Highly Regio- and Stereodivergent Access to 1,2-Amino Alcohols or 1,4-Fluoro Alcohols by NHC-Catalyzed Ring Opening of Epoxy enals

Article abstract of DOI:10.1002/anie.201709823

Described is an unprecedented NHC-catalyzed (NHC=N-heterocyclic carbene), stereoselective ring opening of epoxy and cyclopropyl enals to deliver valuable compounds bearing multiple stereocenters. A straightforward three-step procedure involving two catalytic enantioselective transformations has been developed and leads to a regio- and stereodivergent synthesis of either 1,2-amino alcohols/diamines or 1,4-fluoro alcohols with excellent diastereo- and enantiopurity.

Full text of DOI:10.1002/anie.201709823

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Highly Regio- and Stereodivergent Access to 1,2-Amino Alcohols
or 1,4-Fluoro Alcohols by NHC-Catalyzed Ring Opening of Epoxy
enals
Authors: Yu Zhao, Si Bei Poh, Jun-Yang Ong, and Shenci Lu
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201709823
Angew. Chem. 10.1002/ange.201709823
Link to VoR: http://dx.doi.org/10.1002/anie.201709823
http://dx.doi.org/10.1002/ange.201709823

10.1002/anie.201709823
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COMMUNICATION
Highly Regio- and Stereodivergent Access to 1,2-Amino Alcohols
or 1,4-Fluoro Alcohols by NHC-Catalyzed Ring Opening of Epoxy
enals
Si Bei Poh, Jun-Yang Ong, Shenci Lu* and Yu Zhao*
alcohol synthesis.
A straightforward three-step procedure
Abstract: We describe herein an unprecedented NHC-catalyzed
involving an iminium catalysis-Wittig olefination-NHC catalysis
sequence has been achieved for a regio- and stereodivergent
synthesis of these important compounds with excellent
diastereo- and enantiocontrol (Scheme 1c).[9]
stereoselective ring opening of epoxy- or cyclopropyl-enals to deliver
valuable
compounds
bearing
multiple
stereocenters.
A
straightforward three-step procedure involving two catalytic
enantioselective transformations has been developed that leads to a
regio- and stereodivergent synthesis of 1,2-amino alcohols/diamines
or 1,4-fluoro alcohols with excellent diastereo- and enantiopurity.
The efficient generation of structural diversity in organic
compounds remains an important goal in synthetic chemistry
and other applied areas such as chemical biology.[1] Accordingly,
the development of catalytic regio- or diastereodivergent
transformations from the same starting material has gained
much significance in recent years.[2] For the preparation of
important compounds bearing multiple stereocenters, it is
undoubtedly more attractive to access different isomers in a
highly diastereo- and enantiocontrolled fashion. To achieve this,
however, the catalytic system has to be able to accomodate a
variety of functional groups and overcome the inherent stereo-
control from the existing stereogenic center in the substrate.
Heading towards this goal, we have developed an NHC-
catalyzed process in which essentially complete catalyst control
is realized to deliver valuable compounds in highly diasteo- and
enantiopure fashion.
During the past few decades, N-heterocyclic carbene
catalysis has emerged as a powerful and versatile tool in
stereoselective synthesis.[3] Aldehydes bearing a -leaving
group have found much use as the substrate to access diverse
reactivites under NHC-catalysis. In particular, epoxy- or
cyclopropyl-aldehydes were shown by the Bode group to
undergo ring opening to deliver -functionalized esters (Scheme
1a).[4] Inspired by this, we became interested in the vinylogous
ring opening of cyclopropyl- or epoxy-enals by NHC catalysis;[5]
the intermediate generated this way (Int-A) may undergo
various regio- and diastereodivergent transformations to
produce valuable structures bearing multiple stereocenters
(Scheme 1b).[6] It is noteworthy that elegant examples on NHC-
catalyzed transformations of aldehydes bearing a -carbonate as
the leaving group has been reported by the Sun group and the
Ye group for the synthesis of allenes, -fluoroesters and
dihydropyridazinones.[7] In our proposed transformations, the
incorporation of reactive functionalities such as epoxide in the
substrate may raise serious compatibility issues. As one
particular complication, Int-A may undergo an undesired
intramolecular cyclization leading to lactone side products (when
X = OH).[8] On the other hand, these processes also provides
great opportunities for stereodivergent synthesis of more
functionalized compounds. We present herein the realization of
such strategy in 1,2-amino alcohol/diamine and 1,4-fluoro
Scheme 1. Divergent Reactions Enabled by Dual Catalytic System.
-Amination of epoxy enals for stereodivergent 1,2-amino
alcohol synthesis
We chose epoxy enal 1a as the model substrate for our
investigation, which could be prepared in high enantiopurity
following the previous report on enantioselective epoxidation of
enals via iminium catalysis,[10] followed by Wittig olefination
(Procedure shown in Scheme 2a). We decided to explore
amination of 1a using 2[7c] as such transformation could lead to
stereodivergent synthesis of functionalized 1,2-amino alcohols
that are important structural motifs in synthesis and catalysis.[11]
When the reaction of 1a and 2 was examined using different
azoliums, only the use of 4b led to the formation of the desired
product (R, S)-3a, albeit with a moderate 40% yield (entries 1-4).
To our delight, (R, S)-3a was formed as a single diastereomer
with a high 97% ee. Different reaction conditions were screened
at this stage. The use of other bases such as K2CO3 and DBU
resulted in a complex mixture (results not shown). The different
loading of base or 2 was also examined. 2 equiv. of NaOAc
proved to be a better choice (entry 5 vs entry 4), while a higher
loading of 2 lead to an improved chemical yield (entry 6). Finally,
decreasing the catalyst loading of 4b to 10 mol% led to a
cleaner reaction and further improvement in the chemical yield
to 62% with the same level of stereoselectivity (entry 7).
[]
S. B. Poh, J.-Y. Ong, Dr. S. Lu, Prof. Y. Zhao
Department of Chemistry
National University of Singapore
3 Science Drive 3, Singapore 117543
E-mail: zhaoyu@nus.edu.sg; chmlus@nus.edu.sg
Homepage: zhaoyugroup.sg
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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10.1002/anie.201709823
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COMMUNICATION
Aiming towards a divergent synthesis of the anti-amino
alcohol series, we also examined the amination of 1a by the use
of the enantiomeric azolium ent-4b under otherwise identical
conditions (entry 8). We speculated that the selectivity for this
transformation would be much more challenging to achieve,
considering the catalytic control has to overcome the inherent
substrate control. To our excitement, the reaction proceeded
smoothly with
a complete switch of diastereoselectivity,
producing (S, S)-3a as a single diastreomer in an excellent ee of
99%.[12]
Table 1. Optimization of Enantioselective Amination of 1a[a]
Entry
azolium
20 mol% 4a
20 mol% 4b
20 mol% 4c
20 mol% 4d
20 mol% 4b
20 mol% 4b
10 mol% 4b
10 mol% ent-4b
NaOAc
2 equiv
2 equiv
2 equiv
2 equiv
1 equiv
2 equiv
2 equiv
2 equiv
2
yield[b]
<5%
40
dr[c]
-
ee (%)[d]
n.d.
97
1
2
3
4
5
6
7
8
2 equiv
2 equiv
2 equiv
2 equiv
2 equiv
3 equiv
3 equiv
3 equiv
>20:1
-
<5%
<5%
37
n.d.
n.d.
95
-
>20:1
>20:1
>20:1
1:>20
50
95
62
97
70
99
[a]Unless otherwise specified, the reactions were carried out using 1a (0.1 mmol), 2, chiral
azolium species 4 and base in solvent (0.1 M in 1a) for 48 h at 40 oC with. [b]Isolated yield.
[c]Ratio of (R,S)-3a/(S,S)-3a. [d]Determined by chiral-phase HPLC analysis.
The scope of this stereo-divergent reaction proved to be very
general. As shown in Scheme 2a, a range of epoxy enals 1 were
prepared following the same procedure in good to excellent
enantioselectivity and subjected to the amination reaction
catalyzed by 4b or ent-4b (Scheme 2b). In all the cases, either
the (R, S)- or (S, S)-series of the amino alcohols 3 bearing aryl
(3a-f) or alkyl substituents (3g) could be accessed with uniformly
excellent level of diastereo- and enantioselectivity. Different
functionalities such as nitro, cyano and halogen could be well-
tolerated in the NHC-catalyzed amination step. Such a complete
switch of diastereoselectivity represents a rare example in
asymmetric catalysis. With this catalytic procedure incorporating
two catalytic enantioselective steps, all the possible
stereoisomers of the 1,2-amino alcohols could thus be accessed
in a pure form. The relative and absolute configuration of (R, S)-
3b as well as (S, S)-3f were unambiguously assigned by single
crystal x-ray analysis.
To test the generality and limitations of this catalytic
procedure, we prepared epoxy enal 1h with a tri-substituted
epoxide following the same two-step procedure, although the
enantioselectivity from the asymmetric epoxidation step was
only moderate (62% ee) as shown in Scheme 3. When 1h was
subjected to the standard NHC-catalyzed amination conditions,
the diastereomic ratio of (R, S)-3h: (S, S)-3h dropped to 5:1
when using 4b or 1:4 when ent-4b was employed. Importantly,
the enantioselectivity for the final products were as high as 94-
99% ee. In these transformations, the combination of two
enantioselective steps boosted the enantiopurity of the major
diastereomer of the products to a higher level by the sacrificial
formation of the minor diastereomer.[9a]
Scheme 2. Scope of NHC-Catalyzed Amination of Chiral Epoxy Enals.
Scheme 3. Access to -Amino Tertiary Alcohol.
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To further extend the scope of this catalytic procedure, we
decided to explore substrates bearing other strained ring system
such as cyclopropanes and aziridines. As shown in Scheme 4a,
enals 5a-c bearing a terminal cyclopropane were prepared and
examined in the NHC-catalyzed amination step first. The ring
opening amination reaction proceeded smoothly in these cases;
different electron-withdrawing substitutents on the substrate
including different diesters or diketone were all well-tolerated to
produce 6a-c in moderate to good yields with excellent
enantiopurity. We then turned to the chiral cycloproyl enals such
as 5d, which was prepared in 95% ee via enantioselective
cyclopropanation[13] followed by Wittig olefination. When 5d was
subjected to the NHC-catalyzed amination conditions using 4b
or ent-4b, to our delight, similar level of excellent dr and ee were
obtained to deliver (R, S)-6d or (S, S)-6d as a pure stereoisomer,
albeit with a low chemical yield. In addition, we also prepared
and examined the analogous aziridinyl enal 7,[14] which also
underwent similar stereodivergent amination under the standard
conditions to produce either (R, S)-8 or (S, S)-8 in high dr and
excellent ee (Scheme 4c). These studies dramatically expanded
the scope of this catalytic procedure to prepare more diverse
multi-functional compounds bearing continuous stereogenic
centers.
when the fluorination reactions were performed using ent-4b, we
were delighted to observe a high level of diastereoselectivity (5:1
to >20:1 dr) for the formation of the other diastereomeric series
of the 1,4-fluoro alcohols (shown at the bottom row of Scheme
5). Similarly, good chemical yields and high ee were obtained for
these versatile multi-functionalized products.
Scheme 5. Scope of NHC-Catalyzed Fluorination of Chiral Epoxy Enals.
1,4-Fluoro alcohols 9 possess multiple functionalities and can
serve as a versatile springboard to prepare valuable compounds.
As a representative example, the epoxidation of the allylic
alcohol moiety was attempted using mCPBA (Scheme 6).
Although this acyclic substrate failed to induce
a high
diastereoselectivity for this transformation, the two epoxides 10
and 11 bearing four continuous stereogenic centers could be
accessed in moderate yields and excellent diastereo- and
enantiopurity, respectively. The relative configuration of these
products was assigned by NOE measurements (see SI for
details), which also helped to confirm the configuration of
products 9.
Scheme 4. NHC-Catalyzed Amination of Cyclopropyl and Aziridinyl Enals.
Scheme 6. Functionalization of Fluoro Allylic Alcohols.
-Fluorination of chiral epoxy enals for divergent 1,4-fluoro
alcohol synthesis
A simplified mechanism using ent-4b is proposed to illustrate
the stereocontrol of the reaction process. As illustrated in
Scheme 7, chiral epoxy enal 1 undergoes vinylogous ring
opening in the presence of NHC catalyst to form the key azolium
dienolate intermediate, which likely undergoes fluorination or
amination reaction in different conformations I or I‘. For both
transformations, the chiral catalyst acts as a steric shield to
block the re face of the dienolate. The fluorination or amination
reagents then approaches from the Si face (as shown in TS II or
III) to deliver the products in high diastereoselectivity that is
consistent with the experimental results. It is also noteworthy
that based on the structure and conformation of these
intermediates, the existing stereogenic center from the substrate
does not show significant effect on the stereoselectivity of the
NHC-catalyzed reactions.
In addition to the diastereodivergent synthesis of 1,2-amino
alcohols bearing continuous stereogenic centers, we were also
keen to explore the possibility of establishing the control of
remote stereocenters by a similar three-step procedure that
incorporates a different NHC-catalyzed -functionalization of the
epoxy enals. In particular, we decided to focus on the -
fluorination of 1 in an effort to prepare 1,4-fluoro allylic alcohols.
As shown in Scheme 5a, when epoxy enal 1a was subjected to
the fluorination conditions using NSFI catalyzed by 4b as
previously developed by the Sun group,[7b] the corresponding
1,4-fluoro alcohol (R, R)-9a was obtained in a high yield of 80%
with high dr of 15:1 and excellent 99% ee. This catalytic
procedure also tolerated different substituted aryl or alkyl groups
in the substrate structure to produce (R, R)-9b and (R, S)-9c in
similarly high ee and high to excellent dr. More importantly,
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10.1002/anie.201709823
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Keywords: divergent synthesis · N-heterocyclic carbene
catalysis · fluorination · amination · enantioselectivity
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[3] For selected reviews on NHC catalysis, see: (a) D. Enders, D. O.
Niemeier, A. Henseler, Chem. Rev. 2007, 107, 5606-5655. (b) N. Marion, S.
Díez-González, S. P. Nolan, Angew. Chem., Int. Ed. 2007, 46, 2988-3000. (c)
V. Nair, R. S. Menon, A. T. Biju, C. R. Sinu, R. R. Paul, A. Jose, V. Sreekumar,
Chem. Soc. Rev. 2011, 40, 5336-5346. (d) J. Izquierdo, G. E. Hutson, D. T.
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Candish, D. W. Lupton, Chem. Soc. Rev. 2013, 42, 4906-4917. (g) J. W. Bode,
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Michailidis, N. A. White, T. Rovis, Chem. Rev. 2015, 115, 9307-9387. (k) S. R.
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A. Scheidt, Angew. Chem, Int. Ed. 2016, 44, 14912-14922.
Scheme 7. Proposed mechanism and TS for amination and fluorination of
chiral epoxy enals.
[4] a) K. Y.-K. Chow, J. W. Bode, J. Am. Chem. Soc. 2004, 126, 8126-
8127; b) S. S. Sohn, J. W. Bode, Angew. Chem. Int. Ed. 2006, 45, 6021-6024;
c) H. Lv, J. Mo, X. Fang, Y. R. Chi,Org. Lett. 2011, 13, 5366-5399.
[5] For selected reviews on vinylogous reactions, see: a) G. Casiraghi, F.
Zanardi, G. Appendino, G. Rassu, Chem. Rev. 2000, 100, 1929-1972; b) S. E.
Denmark, J. R. Heemstra, G. L. Beutner, Angew. Chem. Int. Ed. 2005, 44,
4682-4689.
[6] For our group’s previous efforts on catalytic divergent transformations,
see: a) M. Wang, Z.-Q. Rong, Y. Zhao, Chem. Comm. 2014, 50, 15309-
15312.; b) J.-Y. Liao, P.-L. Shao, Y. Zhao, J. Am. Chem. Soc. 2015, 137, 628-
631; c) Y. Huang, C. Ma, Y. X. Lee, R.-Z. Huang, Y. Zhao, Angew. Chem. Int.
Ed. 2015, 54, 13696-13700; d) Z. Q. Rong, M. Wang, C. H. E. Chow, Y. Zhao,
Chem. Eur. J. 2016, 22, 9483-9487; e) T.-L. Liu, J. Wu, Y. Zhao, Chem. Sci.
2017, 8, 3885-3890; f) J.-Y. Liao, Q. Ni, Y. Zhao, Org. Lett. 2017, 19, 4074-
4077.
In conclusion, we have developed a straightforward three-step
procedure that combines the power of enantioselective iminium
catalysis and NHC catalysis to achieve stereodivergent
preparation of functionalized 1,2-amino alcohols and 1,4-fluoro
alcohols. The excellent stereocontrol from the NHC-catalyzed
ring opening of epoxy-, aziridinyl or cyclopropyl enals is
especially noteworthy. Further exploration of new reactivities for
this class of versatile intermediates is under investigation in our
laboratories.
[7] a) Y. Zhao, Y. Tam, Y. Wang, Z. Li, J. Sun, Org. Lett. 2012, 14, 1398-
1401; b) Y.-M. Zhao, M. S. Cheung, Z. Lin, J. Sun, Angew. Chem. Int. Ed.
2012, 51, 10359-10363; c) X.-Y. Chen, F. Xia, J.-T. Cheng, S. Ye, Angew.
Chem. Int. Ed. 2013, 52, 10644-10647.
Acknowledgements
[8] J. Qi, X. Xie, J. He, L. Zhang, D. Ma, X. She, Org. Biomol. Chem. 2011,
9, 5948-5950.
We are grateful for the financial support from A*STAR SERC (R-
143-000-648-305), the Ministry of Education of Singapore (R-
143-000-613-112) and Singapore EDB and GlaxoSmithKline (R-
143-000-564-592).
[9] For selected examples on diastereo- and enantioselective reactions
using two catalysts, see: a) Y. Huang, A. M. Walji, C. H. Larsen, D. W. C.
MacMillan, J. Am. Chem. Soc. 2005, 127, 15051-15053; b) B. Simmons, A. M.
Walji, D. W. C. MacMillan, Angew. Chem. Int. Ed. 2009, 48, 4349-4353;
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Chem. Int. Ed. 2015, 54, 14363-14367; Angew. Chem. 2015, 127, 14571-
14575.
Experimental Section
[10] a) G.-L. Zhao, I. Ibrahem, H. Sundén, A. Córdova, Adv. Synth. Catal.
2007, 349, 1210-1224; b) M. Marigo, J. Franzén, T. B. Poulsen, W. Zhuang, K.
A. Jørgensen, J. Am. Chem. Soc. 2005, 127, 6964-6965.
NHC-catalyzed amination reaction: To a 4 mL vial was added epoxy
enal 1, 5, or 7 (0.10 mmol), 2 (0.20 to 0.30 mmol), triazolium salt 4b (0.01
mmol), NaOAc (0.20 mmol) and 4Å MS (30 mg). The mixture was taken
into the glovebox, where CHCl3 (1.0 mL) was added using a micropipette.
The reaction mixture was taken outside the glovebox and allowed to stir
[11] For selected review on the application of 1,2-amino alcohols, see: D. J.
Ager, I. Prakash, D. R. Schaad, Chem. Rev., 1996, 96, 835-875.
[12] The reaction of enantioenriched 1a and 2 catalyzed by (±)-4b was
carried out, which led to the formation of (R, S)-3a and (S, S)-3a in a 4:3 ratio.
Similarly, the reaction with (±)-1a catalyzed by 4b resulted in 4:5 dr. These
experiment showed that the substrate control in the NHC-catalyzed step is not
significant and the diastereoselectivity is mainly under catalyst control.
[13] H. Xie, L. Zu, H. Li, J. Wang, W. Wang, J. Am. Chem. Soc., 2007, 129,
10886-10894.
o
at 40 C for 48 h. The reaction was cooled to ambient temperature, and
the diastereoselectivity of the reaction was determined by NMR analysis
of an aliquot of the crude reaction mixture. The combined crude reaction
mixture was then directly purified by silica gel column chromatography
with hexanes/ethyl acetate (8:1) as eluent to afford the desired products
in pure form.
[14] Ł. Albrecht, H. Jiang, G. Dickmeiss, B. Gschwend, S. G. Hansen, K. A.
Jørgensen, J. Am. Chem. Soc., 2010, 132, 9188-9196.
NHC-catalyzed -fluorination reaction: To a 4 mL vial was added
epoxy enal 1 (0.10 mmol), NFSI (0.10 mmol), triazolium salt 4b (0.01
mmol), NaOAc (0.20 mmol) and 4Å MS (30 mg). The mixture was taken
into the glovebox, where anhydrous MeOH (20 L) and CHCl3 (1.0 mL),
was added using a micropipette. The reaction mixture was taken outside
the glovebox and allowed to stir at 40 oC for 48 h. The reaction was
cooled to ambient temperature, and the diastereoselectivity of the
reaction was determined by NMR analysis of an aliquot of the crude
reaction mixture. The combined crude reaction mixture was then directly
purified by silica gel column chromatography with hexanes/ethyl acetate
(8:1) as eluent to afford the desired products in pure form.
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Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Si Bei Poh, Jun Yang Ong, Shenci Lu*
and Yu Zhao*
Page No. – Page No.
Highly Regio- and Stereodivergent
Access to 1,2-Amino Alcohols or 1,4-
Fluoro Alcohols by NHC-Catalyzed
Ring Opening of Epoxy enals
We describe herein an unprecedented NHC-catalyzed stereoselective ring opening
of epoxy- or cyclopropyl-enals to deliver valuable compounds bearing multiple
stereocenters. A straightforward three-step procedure involving two catalytic
enantioselective transformations has been developed that leads to a regio- and
stereodivergent synthesis of 1,2-amino alcohols or 1,4-fluoro alcohols with excellent
diastereo- and enantiopurity.
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