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5.01 Carbohydrates Answers.Cdx Chem S-20ab Purple Book Solutions Week 5 - Page 1 of 37 Carbohydrates 1. When glucose is treated with methanol and acid, the methyl glucoside is formed. Provide a complete curved-arrow mechanism for this transformation: OH OH HO CH OH, H+ HO O 3 O H+ HO OH HO OMe OH OH OH OH OH HO HO O HO O O HO OH HO OMe 2 HO OH OH H OH HOCH3 Sol 2. The correct stereochemistry of only one stereocenter is indicated on the above structure. The actual stereochemistry of methyl glucoside is such that, when it is in a chair form, all of the substituents fall in the equatorial positions, except for the –OMe, which is found in the axial position. Given this information, draw a clear, correct representation of methyl glucoside in a chair conformation, with all substituents properly indicated. In addition, explain why the –OMe group preferably adopts an axial conformation; a diagram will be essential. OH This is an example of the anomeric effect. HO OH The –OMe goes axial in order that the lone pair O * OH C–O O fromtheringOcandonateinto *C–O of the –OMe group. OMe OMe 3. When glucose is in its open-chain form, its stereochemistry can H O best be represented using a Fischer projection. On the skeleton Fischer projection shown at right, fill in the H and OH groups of the glucose "backbone" with the correct stereochemistry as H OH determined from your structure shown above. HO H H OH H OH CH2OH Chem S-20ab Purple Book Solutions Week 5 - Page 2 of 37 Carbohydrates: Mechanisms 1. Provide a complete curved-arrow mechanism for all steps in the following transformation. OH HO HO O O OH OH– catalyst HO HO O H OH OH OH OH glucose HO O H—OH O HO OH OH OH H—OH HO O Number the carbons carefully to see where they are going! This O HO O is a tricky mechanism! HO OH OH HO OH OH O HO OH OH O H HO OH HO OH O HO :O H OH OH HO O HO OH H OH O :OH OH OH HO HO OH OH H—OH HO O HO OH OH HOH O Chem S-20ab Purple Book Solutions Week 5 - Page 3 of 37 More Carbohydrates Mechanisms 1. Sorbitol is a sugar alcohol that is used as a sweetener in many low-calorie foods. It can be prepared from glucose. Provide a complete curved-arrow mechanism for the following reaction. You may ignore stereochemistry for the purpose of this mechanism. CH2OH OH OH HO NaBH O 4 HO OH NaOH , H2O HO OH OH OH OH glucose OH sorbitol CH2OH OH OH HO HO O O OH OH HO O OH HO H NaBH4 CH OH CH2OH 2 O H HO H HO O OH "H-" HO O HO O OH OH Chem S-20ab Purple Book Solutions Week 5 - Page 4 of 37 The Anomeric Effect 1. Draw the most stable chair conformations of the following molecules. Explain your choices in terms of either sterics or molecular orbital interactions.Whichwouldyouexpecttohaveashorter C-Cl bond? Cl Cl S Cl Cl prefers to be axial Cl so that S lone pair can * S C–Cl * donate into C–Cl antibonding orbital Cl prefers to be (the anomeric effect). equatorial (sterics) This overrides the steric preference. Slp This one would have a longer C–Cl bond because the donation into C–Cl * antibonding orbital will weaken the C–Cl bond. 2. Explain why the molecule H3C—S—S—CH3, when viewed along the S—S bond, prefers to adopt the gauche conformation. (A Newman projection will be helpful). Describe the orbital interaction(s) involved. CH3 CH3 CH3 CH3 S S There are two interactions: and S lone pair (in the front) Slonepair(intherear) donating into S–C *(intherear) donating into S–C *(inthefront) (Note that only one lobe of the S–C * antibonding orbital is shown.) CH3 S In the anti conformation there are no such interactions: CH3 Chem S-20ab Purple Book Solutions Week 5 - Page 5 of 37 More Anomeric Effect 1. Each of the following observations can be explained by a frontier molecular orbital (FMO) argument. Provide such an explanation for each observation. Be sure to label the relevant donor-acceptor interactions and provide a clear diagram showing the interacting orbitals. (An energy-level diagram is not expected.) a) The following ester prefers conformation A rather than conformation B.(Hint: Consider all thelonepairs in the molecule. A "steric" argument is not acceptable.) O O preferred to CH3 H3C O H3C O CH3 AB O O CH3 H3C O H3C O CH3 In conformation B,thesp2-hybrid In conformation A,thesp2-hybridized oxygen lone pair on the ester oxygen is lone pair is positioned to overlap with *of notabletooverlapwiththe *of the carbonyl (not *).Thisisafavorable the neighboring carbonyl, so there interaction which lowers the overall energy of is no such favorable interaction. this conformation. b) The two conformers of the nitrite ester shown below have slightly different bond lengths (as indicated). (Note that both conformations are entirely planar. How are the bond lengths different between A and B?) O O R 1.17 Å 1.19 Å 1.39 Å NO NO 1.43 Å R A B There is an important orbital interaction in conformation B which is not present Olp O R which makes in conformation A: donating the N=O bond longer and the into NO central N–O bond *N–O shorter In conformation A the orbitals are not aligned properly for this lone pair -> * donation. Chem S-20ab Purple Book Solutions Week 5 - Page 6 of 37 Synthesis and Properties of Amino Acids Here are three of the naturally-occuring amino acids: O O O H2N H2N H2N OH OH OH OH serine NH2 OH lysine O glutamic acid 1. Draw the structures of these three amino acids as they would appear at pH = 7. Classify each one as "neutral," "basic," or "acidic." O + H3N O O O– + + H3N H3N O– O– O– OH acidic + neutral basic NH3 O 2. Match these amino acids with the following isoelectric points: 3.2 5.8 10.1 Explain your reasoning. glutamic serine lysine acid Isoelectric point is the pH at which the species will be neutral. Glutamic acid will only be neutral if its sidechain acid is protonated at low pH. Lysine will only be neutral if its sidechain amino group is unprotonated at high pH. Serine will have an isoelectric point near neutrality. 3. Propose a synthesis of serine (as a racemic mixture) using the Strecker synthesis. + – H O H2N CN + H3N COO NaCN H3O H NH4Cl OH OH OH Chem S-20ab Purple Book Solutions Week 5 - Page 7 of 37 Peptide Synthesis 1. Provide reagents for protecting and deprotecting an amino acid as shown: O O – O MeOH, H+ 1. OH H2N H2N H2N OH OMe 2. H+ workup OH R R R O O O O O tBuO O OtBu H TFA t H2N BuO N H2N OH OH OH R (this is known O R R as "Boc-O-Boc") 2. Using these protecting groups, synthesize the following dipeptide using traditional solution-phase chemistry. You may start with amino acids that are already protected in any way you choose. O Me H tBuO N + OMe OH H2N O O DCC O Me H tBuO N OMe N H O O 1. TFA 2. OH- 3. neutralize O Me H2N OH N H O glycylalanine Chem S-20ab Purple Book Solutions Week 5 - Page 8 of 37 Complete Peptide Synthesis 1. Provide a multistep synthesis for the desired product. You may use any inorganic reagents, plus any organic reagents with 8 carbons or less, the protected amino acid shown below, and the organic reagent DCC. The best answer will require six or fewer steps. (Hint: Think carefully about protecting-group strategy; it is possible to hydrolyze a t-butyl ester using dry acid without disturbing other esters.) For the purpose of this question you may ignore the stereochemistry of the product. Starting material: the following partially-protected amino acid, plus any compounds with 8 carbons or less. O O H O OH H2N O + – 1. NH4 ,CN O O + 2. H3O O O O (Strecker) (Boc-anhydride protecting grp.) O O O H2N OH O H2N OMe OH + O NH MeOH, H+ O (Protect) O DCC coupling O HO O O H N H2N OCH3 O O + H H (dry) O N deprotect O NH OMe aspartame tert-butyls ® O (NutraSweet ) O Chem S-20ab Purple Book Solutions Week 5 - Page 9 of 37 Solid-Phase Peptide Synthesis 1. Now show how you could use solid-phase peptide synthesis to synthesize the same dipeptide, below. Again, you may start with amino acids which are protected in any way you choose. O Me OH Cl tBuO N Merrifield H Resin O Boc-protected Cs2CO3 Alanine O Me O– Cs+ tBuO N H O O Me O tBuO N H O TFA (remove tBoc) Me O H2N O O H t Boc-protected BuO N glycine OH DCC O O Me H tBuO N O N H O O O Me HF H2N OH N (remove H Boc O and detach from glycylalanine resin) Chem S-20ab Purple Book Solutions Week 5 - Page 10 of 37 More Complete Peptide Synthesis 1. Provide a complete synthesis of the dipeptide, lysylglycine, shown below. You may start with unprotected amino acids only. All of the carbon atoms from these amino acids must end up in the product.
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