196
K. Zeitler
original substrate structure—to the formation of two major classes of
catalyst-bound reactive intermediates, i.e., homoenolates and enolates.
In the case of propargylic aldehyde substrates allenolates are proposed
to be the reactive species allowing for the stereoselective formation of
α,β-unsaturated esters (Scheme 8; Zeitler 2006).
Based on this covalent catalyst integration the use of enantiopure
catalysts potentially allows the generation of the corresponding chiral homoenolate and chiral enolate intermediates. In fact, both reactive
species are strongly related as a simple β-protonation (or any other electrophilic attack) of the homoenolate affords the corresponding enolate,
as shown by Bode and coworkers in the context of the enantioselective
hetero Diels–Alder reactions with either aza- or oxodienes (He et al.
2006a,b). Until now, however, in most cases of catalytically generated
homoenolates, after the electrophilic attack the subsequent tautomerization of the initial enolate intermediate affords an activated ester which
is then trapped by suitable nucleophiles. Depending on the nature of
the electrophile acyclic, i.e. simple esters (Chan and Scheidt 2005), or
cyclic products such as γ-lactones or γ-lactams can be formed (Burstein
and Glorius 2004; Sohn et al. 2004; He and Bode 2005). The reaction of
1,2-diketones yields spiro products (Nair et al. 2006a) whereas a formal
[3+3] cycloaddition with azomethines generates bicyclic pyridazinones
(Chan and Scheidt 2007).
Apart form the aforementioned highly enantioselective hetero-Diels–
Alder reactions, that proceed with very low catalyst loadings, the
catalytically accessible enolates have also been used for related intramolecular Michael reactions (Philips et al. 2007) and for the desymmetrization of 1,3-diketones yielding cyclopentenes via an intramolecular aldol reaction (Wadamoto et al. 2007). The formation of
cyclopentenes, however, presents a special case, so—depending on the
stereochemical nature of the enone substrates (s-cis or s-trans) and the
stereochemistry of the final products—two different mechanisms are
discussed in the literature. Whereas trans-cyclopentenes are proposed
to be available upon conjugate addition of a homoenolate to chalcones,
K. Zeitler
original substrate structure—to the formation of two major classes of
catalyst-bound reactive intermediates, i.e., homoenolates and enolates.
In the case of propargylic aldehyde substrates allenolates are proposed
to be the reactive species allowing for the stereoselective formation of
α,β-unsaturated esters (Scheme 8; Zeitler 2006).
Based on this covalent catalyst integration the use of enantiopure
catalysts potentially allows the generation of the corresponding chiral homoenolate and chiral enolate intermediates. In fact, both reactive
species are strongly related as a simple β-protonation (or any other electrophilic attack) of the homoenolate affords the corresponding enolate,
as shown by Bode and coworkers in the context of the enantioselective
hetero Diels–Alder reactions with either aza- or oxodienes (He et al.
2006a,b). Until now, however, in most cases of catalytically generated
homoenolates, after the electrophilic attack the subsequent tautomerization of the initial enolate intermediate affords an activated ester which
is then trapped by suitable nucleophiles. Depending on the nature of
the electrophile acyclic, i.e. simple esters (Chan and Scheidt 2005), or
cyclic products such as γ-lactones or γ-lactams can be formed (Burstein
and Glorius 2004; Sohn et al. 2004; He and Bode 2005). The reaction of
1,2-diketones yields spiro products (Nair et al. 2006a) whereas a formal
[3+3] cycloaddition with azomethines generates bicyclic pyridazinones
(Chan and Scheidt 2007).
Apart form the aforementioned highly enantioselective hetero-Diels–
Alder reactions, that proceed with very low catalyst loadings, the
catalytically accessible enolates have also been used for related intramolecular Michael reactions (Philips et al. 2007) and for the desymmetrization of 1,3-diketones yielding cyclopentenes via an intramolecular aldol reaction (Wadamoto et al. 2007). The formation of
cyclopentenes, however, presents a special case, so—depending on the
stereochemical nature of the enone substrates (s-cis or s-trans) and the
stereochemistry of the final products—two different mechanisms are
discussed in the literature. Whereas trans-cyclopentenes are proposed
to be available upon conjugate addition of a homoenolate to chalcones,