A new strategy for the synthesis of chiral β-alkynyl esters via sequential palladium and copper catalysis

Article abstract of DOI:10.1021/ja203171x

A new strategy for the synthesis of chiral β-alkynyl esters which relies on sequential Pd and Cu catalysis is reported. Terminal alkynes bearing aryl, alkyl, and silyl groups can be employed without prior activation yielding a wide range of important chiral building blocks. The reaction sequence utilizes a robust Pd(II)-catalyzed hydroalkynylation of ynoates with terminal alkynes providing geometrically pure ynenoates which are readily reduced by CuH. In contrast to previous reports, where additions to ynenoates proceed with marginal preference for the 1,6-pathway, this conjugate reduction occurs with high 1,4-selectivity yielding β-alkynyl esters with excellent levels of enantioselectivity. Importantly, the method tolerates a wide range of functionality, including allylic carbonates and carbamates, and thus allows for rapid elaboration of the β-alkynyl esters into a variety of chiral, substituted heterocycles.

Full text of DOI:10.1021/ja203171x

COMMUNICATION
pubs.acs.org/JACS
A New Strategy for the Synthesis of Chiral β-Alkynyl Esters via
Sequential Palladium and Copper Catalysis
Barry M. Trost,* Benjamin R. Taft, James T. Masters, and Jean-Philip Lumb
Department of Chemistry, Stanford University, Stanford, California 94305-5080, United States
S Supporting Information
b
acetylenes to enones, enals, and Meldrum’s acid alkylidenes.7
ABSTRACT: A new strategy for the synthesis of chiral β-
alkynyl esters which relies on sequential Pd and Cu catalysis
is reported. Terminal alkynes bearing aryl, alkyl, and silyl
groups can be employed without prior activation yielding a
wide range of important chiral building blocks. The reaction
sequence utilizes a robust Pd(II)-catalyzed hydroalkynyla-
tion of ynoates with terminal alkynes providing geometri-
cally pure ynenoates which are readily reduced by CuH. In
contrast to previous reports, where additions to ynenoates
proceed with marginal preference for the 1,6-pathway, this
conjugate reduction occurs with high 1,4-selectivity yielding
β-alkynyl esters with excellent levels of enantioselectivity.
Importantly, the method tolerates a wide range of function-
ality, including allylic carbonates and carbamates, and thus
allows for rapid elaboration of the β-alkynyl esters into a
variety of chiral, substituted heterocycles.
While these reports stand out as pioneering efforts, each method
is limited by either the required substitution on the alkyne or
the nature of the olefin wherein the activating group must
subsequently be elaborated into an ester (Scheme 1). Therefore,
the direct synthesis of chiral β-alkynyl esters from readily
accessible reagents remains an unmet challenge.
Scheme 1. Synthesis of Chiral β-Alkynyl Esters
n recent years, transition metal catalysis has increasingly
established atom-economic, efficient processes for the con-
I
struction of functional molecules. Nevertheless, many highly
desirable chemical transformations still require the use of stoi-
chiometric substrate activation. In this regard, the direct catalytic
asymmetric conjugate addition of terminal alkynes to R,β-
unsaturated esters, which exemplifies the ideals of atom and step
economy,1 has not been described. Due to the synthetic utility of
alkynes and esters, these functional molecules are well regarded
as valuable building blocks for further synthetic transformations.
In addition, related chiral β-alkynyl acids have recently been
identified as important pharmacophores in their own right.2
Despite the potential utility of direct catalytic asymmetric
additions of terminal alkynes to activated olefins, progress in this
area has been limited, largely due to the modest nucleophilicity of
transition-metal acetylides.3 This problem has been addressed
through the stoichiometric activation of terminal alkynes as their
boron, aluminum, and zinc acetylides.4 However, such methods
suffer from low functional group tolerance as well as increased
downstream waste by virtue of poor atom economy.5 Recently,
the direct catalytic asymmetric conjugate addition of terminal
alkynes has been realized for a limited set of substrates. By using
Cu catalysis, the Carreira and Shibasaki groups have demon-
strated efficient asymmetric additions of terminal alkynes to
Meldrum’s acid alkylidenes and R,β-unsaturated thioamides,
respectively.6 In addition, Hayashi and Fillion have established
that Rh catalysis can promote the asymmetric addition of silyl
Herein, we describe a new, efficient strategy based on sequen-
tial Pd and Cu catalysis for the enantioselective synthesis of
β-alkynyl esters that achieves this goal. The process takes place
under mild, ambient conditions, occurs with high regio- and
stereoselectivity, and tolerates a variety of substituents on the
terminal alkyne (Scheme 1). Importantly, this method obviates
the need to prepare and isolate geometrically pure enoates via
traditional stereoselective olefination methods.
Since the late 1980s, our lab has been involved in the
development of atom-economical addition reactions that per-
mit the cross-coupling of differentially substituted alkynes.8
Specifically, Pd(OAc)2 ligated to an equimolar amount of
tris(2,6-dimethoxy)phenylphosphine (TDMPP) catalyzes the
hydroalkynylation of ynoates by terminal alkynes, affording a
wide range of stereochemically defined ynenoates in good
yields. This reaction benefits from operational simplicity, as it
is routinely performed with benchtop solvents under an ambi-
ent atmosphere at room temperature. While the ynenoate
products are suitable precursors to a variety of heterocycles,9
we questioned whether these substrates could also participate
Received: April 6, 2011
Published: May 11, 2011
r
2011 American Chemical Society
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COMMUNICATION
Scheme 2. Sequential Pd/Cu Catalysis
Table 1. Synthetic Scopea
yield
(%)c
yield
(%)d (%)e
ee
entry
1
R1
2
R2
3
L
4
1
2
3
4
5
6
7
1a C5H11
1a C5H11
1a C5H11
1b H
2a (CH2)2Ph 3aa 79 L1 4aa 90 99 (S)
2b Ph
3ab 84 L1 4ab 90 99 (S)
3ac 92 L1 4ac 87 99 (S)
3bb 95 L1 4bb 59 99 (S)
2c BDMS
2b Ph
1c OCO2Me 2a (CH2)2Ph 3ca 77 L1 4ca 98 99 (S)
1c OCO2Me 2b Ph
3cb 77 L1 4cb 92 99 (S)
in an asymmetric CuH-catalyzed 1,4-reduction.10 The resulting
two-step process, utilizing readily available alkyne starting
materials, would yield chiral β-alkynyl esters and would provide
an attractive alternative for the direct asymmetric conjugate
addition of terminal alkynes to enoates (Scheme 1).
While attractive, this proposal is not without risk, given the
complexities associated with regiochemical addition/reduction of
extended π-systems. In particular, Cu-mediated conjugate additions
to activated enyne substrates result in competitive 1,4- and 1,6-
addition pathways, where literature precedent suggests a preference
for the latter.11 To our knowledge, there exists only one example of a
copper-catalyzed, asymmetric 1,4-addition of an alkyl aluminum or
zinc reagent to an ynenone that was recently reported by Hoveyda
and co-workers.12 To date, the catalytic asymmetric 1,4-reduction of
an activated enyne has not been described.
To explore the feasibility of this strategy, ynenoate 3aa was
prepared from ynoate 1a and terminal alkyne 2a using 3.0 mol %
of Pd(OAc)2 and monophosphine TDMPP at room temperature
in 79% yield (Scheme 2). Importantly, only the (E)-isomer of
3aa was produced in this Pd-catalyzed hydroalkynylation, as a
mixture of olefin isomers would not be suitable for the ensuing
asymmetric 1,4-reduction (vide infra).10k We envisioned the
asymmetric Cu-catalyzed 1,4-reduction of 3aa taking place in
the presence of the appropriate chiral bis-phosphine and silane,
yielding the corresponding ester 4aa. Initial studies were carried
out using ligands from the SEGPHOS family,10k and while
products resulting from 1,4-reduction were isolated in moderate
yield, they were routinely accompanied by products arising from
competitive 1,6-reduction. Upon screening various ligands, it was
found that CuH ligated to the bis-phosphine (R,R)-WALPHOS
(L1) promoted the 1,4-reduction with high regio- and enantios-
electivity. Optimization of the catalyst loading, temperature, and
reaction time provided satisfactory conditions to achieve com-
plete 1,4-reduction of 3aa. As such, the reduction was carried out
1c OCO2Me 2c BDMS
3cc 67 L1 4cc 81 99 (S)
8b 1d N(H)Boc 2a (CH2)2Ph 3da 91 L2 4da 90 94 (R)
9b 1d N(H)Boc 2b Ph
10b 1d N(H)Boc 2c BDMS
3db 94 L2 4db 94 92 (R)
3dc 93 L2 4dc 85 97 (R)
a Conditions: (i) 1/2 = 1:1.25À1.75, 1.5À3.0 mol % Pd(OAc)2/
TDMPP, 1.0 M in PhMe at 23 °C for 1À18 h. (ii) 5 mol % Cu-
(OAc)2 H2O/L1, DEMS (2 equiv), t-BuOH (2 equiv), 0.2 M in PhMe
3
at 4 °C, 14À20 h. b Conditions (ii) changed to 2 mol % Cu(OAc)2 H2O/
3
L2, DEMS (1.5 equiv), t-BuOH (1.5 equiv), 0.2 M in PhMe/THF (4:1)
at 4 °C, 4 h. c Isolated yield. d Isolated yield based on 3. Enantiomeric
e
excess determined by chiral HPLC using a Daicel CHIRALPAK IB
column. L1 = (R,R)-WALPHOS, L2 = (R,S)-(t-Bu)2-JOSIPHOS.
prepared and subjected to the optimized reduction conditions,
a less regioselective event occurred, affording the desired
product 4bb in 59% yield while maintaining high enantioselec-
tivity (99% ee, entry 4).13 Ester 4bb was subsequently con-
verted to a known compound, and the absolute stereochemistry
was determined to be (S).14 Ynoate 1c was coupled to alkynes
2a, 2b, and 2c in a similar fashion affording ynenoates 3ca, 3cb,
and 3cc bearing allylic carbonates (entries 5À7). Successful Cu-
catalyzed asymmetric 1,4-reduction of all three classes (alkyl,
aryl, and silyl) of this activated enyne afforded β-alkynyl esters
4ca, 4cb, and 4cc with additional functionality while maintain-
ing outstanding regio- and enantioselectivity.15 Notably, both
Pd- and Cu-catalyzed events took place without competitive
ionization or reduction of the allylic carbonate (3ca, 3cb, and
3cc) via a π-allyl type intermediate, further demonstrating the
chemoselectivity of the process. Ynenoates with a pendant
ÀN(H)Boc group could also be prepared in excellent yield
(3da, 3db, and 3dc), although when subjected to asymmetric
1,4-reduction using the combination of Cu(OAc)2 H2O and
3
ligand L1, the regioselectivity of the reduction dropped sig-
nificantly. A further ligand screen revealed that 1,4-selectivity
could be restored by switching to (R,S)-(t-Bu)2-JOSIPHOS
ligand L2 and adding THF as a cosolvent, thus affording
β-alkynyl esters 4da, 4db, and 4dc in excellent yield and
enantioselectivity (entries 8À10). Presumably, when ligand
L1 is employed, an internal coordination event by the ÀN-
(H)Boc group with the catalyst disrupts the preference of
copper to promote selective 1,4-reduction. In contrast, the
catalyst generated from the more electron-rich and sterically
hindered ligand L2 does not suffer from this phenomenon. It
has been noted in prior work that Cu-catalyzed 1,4-reductions
of R,β-unsaturated ketones carried out with L1 and L2 yield
using 5 mol % Cu(OAc)2 H2O/L1 in the presence of diethox-
3
ymethylsilane (DEMS) and t-BuOH at 4 °C, affording ester 4aa
in 90% isolated yield and with a 99% enantiomeric excess.
To demonstrate the synthetic scope of the process, substitu-
tion on both ynoate 1 and terminal alkyne 2 was examined and
the results are summarized in Table 1. Ynenoates 3aa, 3ab, and
3ac, derived from 1a and terminal alkynes bearing alkyl-, aryl-,
and silyl-groups, were synthesized and subjected to the opti-
mized 1,4-reduction conditions affording the desired β-alkynyl
esters 4aa, 4ab, and 4ac (respectively) in excellent yield and
enantioselectivity (entries 1À3). To our surprise, when an
ynenoate bearing a methyl group at the β-position (3bb) was
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COMMUNICATION
Scheme 3. Olefin Isomerization and 1,4-Reduction
Scheme 4. One-Pot Sequential Pd/Cu Catalysis
Scheme 5. Chiral Heterocycle Synthesisa
products with an opposite sense of chirality,10n and therefore
the esters in entries 8À10 have been assigned as (R) by analogy.
To investigate the significance of olefin geometry in the
asymmetric CuH 1,4-reduction, ynenoate (Z)-3db was converted
to its geometric isomer (E)-3db by irradiation in the presence of
diphenyldiselenide.16 As previously described, using 2 mol %
a Conditions: (i) Otera’s catalyst (10 mol %), PhMe, 85 °C, 78%. (ii)
AuCl (10 mol %), PPTS (10 mol %), EtOH, 0 °C, 50%. (iii) AlMe3 (1.5
equiv), PhMe, À20 °C, 81%. (iv) Pd(OAc)2 (7.5 mol %), acrolein, LiBr,
THF, rt, 64%.
Cu(OAc)2 H2O/L2, ynenoate (Z)-3db was reduced with com-
3
plete 1,4-selectivity to provide 4db in 94% yield and with 92%
enantioselectivity as the (R)-enantiomer (Scheme 3). However,
reduction of (E)-3db was distinctly slower and required an
increase in the catalyst loading and reaction time. Even in the
tetrasubstituted chiral dihydro-pyrrole 8 can be prepared from
the corresponding β-alkynyl ester 4db bearing an ÀNHBoc
group.9a The asymmetric synthesis of 8 in good yield and with
high enantioselectivity, utilizing only three transition-metal-
catalyzed steps (in two pots) from phenylacetylene, is particu-
larly noteworthy.
presence of 10 mol % Cu(OAc)2 H2O/L2 for 24 h, only 38%
3
conversion to β-alkynyl ester 4db was observed. Although the
reaction cleanly afforded the complementary (S)-antipode with
92% enantioselectivity as expected, we were surprised to find such
a dramatic drop in the rate of 1,4-reduction. We speculate that the
proximity of the ester to the alkyne in (E)-3db allows a bidentate
chelation with Cu and serves to obstruct the desired 1,4-reduction
pathway. Experiments are currently ongoing to fully elucidate this
interesting observation.
In summary, a new strategy for the synthesis of chiral β-alkynyl
esters from readily available alkyne starting materials has been
developed. We believe that this methodology represents an
attractive alternative to traditional additions of terminal alkynes
to activated olefins. Construction of the chiral propargylic center
is carried out by an initial Pd-catalyzed hydroalkynylation of an
ynoate with a terminal alkyne (CÀC bond) and subsequently an
asymmetric, Cu-catalyzed 1,4-addition of hydride (CÀH bond).
Both steps occur under mild, practical conditions, and the 1,4- vs
1,6-regioselectivity in the asymmetric CuH-reduction is unpre-
cedented. The robust nature of each catalytic step is highlighted
by the development of an efficient one-pot protocol. The method
is highly chemoselective and tolerant of numerous functional
groups, allowing for the rapid construction of chiral functiona-
lized β-alkynyl esters. These substrates can be selectively cy-
clized, providing rapid syntheses of chiral lactones, tetra-
hydrofurans, pyrrolidinones, and dihydropyrroles. Finally, we
note that both Pd-catalyzed hydroalkynylation and Cu-catalyzed
asymmetric 1,4-reduction tolerate a wide variety of electron
withdrawing groups in addition to esters. Therefore, we are
optimistic that future efforts to extend the scope of this new
strategy beyond ynenoates will be realized in due course.
To highlight the robust nature and operational simplicity of these
two catalytic events, a one-pot hydroalkynylation/1,4-reduction
protocol was developed. It quickly became clear that excess terminal
alkyne remaining in solution upon completion of the Pd-catalyzed
hydroalkynylation resulted in attenuated reactivity during the sub-
sequent Cu-catalyzed 1,4-reduction. To circumvent this problem, a
slight excess (1.2 equiv) of ynoate 1d was employed in the
hydroalkynylation with terminal alkyne 2a. Upon complete con-
sumption of 2a, the reaction was cooled to 0 °C and a solution of
Cu(OAc)2 H2O/L2 was introduced followed by DEMS and t-
3
BuOH. The ensuing Cu-catalyzed asymmetric 1,4-reduction took
place smoothly, and the β-alkynyl ester 4da was isolated in 78%
yield and 92% enantiomeric excess from terminal alkyne 2a
(Scheme 4).
An important aspect of this method is that both CÀC and
CÀH bond forming events occur chemoselectively in the pre-
sence of additional functional groups. As a result, functionalized
β-alkynyl esters (4) are produced efficiently and provide an ideal
platform from which to construct a variety of valuable, chiral
heterocycles (Scheme 5). For example, carbonate 4ca is hydro-
lyzed selectively to reveal an ÀOH group that can be differen-
tially cyclized to yield either chiral lactone 5 or chiral
tetrahydrofuran 6.17,18 Similarly, chiral pyrrolidinone 7 and
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
analytical data for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
J. Am. Chem. Soc. 1988, 110, 291. (c) Brestensky, D. M.; Stryker, J. M.
Tetrahedron Lett. 1989, 30, 5677. (d) Koenig, T. M.; Daeuble, J. F.;
Brestensky, D. M.; Stryker, J. M. Tetrahedron Lett. 1990, 31, 3237. For
reviews, see:(e) Rendler, S.; Oestreich, M. Angew. Chem., Int. Ed. 2007,
46, 498. (f) Deutsch, C.; Krause, N.; Lipshutz, B. H. Chem. Rev. 2008,
108, 2916. (g) Lipshutz, B. H. Synlett 2009, 0509. For selected Cu-
catalyzed asymmetric 1,4-reductions, see:(h) Gallagher, B. D.; Taft,
B. R.; Lipshutz, B. H. Org. Lett. 2009, 11, 5374. (i) Lipshutz, B. H.;
Tanaka, N.; Taft, B. R.; Lee, C.-T. Org. Lett. 2006, 8, 1963. (j) Yun, J.;
Kim, D.; Lee, D. Angew. Chem., Int. Ed. 2006, 45, 2785. (k) Lipshutz,
B. H.; Servesko, J. M.; Taft, B. R. J. Am. Chem. Soc. 2004, 126, 8352.
(l) Buchwald, S. L.; Aye, Y.; Rainka, M. P. Proc. Natl. Acad. Sci. U.S.A.
2004, 101, 5821. (m) Czekelius, C.; Carreira, E. M. Angew. Chem., Int.
Ed. 2003, 42, 4793. (n) Lipshutz, B. H.; Servesko, J. M. Angew. Chem., Int.
Ed. 2003, 42, 4789.
(11) (a) Krause, N.; Handke, G. Tetraherdon Lett. 1991, 32, 7229.
(b) Krause, N.; Aksin-Artok, O. In The Chemistry of Organocopper
Compounds; Rappoport, Z., Marek, I., Ed.; Wiley-VCH: Weinheim,
2009; p 857.
(12) (a) Lee, K.-S.; Brown, M. K.; Hird, A. W.; Hoveyda, A. H. J. Am.
Chem. Soc. 2006, 128, 7182. (b) May, T. L.; Brown, M. K.; Hoveyda,
A. H. Angew. Chem., Int. Ed. 2008, 47, 7358.
(13) In addition to the formation of 4bb in 59% isolated yield,
reduction of ynenoate 3bb produced complex, inseparable mixtures of
byproducts consistent with 1,6-reduction that represent the remaining
mass balance.
(14) Alkyne (S)-4bb was readily reduced to ester (R)-SI-3, the
antipode of which has previously been prepared in 87% ee: Lopez, F.;
Harutyunyan, S. R.; Meetsma, A.; Minnaard, A. J.; Feringa, B. L. Angew.
Chem., Int. Ed. 2005, 44, 2752.The absolute stereochemistry of the
current CuH reduction is assigned by analogy.
Corresponding Author
bmtrost@stanford.edu
’ ACKNOWLEDGMENT
We thank the National Science Foundation (CHE 0948222)
and the National Institutes of Health (GM33049) for their
generous support of our programs. B.R.T. and J.-P. L. are grateful
for NIH postdoctoral fellowships (CA137999 and GM083428
respectively). We thank Johnson-Matthey for generously provid-
ing us with palladium salts and Solvias for providing chiral
phosphine ligands.
’ REFERENCES
(1) (a) Trost, B. M. Science 1991, 254, 1471. (b) Wender, P. A.;
Verma, V. A.; Paxton, T. J.; Pillow, T. H. Acc. Chem. Res. 2008, 41, 40.
(2) (a) Shimada, T.; Ueno, H.; Tsutsumi, K.; Aoyagi, K.; Manabe, T.;
Sasaki, S.; Katoh, S. Int. Appl. PCT, WO 2009054479, 2009. (b) Houze,
J.; Liu, J.; Ma, Z.; Medina, J. C.; Schmitt, M. J.; Sharma, R.; Sun, Y.;
Wang, Y.; Zhu, L. U.S. Patent 7,465,804, 2008. (c) Brown, S. P.;
Dransfield, P.; Fu, Z.; Houze, J.; Jiao, X.; Kohn, T. J.; Pattaropong, V.;
Vimolratana, M.; Schmitt, M. J. Int. Appl. PCT, WO 2008130514, 2008.
(d) Xiang, J. N.; Karpinski, J. M.; Christensen, S. B., IV. Int. Appl. PCT,
WO 0009116, 2000. (e) Christensen, S. B., IV; Karpinski, J. M.; Frazee,
J. S. Int. Appl. PCT, WO 9703945, 1997. (f) Bharate, S. B.; Nemmani,
K. V. S.; Vishwakarma, R. A Expert Opin. Ther. Pat. 2009, 19, 237 and
references therein. (g) Woo, J. C. S.; Cui, S.; Walker, S. D.; Faul, M. M.
Tetrahedron 2010, 66, 4730. (h) Yazaki, R.; Kumagai, N.; Shibasaki, M.
Org. Lett. 2011, 13, 952.
(3) (a) Fujimori, S.; Kn€opfel, T. F.; Zarotti, P.; Ichikawa, T.; Boyall,
D.; Carreira, E. M. Bull. Chem. Soc. Jpn. 2007, 80, 1635. (b) Trost, B. M.;
Weiss, A. H. Adv. Synth. Catal. 2009, 351, 963.
We also note that the synthesis of (R)-SI-3, constitutes the formal
addition an alkyl group to an R,β-unsaturated ester. We believe that this
3-step sequence, comprised entirely of catalytic reactions, compares
favorably to other known, asymmetric, alkyl conjugate addition reactions.
(15) While the hydroalkynylations were routinely performed on a
1.0 mmol scale, the CuH reductions were typically performed on a
0.15À0.30 mmol scale. As a preliminary indication of scalability, the
synthesis of alkyne 4ca was performed on a 0.75 mmol scale, and the
reaction maintained excellent yield and enantioselectivity (98% yield,
99% ee).
(16) Trost, B. M.; Hachiya, I.; McIntosh, M. C. Tetrahedron Lett.
1998, 39, 6445.
(17) Lactone 5 was synthesized using Otera’s catalyst (1,3-
diisothiocyanatotetrabutyldistannoxane): Otera, J.; Danoh, N.; Nozaki,
H. J. Org. Chem. 1991, 56, 5307. For a review of transesterification
catalysts, see: Otera, J. Chem. Rev. 1993, 93, 1449.
(4) For boron, see: (a) Chong, J. M.; Shen, L.; Taylor, N. J. J. Am.
Chem. Soc. 2000, 122, 1822. (b) Wu, T. R.; Chong, J. M. J. Am. Chem. Soc.
2005, 127, 3244. (c) Pellegrinet, S. C.; Goodman, J. M. J. Am. Chem. Soc.
2006, 128, 3116. For aluminum, see:(d) Kwak, Y.-S.; Corey, E. J. Org.
Lett. 2004, 6, 3385. (e) Larionov, O. V.; Corey, E. J. Org. Lett. 2010,
12, 300. For zinc see:(f) Yamashita, M.; Yamada, K.; Tomioka, K. Org.
Lett. 2005, 7, 2369. (G) Cui, S.; Walker, S. D.; Woo, J. C. S.; Borths, C. J.;
Mukherjee, H.; Chen, M. J.; Faul, M. M. J. Am. Chem. Soc. 2010,
132, 436.
(5) Li, C.-J.; Trost, B. M. Proc. Natl. Acad. Sci. U.S.A. 2008,
105, 13197.
(6) (a) Kn€opfel, T. F.; Zarotti, P.; Ichikawa, T.; Carreira, E. M. J. Am.
Chem. Soc. 2005, 127, 9682. (b) Yazaki, R.; Kumagai, N.; Shibasaki, M.
J. Am. Chem. Soc. 2010, 132, 10275.
(7) (a) Nishimura, T.; Guo, X.-X.; Uchiyama, N.; Katoh, T.;
Hayashi, T. J. Am. Chem. Soc. 2008, 130, 1576. (b) Nishimura, T.;
Sawano, T.; Hayashi, T. Angew. Chem., Int. Ed. 2009, 48, 8057.
(c) Fillion, E.; Zorzitto, A. K. J. Am. Chem. Soc. 2009, 131, 14608.
(8) (a) Trost, B. M.; Chan, C.; R€uhter, G. J. Am. Chem. Soc. 1987,
109, 3486. (b) Trost, B. M.; Sorum, M. T.; Chan, C.; R€uhter, G. J. Am.
Chem. Soc. 1997, 119, 698. (c) Trost, B. M.; McIntosh, M. C. Tetra-
hedron Lett. 1997, 38, 3207. (d) Trost, B. M.; Gunzner, J. L.; Yasukata, T.
Tetrahedron Lett. 2001, 42, 3775.
(18) For a related gold-catalyzed synthesis of tetrahydrofurans, see:
Belting, V.; Krause, N. Org. Lett. 2006, 8, 4489.
(9) (a) For the synthesis of pyrroles, see: Trost, B. M.; Lumb, J.-P.;
Azzarelli, J. M. J. Am. Chem. Soc. 2011, 133, 740. (b) For the synthesis of
furans and butenolides, see: Trost, B. M.; McIntosh, M. C. J. Am. Chem.
Soc. 1995, 117, 7255. (c) For the synthesis of pyrans and related
7-membered oxygen heterocycles, see: Trost, B. M.; Frontier, A. J.
J. Am. Chem. Soc. 2000, 122, 11727.
(10) For pioneering work in the field of CuH, see: (a) Brestensky,
D. M.; Huseland, D. E.; McGettigan, C.; Stryker, J. M. Tetrahedron Lett.
1988, 29, 3749. (b) Mahoney, W. S.; Brestensky, D. M.; Stryker, J. M.
8505
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