Temperature-Controlled Regioselectivity in the Reductive Cleavage of <i>p</i>-Methoxybenzylidene Acetals
作者:Jesús M. Hernández-Torres、Jihane Achkar、Alexander Wei
DOI:10.1021/jo048999m
日期:2004.10.1
functional groups, including allyl and enol ethers. The presence of water does not interfere with reductive ring opening and may contribute toward in situ generation of H+ as a catalyst for 6-O-PMB ether formation. Reductive cleavage under rigorously aprotic conditions is greatly decelerated, and yields only the 4-O-PMB ether. The temperature-dependent reductive cleavage of the 4,6-acetal can be described in
可以通过调节反应温度和试剂浓度来调节吡咯型4,6-对-甲氧基亚苄基乙缩醛与BH 3 / Bu 2 BOTf在THF中的区域选择性开环。在0还原裂解℃下导致的专属形成4- ø - p甲氧基苄基(PMB)醚,而反应在-78℃下产生的6- ö -PMB醚以高收率。观察到后一种条件与包括烯丙基醚和烯醇醚在内的各种酸敏感性官能团相容。水的存在不会干扰还原性开环,并且可能有助于原位生成H +作为6- O的催化剂-PMB醚的形成。在严格非质子条件下的还原裂解被大大减速,并且仅产生4- O- PMB醚。4,6-乙缩醛的温度依赖性还原裂解可通过动力学与热力学控制来描述:在较高温度下,更易接近的O-6的路易斯酸配位是有利的,而更碱性但空间受限的质子化O-4在低温下占主导地位。