Recently, we reported a new method for stereospecific epimerizations of carbohydrates [I-4]. Thus, unprotected pyranosides or I-thio-pyranosides, having a 2,3-cisΒ° /3,4-transor 2,3-trans-/3,4-cis-arrangement of three contiguous OH groups (i.e., lyxose, arabinose, mannose, or galactose derivatives), react stereospecifically with carbonyl-active aldehydes or ketones in the presence of dicyclohexylcarbodiimide (DCC) us coreagent with inversion at the middle chiral carbon atom; Scheme I. By this route, a suitable monosaccharide moiety of disaccharides c:m also be epimerized selectively [4c].
The key steF of the reactions is the in situ formation of a cyclic imidocarbonic ester intermediate (or sometimes an isourea function) [i,2,4,5], which is attacked by a deprotonated neighbouring hemiacetai moiety in an intramolecular SN2-type reaction forming a cyclic acetal.
The structures of the inverted sugars were supported by NMR data [!-5]. Methyl 6-deoxy-3,4-O-hexafluoroisopropylidene-a-L-altropyranoside, prepared from methyl a'-L-rhamnoside-hexafluoroacetone-DCC [3], was investigated by X-ray crystallography [6]. We have now obtained suitable crystals of the endo-H configured chloral acetals 2-O-cyclohe x ylcarbamoyl-6-O-formyl-[( R )-3,4-O-(2,2,2-trichloroethylidene )]-a-D-altropyranosyl fluoride (1) [I] and methyl [(S).3,4-O-(2,2,2-.richloroethylidene)]-a-o-" Corresponding author. Dedicated to Professor Dr Heinz Delme on the occasion of his 60th birthday.