A sustainable catalytic enantioselective synthesis of norstatine derivatives

Article abstract of DOI:10.1039/c9ob02149c

Norstatine derivatives are of important value in pharmaceutical science. However, their catalytic asymmetric synthesis is rare. We developed a sustainable method via chiral phosphoric acid (CPA)-[Rh(OAc)₂]₂ co-catalyzed multi-component reactions (MCR) of diazoacetates with alcohol/water and imines. This method allows us to synthesize a library of 45 norstatines with excellent enanotioselectivites and broad substrate scope which includes anti-α-aryl-norstatines 11-1, anti-α-alkyl-norstatines 11-2, syn-α-hydro-norstatines 11-3 and syn-α-aryl-norstatines 11-4. The sustainability of this method lies in the reliable scalability, improved safety, and reusable [Rh(OAc)₂]₂ catalyst. The synthetic value of norstatine derivatives was demonstrated by preparing oxazolinone 14, ezetimibe analogue 15, and Taxol C-13 chain 16. Mechanistic study reveals that the synergetic catalysis of CPA and [Rh(OAc)₂]₂ is essential to maintain chemo- and enantioselectivity. Control experiments support the mechanism where the reactions proceed through the trapping of hyper-reactive oxonium ylides with imines. Shortly, we report herein the sustainable catalytic enantioselective synthesis of both syn- and anti-norstatine derivatives. We believe that this method might shed light on the sustainable synthesis of norstatine derivative-based drug candidates.

Full text of DOI:10.1039/c9ob02149c

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Biomolecular Chemistry
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A sustainable catalytic enantioselective synthesis
of norstatine derivatives†
Cite this: Org. Biomol. Chem., 2019,
17, 9792
Taoda Shi, a Changcheng Jing,b Yu Qian, a Xinfang Xu,a
Gopi Krishna Reddy Alavala,a Xinxin Lvb and Wenhao Hu*a
Norstatine derivatives are of important value in pharmaceutical science. However, their catalytic asym-
metric synthesis is rare. We developed a sustainable method via chiral phosphoric acid (CPA)-[Rh(OAc)2]2
co-catalyzed multi-component reactions (MCR) of diazoacetates with alcohol/water and imines. This
method allows us to synthesize a library of 45 norstatines with excellent enanotioselectivites and broad
substrate scope which includes anti-α-aryl-norstatines 11-1, anti-α-alkyl-norstatines 11-2, syn-α-hydro-
norstatines 11-3 and syn-α-aryl-norstatines 11-4. The sustainability of this method lies in the reliable scal-
ability, improved safety, and reusable [Rh(OAc)2]2 catalyst. The synthetic value of norstatine derivatives was
demonstrated by preparing oxazolinone 14, ezetimibe analogue 15, and Taxol C-13 chain 16. Mechanistic
study reveals that the synergetic catalysis of CPA and [Rh(OAc)2]2 is essential to maintain chemo- and
enantioselectivity. Control experiments support the mechanism where the reactions proceed through the
trapping of hyper-reactive oxonium ylides with imines. Shortly, we report herein the sustainable catalytic
enantioselective synthesis of both syn- and anti-norstatine derivatives. We believe that this method might
shed light on the sustainable synthesis of norstatine derivative-based drug candidates.
Received 4th October 2019,
Accepted 22nd October 2019
DOI: 10.1039/c9ob02149c
rsc.li/obc
Challenges associated with MCR are controlling chemo-
selectivity and stereoselectivity.8–10 Theoretically, the number
Introduction
The norstatine skeleton is found to be frequently present in of side reactions and difficulty of stereocontrol increase with
FDA approved drugs and drug-like molecules (Fig. 1). Some of the augmentation of the number of substrates. Aiming to solve
these molecules are Taxols (1 and 2),1 bestatin (3),2 ezetimibe this problem, we adopt a synergetic catalysis strategy.11–17 In
analogues (4),3 metaxalone (5), KNI-227 (6),4 and leuhistin (7).5 the case of the MCR of diazoacetates and alcohol/water with
Taxol, bestatin and ezetimibe, and metaxalone emerge as imines, the cooperation of chiral phosphoric acid (CPA)18–29
blockbuster drugs for the treatment of cancer, obesity, and and [Rh(OAc)2]2 is utilized to chemo-, diastereo- and enantio-
pain, respectively. KNI-227 and leuhistin show anti-HIV and selectively furnish norstatine derivatives. In the catalyst
antibacterial activity, respectively.
system, CPA is known to lower the LUMO of imines via the for-
In light of the pivotal value of norstatine derivatives in mation of a reactive iminium–CPA ion pair. [Rh(OAc)2]2 gener-
pharmaceutical science, synthesis of norstatines has been ates nucleophilic hyper-reactive oxonium ylides. The activation
drawing tremendous interest in the last several decades.
However, examples of catalytic enantioselective synthesis are
rare. The Wang group and Jiang group documented the
Mannich reactions for the synthesis of syn-norstatine deriva-
tives (Scheme 1a and b).6,7 We report herein a sustainable syn-
thesis of syn- and anti-norstatine derivatives via catalytic asym-
metric multi-component reactions (MCR) (Scheme 1c).
aGuangdong Chiral Drug Engineering Laboratory, School of Pharmaceutical Sciences,
Sun Yat-sen University, Guangzhou, 510006, China.
E-mail: huwh9@mail.sysu.edu.cn
bShanghai Engineering Research Center of Molecular Therapeutics and New Drug
Development, School of Chemistry and Chemical Engineering,
East China Normal University, Shanghai, 200062, China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
Fig. 1 Norstatine backbone exists in FDA approved drugs and drug-like
c9ob02149c
molecules.
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mode is eventually realized by narrowing the energy gap
between the LUMO of imines and the HOMO of diazoacetates
and alcohol/water (Fig. 2).
Results and discussion
With the cooperative catalysis mode in mind, we then con-
ducted optimization of the CPA-[Rh(OAc)2]2 co-catalyzed MCR
of diazoacetates and alcohol/water with imines (details are dis-
cussed in the ESI†). Interestingly, by tuning the match of the
oxonium ylides and CPAs, we were able to realize the enantio-
selective synthesis of both anti- and syn-norstatine derivatives.
The combination of oxonium ylide from methyl phenyldiazo-
acetate 8-1a and 9-anthylmethanol 9-1a and CPA (R)-12a, or
ethyl methyldiazoacetate and water-derived oxonium ylide with
CPA (R)-12b accordingly gives anti-norstatine derivatives 11-1a
and 11-2a as the major products with excellent enantio-
selectivities and good to excellent diastereoselectivities
(Table 1, entries 1 and 2). By contrast, syn-norstatin 11-3a or
11-4a was obtained with (R)-12b or (S)-12c as a cocatalyst. In a
word, we established optimal conditions of the catalytic asym-
metric synthesis of both syn- and anti-norstatine derivatives via
CPA-[Rh(OAc)2]2 co-catalyzed MCR of diazoacetates and
alcohol/water with imines.
Scheme 1 Catalytic enantioselective synthesis of norstatine derivatives:
(a) and (b) syn-selective synthesis by Wang et al. and Jiang et al.; (c) our
synthesis of anti- and syn-norstatine derivatives.
With the optimal conditions being established, we sub-
sequently investigated the substrate scope of the enantio-
selective synthesis of both anti- and syn-norstatine derivatives.
With respect to anti-α-aryl-norstatine derivatives 11-1, 11
examples were presented with 34–98% yields, >95 : 5 → 99 : 1
drs, and 49
→
99% ees (Table 2, 11-1a–k). para-
Methoxylphenylamine-derived imines (PMPNH2) were used as
substrates in most of the examples since PMP is an easily
removable group (Table 2, 11-1b–k). Electron-donating groups
or halogen-containing aromatic aldehyde-derived imines show
Fig. 2 Cooperative catalysis mode in the MCR of diazoacetates and
alcohol/water with imines.
Table 1 Optimal conditions of CPA-[Rh(OAc)2]2 co-catalyzed MCR of diazoacetates and alcohol/water with iminesa
11
Optimal conditions
Yieldb/%
anti : sync
eed/%
11-1a
11-2a
11-3a
11-4a
2 mol% [Rh(OAc)2]2, 2 mol% 12a, DCM, −20 °C
2 mol% [Rh(OAc)2]2, 2 mol% 12b, toluene, 20 °C
2 mol% [Rh(OAc)2]2, 2 mol% 12c, DCM, 0 °C
2 mol% [Rh(OAc)2]2, 2 mol% 12b, DCM, 40 °C
86
73
68
75
>99 : 1
71 : 29
32 : 68
8 : 92
93
94
−88
95
a Reactions are performed under optimal conditions. b Isolated yield. c The ratios of anti/syn were obtained via crude 1H NMR. d ees of major dia-
stereomers were obtained via chiral HPLC.
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Table 2 Catalytic enantioselective synthesis of anti-norstatine derivativesa
a Under optimal conditions. b Isolated yield. c The ratios of anti/syn were obtained via crude 1H NMR. d ees were obtained via chiral HPLC.
e Reaction was performed on a gram scale.
excellent
enantioselectivities
and
diastereoselectivities derivatives 11-2b could be prepared on a gram scale without a
(Table 2, 11-1c–g). Notably, even unreactive aliphatic aldehyde- significant loss of enantioinduction (Table 2, 11-2b). Then a
derived imines furnished the desired norstatine derivatives panel of aromatic aldehydes and PMP-NH2 derived imines
with moderate enantioselectivity and excellent diastereo- were evaluated. EDG, EWG, and halogen-bearing aromatic
selectivity (Table 2, 11-1h). Electron-donating groups or halo- groups of the imines are all tolerated in the MCR (Table 2,
gens on the aromatic ring of the diazoesters show good to 11-2f–k). The para-, meta- and ortho-NO2 substituted imines gave
excellent stereoselectivities and reactivities, with 84 → 99% comparable ees (Table 2, 11-2h–j). Notably, the heterocycle,
ees, >99 : 1 drs, and 84–98% yields (Table 2, 11-1i–j). It is inter- furanyl group showed good reactivity and enantioselectivity as
esting to note that the methyl trans-styryldiazoacetate is toler- well (Table 2, 11-2k). Next, a series of other alkyldiazo esters
ated as well, furnishing norstatine derivatives 11-1k with 43% were studied in the MCR. The size of the ester group is not
yield, >99 : 1 dr and 95% ee (Table 2, 11-1k). The single-crystal closely related to the enantioselectivity (Table 2, 11-2a vs. 11-2l
X-ray analysis of 11-1l (CCDC 701595) indicates that the stereo- vs. 11-2m, 11-2p vs. 11-2q). However, the increase in the size of
chemistry is anti-(2S,3S).
the α-alkyl group of the alkyldiazo esters results in a slight
Concisely, the panel of 11-1 norstatines demonstrates the decrease in the enantioselectivity (Table 2, 11-2b vs. 11-2n vs.
efficiency of 12a-[Rh(OAc)2]2 co-catalyzed MCR of aryldiazoace- 11-2o). In short, the 12b-[Rh(OAc)2]2 co-catalyzed MCR of alkyl-
tates and alcohols with imines in the catalytic synthesis of diazo esters, water and imines represents the unprecedented
anti-norstatine derivatives.
catalytic asymmetric synthesis of anti-α-alkyl norstatine deriva-
Then, the scope of catalytic asymmetric synthesis of anti- tives. Though lower diastereoselectivities were obtained in
α-alkyl norstatine derivatives 11-2 was investigated. Overall, 18 comparison with the 12a-[Rh(OAc)2]2 co-catalyzed MCR of aryl-
anti-α-alkyl norstatine derivatives were prepared with 88–95% diazo esters, alcohols and imines, this MCR also gave accept-
ees, 70 : 30–87 : 13 drs, and 58–89% yields (Table 2, 11-2a–r). able enantioselectivities for the syn-α-alkyl norstatine derivatives
Both EDG and halogen substituted aromatic amine-derived in most of the examples (Table 2, 11-2f–r). This allows us to
imines afford excellent enantioselectivities (Table 2, 11-2a–e). access the full set of stereoisomers of α-alkyl norstatine deriva-
Interestingly, the PMP group containing anti-α-alkyl norstatine tives and provides the chance of delineating the relationship
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Table 3 Catalytic enantioselective synthesis of syn-norstatine derivativesa
a Under optimal conditions. b Isolated yield. c The ratios of anti/syn were obtained via crude 1H NMR. d ees were obtained via chiral HPLC. e ees
after recrystallization.
between stereochemistry and bioactivity in the future pharma-
cology study of α-alkyl norstatine derivatives.
Next, 16 examples of the catalytic enantioselective synthesis
of syn-norstatine derivatives were reported with 81–97% ees,
54 : 46–8 : 92 drs and 61–82% yields (Table 3, 11-3a–g and 11-
4a–i). This group of syn-norstatine derivatives can be further
divided into two subgroups which are syn-α-hydrogen-norsta-
tines and syn-α-aryl-norstatine derivatives. Although syn-
α-hydrogen-norstatine derivatives show lower enantioselectivi-
ties than the syn-α-aryl-norstatine derivatives (average ee: 85%
vs. 94%), the single enantiomers can be easily purified by
recrystallization in most of the cases (Table 3, 11-3c–g). In
terms of the electron effect on the enantioselectivity, the EWG
containing R group offers higher ee than EDG and halogen
groups (Table 3, 11-3g vs. 11-3b–f). But the EDG containing R
group results in slightly higher drs than the EWG containing R
group (Table 3, 11-3e–f vs. 11-3g). The moderate diastereo-
selectivity of syn-α-hydrogen-norstatines is likely to relate with
the small size of α-hydrogen which causes poor discrimination
of Re- and Si-face attack on imines by oxonium ylides. This will
be further discussed in detail in the mechanism section.
By contrast, the enantioselectivities of syn-α-aryl-norstatines
are independent of the electron effect of the R group (Table 3,
11-4a–g). But the yields benefit from the EDG containing R
group (Table 3, 11-4f–g). In terms of the R1 group, both EDG
and halogen groups provide excellent enantioselectivities, and
high diastereoselectivities and yields (Table 3, 11-4h–i). The
absolute configuration of 11-4a is assigned as (2R,3S) by com-
paring the reference data.15
Scheme 2 CPA-[Rh(OAc)2]2 co-catalyzed MCR of diazoacetates and
water with imines using in situ-generated diazoacetates.
Then we attempted to improve the safety of the protocol of
the MCR by using in situ generated explosive alkyl diazoesters
(Scheme 2). When adopting the Regitz diazo transfer method,
we could obtain norstatine 11-2b with no loss of enantio-
selectivity. By using (L)-phenylalanine ethyl ester hydrochloride
as a diazo precursor, we could isolate 11-2o in high enantio-
selectivity as well.
Next, we attempted to reuse the noble catalyst [Rh(OAc)2]2
by using a mixture of water and toluene as the solvent system.
[Rh(OAc)2]2 is easily recovered via simple separation of water
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Table 4 One time feeding of 2 mol% [Rh(OAc)2]2 satisfies the pro-
duction of 6 norstatine derivativesa
Scheme 4 Control experiments support the co-catalysis hypothesis
and indicate that the reactions proceed through the trapping of
oxonium ylides with imines.
a Reactions are performed under optimal conditions using a mixture of
toluene : H2O = 1 : 1 as the solvent. b Isolated yield. c The ratios of anti/
syn were obtained via crude 1H NMR. d ees were obtained via chiral
HPLC.
is not capable of inducing asymmetry in the reaction and CPA
by itself cannot generate the essential oxonium ylide inter-
mediate to obtain the desired product. To figure out whether
the reaction proceeds through stable intermediates such as
O–H insertion product 18-2a or aziridination product 19-2a,
control experiments were conducted which showed that both
intermediates don’t generate the desired norstatine 11-4a,
indicating that the reaction might involve the capture of
oxonium ylide by imines (Scheme 4b and c).
Therefore, the mechanism of the reaction is considered as
the cooperative catalysis of [Rh(OAc)2]2 and CPA. [Rh(OAc)2]2
initiates the generation of rhodium carbenoid A which sub-
sequently forms the oxonium ylide B2. By contrast, CPA acti-
vates imine 10 via the formation of iminium which facilitates
its nucleophilic addition by oxonium ylide B2.
For the generation of anti-11, we postulate that the hydroxyl
oxonium ylide B2 attacks iminium on the Re face to avoid the
steric interactions between R3 and the bulky group R′
(Scheme 5, C, arrows colored in red). In the example of anti-11-
1, R3 and R′ are 9-anthrylmethenyl and 9-anthryl, respectively,
both of which are bulky groups. The formation of anti-11-4
might proceed through C because of its less hindered alkyl
group R1 compared to the aromatic group in syn-11-3 which
might adopt D as the transitional state. In the case of D, B2
attacks iminium from the Si face due to the steric interactions
between R′ and R1 (Scheme 5, D, arrows colored in blue). The
production of syn-11-2 might be mediated by D due to its
smaller R3 group in contrast to anti-11-1, allowing for the stabi-
lization of D transitional state. Meanwhile, since it bears
smaller R1 = H as well, the possibility of going through C
exists, resulting in only moderate diastereoselectivities. This
explanation is applied to the moderate to good diastereo-
selectivities of anti-11-2. But the actual energy gap between C
and D of 11-1, 11-2, 11-3, and 11-4, respectively, is not known
yet. The origin of absolute configuration of the norstatines
remains unclear. Future studies with the assistance of calcu-
lational studies might come up with more detailed and
rational models for these observations.
and toluene in a funnel. The aqueous phase with [Rh(OAc)2]2
was used directly in the next run. We ran the reactions with 6
different imines in sequence, providing 11-2a, 11-2b, 11-2d,
11-2g, 11-2h, and 11-2i with maintenance in yields and drs
and around 5% loss in ees (Table 4).
To demonstrate the synthetic application of the product,
2-alkyl norstatine derivatives 11-2a could be transformed into
β-lactam 14 with 72% yield and 93% ee. By comparing the
optical rotation of 14 with the reference, the absolute configur-
ation of the major isomer of 14 was deduced as (3S,4S), and
the other isomer of 14 as (3S,4R).30 Oxazolinone 15 was
obtained with 70% yield and 90% ee from 11-2b upon treat-
ment with triphosgene following cleavage of the PMP group.
Taxol C-13 side chain 16 was prepared from 11-3a in 4 steps
with 18% overall yield and >99% ee. Its optical rotation value
is consistent with the reported data.31 11-1b was converted to
17 with a simple treatment of TMSCl/NaI (Scheme 3).
Mechanism section
To understand the synergy of the rhodium catalyst and
CPA, control experiments were conducted with only
[Rh(2S-MEPY)2]2 or CPA 12b, resulting in racemic norstatine with
42% yield or no desired product, respectively (Scheme 4a). The
result implies that the chiral rhodium catalyst [Rh(2S-MEPY)2]2
Scheme 3 Synthetic applications of norstatine derivatives.
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Scheme 5 Proposed mechanism of CPA-[Rh(OAc)2]2 co-catalyzed MCR of diazoacetates and alcohol/water with imines.
Scholarship Council (TS). We also thank members of the Hu
lab for helpful discussions.
Conclusions
We have documented a sustainable catalytic enantioselective
synthesis of both anti- and syn-norstatine derivatives via MCR
of imines, alcohols/water and diazoacetates. This method
shows excellent enatioselectivities, broad substrate scope,
reliable scalability, improved safety and reusable catalyst,
living up to the principles of green chemistry.32–34 The norsta-
tine derivatives synthesized by the method are proved to be
useful in the synthesis of interesting building blocks in
organic synthesis. Mechanistic study demonstrates that the
synergy of [Rh(OAc)2]2 catalysis and CPA catalysis is critical to
the chemo-and enantioselectivity of the MCR. We believe that
this work might trigger the development of sustainable syn-
thesis of more pharmaceutically valuable norstatine derivative-
based molecules. A medicinal chemistry investigation of the
synthesized norstatines is currently ongoing in our lab.
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