Synthesis of cis -3-acyl-4-alkenylpyrrolidines via Gold(I)-catalyzed cycloisomerization reaction of (Z)-8-aryl-5-tosyl-5-azaoct-2-en-7-yn-1-ols

Article abstract of DOI:10.1021/jo101148e

(Z)-8-Aryl-5-tosyl-5-azaoct-2-en-7-yn-1-ols were cycloisomerized to the corresponding cis-3-acyl-4-alkenylpyrrolidines when treated with a catalytic amount of Ph₃PAuCl/AgOTf in CH₂Cl₂. The reaction proceeded via attack of the hydroxyl group onto the gold-activated alkynes followed by [3,3]-sigmatropic rearrangement to generate cis-3-acyl-4-alkenylpyrrolidines in good yields. This transformation can be applied to the synthesis of cis- and trans-3-acyl-4-alkenylcyclopentanes from (Z)- and (E)-8-aryloct-2-en-7-yn-1-ols, respectively.

Full text of DOI:10.1021/jo101148e

pubs.acs.org/joc
skeletons are present in numerous natural products of bio-
Synthesis of cis-3-Acyl-4-alkenylpyrrolidines via
Gold(I)-Catalyzed Cycloisomerization Reaction of
(Z)-8-Aryl-5-tosyl-5-azaoct-2-en-7-yn-1-ols
logical interest.1 Because the availability of functionalized
nitrogen-containing 5-membered-ring building blocks could
greatly facilitate the elaboration of more complex target
molecules, the design of expedient synthetic routes to such
intermediates has been actively pursued.2-7 Many syn-
thetic methods, as the key step, have been developed in pursuit
of pyrrolidines, including intramolecular hydroamination of
alkenes,2 ring transformation of 2-(haloalkyl)azetidines,3
Lewis acid-promoted [4 þ 2] cycloaddition of 1-nitroalk-
enes with vinyl ethers,4 and palladium-5 and platinum-
catalyzed6 coupling reaction of N-sulfonated enynes.
Among these, transition-metal-catalyzed coupling reac-
tion of N-tethered enynes has received much attention as
an atom-economical and expedient method to the synthe-
sis of nitrogen heterocycles.5,6 However, the Pd-catalyzed
N-sulfonated cycloisomerization reaction of enynes requi-
red heating substrates at elevated temperature, and the Pt-
catalyzed N-sulfonated coupling reaction of enynes gave
only a trace amount of five-membered heterocycles. Re-
cently, cationic phosphine gold(I) complexes have emer-
ged as versatile catalysts for electrophilic activation of alkynes
toward a variety of nucleophiles under mild reaction condi-
tions, allowing numerous synthetic transformations of unsatu-
rated systems into useful structure motifs.7 In particular, gold-
catalyzed intramolecular cyclization of allenes8 and alkenes2,9
tethered with nitrogen nucleophiles represents a common
method for generation of pyrrolidines under mild reaction
conditions. Although gold-catalyzed intramolecular ami-
nation of 1,5-enynes has been reported to give azabicyclic
alkenes containing a pyrrolidine moiety,10 a general gold-
(I)-catalyzed cycloisomerization of tosylamine-tethered 2-
en-7-yn-1-ols to produce 3,4-disubsituted pyrrolidines in
diastereselective fashion has yet to be developed. We have
now demonstrated that phosphine gold(I) can be applied
toward the stereospecific synthesis of pyrrolidines by treat-
ment of (Z)-8-aryl-5-tosyl-5-azaoct-2-en-7-yn-1-ols with a
catalytic amount of Ph3PAuCl/AgOTf. In this transfor-
mation, a nucleophilic 9-endo-dig addition of the hydroxyl
group onto the gold(I)-activated alkyne generated a cationic
allylic vinyl ether gold intermediate. A subsequent Claisen-
type rearrangement of the cyclic transient cationic intermediate
produced a cis-3-acyl-4-alkenylpyrrolidine. Moreover, the
gold(I)-catalyzed Claisen-type rearrangement of (Z)- and
(E)-8-aryloct-2-en-7-yn-1-ols afforded cis- and trans-3-acyl-
4-alkenylcyclopentanes, respectively.
Ming-Chang P. Yeh,* Ming-Nan Lin, Wei-Jong Chang,
Jia-Lun Liou, and Ya-Fon Shih
Department of Chemistry, National Taiwan Normal
University, 88 Ding-Jou Road, Section 4, Taipei 11677,
Taiwan, Republic of China
cheyeh@ntnu.edu.tw
Received June 12, 2010
(Z)-8-Aryl-5-tosyl-5-azaoct-2-en-7-yn-1-ols were cycloiso-
merized to the corresponding cis-3-acyl-4-alkenylpyrro-
lidines when treated with a catalytic amount of Ph3PAuCl/
AgOTf in CH2Cl2. The reaction proceeded via attack of
the hydroxyl group onto the gold-activated alkynes fol-
lowed by [3,3]-sigmatropic rearrangement to generate
cis-3-acyl-4-alkenylpyrrolidines in good yields. This
transformation can be applied to the synthesis of cis-
and trans-3-acyl-4-alkenylcyclopentanes from (Z)- and
(E)-8-aryloct-2-en-7-yn-1-ols, respectively.
The construction of saturated nitrogen heterocyclic build-
ing blocks is an important synthetic goal because such ring
(1) (a) Liddell, J. R. Nat. Prod. Rep. 2002, 19, 773. (b) Michael, J. P. Nat.
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The requisite (Z)-8-aryl-5-tosyl-5-azaoct-2-en-7-yn-1-ol 1
was prepared by addition of the corresponding (3-arylprop-
2- yn-1-yl)tosylamine to (Z)-4-bromobut-2-en-1-yl acetate
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Echavarren, A. M. Chem. Rev. 2008, 108, 3326. (e) Jimenez-Nunez, E.;
Echavarren, A. M. Top. Catal. 2010, 53, 924. (f) Hashmi, A. S. K.; Hutchings,
G. J. Angew. Chem., Int. Ed. 2006, 45, 7896. (g) Hashmi, A. S. K. Nature 2007,
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Organometallics 2008, 27, 5326. (i) Luzung, M. R.; Markham, J. P.; Toste,
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organic synthesis, see: (j) Patil, N. T.; Yamamoto, Y. Chem. Rev. 2008, 108,
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DOI: 10.1021/jo101148e
r
Published on Web 08/10/2010
J. Org. Chem. 2010, 75, 6031–6034 6031
2010 American Chemical Society

Yeh et al.
JOCNote
SCHEME 1. Gold(I)-Catalyzed Synthesis of cis-3-Acyl-4-alkenyl-
pyrrolidines from (Z)-8-Aryl-5-tosyl-5-azaoct-2-en-7- yn-1-ols
FIGURE 1. Compounds 3 and 4.
SCHEME 2. Plausible Formation Mechanism of 2
followed by hydrolysis of the resulting acetates with K2CO3
in MeOH and H2O to afford the desired tosylamine-tethered
2,7-enyn-1-ols 1 in good overall yields (Scheme 1). Treatment
of the parent compound 1a with 10 mol % of Ph3PAuCl/
AgOTf in CH2Cl2 at 25 °C for 2 h afforded cis-3-benzoyl-4-
ethenyl-1-[(4-methylphenyl)sulfonyl]pyrrolidine (2a) as the
sole diastereomer isolated in 87% yield. The relative stereo-
chemistry of the side chains was determined as a cis relation-
ship on the basis of 1H NMR studies. The proton at δ 4.09 as
a quartet in 2a, J = 7.6 Hz, was assigned to H3. The coupling
constant of H3-H4 (J34) of 7.6 Hz agrees with the 7.6-7.8 Hz
coupling constant for the similar cis hydrogens found in the
literature.11 However, PtCl2-catalyzed hydrative cyclization
of tosylamine-tethered allenynes12 and [Rh(COD)2]þ-cata-
lyzed cyclization/hydroboration of 1,6-enynes followed by
oxidation11 were both reported to give 3,4-disubstituted pyr-
rolidines as a mixture of cis and trans isomers. Thus, the
current approach to the diastereoselective synthesis of 3,4-
disubstituted pyrrolidines is achieved without the use of
complex catalysts or critical reaction conditions, only requir-
ing 10 mol % of Ph3PAuCl/AgOTf in CH2Cl2 at room tem-
perature. The investigation of various solvents in the pre-
sence of the gold(I) catalyst revealed that 1,2-dichloroethane
(DCE) was also effective and gave the desired 2a in 83%
yield; the use of toluene did not give a better result (55%).
More coordinating solvent such as THF and acetonitrile
inhibited the reaction, and 2a was isolated in low yields
(6-10%). Moreover, no conversion of 1a into 2a was observed
even after prolonged stirring in the presence of 10 mol % of
PtCl2 at elevated temperature in toluene. It is also known
that TfOH-catalyzed cyclization reaction of a cyclic enynol,
for example, 1-(5-phenylpent-4-yn-1-yl)cyclohex-2-en-1-ol,
However, compound 1a decomposed immediately upon re-
action with 10 mol % of TfOH in CH2Cl2 at room tempera-
ture. The failure of the reaction may provide evidence that the
gold-catalyzed reaction did not proceed via an allylic carboca-
tion. Recently, we have found that the cyclic 2,7-enyn-1-ol 3
(Figure 1), containing a very sensitive allyl propargyl ether
moiety, converted smoothly into oxasiprocyclic ketone 4
using a catalytic amount of Ph3PAuCl/AgOTf (5 mol %,
CH2Cl2, 25 °C, 2 h, 62%),14 while when treated with TfOH,
compound 3 decomposed immediately. Therefore, a gold-
activated intramolecular addition of the hydroxyl group to
the pendant alkyne is likely to proceed. Moreover, treatment
of the terminal alkyne 2k with the gold cation failed to give
pyrrolidines. A complex mixture of unidentified compounds
and starting substrate 1a were observed under the same
reaction conditions. It was suggested that in the reaction of
the terminal alkyne, the addition of the hydroxyl group may
proceed in both 8-exo-dig and 9-endodig manners to give an
unidentified mixture of oils. Thus, an aryl group at the
alkyne terminus is required for the cycloisomerization.
A reaction pathway was suggested in Scheme 2. The gold cata-
lyst coordinated to the triple bond of 1 to give 5a. Because of
the significant carbocationic character at the alkynyl carbon
gave a spiro[5.4]decane.13 The Bronsted acid catalyzed cycli-
€
zation reaction was proposed to proceed by formation of an
allylic cation that reacted as a strong electrophile with the
pendant alkyne. To exclude the possibility of TfOH-cata-
lyzed cyclization of 1a to give 2a, 1a was treated with TfOH.
(11) (a) Miura, T.; Shimada, M.; Murakami, M. J. Am. Chem. Soc. 2005,
127, 1094. (b) Matsuda, T.; Kadowaki, S.; Murakami, M. Helv. Chim. Acta
2006, 89, 1672.
(12) Kinder, R. E.; Widenhoefer, R. A. Org. Lett. 2006, 8, 1967.
(13) Jin, T.; Himuro, M.; Yamamoto, Y. Angew. Chem., Int. Ed. 2009, 48,
5893.
(14) Yeh, M. C. P.; Pai, H. F.; Hsiow, C. Y.; Wang, Y. R. Organometallics
2010, 29, 160.
6032 J. Org. Chem. Vol. 75, No. 17, 2010

Yeh et al.
JOCNote
SCHEME 3. Gold(I)-Catalyzed Synthesis of Cyclopentane
Derivatives from 8-Aryloct-2,7-enyn-1-ols
product 2f (92%). However, treatment of substrates 1g and
1h bearing an electron-withdrawing nitro or ester group at
the para position of the arene was less effective and provided
pyrrolidines 2g (55%) and 2h (63%), respectively. In both
reactions, the starting substrates were recovered in 15-20%
yields. The low yields of 2g-h compared to those of 2a-e
may further support the fact that electron-neutral and -rich
arenes play an important role in stabilizing the C(8). The
relative stereochemistry of products 2a-j was assigned as
the same cis relationship between two hydrogen atoms at the
two adjacent stereogenic centers on the basis of their close
chemical shift values and similar coupling patterns of the
protons in their 1H NMR spectra. Secondary allylic alcohols
1i-j also generated rearrangement products 2i-j as the only
diastereomer in each case and in 71% and 51% yields,
respectively. Structural elucidation of pyrrolidines 2b,c,f,g,j
was achieved by X-ray crystallography. However, the ter-
tiary allylic alcohol with two methyl groups on the C-1
position, for example, (Z)-1,1-dimethyl-8-phenyl-5-tosyl-5-
azaoct-2-en-7- yn-1-ol, failed to generate the corresponding
pyrrolidine using a high loading of the gold catalyst and
elevated temperature. The failure of the cyclization might be
attributed to an increased steric hindrance during the nucleo-
philic addition step of the hydroxyl group onto the gold(I)-
activated alkyne.
The chemistry can be applied to synthesis of 1,2-disubsti-
tuted cyclopentanes from 8-aryloct-2-en-7-yn-1-ols. As shown
in Scheme 3, this reaction proceeds with various (Z)- and (E)-8-
aryloct-2-en-7-yn-1-ols in a stereospecific manner with high
fidelity in transfer of stereochemical information. Thus, (Z)-
8-aryloct-2-en-7-yn-1-ols with malonate as the tether, for
example, 7, were converted using the gold(I) catalyst (10 mol %)
at room temperature for 5 h to the cis-3-acyl-4-ethenylcy-
clopentane derivative 8 in 41% yields. However, the reaction
of (E)-8-aryloct-2-en-7-yn-1-ols 9a-d with the gold(I) cata-
lyst (10 mol %) was less efficient at room temperature.
Thus, the reaction was performed in refluxing toluene for
6 h to give trans-3-acyl-4-ethenylcyclopentanes 10a-d as the
sole diastereomer in each case in 45-55% yields together
with unidentified polymeric oils. The lack of a Thorpe-
Ingold effect in cyclizations of 9a,b to form five-membered
rings17 may allow intermolecular addition of the hydroxyl
group to the gold-activated alkyne and gave fair yields of trans-
1,2-disubstituted cyclopentanes 10a,b. Structural elucidation
FIGURE 2. Compounds 5e, 6c, and 6d.
(C(8)) adjacent to the aryl group, addition of the alcohol
oxygen onto the C(8) in a 9-endo-dig fashion occurred to give
5b. The gold catalyst completed its coordination sphere by
complexation of the alkene to afford 5c, and in this way it
aligned the allylic and vinylic moieties for further conversion.
A similar allylic vinyl gold complex was suggested in the
literature as a low energy intermediate in the gold-catalyzed
rearrangement of allenynes and N-allylic aminonitriles.15 A
subsequent Claisen-type rearrangement of the cyclic transi-
ent intermediate 5c produced the cis-3,4-disubstituted pyr-
rolidine skeleton 6a. Proton transfer (to give 6b) followed by
protodemetalation of 6a afforded the pyrrolidine derivative
2 and regenerated the gold(I) catalyst in the catalytic cycle.
However, in our previous study, attack of the triflate at the
gold center of the cyclic intermediate 6c would release steric
congestion of the spiral skeleton and led to the formation of
the spirocyclic enol 6d (Figure 2). Enol 6d led to spirocyclic
ketones as a 1:1 mixture of diastereomers.14 Although gold
carbenoid 5d has been suggested as a reactive intermediate in
enyne cyclization, the lack of any observed formation of an
alternative [3.1.0] bicyclic skeleton7i indicated that the cycli-
zation is a 9-endo-dig cyclization. Moreover, cyclic substrate
3 gave 4 under the mild reaction conditions, suggesting that
the cyclization was unlikely to proceed via the postulated
steric demanding tricyclic goldcarbenoid intermediate 5e
(Figure 2).
The results of the gold-catalyzed cycloisomerization reac-
tion of cis-8-aryl-5-tosyl-5-azaoct-2-en-7-yn-1-ols 1a-j to
produce cis-3-acyl-4-alkenylpyrrolidines 2a-j are listed in
Scheme 1.16 Electron-neutral and -rich arenes at the alkyne
terminus were proven to be good substrates, as the yields of
the desired pyrrolidines 2a-e ranged from 77% to 87%. In
addition, the substrate with a para bromine atom at the
phenyl ring, for example, 1f, did not inhibit the catalytic
activity of the gold species, as evidenced by a good yield of
(15) (a) Heugebaert, T. S. A.; Stevens, C. V. Org. Lett. 2009, 11, 5018.
(b) Lemiere, G.; Gandon, V.; Agenet, N.; Goddard, J.-P.; de Kozak, A.;
Aubert, C.; Fensterbank, L.; Malacria, M. Angew. Chem., Int. Ed. 2006, 45,
7596.
(16) An iridium-catalyzed coupling reaction of 1a to give exomethylene
€
pyrrolidines was reported: Kummeter, M.; Ruff, C. M.; Muller, T. J. J.
Synlett 2007, 717.
J. Org. Chem. Vol. 75, No. 17, 2010 6033

Yeh et al.
JOCNote
of 10c and 10d was accomplished by X-ray diffraction
analysis.
(q, J = 7.6 Hz, 1H), 3.74-3.60 (m, 3H), 3.28-3.24 (m, 1H),
3.23-3.19 (m, 1H), 2.45 (s, 3H); 13C NMR (100 MHz, CDCl3)
δ 197.4, 143.5, 136.5, 133.7, 133.6, 133.3, 129.6, 128.6, 128.1,
127.5, 117.7, 52.8, 49.0, 48.3, 46.0, 21.4; IR (CH2Cl2) 3064, 2981,
2888, 1681, 1597, 1344, 1163 cm-1; MS (EI) m/e 355.3 (Mþ,
0.15), 223.1 (46), 201.2 (10), 200.2 (68), 155.1 (17), 146.1 (12),
105.1 (100), 91.1 (45), 77.1 (14), 68.1 (11), 67.1 (13), 65.1 (14);
HRMS(EI) m/e calcd for C20H21NO3S 355.1242, found 355.1240.
(()-(1S,2R)-1-Ethenyl-2-(9H-fluorenoyl-2-yl)cyclopentane (10d).
The crude mixture from the cycloisomerization reaction of 9d
(0.28 g, 1.0 mmol) was purified by flash column chromato-
graphy18 (silica gel, 3% ethyl acetate/hexanes) to give 10d (0.16
g, 0.55 mmol, 55%) as a yellow solid: mp 67-69 °C; 1H NMR
(400 MHz, CDCl3) δ 8.12 (s, 1H), 7.99-7.97 (m, 1H), 7.81-7.78
(m, 2H), 7.54 (d, J = 7.2 Hz, 1H), 7.4-7.32 (m, 2H), 5.85 (ddd,
J = 17.4, 10.2, 7.5 Hz, 1H), 5.01 (d, J = 17.1 Hz, 1H), 4.92 (dd,
J = 10.2, 0.7 Hz, 1H), 3.9 (s, 2H), 3.58 (q, J = 8.2 Hz, 1H), 3.06
In summary, a gold(I)-catalyzed cycloisomerization reac-
tion of (Z)-8-aryl-5-tosyl-5-azaoct-2-en-7-yn-1-ols has been
successfully developed. The reaction proceeded via attack of
the hydroxyl group onto the gold-activated alkyne followed
by [3,3]-sigmatropic rearrangement to generate cis-3-acyl-4-
alkenylpyrrolidines. This transformation can be applied to
the synthesis of cis- and trans-3-acyl-4-alkenylcyclopentanes
from (Z)- and (E)-8-aryloct-2-en-7-yn-1-ols, respectively.
Experimental Section
General Procedure for Gold(I)-Catalyzed Cycloisomerization
Reaction of (Z)-8-Aryl-5-tosyl-5-azaoct- 2-en-7-yn-1-ols. A
CH2Cl2 solution (5.6 mL) of (Z)-8-phenyl-5-tosyl-5-azaoct-2-
en-7-yn-1-ol (1a) (0.20 g, 0.56 mmol) were added Ph3PAuCl
(0.028 g, 0.056 mmol) and AgOTf (0.019 g, 0.056 mmol) at room
temperature under an atmosphere of nitrogen. The reaction
mixture was stirred until all 1a was consumed (typically 2 h). The
reaction mixture was filtered through a bed of Celite and
concentrated to give the crude mixture.
(()-(3S,4R)-3-Benzoyl-4-ethenyl-1-[(4-methylphenyl)sulfonyl]-
pyrrolidine (2a). The crude mixture from the cycloisomerization
reaction of 1a (0.20 g, 0.56 mmol) was purified by flash column
chromatography18 (silica gel, 10% ethyl acetate/hexanes) to give
2a (0.17 g, 0.49 mmol, 87%) as a pale yellow oil: 1H NMR (400
MHz, CDCl3) δ 7.81 (d, J = 7.5 Hz, 2H), 7.76 (d, J = 8.1 Hz,
2H), 7.55 (t, J = 7.3 Hz, 1H), 7.43 (t, J = 7.6 Hz, 2H), 7.35 (d,
J = 8.0 Hz, 2H), 5.26 (ddd, J = 16.8 Hz, J = 9.6 Hz, J = 9.6 Hz,
1H), 4.77 (d, J = 10.1 Hz, 1H), 4.72 (d, J = 17.0 Hz, 1H), 4.09
(p, J = 8.0 Hz, 1H), 2.13-1.7 (m, 5H), 1.61-1.52 (m, 1H); 13
C
NMR (100 MHz, CDCl3) δ 202.7, 146.7, 145.0, 143.8, 141.8,
141.1, 136.2, 128.5, 128.4, 127.6, 125.8, 125.7, 121.4, 120.1,
114.6, 52.9, 47.9, 37.4, 33.5, 31.8, 25.6; IR(CH2Cl2) 3074,
2953, 1674, 1639, 1610, 1468, 1424 cm-1; MS (EI) m/e 288.3
(Mþ, 7), 194.2 (26), 193.2 (100), 193.2 (16), 166.2 (11), 165.1 (59);
HRMS (EI) m/e calcd for C21H20O 288.1511, found 288.1514.
Crystals suitable for X-ray diffraction analysis were grown from
CH2Cl2 and hexanes.
Acknowledgment. This work was supported by grants
from the National Science Council (NSC 98-2119-M-003-
004- MY3).
Supporting Information Available: 1H NMR and 13C NMR
spectra of compounds 2a-j, 8, and 10a-d and X-ray data for
compounds 2b,c,f,g, j and 10c-d. This material is available free
of charge via the Internet at http://pubs.acs.org.
(17) Kaneti, J.; Kirby, J. A.; Koedjikov, A. H.; Pojarlieff, I. G. Org.
Biomol. Chem. 2004, 2, 1098.
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6034 J. Org. Chem. Vol. 75, No. 17, 2010