Redox-Neutral α-C-H Functionalization of Pyrrolidin-3-ol

Article abstract of DOI:10.1021/acs.orglett.7b03807

A redox-neutral α-C-H oxygenation of commercially available pyrrolidin-3-ol with a monoprotected p-quinone generated an N-aryliminium ion intermediate, which reacted in situ with boronic acid nucleophiles to produce a series of cis-2-substituted pyrrolidin-3-ols. With this strategy, 8-epi-(-)-lentiginosine was synthesized from (3R,4R)-pyrrolidine-3,4-diol in three steps.

Full text of DOI:10.1021/acs.orglett.7b03807

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Redox-Neutral α‑C−H Functionalization of Pyrrolidin-3-ol
Cheng-Bo Yi, Zhi-Ying She, Yong-Feng Cheng, and Jin Qu*
The State Key Laboratory and Institute of Elemento-organic Chemistry, Collaborative Innovation Center of Chemical Science and
Engineering (Tianjin), College of Chemistry, Nankai University, Tianjin 300071, China
S
* Supporting Information
ABSTRACT: A redox-neutral α-C−H oxygenation of commercially available
pyrrolidin-3-ol with a monoprotected p-quinone generated an N-aryliminium ion
intermediate, which reacted in situ with boronic acid nucleophiles to produce a
series of cis-2-substituted pyrrolidin-3-ols. With this strategy, 8-epi-(−)-lentigino-
sine was synthesized from (3R,4R)-pyrrolidine-3,4-diol in three steps.
he 2-substituted pyrrolidin-3-ol moiety is found in many
atom economy.6,7 In this study, we started from commercially
Tbiologically active natural products, including indolizidine available pyrrolidin-3-ol hydrochloride (1), which was subjected
and pyrrolizidine alkaloids.1 Most of the strategies for synthesis
to redox-neutral α-C−H oxygenation to form an N-aryliminium
ion intermediate that was then allowed to react with boronic
acids or boronates to produce a series of cis-2-substituted
pyrrolidin-3-ols (Scheme 1b).
of this moiety involve pyrrolidine ring closure of open-chain
precursors.1,2 Therefore, to synthesize a series of α-substituted
pyrrolidine derivatives, one must separately prepare precursors
with different α-substituents and then perform the ring closure.
In 1999, Batey and co-workers successfully utilized N-Cbz-
protected 2,3-dihydroxy pyrrolidine as an N-acyliminium ion
precursor for the synthesis of α-substituted pyrrolidines; in the
presence of boron trifluoride, this precursor reacted with various
alkenylboronates or arylboronates to give α-substituted
pyrrolidin-3-ols in very high yields (see Scheme 1a).3a They
further demonstrated the utility of their method for the
simplification of pyrrolidine alkaloid synthesis.3
The direct functionalization of the α-C−H of pyrrolidine or
pyrrolidine derivatives is a concise way to synthesize α-
substituted pyrrolidines. In recent years, many pyrrolidine α-
C−H functionalization strategies have been developed,4,5
including various elegant methods that offer special redox and
In previous work, we showed that pyrrolidine reacts with o-
benzoquinone in 2,2,2-trifluoroethanol (TFE) to give an N,O-
acetal, which can then undergo ring opening by a Grignard
reagent to introduce a substituent at the α-position.7l In this
study, we began by exploring the use of 3,5-di-tert-butyl-1,2-
benzoquinone to oxidize pyrrolidin-3-ol (1) in TFE (Table 1,
entry 1). Although an N,O-acetal was formed, as in our previous
work,5j the acetal did not react with the boron nucleophile (E)-
styrylboronic acid (2a). To prevent generation of the N,O-acetal,
we switched to 2,6-di-tert-butyl-1,4-benzoquinone as the oxidant,
but the reaction gave a complicated mixture of products (entry 2
in Table 1). Next, we tried using a more weakly oxidizing
monoprotected p-quinone, such as 4,4-dimethoxycyclohexa-2,5-
dien-1-one, we successfully obtained desired product 3a in high
yield by using an excessive amount of 1 (entry 3 in Table 1).
However, under this condition, pyrrolidine or piperidin-3-ol did
not react in the same way. Using this oxidant, we evaluated
solvents other than TFE. In THF, the desired reaction did not
occur, and most of the starting material was recovered (entry 4 in
Table 1). In another fluorinated alcohol, 1,1,1,3,3,3-hexafluor-
opropan-2-ol (HFIP), the yield of 3a was only 37% after 24 h
(entry 5 in Table 1); and reaction in a nonfluorinated alcohol
(ethanol) afforded only a 27% yield of the product (entry 6 in
Table 1). The fact that TFE was the best solvent may have been
due to its ability to stabilize the iminium ion intermediate.8 The
Scheme 1. Functionalization of the α-Position of Pyrrolidines
via Iminium Ion Formation
Received: December 6, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03807
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Oxidation/Nucleophilic Addition Reactions of
Pyrrolidin-3-ol (1)
Scheme 2. Proposed Mechanism for Oxidation/Nucleophilic
Addition Reactions of Pyrrolidin-3-ol (1)
a
Scheme 3. Oxidation/Nucleophilic Addition Reactions of
Pyrrolidin-3-ol (1) Using Vinyl Boronic Acids or
a b
,
Boronates
a
Reaction conditions: 1 (0.3 mmol), quinone (0.2 mmol), (E)-
styrylboronic acid (2a, 0.3 mmol), and K2CO3 (0.31 mmol) in solvent
(4 mL) were stirred at room temperature (rt) for 12 h. bIsolated yields
are reported.
reason for the lower yield in the hexafluorinated alcohol, which
has even higher ionizing power than TFE, may have been that its
higher pKa interfered with nucleophilic addition of the amine to
the quinone. Because organic boronates are sometimes easier to
prepare than the corresponding boronic acids, we also evaluated
the use of pinacol boronate as a nucleophile; however, under
these conditions, we observed a slight decrease in the chemical
yield (to 85%, entry 7 in Table 1).
The protons at the 2- and 3-positions of 3a showed correlation
in the two-dimensional NOESY spectrum, indicating a cis
relationship between the substituents at these two positions. We
also prepared trans diastereomer 3a′ by means of a Mitsunobu
reaction of 3a (see the Supporting Information (SI)), and
comparison of the 1H NMR spectrum of the crude product of the
oxidation/nucleophilic addition reaction with the spectrum of
3a′ confirmed that the trans isomer was not produced in the
reaction.
Based on the above-described evidence, we propose the
mechanism outlined in Scheme 2. Nucleophilic attack of the N
atom of pyrrolidin-3-ol on the quinone generates iminium ion A,
and subsequent α-proton abstraction followed by aromatization
of the quinone moiety and loss of methanol produces iminium
ion B.9 The hydroxy group at the 3-position of B coordinates
with styrylboronic acid (2a) to generate transient species C,
which undergoes nucleophilic attack by the double bond from
the same face as the 3-hydroxyl group to afford cis-2-substituted
pyrrolin-3-ol (3a) as the only stereoisomer.3a,10
a
Reaction conditions: 1 (0.3 mmol), quinone (0.2 mmol), vinyl
boronic acid (2, 0.3 mmol), and K2CO3 (0.31 mmol) were stirred in
b
TFE (4 mL) at rt. Pinacol vinyl boronate was used instead of a vinyl
boronic acid.
methoxyphenyl group of products 3 could be removed by
oxidation with CAN to afford the corresponding free amines (see
the SI).
We previously found that, although we could introduce alkyl
groups at the α-position of pyrrolidine via reaction of N,O-acetals
with Grignard reagents,7l α-benzyl-substituted pyrrolidines
generated by reactions with benzyl Grignard reagents underwent
oxidation in air and cyclization to form N,O-acetals. Therefore, in
the current study, we explored α-C−H arylation of pyrrolidin-3-
ol under the same reaction conditions used for the corresponding
alkylation reactions (Scheme 4). Aryl boronic acids with
electron-donating groups gave high yields of the desired products
(3l−3p), and aryl boronic acids with electron-withdrawing
groups also performed well (3q−3s). The cis configuration of the
pyrrolidine substituents was confirmed by X-ray crystal analysis
of 4r, the Ts ester of 3r.12 We found that the 3-hydroxy group was
essential for the relative stability of the product (especially when
an electron-rich aryl group was present), presumably by
Using the optimized reaction conditions (Table 1, entry 3), we
investigated various vinyl boronic acid nucleophiles (Scheme 3).
Substituted styrenyl boronic acids, alkyl boronic acids, and
boronates all gave the desired products in high yields (82%−
95%). Note that our approach to the synthesis of 3j, which is the
key intermediate in a total synthesis of the alkaloid loline,11 is
more simple than the procedure reported in the literature. The p-
B
DOI: 10.1021/acs.orglett.7b03807
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
have been achieved.13 In addition, 11 synthetic routes to 8-epi-
(−)-lentiginosine, which inhibits β-glucosidase and β-xylanase,14
have been reported.14,15 We started a synthesis 8-epi-
(−)-lentiginosine from commercially available (3R,4R)-pyrroli-
dine-3,4-diol hydrochloride, which was allowed to react with 4,4-
dimethoxycyclohexa-2,5-dien-1-one and Ts-protected boronate
6 to give cis-2-substituted pyrrolidine 7 in 83% yield. Removal of
the N-p-methoxylphenyl group with CAN gave the free amine,
which was too polar to be isolated and was therefore trapped with
a readily removable Cbz protecting group to afford 8. Under
hydrogenation conditions, the double bond was reduced and the
Cbz group was removed, at which point the free amine cyclized
spontaneously to give 8-epi-(−)-lentiginosine 9.
Scheme 4. Oxidation/Nucleophilic Addition Reactions of
Pyrrolidin-3-ol (1) Using Aryl Boronic Acids
a
In summary, we have developed a protocol for mild redox-
triggered α-C−H functionalization of pyrrolidin-3-ol to generate
an N-aryliminium ion intermediate, which can then react with
boronic acids or boronates. The protocol was used to synthesize
cis-2-substituted pyrrolidin-3-ols with various substituents in
high yields, and its utility was demonstrated by means of a facile
synthesis of 8-epi-(−)-lentiginosine.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03807.
Experimental details, characterization data for new
compounds, copies of NMR spectra and high-resolution
mass spectra, and X-ray crystallographic data of 4r (PDF)
Accession Codes
CCDC 1510783 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
a
Reaction conditions: 1 (0.3 mmol), quinone (0.2 mmol), aryl boronic
acid (2, 0.3 mmol), and K2CO3 (0.31 mmol) were stirred in TFE (4
mL) at rt.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: qujin@nankai.edu.cn.
ORCID
formation of a hydrogen bond with the N atom. If the 3-hydroxy
group of 3n was protected with a MOM group, the resulting
compound was unstable in air and was quickly oxidized to a
complicated mixture of dark-colored products.
Jin Qu: 0000-0003-0154-7334
Finally, we evaluated the utility of our reaction protocol by
using it in a total synthesis of 8-epi-(−)-lentiginosine (9, Scheme
5). The indolizidine alkaloid (−)-lentiginosine exhibits excellent
anti-HIV, antitumor, and immune-modulating activities, in
addition to inhibiting amyloglycosidases (IC50 = 5 μg mL−1).
More than 40 total syntheses of lentiginosine and its analogues
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by NSFC (Nos. 21372119,
21572107, and 21772105) and Tianjin Natural Science
Foundation (No. 14JCYBJC20200).
Scheme 5. Synthesis of 8-epi-(−)-Lentiginosine
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