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ANSWER ALL QUESTIONS 1. Z-But-2-ene undergoes a 1,3-dipolar cycloaddition with benzenenitrile oxide 2 to give cycloadduct 3. Stereoselective reduction using LiAlH4 of 3 gives 4 (Scheme 1). Scheme 1 i. Draw a structure for cycloadduct 3 and provide a mechanism for its formation. (3 marks) ii. Nitrile oxide 2 is derived from the reaction of benzenaldehyde oxime with hypochlorous acid (HOCl). Provide a detailed mechanism for this transformation into 1,3-dipole 2. (3 marks) iii. Provide detailed mechanisms for the conversion of 3 into 4, and account for the stereochemistry of 4. (4 marks) (Total 10 marks) 2 O O O NH R Fulgimide (E Isomer) 2. The following scheme shows the proposed synthesis of a heliochromic precursor compound D. (i) Suggest how would you convert fulgide D into a fulgimide (3 marks) (ii) What type of catalyst is required for step A? (2 marks) (iii) Compound D undergoes a reversible intramolecular photocyclisation. Show the mechanism for this and indicate what isomerism must occur before this can happen and how this cyclisation can be reversed. (5 marks) (Total 10 marks) C D A B 3 3. Answer ALL parts of this question. Erythromycin A is a macrolide antibiotic isolated from the species Saccharopolyspora erythraea. It is biosynthesised from the macrolide 6- deoxyerythronolide, which is in turn biosynthesised from 7 molecules of propionyl-CoA. It is commonly prescribed for the treatment of penicillin sensitive bacterial infections of the respiratory system in patients with penicillin-allergy, however its use as a more systemic antibiotic are limited by its acid sensitivity. The enzyme pathway responsible for the biosynthesis of 6-deoxyerythronolide from propionyl-CoA utilises the following enzymes: acetal transferase (AT), acyl carrier protein (ACP), dehydratase (DH), enoyl reductase (ER), ketoacylreductase (KR), ketoacylsynthase (KS) and thioesterase (TE). Elucidate and explain the order that these enzymes must be utilised for modules 4 and 5 in order to yield 6-deoxyerthronolide B using the diagram below. The other modules have been completed. 4 (10 marks) 4. Using the Cram chelate model for nucleophilic addition, predict which alcohol isomer will be formed as the major product when Mg(CH3)2 reacts with (2S)-2- methoxy-1-phenyl-propan-1-one, A. A In your answer: (a) Draw a fully labelled Newman projection to represent the reaction transition state, indicating the groups occupying the ‘small’, ‘medium’ and ‘chelating / large’ positions. The nucleophile may be regarded as ‘CH3 - ’. (6 marks) (b) Give the name of the angle of attack at which the nucleophile approaches the carbonyl. (1 mark) (c) Give the name for the category of isomers which describes the two possible products of nucleophilic addition (note: the answer required is not syn and anti). (1 mark) (d) Draw the final product in the extended (zig-zag) conformation and correctly assign the configuration of the newly formed chiral centre using the CIP rules (working must be shown). (2 marks) 5 (Total 10 marks) 5. Cardiac glycosides are natural products isolated from some plants. a. Name one condition that may be treated using cardiac glycosides and define the term cardioactive glycoside. b. Describe the mode of action of cardioactive glycosides (a scheme is not required). (1+2 marks) c. Cardiotonic steroids (CS) are seldom used as drugs, instead another class of drugs is used. Give the reasons why CS aren’t employed as a first line of treatment and name the therapeutic agents that are. (2 marks) d. Scillaren A (1) and oleandrin (2) are each a member of the two main classes of cardiotonic glycosides. Identify those classes referring to the structural moiety specific to each class. (1 mark) OH O O O O HO O OH O HO HO OH OH H OH O H O O H O H O O HO O H 1 2 (1 mark) e. Using a simplified structure, highlight the main structural features found in all cardiotonic glycoside. (1 marks) 6 f. Below is the structure of digoxin (3), a cardiotonic glycoside used as a cardiotonic drug. Referring to its chemical structure, list the main structural characteristics common to all cardiotonic steroids. H OH

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