Approaches to the assembly of the antifungal agent FR-900848: studies on the synthesis of C 2 symmetric tetracyclopropane derivatives and an X-ray crystallographic study of (1R,3S,4S,6R)-bicyclopropyl-1,6-di-{2-[(4R,5R)-di-(isopropyloxycarbonyl)-1,3-dioxolane]}