Total synthesis of LL-Z1640-2 utilizing a late-stage intramolecular Nozaki-Hiyama-Kishi reaction

Article abstract of DOI:10.1016/j.tetlet.2010.10.092

A total synthesis of LL-Z1640-2 (2), a potent and selective kinase inhibitor, has been completed. The key step of the convergent synthesis utilized a late-stage intramolecular Nozaki-Hiyama-Kishi (NHK) reaction to close the macrocycle at the C6'-C7' bond.

Full text of DOI:10.1016/j.tetlet.2010.10.092

Tetrahedron Letters 51 (2010) 6852–6855
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Tetrahedron Letters
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Total synthesis of LL-Z1640-2 utilizing a late-stage intramolecular
Nozaki–Hiyama–Kishi reaction
⇑
Christopher A. LeClair, Matthew B. Boxer, Craig J. Thomas , David J. Maloney
NIH Chemical Genomics Center, National Human Genome Research Institute, National Institutes of Health, 9800 Medical Center Drive, Rockville, MD 20850, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A total synthesis of LL-Z1640-2 (2), a potent and selective kinase inhibitor, has been completed. The key
step of the convergent synthesis utilized a late-stage intramolecular Nozaki–Hiyama–Kishi (NHK) reac-
tion to close the macrocycle at the C60–C70 bond.
Received 15 September 2010
Revised 15 October 2010
Accepted 18 October 2010
Available online 25 October 2010
Published by Elsevier Ltd.
Hypothemycin (1) and LL-Z1640-2 (also known as C292 and 5-
(Z)-7-oxozeaenol) (2) (Fig. 1) are macrolactones of the resorcylic
acid lactone (RAL) class of natural products that have generated
substantial interest due to their potent inhibition of selected ki-
nases.1,2 Both 1 and 2 are produced via the polyketide biosynthetic
pathways of several fungal strains and their structures were first
reported in 1980 and 1978, respectively.3,4 Extensive research on
these agents and related cis-enone RALs (i.e., radicicol A and L-
(1). Additionally, Marquez and co-workers have detailed a synthe-
sis establishing the framework needed to complete the total syn-
thesis of 2 via a late-stage ring-closing metathesis reaction,18
while Hearn et al. recently presented a semisynthetic approach to
analogues of 1.19 Despite the use of different subunits in construct-
ing the carbon framework, all three total syntheses relied upon a
macrolactonization to establish the core ring structure.15–17 Diverg-
ing from previous synthetic strategies toward the preparation of LL-
Z1640-2 (2), we intended to exploit the intramolecular Nozaki–Hiy-
ama–Kishi (NHK) reaction in macrocycle formation at the C60–C70
bond.20,21 Winssinger and co-workers have published several con-
vergent strategies relying on three principle subunits (an aryl sele-
nide, an alkyl iodide (precursor to C10–C60), and a cis-vinyl bromide
(precursor to C70–C100)) that can be coupled in different sequences
to form the cyclic scaffold of 1 and 2.17 We envisioned a synthetic
strategy that, like the Winssinger approach, was modular and al-
lowed insertion of related fragments for rapid library expansion
and SAR explorations. Our route to 2 depended on an aryl selenide
nucleophile for a precedented17a sequence of alkylation, oxidation,
and elimination to generate the requisite C6–C10–C20 trans-olefin
scaffold (Fig. 1). Subsequent Mitsunobu esterification would pro-
vide access to an advanced intermediate poised for macrocycliza-
tion with the intramolecular NHK reaction. The success of this
strategy would allow modified versions of key intermediates 5, 6,
and 7 to enter a similar synthetic sequence providing advanced
SAR explorations around 2 in a rapid manner.
Synthetic elaboration of the aryl selenide 7 is accomplished
starting from commercially available 2,4-dimethoxy-6-methylben-
zoic acid via known methods (2-(trimethylsilyl)ethyl (TMSE) pro-
tection and selenation).17a Alkene 8 was derived in two-steps
from 2-deoxyribose utilizing an established procedure.22 Further
elaboration of 8 involved ozonolysis followed by immediate reduc-
tion of aldehyde 9 with NaBH4 to afford the latent primary alcohol
10, which was ultimately converted to alkyl iodide 6 using Appel
conditions (I2, PPh3, imid.) (Scheme 1). Synthesis of both the bromo
and the iodo versions of the Z-vinyl halide subunit began from
the same silyl protected R-homoallylic alcohol 12.23 Olefin
783277) revealed that 2 is a potent inhibitor of TNFa
production,5
MEK,6 and JNK/p38.7,8 Furthermore, it has also been recognized
that 1 and 2 target selected members of the kinome including
Erk2, TAK1, and Kit.9–11 In 2006, Santi and co-workers showed that
the cis-enone of 1 acted as a Michael acceptor in the presence of
select kinases which contain
a conserved cysteine residue
(Cys166 in Erk2).12 This key interaction was further corroborated
by X-ray crystal structures of hypothemycin (1)13 and LL-Z1640-
2 (2)9 covalently bound within the ATP binding domain of Erk2
through the Cys residue. There are at least 46 kinases that contain
a Cys residue in the hinge region suggesting a high promiscuity of
these important natural products across the kinome. Given the de-
tailed structural understanding of the binding mechanism, obtain-
ing rationally designed analogues that limit binding, covalent
modification, and inhibition to a selected sub-domain of the
kinome are conceivable. Achieving this goal requires efficient and
expedient synthetic methods that afford both the natural products
and the desired analogues. Recently, Winssinger and co-workers
have synthesized a RAL library of hypothemycin, LL-Z-1640-2,
and L-783277 analogues to investigate the structure–activity rela-
tionship of kinase inhibition.14
The first total synthesis of LL-Z1640-2 (2) was published by Tats-
uta et al.15 in 2001 followed by the Lett and co-worker16 and Wins-
singer17 groups completing total syntheses of 2 and hypothemycin
⇑
Corresponding author. Address: NIH Chemical Genomics Center, NHGRI,
National Institutes of Health, 9800 Medical Center Drive, Building B, Room 2055,
MSC: 3370, Bethesda, MD 20892-3370, USA.
E-mail address: craigt@mail.nih.gov (C.J. Thomas).
0040-4039/$ - see front matter Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2010.10.092

C. A. LeClair et al. / Tetrahedron Letters 51 (2010) 6852–6855
6853
Mitsunobu esterification
Me
Me
Me
8'
7'
O
OH
2
10'
OH
O
MeO
O
HO
1
HO
X
H
O
O
O
O
O
O
5'
4
1'
3'
MeO
OH
MeO
OH
MeO
O
3
hypothemycin (1)
LL-Z1640-2 (2)
Nozaki-Hiyama-Kishi (NHK) Reaction
O
O
MeO
O
OH
MeO
O
OPiv
OH
O
Me
TMS
5
O
X
I
MeO
O
4
MeO
O
OPiv
Alkylation
and
SePh
7
6
O
Elimination
Figure 1. Structures of hypothemycin (1) and LL-Z1640-2 (2) with retrosynthetic analysis.
OPiv
OPiv
TBSOTf
2,6-lutidine
O3, Sudan III
OH
OT BS
CH2Cl2, -78 °C;
O
O
O
CH2Cl2
0 °C to rt
(98%)
then Et3N, rt
O
H
O
11
12
8
9
Grubbs' 2nd
O3, Sudan III
NaBH4
MeOH, 0 °C
(71%, 2 steps)
O
B
gen. cat.
CH2Cl2, Δ
(93%)
CH2Cl2, -78 °C;
Et3N, rt
O
(80%)
OPiv
OPiv
I2, PPh3
TBSO
O
H
OTBS
O
B
imidazole
I
HO
O
O
O
THF, 0 °C
(85%)
O
O
14
13
6
10
Ph3P+(CH2I)I-
NaHMDS
THF, -78 °C
(50%)
Scheme 1. Synthesis of alkyl iodide subunit 6.
Br2, THF, 0 °C;
NaOMe, MeOH
THF, 0 °C
(53%)
cross-metathesis with vinyl boronic acid resulted in the desired E-
vinyl boronate ester 13 in high yield (Scheme 2).24 Using condi-
tions developed by Brown et al.,25 13 was subjected to Br2 resulting
in the formation of a dibromide species in situ. Treatment with
NaOMe/MeOH induced elimination and concomitant TBS depro-
tection providing Z-vinyl bromide 5a. Preparation of Z-vinyl iodide
5b26 required an alternate synthetic approach as vinyl boronic es-
ters subjected to I2 under equivalent conditions result in retention
of configuration yielding the undesired E-vinyl iodide.25c,25d As
such, ozonolysis of 12 and subsequent Wittig olefination of the
formed aldehyde 14 using phosphonium methyl iodide provided
Z-vinyl iodide 15, which upon silyl deprotection with HF gave 5b
in good yield.
Treatment of aryl selenide 7 with LDA followed by addition of
alkyl iodide 6 produced a mixture of alkylation products, which
upon stirring with H2O2 afforded trans-olefin product 16 exclu-
sively in high yield (Scheme 3). Removal of the TMSE group with
TBAF furnished the key carboxylic acid intermediate 4 in excellent
yield. The carbon skeleton is completed via Mitsunobu esterifica-
tion with either vinyl bromide 5a or vinyl iodide 5b (yield was
unaffected by the halogen). Pivalate deprotection and Dess–Martin
oxidation occurred in a straightforward manner giving aldehydes
3a and 3b in 90% and 77% yield, respectively.
OTBS
OH
HF (aq.)
MeCN, rt
(76%)
I
X
15
5a, X = Br
5b, X = I
Scheme 2. Synthesis of Z-vinyl halides 5a and 5b.
Initial explorations into the intramolecular NHK reaction to ob-
tain the desired macrocycle 19 utilized vinyl bromide 3a under
various conditions (Table 1). Modifications to solvent system,
equivalents of CrCl2, reaction temperature, and reaction time were
all unsuccessful in improving the low yields. During the course of
the reaction, the starting material was rapidly consumed in agree-
ment with the literature that oxidative addition of Ni to the sub-
strate can be fast.20 While the formation of a Cr–O bond is the
thermodynamic driving force, the formation of product was slow,
which is a common occurrence with the intramolecular NHK reac-
tion.21 The diminished yields might be attributed to decomposition
of the organochromium intermediates despite the tolerance of
numerous functional groups to the reaction conditions. Attempts

6854
C. A. LeClair et al. / Tetrahedron Letters 51 (2010) 6852–6855
Me
O
MeO
O
i. LDA; 6
THF/DMPU (6:1)
-78 °C
MeO
O
MeO
O
5a or 5b
TMS
X
PPh3, DIAD
O
OR
O
THF, rt
ii. H2O2 (30% wt)
THF, rt
MeO
MeO
MeO
7
SePh
(87%)
OR
OPiv
O
O
O
17a, X = Br, R = Piv (91%)
17b, X = I, R = Piv (91%)
16, R = CH2CH2TMS
4, R = H
TBAF
NaOMe
THF, rt
(96%)
MeOH, rt
18a, X = Br, R = H (87%)
18b, X = I, R = H (97%)
Dess-Martin
Periodinane
CH2Cl2, rt
Me
O
1. CrCl2, NiCl2 (cat.)
DMF
Me
O
Me
MeO
O
3a: 50 °C, 48 h (35%)
X
H
OH
MeO
O
O
BCl3
3b: rt, 24 h (61%)
HO
O
O
O
O
MeO
CH2Cl2, 0 °C
(50%)
2. Dess-Martin
Periodinane
CH2Cl2, rtv
(65%)
MeO
OH
MeO
O
3a, X = Br (90%)
3b, X = I (77%)
O
O
2
20
O
Scheme 3. Completion of the synthesis of LL-Z1640-2 (2).
Table 1
Conditions and results of NHK reaction with 3a
Me
Me
MeO
O
Br
H
MeO
O
O
O
NHK Reaction
O
O
OH
O
MeO
3a
MeO
O
19
O
Entry
CrCl2 (equiv)
NiCl2 (mol %)
Solvent
Temp (°C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
5
5
10
10
5
5
5
5
5
5
5
DMF
DMF
DMF
DMF
rt
24
24
48
72
48
48
48
48
48
48
48
15
26
35
34
50
50
50
50
50
50
50
50
50
50
10
10
10
10
10
10
10
10
10
DMF/4-tBuPy (3:1)
DMSO
30
No product
5
No product
19
12
10
DMSO/4-tBuPy (3:1)
DMSO/DMF (20:1)
THF/DMF (2:1)
THF/DMF/4-tBuPy (6:3:1)
DME/DMF (6:1)
10
11
5
5
at further optimization through prolonged reaction times (>48 h)
and specialized workup (aq sodium serinate21d) were unsuccessful.
Based on this study, the best reaction conditions (entry 3, Table 1)
provided the desired macrolactone in a modest 35% yield.
of the protecting groups can affect stereochemical outcome.21
However, we did not investigate this aspect as the subsequent oxi-
dation of the newly formed allylic alcohol renders the diastereose-
lectivity inconsequential to the synthesis. The ensuing penultimate
Dess–Martin oxidation of the allylic alcohol afforded cis-enone 20
without incident. Selective deprotection of the ortho methoxy
group with simultaneous removal of the acetonide was elicited
by subjecting 20 to BCl3 as previously reported to complete the to-
tal synthesis of LL-Z1640-2 (2).17a
In summary, this study has demonstrated the utility of an intra-
molecular Nozaki–Hiyama–Kishi reaction in establishing the mac-
rolactone during the total synthesis of the naturally occurring
macrolide LL-Z1640-2 (2). The resulting synthetic sequence pro-
vides a convergent strategy allowing for rapid exploration of novel
analogues of this important natural product.
Further efforts at optimizing the yield prompted a halogen
switch to vinyl iodide 3b, which was subjected to the best vinyl
bromide reaction conditions (CrCl2 (10 equiv), NiCl2 (5 mol %),
DMF). Gratifyingly, the yield of the intramolecular NHK increased
to an acceptable 61% with reduced reaction temperature (room
temperature) and time (24 h). Exposure to longer reaction times
and increased temperatures were ineffective at increasing the
yield. The macrocyclization appears to have proceeded in a stereo-
selective fashion since macrocycle 19 was isolated as a single
diastereomer. Previous examples of the intramolecular NHK reac-
tion have demonstrated that proximal stereocenters and variation

C. A. LeClair et al. / Tetrahedron Letters 51 (2010) 6852–6855
6855
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Acknowledgment
The authors thank William Leister for his assistance with nor-
mal and reverse phase purification techniques. This research was
supported by the Molecular Libraries Initiative of the National
Institutes of Health Roadmap for Medical Research, the Intramural
Research Program of the National Human Genome Research Insti-
tute, and the National Institutes of Health.
Supplementary data
Supplementary data (experimental procedures and character-
ization data along with NMR spectra are provided) associated with
this article can be found, in the online version, at doi:10.1016/
j.tetlet.2010.10.092.
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