Cascade Skeletal Rearrangement of Gold Carbene Intermediates: Synthesis of Medium-Sized Pyrimidine-Fused Benzolactones

Article abstract of DOI:10.1002/adsc.202100572

A gold-catalyzed cyclization/cascade skeletal rearrangement of o-cyanophenylalkynones with 3-amino-benzo[d]-isoxazoles has been developed, which provides an approach for synthesizing medium-sized benzolactones. Based on the experimental results, we postul

Full text of DOI:10.1002/adsc.202100572

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doi.org/10.1002/adsc.202100572
Cascade Skeletal Rearrangement of Gold Carbene Intermediates:
Synthesis of Medium-Sized Pyrimidine-Fused Benzolactones
Ali Wang,a Xiaoping Hu,a Xin Xie,a and Yuanhong Liua,*
a
State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic
Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032,
People’s Republic of China
E-mail: yhliu@sioc.ac.cn
Manuscript received: May 12, 2021; Revised manuscript received: June 24, 2021;
Version of record online: July 14, 2021
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202100572
argyl carboxylates,[5] furan-yne cyclizations,[6] the
Abstract: A gold-catalyzed cyclization/cascade
skeletal rearrangement of o-cyanophenylalkynones
with 3-amino-benzo[d]-isoxazoles has been devel-
oped, which provides an approach for synthesizing
retro-Buchner reaction of cyclo-heptatrienes,[7]
oxygen[8] and nitrene transfer[9] reactions etc. However,
the reactions involving three or more than three
chemical bonds cleavage are quite rare in gold-
medium-sized benzolactones. Based on the exper-
catalyzed rearrangement reactions.
imental results, we postulate that the initial nucleo-
philic attack occurs preferentially at the keto moiety
instead of the gold-carbene. This reactivity initiates
an attractive cascade process involving carbene
NÀ O-containing heterocycles such as isoxazoles[10]
and benzo[c]isoxazoles (anthranils)[11] are easily avail-
able heterocyclic compounds. Since the pioneering
studies by Ye, Hashmi, Liu, et al., these heterocycles
transfer, 1,2-aryl migration, cycloaddition, ring-
have been disclosed to be efficient nitrene transfer
expansion etc. resulting in multiple bonds cleavage
reagents for the generation of α-imino gold-carbene
of the initial substrates.
intermediates. The related catalytic reactions are
usually initiated through the nucleophilic attack of the
NÀ O-heterocycles to the gold-activated alkyne fol-
Keywords: gold catalysis; ynones; benzo[d]-isoxa-
lowed by ring-opening to give an α-imino gold-carbene
zoles; gold carbene; skeletal rearrangement
intermediate (Scheme 1A). Generally, the subsequent
reactions tend to occur at the carbene center via
nucleophilic attack (type a), or 1,2-CÀ C bond migra-
Transition-metal-catalyzed reactions involving CÀ C tion (type b) (Scheme 1B). As our continuous interests
bonds cleavage have attracted significant attention not in the development of the new strategies for the
only because they can offer efficient and rapid synthesis of heterocycles, we recently found that 1,4,2-
amplification of molecular complexity through reor- dioxazoles,[12a] 4,5-dihydro-1,2,4-oxadiazoles,[12b] ben-
ganization of organic skeletons, but also due to their zo[d]iso-xazoles,[12c,d] benzofurazans[12e] and benzofur-
intriguing reaction mechanisms.[1] Because of the azans N-oxides[12e] (see Scheme 1A) could also be used
inherent strength and stability of CÀ C bond (83– as efficient nitrene transfer reagents[12,13] to promote
85 kcalmol-1),[2] the cleavage of CÀ C bonds has gold-catalyzed cycloaddition reactions with ynamides.
emerged as one of the most challenging and valuable During our studies on ynones,[14] we envisioned that α-
targets in organic synthesis. In this context, gold- keto, α’-imino gold carbenes I would be generated
catalyzed skeletal rearrangements via cationic or gold- from ynones. It is possible that both of the keto and
carbene intermediates have been proved to be a gold-carbene moiety might be attacked by a nucleo-
powerful tool for the cleavage of CÀ C bonds as well as phile. The chemoselectivity should be highly depend-
CÀ X bonds.[3] Owing to the distinguished reactivities ent on the substitution pattern of the substrates. The
and excellent selectivities of these highly electrophilic reaction with the keto moiety would result in the
intermediates, CÀ C or CÀ X bond cleavage reactions formation of a new gold-carbene intermediate II
(e.g., via 1,2-migration) have been widely invoked in poised for further CÀ X bond formations (Scheme 1C,
gold-catalyzed transformations such as in 1,n-enyne type c). However, such reaction pattern has not been
cycloisomerization,[4] 1,2-acyloxy migration of prop- reported yet. We postulate that the reactivity of the
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Table 1. Optimization of the reaction conditions.
Entry Au catalyst
Ag catalyst
(mol %)
Solvent
Yield
(%)[a]
1
2
3
4
catalyst A
catalyst B
PPh3AuNTf2
IPrAuNTf2
AuBr3
–
–
–
–
–
DCE
DCE
DCE
DCE
DCE
34
6
10
53
3
5
6
7
8
9
IPrAuCl
IPrAuCl
AgNTf2 (10) DCE
AgNTf2 (20) DCE
63
65
48
27
51
IPrAuCl
AgNTf2 (5)
DCE
(ArO)3PAuCl[b] AgNTf2 (10) DCE
10
11
12
13
14
15
16
17
18
19
20
IPrAuCl
IPrAuCl
IPrAuCl
IPrAuCl
IPrAuCl
IPrAuCl
IPrAuCl
IPrAuCl
IPrAuCl
IPrAuCl
–
AgNTf2 (10) Toluene
AgNTf2 (10) 1,4-dioxane 43
AgNTf2 (10) MeCN
AgNTf2 (10) MeNO2
AgSbF6 (10) DCE
36
13
61
60
60
18
0
Scheme 1. Gold-catalyzed reactions involving α-imino gold
carbenes.
AgBF4 (10)
AgOTf (10)
AgOTs (10)
AgOAc(10)
–
DCE
DCE
DCE
DCE
DCE
keto moiety might be greatly enhanced by an electron-
poor R1 group. Herein, we report a gold-catalyzed
reaction of cyano-functionalized ynones with 3-amino-
benzo[d]isoxazoles. To our surprise, the ensuing
catalytic reaction proceeds via an unprecedent cascade
skeletal rearrangement with cleavage of two CÀ C
bonds and one CÀ O bond of the starting substrates,
furnishing pyrimidine-fused dibenzo[b,f]oxocin-6-one
derivatives with wide functional group compatibility.
0
0
AgNTf2 (10) DCE
[a] The yields were determined by 1H NMR using 1,3,5-
trimethoxybenzene as an internal standard.
[b] Ar=(2,4-ditBu)C6H3.
The method offers an efficient and straightforward process, and the cyano group in 1a is incorporated into
access for the synthesis of medium-sized heterocycles. the newly-formed pyrimidine ring. The use of more
t
Our studies began with the optimization of gold- hindered BuXPhosAuMeCNSbF6 (catalyst B) led to
catalyzed reactions of o-cyanophenylalkynone 1a with only 6% of 3a (entry 2). Screening of the gold
N,N-dimethylbenzo[d]-isoxazol-3-amine 2a. Gratify- catalysts revealed that N-heterocyclic carbene gold(I)
ingly, in the presence of 5 mol% JohnphosAu(MeCN) complex IPrAuNTf2 (IPr=1,3-bis(2,6-diisopropyl-
SbF6 (catalyst A), an eight-membered pyrimidine-fused phenyl)imidazol-2-ylidene) provided 3a in a higher
benzolactone 3a with blue fluorescent was formed in yield of 53% (entry 4). When the reaction was
°
34% yield in DCE at 120 C for 7 h (Table 1, entry 1). conducted with 5 mol% IPrAuCl and 10 mol%
The structures of 3a and its analogs 3c and 3n (vide AgNTf2, the yield could be further improved to 63%
infra) were unambiguously confirmed by X-ray (entry 6). Increasing the amount of AgNTf2 did not
crystallography.[15] Interestingly, these molecules adopt improve the product yield apparently (entry 7). How-
a highly bent saddle shape, wherein two of the phenyl ever, reducing the amount of AgNTf2 afforded 3a in
rings are in a syn orientation. Based on the X-ray lower yield (48%, entry 8). Other solvents such as
structures of 3c and 3n, we could define that the Ar1 toluene, 1,4-dioxane etc. could not improve the
and Ar2 ring in 3a come from the ynone and reaction efficiency (Àentries 10–13). The counter anions
À
benzoisoxazole, respectively. The results also indicates such as SbF6 , BF4 or OTfÀ were also effective for
that ring-opening of benzoisoxazole occurs during the this transformation, while OTsÀ and OAcÀ were
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significantly less efficient (entries 14–18). The use of was observed. Substrates bearing methoxy or bromo
IPrAuCl or AgNTf2 alone failed to catalyze the group on the parent aryl ring were also compatible
reaction (entries 19–20).
(3n–3o). However, when free amine of benzo[d]
We next investigated the substrate scope of this isoxazol-3-amine was used as a substrate, complicated
reaction under the conditions shown in Table 1, reaction mixture was observed.
entry 6. First, the scope of ynones bearing different
Next, the scope of benzo[d]isoxazoles 2 was
functionalities at their alkyne terminus was examined examined (Scheme 3). The method was general for
using 2a as the reaction partner (Scheme 2). Substrates various benzoisoxazoles. The fluoro or bromo sub-
bearing either electron-donating (p-tBu, p-OMe) or stituent on the phenyl ring were compatible (3p–3r).
electron-withdrawing (p-F, p-CN) groups on the aryl Especially, 3q and 3r with a bromo substituent can be
rings proceeded smoothly to give 3b–3e in 41–62% further elaborated via cross-coupling reactions. The
yields. A 3-Me substituent on the phenyl ring was amino moiety could be azacycles such as azetidine or
compatible (3f). When a sterically demanding o-meth- pyrrolidine (3s, 3t). N-propyl, N-allyl, N-benzyl or N-
ylphenyl-substituted alkyne was employed, no desired thiophen-2-ylmethyl substrates turned out to be suit-
product was formed (3g). However, less hindered 2- able (3u–3x). These results demonstrated the diversity
naphthyl-substituted alkyne cyclized efficiently to and flexibility of this method. Pyrimidines and their
provide 3h in 56% yield. The results indicate that the fused derivatives exist as key structures in numerous
reaction is sensitive to the steric effect. Reaction of the natural products and pharmaceutical agents.[16] There-
heteroaryl-substituted alkyne such as 2-thienyl-substi- fore, the development of efficient route to pyrimidines
tuted one proceeded cleanly to furnish 3i in excellent is highly demanded.
yield. Substrate with a cyclopropyl group also showed
The above results prompted us to investigate the
a high reactivity (3j). Due to the poor solubility of 3j, reactivity of other type of ynone-nitriles. Benzofuro
the NMR yield of this product was also provided. [2,3-d]pyrimidine derivatives 5 were formed in 51–
Aliphatic alkynes with n-butyl or phenylethyl substitu- 60% yields from the reactions of ynone-cyanamides 4
ent converted to 3k–3l in 50–54% yields. With a with 2a (Scheme 4). Among them, 5a displays strong
methoxy group on the alkyl chain, 38% yield of 3m
[a]
Scheme 3. Scope of benzo[d]isoxazoles.
2.5 equiv. 1a was
used.
[a]
[b]
Scheme 2. Scope of ynones. NMR yield. 2.5 equiv. ynone
was used.
Scheme 4. Reaction of ynone-cyanamides with 2a.
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blue fluorescent. Interestingly, during the process, four 9 is possibly formed via O-attack of benzo[d]isoxazole
chemical bonds of the substrates were cleaved includ- to ynone followed by nucleophilic attack of the
ing two CÀ C bonds, one CÀ N bond and one NÀ O nitrogen moiety to the gold-carbene. Recently, O-
bond.
attack of benzisoxazoles to propiolates was reported by
To understand the reaction mechanism, various Liu et al., which is assumed to be reversible with that
control experiments were performed (Scheme 5). It of N-attack. The O or N-attack in these reactions led to
was found that 18O-labeled substrate 18O-1a converted α-imino gold carbenes.[10e,11c] Possibly, both N-attack
to the corresponding lactone 18O-3a in 53% yield and O-attack are involved in the reaction shown in
under the standard conditions. The result implies that eq 4. The results also imply that α-imino gold carbene
the carbonyl oxygen in the product come from ynone may also be formed in our reaction process. By
1a (Scheme 5, eq 1). It was reported that ynones could comparison, we also investigated the reactivity of the
undergo 1,3-oxygen transposition under gold ynone 10 without a cyano group. In this case, most of
catalysis.[17] In fact, in the absence of benzoisoxazole, the substrates remained, and no desired product was
the transposed product 6 was formed from 1a, along formed (Scheme 5, eq 5). The result emphasizes the
with a pyridine derivative 7 through cyclodimerization important role of the cyano group, which can enhance
via 8 (Scheme 5, eq 2). However, the desired product the ynone reactivity by increasing the polarity of the
3a was not formed via gold-catalyzed reaction of 6 alkyne triple bond.
with 2a, indicating that 6 is not the intermediate for 3a
Based on the above results and our previous
(Scheme 5, eq 3). To learn if the gold-carbene is work,[12c] we tentatively propose the following reaction
formed or not during the process, we tried to isolate mechanism for this reaction (Scheme 6). Initially,
the possible intermediates or byproducts under various attack of benzo[d]isoxazole to gold-activated ynone
reaction conditions. Attempt to trap the gold carbene affords vinyl gold intermediate 11 regiospecifically,
intermediate by adding styrene failed to obtain the which fragmentizes into the α-imino gold-carbene 12.
desired products. To our delight, when Me–DalPho- 12 isomerizes to 13. Then the newly formed carbonyl
sAuCl was used as the pre-catalyst, the desired 3a was group attacks the keto moiety to give the cyclized
obtained in 27% yield in PhCF3, along with [5+2] intermediate 14. Subsequently, carbene transfer via
cycloaddition product 9 (Scheme 5, eq 4). The product CÀ C bond cleavage followed by 1,2-aryl migration[18]
generates the intermediate 16. Elimination of gold
catalyst gives 17. 17 undergoes formal [2+2] cyclo-
addition to deliver ring-fused intermediate 18. Ring-
enlargement of 18 affords the final product 3.
When cyanamides 4 are used as the substrates, the
nine-membered heterocyclic intermediate 20 is gener-
ated through the similar reaction pathway. This is
followed by the ring-closure and rearrangement to
afford the products 5 (Scheme 7). The byproduct 23
could not be isolated due to no clean byproducts were
Scheme 5. Control experiments.
Scheme 6. Possible reaction mechanism for the formation of 3.
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Acknowledgements
We thank the National Key Research and Development Program
(2016YFA0202900), the Science and Technology Commission of
Shanghai Municipality (18XD1405000), and the Strategic
Priority Research Program of the Chinese Academy of Sciences
(XDB20000000) for financial support.
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Experimental Section
Typical Procedures for the Synthesis of Pyrimidine-
Fused Benzolactones (3)
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stirred at 120 C for 7 h as monitored by thin-layer chromatog-
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°
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6.75–6.78 (m, 1H), 7.06–7.09 (m, 1H), 7.15–7.24 (m, 4H),
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