Ch 25 notes - Chemistry 140C Spring 2009 (K. Albizati)...

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Chemistry 140C Spring 2009 (K. Albizati) 1 Chapter 25 - Heterocycles Heterocycles are cyclic compounds that have at least one heteroatom as an atom in the ring. Heterocycles are ubiquitous in biochemistry and, not surprisingly, in pharmaceuticals. ( PICTURE OF TABLE 25-1 ). This makes sense because drugs must interact with biomolecules in order to exert a therapeutic effect and frequently this means that drugs must structurally resemble the molecules that key bio-receptor molecules must interact with. Nomenclature Heterocycles can be placed in two categories: aromatic and non-aromatic. There are many common names for classes of heterocycles and for the parent structures. The following prefixes are used to indicate the substitution of a heteroatom for a ring carbon: Aza – nitrogen substitution Oxa – oxygen substitution Thia – sulfur substitution Phospha – phosphorus substitution Examples: However, almost no one uses these names for the parent structures , so substituted analogs are also named using common names.
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Chemistry 140C Spring 2009 (K. Albizati) 2 Common names for important heterocycles: Non-aromatics O O N azetidine tetrahydrofuran H oxirane piperidine N H So……we name compounds using these as parent names when naming substituted rings: Aromatics The common names dominate these ring systems. The most important ones which pertain to Biochemistry/Mol Bio are: However, Biochem/Mol Bio is dominated by nitrogen heterocycles so we will spend the longest with these ring systems.
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Chemistry 140C Spring 2009 (K. Albizati) 3 Reactivity of Non-Aromatic Heterocycles You already know all of this chemistry. Oxygen, sulfur and nitrogen heterocycles react just like ethers, thioethers and amines respectively. A few reminders…… Three-membered heterocycles are highly strained and quite reactive towards nucleophiles in ring opening reactions: Three-membered rings are opened by nucleophiles under either acidic or basic conditions in an S N 2-like sense at the least substituted carbon with inversion of configuration at the reacting center. Example: N H CH 3 H 3 C H CH 2 CH 3 + CH 3 CH 2 NH 2 heat H H 3 C CH 3 CH 3 CH 2 N NCH 2 CH 3 H H H Larger ring amines show their typical chemistry: N H + Cl O Et 3 N N O
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Chemistry 140C Spring 2009 (K. Albizati) 4 Structure and Properties of 5-Membered Ring Aromatic Heterocycles The three main ring systems in this category are: And they have electronic structures which indicate that they are AROMATIC: The compounds fit the 4n+2 rule and show extra stabilization expected of aromatics. Delocalization of the electron system by resonance is extensive: These ring systems are quite electron rich (i.e., nucleophilic) and react very readily in electrophilic aromatic substitution (later).
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Ch 25 notes - Chemistry 140C Spring 2009 (K. Albizati)...

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