Intramolecular stereoselective protonation of aldehyde-derived enolates

Article abstract of DOI:10.1002/anie.201004619

Picking sides: Asymmetric protonation of the titled compounds poses a most significant challenge and has been addressed by taking advantage of internal protonation and subsequent hemiacetal formation to avoid epimerization (see scheme). The substrates employed in these transformations can be easily accessed through a sequence of vinylogous aldol reactions with subsequent conjugate reductions.

Full text of DOI:10.1002/anie.201004619

Angewandte
Chemie
DOI: 10.1002/anie.201004619
Stereoselective Protonation
Intramolecular Stereoselective Protonation of Aldehyde-Derived
Enolates
Anastasie Kena Diba, Claudia Noll, Michael Richter, Marc Timo Gieseler, and Markus Kalesse*
The stereoselective protonation of enolates derived from
aldehydes remains a challenging transformation and only a
limited number of examples for the enantioselective proto-
nation are reported.[1] Nevertheless, it has the potential to
significantly optimize total synthesis by avoiding extensive
protecting and functional group manipulations or changes in
oxidation states.[2] In the course of our synthetic endeavors
towards the natural product angiolam[3] we envisioned
establishing a-chiral centers by an intramolecular protonation
(Scheme 1). Enolate 3 required for this transformation should
the case of angiolam this asymmetric protonation can be
utilized twice for the construction of the southern hemisphere
as depicted in Scheme 1. At the outset of our synthesis we
investigated the stereochemical outcome of the copper
hydride addition to 4, the side-chain segment of angiolam
(Scheme 1).
Unsaturated aldehyde 4 was obtained by Lewis acid
catalyzed addition of 6 to 5. Subsequent cross-coupling and
addition of CuH led to reduction of the activated double bond
and generation of the corresponding enolate. This enolate was
then internally quenched through the secondary alcohol. The
observed selectivity is consistent with protonation via tran-
sition-state A in which the sterically demanding substituents
adopt the equatorial positions. The so-generated alkoxide
then forms hemiacetal 7 which prevents the chiral aldehyde
from epimerization and thus enhances the overall selectivity
observed for this transformation. For the determination of the
relative configuration Dess–Martin oxidation led to lactone 8
(Scheme 2). At this stage the stereochemistry was assigned by
comparison with known compounds[7] and through nOe
experiments. Careful examination of the NMR data indicated
that only one isomer was generated with selectivity higher
than 98% de.
Scheme 1. Retrosynthetic analysis of angiolam. TMS=trimethylsilyl.
be generated by the addition of Strykerꢀs reagent
([{(PPh3)CuH}6])[4] to unsaturated aldehyde 4. We proposed
that using a 1,4-addition to generate the enolate in combina-
tion with internal protonation would circumvent the problems
known for aldehyde-derived enolates such as homoaldol
couplings. Additionally, the a,b-unsaturated aldehyde can be
obtained conveniently by a vinylogous Mukaiyama aldol
reaction[5,6] and therefore both transformations provide an
efficient strategy for the assembly of polyketide segments. In
Scheme 2. Intramolecular protonation and oxidation.
Analysis of the selectivity was performed using alcohol 10
which was obtained from nonselective conjugate reduction of
aldehyde 9 (1:1 mixture of both diastereomers; Scheme 3).
Consequently, lactol 11 derived from the selective proto-
nation protocol was transformed into the TBS-protected
alcohol 12 (Scheme 4). With both isomers in our hands we
were able to determine the selectivity of the intramolecular
protonation through comparison of the NMR spectra of the
mixed samples using different ratios of 10 and 12 (see the
Supporting Information).
[*] A. Kena Diba, C. Noll, M. Richter, M. T. Gieseler, Prof. Dr. M. Kalesse
Centre for Biomolecular Drug Research (BMWZ)
Leibniz Universitꢀt Hannover
Schneiderberg 1B, 30167 Hannover (Germany)
Fax : (+49)511-7623011
E-mail: Markus.Kalesse@oci.uni-hannover.de
Prof. Dr. M. Kalesse
Leiter Medizinische Chemie
Helmholtz Zentrum fꢁr Infektionsforschung
Inhoffenstraße 7, 38124 Braunschweig (Germany)
To show the scope of the substrates that can be trans-
formed under these reaction conditions compounds 13–19,
derived from saturated, unsaturated, and aromatic aldehydes,
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004619.
Angew. Chem. Int. Ed. 2010, 49, 8367 –8369
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8367

Communications
equatorial position. However, the directing effect of 1,3-
relationship is less pronounced. In contrast, with two chiral
centers, as in the case of the matched anti-aldehyde 23, only
one isomer (25) was observed (Scheme 5).
The 1,4- and 1,3-relationships of chiral centers are
prominent features of polyketide natural products, as well
as lactones. We sought to extend this strategy and to perform
the lactonization subsequent to the protonation step. For this
we took advantage of the redox lactonization protocol using
N-heterocyclic carbene (NHC) ligands[8] (Scheme 6).[9] This
umpolung strategy should provide the enolate required for
protonation and additionally lead to the corresponding
lactones as shown for other substrates by Zeitler et al.[10]
This strategy exhibited an additional challenge since NHC
ligands are known to react only sluggishly with unsaturated
aldehydes that exhibit an alpha substituent which result in
unfavorable steric interactions. During the course of our
Scheme 3. Nonselective protonation.
Scheme 4. Alcohol 12 used for the determination of the enantioselec-
tivity. DIBALH=diisobutylaluminum hydride, Piv=pivaloyl, TBS=tert-
butyldimethylsilyl.
were generated using 1 equivalent of Strykerꢀs reagent
(Figure 1). It can be seen that the best selectivity was
observed for unsaturated and aromatic aldehydes.
Figure 1. Lactones generated by intramolecular protonation and
subsequent oxidation.
Scheme 6. Synthesis of lactones by intramolecular protonation and redox
cyclization. Mes=1,3,5-trimethylphenyl.
To evaluate the directing effects of methyl groups that are
in a 1,3-relationship with respect to each other, compound 20
was subjected to intramolecular protonation conditions
(Scheme 5). The obtained syn product is consistent with the
cyclic transition-state B in which the methyl group adopts an
investigations we screened a variety of different ligands
(Scheme 6) and reaction conditions, thereby identifying
immidazolium ligands of the type B and C to be the most
effective catalyst. Highest yields were observed when triazole
catalyst B and benzimidazole catalyst C were used in toluene
at 908C using DBU as the base (Scheme 7a). Surprisingly,
for other substrates subjected to these reaction conditions
only diastereomeric mixtures of both the syn and anti
isomers were observed (Scheme 7b). In the cases where
the reaction time was prolonged the syn isomer became
the prominent product. We rationalize this observation by
suggesting that an epimerization occurs at such high
temperatures in the presence of a base. The slight
preference for the syn product can be explained by small
differences in kinetic acidity for the H in the axial versus
the equatorial position.
Remarkably, the use of D and F provided only the
isomerized products in good yields, which is consistent
with the observations made by Bode and co-workers and
Scheme 5. Intramolecular protonation and oxidation. NMO=N- methyl-
morpholine-N-oxide, TPAP=tetrapropylammonium perruthenate.
8368
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Angewandte
Chemie
Keywords: asymmetric catalysis · organocatalysis ·
.
protonation · natural products · reduction
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selective protonation of aldehyde-derived enolates. The
major advantage is the fact that subsequent transformations
such as olefinations can proceed without the need for
additional functional group manipulations. In cases where
the enolate was generated using Strykerꢀs reagent excellent
selectivity for the anti isomer was observed. The advantage of
using NHC ligands was the direct access to lactones exhibiting
the opposite diastereoisomer, albeit with modest selectivity.
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course.
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Received: July 27, 2010
Published online: September 28, 2010
Angew. Chem. Int. Ed. 2010, 49, 8367 –8369
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