D. A. Evans.J. Johnson Rules for Ring closure: Introduction Chem 206 Ring Closure and Stereoelectronic Considerations C. Nucleophilic ring closures sub-classified according to hybridization An Examination of baldwin's rules (tetrahedral tet; trigonal trig; digonal dig) "Baldwin,'s Rules"provides a qualitative set of generalizations on the D. Nucleophilic ring closures further subclassified according to size of obability of a given ring closure the fomed ring. For example There are circumstances where the"rules"don't apply a They do not apply to non-first-row elements participating in the 5-exo-t cyclization event. The longer bond lengths and larger atomic radii of 2nd row elements result in relaxed geometrical constraints For example, a change in a heteroatom fromo to S could result in relaxation of a given geometric constraint. X=O VS S u The" rules"do not apply to electrocyclic processes Nomenclature Required trajectories Classes of Ring Closing Processes A. EXo-cyclization modes identified by the breaking bond ing positioned exocyclic to the forming cycle 把“4 B. Endo-cyclization modes identified by the breaking bond being positioned endocyclic to the forming cycle X=first-row element N. O Baldwin. J. Chem. Soc. Chem. Commun. 1976. 734D. A. Evans, J. Johnson Rules for Ring Closure: Introduction Chem 206 Ring Closure and Stereoelectronic Connsiderations An Examination of Baldwin's Rules "Baldwin's Rules" provides a qualitative set of generalizations on the probability of a given ring closure. There are circumstances where the "rules" don't apply. ■ They do not apply to non-first-row elements participating in the cyclization event. The longer bond lengths and larger atomic radii of 2nd row elements result in relaxed geometrical constraints. For example, a change in a heteroatom from O to S could result in relaxation of a given geometric constraint. Nomenclature Classes of Ring Closing Processes Y Y – Y Y – exo endo X X X X A. Exo-cyclization modes identified by the breaking bond being positioned exocyclic to the forming cycle. B. Endo-cyclization modes identified by the breaking bond being positioned endocyclic to the forming cycle. X = first-row element N, O C. Nucleophilic ring closures sub-classified according to hybridization state of electrophilic component: (tetrahedral = tet; trigonal = trig; digonal = dig) D. Nucleophilic ring closures further subclassified according to size of the fomed ring. For example: X – Y X Y – X – Y X Y – X – Y X Y – 5-exo-trig 5-exo-dig 5-exo-tet X Y X Y Y X Y X X Y X Y Baldwin, J. Chem. Soc., Chem. Commun., 1976, 734. ■ The "rules" do not apply to electrocyclic processes. Y Y - endo X •• X X = O vs S •• •• Required trajectories: a a a a a a * * a a