130
M. Christmann
Scheme 4. Trituration enrichment at a high eutectic ee and X-ray structures of
(±)-4a (left) and (–)-4a (right)
sition of scalemic and diastereomeric mixtures. In most cases, trituration is used to remove minor impurities (Myers et al. 1997; Srinivasan
et al. 2005), as in our case where we were just aiming separate lactone 10 from the carboxylic acid 4a. Interestingly, we found that when
a weakly enriched scalemic mixture of 4a is triturated in hot n-pentane,
(–)-4a is readily dissolved, while racemic 4a remains as a solid residue
(Scheme 4).
After having developed a reliable route to the enantiopure carboxylic
acid 4a, we next turned our attention to gain access to the enantiopure
decalin fragment. One of the most efficient approaches to this structural
motif is the intramolecular Diels–Alder reaction. The first systematic
studies in the racemic series were performed by Roush and co-workers
in 1981 (Roush and Hall 1981). Evans (Evans et al. 1984, 1988) and
Oppolzer (Oppolzer and Dupuis 1985) showed that chiral auxiliaries
in combination with strong Lewis acids allowed the generation of the
endo adducts with excellent diastereoselectivities (Scheme 5). Recently,
chiral Lewis acid catalysts were successfully used by Evans (Evans et
al. 1999) and Corey (Zhou et al. 2003).
M. Christmann
Scheme 4. Trituration enrichment at a high eutectic ee and X-ray structures of
(±)-4a (left) and (–)-4a (right)
sition of scalemic and diastereomeric mixtures. In most cases, trituration is used to remove minor impurities (Myers et al. 1997; Srinivasan
et al. 2005), as in our case where we were just aiming separate lactone 10 from the carboxylic acid 4a. Interestingly, we found that when
a weakly enriched scalemic mixture of 4a is triturated in hot n-pentane,
(–)-4a is readily dissolved, while racemic 4a remains as a solid residue
(Scheme 4).
After having developed a reliable route to the enantiopure carboxylic
acid 4a, we next turned our attention to gain access to the enantiopure
decalin fragment. One of the most efficient approaches to this structural
motif is the intramolecular Diels–Alder reaction. The first systematic
studies in the racemic series were performed by Roush and co-workers
in 1981 (Roush and Hall 1981). Evans (Evans et al. 1984, 1988) and
Oppolzer (Oppolzer and Dupuis 1985) showed that chiral auxiliaries
in combination with strong Lewis acids allowed the generation of the
endo adducts with excellent diastereoselectivities (Scheme 5). Recently,
chiral Lewis acid catalysts were successfully used by Evans (Evans et
al. 1999) and Corey (Zhou et al. 2003).