Radically Initiated Asymmetric Cyclizations as Model Reactions for Asymmetric Cyclocopolymerizations

Abstract

The two divinyl compounds 3,4-O-cyclohexylidene-d-mannitol 1,2;5,6-bis-O-[(4-vinylphenyl)boronate] (1a) and 3,4-O-cyclohexylidene-d-mannitol 1,2;5,6-bis-O-[(4-vinylnaphthyl-1)boronate] (1b) were cyclized by radical initiation either with an excess of 2,2‘-azobis(isobutyronitrile) (AIBN) or with tributyltin hydride under radical-generating conditions. The cyclization products contained a 19-membered ring and were obtained in yields of around 60%. During this cyclization, two new stereogenic centers are formed and four stereoisomers are therefore possible. The composition could be determined quantitatively after removing the template, 3,4-O-cyclohexylidene-d-mannitol, and deboronation, thus furnishing 5−8. In this way the stereochemical course of the cyclization could be investigated in detail. The reaction is a good model for the stereochemical course of the cyclocopolymerization of monomers 1a or 1b with other comonomers such as methyl methacrylate. It was shown that with both monomers the first step shows low stereoselectivity, whereas the second step is highly stereoselective, which is the cause of the strong optical activity of the copolymers from 1a or 1b after removal of the template.