transfer then affords intermediate 169 in which both the carbonyl and the hydroxy
groups have been activated, leading to the “electrostatically driven” macrolactonization product (Scheme 48).
The most frequently used reagents for thioester formation are depicted in Fig. 3
[100, 103–106]. These variants were developed to increase the yields and/or to
perform the cyclization under milder conditions. For the same reasons metal ions
(Ag, Cu or Hg) were used as chelating agents between the sulfur atom and the
alcoholate in transition state 169. The Gerlach and Thalmann modification introduced in 1974 is probably the most popular one [107]. It uses silver salts (AgClO 4 ,
AgBF 4 , AgOTf) and allows reactions to be carried out at ambient temperature.
14.2 12-Membered Macrocyclic Lactones
14.2.1 Enterobactin
Using the Corey–Brunelle imidazolyl disulfide 172 [104] (Fig. 3), trimer 175 could
be cyclized in only 16 % yield, other methods failed or gave similar low yields. The
Scheme 47 Representative example of a 16-membered macrocycle using the Yamaguchi
protocol
Scheme 48 The Corey–Nicolaou method
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M. Cordes and M. Kalesse
groups have been activated, leading to the “electrostatically driven” macrolactonization product (Scheme 48).
The most frequently used reagents for thioester formation are depicted in Fig. 3
[100, 103–106]. These variants were developed to increase the yields and/or to
perform the cyclization under milder conditions. For the same reasons metal ions
(Ag, Cu or Hg) were used as chelating agents between the sulfur atom and the
alcoholate in transition state 169. The Gerlach and Thalmann modification introduced in 1974 is probably the most popular one [107]. It uses silver salts (AgClO 4 ,
AgBF 4 , AgOTf) and allows reactions to be carried out at ambient temperature.
14.2 12-Membered Macrocyclic Lactones
14.2.1 Enterobactin
Using the Corey–Brunelle imidazolyl disulfide 172 [104] (Fig. 3), trimer 175 could
be cyclized in only 16 % yield, other methods failed or gave similar low yields. The
Scheme 47 Representative example of a 16-membered macrocycle using the Yamaguchi
protocol
Scheme 48 The Corey–Nicolaou method
406
M. Cordes and M. Kalesse