The cephalostatins. 21. Synthesis of bis-steroidal pyrazine rhamnosides (1)

The synthesis of bis-steroidal pyrazines derived from 3-oxo-11,21- dihydroxypregna-4,17(20)-diene (4) and glycosylation of a D-ring side chain with α-L-rhamnose have been summarized. Rearrangement of steroidal pyrazine 10 to 14 was found to occur with boron triflouride etherate. Glycosylation of pyrazine 10 using 2,3,4-tri-O-acetyl-α-L-rhamnose iodide led to 1,2-orthoester-α-L-rhamnose pyrazine 17b. By use of a persilylated α-L-rhamnose iodide as donor, formation of the orthoester was avoided. Bis-steroidal pyrazine 10 and rhamnosides 17b and 21c were found to significantly inhibit cancer cell growth in a murine and human cancer cell line panel. Pyrazine 9 inhibited growth of the nosocomial pathogen Enterococcus faecalis.