New electron deficient dienes and their normal and inverse electron demand Diels-Alder reactions

Pi, Zulan (1996) New electron deficient dienes and their normal and inverse electron demand Diels-Alder reactions. Masters thesis, Memorial University of Newfoundland.

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Abstract

1.3-Butadienes bearing electron donating groups at their 1 and 3 positions (e.g. Danishefsky's diene 50) are popular dienes for the normal Diels-Alder reaction due to their regiochemically predictable cycloaddition with a variety of electron deficient dienophiles and the multifunctional adducts they provide. However, their counterparts which bear electron withdrawing groups at their 1 and 3 positions, and their inverse electron demand Diels-Alder reactions are uncommon. Due to the instability of the few known parent dienes, e.g. dienes 56a-d and 59, the cycloalkane-annulated diene system 63d was chosen for initial study. -- The protected dienes (68, 70, 72) and the deprotected dienes (69, 71, 73) were prepared from the commercially available 2-cyclohexen-1-one in four and five steps, respectively. All these dienes were obtained as pure 2E isomers except 72, which was obtained as a mixture of 2E and 2Z isomers (72a and 72b). The synthetic methodology was also employed to prepare dienes such as 83-86, which feature a five-membered ring. -- The protected dienes 68 and 72 underwent cycloaddition with electron deficient dienophiles (TCNE, PTAD, DMAD, NPM, MA, NQ and BQ) to give endo adducts as the major products. The structures of the adducts 100,103,106,110, 113 and 114 were determined by X-ray crystal structure analysis whereas the structures of other adducts were assigned by analogy. Epimerization was observed in the reactions of 68 with TCNE and with DMAD. Treatment of the protected diene 68 with an electron rich dienophile, 1,1-diethoxyethylene, in refluxing toluene resulted in no reaction. -- The deprotected dienes 69, 71 and 73 participated in cycloaddition with the electron rich dienophiles ethyl vinyl ether, 1,1-diethoxyethylene, 1-ethoxy-1-(trimethylsilyloxy)ethylene and styrene. Nothing less than 100% regioselectivity was ever observed. The resulting adducts incorporated a variety of functionality and offer the potential to be elaborated in a number of ways. -- Dienes 69 and 71 reacted with ethyl vinyl ether with 100% regioselectivity to give the endo adducts 118a and 122. Epimerization of 118a to give 118c was observed during chromatography. -- The reactions of dienes 69 and 71 with 1.1-diethoxyethylene occurred with complete regioselectivity and in 81-86% yield. The structure of the adduct 123a was assigned by X-ray crystallography. Reaction of 73 and the same dienophile resulted the formation of a new diene 125 by the elimination ethanol from the initially formed adduct. -- Diene 69 underwent cycloaddition with styrene to provide the endo adduct 127 as the major product, as determined by X-ray crystallography. The minor product was assigned as the exo adduct 128 by comparing its nmr spectra with those of other adducts. -- Dienes 69 and 73 reacted with enamines to give the aromatized compounds 132 and 138 by elimination of an amine from the initially formed adducts and subsequent dehydrogenation. Both 69 and 79 underwent cycloaddition with PTAD to give crystalline adducts 141 and 143. Epimerization was observed during chromatographic purification of these adducts to give a mixture of them and their epimers 142 and 144. Reaction of 69 with NPM followed by chromatography resulted in one major product in 90% yield, but a conclusive structural assignment of the adduct has not been made.

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