Enantioselective and Diastereoselective Conjugate Radical Additions to α-Arylidene Ketones and Lactones

Article abstract of DOI:10.1055/s-0036-1590930

A highly stereoselective conjugate radical addition to arylidene ketones and lactones has been developed. The conjugate radical additions using chiral salen Lewis acids proceeds with up to 99:1 dr and 87% ee in good to excellent chemical yields.

Full text of DOI:10.1055/s-0036-1590930

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© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–E
letter
A
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C. Zhao, M. P. Sibi
Letter
Synlett
Enantioselective and Diastereoselective Conjugate Radical Addi-
tions to α-Arylidene Ketones and Lactones
Changjia Zhao1
Mukund P. Sibi*
N
O
N
O
Department of Chemistry and Biochemistry, North Dakota
State University, P. O. Box 2735, Fargo, ND, 58108, USA
Mukund.Sibi@ndsu.edu
Al
O
t-Bu
t-Bu
O
R
OTf
t-Bu
t-Bu
Dedicated to Prof. Victor Snieckus on his 80th birthday
X
Ar
X
Ar
RX, Bu3SnH, CH2Cl2, –78 °C
Et3B/O2
X = CH2 or O
43–96% yield
up to 99:1 dr
up to 87% ee
Received: 14.08.2017
Accepted after revision: 12.09.2017
Published online: 20.10.2017
tional groups with chiral activators. Reactions with
monodentate starting materials are attractive in that they
avoid the need for the attachment and detachment of achi-
ral templates or additional manipulations required to arrive
at carbonyl or carboxylic acid functionalities. In this work,
we demonstrate that nucleophilic radical addition to α-
arylidene ketones and lactones mediated by chiral salen
Lewis acids9 proceeds in good yields, high diastereoselectiv-
ity, and good to high enantioselectivity while establishing
two contiguous chiral centers.10
DOI: 10.1055/s-0036-1590930; Art ID: st-2017-r0625-l
Abstract A highly stereoselective conjugate radical addition to
arylidene ketones and lactones has been developed. The conjugate rad-
ical additions using chiral salen Lewis acids proceeds with up to 99:1 dr
and 87% ee in good to excellent chemical yields.
Key words enantioselective radical reaction, arylidene ketones and
lactones, diastereoselectivity, conjugate addition, chiral salen Lewis
acid
Our work began with finding optimal conditions for nu-
cleophilic radical addition to enones11 under chiral Lewis
acid activation. The following conditions were used for ini-
tial Lewis acid screening. α-Benzylidene cyclopentanone
(1) was used as the test substrate, and the reactions were
carried out using isopropyl iodide (4 equiv), tributyltin hy-
dride (3 equiv) in methylenechloride at –78 °C. Triethylbo-
rane/oxygen was used for radical initiation.12 A variety of
chiral Lewis acids were screened (see Table S1 for data)
with variable success.13 We14 and others15 have previously
shown that chiral aluminum Lewis acids are excellent acti-
vators for monodentate substrates. We set out to evaluate
readily available and inexpensive chiral salen Lewis acids.
Data from these investigations are shown in Table 1. The re-
action using the commercially available aluminum salen
catalyst 3 gave the conjugate addition product 2 in high
yield and diastereoselectivity (Table 1, entry 1). The ee for
the product was good. Increasing the catalyst loading did
not lead to improvement in ee for the product (Table 1, en-
tries 2 and 3). Replacing the chloride counterion in 3 with a
triflate (4) showed a small improvement in ee while main-
taining the diastereoselectivity for the reaction (Table 1,
compare entry 1 with 4). Increasing the catalyst loading of 4
did not lead to improvement in selectivity (Table 1, entries
5 and 6). Further changes to the counterion13 (catalysts 5
and 6) did not lead to any improvement in reaction efficien-
Development of new methods for the installation of
contiguous chiral centers with control over relative as well
as absolute stereochemistry is sought after in synthetic or-
ganic chemistry.2 In general, neutral reactions such as cy-
cloadditions or ionic reactions are often the methods of
choice for such processes. In the last two decades, reactions
with radical intermediates have reached prominence for
the development of enantioselective processes.3 Taking les-
sons from stereoselective ionic and cycloaddition methods,
initial development of enantioselective radical reactions
utilized chiral Lewis acids.4 Most of the substrates used in
these studies provided a well-organized chelate with the
chiral Lewis acid for controlling face selectivity.5 In the last
decade, radical processes with organocatalysts have made
spectacular advances for a variety of C–C and C–X bond-
forming reactions.6 In this regard, reactions with photore-
dox reagents have overcome certain limitations associated
with radical reactions such as the use of toxic tin reagents
and the need for large excess of reagents.7
In contrast to many reports on enantioselective radical
reactions with bidentate substrates, reactions with
monodentate substrates such as aldehydes, ketones, or es-
ters have received less scrutiny.8 This is partly due to the
difficulty in discriminating the binding mode of these func-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

B
C. Zhao, M. P. Sibi
Letter
Synlett
cy (Table 1, entries 7 and 8). Use of chromium salen Lewis
acids (7 and 8) gave modest yields of the product with low
ee values (Table 1, entries 9 and 10).
with a more electron-rich p-methoxyphenyl group reduced
the efficiency of the reaction but with little impact on ste-
reoselectivity (Table 2, compare entry 1 with 2).
Table 1 Evaluation of Different Chiral Lewis Acids in Conjugate Addi-
tion of Isopropyl Radical to α-Benzylidene Cyclopentanone.
Table 2 Conjugate Radical Addition to α-Arylidene Cyclopentanones
O
O
O
O
CLA 4 (30 mol%), Et3B/O2
Ar
Ar
CLA (mol%), Et3B/O2
i-PrI (5 equiv), CH2Cl2
Bu3SnH (3 equiv), –78 °C
i-PrI (5 equiv), CH2Cl2
Bu3SnH (3 equiv), –78 °C
2
1
Entry SM
Product
Yield drb
(%)a
ee
(%)c
Ph
N
Ph
N
1
2
3
4
1 Ar = Ph
2 Ar = Ph
81
63
94
75
99:1 75
99:1 70
99:1 66
99:1 79
N
O
N
O
12 Ar = 4-MeOC6H4
14 Ar = 1-naphthyl
16 Ar = 2-naphthyl
13 Ar = 4-MeOC6H4
15 Ar = 1-naphthyl
17 Ar = 2-naphthyl
Al
X
M
X
t-Bu
t-Bu
O
O
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
a Isolated yield after column chromatography.
b Determined by NMR spectroscopy.
c Determined by chiral HPLC analysis.
9
M = Al, X = Cl
3 M = Al, X = Cl
4 M = Al. X = OTf
5 M = Al, X = NTf2
6 M = Al, X = SbF6
7 M = Cr, X = Cl
8 M = Cr, X = OTf
10 M = Al, X = OTf
11 M = Al, X = SbF6
Reactions with naphthyl-substituted enones were also
efficient and had a slight improvement in ee for the 2-
naphthyl group (Table 2, entries 3 and 4). The above results
demonstrate that various α-arylidene cyclopentanones are
competent substrates in conjugate radial addition providing
the products in good yields and selectivities.17
The effect of ring size on stereoselectivity of isopropyl
radical addition was assessed next using an aryldiene cyclo-
hexanone as a substrate. These results are shown in Table 3.
Isopropyl radical addition to α-benzylidene cyclohexanone
(18) using catalyst 4 gave the addition product in good yield
and selectivity (Table 3, entry 1). The ee for the product us-
ing six-membered cyclohexanone as a substrate was similar
to that observed for the five-membered analogue (compare
entry 1 in Tables 2 and 3). Changing the arylidene substitu-
ent from a phenyl to either a p-methoxyphenyl (20) or a
naphthyl group (22) had minimal impact on the overall ef-
ficiency of the conjugate addition (Table 3, entries 2 and 3).
Entry
CLA (mol%)
Yield (%)a
drb
99:1
ee (%)c
1
2
3 (30)
3 (50)
3 (100)
4 (30)
4 (70)
4 (100)
5 (30)
6 (30)
7 (30)
8 (30)
9 (30)
10 (30)
11 (30)
96
92
85
81
90
88
85
89
60
60
89
93
91
73
70
73
75
70
70
70
73
20
5
99:1
99:1
99:1
99:1
99:1
99:1
99:1
95:5
92:8
99:1
99:1
99:1
3
4
5
6
7
8
9
10
11
12
13
36
31
46
a Isolated yield after column purification.
b Determined by NMR spectroscopy.
c Determined by chiral HPLC analysis.
Table 3 Conjugate Radical Addition to α-Arylidene Cyclohexanones
O
O
CLA 4 (30 mol%), Et3B/O2
Catalysts 9–11 gave the product in high yields but low
ee values (Table 1, entries 11–13). From these studies, we
identified the use of chiral Lewis acids derived from
(1R,2R)-(–)-1,2-diaminocyclohexane, commercially avail-
able catalyst 4 (30 mol%) as the optimal catalyst for the con-
jugate additions.16
We next investigated isopropyl radical addition to
arylidene cyclopentanones to assess the impact of aryl sub-
stituents on reactivity and selectivity using salen catalyst 4.
This data is tabulated in Table 2. Replacing a phenyl group
Ar
Ar
i-PrI (5 equiv), CH2Cl2
Bu3SnH (3 equiv), –78 °C
Entry SM
Product
Yield drb
(%)a
ee
(%)c
1
2
3
18 Ar = Ph
19 Ar = Ph
77
91
75
93:7
98:2
97:3
73
74
79
20 Ar = 4-MeOC6H4
22 Ar = 2-naphthyl
21 Ar = 4-MeOC6H4
23 Ar = 2-naphthyl
a Isolated yield after column chromatography.
b Determined by NMR spectroscopy.
c Determined by chiral HPLC analysis.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

C
C. Zhao, M. P. Sibi
Letter
Synlett
The above results demonstrate that conjugate radical addi-
tion to arylidene cyclopentanones and -hexanones pro-
ceeds with good efficiency. Furthermore, ring size or the
nature of the aryl substituent has minimal impact on stere-
oselectivity of the conjugate addition products.
ophilic tertiary radical to 1 was also effective and gave the
product in good yield and selectivity (Table 4, entry 6). Re-
action of 1 with a functionalized tertiary radical (chloro
bromoalkane) was also successful furnishing product 28 in
good yield and a slightly lower ee compared to reaction
with tert-butyl radical (Table 4, entry 7). The addition of 4-
pentenyl radical to 1 was also evaluated (Table 4, entry 8).
The addition of the less nucleophilic primary radical gave
the product in low yield but good stereoselectivity. A prod-
uct from a 5-exo cyclization of the intermediate α-carbonyl
radical was not observed.
Conjugate addition of isopropyl radical to five- and six-
membered benzylidene lactones were investigated next us-
ing catalyst 4. These results are shown in Table 5. Isopropyl
radical addition to 30 (n = 1) using 30 mol% of 4 gave prod-
uct 31 is modest yield and good selectivity (Table 5, entry
1). Increasing the catalyst loading to 100% led to higher
yield of the product with a very slight improvement in ee
(Table 5, compare entry 2 with 1). Reaction with the six-
membered lactone 32 was also effective and gave the prod-
uct in good yield and selectivity (Table 5, entries 3 and 4).
These results show that arylidene lactones are also compe-
tent substrates in enantioselective conjugate radical addi-
tions.
The scope of the chiral-salen-mediated enantioselective
conjugate radical addition to enones (1 and 18) was as-
sessed next using various radicals of differing nucleophilici-
ties. These results are shown in Table 4. As discussed earlier,
isopropyl radical addition to five- and six-membered
enones using 4 as a catalyst proceed with good efficiency
(Table 4, entries 1 and 2). Reactions with cyclic secondary
radicals were investigated next. Cyclopentyl radical addi-
tion to both benzylidene cyclopentanone and -hexanone
proceeded with good efficiency (Table 4, entries 3 and 4).
The products were formed with excellent diastereoselectiv-
ity and the highest enantioselectivities observed in the
present study. Cyclohexyl radical was also effective in the
conjugate addition and gave product 26 in good yield and
selectivity (Table 4, entry 5) similar to that observed for re-
action with cyclopentyl radical. Addition of the more nucle-
Table 4 Conjugate Addition of Different Radicals to α-Benzylidene Cy-
cloalkenones
O
O
R
Table 5 Conjugate Radical Addition to α-Benzylidene Lactones
CLA 4 (30 mol%), Et3B/O2
( )n
( )n
RX (5 equiv), CH2Cl2
O
O
Bu3SnH (3 equiv), –78 °C
CLA 4, Et3B/O2
1
n = 1
2, 19, 24–29
O
O
18 n = 2
i-PrI, Bu3SnH
( )n
( )n
CH2Cl2, –78 °C
30 n = 1
32 n = 2
31 n = 1
33 n = 2
Entry SM
RX
Product Yield (%)a drb
ee (%)c
75
I
I
CLA 4 (mol%) Yield (%)a
drb
ee (%)c
1
1
2
81
99:1
Entry
SM
1
2
3
4
30
30
32
32
30
100
30
43
66
58
66
99:1
73
78
75
75
2
3
4
18
1
19
24
25
77
93
68
93:7
99:1
99:1
73
85
87
99:1
99:1
99:1
I
100
a Isolated yield after column chromatography.
b Determined by NMR spectroscopy.
c Determined by chiral HPLC analysis.
I
18
I
5
6
1
1
26
27
75
89
99:1
99:1
83
81
In this work, we have demonstrated that enantioselec-
tive conjugate radical addition to α-arylidene ketones and
lactones proceed in good yields, very high diastereoselectiv-
ities, and good to high enantioselectivities.18,19 The method-
ology shows good scope for the acceptors as well as the nu-
cleophilic radicals. The methodology also showcases the
use of readily available and cheap chiral salen Lewis acids as
activators for monodentate substrates. The use of chiral
salen Lewis acids in other transformations of significance
are underway in our laboratory.
I
Cl
I
7
8
1
1
28
29
48
48
99:1
72
82
Br
65:35
a Isolated yield after column chromatography.
b Determined by NMR spectroscopy.
c Determined by chiral HPLC analysis.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

D
C. Zhao, M. P. Sibi
Letter
Synlett
Funding Information
(d) Vallavoju, N.; Selvakumar, S.; Jockusch, S.; Sibi, M. P.;
Sivaguru, J. Angew. Chem. Int. Ed. 2014, 53, 5604. (e) Rono, L. J.;
Yayla, H. G.; Wang, D. Y.; Armstrong, M. F.; Knowles, R. R. J. Am.
Chem. Soc. 2013, 135, 17735. (f) Nishida, M.; Hayashi, H.;
Nishida, A.; Kawahara, N. Chem. Commun. 1996, 579. (g) Du, J.;
Skubi, K. L.; Schultz, D. M.; Yoon, T. P. Science 2014, 344, 392.
(h) Lee, S.; Kim, S. Tetrahedron Lett. 2009, 50, 3345. (i) Jang, D.
O.; Kim, S. Y. J. Am. Chem. Soc. 2008, 130, 16152.
This research was partially supported by funds from NIH RO1-54656.
Secinces
N
oaitItGnsueoaitlfe
n
Meraledcai
l
5(4
6
56)
Acknowledgment
We thank North Dakota State University for their support. We thank
Hari Subramanian for technical assistance.
(9) For selected recent reviews, see: (a) Park, J.; Hong, S. Chem. Soc.
Rev. 2012, 41, 6931. (b) Shaw, S.; White, J. D. Synthesis 2016, 48,
2768. (c) Also see: Bandini, M.; Fagioli, M.; Garavelli, M.;
Melloni, A.; Trigari, V.; Umani-Ronchi, A. J. Org. Chem. 2004, 69,
7511. (d) Wu, S.; Tang, J.; Han, J.; Mao, D.; Liu, X.; Gao, X.; Yu, J.;
Wang, L. Tetrahedron 2014, 70, 5986.
Supporting Information
Supporting information for this article is available online at
(10) For the installation of contiguous chiral centers in radical reac-
tions, see: (a) Sibi, M. P.; Chen, J. J. Am. Chem. Soc. 2001, 123,
9472. (b) Sibi, M. P.; Petrovic, G.; Zimmerman, J. J. Am. Chem.
Soc. 2005, 127, 2390. (c) He, L.; Srikanth, G. S. C.; Castle, S. L.
J. Org. Chem. 2005, 70, 8140. (d) Banerjee, B.; Capps, S. G.; Kang,
J.; Robinson, J. W.; Castle, S. L. J. Org. Chem. 2008, 73, 8973.
(e) Lee, J. Y.; Kim, S.; Kim, S. Tetrahedron Lett. 2010, 51, 4947.
(f) Graham, T. H.; Jones, C. M.; Jui, N. T.; MacMillan, D. W. C.
J. Am. Chem. Soc. 2008, 130, 16494.
https://doi.org/10.1055/s-0036-1590930.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
ortioInfgrmoaitn
References and Notes
(1) Beijing Pharmin Technology Company Limited, Room 202, Unit
2, Huilongsen Park, No 99, Kechuang 14th Street, Beijing Eco-
nomic-Technological Development Area, Beijing, 101111, P. R. of
China.
(11) All the starting enones and lactones used in this study are
known in the literature, see Supporting Information.
(2) (a) Asymmetric Synthesis II: The Essentials, 2nd ed.; Christmann,
M.; Bräse, S., Eds.; Wiley-VCH: Weinheim, 2007. (b) Asymmetric
Synthesis II, 2nd ed; Christmann, M.; Bräse, S., Eds.; Wiley-VCH:
Weinheim, 2012. (c) Fundamentals of Asymmetric Catalysis;
Walsh, P. J.; Kozlowski, M. C., Eds.; University Science Books:
Sausalito, CA, 2009.
(12) (a) Yorimitsu, H.; Oshima, K. In Radicals in Organic Synthesis;
Renaud, P.; Sibi, M. P., Eds.; Wiley-VCH: Weinheim, 2001, 11.
(b) Renaud, P. In Encyclopedia of Radicals in Chemistry, Biology
1
V
o.l
and Materials;
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Chatgilialoglu, C.; Studer, A., Eds.; Wiley: New
York, NY, 2012, 601. (c) Curran, D. P.; McFadden, T. R. J. Am.
Chem. Soc. 2016, 138, 7741.
(3) (a) Renaud, P.; Sibi, M. P. Radicals in Organic Synthesis;a1nV2od.
l
Wiley-
VCH: Weinheim, 2001. (b) Stereochemistry of Radical Reactions;
Curran, D. P.; Porter, N. A.; Giese, B., Eds.; VCH: Weinheim,
1995. (c) Encyclopedia of Radicals in Chemistry, Biology and
Materials; Chatgilialoglu, C.; Studer, A., Eds.; Wiley-VCH: Wein-
heim, 2012.
(13) A variety of 3+ and 2+ chiral Lewis acids were screened with
modest success; see Supporting Information for details.
(14) Sibi, M. P.; Nad, S. Angew. Chem. Int. Ed. 2007, 46, 9231.
(15) Urabe, H.; Yamashita, K.; Suzuki, K.; Kobayashi, K.; Sato, F. J. Org.
Chem. 1995, 60, 3576.
(4) (a) Srikanth, G. S. C.; Castle, S. L. Tetrahedron 2005, 61, 10377.
(b) Zimmerman, J.; Sibi, M. P. Top. Curr. Chem. 2006, 263, 107.
(c) Sibi, M. P.; Manyem, S. Tetrahedron 2000, 56, 8033. (d) Sibi,
M. P.; Porter, N. A. Acc. Chem. Res. 1999, 32, 163. (e) Sibi, M. P.;
Manyem, S.; Zimmerman, J. Chem. Rev. 2003, 103, 3263.
(5) For selected examples, see: (a) Sibi, M. P.; Ji, J. G.; Wu, J. H.;
Gürtler, S.; Porter, N. A. J. Am. Chem. Soc. 1996, 118, 9200.
(b) Sibi, M. P.; Ji, J. G. J. Org. Chem. 1997, 62, 3800. (c) Sibi, M. P.;
Zimmerman, J. J. Am. Chem. Soc. 2006, 128, 13346. (d) Ruiz
Espelt, L.; McPherson, I. S.; Wiensch, E. M.; Yoon, T. P. J. Am.
Chem. Soc. 2015, 137, 2452. (e) Zhang, L.; Meggers, E. Acc. Chem.
Res. 2017, 50, 320. (f) Sibi, M. P.; Nie, X.; Shackleford, J. P.;
Stanley, L. M.; Bouret, F. Synlett 2008, 2655.
(6) (a) Asymmetric Organocatalysis: From Biomimetic Concepts to
Applications in Asymmetric Synthesis; Berkessel, A.; Gröger, H.,
Eds.; Wiley-VCH: Weinheim, 2005. (b) Enantioselective Organo-
catalysis: Reactions and Experimental Procedures; Dalko, P. I.,
Ed.; Wiley-VCH: Weinheim, 2007. (c) Prier, C. K.; Rankic, D. A.;
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Lang, S. B.; Heitz, D. R.; Molander, G. A. ACS Catal. 2017, 7, 2563.
(7) Shaw, M. H.; Twilton, J.; MacMillan, D. W. C. J. Org. Chem. 2016,
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(16) For an example of conjugate addition of achiral copper reagent
to α-alkylidene cyclopentanone see: Borner, C.; Dennis, M. R.;
Sinn, E.; Woodward, S. Eur. J. Org. Chem. 2001, 2435.
(17) We have not established the relative stereochemistry of the
major isomer. As noted in ref. 14, we speculate that the newly
formed chiral center controls the stereochemistry of the H-atom-
transfer step.
(18) For stereochemical models of single-point binding substrates to
chiral salens, see: (a) Ref. 14. (b) Sibi, M. P.; Zimmerman, J. J. Am.
Chem. Soc. 2006, 128, 13346. (c) Hutson, G. E.; Turkman, Y. E.;
Rawal, V. H. J. Am. Chem. Soc. 2013, 135, 4988.
(19) Representative Procedure for Radical Addition
To a 6 dram vial were added substrate (0.3 mmol) and Lewis
acid 4 (0.09 mmol, 30 mol%). The vial was sealed with a septum,
the air was removed from the vial via a vacuum pump, nitrogen
was charged. To the mixture was then charged solvent (CH2Cl2,
8 mL), the mixture was stirred at r.t. for 20 min. Then the
mixture was cooled to –78 °C, radical precursor (1.5 mmol, 5
equiv), triethylborane solution (1.0 M in hexane, 1.2 mL, 4
equiv), tributyltin hydride (0.24 mL, 0.9 mmol, 3 equiv), and
oxygen gas (10 mL) were added successively via syringe. The
reaction was stirred at –78 °C for 2–3 h until TLC analysis indi-
cated disappearance of starting material. To the mixture was
added silica gel (3.6 g), the solvent was removed under reduced
pressure, the residue was first washed with hexanes (100 mL),
(8) For selected examples, see: (a) Hepburn, H. B.; Melchiorre, P.
Chem. Commun. 2016, 52, 3520. (b) Uraguchi, D.; Kinoshita, N.;
Kizu, T.; Ooi, T. J. Am. Chem. Soc. 2015, 137, 13768. (c) Guo, H.;
Herdweck, E.; Bach, T. Angew. Chem. Int. Ed. 2010, 49, 7782.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

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C. Zhao, M. P. Sibi
Letter
Synlett
then Et2O (100 mL). To the ether solution was added silica gel
(1.8 g), the solvent was removed under reduced pressure. The
residue was subjected to flash chromatography using hex-
ane/EtOAc (9:1) as eluent to afford the conjugate addition prod-
uct.
2-(2-Methyl-1-phenylpropyl)-cyclopentanone (2)
88 mg (0.5 mmol), yield 81%, dr 99:1, ee 75% by HPLC (210 nm,
25 °C, OJ-H column Chiralcel, 0.46cm × 25 cm, from Daicel
Chemical Ind., Ltd., 1% i-PrOH/hexanes, 0.5 mpm, tR (major) =
18.5 min; tR (minor) = 14.4 min). [α]D +71.3 (c 0.3, CH2Cl2). 1H
NMR (400 MHz, CDCl3): δ = 0.65 (d, J = 6.4 Hz, 3 H), 0.95 (t, J =
6.8 Hz, 3 H), 1.56–2.46 (m, 8 H), 2.61 (dd, J = 10.4, 3.6 Hz, 1 H),
7.11–7.25 (m, 5 H). 13C NMR (100 MHz, CDCl3): δ = 20.9, 22.0,
22.1, 26.9, 29.7, 39.4, 52.7, 53.8, 126.4, 128.5, 128.8, 144.2,
220.3. IR: 3062, 3029, 2960, 2894, 2871, 1733, 1493, 1470,
1407, 1385, 1273, 1152 cm–1. HRMS: m/z calcd for C15H20ONa+:
239.1406; found: 239.1400.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E