Cyclization precursors acyclic dienes

An example of a surprisingly facile and stereoselective formation of an eight-membered lactone from an acyclic precursor diene ester was observed during the total synthesis of the antitumor agent octalactin A (148) (Scheme 27) [81]. The dense substitution pattern in cyclization substrate 146 presumably imposes... 

Silicon has also been employed as a tether for directing ringclosing metathesis (RCM) reactions. The RCM precursors 38 were generated by attachment of chlorodimethylvinylsilane to alkenyl alcohols. An olefin metathesis reaction using Schrock s catalyst provided the cyclized products 39 in excellent yields (eq 18). Although Grubbs catalyst had been successfully employed in the case of allylsiloxy dienes, Schrock s catalyst resulted in better yields in the case of the vinylsiloxy dienes 39. Oxidative cleavage of the silicon tether was then used to provide acyclic hydroxyaldehydes. 

Mercury(II)-mediated cyclization of dienes allows access to bromine-containing natural products. Thus, in the synthesis of aplysistatin, the key step was the cyclization of an acyclic precursor to the required bromoperhydro[ 1 Jbenzoxepine ring system (eq 31). ... 

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