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S. Breitenlechner, P. Selig, T. Bach
ably stronger than complexation to the template (i.e. K dim >K ass ), complexation is always favoured over dimerisation enthalpically. High ee
values could thus be achieved even in reactions using substrates with
unfavourable dimerisation behaviour (Selig and Bach 2006). Although
the use of an excess of template may seem uneconomical at first, it is
important to note that templates (+)-12 and (–)-12, being photochemically unreactive themselves, can be recovered from the reaction mixture
generally in yields between 80% and 99% and used repeatedly.
2.2
[2+2]-Photocycloaddition Reactions
As the synthetically most useful and most frequently used photochemical reactions known, [2+2]-photocycloadditions were conducted enantioselectively in the presence of templates (+)-12 and (–)-12. 2(1H)Quinolones proved to be excellent substrates for this reaction, as they
possess a lactam motif for binding to the template and are well known
for their excellent suitability for both intra- and intermolecular [2+2]photocycloaddition reactions. Initially, 4-alkoxyquinolones were used
in both intramolecular (Bach et al. 2000b) and intermolecular reactions
(Bach and Bergmann 2000), giving enantiomeric excesses between 80%
and 98% ee (Bach et al. 2002a). More recently, templates (+)-12 and
(–)-12 proved to be of general applicability also for structurally more
complex quinolones such as 5 or 13. In all cases, the template had no
observable effect on yields and diastereomeric ratios when compared to
the corresponding racemic reactions. For example, the reaction depicted
in Scheme 2 provided a 78% yield of 6 with 93% ee when conducted in
the presence of 2.3 equivalents of (+)-12 (Brandes et al. 2004). A more
complex example of an intermolecular [2+2]-photocycloaddition using
the cyclic terpene tulipaline (14) resulted in the formation of a spirocyclic cyclobutane 15, which was further converted into the tetracyclic
lactam 16 (Scheme 5) (Selig and Bach 2006).
2.3
Other Cycloaddition Reactions
While the chiral complexing agents (+)-12 and (–)-12 proved to be generally suitable for a wide range of enantioselective [2+2]-photocycloaddition reactions on the c-bond of 2(1H)-quinolones, their applicabil-
S. Breitenlechner, P. Selig, T. Bach
ably stronger than complexation to the template (i.e. K dim >K ass ), complexation is always favoured over dimerisation enthalpically. High ee
values could thus be achieved even in reactions using substrates with
unfavourable dimerisation behaviour (Selig and Bach 2006). Although
the use of an excess of template may seem uneconomical at first, it is
important to note that templates (+)-12 and (–)-12, being photochemically unreactive themselves, can be recovered from the reaction mixture
generally in yields between 80% and 99% and used repeatedly.
2.2
[2+2]-Photocycloaddition Reactions
As the synthetically most useful and most frequently used photochemical reactions known, [2+2]-photocycloadditions were conducted enantioselectively in the presence of templates (+)-12 and (–)-12. 2(1H)Quinolones proved to be excellent substrates for this reaction, as they
possess a lactam motif for binding to the template and are well known
for their excellent suitability for both intra- and intermolecular [2+2]photocycloaddition reactions. Initially, 4-alkoxyquinolones were used
in both intramolecular (Bach et al. 2000b) and intermolecular reactions
(Bach and Bergmann 2000), giving enantiomeric excesses between 80%
and 98% ee (Bach et al. 2002a). More recently, templates (+)-12 and
(–)-12 proved to be of general applicability also for structurally more
complex quinolones such as 5 or 13. In all cases, the template had no
observable effect on yields and diastereomeric ratios when compared to
the corresponding racemic reactions. For example, the reaction depicted
in Scheme 2 provided a 78% yield of 6 with 93% ee when conducted in
the presence of 2.3 equivalents of (+)-12 (Brandes et al. 2004). A more
complex example of an intermolecular [2+2]-photocycloaddition using
the cyclic terpene tulipaline (14) resulted in the formation of a spirocyclic cyclobutane 15, which was further converted into the tetracyclic
lactam 16 (Scheme 5) (Selig and Bach 2006).
2.3
Other Cycloaddition Reactions
While the chiral complexing agents (+)-12 and (–)-12 proved to be generally suitable for a wide range of enantioselective [2+2]-photocycloaddition reactions on the c-bond of 2(1H)-quinolones, their applicabil-