| Abstract
| - Optically active 2-alkoxy-2H-pyran-3(6H)-ones (4a−d) were synthesized in one step by the tin(IV)chloride-promoted glycosylation and rearrangement of the 2-acetoxy-3,4-di-O-acetyl-d-xylal (3)prepared from d-xylose (1). The absolute configuration of the new stereocenter at C-2 was determinedby chemical transformation of the dihydropyranones 4a and 4b into the known alkyl pentopyranosides (7a and 7b, respectively). Also, from 1H NMR experiments using a chiral ytterbium shiftreagent, the enantiomeric excesses for 4a (>86%) and 4b (>77%) were established. Enantiomericallypure 4c and 4d were obtained by reaction of 3 with chiral 2-octanol (R and S, respectively).Dihydropyranones 4a−d were employed as dienophiles in Diels−Alder cycloadditions with 2,3-dimethylbutadiene and butadiene. Under thermal conditions, only moderate yields (∼50%) ofcycloadducts 9a−c and 10a were respectively obtained with good diastereofacial selectivity (>80%).Optimized Lewis acid promoted cycloadditions led to 9a−d and 10a,c in higher yields (∼80%) andwith higher diastereoselectivities (>94%). The major products were formed by approach of the dienesfrom the less hindered face of the dihydropyranones, and the minor products (such as 11a) wereformed by addition from the opposite side. Furthermore, cycloadduct 9a was stable in an alkalinesolution, whereas 11a underwent epimerization under the same conditions.
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