New model compounds were investigated within our efforts to synthesize the natural product heliquinomycin. We could provide good evidence that electronic effects prevent the spiroketal formation of compounds such as 5 incorporating an isocoumarin moiety. As a consequence, precursors 16 and 26 were prepared, which do not exert the strong electron-withdrawing effect of the methoxycarbonyl group present in 5. Dihydroisocumarin derivative 10 and phthalide 25 were coupled by Heck reactions with enone 4 to provide the required precursors 16 and 26. After reductive debenzylation the resulting intermediates were treated with anhydrous hydrochloric acid in alcohols to give spiroketals 18-20 and 28, 29 in reasonable yields. The spiroketalizations occur under thermodynamic control affording products with the hydroxyl group in a trans-relationship with respect to the pyran oxygen atom. Attempts to oxidatively convert dihydroisocumarin 19 into an isocoumarin derivative failed. In one attempt a ring contraction to new spiroketal 24 was observed.
在合成天然产品
氦喹霉素的过程中,我们研究了新的模型化合物。我们可以提供确凿证据,证明电子效应阻止了含有异
香豆素分子的化合物(如 5)形成螺酮。因此,我们制备了前体 16 和 26,它们不会像 5 中的甲氧基羰基那样产生强烈的夺电子效应。二氢异
香豆素衍
生物 10 和
邻苯二甲酸盐 25 与烯酮 4 发生赫克反应,得到所需的前体 16 和 26。还原脱苄基化后,生成的中间体在醇中用无
水盐酸处理,得到螺酮 18-20 和 28、29,收率合理。螺酮化反应是在热力学控制下进行的,生成物中的羟基与
吡喃氧原子呈反式关系。将二氢
异鼠李素 19 氧化转化为异
香豆素衍
生物的尝试失败了。在一次尝试中,观察到环收缩成新的螺酮 24。