Total synthesis of (+)-leucascandrolide A

Article abstract of DOI:10.1002/anie.200462408

A convergent and enantioselective total synthesis of (+)-leucascandrolide (1) was accomplished in 17 steps. Central to this synthesis is the rapid and efficient integration of the bispyran moiety into 1 using two consecutive [4 + 2] annulation reactions b

Full text of DOI:10.1002/anie.200462408

Angewandte
Chemie
Natural Product Synthesis
Total Synthesis of (+)-Leucascandrolide A**
Qibin Su and James S. Panek*
Leucascandrolide A (1) is a bioactive metabolite isolated by
Pietra and coworkers from the calcareous sponge Leucascan-
dra caveolata, found off the east coast of New Caledonia in
the Coral sea.[1] Two-dimensional NMR experiments were
used to determine the relative configuration of 1, while its
absolute configuration was assigned by employing a Mosher
analysis at the C5-hydroxy group. This natural product
possesses an 18-membered macrolide ring that includes two
trisubstituted tetrahydropyran rings, and an unsaturated
oxazole-containing side chain. Leucascandrolide A (1) dis-
plays significant in vitro cytotoxicity against human KB and
P388 tumor cell lines with low IC50 values (0.05 and
0.26 mgmLÀ1, respectively) as well as strong inhibition of
Candida albicans, a pathogenic yeast. Recent reports[2]
indicate that 1 is no longer available from its original natural
source. It has been postulated that 1 is not a metabolite of
Leucascandra caveolata, but rather of an opportunistic
bacteria that colonized the sponge, as evidenced by the
large amounts of dead tissue in the initial harvest of the
marine organism. As a consequence, all known sources of the
natural product have been depleted.[2] This fact, the potent
bioactivity, and the unique structure of 1 have led to much
attention among the synthetic community. Following the first
total synthesis by Leighton et al.,[3] there have been additional
reports detailing total,[4] formal,[5] and fragment syntheses.[6]
Herein, we describe an enantioselective total synthesis of
(+)-leucascandrolide A (1). This synthesis is highlighted by
the rapid and efficient construction of the bispyran 4 which
contains a cis- and a trans-2,6-disubstituted tetrahydropyran
ring. These rings were assembled in two [4+2] annulation
reactions between aldehydes 7 and 6 and our newly intro-
duced chiral allylsilane 8a and crotylsilane 5,[7] respectively.
Our retrosynthetic analysis is illustrated in Scheme 1. Dis-
Scheme 1. Retrosynthesis of 1; SO2Mes=2-mesitylenesulfonate.
that the allylic alcohol could be obtained from the addition of
an alkenyl zinc species to aldehyde 4.
Recently, we have described a highly diastereomerically
and enantiomerically controlled [4+2] annulation between
aldehydes and syn allylsilane 8b, which produces trans-2,6-
disubstituted dihydropyrans (Table 1).[8] Our attempt to
Table 1: Dihydropyran synthesis from aldehydes and allylsilanes 8.
Entry
R
Silane
Product, yield [%][a] d.r. (trans:cis)[b]
1
iPr
8a, X=SO2Mes 9a, 90
1:25
2
PhCH2 8a, X=SO2Mes 9b, 91
1:8
3
C4H9
iPr
8a, X=SO2Mes 9c, 85
1:16
4[8]
5[8]
6[8]
8b, X=Me
9d, 91
9e, 91
9 f, 95
>30:1
>30:1
10:1
À
connection at the C5 ester bond reveals a macrolactone
PhCH2 8b, X=Me
c-Hex 8b, X=Me
containing two pyran rings (2) and an oxazole-containing side
chain 3. Upon further analysis of the macrolide, we envisaged
[a] Yields are based on pure materials isolated after chromatography on
SiO2. [b] The configuration of the pyran products was assigned by NOE
measurements. The product ratio was determined by 1H NMR spectros-
copy (400 MHz).
[*] Q. Su, Prof. Dr. J. S. Panek
Department of Chemistry and
Center for Chemical Methodology and Library Development
Boston University
590 Commonwealth Ave., Boston, MA 02215 (USA)
Fax: (+1)617-353-6466
rationalize such a stereochemical outcome of the annulation
process is depicted in Scheme 2. When allylsilane 8b reacts
with the aldehyde, we suggested that stabilization of the
oxocarbenium cation by the neighboring electron-rich methyl
ether favored a twist-boat intermediate thus accelerating the
formation of the trans-2,6-dihydropyran product (route A in
Scheme 2). If this were the case, the complementary cis-2,6-
dihydropyran adducts could also be obtained from a syn
allylsilane if the ring formation process occurred predom-
inantly through a chair-like transition state. We predicted this
could be achieved by tuning the steric and stereoelectronic
E-mail: panek@chem.bu.edu
[**] Financial support for this research was obtained from the National
Institutes of Health (GM055740), Johnson & Johnson, Merck Co.,
Novartis, Pfizer, and GlaxoSmithKline. The authors are grateful to
Dr. Les A. Dakin and Neil F. Langille for helpful discussions on the
preparation of side chain 3, and to Dr. Julien Beignet for assistance
with the preparation of the manuscript.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200462408
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Communications
Scheme 3. Reagents and conditions: a) TfOH, CH2Cl2, À788C;
b) NaCN, DMF, 608C; c) mercury(ii) trifluoroacetate, THF/H2O, then
NaBH4 in NaOH (aq.); d) TBDPSCl, imidazole, DMF; e) BCl3, CH2Cl2,
À788C; f) PCC, CH2Cl2; g) 5, TMSOTf, CH2Cl2, À508C; h) H2, Pd/C.
TfOH=trifluoromethanesulfonic acid, DMF=dimethylformamide,
TBDPS=tert-butyldiphenylsilyl, PCC=pyridinium chlorochromate,
TMSOTf=trimethylsilyl trifluoromethanesulfonate. [a] Yield based on
recovered starting material.
Scheme 2. Possible transition states for the [4+2] annulation of alde-
hydes with 8.
properties of the X substituent in the starting allylsilane 8, for
example by using a) an electron-withdrawing group X to
minimize anchimeric assistance onto the oxocarbenium ion,
and b) a sterically demanding functional group X to maximize
destabilizing 1,2-diaxial interactions between X and the
allylsilane moiety in the twist-boat conformer (route B in
Scheme 2).
Accordingly, we evaluated the reactivity and selectivity of
allylsilane 8a bearing a mesitylsulfonate group in our [4+2]
annulation (Table 1). The desired cis-2,6-dihydropyran prod-
ucts were obtained in very good yields and with high levels of
diastereoselectivity (entries 1–3);[9] the annulation using
allylsilane 8b to produce trans-2,6-dihydropyrans (entries 4–
6)[8] shows the generality of the methodology for synthesizing
this class of heterocycles. Moreover, the reversal in the sense
of diastereoinduction, resulting from the subtle structural
differences between the two allylsilanes, is in accordance with
our proposed transition states (Scheme 2) and gives further
insight into a plausible mechanism of this interesting [4+2]
annulation.
aldehyde 17 in high yield (Scheme 4). Homologation of the
aldehyde group by a phosphorus-based olefination with
methoxymethylenetriphenylphosphine and subsequent mer-
cury acetate mediated hydrolysis of the resulting enol ether
furnished aldehyde 4 in 87% over two steps.[15] Finally,
addition of an alkenyl zinc species to 4 afforded allylic alcohol
18 in good yield, albeit with disappointing diastereoselectiv-
ity.[16] The diastereomers 18 were separated and the (17-R)
alcohol obtained in 53% yield.
We were then ready to exploit the accessibility of cis-2,6-
dihydropyrans in the synthesis of leucascandrolide A (1).
Gratifyingly, annulation between allylsilane 8a[10] and alde-
hyde 7[11] proceeded smoothly in the presence of TfOH to
afford the desired dihydropyran 10 in good yield and with
good diastereoselectivity (Scheme 3).[12] The presence of the
sulfonate in this product allowed an efficient one-carbon
homologation through SN2 displacement using NaCN to yield
nitrile 11.
Oxymercuration of the double bond of 11 installed the
C5-hydroxy group, as the single regio- and diastereomer 12,[13]
which was protected as the TBDPS ether 13. Subsequent
debenzylation with BCl3 furnished the primary alcohol 14,
which was oxidized to aldehyde 6 using PCC.[14] Next, the
crucial [4+2] annulation between 6 and crotylsilane 5 was
carried out with useful diastereoselectivity and in good yield
to produce dihydropyran 15,[7a] which was hydrogenated to
bispyran 16.
Scheme 4. Reagents and conditions. a) DIBAL-H, Et2O, À788C;
=
b) Ph3P CHOMe, THF, À788C to room temperature, then Hg(OAc)2,
THF/H2O; c) in situ synthesis of the zinc reagent: 4-methyl-1-pentyne,
CH2Cl2, [Cp2Zr(H)Cl], room temperature; then ZnMe2, À60 to 08C;
then reaction with 4, 08C; d) DIBAL-H, CH2Cl2, À788C, then HCl (aq.,
1n); e) PCC, CH2Cl2; f) TBAF, THF. DIBAL-H=diisobutylaluminum
hydride, TBAF=tetrabutylammonium fluoride, Si=TBDPS.
Reduction of the isopropyl ester of 16 in presence of the
nitrile group was conducted with complete chemoselectivity
using DIBAL-H (2.1 equiv.) in Et2O at À788C thus providing
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Chemie
[3] K. R. Hornberger, C. L. Hamblett, J. L. Leighton, J. Am. Chem.
Inspired by a spontaneous macroacetalization reported by
Kozmin et al.,[4a] we decided for a similar transformation of
nitrile 18. Therefore, careful addition of DIBAL-H to a
solution of 18 in CH2Cl2 at À788C followed by acidic
hydrolysis of the resulting imine furnished a transient
hydroxyaldehyde which spontaneously cyclized into the
desired macrolactol. Oxidation to the macrolactone 19 using
PCC followed by a TBAF-promoted deprotection of the C5-
TBDPS ether provided macrolide 2 in excellent yield,
establishing, at this stage, a formal total synthesis of
leucascandrolide A (1).
It has been reported that it was difficult to achieve a direct
acylation of the axially orientated C5-hydroxy group of 2.[4c]
We therefore turned our attention to the Mitsunobu reac-
tion[17] to install the side chain at this center. Inversion of the
configuration at C5 was achieved by a two-step oxidation–
reduction sequence in excellent yield and with excellent
selectivity (Scheme 5). Acid 3[6c] and macrolide 20 were then
united smoothly under Mitsunobu conditions to conclude the
total synthesis of leucascandrolide A (1); the physical and
spectroscopic properties of our compound were identical to
those reported for 1.[1,3]
Soc. 2000, 122, 12894.
[4] a) Y. Wang, J. Janic, S. A. Kozmin, J. Am. Chem. Soc. 2002, 124,
13670; b) A. Fettes, E. M. Carreira, Angew. Chem. 2002, 114,
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[5] a) D. J. Kopecky, S. D. Rychnovsky, J. Am. Chem. Soc. 2001, 123,
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[6] a) M. T. Crimmins, C. A. Carroll, B. W. King, Org. Lett. 2000, 2,
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6812; d) L. A. Dakin, J. S. Panek, Org. Lett. 2003, 5, 3995.
[7] a) H. Huang, J. S. Panek, J. Am. Chem. Soc. 2000, 122, 9836; for
some other examples of pyran syntheses through Prins-type
cyclization, see: b) S. D. Rychnovsky, S. Marumoto, J. J. Jaber,
Org. Lett. 2001, 3, 3815; c) I. E. Markꢁ, D. J. Bayston, Synthesis
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[8] Q. Su, J. S. Panek, J. Am. Chem. Soc. 2004, 126, 2425.
[9] Other electron-withdrawing groups such as acetate, pivaloate,
trifluoroacetate, benzoate, and methyl carbonate as X gave
predominantly the cis isomers, however with lower diastereose-
lectivities and in lower yields.
[10] For a high-yielding preparation of silane 8a, see Supporting
Information.
[11] Aldehyde 7 was prepared in high yield from the known alcohol;
see Supporting Information.
[12] Using a less bulky sulfonate group (p-toluenesulfonate) as X in 8
produced the corresponding cis pyran with lower diastereose-
lectivity (d.r. = 9:1).
[13] H. C. Brown, P. Geoghegan, Jr., J. Am. Chem. Soc. 1967, 89,
1522.
[14] E. J. Corey, J. W. Suggs, Tetrahedron Lett. 1975, 2647.
[15] A. Maercker, Org. React. 1965, 14, 270.
[16] P. Wipf, W. Xu, Tetrahedron Lett. 1994, 35, 5197.
[17] O. Mitsunobu, Synthesis 1981, 1.
Scheme 5. Reagents and conditions. a) Dess–Martin periodinane, pyri-
dine, CH2Cl2; b) NaBH4, MeOH, 08C; c) 3, PPh3, DIAD, THF/benzene.
DIAD=diisopropyl azodicarboxylate.
In summary, we accomplished a convergent and enantio-
selective total synthesis of (+)-leucascandrolide A (1) in 17
steps from available aldehyde 7 and allylsilane 8a. The
present synthesis features an efficient route to 4 using two
consecutive [4+2] annulation reactions between aldehydes
and our chiral allyl- and crotylsilanes for the rapid and
efficient integration of the bispyran moiety into 1. Thus, chiral
organosilane reagents were shown to be of salient utility for
synthesizing complex pyran-containing natural products.
Moreover, studies toward the completion of structural
analogues of 1 using this silane methodology are currently
in progress in our laboratory.
Received: October 22, 2004
Published online: January 17, 2005
Keywords: allylsilanes · annulation · antitumor agents ·
.
asymmetric synthesis · natural products
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