Dibutylsilylene-pentose bis-chelates: on the glycoses' binding sites for strongly Lewis-acidic centres

Excess di(tert-butyl)silylene (DTBS) bis(trifluoromethanesulfonate) formed bis-DTBS derivatives with the four aldopentoses (arabinose, lyxose, ribose and xylose). The structure of the bis-chelates was affected by the bulk of the DTBS groups and the requirement of flat silacycles in the case of five-membered chelate rings. These restrictions resulted in unusual cyclic bis-chelates for ribofuranose (κO1,5,κO2,3 bis-chelate) and lyxopyranose (κO1,4,κO2,3 bis-chelate of a twisted boat conformation). Most importantly, all aldopentoses formed bis-chelates of their open-chain aldehydo isomers. The bis-chelates of aldehydo-arabinose and -xylose were κO2,3,κO4,5-bonded and thus exhibited five-membered chelate rings. The bis-chelates of aldehydo-lyxose and -ribose were κO2,4,κO3,5-bonded and resembled six-membered chelate rings. For lyxose, the aldehydo bis-chelate was isolated as a solid. The molecular structures were assigned by a combined 1H, 13C, and 29Si NMR spectroscopic approach, which was supported by X-ray analyses on crystals of the bis-DTBS chelates of κO1,2,κO3,5-bonded rac-xylofuranose, κO1,5,κO2,3-bonded d-ribofuranose, and κO2,4,κO3,5-bonded aldehydo-d-lyxose.