. In all
Scheme 1. Brønsted Acid Catalyzed Reaction of Propargylic
Alcohols (A) without and (B) with Directing Group via o-
QM
a d
−
Table 2. Substrate Scope
addition event. Accordingly, substrates lacking this o-hydroxy
group or carrying a protected o-hydroxy group (e.g., OMe) do
not react under these conditions (Scheme 1a).
We initiated our studies by investigating the reaction of 1-(o-
hydroxyphenyl)-3-phenylpropargylic alcohol (1a) (1 equiv)
with enamide 2a (1 equiv) in CH2Cl2 in the presence of various
chiral phosphoric acids 3a−e (5 mol %) (Table 1). 7-Alkynyl-
a d
−
Table 1. Optimization Studies
b
c
d
entry
cat.
solvent
time (h)
% yield
64
dr
er
1
2
3
4
5
3a
3b
3c
3d
3e
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
8
16
8
>95:5
>95:5
>95:5
>95:5
96:4
96:4
99:1
99:1
98:2
90:10
c
50
72
70
57
8
16
a
All reactions were carried out with 0.23 mmol (1 equiv) of 1, 0.23
a
All reactions were carried out with 0.42 mmol (1 equiv) of 1, 0.42
mmol (1.0 equiv) of enamide 2a, and 5 mol % of catalyst 3c in 4 mL of
mmol (1.0 equiv) of enamide 2a, and 5 mol % of catalyst 3a−e in 2
b
c
mL of CH2Cl2 at rt for 8−16 h. Isolated yield of the product. dr was
b
c
1
CH2Cl2 at rt for 8−36 h. Isolated yield of the purified product. dr
determined through H NMR analysis of the crude reaction mixture.
1
d
was determined by H NMR analysis of the crude reaction mixture.
er was determined through chiral HPLC analysis (see the Supporting
d
er was determined through chiral HPLC analysis (see the Supporting
Information).
Information).
cases studied, the reaction proceeded smoothly and was
typically completed within 8−36 h at rt. Inspection of Table
2 reveals that the products were obtained typically in good yield
and excellent diastereo- and enantioselectivity, irrespective of
the substitution pattern within the alkynyl or the aryl group. A
broad range of internal alkynes with various substituents like
aromatic, carbocyclic, ethers, aliphatic, long-chain aliphatic, and
TMS moieties were readily tolerated, delivering the products
with excellent results as well. Even a terminal alkyne withstood
the reaction conditions and furnished the desired product 4l in
moderate yield but with excellent diastereo- and enantiose-
lectivity. A crystal structure of 4e (Table 2) and 7 (Table 3)
revealed the absolute configuration which was assigned to all
other products as well (see the SI for details).17
12a-acetamido-substituted benzo[c]xanthene 4a was obtained
as largely a single diastereomer in good to moderate yields in
almost all cases studied within 8−16 h at room temperature.
The results further revealed that ortho-disubstitution within the
3,3′-aryl substituents in the BINOL backbone of the Brønsted
acid catalyst gave superior enantioselectivities. For example,
with phosphoric acids 3a and 3b carrying mesityl and 2,4,6-
triisopropyl phenyl groups as 3,3′-substituents, the product was
obtained with excellent selectivity but only moderate yield
(entries 1 and 2). Phosphoric acid 3d gave rise to an improved
yield while maintaining the excellent enantioselectivity
observed before (entry 4). Phosphoric acid catalyst 3c with
pentamethylated 3,3′-phenyl groups was found to be the
optimal catalyst for this reaction, which gave the highest
enantioselectivity of 99:1 er in combination with excellent
diastereoselectivity of >95:5 and good product yield of 72%
In order to further broaden the scope of this reaction with
respect to the enamide component, we employed some more
carbocyclic as well as heterocyclic enamides in this process, all
B
DOI: 10.1021/ol503662g
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Letter
a d
−
Table 3. Substrate Scope
Scheme 2. Enaminone 10 as Nucleophile
a c
−
Table 4. Hydrogenation Reactions
a
All reactions were carried out with 0.42 mmol (1.0 equiv) of 1, 0.42
mmol (1.0 equiv) of enamides 2b−f, and 5 mol % of catalyst 3c in 4
b
mL of CH2Cl2 at rt for 8−36 h. Isolated yield of the purified product.
c
dr was determined by 1H NMR analysis of the crude reaction mixture.
d
er was determined through chiral HPL analysis (see the Supporting
Information).
of which delivered the desired products in good yield and
excellent selectivity (Table 3).
a
All reactions were carried out with 0.42 mmol (1 equiv) of 4−9 and
12b, 10 mg of Pd/C catalyst in 6 mL of CH3OH at rt under positive
b
c
Quite interestingly, using chromene- and thiochromene-
based enamides 2c and 2d as nucleophiles furnished the
corresponding alkynyl-substituted chromeno[4,3-b]chromenes
6a−c and thiochromeno[4,3-b]chromene 7, respectively, in
good yield, exceptional diastereo- and enantiocontrol. Enamide
2e, lacking the annulated aromatic ring, also conferred the
addition product 8 in good yield, diastereoselectivity, and 88:12
er. With regioisomeric enamide 2f as substrate, alkynylated
benzo[a]xanthene 9 was successfully obtained, albeit with
decreased selectivity. Acyclic enamides, however, currently give
rise to only low enantioselectivity.18
The excellent results obtained with enamides 2a−f
encouraged us to further investigate N-acetylated enaminone
11 as a nucleophile. Under the optimized conditions, 11
underwent nucleophilc substitution with propargylic alcohols
1a and 1h followed by N,O-acetalization to yield the desired
products 11a and 11b, respectively, in good yield and moderate
selectivity (Scheme 2).
To further reveal the synthetic potential of this new process,
some of the products were subsequently subjected to catalytic
hydrogenation using 10% Pd/C to furnish 7-alkyl-12a-
acetamido-substituted tetrahydrobenzo[c]xanthenes 12a−f
with facility (Table 4). This strategy offers an excellent method
of accessing these products which otherwise would be
inaccessible.
In summary, we have developed a highly enantioselective,
Brønsted acid catalyzed propargylic alkylation of 1-(o-
hydroxyphenyl)propargylic alcohols with enamides, a process
pressure of H2 for 2−12 h. Isolated yield of the purified product. dr
was determined through H NMR analysis of the product mixture.
1
which upon subsequent N,O-acetalization furnishes a broad
range of 7-alkynyl-12a-acetamido-substituted benzo[c]-
xanthenes and related heterocycles efficiently. The products
can be easily converted into their highly valuable saturated
analogues by catalytic hydrogenation. This study further
emphasizes the efficacy of phosphoric acid catalyzed,
enantioselective reactions of o-QMs and extends the scope of
synthetic transformations involving enamide nucleophiles.
ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed experimental procedures, spectral data for all new
compounds, and crystallographic data and CIF information for
4e and 7. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: schneider@chemie.uni-leipzig.de.
Notes
The authors declare no competing financial interest.
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Letter
Miyake, Y.; Nishibayashi, Y. Chem.Eur. J. 2012, 18, 3321. (e) Ikeda,
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(f) Motoyama, K.; Ikeda, M.; Miyake, Y.; Nishibayashi, Y. Organo-
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(11) (a) Zhang, C.; Hu, X.-H.; Wang, Y.-H.; Zheng, Z.; Xu, J.; Hu,
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ACKNOWLEDGMENTS
■
We are grateful to the Deutsche Forschungsgemeinschaft for
the generous financial support of this study. We thank Dr. P.
Lonnecke (University of Leipzig) for obtaining the crystal-
structure analysis. We gratefully acknowledge the donation of
chemicals from Evonik and BASF.
̈
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D
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