A tandem annulation with a [1,3]-hydride transfer as the key step leading to isochromans

Article abstract of DOI:10.1039/c7cc06144g

An unprecedented method that enables the direct coupling of an α-C-H bond in alcohols with 2-arylacetaldehydes through a [1,3]-hydride transfer ([1,3]-HT) of oxocarbenium ions catalyzed by a Lewis acid has been developed. The redox neutral preparation of

Full text of DOI:10.1039/c7cc06144g

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DOI: 10.1039/C7CC06144G
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A Tandem Annulation with [1,3]-Hydride Transfer as the Key Step
Leading to Isochromans
Yingwei Wang,a Guangxun Li,b Hongxun Liu,b Zhuo Tang,*b Yuan Cao,a Gang Zhao*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The unprecedented method that enables direct coupling of α-C-H
bond of alcohols with 2-arylacetaldehydes through [1,3]-hydride
transfer ([1,3]-HT) of oxocarbenium ions catalyzed by a Lewis acid
has been developed. The redox neutral preparation of isochroman
derivatives proceeds via tandem condensation/ [1,3]-HT/ Friedel-
Crafts sequence with moderate to good yields. Deuterium-labeling
studies provides mechanistic insights and reveals the redox
functionalization proceeds via [1,3]-HT.
In the past few decades, the tandem [1,n]-hydride
transfer/cyclization process has attracted considerable interest
for its application in selective activation and direct
functionalization of inactive C(sp3)-H.1 This tandem process
that is initiated by an internal hydride transfer and continued
by
a final cyclization step of the presumed dipolar
intermediates represents an intriguing sequential C(sp3)-H
activation/C-X (X = C, N, O) bond formation process and proves
to be a versatile protocol to construct a wide variety of cycles.
In 1969, the [1,6]-hydride transfer was verified in with isotopic
Scheme 1. First examples for [1,n]-hydride transfer/cyclization processes
alkynes to give substituted indene derivatives (Scheme 1c),
which appears to be the first example of C(sp3)-H activation
labelling experiments in the reaction between o-dialkylamino- through [1,4]-hydride transfer.5 In 2008, Donohoe group
anilines and aldehydes by Meth-Cohn group (Scheme 1a).2 This
redox process involving the [1,5]-hydride transfer was initially
clarified with control experiments by Reinhoudt in 1984
(Scheme 1b).3 Despite this transformation mostly undergoes a
[1,5]-hydride transfer to an electrophilic moiety followed by
cyclization,4 resulting in the formation of six-membered or
larger cycles, other hydride transfers have also been realized in
several cases. In 2009, He and co-workers reported the
intramolecular cyclization of o-isopropyl or o-benzyl aryl
developed an elegant strategy in which a [1,2]-hydride transfer
is followed by intramolecular trapping with hydride
a
nucleophile.6 And then, they first realized tandem [1,2]-
hydride transfer/cyclization process for the synthesis of
spirocyclic acetal (Scheme 1d).7 Thus far, the functionalization
of C(sp3)-H through [1,3]-hydride transfer/cyclization has not
been clearly reported although Tverdokhlebov proposed a
[1,3]-hydrogen transfer mechanism in the formation of fused
benzazepines.8 Herein, we present the tandem redox
annulation of alcohols and aldehydes to obtain oxygen
heterocycles through internal cyclization of oxacarbenium
intermediate after [1,3]-hydride transfer.
a. Department of Pharmaceutical & Biological Engineering, School of Chemical
Engineering, Sichuan University, Chengdu, Sichuan 610065 (China)
E-mail: gzhao@scu.edu.cn
Generally, two vital components, hydride acceptor and
hydride donor, form the substrates of the redox-neutral C-H
functionalization reaction that involves an internal hydride
transfer as a key step. In the vast majority cases, the usual
acceptors for hydride transfer are electron-poor alkenes,9
aldehydes,10 iminium ions,11 but also transition-metal-
activated alkynes12 and allenes.13 However, other more
b.Natural Products Research Center Chengdu Institution of Biology Chinese
Academy of Science, Chengdu, Sichuan 610041 (China)
E-mail: tangzhuo@cib.ac.cn
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Table 1. Screening of acid catalystsa.
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DOI: 10.1039/C7CC06144G
Entry
1
2
3
4
5c
6
7
8
Catalyst
-
PhCOOH
AcOH
Yield [%]b
Entry
11
12
13
14
Catalyst
Bi(OTf)3
Ga(OTf)3
Y(OTf)3
Sc(OTf)3
ScCl3.6H2O
Sc(OAc)3.6H2O
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Yield [%][b]
0
0
0
0
22
0
0
0
38
17
45
26
37
53
8
TfOH
BF3.Et2O
TMSOTf
InCl3
MgBr2
In(OTf)3
Al(OTf)3
15
Scheme 2. Designed tandem reaction for [1,3]-HT triggered α-C-H functionalization of
alcohols to ethers.
0
16
17d
18 e
78
82
35
， f
9
10
19d
convenient hydride acceptors that could be applied through
this transformation have rarely been reported.14
c
a Reactions were performed on a 0.2 mmol scale of 1a. b Isolated yield. 1.0 equiv of
A class of important organic intermediates are highly reactive
oxacarbenium ions, which act as excellent electrophilic fragments
attacked by diverse nucleophiles for the synthesis of complex
natural products and bioactive molecules.15 One of multitudinous
measures of forming oxacarbenium ions is acid catalyzed
condensation of alcohols with aldehydes, which has been applied in
a few classical organic reactions, such as Prins cyclization,16
acetalization reaction,17 Oxa-Pictet-Spengler reacion. 18 In view of
the high reactivity of oxacarbenium ions, we designed a new
tandem reaction in which an initial 1,3-hydride transfer would
ultimately result in the formation of oxygen heterocycles. As
outlined in Scheme 2, the acid-catalyzed reaction of alcohol 2 and
aldehyde 1 decorated with pendant nucleophile is envisioned to
initially results in the formation of the oxacarbenium ion A.
BF3.Et2O was used. d Reaction was run with 5.0 equiv of 3-methyl-butanol. e Reaction
was run with 0.25 mL of 3-methyl-butanol in the absence of solvent. 5% amount of
f
catalyst was used.
acid with stronger acidity was still not effective for obtaining target
product 3a (entry 4). After screening several Lewis acids catalysts
(entry 5-9), to our delight, the use of 100% mmol BF3.Et2O and 10%
mmol In(OTf)3 allowed for the isolation of the isochroman product
3a in 22% and 17% yield, respectively (entry 5 and 9). Encouraged
by the result, other Lewis acids metal triflates were evaluated as
the catalysts for the reaction. The addition of all metal triflates to
the reaction led to product formation in 17-53% yields(entry 10-14)
and Sc(OTf)3 was proved the most efficient catalyst in terms of
reaction yield (entry 14). Other Lewis acids containing scandium,
such as ScCl3.6H2O and Sc(OAc)3.6H2O (entry 15 and 16)were tested
with ScCl3.6H2O promoting conversion into 3a in 8% yield. The
increase of the amount of 3-methyl-butanol 2a to 5.0 equiv could
lead to an increase in yield to 78% under toluene as solvent and
Sc(OTf)3 as catalyst (entry 17). Interestingly, attempt to use greatly
excessive alcohol 2a without any solvent was fruitful, affording the
product in 82% yield （entry 18）. Finally, lowering the catalyst
loading of Sc(OTf)3 to 5 mmol% gave a lower conversion but still
provided the desired product (entry 19).
Subsequent to internal [1,3]-hydride transfer
, the resulting
isomerized oxacarbenium ion B could react with the pendant
nucleophile. Proton loss would then result in the formation of
product 3. To the best of our knowledge, the isomerization of
oxacarbenium ions has never been reported although related redox
isomerization process of iminium ions has been developed through
azomethine ylide route19 or [1,3]-proton transfer route. 20
The key to achieving this goal was determining how to trigger
the spatially rougher [1,3]-hydride transfer of oxacarbenium ion A
and selecting suitable pendant nucleophile to selectively capture
new formed oxacarbenium ion B. As arenes with electron-donating
group para to the reaction site has known nucleophilic properties
for oxacarbenium ion and favorable compatibility with aldehyde
group, it occurred to us that arylacetaldehyde with electron-
donating group in para of aromatic ring should be able to serve as
the aldehyde decorated with pendant nucleophile in the proposed
sequence. Such a reaction would lead to the formation of
isochroman template, which constitutes the framework of many
natural products, as well as that of synthetic and semisynthetic
compounds of interest.
Though the desired annulation product was obtained with high
yield under optimized conditions, two different reaction pathways
In order to prevent tandem aldol condensation/intramolecular
Friedel-Crafts cyclization,21 we operated with electron-rich
nonenolizable β-aromatic acetaldehyde 1a and 2a as model
substrates (Table 1). Under initial reaction conditions, no desired
product was observed in the absence of catalyst with toluene as
solvent at 110 ℃ for overnight (entry 1). Addition of 0.1 equiv
benzoic acid or acetic acid, catalysts previously shown to be an
excellent promoter for amine α- functionalization,22 failed to
provide any improvements (entry 2-3). Trifluoromethanesulfonic
Scheme 3. Clarification of the reaction mechanism of the [1,3]-hydride transfer
2 | J. Name., 2012, 00, 1-3
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could be assumed for the formation of key oxacarbenium ion B/E amount of products under the standard conditions. The use of
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DOI: 10.1039/C7CC06144G
A formed by the excess alcohols increased the yields of products to 57% and 31% (3d
(Scheme 3). Because oxacarbenium ion
condensation of aliphatic alcohols 2 and aldehydes 1 has α-H and β- and 3g, respectively) in the absence of solvent. When the primary
H adjacent to oxygen atom (Ha and Hb in Scheme 3), we proposed alcohol containing a chiral center is employed as a substrate, the
the following two reaction paths: (1) the [1,3]-hydride transfer corresponding isochroman 3i is isolated in 47% yield with
pathway accessible to the formation of intermediate B, which diastereoisomeric ratio of 2:1. In contrast with aliphatic alcohols,
maybe subsequently deprotonate in the direct formation of the substrate 2j as a representative of benzyl alcohols unfortunately
vinylether intermediate C (path A) and (2) the pathway involving furnished unsatisfactory result under the optimized protocol. Next,
the [1,4]-hydride transfer followed by [1,2]-hydride transfer (Path we examined the scope of the reaction with a representative set of
B). To clarify the detailed hydride transfer process, we firstly nonenolizable aldehydes bearing electron-donating functional
performed the reaction of α,α-d2-3-methyl-1-butanol [D2]-2a with groups in the meta-position of the aromatic moiety (Table 2).
the substrate 1a under the standard conditions (Eq. 1). The Electron-rich dialkyl-substituted aldehydes were also found to be
corresponding deuterium product [D2]-3a was obtained successfully appropriate substrates for producing annulation products 3k-m in
in 68% yield. The fact that deuterium incorporation of 33% to 72% yields. Lower yields than 3k for compounds 3l and 3m
compoundwas observed only at 1 and 3-position led us to might be caused by some way for decomposition. The substrate
unambiguously rule out the latter pathway. Furthermore, the with a hydroxyl group substitution in the meta-position was
deuterium content of β-position in compound [D5]-3d was no loss likewise reacted to give isochroman 3p, albeit in a diminished yield.
when the reaction of aldehyde 1a and d6-ethanol [D6]-3d was
Moreover, based on the expectation for realizing the tandem
carried out under the neat conditions (Eq 2). Our results clearly redox annulation of benzyl alcohols and aldehyde 1, a range of
indicated that the [1,3]-hydride transfer initiated the cyclization reaction conditions were screened, which confirmed 200 mmol%
reaction.
BF3.Et2O was suitable for catalyzing the redox reaction to obtain the
We then examined the versatility of this [1,3]-HT/cyclization desired product 3j in 25% yield (Scheme 4).23
reaction in the redox α-arylation of diversely aliphatic alcohols 2,
utilizing 1a as substrate. As illustrated in Table 2, The primary
alcohols are compatible with the catalysis conditions to providing
the corresponding cyclic ethers 3a-f in 32% to 76% yields, including
products such as 3e and 3f containing terminal phenyl groups. The
secondary alcohols can be also applied to form tertiary ethers (3g-h)
in good yield. It should be noted that annulations of low boiling-
point substrates (e.g. ethanol and isopropanol) with 1a gave a trace
Scheme 4. Tandem redox annulation of benzyl alcohol.
Though [1,3]-hydride transfer process has been verified by the
deuterium-labeling experiments, the oxocarbenium isomerization/
cyclization sequence could occur through synchronous or stepwise
path theoretically (Scheme 5). We then focused our attention on
the use of chiral alcohol to check whether the 1,3-hydride transfer
in the present reaction occurs simultaneously with nucleophilic
attack. Aldehyde 1e and (S)-2-butanol were subjected to the [1,3]-
hydride transfer/cyclization process to result in racemic 3q in 56%
yield, which indicated the stepwise path seemed plausible.
However, we can’t rule out the possibility of synchronous path due
to the racemization of (R)-3q under the harsh acidic conditions. 24
In conclusion, we have showed here the first examples of
internal redox [1,3]-hydride transfer/ cyclization reaction. Unlike in
Table 2. Scope of alcohols and aldehydesa
a
b
c
Reactions were performed on a 0.2 mmol scale of arylacetaldehyde 2a
.
Reactions were performed with 0.25 mL alcohol in the absence of solvent.
Racemic 2-methyl-1-butanol was used to obtain 3i as a 2:1 mixture of
diastereomers. d The reaction time was extended to 36 h.
Scheme 5. Proposed mechanism.
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Chem. Eur. J. 2013; 19: 5232-5237; (b) C-H. Jiang, X. Lei,
most HT/cyclization reactions, the in-situ forming oxocarbenium ion
(carbocation adjacent to oxygen) was applied as the novel acceptor,
which was attacked by α-hydride from alcohols to realize the
oxocarbenium isomerization. The combination of electron-rich
nonenolizable β-aromatic acetaldehydes and Sc(OTf)3 resulted in α-
arylation of alcohols to isochroman derivatives in moderate to good
yields, in which α-C-H functionalization of alcohols through internal
redox processes is realized firstly.25 Further investigation toward
the development of new reactions through the [1,3]-hydride
shift/functionalization sequence is underway in our laboratory.
View Article Online
L. Zhen, H-J. Du, X. Wen, Q-L. Xu, H. Sun, Beilstein. J.
DOI: 10.1039/C7CC06144G
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23 For details, see Supporting Information.
24 For racemization of 3q, see Supporting Information.
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A table of contents entry
First [1,3]-hydride transfer/cyclization process for oxacarbenium isomerization.