Sulfilimines as Versatile Nitrene Transfer Reagents: Facile Access to Diverse Aza-Heterocycles

Article abstract of DOI:10.1002/anie.201812002

We herein report the unprecedented synthesis of diverse biologically important aza-heterocycles by employing sulfilimines as nitrene transfer reagents. This class of sulfur-based aza-ylides had not been successfully used for gold nitrene transfer before. This work contains an efficient generation of α-imino gold carbenes by N?S cleavage of sulfilimines. These gold carbenes undergo C?H insertion, cyclopropanation, and nucleophilic attack to form indoles (44 examples), 3-azabicyclo[3.1.0]hexan-2-imines (24 examples), and imidazoles (3 examples). Our study represents a unique gold-catalyzed reaction between alkynes and sulfur ylides, and also includes the first aza-heterocycle synthesis that proceeds by intermolecular nitrene transfer followed by cyclopropanation of the α-imino gold carbenes. Moreover, an unexpected synthesis of 4-acylquinolines (3 examples) from 2-acylphenyl sulfilimines and propargylic silyl ether derivatives by a 1,2-hydride shift onto the α-imino gold carbene and a subsequent Mukaiyama aldol cyclization was discovered.

Full text of DOI:10.1002/anie.201812002

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Sulfilimines as Versatile Gold Nitrene Transfer Reagents: Facile
Access to Aza-Heterocyclic Diversity
Authors: Xianhai Tian, Lina Song, Matthias Rudolph, Frank Rominger,
Thomas Oeser, and A. Stephen K. Hashmi
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201812002
Angew. Chem. 10.1002/ange.201812002
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10.1002/anie.201812002
Angewandte Chemie International Edition
COMMUNICATION
Sulfilimines as Versatile Gold Nitrene Transfer Reagents: Facile Access to
Aza-Heterocyclic Diversity
Xianhai Tian,[a] Lina Song,[a] Matthias Rudolph,[a] Frank Rominger,[a] Thomas Oeser,[a] and A. Stephen
K. Hashmi*[a,b]
Abstract: We herein report the unprecedented synthesis of diverse
biologically important aza-heterocycles by employing sulfilimines as
nitrene transfer reagents, this class of sulfur-based aza-ylides has not
been successfully used for gold nitrene transfer before. This work
contains an efficient generation of α-imino gold carbenes via N–S
cleavage of sulfilimines. These gold carbenes undergo C–H insertion,
cyclopropanation and nucleophilic attack to form indoles (44
examples), 3-azabicyclo[3.1.0]hexan-2-imines (24 examples) and
imidazoles (3 examples). This work represents the unique gold-
catalyzed reaction between alkynes and sulfur ylides, and also
includes the first examples of aza-heterocycle syntheses via
intermolecular nitrene transfer followed by cyclopropanation of the α-
imino gold carbenes. Moreover, an unexpected synthesis of 4-
acylquinolines (3 examples) from 2-acylphenyl sulfilimines and
propargylic silyl ether derivatives by using a 1,2-hydride shift onto the
α-imino gold carbene and a subsequent Mukaiyama aldol cyclization
was discovered.
reactivity. For the generation of α-imino gold carbenes the use of
new ylides bearing an aryl group on the anions is highly
challenging and of great importance, such gold carbenes can
efficiently form C-C bonds by insertion into the ortho-C–H bonds.
R1
a
R2
A: Oxygen transfer:
LG = pyridine, sulfide
O
b
O
O
[Au]
LG
O
R2
or
R2
R1
R1
R1
LG
R2
[Au]
B: Nitrene/carbene transfer:
R
X
Ts
Ts
N
Ts
=
N
N
N
N
LG
R2
Ph
N
R1
[Au]
LG
Ts
O
N
Ph
O
R
=
Ph
Ph
Ms
R1
R2
X
R
N
[Au]
X
N
LG
Over the last decade, gold catalysis[1] has been vastly utilized in
the electrophilic activation of alkynes. Especially, the generation
of gold carbenes from alkynes through intra- and intermolecular
atom transfer processes and subsequent trapping of such
Ph
N
N
R = strong acceptors:
Ts, acyl, amidinyl
N
Ph
R
R
LG
LG
R
=
N
Ph
N
Ms
X
X
X
N
O
[3+2]
R2
[Au]
R1
N
N
carbenes has emerged as
a powerful tool for efficient
O
transformations. While N-oxides[2] and sulfoxides[3] were
commonly employed for the formation of α-oxo gold carbenes, the
intramolecular trapping of such α-oxo gold carbenes is restricted
to functional groups in the substituents (Scheme 1, A). By contrast,
the inter- or intramolecular nitrene transfer from nucleophilic
nitrene equivalents such as pyridine-based aza-ylides,[4] azides,[5]
2H-azirines,[6] and isoxazole derivatives,[7] as well as carbene
transfer from sulfonium ylides,[8] extend the opportunities for ring
formation reactions; the generated gold carbenes can be trapped
by functionalities attached to either the transfer reagents or the
alkynes. Among these transfer reagents, ylides like pyridine-
based aza-ylides (N-–N+), sulfonium ylides (C-–S+) have attracted
considerable interests for their distinctive advantages in organic
synthesis because of the functional group tolerance (Scheme 1,
B), e.g. 1,2-hydride shifts[4f] or [3+2] annulations to form
oxazoles,[4a] furans,[8a] and imidazo-diazines.[4c] However, to the
best of our knowledge, these ylides are limited to strong acceptors
(sulfonyls, acyls, amidinyls) on the anions to tune their stablity and
Ph
Ph
Ph
=
S
n-Hexyl
R3
PG
This report:
Sulfilimines as nitrene transfer reagents
R1
N
n
o
R1
R2
i
t
N
r
e
s
n
i
H
H
–
R5
C
R2 PG
N
R4
N
R
S
[Au]
cyclopropanation
1
R3
N
R1
R2
R3
N
N
R
R1
N
RSR
R3
PG [Au]
)
1
,
2
PG
-
h
y
d
r
i
d
e
s
-imino
2
)
M
h
d
u
k
i
f
t
l
a
e
CHO
gold carbenes
i
n
y
a
c
o
m
a
n
d
a
l
s
o
a
t
i
o
n
Ts
N
N
Ph
Ph
Ts
Ph
N
Ph
Ph
N
N
nucleophilic attack
N
N
[Au]
N
Ph
Ph
S
N
N
N
DMS
Ph
[Au]
Ts
[Au]
Ts
Ph
Scheme 1. Previous work on atom transfer processes from ylides and
envisioned α-imino gold carbene formation from sulfilimines via N–S bond
cleavage.
[a]
MSc X. Tian, MSc L. Song, Dr. M. Rudolph, Dr. F. Rominger, Dr. T.
Oeser, Prof. Dr. A. S. K. Hashmi
Sulfur-based ylides are valuable synthetic intermediates due to
their high reactivity, sulfonium ylides[9] have been investigated
with great success with respect to [2+1] annulations for classic
cyclopropane synthesis from alkenes, epoxide synthesis from
carbonyl compounds, and aziridine synthesis from imines.
Recently, even in gold chemistry, acyl-stablized sulfonium ylides
were employed as gold-carbene precusors for 1,3-dipolar [3+2]
annulations[8] and cyclopropanations.[10] In contrast to this, the
Institut für Organische Chemie, Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: hashmi@hashmi.de
Prof. Dr. A. S. K. Hashmi
Chemistry Department, Faculty of Science
King Abdulaziz University
[b]
Jeddah 21589, Saudi Arabia
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201812002
Angewandte Chemie International Edition
COMMUNICATION
study of their nitrogen analogues, sulfilimines,[11] remain
comparatively underdeveloped, especially in gold chemistry.
Compared with other nucleophilic nitrenes, sulfilimines feature
inexpensive and easy synthesis,[12] and due to the strong
polarization towards the nitrogen atom, these buliding blocks can
emerge as valuable synthetic intermediates by N–S bond
cleavage.[13] Nevertheless, to the best of our knowledge, this
versatile and promising metal–nitrene precursor has never been
successfully applied as a gold carbene precusor, although N-
sulfonylsulfilimines were employed by Zhang et al.[4b] for the
synthesis of ,-unsaturated amidines (only 2 examples) in less
than 35% yield and quite moderate E/Z ratios of 1.5/1 to 2/1.
Inspired by previous reports[14] and in line with our interests on
gold-catalyzed C–H annulations (N–O bond cleavage for nitrene
transfer),[7b,d,g] we envisioned the generation of α-imino gold
carbenes from sulfilimines and ynamides, which could be flexibly
and divergently trapped by four types of functionalities attached
to either sulfilimines or ynamides to form four different aza-
heterocycles (Scheme 1).
like methylsulfonyl and easily cleavable 4-nitrophenylsulfonyl
gave 3h–3j in high yields. Ynamides bearing diverse
functionalities including electron-withdrawing, electron-donating,
and halogen substituents on the benzene rings delivered the
desired products 3k–3q in good to high yields, while an alkyl
ynamide (R2 = n-hexyl) reacted unselectively (detected by TLC).
Substrates containing sensitive bromoethynyl motif were
especially well tolerated affording products 3r and 3s in 86% and
77% yield. 3-(Thien-3-yl)-substituted indoles 3t–3u could also be
synthesized.
R1 PG
R2
N
Ms
R1
N
PG
Ms
PicAuCl2
+
S
toluene, 80 °C
R2
2
N
N
1a
H
3
R1 = Me, 3a, 90%
Ph
Ph
R1 = n-Bu, , 82%
R1
Ts
Ms
Ms
3b
R1
Ts
R1 = Ph, , 85%
Ms
3f
R1 = i-Bu, , 84%
N
3c
R1 = 3-Cl-Ph, , 90%
N
3g
R1 = Cyclopropyl, 3d, 81%
N
N
H
R1 = Bn, , 82%
H
3e
R2 = 2-F-Ph, 3k, 79%
R2 = 3-Cl-Ph, , 91%
3l
R2
Ph
R2 = 4-Me-Ph, 3m, 85%
R1 = Me, PG = Ms, , 92%
R1
N
PG
3h
3i
Ms
R2 = 4-t-Bu-Ph, , 83%
R1 = Ph, PG = Ms, , 92%
3n
N
Table 1: Optimization of the reaction conditions[a]
1
3o
N
N
R2 = 4-F-Ph, , 88%
3j
= Me, PG = Ns, , 86%
Ts
R
H
H
R2 = 4-CF3-Ph, 3p, 61%
R2 = 3,5-Di-OMe-Ph, , 97%
Br
3q
R2 = n-Hexyl, not obtained
S
R1 = Me, 3t, 90%
R1
N
Ts
Ms
R1 = Me, 3r, 86%
R1
Ms
R1 = Ph, , 91%
3u
R1 = Ph, , 77%
3s
N
N
N
H
Ts
Entry
Catalyst
IPrAuCl/AgNTf2
PPh3AuCl/AgNTf2
NaAuCl4
Solvent
toluene
toluene
toluene
toluene
THF
Time/h
Yield[b]
17%
13%
77%
90%
67%
72%
n. d.[c]
H
1
2
3
4
5
6
7
24
24
4
Scheme 2. Scope with respect to the ynamides[a, b]. [a] Reaction conditions: 1a
(0.3 mmol, 1.5 equiv), 2 (0.2 mmol, 1.0 equiv), PicAuCl2 (3.8 mg, 5 mol%),
toluene (2 mL, 0.1 M); [b] Yield of isolated product 3.
PicAuCl2
PicAuCl2
4
We then tested the reactivity of N-phenylsulfilimines. Due to the
poor stability of S,S-dimethyl-N-phenylsulfilimines without
electron-withdrawing groups on the benzene ring, we sought to
prepare diverse indoles from S,S-diphenyl-N-phenylsulfilimines
(Scheme 3). To our delight, triphenylsulfilimine 1b reacted
efficiently with a variety of ynamides to afford an array of products
3v–3ac in 82%–99% yields. Ylides 1c–1h bearing methyl, phenyl,
halogen and trifluoromethyl groups at the 4-positions also gave
3ad–3ai in good to excellent yield. The efficiency was contrary to
the electron density on the aromatic ring. 3-Substituted-phenyl
sulfilimines were also found to be suitable for the synthesis of 2-
aminoindoles. The substrate with a chloro substituent at 3-
position could be converted in high efficiency with a poor
ortho/para selectivity (3ak/3aj = 59/33, solid state molecular
structure of 3ak[15] is shown in the supporting information), while
both 3-methoxy- and 4-chloro-3-methoxyphenyl sulfilimines led to
single regioisomers (3al–3am) in high yields. 7-Chloroindole 3an
could be prepared from 2-chlorophenylsulfimine 1l. For 2-
acylphenylsulfilimines, 7-acylindole 3ao was isolated in 93% yield
from normal ynamide 2a, while 4-acylquinolines 5a-c were
prepared from propargylic silyl ethers under standard conditions
through a 1,2-hydride shift onto the α-imino gold carbene and
sequential Mukaiyama aldol cyclization (Scheme 4). Larger
molecules 3ap–3ar containing two indole cores were obtained by
4
PicAuCl2
DCE
4
-
toluene
4
[a] General reaction conditions: 1a (0.3 mmol, 1.5 equiv), 2a (0.2 mmol,
1.0 equiv), catalyst (5 mol%), solvent (2.0 mL, 0.1 M), 80 °C; [b] Isolated
yield of product 3a; [c] n. d.: not detected.
The reaction between S,S-dimethylsulfilimine 1a and ynamide
2a in toluene at 80 °C for 24 h with IPrAuCl/AgNTf2 as catalyst
was explored first, providing 2-aminoindole 3a[15] in 17% yield
(Table 1, entry 1). PPh3AuCl/AgNTf2 as catalyst did not improve
the reaction (entry 2). Further screening showed that gold(III)
complexes are more efficient catalysts (entry 3–4), and
[16]
PicAuCl2 delivered 3a in 90% yield (entry 4). A short solvent
optimization showed that toluene is the best solvent (entry 5–6).
In the absence of any catalyst no desired product was obtained
(entry 7).
Under the optimized conditions, we investigated the reaction
scope. A series of ynamides bearing alkyl and aryl groups on the
nitrogen atom gave the corresponding 2-aminoindoles (3a–3g) in
81-90% yield (Scheme 2). Ynamides with other protective groups
bidirectional reactions of bis-ynamides and
3 equivalents
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10.1002/anie.201812002
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COMMUNICATION
triphenylsulfilimine 1b. Furthermore, a non-sulfonyl-protected
ynamide bearing an indole core is also suitable for this
transformation (3s).
on the nitrogen (4b−4c). S,S-Diphenyl-N-phenyl sulfilimines
bearing other electron-withdrawing groups (NO2, CF3) at either
meta- or para-positions underwent the desired cyclopropanation
process to give the corresponding products in 83%-91% yield.
This reaction was found to be compatible with a series of N-
alkylsulfonyl-substituted ynamides (R4
=
Me, n-Pr, i-Pr,
R1 PG
N
R2
R1
N
PG
cyclopropyl, Bn) and the products 4g−4k were all obtained in
higher than 86% yields. Ynamides with diverse substituents (R5 =
Me, Et, n-Pr, t-Bu, Oet, F, CF3) at the 4-positions of the benzene
rings were also tested. As expected, these ynamides provided the
corresponding cyclopropane compounds 4l−4r in excellent yields.
When R6 was varied from an electron donating group (methyl) to
an electron withdrawing group (chloro or fluoro) at 3-positions of
the benzene rings, the reactions proceeded smoothly and the
corresponding products 4s−4u were achieved in excellent yields.
A bromo substituent also survived in this transformation, and the
target 4v was obtained in 95% yield. Product 4w with two vicinal
quaternary carbon centers could be efficiently synthesized by the
treatment of N-(2-methylallyl)ynamide (R2 = Me) with N-(4-
trifluoroPhenyl)sulfilimine 1h. An internal alkene derivative gave
product 6-methyl-3-azabicyclo[3.1.0]hexan-2-imine 4x in 95%
yield with a diastereomer ratio of 9:1.
Ph
S
PicAuCl2
R3
R3
toluene, 80 °C
N
Ph
N
H
R2
1
3
2
R2
R1
PG = Ts, R1 = Me, , 89%
3v
N
Ph
PG = Ts, R1 = allyl,
, 82%
3w
R1
N
PG
N
Ts
PG = Ts, R1 = Bn, 3x, 93%
H
R2 = Ph, R1 = 4-Me-Ph,
, 91%
PG = Ms, R1 = Me, , 93%
3aa
3y
N
R
2 = Ph, R1 = 4-OMe-Ph,
PG = Ms, R1 = Ph, 3z, 87%
H
3ab
, 99%
R2 = 3-Cl-Ph, R1 = Ph, 3ac, 98%
R3 = Me, 3ad, 76%
Cl
Ph
Ph
R3 = Ph,
, 81%
Ph
3ae
R3
R3 = F, 3af, 94%
N
N
N
R3 = Cl,
R3 = Br,
, 91%
, 89%
R3 = CF3, 3ai, 98%
3ag
N
Ts
N
Ts
N
Cl
Ts
H
H
H
3ah
3aj, 33%
3ak, 59%
H
Ph
Ts
N
Ph
Ph
Cl
N
N
Ts
N
N
N
N
N
H
Ts
Ts
H
O
O
R3
H
N
R3 = Cl, 3an, 95%
R3 = CHO,
3am, 86%
3al
, 84%
N
Ts
H
3ao
, 95%
3ap, 57%[c]
R
PG
R1
N
R2
R3
N
Ph
R
S
PicAuCl2
toluene, 80 °C EWG
R1
EWG
Ms
N
PG
N
R
NH
HN
N
2
NH
Ts
HN
1
N
Ts
3aq
N
Ts
, 81%
4
R3
O
N
H
N
N
[c]
O
Ts
3as, 60%
[c]
3ar
, 63%
Ph
Ph
Ms
N
N
N
Ts
N
, 88%
Scheme 3. Scope of sulfilimines[a, b]. [a] Reaction conditions: 1 (0.3 mmol, 1.5
equiv), 2 (0.2 mmol, 1.0 equiv), PicAuCl2 (3.8 mg, 5 mol%), toluene (2 mL, 0.1
M); [b] Yield of isolated product 3; [c] 3.0 equiv ylide 1a was used.
SO2Ph
4b
, 61%
4a
Ph
Ph
N
Ph
O2N
F3C
Cps[c]
Ms
N
N
N
N
Ts
N
Ts
4e
, 85%
4c, 70%
4d
, 91%
PG
R
N
R
CHO
PG
Ph
S
R4 = Me, , 87%
standard conditions
4g
Ph
N
Ph
N
N
R4 = n-Pr, 4h, 88%
Ph
N
F3C
CHO
4i
N
S
R4 = i-Pr, , 86%
N
Ts
N
TBSO
1
, 0.3 mmol
R4 = cyclopropyl, 4j, 89%
O2N
R4
5a
R = H, PG = Ts, , 63%
R = Cl, PG = Ts, , 64%
O
2
, 0.2 mmol
4f
, 83%
5b
4k
O
R4 = Bn, , 89%
R = H, PG = Ms, 5c, 72%
-DPS
[Au]
R
5 = Me, 4l, 91%
R6
R5
4m
, 92%
R5 = Et,
-TBSOH
PG
R5 = n-Pr, 4n, 94%
F3C
PG [Au]
N
R5 = t-Bu, , 94%
N
N
4o
O
N
OTBS
Ts
R5 = OEt, , 89%
TBS
R6 = Me, , 94%
4p
F3C
4s
R6 = F, 4t, 97%
N
O
N
Ts
R5 = F, 4q, 93%
N
H
N
-[Au]
R5 = CF3, , 95%
R6 = Cl, , 95%
4r
4u
R
CHO
R
Br
Ph
Ph
F3C
F3C
F3C
N
Ts
Scheme 4
.
Synthesis of 4-acylquinolines through gold-catalyzed cascade
[a] Isolated yield of 5.
N
N
4x
, 95%, dr = 9:1
N
, 95%
N
Ts
N
4v, 95%
transformations[a]
.
Ts
4w
Scheme 5. Synthesis of 3-azabicyclo[3.1.0]hexan-2-imines 4 [a, b]. [a] Reaction
conditions: for products 4a-4c, R = Me, 1a (0.3 mmol, 1.5 equiv), 2 (0.2 mmol,
1.0 equiv), PicAuCl2 (3.8 mg, 5 mol%), toluene (2 mL, 0.1 M); for products 4d-
4x, R = Ph, 1 (0.2 mmol, 1.0 equiv), 2 (0.24 mmol, 1.2 equiv), PicAuCl2 (3.8 mg,
5 mol%), toluene (2 mL, 0.1 M); [b] Yield of isolated product 4. [c] Cps = 4-
chlorophenylsulfonyl.
The products are not only confined to quinoline and indole
derivatives. Under the same reaction conditions, by using S,S-
dimethyl sulfilimine 1a and N-allyl ynamide as substrates, we
prepared 3-azabicyclo[3.1.0]hexan-2-imine 4a[15] rather than the
indole product, through cyclopropanation of the corresponding
gold carbene (Scheme 5). Sulfilimine 1a also reacted well with
ynamides bearing either phenylsulfonyl or 4-chlorophenylsulfonyl
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10.1002/anie.201812002
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Angew. Chem. Int. Ed. 2011, 50, 8931; Angew. Chem. 2011, 123,
9093; b) C. Li, L. Zhang, Org. Lett. 2011, 13, 1738; c) M. Garzon,
P. W. Davies, Org. Lett. 2014, 16, 4850; d) R. J. Reddy, M. P. Ball-
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Liao, R.-S. Liu, Adv. Synth. Catal. 2013, 355, 1545.
To completely explore the reactivity pattern between
sulfilimines and ynamides, we further attempted to probe the
feasibility of imidazole synthesis from N-iminylsulfilimine 1p.
Fortunately, under the standard conditions, the fully-substituted
imidazole derivatives 6a-c were formed in 75-95% yields by
employing the second nitrogen atom as a trapping functional
group (Scheme 6).
[5]
Selected examples on -imino gold carbenes from azides, see: a)
D. J. Gorin, N. R. Davis, F. D. Toste, J. Am. Chem. Soc. 2005,
127, 11260; b) B. Lu, Y. Luo, L. Liu, L. Ye, Y. Wang, L. Zhang,
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Zhu, Y. Yu, X. Luo, X. Huang, J. Org. Chem. 2015, 80, 11407.
For examples on -imino gold carbenes from 2H-azirines, see: a)
L. Zhu, Y. Yu, Z. Mao, X. Huang, Org. Lett. 2014, 17, 30; b) A.
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Ts
N
Ar
Ph
Ph
Ph
Ph
6a
Ar = Ph, , 75%
6a
Ar = 4-CH3-Ph, , 77%
6a
Ar = 4-t-Bu-Ph, , 95%
N
PicAuCl2
N
S
N
toluene, 80 °C, 24 h
N
N
Ts
Ar
2
1p
Scheme 6. Synthesis of imidazole 6 through nucleophilic attack of the gold
carbene[a,b]. [a] Reaction conditions: 1p (0.3 mmol, 1.5 equiv), 2 (0.2 mmol, 1.0
equiv), PicAuCl2 (3.8 mg, 5 mol%), toluene (2 mL, 0.1 M); [b] Yield of isolated
product.
[6]
[7]
In conclusion, this successful and broad use of sulfilimines for
the synthesis of biologically important amino-substituted aza-
heterocycles and carbocycles via gold iminocarbene
intermediates clearly demonstrates the synthetic potential of
these reagents and reactions, which certainly is not limited to the
specific examples in this manuscript. Future applications also will
benefit from the fact that the sulfilimine reagents are cheap and
readily available, a future challenge is the use of less activated
alkynes.
Acknowledgements
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Keywords: sulfilimines • gold carbenes • gold catalysis • C–H
annulations • cyclopropanations
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10.1002/anie.201812002
Angewandte Chemie International Edition
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Single crystal X-ray structure analysis: CCDC 1861976 (3a),
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Entry for the Table of Contents
COMMUNICATION
Xianhai Tian, Lina Song, Matthias
Rudolph, Frank Rominger, Thomas
Oeser and A. Stephen K. Hashmi*
Page No. – Page No.
Sulfilimines as Versatile Gold Nitrene
Transfer Reagents: Facile Access to
Aza-Heterocyclic Diversity
Readily available sulfilimines were successfully applied as versatile nitrene transfer
reagents. By trapping the α-imino gold carbenes that were formed with divergent
functionalities, four different types of aza-heterocycles (indoles, 3-
azabicyclo[3.1.0]hexan-2-imines, quinolines, and imidazoles) were prepared in high
efficiency.
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