Acyclic carbohydrate dienophiles

The carbohydrate dienophiles in the above examples are all derived from cyclic structures. Acyclic sugar dienophiles have also been studied. These substrates arc accessible through the reaction of a carbohydrate with a stabilized Witdg reagent or by fragmentation reactions. Two examples of the use of acyclic carbohydrate dienophiles in Diels-Alder reactions are presented below. 

Chiral 1-alkoxy-1,3-dienes derived from carbohydrates have been examined in cycloadditions with azo dienophiles by Stoodley et al. Thereby, reactions with acyclic azo dienophiles were found to be highly selective, leading to optically pure pyridazine derivatives. 

Horton et al. were interested in the synthesis of tetra-C-substituted carbocycles, and extensively studied asymmetric Diels-Alder reactions employing acyclic unsaturated carbohydrate derivatives. Thus, the thermal cycloaddition of the D-arabinose-derived c/i-dienophile (Z)-100 with cyclopentadiene gave endo-adduct 101 in excellent optical purity. The high diastereo-facial differentiation in this reaction arises from a highly favored conformation at the allylic center [76,77] (Scheme 10.33). Therefore, conformer 102 seems to be more favored than 103, where the latter suffers from severe allylic strain between the methoxycarbonyl and the C4-acetoxy group... 

The application of cyclic carbohydrate derivatives as chiral dienophiles has been investigated by several groups. 2,3-Dideoxy-DL-pent-2-enopyranose-4-ulose (17) in the thermal [4+2] cycloaddition to the acyclic dienes 18a-c is reported to produce mixtures of cycloadducts 19a-c epimeric on the hemiacetal center with an all-as-configuration of the other newly formed bonds. The reaction with cyclopentadiene (10) and 1,3-cyclohexadiene (11) yields adducts 20 as the sole products31. It should be emphasized that epimerization may occur on the anomeric center during the protection step (as an equilibrium process), thus leading to a mixture of adducts, while the endolexo ratio remains the same. 

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