Acyclic sugar dienophile

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. 

SCHEME 10.33 Acyclic sugar-derived dienophiles can undergo highly stereoselective cycloadditions with cyclopentadiene. 

Direct Wittig reaction of Ph PCHCOn,Me with the four unsubstituted D-aldopentoses followed by acetylation provides convenient preparative access to acyclic seven-carbon trans-2.3-unsaturated sugar derivatives. These products served as dienophiles for a detailed comparative study in Diels—Alder cycloaddition with cyclopentadiene. Related syntheses afforded analogous cis-dienophiles. Cycloaddition under uncatalyzed thermal conditions gave mixtures of the four possible stereoisomeric norbornene adducts. The endo, exo ratios, and diastereofacial selectivities of the adducts were determined by NMR spectroscopy and by chemical transformations, supplemented by selected X-ray crystallographic analyses. Different distributions of isomers were encountered when a Lewis acid was used to catalyze the cycloaddition. The reaction can be controlled to provide preparative access to selected isomers and thus constitutes a versatile method for chirality transfer from the precursor sugar to four new asymmetric centers in a carbocyclic framework. 

---