Photoisomerization of alfa calcidol by a sensitized quantum chain reaction

Photochem Photobiol. Jul-Aug 2012;88(4):769-73. doi: 10.1111/j.1751-1097.2011.01054.x. Epub 2011 Dec 30.

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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Anthracenes / chemistry*
  • Cholecalciferol / chemical synthesis*
  • Chromatography, High Pressure Liquid
  • Ethylamines / chemistry*
  • Isomerism
  • Kinetics
  • Models, Molecular
  • Oxygen / chemistry*
  • Photochemical Processes
  • Piperazines / chemistry*
  • Quantum Theory
  • Solutions
  • Spectrum Analysis
  • Thermodynamics
  • Ultraviolet Rays

Substances

  • Anthracenes
  • Ethylamines
  • Piperazines
  • Solutions
  • Cholecalciferol
  • anthracene
  • Oxygen
  • triethylamine
  • triethylenediamine