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-2,3-dideoxy-D-arabino-hept-2-enonate (2), under thermal conditions gave essentially a single norbornene adduct, isolated crystalline 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)bic yclo [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 reacts 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)bicycl o [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 and 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 accessible dienophiles 2 and 7, provides a valuable route of access to enantiomerically pure tetra-C-substituted cycloalkanes.
Horton
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Department of Chemistry, Ohio State University, Columbus 43210.
Koh
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D
Takagi
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Y
D002236:Carbohydrate Conformation; D018360:Crystallography, X-Ray; D003517:Cyclopentanes; D007202:Indicators and Reagents; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D015394:Molecular Structure; D009005:Monosaccharides / Q000138:chemical synthesis*; D009904:Optical Rotation; D013237:Stereoisomerism
D003517:Cyclopentanes; D007202:Indicators and Reagents; D009005:Monosaccharides; C004709:1,3-cyclopentadiene