Synthesis of α-substituted iminodiacetate ligands: α-hexadienyl derivatives for the selection of lipoxygenase mimics

Article abstract of DOI:10.1016/j.tet.2004.08.029

Derivatives of iminodiacetic acid (IDA) are important as ligands for metal ions, having numerous applications in separations, sensing, catalysis and medicine. This report describes the preparation of two types of IDA derivatives (1, 2) that could be covalently attached to a polymer or protein surface via a variable length spacer chain. The parent compounds 1 (R′=H) were easily prepared via N-alkylation of dimethyl iminodiacetate with esters of 6-bromo-hexanoic acid and subsequent selective ester hydrolysis. Metal complexes of IDA derivatives having an α-dienyl side chain are required for the selection of histidine-rich proteins with potential lipoxygenase activity. The α-hexadienyl side chain of IDA derivative 2 was selectively introduced in the reaction of (2,4-hexadienyl acetate)Fe(CO)₃ with a glycine-derived TMS-enol ester. Subsequent demetallation, followed by N-carboxymethylation, N-deacylation, N-alkylation with a trichloroethyl 6-halohexanoate, and TCE-ester cleavage provided the desired α-hexadienyl IDA derivative 2. Amide formation with IDA acid 1b demonstrates the feasibility of conjugating the IDA ligands to polymers and proteins while Ni(II)-complexation with the derived IDA triacid 1e shows the complexing ability of the tethered IDA ligand. Graphical Abstract.

Full text of DOI:10.1016/j.tet.2004.08.029

Tetrahedron 60 (2004) 10771–10778
Synthesis of a-substituted iminodiacetate ligands: a-hexadienyl
derivatives for the selection of lipoxygenase mimics*
*
Biswajit Kalita and Kenneth M. Nicholas
Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Norman OK 73019, USA
Received 27 May 2004; revised 12 August 2004; accepted 16 August 2004
Available online 30 September 2004
Abstract—Derivatives of iminodiacetic acid (IDA) are important as ligands for metal ions, having numerous applications in separations,
sensing, catalysis and medicine. This report describes the preparation of two types of IDA deri0vatives (1, 2) that could be covalently attached
to a polymer or protein surface via a variable length spacer chain. The parent compounds 1 (R ZH) were easily prepared via N-alkylation of
dimethyl iminodiacetate with esters of 6-bromo-hexanoic acid and subsequent selective ester hydrolysis. Metal complexes of IDA
derivatives having an a-dienyl side chain are required for the selection of histidine-rich proteins with potential lipoxygenase activity. The
a-hexadienyl side chain of IDA derivative 2 was selectively introduced in the reaction of (2,4-hexadienyl acetate)Fe(CO)3 with a glycine-
derived TMS–enol ester. Subsequent demetallation, followed by N-carboxymethylation, N-deacylation, N-alkylation with a trichloroethyl
6-halohexanoate, and TCE–ester cleavage provided the desired a-hexadienyl IDA derivative 2. Amide formation with IDA acid 1b
demonstrates the feasibility of conjugating the IDA ligands to polymers and proteins while Ni(II)-complexation with the derived IDA triacid
1e shows the complexing ability of the tethered IDA ligand.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
Metalloenzymes catalyze a variety of important hydrolytic,
redox and carbon–carbon bond-forming reactions, some of
which have no efficient synthetic counterparts. In order to
gain a better understanding of the catalytic mechanisms of
metalloenzymes and to develop comparable abiotic
catalysts for synthetic utilization, there have been intensive
studies of metal complexes that simulate the structural and
functional features of metalloenzymes. A common feature
of the active site of many metalloenzymes is a bis- or tris-
imidazole (from histidine) ligand set.1 Accordingly,
numerous synthetic poly(amine)- and some poly(imida-
zole)–metal complexes have been investigated as structural
and electronic models for these metalloenzymes.2 A class of
poly(histidine)-ligated metalloenzymes of special interest
because of their unique and mechanistically intriguing
reactions are the lipoxygenases (LO).3 These iron enzymes
catalyze the regio-, stereo- and enantioselective hydro-
peroxidation of unsaturated fatty acids (Eq. 1).
(1)
The exceptionally high catalytic activity and selectivity of
typical enzymes is thought to result in large measure from
the substrate-binding and the co-catalytic functionality of
the active site’s protein environment (as well as transition
state stabilization). In an effort to incorporate these features
into semi-synthetic metalloenzyme mimics we are investi-
gating various strategies for implanting poly(imidazole)–
metal centers in protein matrices. Our initial efforts in this
area provided a hybrid esterase protein with high activity by
incorporation of a bis(imidazole)-copper cofactor into the
combining site of the 38C2 aldolase antibody.4
In an approach to poly(imidazole)–metal-proteins that
would include a substrate or transition-state binding site at
the metal center we are seeking to select metal/substrate-
binding proteins from libraries, either directly (by panning)
or via immunization/antibody production. For this purpose
we envision employing a strongly, but minimally coordi-
nated metal center that can bind effectively to accessible
bis/tris-histidine sites on peptides/proteins. Inclusion of a
substrate/transition state element situated near to the metal
could select/elicit proteins with complementary binding
sites. The strong metal binding affinity of iminodiacetate
* First presented at the 59th Southwest Regional Meeting of the American
Chemical Society, October 26–28, 2003, Oklahoma City, USA.
Keywords: Iminodiacetate ligands; Catalysis; Lipoxygenase; Regioselec-
tive dienylation; (Diene)iron tricarbonyl complexes.
* Corresponding author. Tel.: C1-405-325-3696; fax: C1-405-325-6111;
e-mail: knicholas@ou.edu
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.08.029

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(IDA) ligands5 and the proven use of M(IDA) complexes for
the purification of histidine-tagged proteins by immobilized
metal-affinity chromatography (IMAC)6 has prompted us to
seek a-functionalized IDA ligands for the selection of his-
rich metal-binding proteins having complementary sub-
strate-binding capability. In the context of our search for LO
mimics we have targeted his-phillic IDA derivatives that
possess an a-tethered dienyl chain 2 to simulate the putative
planar dienyl radical intermediate of the LO enzyme-
catalyzed reactions (Fig. 1). In this contribution we describe
the first synthesis of such compounds, featuring a new and
potentially general method for the a-alkylation of IDA
esters.
IDA ester 3 by TCE-6-bromohexanoate 4a, itself prepared
from inexpensive 6-bromohexanoic acid (Fig. 2). Removal
of the TCE group of 1a to afford acid diester 1b was easily
accomplished under standard reducing conditions. To
establish the capability of conjugating the N-tethered IDA
ligands via an amide linkage, the monoacid 1b was shown to
be converted easily to the benzyl amide 1c using standard
amine/carbodiimide methodology. To initially assess the
metal-binding ability of the new IDA ligands, the triacid 1e
was prepared by N-alkylation of diester 3 to produce triester
1d and subsequent hydrolysis. The triacid 1e formed a green
nickel complex upon treatment with aqueous NiCl2 which is
tentatively formulated as Ni(1e-H)(H2O)n based on its mass
spectrum (ES).
Our original plan for synthesizing the dienyl IDA
derivatives 2 was to install the a-dienyl unit via alkylation
of an IDA-derived enolate with a simple dienyl electrophile
such as sorbyl bromide 5 (Eq. 2). Although seemingly
straightforward, in fact, there are very few literature
precedents for either the alkylation of IDA ester enolates7
or for selective nucleophilic substitutions on the bromide 5.8
To test this approach, alkylation of the BOC-protected IDA
ester 6 was investigated. Treatment of 6 with LDA (THF)
followed by benzyl bromide did afford a modest yield of the
benzyl derivative 7. The reported dienyl bromide 5 needed
for the synthesis of 8 was produced only in about 80%
isomeric purity from the corresponding alcohol under a
variety of conditions.9a,b Finally, use of bromide 5
(1.2 equiv, 80% purity) in the reaction with the enolate
from 6 afforded only 10–15% yield of the alkylated
2. Results and discussion
Two classes of IDA-derivatives were sought, the parent
compounds 1 and the a-hexadienyl derivatives 2, both of
which include a nitrogen-linked spacer chain functionalized
to enable covalent attachment to a polymer support (for
protein binding/selection) or to a protein (for immunization)
via amide formation. To allow orthogonal functionalization/
deprotection of the three carboxyl functions we used the
2,2,2-trichloroethyl (TCE) ester-derived spacer. The mixed
triester 1a was efficiently synthesized by N-alkylation of
Figure 1.
Figure 2.

B. Kalita, K. M. Nicholas / Tetrahedron 60 (2004) 10771–10778
10773
product(s), which was primarily a non-conjugated regio-
isomer (rather than 8) judging by H NMR.
IDA-derivative 14 upon treatment with methyl bromo-
acetate under basic conditions.12 We were pleased to find
that the trifluoroacetamide 14 could be selectively depro-
tected by NaBH4 in methanol (rt, 51%) with no reduction
of the ester functions. Attachment of the hexanoate chain by
N-alkylation of the secondary amine 15 with the TCE
bromoester 4a was found to be very sluggish and inefficient
under a variety of conditions.13 Some improvement was
found using the corresponding iodide 4b (XZI), enabling
the preparation of the triester 2a in moderate yield. The
desired acid diester 2b was obtained when 2a was treated
with excess zinc in glacial acetic acid.14
1
(2)
Seeking a more effective and regioselective dienylation
method we evaluated an approach based on the electrophilic
reactivity of [(pentadienyl)Fe(CO)3]C complexes with mild
carbon nucleophiles.10 The readily available (E,E)-dienyl
acetate complex 9b was selected as the electrophilic
component. The initially targeted nucleophilic partner for
9b, IDA–TMS derivative 10, proved to be extremely labile
and difficult to prepare efficiently. Therefore, we turned to
the known glycine derivative 1111 as the coupling partner
for complex 9b (Fig. 3). The reaction between 9b and 11
proceeded readily and regiospecifically at K78 8C in the
presence of TMS–OTf to afford the dienyl glycine
derivative 12 in excellent yield as a 1:1 diastereomeric
mixture (stereounits at the dienyl-iron and a-amino centers).
To further demonstrate the synthetic potential of the
alkylation of amino acid TMS enol ethers by electrophilic
metal complexes, we also examined the reaction between 11
and the dienyl aldehyde complex 16 (Eq. 3). The anticipated
product (after demetallation) could be useful for selecting/
eliciting metal-binding proteins having a complementary
lipoxygenase late transition state/product binding site. In the
event, the dienyl alcohol complex 17 was obtained as a
partly separable mixture of diastereomers (71% combined),
accompanied by a small amount of the dehydrated complex
18 (single isomer, undetermined geometry). These results
suggest that the coupling of electrophilic metal complexes
with glycine–TMS enol compounds could provide a rather
general entry to a-alkylated amino acid and iminodiacetate
derivatives.
1
Careful comparison of the H NMR spectra of 12 with 9b
and its precursor alcohol 9a revealed characteristic trans
coupling constants for protons of the coordinated diene unit
(JZ8.5 Hz) and highly shielded terminal vinyl protons
(0.5–1.2 ppm) in each case, showing that the E,E-diene
sterochemistry is preserved throughout. Demetallation of
complex 12 with Ce(IV) cleanly produced the correspond-
ing free dienyl glycine derivative 13 as a single regio- and
diastereoisomer (with removal of the stereoinducing
–Fe(CO)3 unit). The latter could be converted to the dienyl
Figure 3.

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B. Kalita, K. M. Nicholas / Tetrahedron 60 (2004) 10771–10778
over Na/benzophenone; dichloromethane and acetonitrile
were distilled from CaH2. Dimethylformamide (DMF) used
was purchased as pre-dried. All other reagents/chemicals
were obtained commercially and were used without any
purification.
1H, 13C and 19F NMR spectra were recorded on a Varian
300 MHz NMR spectrometer. Chemical shifts are reported
in parts per million (ppm, d) relative to TMS (1H, 13C) or
CF3Cl (19F). Infra-red spectra were recorded on a FTS 135-
BioRad FT-IR spectrometer and reported in wavenumbers
(cmK1). Mass spectra are recorded on a Micro-mass
(Q-TOF) spectrometer using electrospray time-of -flight
(ES-TOF). All new compounds were judged to be O95%
pure by NMR and TLC.
(3)
3. Summary/conclusions
4.1.1. TCE bromoester 4a. To a solution of 6-bromo-
hexanoic acid (1.00 g, 5.13 mmol) in 50 mL CCl4 was
added p-TsOH$H2O (1.95 g, 10.3 mmol) and 2,2,2-
trichloroethanol (2.46 mL, 25.6 mmol) under N2 and the
reaction mixture was refluxed for 12 h using a Dean–Stark
apparatus to remove water. After cooling, 10 mL of water
was added and the organic phase was separated. The organic
phase was further washed with water (2!10 mL), dried
over anhydrous MgSO4, and the solvent was removed under
reduced pressure to afford 4a as an oil (1.30 g, 78.0%),
which was pure enough to use directly in the next reaction.
Preparative methods have been developed to access
iminodiacetate derivatives with a functional spacer chain
for immobilization and an a-dienyl chain which may be
useful for selecting complementary metal binding peptides/
proteins with lipoxygenase activity. The a-hexadienyl side
chain of IDA derivatives 2 was selectively introduced in the
reaction of (2,4-hexadienyl acetate)Fe(CO)3 with a glycine-
derived TMS–enol ester. Amide formation with IDA acid
1b demonstrates the feasibility of conjugating the IDA
ligands to polymers and proteins while Ni(II)-complexation
with triacid 1e shows the complexing ability of the tetthered
IDA ligand.
d 1.52–1.53 (m, 2H), 1.71–1.73 (m, 2H), 1.85–1.90 (m, 2H),
2.47 (t, JZ7.4 Hz, 2H), 3.39 (t, JZ6.8 Hz, 2H), 4.72 (s,
2H). 13C NMR (75 MHz, CDCl3): d 24.1 27.7, 32.5, 33.6,
33.9, 74.1, 95.2, 171.9. IR (CHCl3, cmK1): 1755, 2666,
2946. MS (CESI): calcd 325 (M); found 348 (MC23), 350,
352. HRMS (CES): calcd 346.8984 (MCNa); found
346.8982 (MCNa), 348.8925, 350.8921.
Studies of the surface modification and protein conjugation
by IDA derivatives 1b and 2b are underway. These results,
together with investigations of the binding/selection of
peptides and proteins by immobilized complexes of the
ligands, will be reported in due course.
4.1.2. TCE triester 1a. To a solution of 3 (0.085 g,
0.53 mmol) in 5 mL acetonitrile was added anhydrous
Na2CO3 (10 equiv, 0.560 g, 5.28 mmol), followed by
addition of a solution of 4a (1.5 equiv, 0.257 g,
0.792 mmol) in 1 mL acetonitrile under N2 and the mixture
was vigorously stirred at reflux. After 2 days, the solvent
was rotary evaporated and the residual solid was triturated
with ethyl acetate (3!50 mL) and the mixture filtered. The
filtrate was rotary evaporated to give a gummy material
which was purified by flash column chromatography using
mixtures of ethyl acetate and hexane as eluant to give 1a as a
colorless oil (0.120 g, 56.1%).
4. Experimental
4.1. General information
All moisture sensitive reactions were carried out under a dry
N2 atmosphere. All reaction temperatures (8C), except room
temperature (rt), correspond to the external bath
temperatures.
The following compounds were prepared by reported
methods: dimethyl iminodiacetate,15 dimethyl N-t-Boc-
iminodiacetate,15 N-trifluoroacetyl glycine methyl ester,16
2,4-hexadienyl bromide,9a (E,E-2,4-hexadienol)Fe(CO)3,17
1H NMR (300 MHz, CDCl3): d 1.38 (m, 2H), 1.50 (m, 2H),
1.70 (m, 2H), 2.42 (t, JZ8 Hz, 2H), 2.69 (t, JZ8 Hz, 2H),
3.54 (s, 4H), 3.69 (s, 6H), 4.72 (s, 2H). 13C NMR (75 MHz,
CDCl3): d 24.7, 26.6, 27.6, 33.9, 51.7, 54.2, 54.9, 73.9, 95.2,
171.8, 172.1. IR (CHCl3, cmK1): 1443, 1744, 2952. MS
(CESI): calcd 405 (M); found: 428 (MC23), 406 (MC1),
408, 410.
(E,E-2,4-hexadienyl
acetate)Fe(CO)3,18
(E,E-2,4-
hexadienal)Fe(CO)3 and methyl 6-bromohexanoate.20
19
Analytical TLC plates were pre-coated with silica gel 60
F(254). Visualization was accomplished using short wave-
length UV light and/or exposing the plate to an iodine
chamber. Flash column chromatography (FCC) was per-
formed using 200–400 mesh silica gel with nitrogen
pressure. Preparative thin-layer chromatography was per-
formed using Partisil PK6F silica gel plates (Whatman).
Triethylamine was dried by distillation from CaH2 before
use. Tetrahydrofuran (THF) and diethyl ether were distilled
4.1.3. IDA acid 1b. To a solution of 1a (0.116 g,
0.286 mmol) in glacial acetic acid (4 mL) under N2 was
added Zn powder (K100 mesh, 1.86 g, 28.6 mmol) and the
mixture was stirred vigorously at rt. After 10 h, 50 mL of
ethyl acetate was added and the mixture was filtered through
a Celite pad. The residue was washed well with excess ethyl

B. Kalita, K. M. Nicholas / Tetrahedron 60 (2004) 10771–10778
10775
acetate. The combined washings were concentrated by
rotary evaporation and the residual acetic acid was removed
under high vacuum at 50–60 8C leaving a white solid. To
this material was added 50 mL of ethyl acetate and the
solution was washed with saturated aqueous NaHCO3
solution (3!10 mL). The organic phase was separated
and the aqueous phase was acidified to pH 4 with conc. HCl
and then again extracted with ethyl acetate (3!25 mL). The
combined ethyl acetate fractions were dried over anhydrous
MgSO4, filtered and rotary evaporated to give 1b as a gum
(0.052 g, 66.1%).
0.70 mmol) in 1 mL methanol was added 1 mL of 2 N
NaOH and the solution was stirred at rt for 5 h. After
complete disappearance of the starting material (tlc), the
reaction was quenched with 2 N HCl adjusting the pH to 5 at
0–5 8C. Water and methanol were removed by rotary
evaporation and the solid obtained was dried under vacuum,
triturated with methanol (5!25 mL), and the mixture was
filtered. The filtrate was rotary evaporated and vacuum dried
to give 1e as a hygroscopic white solid (0.127 g, 73.4%).
1H NMR (300 MHz, D2O): d 1.3–1.4 (m, 2H), 1.55–1.61
(m, 2H), 1.65–1.75 (m, 2H), 2.33 (t, JZ8 Hz, 2H), 3.19 (t,
JZ8 Hz, 2H), 3.73 (s, 4H). 13C NMR (75 MHz, D2O): 23.4,
23.7, 25.0, 33.6, 55.8, 56.9, 170.2, 178.2. MS (CESI): calcd
247 (M); found 270 (MC23), 293 (270C23), 317 (293C
23C1); (KESI): found 246 (MK1). HRMS (ESC) calcd
270.0953 (MCCNa); found: 270.0966.
1H NMR (300 MHz, CDCl3): d 1.34 (m, 2H), 1.47 (m, 2H),
1.62 (m, 2H), 2.32 (t, JZ7 Hz, 2H), 2.69 (t, JZ7 Hz, 2H),
3.54 (s, 4H), 3.69 (s, 6H), 9.30 (br s, 1H). 13C NMR
(75 MHz, CDCl3): d 24.5, 26.6, 27.4, 33.9, 51.6, 54.1, 54.7,
171.6, 179.3. IR (CHCl3, cmK1): 1699, 1713, 1716, 1732,
1738, 1742. MS (CESI): calcd 275 (M); found 276 (MC1),
298 (MC23); (KESI): 274 (MK1). HRMS (CESI): calcd
298.1267 (MCCNa); found 298.1245 (MCCNa).
4.1.7. Ni complex of 1e. To a solution of the triacid 1e
(0.100 g, 0.405 mmol) in 1.5 mL of distilled water was
added a solution of NiCl2$6H2O (0.096 g, 0.41 mmol) in
0.5 mL of distilled water. The pH of the reaction medium
was adjusted to pH 7 by dropwise addition of 1 M NaOH
and then the mixture was stirred at room temperature for
4 h, followed by heating at 60–65 8C for another 4 h. After
cooling to room temperature, the solution was poured onto a
small crystallization plate. After vacuum evaporation of the
water at rt and drying under high vacuum, 0.143 g of a light
green solid (including NaCl) was obtained. The MS of this
material showed a major ion cluster at 326/328 for
58,60Ni(1e-H)CNaC, consistent with a formulation of
Ni(1e-H)(H2O)n (nZ0–3).
4.1.4. Amide 1c. To a solution of 1b (0.052 g, 0.189 mmol)
in 1 mL dry DMF was added 1-[3-(dimethylamino)propyl]-
3-ethylcarbodiimide hydrochloride (0.036 g, 0.188 mmol)
and N-hydroxysuccinimide (0.022 g, 0.19 mmol) under N2
at room temperature, and the reaction mixture was stirred
for 24 h to make the NHS–ester of 1b. Then a solution of
benzylamine (0.024 g, 0.23 mmol) in 0.5 mL dry DMF was
added to the active ester solution and the reaction mixture
was stirred for another 8 h at room temperature. DMF was
removed under vacuum at 40–50 8C to give a gummy
material which was purified by preparative TLC (1:2 ethyl
acetate/hexane) to afford 1c as a gum (0.042 g, 61%).
MS (CESI): calcd 357 [M(58Ni)]; found 326 [M(58Ni–
3H2OCNa)], 328 [M(60Ni–3H2OCNa)]; (KESI): found
302 (58Ni–3H2O), 304 (60Ni–3H2O). HRMS (CES): calcd
326.0102 (MC–3H2OCNa); found 326.0176.
1H NMR (300 MHz, CDCl3): d 1.35 (m, 2H), 1.50 (m, 2H),
1.65 (m, 2H), 2.20 (t, JZ8 Hz, 2H), 2.68 (t, JZ8 Hz, 2H),
3.51 (s, 4H), 3.67 (s, 6H), 4.41 (d, JZ6 Hz, 2H), 4.49 (d,
JZ6 Hz, 2H), 5.75 (br, s), 5.85 (br, s), 7.23–7.31 (m, 5H).
IR (CHCl3, cmK1): 1617, 1674, 1695, 1700, 1743, 2934,
2985, 3054, 3314, 3441. MS (CESI): calcd: 364; found:
365 (MC1).
4.1.8. Alkylation of 6 with benzyl bromide and sorbyl
bromide. To a solution of diisopropylamine (0.167 mL,
1.19 mmol) in 4.5 mL of anhydrous tetrahydrofuran under
N2 at 0 8C was added n-BuLi (0.75 mL, 1.19 mmol) and the
reaction mixture was stirred for 20 min. The reaction
mixture was then cooled to K78 8C whereupon a solution
of 6 (0.300 g, 1.19 mmol) in 1 mL of tetrahydrofuran was
added dropwise over a period of 7–10 min. The reaction
mixture was then stirred at that temperature for 3 h. Then a
solution of benzyl bromide (0.204 g, 1.19 mmol) in 5.4 mL
tetrahydrofuran was added dropwise at K78 8C and the
mixture was stirred for another 7 h while warming to rt. The
reaction was quenched by addition of saturated aqueous
NH4Cl (5 mL) and then extracted with ethyl acetate (3!
25 mL). The organic extracts were dried over MgSO4,
filtered, and concentrated by rotary evaporation. Purification
of the crude material by FCC (ethyl acetate–hexane) gave
the benzylated product 7 as a gum (0.191 g, 45.5%).
4.1.5. IDA triester 1d. To a solution of dimethyl
iminodiacetate 3 (0.322 g, 2.00 mmol) in 15 mL of dry
acetonitrile under N2 was added a mixture of methyl
6-bromohexanoate (0.627 g, 3.00 mmol) in 0.5 mL aceto-
nitrile and anhydrous Na2CO3 (2.21 g, 20.8 mmol). The
reaction mixture was stirred at 80–85 8C for 2 days.
Acetonitrile was removed by rotary evaporation and the
residue was triturated with ethyl acetate (3!40 mL) and
filtered. The combined organic washings were rotary
evaporated to give a gummy material which was purified
by FCC using a mixture of ethyl acetate and hexane (1:15,
1:10, 1:5) to give the desired 1d as an oil (0.335 g, 58.0%).
1H NMR (300 MHz, CD3OD): d 1.20–1.28 (m, 2H), 1.30–
1.40 (m, 2H), 1.45–1.55 (m, 2H), 2.22 (t, JZ8 Hz, 2H), 2.57
(t, JZ8 Hz, 2H), 3.43 (s, 4H), 3.55 (s, 3H), 3.58 (s, 6H). IR
(CHCl3, cmK1): 1265, 1731, 2952. MS (CESI): calcd 289;
found 312 (MC23), 313 (MC1C23), 290 (MC1). HRMS
(CESI): calcd 312.1424 (MCCNa); found 312.1419.
1H NMR (300 MHz, CDCl3): d 1.37 (s, 9H), 3.00
(complex m, 2H), 3.15 (complex m, 2H), 3.64 (s, 6H),
3.73 (d, JZ3 Hz, 2H), 4.00 and 4.10 (m, 1H, isomeric
mixture), 7.19–7.25 (m, 5H). MS (CESI): calcd: 351 (M);
found: 374 (MC23).
4.1.6. Triacid 1e. To a solution of the triester 1d (0.20 g,

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B. Kalita, K. M. Nicholas / Tetrahedron 60 (2004) 10771–10778
When the same methodology was used for alkylation of 6
with 2,4-hexadienyl bromide 5 (1.2 equiv), a mixture of
isomeric dienyl products was obtained in low yield with the
2,4-dienyl isomer 8 as a minor component.
the aqueous part was further washed with ethyl acetate (2!
25 mL). The combined organic phase was washed with
water (3!10 mL), dried over anhydrous MgSO4, filtered,
and concentrated to give a red gummy material. Purification
by FCC using a mixture of hexane/ethyl acetate as the eluant
afforded 13 as a gum (0.310 g, 59.1%).
4.1.9. TMS–enol ester 11. To a stirred solution of
N-trifluoroacetyl glycine methyl ester (0.550 g, 2.97 mmol)
in 10 mL of dry ether was added anhydrous Et3N (3.70 mL,
26.5 mmol) under N2 at rt and the temperature was lowered
to 0 8C. To this stirred solution was added trimethylsilyl
trifluoromethanesulfonate (1.10 mL, 6.15 mmol) dropwise.
After complete addition the mixture was stirred at rt for 8 h.
The ethereal phase was separated carefully from the lower
salt phase. The salt phase was further washed with 10 mL of
dry ether. The combined organic phase was rotary
evaporated. Drying under high vacuum afforded 11 as a
light red liquid (0.770 g, 94.9%) that was used for the next
step without purification.
1H NMR (300 MHz, CDCl3): d 1.72 (d, JZ7.5 Hz, 3H),
2.60 (m, 2H), 3.78 (s, 3H), 4.61 (m, 1H), 5.30 (m, 1H), 5.62
(m, 1H), 6.00 (m, 2H), 6.80 (br, 1H). 13C NMR (75 MHz,
CDCl3): d 18.0, 34.7, 52.4, 52.9, 115.6 (q, JC–FZ216 Hz),
122.1, 130.1, 130.6, 135.7, 156.7 (q, JC–FZ28 Hz), 170.7.
19F NMR (282 MHz, CDCl3): d K75.9. IR (CHCl3, cmK1):
1178, 1212, 1696, 1700, 1733, 2361, 3326. MS (CESI):
calcd 265 (M); found 288 (MC23), 266 (MC1). HRMS
(CES): calc. 288.0823 (MCNa); found 288.0790.
4.1.12. Dienyl IDA ester 14. To a solution of 13 (0.305 g,
1.15 mmol) in 7 mL of dry DMF was added NaH (0.033 g,
1.38 mmol) under N2 and the mixture was stirred at rt for
10 min. Then a solution of methyl bromoacetate (0.22 mL,
2.3 mmol) in 1 mL dry DMF was added dropwise and the
reaction mixture was stirred at 80 8C while monitoring by
tlc. After disappearance of the starting materials (12 h), the
mixture was cooled to 40 8C and the DMF was removed
under reduced pressure. The residue was triturated with
ethyl acetate (3!50 mL) and the solution was filtered. The
organic phase was rotary evaporated to give a gummy
material that was purified by FCC using hexane/ethyl
acetate to afford 14 (ca. 1.4:1 amide rotomeric mixture) as a
gum (0.190 g, 49.0%).
1H NMR (300 MHz, CDCl3): d 0.26 (s, 9H), 0.27 (s, 9H),
3.61 (s, 3H), 5.64 (s, 1H). 13C NMR (75 MHz, CDCl3): d
0.3, 0.5, 54.6, 88.4, 117.9 (q, JC–FZ203 Hz), 134.8 (q,
JC–FZ29 Hz), 159.0. 19F NMR (282 MHz, CDCl3): d
K71.2.
4.1.10. (E,E)-Dienyl complex 12. To a solution of 11
(0.762 g, 2.79 mmol) in dry CH2Cl2 (15 mL) at K78 8C was
added a solution of 9b (0.876 g, 2.33 mmol) in 1 mL CH2Cl2
under N2. To this stirred solution was added TMSOTf
(84 mL, 20 mol%) and the reaction mixture was stirred while
monitoring reaction progress by tlc. After disappearance of
9b, the reaction was quenched with 2 mL of water and
allowed to warm to rt. The mixture was diluted with 25 mL
CH2Cl2 and the organic phase was dried over anhydrous
MgSO4, filtered, and the solvent rotovapped. Purification of
the crude material by FCC using hexane–ethyl acetate as the
eluant afforded 12 as a yellow gum (0.810 g, 85.8%).
1H NMR (300 MHz, CDCl3): d 1.70 (d, JZ6.6 Hz, 3H),
2.50–2.70 (m, 2H), 3.69 and 3.70 (2s, 3H), 3.73 (s, 3H), 4.10
and 4.20 (complex m, 2H), 4.62 (m, 1H), 5.30–5.40 (m, 1H),
5.60–5.70 (m, 1H), 5.85–6.10 (m, 2H). 13C NMR (75 MHz,
CDCl3): d 18.0, 32.2, 33.5, 45.7, 52.4, 52.8, 115.0, 122.8,
130.0, 130.6, 135.2, 157.5, 167.7, 169.4. 19F (282 MHz,
CDCl3): d K68.15, K69.43. IR (CHCl3, cmK1): 1695,
1699, 1749, 2957. MS (CESI): calcd 337 (M); found 360
(MC23), 338 (MC1), 268 (MKCF3), 697 [(M!2)C23].
1H NMR (300 MHz, CDCl3): (1:1 diastereomeric mixture) d
0.52–0.58 (m, 0.5H), 0.66–0.72 (m, 0.5H), 1.10–1.17 (m,
total 1H), 1.37 (d, JZ6.0 Hz, 3H), 2.03–2.31 (m, 2H), 3.80
and 3.82 (2s, 3H), 4.57–4.64 (m, 1H), 4.89–4.93 (dd,
JZ5.0, 8.5 Hz, 1H), 4.98–5.24 (m, 1H), 6.95 (br s, 1H). 13C
NMR (75 MHz, CDCl3): d 19.3, 35.9, 36.8, 52.4, 53.3, 53.4,
53.7, 53.8, 58.6, 58.7, 83.4, 83.8, 86.4, 86.5, 116.0 (q,
JC–FZ289 Hz), 157.0 (q, JC–FZ21 Hz), 170.7, 170.8,
211.9. 19F NMR (282 MHz, CDCl3): d K75.7, K75.8. IR
(CHCl3, cmK1): 1214, 1440, 1719, 1734, 1971, 2044, 2856,
2923, 2958, 3327, 3405. MS (CESI): calcd 405 (M); found
428 (MCCNa), 429 (MC23C1), 344 (428-3CO), 288.10;
MS (KESI): 404 (MK1). HRMS (CES): calcd 428.0020
(MCCNa); found 428.0019.
4.1.13. IDA amine 15. To a stirred solution of the
N-trifluoroacetyl derivative 14 (0.186 g, 0.551 mmol) in
5 mL dry methanol was added NaBH4 (0.052 g, 1.38 mmol)
in portions under N2 at K5 8C. The reaction was stirred at rt
while monitoring its progress by tlc. After 5.5 h, the reaction
was quenched with glacial acetic acid at 0 8C by lowering
the pH to 6. Methanol was rotary evaporated, 50 mL of ethyl
acetate was added, and the solution was washed with 20 mL
of water. The aqueous phase was further extracted with
ethyl acetate (3!25 mL). The combined organic phase was
dried over anhydrous MgSO4, filtered and rotary evapor-
ated. Purification of the crude material by column
chromatography on silica with ethyl acetate/hexane gave
the 15 as a gum (0.068 g, 51.2%).
4.1.11. (E,E)-Dienyl ester 13. To a stirred solution of
complex 12 (0.802 g, 1.98 mmol) in 30 mL acetone at
K78 8C was added ceric ammonium nitrate (1.63 g,
2.97 mmol) under N2 and the reaction mixture was slowly
allowed to warm to K10 8C. Stirring was continued at this
temperature for 5 h whereupon the starting material was
consumed (tlc). Acetone was removed by rotary evaporation
and the residue was dried under vacuum. Ethyl acetate
(75 mL) was added to the residue followed by addition of
20 mL of water. The ethyl acetate phase was separated and
1H NMR (300 MHz, CDCl3): d 1.69 (d, JZ7 Hz, 3H), 2.30
(br, 1H), 2.40–2.46 (m, 2H), 3.36 (s, 2H), 3.41 (m, 1H), 3.68
(s, 6H), 5.40 (m, 1H), 5.60 (m, 1H), 6.00 (m, 2H). 13C NMR
(75 MHz, CDCl3): d 18.1, 36.3, 48.9, 51.9, 60.5, 125.1,
128.8, 131.1, 134.1, 172.2, 174.1. IR (CHCl3, cmK1): 1739,
1742, 2953, 3019, 3338. MS (CESI): calcd 241 (M); found

B. Kalita, K. M. Nicholas / Tetrahedron 60 (2004) 10771–10778
10777
264 (MC23), 242 (MC1). HRMS (CESI): calcd 264.1211
(MCCNa); found 264.1232.
JZ7 Hz, 2H), 2.51 (m, 2H), 2.73 (t, JZ8 Hz, 2H), 3.47 (d,
JZ6 Hz, 1H), 3.58 (s, 2H), 3.71 and 3.73 (2s, 6H), 5.40–5.50
(m, 1H), 5.60–5.70 (m, 1H), 6.00–6.07 (m, 2H). 13C NMR
(75 MHz, CDCl3, low concentration): 18.1, 23.8, 27.6, 29.7,
32.4, 33.5, 35.8, 48.5, 52.2, 60.3, 124.3, 129.3, 130.9, 134.7,
177.7. IR (CHCl3, cmK1): 1731, 1735, 2253. MS (CESI):
calcd: 355 (M); found: 394 (MCK); (K ESI) 354 (MK1).
4.1.14. 2,2,2-Trichloroethyl-6-iodohexanoate 4b. To a
solution of 2,2,2-trichloroethyl-6-bromohexanoate (0.500 g,
1.53 mmol) in 5 mL acetone was added NaI (0.230 g,
1.53 mmol) at room temperature under N2 and the reaction
mixture was stirred for 10 h during which time the starting
material was completely consumed (tlc). The solvent was
removed by rotary evaporation and the crude material was
triturated with CH2Cl2 (5!15 mL) and then filtered. The
filtrate was concentrated by rotary evaporation and then
dried under vacuum to afford the desired iodo compound 4b
as an oil which was spectroscopically pure (0.540 g, 95%).
4.1.17. (E,E)-Dienyl alcohol complex 17. To a solution of
11 (0.100 g, 0.29 mmol) in 5 mL of dry CH2Cl2 under N2 at
K78 8C was added a solution of aldehyde complex 16
(0.045 g, 0.190 mmol) in 1 mL dry CH2Cl2 followed by
addition of TMSOTf (0.1 mL). The reaction mixture was
stirred at this temperature for 2.5 h, whereupon the aldehyde
was completely consumed (tlc). The reaction was quenched
with 1 mL of water and then extracted with CH2Cl2 (3!
20 mL). The combined organic extracts were dried over
anhydrous MgSO4, filtered, and the solvent was removed by
rotary evaporation. Purification of the residue by preparative
TLC (1:1 diethyl ether/hexane) afforded triene complex 18
(0.016 g, 20.9%), and dienyl alcohol complexes 17a,a0
(0.034 g, 42.5%) and 17b (0.023 g, 28.7%).
1H NMR (300 MHz, CDCl3): d 1.45 (m, 2H), 1.70 (m, 2H),
1.80 (m, 2H), 2.47 (t, JZ7.35 Hz 2H), 3.17 (t, JZ6.9 Hz,
2H), 4.73 (s, 2H). IR (CHCl3, cmK1): 801, 1754, 2856,
2929. MS (ESIC): calcd 372 (MC), 395 (MCC23); found
372.9 (MCC1), 374.9 [(MCC1)C2], 376.9 [(MCC1)C
4], 394.9 (MCC23).
4.1.15. IDA TCE ester 2a. To a solution of 15 (0.065 g,
0.27 mmol) in 5 mL of dry acetonitrile was added
anhydrous Na2CO3 (0.285 g, 2.69 mmol) followed by
addition of bromide 4a (0.13 g, 0.40 mmol) under N2, and
the reaction mixture was stirred at 90 8C while monitoring
its progress by tlc. After 3d, the acetonitrile was removed by
rotary evaporation, and the residue was triturated with ethyl
acetate(3!25 mL)andfiltered.Ethylacetate wasremovedby
rotary evaporation and the residue was purified by preparative
thin layer chromatography (1:2 ethyl acetate/hexane) to afford
2a as a gum (0.037 g, 28%). The corresponding reaction with
iodoester 4b gave 2a in 38% yield.
Compound 17a,a0. (1.3:1 diastereomeric mixture) 1H NMR
(300 MHz, CDCl3): d 0.75 and 0.95 (m, 1H), 1.25 (m, 1H),
1.41 (d, JZ6 Hz, 3H), 1.85 and 2.10 (br s, 1H), 3.78, 3.79
and 3.80 (s, 3H), 3.60 and 3.90 (m, 1H), 4.70 (m, 1H), 5.15
(m, 2H), 7.05 and 7.20 (m, 1H). 19F NMR (282 MHz,
CDCl3): d K75.6, K75.7, K75.8. MS (CESI): calcd: 421
(M); found: 444 (MC23), 404 (MKOH), 865 (2MC23).
1
Compound 17b. H NMR (300 MHz, CDCl3): d 0.78 (m,
1H), 1.23 (m, 1H), 1.40 (d, JZ6 Hz, 3H), 2.30 (br s, 1H),
3.78 (s, 3H), 4.20 (m, 1H), 4.69 (d, JZ6 Hz, 1H), 5.10 (m,
1H), 5.30 (m, 1H), 7.0 (d, JZ15 Hz, 1H). 13C NMR
(300 MHz, CDCl3): d 19.1, 53.0, 57.8, 59.2, 73.1, 86.8,
169.0. 19F NMR (282 MHz, CDCl3): d K75.6. IR (CHCl3,
cmK1): 1179, 1215, 1726, 1758, 1974, 2045, 2349, 3343
(br). MS (CESI): calcd: 421 (M); found: 444 (MC23), 445
(MC1C23), 865 (2MC23), 866 (2MC23C1).
1H NMR (300 MHz, CDCl3): d 1.40–1.50 (m, overlapping,
4H), 1.60–1.70 (m, 2H), 1.70 (t, JZ7 Hz, 3H), 2.30 (m, 2H),
2.40–2.50 (m, overlapping, 4H), 3.40 (m, 1H), 3.66 (s, 6H),
4.05 (t, JZ3 Hz, 2H), 4.73 (s, 2H), 5.40–5.60 (m, 2H),
6.00–6.10 (m, 2H). 13C NMR (75 MHz, CDCl3): d 18.1,
23.9, 27.6, 32.3, 33.4, 33.7, 36.3, 48.9, 51.92, 60.5, 73.9, 95.0,
125.1, 128.8, 131.1, 134.1, 171.8, 172.2, 174.1. IR (CHCl3,
cmK1): 1712, 1730, 1742, 2926. MS (CESI): calcd 485 (M);
found 508 (MC23), 509, 510, 511, 512, 513, 514, 515.
1
Compound 18. (single diastereomer) H NMR (300 MHz,
CDCl3): d 1.20 (m, 1H), 1.48 (d, JZ6 Hz, 3H), 2.85 (m,
1H), 3.74 (s, 3H), 5.20 (m, 2H), 6.40 (d, JZ12 Hz, 1H), 7.23
(br, s, 1H). 19F NMR (CDCl3): K75.3, K75.7. MS (CESI):
calcd: 403; found: 426 (MC23), 427 (MC1C23), 404
(MC1), 829 (2MC23).
4.1.16. IDA acid 2b. To a solution of 2a (0.035 g,
0.072 mmol) in 2 mL glacial acetic acid under N2 was
added Zn powder (K100 mesh, 0.47 g, 7.2 mmol) and the
reaction mixture was stirred vigorously at rt for 14 h. The
mixture was then diluted with 50 mL of ethyl acetate and
filtered through a Celite pad. The combined filtrate was
concentrated by rotary evaporation and the acetic acid left
was removed under high vacuum. To the residue was added
40 mL of ethyl acetate; the solution was then washed with
aqueous NaHCO3 solution (3!5 mL). The organic phase
was separated and the aqueous phase was acidified to pH 4
with conc. HCl and then again extracted with ethyl acetate
(3!30 mL). The combined ethyl acetate phase was dried
over MgSO4, filtered and concentrated to give 2b as a gum
after vacuum drying (0.010 g, 39%).
Acknowledgements
We appreciate helpful discussions with R. W. Taylor (OU),
V. Soloshonok (OU) and K. D. Janda (Scripps Research
Institute), and partial support from the University of
Oklahoma.
References and notes
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