Patent GB-793597-A (GB)
Title: A process for the preparation of polyene aldehydes
IPC: C07C403/00
Filed: 1956-07-23
Abstract: Polyene aldehydes are produced by condensing 2 - [21,61,61 - trimethyl - cyclohexylidene] ethylidene triaryl phosphine, the aryl groups of which may carry alkyl or alkoxy radicals, with an acetal of 2-oxo-propanol-(1), 4-oxo-penten-(2)-al-(1) or 2-methyl-6-oxo-heptadien-(2,4)-al-(1), heating the reaction mixture and hydrolysing the resulting acetal in an acidic medium.  The condensation is preferably effected in an inert solvent, e.g. ether, petroleum ether, benzene, dioxan, tetrahydro-furan or methylene chloride, by mixing the reactants with stirring at room temperature or elevated temperature, suitably in an inert atmosphere, e.g. nitrogen.  The lower acetals such as the dimethyl, diethyl or dibutyl acetals, or cyclic acetals such as ethylene or propylene acetals are preferred.  The condensation product is decomposed by heating for several hours at 35-80 DEG  C. forming the corresponding polyene aldehyde acetals and triaryl-phosphine oxide.  The hydrolysis of the acetal may be effected by stirring in the presence of water in a water-miscible solvent with a mineral acid, e.g. sulphuric acid, or an organic acid, e.g. acetic or p-toluene sulphonic acid, at room temperature or elevated temperature.  The starting materials may be prepared as follows: 2-oxo-propanol-(1)-ethylene acetal is obtained by heating 2-oxo-propanal-(1) and ethylene glycol in the presence of p-toluene-sulphonic acid while continuously distilling off the water formed; 4-oxo-pentene-(2)-al-(1) acetals are prepared by condensing propiolic aldehyde acetals with acetaldehyde, partially hydrogenating the triple bonds and oxidizing the hydroxy group with manganese dioxide; 2 - methyl - 6 - oxo - heptadien - (2,4) - al - (1) acetals are obtained by heating methyl-malonic dialdehyde with an alcohol in an inert solvent with an acid agent and continuously distilling off water, condensing the enol-ether with lithium acetylide and hydrolysing with acid to form 2-methyl - penten - (2) - in - (4) - al - (1) which is then acetalized, condensing the acetal with acetaldehyde, partially hydrogenating the triple bond of the 2-methyl-6-hydroxy-hepten-(2)-in-(4)-al-(1) acetal and oxidizing the hydroxy group with manganese dioxide; 2[21,61,61-trimethylcyclohexylidene]-ethylidene triaryl phosphine is obtained by condensing 2,6,6-trimethyl-cyclohexanone-(1) with lithium acetylide, partially hydrogenating the triple bond to give 2,6,6-trimethyl-1-vinyl-cyclohexanol, reacting the product with phosphorus tribromide and then with triphenyl phosphine to give 2-[21,61,61-trimethyl - cyclohexylidene] - ethyl triphenyl phosphonium bromide and adding to the latter in a nitrogen atmosphere phenyl lithium whereby the desired product is formed.  In examples: (1) 4 - [21,61,61 - trimethyl - cyclohexylidene] - 2 - methyl - buten - (2) - al - (1) is obtained by condensing 2[21,61,61-trimethylcyclohexylidene] - ethylidene triphenyl phosphine with 2-oxo-propanal-(1) ethylene acetal, heating the product to split off triphenylphosphine oxide, and hydrolysing the resulting 4 - [21,61,61 - trimethyl - cyclohexylidene] - 2 - methyl - butene - (2) - al - (1) ethylene acetal with sulphuric acid; (2) the same product is obtained using 1,1-diethoxy-propanone-(2).  The other products of the above process are 6-[21,61,61-trimethylcyclohexylidene] - 4 - methyl hexadien-(2,4)-al-(1) and 8-[21,61,61-trimethylcyclohexylidene] - 2,6 - dimethyl - octatriene - (2,4,6) - al - (1).  4 - [21,61,61 - Trimethyl - cyclohexylidene] - 2 - methyl - buten - (2) - al - (1) is converted to Vitamin A by condensing it with 1 - hydroxy - 3 - methyl - penten - (2) - yne - (4), subjecting the product to acetylation and partial hydrogenation of the triple bond and treating the ester formed with a dehydrating agent.  b -carotene is obtained by condensing 8-[21,61,61-trimethyl - cyclohexylidene] - 2,6 - dimethyloctatriene - (2,4,6) - al - (1) with acetylene, splitting off 2 mols. of water, rearranging the product, partially hydrogenating the triple bond and isomerizing the compound produced.  The 6 - [21,61,61 - trimethyl - cyclohexylidene] - 4 - methyl-hexadiene-(2,4)-al-(1) can be converted into 8-[21,61,61-trimethyl-cyclohexylidene]-2, 6-dimethyl-octatriene-(2,4,6)-al-(1) by acetalization, condensation of the acetal with a propenyl ether and heating the condensation product with acid.