1. Department of Chemistry and Biochemistry, University of Colorado, UCB 215, Boulder, Colorado 80309
2. Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Gilead Sciences and IOCB Research Center, Flemingovo nam. 2, CZ-16610 Prague 6, Czech Republic
3. Department of Chemistry and Biochemistry, University of Denver, Denver, Colorado 80208