ChemInform Abstract: Stereocontrol in Diels-Alder Cycloaddition to Unsaturated Sugars: Reactivities of cis-Dienophiles with Cyclopentadiene

Cycloaddition of cyclopentadiene with a d -arabinose-derived cis -dienophile, methyl ( Z )-4,5,6,7-tetra- O -acetyl-2,32-dideoxy- d - arabino -hept-2-enonate ( 2 ), under thermal conditions gave essentially a single norbornene aduct, isolated crystaline in 81% yield and identified by NMR spectroscopy and X-ray crystallography as methyl ( 5R ,6S)-6- endo -(1,2,3,4-tetra- O -acetyl- d - arabino -tetritol-1-yl) bicyclo[2.2.1]-hept-2-ene-5- endo -carboxylate ( 3 ). The diene adds exclusively from the si -face of the dienophile and give only the endo product. The same sequence starting from l -arabinose gave the enantiomer ( 7 ) of 3 . In contrast, a related cis -dienophile ( 9 ) having a butenolide ring with cyclopentadiene from the opposite ( re ) face giving mainly the endo adduct (5S,6R)-6- endo -(2,3,4-tri- O -acetyl)- d - arabino -tetritol-1-yl) bicyclo[2.2.1]hept-2-ene-5- endo -carboxylic acid 1,4-lactone ( 10 ), isolated crystalline in 70% yield, whose structure was again established by NMR spectroscopy, and firmly consolidated by X-ray crystallography. The minor (11%) product was the exo (5S,6R)isomer 11 . A cis -enonate 14 , analogous to 2 but deoxygenated at the allylic position, showed negligible diastereofacial selectivity ans reacted with cyclopentadiene to give a mixture of all four possible adducts. A 6-membered ring dienophile 16 was also subjected to the same cycloaddition for comparison with the butenolide 9 ; it gave principally the two endo products 17 and 19 in 31 and 38% yields, respectively, accompanied by 12% of a mixture of the two exo products ( 18 and 20 ). The quantitative distribution of cycloaddition products as a function of dienophile stereochemistry is discussed. The high degree of asymmetric induction observed, especially with the readily accesible dienophiles 2 and 7 , providesa valuable route of access to enantiomerically pure tetra- C -substituted cycloalkanes.