Question

3) (20 pts) Assign the indicated carbons as (R), (S), or achiral in each structure below. ...11CH, HE HO

Answer 1

Br 5 1 2 6 114 5 CH3 achiral R (R)-3-bromo-2-methylhexane 4 s (S)-3-methyldihydrofuran-2(3H)-one R 1. 0 4 CH3 achiral 1,2,4-trimethylcyclopent-1-ene U . N2 1 3 2 (R)-2-(dimethylamino)-3-phenylpropan-1-ol S 3 2 HOLA 3a3c02H но ir рны — ОН HOH S- achiral — OH (2R 3S)-2,3,4-trihydroxybutanal 2 HOW HON 3-((59,6S)-5,6-dihydroxy-4,5,6,7-tetrahydro-1H- indol-3-yl)propanoic acid The absolute configuration of the chiral centers (centers that are attached to four different groups) is given by CIP rules. The centers that are not attached to four different groups are known as achiral carbon centers.

To know the absolute configuration at chiral carbons (carbons attached with four different groups), the priority of the attached groups has to given by Chan-Ingold-Prelog (CIP) rules. According to CIP rules the group with highest molecular weight should get first priority (1) followed by the groups with next highest molecular weights. If the attached groups are same in the adjacent atoms, one has to move further to see different atoms After giving priorties according to CIP rules, if the rotation of the priority groups (from 1st priority group to 3rd priority group) is clock wise then the configuration of that chiral center is R if the rotation of the priority groups (from 1st priority group to 3rd priority group) is anti clock wise then the configuration of that chiral center is S If the least priority group (40 is present above the plane then the configuration of the chiral center should be opposite. That means R becomes Sand S becomes R

In Fischer projections, horizontal groups are in above the plane and vertical groups are in below the plane. According to CIP rules, the absolute configurations are given COOH COOH Но, 2: H * OH * HO - chiral carbon (carbon attached to 4 different groups) SH H3C но 3 н. H3C (2R, 3S)-2,3-dihydroxybutanoic acid (2S,3S)-2,3-dihydroxybutanoic acid Since in the given two compounds, the absoulte configuration at the chiral carbons is same at some centers (at 3rd carbon) and different at some centers (at 2 nd carbon). Hence these two are diastereomers to each other