3-alkoxyalkanoic acid derivative, process for preparing the same and herbicide using the same

A herbicide comprising a 3-alkoxyalkanoic acid compound represented by the following formula (I):

• ##STR52## wherein R.sup.1 is an alkyl group having 1 to 6 carbon atoms which may be substituted by a cycloalkyl group having 3 to 8 carbon atoms, an alkenyl group having 2 to 6 carbon atoms, an alkynyl group having 2 to 6 carbon atoms, a cycloalkyl group having 3 to 8 carbon atoms, a haloalkyl group having 2 to 6 carbon atoms, or a cyanoalkyl group having 2 to 6 carbon atoms;
• R.sup.2 is a hydrogen atom or an alkyl group having 1 to 5 carbon atoms;
• R.sup.3 is an alkyl group having 1 to 6 carbon atoms;
• or R.sup.2 and R.sup.3 may be mutually bonded to form a cycloalkyl group having 3 to 8 carbon atoms;
• R.sup.4 is a hydrogen atom, an alkyl group having 1 to 5 carbon atoms or an alkynyl group having 2 to 5 carbon atoms;
• R.sup.5 is an alkoxy group having 1 to 5 carbon atoms or an alkyl group having 1 to 5 carbon atoms;
• R.sup.6 is an alkoxy group having 1 to 5 carbon atoms, an alkyl group having 1 to 5 carbon atoms or a halogen atom;
• and X is an oxygen atom or a sulfur atom,
• or an alkali addition salt thereof as an active ingredient and a herbicidally effective carrier.