10-11 - ISOMERS How are the R and S designations...

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Unformatted text preview: ISOMERS How are the R and S designations determined? A. A. Assign priorities to the substituents about a chirality center. B. With the lowest priority furthest from the viewer, draw a circle from the highest (1) to lowest priority (3). (3). R = clockwise (right) O S = counter clockwise (left) O H H OH O HO O diastereomers = stereoisomers that are not mirror images. enantiomers = mirror images that are not identical 31 32 The R isomer of limonene is found in oranges and the S isomer is found in lemons. Which structure below is the predominant compound found in lemons? How many possible stereoisomers are there for the following compound? Pr Me Pr Pr Et Me Et * * * Et A B Me A. 2 B. 4 C. 6 D. 8 E. 12 Pr Me Et Me Pr Et Pr Et Me Pr Et Me Et Et Pr Me Pr Me 33 2n stereoisomers for n stereogenic centers. (R,R,R) (R,R,S) (R,S,R) (R,S,S) (S.R,R) (S,R,S) (S,S,R) (S,S,S) 34 How many stereoisomers of androsterone are there? A. 8 B. 32 C. 64 D. 128 E. 256 How many stereocenters? How many chair-like rings in this molecule? Tetrodotoxin HO * ** * H* * H* H O androsterone a steroid 27 = 128 = total possible stereoisomers for the above structure. 35 36 1 Tetrodotoxin How many stereocenters? a) b) c) d) e) f) g) h) 4 5 6 7 8 9 10 11 Tetrodotoxin Answer: a) b) c) d) e) f) g) h) 4 5 6 7 8 9 10 11 37 38 Tetrodotoxin How many chair-like rings in this molecule? a) b) c) d) e) f) g) h) 4 5 6 7 8 9 10 11 Molecules with more than one stereocenter. H Cl HH Each of these hydrogens hydrogens are prochiral prochiral. Substituting any of these hydrogens will produce different stereoisomers. stereoisomers. 40 40 39 Molecules with more than one stereocenter. A. R B. S OH Br n=2 22 = 4 stereoisomers OH Br H Cl R R HO H H Cl R H OH H Cl RS * OH Br * R R H OH mirror plane HH mirror plane HO H R S enantiomers diastereomers Br OH enantiomers Br H OH S S HO H HH S H OH HO H S R 41 41 OH S R S S H Cl H Cl H Cl (2S,4R)-4-bromo-2-pentanol bromo- 42 2 What is the name of the enantiomer of (2R,3S,4S)-2-bromo-3-chloro-4-fluoropentane? bromo- chloroCH3 So then how are C and D related? Br CH 3 Br A. B. C. D. E. (2S,3S,4S)-2-bromo-3-chloro-4-fluoropentane bromo- chloro(2R,3R,4S)-2-bromo-3-chloro-4-fluoropentane bromo- chloro(2S,3S,4R)-2-bromo-3-chloro-4-fluoropentane bromo- chloro(2R,3S,4R)-2-bromo-3-chloro-4-fluoropentane bromo- chlorobromo- chloro(2S,3R,4R)-2-bromo-3-chloro-4-fluoropentane Br CH 3 1 C D A. They are diastereomers. B. They are enantiomers. C. They are conformational isomers. D. This still makes my head hurt. 43 44 Give the stereochemical relationship between the following molecules: Give the stereochemical relationship between the following molecules: H H HO Cl Cl H HO H (2R,4aS,8aS)-Decahydro-2-naphthalenol mp = 47-48 °C (2R,4aS,8aR)-Decahydro-2-naphthalenol mp = 67 °C a) b) c) d) Enantiomers Diastereomers Identical None of the above 46 a) b) c) d) Enantiomers Diastereomers Identical None of the above 47 Diastereomers have different physical properties! REMEMBER: Enantiomers have identical physical properties, EXCEPT… O O Pasteur’s Discovery of Enantiomers H (S)-carvone oil of caraway H (R)-carvone oil of spearmint Louis Pasteur (1822-1895) Strasbourg University (picture: Wikipedia) 48 49 3 A model for electromagnetic radiation (light). - A model for plane polarized light. δ OH unpolarized light δ C + 51 52 The Polarimeter An instrument for measuring the plane rotation of polarized light. O H2N H OH O H NH2 OH (S)-phenylalanine = -32.7° 32.7° (R)-phenylalanine = +32.7° +32.7° Jean-Baptiste Biot (17741862, Collège de France) discovered this phenomenon in the early 19th century (picture: Wikipedia) 53 54 End of Monday, Oct. 11 11 Lecture 55 4 ...
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