Practice_Exam_3_Key

Practice_Exam_3_Key - Page 1 PRACTICE EXAM KEY Spring 2009...

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Unformatted text preview: Page 1 PRACTICE EXAM KEY Spring 2009 Exam 3 – CH 0320 NAME ___________________________________ I’m READY! Page 2 1. Give an example for each description. Use Fisher, Haworth, or conformational representations. a) An aldopentose: H H HO CHO OH OH H CH2 OH b) A ketohexose: CH2 OH O H OH HO H H OH CH2 OH c) Methyl β-D-ribopyranoside: O OCH 3 OH OH OH HO H HO CHO H OH H d) L-xylose: CH2 OH e) Xylitol H H O H CH2 OH OH H OH CH2 OH f) Any reducing sugar: A ketose or an aldose, open or closed form. 2. Provide products for each of the following transformations. Page 3 HO HO a CHO H H CH2 OH Br2 /H2 O CHO HO H HO H CH2 OH b CO2 H H H CH2 OH HO HO HCN H HO HO CN OH H H CH2 OH H O c H CH3 NH2 /H O d e f H H H HO H HO HO H H C7 H13 NO O Mannich HS CHO OH OH H OH CH2 OH CHO H H OH OH CH2 OH S SH S BF3 NHNH2 2 equivalents HNO3 /H2 O CN H H H CH2 OH N CH3 + H O HO HO HO NN(H)C6 H5 NN(H)C6 H5 H OH HO H H OH CH2 OH HO HO H H CO2 H H H OH OH CO2 H Page 4 3. For each of the following transformations, provide mechanisms. (8 points each) a) Show each bond forming step, including deprotonations and protonations. b) Show correct formal charges on each atom that is not neutral. c) In base catalyzed reactions, show an appropriate source for protonation. (for example, H2 O or HOR). a. HO OH HO OH HOCH 3/H+ HO O H3 CO O -H+ H+ HO + H2 O H O -H2 O HO + OH O +HOCH3 HO + H3 C(H)O OH O Page 5 3 (continued – Show the mechanism). b. 1) MeI (excess) 2) Ag2 O/H2 O/heat N H MeI H N+ H3 C CH3 OH (H3 C)2 N Page 6 4. Show you would prepare glucose from a pentose using the Kiliani-Fisher method. Show all reagents. Choose the appropriate starting material. Show all intermediates, but it is not necessary to show mechanism. CHO HO H HCN H OH H OH CH2 OH H HO H H CN CN 1) H2 /Pd-BaSO4 CHO HO H OH H OH 2) H2 O/H+ HO H H HO H + H OH OH H OH H OH OH H OH CH2 OH CH2 OH CH2 OH Page 7 5. Devise a preparation of the following amine using the reductive amination conditions. H N CH3 Reductive amination requires a carbonyl compound, and amine, and a reducing agent. The usual process is to use them all at once, in the presence of mild acid (pH about 3.5). H O N CH3 NaCNBH3 + H2 NCH3 An alternative correct answer would form the imine and reduce it with LiAlH4 , catalytic hydrogenation, or other means that have been documented. Page 8 6. a) Draw a hexose that will give an optically active product after treatment with nitric acid. The nitric acid reaction affords an aldaric acid. The product must not have a plane of symmetry. Possible correct answers would be drawings, in any form (either D- or L-) of altrose, glucose, mannose, gulose, idose, or talose. b) Draw a pentose that will afford an optically inactive alditol. Pentose alditols are primary alcohols at C-1 and C-5. We require a plane of symmetry for an optically inactive alditol. Alditols from ribose and xylose have a plane of symmetry. 7. Ruff degradation of Aldose A affords a pentose. The same pentose is formed through a Kiliani-Fisher synthesis beginning with D-threose. Treatment of Aldose A with NaBH4 affords an achiral product. What is Aldose A? Show your reasoning. Aldose A must be a hexose (degradation forms a pentose). Only allose and galactose form achiral products (optically inactive) when treated with NaBH4. Ruff degradation of allose affords ribose. Ruff degradation of galactose affords lyxose. Pentoses that can be formed from D-threose are D-xylose and D-lyxose. Therefore, because the hexose must be allose or galactose, it degrades to ribose or lyxose, and only lyxose can be made from threose, Aldose A must be D-galactose. Page 9 ...
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