Five-membered ring systems 1.2- dioxolanes

With excess HFA a dioxolaneimine 99a is formed from a typical isonitrile reaction mentioned in previous sections. Acidification yields the unsubstituted five-membered ring 99, which is the parent compound of alkylated dioxolanes 99b-e available from isonitriles (R = Me, Et, t-Bu, c-CgHu) 111, 188). Whether the imine structure 99 or the ketone structure 101 has to be assigned to the hydrolysis product is not known. The Chapman rearrangement has been proved in the following system [Eq. (86)] 190). Treatment of 98 with strong bases like l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) yields a spirocyclic compound 100 with elimination of HCN the structure of 100 has been derived by spectroscopic methods 193). The reaction of 98 with HFA X HjO (91) in sulfuric acid produces a l,3-dioxolan-(4)one 112). 

Any factors tending to increase the planarity of the rings in a pair of fused five-membered rings should further increase the stability of the cis isomer relative to the trans. In agreement with this, systems of two five-membered rings in which tetrahydrofuran or 1,3-dioxolane is one of the components have so far been encountered only as cis isomers. This should not be taken as proof that the trans forms are incapable of existence it might be possible to prepare, for example, 1,4 3,6-dianhydrodulcitol, if a suitable method were chosen. 

The sptfo-fused, five-membered, 1,3-dioxolane ring in 40 is thermodynamically less favored than a tmns-fused system of six-membered rings and thus 40 rearranges to 41, as shown in Scheme VII. (For a... 

The NMR spectra display two ABX-spin systems for each compound. One ABX-spin system has only slightly different coupling constants. Furthermore, since one 3-methyl-2-butenyl side chain is still present, this ABX-spin system must be derived from a second oxidized side chain, probably at C-6. A 1,2-dioxolane structure is advanced (52,53). The small variations for the values of the coupling constants are ascribed to the flexibility of the five-membered ring (44). Alternative structures, such as 1,2-dioxetanes or epoxides, are rejected, because trisubstituted 1,2-dioxetanes appear to be unstable (54-57) and because the experimental coupling constants do not agree with those usually observed for epoxides (49,58). 

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