Synthesis of (7S,15S)- and (7R,15S)-dolatrienoic acid

Article abstract of DOI:10.1021/np000502q

The stereospecific synthesis of (7S,15S)- and (7R,15S)-dolatrienoic acids (2) was achieved using an approach consisting of 16 linear steps. The C-11-C-16 unit was prepared in seven steps from ethyl (S)lactate and coupled using a trans-selective Wittig - Schlosser reaction to the C-7-C-10 fragment. Chirality at the C-7 position was introduced using an Evan's-type chiral auxiliary in a cobalt-mediated Reformatsky reaction to give the (3S,11S)-aldehyde 24. Subsequent Wittig reaction with a phosphonium salt derived in three steps from tiglic acid gave (7S,15S)-dolatrienoic acid, one of the four possible diastereoisomers of the nonpeptide portion of the strong Cancer cell growth inhibitory cyclodepsipeptide dolastatin 14 (1). A second diastereoisomer, (7R,15S)-dolatrienoic acid, was synthesized employing chiral oxazolidinone 21 by an analogous synthetic route.

Full text of DOI:10.1021/np000502q

472
J . Nat. Prod. 2001, 64, 472-479
Syn th esis of (7S,15S)- a n d (7R,15S)-Dola tr ien oic Acid 1a
J onathan J . Duffield and George R. Pettit*
Cancer Research Institute and Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona
85287-2404
Received October 27, 2000
The stereospecific synthesis of (7S,15S)- and (7R,15S)-dolatrienoic acids (2) was achieved using an
approach consisting of 16 linear steps. The C-11-C-16 unit was prepared in seven steps from ethyl (S)-
lactate and coupled using a trans-selective Wittig-Schlosser reaction to the C-7-C-10 fragment. Chirality
at the C-7 position was introduced using an Evan’s-type chiral auxiliary in a cobalt-mediated Reformatsky
reaction to give the (3S,11S)-aldehyde 24. Subsequent Wittig reaction with a phosphonium salt derived
in three steps from tiglic acid gave (7S,15S)-dolatrienoic acid, one of the four possible diastereoisomers
of the nonpeptide portion of the strong cancer cell growth inhibitory cyclodepsipeptide dolastatin 14 (1).
A second diastereoisomer, (7R,15S)-dolatrienoic acid, was synthesized employing chiral oxazolidinone
21 by an analogous synthetic route.
In 1972 we began to pursue the Indian Ocean sea hare
Dolabella auricularia as an especially valuable source of
structurally unique and exceptionally active anticancer
constituents.1b,2,3 Later, we isolated dolastatins 1-18 and
elucidated the structures of dolastatins 3 and 10-18 from
this shell-less mollusc.3 The most promising of these,
dolastatin 10, was isolated in 1984.2 We completed the first
total synthesis4 to allow advanced preclinical and clinical
development of this new anticancer drug, which is pres-
ently undergoing a series of phase II human cancer clinical
trials under the auspices of the U.S. National Cancer
Institute.
by comparison with other dolastatins from the same sea
hare,5 but no such ready comparison was apparent for the
C-7 and C-15 chiral centers of dolatrienoic acid. Their
absolute configurations need to be assigned by an X-ray
crystal structure, which so far has eluded us, or by the total
synthesis of dolastatin 14 and comparison with spectral
data acquired from the natural product. Two (the 7R,15R
and 7S,15R) of the four possible diastereoisomers were
recently synthesized.6 Now follows a summary of our
syntheses of the 7S,15S and 7R,15S diastereoisomers of
dolatrienoic acid by procedures suitable for preparation of
the other two isomers.
Retrosynthetic analysis of dolatrienoic acid (2) initially
led us to a disconnection at the C-6-C-7 bond (Scheme 1).
The overall plan was based on a hexadiene ester of the type
3 undergoing a doubly vinylogous aldol reaction with the
optically pure aldehyde 4. We envisaged the preparation
of aldehyde 4 from a suitably protected phosphonium salt
6 and the already known7 aldehyde 5. In turn the phos-
phonium salt 6 would be prepared from the alcohol 7,8 and
the latter might be resolved by using an appropriate chiral
auxiliary and fractional recrystallization. The approach just
outlined was implemented by preparation of bromo alcohol
7 by sodium borohydride reduction of the ketone 8, derived
from ethyl acetoacetate according to the procedure of
Curran and Scholz.8 Esterification with (S)-mandelic acid
gave the corresponding diastereomeric esters (9). Repeated
fractional crystallization of the esters gave a low yield of
the S,S isomer as a pure crystalline solid, but the R,S
isomer resisted crystallization and purification.
Resu lts a n d Discu ssion
Dolastatin 14 was isolated (12 mg from 1600 kg of D.
auricularia in 7.5 × 10-7% yield) and assigned structure 1
on the basis of a series of high-field 2D NMR studies.
Dolastatin 14 consists of a heptapeptide joined by a 16-
carbon olefinic carboxylic acid designated dolatrienoic acid
to form a cyclodepsipeptide. The stereochemical assign-
ments and more extensive biological evaluation of this
unusual peptide have been precluded by its trace occur-
rence in the natural source. However, dolastatin 142 (1)
was found to exhibit exceptional activity against a selection
of human cancer cell lines.3 Thus a practical, total synthesis
of dolastatin 14 has become very necessary, and the present
investigation was undertaken to begin addressing this
research objective. We earlier assumed the stereochemistry
of the dolastatin 14 heptapeptide unit to be exclusively S
In view of these results a different route (Scheme 2) was
chosen to obtain the chiral isomer of phosphonium salt 6.
Starting from the readily available ethyl (S)-lactate (10),
TBDPS protection followed by reduction with borane gave
alcohol 12. An initial attempt to convert this directly to
bromide 14 using N-bromosuccinimide and triphenylphos-
* To whom correspondence should be addressed. Tel: (480)-965-3351.
Fax: (480)-965-8558.
10.1021/np000502q CCC: $20.00
© 2001 American Chemical Society and American Society of Pharmacognosy
Published on Web 04/06/2001

(7S,15S)- and (7R,15S)-Dolatrienoic Acid
J ournal of Natural Products, 2001, Vol. 64, No. 4 473
Sch em e 3a
Sch em e 1
a
(i) 4, Co(PPh3)4, THF, 0 °C; (ii) CH3OH, K2CO3, 0 °C, (iii) (CH3)3OBF4,
proton sponge, molecular sieves, DCM; (iv) DIBAL, DCM, then PDC,
molecular sieves, DCM.
alkenes were formed in a ratio of about 9:1. When the
Schlosser conditions were used, the selectivity of the
reaction was reversed to give mostly the E alkene (17). The
two isomers were easily separable by gravity Si gel column
chromatography, and olefin 17 was converted to aldehyde
4 by treatment with aqueous acetic acid in THF. A small
amount of the product was isolated as the C-9 alcohol (18)
but was easily reprotected.
Sch em e 2a
The proposed C-1-C-6 fragment (3) was prepared (90%
yield) in one step by a Wadsworth-Emmons condensation
between the commercially available triethylphosphono-
acetate and crotonaldehyde. Attempts to couple diene 3 to
aldehyde 4 via an aldol-type reaction were unsuccessful.
Instead we utilized a new cobalt complex based Refor-
matsky reaction10 involving the C-5-C-6 synthon 19
derived from (4R,5S)-4-methyl-5-phenyl-2-oxazolidinone to
give alcohol 20 (Scheme 3). We presumed the Evan’s
oxazolidinone approach would lead to good stereospecificity,
but the reaction led to an approximately 1:1 mixture of
alcohols (20 and 21). Fortunately, the presence of the chiral
oxazolidinone group enabled the separation of the two
diastereoisomers very simply by column chromatography.
Once the oxazolidinone unit was removed by treatment
with potassium carbonate in methanol, the 3R,11S and the
3S,11S diastereoisomers of ester 22 showed identical TLC.
When the absolute stereochemistry of the natural product
has been determined, the use of alternative chiral auxil-
iaries to achieve a stereospecific coupling reaction with
aldehyde 4 will be investigated. The 3S,11S ester 22 was
converted to the key intermediate 24 in two steps in an
overall yield of 81%. The 3-hydroxyl position was first
methylated using Meerwein’s salt to give methyl ester 23,
which was subsequently reduced with DIBAL. Even at -78
°C the ester was rapidly reduced to an approximate 1:1
mixture of aldehyde 24 and the corresponding alcohol.
Immediate treatment of this mixture with pyridinium
dichromate in the presence of molecular sieves led to
aldehyde 24.
a
(i) TBDPSCl, imidazole, DMF; (ii) BH3‚THF, THF, reflux; (iii) TsCl,
pyridine, DCM; (iv) LiBr, THF, reflux; (v) allylmagnesium bromide,
Li2CuCl4, THF; (vi) BH3‚THF, Br2, NaOCH3; (vii) PPh3, toluene, reflux; (viii)
PhLi, 1,3-dioxolane-2-propanal, PhLi, HCl, KOtBu; (ix) 30% aqueous acetic
acid/THF, 1:1, reflux.
phine in DMF gave only a moderate yield of 55%. Finally
the alcohol (12) was converted to bromide 14 via tosylate
13 with an overall yield for the two steps of 93%. Three-
carbon homologation of bromide 14 was achieved by the
lithium tetrachlorocuprate catalyzed addition of allylmag-
nesium bromide. The resulting olefin 15 underwent anti-
Markovnikov bromination to give bromide 16. On treat-
ment with triphenylphosphine, the bromide 16 afforded the
necessary chiral phosphonium salt 6. The overall yield from
ethyl (S)-lactate for the seven steps was 36%. Chiral alde-
hyde 4 was generated from phosphonium salt 6 utilizing
the Schlosser modification of the Wittig reaction.9 However,
by allowing coupling between the ylid derived from phos-
phonium salt 6 and the aldehyde 5 under common Wittig
reaction conditions (1 equiv of n-BuLi) both the Z and E
The 7S,15S diastereoisomer (2a ) of dolatrienoic acid (2)
was prepared by a simple Wittig reaction between aldehyde
24 and the phosphonium salt 29 prepared in three steps
from tiglic acid (25) as follows. Methylation followed by
vinylic bromination with N-bromosuccinimide gave a mix-
ture of two bromo derivatives (27 and 28). While the two
resisted separation by distillation or column chromatog-
raphy, when the mixture was treated with triphenylphos-

474 J ournal of Natural Products, 2001, Vol. 64, No. 4
Duffield and Pettit
Sch em e 4a
a
(i) Methanol, H2SO4, reflux; (ii) NBS, AIBN, CCl4, reflux; (iii) PPh3, toluene, reflux 2 h; (iv) 1 equiv of NaH rt then 1 equiv of 24 -78 °Cfrt; (v) HF‚pyridine,
THF, 117 h; (vi) LiOH‚H2O, 1.3 equiv, THF/water 4:1, 160 h.
Exp er im en ta l Section
phine in refluxing toluene, only one product, the less
hindered salt 29, was isolated. Reaction between the ylid
derived from phosphonium salt 29 (by treatment with
sodium hydride) and aldehyde 24 gave rise to a mixture of
diene 30 and its 4Z isomer (31). This mixture was con-
verted to pure diene 30 by treatment with iodine in
chloroform. Cleavage of the silyl-protecting group was first
attempted using tetrabutylammonium fluoride, but this
proved too basic and caused elimination of methanol to
produce the corresponding very stable conjugated triene.
Reaction of silyl ether 30 with aqueous HF was very slow,
probably due to the biphasic system. Eventually it was
found that deprotection using hydrogen fluoride/pyridine
complex was most satisfactory, yielding alcohol 32 in 89%
yield. Finally ester hydrolysis using lithium hydroxide gave
(7S,15S)-dolatrienoic acid (2a ) in 79% yield along with 14%
of recovered starting material (Scheme 4). The 7R,15S
diastereoisomer (2b) of dolatrienoic acid (2) was prepared
from (4R,5S,3′R,11′S)-3-(11′-(tert-butyldiphenylsilyloxy)-3′-
hydroxy-6′E-dodecenoyl)-4-methyl-5-phenyl-2-oxazolidi-
one (21), which is formed in approximately equal amounts
as the 4R,5S,3′S,11′S isomer (20) during the cobalt-
mediated Reformatsky reaction. The reactions performed
are identical to those used to prepare the 7S,15S diaste-
reoisomer of dolatrienoic acid. The configuration at C-3 for
compounds 20 and 21 was determined by a comparison of
the NMR spectra of 2a and 2b to the corresponding
enantiomers of the Moune et al.6 work, for which absolute
configuration was determined.
Gen er a l Exp er im en ta l P r oced u r es. All reagents were
used as received from Sigma-Aldrich Chemical Co., Acros
Chemical Co., or E.M. Scientific, and solvents were distilled
prior to use. Reactions requiring anhydrous conditions were
conducted in a flame-dried, round-bottom flask, sealed with a
rubber septum under an atmosphere of argon. Evaporation of
solvents was performed under reduced pressure using a rotary
evaporator, with an external bath temperature of 45 °C. For
TLC procedures, Si gel GHLF Uniplates (Analtech, Inc.) were
utilized. Solvents, except aqueous solutions, were dried over
4 Å molecular sieves. Column chromatography (Kieselgel 60)
was performed using Si gel supplied by E. Merck (Darmstadt).
Melting points are uncorrected and were determined em-
ploying an Electrothermal 9100 apparatus. Optical rotations
were recorded using a Perkin-Elmer 241 polarimeter, and [R]D
values are given in 10-1 deg cm2 g-1. IR spectra were obtained
with a Nicolet FT-IR Model MX-1 unit. NMR spectra were
recorded using a Varian Gemini 300 MHz instrument with
residual nondeuterated solvent as an internal standard. The
EIMS mass spectra were from a FINNIGAN-MAT 312 instru-
ment (70 eV). Elemental analyses were determined by Gal-
braith Laboratories, Inc. (Knoxville, TN).
Eth yl 2(S)-(ter t-Bu tyldiph en ylsilyloxy)pr opion ate (11).11
To tert-butylchlorodiphenylsilane (27.48 g, 100 mmol) in dry
DMF (200 mL) was added in one portion imidazole (13.62 g,
200 mmol). The solution was cooled in an ice bath and stirred
for 15 min, after which ethyl (S)-lactate (10, 11.9 mL, 105
mmol) was added (dropwise). After warming to room temper-
ature and stirring for 21 h the reaction was completed by
heating to 60 °C for a further 6 h. Water (1.5 L) was added,
and the mixture was extracted with hexane (4 × 150 mL). The
combined hexane extract was dried, filtered, and concentrated
under reduced pressure to leave a pale yellow oil (35.1 g,
98%): bp 158 °C (1 mm); 1H NMR (CDCl3) δ 7.67 (m, 4 H),
7.33-7.44 (m, 6 H), 4.26 (q, J ) 6.7 Hz, 1 H), 4.02 (q, J ) 7.2
Hz, 2 H), 1.39 (d, J ) 6.7 Hz, 3 H), 1.14 (t, J ) 7.2 Hz, 3 H),
1.11 (s, 9 H).
2(S)-(ter t-Bu tyld ip h en ylsilyloxy)-1-p r op a n ol (12).11 To
the preceding (11, 35.1 g, 98.4 mmol) in anhydrous THF (100
mL) was added borane THF complex (1.0 M in THF, 200 mL)
(dropwise). After completion of the addition the mixture was
stirred at reflux for 2 h. The solution was cooled to room
temperature and the reaction terminated by slow (dropwise)
addition of water (100 mL). The THF was removed under
reduced pressure, and the residue was extracted with DCM.
The combined extract was dried, filtered, and concentrated
In summary the first total syntheses of (7S,15S)-dola-
trienoic (2a ) acid and its 7R,15S isomer were completed
employing a convergent procedure. The longest linear
sequence required 16 steps and provided an overall yield
from ethyl (S)-lactate of ∼1.5%. The 7R,15S diastereoiso-
mer of dolatrienoic acid (2b) was prepared in six steps
using analogous reactions from key intermediate 21. We
are now initiating synthesis of the two remaining diaste-
reoisomers of dolatrienoic acid by the substitution of (S)-
2-(tert-butyldiphenylsilyloxy)-1-propanol (12) by its R enan-
tiomer, which has been prepared11 by Nicolaou et al. in four
steps from ethyl (S)-lactate. Also synthesis of the hep-
tapeptide portion of dolastatin 14 (1) and its coupling to
dolatrienoic acid (2a ) is currently in progress.

(7S,15S)- and (7R,15S)-Dolatrienoic Acid
J ournal of Natural Products, 2001, Vol. 64, No. 4 475
under reduced pressure to leave a colorless oil (30.85 g,
100%): bp 154 °C (1.25 mm); H NMR (CDCl3) δ 7.73 (m, 4
H), 7.38-7.46 (m, 6 H), 4.00 (m, 1 H), 3.55 (dd, J ) 4.0, 12.1
Hz, 1 H), 3.44 (dd, J ) 5.5, 12.1 Hz, 1 H), 2.02 (bs, 1H), 1.12
(s, 9 H), 1.09 (d, J ) 7.6 Hz, 3 H).
24.0 g, 71 mmol) in anhydrous THF (36 mL) at 0 °C was added
borane THF complex (1.0 M solution in THF, 23.7 mL, 71.1
mmol of hydride) (dropwise). After 30 min the reaction was
allowed to warm to room temperature and stirred for a further
30 min. Excess hydride was quenched with MeOH (0.47 mL),
and the reaction was cooled again to -5 °C. Bromine (4.7 mL,
91 mmol) was added dropwise at a rate such that reaction
temperature did not rise above 0 °C. Sodium methoxide (5.56
M solution in MeOH, 21.24 mL, 118 mmol) was next added
(dropwise) keeping the reaction temperature below 5 °C. On
completion of the addition the reaction was allowed to warm
to room temperature and then terminated with sodium
bicarbonate solution. The mixture was extracted with hexane,
and the combined extract was dried, filtered, and concentrated
under reduced pressure to a yellow oil. Separation of the
product by gravity Si gel column chromatography (1.5%
EtOAc-hexane as eluent) gave the required product (16) as a
1
2(S)-(ter t-Bu tyld ip h en ylsilyloxy)-1-(4-m eth ylben zen e-
su lfon yloxy)p r op a n e (13). To a solution of 2(S)-(tert-butyl-
diphenylsilyloxy)-1-propanol (12, 34.5 g, 110 mmol) in dry
DCM (70 mL) was added (one portion) tosyl chloride (31.5 g,
165 mmol). The solution was cooled to 0 °C, and pyridine (17.8
mL, 220 mmol) was added (dropwise) with stirring. A white
precipitate formed, and 30 min later the reaction was allowed
to warm to room temperature and stirred for a further 24 h.
The reaction was stopped with HCl (1.0 M, 200 mL). The
organic layer was removed, and the aqueous phase was
extracted with additional DCM. The combined organic extract
was washed with sodium bicarbonate solution and brine, dried,
filtered, and concentrated under reduced pressure to afford a
pale yellow oil (64.42 g), which was a mixture of the required
product (13) and tosyl chloride and used without further
yellow oil (20.17 g, 68%): bp 151 °C (1 mm); [R]25 -14.5 (c 1,
D
CHCl3); IR (NaCl) 3071, 2961, 2932, 2857, 1260, 1130, 1065,
1038, 822, 702 cm-1 13C NMR (CDCl3) δ 135.9, 134.8, 134.5,
;
purification: bp 180 °C (0.06 mm); [R]27 -16.6 (c 1, CHCl3);
129.5, 129.4, 127.5, 127.4, 69.2, 38.4, 33.7, 32.8, 27.0, 23.8, 23.2,
19.2; 1H NMR (CDCl3) δ 7.68 (m, 4 H), 7.39 (m, 6 H), 3.84 (sx,
J ) 6.0 Hz, 1 H), 3.30 (t, J ) 6.8 Hz, 2 H), 1.72 (m, 2 H), 1.43
(m, 4 H), 1.07 (d, J ) 6.0 Hz, 3 H), 1.05 (s, 9 H); MS m/z (rel
int) 363 (M+ - 57, 15), 361 (M+ - 57, 16), 83 (100). Anal. Calcd
for C22H31BrOSi: C, 62.99; H, 7.45. Found: C, 63.46; H, 7.59.
D
IR (NaCl) 3071, 2957, 1366, 1178, 1109 cm-1
;
13C NMR (CDCl3)
δ 144.6, 135.8, 135.7, 133.7, 133.3, 132.9, 129.7, 127.9, 127.6,
127.5, 73.9, 67.0, 26.8, 21.6, 20.0, 19.0; 1H NMR (CDCl3) δ 7.71
(d, J ) 8.4 Hz, 2 H), 7.62 (m, 4 H), 7.28-7.47 (m, 8 H), 3.98
(sx, J ) 5.6 Hz, 1 H), 3.92 (dd, J ) 5.4, 9.3 Hz, 1 H), 3.83 (dd,
J ) 5.1, 9.3 Hz, 1 H), 2.45 (s, 3 H), 1.06 (d, J ) 6.3 Hz, 3 H),
1.04 (s, 9 H); MS m/z (rel int) 411 (M+ - 57, 23), 353 (100).
5(S)-(ter t-Bu tyld ip h en ylsilyloxy)h exyltr ip h en ylp h os-
p h on iu m Br om id e (6). Triphenylphosphine (15.1 g, 57 mmol)
was added in one portion to 1-bromo-5(S)-(tert-butyldiphenyl-
silyloxy)hexane (16, 20.1 g, 48 mmol) in dry toluene (60 mL),
and the solution was stirred and heated at reflux for 48 h.
After cooling to room temperature the toluene was removed
under reduced pressure and the brown residue was refriger-
ated until it solidified. The glassy solid was triturated with
ether (6 × 50 mL), and the insoluble portion was dried under
high vacuum (0.04 mmHg) to afford the required phosphonium
1-Br om o-2(S)-(ter t-bu tyldiph en ylsilyloxy)pr opan e (14).
Lithium bromide (19.14 g, 220 mmol) was added (in one
portion) to 2(S)-(tert-butyldiphenylsilyloxy)-1-(4-methylbenze-
nesulfonyloxy)propane (13, 51.48 g, 110 mmol) in dry THF (110
mL). The solution was stirred and heated at reflux for 47 h,
during which time a white precipitate formed. The mixture
was cooled to room temperature, diluted with water, and
extracted with DCM. The organic extract was dried, filtered,
and concentrated under reduced pressure to a brown oily
residue. Separation by gravity Si gel column chromatography
(97:3 hexanes-EtOAc as eluent) led to a colorless oil (38.2 g,
93%): bp 162 °C (1.5 mm); [R]24D -14.8 (c 1, CH2Cl2); IR (NaCl)
salt (6) as an off-white foam (24.46 g, 75%): [R]27 -7.5 (c 1,
D
CHCl3); IR (NaCl) 3053, 2961, 2930, 2891, 1437, 1111, 1028,
997, 923 cm-1 13C NMR (CDCl3) δ 135.5, 134.8, 134.6, 134.3,
;
133.4, 133.2, 130.4, 130.2, 129.2, 127.2, 127.2, 68.7, 38.2, 26.8,
3070, 2960, 1191, 1109 cm-1
;
13C NMR (CDCl3) δ 135.8, 133.9,
25.7, 25.5, 22.8, 22.3, 18.9; H NMR (CDCl3) δ 7.66-7.85 (m,
1
133.7, 129.8, 129.7, 127.7, 127.6, 68.8, 39.4, 26.9, 21.9, 19.2;
1H NMR (CDCl3) δ 7.70 (m, 4 H), 7.36-7.46 (m, 6 H), 4.01
(ddq, J ) 6.6, 4.5, 6.0 Hz, 1 H), 3.33 (dd, J ) 10.2, 4.5 Hz, 1
H), 3.26 (dd, J ) 10.2, 6.6 Hz, 1 H), 1.24 (d, J ) 6.0 Hz, 3 H),
15 H), 7.60 (m, 4 H), 7.26-7.39 (m, 6 H), 3.80 (sx, J ) 5.8 Hz,
1 H), 3.60 (m, 2 H), 1.35-1.69 (m, 6 H), 1.01 (d, J ) 6.0 Hz, 3
H), 0.99 (s, 9 H); MS m/z (rel int) 483 (100), 262 (53).
2-(8′(S)-(ter t-Bu tyld ip h en ylsilyloxy)-3′E-n on en yl)-1,3-
d ioxola n e (17). To a solution of 5(S)-(tert-butyldiphenylsily-
loxy)hexyltriphenylphosphonium bromide (6, 24.4 g, 35.8
mmol) in anhydrous THF (50 mL) was added (dropwise)
phenyllithium12 (1.15 M solution in ether, 31.2 mL, 35.9 mmol),
giving a dark orange color. The solution was stirred at room
temperature for 20 min and then cooled to -78 °C. A solution
of 1,3-dioxolane-2-propanal7 (5, 5.12 g, 39.4 mmol) in anhy-
drous ether (30 mL) was added (dropwise) over 30 min, during
which time the reaction was allowed to warm to -40 °C. The
pale orange suspension was stirred at -40 °C for 20 min, and
then additional phenyllithium (1.15 M solution in ether, 31.2
mL, 35.9 mmol) was added dropwise, giving a blood red color.
The reaction was warmed to -30 °C and stirred for a further
20 min. A solution of hydrogen chloride in ether (1.0 M, 40
mL) was added (dropwise) followed by potassium tert-butoxide
(6.02 g, 53.8 mmol) (one portion). After warming to room
temperature the reaction was stirred for an additional 90 min.
Water was added, and the mixture was extracted with DCM.
The combined extract was dried, filtered, and concentrated
under reduced pressure to give a brown oil. Separation by
gravity Si gel column chromatography (24:1 hexanes-EtOAc
as eluent) gave alkene 17 as a colorless oil (8.7 g, 54%): bp
161 °C (1 mm); [R]25D -17.0 (c 1, CHCl3); IR (NaCl) 3071, 3049,
1.09 (s, 9 H); MS m/z (rel int) 321 (M+ - 57, 42), 319 (M+
-
57, 43), 263 (100). Anal. Calcd for C19H25BrOSi: C, 60.47; H,
6.68. Found: C, 60.76; H, 6.92.
5(S)-(ter t-Bu tyld ip h en ylsilyloxy)-1-h exen e (15). To a
solution of 1-bromo-2(S)-(tert-butyldiphenylsilyloxy)propane
(14, 34.60 g, 92 mmol) in anhydrous THF (100 mL) was added
a solution of lithium tetrachlorocuprate (0.1 M, 16 mL, 1.6
mmol) in THF. Allylmagnesium bromide (1.0 M solution in
ether, 160 mL, 160 mmol) was next added over ∼1 h (dropwise)
with cooling (cold water bath). A slight exotherm was observed.
The brown solution was allowed to stir for 17 h at room
temperature. The reaction was terminated by slow addition
of saturated aqueous ammonium chloride solution and ex-
tracted with hexane. The combined extract was dried, filtered,
and concentrated under reduced pressure to a yellow oil.
Separation of the product by gravity Si gel column chroma-
tography (98:2 hexanes-EtOAc as eluent) gave the silyl ether
(15) as a colorless oil (23.6 g, 76%): bp 148 °C (1 mm); [R]25
D
-10.4 (c 1, CHCl3); IR (NaCl) 3070, 3051, 2964, 1645, 1109
cm-1 13C NMR (CDCl3) δ 138.8, 135.9, 134.9, 134.5, 129.5,
;
129.4, 127.5, 127.4, 114.2, 69.1, 38.6, 29.5, 27.1, 23.2, 19.3; 1H
NMR (CDCl3) δ 7.74-7.81 (m, 4 H), 7.39-7.51 (m, 6 H), 5.78
(ddt, J ) 17.1, 9.9, 6.6 Hz, 1 H), 4.92-5.05 (m, 2 H), 3.96 (sx,
J ) 6.0 Hz, 1 H), 2.14 (dt, J ) 7.9, 6.6 Hz, 2 H), 1.52-1.74 (m,
2 H), 1.15 (d, J ) 6.0 Hz, 3 H), 1.14 (s, 9 H); MS m/z (rel int)
281 (M+ -57, 43), 199 (100). Anal. Calcd for C22H30OSi: C,
78.05; H, 8.93. Found: C, 77.66; H, 9.14.
2931, 2888, 2859, 1473, 1427, 1136, 1111, 1036, cm-1 13C NMR
;
(CDCl3) δ 135.8, 134.9, 134.5, 130.7, 129.4, 129.3, 129.1, 127.4,
127.4, 104.1, 69.4, 64.8, 38.9, 33.8, 32.4, 27.0, 25.0, 23.2, 19.2;
1H NMR (CDCl3) δ 7.71 (m, 4 H), 7.36-7.46 (m, 6 H), 5.40 (m,
2H), 4.89 (t, J ) 4.8 Hz, 1H) 3.83-4.00 (m, 4 H), 2.13 (m, 2
H), 1.90 (m, 2 H), 1.69-1.77 (m, 2 H), 1.09 (s, 9 H), 1.09
1-Br om o-5(S)-(ter t-bu tyld ip h en ylsilyloxy)h exa n e (16).
To a solution of 5(S)-(tert-butyldiphenylsilyloxy-1-hexene (15,

476 J ournal of Natural Products, 2001, Vol. 64, No. 4
Duffield and Pettit
(hidden, 3 H); MS m/z (rel int) 451 (M+ - 1, 2), 395 (M+ - 57,
7), 199 (100). Anal. Calcd for C28H40O3Si: C, 74.29; H, 8.91.
Found: C, 74.15; H, 9.45.
(m, 2H), 4.77 (qn, J ) 6.8 Hz, 1 H), 4.10 (m, 1 H), 3.84 (sx, J
) 5.8 Hz, 1 H), 3.09 (m, 2 H), 2.99 (br s, 1 H), 2.12 (m, 2 H),
1.88 (m, 2 H), 1.31-1.71 (m, 6 H), 1.05 (d, J ) 6.0 Hz, 3 H),
1.05 (s, 9 H), 0.91 (d, J ) 6.6 Hz, 3 H); MS m/z (rel int) 570
(M+ - 57, 24), 199 (100). Anal. Calcd for C38H49NO5Si‚1/
2H2O: C, 71.66; H, 7.91; N, 2.20. Found: C, 71.32; H, 8.06; N,
2.18.
9(S)-(ter t-Bu tyld ip h en ylsilyloxy)-4E-d ecen a l (4). To a
solution of 2-(8′(S)-(tert-butyldiphenylsilyloxy)-3′E-nonenyl)-
1,3-dioxolane (17, 8.00 g, 17.7 mmol) in THF (210 mL) was
added aqueous acetic acid (30%, 210 mmol). The solution was
stirred and heated at reflux for 47 h. After cooling to room
temperature the reaction was neutralized with saturated
aqueous sodium bicarbonate solution (∼1.6 L), and the mixture
was extracted with DCM (6 × 100 mL). The combined extract
was dried, filtered, and concentrated under reduced pressure
to a yellow oil. Separation by gravity Si gel column chroma-
tography (24:1 hexanes-EtOAc as eluent) afforded 9(S)-
hydroxy-4E-decenal (18) as a colorless oil (0.38 g, 13%) and
the required silyl ether (4) as a colorless oil (5.75 g, 80%): bp
Isom er 21: bp (260 °C (0.06 mm); [R]25 -10.1 (c 1.5,
D
CHCl3); IR (NaCl) 3445, 3071, 2959, 2932, 2857, 1786, 1697,
1456, 1427, 1371, 1195, 1111, 1037, 968 cm-1 13C NMR
;
(CDCl3) δ 172.5, 153.0, 135.9, 135.0, 134.6, 133.1, 131.1, 129.4,
129.3, 128.9, 128.8, 127.4, 127.4, 125.6, 79.1, 69.5, 67.3, 54.7,
42.7, 38.9, 36.3, 32.5, 28.5, 27.0, 25.1, 23.2, 19.2, 14.6; 1H NMR
(CDCl3) δ 7.68 (m, 4 H), 7.30-7.42 (m, 11 H), 5.68 (d, J ) 7.2
Hz, 1 H), 5.37 (m, 2H), 4.78 (qn, J ) 6.8 Hz, 1 H), 4.13 (m, 1
H), 3.84 (sx, J ) 5.9 Hz, 1 H), 3.18 (dd, J ) 17.7, 2.7 Hz, 1 H),
3.18 (dd, J ) 17.5, 2.7 Hz, 1 H), 3.00 (dd, J ) 17.5, 9.0 Hz, 1
H), 2.88 (br s, 1 H), 2.13 (m, 2 H), 1.88 (m, 2 H), 1.26-1.71
(m, 6 H), 1.05 (s, 9 H), 1.04 (d, J ) 6.0 Hz, 3 H), 0.91 (d, J )
6.6 Hz, 3 H); MS m/z (rel int) 627 (M+, 1), 570 (M+ - 57, 15),
199 (100). Anal. Calcd for C38H49NO5Si‚1/2H2O: C, 71.66; H,
7.91; N, 2.20. Found: C, 71.71; H, 7.94.
117 °C (0.07 mm); [R]22 -17.4 (c 1, CHCl3); IR (NaCl) 3071,
D
2931, 2858, 1728, 1427, 1136, 821 cm-1 13C NMR (CDCl3) δ
;
202.2, 135.8, 134.8, 134.5, 131.8, 129.4, 129.3, 127.7, 127.4,
127.3, 69.4, 43.5, 38.9, 32.4, 27.1, 25.2, 25.0, 23.2, 19.3; 1H
NMR (CDCl3) δ 9.75 (t, J ) 1.7 Hz, 1 H), 7.68 (m, 4 H), 7.34-
7.43 (m, 6 H), 5.37 (m, 2H), 3.84 (sx, J ) 6.0 Hz, 1 H), 2.47
(m, 2 H), 2.31 (m, 2 H), 1.88 (m, 2 H), 1.28-1.50 (m, 4 H),
1.06 (d, J ) 6.0 Hz, 3 H), 1.06 (s, 9 H); MS m/z (rel int) 351-
(M+ - 57, 54), 199(100). Anal. Calcd for C26H36O2Si: C, 76.42;
H, 8.88. Found: C, 76.67; H, 9.24.
Meth yl 11S-(ter t-Bu tyld ip h en ylsilyloxy)-3S-h yd r oxy-
6E-d od ecen oa te (22). A saturated (ca. 0.056 M) methanolic
solution of potassium carbonate (10.5 mL, 0.59 mmol) was
added (dropwise) over about 30 min to a solution of 3-(11′S-
(tert-butyldiphenylsilyloxy)-3′S-hydroxy-6′E-dodecenoyl)-4R-
methyl-5S-phenyl-2-oxazolidione (20, 1.00 g, 1.6 mmol) in
anhydrous methanol (5 mL) at 0 °C. After completion of the
addition the reaction was stirred for a further 30 min. The
reaction was stopped with saturated aqueous ammonium
chloride, and the mixture was extracted with DCM. The
combined extract was dried, filtered, and concentrated under
reduced pressure to leave a yellow oil. Separation by gravity
Si gel column chromatography (82:18 hexanes-EtOAc as
eluent) afforded the required ester as a colorless oil (0.58 g,
3-(1-Br om oa cet yl)-4R-m et h yl-5S-p h en yl-2-oxa zolid i-
n on e (19). To a stirred solution of 4R-methyl-5S-phenyl-2-
oxazolidinone (5.00 g, 28.2 mmol) in THF (40 mL) at -78 °C
was added (dropwise) n-BuLi (2.5 M solution in hexanes, 12.4
mL, 31 mmol). The solution became dark red. After 20 min
bromoacetyl bromide (3.05 mL, 35 mmol) was added (drop-
wise), turning the reaction yellow. After 1 h at -78 °C the
reaction was warmed to room temperature and stirred for a
further 3 h. The reaction was stopped with water and extracted
with DCM. The combined extract was washed with 1 N sodium
hydroxide and then brine, dried, filtered, and concentrated
under reduced pressure to a brown oil. Separation by gravity
Si gel column chromatography (9:1 hexanes-EtOAc as eluent)
gave the required oxazolidinone (19) as a clear, colorless oil
75%): [R]25 -8.5 (c 1.7, CHCl3); IR (NaCl) 3447, 3071, 2932,
D
2858, 1736, 1429, 1109, 821 cm-1
;
13C NMR (CDCl3) δ 173.3,
135.8, 134.9, 134.5, 131.0, 129.4, 129.3, 129.2, 127.4, 127.3,
69.4, 67.4, 51.6, 41.1, 38.8, 36.3, 32.4, 28.5, 27.0, 25.0, 23.2,
19.2; 1H NMR (CDCl3) δ 7.67 (m, 4 H), 7.33-7.42 (m, 6 H),
5.36 (m, 2H), 4.00 (m, 1 H), 3.84 (sx, J ) 5.6 Hz, 1 H), 3.69 (s,
3 H), 2.50 (dd, J ) 16.2, 3.6 Hz, 1 H), 2.41 (dd, J ) 16.2, 8.4
Hz, 1 H), 2.09 (m, 2 H), 1.88 (m, 2 H), 1.26-1.61 (m, 6 H),
(6.31 g, 75%): bp 141 °C (0.07 mm); [R]25 +19.8 (c 2, CHCl3);
D
IR (NaCl) 3065, 3034, 2986, 2936, 1782, 1705, 1356, 1200,
1123, 1040, 970 cm-1 13C NMR (CDCl3) δ 165.8, 152.6, 132.9,
;
129.0, 128.8, 125.7, 79.5, 55.2, 28.4, 14.27; 1H NMR (CDCl3) δ
7.27-7.43 (m, 5 H), 5.73 (d, J ) 7.5 Hz, 1 H), 4.76 (qn, J ) 6.8
Hz, 1 H), 4.55 (d, J ) 12.6 Hz, 1 H), 0.92 (d, J ) 6.6 Hz, 3 H);
MS m/z (rel int) 299 (M+, 27), 297 (M+, 30), 107 (100).
1.05 (s, 9 H), 1.05 (hidden 3 H); MS m/z (rel int) 425 (M+
57, 7), 199 (100). Anal. Calcd for C29H42O4Si: C, 72.16; H, 8.77.
Found: C, 72.11; H, 8.97.
-
Meth yl 11S-(ter t-Bu tyld ip h en ylsilyloxy)-3S-m eth oxy-
6E-d od ecen oa te (23). To a solution of methyl 11S-(tert-
butyldiphenylsilyloxy)-3S-hydroxy-6E-dodecenoate (22, 1.43 g,
2.97 mmol) in anhydrous DCM (22 mL) was added 4 Å
molecular sieves (1.8 g). After stirring at room temperature
for 20 min Proton Sponge (1.88 g, 8.8 mmol) and trimethy-
loxonium tetrafluoroborate (1.27 g, 8.6 mmol) were added. The
solution was stirred at room temperature for a further 3 h,
during which time a white precipitate formed and the reaction
became yellow. The suspension was diluted with DCM and
filtered through a sintered funnel. The solid was rinsed with
more DCM, and the combined filtrate and washings were
concentrated under reduced pressure to a yellow oil. Purifica-
tion by gravity Si gel column chromatography (19:1 hexanes-
EtOAc as eluent) afforded ester 23 as a clear, colorless oil (1.40
g, 95%): [R]25D -11.0 (c 1, CHCl3); IR (NaCl) 3071, 2932, 2857,
3-(11′S-(ter t-Bu t yld ip h en ylsilyloxy)-3′S-h yd r oxy-6′E-
d od ecen oyl)-4R-m et h yl-5S-p h en yl-2-oxa zolid ion e (20).
Magnesium turnings (2.16 g, 89 mmol) were washed in dry
DCM (20 mL) for 1 h. The DCM was removed via syringe, and
triphenylphosphine (3.82 g, 14.6 mmol) was added followed
by anhydrous cobalt chloride (0.47 g, 3.65 mmol). THF (37.5
mL) was added, and the mixture was stirred at room temper-
ature for 24 h, by which time it had become dark brown. The
reaction was cooled to 0 °C, and a solution of 3-(1-bromoacetyl)-
4R-methyl-5S-phenyl-2-oxazolidinone (19, 4.56 g, 15.3 mmol)
and 9(S)-(tert-butyldiphenylsilyloxy)-4E-decenal (4, 6.24 g, 15.3
mmol) in THF (75 mL) was added dropwise over 3 h. After a
further 10 min at 0 °C the reaction was terminated by addition
of 0.1 N HCl (375 mL). The mixture was extracted with EtOAc
(5 × 100 mL), and the combined extract was dried, filtered,
and concentrated under reduced pressure to a brown oil.
Separation of the products by gravity Si gel column chroma-
tography (4:1 hexanes-EtOAc as eluent) afforded the 3′R,-
11′S isomer (21) as a colorless oil (3.1 g, 32%). The title
compound (20) was also obtained as a colorless oil (3.2 g,
1740, 1109, 822, 704 cm-1 13C NMR (CDCl3) δ 172.0, 135.8,
;
134.8, 134.5, 130.8, 129.3, 129.3, 129.2, 127.3, 127.3, 77.0, 69.3,
56.8, 51.4, 39.1, 38.8, 33.7, 32.4, 28.0, 26.9, 25.0, 23.1, 19.1;
1H NMR (CDCl3) δ 7.68 (m, 4 H), 7.32-7.40 (m, 6 H), 5.35 (m,
2H), 3.84 (sx, J ) 5.9 Hz, 1 H), 3.66 (s, 3 H), 3.64 (m, 1 H),
3.33 (s, 3 H), 2.54 (dd, J ) 15.0, 7.2 Hz, 1 H), 2.41 (dd, J )
15.0, 5.4 Hz, 1 H), 2.03 (m, 2 H), 1.88 (m, 2 H), 1.31-1.65 (m,
6 H), 1.05 (s, 9 H), 1.04 (hidden 3 H); MS m/z (rel int) 439 (M+
- 57, 77), 135 (100). Anal. Calcd for C30H44O4Si: C, 72.54; H,
8.93. Found: C, 72.63; H, 9.16.
34%): bp 282 °C (1.5 mm); [R]25 +14.2 (c 1.3, CHCl3); IR
D
(NaCl) 3445, 3071, 2960, 2932, 2857, 1786, 1699, 1471, 1427,
1371, 1350, 1219, 1111, 702 cm-1 13C NMR (CDCl3) δ 172.7,
;
153.0, 135.9, 135.0, 134.6, 133.1, 131.0, 129.4, 129.3, 128.9,
128.7, 127.4, 127.3, 125.6, 79.2, 69.5, 67.5, 54.7, 42.7, 38.9, 36.4,
32.5, 28.5, 27.0, 25.1, 23.2, 19.3, 14.5; 1H NMR (CDCl3) δ 7.68
(m, 4 H), 7.29-7.45 (m, 11 H), 5.66 (d, J ) 7.2 Hz, 1 H), 5.37
11S-(ter t-Bu tyld ip h en ylsilyloxy)-3S-m eth oxy-6E-d od e-
cen a l (24). A solution of di-iso-butylaluminum hydride

(7S,15S)- and (7R,15S)-Dolatrienoic Acid
J ournal of Natural Products, 2001, Vol. 64, No. 4 477
(1.0 M in DCM, 5.4 mL, 5.4 mmol) was added (dropwise) at
-78 °C to a solution of methyl 11S-(tert-butyldiphenylsilyloxy)-
3S-methoxy-6E-dodecenoate (23, 1.38 g, 2.78 mmol) in anhy-
drous DCM (17 mL). After 5 min the cold reaction mixture
was poured into 1 N HCl (50 mL), and the organic layer
removed. The aqueous phase was extracted with DCM, and
the combined extract was dried, filtered, and concentrated
under reduced pressure to a yellow oil. Proton NMR revealed
this to be a mixture of aldehyde 24 and the corresponding
alcohol. This mixture was dissolved in anhydrous DCM (17
mL) and powdered 4 Å molecular sieves (1.53 g), and PDC
(1.53 g, 3.8 mmol) was added in one portion. The suspension
was stirred at room temperature for 1 h, during which time it
became dark brown. The thick suspension was diluted with
DCM, filtered through Celite, and concentrated to a brown oil.
Separation by gravity Si gel column chromatography (9:1
hexanes-EtOAc as eluent) gave aldehyde 24 as a colorless oil
concentrated under reduced pressure to a yellow oil. Purifica-
tion by gravity Si gel column chromatography (93:7 hexanes-
EtOAc as eluent) afforded a mixture of E,E-diene 30 and the
4Z isomer (31) as a clear colorless oil (0.46 mg, 67%). Also
isolated was the starting material (24, 183 mg, 32%). The
mixture of isomers (30 and 31) was dissolved in chloroform
(20 mL), and iodine (0.21 g, 0.82 mmol) was added. The purple
solution was stirred at room temperature for 24 h. The reaction
mixture was washed with 10% aqueous sodium bisulfite
solution (2 × 10 mL) and brine (10 mL). The organic phase
was dried, filtered, and concentrated under reduced pressure
to leave the diene 30 as a colorless oil (0.46 g, 67%): [R]26
D
-12.6 (c 1, CHCl3); IR (NaCl) 3069, 3048, 2934, 2859, 1709,
1640, 1462, 1244, 1109, 704 cm-1 13C NMR (CDCl3) δ 169.0,
;
138.7, 138.5, 135.8, 134.9, 134.6, 130.7, 129.6, 129.4, 129.3,
127.9, 127.4, 127.3, 125.3, 79.7, 69.4, 56.7, 51.7, 38.9, 37.3, 33.6,
32.4, 28.3, 27.0, 25.1, 23.2, 19.2, 12.6; 1H NMR (CDCl3) δ 7.67
(m, 4 H), 7.32-7.40 (m, 6 H), 7.18 (d, J ) 11.4 Hz, 1 H), 6.39
(dd, J ) 15.1, 11.4 Hz, 1 H), 6.07 (dt, J ) 15.1, 7.4 Hz, 1 H),
5.34 (m, 2 H), 3.84 (sx, J ) 6.0 Hz, 1 H), 3.73 (s, 3 H), 3.33 (s,
3 H), 3.25 (qn, J ) 5.6 Hz, 1 H), 2.39 (t, J ) 6.3 Hz, 2 H), 2.04
(m, 2 H), 1.93 (s, 3 H), 1.88 (m, 2 H), 1.31-1.55 (m, 6 H), 1.05
(s 9 H), 1.03 (d, hidden, 3 H); MS m/z (rel int) 505 (M+ - 57,
100). Anal. Calcd for C35H50O4Si‚1/2H2O: C, 73.51; H, 8.99.
Found: C, 73.65; H, 8.78.
(1.10 g, 85%): bp 187 °C (0.05 mm); [R]25 -12.6 (c 1, CHCl3);
D
IR (NaCl) 3069, 2932, 2859, 1726, 1554, 1460, 1427, 1375, 1105
cm-1 13C NMR (CDCl3) δ 201.5, 135.8, 134.8, 134.5, 131.1,
;
129.4, 129.3, 129.1, 127.4, 127.3, 75.6, 69.4, 56.7, 47.8, 38.8,
33.7, 32.4, 28.0, 27.0, 25.0, 23.2, 19.2; 1H NMR (CDCl3) δ 9.78
(t, J ) 2.1 Hz, 1 H), 7.68 (m, 4 H), 7.32-7.40 (m, 6 H), 5.34
(m, 2H), 3.84 (sx, J ) 6.0 Hz, 1 H), 3.70 (qn, J ) 5.4 Hz, 1 H),
3.33 (s, 3 H), 2.59 (ddd, J ) 15.9, 6.6, 2.1 Hz, 1 H), 2.50 (ddd,
J ) 15.9, 5.1, 2.0 Hz, 1 H), 2.03 (m, 2 H), 1.88 (m, 2 H), 1.32-
1.67 (m, 6 H), 1.05 (s, 9 H), 1.04 (hidden 3 H); MS m/z (rel int)
409 (M+ - 57, 3), 199 (100). Anal. Calcd for C29H42O3Si: C,
74.63; H, 9.07. Found: C, 74.61; H, 9.31.
Meth yl 15S-Hyd r oxy-7S-m eth oxy-2-m eth yl-2E,4E,10E-
h exa d eca tr ien oa te (32). To a stirred solution of methyl 15S-
(tert-butyldiphenylsilyloxy)-7S-methoxy-2-methyl-2E,4E,6E-
hexadecatrienoate (30, 0.26, 0.47 mmol) in anhydrous THF
(25 mL) was added (dropwise) hydrogen fluoride pyridine
complex (70%, 3.8 mL). After stirring at room temperature for
117 h the reaction was diluted with EtOAc and washed with
saturated aqueous sodium bicarbonate solution and then
saturated aqueous ammonium chloride solution. The organic
phase was dried, filtered, and concentrated under reduced
pressure to a yellow oil. Separation by gravity Si gel column
chromatography (4:1 hexanes-EtOAc as eluent) afforded the
3-Met h oxyca r b on yl-2E-b u t en ylt r ip h en ylp h osp h on i-
u m Br om id e (29). To a solution of tiglic acid (25, 5.67 g, 57
mmol) in methanol (60 mL) was added concentrated sulfuric
acid (1 mL), and the solution was stirred and heated at reflux
for 15 h. After cooling to room temperature the reaction was
basified to pH 9 with sodium bicarbonate solution and
extracted with DCM. The combined extract was washed with
brine, dried, filtered, and concentrated to a yellow liquid (5.7
g), which was used without further purification. The mixture
containing ester 26 was dissolved in carbon tetrachloride (50
mL), and N-bromosuccinimide (8.2 g, 50 mmol) was added
followed by AIBN (16 mg, 0.1 mmol). The suspension was
stirred and heated at reflux for 18 h. After cooling to room
temperature the mixture was filtered and concentrated to a
brown oil. Purification by gravity Si gel column chromatog-
raphy (97:3 hexanes-EtOAc as eluent) gave a mixture of
isomeric bromides (27 and 28) as a colorless liquid (7.8 g). To
the mixture in toluene (50 mL) was added triphenylphosphine
(8.5 g, 32 mmol). The reaction was stirred and heated at reflux
for 2 h and then cooled to room temperature. The phosphonium
salt was collected and washed with ether to give a light brown
solid (12.1 g, 47%) (overall from tiglic acid). The solid was
recrystallized three times from ethanol-ether to leave phos-
phonium salt 29 as colorless plates (6.30 g, 24%): mp 172 °C
(dec); IR (KBr disk) 3034, 2978, 2909, 2860, 1711, 1645, 1439,
alcohol 32 as a clear colorless oil (0.14 g, 89%): [R]25 -1.7 (c
D
1.6, CHCl3); IR (NaCl) 3450, 3030, 2930, 2857, 1709, 1640,
1435, 1244, 1107, 972 cm-1 13C NMR (CDCl3) δ 169.0, 138.7,
;
138.5, 130.5, 129.8, 127.9, 125.2, 79.7, 67.8, 56.6, 51.6, 38.7,
37.2, 33.5, 32.4, 28.2, 25.6, 23.4, 12.5; 1H NMR (CDCl3) δ 7.17
(d, J ) 11.4 Hz, 1 H), 6.39 (dd, J ) 15.0, 11.3 Hz, 1 H), 6.07
(dt, J ) 15.0, 7.4 Hz, 1 H), 5.41 (m, 2 H), 3.77 (m, 1 H), 3.75
(s, 3 H), 3.34 (s, 3 H), 3.26 (qn, J ) 5.9 Hz, 1 H), 2.39 (t, J )
6.3 Hz, 2 H), 2.01 (m, 4 H), 1.93 (s, 3 H), 1.36-1.60 (m, 6 H),
1.18 (d, J ) 6.0 Hz, 3 H); MS m/z (rel int) 324 (M+, 2), 292
(M+ - 32, 5), 67 (100). Anal. Calcd for C19H32O4: C, 70.33; H,
9.94. Found: C, 70.05; H, 10.26.
15S-H yd r oxy-7S-m et h oxy-2-m et h yl-2E,4E,10E-h exa -
d eca tr ien oic Acid (2a ). To a solution of methyl 15S-hydroxy-
7S-methoxy-2-methyl-2E,4E,6E-hexadecatrienoate (32, 0.14 g,
0.42 mmol) in THF (7.5 mL) and water (1.9 mL) was added
lithium hydroxide monohydrate (24 mg, 0.56 mmol) at 0 °C.
The reaction was allowed to warm to room temperature and
stirred for 160 h. The mixture was acidified to pH 2 with 2 M
HCl, diluted with water, and extracted with DCM. The
combined extract was dried, filtered, and concentrated under
reduced pressure to a yellow oil. Separation by gravity Si gel
column chromatography afforded the starting material (32)
as a colorless oil (19 mg, 14%, following elution with 4:1
hexanes-EtOAc) and the acid 2a as a colorless oil (103 mg,
1271, 1113, 1067, 883, 729, 687, 544, 507 cm-1 13C NMR
;
(CDCl3) δ 166.2, 136.1, 135.9, 134.9, 133.3, 133.2, 130.1, 129.9,
125.0, 124.9, 117.6, 116.4, 51.7, 25.3, 24.6, 13.0; 1H NMR
(CDCl3) δ 7.47-7.72 (m, 15 H), 6.44 (dt, J ) 8.1, 7.2 Hz, 1 H),
4.76 (dd, J ) 15.9, 8.1 Hz, 2 H), 3.46 (s, 3 H), 1.41 (d, J ) 3.3
Hz, 3 H); MS m/z (rel int) 374 (M+ - 81, 48), 262 (100). Anal.
Calcd for C24H24BrO2P: C, 63.31; H, 5.31; Br, 17.55; P, 6.80.
Found: C, 63.13; H, 5.54; Br, 17.49; P, 6.91.
Meth yl 15S-(ter t-Bu tyld ip h en ylsilyloxy)-7S-m eth oxy-
2-m eth yl-2E,4E,10E-h exa d eca tr ien oa te (30). To 3-meth-
oxycarbonyl-2E-butenyltriphenylphosphonium bromide (29,
0.58 g, 1.22 mmol) and NaH (60% dispersion in oil, 49 mg,
1.22 mmol) was added THF (20 mL). The reaction mixture was
stirred at room temperature for 2 h, over which time an intense
orange color formed. The ylid was added slowly (dropwise) to
a solution of 11S-(tert-butyldiphenylsilyloxy)-3S-methoxy-6E-
dodecenal (24, 0.57 g, 1.22 mmol) in THF (12 mL) at -78 °C.
The orange solution was allowed to warm to room temperature
over 4 h, and stirring was continued for a further 16 h. The
reaction was terminated with water, and the mixture extracted
with DCM. The combined extract was dried, filtered, and
79%, by elution with 4:1 DCM-CH3OH): [R]25 -1.4 (c 1,
D
CHCl3); IR (NaCl) 3422, 3135, 2930, 2859, 1682, 1422, 1260,
1103, 974, 814 cm-1 13C NMR (CDCl3) δ 173.5, 140.2, 139.7,
;
130.5, 129.9, 128.0, 124.8, 79.7, 68.0, 56.7, 38.7, 37.3, 33.6, 32.4,
28.3, 25.6, 23.4, 12.2; 1H NMR (CDCl3) δ 7.26 (d, J ) 11.4 Hz,
1 H), 6.39 (dd, J ) 15.0, 11.4 Hz, 1 H), 6.10 (dt, J ) 15.0, 7.4
Hz, 1 H), 5.41 (m, 2 H), 3.79 (m, 1 H), 3.34 (s, 3 H), 3.26 (qn,
J ) 6.0 Hz, 1 H), 2.40 (t, J ) 6.0 Hz, 2 H), 2.01 (m, 4 H), 1.92
(s, 3 H), 1.39-1.53 (m, 6 H), 1.18 (d, J ) 6.0 Hz, 3 H); MS m/z
(rel int) 292 (M+ - 18, 3), 67 (100); HRMS m/z calcd for
C18H30O4Na, (M + Na)+, 333.2042; found, (M + Na)+, 333.2054.
Meth yl 11S-(ter t-Bu tyld ip h en ylsilyloxy)-3R-h yd r oxy-
6E-d od ecen oa te (22b). A saturated (ca. 0.056 M) methanolic

478 J ournal of Natural Products, 2001, Vol. 64, No. 4
Duffield and Pettit
solution of potassium carbonate (26.5 mL, 1.48 mmol) was
added (dropwise) over about 30 min to a solution of 3-(11′S-
(tert-butyldiphenylsilyloxy)-3′R-hydroxy-6′E-dodecenoyl)-4R-
methyl-5S-phenyl-2-oxazolidione (21, 2.50 g, 3.99 mmol) in
anhydrous methanol (5 mL) at 0 °C. After completion of the
addition the reaction mixture was stirred for a further 30 min.
The reaction was stopped with saturated aqueous ammonium
chloride, and the mixture was extracted with DCM. The
combined extract was dried, filtered, and concentrated under
reduced pressure to leave a yellow oil. Separation by gravity
Si gel column chromatography (82:18 hexanes-EtOAc as
eluent) gave the required ester as a colorless oil (1.32 g, 69%):
J ) 5.6 Hz, 1 H), 3.70 (qn, J ) 6.1 Hz, 1 H), 3.34 (s, 3 H), 2.61
(ddd, J ) 16.2, 6.9, 2.2 Hz, 1 H), 2.51 (ddd, J ) 16.2, 5.3, 2.2
Hz, 1 H), 2.03 (m, 2 H), 1.88 (m, 2 H), 1.29-1.72 (m, 6 H),
1.05 (s, 9 H), 1.04 (hidden 3 H); MS m/z (rel int) 409 (M+
-
57, 3), 199 (100). Anal. Calcd for C29H42O3Si: C, 74.63; H, 9.07.
Found: C, 74.26; H, 9.36.
Meth yl 15S-(ter t-Bu tyld ip h en ylsilyloxy)-7R-m eth oxy-
2E,4E,10E-h exa d eca tr ien oa te (30b). To 3-methoxycarbonyl-
2E-butenyltriphenylphosphonium bromide (29, 96 mg, 0.21
mmol) and NaH (60% dispersion in oil, 8.4 mg, 0.21 mmol)
was added THF (3.5 mL). The reaction was stirred at room
temperature for 2 h, over which time an intense orange color
formed. The ylid was added slowly (dropwise) to a solution of
11S-(tert-butyldiphenylsilyloxy)-3R-methoxy-6E-dodecenal (24b,
98 mg, 0.21 mmol) in THF (2 mL) at -78 °C. The orange
solution was allowed to warm to room temperature over 4 h,
and stirring was continued for a further 16 h. The reaction
was terminated with water, and the mixture was extracted
with DCM. The combined extract was dried, filtered, and
concentrated under reduced pressure to a yellow oil. Purifica-
tion by gravity Si gel column chromatography (93:7 hexanes-
EtOAc as eluent) afforded a mixture of E,E-diene (30b) and
the 4Z isomer (31b) as a clear, colorless oil (77 mg, 65%). Also
isolated was significant starting material (24b, 26 mg, 26%).
The mixture of isomers (30b and 31b) was dissolved in
chloroform (7 mL), and iodine (35 mg, 0.138 mmol) was added.
The purple solution was stirred at room temperature for 24
h. The reaction mixture was washed with 10% aqueous sodium
bisulfite solution (2 × 10 mL) and brine (10 mL). The organic
phase was dried, filtered, and concentrated under reduced
pressure to leave the diene 30b as a colorless oil (77 mg,
65%): [R]26D -11.8 (c 1.3, CHCl3); IR (NaCl) 3069, 3042, 2934,
[R]25 -23.5 (c 1, CHCl3); IR (NaCl) 3396, 3071, 2932, 2857,
D
1734, 1427, 1375, 1109, 822 cm-1 13C NMR (CDCl3) δ 173.3,
;
135.8, 134.9, 134.6, 131.0, 129.4, 129.3, 129.3, 127.4, 127.3,
69.4, 67.5, 51.6, 41.1, 38.8, 36.3, 32.4, 28.5, 27.0, 25.0, 23.2,
19.2; 1H NMR (CDCl3) δ 7.67 (m, 4 H), 7.32-7.41 (m, 6 H),
5.35 (m, 2H), 4.00 (m, 1 H), 3.84 (sx, J ) 5.8 Hz, 1 H), 3.69 (s,
3 H), 2.50 (dd, J ) 16.0, 3.6 Hz, 1 H), 2.41 (dd, J ) 16.0, 8.4
Hz, 1 H), 2.09 (m, 2 H), 1.87 (m, 2 H), 1.30-1.59 (m, 6 H),
1.05 (s, 9 H), 1.05 (hidden 3 H); MS m/z (rel int) 425 (M+
57, 4), 199 (100). Anal. Calcd for C29H42O4Si: C, 72.16; H, 8.77.
Found: C, 72.68; H, 9.00.
-
Meth yl 11S-(ter t-Bu tyld ip h en ylsilyloxy)-3R-m eth oxy-
6E-d od ecen oa te (23b). After adding 4 Å molecular sieves
(1.36 g) to a solution of methyl 11S-(tert-butyldiphenylsilyloxy)-
3R-hydroxy-6E-dodecenoate (22b, 1.30 g, 2.7 mmol) in anhy-
drous DCM (20 mL) and stirring at room temperature for 20
min, Proton Sponge (1.71 g, 8.0 mmol) and trimethyloxonium
tetrafluoroborate (1.15 g, 7.8 mmol) were added. The solution
was stirred at room temperature for a further 3 h, during
which time a white precipitate formed and the reaction
mixture became yellow. The suspension was diluted with DCM
and filtered through a sintered funnel. The solid was rinsed
with more DCM, and the combined filtrate and washings were
concentrated under reduced pressure to a yellow oil. Purifica-
tion by gravity Si gel column chromatography (19:1 hexanes-
EtOAc as eluent) afforded ester 23b as a clear, colorless oil
(1.33 g, 99%): [R]27D -16.9 (c 1, CHCl3); IR (NaCl) 3071, 2932,
2859, 1709, 1640, 1432, 1242, 1107, 972, 824 cm-1 13C NMR
;
(CDCl3) δ 169.0, 138.7, 138.5, 135.8, 134.9, 134.6, 130.8, 129.6,
129.4, 129.3, 128.0, 127.4, 127.3, 125.3, 79.7, 69.5, 56.7, 51.7,
38.9, 37.3, 33.6, 32.5, 28.3, 27.0, 25.1, 23.2, 19.2, 12.6; 1H NMR
(CDCl3) δ 7.67 (m, 4 H), 7.32-;7.40 (m, 6 H), 7.18 (d, J ) 11.5
Hz, 1 H), 6.39 (dd, J ) 15.2, 11.5 Hz, 1 H), 6.07 (dt, J ) 15.2,
7.4 Hz, 1 H), 5.34 (m, 2 H), 3.84 (sx, J ) 5.8 Hz, 1 H), 3.74 (s,
3 H), 3.34 (s, 3 H), 3.24 (qn, J ) 6.0 Hz, 1 H), 2.39 (t, J ) 6.6
Hz, 2 H), 2.04 (m, 2 H), 1.93 (s, 3 H), 1.89 (m, 2 H), 1.31-1.54
(m, 6 H), 1.05 (s 9 H), 1.04 (d, hidden, 3 H); MS m/z (rel int)
505 (M+ - 57, 8), 199 (100); HRMS m/z calcd for C34H41O4Si,
(M - C4H9)+, 505.2774; found, (M - C4H9)+, 505.2780.
2859, 1742, 1429, 1375, 1109, 704 cm-1 13C NMR (CDCl3) δ
;
172.0, 135.8, 134.8, 134.5, 130.8, 129.3, 129.3, 129.2, 127.4,
127.3, 77.1, 69.4, 56.9, 51.4, 39.1, 38.8, 33.7, 32.4, 28.0, 26.9,
1
25.0, 23.1, 19.1; H NMR (CDCl3) δ 7.68 (m, 4 H), 7.33-7.40
(m, 6 H), 5.35 (m, 2H), 3.85 (sx, J ) 5.5 Hz, 1 H), 3.69 (s, 3 H),
3.64 (m, 1 H), 3.34 (s, 3 H), 2.54 (dd, J ) 15.0, 7.4 Hz, 1 H),
2.41 (dd, J ) 15.0, 5.6 Hz, 1 H), 2.04 (m, 2 H), 1.88 (m, 2 H),
1.27-1.65 (m, 6 H), 1.06 (s, 9 H), 1.05 (hidden 3 H); MS m/z
(rel int) 439 (M+ - 57, 17), 199 (100). Anal. Calcd for C30H44O4-
Si: C, 72.54; H, 8.93. Found: C, 72.95; H, 9.21.
Meth yl 15S-Hyd r oxy-7R-m eth oxy-2-m eth yl-2E,4E,10E-
h exa d eca tr ien oa te (32b). To a stirred solution of methyl
15S-(tert-butyldiphenylsilyloxy)-7R-methoxy-2-methyl-2E,4E,6E-
hexadecatrienoate (30b, 61 mg, 0.11 mmol) in anhydrous THF
(6 mL) was added (dropwise) hydrogen fluoride pyridine
complex (70%, 0.9 mL). After stirring at room temperature for
88 h the reaction was diluted with EtOAc and washed with
saturated aqueous sodium bicarbonate solution and then
saturated aqueous ammonium chloride solution. The organic
phase was dried, filtered, and concentrated under reduced
pressure to a yellow oil. Separation by gravity Si gel column
chromatography (4:1 hexanes-EtOAc as eluent) provided the
11S-(ter t-Bu tyld ip h en ylsilyloxy)-3R-m eth oxy-6E-d od e-
cen a l (24b). A solution of di-iso-butylaluminum hydride (1.0
M in DCM, 0.48 mL, 0.48 mmol) was added (dropwise) at -78
°C to a solution of methyl 11S-(tert-butyldiphenylsilyloxy)-3R-
methoxy-6E-dodecenoate (23b, 0.12 g, 0.12 mmol) in anhy-
drous DCM (5 mL). After 5 min the cold reaction mixture was
poured into 1 N HCl (20 mL), and the organic layer was
removed. The aqueous phase was extracted with DCM, and
the combined extract was dried, filtered, and concentrated
under reduced pressure to a yellow oil. Proton NMR revealed
this to be a mixture of aldehyde 24b and the corresponding
alcohol. The product was dissolved in anhydrous DCM (5 mL),
and a mixture of powdered 4 Å molecular sieves (0.14 g) and
PDC (0.14 g, 0.34 mmol) was added in one portion. The
suspension was stirred at room temperature for 1 h, during
which time it became dark brown. The thick suspension was
diluted with DCM, filtered through Celite, and concentrated
to a brown oil. Separation by gravity Si gel column chroma-
tography gave aldehyde 24b as a colorless oil (98 mg, 87%):
bp 184 °C (0.06 mm); [R]25D -19.6 (c 1, CHCl3); IR (NaCl) 3069,
alcohol 32b as a clear colorless oil (0.14 g, 89%): [R]27 +8.0
D
(c 1.2, CHCl3); IR (NaCl) 3412, 3027, 2930, 2857, 1707, 1640,
1437, 1244, 1105, 972 cm-1 13C NMR (CDCl3) δ 169.0, 138.7,
;
138.5, 130.5, 129.9, 128.0, 125.3, 79.8, 68.0, 56.7, 51.7, 38.8,
37.3, 33.6, 32.4, 28.3, 25.6, 23.5, 12.6; 1H NMR (CDCl3) δ 7.17
(d, J ) 11.2 Hz, 1 H), 6.39 (dd, J ) 15.0, 11.2 Hz, 1 H), 6.07
(dt, J ) 15.0, 7.4 Hz, 1 H), 5.41 (m, 2 H), 3.78 (m, 1 H), 3.75
(s, 3 H), 3.34 (s, 3 H), 3.26 (qn, J ) 6.0 Hz, 1 H), 2.39 (t, J )
6.6 Hz, 2 H), 2.01 (m, 4 H), 1.93 (s, 3 H), 1.40-1.55 (m, 6 H),
1.19 (d, J ) 6.0 Hz, 3 H); MS m/z (rel int) 324 (M+, 1), 292
(M+ - 32, 4), 67 (100); HRMS m/z calcd for C19H32O4, (M+),
324.2301; found, (M+), 324.2313.
15S-Hyd r oxy-7R-m eth oxy-2-m eth yl-2E,4E,10E-h exa d e-
ca tr ien oic a cid (2b). To a solution of methyl 15S-hydroxy-
7R-methoxy-2-methyl-2E,4E,6E-hexadecatrienoate (32b, 20
mg, 0.062 mmol) in THF (1.1 mL) and water (0.28 mL) was
added lithium hydroxide monohydrate (3.5 mg, 0.083 mmol)
at 0 °C. The reaction mixture was allowed to warm to room
temperature and stirred for 90 h. Following acidification to
2932, 2859, 1726, 1460, 1429, 1373, 1107 cm-1 13C NMR
;
(CDCl3) δ 201.4, 135.8, 134.9, 134.5, 131.1, 129.4, 129.3, 129.1,
127.4, 127.4, 75.7, 69.4, 56.7, 47.9, 38.9, 33.7, 32.4, 28.0, 27.0,
1
25.0, 23.2, 19.2; H NMR (CDCl3) δ 9.79 (t, J ) 2.2 Hz, 1 H),
7.68 (m, 4 H), 7.33-7.41 (m, 6 H), 5.34 (m, 2H), 3.84 (sx,

(7S,15S)- and (7R,15S)-Dolatrienoic Acid
J ournal of Natural Products, 2001, Vol. 64, No. 4 479
Refer en ces a n d Notes
pH 2 with 2 M HCl and dilution with water, the solution was
extracted with DCM. The combined extract was dried, filtered,
and concentrated under reduced pressure to a yellow oil.
Separation by gravity Si gel column chromatography afforded
the starting material (32b) as a colorless oil (3.5 mg, 18%,
elution with 4:1 hexanes-EtOAc) and the acid 2b as a colorless
(1) (a) Contribution 448 in the series Antineoplastic Agents. For part 447
see: Beutler, J . A.; McCall, K. L.; Herbert, K.; Herald, D. L.; Pettit,
G. R.; J ohnson, T.; Shoemaker, R. H.; Boyd, M. R. J . Nat. Prod. 2000,
63, 657-661. (b) Pettit, G. R.; Ode, R. H.; Herald, C. L.; Von Dreele,
R. B.; Michel, C. J . Am. Chem. Soc. 1976, 98, 4677-4678.
(2) Pettit, G. R.; Kamono, Y.; Herald, C. L.; Tuinman, A. A.; Boetner, F.
E.; Kizu, H.; Schmidt, J . M.; Baczynskyj, L.; Tomer, K. B.; Bontems,
R. J . J . Am. Chem. Soc. 1987, 109, 6883-6885. Pettit, G. R. The
Dolastatins. In Progress in the Chemistry of Organic Natural Prod-
ucts; Herz, W., Kirby, G. W., Moore, R. E., Steglich, W., Tamm, C.,
Eds.; 1997; Vol. 70, pp 1-79.
(3) (a) Pettit, G. R. Anticancer Drugs from Animals, Plants and Micro-
organisms; J . Wiley and Sons: New York, 1994. (b) Pettit, G. R.; Xu,
J .; Boyd, M. R. J . Nat. Prod. 1997, 60, 752-754. (c) Pettit, G. R.; Xu,
J .; Cerny, R. L. Bioorg. Med. Chem. Lett. 1997, 7, 827-832.
(4) Pettit, G. R.; Singh, S. B.; Hogan, F.; Lloyd-Williams, P.; Burkett, D.
D.; Clewlow, P. J . J . Am. Chem. Soc. 1989, 111, 5463-5465.
(5) Pettit, G. R.; Kamono, Y.; Herald, C. L.; Dufresne, C.; Bates, B. R.;
Schmidt, J . M.; Cerny, R. L.; Kizu, H. J . Org. Chem. 1990, 55, 2989-
2990.
oil (15 mg, 79%, elution with 4:1 DCM-CH3OH): [R]27 +6.4
D
(c 1.5, CHCl3); IR (NaCl) 3414, 2930, 2857, 1682, 1640, 1422,
1377, 1254, 1105, 974, 812, 750 cm-1 13C NMR (CDCl3) δ
;
173.3, 140.4, 139.8, 130.5, 129.9, 128.0, 124.7, 79.7, 68.0, 56.8,
38.7, 37.3, 33.6, 32.4, 28.3, 25.6, 23.4, 12.2; H NMR (CDCl3)
1
δ 7.28 (d, J ) 11.1 Hz, 1 H), 6.41 (dd, J ) 14.8, 11.1 Hz, 1 H),
6.12 (dt, J ) 14.8, 7.8 Hz, 1 H), 5.41 (m, 2 H), 3.80 (m, 1 H),
3.35 (s, 3 H), 3.27 (qn, J ) 5.9 Hz, 1 H), 2.41 (t, J ) 6.5 Hz, 2
H), 2.02 (m, 4 H), 1.93 (s, 3 H), 1.36-1.60 (m, 6 H), 1.19 (d, J
) 6.0 Hz, 3 H); MS m/z (rel int) 292 (M+ - 18, 3), 67 (100);
HRMS m/z calcd for C18H30O4Na, (M + Na)+, 333.2042; found,
(M + Na)+, 333.2033.
(6) Moune, S.; Niel, G.; Busquet, M.; Eggleston, E.; J ouin, P. J . Org.
Chem. 1997, 62, 3332-3339.
(7) Lucchesini, F. Tetrehedron 1992, 9951-9966.
Ack n ow led gm en t. We very much appreciate the financial
support provided by the following: Outstanding Investigator
Grant CA44344-06-12 awarded by the Division of Cancer
Treatment and Diagnosis, National Cancer Institute, DHHS;
The Arizona Disease and Control Research Commission; Gary
L. and Diane R. Tooker; Lottie Flugel; J ohn and Edith Reyno;
Dr. J ohn Budzinski; Polly Trautman; the Ladies Auxiliary to
the Veterans of Foreign Wars; and the Robert E. Dalton
Endowment Fund.
(8) Curran, D. P.; Scholz, D. Monatsh. Chem. 1977, 1401-1411.
(9) Schlosser, M.; Christmann, K. F. Angew. Chem. 1966, 78, 115.
(10) Orsini, F. J . Org. Chem. 1997, 62, 1159-1163.
(11) Nicolaou, K. C.; Randall, J . L.; Furst, G. T. J . Am. Chem. Soc. 1985,
107, 5556-5558.
(12) Phenyllithium was freshly prepared before each reaction by the
addition of an ethereal solution of bromobenzene to 2 equiv of lithium
in ether. Excess lithium was removed by filtration through glass wool
under an argon atmosphere. The solution was titrated against
diphenylacetic acid.
NP000502Q