Pd-Catalyzed Asymmetric Allylic Cycloaddition of Vinyloxetanes with Formaldehyde

Article abstract of DOI:10.1021/acs.orglett.8b03665

An efficient method for the enantioselective construction of tertiary 1,3-diols via Pd-catalyzed asymmetric allylic cycloaddition of vinyloxetanes with an abundant feedstock, formaldehyde, is developed. Using the palladium complex generated in situ from P

Full text of DOI:10.1021/acs.orglett.8b03665

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Pd-Catalyzed Asymmetric Allylic Cycloaddition of Vinyloxetanes
with Formaldehyde
Hang Xu,† Sardaraz Khan,† Hongfang Li,†,‡ Xue Wu,‡ and Yong Jian 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, People’s Republic of China
‡Key Laboratory for Organism Resources of the Changbai Mountain and Functional Molecules, Ministry of Education, and
Department of Chemistry, College of Science, Yanbian University, 977 Gongyuan Road, Yanji, Jilin 133002, People’s Republic of
China
S
* Supporting Information
ABSTRACT: An efficient method for the enantioselective
construction of tertiary 1,3-diols via Pd-catalyzed asymmetric
allylic cycloaddition of vinyloxetanes with an abundant feed-
stock, formaldehyde, is developed. Using the palladium complex
generated in situ from Pd2(dba)3·CHCl3 and phosphoramidite
L3 as a catalyst under mild conditions, the process allows one to
convert racemic 2-substituted 2-vinyloxetanes (1) to the
corresponding 1,3-dioxanes (2) as methylene acetal protected tertiary 1,3-diols in high yields with good to excellent
enantioselectivities.
hiral tertiary 1,3-diols are prevalent motifs in a wide
variety of medicinally relevant agents and natural
Michael addition to β,β-disubstituted Michael acceptors for the
construction of tertiary C−O bond are largely under-
developed.7 Transition-metal-catalyzed asymmetric allylic
etherification is one of the most powerful methods for the
C−O bond formation to provide chiral allylic ethers,8 which
could be converted to 1,3-diols by further manipulations.
However, regioselective construction of tertiary C−O bond
using this transformation remains a significant challenge.9 In
contrast, only a few methods for more directly accessing chiral
tertiary 1,3-diols have been reported, including asymmetric
dihydroxylation of functionalized olefins10 and kinetic
resolution of diols via asymmetric acetalization.11 Therefore,
the development of efficient catalytic methods, which allows
rapid access to enantioenriched tertiary 1,3-diols, is highly
appealing.
C
products, such as phoslactomycin B deamino precursor,1
convolutamydine E isolated from the Floridian marine
bryozoan Amathia convoluta,2 potential anticancer agent a,3
and potent 11β-HSD1 inhibitor b4 (Figure 1). The importance
Pd-catalyzed cycloaddition reactions through zwitterionic
allylpalladium intermediates have recently emerged as an
important strategy for the construction of various cyclic
frameworks.12 Various allylic donors have successfully been
applied to this transformation with diverse unsaturated
electrophiles. Among them, vinyl epoxides13 and vinylethylene
carbonates (VECs)14,15 have served as efficient 1,3- or 1,5-C,O-
dipoles in the Pd-catalyzed allylic cycloaddition reactions to
afford oxo-cyclic compounds. (See Scheme 1.) However, the
analogous vinyl oxetanes16 as C,O-dipoles for the cycloaddition
have been largely underdeveloped.17
Figure 1. Representative medicinally relevant agents and natural
products containing tertiary 1,3-diols.
of this particular framework has prompted chemists to develop
efficient asymmetric catalytic methods for the synthesis of
chiral tertiary alcohols bearing useful functionalities. Never-
theless, multistep transformations are generally required to
access chiral tertiary 1,3-diols. Most common approaches to
chiral tertiary 1,3-diols involve the asymmetric Aldol reaction
of ketones, followed by a reduction or asymmetric allylation/
alkenylation of ketones and further manipulations.5 Oxa-
Michael addition followed by reduction is also an effective
protocol for the synthesis of chiral 1,3-diols.6 However, oxa-
There is only one report of Pd-catalyzed allylic cycloaddition
of vinyl oxetanes with isocyanates/carbodiimides to afford 1,3-
oxazines in a racemic form.18 During our studies on this work,
Received: November 16, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03665
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Enantioselective Cycloaddition of Vinyloxetanes
with Formaldehyde
Table 1. Condition Optimizations
b
c
entry ligand
solvent
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
CH2Cl2
toluene
cyclohexane
Et2O
temperature, T (°C) yield (%) ee (%)
Zhao and co-workers reported Pd-catalyzed asymmetric formal
[6 + 4] cycloaddition of vinyl oxetanes with azadienes to
prepare 10-membered heterocycles.19 Most recently, we
developed VECs as more stable and readily accessible allylic
donors and achieved the Pd-catalyzed asymmetric decarbox-
ylative allylic cycloaddition of VECs with various unsaturated
electrophiles to afford diverse heterocycles with tetrasubsti-
tuted stereocenters in high efficiencies.14 In our previous
studies, we developed an efficient process of Pd-catalyzed
asymmetric allylc cycloaddition of VECs with formaldehyde to
access chiral 1,3-dioxolanes as methylene acetal protected
tertiary 1,2-diols.14e Based on our continuous efforts to
develop efficient methods for the enantioselective construction
of tertiary alcohols with diverse functionalities, we are
interested in the possibility of using vinyl oxetanes in an
enantioselective allylic cycloaddition with formaldehyde to
access enantioenriched tertiary 1,3-diol derivatives. In this
communication, we report Pd-catalyzed asymmetric allylic
cycloaddition of vinyl oxetanes with formaldehyde, which is an
efficient method that allows rapid access to valuable chiral 1,3-
dioxanes as methylene acetal protected tertiary 1,3-diols in
high yields with high levels of enantioselectivities.
Initial studies focused on the cycloaddition of vinyloxetane
1a as a standard substrate with formaldehyde (Table 1). Based
on our previous studies on the cycloaddition of VECs,14 we
began our investigation by examining the cycloaddition of 1a
with formaldehyde (37% aqueous solution) in the presence of
a Pd(0)-catalyst with different phosphoramidite ligands. First,
we tried the reactions using phosphoramidites derived from
binol20 in tetrahydrofuran (THF) at 20 °C for 17 h. The
reactions proceeded well to give desired 1,3-dioxane 2a in
good yields (Table 1, entries 1−3), and the reaction with
ligand L3 revealed the best enantioselectivity (80% enantio-
meric excess (ee)). The Zhou ligand21 L4 is also effective for
the reaction, but the yield slightly decreased (Table 1, entry 4).
The reaction using ligand L6 gave the cycloadduct 2a in 98%
ee, whereas the yield is poor (Table 1, entry 6). The yields can
be improved without significant deterioration of enantiose-
lectivities when the reactions proceeded at 40 °C (Table 1,
entries 7−9). Further increasing the reaction temperature to
60 °C (Table 1entries 10 and 11), the yields decreased instead.
Based on these results, we tried to further screen reaction
solvent with L3 as a ligand at 40 °C (Table 1entries 12−18).22
As a result, we found that the reaction proceeded smoothly in
the different solvents, and the reaction in diethyl ether showed
the best results, affording 1,3-dioxane 2a in 86% yield with 92%
ee (Table 1, entry 15).
1
2
L1
L2
L3
L4
L5
L6
L3
L4
L6
L3
L6
L3
L3
L3
L3
L3
L3
L3
20
20
20
20
20
20
40
40
40
60
60
40
40
40
40
40
40
40
79
72
75
63
−
−35
44
80
81
−
−98
80
70
−94
79
−97
68
86
89
92
74
93
77
3
4
5
6
7
8
9
8
91
67
49
75
12
82
63
53
86
79
66
68
10
11
12
13
14
15
16
17
18
1,4-dioxane
d
CPME
MeCN
a
Reaction conditions: Pd2(dba)3·CHCl3 (2.5 mol %), ligand (10
mol %), 1a (0.2 mmol), formaldehyde (2.0 mmol, 37% aqueous
b
c
solution), solvent (1.0 mL), 17 h. Isolated yields. ee = enantiomeric
excess. Determined by chiral HPLC. The absolute configuration was
confirmed by X-ray crystallography of 4 elaborated from 2a (Scheme
d
3). CPME = cyclopentyl methyl ether.
various substituted vinyl oxetanes 1 with formaldehyde. As
shown in Scheme 2 , a wide range of aryl-substituted vinyl
oxetanes bearing different electronic and steric properties was
tolerated under the reaction conditions, affording the
corresponding 1,3-dioxanes 2a−2l in moderate to high yields
with good to high enantioselectivities. For the cycloaddition of
2-(1-naphthyl)-2-vinyloxetane (1m), high enantioselectivity
(>99% ee) was observed, but the yield was poor. The
cycloaddition of 2-(2-naphthyl)-2-vinyloxetane (1n) pro-
ceeded smoothly to afford corresponding 1,3-dioxane 2n in
high yield with moderate enantioselectivity (52% ee).
Unfortunately, the reaction conditions were not compatible
for the aliphatic-substituted vinyloxetane 1o: only poor yield
and enantioselectivity were observed. When using ligand L4
instead of L3 under otherwise identical conditions, the reaction
efficiency can be improved to afford corresponding 1,3-dioxane
2o in 58% yield with moderate enantioselectivity (64% ee).
The reaction of 2-cyclohexyl-2-vinyloxetane (1p) was
ineffective under these reaction conditions.
Next, we tried vinyl carbonate 3a, which is synthesized from
the corresponding 1,3-diol, instead of vinyl oxetane 1a for this
cycloaddition reaction. As shown in Scheme 3, the reaction
conditions were also effective for the cycloaddition of vinyl
With the optimal conditions in hand (Table 1, entry 15), the
generality of this process was evaluated by the reaction of
B
DOI: 10.1021/acs.orglett.8b03665
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Substrate Scope for Pd-Catalyzed Asymmetric
Cycloaddition of Vinyloxetanes 1 with Formaldehyde
Scheme 4. Elaboration of 2a To Form Triol 5
a
unambiguously assigned by X-ray crystallography (see the
Supporting Information). Deprotection of methylene acetal of
compound 4 was performed with trifluoroacetic anhydride and
acetic acid, followed by transesterification under basic
conditions in methanol to give triol 5 in 86% yield without a
significant loss of enantioselectivity (90% ee).
In conclusion, we have developed an efficient method for the
enantioselective construction of methylene acetal protected
tertiary 1,3-diols via Pd-catalyzed asymmetric allylic cyclo-
addition of vinyloxetanes with an abundant feedstock:
formaldehyde. The reactions proceeded smoothly in the
presence of Pd2(dba)3·CHCl3 and phosphoramidite L3
under mild conditions, providing 4-substituted 4-vinyl-1,3-
dioxanes 2 in high yields with good to excellent enantiose-
lectivities. The synthetic utility was demonstrated by the
elaboration of 1,3-dioxane 2a into corresponding triol 5
bearing a tetrasubstituted stereocenter. Further studies on the
extending of the scope of the cycloaddition of vinyloxetanes
with other unsaturated electrophiles are currently underway
and will be reported in due course.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03665.
Detailed experimental procedures; characterization data
of all the new compounds; copies of HPLC chromatog-
raphies, 1H and 13C NMR spectra of the products
(PDF)
a
Reaction conditions: Pd2(dba)3·CHCl3 (2.5 mol %), ligand (10
mol %), 1a (0.2 mmol), formaldehyde (2.0 mmol, 37% aqueous
solution), solvent (1.0 mL), 17 h. Yields are isolated yields. The
enantioselectivities were determined by chiral HPLC. The absolute
configuration was confirmed by X-ray crystallography of 4 elaborated
from 2a (Scheme 3); those of the other products were assigned by
analogy. The results were obtained by using L4 as a ligand under
otherwise identical conditions.
Accession Codes
CCDC 1876159 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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
b
Scheme 3. Pd-Catalyzed Asymmetric Cycloaddition of Vinyl
Carbonate 3a with Formaldehyde
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yjian@sjtu.edu.cn.
ORCID
Yong Jian Zhang: 0000-0001-5808-1745
Notes
The authors declare no competing financial interest.
carbonate 3a with formaldehyde to furnish 1,3-dioxane 2a in
60% yield with high enantioselectivity (95% ee).
The synthetic versatility of the present protocol was
demonstrated by the elaboration of 1,3-dioxane 2a into triol
5 (Scheme 4). Oxidation of 2a with RuCl3 and NaIO4 and
subsequent reduction with LiAlH4 gave compound 4 in 60%
yield for two steps. The absolute configuration of 4 was
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Nos. 21572130 and 21871179). We
thank the Instrumental Analysis Center of Shanghai Jiao Tong
University for HRMS analysis.
C
DOI: 10.1021/acs.orglett.8b03665
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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D
DOI: 10.1021/acs.orglett.8b03665
Org. Lett. XXXX, XXX, XXX−XXX