Some serious and unexpected complications emerged in the key annulation step required to reach 1, the core of guanacastepene A (see scheme). The solution to these problems concurrently solved the major issues associated with the different oxidation states of C14 and C15. Although the authors have not generalized this type of annulation, it could have broad applications to building incrementally oxygenated polycyclic natural products.
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Tan et al. (2002) studied this question.