Patent ID: 7759514

Claim:
A compound of the formula (I): wherein, each R 1 , R 2 , and R 3 are independently alkyl substituted with aryl or heteroaryl, each of which is optionally substituted with 1-5 substituents selected from OR 6 , CN, NO 2 , NHR 7 , N(R 7 ) 2 , halo, CONHR 7 , CON(R 7 ) 2 , CO 2 R 8 , or C 1 alkyl; X is O, or S; R 4 is H, CON(R 7 ) 2 , CONHR 7 , CH 2 OH, CH(OH)CH═CH 2 , or C(O)NHCHR 10 CO 2 H; each R 5 is independently H, alkyl, alkenyl, aryl, heteroaryl, acyl, P 1 , or C(O)CHR 10 NH 2 ; each R 6 is independently H, alkyl, or P 3 ; each R 7 is independently H, alkyl, acyl, or P 2 ; each R 8 is independently H, alkyl, aralkyl, or heteroaralkyl; each R 10 is independently an amino acid side chain; each P 1 and P 2 is independently a nitrogen protecting group; and each P 3 is independently an oxygen protecting group; or pharmaceutically acceptable salts thereof.