INQUIMAE and Departamento de Química Inorgánica, Analítica y Química Física, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad de Buenos Aires, Argentina.
Photochem Photobiol. 2012 Jul-Aug;88(4):769-73. doi: 10.1111/j.1751-1097.2011.01054.x. Epub 2011 Dec 30.
The production of vitamin D3 is a pharmaceutically relevant process, producing high added-value products. Precursors are extracts from vegetal origin but bearing mainly an E geometry in the 5,6 double bond. The synthesis of vitamin D3 (5-E-α-calcidol) with the correct Z stereochemistry in the 5,6 double bond from the E isomer using anthracene and triethylamine (TEA) as the sensitizer system was studied from the kinetic and mechanistic point of view. The sensitized isomerization of E-calcidol by irradiation of anthracene takes place only in deoxygenated solution and yields the Z isomer in ca 5% yield in the photostationary state. When TEA is added to the system, the E-Z reaction is not inhibited by oxygen any more, the quantum yield of photoisomerization to the Z isomer grows linearly with the concentration of E-calcidol, while conversions higher than 95% to the Z isomer are reached in the photostationary state and E-Z quantum yields as high as 45 at [E-calcidol] = 25 mM are reached. If TEA is replaced by 1,4-diazabicyclo[2.2.2]octane, the reaction rate drops to one-third at the same amine concentration. The observations can be explained by a quantum chain reaction mechanism. The high conversion achieved eliminates the need of isomer separation.
维生素 D3 的生产是一个具有药学相关性的过程，可生产高附加值的产品。前体是从植物中提取的，但主要在 5,6 双键中具有 E 几何形状。从 E 异构体使用蒽和三乙胺 (TEA) 作为敏化剂体系合成维生素 D3（5-E-α-钙化醇），其 5,6 双键中具有正确的 Z 立体化学，从动力学和机理的角度进行了研究。蒽辐照下 E-钙化醇的敏化异构化仅在脱氧溶液中发生，在光稳定态下以约 5%的产率得到 Z 异构体。当向体系中添加 TEA 时，氧不再抑制 E-Z 反应，Z 异构体的光异构化量子产率随 E-钙化醇浓度线性增长，而在光稳定态下转化率高于 95%，达到 Z 异构体，E-Z 量子产率高达 45 在 [E-钙化醇] = 25 mM 时达到。如果用 1,4-二氮杂二环[2.2.2]辛烷代替 TEA，在相同的胺浓度下，反应速率下降至三分之一。这些观察结果可以用量子链反应机制来解释。高转化率消除了异构体分离的需要。
