Synthesis of the C1-C13 subunit of spirastrellolides A and B by prins cyclization

Article abstract of DOI:10.1055/s-0029-1218588

The C1-C13 subunit of the marine natural products spirastrellolides A and B, which contains a cis-substituted tetrahydropyran ring, was prepared by using the Prins cyclization strategy. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0029-1218588

PAPER
505
Synthesis of the C1–C13 Subunit of Spirastrellolides A and B by Prins
Cyclization
C1–C13Subunit
o
o
f
S
pirastrello
w
lide
s
AandB ravaram Sabitha,* A. Senkara Rao, Jhillu S. Yadav
Organic Division I, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Fax +91(40)27160512; E-mail: gowravaramsr@yahoo.com
Received 26 August 2009; revised 8 September 2009
been determined by X-ray diffraction analysis.4 A number
of elegant syntheses of major fragments of 1 and 2 have
been reported along with total syntheses.5 Interest in the
synthesis of spirastrellolides derives not only from their
unique molecular architecture, but also from their poten-
tial as lead structures for the development of novel antitu-
mor therapeutic agents.6
Abstract: The C1–C13 subunit of the marine natural products spi-
rastrellolides A and B, which contains a cis-substituted tetrahydro-
pyran ring, was prepared by using the Prins cyclization strategy.
Key words: tetrahydropyrans, diastereoselectivity, homoallylic al-
cohols, aldehydes, cyclizations
Prins cyclization has emerged as a powerful tool for the
formation of tetrahydropyran rings of natural products.7 In
continuation of our interest in the Prins reaction8 and its
applications in synthesis,9 we present preliminary results
on our synthesis of the C1–C13 tetrahydropyran subunit
of spirastrellolides A and B through application of the
Prins cyclization approach. A retrosynthetic analysis for 1
and 2 is shown in Scheme 1. The Prins reaction of ho-
moallylic alcohol 4 with the aldehyde 5, followed by rad-
ical dehalogenation, should provide subunit 3. In turn,
fragments 4 and 5 could be prepared from the known
chiral epoxides 6 and 7.
Marine organisms, particularly sponges, are a prolific
source of novel biologically active compounds with un-
usual, often complex, structures.1 Spirastrellolides A and
B (1 and 2, respectively) are complex polyketide natural
products, isolated in 2003 by Andersen and co-workers
from a Caribbean marine sponge Spirastrella coccinea
collected in Dominica.2 Besides 1 and 2, five new mac-
rolides, the spirastrellolides C through G, have also been
isolated from the same species.3 Spirastrellolides repre-
sent an entirely new structural class of potent antimitotic
agents, and preliminary biological studies have shown
that they act as a specific inhibitors of protein phosphatase
2A,2b and cause premature entry of cells into mitosis,
which may have chemotherapeutic utility. The absolute
configuration of the spirastrellolide macrocyclic core has
The homoallylic alcohol fragment 4 was prepared from
the known epoxide 6,10 either by ring opening with vinyl-
magnesium bromide11 in the presence of copper(I) cya-
OH
HO
O
OMe
O
O
1
O
O
O
R
7
3
9
11
O
OBn
BnO
3
1
13
5
O
HO
OH OH
HO
7
5
11
3
13
9
OH
O
X
O
HO
X
BnO
OH
OBn
CHO
spirastrellolide A (1): X–X = CH=CH, R = Cl
spirastrellolide B (2): X–X = CH2CH2, R = H
+
O
O
5
4
O
O
BnO
BnO
6
7
Scheme 1 Retrosynthetic analysis
SYNTHESIS 2010, No. 3, pp 0505–0509
x
x
.
x
x
.2
0
0
9
Advanced online publication: 07.12.2009
DOI: 10.1055/s-0029-1218588; Art ID: Z17809SS
© Georg Thieme Verlag Stuttgart · New York

506
G. Sabitha et al.
PAPER
nide or, in two steps, by treatment with lithium acetylide12 acetonide 11 was prepared under conventional reaction
in dimethyl sulfoxide at room temperature to give the cor- conditions by using 2,2-dimethoxypropane in dichlo-
responding homopropargylic alcohol, and subsequent romethane containing pyridinium p-toluenesulfonate as a
controlled hydrogenation with Lindlar’s catalyst catalyst. The stereochemical assignment of the newly cre-
(Scheme 2).
ated center was made on the basis of Rychnovsky’s anal-
ogy,15 as the 13C NMR spectrum of 11 exhibited peaks for
the acetonide methyl carbons at d = 24.4 and 24.5 and for
the quaternary carbon at d = 100.5, which are characteris-
tic of the acetonide of an anti-1,3-diol moiety. The re-
quired aldehyde 5 was prepared in 86% yield in two steps
by reduction with lithium aluminum hydride (95%) fol-
lowed by oxidation with 2-iodobenzoic acid (Scheme 3).
OH
O
a
BnO
BnO
4
6
Scheme 2 Reagents and conditions: a) vinylmagnesium bromide,
CuCN, THF, –78 to –40 °C, or 1. lithium acetylide, DMSO, 0 °C to
r.t., 1 h, 92%; 2. H2, Pd/CaCO3 (cat.), quinoline, EtOAc, r.t., 95%.
Prins cyclization of homoallylic alcohol 4 with aldehyde
5 (which has the required chiral centers) in the presence of
iron(III) chloride in dichloromethane gave a 4-chlorotet-
rahydropyran intermediate (62%), the 2,4,6-syn-isomer
12 of which was the major product. The reaction simulta-
neously resulted in the deprotection of the acetonide
group (Scheme 4). Finally, the product was subjected to
radical dehalogenation9a by using by treatment with 2,2-
azobis(isobutyronitrile) and tributylstannane, which com-
pleted the synthesis of the C1–C13 subunit 3 containing
the cis-substituted tetrahydropyran ring.
The aldehyde fragment 5 was prepared from the known
chiral epoxide 7.10 Accordingly, the opening of epoxide 7
was carried out as described above for the opening of 6 to
provide the homoallylic alcohol 8 in 95% yield for the two
steps. After the oxidative cleavage of the olefin (osmium
tetroxide/N-methylmorpholine N-oxide then sodium per-
iodate), the resulting aldehyde was converted directly into
the b-keto ester 9 by treatment with ethyl diazoacetate.
The anti-selective reduction of 9 with tetramethylammo-
nium triacetoxyborohydride in acetone–acetic acid (1:1)
at 25 °C, resulted in selective formation of the anti 3,5-di-
hydroxy ester 10 in 79% yield (syn/anti 1:9). The mixture
was separated by flash column chromatography. The
In conclusion, we have accomplished a concise synthesis
of the C1–C13 tetrahydropyran subunit of spira-
strellolides A and B. This work highlights the utility of the
OH
OH
O
O
O
a
b
BnO
BnO
BnO
O
OEt
7
9
8
OH OH
O
O
O
c
d
BnO
OEt
BnO
OEt
11
10
O
O
e
CHO
BnO
5
Scheme 3 Reagents and conditions: a) vinylmagnesium bromide, CuCN, THF, –78 to –40 °C, 95%, or 1. lithium acetylide, DMSO, 0 °C to
r.t., 1 h, 92%; 2. Pd/CaCO3, H2, quinoline, EtOAc, r.t., 95%; b) 1. OsO4/NMO, acetone–H2O (4:1) then NaIO4, THF–H2O (2:1), 75%. 2.
N2CHCO2Et, SnCl2, CH2Cl2, 0 °C to r.t, 55%; c) Me4NBH(OAc)3, 25 °C, 1 h, acetone–AcOH (1:1), 85%; d) Me2C(OMe)2, PPTS (cat.),
CH2Cl2, 95%; e) 1. LAH, THF, r.t., 95%; 2. 2-iodobenzoic acid, DMSO, CH2Cl2, 86%.
OH
OHC
OBn
a
BnO
+
O
O
4
5
BnO
O
BnO
O
b
OBn
OBn
OH OH
OH OH
12
Cl
3
Scheme 4 Reagents and conditions: a) FeCl3, CH2Cl2, 0 °C to 20 °C, 1 h, 62%; b) Bu3SnH/AIBN, benzene, reflux, 12 h, 90%.
Synthesis 2010, No. 3, 505–509 © Thieme Stuttgart · New York

PAPER
C1–C13 Subunit of Spirastrellolides A and B
507
by column chromatography (silica gel) to give the corresponding di-
ol. NaIO4 (2.48 g, 11.64 mmol) was then added to a stirred soln of
the diol in 2:1 THF–H2O (20 mL) at 0 °C. After 2 h, the mixture was
filtered, and the filter was rinsed with Et2O (3 × 30 mL). The filtrate
was washed with H2O and brine, then dried (Na2SO4) and concen-
trated to give the aldehyde, which was used in the next reaction.
Prins and radical reactions. Elaboration of this fragment is
underway, and the results will be published in due course.
Reactions were conducted under N2 in anhydrous solvents such as
CH2Cl2, THF, or EtOAc. All reactions were monitored by TLC [sil-
ica-coated plates visualized under UV light; hexane (bp 60–80 °C].
Yields refer to chromatographically and spectroscopically (1H and
13C NMR) homogeneous materials. Air-sensitive reagents were
transferred by syringe or double-ended needle. Solvents were evap-
orated under reduced pressure in a Buchi rotary evaporator. 1H and
13C NMR spectra of samples in CDCl3 were recorded on Varian FT-
200 MHz (Gemini) and Bruker UXNMR FT-300 MHz (Avance)
spectrometers. The chemical shift d is reported relative to TMS
(d = 0.0 ppm) as an internal standard. Mass spectra were recorded
under EI conditions at 70 eV on an LC-MSD (Agilent Technolo-
gies, Santa Clara, CA, USA) spectrometer. All high-resolution
spectra were recorded on a QSTAR XL hybrid MS/MS system (Ap-
plied Biosystems/MDS Sciex, Foster City, CA, USA), equipped
with an ESI source (IICT, Hyderabad, India). Column chromatog-
raphy was performed on silica gel (60–120 mesh; Acme Chemical
Co., India). TLC was performed on Merck 60 F-254 silica gel
plates. Optical rotations were measured with JASCO DIP-370 pola-
rimeter at 20 °C. The ee of the products was determined by chiral
HPLC [Dionex, Chromeleon PDA].
Anhyd CH2Cl2 (25 mL) and N2CHCO2Et (1.11 mL, 10.52 mmol)
were added sequentially to anhyd SnCl2 (1.99 g, 10.52 mmol) with
stirring at 0 °C. A few drops of a soln of the crude aldehyde (2.0 g,
9.61 mmol) in anhyd CH2Cl2 (3 mL) were slowly added to this sus-
pension. When evolution of N2 began, the remainder of the alde-
hyde soln was added dropwise over 10 min. When the evolution of
N2 stopped (30 min), the mixture was transferred to a separatory
funnel with sat. brine (30 mL) and extracted with Et2O (2 × 20 mL).
The combined organic layers were dried (Na2SO4) and concentrat-
ed, and the residue was purified by column chromatography to give
25
a yellow liquid; yield: 1.99 g (70%); [a]D –12.94 (c 0.0085,
CHCl3).
IR (neat): 3447, 2925, 2860, 1739, 1713, 1631, 1452, 1409, 1313,
1207, 1097, 1028, 747, 699 cm–1.
1H NMR (CDCl3, 400 MHz): d = 1.28 (t, J = 7.3 Hz, 3 H), 1.72–
1.82 (m, 2 H), 2.68 (d, J = 6.6, 2 H), 3.41 (s, 2 H), 3.59–3.72 (m, 2
H), 4.18 (q, J = 7.3 Hz, 2 H), 4.21–4.27 (m, 1 H), 4.49 (s, 2 H),
7.21–7.37 (m, 5 H).
13C NMR (CDCl3, 75 MHz): 13.9, 35.7, 49.7, 49.9, 61.4, 66.7, 67.9,
73.3, 127.6, 127.7, 128.0, 137.9, 166.9, 202.8.
ESI-MS: m/z 317 [M + Na]+.
HRMS-ESI: m/z [M + Na]+ calcd for C16H22NaO5: 317.1335; found:
315.1338.
(3R)-1-(Benzyloxy)hex-5-en-3-ol (4)
Br(CH2)2Br (2 drops) and freshly prepared CH2=CHBr (1.58 mL,
22.5 mmol) were sequentially added in a dropwise manner to Mg
turnings (0.54 g, 22.5 mmol) in anhyd THF (25 mL) at r.t., and the
mixture was stirred for 0.5 h. CuCN (10 mg) was added and the
mixture was cooled to –78 °C. A soln of epoxide 6 (2 g, 11.23
mmol) in THF (5 mL) was added. The mixture was allowed to warm
to –40 °C and stirred for 4 h. The reaction was quenched with sat.
aq NH4Cl (20 mL), and the mixture was extracted with EtOAc (2 ×
20 mL). The combined organic layers were washed with brine (20
mL), dried (Na2SO4; 1 g), and concentrated under reduced pressure.
The residue was purified by column chromatography to give a yel-
low liquid; yield: 2.1 g (95%); [a]D25 +5.98 (c 0.01, CHCl3).
Ethyl (3S,5R)-7-(Benzyloxy)-3,5-dihydroxyheptanoate (10)
A soln of the hydroxy ketone 9 (1.70 g, 5.78 mmol) in acetone (2
mL) was added to a soln of Me4NBH(OAc)3 (1.67 g, 6.34 mmol) in
1:1 AcOH–acetone (12 mL), and the mixture was stirred at r.t. for 2
h. The reaction was then quenched with sat. aq NH4Cl (10 mL).
When effervescence ceased, the soln was treated with 1.0 M aq po-
tassium tartrate (5 mL) and stirred for 20 min. The aqueous soln was
then extracted with EtOAc (10 × 2 mL). The combined organic lay-
ers were washed with sat. brine (10 mL), dried (Na2SO4), filtered,
and concentrated in vacuo. The residue was purified by column
IR (neat): 3423, 2924, 2865, 1717, 1276, 1099, 741, 700 cm–1.
1H NMR (CDCl3, 300 MHz): d = 1.67–1.80 (m, 2 H), 2.16–2.28 (m,
2 H), 3.54–3.75 (m, 1 H), 3.83 (t, J = 5.3 Hz, 2 H), 4.51 (s, 2 H),
5.00–5.14 (m, 2 H), 5.70–5.90 (m, 1 H), 7.17–7.40 (m, 5 H).
13C NMR (CDCl3, 75 MHz): 35.7, 41.8, 69.0, 70.3, 73.2, 117.5,
127.6, 127.7, 128.4, 134.7, 137.8.
ESI-MS: m/z 229 [M + Na]+.
HRMS-ESI: m/z [M + Na]+ calcd for C13H18NaO2: 229.1204; found:
25
chromatography to give a yellow liquid; yield 1.62 g (85%); [a]D
–2.35 (c 0.0085, CHCl3).
IR (neat): 3437, 2929, 2864, 1728, 1371, 1093, 1028, 742, 699 cm–1.
1H NMR (CDCl3, 300 MHz): d = 1.27 (q, J = 7.3 Hz, 3 H), 1.58–
1.91 (m, 4 H), 2.40–2.53 (m, 2 H), 3.60–3.74 (m, 2 H), 4.07–4.18
(m, 3 H), 4.25–4.33 (m, 1 H), 4.51 (s, 2 H), 7.23–7.34 (m, 5 H).
13C NMR (CDCl3, 75 MHz): d = 18.0, 25.7, 37.0, 52.0, 55.2, 55.7,
70.2, 71.7, 72.7, 127.3, 127.5, 128.2, 138.5, 168.1.
ESI-MS: m/z 297 [M + H]+.
HRMS-ESI: m/z [M + Na]+ calcd for C16H24NaO5: 319.1521; found:
229.1208.
(3S)-1-(Benzyloxy)hex-5-en-3-ol (8)
This compound was prepared in an identical manner to homoallylic
alcohol 4, except that the stereoisomeric epoxide 7 was used instead
of 6. The spectroscopic properties of the product were identical to
those of 4.
319.1523.
Yellow liquid; yield: 2.1 g (95%); [a]D25 –10.00 (c 0.01, CHCl3).
Ethyl 2-{(4R,6S)-6-[2-(Benzyloxy)ethyl]-2,2-dimethyl-1,3-diox-
an-4-yl}acetate (11)
Ethyl (5R)-7-(Benzyloxy)-5-hydroxy-3-oxoheptanoate (9)
NMO (1.24 g, 10.59 mmol) and the homallylic alcohol 8 (2.0 g, 9.70
mmol) were dissolved in 4:1 acetone–H2O (20 mL). A 0.02 M soln
of OsO4 in toluene (0.4 mL) was added, and the soln was stirred for
12 h at r.t., then cooled in an ice bath. The reaction was quenched
by addition of sat. aq NaHCO3 (15 mL). Most of the acetone was re-
moved by rotary evaporation, and the aqueous mixture was extract-
ed with EtOAc (3 × 40 mL). The combined extracts were
concentrated under reduced pressure, and the residue was purified
Me2C(OMe)2 (0.94 mL, 7.6 mmol) and a catalytic amount of PPTS
(0.127 g, 0.506 mmol) were added to a soln of hydroxy ester 10
(1.50 g, 5.06 mmol) in anhyd CH2Cl2 (10 mL), and the mixture was
stirred at r.t. for 1 h. NaHCO3 was added to neutralize the PPTS, and
the mixture was filtered and concentrated to give a residue that was
purified by column chromatography (silica gel) to give a brown liq-
uid; yield: 1.61 g (95%); [a]D25 –25.0 (c 0.008, CHCl3).
IR (neat): 3446, 2986, 2928, 2858, 1737, 1223, 1167, 1121, 1030,
740 cm–1.
Synthesis 2010, No. 3, 505–509 © Thieme Stuttgart · New York

508
G. Sabitha et al.
PAPER
1H NMR (CDCl3, 300 MHz): d = 1.26 (t, J = 7.1 Hz, 3 H), 1.28 (s,
3 H), 1.31 (s, 3 H), 1.52–1.68 (m, 2 H), 1.73 (q, J = 6.4 Hz, 2 H),
2.35 (dd, J = 5.4, 15.4 Hz, 1 H), 2.47 (dd, J = 8.1, 15.4 Hz, 1 H),
3.45–3.55 (m, 2 H), 3.91–4.03 (m, 1 H), 4.12 (q, J = 6.2 Hz, 2 H),
4.17–4.28 (m, 1 H), 4.46 (s, 2 H), 7.20–7.34 (m, 5 H).
13C NMR (CDCl3, 75 MHz): d = 14.1, 24.4, 24.5, 35.8, 37.8, 40.7,
60.3, 63.4, 66.4, 73.0, 100.5, 127.5, 127.6, 128.3, 138.3, 170.8.
ESI-MS: m/z 499 [M + Na].
HRMS-ESI: m/z [M + Na]+ calcd for C27H37ClNaO5: 499.2227;
found: 400.2229.
(2S,4R)-6-(Benzyloxy)-1-{(2S,6R)-6-[2-(benzyloxy)ethyl]tet-
rahydro-2H-pyran-2-yl}hexane-2,4-diol (3)
Bu3SnH (1.41 mL 5.25 mmol) and a catalytic amount of AIBN
(0.018 g, 0.105 mmol) were added to soln of 12 (0.5 g, 1.05 mmol)
in anhyd benzene (20 mL) under N2, and the soln was refluxed for
12 h. The mixture was then allowed to cool to r.t. and concentrated
in vacuo. The residue was purified by flash column chromatography
ESI-MS: m/z 359 [M + Na].
HRMS-ESI: m/z [M + Na]+ calcd for C19H28NaO5: 359.1834; found:
359.1830.
25
(silica gel) to give a colorless liquid; yield: 0.41 g (90%); [a]D
–15.0 (c 0.018, CHCl3).
{(4S,6R)-6-[2-(Benzyloxy)ethyl]-2,2-dimethyl-1,3-dioxan-4-
yl}acetaldehyde (5)
IR (neat): 3457, 2928, 2858, 1719, 1452, 1368, 1273, 1200, 1098,
740, 700 cm–1.
1H NMR (CDCl3, 400 MHz): d = 1.14–1.92 (m, 14 H), 3.45–3.72
(m, 6 H), 4.01–4.11 (m, 1 H), 4.12–4.21 (m, 1 H), 4.46 (s, 2 H), 4.50
(s, 2 H), 7.19–7.36 (m, 10 H).
13C NMR (CDCl3, 75 MHz): d = 23.4, 30.9, 31.4, 36.4, 36.6, 42.1,
43.0, 66.1, 66.7, 68.5, 68.9, 72.9, 73.2, 75.1, 75.6, 127.4, 127.5,
128.2, 128.3.
A soln of compound 11 (1.55 g, 4.61 mmol) in anhyd THF (20 mL)
was added dropwise to a suspension of LAH (1.17 g, 9.2 mmol) in
anhyd THF (3 mL) at 0 °C, and the mixture was stirred at r.t. for 1
h then cooled to 0 °C. H2O (1.6 mL), 15% aq NaOH (1.6 mL), and
H2O (4.2 mL) were added successively, and the mixture was stirred
for 2 h then filtered through Celite. The filtrate was concentrated in
vacuo, and the residue was purified by column chromatography to
afford a primary alcohol as a yellow liquid that was used in the next
reaction; yield: 1.28 g (95%).
ESI-MS: m/z 443 [M + 1].
A soln of the primary alcohol (1.28 g, 4.35 mmol) in anhyd CH2Cl2
(20 mL) at 0 °C was added to a stirred solution of 2-iodoxybenzoic
acid (2.52 g, 8.70 mmol) in anhyd DMSO (5 mL), and the mixture
was kept at r.t. for 3 h. When the reaction was complete (TLC), the
mixture was filtered through Celite using Et2O (2 × 20 mL). The fil-
trate was washed with H2O (20 mL) and brine (20 mL) then dried
(Na2SO4). The Et2O layer was concentrated under reduced pressure,
and the crude product was purified by column chromatography to
give aldehyde 5 as a viscous liquid; yield: 1.15 g (86%); [a]D
–8.75 (c 0.004, CHCl3).
IR (neat): 2936, 2860, 1726, 1376, 1223, 1102, 1029, 740, 698 cm–1.
1H NMR (CDCl3, 300 MHz): d = 1.29 (s, 3 H), 1.32 (s, 3 H), 1.55–
1.69 (m, 2 H), 1.74 (q, J = 6.2 Hz, 2 H), 2.46 (dd, J = 1.5, 4.7 Hz,
1 H), 2.56 (dd, J = 2.2, 8.3 Hz, 1 H), 3.43–3.58 (m, 2 H), 3.94–4.05
(m, 1 H), 4.25–4.36 (m, 1 H), 4.46 (s, 2 H), 7.20–7.34 (m, 5 H), 9.72
(t, J = 1.7 Hz, 1 H).
13C NMR (CDCl3, 75 MHz): d = 24.5, 24.7, 36.0, 38.0, 49.1, 62.2,
63.6, 66.5, 73.1, 100.7, 127.6, 127.7, 128.3, 138.4, 201.0.
HRMS-ESI: m/z [M + Na]+ calcd for C27H38O5Na: 465.2616; found:
465.2619.
Acknowledgment
A.S.R. thanks CSIR, New Delhi for the award of a fellowship.
25
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(2S,4R)-6-(Benzyloxy)-1-{(2R,4S,6S)-6-[2-(benzyloxy)ethyl]-4-
chlorotetrahydro-2H-pyran-2-yl}hexane-2,4-diol (12)
Anhyd FeCl3 (0.39 g, 2.40 mmol) was added in one portion to a soln
of homoallylic alcohol 4 (0.5 g, 2.42 mmol) and aldehyde 5 (0.70 g,
2.39 mmol) in anhyd CH2Cl2. The reaction, which was complete in
~10 min, was quenched by addition of H2O (5 mL) with stirring, and
the mixture was extracted with CH2Cl2 (3 × 25 mL). The combined
organic layers were dried (Na2SO4) and concentrated under reduced
pressure. The crude product was purified by flash column chroma-
tography (silica gel) to give a viscous liquid; yield: 0.71 g (62%);
[a]D25 –10.5 (c 0.0085, CHCl3).
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2525. (h) Paterson, I.; Anderson, E. A.; Dalby, S. M.; Lim,
J. H.; Maltas, P.; Moessner, C. Chem. Commun. 2006, 4186.
(i) Pan, Y.; De Brabander, J. K. Synlett 2006, 853.
(j) Wang, C.; Forsyth, C. J. Org. Lett. 2006, 8, 2997.
(k) Paterson, I.; Anderson, E. A.; Dalby, S. M.; Genovino, J.;
Lim, J. H.; Moessner, C. Chem. Commun. 2007, 1852.
(l) Fürstner, A.; Fenster, M. D. B.; Fasching, B.; Godbout,
C.; Radkowski, K. Angew. Chem. Int. Ed. 2006, 45, 5506.
(m) Fürstner, A.; Fasching, B.; O’Neil, G. W.; Fenster, M.
IR (neat): 3452, 2922, 2853, 1454, 1373, 1223, 1101, 739 cm–1.
1H NMR (CDCl3, 300 MHz): d = 1.35–2.12 (m, 12 H), 3.32–3.74
(m, 6 H), 3.88–4.20 (m, 3 H), 4.45 (s, 2 H), 4.50 (s, 2 H), 7.21–7.32
(m, 10 H).
13C NMR (CDCl3, 75 MHz): d = 35.8, 36.4, 42.2, 42.3, 43.0, 43.3,
55.4, 65.7, 66.3, 69.0, 69.2, 73.0, 73.3, 73.8, 74.0, 127.6, 127.7,
128.3, 128.4.
Synthesis 2010, No. 3, 505–509 © Thieme Stuttgart · New York

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Synthesis 2010, No. 3, 505–509 © Thieme Stuttgart · New York