The α-substituted enecarbamates and enamides 5a-d, 5f and 6 were prepared in two steps from the corresponding ketone, N-benzylamine and an appropriate acylating agent (Boc2O, Ac2O). The [2+2] photocycloaddition reactions of benzaldehyde to the alkenes 5a-d which bear a primary or secondary alkyl substituent proceeded smoothly and gave the 3-aminooxetanes 8a-d in moderate to good yields (46-71 %). The α-phenyl-substituted enecarbamate 5f did not produce a photocycloaddition product presumably due to rapid energy transfer (triplet sensitization) from the photoexcited aldehyde. For less obvious reasons the tert-butyl-substituted enamide 6 did not react in the Paternò-Büchi reaction either. The 3-alkyl-3-aminooxetanes 8 were obtained as a mixture of cis- and trans-diastereoisomers. An increase in the steric bulk of the alkyl substituent R shifted the diastereomeric ratio (cis-8/trans-8) in the direction of the thermodynamically more stable cis-product (29:71 for R = CH3 up to 57:43 for R = cyclohexyl). The separated oxetane diastereoisomers cis-8a and trans-8a (R = CH3) underwent a smooth ring opening/cyclization reaction upon treatment with trifluoroacetic acid. Oxetane trans-8a yielded the oxazolidinones 9 and trans-10 (92 %), oxetane cis-8a gave exclusively the oxazolidinone cis-10 (54 %).
α-取代的邻炔
氨基甲酸酯和烯酰胺5a-d、5f和6是通过两步反应从相应的酮、N-
苄胺和适当的酰化试剂(Boc2O、Ac2O)制备的。
苯甲醛与具有一级或二级烷基取代基的烯烃5a-d的[2+2]光环加成反应顺利进行,得到3-
氨基
氟烯烃8a-d,产率中等到良好(46-71%)。α-苯基取代的邻炔
氨基甲酸酯5f未能生成光环加成产物,这可能是由于光激发的醛迅速发生能量转移(三重态敏化)。由于不太明显的原因,叔丁基取代的烯酰胺6在Paternò-Büchi反应中也未发生反应。所得到的3-烷基-3-
氨基
氟烯烃8是顺式和反式二叠体异构体的混合物。烷基取代基R的立体阻碍增强使得二叠体比率(cis-8/trans-8)向热力学更稳定的顺式产物移动(R =
CH3时为29:71,R = 环己基时为57:43)。分离得到的
氟烯烃二叠体cis-8a和trans-8a(R = )在
三氟乙酸处理下顺利发生开环/环化反应。
氟烯烃trans-8a产生了氧杂
二氢吡啶酮9和trans-10(92%),而
氟烯烃cis-8a则专一获得氧杂
二氢吡啶酮cis-10(54%)。