Polyfunctional Lithium, Magnesium, and Zinc Alkenyl Reagents as Building Blocks for the Synthesis of Complex Heterocycles

Article abstract of DOI:10.1002/anie.201600961

New conjunctive β-silylated organometallic reagents of Li, Mg, and Zn have been prepared and used for an expeditive construction of various polyfunctionalized 5-, 6-, and 7-membered heterocycles, such as furans, pyrroles, quinolines, benzo[b]thieno-[2,3-b

Full text of DOI:10.1002/anie.201600961

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International Edition: DOI: 10.1002/anie.201600961
German Edition: DOI: 10.1002/ange.201600961
Conjunctive Reagents
Polyfunctional Lithium, Magnesium, and Zinc Alkenyl Reagents as
Building Blocks for the Synthesis of Complex Heterocycles
Zhi-Liang Shen+, Vasudevan Dhayalan+, Andreas D. Benischke+, Robert Greiner,
Konstantin Karaghiosoff, Peter Mayer, and Paul Knochel*
Dedicated to Professor Manfred Heuschmann
Abstract: New conjunctive b-silylated organometallic reagents
of Li, Mg, and Zn have been prepared and used for an
expeditive construction of various polyfunctionalized 5-, 6-,
and 7-membered heterocycles, such as furans, pyrroles, quino-
lines, benzo[b]thieno-[2,3-b]pyridine, naphthyridines, fused
pyrazoles, and 2,3-dihydro-benzo[c]azepines. The latent silyl
group has been converted into various carbon–carbon bonds in
most heterocycle types.
Thus, the trialkylsilyl-substituted[10] propargyl alcohols
(6a–b) were hydroaluminated with sodium bis(2-methoxy-
ethoxy)aluminum hydride (Red-Al)[11] followed by iodolysis,
providing the Z-allylic alcohols 7a–b in 60–61% overall yield
(Scheme 2). After MnO2 oxidation[12] and standard acetal
M
ain-group polyfunctional organometallic reagents[1] of
lithium,[2] magnesium,[3] and zinc[4] have found many synthetic
applications. The presence of electrophilic functional groups
in close proximity to a reactive carbon–metal bond opens
numerous synthetic opportunities to perform cyclizations and
therefore to construct new heterocyclic scaffolds of high
interest for the pharmaceutical and agrochemical industry.[5]
Herein, we wish to report the synthesis of the conjunctive
reagents 1–3 (Met = Li, MgCl, ZnCl).[6] These b-silylated[7] b-
metalated unsaturated acetals provide an entry to important
functionalized 5-, 6-, and 7-membered heterocycles and are
synthetic equivalents of the two allylic conjunctive synthons[8]
4 and 5 (Scheme 1). These reagents combine an electrophilic
Scheme 2. Preparation of acetal-containing organometallic reagents 1–
3 from trialkylsilyl-substituted propargylic alcohols 6a–b.
formation,[13] the Z-alkenyl iodides 8a–b were obtained in 81–
86% yield. Treatment of 8a–b with nBuLi[14] (1.1 equiv, THF,
À788C, 0.5 h) furnished the expected lithium reagents 1a–b in
70–90%. The reactions of 8a–b with iPrMgCl·LiCl[14]
(1.2 equiv, THF, 08C, 0.5 h) gave the corresponding magne-
sium reagents (2a–b) in 85–92% yield. Further transmetala-
tion of 2a–b with ZnCl2 led to the corresponding alkenylzinc
reagents 3a–b in > 98% yield.[15] With these six alkenylme-
tallic reagents 1a/b–3a/b in hand, we have prepared a range of
valuable heterocycles.
Scheme 1. The conjunctive reagents 1–3 (Met=Li, MgCl, and ZnCl).
To test our concept, we have prepared 1,2-disubstituted
furans and pyrroles. Thus, after treating the alkenylmagne-
sium reagents (2a–b) with various aryl and heteroaryl
aldehydes bearing either electron-donating or -withdrawing
substituents followed by an acid-mediated deacetalization, we
observed a spontaneous cyclization, leading to a variety of
1,2-di-substituted furans[16] 9a–f in 68–92% overall yield
(Scheme 3).
acetal function with two 1,1-bimetallic[9] nucleophilic entities
of well-differentiated reactivity. Furthermore, the silyl group
may be converted into various carbon–carbon bonds.
[*] Dr. Z.-L. Shen,[+] Dr. V. Dhayalan,[+] M. Sc. A. D. Benischke,[+]
M. Sc. R. Greiner, Prof. Dr. K. Karaghiosoff, Dr. P. Mayer,
Prof. Dr. P. Knochel
Importantly, the TMS group (TMS = SiMe3) present in 9d
can be readily converted into an iodide with ICl, affording the
3-iodo-furan (10) in 71% yield. After an I/Mg-exchange with
iPrMgCl·LiCl[14] leading to an intermediate Grignard reagent,
the addition of a second aldehyde provides the hydroxyary-
lated product 11 in 94% yield (Scheme 4).
Department Chemie, Ludwig-Maximilians-Universität München
Butenandtstrasse 5–13, Haus F, 81377 München (Germany)
E-mail: paul.knochel@cup.uni-muenchen.de
[+] These authors contributed equally to this work.
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201600961.
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reagents 1–3 prove also their utility in the synthesis of
annelated 6-membered heterocycles such as quinolines,
benzo[b]thieno-[2,3-b]pyridine, and naphthyridines,[19] which
are relevant heterocycles for pharmaceutical applications.[20]
Thus, the alkenylzincs (3a–b) underwent Pd-catalyzed
Negishi[21] cross-couplings with various 1-halo-2-nitroarenes
(15a–g), providing alkenylated nitroarenes of type 16, which
after indium- or zinc-mediated reduction[22] and acidic acetal
cleavage gave the annelated pyridines 17a–i in 41–70% yield
(Scheme 6).
Scheme 3. Preparation of furans 9a–f using alkenylmetallic reagents
2a–b.
Scheme 4. Conversion of the 3-TMS-furan (9d) to 3-iodo-furan (10)
followed by an I/Mg-exchange.
Our method was extended to a pyrrole synthesis by using
the Li-conjunctive reagent 1a instead of 2a–b.[17] Thus, the
alkenyllithium (1a) adds to various N-sulfonylaldimines or N-
(diethoxyphosphoryl)aldimines at À788C, providing after an
acidic deacetalization and spontaneous cyclization various
1,2-disubstituted pyrroles (12a–e) in 74–93% yield. Similarly,
the 3-TMS-pyrrole 12 f (which was prepared from 1b in 52%
yield)[10] was converted into an iodide using N-iodosuccini-
mide (NIS) and Ag2CO3 ((CF3)2CHOH, 08C, 2 h).[18] After an
I/Mg-exchange and addition to 3-bromo-benzaldehyde, the
pyrrole 14 is obtained in 74% yield (Scheme 5). The versatile
Scheme 6. Pd-catalyzed cross-couplings of alkenylzinc reagents 3a–b
with 1-halo-2-nitroarenes 15a–g for the preparation of annelated
pyridines 17a–i.
Using 3-bromo-2-nitrobenzo[b]thiophene or bromo-
nitro-pyridines as coupling reagents provides a short access
to the valuable benzo[b]thieno[2,3-b]pyridine (17 f, 54%
yield) and the 1,5- and 1,6-naphthyridines (17g–i, 60–65%
yield).[23]
Performance of an I/Si-exchange on the quinolines 17a–b
proved difficult both with 4-TMS- or 4-TBS-substituted
quinolines owing to the electron-deficient nature of this
heterocyclic ring. However, dearomatization of the N-hetero-
cycles (17a–b) by performing a 1,2-addition of an organo-
lithium (nBuLi or PhLi) provides after a treatment with
ClCO2Et the alkenylsilanes (18a–c) in 77–94% yield. Iodi-
nation of 18a–c using NIS and Ag2CO3[18] furnishes the 4-iodo
derivatives (19a–b) in 95–96% yield.[10] Negishi cross-cou-
pling with phenylzinc chloride or 1-methyl-1H-indol-5-ylzinc
bromide gives the desired coupling products 20a–b (64–92%
yield). Rearomatization of 20a–b using KOH/N2H4 in ethyl-
ene glycol (1908C, 2 h) provides the 2,4-difunctionalized
quinolines 21a–b in 76–81% yield (Scheme 7). Furthermore,
the naphthyridine 17i was readily magnesiated with
TMPMgCl·LiCl (À408C, 0.5 h).[24] Negishi cross-coupling
with ethyl 3-iodo-benzoate provides the 4-substituted naph-
thyridine 22 in 72% yield (Scheme 8).
Novel heterocycles were prepared using the conjunctive
reagent 3a. Thus, the cross-coupling of 3a with the 2-
Scheme 5. Preparation of pyrroles 12a–f using alkenylmetallic reagents
1a–b and further functionalization.
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Scheme 7. I/Si-exchange of the quinolines 17a–b, Negishi cross-cou-
pling, and rearomatization leading to functionalized quinolones 21a–b.
Scheme 10. Proposed mechanism for the formation of heterocycles 26.
(Scheme 10). Interestingly, performing this reaction using 1,6-
dialdehyde 25d gives a similar fused pyrazole 26d (65%
yield), in which the TBS-group has migrated formerly from
the pyrazole ring to a benzylic position. The structures of 26c–
d were unambiguously confirmed by X-ray diffraction
Scheme 8. Directed metalation of 1,5-naphthyridine (17i) followed by
Negishi cross-coupling.
halogeno-benzaldehydes (23a–d) provides the polyfunc-
tionalized arenes 24a–d in 51–77% yield. After acid hydrol-
ysis, the 1,6-dialdehydes (25a–d) were obtained in 44–84%
yield (Scheme 9). Remarkably, the 1,6-dialdehydes (25a–c)
readily underwent a cyclization after treatment with hydra-
zine monohydrate in a 1:4 mixture of acetic acid and ethanol,
leading to the tricyclic fused pyrazoles 26a–c in 62–77% yield,
instead of the expected 8-membered ring system of type 29
analysis.[23] The C Si bond of these fused pyrazoles can be
À
À
readily converted in two steps into a new C C bond. Thus, an
I/Si-exchange of 26c using NIS and Ag2CO3 provides the
iodide 27 in 84% yield.[18] An I/Mg-exchange with iPrMgCl·-
LiCl[14] and subsequent quenching with 4-cyano-benzalde-
hyde leads to the pyrazole derivative 28 in 95% yield
(Scheme 9).
A tentative mechanism for the formation of compounds
26 may involve the formation of an 8-membered ring of type
29, which undergoes an electrocyclization, leading to the
tricyclic ring system 30. Acidic rearrangement of 30 provides
the iminium intermediate 32a, which leads to the formation of
products 26a–c or 26d (Scheme 10). Thus, the iminium 32a
may undergo a proton migration, leading to 33, which
undergoes a 1,5-silyl migration,[25] affording the intermediate
34, which after aromatization furnishes 26a–c. On another
hand, if a bulky substituent (annelated ring) is present in the
intermediate 32a, the steric hindrance favors a double 1,5-silyl
migration, affording intermediate 35 via the silylammonium
ion 36. After proton loss, the desired silyl derivative 26d is
obtained.
Furthermore, the 1,6-dialdehydes of type 25 were also
converted into benzo[c]azepine derivatives of type 37 via
a double reductive amination[26] using NaBH(OAc)3 and an
aniline derivative, affording the corresponding 2,3-dihydro-
benzo[c]-azepines[27] 37a–c in 63–89% yield (Scheme 11).
In conclusion, we have described the preparation of new
conjunctive alkenyl-Li, -Mg, and -Zn reagents (1–3), bearing
a latent aldehyde function and a silyl group facilitating further
functionalizations. These versatile building blocks allow the
synthesis of various classes of important heterocycles (includ-
ing furans, pyrroles, quinolines, a benzo[b]thieno[2,3-b]pyri-
dine, a 1,5-naphthyridine, and a 1,6-naphthyridine), as well as
Scheme 9. Preparation of fused pyrazoles of type 26 using the
conjunctive reagent 3a and further functionalization.
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Keywords: conjunctive reagents · lithium · magnesium ·
N-heterocycles · zinc
How to cite: Angew. Chem. Int. Ed. 2016, 55, 5332–5336
Angew. Chem. 2016, 128, 5418–5422
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Received: January 27, 2016
Published online: March 16, 2016
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