Two new chlorocyclopropane macrolides, phorbasides A and B, have been characterized from the sponge Phorbas sp. that previously yielded phorboxazoles A and B. We describe the assignment of the absolute configuration of the trans-chlorocyclopropane ring that exploits a CD Cotton effect arising from hyperconjugation to the ene-yne chromophore. Phorbasides and callipeltoside A share the same macrolide configurations but, unexpectedly, opposite cyclopropane configurations.
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