Gold(I)-Catalyzed Tandem Reaction of γ-Amino-Substituted Propargylic Esters to Pyrrolines and its Application in the Formal Synthesis of (±)-Aphanorphine

Article abstract of DOI:10.1002/adsc.201401161

An effective synthesis of structurally diverse pyrroline derivatives has been accomplished by a gold(I)-catalyzed tandem 1,3-acyloxy rearrangement/intramolecular azacylization reaction of γ-amino-substituted propargylic esters in good to excellent chemica

Full text of DOI:10.1002/adsc.201401161

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DOI: 10.1002/adsc.201401161
Gold(I)-Catalyzed Tandem Reaction of g-Amino-Substituted
Propargylic Esters to Pyrrolines and its Application in the Formal
Synthesis of (Æ)-Aphanorphine
Zhengshen Wang,a Huaiji Zheng,a Juan Yang,a Xingang Xie,a,*
and Xuegong Shea,b,*
a
State Key Laboratory of Applied Organic Chemistry, Department of Chemistry, Lanzhou University, Lanzhou 730000,
Peopleꢀs Republic of China
E-mail: xiexg@lzu.edu.cn or shexg@lzu.edu.cn
Collaborative lnnovation Center of Chemical Science and Engineering, Tianjin, Peopleꢀs Republic of China
b
Received: December 14, 2014; Revised: March 20, 2015; Published online: June 10, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201401161.
Abstract: An effective synthesis of structurally di- of (Æ)-aphanorphine with a catalyst loading as low
verse pyrroline derivatives has been accomplished by as 0.5 mol% to provide the key intermediate 5-(4-
a gold(I)-catalyzed tandem 1,3-acyloxy rearrange- methoxybenzyl)-1-tosyl-2,5-dihydro-1H-pyrrol-3-yl
ment/intramolecular azacylization reaction of g- pivalate on a gram scale.
amino-substituted propargylic esters in good to ex-
cellent chemical yields (52–98%). The reaction pro-
ceeds under extremely mild conditions and has also Keywords: 1,3-acyloxy rearrangement; azacylization;
demonstrated its potential in a concise formal syn- gold(I)-catalyzed reaction; propargylic esters; pyrro-
thesis
lines
Introduction
lidines is of considerable importance. Various meth-
ods have been developed, especially those based on
Nitrogen-containing heterocycles, especially of the transition metal catalysis including Pd, Ru, Rh, Fe,
pyrroline and pyrrolidine types, are particularly ubiq- Cu and so on.[5] At the end of the 20th century,
uitous and essential structural elements for pharma- Fukuda, Utimoto and Teles et al. reported that gold
ceutical agents and biologically active natural alka- salts could efficiently catalyze the addition reactions
loids, such as preussin,[1] codonopsinine,[2] aphanor- of O- and N-nucleophiles to alkenes or alkynes due
phine,[3] and pretomaymycin[4] (Figure 1). Consequent- to their incredible and unique reactivity for the elec-
ly, the development of efficient methodologies for the trophilic activation of carbon-carbon multiple
synthesis of structurally diverse pyrrolines and pyrro- bonds.[6] Ever since, the applications of gold com-
plexes as p-acid catalysts for organic transformations
have increased dramatically.[7]
As well, the metal, especially gold- and platinum-
catalyzed isomerization of propargylic esters has
become a powerful tool to obtain valuable functional-
ized synthons for organic chemistry. The isomeriza-
tion of propargylic esters proceeds via two routes:
1,2-acyloxy migration and 1,3-acyloxy rearrangement,
which differ mainly in the initial carbonyl group cycli-
zation to the activated carbon-carbon triple bond in
either the 5-exo-dig or 6-endo-dig model[8]
(Scheme 1). Recently, the Diver group developed an
elegant gold-promoted heterocyclization of internal
alkynes for the generation of dehydropyrrolidines or
dehydropiperidine. However, the ratio of the products
Figure 1. Natural products with a substituted pyrroline subu-
nit.
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Gold(I)-Catalyzed Tandem Reaction of g-Amino-Substituted Propargylic Esters
Table 1. Optimization of the reaction conditions.[a]
Scheme 1. Activation of the carbon-carbon multiple bond.
depends on the alkyne substrates.[9] To the best of our
knowledge, a cascade transformation between the in
situ formed metal-allene complex intermediates and
an internal nucleophilic species would be synthetically
valuable, as it would lead to an overall reductive sub-
stitution process of the starting propargylic esters. In
this manuscript, we describe our efforts to expand the
scope of this reaction via gold-catalyzed tandem 1,3-
acyloxy rearrangement/intramolecular azacylization,
which provides an efficient synthesis of 2,5-disubsti-
tuted pyrrolines from g-amino-substituted propargylic
esters. As well, its potential was also demonstrated in
a concise formal synthesis of (Æ)-aphanorphine.
[a]
Reaction conditions: room temperature, freshly distilled
solvent, in the air.
Isolated yields.
The remaining 1a was recovered.
DCE=1,2-dichloroethane.
Ts=p-methylbenzenesulfonyl, Piv=pivaloyl.
[b]
[c]
[d]
[e]
À
or BF4 could also provide the desired product with
Results and Discussion
slightly lower yields (entries 5 and 6). Using
AuPPh3Cl/AgSbF6 as the catalyst system, the reaction
Initially, propargylic ester 1a was selected as a model also proceeded well in tetrahydrofuran (THF) or 1,2-
substrate to investigate this proposed tandem trans- dichloroethane (DCE) as solvent (entries 7 and 8).
formation. The isomerization of g-amino-substituted When the reaction was carried out in CH3NO2, tolu-
propargylic ester 1a was examined under a variety of ene, wet DCM or CH3CN, no reaction was observed
conditions (Table 1). Pleasingly, the reaction proceed- (entries 9–12). In addition to Au catalysts, PtCl2 in tol-
ed smoothly in the presence of AuCl3·2H2O uene was also examined, which was ineffective for
(10 mol%) in dichloromethane (DCM) at room tem- this transformation (entry 13).
perature for 0.5 h, leading to 2a in 61% yield
Having established the optimal reaction conditions,
(Table 1, entry 1), while AuBr3 (10 mol%) or a series of bifunctional propargylic esters was then
AuPPh3Cl (10 mol%) (entries 2 and 3) proved to be easily prepared in a modular way either from the cor-
ineffective. To our delight, the yield was readily in- responding imines in 3 steps or from the correspond-
creased to 96% in the presence of 5 mol% AuPPh3Cl/ ing propargylic alcohols in 6 steps (Scheme 2). The
AgSbF6 (entry 4). Likewise, employing gold(I) comÀ- details of their preparation and characterization data
plexes with alternative counter ions such as SO3CF3
can be found in the Supporting Information.
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Zhengshen Wang et al.
Scheme 2. Synthesis of the key substrate for the starting materials.
With the propargylic esters in hand, we then inves- methane and were also found to be suitable for this
tigated the scope of this reaction. Firstly, the sub- gold-catalyzed reaction, but needing much more time
strates 1b and 1c with the different N-protecting and the yields were slightly lower (Table 2, entries 1
group or the ester group were treated with 5 mol% and 2), which also demonstrated that the substrate
AuPPh3Cl/AgSbF6 at room temperature in dichloro- with the p-methylbenzenesulfonyl group as the N-pro-
Table 2. Gold(I)-catalyzed synthesis of substituted pyro-
Table 2. (Continued)
lines.[a]
[a]
Reaction conditions: catalyst (5 mol%), room tempera-
ture, dry dichloromethane, in the air.
Yield of isolated product.
Ts=p-methylbenzenesulfonyl, Bz=benzoyl, Piv=pivalo-
[b]
[c]
yl.
tecting group and pivaloyl as the ester was the most
suitable one for this gold-catalyzed tandem reaction.
The generality of this gold-catalyzed cyclization re-
action was then examined. As revealed in Table 2, the
reaction of the g-amino-substituted propargylic esters
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Gold(I)-Catalyzed Tandem Reaction of g-Amino-Substituted Propargylic Esters
with another alkyl or aryl substituent at the g-position which was not beneficial for the formation of pyrro-
proceeded in the presense of AuPPh3Cl (5 mol%) and lines.
AgSbF6 (5 mol%) in dichloromethane (DCM) at
For substrates substituted in the a- or g-position,
room temperature for 0.5 h, leading to the corre- the transformation was expected to be more favorable
sponding pyrolines in good yields ranging from 78 to because of a Thorpe–Ingold effect.[10] To our delight,
98%. During these runs, the substrates with a strong all the gold-catalyzed cyclization reactions led to the
electron-donating group (OMe) at the para position corresponding 2,5-disubstituted pyrrolines (entries 7–
of the phenyl ring required a longer reaction time 9) in 75–84% yield and the relative configuration of
(Table 2, entry 6 vs. entries 4 and 5) probably due to 2h was confirmed by X-ray crystallographic analy-
electronic activation of the double bond of the allene sis.[11] Especially, the cyclization of substrate 1j pro-
vides a facile construction of the essential structural
element, the azaspiro[4,5]decane ring system, which
could be found in many kinds of natural alkaloids.
Also, a g-amino-substituted propargylic ester with
a protected hydroxy group, 1k, was tolerated under
our reaction conditions and the corresponding pyrro-
line 2k was obtained in high 98% yield, which could
be further converted to the bicyclic amine and used
for the synthesis of biologically active alkaloids, such
as castanospermine[12] and serratinine[13] (Figure 1).
The easily modifiable functional groups (Br and
OPiv) on the alkyl chain are well-tolerated and the
desired products 2l and 2m (entries 11 and 12) are ob-
tained in attractive yields, demonstrating the mild
nature of the catalytic conditions.
Our present understanding of the mechanism of
Scheme 3. Proposed mechanism for gold-catalyzed pyrroline
formation.
this gold-catalyzed cycloisomerization reaction of g-
Scheme 4. The favorable route for 2,5-disubstituted pyrrolines (2h).
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Zhengshen Wang et al.
amino-substituted propargylic esters is shown in tention and a number of syntheses have been report-
Scheme 3.[14] Firstly, in the presence of AgSbF6, ed,[16] was chosen as our target molecule.
gold(I) coordinated to the p-system of the alkyne and
Our synthesis commenced from the addition reac-
initiated the 3,3-rearrangement of the pivaloate group tion of 4-methoxyphenylacetaldehyde 3 and Ts-pro-
to form p-complex C. Subsequently, an intramolecular tected propargylic amine 4.[17] The obtained secondary
SN2-type reaction occurred to give the cyclic vinylgold alcohol was subsequently protected as pivolate 5, and
intermediate D which underwent an elimination reac- the overall yield of the two steps was 59%. As we ex-
tion to produce the pyrroline product and regenerate pected, the gold-catalyzed tandem reaction of propar-
the catalytic gold(I) species.[15] During this course, p- gylic amine 5 proceeded smoothly in the presence of
complex C was produced from isomerization of the AuPPh3Cl and AgSbF6 as the catalyst system in di-
alkyne, and the conformation of the allene intermedi- chloromethane at ambient temperature to give pyrro-
ate C could be (R)- or (S)- which depended on the line 6 in 95% yield. It was worthy to note here that
difference of its substituents.
this step could be executed on a gram-scale together
The observed 2h as the cis-product could be ex- with the catalyst loading being reduced to 0.5 mol%.
plained by an envelope-like transition state in which Subsequently hydrolysis of the pivaloyl followed by
the p–p interaction between the aryl substituent and a nucleophilic methylation produced the tertiary alco-
the p-methylbenzenesulfonyl group to lead the gold hol 7 in 77% yield. Then an AlCl3-promoted intramo-
coordinated to the upside of the allene intermediate, lecular Friedel–Crafts alkylative cyclization of alcohol
and the aryl groupꢀs preference for the equatorial po- 7 was effected to furnish the desired tricyclic com-
sition rather than the axial one. Next, an intramolecu- pound 8 as colorless needles,[18] for which the relative
lar SN2-type reaction takes place to produce the pyr- structure was confirmed by X-ray crystallographic
roline as the cis-product. (Scheme 4)
analysis (Figure 2). The synthesis of (Æ)-aphanor-
In light of this novel gold-catalyzed cyclization, we phine could be accomplished from the tricyclic pre-
became interested in exploring its application to syn- cursor 8 in three additional steps (desulfurization, N-
thesize structurally diverse natural alkaloids. (Æ)- methylation, and O-demethylation), which has been
Aphanorphine (Figure 1), an alkaloid with potential reported by Ogasawara and co-workers[19] (Scheme 5).
biological activity and a unique tricyclic 3-benzaze-
pine framework which has attracted considerable at-
Conclusions
In conclusion, we have accomplished an effective syn-
thesis of structurally diverse pyrroline derivatives via
a novel gold(I)-catalyzed tandem 1,3-acyloxy rear-
rangement/intramolecular azacylization reaction of g-
amino-substituted propargylic esters. Our reaction
proceeds under mild conditions, features low catalyst
loading, short reaction time, and its potential in or-
ganic synthesis has also been demonstrated in the
concise formal synthesis of (Æ)-aphanorphine. A fur-
Figure 2. X-ray single crystal structure of compound 8.
ther modification of this new tandem reaction and its
Scheme 5. Formal synthesis of (Æ)-aphanorphine.
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Gold(I)-Catalyzed Tandem Reaction of g-Amino-Substituted Propargylic Esters
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applications to the syntheses of other bioactive natu-
ral products containing pyrroline motifs is currently
underway in our lab and will be reported in due
course.
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Experimental Section
Typical Procedure for the Cyclization Reaction
To a solution of 1a (156 mg, 0.48 mmol) and AuClPPh3
(12 mg, 5 mol%) in DCM (10 mL), was added AgSbF6
(8 mg, 5 mol%), the mixture was stirred at room tempera-
ture for 30 min until the starting material was completely
consumed. The mixture was concentrated under reduced
pressure. Purification of the residue by flash chromatogra-
phy on silica gel (petrol ether/ethyl acetate 3:1) gave 2a as
a white crystalline solid; yield: 150 mg (96%); mp 81–828C.
Acknowledgements
We are greatful for the financial support by the NSFC
(21125207, 21372103, 21472079), SRFDP (20130211110018)
and Program 111.
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