Arabinose-derived dienophile

A final example of the use of carbohydrate-derived dienophiles in Diels-Alder reactions involves the role of the carbohydrate as a chiral auxiliary. Shing and coworkers have described the Diels-Alder reaction of arabinose-derived dienophile 31 with butadiene under Lewis acid catalyzed conditions (Scheme 5). A mixture of cycloadducts 32 with R and S configurations at the cyclohexane ring stereocenter was obtained in 68% yield. The ratio was 73 27 in favor of the / -product when a benzyl glycoside was used lower selectivities were observed with the corresponding methyl glycoside (77). 

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... 

A very high degree of asymmetric induction was evident in the thermal reaction of the D-arabinose-derived cis-dienophile 20 with cyclopentadiene, and the crystalline adduct isolated in 95% yield was identified as methyl (5R,6S)-6-endo-(1,2,3,4-tetra-O-acetvl-D-arabino-tetritol-l-yl)bicyclo[2,2.1]hept-2-eno-5-endp-carboxylate (31), mp 103 by NMR spectroscopy and also by X-ray crystallography. Use of the same sequence starting from L-arabinose gave the enantiomer (32) of 31. 

The pentoses may be converted by the conventional sequence (5) of dithioacetal formation, acetylation, and demercaptalation into the respective aldehydo-oentose 2,3,4,5-tetraacetates, which afford by Wittig chain-extension (2) the desired dienophiles. Thus aldehydo-D-arabinose 2,3,4,5-tetraacetate reacted with Ph3PCHC02Me in boiling benzene to give 90% of the pure, crystalline E-unsaturated sugar derivative 6, and the corresponding enantiomer was likewise prepared from L-arabinose. 

Preparative Utility. Because of the ready availability of both enantiomers of arabinose, the reactions of the derived Cj dienophiles were evaluated in particular detail for potential preparative utility, with emphasis on the ability to separate by crystallization the major isomer from the reaction. Thus dienophile 6 reacted to give, by direct isolation, a 40% yield of pure, crystalline methyl (5Sf 6S)-6 endo-(1,2,3,4 tetra"Q"acetvl"D-arabino-tetritol-1-yl)bicyclo[2.2.l]hept-2-eno-5-sxo-carboxylate (22), mp 103.5—104.5 , [cr]n +73 , and likewise the L enantiomer of 6 gave methyl (5R,6E)-6-enfl2-(1,2,3,4-tetra-O-acetyl-L-arabino-tetritol-l-vl)bicyclo[2.2.11heDt-2-eno-5-exo-carboxylate (23). 

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