A one-pot deprotection and intramolecular oxa-michael addition to access angular trioxatriquinanes

Article abstract of DOI:10.1055/s-0030-1289518

An efficient one-pot deprotection-oxa-Michael addition strategy has been used to synthesize a few trioxatriquinanes starting from commercially available sugars. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0030-1289518

2580
LETTER
A One-Pot Deprotection and Intramolecular Oxa-Michael Addition to Access
Angular Trioxatriquinanes
O
xa-Michae
l
A
ddi
o
tion to
A
cce
s
s
Angu
a
lar
T
rioxatriquin
R
anes amakrishna, Krishna P. Kaliappan*
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India
Fax +91(22)25723480; E-mail: kpk@chem.iitb.ac.in
Received 12 July 2011
H
O
Abstract: An efficient one-pot deprotection–oxa-Michael addition
O
O
O
COOH
strategy has been used to synthesize a few trioxatriquinanes starting
O
from commercially available sugars.
HO
O
O
H
Key words: triquinane, RCM, allylic oxidation, deprotection
5
6
7
hirsutic acid (4)
angular oxatriquinane,
dioxatriquinane, trioxatriquinane
O
The triquinanes,1 a unique substructural class of a family
of sesquiterpenoid natural products, are endowed with
three five-membered fused rings and can be classified into
three broad categories: linear 1, angular 2, and modephane
3 depending upon the mode of their ring fusion (Figure 1).
The synthesis of triquinanes has been a vibrant area of or-
ganic synthesis ever since the structural elucidation of a
carbocyclic linear triquinane, hirsutic acid (4).
H
H
O
O
O
H
O
O
HO
O
O
H
OH
HO
H
merrilactone A (8)
(modephane oxatriquinane)
isogenine (9)
(linear dioxatriquinane)
H
O
O
O
O
O
H
H
H
H
2 angular
3 modephane
1 linear
H
O
H
OH
Figure 1 Classification of triquinanes
AcO
HO
H
C-norcardanolide (10)
(linear dioxatriquinane)
hirundigenin (11)
(angular trioxatriquinane)
The enticing molecular architecture of triquinanes cou-
pled with their wide spectrum of biological properties has
made them attractive synthetic targets. Several methods,
especially cascade radical methods,2 transannulation reac-
tions,3 alkene-arene and photocycloaddition reactions4
have been developed to synthesize the core framework as
well as several natural products belonging to this catego-
ry. The diversity of carbocyclic triquinanes extends to
their hetero analogues, namely the azatriquinanes5 and ox-
atriquinanes.6 However, these hetero variants of the
triquinanes have received much less attention than their
carbon analogues, which is evident from the limited re-
ports appearing in the literature. The activity of a natural
product can often be attributed to the presence of a hetero-
cycle. For example, the biological activity of merrilactone
A (8), isolated from Illicium merrillianum,7 is attributed to
the presence of the oxygenated pentacyclic architecture.
In addition, there are steroid-based hybrid natural prod-
ucts, such as isogenine (9)8 and C-norcardanolide (10),9
and hirundigenin (11)10 having dioxatriquinane and trioxa-
Figure 2
triquinane subunits with interesting biological profiles
(Figure 2).
The total synthesis of ( )-hirsutene reported by
Fukumoto11 and co-workers particularly drew our atten-
tion to our current objective of synthesizing angular triox-
atriquinane frameworks, as some of the synthetic
intermediates possessing oxa-triquinane systems were
proven to exhibit potent in vitro cytotoxic activity.
In continuation12 of our interest in exploring these aesthet-
ically pleasing molecules, we developed interest in de-
signing the angular trioxatriquinane framework 7 from
commercially available carbohydrate precursors. Our tar-
get, the angular trioxatriquinane framework 12 can be
considered as a template of the syringolides13 1 (13) and
2 (14) produced by Pseudomonas syringae pv. tomato,
which elicit a hypersensitive defense response (HR) in
specific soybean cultivars (Figure 3). Herein, we describe
the synthesis of an angular trioxatriquinane framework
using one-pot deprotection and intramolecular oxa-
Michael addition as key steps, starting from sugars such as
D-xylose, L-xylose, and D-glucose.
SYNLETT 2011, No. 17, pp 2580–2584
x
x
.x
x
.2
0
1
1
Advanced online publication: 06.10.2011
DOI: 10.1055/s-0030-1289518; Art ID: D22311ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Oxa-Michael Addition to Access Angular Trioxatriquinanes
2581
O
ring-closing metathesis precursor diene 18. However, all
attempts to accomplish the key ring-closing metathesis
(RCM)15 of diene 18 with both Grubbs first- and second-
generation catalysts (19 and 20) resulted mainly in recov-
ery of the starting material even after prolonged periods of
refluxing in CH2Cl2–toluene. Then, prompted by litera-
ture reports16 and based on our own experience,17 we per-
formed the RCM of diene 18 by employing 10 mol% of
Grubbs first-generation catalyst with 15 mol% of Ti(Oi-
Pr)4 in dichloromethane. Much to our disappointment, this
modification did not have significant impact on the out-
come with only trace amounts of cyclized product 21 be-
ing obtained now (Scheme 2).
H
HO
O
O
O
R
O
O
O
O
O
H
O
HO
12
13 R = C5H11 syringolide 1
14 R = C7H15 syringolide 2
Figure 3
As outlined in Scheme 1, we envisaged that angular trioxa-
triquinane 12 could be obtained from the hydroxyspiro-
lactone 15 by an intramolecular oxa-Michael addition,
which, in turn, could be accessed from allyl alcohol 16.
This allyl alcohol could be easily obtained from D-xylose
in a few steps.
In view of this unexpected difficulty, we next focused on
synthesizing the spirolactone 21 by subjecting diene 22 to
RCM followed by allylic oxidation (Scheme 3). Accord-
ingly, diene 2218 was obtained by allylation of alcohol 16
using NaH and allyl bromide. Now, diene 22 underwent
O
O
O
O
HO
O
O
O
O
O
O
O
O
O
TBDPSO
a
TBDPSO
O
O
O
O
12
15
OH
O
O
16
22
O
TBDPSO
D-xylose
O
O
O
O
O
OH
16
c
b
TBDPSO
TBDPSO
O
O
O
O
Scheme 1 Retrosynthetic analysis of angular trioxatriquinane
O
23
21
O
O
Thus, our synthesis embarked with the TBDPS-protected
ketone 17, readily available from D-xylose by a three-step
protocol.14 Addition of vinyl magnesium bromide to ke-
tone 17 afforded the alcohol 16 stereoselectively which,
upon treatment with acryloyl chloride and NaH, gave the
O
O
intramolecular
d
O
O
O
O
O
oxa-Michael
addition
O
O
O
12
24
Scheme 3 Reagents and conditions: (a) NaH, allyl bromide, THF,
0 °C to reflux, 4 h, 76%; (b) Grubbs I (19), CH2Cl2, reflux, 12 h, 91%;
(c) PDC, Celite, t-BuOOH, PhH, 8 h, 75%; (d) TBAF, 0 °C, 1 h, 60%.
O
O
O
a
O
TBDPSO
TBDPSO
TBDPSO
O
O
OH
O
O
O
O
17
O
16
O
TBDPSO
TBDPSO
a
O
O
O
O
O
O
c
b
TBDPSO
OH
O
O
25
26
O
O
O
O
O
O
O
18
O
c
b
21
TBDPSO
TBDPSO
O
O
O
O
MesN
NMes
Ph
O
PCy3
Ru
27
28
Cl
Ph
Cl
Cl
O
O
Ru
O
O
intramolecular
Cl
Cy3P
d
O
O
O
O
O
oxa-Michael
addition
O
Cy3P
O
20
19
O
29
30
Scheme 2 Reagents and conditions: (a) vinyl magnesium bromide,
THF, –10 °C to r.t., 3 h, 85%; (b) NaH, acryloyl chloride, THF, r.t.,
2 h, 89%; (c) various conditions.
Scheme 4 Reagents and conditions: (a) NaH, allyl bromide, THF,
0 °C to reflux, 4 h, 76%; (b) Grubbs I (19), CH2Cl2, reflux, 12 h, 91%;
(c) PDC, Celite, t-BuOOH, PhH, 8 h, 75%; (d) TBAF, 0 °C, 1 h, 60%.
Synlett 2011, No. 17, 2580–2584 © Thieme Stuttgart · New York

2582
K. Ramakrishna, K. P. Kaliappan
LETTER
addition in the synthesis of secosyrins by Mukai23 and co-
workers under several acidic and basic conditions were
futile.
O
O
O
O
O
O
O
O
3 steps
a
D-glucose
Having optimized the reaction conditions with D-xylose,
the same series of transformations was carried out on L-
xylose-derived alcohol 25 to synthesize the trioxa-
triquinane 30, as shown in Scheme 4.
O
O
OH
O
31
32
O
O
O
O
O
O
As D-glucose diacetonide has a similar stereochemical re-
lationship to D-xylose monoacetonide, it seemed logical
to this sequence. Accordingly, the known alcohol 31,24
readily available from D-glucose diacetonide, was con-
verted into the RCM precursor 32 by a base-mediated
alkylation with allyl bromide.
O
O
O
c
b
O
O
O
O
33
34
HO
OH
HO
O
The key RCM of diene 32 with 5 mol% of Grubbs first-
generation catalyst furnished the spiro ether 33 in 90%
yield. This spiro ether was relatively stable, unlike the
spiro ethers derived from D- and L-xylose. Allylic oxida-
tion of 33 under PDC and Celite conditions furnished the
spiro lactone 34 in 70% yield. Finally, selective deprotec-
tion of the more exposed acetonide group of 34, with 60%
AcOH, directly furnished the anticipated cyclized product
36 (Scheme 5).
O
O
O
d
intramolecular
O
O
O
O
oxa-Michael
addition
O
O
36
O
35
Scheme 5 Reagents and conditions: (a) NaH, allyl bromide, THF,
0 °C to reflux, 6 h, 90%; (b) Grubbs I (19), CH2Cl2, reflux, 12 h, 89%;
(c) PDC, Celite, t-BuOOH, PhH, 18 h, 70%; (d) 60% aq AcOH, r.t.,
16 h, 74%.
The formation of the five-membered cyclized product 36
over the other possible six-membered cyclized product 37
was confirmed by carrying out the oxidation of the free
hydroxyl group. Interestingly, oxidation of compound 36
with O-iodoxybenzoic acid (IBX) furnished aldehyde 38,
which was unambiguously characterized from its 1H
NMR spectroscopic data. We attributed this outcome to
the stability of the five-five-five-membered ring fusion
over the five-six-five-membered ring fusion (Scheme 6).
RCM reaction smoothly, unlike diene 18, leading to the
formation of the spiro ether 23 in 91% yield with 5 mol%
of Grubbs first-generation catalyst.19 It is important to
note that the spiro ether was quite unstable, and further
transformations needed to be carried out immediately.
Thus, allylic oxidation20 of 23 with pyridinium dichro-
mate (PDC) and tert-butylhydroperoxide (TBHP) was
carried out to furnish the required spiro lactone 21 in 85%
yield.21 Having compound 21 in hand, the stage was then
set for the deprotection of the TBDPS group by treating
with tetra-n-butylammonium fluoride (TBAF) at 0 °C for
one hour.
In summary, we have synthesized a few angular trioxa-
triquinanes from readily available carbohydrate starting
materials. The key steps involved in this strategy are RCM
followed by allylic oxidation to construct the spiro lactone
and formation of trioxatriquinane in a single step by desi-
lylation with concomitant intramolecular oxa-Michael ad-
dition. Our efforts in the application of this method to
construct azadioxatriquinane and tetraoxatetraquinane
frameworks are underway.
Gratifylingly, this resulted in the direct formation of trioxa-
triquinane 12 in 60% yield via deprotection of the silyl
group with concomitant intramolecular oxa-Michael addi-
tion in a single step.22 It should be noted that attempts to
accomplish a similar kind of intramolecular oxa-Michael
O
HO
H
O
O
O
O
b
O
O
a
O
O
O
O
O
O
O
O
O
O
O
36
38
O
HO
O
a
O
O
O
34
O
O
O
37
Scheme 6 Reagents and conditions: (a) 60% aq AcOH, r.t., 16 h, 74%; (b) IBX, EtOAc, reflux, 6 h.
Synlett 2011, No. 17, 2580–2584 © Thieme Stuttgart · New York

LETTER
Oxa-Michael Addition to Access Angular Trioxatriquinanes
2583
Mazzola, E. P.; Graham, K. J.; Clardy, J. J. Org. Chem.
1993, 58, 2940.
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Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
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Soc. 1993, 115, 9856. For some recent reviews related to
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Chem. Rev. 2004, 104, 2199.
Acknowledgment
We thank IRCC, IIT Bombay for financial support and KR thanks
CSIR, New Delhi for a Fellowship.
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References and Notes
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(18) A 60% oil dispersion of NaH (75 mg, 1.84 mmol, 3 equiv),
after washing with anhyd hexanes (3 × 5 mL), was
suspended in anhyd THF (5 mL), and the reaction mixture
was cooled to 0 °C. To this, a solution of compound 16 (280
mg, 0.61 mmol, 1 equiv) in anhyd THF (5 mL) was added
dropwise and allowed to stir at r.t. for 1 h. Allyl bromide (0.1
mL, 1.23 mmol, 2 equiv) and TBAI (cat.) were added to the
mixture at 0 °C, which was further stirred for 1 h at r.t.
followed by heating to reflux for 4 h. The mixture was
quenched by the careful addition of sat. aq NH4Cl and
extracted with EtOAc (3 × 10 mL). The combined organic
layers were dried over Na2SO4 and concentrated in vacuo to
give the crude product. Silica gel column chromatography of
the residue, eluting with 4% EtOAc in hexanes provided 22
(230 mg, 76%) as a colorless syrup. Rf = 0.45 (10% EtOAc
in hexanes); [a]D20 28.9 (c 1.14, CHCl3). 1H NMR (400
MHz, CDCl3): d = 7.70–7.67 (m, 4 H), 7.42–7.33 (m, 6 H),
5.94–5.85 (m, 1 H), 5.81 (d, J = 3.6 Hz, 1 H), 5.58 (dd,
J = 17.8, 11.3 Hz, 1 H), 5.25 (dd, J = 11.3, 0.9 Hz, 1 H), 5.23
(dq, J = 17.2, 1.8 Hz, 1 H), 5.16 (dd, J = 17.8, 0.9 Hz, 1 H),
5.11 (dq, J = 10.4, 1.5 Hz, 1 H), 4.45 (d, J = 3.6 Hz, 1 H),
4.33 (dd, J = 7.1, 3.5 Hz, 1 H), 4.10 (ddt, J = 12.4, 5.1, 1.6
Hz, 1 H), 3.98 (ddt, J = 12.4, 5.3, 1.6 Hz, 1 H), 3.78 (dd,
J = 11.4, 3.5 Hz, 1 H), 3.70 (dd, J = 11.4, 7.2 Hz, 1 H), 1.61
(s, 3 H), 1.36 (s, 3 H), 1.03 (s, 9 H). 13C NMR (100 MHz,
CDCl3): d = 135.9, 135.8, 135.1, 134.6, 133.8, 133.7, 129.7,
129.6, 127.8, 127.7, 117.9, 116.0, 112.9, 104.5, 84.9, 82.8,
81.8, 66.3, 63.5, 27.2, 27.0, 26.9, 19.4. IR (neat): n = 3072,
2932, 2858, 1646, 1462, 1428, 1383, 1217, 1121, 1042, 927,
878, 823, 758, 704, 612, 548, 505 cm–1. ESI-HRMS: m/z
calcd for C29H38O5SiNa [M + Na]+: 517.2386; found:
517.2380.
(3) (a) Mehta, G.; Srikrishna, A.; Reddy, A. V.; Nair, M. S.
Tetrahedron 1981, 37, 4543. (b) Mehta, G.; Reddy, A. V.;
Murthy, A. N.; Reddy, D. S. J. Chem. Soc., Chem. Commun.
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Correia, C. R. D. J. Am. Chem. Soc. 1987, 109, 2523.
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(4) (a) Wender, P. A.; Sigget, L.; Nuss, J. M. In Comprehensive
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Hursthouse, M. B.; Shishkin, O. V.; Mascal, M. J. Org.
Chem. 1998, 63, 6016. (b) Li, Y.; Meng, Y.; Meng, X.; Li,
Z. Tetrahedron 2011, 67, 4002.
(6) (a) Yadav, J. S.; Kumar, P. T. K.; Gadgil, V. R. Tetrahedron
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Hoffmann, H. M. R. Tetrahedron 1995, 51, 7389.
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Li, T.; Lim, Y.-H.; Rucando, D.; Sieburth, S. McN. Org.
Lett. 2001, 3, 2165. (f) Gurjar, M. K.; Ravindranadh, S. V.;
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Tetrahedron Lett. 2000, 41, 6111. (b) Huang, J.-M.; Yang,
C.-S.; Tanaka, M.; Fukuyama, Y. Tetrahedron 2001, 57,
4691.
(19) To a solution of diene 22 (210 mg, 0.42 mmol, 1 equiv) in
argon-purged CH2Cl2 was added Grubbs first-generation
catalyst (5 mol%), and the mixture was refluxed for 12 h.
DMSO (2–3 drops) was added and the mixture stirred for
6 h to remove metal impurities. The mixture was then
concentrated in vacuo to give a residue which, upon
purification by silica gel column chromatography with 14%
EtOAc in hexane, afforded compound 23 (180 mg, 91%) as
(8) Krasso, A. F.; Binder, M.; Tamm, C. h. Helv. Chim. Acta
1972, 55, 1352.
(9) Pettit, G. R.; Kasturi, T. R.; Knight, J. C.; Occolowitz, J.
J. Org. Chem. 1970, 35, 1404.
(10) Gui, J.; Wang, D.; Tian, D. Angew. Chem. Int. Ed. 2011, 50,
20
a colorless syrup. Rf = 0.26 (16% EtOAc in hexanes); [a]D
7093.
+27.6 (c 1.56, CHCl3). 1H NMR (400 MHz, CDCl3):
(11) (a) Toyota, M.; Nishikawa, Y.; Motoki, K.; Yoshida, N.;
Fukumoto, K. Tetrahedron Lett. 1993, 34, 6099.
(b) Toyota, M.; Nishikawa, Y.; Motoki, K.; Yoshida, N.;
Fukumoto, K. Tetrahedron 1993, 49, 11189.
(12) (a) Kaliappan, K. P.; Nandurdikar, R. S. Chem. Commun.
2004, 2506. (b) Kaliappan, K. P.; Nandurdikar, R. S.;
Shaikh, M. M. Tetrahedron 2006, 62, 5064.
(13) (a) Smith, M. J.; Mazzola, E. P.; Sims, J. J.; Midland, S. L.;
Keen, N. T.; Burton, V.; Stayton, M. M. Tetrahedron Lett.
1993, 34, 223. (b) Midland, S. L.; Keen, N. T.; Sims, J. J.;
Midland, M. M.; Stayton, M. M.; Burton, V.; Smith, M. J.;
d = 7.69–7.65 (m, 4 H), 7.43–7.34 (m, 6 H), 5.98 (dt, J = 6.2,
1.3 Hz, 1 H), 5.80 (d, J = 3.6 Hz, 1 H), 5.67 (dt, J = 6.2, 1.5
Hz, 1 H), 4.69 (ddd, J = 13.4, 2.5, 1.7 Hz, 1 H) 4.57 (ddd,
J = 13.4, 2.4, 1.7 Hz, 1 H), 4.32 (t, J = 4.5 Hz, 1 H), 4.22 (d,
J = 3.7 Hz, 1 H), 3.68, 3.64 (ABq, JAB = 11.4 Hz, 1 H), 3.66,
3.63 (ABq, JAB = 11.4 Hz, 1 H), 1.63 (s, 3 H), 1.36 (s, 3 H),
1.03 (s, 9 H). 13C NMR (100 MHz, CDCl3): d = 135.9, 135.8,
133.6, 133.5, 129.8, 129.6, 127.8, 125.6, 113.2, 103.5, 95.8,
83.0, 80.4, 75.9, 63.0, 27.0, 26.9, 26.6, 19.3. IR (neat):
n = 3019, 2930, 2857, 1597, 1216, 1113, 1082, 1042, 758,
Synlett 2011, No. 17, 2580–2584 © Thieme Stuttgart · New York

2584
K. Ramakrishna, K. P. Kaliappan
LETTER
668 cm–1. ESI-HRMS: m/z calcd for C27H34O5SiNa [M +
1013, 759, 670 cm–1. ESI-HRMS: m/z calcd for
Na]+: 489.2073; found: 489.2081.
C27H32O6SiNa [M + Na]+: 503.1866; found: 503.1879.
(22) To a solution of spirolactone 21 (270 mg, 0.56 mmol, 1
equiv) in anhyd THF (6 mL) was added TBAF solution in
THF (1 M, 1.12 mL, 2 equiv) at 0 °C, and the mixture was
stirred at the same temperature for 1 h. The reaction mixture
was diluted with EtOAc, and solvents were removed in
vacuo to obtain the crude product, which was purified by
silica gel chromatography, eluting with 32% EtOAc in
hexanes to afford the trioxatriquinane 12 (80 mg) as a white
low-melting solid in 60% yield. Rf = 0.19 (40% EtOAc in
hexanes); [a]D20 +27.8 (c 1.29, CHCl3). 1H NMR (400 MHz,
CDCl3): d = 5.99 (d, J = 3.6 Hz, 1 H), 4.82 (d, J = 2.9 Hz, 1
H), 4.59 (d, J = 3.6 Hz, 1 H), 4.55 (dd, J = 6.7, 1.3 Hz, 1 H),
4.13 (d, J = 11.0 Hz, 1 H), 4.03 (dd, J = 11.0, 2.9 Hz, 1 H),
2.88 (dd, J = 19.0, 6.7 Hz, 1 H), 2.74 (dd, J = 19.0, 1.3 Hz, 1
H), 1.63 (s, 3 H), 1.39 (s, 3 H). 13C NMR (100 MHz, CDCl3):
d = 173.6, 114.6, 105.9, 97.7, 83.0, 79.6, 79.2, 72.3, 35.5,
27.2, 27.0. IR (neat): n = 3021, 2937, 1799, 1731, 1376,
1217, 1106, 1012, 909, 870, 758, 712, 669 cm–1. ESI-
HRMS: m/z calcd for C11H14O6Na [M + Na]+: 265.0688;
found: 265.0682.
(20) Chidambaram, N.; Chandrasekaran, S. J. Org. Chem. 1987,
52, 5048.
(21) To a solution of spiroether 23 (260 mg, 0.56 mmol, 1 equiv)
in benzene (20 mL) was added Celite (1.2 g/mmol) at r.t. and
the mixture cooled to 0–10 °C. To this reaction mixture was
added PDC (1.06 g, 2.8 mmol, 5 equiv), followed by t-
BuOOH (0.39 mL, 2.8 mmol, 5 equiv), and the whole was
stirred for 8 h at r.t. The reaction mixture was filtered
through a pad of Celite and washed with EtOAc, the filtrate
was concentrated in vacuo to furnish the crude product as a
yellow syrup. Silica gel column chromatography, eluting
with 20% EtOAc in hexanes afforded the spirolactone 21
(230 mg) as a colorless syrup in 85% yield. Rf = 0.39 (40%
EtOAc in hexanes); [a]D20 +52.2 (c 1.41, CHCl3). 1H NMR
(400 MHz, CDCl3): d = 7.66–7.59 (m, 4 H), 7.53 (d, J = 5.7
Hz, 1 H), 7.46–7.36 (m, 6 H), 6.14 (d, J = 5.7 Hz, 1 H), 5.98
(d, J = 3.7 Hz, 1 H), 4.52 (t, J = 3.7 Hz, 1 H), 4.49 (d, J = 3.7
Hz, 1 H), 3.73 (dd, J = 11.9, 4.1 Hz, 1 H), 3.61 (dd, J = 11.9,
3.6 Hz, 1 H), 1.63 (s, 3 H), 1.36 (s, 3 H), 1.03 (s, 9 H). 13
C
NMR (100 MHz, CDCl3): d = 170.9, 153.8, 135.7, 135.6,
132.7, 132.6, 130.1, 130.0, 128.0, 127.9, 121.3, 114.3,
103.5, 91.4, 81.6, 79.2, 61.0, 26.9, 26.8, 26.7, 19.3. IR
(neat): n = 3021, 2933, 2860, 1774, 1601, 1216, 1166, 1106,
(23) Mukai, C.; Moharram, S. M.; Azukizawa, S.; Hanaoka, M.
J. Org. Chem. 1997, 62, 8095.
(24) Nielsen, P.; Petersen, M.; Jacobsen, J. P. J. Chem. Soc.,
Perkin Trans. 1 2000, 3706.
Synlett 2011, No. 17, 2580–2584 © Thieme Stuttgart · New York

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