SYNTHESIS AND THERMOLYSIS OF KETAL DERIVATIVES OF 3-HYDROXY-1,2-DIOXOLANES
SYNTHESIS AND THERMOLYSIS OF KETAL DERIVATIVES OF 3-HYDROXY-1,2-DIOXOLANES
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DOI：
10.1021/jo00101a030
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发表时间：
1994-11-04
影响因子：
3.6
通讯作者：
CHEN, YX
中科院分区：
文献类型：
--
作者：
BAUMSTARK, AL;VASQUEZ, PC;CHEN, YX
3-[(三甲基甲硅烷基)氧基]-3,4,4,5-四甲基-5-苯基-1,2-二氧戊环(2)、3-甲氧基-3,4,4,5-四-甲基-5-苯基-1,2-二氧戊环(3)和3-乙酰氧基-3,4,4,5-四甲基-5-苯基-1(4)由相应的碱性条件下的 3-羟基-1,2-二氧戊环 (1a)。 3-乙酰氧基-4,4-二甲基-3,5,5-三苯基-1,2-二氧戊环(5)也通过该方法合成。在酸性条件下，3-羟基-1,2-二氧戊环1a定量分解为苯酚和3,3-二甲基-2,4-戊二酮。这种竞争性降解取决于 5 位取代基的性质。 5 位甲基减缓了降解重排，而苯基则有利于降解重排。在酸性条件下，由适当的 1,2-二氧戊环前体和相应的醇合成 3-甲氧基-和 3-(烯丙氧基)-4,4,5,5-四甲基-3-苯基二氧戊环 (6, 7)。在 60 摄氏度下，衍生的 1,2-二氧戊环 2-7 被发现比相应的 3-羟基-1, 2-二氧戊环更稳定。测定了 1,2-二氧戊环 2-7 热解的一级速率常数。产品研究表明，2-5 的热分解产生成对的酮和衍生的羧酸。除了R基团迁移产物外，还观察到4的热解产生乙酰氧基迁移产物。6在60℃下在苯中热解，定量产生苯甲酸甲酯和频哪酮。 7 的热分解产生与6 类似的产物。没有发现7 中烯丙氧基的碳-碳双键内部捕获自由基的证据。热分解似乎以过氧键均裂作为速率决定步骤进行。随后中间体 1,5-氧双自由基的 β-断裂以及有趣的重排显示出对烷基与苯基迁移的高度偏好，解释了观察到的产物分布。结果表明，β-断裂/重排机制可能不是一致的，而是逐步产生 1,3-二自由基和羰基片段。
3-[(Trimethylsilyl)oxy]-3,4,4,5-tetramethyl-5-phenyl-1,2-dioxolane (2), 3-methoxy-3,4,4,5-tetra-methyl-5-phenyl-1,2-dioxolane (3), and 3-acetoxy-3,4,4,5-tetramethyl-5-phenyl-1 (4) were synthesized from the corresponding 3-hydroxy-1,2-dioxolane (1a) under basic conditions. 3-Acetoxy-4,4-dimethyl-3,5,5-triphenyl-1,2-dioxolane (5) was also synthesized via this approach. Under acidic conditions, 3-hydroxy-1,2-dioxolane la underwent quantitative decomposition to phenol and 3,3-dimethyl-2,4-pentanedione. This competing degradation was dependent on the nature of the substituents at position-5. Methyl groups at position-5 slowed the degradative rearrangement whereas phenyl groups favored it. 3-Methoxy- and 3-(allyloxy)-4,4,5,5-tetramethyl-3-pheny dioxolanes (6, 7) were synthesized under acidic conditions from the appropriate 1,2-dioxolane precursors and the corresponding alcohols. At 60 degrees C, derivatized 1,2-dioxolanes 2-7 were found to be more stable than the corresponding 3-hydroxy-1, 2-dioxolanes. The first order rate constants for the thermolysis of 1,2-dioxolanes 2-7 were determined. Product studies showed that thermolysis of 2-5 yielded pairs of ketones and derivatized carboxylic acids. In addition to R-group migration products, an acetoxy migration product was observed for the thermolysis of 4. Thermolysis of 6 at 60 degrees C in benzene yielded methyl benzoate and pinacolone, quantitatively. Thermolysis of 7 yielded products analogous to those for 6. No evidence for internal trapping of radicals by the carbon-carbon double bond of the allyloxy group in 7 was found. The thermolyis appeared to proceed with peroxy bond homolysis as the rate-determining step. Subsequent beta-scissions of the intermediate 1,5-oxygen diradical with interesting rearrangements that show a high preference for alkyl vs phenyl migration account for the observed product distributions. The results suggest that the beta-scission/ rearrangement mechanism may not be concerted but rather stepwise to yield 1,3-diradical and carbonyl fragments.