γ-allenyl allyl benzothiazole sulfonyl anions undergo cis-selective (Sylvestre) Julia olefinations

Article abstract of DOI:10.1055/s-2004-837214

γ-Allenyl allyl benzothiazolyl sulfones 8 and ent-8 provided allenyl trienes and polyenes with cis-selectivities ranging from 71:29 to 100:0 upon condensation (NaHMDS, THF, -78 °C to 25 °C) with a variety of unsaturated aldehydes.

Full text of DOI:10.1055/s-2004-837214

294
LETTER
g-Allenyl Allyl Benzothiazole Sulfonyl Anions Undergo cis-Selective
(Sylvestre) Julia Olefinations
c
is-Selective (Sy
e
l
a
O
lef
i
é
nations n Vaz, Rosana Alvarez, José A. Souto, Angel R. de Lera*
Departamento de Química Orgánica, Universidade de Vigo, 36310 Vigo, Spain
Fax +34(986)812556; E-mail: qolera@uvigo.es
Received 28 October 2004
ratios.2 Notwithstanding the general trends, many excep-
tions to these predictions have been reported.1
Abstract: g-Allenyl allyl benzothiazolyl sulfones 8 and ent-8 pro-
vided allenyl trienes and polyenes with cis-selectivities ranging
from 71:29 to 100:0 upon condensation (NaHMDS, THF, –78 °C to
25 ºC) with a variety of unsaturated aldehydes.
It was recognized in a recent review article1 that the com-
bination of b,g-unsaturated a-metallated sulfones and a,b-
unsaturated aldehydes has been seldom exploited in syn-
thesis of trienes (and higher polyenes) despite its conver-
gent nature. Only the carotenoid field has witnessed the
extension of the seminal studies reported by Kocienski et
al. on the synthesis of the triene fragment of rapamycin.8
Our group disclosed the preparation of a pentaenyl bis-
stannane for symmetrical carotenoid construction,9 and
Katsumura’s group described the synthesis of the entire
polyene system10 of the carotenoid butenolide peridinin
using in both cases BT-sulfone condensation reactions. In
our approach to the latter compound,11 we prepared alle-
nyl triene 3 by coupling of 1 and 2. As in the above cases
and in keeping with other recent reports on trienes,12 the
major product had the Z-geometry (Scheme 1).
Key words: Julia olefination, stereoselectivity, allenes, polyenes
The one-pot (Sylvestre) Julia olefination,1 first reported in
1991 by Sylvestre Julia and coworkers,2 is a variant of the
classical (Marc) Julia3 connective olefination reaction that
uses heteroaryl (the original benzothiazole-2-yl,2 BT, and
the more recently reported 1-phenyl-1H-tetrazol-5-yl,
PT;4a 1-tert-butyl-1H-tetrazol-5-yl, TBT4b and pyridin-2-
yl, PYR4c,d) instead of arylsulfones. The intermediate b-
alkoxysulfones in these heterocyclic analogues experi-
ence a Smiles rearrangement via a spirocyclic intermedi-
ate resulting in the transfer of the heterocycle from S to O
to yield a sulfinate salt. Spontaneous elimination of SO2
and lithium benzothiazolone (or the like) yields the alkene
product. The (Sylvestre) Julia reaction thus circumvents
the (recommended) isolation of the b-alkoxy arylsulfone
intermediates, as well as their subsequent treatment with
an acylating agent and a one-electron reductant, of the
classical version. It is precisely the intermediacy of a rad-
ical species (of alkyl or vinyl nature when using SmI2/
HMPA5 or Na/Hg,6 respectively) what determines the
high trans-stereoselectivity of the (Marc) Julia olefina-
tion.7 On the other hand, being frequently substrate-de-
pendent and further influenced by reactions conditions,
predictions on the stereochemical outcome of the Sylves-
tre Julia (or Julia–Kocienski) olefination are problematic.
Trends are nevertheless emerging. Trans-stereoselectivi-
ties are generally obtained for allyl BT- and benzyl BT-
sulfones reacting with (branched) aldehydes.2 Reversible
addition–retroaddition (fragmentation) processes of the b-
alkoxy BT-sulfone into resonance-stabilized a-metallated
BT-sulfone and aldehyde as well as loss of lithium ben-
zothiazolone to afford zwitterionic intermediates have
been proposed to mechanistically justify the E-geometry
of the formed olefin.1 In the absence of the extra reso-
nance stabilizing effect, as in alkyl BT-sulfones, cis/trans
ratios favoring the former are obtained, which likely re-
flect the (kinetic) b-alkoxysulfone diastereomeric syn/anti
SO2BT
H
H
OH
AcO
H
1
+
OH
SnBu3
SnBu3
AcO
3
OHC
2
Scheme 1 Reagents and conditions: NaHDMS, THF, –78 °C; then
2, –78 °C to 25 °C; 70% of a 75:25 Z/E-3 mixture.
This finding led to question whether this stereochemical
outcome was general for polyenes. We wish to report the
general finding that g-allenyl allyl sulfones indeed afford
cis-polyenes upon reaction with unsaturated aldehydes. In
addition, we correct our assignment of the geometry of
symmetrical 1,w-bis(tributylstannyl)-1,3,5,7,9-decapen-
taene (16) obtained by the same method and previously
reported as trans.9
The g-allenyl allyl BT-sulfone 8 and its enantiomer ent-8
were prepared, pairwise along the sequence described in
Scheme 2, by Stille cross-coupling13 of bromoallenes 7
with stannylated allyl BT-sulfone 11.14 We have demon-
strated that Stille cross-coupling reaction conditions in-
duce an anti-selective SN2¢ displacement by palladium on
SYNLETT 2005, No. 2, pp 0294–0298
0
1.
0
2.
2
0
0
5
Advanced online publication: 17.12.2004
DOI: 10.1055/s-2004-837214; Art ID: G39104ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
cis-Selective (Sylvestre) Julia Olefinations
295
CHO
O
c
CHO
c
a,b
a,b
OH
O
O
O
84%
80%
88% for a
93% for b
95% for a
84% for b
4
ent-5
ent-6
6
5
d
86%
d
87%
SO2BT
SO2BT
H
Br
Br
H
H
H
e
e
2
2
2
2
62%
OH
OH
65%
OH
ent-8
OH
8
7
ent-7
N
N
f
g
S
S
Bu3Sn
56%
98%
SO2
Bu3Sn
OH
Bu3Sn
S
11
9
10
Scheme 2 g-Allenyl allyl benzothiazolyl sulfone synthesis. Reagents and conditions: a. D-(–)-DET or L-(+)-DET, Ti(Oi-Pr)4, t-BuOOH,
CH2Cl2, –20 °C, 12 h; b. DMSO, (COCl)2, –60 °C, 15 min, then Et3N, 10 min; c. LDA, TMSCHN2, THF, –78 °C, 1 h; then 25 °C, 2 h; d. HBr,
CuBr, NH4Br, Et2O, –10 °C, 2.5 min; e. 11, PdCl2(PhCN)2, i-Pr2NEt, DMF–THF, 45 °C, 2 h; f. BTSH, DIAD, PPh3, THF, 0 °C to 25 °C, 30
min; g. (NH4)6Mo7O24·4H2O, 35% H2O2, EtOH, 25 °C, 2.5 h.
these bromoallenes, followed by a [1,3]-metallotropic Table 1. Reactions and purifications were carried out in
shift before transmetallation with the stannanes to finally light-protected glassware to minimize isomerizations.
afford the corresponding substituted vinylallenes with in-
Regardless of the aldehyde structure, the stereoselectivity
is good to excellent and favors the Z-isomer. Entries 1–5
version of configuration.11
The synthesis of sulfone 11 started with alkenylstannane include a variety of a,b-unsaturated aldehydes 1224 which
9,15 which was converted into sulfide 10 using the afforded allenyl trienes with Z/E ratios ranging from
Mitsunobu variant16 (BTSH, PPh3, DIAD, 0 – 25 °C, 71:29 (13d) to 92:8 (13c). Assignment of the geometries
98%). Oxidation of 10 with a peroxymolybdate(VI)17 relied on the values of proton coupling constants (i.e., for
reagent in the presence of H2O2 [(NH4)6Mo7O24·4H2O, the newly formed double bond of 13b Z: J = 10.4 Hz; E:
35% H2O2, EtOH, 25 °C, 56%] delivered 11. Enantiopure J = 14.1 Hz) and were further confirmed by NOE studies.
bromoallenes 7 and ent-7 were in turn acquired by the Crotonaldehyde 12e, on the other hand, provided an in-
regio- and stereoselective displacement (SN2¢ anti) of tractable mixture of products, perhaps due to the special
alkynyl oxiranes 6 under Chemla’s18 conditions (48% lability of the allenyl triene formed. The polyenyl alde-
HBr, CuBr, NH4Br, Et2O, –10 °C, 2.5 h). Alkynyl hydes 12f and 12g afforded single products of Z geome-
oxiranes 6 and ent-6 were traced back to the Sharpless tries. Control of aldehyde reactivity in symmetrical
asymmetric epoxidation19 of b-cyclogeraniol 420 by a octadienal 12f could be made possible by the use of 1.1
modification (use of stoichiometric quantities) of the re- mol equivalents of 12f. Allenyl pentaenal 13f25 (65%) was
ported procedure,21 oxidation of the epoxyalcohols to 5 accompanied by a small amount (10%) of the symmetrical
(Swern conditions)22 and Colvin rearrangement23 of the carotenoid, the product of two-fold Julia condensation.
vinylcarbenes obtained upon treatment of aldehydes 5 Lastly, the entire C37-norcarotenoid skeleton of the caro-
with the anion of trimethylsilyldiazomethane (Scheme 2). tenoid butenolides, such as peridinin, could be prepared
The enantiomeric purity of the bromoallenes was assessed stereoselectively (only the Z-isomer was obtained) using
by chiral HPLC [Chiralcel OD-H, 5 mm, 0.46 × 15 cm, aldehyde 12g.26
97.5:2.5 hexane–EtOH, 0.15 mL/min; tR(1S,2aS) = 20.15
min; tR(1R,2aR) = 21.14 min], and ee values of 99% were
measured for each enantiomer. We surmise that the high
enantioselectivity was preserved along the sequence from
In view of these results, we re-examined the reaction of
stannylated dienal 14 and dienyl BT-sulfone 15.9 Under
the same general conditions, the symmetrical 1,w-
bis(tributylstannyl)-1,3,5,7,9-decapentaene (16) of Z-
that enforced by the reagent in the SAE of 4.
geometry12 was obtained as single product (Scheme 3).27
The anions of g-allenyl allyl BT-sulfones 8 were generat-
ed with NaHMDS in THF at –78 °C, as optimized for
carotenoid synthesis.11 Aldehydes 12a–g24 were added at
–78 °C, and the temperature was raised to 25 °C before
quenching. Purification by chromatography after work-up
afforded the allenyl trienes (or polyenes) 13 in the yields
and isomer ratio at the newly formed double bond listed in
The Z>E stereoselectivity in the connective olefination re-
action of allenyl allyl BT-sulfones and unsaturated alde-
hydes to afford polyenes appears, therefore, to be general.
If the Julia reaction is to rival other double-bond forming
processes (Wittig and Horner–Wadsworth–Emmons con-
densations), then substrates and/or reaction conditions
must be found that afford predictable and stereocomple-
Synlett 2005, No. 2, 294–298 © Thieme Stuttgart · New York

296
B. Vaz et al.
LETTER
Table 1 Yields (%) and Z/E Isomer Ratio for the (Sylvestre) Julia Olefination of 8 and ent-8 with Aldehydes 12a–g
SO2BT
R
R' CHO
H
R'
R
12
R
+
OH
R'
NaHMDS, THF, –78 °C
E-13
Z-13
8
Entry BT-sulfone
R¢CHO
Polyene
Yield (%)
82
Z:E ratioa
1
8
13a
86:14
CHO
Bu3Sn
12a
2
3
8
8
13b
13c
83
86
78:22
92:8
Ph
OHC
12b
OHC
12c
4
ent-8
13d
78
71:29
OHC
OTBDPS
12d
5
6
ent-8
ent-8
13e
Decomposition
65b
–
OHC
12e
13f25
100:0
CHO
OHC
12f
7
8
13g26
63
100:0
O
OHC
O
O
12g
a Determined by integration of the 1H NMR spectra of the reaction mixtures, which were separated by HPLC (preparative Nova-Pak, HR Silica,
1.9 × 30 cm, 6 mm; 95:5 hexane–EtOAc; flow rate: 3.5 mL/min).
b Yield of 13f. The bis-olefination polyene (10%) was also obtained.
Me
Acknowledgment
Me
CHO
We thank the European Commission (QLK3-2002-02029 ‘Anti-
cancer Retinoids’), the Spanish Ministerio de Ciencia y Tecnología
(Grant SAF01-3288, Ramón y Cajal Research Contract to R.A. and
FPU fellowship to B.V.) and Xunta de Galicia (Grant
PGIDIT02PXIC30108PN) for financial support
Bu3Sn
Bu3Sn
a
14
Me
+
Me
16
BTO2S
SnBu3
SnBu3
15
References
Scheme 3 Reagents and conditions: a) NaHMDS, THF, –78 °C;
then 15, –78 °C to 25 °C, 12 h; 83%.
(1) For a review, see: Blakemore, P. R. J. Chem. Soc., Perkin
Trans. 1 2002, 2563.
(2) (a) Baudin, J. B.; Hareau, G.; Julia, S. A.; Ruel, O.
Tetrahedron Lett. 1991, 32, 1175. (b) Subsequent studies:
Baudin, J. B.; Hareau, G.; Julia, S. A.; Ruel, O. Bull. Soc.
Chim. Fr. 1993, 130, 336. (c) Baudin, J. B.; Hareau, G.;
Julia, S. A.; Lorne, R.; Ruel, O. Bull. Soc. Chim. Fr. 1993,
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(3) Julia, M.; Paris, J.-M. Tetrahedron Lett. 1973, 4833.
(4) (a) Blakemore, P. R.; Cole, W. J.; Kocienski, P. J.; Morley,
A. Synlett 1998, 26. (b) Kocienski, P. J.; Bell, A.;
Blakemore, P. R. Synlett 2000, 365. (c) Charette, A. B.;
mentary outcomes. In this regard, at least for trienes,
reversal (E>Z) of the stereoselectivity shown here12 has
been reported to result from the condensation of alkyl
BT-sulfones and dienals (cf. rapamycin8 and tiacynomicin
synthesis28).
Synlett 2005, No. 2, 294–298 © Thieme Stuttgart · New York

LETTER
Bethelette, C.; St-Martin, D. Tetrahedron Lett. 2001, 42,
5149. (d) Corrigendum: Charette, A. B.; Bethelette, C.; St-
Martin, D. Tetrahedron Lett. 2001, 42, 6619.
(5) (a) Kende, A. S.; Mendoza, J. S. Tetrahedron Lett. 1990, 31,
7105. (b) See also: Markó, I. E.; Murphy, F.; Dolan, S.
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cis-Selective (Sylvestre) Julia Olefinations
297
CO2Et
OHC
O
(CH2OH)2, PPTS
C6H6, 100 °C (91%)
p-TsOH
R
OHC
OTBDPS
(6) Keck, G. E.; Savin, A.; Weglarz, M. A. J. Org. Chem. 1995,
60, 3194.
H2O, acetone (90%)
O
12d
(7) (a) Kocienski, P. J.; Lythgoe, B.; Ruston, S. J. Chem. Soc.,
Perkin Trans. 1 1978, 829. (b) Kocienski, P. J.; Lythgoe, B.;
Waterhouse, I. J. Chem. Soc., Perkin Trans. 1 1980, 1045.
(8) Bellingham, R.; Jarowicki, K.; Kocienski, P. J.; Martin, V.
Synthesis 1996, 285.
R = CO2Et
R = OH
DIBAL-H, THF
–78 °C (85%)
TBDPSCl, Imidazole
DMF, –25 °C (90%)
R = OTBDPS
Scheme 4
(9) Vaz, B.; Alvarez, R.; de Lera, A. R. J. Org. Chem. 2002, 67,
5040.
(10) Furuichi, N.; Hara, H.; Osaki, T.; Mori, H.; Katsumura, S.
Angew. Chem. Int. Ed. 2002, 41, 1023.
(11) Vaz, B.; Alvarez, R.; Brückner, R.; de Lera, A. R.,
submitted.
Pd2dba3,
I
OHC
+
AsPh3
(12) Sorg, A.; Brückner, R. Synlett 2005, preceding paper.
(13) (a) Stille, J. K. Pure Appl. Chem. 1985, 57, 1771. (b) Stille,
J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508.
(c) Farina, V. In Comprehensive Organometallic Chemistry
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Advances in Metal-Organic Chemistry, Vol. 5; Liebeskind,
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(g) Duncton, M. A. J.; Pattenden, G. J. Chem. Soc., Perkin
Trans. 1 1999, 1235.
CHO
NMP, 25 °C OHC
(80%)
CHO
12f
Bu3Sn
Scheme 5
(25) Data for 13f: 1H NMR (600 MHz, CDCl3): d = 9.45 (s, 1 H,
CHO), 6.98 (dd, J = 14.4, 11.8 Hz, 1 H, H15), 6.95 (d,
J = 11.9 Hz, 1 H, H14¢), 6.68 (dd, J = 14.4, 11.7 Hz, 1 H,
H
15¢), 6.59 (d, J = 12.1 Hz, 1 H, H10), 6.38 (t, J = 12.0 Hz, 1
H, H11), 6.33 (d, J = 12.1 Hz, 1 H, H14), 6.11 (s, 1 H, H8),
5.95 (d, J = 11.9 Hz, 1 H, H12), 2.13 (s, 3 H, C13-CH3), 1.90–
1.80 (m, 1 H, H3), 1.87 (s, 3 H, C13¢-CH3), 1.85 (s, 3 H, C9-
CH3), 1.80–1.70 (m, 1 H, H2 or H4), 1.60–1.50 (m, 3 H, H2 +
H3 + H4), 1.40–1.30 (m, 1 H, H2 or H4), 1.32 (s, 3 H, C5-
CH3), 1.27 (s, 3 H, C1-CH3), 1.04 (s, 3 H, C1-CH3) ppm. MS
(EI+): m/z (%) = 367 (27) [M+ + 1], 366 (100) [M+], 322 (35),
281 (41), 202 (28), 157 (36), 111 (37), 109 (32), 99 (27), 97
(60), 95 (27), 85 (61), 83 (62), 81 (30), 71 (80), 69 (85).
HRMS (EI+): m/z calcd for C25H34O2: 366.2559; found:
366.2555. FT-IR (NaCl): n = 3600–3400 (br, OH), 2960 (s,
C-H), 2923 (s, C-H), 2853 (m, C-H), 1930 (w, C=C=C),
1731 (s, C=O), 1662 (s), 1552 (m), 1261 (s) cm–1. UV
(MeOH): lmax = 294, 416 nm (Figure 1).
(14) All new compounds gave satisfactory spectroscopic data and
correct combustion analysis or HRMS.
(15) Lipshutz, B. H.; Kozlowski, J. A.; Wilhelm, R. S. J. Org.
Chem. 1984, 49, 3943.
(16) Blakemore, P. R.; Kocienski, P. J.; Morley, A.; Muir, K. J.
Chem. Soc., Perkin Trans. 1 1999, 955.
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1963, 28, 1140.
(18) (a) Bernard, N.; Chemla, F.; Normant, J. F. Tetrahedron
Lett. 1998, 39, 8837. (b) For propargyl alcohols, see: Black,
D. K.; Landor, S. R.; Patel, A. N.; Whiter, P. F. Tetrahedron
Lett. 1963, 483.
(19) Sharpless, K. B. In Comprehensive Organic Synthesis, Vol.
7; Trost, B. M.; Fleming, I., Eds.; Pergamon Press: Oxford,
1991, 389–436.
11
(20) Crombie, B. S.; Smith, C.; Varnavas, C. Z.; Wallace, T. W.
12
J. Chem. Soc., Perkin Trans. 1 2001, 2, 206.
8
(21) Abad, A.; Agulló, C.; Arnó, M.; Cuñat, A. C.; Zaragoza, R.
J. Synlett 1993, 895.
(22) (a) Mancuso, A. J.; Swern, D. Synthesis 1981, 165.
(b) Tidwell, T. T. Org. React. 1990, 39, 297.
(23) Colvin, E. W.; Hamill, B. J. J. Chem. Soc., Chem. Commun.
1973, 151.
13
H
14
5
15
15'
OH
13f
14'
CHO
(24) Synthesis of non-commercial aldehydes 12: (a) See for
12a: Fliegel, F.; Beaudet, I.; Quintard, J.-P. J. Organomet.
Chem. 2001, 624, 383. (b) See for 12d: The sequence is
provided below (Scheme 4); alternative preparation: Cole,
K. P.; Hsung, R. P. Org. Lett. 2003, 5, 4843. (c) See for
12f: The sequence is provided below (Scheme 5); alternative
preparation: van Wijk, A. A. C.; Lugtenburg, J. Eur. J. Org.
Chem. 2002, 4217. (d) See for 12g: This vinylogous epoxy-
retinal butenolide was acquired by inversion of the steps
leading to the ring-deoxygenated peridinin skeleton (see ref.
11): Vaz, B.; Alvarez, R.; de Lera, A. R., manuscript in
preparation.
Figure 1
(26) Data for 13g: 1H NMR [600 MHz, (CD3)2CO]: d = 7.50 (s, 1
H, H10), 7.17 (d, J = 15.6 Hz, 1 H, H7), 7.10 (t, J = 12.8 Hz,
1 H, H15¢), 6.76 (t, J = 12.3 Hz, 1 H, H15), 6.70–6.60 (m, 2 H,
H
10¢ + H14), 6.47 (t, J = 10.4 Hz, 1 H, H11¢), 6.42 (d, J = 15.6
Hz, 1 H, H8), 6.22 (t, J = 11.5 Hz, 1 H, H14¢), 6.18 (s, 1 H,
H8¢), 6.00 (s, 1 H, H12), 3.54 (s, 1 H, OH), 2.21 (d, J = 6.0 Hz,
3 H, C13-CH3), 2.00 (m, 1 H, H3¢), 1.89 (s, 3 H, C9¢-CH3),
1.90–1.80 (m, 2 H, H4¢ + H4), 1.60–1.50 (m, 1 H, H2¢), 1.50–
1.30 (m, 7 H, H3¢ + H2¢ + H4¢ + H2 + 2 H3 + H4), 1.36 (s, 3 H,
C1¢-CH3), 1.28 (s, 3 H, C5¢-CH3), 1.16 (s, 3 H, C1-CH3), 1.13
Synlett 2005, No. 2, 294–298 © Thieme Stuttgart · New York

298
B. Vaz et al.
LETTER
(s, 3 H, C5-CH3), 1.10–1.00 (m, 1 H, H2), 1.01 (s, 3 H, C1¢-
CH3), 0.91 (s, 3 H, C1-CH3) ppm. 13C NMR [100 MHz,
(CD3)2CO]: d = 204.4 (s, C7¢), 170.2 (s, C=O), 149.1 (s, C11),
139.8 (d, C14), 138.7 (d, C10), 137.9 (s, C9¢), 135.7 (s, C13),
135.3 (d, C7), 134.1 (d, C15¢), 131.5 (d, C15), 130.4 (d, C12¢),
129.6 (d, C11¢), 126.5 (s, C9), 124.4 (d, C10¢), 123.6 (d, C8),
121.3 (s, C6¢), 120.5 (d, C12), 104.4 (d, C8¢), 72.7 (s, C6), 71.7
(s, C5¢), 67.3 (s, C5), 42.6 (t, C4¢), 42.4 (t, C2¢), 37.5 (t, C2),
36.2 (s, C1¢), 35.3 (s, C1), 33.6 (q, C1¢-CH3), 32.6 (q, C5¢-
CH3), 31.7 (t, C4), 30.2 (q, C1¢-CH3), 27.4 (q, C1-CH3), 27.1
(q, C1-CH3), 22.0 (q, C5-CH3), 20.0 (t, C3¢), 18.7 (t, C3), 16.5
(q, C13-CH3), 15.2 (q, C9¢-CH3) ppm. MS (FAB+): m/z (%) =
558 (10) [M+ + 2], 557 (11) [M+ + 1], 556 (85) [M+], 540
(12), 539 (26), 394 (15), 393 (26), 322 (27), 307 (29), 289
(20), 241 (12), 165 (27). HRMS (FAB+): m/z calcd for
C37H49O4: 557.3631; found: 557.3613. FT-IR (NaCl): n =
3600–3400 (br, OH), 2961 (s, C-H), 2923 (s, C-H), 2849 (m,
C-H), 1926 (w, C=C=C), 1749 (s, C=O), 1521 (w), 1449 (w)
cm–1 (Figure 2).
11'
8'
12'
14
10'
H
15'
15
6'
5'
OH
12
O
13g
O
6
10
8
O
5
Figure 2
(27) Determined by 2D HMQC-TOCSY. Although the geometry
of 1,w-bis(tributylstannyl)-1,3,5,7,9-decapentaene (16)
reported by our group was in error (ref.9), the structures of
the final carotenoids b,b-carotene and (3R,3¢R)-zeaxanthin
obtained by Stille coupling of 16 with the corresponding
trienyliodides are correct. Isomerization takes place at the
carotenoid stage by the action of palladium, since reagent 16
is stable to the Stille coupling reaction conditions.
(28) Smith, A. B. III; Wan, Z. J. Org. Chem. 2000, 65, 3738.
Synlett 2005, No. 2, 294–298 © Thieme Stuttgart · New York