A facile cross-metathesis-radical-cyclisation approach to monobenzannulated spiroketals

Article abstract of DOI:10.1055/s-0028-1087942

The synthesis of a series of 5,5-, 5,6-, and 6,6-monobenzannulated spiroketals using a novel cross-metathesis-radical-cyclisation approach is reported. Cross metathesis between two olefin coupling partners resulted in the formation of a heterodimer which

Full text of DOI:10.1055/s-0028-1087942

LETTER
793
A Facile Cross-Metathesis–Radical-Cyclisation Approach to Monobenz-
annulated Spiroketals
A
Y
pproachtoMono
e
benzannula
n
tedSpiroket
-
als Cheng (William) Liu, Jonathan Sperry, Dominea C. K. Rathwell, Margaret A. Brimble*
Department of Chemistry, University of Auckland, 23 Symonds St., Auckland 1142, New Zealand
Fax +64(9)3737422; E-mail: m.brimble@auckland.ac.nz
Received 24 November 2008
Our research group has a longstanding interest in the syn-
thesis of simplified analogues of the unique aryl spiroketal
Abstract: The synthesis of a series of 5,5-, 5,6-, and 6,6-mono-
benzannulated spiroketals using a novel cross-metathesis–radical-
cyclisation approach is reported. Cross metathesis between two ole-
moiety of the rubromycins. For example, we reported the
6
fin coupling partners resulted in the formation of a heterodimer synthesis of a series of 5,6-bisbenzannulated and 6,6-
7
which upon hydrogenation furnished a saturated alcohol product. bisbenzannulated spiroketals as well as a series of 6,6-
Oxidative radical cyclisation then afforded the desired monobenz-
annulated spiroketals in good overall yield.
bisbenzannulated spiroketals containing additional oxy-
8
gen atoms by a common acid-catalysed cyclisation of
Key words: cross metathesis, radical cyclisation, spiroketal, rubro- their respective dihydroxyketone precursors.
mycins, berkelic acid
HO
O
OH
O
MeO
O
HO
Nature has historically provided the most important
source of lead compounds for the development of new
therapeutic agents. By modification of a biologically ac-
tive lead compound, libraries of simplified analogues can
be synthesised that maintain or improve the desired bio-
O
CO2Me
HO
O
α-rubromycin (1)
O
OH
O
1
HO
O
logical activity.
CO2Me
MeO
O
O
Spiroketals are a common structural element in a plethora
of natural products of medicinal and environmental im-
β-rubromycin (2)
2
portance. One such set of compounds, the rubromycins
O
MeO
O
O
(
Figure 1) are a class of natural antibiotics that display a
HO
O
OH
3
CO Me
broad range of biological activity that includes inhibition
of human telomerase by b-rubromycin (2) and g-rubromy-
cin (3, IC 3.06 ± 0.85 mM and 2.64 ± 0.09 mM, respec-
2
MeO
O
O
γ-rubromycin (3)
5
0
tively). Intriguingly, a-rubromycin (1), derived from ring
opening of the spiroketal core of b-rubromycin (2), exhib-
its much lower inhibition of telomerase (IC > 200 mM)
O
OH
O
CO2H
O
O
OH
5
0
than other members of the family. The absence of a
spiroketal moiety in a-rubromycin (1) implies that the
spiroketal unit is an essential structural unit for inhibition
H
H
CO2Me
berkelic acid (4)
O
4
of telomerase. Another natural product, berkelic acid (4),
possesses a monobenzannulated 6,5-spiroketal as well as
an additional oxygen-containing six-membered ring
which is fused to both rings of the chroman unit. It is an
inhibitor of matrix metalloproteinase-3 (MMP-3) and
caspase-1, and was shown to exhibit selectivity toward
Figure 1 Spiroketal-containing natural products
Recent novel methods for the synthesis of monobenzan-
nulated spiroketals include montmorillonite K-10 clay
mediated formation of spirochromans from substituted 2-
vinylpyrans via a glycosylation, Claisen rearrangement
5
ovarian cancer cell line OVCAR-3 (GI50 91 nM). In-
spired by the biological activity of compounds 1–4, we
envisioned building simplified analogues of the central
benzannulated spiroketal core of b-rubromycin (2), g-rubro-
mycin (3), and berkelic acid (4) in order to see if the bio-
logical activity was maintained.
9
and intramolecular ring-closure sequence, gold-catalysed
double intramolecular alkyne hydroalkoxylation,10 lac-
tone alkylidenation with a functionalised titanium car-
benoid bearing a protected hydroxyl group, followed by
acid-mediated intramolecular cyclisation of the formed
1
1
exocyclic enol ether, and a hetero-Diels–Alder cycload-
dition between an o-quinone methide and an exo-enol
ether.12 We now report the synthesis of a series of 5,5-,
SYNLETT 2009, No. 5, pp 0793–0797
x
x.
x
x.
2
0
0
9
Advanced online publication: 24.02.2009
DOI: 10.1055/s-0028-1087942; Art ID: D39308ST
5
,6-, and 6,6-monobenzannulated spiroketals related to
©
Georg Thieme Verlag Stuttgart · New York

7
94
Y.-C. Liu et al.
LETTER
the central spiroketal core of b-rubromycin (2), g-rubro- desired heterodimer 16 as an inconsequential mixture of
mycin (3), and berkelic acid (4) using a novel cross- E/Z isomers.
metathesis–radical-cyclisation approach.
Based on this result, the synthesis of heterodimers 17–19
could thus be carried out in a similar fashion in moderate
yields. Thus, 17 was obtained from olefin coupling part-
ners 20 and 24, 18 from 21 and 23, and 19 from 21 and 24,
respectively. Heterodimers 16–19 were all produced as a
mixture of E/Z isomers (Scheme 2).
radical cyclisation
O
O
O
m
OH
n
m
n
R
R
5
6
7
8
9
: R = H; n = 1; m = 2
: R = Me; n = 1; m = 2
: R = Me; n = 1; m = 1
: R = Me; n = 2; m = 2
: R = Me; n = 2; m = 1
10: R = H; n = 1; m = 2
11: R = Me; n = 1; m = 2
12: R = Me; n = 1; m = 1
13: R = Me; n = 2; m = 2
14: R = Me; n = 2; m = 1
O
n
2
2
+
0: n = 1
1: n = 2
O
(i)
O
OBn
R
n
m
OBn
R
n
m
15: R = H; n = 1; m = 2
16: R = Me; n = 1; m = 2
17: R = Me; n = 1; m = 1
18: R = Me; n = 2; m = 2
2
2
0: n = 1
1: n = 2
cross metathesis
22: R = H, m = 2
3: R = Me; m = 2
4: R = Me; m = 1
O
2
2
+
OBn
1
9: R = Me; n = 2; m = 1
R
n
m
R
OBn
m
1
1
1
1
1
5: R = H; n = 1; m = 2
6: R = Me; n = 1; m = 2
7: R = Me; n = 1; m = 1
8: R = Me; n = 2; m = 2
9: R = Me; n = 2; m = 1
(ii)
2
2
2
2: R = H, m = 2
3: R = Me; m = 2
4: R = Me; m = 1
O
OH
n
m
Scheme 1 Retrosynthesis of monobenzannulated spiroketals 5–9
R
1
1
0: R = H; n = 1; m = 2
1: R = Me; n = 1; m = 2
It was envisaged that monobenzannulated spiroketals 5–9
12: R = Me; n = 1; m = 1
13: R = Me; n = 2; m = 2
1
3
could be synthesised by oxidative radical cyclization of
alcohols 10–14 which in turn could be formed by concom-
itant hydrogenation of the olefin and hydrogenolysis of
the benzyl group from olefin adducts 15–19. Each of the
olefin adducts 15–19 could in turn be assembled via cross
metathesis of an appropriate terminal olefin 20 or 21 with
an appropriate coupling partner 22–24 (Scheme 1). This
convergent approach was envisaged to provide rapid ac-
cess to a range of monobenzannulated spiroketals that
would be amenable to analogue design.
1
4: R = Me; n = 2; m = 1
Scheme 2 Reagents and conditions: (i) Grubbs II, 60 °C, 14–70 h,
1
3
5, 19% (E/Z = 1:0); 16, 58%, (E/Z = 2.1:1); 17, 57% (E/Z = 4:1); 18,
8% (E/Z = 1.1:1); 19, 51% (E/Z = 3.5:1); (ii) H , Pd(OH) /C, MeOH,
2
2
r.t., 2.5–5 h, 10, 44%; 12, 83%; 14, 76%, or H , 10% Pd/C, EtOAc,
2
r.t., 3.5 h, 11, 100%; 13, 63%.
Having synthesised the required heterodimer products
5–19, hydrogenation of the double bond and hydrogeno-
lysis of the benzyl ethers could be conducted (Scheme 2).
Thus, alcohols 10,20 12, and 14 were prepared from
heterodimers 15, 17, and 19 respectively, using Pearl-
man’s catalyst in methanol under an atmosphere of hydro-
gen. Alcohols 11 and 13 were prepared from heterodimers
1
Initially, the synthesis of the two heterocyclic alkenes 20
and 21 was conducted. Olefin 20 was synthesised from
phenol in two steps using the procedure reported by Stoltz
1
4
et al., and olefin 21 was readily available by the palladi-
um-catalysed annulation of 2-iodophenol with 1,4-penta-
1
6 and 18 in a similar fashion; however, it was necessary
1
5
diene. Benzyl-protected olefin coupling partners 22–24
to change the catalyst system to 10% palladium on carbon
in ethyl acetate. This was done to prevent further hydro-
genolysis of the long-chain alcohols 10, 11, and 13
were obtained by benzylation of their appropriate alcohol
1
6
precursors under standard conditions; thus, olefin 22
1
7
was prepared from 3-buten-1-ol, olefin 23 from 3-meth-
yl-3-buten-1-ol, and olefin 24 from 2-methyl-2-propen-
-ol.
(
m = 2) frequently observed using Pearlman’s catalyst, in-
1
8
terestingly not observed with the short-chain alcohols 12
and 14 (m = 1). Alcohols 11–14 were all isolated as an in-
separable 1:1 mixture of diastereomers as determined by
1
With the appropriate starting materials to hand, attention
turned to the cross-metathesis step (Scheme 2). Unfortu-
nately, cross metathesis of olefin 20 with olefin 22 only
afforded a low yield of heterodimer 15 exclusively as the
E-isomer, unsurprisingly as both 20 and 22 are both clas-
1
H NMR.
Next, the key oxidative radical cyclisation of alcohols 10–
4 was undertaken (Table 1). Using a procedure devel-
oped in our laboratory, irradiation of alcohols 10–14
with a desk lamp (60 W) in the presence of iodobenzene
diacetate and iodine in cyclohexane at 7 °C delivered
spiroketals 5–9 in variable yields.21 Higher yields were
1
1
3c
1
9
sified as type I olefins. As a result of this observation,
disubstituted type III olefins 23 and 24 were envisioned as
suitable coupling partners for heterocyclic olefins 20 and
2
1. It was found that using one equivalent of olefin 20 and
six equivalents of olefin 23 provided the best yield of the
Synlett 2009, No. 5, 793–797 © Thieme Stuttgart · New York

LETTER
Approach to Monobenzannulated Spiroketals
795
Table 1 Oxidative Radical Cyclisation of Alcohols 10–14
O
O
O
PhI(OAc)2, I2, hν
m
OH
Cyclohexane,
7 °C, 2–3 h
n
m
n
R
R
n, m = 1, 2; R = H, Me
Starting material
Product
Yield (%)
dr
O
δC 109.6
O
OH
OH
4
4
0
2
O
1
0
5
O
O
δC 109.8
O
O
Me
Me
11
6
O
O
OH
+
Me
73
1.4:1
O
O
Me
4'
4'
δC 118.7 Me
δC 118.6
12
7
a (major)
7b (minor)
O
OH
δC 96.3
O
5
8
2
Me
O
13
Me
8
O
OH
O
+
O
δC 107.2
δC 107.2
9
1.4:1
Me
Me
O
O
14
Me
9
a (major)
9b (minor)
observed for the formation of the five-membered rings
compared to their six-membered counterparts.
2.69–2.82
1.20
O
O
Me
H
Spiroketal 5 was isolated as a single isomer wherein the
C–O bond of the five-membered ring adopts an axial po-
sition with respect to the six-membered ring. For spiro-
ketals 7 and 9, two inseparable racemic diastereomers
were observed for each spiroketal. With respect to diaste-
reomeric spiroketals 7a and 7b (Figure 2), it was found
that the methyl group at C-4¢ in spiroketal 7a was more
4.25
3.68
O
O
H
H
H
H
2.51
2.12
Me
H
H
H
H
H
4
.13
1
.73
1.13
3.56
2.38
2.45–2.54
7b (minor)
7a (major)
=
NOE correlation
upfield than the corresponding methyl group in 7b where Figure 2 Selected d values (ppm) and NOE correlations in spiro-
it occupied a position syn to the oxygen atom of the dihy-
H
ketals 7a and 7b
drobenzofuran moiety. Similarly, the signal for H-4¢ in
spiroketal 7a was further downfield than that for H-4¢ in
Surprisingly, spiroketals 6 and 8 were obtained as a single
racemic diastereomer. It was established that the methyl
substituent adopted an equatorial position on the six-
membered ring, and the corresponding spiroketal bearing
a methyl group in the axial position was not observed.
7
b due to H-4¢ in 7a being syn to the oxygen atom of the
dihydrobenzofuran moiety. Based on this and NOE evi-
dence, spiroketal 7a was assigned as the major diastere-
omer with the methyl group adopting a pseudoequatorial
position.
Synlett 2009, No. 5, 793–797 © Thieme Stuttgart · New York

7
96
Y.-C. Liu et al.
LETTER
Presumably, once spiroketal formation had taken place,
ring opening and ring closure occurred leading to forma-
tion of the thermodynamically favoured spiroketal in
which the methyl group is equatorial and the C–O bond of
the five-membered ring is axial with respect to the six-
membered ring thus gaining maximum stability from the
anomeric effect. Thus, the formation of two racemic dia-
stereomers for each of spiroketals 7 and 9 can be rationa-
lised by the fact that the anomeric effect was weaker in
five-membered ring systems (for spiroketals 7 and 9) than
in six-membered ring systems (for spiroketals 6 and 8).22
(14) Trend, R. M.; Ramtohul, Y. K.; Stoltz, B. M. J. Am. Chem.
Soc. 2005, 127, 17778.
(
(
(
15) Larock, R. C.; Berrios-Peña, N. G.; Fried, C. A.; Yum, E. K.;
Tu, C.; Leong, W. J. Org. Chem. 1993, 58, 4509.
16) Westwell, A. D.; Williams, J. M. J. Tetrahedron 1997, 53,
13063.
17) Cleary, P. A.; Woerpel, K. A. Org. Lett. 2005, 7, 5531.
2
2
(18) Waser, J.; Gaspar, B.; Nambu, H.; Carreira, E. M. J. Am.
Chem. Soc. 2006, 128, 11693.
(
19) Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs, R. H.
J. Am. Chem. Soc. 2003, 125, 11360.
(
(
20) Normant, A. Bull. Soc. Chim. Fr. 1940, 7, 37.
21) General Procedure – Oxidative Radical Cyclisation
In conclusion, the synthesis of substituted monobenzan-
nulated spiroketals 5–9 was achieved using two key reac-
tions, namely cross metathesis between two olefin
coupling partners using Grubbs second-generation cata-
lyst and intramolecular oxidative radical cyclisation of a
tethered alcohol. Studies toward the synthesis of the
rubromycins 1–3, berkelic acid (4) and analogues using
this methodology are ongoing.
A mixture of alcohol (0.052 mmol), PhI(OAc) (0.106
2
mmol), and I (0.118 mmol) in anhyd cyclohexane (4.3 mL)
2
was degassed with argon at r.t. for 15 min. The resulting
solution was cooled in an ice–water bath (7 °C) and
irradiated with a desk lamp (60 W) for 2–3 h after which it
was diluted with Et O (10 mL), then sat. Na S O (10 mL)
2
2
2
3
and sat. NaHCO (10 mL) were added. After separation of
3
both phases, the aqueous phase was extracted with Et O
2
(4 × 15 mL). The organic phases were combined, dried over
anhyd MgSO , filtered, and the solvents concentrated in
4
vacuo. The crude product was purified by column
Acknowledgment
chromatography on SiO (100% n-pentane, then n-pentane–
2
Et O, 12:1) to give the spiroketal product.
2
We thank the Tertiary Education Committee, New Zealand for the
award of a Bright Futures Top Achiever Doctoral Scholarship
(
±)-4¢-Methyl-3¢,4¢5¢,6¢tetrahydro-3H-spiro(benzofuran-
,2¢-pyran) (6)
Pale yellow oil (7 mg, 0.034 mmol, 42%); R = 0.31
2
(
Y.-C. Liu).
f
(
hexanes–Et O, 10:1). IR (film): 2946, 2925, 2869, 1597,
2
1
479, 1461, 1377, 1238, 1216, 1121, 1096, 1084, 1034, 869,
References and Notes
–
1 1
8
26, 810, 791, 776, 747, 706 cm . H NMR (300 MHz,
): d = 0.98 (d, J = 6.6 Hz, 3 H, C-4¢-CH ), 1.33 (qd,
(
(
1) Paterson, I.; Anderson, E. A. Science 2005, 310, 451.
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Total Synthesis of Spiroketal-Containing Natural Products,
In Total Synthesis of Natural Products, Vol. 8; ApSimon, J.,
Ed.; John Wiley and Sons, 2007.
CDCl
3
3
J = 12.8, 4.9 Hz, 1 H, Hax-5¢), 1.47 (dd, J = 13.1, 12.5 Hz, 1
H, Hax-3¢), 1.64 (dtd, J = 13.2, 3.8, 1.9 Hz, 1 H, Heq-5¢), 2.03
(ddd, J = 13.4, 3.8, 1.8 Hz, 1 H, Heq-3¢), 2.08–2.24 (m, 1 H,
H
1 H, H
ax-4¢), 3.05 (d, J = 16.3 Hz, 1 H, H
-3), 3.12 (d, J = 16.3 Hz,
-3), 3.74 (ddd, J = 11.4, 4.9, 1.5 Hz, 1 H, Heq-6¢), 4.06
b
a
(
(
3) Brasholz, M.; Sörgel, S.; Azap, C.; Reißig, H.-U. Eur. J.
Org. Chem. 2007, 3801.
4) Ueno, T.; Takahashi, H.; Oda, M.; Mizunuma, M.;
Yokoyama, A.; Goto, Y.; Mizushina, Y.; Sakaguchi, K.;
Hayashi, H. Biochemistry 2000, 39, 5995.
(ddd, J = 11.3, 13.0, 2.4 Hz, 1 H, Hax-6¢), 6.80 (d, J = 7.9 Hz,
1 H, H-7), 6.85 (td, J = 7.4, 0.9 Hz, 1 H, H-5), 7.10–7.17 (m,
2 H, H-4 and H-6). C NMR (75 MHz, CDCl
(CH
(CH
7), 109.8 (C, C-2), 120.6 (CH, C-5), 124.9 (CH, C-4), 126.0
(C, C-3a), 127.9 (CH, C-6), 158.2 (C, C-7a). MS (EI, 70 eV):
m/z (%) = 41 (30), 51 (12), 55 (15), 69 (16), 78 (41), 91 (12),
97 (70), 107 (29), 115 (4), 121 (4.5), 131 (21), 134 (10), 145
(3), 159 (2.5), 171 (1.5), 189 (59.5), 203 (5), 204 (100) [M] .
HRMS (EI): m/z [M] calcd for C13
1
3
): d = 22.1
, C-5¢), 42.5
, C-6¢), 109.7 (CH, C-
2
3
, C-4¢-CH
), 26.2 (CH, C-4¢), 33.3 (CH
, C-3¢) 42.8 (CH , C-3), 62.6 (CH
3
3
2
(
5) Stierle, A. A.; Stierle, D. B.; Kelly, K. J. Org. Chem. 2006,
2
2
7
1, 5357.
6) (a) Tsang, K. Y.; Brimble, M. A.; Bremner, J. B. Org. Lett.
003, 5, 4425. (b) Tsang, K. Y.; Brimble, M. A.
(
2
Tetrahedron 2007, 63, 6015.
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Tetrahedron 2006, 62, 5883.
+
(
(
(
+
H
16
O
2
: 204.1150; found:
8) Brimble, M. A.; Liu, Y.-C.; Trzoss, M. Synthesis 2007,
204.1146.
1392.
(±)-4¢-Methyl-4¢,5¢-dihydro-3H,3¢H-spiro(benzofuran-
9) van Hooft, P. A. V.; van Swieten, P. F.; van der Marel,
2,2¢-furan) (7a,b)
G. A.; van Boeckel, C. A. A.; van Boom, J. H. Synlett 2001,
Pale yellow oil (8 mg, 0.042 mmol, 73%); 7a/7b = 1.4:1,
mixture of inseparable major (7a) and minor* (7b)
269.
(
(
10) Zhang, Y.; Xue, J.; Xin, Z.; Xie, Z.; Li, Y. Synlett 2008, 940.
11) Main, C. A.; Rahman, S. S.; Hartley, R. C. Tetrahedron Lett.
diastereomers; R
2954, 2924, 2855, 1598, 1479, 1462, 1377, 1241, 1120,
f
= 0.33 (hexanes–EtOAc, 9:1). IR (film):
–
1 1
2
008, 49, 4771.
12) (a) Huang, Y.; Pettus, T. R. R. Synlett 2008, 1353.
b) Marsini, M. A.; Huang, Y.; Lindsey, C. C.; Wu, K.-L.;
1082, 1010, 830, 779, 747, 706 cm . H NMR (400 MHz,
CDCl ): d = 1.13 (d, J = 6.8 Hz, 3 H, C-4¢-CH ), 1.20 (d,
J = 6.6 Hz, 2.1 H, C-4¢-CH *), 1.73 (dd, J = 12.9, 10.1 Hz, 1
H, H -3¢), 2.12 (dd, J = 13.6, 6.0 Hz, 0.7 H, H -3¢*), 2.38
(dd, J = 13.4, 9.4 Hz, 0.7 H, H -3¢*), 2.45–2.54 (m, 0.7 H,
H-4¢*), 2.51 (dd, J = 12.9, 7.0 Hz, 1 H, H -3¢), 2.69–2.82 (m,
1 H, H-4¢), 3.23–3.32 (m, 3.4 H, H-3 and H-3*), 3.56 (t,
J = 8.0 Hz, 1 H, H -5¢), 3.68 (t, J = 8.2 Hz, 0.7 H, H -5¢*),
4.13 (t, J = 7.9 Hz, 0.7 H, H -5¢*), 4.25 (t, J = 8.0 Hz, 1 H,
-5¢), 6.76 (d, J = 8.4 Hz, 1 H, H-7), 6.79 (d, J = 8.9 Hz, 0.7
H, H-7*), 6.85 (t, J = 7.4 Hz, 1.7 H, H-5 and H-5*), 7.11 (t,
(
3
3
(
3
Pettus, T. R. R. Org. Lett. 2008, 10, 1477. (c) Bray, C. D.
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2
B
A
B
008, 2500.
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(
6
B
A
R. J. Aust. J. Chem. 1996, 49, 189. (c) Brimble, M. A.
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B
H
A
Synlett 2009, No. 5, 793–797 © Thieme Stuttgart · New York

LETTER
J = 7.9 Hz, 1.7 H, H-6 and H-6*), 7.16 (d, J = 7.4 Hz, 1.7 H,
Approach to Monobenzannulated Spiroketals
797
125.8 (C, C-3a*), 127.89 (CH, C-6*), 127.94 (CH, C-6),
157.7 (C, C-7a), 158.0 (C, C-7a*). MS (EI, 70 eV): m/z
(%) = 37 (21), 47 (41.5), 78 (10), 83 (100), 85 (65.5), 107
1
3
H-4 and H-4*). C NMR (100 MHz, CDCl ): d = 17.7 (CH ,
3
3
C-4¢-CH ), 18.0 (CH , C-4¢-CH *), 32.3 (CH, C-4¢), 32.9
3
3
3
+
(
CH, C-4¢*), 39.0 (CH , C-3), 40.1 (CH , C-3*), 44.6 (CH ,
(21), 131 (6), 134 (3.5), 175 (9), 190 (30) [M] . HRMS (EI):
2
2
2
+
C-3¢*), 45.2 (CH , C-3¢), 75.3 (CH , C-5¢*), 75.5 (CH , C-
m/z [M] calcd for C H O : 190.0994; found: 190.0990.
2
2
2
12 14
2
5¢), 109.4 (CH, C-7), 109.5 (CH, C-7*), 118.6 (C, C-2*),
118.7 (C, C-2), 120.46 (CH, C-5), 120.51 (CH, C-5*),
124.49 (CH, C-4*), 124.53 (CH, C-4), 125.7 (C, C-3a),
(22) Deslongchamps, P. Stereoelectronic Effects in Organic
Chemistry, In Organic Chemistry Series, Vol. 1; Baldwin,
J. E., Ed.; Pergamon: Oxford, 1983, 4.
Synlett 2009, No. 5, 793–797 © Thieme Stuttgart · New York

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