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Solutions to Problems 107 65. (a) HO H Careful! The = group is higher C C priority than the benzene ring, even though C C CH₂ N the latter translates into C C The first point of difference is N > C! a d a C III R is the answer. c b c b (b) Equal energy: The transition states are mirror images of each other (they are enantiomeric). (c) The enzyme must be lowering the energy of the transition state that leads to the (-) isomer, relative to the one that leads to the (+) isomer. To do this, the enzyme must be chiral, and optically pure as well, so that the two transition states in the presence of the enzyme become diastereomeric and, therefore, different in energy from each other. 66. (a) The four structures on which your models should be based are as follows. In each molecule the chiral carbons are the two ring fusion carbons. For each, the highest priority substituent is the other ring fusion; the ring bond closer to the N is second. H H H H R NH NH R NH S NH S R R S H H H H Cis Trans (enantiomers) - (enantiomers) Cis and trans are diastereomers of one another. (b) Ring bond priorities are not changed by the additional groups because they are beyond the first points of difference with respect to the ring fusion chiral carbon atoms. H H R NH R NH S S CO₂H R CO₂H H H H H S NH S NH R R HO₃P S CO₂H R CO₂H H H