The functionalised spiro [4.4] nonane 30 is available from tert-butylcyclopentadiene 16via spiroalkylation with 12b to give 17/18, followed by singlet oxygenation/aldolisation to the mixture of 21, 22, 25 and 26. Oxidation of the mixture gave two enediones 28/29, which were reduced with K-Selectride and the diastereoisomer 26 was separated and hydrogenated to give 30.
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DeLuca et al. (1991) studied this question.
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