RADICALLY INITIATED ASYMMETRIC CYCLIZATIONS AS MODEL REACTIONS FOR ASYMMETRIC CYCLOCOPOLYMERIZATIONS

The two divinyl compounds 3,4-O-cydohexylidene-D-mannitol 1,2;5,6-bis- O-[(4-vinylphenyl)boronate] (la) and 3,4-O-cyclohexylidene-D-mannitol 1,2;5,6-bis-O-[(4-vinylnaphthyl-1)boronate](lb) were cyclized by radic al 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 obtain ed in yields of around 60%. During this cyclization, two new stereogen ic centers are formed and four stereoisomers are therefore possible. T he composition could be determined quantitatively after removing the t emplate, 3,4-O-cyclohexylidene-D-mannitol, and deboronation, thus furn ishing 5-8. In this way the stereochemical course of the cyclization c ould be investigated in detail. The reaction is a good model for the s tereochemical course of the cyclocopolymerization of monomers la or Ib 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 la or Ib after removal, of the template.