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 (κO(1,5),κO(2,3) bis-chelate) and lyxopyranose (κO(1,4),κO(2,3) 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 κO(2,3),κO(4,5)-bonded and thus exhibited five-membered chelate rings. The bis-chelates of aldehydo-lyxose and -ribose were κO(2,4),κO(3,5)-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 (1)H, (13)C, and (29)Si NMR spectroscopic approach, which was supported by X-ray analyses on crystals of the bis-DTBS chelates of κO(1,2),κO(3,5)-bonded rac-xylofuranose, κO(1,5),κO(2,3)-bonded d-ribofuranose, and κO(2,4),κO(3,5)-bonded aldehydo-d-lyxose.