New Developments in EnantioselectiveBrønsted Acid Catalysis
243
Fig. 12. Brønsted-acid catalyzed enantioselective Nazarov cyclization
enantioselective Nazarov reaction procedure (Table 11; Rueping et al.
2007c). In general it was possible to successfully transfer differently
substituted dienones to the corresponding cyclopentenones 32a–h in
good yields and with excellent enantioselectivities (86%–99% ee). As
shown in Table 11, the reaction is not only applicable to the alkyl-, arylsubstituted, but also to the dialkyl-substituted dienones.
The absolute configuration of the products was obtained from the
X-ray crystal structure analysis. The cis-products of compound 32e exhibits the (S)-configuration at both stereogenic centers. This is in agreement with a kinetic protonation through Brønsted acid 33.
While the newly developed Brønsted acid catalyzed Nazarov reaction primarily generates the cis-cyclopentenones, the asymmetric metal
catalyzed variations described so far often provide the trans-product
(Liang et al. 2003; Aggarwal and Belfield 2003; Liang and Trauner
2004). To demonstrate that a route to these isomers is also possible we
effectively isomerized the cyclopentenone cis-32a to the corresponding
cyclopentenone trans-32a without loss of enantiomeric purity
(Scheme 8).
243
Fig. 12. Brønsted-acid catalyzed enantioselective Nazarov cyclization
enantioselective Nazarov reaction procedure (Table 11; Rueping et al.
2007c). In general it was possible to successfully transfer differently
substituted dienones to the corresponding cyclopentenones 32a–h in
good yields and with excellent enantioselectivities (86%–99% ee). As
shown in Table 11, the reaction is not only applicable to the alkyl-, arylsubstituted, but also to the dialkyl-substituted dienones.
The absolute configuration of the products was obtained from the
X-ray crystal structure analysis. The cis-products of compound 32e exhibits the (S)-configuration at both stereogenic centers. This is in agreement with a kinetic protonation through Brønsted acid 33.
While the newly developed Brønsted acid catalyzed Nazarov reaction primarily generates the cis-cyclopentenones, the asymmetric metal
catalyzed variations described so far often provide the trans-product
(Liang et al. 2003; Aggarwal and Belfield 2003; Liang and Trauner
2004). To demonstrate that a route to these isomers is also possible we
effectively isomerized the cyclopentenone cis-32a to the corresponding
cyclopentenone trans-32a without loss of enantiomeric purity
(Scheme 8).