Au(I)-Catalyzed Cyclization/Semipinacol Rearrangement Reaction of Allenes to Construct Quaternary Carbon-Containing Scaffolds

Allenes to Construct Quaternary Carbon-Containing Sca ﬀ olds

Letter

Au(I)-Catalyzed Cyclization/Semipinacol Rearrangement Reaction of

Tian-Lu Zheng, Ye Zhang, A-Long Gou, Fu Cheng, Si-Zhan Liu, Lan Yu,* Ming-Yue Cui, Xue-Tao Xu,

Kun Zhang, and Shao-Hua Wang*

Cite This: https://dx.doi.org/10.1021/acs.orglett.0c02262

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sı Supporting Information

ABSTRACT: The development of methods toward the construction of

quaternary carbon centers has been a hot topic in recent years. In this

work, an Au(I)-catalyzed intramolecular cyclization/semipinacol

rearrangement of allene-containing allylic silyl ether was developed to

provide a direct strategy for the construction of multisubstituted

cyclohexene-type compounds with a quaternary carbon center in

moderate to good yields. In particular, this method provides an

alternative synthetic strategy for the construction of a multisubstituted spirocyclo[4.5]decane skeleton and may be applied to the

synthesis of related bioactive molecules and their derivatives, thus facilitating the corresponding functional studies.

he quaternary carbon center is a highly important key

structural moiety that is broadly found in bioactive

in situ generated dipoles from acetals, formal [4 + 2] reaction

between benzofulvenes and 2,4-dienals through trienamine

natural products and pharmaceutical molecules and is essential

catalysis, transition-metal-catalyzed C−H functionalization of

aromatic compounds with alkynes, intramolecular α-arylation

of α-substituted cyclic ketones, photo-induced intramolecular

2 + 2] cyclization reaction, and intramolecular dearomatiza-

tion cyclization of phenols have been developed (Scheme 1).

Despite these achievements, it is still highly desirable to further

for their corresponding biological activities. It is also clear that

this moiety is also diﬃcult to construct because of the special

steric repulsion generated by the four substituents bonded to

[

the carbon. Therefore, the development of eﬃcient methods

for the construction of this structural moiety is a well-known

synthetic challenge that has attracted extensive attention of the

synthetic chemistry community. For instance, for the

construction of spirocyclo[4.5]decane skeleton which is the

key motif of a wide variety of bioactive natural products and

synthetic intermediates (Figure 1), a series of synthetic

Scheme 1. Selected Strategies for the Construction of a

Spirocyclo[4.5]decane Skeleton

methods have been developed by organic chemists to conﬁrm

the corresponding chemical structures and carry out related

molecular function studies, in particular, the screening of

druglike molecules. Selected strategies such as the intermo-

lecular [3 + 2] cycloaddition reaction of N -benzylindoles with

Received: July 8, 2020

Figure 1. Selected natural products bearing spirocyclo[4,5]decane

cores.

XXXX American Chemical Society

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

Letter

investigate this topic for the pursuit of reaction eﬃciency and

structural diversity of the corresponding products.

Table 1. Optimization of Reaction Conditions

In addition to the strategies mentioned above, semipinacol

rearrangement has also been recognized as a practical approach

for the construction of quaternary carbon-containing scaﬀolds

through an appropriate migration or reorganization proce-

dure. Accordingly, based on the characteristics of semi-

pinacol rearrangement, the exploration of new initiators to

trigger the rearrangement process is invariably a key point for

the corresponding methodology design and has attracted broad

attention of synthetic chemists, leading to a variety of diﬀerent

reaction patterns as well as synthetic strategies for obtaining

entry

Ag salt

solvent

DCE

time (h)

yield (%)

AgNO₃

AgOTf

AgTFA

AgOAc

trace

Ag PO₄

1a,c

bioactive natural products.

In the case of the

AgSbF₆

spirocyclo[4.5]decane skeleton, although both α-hydroxy

epoxide alcohol and vinylogous α-ketol have been used as

precursors to construct this type of moiety (Scheme 2, eqs 1

Ag CO₃

AgNTf₂

AgBF₄

DCE

toluene

Dioxane

THF

Scheme 2. Design of Tandem Semipinacol Rearrangement

Reaction

AgBF

AgBF₄

MeOH

EtOH

(CH ) CO

CH CN

DCE

CHCl₃

DMF

DMSO

trace

CH CN

Unless speciﬁed, all reactions were running at 1a (0.1 mmol, 1

equiv), Au cat. (15 mol %), and Ag salt (15 mol %) in a reaction tube

in solvent (2 mL) at room temperature. Isolated yield of 2a. No Au

catalyst was added. nd = not detected.

in the absence of Au(PPh )Cl, and only 1a was recovered

(

Table 1, entry 22). Therefore, the use of Au(PPh )Cl and

AgBF as the catalyst in CH CN at room temperature was

selected as the optimal reaction conditions.

and 2), it is still important to further explore new

rearrangement patterns that can eﬃciently introduce the

needed functional groups on the products for subsequent

transformations. Therefore, in connection with our interests in

the semipinacol rearrangement-based synthetic strategy

development and inspired by the chemical property of allene

With the optimized reaction conditions in hand (Table 1,

entry 16), we next investigated the substrate scope of this

reaction. As shown in Table 2, most of the tested substrates

gave the expected products in good to excellent yields. For R

and R being methyl and hydrogen, respectively, the expected

product 2b was obtained in 75% yield and a dr ratio of 1:1.

functional group in cyclization reaction, we envisioned that

an Au(I)-catalyzed intramolecular cyclization/semipinacol

rearrangement reaction of allylic alcohols with a properly

installed allene moiety may be viable to aﬀord spirocyclo[4.5]-

When a cyclopentane or cyclohexane ring was formed by R

and R , the desired tricyclic products 2c and 2d were produced

in 65% and 70% yields, respectively. Furthermore, the reaction

was applicable for the substrate with a cyclopentanol formed

14,15

decane and related skeletons (Scheme 2, eq 3).

Herein, we

by R and R to aﬀord product 2e through a ﬁve-membered to

six-membered ring expansion. To support the utility of this

reaction in the construction of more complex skeletons, two

substrates 1f and 1g with fused and spirocyclic ring systems,

respectively, were subjected to this transformation giving the

expected products 2f and 2g in good yields. The product 2g

was also obtained, conﬁrming that a protected amine moiety

was amenable to this reaction. Based on the above results, we

further applied the reaction to the substrates with an aryl group

demonstrate such a tandem reaction as a new reaction pattern

for the semipinacol rearrangement involved transformations.

Following the original assumption, we ﬁrst tested the

feasibility of the designed tandem reaction using 1a as the

model substrate. Fortunately, the ﬁrst use of the combination

of Au(PPh )Cl and AgNO , a common catalyst system for the

activation of allene moiety, gave the expected spirocyclic

product 2a in DCE with a yield of 82% (Table 1, entry 1).

Next, the counterion eﬀect was further tested with other silver

salts, and AgBF₄was found to be the best choice.

Subsequently, under the catalysis of Au(PPh )Cl and AgBF ,

as R , and found that most of the substrates were converted to

the desired products. According to the results, it is very clear

that the corresponding migration step is in good agreement

with a common feature of semipinacol rearrangement; i.e., the

aryl group tends to migrate more preferentially than the alkyl

group and hydrogen. Therefore, the corresponding methyl

the reaction was carried out with diﬀerent solvents, among

which the best result was obtained in CH CN to aﬀord 2a in

5% yield (Table 1, entry 16). Additionally, no 2a was formed

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

Letter

Table 2. Au(I)-Catalyzed Tandem Synthesis of Quaternary

Carbon-Containing Scaﬀolds

2o−2q in good yields. Other than the phenyl group, substrate

1s with a thiophene-2-yl group was also applicable to this

reaction, leading to product 2s in 67% yield. Other than the

MOMOCH CH group, another two substrates 1w and 1x

with R being methyl and phenyl, respectively, were also

amenable to this reaction, aﬀording the desired products 2w

and 2x. It should be noted that the use of substrates 1y and 1z

did not give any desired products. Furthermore, to

demonstrate the feasibility of this reaction for practical use, a

gram-scale reaction of substrate 1a was carried out to give

product 2a in 86% yield.

As a support of the potential utility of this method, the

construction of the angular triquinane type of ring system was

attempted using compound 2w as the starting material

(

Scheme 3). Ozonolysis of 2w followed by an acid-promoted

Scheme 3. Synthetic Utility of the Tandem Reaction

intramolecular cyclization gave 3w in 34% yield over two steps.

Next, hydrogenation of 3w could aﬀord 4w, which could be

converted to 5w with the expected tricyclic ring system in the

presence of KO Bu.

To better understand the reaction mechanism, a control

experiment of 1a was carried out under optimized conditions

along with 1 equiv of H O, and 2a could be obtained in 90%

yield in 3 h (Scheme 4, upper part). Therefore, it is very likely

Scheme 4. Control Experiment and Plausible Reaction

Mechanism

Unless speciﬁed, all reactions were running at 1 (0.1 mmol, 1 equiv),

Au cat. (15 mol %), and Ag salt (15 mol %) in a reaction tube in

CH CN (2 mL) at room temperature. 1 g of 1a was used.

ketone and aldehydes with an aryl-substituted quaternary

carbon center can be selectively obtained by choosing the

proper substrates. Additionally, the steric eﬀects and electronic

eﬀects of the substituent on the phenyl ring of the substrates

were also observed for this reaction. For example, when the

substituent on the phenyl was a methoxy group (substrates 2i

to 2k), its presence at the para-position aﬀorded the expected

product 2k in the highest yield (70%), and the lowest yield of

that H O is involved in this reaction. On the basis of the above

experimental results, a plausible reaction mechanism was

proposed using the reaction of substrate 1a as the example

(Scheme 4, lower part). First, with the activation of the Au(I)

catalyst, an intramolecular cyclization/semipinacol rearrange-

ment occurs to give intermediate I that then aﬀords the

product 2a through the release of the Au(I) catalyst and

8% was obtained for product 2i. When the substituent on the

TESOH in the presence of a trace amount of H O from the

phenyl was at the para-position, the use of an electron-

donating group such as a methyl or methoxy group led to

products 2k and 2m being obtained in higher yields than 2n

with a chloro group. Furthermore, three typical substrates 1o−

solvent.

In conclusion, we have successfully developed an Au(I)-

catalyzed cyclization/semipinacol rearrangement of allenes

providing a direct synthetic method for the synthesis of

multisubstituted cyclohexene type compounds with a quater-

nary carbon center. This method not only further enriches the

q where R and R were phenyl-based substituents also

underwent the reaction smoothly to give the expected products

Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters

1) (a) Saraswat, P.; Jeyabalan, G.; Hassan, M. Z.; Rahman, M. U.;

Letter

semipinacol rearrangement-based tandem reaction model but

also may provide an alternative synthetic strategy for the

synthesis of related bioactive molecules and their derivatives to

facilitate the corresponding functional studies.

Guangdong Province (No. 2017KTSCX185, 2017KSYS010,

2016KCXTD005, and 2019KZDXM035).

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ASSOCIATED CONTENT

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The Supporting Information is available free of charge at

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AUTHOR INFORMATION

■

Corresponding Authors

Lan Yu − School of Pharmacy & State Key Laboratory of

Applied Organic Chemistry, Lanzhou University, Lanzhou

30000, P.R. China; Email: yul@lzu.edu.cn

Shao-Hua Wang − School of Pharmacy & State Key Laboratory

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Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.orglett.0c02262

(

Author Contributions

T.-L.Z. and Y.Z. contributed equally to this work.

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ACKNOWLEDGMENTS

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This work is supported by the National Natural Science

Foundation of China (21472077 and 21772071), the

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