Premium
Photoisomerization of Alfa Calcidol by a Sensitized Quantum Chain Reaction †
Author(s) -
Estruch Gastón A.,
Aramendía Pedro F.
Publication year - 2011
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.2011.01054.x
Subject(s) - photostationary state , photoisomerization , chemistry , quantum yield , isomerization , double bond , yield (engineering) , photochemistry , anthracene , medicinal chemistry , organic chemistry , catalysis , fluorescence , thermodynamics , physics , quantum mechanics
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 m m 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.