Stereoselective synthesis of the C11-N26 fragment of griseoviridin

Article abstract of DOI:10.1055/s-2002-20037

The C₁₁-N₂₆ fragment of griseoviridin has been prepared enantioselectively. The 1,3-syn diol synthon was derived from lipase-catalyzed selective acylation of (±)-1-(benzyloxy)pent-4-en-2-ol. The conjugated (E,E)-dienylmethyl azide fu

Full text of DOI:10.1055/s-2002-20037

PAPER
371
Stereoselective Synthesis of the C11–N26 Fragment of Griseoviridin
Stereoselective
Sy
r
nthesis
o
u6
f
the
C
11–NnFragment of Grise
K
oviridin . Ghosh,* Hui Lei
2
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, USA
Fax +1(312)9962183; E-mail: arunghos@uic.edu
Received 2 November 2001; revised 7 January 2002
Me
O
Abstract: The C11–N26 fragment of griseoviridin has been prepared
H
O
H
N
enantioselectively. The 1,3-syn diol synthon was derived from li-
pase-catalyzed selective acylation of ( )-1-(benzyloxy)pent-4-en-
2-ol. The conjugated (E,E)-dienylmethyl azide functionality was in-
stalled by a Pd-catalyzed allylic azidation reaction.
O
S
11
H
O
N
O
N H
25
Key words: alkenes, azides, antibiotics, catalysis, palladium
OH OH
Griseoviridin (1)
Griseoviridin (1), a member of the streptogramin family
of antibiotics, is a broad-spectrum antibiotic produced by
a strain of Streptomyces griseus.1 It has exhibited in vitro
inhibitory properties against various pathogenic bacteria
and fungi. Griseoviridin (1) inhibits protein synthesis by
interfering with bacterial ribosomal function.2 The struc-
ture of 1 and its absolute configuration have been deter-
mined by X-ray crystallography.3 The unique structural
features of griseoviridin (1) and its biological properties
have attracted much synthetic attention. Retrosynthetical-
ly, griseoviridin can be assembled in a convergent manner
from two appropriately protected fragments: a C11–N26 di-
enyl azide fragment 2 and a nine-membered macrocycle 3
(Figure). To date, the only total synthesis of griseoviridin
(1) has been recently achieved by Meyers.4 Several ap-
proaches for the synthesis of the oxazole or macrocyclic
fragments of 1 have also been published over the years.5
Herein we report an enantioselective synthesis of the pro-
tected C11–N26 fragment 2 of griseoviridin (1) utilizing an
optically active 1,3-syn diol synthon derived from lipase-
catalyzed selective acylation of ( )-1-benzyloxypent-4-
en-2-ol. A Pd-catalyzed allylic azidation has been utilized
to install the conjugated (E,E)-dienylmethyl azide func-
tionality selectively.
CO2Me
Me
O
O
N
O
O
S
N3
H2N
+
H
COOH
O
3
2
Ph
Figure
analysis). To install the conjugated primary (E,E)-dienyl-
methyl azide we have elected to carry out a palladium-cat-
alyzed azidation of the corresponding allyl ester. While
Murahashi and co-workers have extensively investigated
Pd(0)-catalyzed azidation of allyl esters to primary allyl
azides, selective formation of dienylmethyl azides from
the corresponding diene acetates has not been examined.7
Such conjugated allylamines are structural features of
many biologically important natural products.8 Thus,
treatment of 7 with vinylmagnesium bromide in diethyl
ether at 0 °C provided the allylic alcohol which was acety-
lated with acetic anhydride and triethylamine in the pres-
ence of a catalytic amount of DMAP to furnish acetate
derivative 8 as a 1:1 mixture in 74% yield for two steps.
Exposure of 8 to NaN3 in a mixture (1:1) of THF and wa-
ter at 65 °C in the presence of a catalytic amount (5 mol%)
of Pd(PPh3)4 afforded (E,E)-dienylmethyl azide 9 exclu-
sively in 81% isolated yield. The corresponding reaction
with Pd(OAc)2 and PBu3-derived catalyst was less effec-
tive forming azide 9 in only 71% yield. The configuration
of the newly formed double bond was confirmed to be E
by a proton decoupling experiment which showed the
coupling constant of the olefinic protons to be 14.7 Hz. To
elaborate the oxazole functionality, the isopropyl ester (9)
was saponified by treatment with aqueous sodium hy-
droxide in methanol at 23 °C. The resulting acid was cou-
pled with L-serine methyl ester in the presence of isobutyl
chloroformate and N-methylmorpholine to afford amide
The synthesis of C11–N26 fragment 2 is illustrated in the
Scheme. The known optically active diol 5 was prepared
in multigram quantities by enzymatic acylation of ( )-1-
benzyloxypent-4-en-2-ol as described previously.6 Re-
moval of the benzyl group by catalytic hydrogenation of 5
over Pearlman’s catalyst provided a triol which was react-
ed with dimethylbenzylidene acetal in CH2Cl2 in the pres-
ence of p-TsOH at 23 °C to afford benzylidene derivative
6 in 83% yield over 2 steps. Swern oxidation of 6 followed
by Wittig olefination of the resulting aldehyde with (triph-
enylphosphoranylidene)acetaldehyde in benzene at reflux
provided a mixture of , -unsaturated aldehyde 7 being
1
the major isomer (E/Z ratio of 12:1 by H and 13C NMR
Synthesis 2002, No. 3, 18 02 2002. Article Identifier:
1437-210X,E;2002,0,03,0371,0374,ftx,en;M05201SS.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881

372
A. K. Ghosh, H. Lei
PAPER
(4S,6R)-6-Isopropyloxycarbonylmethyl-4-hydroxymethyl-2-
10 in 80% yield for two steps. The serine amide 10 was
converted to the corresponding oxazoline by treatment
with Burgess’ salt in THF at reflux for 6 hours.9 Subse-
quent oxidation of the oxazoline with bromotrichlor-
omethane and 1,8-diazabicyclo[5.4.0]undec-7-ene as
described by Williams furnished 2 in 62% yield for two
steps.10
phenyl-1,3-dioxane (6)
To a stirred solution of 5 (804 mg, 2.7 mmol) in EtOAc (30 mL) and
MeOH (15 mL) was added Pd(OH)2 (161 mg). The resulting solu-
tion was placed under a H2 balloon and stirred at 23 °C for 2 h. The
mixture was filtered and the filtrate was evaporated under reduced
pressure to provide the crude triol (519 mg, 91%), which was used
directly for the next step without further purification.
1H NMR (CDCl3): = 5.01–5.07 (m, 1 H), 4.29 (m, 1 H), 3.98 (m,
1 H), 3.63 (dd, 1 H, J = 3.5, 11.2 Hz), 3.49 (dd, 1 H, J = 6, 11.2 Hz),
3.30 (br, 3 H), 2.45 (d, 2 H, J = 6 Hz), 1.55–1.72 (m, 2 H), 1.24 (d,
6 H, J = 6 Hz).
OH
O
OBn
ref. 6
CHO
OAc
O
OH OH
BnO
(±)-4
5
HRMS (EI): m/z calcd for C9H19O5 (M + H+) 207.1232, found (M +
a, b
H+) 207.1236.
To a stirred solution of the above triol (992 mg, 4.81 mmol) and
benzaldehyde dimethyl acetal (1.8 mL, 12 mmol) in CH2Cl2 (40
mL) was added p-TsOH (46 mg, 0.24 mmol). The resulting mixture
was stirred at r.t. for 1 h and was quenched by sat. aq NaHCO3. The
layers were separated and the aqueous layer was extracted with
CH2Cl2 (2 30 mL). The combined organic layers were washed
with brine and dried (Na2SO4). Evaporation of the solvent under re-
duced pressure provided a residue which was purified by column
chromatography on silica gel (30% EtOAc in hexanes) to afford the
alcohol 6 (1.28 g, 91%); [ ]D23 –3.64 (c = 2.75, CHCl3).
iPrO2C
iPrO2C
c, d
OH
O
O
O
O
7
6
Ph
Ph
e, f
N3
iPrO2C
iPrO2C
g
IR (film): 3450, 2979, 2924, 1729, 1106, 1023 cm–1.
O
O
O
O
Ph
9
Ph
1H NMR (400 MHz, CDCl3): = 7.47–7.50 (m, 2 H), 7.34–7.36 (m,
3 H), 5.59 (s, 1 H), 5.04 (m, 1 H), 4.35 (m, 1 H), 4.03 (m, 1 H), 3.60–
3.74 (m, 2 H), 2.70 (dd, 1 H, J = 7, 15.6 Hz), 2.50 (dd, 1 H, J = 6,
15.6 Hz), 2.11 (br, 1 H), 1.51–1.66 (m, 2 H), 1.23 (d, 6 H, J = 6.6
Hz).
13C NMR (100 MHz, CDCl3): = 170.1, 138.0, 128.8, 128.1, 126.1,
100.5, 77.4, 72.9, 68.0, 65.3, 41.2, 31.9, 21.7.
8
h, i
CO2Me
MeO2C
O
OH
N3
O
N
N3
NH
O
j, k
HRMS (EI): m/z calcd for C16H22O5Na (M + Na+) 317.1365, found
O
O
2
(M + Na+) 317.1382.
O
10
Ph
Ph
(4S,6R)-6-Isopropyloxycarbonylmethyl-2-phenyl-4-[(E)-3-
oxoprop-1-enyl]-1,3-dioxane (7)
Scheme (a) H2, Pd(OH)2, EtOAc, 91%; (b) PhCH(OMe)2, p-TsOH,
CH2Cl2, 91%; (c) (COCl)2, DMSO, Et3N, CH2Cl2, –78 °C; (d)
Ph3P=CHCHO, benzene, reflux, 70% for 2 steps; (e) CH2=CHMgBr,
Et2O, 0 °C ; (f) Ac2O, Et3N, DMAP, CH2Cl2, 74% for 2 steps ; (g)
Pd(PPh)4, NaN3, THF–H2O, 65 °C, 81%; (h) NaOH, MeOH–H2O; (i)
N-methylmorpholine, isobutyl chloroformate, serine methyl
ester HCl, THF, –30 to +23 °C, 80% 2 for steps; (j) Burgess salt, THF,
reflux; (k) BrCCl3, DBU, CH2Cl2, 0 to 23 °C, 62% for 2 steps
Oxalyl chloride (0.2 mL, 2.2 mmol) was added to CH2Cl2 (30 mL)
with stirring at –78 °C. The resulting mixture was stirred for 5 min
and DMSO (0.3 mL, 4.28 mmol) was added. After 15 min, alcohol
6 (210 mg, 0.71 mmol) in CH2Cl2 (5 mL) was added dropwise. The
resulting mixture was stirred at –78 °C for 30 min and Et3N (1 mL,
7.1 mmol) was added. The solution was warmed to 23 °C and
quenched with aq sat. NH4Cl solution. The layers were separated
and the aqueous layer was extracted with CH2Cl2 (2 15 mL). The
combined organic layers were washed with brine, dried (Na2SO4)
and evaporated under reduced pressure to provide the crude alde-
hyde, which was used directly for next step without further purifi-
cation. The crude aldehyde and (triphenylphosphoranylidene)-
acetaldehyde (216.1 mg, 0.71 mmol) were dissolved in CHCl3 and
heated to reflux for 15 h. The resulting mixture was cooled to r.t.
and the solvent was removed under reduced pressure. The residue
was purified by column chromatography on silica gel (30% EtOAc
in hexanes) to provide aldehyde 7 (156 mg, 70%); [ ]D23 –13.2 (c =
1.0, CHCl3).
In conclusion, an enantioselective synthesis of the C11–
N26 fragment of griseoviridin (1) has been achieved. Effi-
cient installation of the conjugated (E,E)-dienylmethyl
azide functionality by a Pd-catalyzed allylic azidation is
noteworthy.
All moisture sensitive reactions were carried out under N2. Anhyd
solvents were obtained as follows: THF, distilled from sodium and
benzophenone; CH2Cl2, distilled from P2O5; Et3N, distilled from
CaH2. All other solvents were HPLC grade. Column chromatogra-
phy was performed with Whatman 240–400 mesh silica gel under
low pressure of 5–10 psi. TLC was carried out with E.Merck silica
IR (film): 2980, 1728, 1691, 1375, 1147, 1109, 1017 cm–1.
1H NMR (400 MHz, CDCl3): = 9.58, (d, 1 H, J = 8 Hz), 7.49–7.51
(m, 2 H), 7.34–7.38 (m, 3 H), 6.81 (dd, 1 H, J = 4.1, 16 Hz), 6.40
(ddd, 1 H, J = 1.6, 7.9, 16 Hz), 5.67 (s, 1 H), 5.04 (m, 1 H), 4.70 (m,
1 H), 4.40 (m, 1 H), 2.72 (dd, 1 H, J = 7, 15.7 Hz), 2.52 (dd, 1 H,
J = 6.4, 15.7 Hz), 1.93 (m, 1 H), 1.60 (m, 1 H), 1.24 (d, 6 H, J = 6.3
Hz).
1
gel 60-F254 plates. H and 13C NMR spectra were recorded on
Bruker AM 400 (400 MHz), Avance 400 (400 MHz) and Avance
500 (500 MHz) spectrometers.
Synthesis 2002, No. 3, 371–374 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
Stereoselective Synthesis of the C11–N26 Fragment of Griseoviridin
373
13C NMR (100 MHz, CDCl3): = 193.3, 169.9, 153.9, 137.6, 131.1,
129.0, 128.3, 120.0, 100.6, 74.8, 73.2, 68.2, 40.9, 35.4, 21.8
(1S,4S,6R)-4-[(1E,3E)-5-Azidopenta-1,3-dienyl)-6-[N-(1-hy-
droxylmethyl-2-methoxycarbonyl)acetylamino]-2-phenyl-1,3-
dioxane (10)
HRMS (EI): m/z calcd for C18H22O5Na (M + Na+) 341.1365, found
Azide 9 (18 mg, 0.05 mmol) in a mixture of MeOH (3 mL) and 4 N
aq NaOH (3 mL) was stirred at r.t. for 12 h. After this period, the
mixture was concentrated under reduced pressure. The resulting
residue was cooled to 0 °C and acidified to pH 3 with 25% citric ac-
id. The mixture was extracted with EtOAc (3 5 mL). The com-
bined organic layers were dried (Na2SO4) and evaporated under
reduced pressure to provide the crude acid, which was used directly
without further purification. To a solution of the above acid in THF
(5 mL) cooled at –30 °C was added N-methylmorpholine (17 L,
0.16 mmol) and isobutyl chloroformate (7 L, 0.05 mmol). The re-
sulting mixture was stirred at –30 °C for 30 min. L-Serine methyl es-
ter (8 mg, 0.05 mmol) was added and the reaction mixture was
warmed to 23 °C and stirred for 15 h. After this period, the mixture
was evaporated under reduced pressure and purified by column
chromatography on silica gel (80% EtOAc in hexanes) to provide
amide 10 (17 mg, 80% 2 steps); [ ]D23 +11.5 (c = 2.26, CHCl3).
(M + Na+) 341.1357.
(4S,6R)-4-[(1E)-3-Acetyloxypenta-1,4-dienyl]-6-isopropoxycar-
bonylmethyl-2-phenyl-1,3-dioxane (8)
To a stirred solution of aldehyde 7 (156.1 mg, 0.49 mmol) in Et2O
(10 mL) cooled at 0 °C was added a 1 M solution of vinylmagne-
sium bromide in THF (1 mL, 1 mmol) dropwise. The resulting mix-
ture was stirred at 0 °C for 30 min and was quenched by sat. aq
NH4Cl solution. The layers were separated and the aqueous layer
was extract with EtOAc (2 10 mL). The combined organic layers
were washed with brine, dried (Na2SO4) and evaporated under re-
duced pressure to provide the crude alcohol, which was used direct-
ly for next step without further purification. To a stirred solution of
the above alcohol in CH2Cl2 (10 mL) at 0 °C was added Ac2O (92
L, 1 mmol), Et3N (140 L, 1 mmol) and DMAP (6 mg, 0.05
mmol). The resulting mixture was stirred for 1 h and was quenched
by sat. aq NH4Cl solution. The layers were separated and the aque-
ous layer was extracted with CH2Cl2 (2 10 mL). The combined or-
ganic layers were washed with brine, dried (Na2SO4) and
evaporated and the residue was purified by column chromatography
on silica gel (10% EtOAc in hexanes) to provide a mixture of ace-
tates 8 as a colorless oil (146 mg, 74% 2 steps).
IR (film): 3367, 2952, 2100, 1744, 1653, 1533, 1217 cm–1.
1H NMR (400 MHz, CDCl3): = 7.49 7.52 (m, 2 H), 7.35–7.39 (m,
3 H), 6.95 (d, 1 H, J = 7.5 Hz), 6.23–6.37 (m, 2 H), 5.71–5.83 (m, 2
H), 5.63 (s, 1 H), 4.65 (m, 1 H), 4.44 (m, 1 H), 4.34 (m, 1 H), 3.79–
3.91 (m, 4 H), 3.73 (s, 3 H), 2.51–2.62 (m, 2 H), 1.60–1.80 (m, 2 H).
13C NMR (100 MHz, CDCl3): = 170.7, 170.4, 137.9, 133.7, 133.4,
129.7, 129.1, 128.3, 126.8, 126.2, 100.9, 76.3, 73.6, 63.1, 54.7,
52.7, 52.5, 42.8, 36.2.
IR (film): 2961, 1734, 1371, 1234, 1105, 1019 cm–1.
1H NMR (400 MHz, CDCl3): = 7.48–7.51(m, 2 H), 7.33–7.35 (m,
3 H), 5.77–5.87 (m, 3 H), 5.73 (m, 1 H), 5.61 (s, 1 H), 5.28 (m, 2 H),
5.04 (m, 1 H), 4.43 (m, 1 H), 4.34 (m, 1 H), 2.69 (dd, 1 H, J = 7,
15.6 Hz), 2.49 (dd, 1 H, J = 6.3, 15.6 Hz), 2.08 (s, 3 H), 1.78–1.83
(m, 1 H), 1.53–1.60 (m, 1 H), 1.24 (d, 6 H, J = 6.3 Hz).
13C NMR (100 MHz, CDCl3): = 170.1, 169.9, 138.2, 134.9, 132.6,
132.5, 128.8, 128.2, 128.0, 127.9, 126.1, 117.6, 100.6, 75.9, 75.8,
74.2, 74.1, 73.2, 68.1, 41.2, 36.4, 21.8, 21.2.
HRMS (EI): m/z calcd for C21H26N4O6Na (M + Na+) 453.1750,
found (M + Na+) 453.1755.
(4S,6R)-4-[(1E,3E)-5-Azidopenta-1,3-dienyl)-6-[(4-(methoxy-
carbonyl)-2-oxazolyl)methyl]-2-phenyl-1,3-dioxane (2)
To a stirred solution of the amide 10 (6 mg, 0.01 mmol) in THF (5
mL) was added methyl-N-(triethylammoniosulfonyl)carbamate
(Burgess’ reagent) (6 mg, 0.03 mmol). The resulting mixture was
heated to 60 ºC for 2 h. After this period, the mixture was cooled to
r.t. and passed through a short silica gel column. Evaporation of the
solvent under reduced pressure provided the crude oxazoline, which
was used directly without further purification. To a stirred solution
of the above crude oxazoline in CH2Cl2 (3 mL) cooled at 0 °C was
added DBU (6 L, 0.04 mmol) and BrCCl3 (4 L, 0.04 mmol). The
mixture was warmed to 23 °C and stirred for 15 h. After this period,
the mixture was quenched with sat. aq NH4Cl. The layers were sep-
arated and the aqueous layer was extracted with CH2Cl2 (3 3 mL).
The combined organic layers were washed with brine, dried
(Na2SO4) and evaporated under reduced pressure. The residue was
purified by column chromatography on silica gel (50% EtOAc in
hexanes) to provide oxazole 2 (3.4 mg, 62% 2 steps); [ ]D23 –6.25
(c = 0.48, CHCl3).
+
HRMS (EI): m/z calcd for C22H32NO6 (M + NH4 ) 406.2230, found
+
(M + NH4 ) 406.2235.
(4S,6R)-4-[(1E,3E)-5-Azidopenta-1,3-dienyl)-6-isopropoxycar-
bonylmethyl-2-phenyl-1,3-dioxane (9)
To a stirred solution of acetate 8 (129 mg, 0.33 mmol) in THF (10
mL) and H2O (4 mL) was added NaN3 (43 mg, 0.7 mmol) and
Pd(PPh3)4 (19 mg, 0.016 mmol). The resulting mixture was heated
to reflux for 1 h and cooled to r.t. The layers were seperated and the
aqueous layer was extracted with EtOAc (3 5 mL). The combined
organic layers were washed with brine, dried (Na2SO4) and evapo-
rated under reduced pressure. The residue was purified by column
chromatography on silica gel (5% EtOAc in hexanes) to provide
23
azide 9 (99 mg, 81%) as a colorless oil; [ ]D +0.76 (c = 1.3,
CHCl3).
IR (film): 2923, 2102, 1732, 1112 cm–1.
1H NMR (400 MHz, CDCl3): = 7.49–7.51 (m, 2 H), 7.34–7.36 (m,
3 H), 6.24–6.38 (m, 2 H), 5.73–5.86 (m, 2 H), 5.63 (s, 1 H), 5.05 (m,
1 H), 4.46 (m, 1 H), 4.35 (m, 1 H), 3.81 (d, 2 H, J = 6.7 Hz), 2.71
(dd, 1 H, J = 7, 15.6 Hz), 2.50 (dd, 1 H, J = 6.3, 15.6 Hz), 1.80 (m,
1 H), 1.60 (m, 1 H), 1.24 (d, 6 H, J = 6.2 Hz).
13C NMR (100 MHz, CDCl3): = 170.1, 138.1, 133.8, 133.7, 129.4,
128.7, 128.1, 126.5, 126.1, 100.6, 76.2, 73.1, 67.9, 52.4, 41.1, 36.3,
21.7.
1H NMR (400 MHz, CDCl3): = 8.19, (s, 1 H), 7.46–7.52 (m, 2 H),
7.31–7.36 (m, 3 H), 6.23–6.37 (m, 2 H), 5.71–5.85 (m, 2 H), 5.62
(s, 1 H), 4.40–4.48 (m, 2 H), 3.92 (s, 3 H), 3.80 (d, 2 H, J = 6.4 Hz),
3.24 (dd, 1 H, J = 6.5, 15 Hz), 3.08 (dd, 1 H, J = 6.5, 15 Hz), 1.78–
1.95 (m, 2 H).
13C NMR (100 MHz, CDCl3): = 162.1, 161.5, 144.0, 137.9, 133.6,
133.3, 129.6, 128.8, 128.1, 126.7, 126.1, 125.9, 100.7, 73.9, 52.4,
52.1, 36.2, 34.6, 29.6.
HRMS (EI): m/z calcd for C20H25N3O4Na (M + Na+) 394.1743,
found (M + Na+) 394.1732.
HRMS (EI): m/z calcd for C21H22N4O5K (M + K+) 449.1227, found
(M + K+) 449.1240.
Synthesis 2002, No. 3, 371–374 ISSN 0039-7881 © Thieme Stuttgart · New York

374
A. K. Ghosh, H. Lei
PAPER
(4) Dvorak, C. A.; Schmitz, W. D.; Poon, D. J.; Pryde, D. C.;
Lawson, J. P.; Amos, R. A.; Meyers, A. I. Angew. Chem.,
Int. Ed. 2000, 39, 1664.
(5) (a) Meyers, A. I.; Amos, R. A. J. Am. Chem. Soc. 1980, 102,
870. (b) Butera, J.; Rini, J.; Helquist, P. J. Org. Chem. 1985,
50, 3676. (c) Liu, L.; Tanke, R. S.; Miller, M. J. J. Org.
Chem. 1986, 51, 5332. (d) Marcantoni, E.; Massaccesi, M.;
Petrini, M. J. Org. Chem. 2000, 65, 4553.
(6) Ghosh, A. K.; Lei, H. J. Org. Chem. 2000, 65, 4779.
(7) Murahashi, S.; Taniguchi, Y.; Imada, Y.; Tanigawa, Y. J.
Org. Chem. 1989, 54, 3292.
(8) (a) Cocito, C. Microbiol. Rev. 1979, 43, 145. (b)Service,R.
F. Science 1995, 270, 724.
Acknowledgment
Financial support of this work by the National Institutes of Health
(GM 55600) is gratefully acknowledged.
References
(1) Ehrlich, J.; Coffey, G. L.; Fisher, M. W.; Galbraith, M. M.;
Knudsen, M. P.; Sarber, R. W.; Schlingman, A. S.; Smith, R.
M.; Weston, J. K. Antibiot. Annu. 1955, 790.
(2) Vazquez, D. Antibiotics, Vol. III; Corcoran, J. W.; Hahn, F.
E., Eds.; Springer Verlag: New York, 1975.
(3) Birubaum, G. I.; Hall, S. R. J. Am. Chem. Soc. 1976, 98,
1926.
(9) Burgess, E. M.; Penton, H. R.; Taylor, E. A.; Williams, W.
M. Org. Synth. Coll. Vol. VI; Wiley: New York, 1988, 788.
(10) Williams, D. R.; Lowder, P. D.; Gu, Y. G.; Brooks, D. A.
Tetrahedron Lett. 1997, 38, 331.
Synthesis 2002, No. 3, 371–374 ISSN 0039-7881 © Thieme Stuttgart · New York