N-heterocyclic carbene-catalyzed stereoselective cascade reaction: Synthesis of functionalized tetrahydroquinolines

Article abstract of DOI:10.1021/ol4024985

The first N-heterocyclic carbene-catalyzed stereoselective aza-Michael-Michael-lactonization cascade reaction of 2′-aminophenylenones and 2-bromoenals for the construction of chiral functionalized tetrahydroquinolines with three consecutive stereogenic ce

Full text of DOI:10.1021/ol4024985

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
N‑Heterocyclic Carbene-Catalyzed
Stereoselective Cascade Reaction:
Synthesis of Functionalized
Tetrahydroquinolines
Hai-Rui Zhang, Zhen-Wen Dong, Yu-Jie Yang, Ping-Luan Wang, and Xin-Ping Hui*
State Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou 730000, P. R. China
huixp@lzu.edu.cn
Received July 25, 2013
ABSTRACT
The first N-heterocyclic carbene-catalyzed stereoselective aza-MichaelꢀMichaelꢀlactonization cascade reaction of 20-aminophenylenones and
2-bromoenals for the construction of chiral functionalized tetrahydroquinolines with three consecutive stereogenic centers has been achieved in
high yields (up to 98%) with excellent diastereo- (>25:1) and enantioselectivities (up to 98.7% ee).
The tetrahydroquinolines are very attractive targets
because of their biological activities. This structural motif
naturally occurs in several biologically active natural
products and pharmaceutics. In recent years, interest in
the synthesis of chiral tetrahydroquinolines has increased
dramatically.1 Until now, many synthetic methods
for accessing such compounds have been developed,
such as enantioselective hydrogenation of quinolines,2
aza-DielsꢀAlder reactions,3 Reissert-type reactions,4
NHC-catalyzed intramolecular aldehydeꢀketone benzoin
reactions,5 etc.
N-Heterocyclic carbene (NHC)-catalyzed umpolung
reactions have received great attention and contributed
tremendously to progress in recent decades.6,7 Since Glor-
ius et al.8a and Bode et al.8b independently reported the
NHC-catalyzed reaction of enals, all kinds of 2-functiona-
lized aldehydes9ꢀ14 have been explored in a large number
of umpolung reactions. However, the value of NHC-
catalyzed cascade reactions has only received growing
(1) For reviews, see: (a) Katritzky, A. R.; Rachwal, S.; Rachwal, B.
Tetrahedron 1996, 52, 15031. (b) Sridharan, V.; Suryavanshi, P. A.;
Menendez, J. C. Chem. Rev. 2011, 111, 7157.
(6) For selected reviews on NHC catalysis, see: (a) Balensiefer, T.
Acc. Chem. Res. 2004, 37, 534. (b) Zeitler, K. Angew. Chem., Int. Ed.
2005, 44, 7506. (c) Enders, D.; Niemeier, O.; Henseler, A. Chem. Rev.
ꢀ
(2) Selected examples: (a) Wang, W.-B.; Lu, S.-M.; Yang, P.-Y.; Han,
X.-W.; Zhou, Y.-G. J. Am. Chem. Soc. 2003, 125, 10536. (b) Rueping,
M.; Antonchick, A. P.; Theissmann, T. Angew. Chem., Int. Ed. 2006, 45,
3683. (c) Han, Z.-Y.; Xiao, H.; Chen, X.-H.; Gong, L.-Z. J. Am. Chem.
Soc. 2009, 131, 9182. (d) Wang, T.; Zhuo, L.-G.; Li, Z.; Chen, F.; Ding,
Z.; He, Y.; Fan, Q.-H.; Xiang, J.; Yu, Z.-X.; Chan, A. S. C. J. Am. Chem.
Soc. 2011, 133, 9878. (e) Chen, Q.-A.; Gao, K.; Duan, Y.; Ye, Z.-S.; Shi,
L.; Yang, Y.; Zhou, Y.-G. J. Am. Chem. Soc. 2012, 134, 2442.
ꢀ
2007, 107, 5606. (d) Marion, N.; Dıez-Gonzalez, S.; Nolan, S. P. Angew.
Chem., Int. Ed. 2007, 46, 2988. (e) Nair, V.; Vellalath, S.; Babu, B. P.
Chem. Soc. Rev. 2008, 37, 2691. (f) Phillips, E. M.; Chan, A.; Scheidt,
K. A. Aldrichimica Acta 2009, 42, 55. (g) Moore, J. L.; Rovis, T. Top.
Curr. Chem. 2010, 291, 77. (h) Biju, A. T.; Kuhl, N.; Glorius, F. Acc.
Chem. Res. 2011, 44, 1182. (i) Nair, V.; Menon, R. S.; Biju, A. T.; Sinu,
C. R.; Paul, R. R.; Jose, A.; Sreekumar, V. Chem. Soc. Rev. 2011, 40,
5336. (j) Rong, Z. Q.; Zhang, W.; Yang, G. Q.; You, S.-L. Curr. Org.
Chem. 2011, 15, 3077. (k) Vora, H. U.; Rovis, T. Aldrichimica Acta 2011,
44, 3. (l) Cohen, D. T.; Scheidt, K. A. Chem. Sci. 2012, 3, 53. (m) Bugaut,
X.; Glorius, F. Chem. Soc. Rev. 2012, 41, 3511.
€
(3) (a) Keck, D.; Vanderheiden, S.; Brase, S. Eur. J. Org. Chem. 2006,
4916. (b) Xu, H.; Zuend, S. J.; Woll, M.; Tao, G. Y.; Jacobsen, E. N.
Science 2010, 327, 986. (c) Dagousset, G.; Zhu, J.; Masson, G. J. Am.
Chem. Soc. 2011, 133, 14804.
(4) (a) Takamura, M.; Funabashi, K.; Kanai, M.; Shibasaki, M.
J. Am. Chem. Soc. 2001, 123, 6801. (b) Sun, Z.; Yu, S.; Ding, Z.; Ma, D.
J. Am. Chem. Soc. 2007, 129, 9300.
(7) (a) Kuwano, S.; Harada, S.; Kang, B.; Oriez, R.; Yamaoka, Y.;
Takasu, K.; Yamada, K. J. Am. Chem. Soc. 2013, 135, 11485. (b)
Kuwano, S.; Harada, S.; Oriez, R.; Yamada, K. Chem. Commun.
2012, 48, 145. (c) Kang, Q.; Zhang, Y. Org. Biomol. Chem. 2011, 9, 6715.
(5) Jia, M.-Q.; You, S.-L. ACS Catal. 2013, 3, 622.
r
10.1021/ol4024985
XXXX American Chemical Society

the carbonyl and β-carbon atoms of the 1,3-(bis)-
electrophile are used. In addition, little effort has been
focused on the use of the R,β-unsaturated acylazolium
intermediate in enantioselective cascade reactions.16d In
continuation of our investigations into enantioselective
reactions, we designed an efficient stereoselective aza-
MichaelꢀMichaelꢀlactonization cascade reaction of
2-bromoenals and 20-aminophenylenones catalyzed by
NHC for the synthesis of chiral functionalized tetrahydro-
quinolines (Scheme 1). This strategy allows three new
bonds and three stereocenters to be constructed in a single
cascade sequence.
Scheme 1. NHC-Catalyed Stereoselective Cascade Reaction
attention in the past four years.15 R,β-Unsaturated acyla-
zolium intermediates, which are efficient 1,3-(bis)-
electrophiles used for 1,4- or 1,2-addition reactions, can
be formed from R,β-unsaturated acyl fluorides,16 ynals,10
enals with an oxidant,17 and 2-haloenals.18 However, only
In the first stage of the study, we investigated the
stereoselective aza-MichaelꢀMichaelꢀlactonization cas-
cade reaction of 2-bromoenal 1a and 20-aminophenyle-
none 2a catalyzed by chiral N-heterocyclic carbene 3a
(10 mol %) in toluene at rt. The NHC-catalyst 3a provided
the product 4a in moderate yield and enantioselectivity
(Table 1, entry 1). The MoritaꢀBaylisꢀHillman reaction
did not proceed under these conditions probably due to the
increased degree of conjugation of 20-aminophenylenones
which did not favor conjugate addition of the DABCO
to the Michael acceptor. Furthermore, a series of chiral
NHC-catalysts 3bꢀ3g were tested in the cascade reaction
and the results showed that NHC-catalyst 3d was the best
catalyst for this transformation (entry 4). When chiral
NHC-catalysts 3eꢀ3g were used, compound 5 was pro-
vided instead of the desired product 4a. A screen of bases
revealed DABCO to be the preferred base (entry 4). Other
solvents were also investigated, and dichloromethane was
found to be the best reaction solvent (entry 10). To increase
the yield of this cascade reaction, the ratio of 2-bromoenal
1a and 20-aminophenylenone 2a was optimized (entries
12ꢀ14). To our delight, the product 4a was obtained with
98% yield, >25:1 dr, and 97% ee when 1.5 equiv of the
2-bromoenal was used (entry 13). When the amount of
NHC-catalyst 3d was reduced to 7.5 mol %, a high yield,
excellent diastereo- and enantioselectivity, and a fast reac-
tion rate were maintained (entry 15). Upon further reduc-
tion to 5 and 3 mol %, slightly lower yields and enan-
tioselectivities were obtained, respectively (entries 16ꢀ17).
When 2-chloro-3-phenylacrylaldehyde (1a0) was used, the
product 4a was isolated in 91% yield and 93% ee (entry 18).
To demonstrate the generality of the NHC-catalyst 3d
promoted stereoselective aza-MichaelꢀMichaelꢀlactoniza-
tion cascade reaction, a variety of 2-bromoenals and 20-
aminophenylenones were explored (Table 2). When the
cascade reaction of 2-bromoenals 1aꢀ1f and 20-amino-
phenylenone 2a were examined under the optimized
conditions, products 4aꢀ4f were obtained in high yield
(8) (a) Burstein, C.; Glorius, F. Angew. Chem., Int. Ed. 2004, 43, 6205.
(b) Sohn, S. S.; Rosen, E. L.; Bode, J. W. J. Am. Chem. Soc. 2004, 116,
14370.
(9) (a) He, M.; Bode, J. W. Org. Lett. 2005, 7, 3131. (b) Chan, A.;
Scheidt, K. A. Org. Lett. 2005, 7, 905. (c) Nair, V.; Vellalath, S.;
Poonoth, M.; Suresh, E. J. Am. Chem. Soc. 2006, 128, 8736. (d) He,
M.; Struble, J. R.; Bode, J. W. J. Am. Chem. Soc. 2006, 128, 8418. (e)
Phillips, E. M.; Wadamoto, M.; Chan, A.; Scheidt, K. A. Angew. Chem.,
Int. Ed. 2007, 46, 3107. (f) Chan, A.; Scheidt, K. A. J. Am. Chem. Soc.
2007, 129, 5334. (g) Chiang, P.-C.; Kaeobamrung, J.; Bode, J. W. J. Am.
Chem. Soc. 2007, 129, 3520. (h) Phillips, E. M.; Reynolds, T. E.; Scheidt,
K. A. J. Am. Chem. Soc. 2008, 130, 2416. (i) Chan, A.; Scheidt, K. A.
J. Am. Chem. Soc. 2008, 130, 2740. (j) Hirano, K.; Piel, I.; Glorius, F.
Adv. Synth. Catal. 2008, 350, 984. (k) Li, Y.; Zhao, Z.-A.; He, H.; You,
S.-L. Adv. Synth. Catal. 2008, 350, 1885. (l) Yang, L.; Tan, B.; Wang, F.;
Zhong, G. J. Org. Chem. 2009, 74, 1744. (m) Li, Y.; Wang, X.-Q.; Zhang,
C.; You, S.-L. Chem. Commun. 2009, 5823. (n) Kaeobamrung, J.; Bode,
J. W. Org. Lett. 2009, 11, 677. (o) Lathrop, S. P.; Rovis, T. J. Am. Chem.
Soc. 2009, 131, 13628. (p) Cardinal-David, B.; Raup, D. E. A.; Scheidt,
K. A. J. Am. Chem. Soc. 2010, 132, 5345. (q) Jiang, H.; Gschwend, B.;
Albrecht, L.; Jøgensen, K. A. Org. Lett. 2010, 12, 5052. (r) Kaeobamrung,
J.; Kozlowaki, M. C.; Bode, J. W. Proc. Natl. Acad. Sci. U.S.A. 2010,
107, 20661. (s) Chen, Z.; Yu, X.; Wu, J. Chem. Commun. 2010, 46, 6356.
(t) Cohen, D. T.; Cardinal-David, B.; Scheidt, K. A. Angew. Chem., Int.
Ed. 2011, 50, 1678. (u) Fang, X.; Chen, X.; Chi, Y. R. Org. Lett. 2011, 13,
4708. (v) Lu, S.; Poh, S. B.; Siau, W.-Y.; Zhao, Y. Angew. Chem., Int. Ed.
2013, 52, 1731.
(10) (a) Zeitler, K. Org. Lett. 2006, 8, 637. (b) Kaeobamrung, J.;
Mahatthananchai, J.; Zheng, P.; Bode, J. W. J. Am. Chem. Soc. 2010,
132, 8810. (c) Mahatthananchai, J.; Zheng, P.; Bode, J. W. Angew.
Chem., Int. Ed. 2011, 50, 1673. (d) Zhu, Z.-Q.; Xiao, J.-C. Adv. Synth.
Catal. 2010, 352, 2455. (e) Mahatthananchai, J.; Kaeobamrung, J.;
Bode, J. W. ACS Catal. 2012, 2, 494. (f) Qi, J.; Xie, X.; Han, R.; Ma,
D.; Yang, J.; She, X. Chem.;Eur. J. 2013, 19, 4146. (g) Du, D.; Hu, Z.;
Jin, J.; Lu, Y.; Tang, W.; Wang, B.; Lu, T. Org. Lett. 2012, 14, 1274. (h)
Romanov-Michailidis, F.; Besnard, C.; Alexakis, A. Org. Lett. 2012, 14,
4906. (i) Zhou, B.; Luo, Z.; Li, Y. Chem.;Eur. J. 2013, 19, 4428.
(11) (a) Kawanaka, Y.; Phillips, E. M.; Scheidt, K. A. J. Am. Chem.
Soc. 2009, 131, 18028. (b) Phillips, E. M.; Wadamoto, M.; Roth, H. S.;
Ott, A.; Scheidt, K. A. Org. Lett. 2009, 11, 105.
(12) (a) Reynolds, N. T.; Read de Alaniz, J.; Rovis, T. J. Am. Chem.
Soc. 2004, 126, 9518. (b) Reynolds, N. T.; Rovis, T. J. Am. Chem. Soc.
2005, 127, 16406. (c) He, M.; Uc, G. J.; Bode, J. W. J. Am. Chem. Soc.
2006, 128, 15088. (d) Vora, H. U.; Rovis, T. J. Am. Chem. Soc. 2010, 132,
2860.
(13) (a) Chow, K. Y.; Bode, J. W. J. Am. Chem. Soc. 2004, 126, 8126.
(b) Sohn, S. S.; Bode, J. W. Angew. Chem., Int. Ed. 2006, 45, 6021. (c)
Bode, J. W.; Sohn, S. S. J. Am. Chem. Soc. 2007, 129, 13798. (d) Vora,
H. U.; Rovis, T. J. Am. Chem. Soc. 2007, 129, 13796. (e) Li, G.-Q.; Li, Y.;
Dai, L.-X.; You, S.-L. Org. Lett. 2007, 9, 3519. (f) Thai, K.; Wang, L.;
Dudding, T.; Bilodeau, F.; Gravel, M. Org. Lett. 2010, 12, 5708. (g)
Wang, L.; Thai, K.; Gravel, M. Org. Lett. 2009, 11, 891.
(17) (a) Guin, J.; De Sarkar, S.; Grimme, S.; Studer, A. Angew.
Chem., Int. Ed. 2008, 47, 8727. (b) De Sarkar, S.; Studer, A. Angew.
Chem., Int. Ed. 2010, 49, 9266. (c) Wanner, B.; Mahatt-hananchai, J.;
Bode, J. W. Org. Lett. 2011, 13, 5378. (d) Rong, Z.-Q.; Jia, M.-Q.; You,
S.-L. Org. Lett. 2011, 13, 4080. (e) Kravina, A. G.; Mahatthananchai, J.;
Bode, J. W. Angew. Chem., Int. Ed. 2012, 51, 9433.
(18) (a) Sun, F.-G.; Sun, L.-H.; Ye, S. Adv. Synth. Catal. 2011, 353,
3134. (b) Yao, C.; Wang, D.; Lu, J.; Li, T.; Jiao, W.; Yu, C. Chem.;Eur.
J. 2012, 18, 1914. (c) Yetra, S. R.; Bhunia, A.; Patra, A.; Mane, M. V.;
Vanka, K.; Biju, A. T. Adv. Synth. Catal. 2013, 355, 1089. (d) Xiao, Z.;
Liu, R.; Li, T.; Jiao, W.; Yu, C.; Yao, C. Chem.;Eur. J. 2013, 19, 456.
(14) Wu, Y.; Yao, W.; Pan, L.; Zhang, Y.; Ma, C. Org. Lett. 2010, 12,
640.
(15) Grossmann, A.; Enders, D. Angew. Chem., Int. Ed. 2012, 51, 314.
(16) (a) Ryan, S. J.; Candish, L.; Lupton, D. W. J. Am. Chem. Soc.
2009, 131, 14176. (b) Candish, L.; Lupton, D. W. Org. Lett. 2010, 12,
4836. (c) Ryan, S. J.; Candish, L.; Lupton, D. W. J. Am. Chem. Soc.
2011, 133, 4694. (d) Candish, L.; Lupton, D. W. J. Am. Chem. Soc. 2013,
135, 58.
B
Org. Lett., Vol. XX, No. XX, XXXX

Table 1. Optimization of Reaction Conditionsa
Table 2. Substrate Scope of the NHC-Catalyzed Cascade
Reactiona
time yield
ee
R1 (1)
Ph (1a)
R2 (2)
Ph (2a)
prod (h) (%)b drc (%)d
4a
4b
4c
3.5
3.5
4
98 >25:1 97.2
98 >25:1 98.3
98 >25:1 97.3
98 >25:1 95.1
98 >25:1 95.0
93 >25:1 96.7
98 >25:1 97.6
95 >25:1 94.0
93 >25:1 97.7
93 >25:1 89.7
48 >25:1 86.9
93 >25:1 97.6
95 >25:1 89.2
98 >25:1 94.8
97 >25:1 98.0
75 >25:1 98.7
4-FC6H4 (1b)
2-ClC6H4 (1c)
3-ClC6H4 (1d)
4-ClC6H4 (1e)
4-BrC6H4 (1f)
4-MeC6H4 (1g)
Ph (2a)
Ph (2a)
Ph (2a)
Ph (2a)
Ph (2a)
Ph (2a)
4d
4e
3.5
3.5
3
4f
4ge
4he
4ie
4je
4ke
4le
4
catalyst
time yield
(h) (%)b
ee
(%)d
4-MeOC6H4 (1h) Ph (2a)
4-NO2C6H4 (1i) Ph (2a)
4-CF3C6H4 (1j) Ph (2a)
6
entry (mol %)
base
solvent
drc
3.5
3.5
8
1
2
3
4
5
6
7
8
3a (10) DABCO toluene
3b (10) DABCO toluene
3c (10) DABCO toluene
3d (10) DABCO toluene
3d (10) Cs2CO3 toluene
7
3
3
3
3
3
3
3
3
3
3
3.5
3.5
3.5
3.5
4.5
9
62 >25:1 73
65 >25:1 93
65 >25:1 94
76 >25:1 94
63 >25:1 96
53 >25:1 98
46 >25:1 94
70 >25:1 97
62 >25:1 94
79 >25:1 97
48 >25:1 97
94 >25:1 97
98 >25:1 97
98 >25:1 94
98 >25:1 97
97 >25:1 96
93 >25:1 95
91 >25:1 93
Me, (1k)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (1a)
Ph (2a)
4-ClC6H4 (2b)
4-BrC6H4 (2c)
4-MeC6H4 (2d) 4ne 3.5
4-MeOC6H4 (2e) 4oe 2.5
2.5
4me 2.5
3d (10) K2CO3
3d (10) DBU
3d (10) Et3N
toluene
toluene
toluene
Me (2f)
4pe 3.5
a Reaction conditions: 1/2/catalyst 3d/DABCO = 1.5:1:0.075:1.65
(molar ratio). b Isolated yield. c Determined by 1H NMR (400 MHz) of
the crude products. d Determined by chiral HPLC. e Catalyst 3d (10 mol %)
was used.
9
3d (10) DABCO THF
10
11
12e
13f
14g
15f
16f
17f
3d (10) DABCO CH2Cl2
3d (10) DABCO CH3CO2Et
3d (10) DABCO CH2Cl2
3d (10) DABCO CH2Cl2
3d (10) DABCO CH2Cl2
3d (7.5) DABCO CH2Cl2
The possible mechanism for this cascade reaction is
illustrated in Figure 1. Addition of the N-heterocyclic car-
bene, generated by deprotonation of precursor 3d, to the
2-bromoenal 1 forms the Breslow intermediate A. Inter-
mediate A is transformed into C through tautomerization
and debromination. The aza-Michael addition of 20-ami-
nophenylenone 2 to C provides enolate D. Enolate D can
itself undergo intramolecular Michael addition to give E,
which can participate in intramolecular lactonization to
furnish the desired product 4 and regenerate the N-hetero-
cyclic carbene catalyst.
The structures of the products 4aꢀ4p were characterized
by spectroscopic data, and the absolute configuration of
the product was determined to be as in (4R,5R,10S)-4f by
X-ray crystal analysis (Figure 2).19 The absolute config-
uration of the other products was assigned tentatively by
analogy.
The synthetic versatility of the adducts in this stereo-
selective cascade reaction was illustrated by a further
transformation. The adduct 4a could be easily ring opened
by benzylamine to the tetrahydroquinoline derivative 6 in
96% yield and 95.5% ee (Scheme 2).
In conclusion, we have developed an efficient NHC-
catalyzed stereoselective aza-MichaelꢀMichaelꢀlactonization
3d (5)
3d (3)
DABCO CH2Cl2
DABCO CH2Cl2
DABCO CH2Cl2
18f,h 3d (3)
3.5
a Reaction conditions: 1a/2a/base = 1:1:1.1 (molar ratio). b Isolated
yield. c Determined by 1H NMR (400 MHz) of crude product. d Deter-
mined by chiral HPLC. e 1a/2a = 1.25:1 (molar ratio). f 1a/2a = 1.5:1
(molar ratio). g 1a/2a = 1.75:1 (molar ratio). h 2-Chloro-3-phenylacry-
laldehyde (1a0) was used.
and excellent enantioselectivity. However, when the reac-
tions of 20-aminophenylenone 2a and 2-bromoenals
1gꢀ1j, which contain electron-donating or -withdrawing
groups, were carried out under the optimized conditions,
the products 4gꢀ4j were obtained in moderate yields
and excellent enantioselectivities. It should be noted
that 2-bromoenals 1j with the CF3ꢀ group exhibited
slightly low enantioselectivity. With respect to the alkyl-
substituted 2-bromoenal 1k, product 4k was afforded in
48% yield with 86.9% ee.
Several 20-aminophenylenones were also examined for
this reaction. In the course of the experiment, we found
thatboth electron-withdrawing and -donating substituents
on the aromatic ring of the 20-aminophenylenones 2bꢀ2e
provided the products 4lꢀ4o in high yield and excellent
enantioselectivity. The reaction of alkyl substituted 20-
aminophenylenone 2f also worked well to give product
4p with 75% yield and 98.7% ee.
(19) CCDC 950181 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
daa_request/cif.
Org. Lett., Vol. XX, No. XX, XXXX
C

Figure 2. X-ray crystallography of compound 4f.
Scheme 2. Transformation of Compound 4a
Figure 1. Possible catalytic cycle.
Acknowledgment. We are grateful for the financial
support of the National Natural Science Foundation
(21172099, 21372106), Program “111”, and J1103307.
cascade reaction of 20-aminophenylenones and 2-
bromoenals. Functionalized tetrahydroquinolines with
three consecutive stereogenic centers were obtained in high
yield with excellent diastereo- and enantioselectivities. This
approach is attractive due to the mild reaction conditions
and the potential utilization value of the products in mole-
cular biology and pharmacy. In addition, this strategy
extends the scope of NHC-catalysis and provides a simple
protocol for the NHC-catalyzed cascade reaction.
Supporting Information Available. Experimental pro-
cedure, characterization data for the products, crystal-
lographic data of compound 4f (CIF). This material
is available free of charge via the Internet at http://
pubs.acs.org.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX