How many secondary carbocations are involved in the biosynthesis of avermitilol?

Quantum chemical calculations were used to assess the viability of proposed secondary carbocations as intermediates in the biosynthesis of avermitilol. One, a cyclopropylcarbinyl cation, was found to be a true minimum, while another, a simple secondary cation, was found to exist only as part of a transition structure for water capture.