CYCLIZATION OF HYDROXYENOL ETHERS INTO SPIROACETALS - EVIDENCE FOR THE POSITION OF THE TRANSITION-STATE AND ITS IMPLICATION ON THE STEREOELECTRONIC EFFECTS IN ACETAL FORMATION

Acid-catalyzed cyclizations under thermodynamically and kinetically controlled conditions of the hydroxyenol ethers 18-21 are reported. Thermodynamically controlled cyclizations of 18, 19, and 21 produced only the more stable corresponding spiroacetals 22 and 27. Thermodynamically controlled cyclization of compound 20 produced a 1:1 mixture of non-epimerisable spiroacetals 24 and 26. On the other hand, kinetically controlled cyclizations of the same four hydroxyenol ethers produced, along with the more stable spiroacetals mentioned above, the less stable spiroacetals 23, 25, and 28. These results show that the kinetically controlled cyclization takes place via an early transition state which produces a mixture of the less stable and the more stable isomers. These results are explained by an early transition state taking into account the principle of stereoelectronic control while following the antiperiplanar lone-pair hypothesis (Burgi-Dunitz angle of attack of a nucleophile on a pi system).