Assigning Stereochemistry IV Fischer Projections

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1 or use in UCDavis Chemistry 8/118 Series Assigning Stereochemistry IV ischer Projections ischer Projections were invented by Dr. Emil ischer as a way of visualizing the stereochemistry of a three-dimensional molecule on a flat surface such as a piece of paper. While ischer projections are not the only way to look at chiral molecules they are often useful for comparison of molecules with multiple contiguous stereocenters and are still used regularly when looking at biological molecules such as sugars and amino-acids. ischer Projection Dash-Wedge (Bowtie) Structure Bond-line Structure IUPAC Name C 3 C 2 C 3 C 3 C 2 C 3 (2R)-2-bromobutane C 3 C 3 C 3 C 3 (2S,3R)-2-bromo-3-chlorobutane C C 2 C C 2 (2R,3S,R,5R)-2,3,,5,6-pentahydroxyhexanal or (D)-glucose Reading ischer Projections: The bonds that are written horizontally are pointed towards the viewer (wedge) The bonds that are written vertically are pointed away from the viewer (dash). Typically the carbon chain is written vertically with C1 of the chain at the top. Molecules with multiple chiral centers are written in an all eclipsed conformation. When looking at carbohydrates (sugars) points to the right when the center is R and points to the left when the center is S. 11

2 or use in UCDavis Chemistry 8/118 Series Writing ischer Projections - Single Stereocenter: 1) ind the stereocenter in the molecule. If there are multiple stereocenters see below. 2) Write in the fourth bond to if necessary. The chiral center should have two in-plane, one dash, and one wedge groups. Whichever one is missing will be the bond to the. If more than one group is not written you have at least two attached and itʼs not a stereocenter. 3) Pick two groups to be the horizontal groups (wedges) on your ischer projection. Usually itʼs easiest to visualize using the group thatʼs already a wedge and one of the in-plane (line) groups. ) Imagine grabbing one group with your left hand and the other with your right hand. 5) Imagine pulling on the molecule and rotating it so the group in your left hand is pointing left and a bit towards you and the group in your right hand is pointing right and a bit towards you. Imagine youʼre reaching forward with both hands like a zombie to grab the molecule. 6) The molecule should now be oriented with wedges left/right and dashes up/down. If you have a group still in the plane and one back tip the two groups youʼre ʻholdingʼ to turn the in-plane group backwards a bit. 7) SMS it flat and youʼre done! Example: (D)-Alanine stereocenter N 2 N 2 th group () is a dash grab in Left and Turn 2 N counterclockwise grab in Right and C 3 N 2 Tip slightly upwards C C 3 N 2 Same as C 3 N 2 SMS! C 3 N 2 Note: Picking different groups to be wedges will result in different though still correct ischer projections. 12

3 or use in UCDavis Chemistry 8/118 Series Writing ischer Projections - Multiple Stereocenters: NTE: To write a ischer projection with multiple stereocenters a molecule needs to be in a fully eclipsed conformation, which is different from how molecules are usually drawn in bond-line structures. 1) ind the stereocenters in the molecule. 2) Write in the fourth bond to if necessary. Each chiral center should have two in-plane, one dash, and one wedge groups. Whichever one is missing will be the bond to the. If more than one group is not written you have at least two attached and itʼs not a stereocenter. Method A: 3) Rotate the molecule so the bond between the two stereocenters is vertical. ) Rotate the top stereocenter so the dash is pointing up and back and the other two groups are pointing left and forward or right and forward. This should be about a 30 clockwise or counterclockwise turn. 5) Rotate the lower stereocenter so the dash is pointing down and back and the other two groups are pointing left and forward or right and forward. This should be about a 30 turn in the opposite direction of the top center. 6) SMS it flat and youʼre done! Method B: 3) Rotate the bond between the stereocenters so the molecule is eclipsed. This should be about a 60 turn leaving one side unmoved. ) Rotate the molecule so the bond between the two stereocenters is vertical. (ne side will be in plane, the top and bottom groups will be back and the other side will be wedges. 5) If desired, tip the molecule slightly counterclockwise so that both centers are oriented with wedges left/right and dashes up/down. 6) SMS it flat and youʼre done! NTE: or more than two stereocenters rotate the all centers eclipsed before flattening. 13

4 or use in UCDavis Chemistry 8/118 Series Example: (2R,3S)-2-bromo-3-pentanol Add th group () as necessary rotate so C-C is vertical Turn top C so dash is vertical (counterclockwise) Rotate C-C into eclipsed structure rotate so C-C is vertical SMS! 3 C C 2 C 3 Turn bottom C so dash is vertical (clockwise) SMS! 3 C C 2 C 3 Rotate upper stereocenter C 3 C 2 C 3 Rotate lower stereocenter C 3 C 2 C 3 Projection A... projection B... projection C. Example: (D)-glucose Add th group () as necessary rotate so C-C is vertical Rotate C-C into eclipsed structure C C 2 SMS! C 2 C C C Rotate C-C into eclipsed structure C 2 C 2 1

5 or use in UCDavis Chemistry 8/118 Series Rotating ischer Projections: Rotating a ischer projection 90 will give the enantiomer. 90 o Turn S center Rotating a ischer projection 180 will give the original molecule. 180 o Turn Switching two groups in a ischer projection will give the enantiomer. dd number of pair switches S center A second switch in the ischer projection will go back to the original molecule. Even number of pair switches Rotation around a bond will also give the original molecule. Rotate 15

6 or use in UCDavis Chemistry 8/118 Series Assigning Stereochemistry using ischer Projections: ischer projections can be used to assign stereochemistry. If the th (lowest) priority group is vertical the other three groups will show clockwise (R) or counterclockwise (S) rotation or or ,2,3 - ockwise 1,2,3 - Counterclockwise R S 16