π-Facial diastereoselectivity in the Diels-Alder reactions of substituted cyclohexadienes

Ogbomo, Sunny M. (1995) π-Facial diastereoselectivity in the Diels-Alder reactions of substituted cyclohexadienes. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Diels-Alder reaction of cis-cyclohexa-3,5-diene-1,2-diol derivatives 50, 51, 40a, 41a and 42a with diethyl azodicarboxylate (DEAD) gave very predominantly products 56, 58, 59, 60 and 62, which arose by addition to the oxygen functions. -- Similarly, cis-cyclohexa-3,5-diene-1,2-diol derivatives 50, 51, 40a, 41a, 42a, and benzene oxide-oxepin 48 reacted with 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) 109 to afford anti adducts 70, 68, 70, 71 and 69 exclusively, and the anti adduct 66 was the predominant product in the case of diene 50. Moreover, there was a reversal of selectivity when the cis-diol 49 reacted with PTAD, the predominant product 64 arose by addition syn to the oxygen function. The structures of the adducts were determined by nmr, and X-ray crystallography in the cases of 72, 67, and 64. While the anti selectivity was rationalized in terms of electronic effects, hydrogen bonding explained the syn selectivity obtained with diene 49. -- The acetonide 40a and its Diels-Alder reaction with dienophiles are described. The experiments indicated that the acetonide dimerized in a Diels-Alder manner affording two products in a ratio of 6:1. The major dimer was that in which addition occurred to the anti face of both reacting partners. -- The results of Diels-Alder reactions of acetonide 40a with PTAD, DEAD, dimethyl acethylenedicarboxylate (DMAD), tetracyanoethylene (TCNE) and ethyl propiolate (EP) afforded exclusively anti products 69, 59, 73, 81 and 86, respectively. When butenone, benzoquinone, maleimide and N-methylmaleimide were used, the predominant products were anti adducts, but selectivity was in favor of syn with vinylene carbonate and there was no selectivity when dimethyl maleate was used. Lone pair-lone pair i.e. electronic factor is invoked to explain these selectivities, an alternative rationalization is in terms of electron-donating and electron-withdrawing effects. -- A study of the influence of solvent on facial selectivity was carried out using the acetonide 40a with maleimide. The kinetic results indicated that there was significant facial selectivity when water or a polar solvent was used but selectivity was less in a non-polar solvent. A 1M solution of LiCI in 100 mL water gave higher selectivity than a 5M solution of LiCIO₄ in ether. However, selectivity was with LiCIO₄ in water. Solubility and concentration phenomena are used to rationalize these selectivities.

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