Studies on the α-acetylation of δ-valerolactone and ε-caprolactone

Abstract

The synthetic approach to α-acetylated δ-valero- ( 7a ) and ε-caprolactone ( 7b ) is reported. While 7a was isolated in 21% yield from the respective iodoester 5a by an alkylation sequence involving transesterification and Finkelstein reaction, 7b was not obtained from 5b but the dimer 8 . Also transesterification and olefin ring closing metathesis (RCM) failed to prepare 7b . RCM resulted in the dimeric lactone 10 , showing that the formation of 14-membered rings is favored over that of seven-membered rings. A Mukaiyama-type Claisen reaction finally gave α-acetyl lactones 7a and 7b in practically useful quantity of about 10 g (62% yield): starting lactones 13a, b were converted to silylenolethers 14a, b , which were acetylated with acetic anhydride in presence of the Lewis acid catalyst TiCl 4. However, acetylation depends on the addition sequence of starting materials: if the mixture of Ac 2O/TiCl 4 is added to 14b , lactone 7b can be further converted to give bis-oxepanonyl ethanols 15a, b . Both compounds 15 were characterized by X-ray crystallography and NMR.