Questions of the Day: a) Diels-Alder reactions are what type of reactions? b) Which structural feature is always formed as the result of Diels-Alder reactions? c) What is a “good diene” (i.e. one that reacts fast in Diels- Alder)? d) What would be a good dienophile?

Today: Diels-Alder (cont.), NMR Esterification (Exp.5), a first introduction MiniQuiz

Diels-Alder reactions require a 1,3-diene (in s-cis) and a dienophile (an alkene, best when EWG is attached). Reaction equations for our experiment: Retro Diels Alder: Cracking of dimer Diels Alder: Reaction of 1,3-cyclopentadiene with maleic anhydride Hydrolysis of Diels-Alder product Dimer D.-A. adduct Cis endo Compare: Polarity of I versus II Mp. of I versus II

Diels – Alder Reactions Which of the following dienes would be most reactive? why? 1,3-cyclopentadiene, 1,3-butadiene (see problems on Report Form) Which of the following dienophiles would be more reactive? Ethene or maleic anhydride

Diels – Alder Reactions Synthesis of Natural Products AB C D Steroids!

Cholesterol AB C D CH 3 HO

Diels Alder The H-NMR of the dimer of 1,3-cyclopentadiene is quite complex, with many signals. In contrast, the proton NMR of our Diels-Alder product is simple, with few signals; the same is true for its hydrolysis product. How can you explain this?

In order to increase the yield of the ester, which of the following would help? use an excess of methanol use an excess of conc. sulfuric acid Calculations …! Methyl Benzoate Synthesis (Exp.5) the first step of a two- step synthesis You will heat your reaction under reflux for 1 hour! Do Practice Problems, calculations … Why sulfuric acid and not HCl?

1.General properties of esters? 2.Compare the boiling points of carboxylic acids and esters of similar molecular weight and explain. 3.Examples of esters in biological systems? Esters and Esterifications Fats and Oils Waxes Ripening process in fruits phosphate esters in DNA

Salicin and Aspirin Salicin in willow bark (Salix)

Next time: Conclusion of “Esterification” (Exp. 5) Grignard reactions (Exp. 6) MiniQuiz on today’s class

MiniQuiz 4 Problems: 1)Draw a simple, but clear H-NMR for alpha - monobromoethylbenzene 2)Draw a simple, but clear H-NMR for alpha - dibromoethylbenzene