Exploitation of sugar ring flipping for a hinge-type tether assisting a [2 + 2] cycloaddition
作者:Hideya Yuasa、Masatoshi Nakatani、Hironobu Hashimoto
DOI:10.1039/b609115f
日期:——
Methyl 3-O-p-methoxybenzyl-β-D-xylopyranoside (2) was exploited as a novel hinge-type tether for the [2 + 2] cycloaddition of cinnamate. The major ring conformation occupied by the 2,4-dicinnamate derivative of 2 was 4C1, which extends two cinnamates along a diequatorial orientation. However, 3-O-deprotected dicinnamate 5, when in a non-polar solvent, favours the 1C4 conformation, which assists the approach of two cinnamates with the 1,3-diaxial scaffold. Photoirradiation of compound 5 at 313 nm in CHCl3 afforded the intramolecular cycloaddition of cinnamates to give methyl β-, δ-, and ξ-truxinates in a 86 : 8 : 6 ratio after transesterification with methanol. The regio- and stereoselectivities are comparable to those reported by others for tethered cinnamates. The per-deuterated dicinnamate derivative of 5 facilitated the conformation analyses of the pyranoside rings by 1H NMR, indicating that all the products of photoirradiation had 1C4-fixed pyranosides. Excellent β-selectivity was achieved when m-bromocinnamate was subjected to hinge-tethered [2 + 2] cycloaddition.
3-O 对甲氧基苄基-β-D-吡喃木糖苷甲酯(2)被用作肉桂酸盐[2 + 2]环加成反应的新型铰链型系链。2 的 2,4-二肉桂酸酯衍生物占据的主要环构象为 4C1,它沿着死角方向延伸出两个肉桂酸酯。然而,在非极性溶剂中,3-O-脱保护二肉桂酸酯 5 倾向于 1C4 构象,这有助于两个肉桂酸酯与 1,3 轴支架的接近。在 CHCl3 中于 313 纳米波长下对化合物 5 进行光照射,可使肉桂酸盐发生分子内环加成反应,在与甲醇发生酯交换反应后,可得到比例为 86 : 8 : 6 的 β-、δ- 和 ξ-三羟甲基化合物。其区域选择性和立体选择性与其他报告的系留肉桂酸酯相当。5 的过氚代双肉桂酸酯衍生物有助于通过 1H NMR 对吡喃苷环进行构象分析,表明所有光照射产物都具有 1C4 固定的吡喃苷。当 m-溴肉桂酸酯进行铰链系[2 + 2]环加成反应时,可获得极佳的 β 选择性。