Wipf Peter, Graham Thomas H
Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA. pwipf+@pitt.edu
J Org Chem. 2003 Nov 14;68(23):8798-807. doi: 10.1021/jo034813s.
The photoactivated W(CO)(6)/DABCO/THF system has been used for the formal endo-cyclization of alkynes to pyran rings. We found that the regioselectivity of ring closure depends on the relative configuration of the 3,5-dihydroxy-1-alkynes, as well as, more decisively, on the type of O-protective group. Oxygen substitution at the propargylic carbon slows the rate of alkyne insertion and allows for dihydrofuran formation through exo-cyclization. In contrast, the use of bulky silyl ethers or carbon substituents leads to dihydropyrans through endo-cyclization. Substrates bearing leaving groups such as esters, phenols, or thiophenols at the propargylic site eliminate and thus represent a limitation to the cycloisomerization methodology. Propargyl vinyl ethers will rearrange to give dienals instead of glycals. 1,2-Wittig rearrangement products of dihydropyrans are readily prepared and converted to complex bicyclic building blocks for organic synthesis.
光活化的W(CO)₆/DABCO/THF体系已用于炔烃向吡喃环的形式内环化反应。我们发现，环化的区域选择性取决于3,5 - 二羟基 - 1 - 炔烃的相对构型，更关键的是，还取决于O - 保护基的类型。炔丙基碳上的氧取代减缓了炔烃插入的速率，并允许通过外环化形成二氢呋喃。相比之下，使用庞大的硅基醚或碳取代基会通过内环化生成二氢吡喃。在炔丙基位点带有酯、酚或硫酚等离去基团的底物会消除，因此是环异构化方法的一个限制。炔丙基乙烯基醚会重排生成二烯醛而不是缩水甘油醛。二氢吡喃的1,2 - 维蒂希重排产物很容易制备，并可转化为用于有机合成的复杂双环结构单元。
