Patent GB-704983-A (GB)
Title: Synthesis of material rich in l-isoleucine
Filed: 1951-04-03
Abstract: An amino acid product rich in L-isoleucine is obtained by reacting D-alpha-methylbutyraldehyde with an aqueous solution of an inorganic cyanide and a carbonate of ammonia at elevated temperature to form 5-(sec-butyl) hydantoin and hydrolyzing the latter to obtain an aqueous solution of L-isoleucine and D-allo isoleucine or salts thereof.  The D-alpha-methylbutyraldehyde may be obtained by oxidizing optically active amyl alcohol (D-2-methyl-1-butanol) by passing a mixture thereof with an oxygen-containing gas in vapour phase at temperatures of 400 DEG  to 550 DEG C. into contact with metallic silver.  The oxygen-containing gas is preferably used in an amount corresponding to from 0.2 to 0.8 molecular proportions of free oxygen per molecular proportion of the alcohol and the vapours are usually passed first through a preheating zone and then through a body of small pieces, or filaments, of metallic silver.  The silver is usually employed in granular form but it may also be used as metallic shavings or turnings or as one or more silver screens.  The vapourous products are condensed and the aldehyde separated by fractional distillation usually in the form of an azeotrope with water.  The inorganic cyanide used in the hydrolysis of the aldehyde to the hydantoin may be an alkali metal cyanide or other water-soluble and ionisable inorganic cyanide such as hydrogen cyanide or calcium cyanide and either ammonium carbonate or bicarbonate may be used whilst other ammonium salts of simple acids, e.g. ammonium chloride, bromide, or acetate may be used together with the carbonate or bicarbonate, provided the mixture contains in chemically combined form, e.g. as ammonium carbonate or bicarbonate, at least two molecular proportions of ammonia and at least 1, preferably 1.5 or more, molecular proportions of carbon dioxide per molecular proportion of the aldehyde.  The hydrolysis may be effected by heating the aqueous mixture to between 40 DEG  and 120 DEG C. at atmospheric or superatmospheric pressure.  The hydantoin may be isolated from the reaction liquor or may be hydrolyzed while dissolved in the liquor.  The hydrolysis is usually effected by heating the hydantoin with an aqueous alkali, e.g. sodium, potassium, or barium hydroxide, or lime, and an excess of water and of the alkali metal hydroxide should be used.  The mixture is preferably heated under pressure in a bomb or autoclave at between 115 DEG C. and 225 DEG C. although the hydrolysis can also be effected by boiling the mixture at atmospheric pressure.  The amino acid product may be isolated by acidifying the hydrolysis liquor to a pH value between 4.5 and 8.5 e.g. with hydrochloric or sulphuric acid, evaporating to dryness, washing with alcohol and then purifying e.g. by digestion with an aqueous suspension of lead oxide, treatment of the mixture with hydrogen sulphide to precipitate the lead as sulphide, and then converting the remaining amino acids to their copper salts, washing the salts with alcohol and finally liberating the amino acids from the copper salts.  Alternatively the alkali metal salts of the amino acid may be extracted from the crude hydrolysis liquor with methanol or ethanol and the extract acidified to form the free acid, or the crude alkaline hydrolysis liquor may be treated with the acidic form of a cation exchange agent to liberate the free amino acids.  The product recovered in any of the above ways consists mainly of a mixture of the two amino acids.  When L-isoleucine itself is desired as the product the mixture may be reacted with formic acid to form the N-formyl derivatives of the amino acids e.g. by heating with formic acid to 90 DEG C. or above and usually to boiling so as to distil unreacted formic acid from the mixture.  The residual mixture of the N-formyl derivatives may then be dissolved in a solvent, e.g. water, acetone, dioxane, or ethyl acetate, and a portion of the N-formyl-L-isoleucine crystallized from the solution.  The crystals are separated and if necessary are further purified by recrystallization from methyl ethyl ketone and are then hydrolyzed to the amino acid e.g. by heating with an aqueous solution of a mineral acid such as hydrochloric, hydrobromic, or sulphuric acid.  The mother liquor from which the N-formyl L-isoleucine was crystallized still retains some of the latter in admixture with N-formyl alloisoleucine and may be heated with an alkali e.g. sodium, potassium or barium hydroxide so as to effect both hydrolysis and racemization of the amino acid derivatives. Alternatively the mother liquor may be evaporated to crystallize the N-formyl derivatives and the crystalline material treated with aqueous alkali.  The resultant mixture is then neutralized and the racemized mixture of L-isoleucine and D-alloisoleucine separated. This mixture may again be converted to the N-formyl derivatives and a further amount of L-isoleucine recovered by repeating the above procedure.  Examples are given in which (1) a mixture of the isomeric amino acids is obtained from D-2-methyl-1-butanol and (2) L-isoleucine itself is obtained from such a mixture of the isomeric acids by forming the N-formyl derivative as above.  The oxidation of the D-2-methyl-1-butanol is effected with air at 480 DEG -520 DEG C. in a stainless steel tube packed with silver pellets whilst the aldehyde obtained is converted to the hydantoin by heating with an aqueous solution of sodium cyanide and ammonium bicarbonate.  The hydrolysis of the hydantoin is effected by heating with a 50 per cent. aqueous sodium hydroxide solution in a steel bomb at 170 DEG C. and the mixture of amino acids isolated by treating the hydrolysis liquor, after filtration, with the acidic form of a granular anion exchance agent consisting of a sulphonated copolymer of styrene, ethyl vinyl benzene and divinyl benzene.  Specification 693,522 is referred to.ALSO:An amino acid product in L-isoleucine and obtained by reacting D-alpha-methylbutyralde-hyde with an aqueous solution of an inorganic cyanide and a carbonate of ammonia at elevated temperature to form 5-(sec-butyl) hydantoin and hydrolysing the latter to obtain an aqueous solution of L-isoleucine and D-alloisoleucine, or salts thereof is treated for the isolation of L-isoleucine itself (see Group IVb) and either the mixture of amino acids or the L-isoleucine itself are stated to be suitable for use in animal feeding stuffs.  The D-alpha-methylbutyraldehyde may be ob-tained by oxidizing D-2-methyl-1-butanol in the vapour phase with an oxygen-containing gas at 400-550 DEG C. in the presence of metallic silver.