Lewis-Acid-Mediated Thiocyano Semipinacol Rearrangement of Allylic Alcohols for Construction of α-Quaternary Center β-Thiocyano Carbonyls

Article abstract of DOI:10.1021/acs.orglett.9b03722

An electrophilic thiocyano semipinacol rearrangement of allylic alcohols has been achieved for the first time by using N-thiocyano-dibenzenesulfonimide (NTSI). This approach provides a direct, simple, and efficient strategy for the formation of thiocyano

Full text of DOI:10.1021/acs.orglett.9b03722

Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Lewis-Acid-Mediated Thiocyano Semipinacol Rearrangement of
Allylic Alcohols for Construction of α‑Quaternary Center
β‑Thiocyano Carbonyls
Xu-Feng Song,† Ai-Hui Ye,† Yu-Yang Xie, Jia-Wei Dong, Chao Chen, Ye Zhang,* and Zhi-Min Chen*
School of Chemistry and Chemical Engineering, Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs, & Shanghai
Jiao Tong University Affiliated Sixth People’s Hospital South Campus, Shanghai Jiao Tong University, 800 Dongchuan Road,
Shanghai 200240, P. R. China
S
* Supporting Information
ABSTRACT: An electrophilic thiocyano semipinacol rear-
rangement of allylic alcohols has been achieved for the first
time by using N-thiocyano-dibenzenesulfonimide (NTSI).
This approach provides a direct, simple, and efficient strategy
for the formation of thiocyano carbonyl compounds with
moderate to excellent yields. Meanwhile, an all-carbon
quaternary center was rapidly constructed. In addition, an asymmetric version of this tandem reaction was preliminarily
investigated.
rganic thiocyanates are not only a class of important and
versatile synthetic intermediates, which could be easily
transformed into other valuable compounds bearing sulfur
group such as aryl sulfur compounds, trifluoromethylthiol
compounds, and thioethers,1 but also exist in many bioactive
natural products, for example, 9-thiocyanatopupukeanane,
fasicularin, and welwitindolinone C (Figure 1).2 Consequently,
Scheme 1. Design of Electrophilic Thiocyano Semipinacol
Rearrangement of Allylic Alcohols
O
Figure 1. Representative bioactive natural products containing SCN
group and quaternary center.
the development of synthetic methods for the introduction of
SCN group into organic compounds has attracted more and
more attention, and many methodologies have been developed.3
Recently, some radical difunctionalizations of alkenes involving
thiocyanation have been disclosed, which provide a viable route
to thiocyanates.4 On the other hand, electrophilic thiocyanation
of olefins has also been shown to be an efficient, straightforward,
and promising method for the synthesis of SCN-containing
compounds.5 For instance, our group has reported a series of
thiocyano oxyfunctionalization reactions of alkenes for the
construction of SCN-containing heterocycles with an oxa-
quaternary center using NTSI, which was independently
developed by our group and Chen’s group (Scheme 1a).5a,b
More recently, a selenide-catalyzed thiocyanoaminocyclization
of alkenes has been achieved by Zhao’s group.5c In addition,
some catalytic asymmetric thiocyanation reactions have been
documented.6,5c Despite these advances, electrophilic thiocya-
nations of alkenes are still limited. To the best of our knowledge,
no examples of electrophilic thiocyanation of alkenes including
tandem C−SCN and C−C bonds formation have been
reported. Therefore, the development of new and useful
methods for the synthesis of various organic thiocyanates is
still in high demand.
All-carbon quaternary centers are key and widespread
structural motifs in tremendous biologically active natural
products and pharmaceuticals.7 Accordingly, many efforts to
develop synthetic methods for the construction of all-carbon
quaternary centers have been made.8 However, highly efficient
formation of quaternary centers is still one of the most
Received: October 21, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03722
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
challenging topics in organic synthesis. The electrophile-
initiated semipinacol rearrangement of allylic alcohols has
proven to be a powerful strategy for the formation of α-
quaternary center carbonyl compounds.9,10 In this context, Tu
and co-workers have developed a number of electrophilic
semipinacol rearrangements of allylic alcohols, which have also
been successfully applied to synthesize natural products.11
Combined with our research interests in the synthesis of
organosulfur compounds12,5a and rearrangement reactions, we
aimed to investigate thiocyano semipinacol rearrangement of
allylic alcohols, which could rapidly afford α-quaternary center
β-thiocyano carbonyl compounds. Furthermore, C−SCN and
C−C bonds were simultaneously formed. Herein, we present
our preliminary results for a Lewis-acid-mediated thiocyano
semipinacol rearrangement of allylic alcohols and its synthetic
applications (Scheme 1b).
With the optimal reaction conditions in hand, the scope of this
rearrangement reaction was first evaluated with a number of 1,1-
disubstituted alkene alcohols (Scheme 2). In general, the desired
a b
,
Scheme 2. Scope of Allylic Alcohols
Initially, 1,1-disubstituted alkene alcohol 2a was selected as a
model substrate to study the rearrangement reaction. As shown
in Table 1, three different electrophilic SCN reagents were first
a
Table 1. Conditions Optimization
b
entry
R−SCN
solvent
acid
yield (%)
1
2
3
4
5
6
7
8
9
1a
1b
1c
1c
1c
1c
1c
1c
1c
1c
CH2Cl2
CH2Cl2
CH2Cl2
MeCN
CHCl3
toluene
THF
CH2Cl2
CH2Cl2
CH2Cl2
AcCl
AcCl
AcCl
AcCl
AcCl
AcCl
AcCl
TMSCl
MsOH
AcCl
93
95
92
89
79
52
95
74
a
Reaction conditions: allylic alcohol 2 (0.1 mmol), 1c (0.13 mmol),
AcCl (0.13 mmol) in CH2Cl2 (1 mL) stirred at 25 °C for 1−22 h
b
c
under Ar. Isolated yield. Reaction was conducted at 0 °C.
rearranged products were obtained with moderate to excellent
yields. Compared with 2a, the fluorine group at ortho-position of
phenyl ring obviously decreased the yield (67%, 3b). The
methyl group at different positions of phenyl ring has only a
small impact on the yield, affording nearly the same yields (3c−
e). Next, the electronic effect of substituents was tested. It was
found that the relatively neutral and electron-rich groups did not
evidently affect the yield (3f−h), whereas the strong electron-
poor group at para-position of aryl ring distinctly decreased the
yield (3i). It should be noted that the use of 4-chlorophenyl
substrate gave the product in 99% yield. Biphenyl substrate was
also suitable for this reaction (3j). To our satisfaction,
multisubstituted phenyl substrates were also well-tolerable in
this reaction, furnishing the desired products in moderate to
good yields (3k−n). In particular, unactivated alkene 2o was
also subjected to this transformation and successfully delivered
the corresponding product 3o in 76% yield.
c
10
trace
a
Reaction conditions: 2a (0.1 mmol), SCN source (0.13 mmol), acid
(0.13 mmol) in solvent (1 mL) stirred at 25 °C for 4−24 h under Ar.
Isolated yield. 0.1 equiv of AcCl was used.
b
c
screened with acetyl chloride (AcCl) as the acid promoter and
dichloromethane as the solvent at room temperature. To our
delight, 2a smoothly converted to the rearranged product 3a
with 93% and 95% yield, respectively, using 1b developed by
Chen group3o and 1c (entries 2−3), whereas no desired product
was obtained using 1a as the SCN reagent (entry 1). Hence, 1c
was identified as the suitable SCN reagent. Next, some other
solvents such as acetonitrile, chloroform, toluene, and
tetrahydrofuran were also tested; however, no better yield was
obtained (entries 4−7). It was found that using trimethyl-
chlorosilane also gave 95% yield, but the use of methanesulfonic
acid (MsOH) decreased the yield (entries 8−9). Finally,
decreasing the amount of AcCl to 0.1 equiv resulted in very
low conversion and only trace desired product was obtained
(entry 10). Therefore, the conditions of entries 3 or 8 were chose
as the optimal reaction conditions.
Subsequently, a variety of substrates with different migration
groups were examined under the standard conditions (Scheme
3). Substrate with diphenyls at cyclobutanol group proceeded
smoothly, giving the desired ring expanding product in 84%
yield, whereas oxa-cyclobutanol evidently decreased the yield
B
DOI: 10.1021/acs.orglett.9b03722
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Organic Letters
Letter
a b
,
a b
,
Scheme 3. Scope of Secondary and Tertiary Alcohols
Scheme 4. Scope of Trisubstituted Alkenes
a
Reaction conditions: trisubstituted alkene 2 (0.1 mmol), 1c (0.13
mmol), AcCl (0.13 mmol) in CH2Cl2 (1 mL) stirred at 25 °C for 2−
b
11 h under Ar. Isolated yield and dr values were determined by crude
c
d
1H NMR. TMSCl was used instead of AcCl. TMSCl and CH3CN
were used.
Scheme 5. Synthetic Applications
a
Reaction conditions: secondary and tertiary alcohol 2 (0.1 mmol),
1c (0.13 mmol), TMSCl (0.13 mmol) in CH2Cl2 (1 mL) stirred at 25
b
c
°C for 1−12 h under Ar. Isolated yield. AcCl was used instead of
TMSCl.
(3p, 3q). Nicely, cyclopentanol and 9-fluorenol substrates
worked well, forming the corresponding products with 76% and
65% yields, respectively (3r, 3s). To our delight, acyclic
secondary and tertiary alcohols were also accessible and
delivered the desired products in good to excellent yields (3t−
x). It is worth noting that some α-quaternary center β-thiocyano
aldehydes were obtained when secondary alcohols were used
(3v−x).
To further extend the scope of this tandem reaction, we also
examined some trisubstituted alkenes (Scheme 4). First, (Z)-1-
(1-phenylprop-1-en-1-yl)cyclobutan-1-ol 2y was used, which
proceeded successfully using TMSCl as the acid promotor and
furnished the product 3y in 85% yield with 8:1 dr value. Inspired
by this result, two indene-type alkene substrates were subjected
to the reaction (2z, 2aa). As expected, the corresponding
products were obtained with good yields and acceptable
diastereoselectivities. It was found that 1,2-dihydronaphthalene
and chromene substrates were tolerated affording the products
in moderate yields and diastereoselectivities (3ab−ac). The
relative configuration of 3ab (main product) was determined by
X-ray crystallography. Finally, unactivated cyclic trisubstituted
alkene 2ad was also tested, and unfortunately the desired
product 3ad was only obtained in 27% yield with 3:1 dr value.
To demonstrate the synthetic utility of this rearrangement
reaction, some conversions were further accomplished for the
synthesis of other useful organic sulfur compounds. As shown in
Scheme 5, a gram-scale reaction of 2a was first carried out using
TMSCl as the acid and the product 3a was readily obtained with
maintained yield. Next, treatment of 3a with inorganic base
Cs2CO3 in CH3CN produced cyclization product 4 in 64% yield,
which showed that the CN group is a good leaving group.
Similarly, further cyclization of compound 3ab was also
conducted, delivering the desired polycyclic thioether com-
pounds 5 in 90% yield, which is difficult to be obtained through
other synthetic method. Compound 3a could undergo
nucleophilic substitution to produce β-phenylthio ketone 6 in
moderate yield. Reaction with diphenylphosphine oxide in the
presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) resulted
in the construction of phosphonothioate 7 in 53% yield.
A catalytic asymmetric version of this rearrangement reaction
was also performed (Table 2). On the basis of Denmark and our
previous works on Lewis base catalyzed sulfenylation of
alkenes,13,12 chiral BINAM-derived selenide (S)-8a developed
by the Denmark group was first chosen as the catalyst. The acids
mentioned above were seriatim screened with 1b as the
thiocyano source. The use of MsOH (0.1 equiv) gave the best
results and produced the product 3a in 65% yield with 19% ee
(entries 1−3). With use of 1c instead of 1b in the presence of
MsOH, the enantioselectivity decreased to 9% ee. Although
some other conditions have been attempted, the results were
unsatisfactory.
On the basis of previous works and our results, a possible
mechanism for the thiocyano semipinacol rearrangement is
proposed in Scheme 6. The allylic alcohol 2a first reacts with 1c
to easily form a thiiranium ion intermediate 9 in the presence of
C
DOI: 10.1021/acs.orglett.9b03722
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Organic Letters
Letter
*E-mail: zhangye2997@163.com.
ORCID
Table 2. Preliminary Study on Catalytic Asymmetric Version
of This Reaction
a
Zhi-Min Chen: 0000-0002-6988-8955
Author Contributions
†X.-F.S. and A.-H.Y. contributed equally.
Notes
The authors declare no competing financial interest.
b
b
entry
acid
equiv
temp. (°C)
yield (%)
ee (%)
ACKNOWLEDGMENTS
1
2
3
MsOH
AcCl
TMSCl
MsOH
0.1
1.3
1.3
0.1
−10
0
0
65
65
73
64
19
11
1
■
This work was supported by the National Natural Science
Foundation of China (21871178, 21702135) and the
Innovation Fund from Joint Research Center for Precision
Medicine set up by Shanghai Jiao Tong University & Affiliated
Sixth People’s Hospital South Campus (IFPM2017B009). We
gratefully thank Prof. Yong-Qiang Tu (Shanghai Jiao Tong
University) for helpful suggestions and comments on this
manuscript.
c
4
−10
9
a
Reaction conditions: 2a (0.1 mmol), 1b (0.13 mmol), (S)-8a (0.01
mmol), and acid in CH2Cl2 (1 mL) stirred for 2−24 h under Ar.
Isolated yield and ee values were determined by chiral HPLC. 1c
b
c
was used instead of 1b.
Scheme 6. Proposed Mechanism for Thiocyano Semipinacol
Rearrangement of Allylic Alcohols
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b03722.
Experimental details, X-ray crystallography structures of
compounds 3ab and 5, analytical data for new
compounds, NMR spectra of new compounds (PDF)
Accession Codes
CCDC 1956168 and 1959408 contain 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
e-mailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: chenzhimin221@sjtu.edu.cn.
D
DOI: 10.1021/acs.orglett.9b03722
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