CONJUGATE ADDITION OF METHANOL TO ALPHA-ENONES - PHOTOCHEMISTRY AND STEREOCHEMICAL DETAILS
CONJUGATE ADDITION OF METHANOL TO ALPHA-ENONES - PHOTOCHEMISTRY AND STEREOCHEMICAL DETAILS
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DOI：
10.1021/jo00244a039
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发表时间：
1988-04-29
影响因子：
3.6
通讯作者：
BECKWITH, ALJ
中科院分区：
文献类型：
--
作者：
BENKO, Z;FRASERREID, B;BECKWITH, ALJ
先前的研究似乎表明，二苯甲酮引发的甲醇与碳水化合物衍生的烯酮 1 的光加成比类似的碳环烯酮更容易发生。因此，进行了机理研究（a）以确定光加成反应的机理细节，以及（b）比较碳水化合物和碳环底物的量子产率。有证据表明，唯一重要的光化学事件是夺氢（产生“CH2OH”），而向烯酮底物的能量转移不是一个因素。ESR光谱法监测了“CH2OH加成1”的过程，并且该光谱最好解释为分别涉及假椅和假船中间体26和27的平衡混合物的初始形成。确定了 1、5 和 11 的量子产率，以便发现 (a) 侧氧（1 vs 5）和 (b) 环氧（5 vs 11）的影响。后两者给出轴向和赤道光加合物，并且在实验误差内，它们的反应性没有可检测到的差异。然而，对于轴向加合物的形成，发现碳水化合物烯酮 1 比氧杂环己烷 5 或碳环 11 更具反应性。 方案 I 1972 年，Fraser-Reid 及其同事报道，碳水化合物衍生的烯酮 1 经历有效的二苯甲酮引发的甲醇光加成，得到羟甲基加合物 2（方案 I）。 2 在旨在扩大这一过程范围的后续研究中，3、4 提出了两个一般性观察结果。首先，RCH2OH、RCH (OR') 2 和 RCHO 类型的多种其他含氧化合物也可作为出色的添加物。
Previous studies seemed to indicatethat the benzophenone-initiated photoaddition of methanolto the car-bohydrate-derived-enone 1 occurred more readily than to comparable carbocyclic-enones. A mechanistic study was therefore undertaken (a) toestablish the mechanistic details of the photoaddition reaction and (b) to compare thequantum yields of the carbohydrate and carbocyclic substrates. Evidence is presented that shows that the only important photochemicalevent is hydrogen abstraction (to give" CH2OH) and that energy transfer to the enone substrate is not a factor. The course of addition of" CH2OH to 1 has been monitored by ESR spectroscopy, and the spectrum is best interpreted as involving the initial formation of an equilibrium mixture of the pseudochair and pseudoboat intermediates 26 and 27, respectively. The quantum yields were determined for 1, 5, and 11 in order to discover the effect of (a) the pendant oxygen (1 vs 5) and (b) the ring oxygen (5 vs 11). The latter two give both axial and equatorial photoadducts, and within experimentalerror, there was no detectable difference in their reactivities. However, for the axial adduct formation, the carbohydrate enone 1 was found to be more reactive than oxane 5 or carbocycle 11.Introduction Scheme I In 1972, Fraser-Reid and co-workers reported that the carbohydrate-derived-enone 1 undergoes efficient benzophenone-initiated photoaddition of methanol to give the hydroxymethyl adduct 2 (Scheme I). 2 In subsequent studies aimed at extending the scope of this process, 3, 4 two general observations were made. Firstly, a wide variety of other oxygenated compounds of the type RCH2OH, RCH (OR') 2, and RCHO also served as excellent addends.