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During the hetero Diels-Alder reaction (HAD) between 4-(4-methoxyphenyl)-1-phenyl-3- (trimethylsilyloxy)-2-aza-butadiene (2-ABDE) and hexanone, a unimolecular [2+2] cyclization of 2-ABDE forming a four-membered monocyclic β-lactam took place. The reaction parameters such as Lewis acid catalysts, temperature and microwave irradiation on the HDA and [2+2] reactions have been thoroughly studied. The results show that the competition between the [2+2] and [4+2] reactions is largely dependent of temperature. At low temperature as -78 ℃, the product of [4+2] reaction predominated, while at high temperature as 135 ℃, the product of [2+2] reaction predominated. Microwave irradiation improved the yield of the [2+2] reaction significantly. It was also found that the strength and the hardness of the Lewis acid affected the catalytic ability for the [2+2] reaction of 2-azabutadiene.
Racemic diastereoisomeric spiro-phosphonate derivatives 5~6 were obtained by tandem double Michael addition and intramolecular nucleophilic substitution of 3-bromo-5-methoxy-5H-furan-2-one and α-hydroxylbenzylphosphonate under a warm condition. The target compounds 5~6 were identified by the elemental analysis, IR, 1H NMR, 13C NMR and MS techniques. The stereochemistry and configuration of 5c and 6c were confirmed further by X-ray crystallography. This result could lead to a new strategy for the synthesis of some complicated compounds containing a phosphorus functional group.