Depicting Carbohydrate Stereochemistry: Fischer Projections Three steps for assigning R,S stereochemical designations in Fischer projections Step 1 Assign priorities to the four substituents in the usual way Step 2 Place the group of lowest priority,usually H,at the top of the Fischer projection by using one of the allowed motions The lowest-priority group is thus oriented back away from viewer Step 3 Determine the direction of rotation 1-2-3 of the remaining three groups and assign R or S configuration Three steps for assigning R,S stereochemical designations in Fischer projections Step 1 Assign priorities to the four substituents in the usual way Step 2 Place the group of lowest priority, usually H, at the top of the Fischer projection by using one of the allowed motions ▪ The lowest-priority group is thus oriented back away from viewer Step 3 Determine the direction of rotation 1→2→3 of the remaining three groups and assign R or S configuration Depicting Carbohydrate Stereochemistry: Fischer Projections