D A Evans,J. Johnson Rules for Ring closure: SP2 Carbon Related Systems Chem 206 Trigonal Carb Endo cyclization modes that are disallowed (3 to 5-endo-trig) n-endo-tng X= first-row element The 5-endo-trig cyclization is a watershed case distance from reacting centers: 2.77A Case 1: Baldwin. J. Chem. Soc. Chem. Commun., 1976. 734 CO,Me base It is possible that a"nonvertical 5-endo-trig trajectory is operational like that suspected in C=o addition however cO, Me base COmE Second row atom relaxes the cyclization geometrical requirement Case 2: Baldwin. J. chem. Soc. Chem. Commun. 1976 736 CO,Me MeO2C 5-exo-trigD. A. Evans, J. Johnson Rules for Ring Closure: SP2 Carbon & Related Systems Chem 206 Trigonal Carbon Endo cyclization modes that are disallowed (3 to 5-endo-trig) n-endo-trig The 5-endo-trig cyclization is a watershed case X = first-row element Case 1: Baldwin, J. Chem. Soc., Chem. Commun., 1976, 734. OH CO2Me O CO2Me 5-endo-trig Disfavored base X however SH CO2Me S base CO2Me X Second row atom relaxes the cyclization geometrical requirement Case 2: Baldwin, J. Chem. Soc., Chem. Commun., 1976, 736. NH2 MeO2C CO2Me HN HN MeO2C MeO2C CO2Me O 5-endo-trig 0% 5-exo-trig 100% NH2 MeO2C CO2Me distance from reacting centers: 2.77 Å It is possible that a "nonvertical" trajectory is operational like that suspected in C=O addition Y C Y – C X •• X