Ring interconversion synthesis

An interesting example of the synthesis of a pyrazoline via ring interconversion involving a saturated side chain has been studied in detail (113- 114).65,66 This reaction is unlike the classical mhrs in this case, the starting ring and side chain do not form a continuous 7r-electron system, and the conversion is azole-to-azoline and not azole-to-azole, as in the original scheme 1 - 2. 

Heterocyclic synthesis can also be done by ring interconversion. These processes are treated in more detail in the reactivity sections since they involve reaction at the heteroatom. Here, they are only briefly illustrated to provide examples of the transformations. 

Colquhoum, H. M. Lewis, D. E Ben-Haida, A. Hodge, P. Ring-chain interconversion in high-performance polymer system. 2. Ring-opening polmerization-copolyetherification in the synthesis of aromatic poly(ether sulfones). Macromolecules 2003, 36, 3775-3778. 

In Fig. 1 various targets of some important cytostatic agents are depicted. Their main mechanisms of action can be briefly summarized as follows. Pentostatin blocks purine nucleotides by inhibiting adenosine deaminase. 6-Mercaptopurine and 6-thioguanine inhibit purine ring biosynthesis and they inhibit nucleotide interconversions. Methotrexate by inhibiting dihydrofolate reduction blocks thymidine monophosphate and purine synthesis. 5-Fluorouracil also blocks thymidine monophosphate synthesis. Dactinomycin, daunorubicin, doxorubicin and mitoxantrone intercalate with DNA and inhibit RNA synthesis. L-asparaginase deaminates... 

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