Toxin and method for preparing intermediate thereof

A compound of formula (IV),

• wherein:
• A is a 3 to 8 membered ring;
• R1 is hydrogen or an amino protecting group;
• R2 is a carboxyl protecting group;
• and
• Rx is selected from the group consisting of alkyl and cycloalkyl, wherein the alkyl and cycloalkyl are each independently and optionally substituted by one or more groups selected from the group consisting of alkyl, haloalkyl, halogen, hydroxy, amino, nitro, cyano, alkoxy, hydroxyalkyl, cycloalkyl, heterocyclyl, aryl and heteroaryl.

A process for preparing a compound of formula (I),

• comprising hydrolyzing a chiral compound of formula (II) comprising a chiral auxiliary group R3 under an alkaline condition and optionally adding a carboxyl protecting group to obtain a compound of formula (III);
• alkylating the compound of formula (III) with an alkylating agent to obtain a compound of formula (IV);
• removing the carboxyl protecting group on the compound of formula (IV) to obtain a compound of formula (V);
• and preparing the compound of formula (I) from the compound of formula (V),
• wherein:
• A is a 3 to 8 membered ring;
• R is halogen;
• R1 is an amino protecting group;
• R2 is a carboxyl protecting group;
• Rx is selected from the group consisting of alkyl and cycloalkyl, wherein the alkyl and cycloalkyl are each independently and optionally substituted by one or more groups selected from the group consisting of alkyl, haloalkyl, halogen, hydroxy, amino, nitro, cyano, alkoxy, hydroxyalkyl, cycloalkyl, heterocyclyl, aryl and heteroaryl;
• and
• n is 1, 2, 3, 4 or 5.