Diastereoselective tetrahydropyrone synthesis through transition-metal-free oxidative carbon-hydrogen bond activation

Article abstract of DOI:10.1002/anie.200706002

(Chemical Equation Presented) C-C bonds from C-H bonds: Tethering nucleophilic groups to benzylic and allylic ethers allows cyclization reactions by DDQ-mediated oxidative carbon-hydrogen bond activation (see scheme; DDQ = 2,3-dichloro-5,6-dicyano-1,4-ben

Full text of DOI:10.1002/anie.200706002

Communications
DOI: 10.1002/anie.200706002
ꢀ
C H Bond Activation
Diastereoselective Tetrahydropyrone Synthesis through Transition-
Metal-Free Oxidative Carbon–Hydrogen Bond Activation**
Wangyang Tu, Lei Liu, and Paul E. Floreancig*
Selectively activating chemical bonds that are generally
considered to be inert is an attractive strategy for introducing
functionality into and enhancing the structural complexity of
easily-prepared substrates, particularly when bond activation
ultimately leads to carbon–carbon bond formation.[1] We have
reported[2] several examples in which oxidative carbon–
carbon bond activation can be used to initiate cyclization
reactions through carbon–carbon bond formation. Reaction
initiation through single-electron oxidation[3] alleviates che-
moselectivity problems that can arise from conventional
Lewis acid initiated methods for electrophile formation. To
facilitate substrate synthesis and improve reaction atom
economy[4] we have initiated a program that is directed
toward promoting oxidative electrophile formation by
carbon–hydrogen bond activation. Toward this objective we
initially chose to exploit the propensity of 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (DDQ) to form aryl-substituted
oxocarbenium ions from benzylic ethers.[5] This process has
been utilized for bimolecular carbon–carbon bond forma-
tion,[6] but these reactions proceed efficiently only with
electron-rich arenes, and either require high temperatures
with ketone nucleophiles or dicarbonyl/Lewis acid mixtures,
or the addition of pregenerated nucleophiles such as enolsi-
lanes after cation formation.[7]
strate that the scope of the process can be dramatically
expanded by applying the protocol in an efficient approach to
ring formation through allylic carbon–hydrogen bond activa-
tion. The incorporation of oxygen-containing groups into the
substrates and the impact of arene or alkene substitution on
the reaction rate is also discussed.
Our initial studies focused on the conversion of para-
methoxybenzyl (PMB) ether 1 into tetrahydropyrone 2
(Scheme 1) by DDQ-mediated oxocarbenium ion formation.
Successful and broad application of DDQ-mediated
carbon–hydrogen bond activation and subsequent carbon–
carbon bond formation to annulation reactions requires that
the nucleophiles be stable toward DDQ, that the reaction
products not be subject to additional oxidation, and that a
wide range of ethers serve as substrates. Herein we report that
DDQ promotes the formation of stabilized carbocations by
benzylic carbon–hydrogen bond activation under ambient
conditions in the presence of appended nucleophilic groups
and leads to diastereoselective carbon–carbon bond forma-
tion. Particularly important is the observation that, relative to
bimolecular addition reactions, appending nucleophilic
groups to the ether enhances the range of the benzylic
groups that can serve as cation precursors. We also demon-
ꢀ
Scheme 1. Cyclization through oxidative C H activation.
This process proceeds most readily in 1,2-dichloroethane and
in the presence of 2,6-dichloropyridine and powdered 4 Š
molecular sieves (M.S.), which inhibit oxidative cleavage of
the PMB group. Under these conditions the reaction was
complete within 10 minutes at room temperature to provide 2
in 77% yield as a single diastereomer. This reaction
proceeds[8] by electron transfer from 1 to DDQ to form
radical-ion pair 3. Because of the substantially weakened
carbon–hydrogen bonds in alkylarene radical cations,[9] the
formation of benzylic cation 4, shown in the chair conforma-
tion that is relevant to cyclization,[10] either undergoes
[*] W. Tu, L. Liu, Prof. P. E. Floreancig
Department of Chemistry
University of Pittsburgh
Pittsburgh, PA 15260 (USA)
Fax: (+1)412-624-8611
E-mail: florean@pitt.edu
[**] This work was supported by grants from the National Institutes of
Health (GM-62924) and the National Science Foundation (CHE-
0139851).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
subsequent direct hydrogen atom transfer,[11] or a sequence
including proton transfer and rapid benzylic radical oxida-
tion.[8] Cyclization and acetyl group cleavage yield 2.
Although 2 is a benzylic ether, and therefore a potential
oxidation substrate, overoxidation can be completely sup-
pressed by suitable reaction monitoring. This useful result can
be attributed to the steric interactions that limit access to
radical-cation conformer 5 in which the benzylic hydrogen
atom is aligned appropriately with the p system of the arene
to effect efficient benzylic oxidation.[12]
methyl ethers (Table 1, entry 6), which are expected to have
oxidation potentials that are approximately the same as 1,[17]
also served as substrates for the reaction. These results
indicate that this process could be applied to the construction
of a diverse set of aryl tetrahydropyrans, including biologi-
cally interesting structures related to the glycosidic anti-
biotics.[18] To explain the successful cyclizations of the benzylic
ethers that serve as poor substrates in bimolecular reactions,
we propose that oxidative carbon–hydrogen bond activation
(3!4 in Scheme 1), in contrast to postulates regarding
electron-rich arenes,[7] is reversible when the intermediate
oxocarbenium ion is not strongly stabilized by an electron-
donating group on the arene. The rate enhancement derived
from intramolecular nucleophilic attack on transiently-
formed oxocarbenium ions promotes a successful cyclization
in this system under conditions in which intermolecular
nucleophilic attack is rarely observed.
The scope of the method is shown in Table 1. Remarkably,
although it reacts relatively slowly, a nonsubstituted benzyl
ether (Table 1, entry 1) proved to be a suitable substrate for
Table 1: Arene and alkyl chain scope.[a]
To determine whether the process is tolerant of functional
groups, we prepared substrates that bear oxygen-containing
substituents in the alkyl side chain. Silyl ethers are tolerated
and do not lead to undesired acetal formation (Table 1,
entry 7). Electrophilic groups such as acetate (Table 1,
entry 8) and isopropyl ester (Table 1, entry 9) groups are
also tolerated, highlighting the capacity of oxidative cleavage
reactions in creating unique and useful chemoselectivity
patterns in functionalized molecules. Notably, the yields of
cyclizations that employ functionalized alkyl chains are
comparable to those that employ unfunctionalized alkyl
chains and, though oxidations produce oxocarbenium ions
in which the adility of the oxygen atom to stabilize the cation
is diminished by the electron-withdrawing substituents, the
reaction rates drop only slightly.
Entry
Ar
R
t [h]
Yield [%][b]
1
2
Ph
4-MePh
C5H11
C5H11
C5H11
C5H11
C5H11
C5H11
CH2OTBS
CH2OAc
CO2iPr
14
0.5
0.1
1.5
12
4
0.75
0.75
0.75
63
82
83
57
63
84
74
74
68
3[c]
4
3,4-(MeO)2Ph
3,5-(MeO)2Ph
2-furyl
1-naphthyl
4-MeOPh
4-MeOPh
4-MeOPh
5
6
7
8
9
[a] Representative procedure: Substrate, 2,6-dichloropyridine (4 equiv),
and 4 Š M.S. were stirred in DCE for 15-30 min. DDQ (2 equiv) was then
added and the reaction mixture was stirred for the indicated time.
[b] Yields are reported for isolated, purified products. [c] Reaction was
conducted at ꢀ208C. TBS=tert-butyldimethylsilyl.
To expand the scope of potential substrates with the
objective of preparing products that do not contain an
aromatic ring, we turned our attention to allylic ethers.
Whereas Yadav et al. has shown[19] that primary, but not
secondary, allyl ethers slowly undergo DDQ-mediated oxi-
dative cleavage in the presence of water, we found only a
single precedent for carbon–carbon bond-forming reac-
tions[2d] from allylic ethers. Inspired by the successful cycliza-
tion of the unsubstituted benzyl ether, we subjected a series of
allylic ethers to a modification of the reaction conditions in
which a catalytic amount of LiClO4 was added to inhibit
overoxidation (Table 2).[20] Reactivity in this series again
followed expected trends based on the substrate oxidation
potentials[21] and the stability of the intermediate cations. (E)-
1,2-Disubstituted allylic ether 6 reacted smoothly to form 7.
When mixtures of 6 and its corresponding Z isomer were
subjected to the reaction conditions the E isomer was
consumed much more rapidly because of the heightened
steric interactions that destabilize the oxocarbenium ion of
the Z isomer.[22] 1,1-Disubstituted allylic ether 8 reacted more
slowly than 6 because the vinyl methyl group does not directly
stabilize the intermediate cation. Trisubstituted alkenes 10
and 12 reacted quite quickly,[23] with cyclizations being
complete within 1 hour at or below room temperature. No
alkene isomerization was noted in the formation of 11 and 13.
While reacting slowly and requiring slightly elevated temper-
atures, unsubstituted allyl ether 14 provides 15 in good yield
the cyclization despite the observation that electron-donating
groups are required for related bimolecular oxidative alkyla-
tion reactions.[6] The reaction rate increases as expected for
the p-methylbenzyl ether (Table 1, entry 2), consistent with its
lower oxidation potential[13] and greater capacity for stabili-
zation of the intermediate cation. In this example, oxidation
was observed only at the alkoxyalkyl group, demonstrating
that the regiochemistry of carbon–hydrogen bond activation
in arenes that contain multiple alkyl groups is determined by
the stability of the intermediate cation or carbon–hydrogen
bond strength.[14] The easily-oxidized 3,4-dimethoxybenzyl
ether[15] was exceedingly reactive toward DDQ at room
temperature, and efficient cyclization proceeded within
minutes at ꢀ208C (Table 1, entry 3). The 3,5-dimethoxyben-
zyl ether (Table 1, entry 4), while forming the cyclized
product in acceptable yield, was far less reactive than the
3,4-disubstituted isomer despite having an oxidation potential
that would be expected[15] to be similar. This result is
consistent with literature reports[16] in which 3,5-dimethoxy-
benzyl ethers undergo oxidative cleavage reactions more
slowly than the corresponding 3,4-dimethoxy isomers, pro-
viding additional evidence for the importance of stability of
the intermediate cation in determining the reaction rates.
Furanylmethyl ethers (Table 1, entry 5) and 1-naphthyl-
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Table 2: Cyclization through vinyl-substituted oxocarbenium ions.[a]
the unsaturated fragment of ether 22 (Table 2, entry 9) slows
the reaction to a greater extent than incorporating an
electron-withdrawing group on the aliphatic fragment, the
reaction still proceeds efficiently and stereoselectively. The
efficient cyclization of a substrate in which both chains in the
ether contain functional groups indicates that the method
should be applicable to complex molecule synthesis.
Entry
Substrate
Product
t [h] Yield
[%]
1
2
81
77
We have demonstrated that a range of benzylic and allylic
ethers, which contain enol acetate nucleophiles, serve as
substrates for DDQ-mediated oxidative cyclization reactions.
These processes proceed through efficient carbon–hydrogen
bond activation and result in diastereoselective tetrahydro-
pyrone formation. Appending a nucleophilic group to the
ether allows the use of arenes and alkenes with a wide
assortment of substitution patterns and oxidation potentials.
The method is tolerant of commonly encountered functional
groups on either side of the ether linkage, making it
applicable to structurally complex substrates. Strategically,
the use of allylic and benzylic ether formation as a facile
method for fragment coupling, and the capacity of ether
groups to serve as effective hydroxy-protecting groups prior
to oxidative cyclization coupled with the unique chemo-
selectivity patterns that can be accessed by oxidative pro-
cesses make this an attractive method for ring formation.
Studies directed toward expanding the scope of the nucleo-
phile and elucidating the mechanistic nuances of the process
are in progress and will be reported in due course.
2
17
3[b]
4[b]
5[c]
6
1
1
82
85
80
75
48
24
Received: December 31, 2007
Revised: February 29, 2008
Published online: April 21, 2008
7
30
12
73
77
ꢀ
Keywords: C H activation · cyclization · oxidation ·
.
substituent effects · synthetic methods
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