An Astonishingly Easy Method for Determining Rand Sfor Fischer Projections Ben Ruekbetg The University of Rhode Island, Kingston, RI 02881 Once students master the concent Cahn-Ineold-Preloe sequencing as applied to wedge anddash representations of chiral centers. thev mav still have difficultv - aoolvine .. . the concept to Fischer projections. This is unnecessary! ~ k e ar brief explanation of the rationalization of the system, a few very simple rules will allow students to identify the absolute configuration of Fischer projections correctly with minimal mental trauma. Imagine a chiral center as represented by aFischer projection on a page, with the number 4 group in the upper position. You are looking into the chiral center from the perspective of aline perpendicular to the page a t the chiral center. If you now imagine that you are looking a t the chiral center alone a line 54.75" below the oernendicular line (directlv - awaifrom the number4 group), the chiralcenter wili exact& eclipse the numher 4 croup. This will he referred to as the "gr&p-4-eclipsed perspective" and is the very perspective from which the RIS designation is to be determined. The remaining three groups are now arrayed as a triangle with the vertex down. If they are counted 1-2-3 in a clockwise direction, the chiral center is designated "R". If they are counted 1-2-3 in a counterclockwise direction, then the chiral center is designated "S". Now recall that the spacial arrangement of groups 1 through 3 in the original Fischer projection was in the same way around a vertex-down triangle as they would be in the group-4-eclipsed perspective. Therefore. the direction in which erouns . 1-3 are "counted" in a ~ i s c h e projection, r with group 4 in the upper position, will he the same as the direction thev would be counted in the group-4-eclipsed perspective giving the correct RIS designation. Similarly, if group 4 is in the lower position, shifting the perspective u p by 54.75O gives the group-4-eclipsed perspec- . tive kith the remaining three groups in a vertex-up triangle. Again, the direction in which groups 1 through 3 would he "counted" (rigorously correctly giving the RIS designation) is the same direction in which they would be counted as they anoear in the orieinal Fischer oroiection. This means that if t& numher 4 is either on tge top or the bottom of the Fischer oroiection. then "countins" the remainins three groups in t i e ord, they appear on the page uill &e the
1034
Journal of Chemical Education
correct R or S assignment. If the number 4 group is either on the right or the left side of the Fischer projection, then the perspective from 54.75O to the right or left of the perpendicular line, respectively, will represent looking down the carbon-group 4 bond in the wrong direction, with group 4 eclipsing the chiral center instead of the other way around. From this perspective, the trianeular arrav of the remainine erouns (which are still in the same orderas they appear i n i c e ~ i s c h eprojection) r will give the wrong answer. This is because the chiral center is being examined from the wrong direction. If one viewed a transparent clock from behind, the hands would appear to move-in a counterclockwise direction. If looked ;
