Asymmetric cinnamylation of N-tert-butanesulfinyl imines with cinnamyl acetates: total syntheses of (+)-lycoricidine and (+)-7-deoxypancratistatin

Article abstract of DOI:10.1039/c7cc00108h

Highly diastereoselective palladium catalyzed cinnamylation of N-tert-butanesulfinyl imines with cinnamyl acetates has been established to provide enantioenriched β-aryl homoallylic amines. The synthetic application of this stragety has been successfully

Full text of DOI:10.1039/c7cc00108h

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Asymmetric Cinnamylation of N-tert-Butanesulfinyl Imines with
Cinnamyl Acetates: Total Syntheses of (＋)-Lycoricidine and (＋)-
7-Deoxypancratistatin
Received 00th January 20xx,
Accepted 00th January 20xx
Sen-Lin Cai, Bin-Hua Yuan, Yi-Xiang Jiang, Guo-Qiang Lin and Xing-Wen Sun*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A highly diastereoselective palladium catalyzed cinnamylation of Interestingly, the isocarbostyril alkaloid family has a highly
N-tert-butanesulfinyl imines with cinnamyl acetates has been functionalized β-aryl ethylamine motif (2-aryl ethylamine) as
established to provide enantioenriched β-aryl homoallylic amines. common subunit. Accordingly, we envisioned to synthesize
The synthetic application of this stragety has been successfully these isocarbostyril alkaloids with
a late-stage common
demonstrated in the concise total syntheses of antitumor natural intermediate utilizing a similar key cinnamylation strategy.
products (+)-lycoricidine and (+)-7-deoxypancratistatin.
During our previous studies, we found that some cinnamyl
halides such as and II (Fig. 2) are unstable. Madsen also found
I
The Amaryllidaceae isocarbostyril alkaloids such as lycoricidine
that the decomposition of halide II (Fig. 2) complicated the
operation of the Zn-mediated addition and severely affected
the yield of the reaction.7
(
(
1
), narciclasine (
2
), 7-deoxypancratistatin (
5
3), pancratistatin
) (Fig. 1), exhibit high levels
4) and trans-dihydrolycoricidine (
of antitumor, antiviral, and anti-Alzheimer’s disease activities
and serve as useful lead compounds for drug discovery.1 Due
to the low natural abundance,1b significant efforts have thus
been devoted to their chemical syntheses in the past 30 years,
and many different approaches have been reported.2 In all
these works, to the best of our knowledge, there are only two
asymmetric total syntheses examples giving both saturated
and 1,10a-dehydro- products with a general intermediate.3
As a part of the ongoing efforts to synthesize biologically
active chiral nitrogenous compounds, we4 recently established
efficient approaches by Zn-mediated cinnamylation of Ellman’s
imine5 with cinnamyl halides for the preparation of chiral β-
aryl homoallylic amines.6 This strategy has successfully been
applied in the divergent total synthesis of β-aryl ethylamine
motif containing alkoloids, such as amathaspiramides.4b
Masuyama and Tamaru et al. initiated the umpolung
allylation using more stable allylic alcohols and esters.8 Various
excellent advances have been achieved in this field,5f,9 but
cinnamylations of imines providing optically pure β-aryl
homoallylic amines are still rare.10 We herein established a
more general asymmetric cinnamylation of imines using
readily available cinnamyl acetates.11
A model reaction of (R)-imine 6a and cinnamyl acetate 7a in
the presence of ZnEt2 and Pd(PPh3)4 was conducted to
optimize the reaction conditions. The results are shown in
Table 1. When the reaction was carried out in N,N-dimethyl
formamide (DMF) at room temperature, two syn-isomers, (Rs,
1R, 2R)-8a’ and (Rs, 1S, 2S)-8a (8a’:8a=56:44.) were afforded in
89% combined yield (Table 1, entry 1). Under such conditions,
the formation of anti-isomers was not observed. When 2 equiv
of LiCl4a was applied as an additive, the ratio of 8a’:8a
increased to 88:12 (Table 1, entry 2). Use of more LiCl did not
further improve the selectivity (Table 1, entry 3). The reaction
OH
C
OH
OH
OH
R2
OH
OH
1
10a
1
C
10a
4
O
O
O
O
4a
NH
A
A
B
in tetrahydrofuran (THF) resulted in only one syn-isomer, 8a,
B
NH
7
together with a small amount of anti-isomer. The observed
stereoselectivity is consistent with the proposed cyclic
chelation model (Fig. 3).4a,9b,10b The reaction at room
temperature gave the best result, providing 97% yield and 92:8
dr (Table 1, entry 4). LiCl did not improve selectivity in THF
system (Table 1, entry 5). Lowering the temperature to 0 oC
R1
O
R1
O
lycoricidine(1), R1=H
1
7-deoxypancratistatin ( ): R = H, R2 = OH
3
pancratistatin ( ): R1 = R2 = OH
1
4
2
narciclasine ( ), R =OH
trans-dihydrolycoricidine (5): R1 = R2 = H
Fig. 1 Structures of amaryllidaceae isocarbostyril alkaloids
O
O
Br
Br
MeO
COOMe
I
II
Fig. 2 Some sensitive cinnamyl halides.
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Table 1. Optimization of reaction conditions
Table 2. Substrate scope for the developed methodologya
Entrya
Solvent
DMF
DMF
DMF
THF
Additive
Yield (%)b
drc
1
-
89
84
74
97
97
16
44:56
12:88
14:86
92:8
2
2 eq LiCl
3
5 eq LiCl
4
-
5
6d
THF
2 eq LiCl
-
91:9
THF
>98:2
a Reaction was run with 6a (0.2 mmol), 7a (0.4 mmol), ZnEt2 (0.8 mmol) and
Pd(PPh3)4 (5 mol%) in 2 mL of solvent at rt. b Isolated yield. c Determined by crude
1H NMR; see the Supporting Information for details. d Reaction was run at 0 oC.
Fig. 3 Proposed reaction mechanism. 4a, 9b, 10b
a
Reaction was run with 6 (0.2 mmol), 7 (0.4 mmol), ZnEt2 (0.8 mmol) and
Pd(PPh3)4 (5 mol%) in 2 mL of THF at rt. b Isolated yield. c Determined by crude 1H
NMR; see the Supporting Information for details. d Reaction was run at 15 oC.
gave excellent dr, but the yield declined dramatically.
With the optimized conditions (Table 1, entry 4) in hand, we
next studied the substrate scope. First, a wide variety of
cinnamyl acetates were investigated (Table 2). The substituted
cinnamyl acetates all worked well with 6a
,
giving
corresponding homoallylic amines 8a-e in excellent yields (95-
99%) and with high diastereoselectivities (91:9 to 95:5 dr). It is
worth mentioning that the preparations of product of 8b-c are
difficult using previous methods due to the corresponding
halides are unstable. Heterocycle substituted allylic acetates
also are suitable substrates, affording 8f-h in good yields and
good diastereoselectivities (88:12-91:9 dr). Not surprisingly,
unsubstituted allylic acetate 7i reacted well with imine 6a and
give 8i in 84% yield and with 98:2 dr. Next, the applicable
scope of imines was explored (Table 2). In all cases, the
Scheme 1 Retrosynthetic analysis of 1, 3, and 5
reaction proceeded effectively with cinnamyl acetate 7a
Substituted aryl imines, heterocyclic imines, as well as aliphatic
imines gave a variety of -aryl homoallylic amines in high yields
.
β
better electrophiles than epoxides in terms of ring- opening by
nucleophiles,16 we intended to employ cyclic sulfate
9
as the
. The corresponding cis-diol of
can be prepared by dihydroxylation of
olefin 10, which is readily available through ring-closing
metathesis (RCM) of -aryl homoallylic amine 11. Compound
and with high diastereoselectivities (8j-u). The absolute
configuration of the major products were assigned by X-ray
analysis of 8n and the configuration of 8a’ was identified by
comparing its NMRs to those reported in literature.4a
common precursor to
the cyclic sulfate
1 and 3
9
After the method was established, we started to investigate
its application in isocarbostyril alkaloid syntheses. Scheme 1
β
11 could be prepared in a tandem reaction by cinnamylation of
imine 13 with acetate 12 followed by an intramolecular
cyclization. According to the stereochemical outcomes of our
developed methodology, (S)-imine 13 was used to generate
the desired stereocenters.
shows the retrosynthetic analyses of the 7-deoxy members,
7,3a,13
lycoricidine (
dihydrolycoricidine (
been reported previously,15 we focused on the total syntheses
of compounds and . Because cyclic sulfates are usually
1
),12 7-deoxy-pancratistatin (
3
)
and trans-
5
)14. As the hydrogenation of
1
to had
5
1
3
Cinnamyl acetate 12 was conveniently synthesized from
bromide 147 (Scheme 2), under Ag2CO3-assisted Heck reaction
2 | J. Name., 2012, 00, 1-3
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condition established by Jiao.11 Imine 13 was prepared from yield by a simple filtration. With the establishment of the
iodoribose derivative 15 17 Elimination of 15 with n-BuLi,18 three-ring framework, we turned to the installation of the
.
followed by the condensation with (S)-tert-butanesulfinamide hydroxyl groups.
in the presence of Ti(OEt)4,19 afforded the (S)-N-tert-
The tert-butanesulfinyl was removed in order to avoid
butanesulfinyl imine 13 in 65% yield (Scheme 3). With the potential oxidation during the ensuing dihydroxylation. The
intermediates 12 and 13 in hand, we began to study the selective de-protection was achieved by using acetone as the
construction of B ring (Scheme 3). As we expected, the tandem co-solvent under the acidic condition. The presence of acetone
sequence of cinnamylation and cyclization worked efficiently, in the reaction mixture fully suppressed removal of the
giving the desired trans-fused lactam 11 as a single acetonide motif leading to the formation of 10 in a
diastereoisomer
in
91%
yield.
The
excellent quantitative yield. The subsequent dihydroxylation to install
diastereoselectivity might arise from a beneficial matching of the C1,C2-diol occurred with good diastereoselectivity (dr 13:1)
the chiral sulfinamide auxiliary and the existing carbon due to the adjacent bulky groups, producing the exo-diol 17 in
stereocenters. For the formation of C ring through RCM
86% yield. The structure of 17 was confirmed by X-ray
crystallography. Treatment of diol 17 with SOCl2 and the
subsequent oxidation with RuCl3/NaIO4 led to 67% yield of the
cyclic sulfate
9, which could serve as the common precursor
for the syntheses of
1
and 3. Upon the sulfate ring opening by
n-Bu4NI at the less hindered C1-position, the resultant iodide
went through anti-elimination and acid hydrolysis,20 affording
Scheme 2. Synthesis of cinnamyl acetate (12).
the target lycoricidine (
1) in 48% yield.
For the synthesis of 7-deoxypancratistatin (
3), a similar
trans-diaxial attack of cyclic sulfate 9 with PhCOONa at C1-
position,21 followed by the acid hydrolysis, could give the
benzoate 18 in 87% yield. Upon the treatment of 18 with
MeONa, 7-deoxypancratistatin (3) was obtained in 97% yield
(Scheme 3). The analytical data of compounds
consistent with those in previous reports. 12, 13
1 and 3 are
In summary, we established a highly diastereoselective
palladium catalyzed cinnamylation of N-tert-butanesulfinyl
imines with cinnamyl acetates. Circumventing the use of
unstable cinnamyl halides, this method largely extends the
substrate scope for the preparation of optically pure
homoallylic amines. Utilizing this key strategy, (+)-lycoricidine
) and (+)-7-deoxypancratistatin ( ) have been synthesized
β-aryl
(
1
3
through a common late-stage intermediate. The longest linear
routes from commercially available 6-bromopiperonal (2 steps
to known compound 147) to intermediate
9
were achieved in 9
steps and 25% overall yield. The final targets and were
obtained from in only 2 and 3 steps, respectively. It is also
possible to access trans-dihydrolycoricidine ( ) through the
hydrogenation of (+)-lycoricidine (
).15 These results will
1
3
9
5
1
promote the divergent syntheses of the promising
isocarbostyril alkaloids and their analogues.
The authors gratefully acknowledge the financial support
from the National Natural Science Foundation of China
(21472019) and thank Dr. Søren Kramer and Tiezheng Jia for
their generous help.
Notes and references
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