Synthesis and Diels-Alder reactions of carbohydrate-derived dienes and their carbocyclic analogs The effects of allylic and remote substituents on facial selectivity

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Description: The Diels-Alder reactions of two carbohydrate-derived dienes and their carbocyclic analogs have been examined. The objectives of this study were: (1) to determine the diastereofacial selectivity of Diels-Alder reactions of these dienes and (2) to determine the relationship between the substituents on the dienes and the degree of diastereofacial selectivity. The dieno-pyranosides (4R)-3,4-dihydro-4-methoxy-6-ethenyl-2H-pyran (53) and (2S)-3,4-dihydro-2-methoxy-6-ethenyl-2H-pyran (54) were both synthesized from tri-O-acetyl- scD-glucal in 10 and eight steps, respectively. The addition of a hindered amine base to the Diels-Alder reactions of the pyranosidic dienes was found to be vital for the formation of good yields of unrearranged cycloaddition reaction products. The dominant trend for anti addition in the Diels-Alder reaction is attributed to a combination of steric and electronic effects that influence the transition state. The carbohydrate-derived dienes 53 and 54 exhibited a greater diastereofacial selectivity than their carbocyclic analogs 55 and 56 in cycloadditions reactions with cyclic as well as acyclic dienophiles. The cycloaddition of a linear dienophiles to the dienes studied was found to proceed with decreased anti diastereofacial selectivity as compared to a cyclic dienophile. The Diels-Alder reactions of dienes 53-56 with dienophile, N-phenylmaleimide proceeded at a faster rate in the presence of 5 M ethereal lithium perchlorate as compared with the uncatalyzed reaction. A marked increase in the formation of the syn addition was observed in all reactions conducted and the ratio of anti to syn cycloadducts varied with the concentration of lithium perchlorate, the reaction medium and the reaction temperature. (Abstract shortened by UMI.)
Language: English
Format: Degree Work