Retinyl acetate isomerization

Some conversion into the anhydrovitamin (112) occurs during silica gel t.l.c. of retinyl palmitate in non-polar solvents. Some new colour reactions of vitamin A are reported to be better than the Carr-Price reaction. The kinetics and mechanism of acid-catalysed isomerization of retinyl acetate into the trans-retro-derivative (113) have been studied. Oppenauer oxidation of kitol (39) results in specific cyclopentanol-cyclopentanone oxidation. ... 

Electronic Absorption Spectroscopy. The absorption spectra of some polyenes in the crystalline state have been studied. On adsorption of certain gases on the crystallite surfaces, a new band appears on the low-energy side of the long-wavelength band in the spectra of these polyenes, e.g. at 536, 537, and 375 nm for all- trans-ft-carotene, 15-cis-/3-carotene, and retinyl compounds, respectively.92 The surface pressure vs. area isotherms and absorption spectra of all-trans-, 9-cis-, and 13-cis-retinal, all-trans-retinol, and all- trans-retinyl acetate have been studied at air-water and air-solid interfaces, respectively. The spectra of monolayers of the isomeric retinals showed a red shift of 15 lnm compared with the solution spectra, whereas... 

Separation and Assay. Procedures for the separation, purification, and assay of carotenoids and retinoids by h.p.l.c., g.c., and g.c.-m.s. are given in an extensive article." Another, general, review includes information on the h.p.l.c. separation of retinoids.A particularly useful method has been developed for resolution and analysis of some carotenoid optical isomers.For example, (3R,3 R)-, (3S,3 S)-, and (3/ ,3 5)-astaxanthin were converted into the diastereomeric (-)-camphanic acid diesters, which were separated by h.p.l.c. This procedure has been used to analyse the isomeric composition of a natural astaxanthin sample. An h.p.l.c. procedure for separation of a-, P-, and y-carotenes (173)—(175) and lycopene (176) has been described." Several papers describe methods for the h.p.l.c. separation and purification of various retinal and retinol isomers and derivatives.A procedure for the preparative t.l.c. of oxidation products of retinyl acetate has been described,and a competitive protein-binding radioassay for retinoic has been reported. 

Cisoid retinoids have been isomerized to give the corresponding trans compounds by catalysis with iodine (Cainelli etal, 1973 Reif and Grassner, 1973). Methyl (13Z)-retinoate (176) has been converted to (all- )-retinoic acid (3) by being treated at room temperature with potassium amide in toluene and hydrolysis of the product obtained (Matsui, 1962). Homogeneous catalysis of a mixture of (9Z)-retinyl acetate (366) and (all- )-retinyl acetate (9) in the presence of palladium(II) chloride/acetonitrile adducts gave, by isomerization, a mixture more concentrated in (9), and pure (all- )-retinyl acetate (9) was then isolated in crystalline form from the latter mixture (Stoller and Wagner, 1975 Fischli et al., 1976). 

A number of methods for the photochemical conversion of (1 lZ)-retinyl acetate (12) or (9Z)-retinyl acetate (366) to the commercial product (all- )-retinyl acetate (9) have been described (Dalle and Rosenberg, 1970 Fischer, 1978). A very mild photochemical method consists in irradiating the E Z isomer mixture (9/12) with visible light in the presence of a colored sensitizer that absorbs light at wavelengths > 350 nm. Chlorophyll has been successfully used for this purpose (Fischer, 1978). (9Z)-Retinyl acetate (366) was isomerized in the presence of a small amount of a tertiary amine, using a 150-W xenon lamp, to give (all- )-retinyl acetate (9), which separated in crystalline form from the irradiated hexane solution (LaLonde and Stoller, 1976, 1978). 

Dissolve the appropriate methyl ester in methanol, and saponify as previously described for preparation of retinol from retinyl acetate After saponification, add water, and then acidify with dilute glacial-acetic acid make sure the solution is acidic to litmus paper. (In aqueous-alkaline solution, retinoid carboxylic acids remain as sodium salts, and are not extracted by organic solvents.) Extract the retinoid-carboxylic acid with diethyl ether two or three times (Note that hexane is not a good solvent for these polar retinoids.) Then wash the ether extract with water, and dry it over anhydrous sodium sulfate. Alternatively, if the volume is small, vortex and centrifuge the sample, remove any water, and evaporate the solvent The retinoid-carboxylic acids usually are obtained as yellow solids. Do not add any (not even a trace) HCl to 5,6-epoxy retinoids, because they instantaneously undergo isomerization to 5,8-epoxy retinoids, this change in structure is readily confirmed by the change m absorption spectrum (Table 1). 

---