The invention is directed to a cyclic acetal of N-acylglutamic acid-.gamma.-semialdehyde of the formula ##STR1## in which A is an unsubstituted alkylene group having 2 to 3 carbon atoms or such an alkylene group substituted by 1 to 2 methyl groups and R is a methyl, methoxy, phenyl, or benzyloxy group and to a method of producing a compound of formula (I) by reaction of a compound of the general formula ##STR2## in which A is as defined above, with hydrogen cyanide or a cyanide ion supplying compound, ammonia or an ammonium ion supplying compound and carbon dioxide or a carbonate ion supplying compound and basic hydrolysis of the reaction mixture obtained and reaction of the hydrolysis mixture with a suitable acylating compound and use of the compounds of formula (I) to produce .alpha.-N-acyltryptophane.