Evans. Breit Conformational Analysis: Cyclic Systems-5 Chem 206 The impact of trigonal Carbon Polysubstituted Cyclohexane A Values Lets now compare look at the carbonyl analog in the 3-position L As long as the substituents on the ring do not interact in either conformation, their A-values are roughly additive 1, 4 Disubstitution: A Values are roughly additive AG°=-1.36 kcal/mol versus-174 for cyclohexane AG°=0 kcal/mol Let's now compare look at the carbonyl analog in the 2-position epimerization △G°=-2(1.74)=-348 kcal/mol 1, 3 Disubstitution A Values are only additive in the trans diastereomer AG°=-1.56 kcal/mol ersus-174 for cyclohexane However, the larger alkyl groups do not follow the expected trend △G°=AMe)-A() Can you explain?(see Eliel, page 732) CHMe? CHMe? 小个x= AG°=-059 kcal/mol versus-2. 15 for cyclohexane The new interaction! For X= M +088 △G°=-162 kcal/mol +088 versus-5.0 for cyclohexane △G°=2(9)+1(+37)=5.5 kcal/mol +3.7CMe3 H O Me3C CHMe2 H O Me3C Me H O Me3C Me H O H Me O H Me O Me3C H CMe3 O Me3C H CHMe2 O Me3C Me Me Me Me H X Me H X H Me H H Me Me H Me Me Me Me Me X H H Me H X H Me Me H H H H Evans, Breit Conformational Analysis: Cyclic Systems-5 Chem 206 ■ Let's now compare look at the carbonyl analog in the 3-position The impact of trigonal Carbon DG° = –1.36 kcal/mol versus –1.74 for cyclohexane ■ Let's now compare look at the carbonyl analog in the 2-position DG° = –1.56 kcal/mol versus –1.74 for cyclohexane DG° = –0.59 kcal/mol versus –2.15 for cyclohexane However, the larger alkyl groups do not follow the expected trend. Can you explain? (see Eliel, page 732) DG° = –1.62 kcal/mol versus –5.0 for cyclohexane Polysubstituted Cyclohexane A Values 1,4 Disubstitution: A Values are roughly additive. DG° = –2(1.74) = –3.48 kcal/mol 1,3 Disubstitution: A Values are only additive in the trans diastereomer DG° = 0 kcal/mol ■ As long as the substituents on the ring do not interact in either conformation, their A-values are roughly additive DG° = A(Me) – A(X) The new interaction! For X = Me + 3.7 + 0.88 + 0.88 DG° = 2(.9) + 1(+3.7)= 5.5 kcal/mol base epimerization base epimerization base epimerization