Mal Kanchan, Das Supriya, Maiti Nakul C, Natarajan Ramalingam, Das Indrajit
†Chemistry Division, CSIR-Indian Institute of Chemical Biology, 4, Raja S. C. Mullick Road, Jadavpur, Kolkata 700 032, India.
J Org Chem. 2015 Mar 20;80(6):2972-88. doi: 10.1021/jo5024766. Epub 2015 Mar 9.
The potential of β,γ-unsaturated α-ketothioesters participating in hetero-Diels-Alder reaction has remained unexplored. We report herein the first study of a ZnI2-catalyzed highly diastereoselective inverse electron demand hetero-Diels-Alder reaction of β,γ-unsaturated α-ketothioesters with olefins to access highly substituted 3,4-dihydro-2H-pyrans. All the reactions proceed with cis-selectivity in moderate to excellent yields. Under similar reaction conditions, terminal alkynes undergo direct conjugate 1,4-addition to yield δ,ε-acetylenic α-ketothioesters. Furthermore, the utility of these cycloadducts has been demonstrated by an NBS-MeOH mediated stereospecific efficient access to fully substituted pyran rings. The product bromoethers undergo E2 elimination with DBU, resulting in substituted 3,6-dihydro-2H-pyrans. In addition, the thioester moiety of the products has been used for further transformations, such as amidations and Fukuyama coupling reactions.
β,γ-不饱和α-酮硫酯参与杂环狄尔斯-阿尔德反应的潜力尚未得到探索。我们在此报告了第一项关于ZnI₂催化的β,γ-不饱和α-酮硫酯与烯烃的高度非对映选择性逆电子需求杂环狄尔斯-阿尔德反应的研究，以获得高度取代的3,4-二氢-2H-吡喃。所有反应均以顺式选择性进行，产率中等至优异。在相似的反应条件下，末端炔烃进行直接共轭1,4-加成，生成δ,ε-炔基α-酮硫酯。此外，这些环加成产物的实用性已通过NBS-MeOH介导的立体定向有效合成完全取代的吡喃环得到证明。产物溴醚与DBU发生E2消除反应，生成取代的3,6-二氢-2H-吡喃。此外，产物的硫酯部分已用于进一步转化，如酰胺化反应和福山偶联反应。
