Dang Hai-Shan, Roberts Brian P, Tocher Derek A
Christopher Ingold Laboratories, Department of Chemistry, University College London, 20 Gordon Street, London, UK WC1H 0AJ.
Org Biomol Chem. 2003 Nov 21;1(22):4073-84. doi: 10.1039/b309060b.
The thiol-catalysed radical-chain redox rearrangement of cyclic benzylidene acetals derived from 1,2- and 1,3-diols of terpene origin has been investigated from both synthetic and mechanistic standpoints. The redox rearrangement was carried out either at ca. 70 degrees C (using Bu(t)ON=NOBu(t) as initiator) or at ca. 130 degrees C (using Bu(t)OOBu(t) as initiator) in the presence of triisopropylsilanethiol or methyl thioglycolate as catalyst; the silanethiol was usually more effective. This general reaction affords the benzoate ester of the monodeoxygenated diol, unless rearrangement of intermediate carbon-centred radicals takes place prior to final trapping by the thiol to give the product, in which case structurally rearranged esters are obtained. For the benzylidene acetals of 1,2-diols prepared by vicinal cis-dihydroxylation of 2-carene, alpha-pinene or beta-pinene, intermediate cyclopropylcarbinyl or cyclobutylcarbinyl radicals are involved and ring opening of these leads ultimately to unsaturated monocyclic benzoates. 1,2-Migration of the benzoate group in the intermediate beta-benzoyloxyalkyl radical sometimes also competes with thiol trapping during the redox rearrangement of benzylidene acetals derived from 1,2-diols. Redox rearrangement of the benzylidene acetal from carane-3,4-diol, obtained by cis-dihydroxylation of 3-carene, does not involve intermediate cyclopropylcarbinyl radicals and leads to benzoate ester in which the bicyclic carane skeleton is retained. The inefficient redox rearrangement of the relatively rigid benzylidene acetal from exo,exo-norbornane-2,3-diol is attributed to comparatively slow chain-propagating beta-scission of the intermediate 2-phenyl-1,3-dioxolan-2-yl radical, probably caused by the development of adverse angle strain in the transition state for this cleavage. Similar angle strain effects are thought to influence the regioselectivities of redox rearrangement of bicyclic [4.4.0]benzylidene acetals resulting from selected 1,3-diols, themselves prepared by reduction of aldol adducts derived from reactions of aldehydes with the kinetic lithium enolates obtained from menthone and from isomenthone.
从合成和机理的角度，对源自萜烯类1,2 - 二醇和1,3 - 二醇的环状亚苄基缩醛的硫醇催化自由基链式氧化还原重排反应进行了研究。该氧化还原重排反应在约70℃（使用叔丁基亚硝酸酯作为引发剂）或约130℃（使用二叔丁基过氧化物作为引发剂）下进行，以三异丙基硅烷硫醇或硫代乙醇酸甲酯作为催化剂；硅烷硫醇通常更有效。除非在硫醇最终捕获中间体碳中心自由基生成产物之前发生重排，否则该一般反应会生成单脱氧二醇的苯甲酸酯，在这种情况下会得到结构重排的酯。对于通过2 - 蒈烯、α-蒎烯或β-蒎烯的邻位顺式二羟基化制备的1,2 - 二醇的亚苄基缩醛，涉及中间体环丙基甲基或环丁基甲基自由基，这些自由基的开环最终导致不饱和单环苯甲酸酯。在源自1,2 - 二醇的亚苄基缩醛的氧化还原重排过程中，中间体β-苯甲酰氧基烷基自由基中苯甲酸酯基团的1,2 - 迁移有时也会与硫醇捕获竞争。通过3 - 蒈烯的顺式二羟基化得到的蒈烷 - 3,4 - 二醇的亚苄基缩醛的氧化还原重排不涉及中间体环丙基甲基自由基，并生成保留双环蒈烷骨架的苯甲酸酯。外型，外型 - 降冰片烷 - 2,3 - 二醇相对刚性的亚苄基缩醛的低效氧化还原重排归因于中间体2 - 苯基 - 1,3 - 二氧戊环 - 2 - 基自由基相对较慢的链增长β-断裂，这可能是由于该裂解过渡态中不利的角应变发展所致。类似的角应变效应被认为会影响由选定的1,3 - 二醇生成的双环[4.4.0]亚苄基缩醛的氧化还原重排的区域选择性，这些1,3 - 二醇本身是通过还原醛与从薄荷酮和异薄荷酮得到的动力学锂烯醇盐反应生成的羟醛加成物制备的。
