A compound having structure (4) wherein each Ra independently is C1-C4 alkyl, or benzyl, each Rb independently is C1-C4 alkyl, R10 is C1-C8 alkyl, or C2-C8 alkenyl, or C2-C8 alkynyl, wherein C1-C8 alkyl is optionally substituted by one C1-C4 alkoxy group and/or pharmaceutically acceptable salts or enriched optical isomers thereof. These compounds are provided for use in the treatment of tumors cancer and the prophylaxis or treatment of viral infections.
