Acetylated dienophiles, diastereofacial

The preference for formation of exo products is again evident throughout. As for diastereofacial selectivty, the pattern shown with the acetylated dienophiles 5—8 was again evident, with favored re-face attack for those compounds (9 and 12) having the R configuration at the allylic center and si-face attack for those (10 and 11) having the S configuration at the allylic center. 

Gupta, R C, Raynor, C M, Stoodley, R J, Slawin, A M Z, Williams, D J, Asymmetric Diels-Alder reactions. Part 1. Diastereofacial reactivity of ( )-3-trimethylsilyloxybuta-l,3-dienyl 2,3,4,6-tetra-O-acetyl-(3-D-glucopyranoside towards cyclic dienophiles, J. Chem. Soc. Perkin. Trans 1, 1773-1785, 1988. 

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. 

However, with these isopropylidenated dienophiles, the diastereofacial selectivity was much higher than with the acetylated analogues, especially in the ribo and Ivxo isomers, a factor of importance in any proposed application in chiral synthesis. 

The observed high diastereofacial selectivity is attributable to conformational restriction at the allylic center in dienophile 20. The steric bulk of the sugar chain and the acetyl group effectively limit the C-3—C-4 rotamers to a single conformer, as shown in the following scheme. The diene attacks almost exclusively from the same side as the allylic oxygen atom fsi-face for the D-enonate 20), where the steric hindrance is lowest. 

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