title: Photoisomerization of alfa calcidol by a sensitized quantum chain reaction.
abstract: 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.
authors: 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.
authors: Gastón A
authors: GA
authors: Estruch
authors: Pedro F
authors: PF
authors: Aramendía
mesh_terms: D000873:Anthracenes / Q000737:chemistry*; D002762:Cholecalciferol / Q000138:chemical synthesis*; D002851:Chromatography, High Pressure Liquid; D005021:Ethylamines / Q000737:chemistry*; D007536:Isomerism; D007700:Kinetics; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry*; D055668:Photochemical Processes; D010879:Piperazines / Q000737:chemistry*; D011789:Quantum Theory; D012996:Solutions; D013057:Spectrum Analysis; D013816:Thermodynamics; D014466:Ultraviolet Rays
chemical_list: D000873:Anthracenes; D005021:Ethylamines; D010879:Piperazines; D012996:Solutions; D002762:Cholecalciferol; C034020:anthracene; D010100:Oxygen; C016162:triethylamine; C007306:triethylenediamine