Organic Chemistry Task

Redraw structures and identify enantiotopic or diastereotopic protons

a) Carefully redraw the three structures below and clearly identify on them any enantiotopic or diastereotopic protons. [10%]

                                           
b) What is the missing reagent - ‘?’ over the first arrow? This generates (in situ) a new reactant which reacts with (S)-phenylalanine 1. With the aid of curly arrows, provide a comprehensive overview of all the mechanistic considerations important for the formation of the unidentified intermediate in the bracket, its transformation into the strained lactone, and its reaction with water to afford product 2. [10%]

                                                     
c) Using N-Boc-(S)-tert-leucine (3) as your starting material, outline a suitable sequence of reactions, reagents and a full mechanism using curly arrows that accounts for the synthesis of (S)-tert-leucine SuperQuat 4. [10%]

                                                                             
d) Using N-acylated SuperQuat auxiliary 5, LDA, and allyl bromide (6) sketch a full mechanistic rational which explains the formation of your new product (indicated by the first ‘?’) and any new stereochemical centers within it. Explain why the introduction of the gem-dimethyl substituents on the SuperQuat are key to excellent levels of stereoinduction. When the product of your first reaction is reacted with lithium alkoxide 7 what happens? Using curly arrows show the mechanism and product of this reaction. [15%]

                                                           
e) Using N-acylated SuperQuat auxiliary 8, dibutylboron triflate, diisopropylethylamine and 3-nitrobenzaldehyde sketch a full mechanistic rational which uses all these and the structure of the product you would expect. What is the name of this reaction? Your answer should include a model to explain the stereoselectivity.  

[15%]

                           
f) Using the Schollköpf chiral auxiliary 9 how would you synthesize a-amino acid 10 with its quaternary stereocenter. Be sure to sketch a full multi-step reaction protocol and mechanistic rational explaining how asymmetry is induced at the quaternary center in 10 using 9.  Assign the stereochemistry of the new stereochemical center in 10. Suggest appropriate reagents, bases, electrophiles etc you need to effect the asymmetric synthesis of 10. [20%]

                                                             
g) Using a,b-unsaturated aldehyde 11, N-methylindole 12, catalyst 13 and the chlorinated ketone 14 outline the synthesis of optically active 15. Be sure to sketch a full mechanistic rational that includes curly arrows and accounts for the stereochemistry in the optically active product 15. [20%]