Palladium-catalyzed enantioselective decarboxylative cycloaddition of vinylethylene carbonates with isocyanates

Article abstract of DOI:10.1002/chem.201405830

An efficient method for the enantioselective construction of β-substituted β-vinylglycinol derivatives through palladium-catalyzed decarboxylative cycloaddition of vinylethylene carbonates with isocyanates was developed. By using a palladium complex generated in situ from [Pd₂(dba)₃] · CHCl₃ (dba=dibenzylideneacetone) and (S)-Segphos as a catalyst under mild reaction conditions, the process provided 4-substituted-4-vinyloxazolidin-2-ones in high yields with a high level of enantioselectivity.The stereochemical outcome of the reaction was explained by DFT calculations and the synthetic utility of the process was demonstrated by the gram-scale transformation and formal synthesis of MK-0731 as a kinesin spindle protein inhibitor.

Full text of DOI:10.1002/chem.201405830

DOI: 10.1002/chem.201405830
Communication
&
Asymmetric Catalysis
Palladium-Catalyzed Enantioselective Decarboxylative
Cycloaddition of Vinylethylene Carbonates with Isocyanates
Ajmal Khan,[a] Juxiang Xing,[a] Jingming Zhao,[a] Yuhe Kan,[b] Wanbin Zhang,[a] and Yong
Jian Zhang*[a]
Abstract: An efficient method for the enantioselective
construction of b-substituted b-vinylglycinol derivatives
through palladium-catalyzed decarboxylative cycloaddi-
tion of vinylethylene carbonates with isocyanates was de-
veloped. By using a palladium complex generated in situ
from [Pd2(dba)3]·CHCl3 (dba=dibenzylideneacetone) and
(S)-Segphos as a catalyst under mild reaction conditions,
the process provided 4-substituted-4-vinyloxazolidin-2-
ones in high yields with a high level of enantioselectivity.
The stereochemical outcome of the reaction was ex-
Figure 1. Representative medicinally relevant agents and natural products
plained by DFT calculations and the synthetic utility of the
with b,b-disubstituted b-amino alcohols.
process was demonstrated by the gram-scale transforma-
tion and formal synthesis of MK-0731 as a kinesin spindle
protein inhibitor.
tive routes to these compounds are largely unexplored.[8] Pd-
catalyzed cycloaddition of vinyloxiranes with isocyanates[9] and
intramolecular cycloaddition of 2-buten-1,4-diol carbamates,[10]
The development of enantioselective transformations that
enable rapid access to multifunctional chiral molecules is a prin-
cipal goal in asymmetric catalysis. Optically active b-substituted
b-vinylglycinols are important chiral building blocks for a wide
variety of medicinally relevant agents and natural products,
such as the immunosuppressant myriocin,[1] manzacidins,[2]
neurokinin receptor antagonist rolapitant,[3] and a kinesin spin-
dle protein inhibitor MK-0731[4] as a potential drug for the
treatment of taxane-refractory cancers (Figure 1). They are also
highly useful precursors of biologically valuable quaternary
amino acids, such as quaternary vinylglycines[5] and a-substi-
tuted serines.[6] In this context, various methods for the asym-
metric synthesis of b-substituted b-vinylglycinol derivatives
have been developed. However, most of them are based on
the stoichiometric use of a chiral auxiliary, chiral pool, or ste-
reospecific methods.[7] In contrast, the catalytic enantioselec-
pioneered by Trost and Hayashi, respectively, are efficient
methods for the synthesis of vinylglycinols. However, the
asymmetric variants of the reactions are mostly unsatisfacto-
ry.[11] The related asymmetric transformations, such as the cy-
cloaddition of vinyloxiranes with carbodiimides[11c] and
imines[12] as well as direct amination of vinyloxiranes,[13] which
have also been developed for the synthesis of enantioenriched
vinylglycinol derivatives. Nevertheless, limited progress has
been accomplished for the construction of b-substituted b-vi-
nylglycinol derivatives,[11c,12,13] and the substrate of which has
been limited to isoprene oxide. Therefore, the development of
an efficient method for the enantioselective construction of b-
vinylglycinols with diverse b-substituents is highly appealing.
Transition-metal-catalyzed asymmetric interceptive decar-
boxylative allylations of allylic partners by unsaturated electro-
philes are useful methods for the formation of multifunctional-
ized chiral building blocks.[14] Most recently, we disclosed
a useful interceptive decarboxylative allylation processes using
stable vinylethylene carbonates (VECs) as allylic donors and
formaldehyde and activated Michael acceptors as intercep-
tors.[15] The processes in the presence of chiral palladium–phos-
phoramidite complex as a catalyst afforded tertiary vinylglycol-
s[15a] and multisubstituted tetrahydrofurans with vicinal all-
carbon stereocenters[15b] in high yields with a high level of ste-
reoselectivity. Based on these research results, we envisioned
that b-substituted b-vinylglycinols could be constructed
through the decarboxylative cycloaddition of VECs with isocya-
nates.[16] As illustrated in Scheme 1, the zwitterionic p-allylpalla-
dium intermediate A, which is generated from VECs 1 through
[a] A. Khan,+ J. Xing,+ J. Zhao, Prof. W. Zhang, Prof. Y. J. Zhang
School of Chemistry and Chemical Engineering
and Shanghai Key Laboratory of Electrical Insulation and Thermal Aging
Shanghai Jiao Tong University
800 Dongchuan Road, Shanghai 200240 (P. R. China)
Fax: (+86)21-5474-1297
E-mail: yjian@sjtu.edu.cn
[b] Prof. Y. Kan
Jiangsu Key Laboratory for Chemistry of Low-Dimensional Materials
School of Chemistry and Chemical Engineering
Huaiyin Normal University, Huaian 223300 (P. R. China)
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405830.
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Table 1. Palladium-catalyzed asymmetric decarboxylative cycloaddition
of 1a with 2.[a]
Scheme 1. Strategy for Pd-catalyzed decarboxylative cycloaddition of VECs
1 with isocyanates 2.
Entry
2
Ligand
3
Yield [%][b]
ee [%][c]
a decarboxylative process, could attack isocyanates 2 to afford
intermediate B. The subsequent cycloaddition could be feasi-
ble to form favored vinyloxazolidinones 3 as carbonyl-protect-
ed b-substituted b-vinylglycinols. Herein, we report the palladi-
um-catalyzed asymmetric decarboxylative cycloaddition of
VECs with isocyanates to construct 4-substituted-4-vinyloxazo-
lidin-2-ones in high yields with a high level of enantioselectivi-
ty.
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2c
2d
2e
2 f
2g
2h
2i
L1
L2
L3
(R)-BINAP
3aa
3aa
3aa
3aa
3aa
3aa
3aa
3aa
3aa
3ab
3ac
3ad
3ae
3af
88
49
39
97
95
95
93
99
95
92
93
93
95
93
89
94
90
94 (R)
74 (R)
14 (R)
95 (S)
94 (R)
87 (R)
85 (R)
92 (S)
99 (S)
74 (R)
92 (R)
84 (S)
92 (S)
88 (S)
95 (S)
37 (R)
40 (R)
(S)-Tol-BINAP
(S)-xylyl-BINAP
(S)-H8-BINAP
(R)-Cl-MeO-BIPHEP
(R)-Segphos
(S)-Segphos
(S)-Segphos
(R)-Segphos
(R)-Segphos
(R)-Segphos
(R)-Segphos
(S)-Segphos
(S)-Segphos
9
10
11
12
13
14
15
16
17
Initial studies focused on finding an effective chiral ligand
for promoting the palladium-catalyzed decarboxylative cyclo-
addition. For this purpose, VEC 1a and 2-methoxyphenyliso-
cyanate (2a) were chosen as standard reaction partners be-
cause they are readily available commercial sources and the cy-
cloadduct vinyloxazolidinone 3aa is also a quite useful chiral
building block.[11] Based on our previous research results,[15] we
began our investigations by examining the cycloaddition of 1a
with 2a by using phosphoramidite ligands (Table 1). To our de-
light, the reaction proceeded smoothly with phosphoramidite
L1 as a ligand to afford vinyloxazolidinone 3aa in high yield
with 94% ee (Table 1, entry 1). However, the diastereomeric
phosphoramidite ligands L2 and L3 were less effective for the
reaction (entries 2 and 3). Remarkably, the reaction could be
further improved when using (R)-BINAP as a ligand, affording
vinyloxazolidinone 3aa in 97% yield with high enantioselectivi-
ty (95% ee, entry 4). By means of further screening of bisphos-
phine ligands (entries 5–9), we found that the reaction using
(R)-Segphos as a ligand provided the best enantioselectivity
(99% ee, entry 9). With the optimized conditions in hand, vari-
ous phenylisocyanates containing different substituents was
next examined. All of the reactions performed well, leading to
the corresponding vinyloxazolidinones 3ab–ag in good to ex-
cellent enantioselectivities (entries 10–15). Sterically demand-
ing 2-substituted phenylisocyanates provided higher enantio-
selectivity than the 3- or 4-substituted analogues. However,
low enantioselectivities were observed when the reactions
were carried out with benzoylisocyanate 2h and tosylisocya-
nate 2i (entries 16 and 17).
3ag
3ah
3aj
[a] Reaction conditions: [Pd2(dba)3]·CHCl3 (0.5 mol%), ligand (1 mol% for
bisphosphanes; 2 mol% for phosphoramidites), 1a (0.2 mmol),
2
(0.2 mmol), THF (1.0 mL), 408C, 17 h. [b] Yields are of isolated materials.
[c] Determined by HPLC analysis using a chiral stationary phase. The ab-
solute configurations of 3aa–aj were determined by comparison of the
sign of optical rotation with that in the reported data,[11e,f] or surmised by
analogy. BINAP=2,2’-bis(diphenylphosphino)-1,1’-binaphthyl; H8-BINAP=
2,2’-bis(diphenylphosphino)-5,5’,6,6’,7,7’,8,8’-octahydro-1,1’-binaphthyl; Cl-
MeO-BIPHEP=5,5’-dichloro-6,6’-dimethoxy-2,2’-bis(diphenylphosphino)-
1,1’-biphenyl;
dioxole; dba=dibenzylideneacetone.
Segphos=5,5’-bis(diphenylphosphino)-4,4’-bi-1,3-benzo-
[Pd2(dba)3]·CHCl3 (dba=dibenzylideneacetone) and (S)-Seg-
phos as a catalyst afforded cycloadduct 3ba in 93% yield with
91% ee.[17] Interestingly, the absolute configuration of 3ba is S-
form which is an opposition configuration in comparison with
the cycloaddition product 3aa from VEC 1a. The reactions of
4-alkyl substituted VECs 1c and 1d provided the correspond-
ing vinyloxazolidinones 3ca and 3da in high yields with good
enantioselectivities. Meaningfully, the cycloaddition reaction
was also effective for more functionalized VECs bearing benzyl-
protected hydroxymethyl 1e and hydroxyethyl 1 f, as well as
3-butenyl 1g, providing the corresponding highly functional-
ized vinyloxazolidinones 3ea, 3 fa, and 3ga, in which four dif-
ferent functional groups were located at one carbon stereo-
genic center. 4-Aryl VECs 1h–q were also found to be suitable
substrates, affording 4-aryl vinyloxazolidinones 3ha–qa in
good to excellent yields (61–95%) with high levels of enantio-
With established 2-methoxyphenylisocyanate (2a) as the op-
timal electrophile, we subsequently turned our attention to
the construction of vinyloxazolidinones bearing a quaternary
stereocenter by the palladium-catalyzed decarboxylative cyclo-
addition of 4-substituted VECs with isocyanate 2a. Gratifyingly,
a wide range of 4-alkyl- and aryl-substituted VECs were tolerat-
ed in the reaction conditions affording the corresponding 4-
substituted-4-vinyloxazolidin-2-ones 3 in acceptably high yields
and enantioselectivities (Table 2). The reaction of 4-Me-VEC 1b
with 2a by using a palladium complex generated in situ from
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enantioselectivities. In addition, the VECs with versatile furan
1t and thiophene 1u moieties were also suitable for the reac-
tion conditions to afford the cycloadducts 3ta and 3ua in high
yields with good enantioselectivities.
Table 2. Palladium-catalyzed asymmetric decarboxylative cycloaddition
of 4-substituted VECs 1 with 2a.[a]
According to the proposed reaction pathway as revealed in
Scheme 1, the final cycloaddition step might be the stereo-
chemistry-determining step. To gain more information on the
stereochemical outcome of the present process, DFT calcula-
tions were carried out by using the B3LYP/def2-TZVP
method.[18] Considering that the cycloaddition products from
4-substituted VECs gave opposite stereochemistry with that
obtained from H-VEC (1a), four plausible conformers of the p-
allylpalladium-(S)-Segphos intermediate B from H-VEC (1a)
(Figure 2) and that from Me-VEC (1b) (Figure 3) were chosen
for the geometry optimizations (see the Supporting Informa-
tion for details). In the case of H-VEC (1a), the p-allylpalladium
intermediate BH-VEC-syn-a bearing a syn-allylic unit (syn position
between oxymethylene group and C2-proton of the allylic
group) revealed much lower energy than the intermediate BH-
Figure 2. Stereochemical outcome by geometric optimization of plausible al-
lylpalladium intermediates B from H-VEC (1a).
[a] Reaction conditions: [Pd2(dba)3]·CHCl3 (2.5 mol%), (S)-Segphos
(5 mol%), 1 (0.2 mmol), 2a (0.2 mmol), THF (1.0 mL), 408C, 17 h. Yields
are of isolated materials. Enantiomeric excesses were determined by
HPLC analysis by using a chiral stationary phase. The determination of ab-
solute configurations of 3ba and 3ha were described in the Supporting
Information. Those of the other products were assigned by analogy. 1b:
R=Me, 1c: R=PhCH2CH2, 1d: R=cyclohexyl, 1e: R=BnOCH2, 1 f: R=
BnOCH2CH2, 1g: R=but-3-enyl, 1h: R=Ph, 1i: R=2-MeOC6H4, 1j: R=3-
MeOC6H4, 1k: R=4-MeOC6H4, 1l: R=4-MeC6H4, 1m: R=4-tBuC6H4, 1n:
R=4-ClC6H4, 1o: R=4-BrC6H4, 1q: R=2,4-diFC64, 1r: R=1-naphthyl, 1s:
R=2-naphthyl, 1t: R=2-furanyl, 1u: R=3-thiophenyl.
selectivity (90–99% ee). The absolute configuration of 3ha was
determined as R-form, which indicated that it is formed
through the same stereochemical mode with methyl-substitut-
ed 3ba because of the change in priorities. The reaction of
VECs with naphthyl group 1r and 1s were also effective to fur-
nish the cycloadducts 3ra and 3sa in good yields with high
Figure 3. Stereochemical outcome by geometric optimization of plausible al-
lylpalladium intermediates B from Me-VEC (1b).
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bearing an anti-allyl unit (Figure 2). Thus, the reaction
with NaOH in ethanol gave quaternary vinylglycinols 5 in high
yields. More significantly, a short formal synthesis of MK-0731
was accomplished by using vinyloxazolidinone (S)-3ha, which
was obtained from the cycloaddition of 1h and 2a with (R)-
Segphos in 90% yield with 91% ee. As illustrated in Scheme 4,
VEC-anti-b
of 1a with 2a afforded (R)-3aa as the major enantiomer
through a backside SN2-type attack in the cycloaddition.[19] In
contrast, for the p-allylpalladium intermediates from Me-VEC
1b, the corresponding syn intermediates revealed much higher
energy than the anti intermediates, and the anti intermediate
BMe-VEC-anti-b showed the lowest energy. Thus, affording (S)-3ab
as the major enantiomer for the cycloaddition of Me-VEC (1b)
is favorable. Although the true stereochemical outcome would
be more complicated and the kinetic scenario of the reaction
must be considered, opposite stereochemistry for the reaction
of 4-substituted VECs in comparison with the reaction of H-
VEC (1a) is convincing based on these DFT calculation re-
sults.[20]
The synthetic versatility of the present protocol was demon-
strated through the scale-up transformations, the conversion
of cycloadducts into corresponding b-substituted b-vinylglyci-
nols, and the formal synthesis of MK-0731 as a potential drug
for the treatment of taxane-refractory cancers.[4] The present
asymmetric decarboxylative cycloaddition reaction is quite
scalable as demonstrated by the gram-scale syntheses of vinyl-
oxazolidinones 3aa, 3ba, and 3ha in high yields with high
levels of enantioselectivity (Scheme 2). In the case of 3aa, re-
Scheme 4. Formal synthesis of MK-0731.
oxidative deprotection of (S)-3ha by using the above-men-
tioned method afforded (S)-4h in 60% yield, and subsequent
N-alkylation with allylic bromide 6 furnished compound 7 in
high yield. Straightforward ring-closing metathesis of 7 with
the first generation of Grubbs catalyst provided bicyclic com-
pound 8 in 95% yield. The compound 8 would be an ad-
vanced chiral intermediate in Merck’s synthesis of MK-0731.[4]
In conclusion, we have developed an efficient method for
the enantioselective construction of 4-substituted-4-vinyloxa-
zolidin-2-ones as carbonyl-protected b-substituted b-vinylglyci-
nols by Pd-catalyzed asymmetric decarboxylative cycloaddition
of VECs with isocyanates. The reactions proceeded smoothly in
the presence of [Pd2(dba)3]·CHCl3 and (S)-Segphos under the
mild reaction conditions, providing vinyloxazolidinones 3 in
high yields (up to 95%) with good to excellent enantioselectiv-
ities (81–99% ee). The stereochemical outcome of the process
was explained by DFT calculations on the geometry optimiza-
tions of plausible reaction intermediates. In addition, the syn-
thetic utility of the process was demonstrated by the gram-
scale transformations, the deprotection of the cycloaddition
products to b-substituted b-vinylglycinols, and the formal syn-
thesis of MK-0731 as a potential drug for the treatment of
taxane-refractory cancers. Further studies to extend the scope
of the decarboxylative cycloaddition of VECs are currently un-
derway and will be reported in due course.
Scheme 2. Scaled-up Pd-catalyzed asymmetric decarboxylative cycloaddition
of VECs 1 with 2a.
ducing the catalyst loading to 0.2 mol% still gave excellent ef-
ficiency. The vinyloxazolidinones 3 can be converted into the
corresponding b-substituted b-vinylglycinols 5 through two-
step deprotection processes (Scheme 3). The oxidative depro-
tection[21] of the N-aryl group of 3 was achieved with excess
ammonium persulfate in the presence of a catalytic amount of
AgNO3 to afford vinyloxazolidinones 4 in moderate yields with-
out deterioration of optical purity. Straightforward hydrolysis
Acknowledgements
This work was supported by the Innovation Program of Shang-
hai Municipal Education Commission (14ZZ023), National Key
Basic Research Program of China (2013CB934101), Shanghai
Pujiang Program (11PJD012), and Shanghai Jiao Tong Universi-
ty. We thank the Instrumental Analysis Center of Shanghai Jiao
Tong University for HRMS analysis.
Scheme 3. Deprotection of 3 to b-substituted b-vinylglycinols 5. [a] Yields of
recovered starting materials within parenthesis.
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Keywords: amino alcohols
cycloaddition · palladium · substituted vinylglycinols
·
asymmetric catalysis
·
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Received: October 27, 2014
Published online on November 14, 2014
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