Conformationally constrained analogues of diacylglycerol. 24. Asymmetric synthesis of a chiral (R)-DAG-lactone template as a versatile precursor for highly functionalized DAG-lactones

Article abstract of DOI:10.1021/ol0492041

(Equation Presented) Commercially available 2-methylenepropane-1,3-diol was converted to chiral epoxide (R)-2 via Sharpless asymmetric epoxidation in >96% ee. Regiospecific epoxide ring opening and reduction of the intermediate alkyne set the stage for a

Full text of DOI:10.1021/ol0492041

ORGANIC
LETTERS
2004
Vol. 6, No. 14
2413-2416
Conformationally Constrained
Analogues of Diacylglycerol. 24.
Asymmetric Synthesis of a Chiral
(R)-DAG-Lactone Template as a
Versatile Precursor for Highly
Functionalized DAG-Lactones
Ji-Hye Kang,† Maqbool A. Siddiqui,† Dina M. Sigano,† Krzysztof Krajewski,†
,§
Nancy E. Lewin,‡ Yongmei Pu,‡ Peter M. Blumberg,‡ Jeewoo Lee,* and
,†
Victor E. Marquez*
Laboratory of Medicinal Chemistry, Center for Cancer Research, National Cancer
Institute-Frederick, National Institutes of Health, Frederick, Maryland 21702,
Laboratory of Cellular Carcinogenesis & Tumor Promotion, Center for Cancer
Research, National Cancer Institute, National Institutes of Health,
Bethesda, Maryland 20892, and Laboratory of Medicinal Chemistry,
College of Pharmacy, Seoul National UniVersity, Shinlin Dong, Kwanak-ku,
Seoul 151-742, South Korea
marquezV@dc37a.nci.nih.goV; jeewoo@snu.ac.kr
Received April 29, 2004
ABSTRACT
Commercially available 2-methylenepropane-1,3-diol was converted to chiral epoxide (R)-2 via Sharpless asymmetric epoxidation in >96% ee.
Regiospecific epoxide ring opening and reduction of the intermediate alkyne set the stage for a one-pot lactonization to give (R)-6, a convenient
precursor for all functionalized chiral DAG-lactones used as potent PK-C ligands. The synthesis of the most potent DAG-lactones known to
date, (Z)-10 and (E)-10, served to confirm PK-C’s exclusive preference for the (R)-stereochemistry in this class of compounds.
The central role of protein kinase C in cell signal transduction
has been well established since its discovery more than 2
decades ago.1 The classic (R, â1, â2, and γ) and novel (δ,
ꢀ, η, and θ) PK-C isozymes contain in their regulatory
domain two copies of a cysteine-rich motif (C1 domains)
about 50 amino acids long, which are the receptors for the
phorbol ester tumor promoters and the second messenger
diacylglyerol (DAG). Over the past several years, we have
synthesized a number of potent PK-C ligands based on a
constrained glycerol scaffold (DAG-lactone) that bind to
these C1 domains with high affinity.2,3 During our investiga-
tions, we have demonstrated the importance of the alkyl
chains in controlling binding affinity as a function of size,
† Laboratory of Medicinal Chemistry, National Institutes of Health.
‡ Laboratory of Cellular Carcinogenesis & Tumor Promotion, National
Institutes of Health.
§ Laboratory of Medicinal Chemistry, Seoul National University.
(1) Castagna, M.; Takai, Y.; Kaibuchi, K.; Sano, K.; Kikkawa, U.;
Nishizuka, Y. J. Biol. Chem. 1982, 257, 7847-7851.
10.1021/ol0492041 CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/17/2004

position on the glycerol scaffold, and degree of branching.4
The evolution of this process can be visualized in Figure 1,
hydrate precursors, such as 1,2:3,5-di-O-isopropylidene-R-
D-threo-apiofuranose5 and D-arabinose.6 Furthermore, because
compounds having a type III scaffold have not yet been
synthesized in pure enantiomeric form, we chose to dem-
onstrate the utility of our approach by synthesizing type III
molecules, such as compounds (E)-10 and (Z)-10, as (R)-
enantiomers. As anticipated, the binding affinities of both
geometric isomers confirmed the preference of PK-C for the
(R)-enantiomers.
The synthesis began with protection of commercially
available 2-methylenepropane-1,3-diol as its monotrityl ether
1 (Scheme 1). Use of Sharpless mnemonic rules led to the
Scheme 1a
Figure 1. Structures and PK-CR binding affinity of racemic and
(R)-enantiomeric DAG-lactones with templates of increasing com-
plexity (I, II, and III).
a Reagents and conditions: (a) Et3N, TrCl, CH2Cl2 (52%); (b)
(+)-DET (2% mol), Ti(Oi-Pr)4 (10% mol), t-BuOOH, CH2Cl2, -20
°C (80%); (c) NaH, BnBr, DMF (86%); (d) LiCtCH‚EDA, DMSO
(79%); (e) Lindlar cat. (50% w), H2, quinoline (50% w), hexane
(97%); (f) BH3‚SMe2, THF, -78 °C; then PCC, CH2Cl2 (48% for
2 steps).
where it can also be seen that for scaffolds I and II the active
enantiomer has the (R)-stereochemistry. In the few cases
where both (R)- and (S)-enantiomers were synthesized,
binding affinity differences greater than two orders of magni-
tude were observed with activity residing exclusively with
the (R)-enantiomer.5 An important improvement in potency
occurred when transferring the bulk of the alkyl group from
the acyl position in I to the R-alkylidene position in II, which
reduced the acyl group to the simplest acetyl moiety and
resulted in a 5- to 8-fold increase in binding affinity.6
Because our most recent potent compounds belong to
scaffold type III, where the alkyl chains are distributed
between both acyl and R-alkylidene positions, we wanted
to confirm that, in agreement with scaffolds I and II, the
required stereochemistry for III was also (R). This confirma-
tion was considered important because normally for the sake
of convenience the search for novel compounds is initially
conducted with racemic DAG-lactones, with the synthesis
of the pure enantiomer postponed until after the initial screen.
In the present manuscript, we wish to present a general
approach to a simple, chiral DAG-lactone [(R)-6] that serves
as chiral precursor for all three scaffolds (I-III). This new
process appears to be vastly superior in efficiency and
simplicity when compared to previous methods of syntheses
of (R)-DAG-lactones (scaffolds I and II) from chiral carbo-
selection of L-(+)-diethyl tartrate as the optically active
reagent for the chiral epoxidation of alkene 1 to produce the
desired DAG-lactone (R)-6. Epoxidation of 1 with t-BuOOH
in the presence of catalytic amounts of titanium tetraisopro-
poxide and L-(+)-diethyl tartrate gave an 80% yield of the
desired, chiral epoxide (R)-2 with >96% ee as confirmed
by its Mosher ester. Protection of the remaining free alcohol
as a benzyl ether provided compound (R)-3 and set the stage
for the ensuing nucleophilic opening of the epoxide moiety
with lithium acetylide (ethylenediamine complex) to give the
key intermediate (R)-4. In the presence of Lindlar catalyst
(Pd-CaCO3-PbO), the alkyne group in (R)-4 was successfully
reduced to the alkene to give the tertiary homoallylic alcohol
(R)-5. As was the case for the synthesis of racemic lactones,
formation of lactone (R)-6 was achieved in “one pot” after
hydroboration of the olefin and immediate oxidation with
pyridinium chlorochromate.7
From lactone (R)-6, the synthesis of chiral (R)-DAG-
lactones (E)-10 and (Z)-10 was completed using a well-
established methodology developed in our laboratory2 in-
volving aldol condensation with 5-methyl-3-(2-methylpropyl)-
hexan-1-one followed by olefination (Scheme 2). Separation
of geometric isomers (E)-7 and (Z)-7 was achieved at this
stage by column chromatography, and completion of the
synthesis was performed individually for each isomer.
Removal of the trityl ether gave the free alcohols (E)-8 and
(2) Nacro, K.; Bienfait, B.; Lee, J.; Han, K. C.; Kang, J. H.; Benzaria,
S.; Lewin, N. E.; Bhattacharyya, D. K.; Blumberg, P. M.; Marquez, V. E.
J. Med. Chem. 2000, 43, 921-944.
(3) Marquez, V. E.; Blumberg, P. M. Acc. Chem. Res. 2003, 36, 434-
443.
(4) Marquez, V. E.; Nacro, K.; Benzaria, S.; Lee, J.; Sharma, R.; Teng,
K.; Milne, G. W.; Bienfait, B.; Wang, S.; Lewin, N. E.; Blumberg, P. M.
Pharmacol. Ther. 1999, 82, 251-261.
(5) Lee, J.; Sharma, R.; Wang, S.; Milne, G. W.; Lewin, N. E.; Szallasi,
Z.; Blumberg, P. M.; George, C.; Marquez, V. E. J. Med. Chem. 1996, 39,
36-45.
(6) Lee, J.; Wang, S.; Milne, G. W.; Sharma, R.; Lewin, N. E.; Blumberg,
P. M.; Marquez, V. E. J. Med. Chem. 1996, 39, 29-35.
(7) Sharma, R.; Lee, J.; Wang, S.; Milne, G. W.; Lewin, N. E.; Blumberg,
P. M.; Marquez, V. E. J. Med. Chem. 1996, 39, 19-28.
2414
Org. Lett., Vol. 6, No. 14, 2004

(CH3)3 side chain. This proton’s signal, which in rac-(Z)-10
resonates at δ ) 4.08 (d, Jgem ) 11.8 Hz) shifted to δ )
4.20 and was split into two doublets with identical geminal
coupling constants. A similar shift to lower field occurred
for rac-(E)-10, which showed the same splitting pattern.
When the same experiment was performed with enantiopure
(E)-10 and (Z)-10, the Eu chiral reagent induced the expected
downfield shift, and a second set of very minor signals was
observed. Peak areas measured for the newly resolved signals
for both (E)-10 and (Z)-10 enantiomers corresponded to
optical purities >90%. Enantiomeric purity was also assessed
by chiral HPLC analysis on a ChiraCel OD column (Figure
2). Whereas the racemate could be partially resolved into
two distinct peaks, both (E)-10 and (Z)-10 enantiomers eluted
essentially as single peaks. Although the poor resolution
precluded accurate integration of peak areas, the lack of any
“shoulders” present on the peaks by visual inspection allowed
us to estimate an enantiomeric purity of >90%.
Scheme 2a
a Reagents and conditions: (a) LHMDS, (i-Bu)2CHCH2CHO,
THF, -78 °C; (b) Et3N, MsCl, CH2Cl2; (c) DBU, CH2Cl2 (Z 43%,
E 40%); (d) formic acid, CH2Cl2 (Z 60%, E 92%); (e) Et3N, pivaloyl
chloride, DMAP, CH2Cl2 (Z 100%, E 100%); (f) BCl3, CH2Cl2,
-78 °C (Z 79%, E 82%).
The PK-C binding affinities for (E)-10 and (Z)-10 are
expressed in terms of the parameter Ki, which measures the
ability of the ligand to displace PK-CR-bound-[20-3H]-
phorbol-12,13-dibutyrate (PDBU) in the presence of phos-
phatidylserine. The inhibition curves obtained were of the
type expected for competitive inhibition, and the Ki values
were calculated from the ID50 values.10 The Ki values for
(E)-10 and (Z)-10 were 2.4 ( 0.34 and 1.45 ( 0.2 nM,
respectively. These values are almost exactly half of the Ki
values for the racemates (Figure 3),9,11 thus confirming that
(Z)-8 with the (S)-stereochemistry. Acylation of the free
alcohol with pivalolyl chloride gave compounds (E)-9 and
(Z)-9, which after final removal of the benzyl group afforded
the target compounds (E)-10 and (Z)-10 with the (R)-
stereochemistry. Consistent with previously synthesized
DAG-lactones, the vinyl proton of the (Z)-isomer displayed
a characteristic signal at δ ) 6.0 in its H NMR spectrum,
while the corresponding signal of the (E)-isomer appeared
more downfield at δ ) 6.7.2
Although the pivaloyl group is sufficiently hindered to
prevent spontaneous racemization by acyl migration, as we
have previously observed with linear chains,5 we wanted to
confirm this by authenticating the structural integrity of the
targets. First, we used the chiral NMR shift reagent europium
tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorate] to
determine enantiomeric purity.8 In the presence of the Eu
chiral reagent, both racemic samples of (E)-109 and (Z)-109
showed the largest pseudocontact-shift difference for one of
the enantiotopic protons (underscored) on the CHHOCOC-
1
Figure 3. Comparison of PK-CR binding affinities between
racemic and chiral DAG-lactones with (Z)- and (E)-geometries.
the (R)-enantiomers uniformly represent the “active” template
regardless of the pattern of substitution or balance of alkyl
groups at either the acyl or R-alkylidene positions on the
DAG-lactones. These results also confirm the typically
observed trend of a ca. 2-fold difference in favor of the (Z)-
isomers.
Figure 2. (A) Racemic (E)-10 (100 µL, 1.8 mg/mL). (B)
Enantiopure (R)-(E)-10 (50 µL, 1.7 mg/mL). (C) Racemic (E)-10
(50 µL) and enantiopure (R)-(E)-10 (50 µL).
Org. Lett., Vol. 6, No. 14, 2004
2415

In conclusion, we have developed a general approach to
a simple, chiral DAG-lactone [(R)-6] that serves as a con-
venient precursor for all three types of functionalized, chiral
DAG-lactones used as potent PK-C ligands. In addition, the
synthesis of (Z)-10 and (E)-10 confirm the exclusive prefer-
ence for the (R)-stereochemistry in this class of compounds
by the PK-C enzyme.
Acknowledgment. The authors thank Dr. James A.
Kelley from this laboratory for mass spectral analysis and
interpretation. This work was supported partly by the Korea
Research Foundation Grant (KRF-2003-015-E00229).
Supporting Information Available: General experimen-
tal procedures and complete characterization data for all new
compounds, plus 1H NMR spectra of racemic and optically
pure samples of (E)-10 and (Z)-10 with the chiral shift
reagent. This material is available free of charge via the
Internet at http://pubs.acs.org.
(8) Goering, H. L.; Eikenberry, J. N.; Koerner, G. S. J. Am. Chem. Soc.
1971, 93, 5913-5914.
(9) Lee, J.; Han, K.-C.; Kang, J.-H.; Pearce, L. L.; Lewin, N. E.; Yan,
S.; Benzaria, S.; Nicklaus, M. C.; Blumberg, P. M.; Marquez, V. E. J. Med.
Chem. 2001, 44, 4309-4312.
(10) Lewin, N. E.; Blumberg, P. M. Methods Mol. Biol. 2003, 233, 129-
156.
(11) Choi, Y. S.; Kang, J.-H.; Lewin, N. E.; Blumberg, P. M.; Lee, J.;
Marquez, V. E. J. Med. Chem. 2003, 46, 2790-2793.
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