| abstract |
The present invention provides a chemical compound having the structure being one selected from a group consisting of n n n n n n n n n n wherein R 1 is one selected from a group consisting of COOCH 3 , COOCH 2 Ph, CONHCH(CH 3 ) 2 and CONHC 6 H 5 , R 2 is one selected from a group consisting of H, CH 3 and CH(CH 3 ) 2 , R 3 is one selected from a group consisting of H, CH 3 , CH(CH 3 ) 2 and CH 2 Ph, and R 4 is one of CH(CH 3 ) 2 and C 6 H 5 . |