Open-Closed Ring Systems

The dihydroazulene vinylheptafulvene couple (9) [85, 98], dihydroindolizine (10) [95], and fulgides are further examples of photochromic open-closed ring systems... 

Some Senecio alkaloids have opened pyrrolizidine ring systems which show transannular interactions and are closely related in structure to the compounds discussed above. The alkaloid senkirkine is a cyclic ester of (37) (74JA6165) and its X-ray structure shows a rather short transannular contact of 2.23 A. 

The atoms of organic compounds are arranged as either open chains (acyclic or aliphatic) or closed ring systems (cyclic). Each corner or kink in the ring (or chain) indicates a CH2 group, although these are usually abbreviated to C or omitted. Each line represents a bond. 

Geometrical parameters provide much more information than simple identification. The phosphorus-carbon distance of 1.686(6) A in this tungsten complex is consistent with there being a double bond, as it is close to those observed in open chain phosphaalkenes. But how much better it would have been if the parent compound could have been studied uncomplexed, so that the strain in this unusual ring system could have been investigated. The geometrical parameters would then have provided very useful information about the likely reactivity of the compound, and information about the changes on... 

The chemical structure of vitamin D3 is closely related to its precursor, 7-dehydrocholesterol, from which it is produced by a photochemical reaction. Therefore, vitamin Do is closely related structurally to the four-ring nucleus of steroids derived from the cyclopentanoperhydro-phenanthrene ring system. No vitamin D activity is noticed until the B ring of 7-dehydrocholesterol is opened between C-9 and C-10. Thus, vitamin D3 is a 9,10-seco steroid and its carbon skeleton is numbered accordingly (Scheme I). The important aspects of its chemistry center about the 5,6,7-cis-triene structure. The formula for vitamin D3 is C27H44O and its formula weight is 384.64. 

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