Dang Hai-Shan, Roberts Brian P, Sekhon Jasmeet, Smits Teika M
Christopher Ingold Laboratories, Department of Chemistry, University College London, 20 Gordon Street, London, UK WC1H 0AJ.
Org Biomol Chem. 2003 Apr 21;1(8):1330-41. doi: 10.1039/b212303g.
Five- or six-membered cyclic benzylidene acetals, derived from 1,2- or 1,3-diol functionality in carbohydrates, undergo an efficient thiol-catalysed radical-chain redox rearrangement resulting in deoxygenation at one of the diol termini and formation of a benzoate ester function at the other. The role of the thiol is to act as a protic polarity-reversal catalyst to promote the overall abstraction of the acetal hydrogen atom by a nucleophilic alkyl radical. The redox rearrangement is carried out in refluxing octane and/or chlorobenzene as solvent at ca. 130 degrees C and is initiated by thermal decomposition of di-tert-butyl peroxide (DTBP) or 2,2-bis(tert-butylperoxy)butane. The silanethiols (Bu(t)O)3SiSH and Pr(i)3SiSH (TIPST) are particularly efficient catalysts and the use of DTBP in conjunction with TIPST is generally the most effective and convenient combination. The reaction has been applied to the mono-deoxygenation of a variety of monosaccharides by way of 1,2-, 3,4- and 4,6-O-benzylidene pyranoses and a 5,6-O-benzylidene furanose. It has also been applied to bring about the dideoxygenation of mannose and of the disaccharide alpha,alpha-trehalose. The use of p-methoxybenzylidene acetals offers no great advantage and ethylene acetals do not undergo significant redox rearrangement under similar conditions. Functional group compatibility is good and tosylate, epoxide and ketone functions do not interfere; it is not necessary to protect free OH groups. Because of the different mechanisms of the ring-opening step (homolytic versus heterolytic), the regioselectivity of the redox rearrangement can differ usefully from that resulting from the Hanessian-Hullar (H.-H.) and Collins reactions for brominative ring opening of benzylidene acetals. When simple deoxygenation of a carbohydrate is desired, the one-pot redox rearrangement offers an advantage over H.-H./Collins-based procedures in that the reductive debromination step (which often involves the use of toxic tin hydrides) required by the latter methodology is avoided.
由碳水化合物中的1,2 - 或1,3 - 二醇官能团衍生而来的五元或六元环状亚苄基缩醛，会发生高效的硫醇催化自由基链氧化还原重排反应，导致二醇一端脱氧，另一端形成苯甲酸酯官能团。硫醇的作用是作为质子极性反转催化剂，促进亲核烷基自由基对缩醛氢原子的整体夺取。氧化还原重排反应在约130℃下于回流的辛烷和/或氯苯作为溶剂中进行，由二叔丁基过氧化物（DTBP）或2,2 - 双（叔丁基过氧）丁烷的热分解引发。硅烷硫醇（Bu(t)O）3SiSH和Pr(i)3SiSH（TIPST）是特别有效的催化剂，DTBP与TIPST结合使用通常是最有效且方便的组合。该反应已通过1,2 - 、3,4 - 和4,6 - O - 亚苄基吡喃糖以及5,6 - O - 亚苄基呋喃糖应用于多种单糖的单脱氧反应。它还被用于实现甘露糖和二糖α,α - 海藻糖的双脱氧反应。对甲氧基亚苄基缩醛的使用没有太大优势，并且乙烯缩醛在类似条件下不会发生显著的氧化还原重排。官能团兼容性良好，甲苯磺酸酯、环氧化物和酮官能团不产生干扰；无需保护游离的OH基团。由于开环步骤的机制不同（均裂与异裂），氧化还原重排的区域选择性可能与亚苄基缩醛溴化开环的Hanessian - Hullar（H.-H.）反应和Collins反应所产生的区域选择性有所不同。当需要对碳水化合物进行简单脱氧时，一锅法氧化还原重排相对于基于H.-H./Collins的方法具有优势，因为后者方法所需的还原脱溴步骤（通常涉及使用有毒的氢化锡）被避免了。
