Synthesis of β-prolinols via [3+2] cycloaddition and one-pot programmed reduction: Valuable building blocks for polyheterocycles

Article abstract of DOI:10.1016/j.tetlet.2016.11.035

A novel two-step synthesis of multisubstituted β-prolinols has been developed, featuring a [3+2] cycloaddition of azomethine ylides and a programmed reduction triggered by the combination of borane and lithium aluminum hydride (LAH). β-Prolinols are shown to be valuable building blocks for polyheterocycles.

Full text of DOI:10.1016/j.tetlet.2016.11.035

Tetrahedron Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of b-prolinols via [3+2] cycloaddition and one-pot
programmed reduction: Valuable building blocks for polyheterocycles
⇑
Jundong Li, Na Lin, Lei Yu, Yandong Zhang
Department of Chemistry and Fujian Provincial Key Laboratory of Chemical Biology, Collaborative Innovation Center of Chemistry for Energy Materials, College of Chemistry
and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel two-step synthesis of multisubstituted b-prolinols has been developed, featuring a [3+2] cycload-
dition of azomethine ylides and a programmed reduction triggered by the combination of borane and
lithium aluminum hydride (LAH). b-Prolinols are shown to be valuable building blocks for
polyheterocycles.
Received 19 October 2016
Revised 4 November 2016
Accepted 7 November 2016
Available online xxxx
Ó 2016 Elsevier Ltd. All rights reserved.
Keywords:
Heterocycles
b-Prolinols
[3+2] cycloaddition
Programmed reduction
Introduction
Moreover, the resulting products were utilized for the facile syn-
thesis of several polyheterocycles, including the core of martinellic
The
a
-prolinols have been widely used as heterocyclic building
acid.
By taking advantage of the technologies developed by Car-
retero12 and us9 for the synthesis of
-cyanopyrrolidines through
the [3+2] cycloaddition of -iminonitriles and electron-deficient
blocks,1 organic catalysts2 and ligands3 for asymmetric catalysis.
Various methods have been developed for the synthesis of
a-proli-
a
nols.4 Although their constitutional isomers, b-prolinol derivatives
frequently occur as structural motifs in natural products5 and
bioactive molecules6 (Fig. 1) and also serve as important building
blocks in organic synthesis,7 a convenient access to such small
heterocycles remains elusive.8
a
olefins, the current proposal still poses a challenge in the second
step (Scheme 1b). In this step, we designed a one-pot reduction
including reductive removal of a cyano group next to an amino
group13 and a reduction of an ester group to primary alcohol. How-
ever, commonly-used powerful reductants (e.g., LAH or DIBAL-H)
for the reduction of an ester group usually inevitably caused the
direct reduction of the cyanide to a primary amine as a competitive
reaction.14 To date, this long-standing problem has yet to be
solved. We envisioned that the desired chemoselectivity15 could
be achieved through a meticulous control of reaction pathway by
the employment of Lewis acid activators. To our knowledge, such
programmed reduction is unprecedented. Hence, the development
of reductive system to realize a well-controlled reduction is pivotal
for this two-step synthesis of b-prolinols.
In our recent study on the synthesis of pyrrolidine natural prod-
ucts,9 we developed an efficient synthesis of a broad spectrum of 5-
unsubstituted pyrrolidines through a two-step strategy, which
involved a 1,3-dipolar cycloaddition of a-iminonitriles and a novel
reductive decyanation reaction with borane and sodium borohy-
dride (Scheme 1a). Notably, in the later transformation, we found
that the borane played a dual role as Lewis acid activator and
reducing agent while a catalytic amount of sodium borohydride
functioned as a basic initiator for an anionic chain reaction.
Inspired by these results, we envisioned that a similar two-step
strategy might be feasible for accessing the multisubstituted b-
prolinols (Scheme 1b), which could be elaborated into valuable
polyheterocyclic compounds. Herein, we report a general approach
to the synthesis of multifunctionalized b-prolinols via [3+2]
cycloaddition followed by one-pot10 programmed reduction.11
Results and discussion
At the outset,
a-cyanopyrrolidine 1a was chosen as a model
substrate for a survey of reductants and additives, and the results
are outlined in Table 1. Not surprisingly, treatment of 1a with
LAH (2.0 equiv) at 0 °C afforded the desired b-prolinol 2a in 48%
1H NMR yield together with a significant amount of diamine 3
⇑
Corresponding author.
E-mail address: ydzhang@xmu.edu.cn (Y. Zhang).
http://dx.doi.org/10.1016/j.tetlet.2016.11.035
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.
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2
J. Li et al. / Tetrahedron Letters xxx (2016) xxx–xxx
the cooperation of borane and LAH delivered the desired b-
prolinol 2a in high yield (Table 1, entry 7) and only a trace
amount of diamine was observed. In this programmed
reduction, reductive decyanation and ester reduction proceeded
harmoniously. Importantly, in contrast to the results of entry 1,
HN
HN
H
H
N
H
O
N
NMe2
OMe
HO2C
CH3
H
N
3
HO2C
HN NH
NH
N
R
H
H
N
Cy
H
martinellic acid
acetylcholinesterase HCV RNA polymerase
the
introduction
of
borane
significantly
enhanced
the
inhibitors inhibitor
chemoselectivity (the reductive decyanation v.s. direct reduction of
cyano group) of powerful LAH. To our knowledge, a combination
of borane and LAH has not been employed previously in a
reductive decyanation reaction.
Fig. 1. Selected bioactive heterocycles containing masked b-prolinols.
With the optimal reaction conditions in hand, we next exam-
ined the scope of this two-step transformation. Cyanopyrrolidines
a) Our previous work:
H
N
R1
H
N
R2
CN
R3
CN
H
H
R3
R1
1a–1z were conveniently prepared from corresponding
a-iminon-
BH3 (1.2 equiv)
NaBH4 (0.2 equiv)
R1
N
itriles and ,b-unsaturated esters via AgOAc catalyzed [3+2]
a
[3+2]
R2
cycloaddition in good yields.23 As shown in Table 2, a variety of
multisubstituted b-prolinols were efficiently prepared via the
newly developed programmed reduction protocol. The cyanopy-
rrolidines bearing various substituted phenyl groups or heterocy-
cles were well-tolerated substrates to deliver b-prolinols 2a–2n
in good to high yields. Interestingly, the styrenyl double bond of
cyanopyrrolidine 1o survived this double-site reduction with bor-
ane and LAH without the occurrence of evident hydroboration
reaction. However, the cyanopyrrolidine with a terminal double
bond (1p) failed to give pure desired b-prolinol 2p because of
excessive hydroboration in the presence of borane and hydrogena-
tion in the workup operation with Pd/C.16 A series of b-prolinols
bearing aliphatic chains (2q–2t) or rings (2u–2w) were efficiently
prepared through current protocol. Steric hindrance was proved
to have little effect on the selective reduction and three multisub-
stituted b-prolinols (2x–2z) were synthesized in good yields. Nota-
bly, the current procedure could be also conducted on a gram scale
with similar efficiency (2a and 2m).
R2
R3
5-unsubstitued pyrrolidines
b) This Work:
R1
H
H
R1
N
H
N
One-pot
Programmed reduction
[3+2]
CN
HO
H
MeO2C
R2
R3
R2
R3
multisubstituted β-prolinols:
valuable heterocyclic building blocks
Scheme 1. Construction of Multisubstituted b-Prolinols via [3+2] Cycloaddition and
Programmed Reduction.
(49% yield) (Table 1, entry 1). Interestingly, reaction of 1a with
2.2 equiv of DIBAL-H at À78 °C produced prolinol 4 in 51% yield
and a trace amount of 2a (Table 1, entry 2). However, when 1a
was treated with 5.0 equiv of DIBAL-H at 0 °C then rt, diamine 3
was obtained as a major product (64% of 3 and 20% of 2a; Table 1,
entry 3). To our delight, sequentially running the reductive decya-
nation under our previously developed conditions9a (1.2 equiv of
borane and 0.2 equiv of NaBH4) and ester reduction (2.0 equiv of
LAH) either in a two-pot or in a one-pot manner gave 2a in high
In our previous studies on the reductive decyanation of
a-
cyanopyrrolidines with borane and NaBH4, the preliminary mech-
anistic study showed that borane acted not only as a Lewis acid
activator but also as a major hydride source (Scheme 2a).9a In
order to gain some mechanistic insight on the new programmed
reduction, a reduction of 1a with borane (1.3 equiv) and LiAlD4
(2.0 equiv) was conducted (Scheme 2b). The results revealed that
borane was still the major hydride source of the reductive
decyanation on C5, albeit in a lower percentage of 62%. Notably,
yields (Table 1, entries
4 and 5). Encouraged by the good
performance of borane in reductive decyanation,9a we next
explored the possibility to realize the two aforementioned
reductions in single-step operation based on
programmed reduction. Although the combination of borane and
DIBAL-H did not improve the desired reaction (Table 1, entry 6),
a concept of
Table 1
Optimization of Reaction Conditions.a
H
H
N
H
N
NH2
H
N
N
Ph
CN conditions Ph
HOH2C
Ph
Ph
CN
MeO2C
HOH2C
HOH2C
1a
2a
3
4
Entry
Reductants(equiv)
T(°C)
Yield%b
t(h)
1
2
3
4
5
6
7
LAH (2.0)
DIBAL-H (2.2)
DIBAL-H (5.0)
0
48% (2a), 49% (3)
51% (4)
1
6
3
–
3
5
1
À78
0 ? 25
0
25 ? 0
0 ? 25
0
20% (2a), 64% (3)
BH3 (1.2), NaBH4 (0.2), workup; then LAH (2.0)d
BH3 (1.2), NaBH4 (0.2), then LAH (2.0)
BH3 (1.3), DIBAL-H (5.0)
86% (2a)c
93% (2a)c
33% (2a), 31% (3)
BH3 (1.3), LAH (2.0)
92% (2a)c
a
Reactions were performed with 1.0 mmol of 1a in 10 mL of THF (0.1 M). The reactions of entries 4–7 were quenched with aqueous NaOH (20%) and crude products was
later treated with Pd/C (10 wt%) in methanol.
b
NMR yield with triphenylmethane as an internal standard.
Isolated yield.
Through a stepwise manner. BH3 refers to BH3ÁTHF. DIBAL-H = diisobutyl aluminum hydride, LAH = lithium aluminum hydride.
c
d
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J. Li et al. / Tetrahedron Letters xxx (2016) xxx–xxx
3
Table 2
CuI, Cs2CO3
HN
TsN
TsN
Substrate Scope for Two-Step Synthesis of b-Prolinols.a
TsCl, Pyr.
88%
H
8-hydroxyquinoline
a)
b)
Toluene, 110 °C
H
R2
CO2Me
R2
R3
R3
Br OH
2i
Br OH
7
91%
O
8
BH ·THF (1.3 equiv)
CH2OH
R1
3
LAH (2.0 equiv)
H
NC
N
H
R1
N
THF (0.1M), 0 °C
then workup
H
TsN
H
HN
(HCHO)n
TFAA
TsN
1) TsCl, Pyr. 91%
2) TsNHBoc, DIAD
H
1
2
H
MsOH, DCE
2a R=H, 92%
(gram-scale)
2b R=p-F, 97%
2c R=p-Cl, 91%
2d R=p-CF3, 85%
2e R=p-Me, 86%
OH
2f R=p-OMe, 91%
NTs
OH
OH
PPh3; then TFA
92%
93%
R
NHTs
2g[b] R= -Br, 84%
p
2a
9
10
2h[b] R=m-Br, 83%
N
H
HN
TsN
TsN
2i R=o-Br, 67% +26% 2a
TsCl, Et3N
Li
c)
DMAP, DCM
DMSO/THF
OTs
30%
OH
X
OH
OH
92%
2a
11
12
N
N
H
N
TsHN
H
NBoc
N
H
TsN
2j X=O, 80%
2k X=S, 87%
2l X=NTs, 79%
H
TfOH, DCM
–78 °C
83%
2n[c] 71%
2m 90%, (gram-scale)
H
OH
Me
Me
OH
13
14
OH
)
(
6
Ph
Ph
N
H
Scheme 3. Synthesis of Heterotricycles from b-Prolinols.
N
H
N
H
2o 77%
2p[d] complex
2q 82%
OH
TsN
OH
OH
TsN
H
X-ray
OMe
OTBS
DIH, Na2SO3
H
DCE, 60 °C
72%
N
H
N
H
OMe
N
N
H
NHTs
Ts
2r 75%
2s 78%
2t 54%
9
15
NBS, CH3CN
reflux
OH
OH
OH
Pd(OAc)2
Xantphos
91%
TsN
N
HN
N
N
H
O
NTs
H
H
1) TsCl, Pyr. 89%
2) TsNHBoc, DIAD
CO (1 atm), MeOH
Et3N, 70 °C
H
Br
Br
2u 87%
2v 81 %
2w 85%
H
85%
N
PPh3; then TFA, 93%
3) DIH, Na2SO3, DCE
reflux, 76%
HO
2h
Me
Me
Ts
Ph
OH
OH
OH
16
N
H
N
N
H
HN
TsN
H
H
2x 81%
2y 77%
2z 85%
MeO2C
HN
N
H
H
a
HO2C
HN NH
Unless otherwise noted, all reactions were performed with the standard pro-
cedure; isolated yields are reported.
N
Ts
H
NH
17
b
N
A tiny amount (around 10%) of 2a was also isolated.
Column chromatography was necessary before the treatment with Pd/C in
H
c
Core of martinellic acid
martinellic acid
methanol.
d
The amino-borane complex of 2p could be obtained in 13% yield. Boc = tert-
butoxycarbonyl, Ts = p-toluenesulfonyl, TBS = tert-butyldimethylsilyl.
Scheme 4. Synthesis of the Tricyclic Core of Martinellic Acid.
tosylation of secondary amine allowed the rapid access to the core
skeleton (8) of
series of acetylcholinesterase inhibitors6b
CO2Me
a
CO2Me
3
3
(Scheme 3a). Furthermore, Pictet-Spengler cyclization18 over the
tosylated b-prolinamine 9, which was prepared from b-prolinol
2a in two steps, afforded tetrahydrobenzazepine 10 in high yield
(Scheme 3b). Although an attempt to construct tricyclic compound
13 failed,19 interestingly, an otherwise difficult-to-make 2-(2-
naphthyl)ethylamine derivative 14 was formed probably through
a cyclization-elimination/aromatization cascade (Scheme 3c).
Finally, as a showcase of b-prolinols’ application in natural pro-
duct synthesis, the core of martinellic acid20 was constructed in
only four steps from 2h (53% overall yield), featuring a direct aro-
matic CAH amination mediated by 1,3-diiodo-5,5-dimethylhydan-
toin (DIH)18a,21 and palladium-catalyzed carbonylation22
(Scheme 4). An alternative route starting from compound 9 was
also viable.
BH3·THF (1.2 equiv)
2
Hα/Dα
Hβ
5
5
2
5N
a)
b)
Ph
NC
NC
N
H
1a
NaBD4 (2.0 equiv)
80%
Hα/Dα = 85:15
Ph
H
5
36
CO2Me
HO
H/D
6
H/D
BH3·THF (1.3 equiv)
LiAlD 4 (2.0 equiv)
2
Hα/Dα
Hβ
3
2
5
N
H
Ph
N
Ph
H
91%
1a
C5: Hα/Dα= 62:38
C6: H/D = 34:66
6
Scheme 2. Determination of the Hydride Source via Deuteration Experiments.
borane was also partially involved in the reduction of the ester
group (C6).23
To further demonstrate the synthetic value of the b-prolinols
obtained through this new two-step protocol, a couple of polyhete-
rocycles were efficiently constructed (Scheme 3). As exemplified
with 2i, copper-catalyzed CAO bond formation17 after a selective
Conclusions
In summary, a general two-step synthesis of multisubstituted
b-prolinols through a [3+2] cycloaddition of azomethine ylides
Please cite this article in press as: Li J., et al.. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.11.035

4
J. Li et al. / Tetrahedron Letters xxx (2016) xxx–xxx
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and a programmed reduction triggered by the combination of bor-
ane and LAH has been developed. The resulting b-prolinols were
utilized for the rapid construction of a couple of polyheterocycles,
including the core of the martinellic acid. We expect this method-
ology will find more use in the synthesis of natural products and
pharmaceuticals. Further studies on the asymmetric version of this
strategy are ongoing in our laboratory.
Acknowledgments
(b) Calder EDD, Grafton MW, Sutherland A. Synlett. 2014;25:1068;
(c) Albrecht Ł, Jiang H, Jørgensen KA. Angew Chem Int Ed. 2011;50:8492;
(d) Broadwater SJ, Roth SL, Price KE, Kobaslija M, McQuade DT. Org Biomol
Chem. 2005;3:2899.
We gratefully acknowledge the Natural Science Foundation of
China for financial support of this work by grants (nos.
21142007, 21272194, 21572187 and U1405228). We thank the
support from SRF for ROCS of SEM, FRFCU (no. 20720160025),
NFFTBS (no. J1310024) and the opening foundation from Beijing
National Laboratory for Molecular Science (BNLMS). We also thank
Ms. Xiuying Zheng of Xiamen University for solving the X-ray
structure.
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Please cite this article in press as: Li J., et al.. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.11.035