D.A. Evans, J. Johnson Rules for Ring Closure: SP2& SP Carbon Related Systems Chem 206 Trigonal Carbon: Intramolecular Aldol Condensations Digonal Carbon: Cyclizations on to Acetylenes Baldwin. Tetrahedron 1982. 38. 2939 DIGONAL: Angle of approach for attack on triple bonds molenda)-EXo- aldwin 120 3 and 4-EXo-dig are disfavored Favored: 6-7-enolendo) -exo-trig Disfavored: 3-5-enolendo)-exo-trig 5 to 7-Exo-dig are favored 3 to 7-Endo-dig are favored (Enolexo) -exo-trig Ab initio scf 4-31G calculations for the interaction of hydride with acetylene Favored: 3-7-enolexo)-exo-trig H、 4-31G basis set 0 5(Enolendo)-Exo-trig 6-(Enolendol-Exo-trig 712 Houk. J.AcS1979,101,1340 H STo-3G minimal basis 110°120 Crystal Structures do not support Baldwin stical Distribution, (+ Ii)/=2: 1 erimental distribution (KOH, MeOH, r t, 5 min, 77%y) Caution: Baldwins conclusions assume that the RDs is ring closure however, it is well known(by some! that the rate determining step is dehydration in a base-catalyzed aldol condensation J Dunitz and j. wallis J. C. S. Chem. Comm. 1984. 671D. A. Evans, J. Johnson Rules for Ring Closure: SP2 & SP Carbon & Related Systems Chem 206 Trigonal Carbon: Intramolecular Aldol Condensations R MO X Y R O X YM R MO X Y X YM R O O Me Me O Me O Me O Me O Me Me O O Me O Me O (Enolendo)-Exo-trig (Enolexo)-exo-trig Favored: 3-7-(enolexo)-exo-trig Favored: 6-7-(enolendo)-exo-trig Disfavored: 3-5-(enolendo)-exo-trig 5-(Enolendo)-Exo-trig 6-(Enolendo)-Exo-trig Statistical Distribution, (I + II)/III = 2:1 Experimental Distribution, = 0:100 (KOH, MeOH, r.t., 5 min, 77% y.) Baldwin, Tetrahedron 1982, 38, 2939 favored Caution: Baldwin's conclusions assume that the RDS is ring closure; however, it is well known (by some!) that the rate determining step is dehydration in a base-catalyzed aldol condensation. Digonal Carbon: Cyclizations on to Acetylenes DIGONAL: Angle of approach for attack on triple bonds - 3 and 4-Exo-dig are disfavored - 5 to 7-Exo-dig are favored - 3 to 7-Endo-dig are favored Baldwin: Ab initio SCF 4-31G calculations for the interaction of hydride with acetylene: J. Dunitz and J. Wallis J. C. S. Chem. Comm. 1984, 671. N N N + O- + Crystal Structures do not support Baldwin H H C C H H H C C H H H _ 127 o 156o 148o 2.13 4-31G basis set Houk, J.ACS.1979, 101, 1340. 1.22 110o -120o 1.5-2.0 STO-3G minimal basis set Dunitz, Helv Chim. Acta 1978, 61, 2538. N O N N 104o 93o 2.44 2.92 86o 120° 120° E + Nu￾I II III H