Enantiomerically Pure 7-Oxabicyclo[2.2.1]Hept-5-En-2-Yl Derivatives (Naked Sugars) as Synthetic Intermediates .12. Highly Stereoselective Total Syntheses of Octoses and Derivatives

Mukaiyama cross aldolizations of (R)-2,3-O-isopropylideneglyceraldehyde (10) with (1R,4S,5R,6R)-5-exo, 6-exo-(isopropylidenedioxy)-7-oxabicyclo[2.2.1]heptan-2-one ((+)-8) were highly diastereoselective and led to the corresponding u,u,l or SYNCAT ((+)-11) and u,u,u or ANCAT ((-)-21) aldols, respectively. The results were interpreted in terms of extended open transition state models with (ul,lk) and (ul,ul) topicities, respectively, which minimize steric repulsions. Aldols (+)-11 and (-)-21 were converted into (tert-butyl)dimethylsilyl 6-O-acetyl-2,3:7, 8-di-O-isopropylidene-D-glycero-L-talo-alpha-octofuranosid-5-ulose ((-)-18) and its D-talo diastereomer ((+)-28), respectively. Reduction of (-)-18 with LiEt3BH in THF gave, after deprotection, the known D-threo-L-talo-octose ((-)-4). Reduction of (-)-18 with (i-Bu)2AlH/THF gave, after deprotection, the unknown D-threo-D-allo-octose ((+)-5) with high stereoselectivity. Similarly, the unknown D-erythro-D-talo-octose ((+)-6) and D-erythro-L-allo-octose ((-)-7) were derived from (+)-28 through reduction with LiB(s-Bu)3H and (i-Bu)2AlH, respectively.