Homologation of Protected Hexoses with Grignard C1 Reagents

Derivatives of three stereoisomeric hexodialdo-1,5-pyranosides were reacted with four Grignard C1 reagents: methoxymethyl-, allyloxymethyl-, benzyloxymethyl, and dimethylphenylsilylmethyl-magnesium chlorides. Two stereoisomeric heptoses were obtained in each case in a good yield. The methyl alloside-derived heptosides were accompanied by C-5 inverted products. The addition of Grignard reagents to aldehydes 5–8 has been discussed in terms of parallel α- or β-chelated and Felkin–Anh transition states. It has been found that the silyl Grignard reagent 12 exhibits a strong preference for the formation of heptose derivatives of l-configuration at C-6.