Three new cryptophycins from Nostoc sp. GSV 224.

Cryptophycin 46 (2), -175 (3), and -176(4) have been identified as three new trace constituents of Nostoc sp. GSV 224. Cryptophycin-46 is an epimer of cryptophycin-3 (5) and to date is the only naturally occurring analogue having the S configuration at C-10 (C-2 in Unit B). Cryptophycins-175 and -176 also differ in unit B where 3 is the O-methyl analogue of cryptophycin-45 (6) and 4 is the O-desmethyl analogue of cryptophycin-21 (8). The relative and absolute stereochemistries of the three new analogues have been related to known cryptophycins by synthesis.