Enantioselective Diels-Alder reaction of α-Acyloxyacroleins catalyzed by chiral 1,1′-binaphthyl-2,2′-diammonium salts

Article abstract of DOI:10.1002/adsc.200600322

A diammonium salt of chiral 1,1′-binaphthyl-2,2′-diamine (2a) and trifluoromethanesulfonimide (Tf₂NH) shows excellent catalytic activity and enantioselectivity for the Diels-Alder reaction of α-acyloxyacroleins. For example, in the presence of

Full text of DOI:10.1002/adsc.200600322

FULL PAPERS
DOI: 10.1002/adsc.200600322
Enantioselective Diels–Alder Reaction of a-Acyloxyacroleins
Catalyzed by Chiral 1,1’-Binaphthyl-2,2’-diammonium Salts
Akira Sakakura,a Kenji Suzuki,a and Kazuaki Ishiharaa,*
a
Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 464-8603, Japan
Fax : (+81)-52-789-3222; e-mail: ishihara@cc.nagoya-u.ac.jp
Received: June 30, 2006; Accepted: September 12, 2006
Abstract: A diammonium salt of chiral 1,1’-binaphth- the enantioselectivity due to the formation of strong
yl-2,2’-diamine (2a) and trifluoromethanesulfonimide intramolecular hydrogen bonding of the acyl group
(Tf2NH) shows excellent catalytic activity and enan- with a proton of the ammonium group in the transi-
tioselectivity for the Diels–Alder reaction of a-acyl- tion state. This catalyst can be easily prepared in situ
oxyacroleins. For example, in the presence of 5 by mixing the commercially available chiral diamine
mol% of 2a and 9.5 mol% of Tf2NH, the Diels– and Tf2NH.
Alder reaction of a-(cyclohexanecarbonyloxy)acro-
lein with cyclopentadiene proceeded in EtCN at
À758C to give the adducts in 88% yield with 92% Keywords: a-acyloxyacrolein; ammonium salts; 1,1’-
exo and 91% ee. The electron-donating property of binaphthyl-2,2’-diamine; cycloaddition; Diels–Alder
the acyl group of the a-acyloxyacroleins increases reaction
Introduction
in an enantiomerically pure form is strongly needed.
Recently, we reported the enantioselective organoca-
The enantioselective Diels–Alder reaction is one of talytic Diels–Alder reaction with a-acyloxyacrolein.[9]
the most powerful organic transformations, and is a Our catalyst is the chiral ammonium salt of the chiral
versatile method for the synthesis of many important triamine 1 bearing a primary aliphatic amino group
chiral building blocks for the total synthesis of bioac- with pentafluorobenzenesulfonic acid (C6F5SO3H).[10]
tive natural products.[1] Recently, the use of organoca- The Diels–Alder reactions of acyclic dienes and cyclo-
talysts for the enantioselective Diels–Alder reaction hexadiene with a-(p-methoxybenzoyloxy)acrolein
has attracted great attention and has been extensively give the adducts with high enantioselectivities. Un-
studied with great success.[2–4] Most of the reported or- fortunately, however, the enantioselectivity for the re-
ganocatalysts for the enantioselective Diels–Alder re- action of cyclopentadiene is up to 83% ee. In addi-
action are ammonium salts of secondary amines, and tion, the catalytic activity is not so high (10–20 mol%
their general mode of catalysis is the generation of of catalyst is loaded at À208C to room temperature)
iminium cations as active intermediates. While these because of its relatively weak acidity.
organocatalysts give good results for the Diels–Alder
To improve the catalytic activity and enantioselec-
reaction of a-unsubstituted acroleins, it is difficult to tivity for the Diels–Alder reaction of cyclopentadiene,
activate a-substituted acroleins, probably because of we planned to use the ammonium salt of a weakly
poor generation of the corresponding iminium cat- basic aromatic amine with a super Brønsted acid[10] as
ions. The greater bulkiness of the secondary amines a catalyst at a lower reaction temperature. The strong-
has been considered to be unfavorable for generating
an iminium ion with an a-substituted acrolein.[5]
a-Acyloxyacrolein is an important alternative to a-
haloacrolein,[6] and is very versatile in synthesis as a
dienophile.[7] For example, the adducts of cyclopenta-
diene with a-acyloxyacrolein can be converted to
bicyclo[2.2.1]hept-5-en-2-one, which is a promising
G
synthetic intermediate for the total synthesis of sever-
al bioactive compounds,[8] and the development of a
practical method for the synthesis of this compound
Adv. Synth. Catal. 2006, 348, 2457 – 2465
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Table 1. Diels–Alder reaction of cyclopentadiene with meth-
acrolein.[a]
Entry HX
Time Yield
[h]
exo/
ee
[%][b]
endo[b] [%][c]
1
2
C6F5SO3H
TfOH
C6F13SO3H
(+)-CSA
2,4,6-(i-
6
6
6
3
3
8
90:10
97:3
93:7
87:13
88:12
45
39
0
13
13
Scheme 1. Chiral 1,1’-binaphthyl-2,2’-diammonium salt cata-
lysts for the enantioselective Diels–Alder reaction of cyclo-
pentadiene with a-substituted acroleins.
27
99
99
85
3
4[d]
5[d]
Pr)3C6H2SO3H
Tf2NH
ly acidic ammonium salt of chiral aromatic diamine
2a was thought to react easily with an a-substituted
acrolein to form an iminium cation as an active inter-
mediate, which would smoothly react with cyclopenta-
diene even at lower temperature (Scheme 1). We de-
scribe here chiral 1,1’-binaphthyl-2,2’-diammonium
salt catalysts for the enantioselective Diels–Alder re-
actions of a-substituted acroleins.[11]
6
7
6
6
13
30
97:3
97:3
61
60
A
[a]
Unless otherwise noted, the Diels–Alder reaction of cy-
clopentadiene (4mmol) with methacrolein (1 mmol) in
EtCN (2 mL) was carried out at À758C.
[b]
[c]
[d]
1
Determined by H NMR analysis.
Enantiomeric excess of the exo-adduct.
The reaction was conducted at 08C.
Results and Discussion
ly) (entries 7 and 8). The suitably strong acidities of
First, we examined the Diels–Alder reaction of cyclo- the ammonium catalysts promoted the reactions well,
pentadiene (4equivs.) with methacrolein using chiral and the bulkiness of the counter anions of the ammo-
ammonium salts of commercially available 2a (5 nium catalysts might increase the enantioselectivities.
mol%) and a variety of Brønsted acids (HX, 9.5 Therefore, we considered that commercially available
mol%) in propionitrile (EtCN) at À758C (Table 1). Tf2NH was the most suitable Brønsted acid for the
The ammonium salt of 2a and C6F5SO3H gave the present Diels–Alder reaction.
(2R)-exo-adduct as a major diastereomer in a conver-
Next, we investigated the catalytic activities of am-
sion yield of 8% with 45% ee (entry 1). The use of monium salts of the binaphthyldiamines 2 and octahy-
trifluoromethanesulfonic acid (TfOH) gave an enan- drobinaphthyldiamines 3 with Tf2NH (Table 2). The
tioselectivity (39% ee) similar to that with C6F5SO3H ammonium salt of 6,6’-dibromo derivative 2b showed
(entry 2). The ammonium salt of the much stronger good catalytic activity (82% yield) with an enantiose-
perfluorohexanesulfonic acid (C6F13SO3H) gave the lectivity similar to that with 2a (62% ee) (entry 1).
adduct in quantitative yield with no enantioselectivity, The introduction of bromo substituents at the 6,6’-po-
since the acidity of the ammonium salt was too strong sitions would enhance the acidity of the ammonium
to control the reaction (entry 3). Since the catalytic salt to result in higher catalytic activity. The ammoni-
activities of the ammonium salts of (+)-10-camphor- um salts of N-monosubstituted or N,N’-disubstituted
sulfonic acid [(+)-CSA] and 2,4,6-triisopropylbenz- binaphthyldiamines 2c–g all gave the adduct with
enesulfonic acid [2,4,6-(i-Pr)3C6H2SO3H], which were quite lower enantioselectivities (<13% ee) (entries 2–
weaker sulfonic acids than TfOH, were quite low, the 6). The N-substituents might hinder the formation of
reactions catalyzed by them required an increased re- the aldimine intermediate. The ammonium salt of oc-
action temperature (08C) and gave the exo-adduct tahydrobinaphthyldiamine 3a showed moderate cata-
with quite low enantioselectivities (13% ee) (entries 4 lytic activity (52% yield) with low enantioselectivity
and 5). Importantly, ammonium salts of trifluorome- (33% ee) (entry 7). The introduction of bromo sub-
thanesulfonimide (Tf2NH) and perfluorobutanesulfon- stituents at the 3,3’-positions led to the opposite enan-
imide [(C4F9SO2)2NH] gave the (2R)-exo-adduct with tioselectivity with low enantiomeric excess (18%)
better enantioselectivities (61 and 60% ee, respective- (entry 8). Among the chiral diamines we examined,
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Adv. Synth. Catal. 2006, 348, 2457 – 2465

Enantioselective Diels–Alder Reaction of a-Acyloxyacroleins
Table 2. Substituent effect of binaphthyldiamines 2 and octahydrobinaphthyldiamines 3 on the Diels–Alder reaction.[a]
FULL PAPERS
Entry
2 [R1, R2, R3] or 3 [R4]
X [mol%]
Conditions [8C, h]
Yield [%][b]
exo/endo[b]
ee [%][c] (config)
1
2
3
4
5
6
7
8
2b [Br, H, H]
2c [H, Me, Me]
2d [H, Bn, H]
2e [H, Ph, H]
2f [H, Bz, H]
2g [H, Ts, H]
3a [H]
9.5
9.5
9.5
4.8
4.8
4.8
9.5
9.5
À75, 22
À20, 2
82
93
99
59
29
20
52
19
99:1
62 (2R)
0
0
0
13 (2R)
0
33 (2R)
S)
82:18
90:10
91:9
97:3
99:1
À45, 4.5
À75, 20
À40, 72
À45, 15
À40, 10
À75, 25
90:10
96:418 (2
3b [Br]
[a]
The Diels–Alder reaction of cyclopentadiene (4mmol) with methacrolein (1 mmol) in EtCN (2 mL) was carried out.
[b]
[c]
1
Determined by H NMR analysis.
Enantiomeric excess of the exo-adduct.
2a and 2b were found to be quite effective for the
present Diels–Alder reaction.
Therefore, we investigated the Diels–Alder reaction
of cyclopentadiene with several a-acyloxyacroleins,
With these results in hand, we examined the Diels– which had an acyloxy group with electron-donating
Alder reaction of cyclopentadiene (4equivs.) with a- substituents and could be dissolved in EtCN at
acyloxyacroleins catalyzed by the ammonium salt of À758C. As a result, we found that the reaction with
2a or 2b (5 mol%) with Tf2NH (9.5 mol%) (Table 3). a-(cyclohexanecarbonyloxy)acrolein catalyzed by the
In the presence of the ammonium salt of 2a, the ammonium salt of 2a gave the corresponding (2S)-
Diels–Alder reaction with a-(p-methoxybenzoyloxy)- exo-adduct with good enantioselectivity (86% ee) and
acrolein, which gave the highest enantiomeric excess in good yield (80%) (entry 3). Moreover, when the
(83%) in the Diels–Alder reaction of cyclopentadiene reaction was conducted in the presence of 10 mol%
catalyzed by the ammonium salt of 1 and C6F5SO3H, of water, the yield and enantioselectivity were in-
gave the corresponding (2S)-exo-adduct as a major creased[12] (88% yield and 91% ee) (entry 4). Water
diastereomer with remarkably high enantiomeric might promote the hydrolysis of the aldimine inter-
excess (94%) in moderate yield (48%) (entry 1). The
mediate to increase the catalytic turnover rate. The
electron-donating property of the p-methoxybenzoyl- use of the ammonium salt of 2a (5 mol%) and Tf2NH
oxy group was thought to increase the enantioselec- (4.8 mol%) as a catalyst gave a lower yield (72%)
tivity (see Figure 1). However, the poor solubility of and enantioselectivity (71%) (entry 5). a-(Cyclopen-
a-(p-methoxybenzoyloxy)acrolein in EtCN at À758C tanecarbonyloxy)acrolein and a-(diphenylacetyloxy)-
resulted in poor reactivity. The use of the ammonium acrolein also gave good results (entries 7 and 8). Im-
salt of 2b as a catalyst gave the (2S)-exo-adduct in portantly, a-[p-(triisopropylsilyloxy)benzoyloxy]acro-
quantitative yield, although the enantioselectivity was lein, which had good electron-donating ability and
slightly decreased (87% ee) (entry 2). The stronger could be dissolved in EtCN, exhibited reactivity
acidity of the ammonium salt of 2b would result in a higher than a-(p-methoxybenzoyloxy)acrolein (76%
higher reaction rate but lower enantioselectivity than yield) with excellent enantioselectivity (94% ee)
that of 2a.
(entry 9).
Adv. Synth. Catal. 2006, 348, 2457 – 2465
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Akira Sakakura et al.
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Table 3. Diels–Alder reaction of cyclopentadiene with a-acyloxyacroleins.[a]
Entry
R
Diamine
Time [h]
Yield [%][b]
exo/endo[b]
ee [%][c]
1
2
3
p-MeOC6H4
p-MeOC6H4
c-C6H11
c-C6H11
c-C6H11
c-C6H11
c-C5H9
Ph2CH
p-TIPSOC6H4
2a
2b
2a
2a
2a
2b
2a
2a
2a
28
21
48
99
93:7
87:13
94
87
2480
2488
2472
25
2495
2499
2476
:6
94 86
92:8
91:9
78
4[d]
5[e]
6
91
71
86:1483
88
7[d]
8[f]
9
92:8
95:5
93:7
91
94
[a]
Unless otherwise noted, the Diels–Alder reaction of cyclopentadiene (1.6 mmol) with a-acyloxyacrolein (0.4mmol) in
EtCN (0.8 mL) was carried out at À758C.
[b]
[c]
[d]
[e]
[f]
1
Determined by H NMR analysis.
Enantiomeric excess of the exo-adduct.
The reaction was conducted in the presence of H2O (10 mol%).
The reaction was conducted in the presence of 2a (5 mol%) and Tf2NH (4.8 mol%).
The reaction was carried out at À408C.
Table 4. Diels–Alder reaction of 2,3-dimethylbutadiene with
a-acyloxyacroleins.[a]
Figure 1. Proposed transition-state assembly. Counter anions
(Tf2NÀ) are omitted for clarity.
Entry
Solvent
Time [h]
Yield [%][b]
ee [%][c]
1
2
3
4Et
5
EtNO2
EtCN
CH2Cl2
2O
72
65
72
15
15
60
52
59
3
71
54
49
ND
ND
As described above, we succeeded in the Diels–
Alder reaction of cyclopentadiene with high enantio-
selectivity. However, when the reaction of acyclic 2,3-
dimethylbutadiene with a-(cyclohexanecarbonyloxy)-
acrolein was conducted under the same reaction con-
ditions, the yield and enantioselectivity of the corre-
sponding Diels–Alder adduct were moderate (52%
yield and 54% ee) (Table 4, entry 2). Therefore, we
screened a variety of solvents (Table 4), and found
that the use of more polar nitroethane (EtNO2) as a
solvent resulted in a higher enantioselectivity (60%
Toluene
3
[a]
The Diels–Alder reaction of cyclopentadiene (1.6 mmol)
with a-acyloxyacrolein (0.4mmol) in solvent (0.8 mL)
was carried out at À758C.
[b]
[c]
1
Determined by H NMR analysis.
Enantiomeric excess of the exo-adduct.
yield and 71% ee) (entry 1).[13] Dichloromethane use of
a
less polar solvent such as dieth
(CH2Cl2) gave an enantioselectivity (49% ee) that yl ether (Et2O) or toluene resulted in low reactivity
was slightly lower than that with EtCN (entry 3). The (entries 4and 5).
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Enantioselective Diels–Alder Reaction of a-Acyloxyacroleins
Table 5. Diels–Alder reaction of dienes with a-acyloxyacroleins.[a]
FULL PAPERS
Entry
Diene
Product
R
Time [h]
Yield [%][b]
eo/endo[b]
ee [%][c] (config)
1
c-C6H11
c-C5H9
Ph2CH
p-TIPSOC6H4
c-C6H11
Ph2CH
p-TIPSOC6H4
48
48
10
48
4
88
98
99
85
8
99
-
-
-
-
70 (ND)
65 (ND)
70 (ND)
85 (ND)
71 (ND)
67 (ND)
82 (ND)
2
3[d]
4
5
67 >99:<1[e]
>99:<1[e]
85 >99:<1[e]
6[d]
7
16
4
8
8
9
c-C6H11
p-TIPSOC6H4
48
48
65
77
-
-
68 (ND)
82 (ND)
10[f]
11[f]
c-C6H11
c-C5H9
4
4
8
8
90 <1:>99
97 <1:>99
91 (2S)
87 (2S)
[a]
Unless otherwise noted, the Diels–Alder reaction of dienes (1.6 mmol) with a-acyloxyacrolein (0.4mmol) in EtNO 2
(0.8 mL) was carried out at À758C.
[b]
[c]
[d]
[e]
[f]
1
Determined by H NMR analysis.
Enantiomeric excess of the major diastereomer.
The reaction was carried out at À408C.
The molar ratio of 4-methyl and 3-methyl isomers is indicated.
The reaction was conducted in the presence of 2a (5 mol%) and Tf2NH (9.5 mol%).
With the optimized reaction conditions in hand, the while the reaction of cyclopentadiene is exo-selec-
reaction of various dienes was examined to explore tive.[6i,j] The Diels–Alder adducts of cyclohexadiene
the generality and scope of the present Diels–Alder with a-acyloxyacrolein can be converted to bicyclo-
reaction (Table 5). Since acyclic dienes such as 2,3-di-
A
methylbutadiene, isoprene and butadiene were less common intermediate for the total syntheses of sever-
reactive, 10 mol% of the catalyst was used for the re- al bioactive compounds.[15]
action of acyclic dienes. For the Diels–Alder reaction
The sense of asymmetric induction observed in the
of 2,3-dimethylbutadiene and isoprene, the use of a- present Diels–Alder reaction is considered to be as
(diphenylacetyloxy)acrolein resulted in an enantiose- follows (Figure 1).[11] The aldimine would be formed
lectivity similar to that with a-(cyclohexanecarbony- from an a-acyloxyacrolein and the ammonium salt of
loxy)acrolein and a-(cyclopentanecarbonyloxy)acro- 2a as an active intermediate. In the active intermedi-
lein (entries 1–3, 5 and 6). As expected, the reaction ate, the aldimine moiety is activated by Tf2NH and
of 2,3-dimethylbutadiene with a-[p-(triisopropylsilyl- the acyloxy group forms an intramolecular hydrogen
oxy)benzoyl]acrolein gave much better results (85% bonding with a proton of the ammonium group. The
yield, 85% ee) (entry 4).[14]
diene should approach the si-face of the s-trans acro-
The Diels–Alder reaction of cyclohexadiene with a- lein moiety of the active intermediate from the less-
(cyclohexanecarbonyloxy)acrolein was conducted in hindered side, to give the (2S)-adduct.
the presence of 5 mol% of the ammonium catalyst
and gave the corresponding (2S)-endo-adduct as a
a-[p-(Triisopropylsilyloxy)benzoyl]acrolein bearing
a good electron-donating p-(triisopropylsilyloxy)-
single diastereomer in 90% yield with 91% ee phenyl group exhibited very high enantioselectivity,
(entry 10). a-(Cyclopentanecarbonyloxy)acrolein also which would be attributed to the formation of the
gave good results (97% yield, 87% ee) (entry 11). In strong intramolecular hydrogen bonding in the transi-
general, the Diels–Alder reaction of cyclohexadiene tion state, due to the higher basicity of the carbonyl
with a-substituted acroleins is highly endo-selective, oxygen of the acyloxy group.
Adv. Synth. Catal. 2006, 348, 2457 – 2465
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Akira Sakakura et al.
FULL PAPERS
Conclusions
5-ene (Tables 1 and 2),[9,11] (À)-(1S,2S,4S)-2-formylbicyclo-
A
(1S,2S,4S)-2-formylbicyclo
[2.2.1]hept-5-ene-2-yl
cyclohex-
In conclusion, we have developed the enantioselective
Diels–Alder reaction with a-acyloxyacroleins cata-
lyzed by the chiral ammonium salt of binaphthyl-
based diamine 2a with Tf2NH. The use of a small
amount (5–10 mol%) of this chiral ammonium cata-
lyst at a low reaction temperature (À758C) gave ex-
cellent reactivity and enantioselectivity in the Diels–
Alder reaction with a-acyloxyacroleins. The electron-
donating property of the acyl group of the a-acylox-
yacroleins increased the enantioselectivity due to the
formation of strong intramolecular hydrogen bonding
between the acyl group and a proton of the ammoni-
um group in the transition state. The ammonium cata-
lyst could be easily prepared in situ by mixing com-
mercially available 2,2’-diamino-1,1’-binaphthyl (2a)
and Tf2NH.
anecarboxylate (Tables 3),[11] (À)-(1S,2S,4S)-2-formylbicyclo-
[2.2.1]hept-5-ene-2-yl cyclopentanecarboxylate (Table 3),[11]
(+)-(1R,2S,4R)-2-formylbicyclo
hexanecarboxylate
(Table 5),[11]
formylbicyclo[2.2.2]oct-5-ene-2-yl cyclopentanecarboxylate
A
cyclo-
(+)-(1R,2S,4R)-2-
U
(Table 5),[11] 1-formyl-3,4-dimethylcyclohex-3-enyl cyclohex-
anecarboxylate (Table 4),[11] 1-formyl-3,4-dimethylcyclohex-
3-enyl cyclopentanecarboxylate (Table 5)[11] were previously
known.
(S)-6,6’-Dibromo-1,1’-binaphthyl-2,2’-diamine (2b)
To a solution of 2a (284mg, 1.0 mmol) in 1,4-dioxane
(10 mL) was added N-bromosuccinimide (534mg,
3.0 mmol), and the mixture was stirred at ambient tempera-
ture for 5 h. The reaction was quenched with 10% aqueous
Na2S2O3 (10 mL), and the aqueous layer extracted with
EtOAc (50 mL”3). The combined organic layer and ex-
tracts were washed with H2O and brine, dried (MgSO4), and
concentrated. The residue was purified by column chroma-
tography on silica gel using hexane-EtOAc as the eluent, to
give 2b; yield: 173 mg (41%). [a]2D6: 0.400 (c 1.0, CHCl3); IR
Experimental Section
(KBr): n=1617, 1492, 1383, 1350, 1068 cmÀ1
;
1H NMR
General Remarks
(300 MHz, CDCl3): d=3.73 (s, 4H), 6.90 (d, J=8.7 Hz, 2H),
7.16 (d, J=8.7 Hz, 2H), 7.26 (dd, J=2.1, 8.7 Hz, 2H), 7.72
(d, J=8.7 Hz, 2H), 7.94(d, J=2.1 Hz, 2H); 13C NMR
(75 MHz, CDCl3): d=111.9, 116.2, 119.4, 125.7, 128.9, 129.6,
130.2, 130.3, 132.2, 143.2; HR-MS (FAB): m/z=442.9568,
calcd. for C20H15Br2N2 [M+H]+: 442.9583.
IR spectra were recorded on a JASCO FT/IR-460 plus spec-
trometer. 1H NMR spectra were measured on a Varian
Gemini-2000 spectrometer (300 MHz) at ambient tempera-
ture. Data were recorded as follows: chemical shift in ppm
from internal tetramethysilane on the d scale, multiplicity
(s=singlet; d=doublet; t=triplet; m=multiplet), coupling
constant (Hz), and integration. 13C NMR spectra were mea-
sured on a Varian Gemini-2000 spectrometer (75 MHz).
Chemical shifts were recorded in ppm from the solvent reso-
nance employed as the internal standard (CDCl3 at
77.0 ppm). Analytical HPLC was performed on a Shimadzu
LC-10 coupled diode array-detector SPD-MA-10 A-VP and
column of Daicel CHIRALCEL OD-H, AD-H or OJ-H (Ø
4.6 mm”250 mm). Optical rotations were measured on a
RUDOLPH AUTOPOL IV digital polarimeter. GC analysis
was performed with Shimadzu 17 A instruments with a
flame-ionization detector and a capillary column of PEG-
HT Bonded (25 m”0.25 mm) using nitrogen as carrier gas.
All experiments were carried out under an atmosphere of
dry nitrogen. For TLC analysis, Merck precoated TLC
plates (silica gel 60 F254 0.25 mm) were used. For preparative
column chromatography, Merck silica gel 60 (0.040–
0.063 mm) was used. High resolution mass spectral analysis
(HR-MS) was performed at Chemical Instrument Room,
Research Center for Materials Science, Nagoya University.
Dry toluene and THF was purchased from Wako as “anhy-
drous” and stored under nitrogen. CH2Cl2, EtCN and
EtNO2 were freshly distilled from calcium hydride. Other
materials were obtained from commercial supplies and used
without further purification. (S)-2,2’-diamino-1,1’-binaphthyl
(2a) was purchased from Wako. Tf2NH was purchased from
Aldrich. Diamines 2c,[16] 2d,[17] 2e,[18] 3a[19] were prepared ac-
cording to the reported procedure. a-(p-Methoxybenzoyloxy)-
acrolein (Table 3),[9,11] a-(cyclohexanecarbonyloxy)acrolein
(Tables 3–5),[9] a-(cyclopentanecarbonyloxy)acrolein (Tables
(S)-N-Benzoyl-1,1’-binaphthyl-2,2’-diamine (2f)
To a solution of 2a (100 mg, 0.35 mmol) and pyridine
(85 mL, 1.05 mmol) in acetnitrile (3.5 mL) was added benzo-
yl chloride (45 mL, 0.39 mmol), and the mixture was refluxed
for 12 h. After cooling to ambient temperature, the reaction
was quenched with brine (10 mL) and extracted with EtOAc
(15 mL”3). The combined organic extracts were dried
(MgSO4) and concentrated. The residue was purified by
column chromatography on silica gel using hexane-EtOAc
as the eluent, to give 2f; yield: 26 mg (19%);. [a]2D6: À103.7
(c 1.00, CHCl3); IR (KBr): n=1619, 1596, 1500, 1426,
1080 cmÀ1 1H NMR (300 MHz, CDCl3): d=3.74(s, 2H),
;
7.01 (d, J=8.4Hz, 1H), 7.18 (d, J=8.7 Hz, 1H), 7.19–7.48
(m, 10H), 7.84(d, J=8.4Hz, 1H), 7.89 (d, J=8.7 Hz, 1H),
7.94(s, 1H), 7.95 (d, J=8.1 Hz, 1H), 8.08 (d, J=9.0 Hz,
1H), 8.91 (d, J=9.0 Hz, 1H); 13C NMR (75 MHz, CDCl3):
d=110.3, 118.2, 120.4, 120.7, 123.0, 123.8, 125.3, 125.6, 126.9,
127.1, 127.7, 128.3, 128.4, 128.5, 128.7, 129.6, 130.7, 131.4,
131.7, 132.5, 133.6, 134.9, 135.3, 143.0, 165.4; HR-MS (FAB):
m/z=389.1656, calcd for C27H21ON2 [M+H]+: 389.1654.
(S)-N-(p-Toluenesulfonyl)-1,1’-binaphthyl-2,2’-
diamine (2g)
To a solution of 2a (100 mg, 0.35 mmol) and pyridine
(85 mL, 1.05 mmol) in acetnitrile (3.5 mL) was added p-tol-
uenesulfonyl chloride (73 mg, 0.38 mmol), and the mixture
was refluxed for 12 h. After cooling to ambient temperature,
the reaction was quenched with brine (10 mL) and extracted
3 and 5),[11] (1S,2R,4S)-2-formyl-2-methylbicyclo
[2.2.1]hept-
A
2462
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Adv. Synth. Catal. 2006, 348, 2457 – 2465

Enantioselective Diels–Alder Reaction of a-Acyloxyacroleins
FULL PAPERS
with EtOAc (15 mL”3). The combined organic extracts
were dried (MgSO4) and concentrated. The residue was pu-
rified by column chromatography on silica gel using hexane-
EtOAc as the eluent, to give 2g; yield: 130 mg (85%); [a]2D7:
9.61 (c 1.00, CHCl3); IR (KBr): n=1620, 1596, 1509, 1401,
trated under vacuum. The crude product was purified by
silica gel chromatography using hexane-EtOAc as the
eluent.
(À)-(1S,2S,4S)-2-Formylbicyclo
C
1
1318, 1163, 1091 cmÀ1; H NMR (300 MHz, CDCl3): d=2.30
1454, 1335, 1192, 1153, 1138, 1046 cmÀ1; H NMR (300 MHz,
(s, 3H), 3.40 (s, 2H), 6.40 (d, J=8.4Hz, 1H), 6.67 (s, 1H),
6.94(m, 1H), 6.99 (d, J=8.1 Hz, 2H), 7.03 (m, 1H), 7.07 (d,
J=8.7 Hz, 1H), 7.21 (m, 2H), 7.34–7.45 (m, 1H), 7.40 (d,
J=8.1 Hz, 2H), 7.76 (d, J=8.1 Hz, 1H), 7.83 (d, J=8.7 Hz,
1H), 7.86 (d, J=8.4Hz, 1H), 7.94 (d, J=8.7 Hz, 1H), 8.12
(d, J=8.7 Hz, 1H); 13C NMR (75 MHz, CDCl3): d=21.6,
109.6, 118.1, 119.6, 121.7, 122.5, 123.4, 125.5, 125.8, 127.2,
127.3, 128.2, 128.2, 129.5, 129.8, 130.8, 131.4, 132.8, 133.6,
133.7, 136.2, 142.8, 143.8; HR-MS (FAB): m/z=439.1450,
calcd. for C27H23O2N2S [M+H]+: 439.1480.
CDCl3): d=1.17 (dd, J=3.9, 12.9 Hz, 1H), 1.41 (m, 1H),
1.67 (d, J=9.6 Hz, 1H), 2.48 (dd, J=3.9, 12.9 Hz, 1H), 2.93
(s, 1H), 3.15 (s, 1H), 5.00 (s, 1H), 5.90 (dd, J=3.0, 5.4Hz,
1H), 6.31 (dd, J=3.0, 5.7 Hz, 1H), 7.20–7.40 (m, 10H), 9.60
(s, 1H); 13C NMR (75 MHz, CDCl3): d=38.0, 42.2, 45.8,
48.7, 56.6, 92.2, 127.5, 128.7, 128.8, 132.5, 138.0, 140.7, 172.7,
198.6; HR-MS (FAB): m/z=333.1494, calcd. for C22H21O3
[M+H]+: 333.1491. The exo-endo ratio was determined by
1H NMR analysis: d=9.39 [s, 1H, CHO (endo-isomer)] and
9.60 [s, 1H, CHO (exo-isomer)]. The ee and the absolute
configuration of major enantiomer were established accord-
ing to the reported procedure[9,11] by HPLC analysis (Daicel
Chiralcel OJ-H column, hexane-i-PrOH=40:1, flow rate=
1 mLminÀ1) after conversion to (2-hydroxybicyclo-
(S)-3,3’-Dibromo-5,5’,6,6’,7,7’,8,8’-octahydro-1,1’-
binaphthyl-2,2’-diamine (3b)
Compound 3b was prepared according to the same manner
as 2b using (S)-3a instead of (S)-2a. [a]2D7: À26.8 (c 1.00,
A
1
CHCl3); IR (KBr): n=2928, 1602, 1458, 1094 cmÀ1; H NMR
LiAlH4 (2 equivs.) in THF at ambient temperature and sub-
sequent selective benzoylation of the primary hydroxy
group of the corresponding 1,2-diol with benzoyl chloride
(1.3 equivs.) in the presence of N,N-diisopropylethylamine
(2 equivs.) in chloroform at 08C: tR =15.9 (minor endo-enan-
tiomer), 17.6 (minor endo-isomer), 20.9 min (minor exo-
isomer), and 23.4(major exo-isomer) min.
(300 MHz, CDCl3): d=1.50–1.80 (m, 8H), 2.00–2.30 (m,
4H), 2.70 (t, J=6.0 Hz, 4H), 3.72 (s, 4H), 7.21 (s, 2H);
13C NMR (75 MHz, CDCl3): d=23.1, 23.3, 26.9, 29.2, 107.2,
122.5, 129.2, 132.4, 135.8, 139.3; HR-MS (FAB): m/z=
451.0217, calcd. for C20H23Br2N2 [M+H]+: 451.0209.
a-(Diphenylacetyloxy)acrolein (Tables 3 and 5)
(À)-(1S,2S,4S)-2-Formylbicyclo
C
p-
This compound was prepared according to the reported pro-
cedure.[9,11] IR (KBr): n=2880, 1766, 1687, 1644, 1496, 1455,
1710, 1603, 1509, 1463, 1272, 1162, 1111, 1095 cmÀ1
;
1347, 1272, 1191, 1142, 1084 cmÀ1
;
1H NMR (300 MHz,
1H NMR (300 MHz, CDCl3): d=1.10 (d, J=7.2 Hz, 1H),
1.20–1.37 (m, 4H), 1.48 (m, 1H), 1.76 (d, J=9.0 Hz, 1H),
2.64(d, J=3.6, 12.9 Hz, 1H), 6.22 (dd, J=3.0, 5.4Hz, 1H),
6.47 (dd, J=3.0, 5.7 Hz, 1H), 6.89 (d, J=8.7 Hz, 2H), 7.87
(d, J=8.7 Hz, 2H), 9.73 (s, 1H); 13C NMR (75 MHz,
CDCl3): d=12.8, 18.0, 38.1, 42.4, 45.7, 48.8, 91.7, 119.9,
121.9, 132.0, 132.5, 140.1, 161.2, 166.5, 198.9; HR-MS (FAB):
m/z=415.2309, calcd for C24H35O4Si [M+H]+: 415.2305.
The exo-endo ratio was determined by 1H NMR analysis:
d=9.53 [s, 1H, CHO (endo-isomer)] and 9.73 [s, 1H, CHO
(exo-isomer)]. The ee and the absolute configuration of
major enantiomer were established by the same procedure
as described above for (À)-(1S,2S,4S)-2-formylbicyclo-
CDCl3): d=5.24(s, 1H), 5.94(d, J=2.4Hz, 1H), 6.04 (d,
J=2.4 Hz, 1H), 7.25–7.41 (m, 10H), 9.40 (s, 1H); 13C NMR
(75 MHz, CDCl3): d=56.6, 121.8, 127.7, 128.8, 128.8, 137.9,
152.7, 170.1, 185.1; HR-MS (FAB): m/z=267.1025, calcd.
for C17H15O3 [M+H]+: 267.1021.
a-[p-(Triisopropylsilyloxy)benzoyloxy]acrolein
(Tables 3 and 5)
This compound was prepared according to the reported pro-
cedure.[9,11] IR (neat): n=2869, 1741, 1602, 1509, 1464, 1252,
1164, 1077 cmÀ1 1H NMR (300 MHz, CDCl3): d=1.11 (d,
;
J=6.9 Hz, 18H), 1.22–1.35 (m, 3H), 6.03 (d, J=2.1 Hz,
1H), 6.19 (d, J=2.1 Hz, 1H), 6.93 (d, J=8.7 Hz, 2H), 8.02
(d, J=8.7 Hz, 2H), 9.48 (s, 1H); 13C NMR (75 MHz,
CDCl3): d=12.8, 17.9, 120.0, 120.9, 121.5, 132.6, 153.0, 161.5,
163.7, 185.7; HR-MS (FAB): m/z=349.1832, calcd for
C19H29O4Si [M+H]+: 349.1835.
A
(À)-1-Formyl-3,4-dimethylcyclohex-3-enyl diphenylacetate
(Table 5): [a]2D7: À3.60 (c 1.00, CHCl3) for 70% ee; IR
(neat): n=2915, 1734, 1601, 1496, 1452, 1227, 1188, 1150,
1068 cmÀ1 1H NMR (300 MHz, CDCl3): d=1.59 (s, 6H),
;
1.63–1.74(m, 1H), 1.85–2.05 (m, 3H), 2.19 (br d, J=
17.7 Hz, 1H), 2.55 (br d, J=17.7 Hz, 1H), 5.05 (s, 1H),
7.20–7.40 (m, 10H), 9.53 (s, 1H); 13C NMR (75 MHz,
CDCl3): d=18.7, 18.9, 26.8, 27.6, 34.9, 56.9, 84.2, 121.1,
125.0, 127.5, 127.5, 128.6, 128.7, 128.8, 138.1, 138.2, 172.2,
198.5; HRMS (FAB): m/z=349.1809, calcd. for C23H25O3
[M+H]+: 349.1804. The ee was determined, according to the
reported procedure[9,11] by chiral HPLC (Daicel OD-H,
hexane-i-PrOH=40:1, flow rate 0.5 mLminÀ1) after conver-
sion to (1-hydroxy-3,4-dimethylcyclohex-3-enyl)methyl ben-
zoate by reduction with LiAlH4 (2 equivs.) in THF at ambi-
Representative Procedure for the Enantioselective
Diels–Alder Reaction:[11]
To a solution of (S)-2a (5.7 mg, 0.02 mmol) and trifluorome-
thanesulfonimide (10.7 mg, 0.038 mmol) in propionitrile or
nitroethane (0.8 mL) was added a-acyloxyacrolein
(0.4mmol). After cooling to À758C, diene (1.6 mmol) was
added to the solution, and the reaction mixture was stirred
at À758C for several hours. Upon consumption of a-acyloxy-
acrolein, the reaction was quenched with Et3N, and concen-
Adv. Synth. Catal. 2006, 348, 2457 – 2465
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.asc.wiley-vch.de
2463

Akira Sakakura et al.
FULL PAPERS
ent temperature and subsequent selective benzoylation of
the primary hydroxy group of the corresponding 1,2-diol
with benzoyl chloride (1.3 equivs.) in the presence of N,N-
diisopropylethylamine (2 equivs.) in chloroform at 08C: tR =
31.2 (major) and 35.7 (minor) min. The absolute configura-
tion was not established.
1.72 (s, 3H), 1.79–1.92 (m, 1H), 1.98–2.13 (m, 1H), 2.13–
2.33 (m, 2H), 2.44 (br d, J=18.0 Hz, 1H), 2.69 (br d, J=
18.0 Hz, 1H), 5.35 (s, 1H), 6.90 (d, J=8.7 Hz, 2H), 7.91 (d,
J=8.7 Hz, 2H), 9.67 (s, 1H); 13C NMR (75 MHz, CDCl3):
d=12.8, 18.0, 23.4, 26.4, 26.9, 29.8, 82.4, 116.7, 119.9, 122.0,
132.0, 133.4, 161.2, 165.9, 198.9; HR-MS (FAB): m/z=
417.2442, calcd. for C24H37O4Si [M+H]+: 417.2461. The ee
and the absolute configuration of major enantiomer were es-
tablished by the same procedure as described above for (À)-
1-formyl-4-methylcyclohex-3-enyl cyclohexanecarboxylate.
The absolute configuration was not established.
(À)-1-Formyl-3,4-dimethylcyclohex-3-enyl p-(triisopropyl-
silyloxy)benzoate (Table 5): [a]27: À8.81 (c 1.00, CHCl3) for
85% ee; IR (neat): n=2868, 1D737, 1708, 1602, 1509, 1464,
1
1289, 1248, 1163, 1112, 1096, 1069 cmÀ1; H NMR (300 MHz,
CDCl3): d=1.10 (d, J=7.2 Hz, 18H), 1.20–1.37 (m, 3H),
1.65 (s, 3H), 1.67 (s, 3H), 1.75–1.87 (m, 1H), 1.98–2.30 (m,
3H), 2.34(br d, J=18.6 Hz, 1H), 2.69 (br d, J=18.6 Hz,
1H), 6.90 (d, J=8.7 Hz, 2H), 7.91 (d, J=8.7 Hz, 2H), 9.64
(s, 1H); 13C NMR (75 MHz, CDCl3): d=12.8, 18.0, 18.8,
19.0, 27.2, 27.8, 35.4, 83.4, 119.9, 121.5, 122.0, 124.9, 132.0,
161.2, 165.9, 198.8; HR-MS (FAB): m/z=431.2604, calcd.
for C25H39O4Si [M+H]+: 431.2618. The ee and the absolute
configuration of major enantiomer were established by the
same procedure as described above for (À)-1-formyl-3,4-di-
methylcyclohex-3-enyl diphenylacetate. The absolute config-
uration was not established.
(À)-1-Formylcyclohex-3-enyl
cyclohexanecarboxylate
(Table 5): [a]2D3: À5.21 (c 1.00, CHCl3) for 68% ee; IR
(neat): n=2855, 1732, 1451, 1375, 1362, 1309, 1216, 1164,
1096 cmÀ1 1H NMR (300 MHz, CDCl3): d=1.14–1.53 (m,
;
6H), 1.54–1.81 (m, 4H), 1.91 (br d, J=12.3 Hz, 2H), 1.97–
2.42 (m, 4H), 2.62 (br d, J=18.0 Hz, 1H), 5.63 (d, J=
9.6 Hz, 1H), 5.75 (d, J=9.6 Hz, 1H), 9.51 (s, 1H); 13C NMR
(75 MHz, CDCl3): d=21.5, 25.4, 25.8, 26.2, 29.0, 29.3, 42.9,
81.8, 122.5, 126.2, 175.9, 198.7; HR-MS (FAB): m/z=
237.1494, calcd. for C14H21O3 [M+H]+: 237.1491. The ee was
determined, according to the reported procedure[9,11] by
chiral HPLC (Daicel AD-H, hexane-i-PrOH=40:1, flow
rate 1 mLminÀ1) after conversion to (1-hydroxycyclohex-3-
enyl)methyl benzoate by reduction with LiAlH4 (2 equivs.)
in THF at ambient temperature and subsequent selective
benzoylation of the primary hydroxy group of the corre-
sponding 1,2-diol with benzoyl chloride (1.3 equivs.) in the
presence of N,N-diisopropylethylamine (2 equivs.) in chloro-
form at 08C: tR =38.3 (major) and 40.9 (minor) min. The ab-
solute configuration was not established.
(À)-1-Formyl-4-methylcyclohex-3-enyl
cyclohexanecar-
boxylate (Table 5): [a]D26: À8.01 (c 1.00, CHCl3) for 71% ee;
IR (neat): n=2856, 1728, 1450, 1379, 1315, 1244, 1167, 1134,
1
1071, 1026 cmÀ1; H NMR (300 MHz, CDCl3): d=1.14–1.53
(m, 6H), 1.53–2.21 (m, 8H), 1.69 (s, 3H), 2.27 (br d, J=
18.0 Hz, 1H), 2.36 (tt, J=3.6, 11.1 Hz, 1H), 2.57 (br d, J=
18.0 Hz, 1H), 5.30 (s, 1H), 9.52 (s, 1H); 13C NMR (75 MHz,
CDCl3): d=23.3, 25.4, 25.4, 25.8, 26.2, 26.6, 28.9, 29.0, 29.7,
42.8, 81.8, 116.4, 133.5, 175.9, 199.0; HR-MS (FAB): m/z=
251.1637, calcd for C15H23O3 [M+H]+: 251.1647. The ee was
determined, according to the reported procedure,[9,11] by
chiral HPLC (Daicel AD-H, hexane-i-PrOH=40:1, flow
rate 1 mLminÀ1) after conversion to (1-hydroxy-4-methylcy-
clohex-3-enyl)methyl benzoate by reduction with LiAlH4
(2 equivs.) in THF at ambient temperature and subsequent
selective benzoylation of the primary hydroxy group of the
corresponding 1,2-diol with benzoyl chloride (1.3 equivs.) in
the presence of N,N-diisopropylethylamine (2 equivs.) in
chloroform at 08C: tR =37.8 (major) and 42.7 (minor) min.
The absolute configuration was not established.
(À)-1-Formylcyclohex-3-enyl p-(triisopropylsilyloxy)ben-
zoate (Table 5): [a]2D3: À12.4( c 1.00, CHCl3) for 82% ee; IR
(neat): n=2869, 1735, 1701, 1603, 1509, 1228, 1171, 1133,
1064, 1036 cmÀ1 1H NMR (300 MHz, CDCl3): d=1.10 (d,
;
J=7.2 Hz, 18H), 1.20–1.36 (m, 3H), 1.76–1.88 (m, 1H),
2.12–2.40 (m, 3H), 2.47 (br d, J=18.3 Hz, 1H), 2.73 (br d,
J=18.6 Hz, 1H), 5.66 (d, J=10.2 Hz, 1H), 5.80 (d, J=
10.2 Hz, 1H), 6.90 (d, J=9.0 Hz, 2H), 7.92 (d, J=9.0 Hz,
2H), 9.66 (s, 1H); 13C NMR (75 MHz, CDCl3): d=12.8,
18.0, 21.6, 26.4, 29.4, 82.4, 119.9, 121.9, 122.8, 126.2, 132.1,
161.2, 165.9, 198.6; HR-MS (FAB): m/z=403.2302, calcd.
for C23H35O4Si [M+H]+: 403.2305. The ee and the absolute
configuration of major enantiomer were established by the
same procedure as described above for (À)-1-formylcyclo-
hex-3-enyl cyclohexanecarboxylate. The absolute configura-
tion was not established.
(À)-1-Formyl-4-methylcyclohex-3-enyl
diphenylacetate
(Table 5): [a]2D6: À15.2 (c 1.00, CHCl3) for 67% ee; IR
(neat): n=2829, 1732, 1720, 1598, 1496, 1454, 1312, 1238,
1187, 1162, 1074, 1026 cmÀ1 1H NMR (300 MHz, CDCl3):
;
d=1.61 (s, 3H), 1.69–1.96 (m, 3H), 1.96–2.10 (m, 1H), 2.31
(br d, J=18.3 Hz, 1H), 2.55 (br d, J=18.3 Hz, 1H), 5.06 (s,
1H), 5.24(s, 1H), 7.20–7.04 (m, 10H), 9.56 (s, 1H);
13C NMR (75 MHz, CDCl3): d=23.2, 26.1, 26.5, 29.6, 56.9,
83.1, 116.1, 127.5, 127.6, 128.7, 128.7, 128.8, 128.8, 133.6,
138.2, 138.2, 172.3, 198.6; HR-MS (FAB): m/z=335.1646,
calcd. for C22H23O3 [M+H]+: 335.1647. The ee and the abso-
lute configuration of major enantiomer were established by
the same procedure described above for (À)-1-formyl-4-
methylcyclohex-3-enyl cyclohexanecarboxylate. The abso-
lute configuration was not established.
Acknowledgements
Financial support for this project was provided by JSPS.KA-
KENHI (15205021) and the 21 st Century COE Program
“Nature-Guided Materials Processing” of MEXT. We thank
Mr. Kazuhiko Nakano for his helpful advice.
(À)-1-Formyl-4-methylcyclohex-3-enyl p-(triisopropylsilyl-
oxy)benzoate (Table 5): [a]26: À25.6 (c 1.00, CHCl3) for
85% ee; IR (neat): n=2868D, 1738, 1708, 1603, 1509, 1464,
References
1
1280, 1244, 1163, 1112, 1096, 1069 cmÀ1; H NMR (300 MHz,
[1] For recent reviews, see: a) K. C. Nicolaou, S. A.
CDCl3): d=1.10 (d, J=7.2 Hz, 18H), 1.20–1.37 (m, 3H),
Snyder, T. Montagnon, G. Vassilikogiannakis, Angew.
2464
www.asc.wiley-vch.de
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 2457 – 2465

Enantioselective Diels–Alder Reaction of a-Acyloxyacroleins
FULL PAPERS
Chem. Int. Ed. 2002, 41, 1668–1698; b) E. J. Corey,
Angew. Chem. Int. Ed. 2002, 41, 1650–1667.
Wang, S. Wang, Y. Chen, R. Gentles, T. Sowin, J. Org.
Chem. 2001, 66, 2052; e) X.-F. Zhu, F. Nydegger, A.
Gossauer, Helv. Chim. Acta 2004, 87, 2245–2265.
[9] K. Ishihara, K. Nakano, J. Am. Chem. Soc. 2005, 127,
10504–10505.
[10] C6F5SO3H is a weaker Brønsted acid than TsOH; a) K.
Ishihara, S. Nakagawa, A. Sakakura, J. Am. Chem. Soc.
2005, 127, 4168–4169; b) A. Sakakura, S. Nakagawa, K.
Ishihara, Tetrahedron 2006, 62, 422–433.
[11] Part of this work has been published as a preliminary
communication, see: A. Sakakura, K. Suzuki, K.
Nakano, K. Ishihara, Org. Lett. 2006, 8, 2229–2232.
[12] The reaction conducted in the presence of water (100
mol%) gave similar results. When the reaction was car-
ried out in the presence of more than 500 mol% of
water, the solvent was partially frozen and the reactivi-
ty was decreased.
[13] Unfortunately, when the Diels–Alder reaction of cyclo-
pentadiene was conducted in EtNO2, cyclopentadiene
polymerized and the Diels–Alder adducts were ob-
tained with low yield and enantioselectivity.
[14] The absolute stereochemistries of the Diels–Alder ad-
ducts of 2,3-dimethylbutadiene, isoprene and butadiene
were proposed to be 1S by analogy with cyclopentadi-
ene.[6a]
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