Diastereoselective oxidative α-amination of aliphatic aldehydes catalyzed by iodine: Synthesis of syn-γ-hydroxy-α-amino acetals

Article abstract of DOI:10.1039/c3ob42043d

Aldehydes can react with secondary amines to give α-amino acetals via the α-amination of aliphatic aldehydes catalyzed by iodine. The presence of an asymmetric hydroxylated center at the γ-position of the aldehyde was found to induce the stereoselective a

Full text of DOI:10.1039/c3ob42043d

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Diastereoselective oxidative α-amination of aliphatic
aldehydes catalyzed by iodine: synthesis of
syn-γ-hydroxy-α-amino acetals†
Cite this: Org. Biomol. Chem., 2013, 11,
8387
Yun-Xiao Zhang,a An-Qi Zhang,a Jie-Sheng Tianb and Teck-Peng Loh*b
Aldehydes can react with secondary amines to give α-amino acetals via the α-amination of aliphatic alde-
hydes catalyzed by iodine. The presence of an asymmetric hydroxylated center at the γ-position of the
aldehyde was found to induce the stereoselective amino group. This method represents a stereoselective
α-amination of γ-hydroxyaldehydes for the synthesis of syn-γ-hydroxy-α-amino acetals in good yields and
reasonable diastereoselectivities under very mild conditions.
Received 12th October 2013,
Accepted 17th October 2013
DOI: 10.1039/c3ob42043d
www.rsc.org/obc
Among 1,3-difunctionalized compounds, the chiral 1,3-amino- synthesis of chiral γ-hydroxy-α-amino acid derivatives. Herein
alcohol motif is one of the most important structural units in we report the preparation of syn-γ-hydroxy-α-amino acetals via
biologically active natural products and potential drugs such iodine-catalyzed α-amination of the aliphatic aldehyde.
as (−)-5-epi-Negamycin,1,2 Hyperaspine3 and HAP-12 (Fig. 1).4
Previous studies had shown that dibenzylamine is an ideal
Therefore, the development of stereo-controlled access to this amination reagent because α-amino acid derivatives bearing a
motif has attracted much attention in recent years. Although N,N-dibenzyl group do not epimerize easily.7 A preliminary
various methods for their construction have been established, investigation on the catalyst system using TBDPS-protected
most of these methods are based on stereoselective reduction γ-hydroxyaldehyde 1 and dibenzylamine as model substrates
of the corresponding chiral β-amino carbonyl compounds.5
revealed that I2/H2O2 in 1,2-dichloroethane gave moderate
Recently, oxidative rearrangement of tertiary amines for the yields with reasonable diastereoselectivities (Table 1, entry 1).
synthesis of α-amino acid derivatives was developed by our On the other hand, CuI/t-BuOOH in MeCN afforded low yield
group.6 We envisaged that introduction of a suitably protected but good diastereoselectivity.8 As such, we decided to use the
chiral γ-hydroxy group may serve to induce asymmetric α-ami- I2/H2O2 system for further studies.
nation of the aldehyde, to provide a novel strategy for the
Next, our investigation showed that the identity of the pro-
tecting groups of the γ-hydroxy group, including benzyl,
benzoyl, acetyl and t-butyldiphenylsilyl, influences the
diastereomeric ratio of the corresponding products, using the
I2/H2O2/DCE system. We reckoned that bulky protecting
groups such as the t-butyldiphenyl-silyl (TBDPS) group induce
good diastereoselectivity. It is noteworthy that low reaction
temperature did not affect the diastereoselectivities (Table 2,
entry 2 vs. 3, 6 vs. 7), whereas running the reaction at higher
temperature slightly reduced the diastereoselectivities,
although the reaction times were shortened and the overall
yields were improved (Table 2, entries 1 and 6).
Fig. 1 Examples of 1,3-aminoalcohol derivatives.
Subsequently, the scope of t-butyldiphenylsilyl protected
γ-hydroxyaldehydes for diastereoselective α-amination was
investigated, as shown in Table 3. All the γ-(tert-butyldiphenyl-
silyl)oxy aldehydes screened gave the corresponding products
in moderate to good yields under the standard reaction con-
ditions. It is worth mentioning that γ-aliphatic substrates gave
good diastereoselectivities (Table 3, entries 1, 9 and 10) and
the diastereomeric ratio of their α-aminated products corr-
elates with the steric size of the γ-aliphatic group. Low
aSchool of Chemistry and molecular Engineering, Zhengzhou University, Zhengzhou
450052, P.R. China
bDivision of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, Singapore 637616.
E-mail: teckpeng@ntu.edu.sg; Fax: +65 6515 8229; Tel: +65 6513 8475
†Electronic supplementary information (ESI) available: Detailed experimental
procedures and analytical data. CCDC 965932. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c3ob42043d
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Table 1 Optimization of the catalytic systemsa
Entry
Catalyst system
Solvent
Time (h)
Temp. (°C)
Isolated yield (%)
d.r.b (syn/anti)
1
2
I2 (0.2 equiv.), Na2CO3·1.5H2O2 (1.0 equiv.)
CuI (0.4 equiv.), 2 t-BuOOH (1.1 equiv.)
DCE
MeCN
72
72
r.t.
r.t.
62
31
85 : 15
82 : 18
a Reactions were performed with 1 (0.65 mmol, dissolved in 0.5 mL methanol and the corresponding solvent), dibenzylamine (0.78 mmol)
and the corresponding catalytic system. b Diastereomeric ratios were determined from crude 1H NMR.
Table 2 Optimization of the reaction conditionsa
Table 3 Scope of protected γ-hydroxyaldehydesa
Time
(h)
Temp.
(°C)
Isolated
yield (%)
d.r.b
(syn/anti)
Entry
R′
Product
1
2
3
4
5
6
7
Bn–
Bn–
Bn–
Ac–
PhCO–
TBDPS–
TBDPS–
24
72
72
24
24
24
72
40
r.t.
0
40
40
40
r.t.
1b
1b
1b
1c
1d
1a
1a
57
46
42
63
74
70
62
80 : 20
82 : 18
82 : 18
77 : 23
75 : 25
85 : 15
85 : 15
a Reactions were performed with the corresponding aldehyde
(0.65 mmol, dissolved in 2.5 mL (methanol–dichloroethane = 1 : 4)),
dibenzylamine (0.78 mmol), I2 (0.13 mmol), and Na2CO3·1.5H2O2
(0.65 mmol). b Diastereomeric ratio calculated by crude 1H NMR.
diastereoselectivities were obtained from γ-aromatic substrates
(Table 3, entries 2–8). The electronic nature of substituents on
the phenyl ring played only a minor role in determining the
diastereoselectivity of this reaction (Table 3, entry 6 vs. 8). On
the other hand, the position of these substituents on the
phenyl ring exerted a significant effect on the diastereomeric
ratio. ortho-Substituted γ-aromatic substrates proved to be
more sterically encumbered and hence induced higher
diastereoselectivities compared to the para-substituted ones
(Table 3, entry 5 vs. 3, 7 vs. 6). Moreover, we managed to
obtain the crystal structure of product 10ab successfully
(Fig. 2), which was confirmed to be the syn-isomer of 10a,
while the structures of other pairs of diastereoisomers were
assigned by comparison of their NMR data.
Based on the results shown in Table 3, we attempted to
understand the mechanism of the oxidative rearrangement
reaction as shown in Scheme 1. Enamine B is a crucial inter-
mediate in the reaction,6 while the I2/H2O2 catalyst system con-
stitutes a secondary cycle for (re)generation of the active
electrophilic iodine to react with B.9 The rearrangement prob-
ably involves the formation of aziridinium ions F and G from
B, triggered by iodination. The fact that the diastereomeric
ratio is dependent on the steric size of the γ substituent, with
a Reactions were performed with the corresponding aldehyde
(0.65 mmol, dissolved in 2.5 mL (methanol–dichloroethane = 1 : 4)),
dibenzylamine (0.78 mmol), I2 (0.13 mmol), and Na2CO3·1.5H2O2
(0.65 mmol). Diastereomeric ratios (syn/anti) were determined from
crude 1H NMR. The diastereoisomers were separated easily by silica
gel column chromatography except 5a. b The X-ray crystal structure of
10ab (syn-10a) is shown in Fig. 2.
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Fig. 2 The single crystal X-ray structure of syn-10a (10ab).10
Experimental section
Preparation of substrates: γ-protected hydroxyl aldehydes
All γ-protected hydroxyl aldehydes were made from γ-protected
hydroxyl primary alcohols by Swern oxidation. These alcohols
were synthesized via allylation of aldehydes, protection of
hydroxyl group of these homoallylic alcohols, and Brown
hydroboration reaction (see ESI†).
General procedure for Swern oxidation of γ-protected hydroxyl
primary alcohols (compound 1 as an example)
To a solution of oxalyl dichloride (0.74 mL, 8.5 mmol) in DCM
(20 mL) cooled to −78 °C was added dropwise a solution of
DMSO (0.65 mL, 9.2 mmol) in DCM (5.0 mL) and stirred for
10 min. A solution of 4-((tert-butyldiphenylsilyl)oxy)-6-phenyl-
hexan-1-ol (3.06 g, 7.1 mmol) in DCM (5.0 mL) was added
dropwise and stirred for 30 min at −78 °C. Et3N (4.95 mL,
35 mmol) was added in one portion and kept for 20 min and
then warmed to room temperature for 2 h. The reaction
mixture was washed with saturated NH4Cl, dried over MgSO4,
filtered and concentrated in vacuo. The crude product was pur-
ified via silica gel chromatography to afford the desired
compound.
Scheme 1 Proposed mechanism for the diastereoselective formation of
γ-((tert-butyldiphenylsilyl)oxy)-α-dibenzylamino acetals.
a preference for the syn isomer as established for 10a (Table 3),
suggests that F is the preferred conformation over G, in the
case when R is bulkier than R′O, on steric grounds.
4-((tert-Butyldiphenylsilyl)oxy)-6-phenylhexanal (1). Color-
less oil, 80% yield.
1H NMR (400 MHz, CDCl3): δ 9.62 (s, 1H), 7.65–7.68 (m,
4H), 7.35–7.45 (m, 6H), 7.11–7.22 (m, 3H), 6.96 (d, J = 7.2, 2H),
Conclusions
In summary, this method represents a stereoselective α-ami- 3.82–3.87 (m, 1H), 2.50 (t, J = 8.0, 2H), 2.44 (t, J = 7.2, 2H),
nation of γ-hydroxyaldehydes via the I2-catalyzed α-amination of 1.71–1.92 (m, 4H), 1.06 (s, 9H).
the aliphatic aldehyde, to give syn-γ-hydroxy-α-amino acetals, a
13C NMR (100 MHz, CDCl3): δ 202.2, 141.8, 135.9, 135.8,
1,3-aminoalcohol derivative. The reaction can be carried out 134.1, 134.0, 129.71, 129.67, 128.3, 128.2, 127.64, 127.59,
under very mild conditions, affording mainly the syn products 125.7, 71.8, 39.4, 38.0, 31.4, 28.1, 27.1, 19.4.
in moderate to good yields. Further studies on enhancing the
stereoselectivity and the application of this methodology to the
synthesis of complex molecules are in progress.
FTIR (neat): 1722 cm−1
.
HRMS: Calcd. for C28H35O2Si [M + H+]: 431.2406. Found:
431.2419.
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4-((tert-Butyldiphenylsilyl)oxy)-4-phenylbutanal (2). Color-
less oil, 70% yield.
1H NMR (400 MHz, CDCl3): δ 9.49 (s, 1H), 7.67–7.69 (m, 433.2197.
FTIR (neat): 1722 cm−1
HRMS: Calcd. for C27H33O3Si [M + H+]: 433.2199. Found:
.
2H), 7.32–7.45 (m, 6H), 7.21–7.28 (m, 7H), 4.80 (t, J = 5.6, 1H),
2.16–2.34 (m, 2H), 1.93–1.98 (m, 2H), 1.04 (s, 9H).
4-((tert-Butyldiphenylsilyl)oxy)-4-(2-methoxyphenyl)butanal
(7). Colorless oil, 71% yield.
13C NMR (100 MHz, CDCl3): δ 202.0, 143.4, 135.9, 135.8,
1H NMR (400 MHz, CDCl3): δ 9.52 (s, 1H), 7.67–7.69 (m,
134.0, 133.3, 129.8, 129.6, 128.1, 127.6, 127.4, 127.2, 126.1, 2H), 7.59 (d, J = 7.6, 1H), 7.31–7.46 (m, 6H), 7.16–7.23 (m, 3H),
74.3, 39.0, 32.1, 27.0, 19.3.
6.95 (t, J = 7.6, 1H), 6.70 (d, J = 8.0, 1H), 5.31 (t, J = 5.2, 1H),
3.59 (s, 3H), 2.17–2.36 (m, 2H), 1.91–2.04 (m, 2H), 1.05 (s, 9H).
13C NMR (100 MHz, CDCl3): δ 202.7, 155.3, 135.78, 135.76,
134.1, 133.5, 131.7, 129.6, 129.3, 127.9, 127.5, 127.3, 120.4,
FTIR (neat): 1722 cm−1
.
HRMS: Calcd. for C26H31O2Si [M + H+]: 403.2093. Found:
403.2087.
4-((tert-Butyldiphenylsilyl)oxy)-4-(p-tolyl)butanal (3). Color- 109.9, 67.8, 55.0, 38.8, 30.3, 27.0, 19.3.
less oil, 66% yield.
FTIR (neat): 1722 cm−1
HRMS: Calcd. for C27H33O3Si [M + H+]: 433.2199. Found:
.
1H NMR (400 MHz, CDCl3): δ 9.47 (s, 1H), 7.67–7.69 (m,
2H), 7.33–7.45 (m, 6H), 7.24 (t, J = 7.6, 2H), 7.11 (d, J = 8.0, 433.2195.
2H), 7.07 (d, J = 8.4, 2H), 4.77 (t, J = 5.6, 1H), 2.32 (s, 3H),
2.15–2.30 (m, 2H), 1.90–1.95 (m, 2H), 1.04 (s, 9H).
4-((tert-Butyldiphenylsilyl)oxy)-4-(4-fluorophenyl)butanal (8).
Colorless oil, 70% yield.
13C NMR (100 MHz, CDCl3): δ 202.1, 140.4, 136.7, 135.9,
1H NMR (400 MHz, CDCl3): δ 9.48 (s, 1H), 7.67 (d, J = 7.6,
135.8, 134.1, 133.4, 129.7, 129.5, 128.8, 127.6, 127.4, 126.1, 2H), 7.31–7.44 (m, 6H), 7.22 (t, J = 7.6, 2H), 7.15–7.18 (m, 2H),
74.2, 39.1, 32.2, 27.0, 21.1, 19.3.
FTIR (neat): 1722 cm−1
HRMS: Calcd. for C27H33O2Si [M + H+]: 417.2250. Found:
417.2238.
6.92 (d, J = 8.8, 2H), 4.78 (t, J = 5.6, 1H), 2.18–2.33 (m, 2H),
1.92–1.98 (m, 2H), 1.05 (s, 9H).
.
13C NMR (100 MHz, CDCl3): δ 202.6, 161.9 (d, J = 244.0),
139.3 (d, J = 3.1), 135.75, 135.72, 133.7, 133.1, 129.8, 129.6,
4-((tert-Butyldiphenylsilyl)oxy)-4-(4-ethylphenyl)butanal (4). 127.6 (d, J = 7.4), 127.4, 114.8 (d, J = 21.0), 73.8, 39.0, 32.1,
Colorless oil, 68% yield.
26.9, 19.2.
FTIR (neat): 1724 cm−1
HRMS: Calcd. for C26H30FO2Si [M + H+]: 421.1999. Found:
1H NMR (400 MHz, CDCl3): δ 9.48 (s, 1H), 7.68 (d, J = 6.8, 2H),
7.31–7.44 (m, 6H), 7.22 (t, J = 7.2, 2H), 7.13 (d, J = 8.0, 2H), 7.08
.
(d, J = 8.0, 2H), 4.77 (t, J = 5.6, 1H), 2.62 (q, J = 7.6, 2H), 2.16–2.33 421.1990.
(m, 2H), 1.92–1.97 (m, 2H), 1.22 (t, J = 7.6, 3H), 1.04 (s, 9H).
4-((tert-Butyldiphenylsilyl)oxy)undecanal (9). Colorless oil,
13C NMR (100 MHz, CDCl3): δ 202.1, 143.1, 140.6, 135.9, 68% yield.
135.8, 134.1, 133.4, 129.7, 129.5, 127.6, 127.5, 127.3, 126.1,
74.2, 39.1, 32.1, 28.5, 27.0, 19.3, 15.6.
1H NMR (400 MHz, CDCl3): δ 9.63 (s, 1H), 7.65–7.67 (m,
4H), 7.35–7.44 (m, 6H), 3.74–3.80 (m, 1H), 2.43 (t, J = 7.6, 2H),
1.77–1.85 (m, 1H), 1.64–1.73 (m, 1H), 1.37–1.42 (m, 2H),
FTIR (neat): 1722 cm−1
.
HRMS: Calcd. for C28H35O2Si [M + H+]: 431.2406. Found: 1.08–1.28 (m, 10H), 1.05 (s, 9H), 0.86 (t, J = 7.2, 3H).
431.2411.
13C NMR (100 MHz, CDCl3): δ 202.6, 135.9, 135.8, 134.3,
4-((tert-Butyldiphenylsilyl)oxy)-4-mesitylbutanal (5). Color- 134.2, 129.63, 129.58, 127.6, 127.5, 72.2, 39.4, 36.2, 31.7, 29.4,
less oil, 65% yield.
29.1, 28.1, 27.1, 25.0, 22.6, 19.4, 14.1.
FTIR (neat): 1722 cm−1
HRMS: Calcd. for C27H41O2Si [M + H+]: 425.2876. Found:
1H NMR (400 MHz, CDCl3): δ 9.37 (s, 1H), 7.70 (d, J = 6.8,
2H), 7.32–7.47 (m, 6H), 7.20 (t, J = 7.6, 2H), 6.83 (s, 1H), 6.63
.
(s, 1H), 4.98–5.01 (m, 1H), 2.65 (s, 3H), 2.24 (s, 3H), 2.05–2.21 425.2857.
(m, 4H), 1.80 (s, 3H), 1.04 (s, 9H).
4-((tert-Butyldiphenylsilyl)oxy)-4-cyclohexylbutanal
(10).
13C NMR (100 MHz, CDCl3): δ 201.7, 137.0, 136.3, 135.93, Colorless oil, 65% yield.
135.90, 134.9, 134.2, 133.3, 131.2, 129.7, 129.5, 128.6, 127.7,
127.3, 71.4, 40.4, 29.1, 27.0, 20.8, 20.3, 19.3.
1H NMR (400 MHz, CDCl3): δ 9.47 (s, 1H), 7.67 (d, J = 7.6,
4H), 7.35–7.44 (m, 6H), 3.53–3.56 (m, 1H), 2.19–2.39 (m, 2H),
1.62–1.70 (m, 7H), 1.33–1.38 (m, 1H), 1.04–1.09 (m, 5H), 1.06
FTIR (neat): 1722 cm−1
.
HRMS: Calcd. for C29H37O2Si [M + H+]: 445.2563. Found: (s, 9H).
445.2545.
13C NMR (100 MHz, CDCl3): δ 202.5, 136.0, 135.9, 134.4,
4-((tert-Butyldiphenylsilyl)oxy)-4-(4-methoxyphenyl)butanal
(6). Colorless oil, 88% yield.
134.2, 129.60, 129.59, 127.52, 127.49, 76.73, 43.2, 40.3, 28.6,
28.0, 27.2, 26.6, 26.45, 26.35, 25.2, 19.6.
1H NMR (400 MHz, CDCl3): δ 9.49 (s, 1H), 7.67 (d, J = 6.8,
2H), 7.33–7.45 (m, 6H), 7.24 (t, J = 7.2, 2H), 7.12 (d, J = 8.8,
FTIR (neat): 1722 cm−1
.
HRMS: Calcd. for C26H37O2Si [M + H+]: 409.2563. Found:
2H), 6.80 (d, J = 8.8, 2H), 4.77 (t, J = 5.6, 1H), 3.79 (s, 3H), 409.2556.
2.16–2.32 (m, 2H), 1.94 (dd, J1 = 7.2, J2 = 6.0, 2H), 1.03 (s, 9H).
γ-Protected hydroxy-α-amino acetals
13C NMR (100 MHz, CDCl3): δ 202.1, 158.7, 135.84, 135.76,
135.6, 134.1, 133.4, 129.7, 129.5, 127.6, 127.4, 127.2, 113.5,
74.0, 55.2, 39.2, 32.2, 27.0, 19.3.
Typical procedure for the diastereoselective synthesis of
γ-hydroxy-α-amino acetals via iodine-catalyzed (1a as an
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example). 4-((tert-Butyldiphenylsilyl)oxy)-6-phenylhexanal
1
127.0, 126.4, 107.6, 74.2, 55.5, 54.2, 54.0, 51.7, 36.3, 30.8, 27.0,
(0.279 g, 0.65 mmol) was added to a mixture of HNBn2 19.3.
(0.153 g, 0.78 mmol) and I2 (33 mg, 0.13 mmol) in methanol
(0.5 mL, 19 equiv.)–dichloroethane (2.0 mL), following the
Syn-2a (2ab):
1H NMR (400 MHz, CDCl3): δ 7.68 (d, J = 7.6, 2H), 6.95–7.44
addition of NaCO3·1.5H2O2 (0.101 g, 0.65 mmol). The mixture (m, 21H), 6.54 (d, J = 8.0, 2H), 4.60 (dd, J1 = 10.0, J2 = 4.0, 1H),
was stirred for 72 h at room temperature. The resulting reac- 4.18 (d, J = 4.8, 1H), 3.61 (d, J = 13.6, 2H), 3.48 (d, J = 13.6, 2H),
tion mixture was mixed with a small amount of silica gel and 2.95 (s, 3H), 2.88 (s, 3H), 2.36–2.39 (m, 1H), 2.12–2.19 (m, 1H),
concentrated. The crude product was purified by flash silica 1.98–2.04 (m, 1H), 0.94 (s, 9H).
gel column chromatography (triethylamine–diethyl ether–
13C NMR (100 MHz, CDCl3): δ 144.0, 140.5, 136.0, 135.94,
hexane = 0.1 : 1 : 100, v/v/v) to afford the desired product 1aa as 135.88, 133.9, 129.4, 129.2, 128.1, 127.6, 127.4, 127.21, 127.19,
a light yellowish oil and 1ab as a colorless crystal. The d.r. 126.65, 126.57, 106.1, 74.4, 54.5, 54.4, 53.9, 52.4, 38.0, 26.9, 19.3.
1
(1ab(syn)/1aa(anti)) was detected by crude H NMR before iso-
FTIR (neat): 3053, 1454, 1427, 1265 cm−1
.
lation. It is necessary to point out that most of the diastereoi-
HRMS: Calcd. for C42H50NO3Si [M + H+]: 644.3560. Found:
somers were separated easily by silica gel column 644.3555.
chromatography. The structure of compound 10ab was con-
N,N-Dibenzyl-4-((tert-butyldiphenylsilyl)oxy)-1,1-dimethoxy-
formed as a syn-isomer according to its crystal structure 4-(p-tolyl)butan-2-amine (3a). 76% Total yield.
(Fig. 2) and the structures of other pairs of diastereoisomers
were obtained by comparing their NMR data. For information
on 1b, 1c and 1d, see ESI.†
Anti-3a (3aa):
Light yellowish oil.
1H NMR (400 MHz, CDCl3): δ 7.70 (d, J = 7.6, 2H), 7.46 (d,
N,N-Dibenzyl-4-((tert-butyldiphenylsilyl)oxy)-1,1-dimethoxy- J = 7.6, 2H), 7.29–7.39 (m, 4H), 7.19–7.23 (m, 2H), 7.09–7.11
6-phenylhexan-2-amine (1a). Light yellowish oil, 62% total (m, 8H), 7.03 (d, J = 8.0, 2H), 6.90–6.92 (m, 4H), 4.90 (dd, J1 =
yield.
Anti-1a (1aa):
8.8, J2 = 5.6, 1H), 4.09 (d, J = 5.6, 1H), 3.54 (d, J = 13.6, 2H),
3.14 (d, J = 13.6, 2H), 3.05 (s, 3H), 2.75 (s, 3H), 2.38 (s, 3H),
1H NMR (400 MHz, CDCl3): δ 7.69–7.71 (m, 4H), 7.12–7.41 2.29–2.33 (m, 1H), 2.06–2.12 (m, 2H), 1.02 (s, 9H).
(m, 19H), 6.88 (d, J = 7.2, 2H), 4.10 (d, J = 4.4, 1H), 4.04–4.08
13C NMR (100 MHz, CDCl3): δ 141.5, 140.1, 136.3, 135.93,
(m, 1H), 3.64 (d, J = 13.6, 2H), 3.52 (d, J = 13.6, 2H), 3.122 (s, 135.89, 134.8, 134.2, 129.3, 129.2, 129.0, 128.7, 127.7, 127.4,
3H), 3.115 (s, 3H), 2.72–2.77 (m, 1H), 2.46–2.61 (m, 2H), 127.2, 127.1, 126.4, 107.5, 74.0, 55.5, 54.1, 54.0, 51.6, 36.4,
1.76–1.89 (m, 2H), 1.56–1.70 (m, 2H), 1.05 (s, 9H).
13C NMR (100 MHz, CDCl3): δ 142.7, 140.4, 136.03, 135.95,
135.0, 134.8, 129.4, 128.8, 128.4, 128.08, 128.07, 127.43,
127.39, 126.7, 125.4, 107.9, 71.4, 55.9, 55.2, 54.6, 54.2, 38.0,
34.1, 30.8, 27.1, 19.4.
27.0, 21.2, 19.3.
Syn-3a (3ab):
Colorless crystal, mp: 100.2 °C.
1H NMR (400 MHz, CDCl3): δ 7.68 (d, J = 7.6, 2H), 7.44 (d,
J = 8.0, 2H), 7.06–7.40 (m, 18H), 6.78 (d, J = 7.6, 2H), 6.45 (d,
J = 7.6, 2H), 4.59 (dd, J1 = 10.0, J2 = 4.0, 1H), 4.17 (d, J = 5.2,
Syn-1a (1ab):
1H NMR (400 MHz, CDCl3): δ 7.68–7.72 (m, 4H), 7.10–7.42 (m, 1H), 3.58 (d, J = 13.6, 2H), 3.48 (d, J = 13.6, 2H), 2.97 (s, 3H),
19H), 6.87 (d, J = 7.6, 2H), 4.26 (d, J = 5.2, 1H), 4.108–4.125 (m, 2.90 (s, 3H), 2.38–2.40 (m, 1H), 2.22 (s, 3H), 2.09–2.16 (m, 1H),
1H), 3.52 (d, J = 13.2, 2H), 3.46 (d, J = 13.2, 2H), 3.31 (s, 6H), 1.96–2.02 (m, 1H), 0.94 (s, 9H).
2.66–2.69 (m, 1H), 2.52–2.56 (m, 1H), 2.38–2.41 (m, 1H), 1.78–1.84
(m, 1H), 1.63–1.69 (m, 1H), 1.11–1.34 (m, 2H), 1.05 (s, 9H).
13C NMR (100 MHz, CDCl3): δ 141.0, 140.6, 136.05, 136.02,
136.0, 134.6, 134.0, 129.3, 129.2, 128.2, 128.0, 127.2, 127.1,
13C NMR (100 MHz, CDCl3): δ 143.0, 140.3, 136.05, 136.01, 126.5, 106.2, 74.2, 54.5, 54.4, 54.1, 52.5, 37.9, 26.9, 21.1, 19.3.
134.8, 134.6, 129.45, 129.42, 129.1, 128.4, 128.0, 127.5, 127.4,
126.7, 125.3, 107.2, 70.8, 55.2, 54.9, 54.4, 54.1, 36.9, 33.5, 30.1,
27.1, 19.4.
FTIR (neat): 3053, 1265 cm−1
HRMS: Calcd. for C43H52NO3Si [M + H+]: 658.3716. Found:
658.3716.
N,N-Dibenzyl-4-((tert-butyldiphenylsilyl)oxy)-4-(4-ethylphenyl)-
HRMS: Calcd. for C44H54NO3Si [M + H+]: 672.3873. Found: 1,1-dimethoxybutan-2-amine (4a). 65% Total yield.
.
FTIR (neat): 3053, 2305, 1636, 1422, 1265 cm−1
.
672.3870.
Anti-4a (4aa):
N,N-Dibenzyl-4-((tert-butyldiphenylsilyl)oxy)-1,1-dimethoxy-
4-phenylbutan-2-amine (2a). Light yellowish oil, 62% total
yield.
Light yellowish oil.
1H NMR (400 MHz, CDCl3): δ 7.69 (d, J = 7.2, 2H), 7.28–7.44
(m, 6H), 7.18–7.21 (m, 2H), 7.04–7.12 (m, 10H), 6.90–6.92 (m,
4H), 4.89–4.93 (m, 1H), 4.10 (d, J = 5.2, 1H), 3.55 (d, J = 13.6,
Anti-2a (2aa):
1H NMR (400 MHz, CDCl3): δ 7.69 (d, J = 6.8, 2H), 7.19–7.45 2H), 3.15 (d, J = 13.6, 2H), 3.06 (s, 3H), 2.74 (s, 3H), 2.38 (s,
(m, 13H), 7.09–7.13 (m, 6H), 6.90–6.92 (m, 4H), 4.92–4.96 (m, 3H), 2.67 (q, J = 7.6, 2H), 2.31–2.35 (m, 1H), 2.10–2.13 (m, 2H),
1H), 4.10 (d, J = 5.2, 1H), 3.56 (d, J = 13.6, 2H), 3.14 (d, J = 13.6, 1.28 (t, J = 7.6, 3H), 1.02 (s, 9H).
2H), 3.07 (s, 3H), 2.76 (s, 3H), 2.31–2.36 (m, 1H), 2.09–2.14 (m,
13C NMR (100 MHz, CDCl3): δ 142.8, 141.6, 140.2, 135.9,
2H), 1.02 (s, 9H).
134.8, 134.2, 129.3, 129.1, 129.0, 127.7, 127.5, 127.4, 127.2,
13C NMR (100 MHz, CDCl3): δ 144.5, 140.1, 135.9, 134.7, 127.1, 126.4, 107.6, 74.0, 55.5, 54.2, 54.0, 51.6, 36.3, 28.6, 27.0,
134.1, 129.4, 129.2, 129.0, 128.1, 127.8, 127.4, 127.3, 127.2, 19.3, 15.6.
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Syn-4a (4ab):
Colorless crystal, mp: 95.8 °C.
J = 13.2, 2H), 3.48 (d, J = 13.2, 2H), 2.99 (s, 3H), 2.93 (s, 3H),
2.36 (brs, 1H), 2.11–2.16 (m, 1H), 1.96–2.04 (m, 1H), 0.94 (s,
1H NMR (400 MHz, CDCl3): δ 7.68 (d, J = 7.2, 2H), 7.06–7.43 9H).
(m, 18H), 6.79 (d, J = 7.6, 2H), 6.49 (d, J = 7.6, 2H), 4.60 (dd,
13C NMR (100 MHz, CDCl3): δ 158.3, 140.5, 136.3, 136.0,
J1 = 10.0, J2 = 4.0, 1H), 4.16 (d, J = 5.2, 1H), 3.58 (d, J = 13.2, 135.9, 134.6, 134.0, 129.4, 129.23, 129.2, 128.3, 128.0, 127.4,
2H), 3.49 (d, J = 13.2, 2H), 2.94 (s, 3H), 2.90 (s, 3H), 2.51 (q, J = 127.2, 126.5, 112.9, 106.1, 73.9, 55.1, 54.5, 54.48, 54.0, 52.5,
7.6, 2H), 2.38–2.40 (m, 1H), 2.10–2.17 (m, 1H), 1.98–2.03 (m, 37.9, 26.9, 19.3.
1H), 1.14 (t, J = 7.6, 3H), 0.95 (s, 9H).
FTIR (neat): 3053, 1610, 1512, 1427, 1265 cm−1
HRMS: Calcd. for C43H52NO4Si [M + H+]: 674.3666. Found:
.
13C NMR (100 MHz, CDCl3): δ 142.6, 141.2, 140.6, 136.02,
135.96, 134.6, 134.0, 129.3, 129.18, 129.15, 128.0, 127.4, 127.2, 674.3667.
127.0, 126.5, 106.3, 74.3, 54.5, 54.4, 54.0, 52.5, 37.8, 28.5, 26.9,
19.3, 15.9.
N,N-Dibenzyl-4-((tert-butyldiphenylsilyl)oxy)-1,1-dimethoxy-
4-(2-methoxyphenyl)butan-2-amine (7a). Light yellowish oil,
47% total yield.
FTIR (neat): 3053, 1265 cm−1
.
HRMS: Calcd. for C44H54NO3Si [M + H+]: 672.3873. Found:
672.3870.
Anti-7a (7aa):
1H NMR (400 MHz, CDCl3): δ 7.67 (d, J = 7.2, 2H), 7.60 (brs,
N,N-Dibenzyl-4-((tert-butyldiphenylsilyl)oxy)-4-mesityl-1,1- 1H), 6.94–7.43 (m, 20H), 6.53 (d, J = 8.0, 1H), 5.38–5.41(m, 1H),
dimethoxybutan-2-amine (5a). Light yellowish oil, 61% total 4.03 (d, J = 4.4, 1H), 3.63 (d, J = 14.0, 2H), 3.27 (s, 3H), 3.22 (d,
yield.
Anti-5a (5aa) mixed with syn-5a (5ab):
1H NMR (400 MHz, CDCl3): δ 7.70 (d, J = 6.8), 7.09–7.40
J = 14.0, 2H), 3.02 (s, 3H), 2.95 (s, 3H), 2.52–2.56 (m, 1H),
2.19–2.26 (m, 2H), 0.99 (s, 9H).
13C NMR (100 MHz, CDCl3): δ 140.6, 135.9, 135.89, 134.7,
(overlap), 6.87–6.90 (overlap), 6.54 (s), 5.26 (dd, J1 = 10.8, J2 = 134.2, 129.1, 129.0, 128.9, 127.9, 127.7, 127.2, 127.0, 126.2,
6.4), 4.16 (d, J = 6.4), 3.57 (d, J = 13.6), 3.13 (d, J = 13.6), 120.6, 110.1, 107.6, 55.1, 54.6, 54.4, 54.2, 53.6, 27.0, 19.3.
3.01 (s), 2.86 (s), 2.58–2.77 (overlap), 1.98–2.37 (overlap),
Syn-7a (7ab):
1.54 (s), 1.01 (s).
1H NMR (400 MHz, CDCl3): δ 7.68 (d, J = 6.8, 2H), 7.10–7.39
13C NMR (100 MHz, CDCl3): δ 140.7–126.4 (overlap), 106.6, (m, 20H), 6.82 (brs, 1H), 6.61 (d, J = 8.4, 1H), 5.39 (brs, 1H),
71.17, 55.6–54.0 (overlap), 51.0, 34.2–14.1 (overlap).
Syn-5a (5ab) mixed with anti-5a (5aa):
4.02 (d, J = 4.0, 1H), 3.60 (d, J = 13.6, 2H), 3.41 (s, 3H), 3.38 (d,
J = 13.6, 2H), 3.08 (s, 3H), 2.97 (s, 3H), 2.63–2.66 (m, 1H),
1H NMR (400 MHz, CDCl3): δ 7.66 (d, J = 6.8, 2H), 7.09–7.40 2.17–2.20 (m, 2H), 0.95 (s, 9H).
(overlap), 6.87–6.90 (overlap), 6.72 (s, 1H), 6.50 (s, 1H), 5.21 (t,
13C NMR (100 MHz, CDCl3): δ 140.8, 136.0, 135.9, 134.5,
J = 7.2, 1H), 3.78 (d, J = 4.0, 1H), 3.71 (d, J = 14.4, 2H), 3.45 (d, 134.2, 129.3, 129.0, 128.8, 128.2, 128.0, 127.8, 127.3, 126.9,
J = 14.4, 2H), 2.98 (s, 1H), 2.96 (s, 1H), 2.58–2.77 (overlap), 126.2, 120.3, 109.9, 108.0, 54.93, 54.88, 54.6, 54.5, 34.7, 27.0,
1.98–2.37 (overlap), 1.59 (s, 3H), 0.92 (s, 9H).
19.3.
FTIR (neat): 3053, 1636, 1265 cm−1
HRMS: Calcd. for C43H52NO4Si [M + H+]: 674.3666. Found:
674.3668.
N,N-Dibenzyl-4-((tert-butyldiphenylsilyl)oxy)-4-(4-fluorophenyl)-
1,1-dimethoxybutan-2-amine (8a). 82% Total yield.
Anti-8a (8aa):
13C NMR (100 MHz, CDCl3): δ 140.7–126.4 (overlap), 108.3,
71.12, 55.6–54.0 (overlap), 51.0, 34.2–14.1 (overlap).
.
FTIR (neat): 3053, 1265 cm−1
.
HRMS: Calcd. for C45H56NO3Si [M + H+]: 686.4029. Found:
686.4032.
N,N-Dibenzyl-4-((tert-butyldiphenylsilyl)oxy)-1,1-dimethoxy-
4-(4-methoxyphenyl)butan-2-amine (6a). 74% Total yield.
Anti-6a (6aa):
Light yellowish oil.
1H NMR (400 MHz, CDCl3): δ 7.66–7.68 (m, 2H), 7.30–7.43
(m, 6H), 7.19–7.15 (m, 2H), 7.10–7.15 (m, 8H), 6.96–6.97 (m,
Light yellowish oil.
1H NMR (400 MHz, CDCl3): δ 7.69 (d, J = 6.8, 2H), 7.45 (d, 4H), 6.86 (t, J = 8.8, 2H), 4.92 (dd, J1 = 10.4, J2 = 4.4, 1H), 4.13
J = 7.2, 2H), 7.29–7.41 (m, 4H), 7.22–7.23 (m, 2H), 7.07–7.11 (d, J = 4.8, 1H), 3.58 (d, J = 13.6, 2H), 3.18 (d, J = 13.6, 2H), 3.10
(m, 8H), 6.94–6.96 (m, 4H), 6.74 (d, J = 8.4, 2H), 4.89 (dd, J1 = (s, 3H), 2.83 (s, 3H), 2.26–2.30 (m, 1H), 2.12–2.19 (m, 1H),
10.0, J2 = 4.8, 1H), 4.11 (d, J = 5.2, 1H), 3.83 (s, 3H), 3.57 (d, J = 1.98–2.05 (m, 1H), 1.02 (s, 9H).
13.6, 2H), 3.16 (d, J = 13.6, 2H), 3.07 (s, 3H), 2.79 (s, 3H),
2.30–2.33 (m, 1H), 2.02–2.15 (m, 2H), 1.02 (s, 9H).
13C NMR (100 MHz, CDCl3): δ 140.3, 139.9, 135.7, 134.4,
133.9, 129.4, 129.3, 128.8, 128.7, 128.6, 127.8, 127.4, 127.2,
13C NMR (100 MHz, CDCl3): δ 158.7, 140.1, 136.7, 135.91, 126.5, 114.8, 114.6, 107.8, 73.4, 55.7, 54.2, 52.2, 36.4, 27.0,
135.88, 134.8, 134.2, 129.3, 129.2, 128.9, 128.3, 127.8, 127.4, 19.3.
127.2, 113.4, 107.7, 73.7, 55.6, 55.2, 54.2, 54.1, 51.9, 36.4, 27.0,
Syn-8a (8ab):
19.3.
Colorless crystal, mp: 112.0 °C.
Syn-6a (6ab):
Colorless crystal, mp: 96.4 °C.
1H NMR (400 MHz, CDCl3): δ 7.65–7.67 (m, 2H), 7.30–7.42
(m, 6H), 7.22–7.24 (m, 2H), 7.14–7.19 (m, 6H), 7.06–7.07 (m,
1H NMR (400 MHz, CDCl3): δ 7.67 (d, J = 6.8, 2H), 7.06–7.44 4H), 6.61 (t, J = 8.8, 2H), 4.92 (dd, J1 = 8.4, J2 = 5.6, 2H), 4.55
(m, 18H), 6.50 (d, J = 8.0, 2H), 6.45 (d, J = 8.0, 2H), 4.57 (dd, (dd, J1 = 10.8, J2 = 4.0, 1H), 4.23 (d, J = 5.2, 1H), 3.64 (d, J =
J1 = 10.8, J2 = 2.8, 1H), 4.20 (d, J = 4.8, 1H), 3.72 (s, 3H), 3.62 (d, 13.6, 2H), 3.46 (d, J = 13.6, 2H), 3.00 (s, 3H), 2.94 (s, 3H),
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2.27–2.32 (m, 1H), 2.12–2.18 (m, 1H), 1.94–2.01 (m, 1H), 0.94 54.4, 54.3, 53.9, 39.6, 31.2, 30.7, 27.1, 26.8, 26.4, 26.3, 24.3,
(s, 9H). 19.6.
13C NMR (100 MHz, CDCl3): δ 140.4, 139.8, 135.94, 135.89, FTIR (neat): 3053, 1634, 1265 cm−1
134.3, 133.8, 129.4, 129.32, 129.27, 128.8, 128.7, 128.1, 127.5,
HRMS: Calcd. for C42H56NO3Si [M + H+]: 650.4029. Found:
.
127.2, 126.6, 114.3, 114.1, 105.8, 73.7, 54.5, 54.4, 53.8, 52.4, 650.4033.
38.0, 26.9, 19.2.
FTIR (neat): 3053, 1634, 1265 cm−1
.
HRMS: Calcd. for C42H49FNO3Si [M + H+]: 662.3466. Found:
622.3467.
Acknowledgements
N,N-Dibenzyl-4-((tert-butyldiphenylsilyl)oxy)-1,1-dimethoxy-
undecan-2-amine (9a). Light yellowish oil, 67% total yield.
Anti-9a (9aa):
We gratefully acknowledge the Nanyang Technological
University, the Singapore Ministry of Education Academic
Research
Fund
AcRF
Tier
2
(MOE2010-T2-2-067,
1H NMR (400 MHz, CDCl3): δ 7.67–7.69 (m, 4H), 7.32–7.40
(m, 6H), 7.15–7.28 (m, 10H), 4.11 (d, J = 4.4, 1H), 3.97–4.01 (m,
1H), 3.64 (d, J = 14.0, 2H), 3.55 (d, J = 13.6, 2H), 3.16 (s, 3H),
3.15 (s, 3H), 2.75–2.80 (m, 1H), 1.74 (t, J = 6.4, 2H), 1.01–1.26
(m, 21H), 0.86 (t, J = 7.2, 3H).
MOE2011-T2-1-013) and ETRP 1002111 for the financial
support of this research.
13C NMR (100 MHz, CDCl3): δ 140.5, 136.0, 135.9, 135.1,
129.3, 129.2, 128.9, 128.0, 127.32, 127.3, 126.6, 107.8, 71.7,
56.0, 55.0, 54.7, 54.1, 36.3, 34.2, 31.8, 29.6, 29.2, 27.1, 24.6,
22.7, 19.4, 14.1.
Notes and references
1 J. J. Masters and L. S. Hegedus, J. Org. Chem., 1993, 58,
4547.
Syn-9a (9ab):
2 (a) M. Hamada, T. Takeuchi, S. Kondo, Y. Ikeda,
H. Naganawa, K. Maeda, Y. Okami and H. Umezawa, J. Anti-
biot., 1970, 23, 170; (b) S. Shibahara, S. Kondo, K. Maeda,
H. Umezawa and M. Ohno, J. Am. Chem. Soc., 1972, 94,
4353; (c) S. Kondo, H. Yamamoto, K. Maeda and
H. Umezawa, J. Antibiot., 1971, 24, 732; (d) S. Kondo,
K. Yoshida, T. Ikeda, Y. Iinuma, Y. Honma, M. Hamada
and H. Umezawa, J. Antibiot., 1977, 30, 1137.
3 (a) W. Zhu and D. Ma, Org. Lett., 2003, 5, 5063;
(b) B. Lebrun, J.-C. Braekman, D. Daloze, P. Kalushkov and
J. M. Pasteels, Tetrahedron Lett., 2001, 42, 4621.
4 (a) S. Yasuda, H. Kitagawa, M. Ueno, H. Ishitani,
M. Fukasawa, M. Nishijima, S. Kobayashi and K. Hanada,
J. Biol. Chem., 2001, 276, 43994; (b) N. Kudo, K. Kumagai,
R. Matsubara, S. Kobayashi, K. Hanada, S. Wakatsuki and
R. Kato, J. Biol. Chem., 2010, 396, 245; (c) A. Ďuriš,
T. Wiesenganger, D. Moravčíková, P. Baran, J. Kožíšek,
A. Daïch and D. Berkeš, Org. Lett., 2011, 13, 1642.
1H NMR (400 MHz, CDCl3): δ 7.65–7.69 (m, 4H), 7.11–7.40
(m, 16H), 4.24 (d, J = 5.2, 1H), 4.01–4.04 (m, 1H), 3.44–3.52 (m,
4H), 3.294 (s, 3H), 3.291 (s, 3H), 2.62–2.66 (m, 1H), 1.69–1.76
(m, 1H), 1.54–1.61 (m, 1H), 1.06–1.28 (m, 9H), 1.01 (s, 9H),
0.86–0.96 (m, 6H).
13C NMR (100 MHz, CDCl3): δ 140.4, 136.01, 135.99, 135.0,
134.8, 129.3, 129.1, 127.9, 127.34, 127.3, 126.6, 107.3, 71.2,
55.1, 54.7, 54.12, 54.07, 35.0, 33.7, 31.8, 29.6, 29.2, 27.0, 23.8,
22.6, 19.3, 14.1.
FTIR (neat): 3053, 1634, 1421, 1265 cm−1
HRMS: Calcd. for C43H60NO3Si [M + H+]: 666.4342. Found:
666.4345.
N,N-Dibenzyl-4-((tert-butyldiphenylsilyl)oxy)-4-cyclohexyl-
1,1-dimethoxybutan-2-amine (10a). 70% Total yield.
Anti-10a (10aa):
.
Light yellowish oil.
1H NMR (400 MHz, CDCl3): δ 7.69–7.72 (m, 4H), 7.32–7.41
(m, 6H), 7.14–7.23 (m, 10H), 3.87–3.92 (m, 2H), 3.55 (d, J =
14.0, 2H), 3.39 (d, J = 14.0, 2H), 3.09 (s, 3H), 3.06 (s, 3H),
2.66–2.70 (m, 1H), 1.55–1.79 (m, 8H), 1.20–1.39 (m, 5H), 1.05
(s, 9H).
5 (a) R. Millet, A. M. Träff, M. L. Petrus and J. E. Bäckvall,
J. Am. Chem. Soc., 2010, 132, 15182; (b) J. Barluenga,
A. L. Viado, E. Aguilar, S. Fustero and B. Olano, J. Org.
Chem., 1993, 58, 5972.
13C NMR (100 MHz, CDCl3): δ 140.4, 136.14, 136.1, 135.5,
134.8, 129.3, 129.2, 128.9, 128.0, 127.3, 127.2, 126.6, 108.0,
75.6, 56.0, 55.2, 54.3, 54.2, 41.6, 31.0, 30.3, 29.9, 27.2, 26.8,
26.6, 26.5, 25.9, 19.6.
6 (a) J. S. Tian and T. P. Loh, Angew. Chem., Int. Ed., 2010, 49,
8417; (b) J. S. Tian and T. P. Loh, Chem. Commun., 2011, 47,
5458; (c) J. S. Tian, K. W. J. Ng, J. R. Wong and T. P. Loh,
Angew. Chem., Int. Ed., 2012, 51, 9105.
Syn-10a (10ab):
Colorless crystal, mp: 122.3 °C.
7 (a) M. T. Reetz, M. W. Drewes and A. Schmitz, Angew.
Chem., Int. Ed. Engl., 1987, 26, 1141; (b) M. T. Reetz,
M. W. Drewes, K. Lennick, A. Schmitz and X. Holdgrün,
Tetrahedron: Asymmetry, 1990, 1, 375.
8 (a) X. Li, L. Huang, H. Chen, W. Wu, H. Huang and
H. Jiang, Chem. Sci., 2012, 3, 3463; (b) H. Jiang, W. Zeng,
Y. Li, W. Wu, L. Huang and W. Fu, J. Org. Chem., 2012, 77,
5179; (c) K. Xu, G. Lai, Z. Zha, S. Pan, H. Chen and
Z. Wang, Chem.–Eur. J., 2012, 18, 12357.
1H NMR (400 MHz, CDCl3): δ 7.64–7.68 (m, 4H), 7.09–7.35
(m, 16H), 4.11 (d, J = 5.6, 1H), 3.92 (dd, J1 = 10.0, J2 = 4.4, 1H),
3.40 (d, J = 13.6, 2H), 3.35(d, J = 13.6, 2H), 3.28 (s, 3H), 3.24 (s,
3H), 2.59–2.64 (m, 1H), 1.39–1.61 (m, 8H), 1.18–1.26 (m, 3H),
1.01 (s, 9H), 0.56–0.79 (m, 2H).
13C NMR (100 MHz, CDCl3): δ 140.3, 136.12, 136.1, 135.4,
134.5, 129.3, 129.2, 128.0, 127.3, 127.2, 126.6, 107.6, 75.0, 55.1,
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Org. Biomol. Chem., 2013, 11, 8387–8394 | 8393

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8394 | Org. Biomol. Chem., 2013, 11, 8387–8394
This journal is © The Royal Society of Chemistry 2013