A set of novel borrelidin analogues have been prepared by precursor-directed biosynthesis. Structure-activity relationship analysis suggests that steric structural arrangement within the C17 side chain is important for differentiating cytotoxic and anti-angiogenic activities. A C17-cyclobutyl analogue 3 was found to have markedly increased selectivity for in vitro angiogenesis inhibition over cytotoxicity and is therefore potentially useful as an anticancer agent.