You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

Synthesis and biological evaluation of xanthine derivatives on dipeptidyl peptidase 4. A series of xanthine derivatives in which a methylene was inserted at position 8 of xanthine scaffold was synthesized and evaluated as inhibitors of dipeptidyl peptidase 4 (DPP-4) for the treatment of type 2 diabetes. As the results of structure-activity relationship (SAR) study of the series, the compounds with 4-methyl-quinazoline-2-yl-methyl group at N-1 position and 2-aminoethylaminomethyl group gave better activities. Compounds H4 and H9 showed good DPP-4 inhibition and more than 100-fold selectivity over DPP-7 and DPP-8.
{"Chemical": ["methylene", "xanthine", "4-methyl-quinazoline-2-yl-methyl", "2-aminoethylaminomethyl"]}