Synthesis of 4-aza analogs of epothilone D

Article abstract of DOI:10.1055/s-2004-835665

An efficient synthesis of epothilone D analogs of type 1 has been developed, which is based on a highly diastereoselective Evans aldol reaction as one of the key steps. A profound difference in the relative stabilities of TBS-protecting groups on C7-O and C15-O was observed between secondary and primary amide groups at C5-N4. Although modeling studies had suggested analogs of type 1 to assume a conformation similar to the NMR-derived conformation of tubulin-bound epothilone A, compounds 1a-d were found to be significantly less active than the parent compound epothilone D.

Full text of DOI:10.1055/s-2004-835665

LETTER
2709
Synthesis of 4-Aza Analogs of Epothilone D
Synthesisof
4r
é
logsof
E
po
d
thilone
D
éric Cachoux, Franck Schaal, Antje Teichert, Trixie Wagner, Karl-Heinz Altmann*1
Novartis Institutes for Biomedical Research, 4002 Basel, Switzerland
Fax +41(44)6331360; E-mail: karl-heinz.altmann@pharma.ethz.ch
Received 17 August 2004
in a rather straightforward manner (e. g. through amide
bond formation or reductive amination in the case of
nitrogen).
Abstract: An efficient synthesis of epothilone D analogs of type 1
has been developed, which is based on a highly diastereoselective
Evans aldol reaction as one of the key steps. A profound difference
in the relative stabilities of TBS-protecting groups on C7-O and
C15-O was observed between secondary and primary amide groups
at C5-N4. Although modeling studies had suggested analogs of type
1 to assume a conformation similar to the NMR-derived conforma-
tion of tubulin-bound epothilone A, compounds 1a–d were found to
be significantly less active than the parent compound epothilone D.
Our laboratory has recently been involved in the determi-
nation of the bioactive conformation of epothilone A and
one of the characteristic features of the structure is the
presence of a syn-periplanar conformation about the C4-
C5 bond.5 The same geometry would be enforced in ana-
logs of type 1, provided that the amide bond between N4
and C5 would be present in a cis conformation. At the
same time preliminary modeling studies indicate that the
presence of a cis amide bond in this position should allow
replacement of the C1-C4 segment by various types of b-
amino acids without causing significant distortions in the
bioactive conformation of the C5-O16 segment. Based on
these considerations and given the fact that structures of
type 1 would lend themselves to an efficient combinatori-
al chemistry approach employing a single advanced inter-
mediate [i. e. carboxylic acid 10 (21), vide infra] we
embarked on the synthesis of some prototypic examples
of analogs of type 1.6
Key words: epothilones, total synthesis, aldol reaction, microtu-
bule inhibitors, natural products
Epothilones are natural products that were first isolated
from culture extracts of the cellulose-degrading myxo-
bacterium Sorangium cellulosum.2 These substances
(Figure 1) are highly potent antiproliferative agents,
which inhibit tumor cell growth through the same mecha-
nism of action as the clinical anticancer drug Taxol®, i. e.
the stabilization of cellular microtubules and interference
with microtubule dynamics.
Our retrosynthetic analysis for these target structures
(Scheme 1) was based on a prospective endgame involv-
ing amide bond formation between 10 and 11 followed by
macrolactonization and deprotection. Carboxylic acid 10
would be the result of a Pd(0)-catalyzed coupling between
vinyl iodide 8 and the zincate derived from alkyl iodide 7.
Alkyl iodide 7 in turn would be derived from the aldol
product of aldehyde 27 and the propionyl Evans-oxazolid-
inone 3 (vide infra).
As for other syntheses of epothilones and their analogs,
one of the key challenges in the implementation of this
synthetic scheme was the generation of the C6-C7 bond in
a highly stereoselective manner. In order to establish this
bond in the most efficient way possible, we carried out an
extensive investigation of the reaction between aldehyde
2 and propionyl oxazolidinone 3 (Scheme 2, Table 1).
Figure 1 Structures of epothilones A (I, Z = H), B (I, Z = CH3), C
(II, Z = H), and D (II, Z = CH3) and of target structures 1.
Numerous syntheses of natural epothilones have been re-
ported and a large number of analogs have been prepared
for SAR studies.3 Out of these structures, only few are
characterized by the replacement of carbon atoms in the
macrocyclic skeleton by heteroatoms and those investi-
gated to date were found to be poorly active.4 Overall,
however, the potential of such modifications remains
largely unexplored, which is in spite of the fact that the re-
placement of carbon by hetero-atoms in complex struc-
tures could lead to improved synthetic accessibility and
offer the potential to generate large sets of diverse analogs
As illustrated by the data shown in Table 1, this study re-
sulted in the identification of a set of optimized reaction
conditions which provided the desired aldol product 4a in
90% isolated yield with none of the other isomers being
detectable in the crude product by TLC analysis. Com-
pound 4a was converted into its TBS ether 5 by treatment
with TBSOTf and 2,6-lutidine in 91% yield. Hydro-
genolytic removal of the benzyl group followed by stan-
dard functional group transformation then gave the
desired iodide 7 in 81% yield (Scheme 3).
SYNLETT 2004, No. 15, pp 2709–2712
0
7
.1
2
.2
0
0
4
Advanced online publication: 12.11.2004
DOI: 10.1055/s-2004-835665; Art ID: G32004ST
© Georg Thieme Verlag Stuttgart · New York

2710
F. Cachoux et al.
LETTER
Scheme 1 Retrosynthetic analysis for 4-aza epothilones 1.
been observed for other substrates, the excess of Lewis
acid employed in the aldol reaction between 2 and 3 does
not lead to preferential formation of the anti isomer with
respect to the substituents at C6 and C7.9
b-Amino acid esters 11b and 11d were prepared from
commercially available b-glycine methyl ester hydrochlo-
ride 11a and DL-3-aminobutyric acid, respectively, as out-
lined in Scheme 4 and Scheme 5. Racemic 11c
(Scheme 1, R = H; R¢ = CH3) was obtained through direct
acid-catalyzed esterification of DL-3-aminobutyric acid.
Scheme 2 Aldol reaction between aldehyde 2 and propionyl oxazo-
lidinone 3.
Scheme 4 Reaction conditions: (i) (CF3CO)2O, i-Pr2EtN, CH2Cl2,
0 °C, 1.5 h, 95%; (ii) K2CO3, MeI, DMF, 0 °C → r.t., 24 h, 91%; (iii)
NH4OH (2 M), MeOH, r.t., 4 h, 76%.
Scheme 3 Reaction conditions: (i) TBSOTf, 2,6-lutidine, 0 °C →
r.t., 4 h, 91%; (ii) H2, Pd/C (10%), MeOH, r.t., 6 h, 82%; (iii) a) Me-
sCl, Et3N, 0 °C, 30 min; b) NaI, acetone, 50 °C, 3 h, 81% (two steps).
The syn/anti stereochemistry about the C6-C7/C7-C8
bonds was unequivocally established by X-ray crystallo-
graphy at the stage of iodide 7.8 Thus, contrary to what has
Table 1 Variation of Reaction Conditions in the Aldol Reaction
between 2 and 3: Effects on Selectivity and Yield
Scheme 5 Reaction conditions: i) BOC2O, Et3N, H2O–dioxane, r.t.,
12 h, 50%; (ii) NaH, MeI, THF, 0 °C → r.t., 4 h; (iii) MeOH, DCC,
DMAP, 0 °C, 2 h, 68% (two steps); (iv) HCl–dioxane (4 N), r.t., 30
min, 96%.
Entry Reagents (equiv)a
Proce- Ratio
Yield
(%)
dureb
4a:4b
3
Bu2BOTf Et3N
Pd(0)-catalyzed coupling7,10,11 between the zincate de-
rived from alkyl iodide 7 and vinyl iodide 8 provided the
protected olefin 9 as a single double bond isomer in a good
71% yield (Scheme 6).
1
2
3
4
5
6
0.9
0.9
0.9
0.9
1.11
1.3
1.2
1.2
1.3
1.4
1.5
1.5
1.4
1.4
1.5
1.6
1.7
1.7
A
B
B
C
C
C
90:10
100:0
100:0
100:0
100:0
100:0
38/4
43
46
Vinyl iodide 8 was prepared by a modification of the lit-
erature procedure,6 which involves the use of the isolated
Wittig salt [CH3CH(I)PPh3]+I– in the olefination step,
rather than preparing this reagent in situ. Although this ap-
proach did not lead to significantly improved yields for
the olefination reaction, it proved to be highly advanta-
geous upon scale-up and gave more reproducible results.
The analogous TES-protected vinyl iodide 19 was ob-
tained from 8 via CSA-catalyzed TBS-removal and re-
protection of the free OH group by reaction with TESOTf.
60
78
90
a All reactions were performed with 1 equiv of aldehyde 2.
b Oxazolidinone 3, Bu2BOTf, Et3N, CH2Cl2, 0 °C, 1 h; then addition
of 2 at –78 °C and A, B, or C. Procedure A: –78 °C for 15 min, 0 °C
for 2 h; procedure B: –78 °C for 1 h, –20 °C for 1 h, 0 °C for 1 h;
procedure C: –78 °C for 3 h.
Synlett 2004, No. 15, 2709–2712 © Thieme Stuttgart · New York

LETTER
Synthesis of 4-Aza Analogs of Epothilone D
2711
supra). Coupling of 7 with 19 gave 20 (Scheme 6), which
was further transformed into the corresponding C15-
OTES derivative 21. Coupling of 21 with 11a and 11c
gave the corresponding amides, from which the TES-
group on C15-O could be removed selectively with an
equimolar mixture of TBAF and HOAc to give the desired
seco-esters 14a and 14c in 86% and 84% yield, respec-
tively.
Scheme 6 Reaction conditions: (i) a) 7, Zn-Cu, 1,2-dibromoethane,
TMSCl, DMA, TMSOTf; b) Pd(PPh3)4, 8 (19), benzene, 65 °C; 9:
85%, 20: 71%; (ii) LiOH, H2O2, THF–H2O 4:1, r.t., 3 h; 10: 78%;
21: 70%.
Figure 2 C7-O- and C15-O-deprotected seco-esters 13 and 14.
Cleavage of the chiral auxiliary with LiOOH (Scheme 6)
followed by HBTU-mediated coupling12 of the resulting
acid 10 with amino acid esters 11a–d gave the protected
seco esters 12a–d in good to excellent yields (Scheme 7).
Table 2 Selective Deprotection of Tris-TBS-Protected seco-Acid
Methyl Esters 12
Entry Ester TBAF Temp Product Deprotection Yield
(equiv) (°C)
(position C-O) (%)
Progression of the synthesis then required the selective re-
moval of the TBS protecting group from the allylic hy-
droxyl group at position C15 (Figure 2). Numerous
examples exist in the literature where this step has been
successfully carried out as part of the total synthesis of
natural epothilones or different analog structures.13 For
example, Nicolaou et al. have described the selective
cleavage of the C15-OTBS ether in tris-TBS protected
precursors of epothilone A or B by means of 6 equivalents
of TBAF at room temperature.13 Surprisingly, however,
treatment of 12a with three equivalents of TBAF at 0 °C
resulted in the selective cleavage of the C7-OTBS group
rather than the desired cleavage at C15-O. The same result
was obtained with 12c, whereas N-methylated derivatives
12b and 12d gave the desired C15 alcohols without prob-
lems (Figure 2, Table 2).
1
2
3
4
12a
12b
12c
12d
3
6
3
6
0
13a
14b
13c
14d
7
15
7
65
65
74
70
r.t.
0
r.t.
15
Completion of the syntheses through (i) ester saponifica-
tion, (ii) Yamaguchi macrolactonization (trichloro-
benzoyl chloride and DMAP), and (iii) TBS-removal
from C7-O with HF·pyridine was then straightforward
and gave the desired epothilone analogs 1 in 49–57%
yield from 14 (3 steps; Scheme 8).14
These observations may be rationalized by neighboring
group participation in the case of the secondary amide
groups (12a and 12c), which could involve initial transfer
of the TBS group to the amide oxygen (through a five-
membered transition state) and the formation of a labile
TBS-imidate. In order to resolve the selectivity problem
in this crucial deprotection step, the TBS group in 8 was
replaced by a more labile TES group to produce 19 (vide
Scheme 8 Reaction conditions: (i) LiOH, THF–H2O 7:1, r.t., 5 h,
64–90%; (ii) 2,4,6-Cl3PhCOCl, Et3N, THF, r.t., 30 min, then DMAP,
toluene, r.t., 3 h, 74–90%; (iii) HF·pyridine, MeCN, r.t., 6 h, 67–86%.
In the case of 16c and 16d isomers at C3 were readily sep-
arable by flash column chromatography. The individual
diastereoisomers were obtained in 25–30% yield (yields
in Scheme 8 refer to the combined yields for the pure
diastereoisomers) and were then deprotected separately.
Scheme 7 Reaction conditions: i-Pr2EtN, HBTU, DMF, r.t., 3 h;
12a: 83%; 12b: 78%; 12c: 94%; 12d: 72%.
Synlett 2004, No. 15, 2709–2712 © Thieme Stuttgart · New York

2712
F. Cachoux et al.
LETTER
(5) Carlomagno, T.; Blommers, M. J. J.; Meiler, J.; Jahnke, W.;
Schupp, T.; Petersen, F.; Schinzer, D.; Altmann, K.-H.;
Griesinger, C. Angew. Chem. Int. Ed. 2003, 42, 2511.
(6) The target structures are analogs of epothilone D (Figure 1),
which promotes tubulin polymerization to the same degree
as epothilone B. Although less active than epothilone B,
epothilone D is also a highly active antiproliferative agent,
which inhibits human cancer cell growth in vitro with low
nM IC50 values (cf. ref.3).
(7) Aldehyde 2 was obtained through Swern oxidation of the
corresponding primary alcohol: Schinzer, D.; Bauer, A.;
Schieber, J. Chem.–Eur. J. 1999, 9, 2492.
(8) Crystallographic data for 7 have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication number CCDC 234369.
(9) (a) Walker, M. A.; Heathcock, C. H. J. Org. Chem. 1991, 56,
5747. (b) Danda, H.; Hansen, M. M.; Heathcock, C. H. J.
Org. Chem. 1990, 55, 173.
(10) Altmann, K.-H.; Bold, G.; Caravatti, G.; Denni, D.;
Flörsheimer, A.; Schmidt, A.; Rihs, G.; Wartmann, M. Helv.
Chim. Acta 2002, 85, 4086.
(11) (a) Schinzer, D.; Bauer, A.; Schieber, J. Synlett 1998, 861.
(b) Altmann, K.-H.; Bold, G.; Caravatti, G.; End, N.;
Flörsheimer, A.; Guagnano, V.; O’Reilly, T.; Wartmann, M.
Chimia 2000, 54, 612.
(12) Knorr, R.; Trzeciak, A.; Bannworth, W.; Gillessen, D.
Tetrahedron Lett. 1989, 28, 1917.
(13) (a) Nicolaou, K. C.; Vourloumis, D.; Vallberg, H.;
Roschangar, F.; Sarabia, F.; Ninkovic, S.; Yang, Z.; Trujillo,
J. I. J. Am. Chem. Soc. 1997, 119, 7960. (b) Nicolaou, K.
C.; Ninkovic, S.; Sarabia, F.; Vourloumis, D.; Vallberg, H.;
Finlay, M. R. V.; Yang, Z. J. Am. Chem. Soc. 1997, 119,
7974.
(14) As the first compound synthesized in this series, 1b was
obtained from 12b by removal of both TBS-groups with
HF·pyridine, ester saponification, and macrolactonization of
the fully deprotected seco-acid. This approach gave pure 1b
in only 13% isolated yield (for the cyclization step) after
purification by preparative HPLC. Although clearly
unsatisfactory, the amount of material obtained through this
process was sufficient for complete in vitro biological
profiling of 1b and the compound proved to possess only
low biological activity. In light of this result resynthesis of
1b from 14b was deemed insensible.
In conclusion, we have developed an efficient stereoselec-
tive route to a new class of aza analogs of epothilones,
which is based on a highly diastereoselective aldol reac-
tion as one of the key steps. Biological evaluation of com-
pounds 1 revealed a significant loss in biological potency
compared with epothilone D.15 However, only a limited
number of building blocks have been investigated as po-
tential C1-C4 replacements in this pilot study. The meth-
odology developed in the course of this work sets the
stage for a more comprehensive exploration of different
amino acids, which may yet result in the discovery of po-
tent analogs of epothilones. In addition, intermediates 10
and 21 should also be valuable building blocks for the
preparation of other types of epothilone analogs.
References
(1) Current address: ETH Zürich, Department of Chemistry and
Applied Biosciences, Institute of Pharmaceutical Sciences,
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(15) The results of the biological evaluations will be published
separately.
Synlett 2004, No. 15, 2709–2712 © Thieme Stuttgart · New York