Electronically unfavorable addition of electron-deficient olefins to isochromenylium tetrafluoroborates initiated by C-1 O-glycosylation

Article abstract of DOI:10.1016/j.tetlet.2012.03.106

Electronically unfavorable reactions between isochromenylium tetrafluoroborates and electron-deficient olefins have been studied and achieved by assistance of a phenolic hydroxyl group in the olefin substrates, providing the corresponding 2-oxabicyclo[3.3.1]nonane derivatives diastereoselectively in moderate to satisfactory yields. The new methodology is initiated by an intermolecular C-1 O-glycosylation, and completed with an intramolecular Michael addition and an aldol condensation in cascade fashion.

Full text of DOI:10.1016/j.tetlet.2012.03.106

Tetrahedron Letters 53 (2012) 2765–2768
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Tetrahedron Letters
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Electronically unfavorable addition of electron-deficient olefins to
isochromenylium tetrafluoroborates initiated by C-1 O-glycosylation
⇑
⇑
Shu-Yan Yu, Zhi-Long Hu, Hao Zhang, Shaozhong Wang , Zhu-Jun Yao
State Key Laboratory of Coordination Chemistry and Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing,
Jiangsu 210093, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Electronically unfavorable reactions between isochromenylium tetrafluoroborates and electron-deficient
olefins have been studied and achieved by assistance of a phenolic hydroxyl group in the olefin sub-
strates, providing the corresponding 2-oxabicyclo[3.3.1]nonane derivatives diastereoselectively in mod-
erate to satisfactory yields. The new methodology is initiated by an intermolecular C-1 O-glycosylation,
and completed with an intramolecular Michael addition and an aldol condensation in cascade fashion.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 12 January 2012
Revised 15 March 2012
Accepted 27 March 2012
Available online 30 March 2012
Keywords:
Isochromenylium tetrafluoroborate
Michael addition
Aldol condensation
Cascade reaction
Diastereoselectivity
Isochromenylium tetrafluoroborates (ICTBs) are a new type of
air-stable organic salts in the solid state, while they exhibit high
reactivity when exposure to the electron-rich substrates in the
solutions without any assistance of catalysts or promoters.1,2 Our
recent investigations indicated that these organic salts were prone
to undergo diverse cascade transformations under mild metal-free
conditions,2–5 showing discriminated chemical behaviors from
those corresponding organometallic intermediates in situ pro-
duced in those metal-catalyzed or promoted reactions of o-alky-
nylbenzaldehydes.6 For example, ICTBs were able to smoothly
react with the nucleophiles at the C-1 position,3 and with the elec-
tron-rich olefins through [4+2]-cycloaddition pathways under mild
metal-free conditions.4 Three-component reactions of ICTBs with
enol (in situ converted from aliphatic aldehyde), and aldehyde or
nitriles were also discovered by us recently, providing a variety
of hydroxyl- and amino-substituted dihydronaphthalene and
tetrahydronaphthalene derivatives diastereoselectively.5
Diels–Alder (DA) cycloaddition is one of the most powerful
[4+2]-methodologies in organic synthesis. Both normal DA reac-
tions (between electron-rich diene and electron-deficient dieno-
phile)7 and inverse electron-demand DA reactions (between
electron-deficient diene and electron-rich dienophile)8 require
molecular orbital matching between the dienes and the dieno-
philes. To the best of our knowledge, all the reported Diels–Alder
cycloadditions involving isochromenylium intermediates take
place with the electron-rich olefins.2–6 ICTBs therefore can be re-
garded as an electron-deficient non-classical oxa-diene equivalent.
Theoretically, they should be chemically inert to those electron-
deficient olefins in the [4+2]-cycloaddition. This was verified in
our examinations (Fig. 1a). Either at room temperature or under
higher-temperature conditions, ICTBs 1 were unable to react with
the electron-deficient olefins 2. However, realization of such
‘impossible’ [4+2]-additions is not only important in expanding
the reaction scope of ICTBs, but also is of value for the convenient
synthesis of a number of bioactive natural products and drugs9–11
having a functionalized dihydronaphthalene and tetrahydronaph-
thalene core (Fig. 1b). Herein, we wish to report our study on the
unfavorable [4+2]-addition between ICTBs and electron-deficient
olefins.
In order to achieve such an electronically unfavorable [4+2]-
addition, proper manipulation of the reaction pathway is needed.
As indicated in our previous work,3 the C-1 position of ICTBs 1
allows quick O-, N-, or C-glycosylation12 under mild conditions.
This reaction property of ICTBs hinted us that a possible multistep
cascade process, instead of the direct [4+2]-cycloaddition, might
resolve the above problem ( Fig. 2). In other words, a formal
[4+2]-adduct could be generated by an alternative multistep cas-
cade process. Following such an idea, we proposed that the Hückel
aromaticity of ICTB could be destroyed with an intermolecular
O- or N-acetalization at its C-1 position. Such initiation would pro-
vide an opportunity for a further intramolecular Michael addition
under acid-catalyzed conditions because the remaining C–C double
⇑
Corresponding authors. Tel./fax: +86 25 8359 3732.
E-mail addresses: wangsz@nju.edu.cn (S. Wang), yaoz@nju.edu.cn, yaoz@
mail.sioc.ac.cn (Z.-J. Yao).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.03.106

2766
S.-Y. Yu et al. / Tetrahedron Letters 53 (2012) 2765–2768
produced in the initial N-acetalization might block the further pro-
cess by protonating the aniline nitrogen. To avoid such a problem,
an ortho-phenolic hydroxyl group (Fig. 2, olefin 3, X = O) was con-
sidered to be introduced into the olefin substrate as the stimulat-
ing group (Table 1). To our delight, olefin 3a could smoothly
react with ICTB 1a in acetonitrile at room temperature, affording
2-oxabicyclo[3.3.1]nonane 4aa in 12 h (69% yield, dr >20:1) (Table
1, entry 1). Raising the temperature could shorten the reaction
time but lower the diastereoselectivity (entries 2–3). A parallel
reaction of ICTB 1b also gave satisfactory results (entry 4).
Using the above optimized conditions, a number of additional
electron-deficient olefins 3b–3q were examined (Fig. 3 and Table
2). Most reactions could complete at room temperature affording
multiring or bicyclo-products 4 in moderate to satisfying yields,
and some of them exhibited good to excellent diastereoselectivi-
ties. These reactions also could tolerate a variety of electron-with-
drawing groups, including ester, nitro, ketone, and cyano groups.
Again, alternative of ICTB 1a with 1b did not significantly affect
O
R
MeCN, THF,
O
R
toluene, or DCE
R1
O
O
R1
R2
R2
(2.0 equiv)
+
BF4
O
O
rt to 80 °C
1a
(R = Ph)
2a (R1 = AcO(CH2)2; R2 = CO2Me)
2b
(R1 = Me; R2 = CO2Me)
1b
(R = n-C6H13
)
expected product(s)
2c (R1 = Me; R2 = CN)
(R1 = Ph; R2 = NO2)
2d
HO
O
O
O
O
CH3
CH3
N
H
N
H
H
O
CH3
OH
O
N
OH
O
O
O
Chelidonine
Corynoline
Lasofoxifene
Figure 1. (a) Examination of the direct reaction between ICTBs 1 and electron-
deficient olefins 2a–2d; and (b) several representative natural and unnatural
bioactive products bearing 2-phenyl-1,2,3,4-tetrahydronaphthalene core.
Reactants
R1
O
R
O
O
R
Electron-rich
R2
3d
R1
O
O
O
R
HX
R1
BF4
O
- HBF4
OH
4
+
(R1 = COCH3, R2=H)
O
1
O
O
1a
(R = Ph)
O
BF4
1b
3e (R1 = CO2Et, R2 = H)
(R = nC6H13
)
X
Electron-poor
3f (R1 = CO2Me, R2 = 5-Cl)
3g (R1 = CO2Me, R2= 3,5-di-t-Bu)
2e (X = NH; R1 = Ph)
O
1a
(R = Ph)
3
(X = O)
- HBF4
R1
(R1 = CO2Me, R2 = 4-OH)
3h
3i
(R1 = CN, R2 = 3,5-di-t-Bu)
O
R
OH
R1
O
O
O
O
R
(R1 = p-CH3OC6H4)
3b
O
O
R1
CO2Me
3c (R2 = 2-furyl)
N
O
O
R2
OH
OH
OH
R1
X
OH
+X
3j
R2
R3
A1
(R = (R)-Bn, R2 = H)
1
B1
3m
HO
R1
3n (R1 = (S)-Bn, R2 = H)
3o (R1 = (S)-Ph, R2 = H)
(R1 = H, R2 = Me, R3 = CO2Et)
3k
3p (R1 = (S)-t-Bu, R2 = H)
Figure 2. A proposed multistep cascade process for the reaction between ICTB 1a
and enone 2e or 3.
3l (R1 = CH3, R2 = H, R3 = CO2Et)
(R1 = (S)-t-Bu, R2 = 5-Br)
3q
Products
Table 1
R
O
R
O
O
Optimization of reaction conditions of ICTBs 1 with olefin 3aa
R
O
O
R1
O
O
R1
O
NO2
R
O
O
O
O
O
O
R1
O
O
R
R2
O
OH
3a
O2N
O
O
O
O
BF4
conditions
4d
4ea 4eb
, ,
(X-ray),
see Table 1
4b (X-ray), 4c
4b'
4c'
(X-ray),
4aa (R = Ph)
4f 4g 4h 4i
,
,
,
1a
(R = Ph)
4ab (R = n-C6H13
)
1b (R = n-C6H13
)
O
Ph
O
Ph
Entry
1
Solvent
T (°C)
Time
4
Yieldb (%)
R1
O
O
O
R2
R3
O
1
2
3
4
1a
1a
1a
1b
MeCN
MeCN
MeCN
MeCN
25
50
80
25
12 h
3 h
20 min
12 h
4aa
4aa
4aa
4ab
69 (dr >20:1)
66 (dr 14:1)
71 (dr 11:1)
63 (dr >20:1)
O
O
O
O
4j
4k, 4l
a
Reaction conditions: 1 (0.1 mmol) and 3a (0.12 mmol) in CH3CN (5 mL) under
O
O
Ph
nitrogen atmosphere.
Ph
O
b
Isolated yields; drs were determined by 1H NMR methods.
O
O
O
R2
R1
R2
O
O
R1
O
O
bond (enol ether) of intermediate A1 is electron-rich. After forma-
tion of the new C–C bond, a subsequent intramolecular Mannich or
aldol condensation would complete the final ring-closure.
Enone 2e (Fig. 2, X = NH) was initially designed and examined as
the electron-deficient olefin substrate. Unfortunately, the reaction
did not provide any desired product. We speculated that HBF4
N
N
O
O
O
O
,
4m 4n 4o 4p 4q
,
,
,
4m' 4n' 4o' 4p' 4q'
, , , , (X-ray)
Figure 3. Reactants and products appeared in this work (see Table 2 for the details,
and see Table 1 and Scheme 1 for additional examples).

S.-Y. Yu et al. / Tetrahedron Letters 53 (2012) 2765–2768
2767
O
Ph
the reaction behaviors with the same olefin (entries 4 and 5), and
a
Ph
variation of reaction temperature was observed to affect the rate
and the diastereoselectivity of the reaction (entries 1 and 2). The
reactions with tri-substituted olefins 3k and 3l (entries 11 and
12) needed higher temperatures and longer reaction times, giving
corresponding bridged-ring products 4k and 4l bearing a newly
born quaternary carbon, respectively. Unfortunately, this reaction
did not take place with the tetra-substituted electron-deficient ole-
fins. This might be caused by steric effects in the transition states
and higher energy barrier to form two contiguous quaternary car-
bons. The relative configurations of products 4b, 4b0, 4d, and 4q0
were determined by the corresponding 1D and 2D NMR experi-
O
O
Ph
O
1a
O
NH
OH
MeCN, 80 °C
5h, 73%
NH
O
3r
4r'
O
Ph
O
O
Ph
O
NH
OH
4r
Table 2
Results of the additions of olefins 3 to ICTBs 1
O
b
Ph
O
Entry
1
3
T (°C)
Time (h)
4
Yielda (%)
Br
CO2Me
1b
2
1a
1a
1a
1a
1b
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
3b
3c
3d
3e
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
25
25
25
25
25
50
50
25
50
50
70
80
25
25
25
25
25
8
8
8
8
8
3
3
10
4
2
16
7
12
10
10
12
9
4b + 4b0
4c + 4c0
4d
4ea
4eb
4f
4g
4h
4i
4j
4k
4l
4m + 4m0
4n + 4n0
4o + 4o0
4p + 4p0
4q + 4q0
83 (6:1)c
86 (5:1)d
81
80
84
O
O
1a
, MeCN
Br
3b
4
OH
40 °C
Br
O
O
5
6
7
8
4s'
3s
Br
82
Br
78 (17:1)e
72
Ph
O
9
71 (15:1)e
85
K2CO3
O
O
10
11
12
13
14
15
16
17b
Br
46 (8:1)e
44
DCM, 40 °C
OH
CO2Me
69%
86 (1.6:1)f
87 (1.7:1)f
90 (2.7:1)
88 (4:1)
83 (3.7:1)
4s
Scheme 1. b-Elimination of addition products to dihydronaphthalene derivatives.
ments and confirmed by their X-ray single crystal analyses
(Fig. 4, see Supplementary data for the details), and other products
were elucidated by spectral comparison accordingly.
a
b
c
d
e
f
Isolated yields.
The product(s) was confirmed by X-ray single crystal analysis.
The reaction was completed in 30 min at 80 °C with 85% yield (dr = 3:1).
The reaction was completed in 30 min at 60 °C with 88% yield (dr = 1.5:1).
The major diastereoisomer is shown.
To obtain enantiopure products, introduction of chiral auxilia-
ries to the olefin substrates 3m–3q was also attempted (Table 2,
entries 13–17). Two diastereomeric products formed, which could
be separated by silica gel chromatography (entries 15–17). Differ-
ent auxiliaries showed slight difference in the control of diastere-
oselectivity. Among these, the reaction with olefin 3p gave the
best diastereoselectivity (dr 4:1, entry 16). The corresponding
NOESY experiments and a single crystal X-ray diffraction experi-
ment of 4q0 enabled us to elucidate the relative and absolute
stereochemistries of these products. With their structural informa-
tion, we can easily understand that the two diastereomeric prod-
ucts were provided from the different-face additions to the ICTB,
respectively (Fig. 2). It also indicates that the final stereoselectivity
of product(s) is determined by the facial selectivity of the intermo-
lecular C-1 attack by the phenol hydroxyl group at the first step.
After the O-glycosylation, the following intramolecular Michael
addition and aldol cyclization are believed to be highly
stereoselective.
These two products are inseparable and their ratios were determined by 1H NMR
method.
A formal [4+2]-adduct, dihydronaphthalene derivative 4r, was
also observed in the reaction between ICTB 1a and olefin 3r (Scheme
1a). It is believed that a subsequent b-elimination happened after
the addition of ICTB 1a and olefin 3r under the described conditions
via the 2,3-dihydrobenzo[b][1,4]oxazepin-4(5H)-one intermediate
4r0. Such a phenomenon indicated that the products 4 in Table 2,
with proper treatment, could be further transformed into the corre-
sponding dihydronaphthalene derivatives. Indeed, treatment of the
crude product 4s0 (without purification) with K2CO3 in DCM affor-
ded the expected 2-phenyl-1,2-dihydronaphthalene derivative 4s
in 69% overall yield from 3s (Scheme 1b).
In summary, the electronically unfavorable reactions between
isochromenylium tetrafluoroborates and electron-deficient olefins
have been studied and achieved in this work.13 By introducing a
Figure 4. X-ray crystal structures of products 4b, 4b0, 4d, and 4q0.

2768
S.-Y. Yu et al. / Tetrahedron Letters 53 (2012) 2765–2768
Chem. 2000, 1019–1025; (h) Ezquerra, J.; Pedregal, C.; Lamas, C. J. Org. Chem.
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Iwasawa, N. Angew. Chem., Int. Ed. 2008, 47, 4903–4905; (k) Barluenga, J.;
Ballesteros, A.; González, J. M. J. Am. Chem. Soc. 2003, 125, 9028–9029; (l) Asao,
N.; Nogami, T.; Lee, S.; Yamamoto, Y. J. Am. Chem. Soc. 2003, 125, 10921–10925;
(m) Hsu, Y. C.; Ting, C. M.; Liu, R. S. J. Am. Chem. Soc. 2009, 131, 2090–2091.
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1064–1073.
phenolic hydroxyl group into the olefin substrates, additions of
electron-deficient olefins to isochromenylium tetrafluoroborates
1 were successfully stimulated by the C-1 O-glycosylation and
completed with subsequent intramolecular Michael addition and
aldol condensation, providing 2-oxabicyclo[3.3.1]nonane deriva-
tives 4 diastereoselectively in moderate to satisfactory yields. The
developed cascade methodology is advantageous in ease of opera-
tions, metal-free conditions, and high efficiency of generating mul-
tiple stereogenic centers including the quaternary carbons, and
thus will be useful in future applications to the synthesis of bioac-
tive natural products and drug-like molecules.
9. Liao, H.-Z.; Hu, Z.-L.; Cui, K.; Jiao, J.; Qin, Y.; Yao, Z.-J. Synthesis 2010, 3474–
3480.
10. Gennari, L.; Merlotti, D.; Martini, G.; Nuti, R. Expert Opin. Investig. Drugs 2006,
15, 1091–1103.
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Rudolph, A.; Chan, W. H.; Ruiz, J.; Dockendorff, C.; Lautens, M. Chem. Eur. J.
2008, 14, 2112–2124. and references cited therein..
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M.; Iwasa, K. Tetrahedron 1979, 35, 1977–1979; (c) Fang, Z. Z.; Zhang, Y. Y.; Ge,
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Acknowledgments
This project is financially supported by MOST 973 Program
(2010CB833200), NSFC (21032002), the Fundamental Research
Funds for the Central Universities (No. 1082020502) and the State
Key Laboratory of Phytochemistry and Plant Resources in West
China.
13. Representative procedure: To a solution of ICTB 1a (0.1 mmol) in dry CH3CN
(5 mL) was added electron-deficient olefin 3b (0.12 mmol). The reaction was
stirred at 25 °C for 8 h until the starting material was consumed. Saturated
aqueous NaHCO3 (5 mL) was added, and the mixture was extracted with ethyl
acetate (15 mL ꢀ 3). The combined organic extracts were washed with brine,
dried over anhydrous Na2SO4 and concentrated. The residue was purified by
flash chromatography on silica gel (petroleum ether/ethyl acetate = 3:1) to give
4b and 4b0 (83% combined yield, ratio 6:1). Data of 4b: white solid; mp 111–
113 °C; 1H NMR (CDCl3, 300 MHz): d 7.72 (d, J = 7.5 Hz, 2H), 7.67 (t, J = 8.1 Hz,
1H), 7.43 (dd, J = 8.4, 17.5 Hz, 2H), 7. 31 (d, J = 7.5 Hz, 2H), 7.08 (d, J = 8.4 Hz,
1H), 7.05 (s, 1H), 6.93 (m, 3H), 6.67 (d, J = 8.4 Hz, 2H), 6.49 (s, 1H), 5.45 (d,
J = 1.8 Hz, 1H), 4.88 (d, J = 11.2 Hz, 1H), 4.14 (t, J = 11.2 Hz, 1H), 3.90 (s, 3H),
3.63 (s, 3H), 3.60 (s, 3H), 3.16 (dd, J = 1.8, 11.2 Hz, 1H); 13C NMR (CDCl3,
75 MHz): 200.9, 192.3, 160.3, 158.6, 150.3, 148.5, 137.1, 136.1, 132.8, 130.2,
129.2, 128.8, 128.4, 128.0, 127.3, 124.0, 121.6, 119.8, 117.8, 113.8, 113.0, 110.1,
76.8, 56.4, 55.9, 55.7, 54.8, 50.7, 40.3; IR (KBr) vmax 1690, 1603, 1513, 1423,
1295, 1255, 1216, 1127, 694 cm–1; HRMS (ESI) calcd for C33H28O6Na [M+Na+]:
543.1778; found: 543.1780. Data of 4b0: white solid; mp 112–114 °C; 1H NMR
(CDCl3, 300 MHz): d 7.74 (dd, J = 0.9, 4.8 Hz, 1H), 7.68 (d, J = 4.2 Hz, 2H), 7.43–
7.47 (m, 2H), 7.33 (t, J = 4.8 Hz, 2H), 7.05 (s, 1H), 7.01 (d, J = 4.8 Hz, 1H), 6.99 (d,
J = 5.1 Hz, 2H), 6.93 (t, J = 4.8 Hz, 1H), 6.59 (d, J = 5.1 Hz, 2H), 6.47 (s, 1H), 5.64
(d, J = 2.4 Hz, 1H), 5.28 (d, J = 3.9 Hz, 1H), 4.34 (dd, J = 3.9, 5.1 Hz, 1H), 3.89 (s,
3H), 3.86–3.88 (m, 1H), 3.71 (s, 3H), 3.64 (s, 3H); 13C NMR (CDCl3, 100 MHz): d
201.1, 192.5, 159.9, 158.5, 149.4, 148.4, 137.7, 136.1, 132.7, 130.3, 129.1, 128.3,
128.2, 128.1, 127.2, 125.7, 121.4, 120.2, 117.9, 113.3, 111.4, 110.8, 76.2, 55.8,
55.7, 55.0, 48.4, 47.3, 40.6; IR (KBr) vmax 1686, 1607, 1515, 1463, 1305, 1255,
1216, 1127, 691 cmꢁ1; ESI-MS (m/z): 543 [M+Na+]; HRMS (ESI) calcd for
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.03.
106.
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