Cyclohexane derivatives epoxide opening

The first reported synthesis of a member of this class, that of erythronolide B, was completed by Corey and is depicted in Scheme 2.14. The retrosynthetic strategy involved the use of two segments, C-10 to C-13 and C-1 to C-9, the former arising from selective ring opening of an epoxide and the latter from a series of manipulations on a rigid cyclohexane derivative. 

A similar cyclisation of an alkene derived from geranyl acetate 24 by dihydroxylation and formation of the epoxide 26 leads to a substituted cyclohexane 28. The Lewis acid ZrCU is used to open the epoxide and the alkene attacks intramolecularly 27 to give eventually the ryn-compound 28 with both substituents equatorial. The alignment of the alkene and the epoxide in a chair conformation 27a is responsible for the diastereoselectivity Note the regioselectivity the less substituted end of the alkene attacks the more substituted end of the epoxide 27. These are just two examples of the very many ordinary ionic reactions that can be used to make six-membered rings. 

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