IODOALKOXYLATION OF 1,5-ANHYDRO-2-DEOXY-HEX-1-ENITOLS (GLYCALS)

Exclusive trans-addition is observed in the iodoalkoxylation of the 6-membered cyclic enol ethers 3,4-di-O-acetyl-l-rhamnal (1), 3,4-di-O-acetyl-l-fucal (2), and related glycal derivatives. The main products from 1 and from 3,4,6-tri-O-acetyl-d-glucal (3) thus had the α-manno configuration. Similarly, α-talo products were obtained from 2 in 87-93% yield. The product distribution is not affected by the electronegativity of the 5-substituent. It is concluded that steric factors in the glycal and the nucleophile affect only the step of trans-diaxial opening of the intermediate iodonium ion. Enhanced yields of the desired trans-diaxial products were observed in reactions of glycals having the lyxo configuration when the reactions were conducted in tetrahydrofuran or methanol. © 1990.