Enantioselective total synthesis and determination of the absolute configuration of the 4,6,8,10,16,18-hexamethyldocosane from Antitrogus parvulus

Article abstract of DOI:10.1002/anie.200501612

(Chemical Equation Presented) A copper-catalyzed sp³-sp ³ cross-coupling for fragment coupling in a total synthesis is demonstrated. The diastereo- and enantioselective total synthesis of the 4,6,8,10,16,18-hexamethyldocosanes highli

Full text of DOI:10.1002/anie.200501612

Angewandte
Chemie
Asymmetric Synthesis
amount of material isolated from the natural source has not
yet allowed the determination of their biological role.
Combined spectroscopic and synthetic efforts have elucidated
the relative anti configuration of the four methyl-bearing
stereocenters in the tetrad unit of 1 and the relative
syn configuration within the methyl diad region. However,
so far the stereochemical relation between the all-anti tetrad
and the syn diad remains unknown (1a or 1b), as does the
absolute configuration (Scheme 1).[2]
DOI: 10.1002/anie.200501612
Enantioselective Total Synthesis and
Determination of the Absolute Configuration of
the 4,6,8,10,16,18-Hexamethyldocosane from
Antitrogus parvulus**
We herein report on the total synthesis of both diaster-
eomers 1a and 1b in enantiomerically pure form, thus
enabling the determination of the relative and absolute
configurations of the natural product. The synthesis relies
heavily on our recently developed copper-mediated and
ortho-diphenylphosphanylbenzoyl (o-DPPB)-directed syn-
allylic substitution with Grignard reagents for iterative
deoxypropionate synthesis.[3] Additionally, we demonstrate
the power of copper-catalyzed sp3–sp3 cross coupling by
employing it for building-block construction and as the
fragment-coupling step in a convergent total synthesis.
Christian Herber and Bernhard Breit*
The larvae of the cane beetle Antitrogus parvulus (known as
cane grubs) are a source of major damage in sugar-cane crops
in Australia.[1] On searching for an environmentally benign
plant-protection strategy, recent research has focused on the
identification of sex pheromones of the cane beetle. As a
result hydrocarbons, such as 4,6,8,10,16-penta- and
4,6,8,10,16,18-hexamethyldocosanes (1), that feature an
unprecedented anti-anti-anti stereochemistry in the 4,6,8,10-
methyl tetrad have been discovered.[2] However, the small
Scheme 1. Retrosynthesis for 1a and 1b. PG=protecting group, LG=leaving group, M=metal.
[*] Dipl.-Chem. C. Herber, Prof. Dr. B. Breit
Our synthetic plan is outlined in Scheme 1. Fragment
Institut für Organische Chemie und Biochemie
Albert-Ludwigs-Universität Freiburg
Albertstrasse 21, 79104 Freiburg (Germany)
Fax: (+49)761-203-8715
coupling of the methyl tetrad and the methyl diad by
employing a catalytic sp3–sp3 cross-coupling reaction[4] was
envisioned as an attractive final step of the synthesis to allow
the flexible construction of both diastereomers 1a and 1b.
The corresponding tetradeoxypropionate building block
should be assembled by our iterative deoxypropionate
synthetic strategy employing enantiomerically pure allylic o-
DPPB building blocks 2 and 3 as propionate acetate and
propionate units, respectively.[3] The two optical antipodes of
the methyl diad building block should be readily accessible by
similar chemistry.
E-mail: bernhard.breit@organik.chemie.uni-freiburg.de
[**] This work was supported by the Fonds der Chemischen Industrie,
the Deutsche Forschungsgemeinschaft, and the Alfried Krupp
Award for young university teachers of the Krupp foundation (to
B.B.). We thank Y. Schmidt for preliminary experiments, C. Steinger
for technical assistance, and Novartis AG and BASF AG for
generous gifts of chemicals. We particularly thank Prof. W. Kitching
(University of Queensland, Brisbane, Australia) for providing a
sample of the natural product.
The synthesis began with construction of the tetrad
building block A (Scheme 2). Thus, iodide (+)-4 (available
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2005, 44, 5267 –5269
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standard functional-group manipulation led
to building blocks (À)-B and (+)-B.
The final fragment-coupling step
(Scheme 4) employing a copper-catalyzed
sp3–sp3 cross coupling of the Grignard
reagent derived from (À)/(+)-B with triflate
A required some optimization because of
problems that arose from the small scale of
the reaction (0.1 mmol). It was found that
generation of a Grignard reagent on this
small scale was successful in the presence of
an excess of dibromoethane. However, the
thus-formed excess of magnesium bromide
also reacted with the triflate electrophile to
yield the corresponding bromide as a by-
product (which unfortunately did not
undergo further cross coupling). This prob-
lem could be circumvented by the addition of
an ethereal solution of triflate A together
with 4 mol% of the catalyst [Li2CuCl4] to an
ethereal solution of the Grignard-reagent-
derived form of B. Excellent yields of both
diastereomers 1a and 1b were obtained
under these conditions. Purification of the
final product was achieved through kugelrohr distillation,
which, as an additional benefit, allowed the boiling points of
1a and 1b (2108C/20 mbar) to be determined.
Scheme 2. Synthesis of tetradeoxypropionate building block A. Im=imidazole, Tf=tri-
fluoromethanesulfonyl.
in three steps from the Roche ester)[5] was transformed into a
Grignard reagent and subjected to o-DPPB-directed syn-
allylic substitution with allylic o-DPPB ester (R)-(+)-3 in the
presence of 0.5 equivalents of copper bromide·dimethyl
sulfide to give the dideoxypropionate (À)-5 with a complete
1,3-chirality transfer.[3] Two separate iterations consisting of
three steps (alkene ozonolysis with a reductive workup
(NaBH4), transformation to the iodide, and directed syn-
allylic substitution with (S)-(À)-3 and (R)-(+)-2, respectively)
furnished the tetradeoxypropionate (À)-7 with all the carbon
atoms and stereocenters in place. Alkene hydrogenation and
reductive cleavage of the para-methoxybenzyl (PMB) ether
group occurred upon heterogenous catalytic hydrogenation.
The thus-derived building block A was stored as the alcohol
and activated prior to fragment coupling as the corresponding
triflate.
Synthesis of the dideoxypropionate building blocks B
commenced from chloride 8 (Scheme 3).[6] For the expansion
of the carbon skeleton, we chose a copper-catalyzed sp3–sp3
cross-coupling reaction with a three-carbon electrophile of
type 9 derived from the Roche ester.[7] Orientating experi-
ments with the corresponding bromide and iodide derivatives,
however, showed that significant chemoselectivity problems
occurred because of elimination and reduction processes.
These problems could be circumvented by employing the
triflates (+)-9 and (À)-9 at À208C. In these cases, clean cross
coupling in the presence of 4 mol% of [Li2CuCl4] was
observed. Exposure of the crude coupling product to meth-
anolic hydrogen chloride gave the primary alcohols (À)-10
and (+)-10 in 82% yield (two steps). The Mukaiyama redox
condensation furnished the corresponding iodides, which
were subjected to the above protocol for the directed syn-
allylic substitution with (R)-(+)-3 and (S)-(À)-3, respectively,
to furnish the dideoxypropionates (À)-11 and (+)-11 in
excellent yield and complete diastereoselectivity. Minimal,
Scheme 3. Synthesis of the enantiomeric dideoxypropionate building
blocks (À)-B/(+)-B. Bn=benzyl, TBS=tert-butyldimethylsilyl, NBS=
N-bromosuccinimide.
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Angewandte
Chemie
[5] Prepared analogously to (R)-3, see: T. J. Heckrodt,
J. Mulzer, Synthesis 2002, 1857 – 1866.
[6] E. Dimitriadis, R. A. Massy-Westropp, Aust. J.
Chem. 1984, 37, 619 – 627.
[7] G. F. Solberghe, I. E. Marko, Tetrahedron Lett.
2002, 43, 5061 – 5066.
[8] A comparison of the analytical data of the natural
material and synthetic compounds 1a and 1b can be
found in the Supporting Information.
Scheme 4. Fragment coupling through Cu-catalyzed sp3–sp3 cross coupling. Synthesis
of (+)-1a and (+)-1b.
To prove the identity of the natural and synthetic material
13C NMR spectra of 1a and 1b were recorded in the presence
of an internal glass-capillary tube containing a solution of the
natural product in CDCl3.[8] Comparison of these spectra
showed a perfect match for diastereomer 1b. Finally, compar-
ison of the optical-rotation studies of the natural material
([a]2D0 + 10.7 (c = 0.44, CHCl3)) and the synthetic product 1b
([a]2D0 + 8.5 (c = 1.21, CHCl3)) determined the absolute
configuration of the natural product as that depicted in
Scheme 4.
In conclusion, the enantioselective total synthesis of the
(4S,6R,8R,10S,16R,18S)-hexamethyldocosane from Antitro-
gus parvulus has been accomplished, thus enabling the
relative and the absolute configuration of the natural product
to be determined. The successful enantioselective synthesis of
1b (and its diastereomer 1a) highlights the synthetic power of
our recent methodology for deoxypropionate construction,
which relies on an o-DPPB-directed and copper-mediated
allylic substitution by Grignard reagents. Furthermore, the
synthetic utility of copper-catalyzed sp3–sp3 cross-coupling for
fragment coupling in a total synthesis has been demonstrated.
Received: May 11, 2005
Published online: July 20, 2005
À
Keywords: asymmetric synthesis · C C coupling · Grignard
reaction · organocopper reagents · total synthesis
.
[1] P. G. Allsopp, K. J. Chandler, Proc. Int. Soc. Sugar-Cane Technol.
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Cribb, P. G. Allsopp, C. J. Moore, W. Kitching, Org. Lett. 2003, 5,
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[4] For previous reports on copper-catalyzed sp3–sp3 cross-coupling
reactions, see: a) M. Tamura, J. Kochi, Synthesis 1971, 303 – 305;
b) G. Fouquet, M. Schlosser, Angew. Chem. 1974, 86, 82 – 83;
Angew. Chem. Int. Ed. Engl. 1974, 13, 701 – 702; c) H. Kotsuki, I.
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M. Jezequel, Tetrahedron 2000, 56, 2733 – 2737.
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