91Ó°ÊÓ

Diels-Alder reaction of a monosubstituted diene (such as \(\mathrm{CH}_{2}=\mathrm{CH}^{ \left.-\mathrm{CH}=\mathrm{CHOCH}_{3}\right)}\) with a monosubstituted dienophile (such as \(\mathrm{CH}_{2}=\mathrm{CHCHO}\) ) gives a mixture of products, but the 1,2 -disubstituted product often predominates. Draw the resonance hybrid for each reactant and use the charge distribution of the hybrids to explain why the 1,2 -disubstituted product is the major product.

Like alkenes, conjugated dienes can be prepared by elimination reactions. Draw a stepwise mechanism for the acid-catalyzed dehydration of 3 -methyl-2-buten-1-ol \(\left[\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CHCH}_{2} \mathrm{OH}\right]\) to isoprene \(\left[\mathrm{CH}_{2}=\mathrm{C}\left(\mathrm{CH}_{3}\right) \mathrm{CH}=\mathrm{CH}_{2}\right]\).

(a) Draw the two isomeric dienes formed when \(\mathrm{CH}_{2}=\mathrm{CHCH}_{2} \mathrm{CH}(\mathrm{Cl}) \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\) is treated with an alkoxide base. (b) Explain why the major product formed in this reaction does not contain the more highly substituted alkene.

The treatment of isoprene \(\left[\mathrm{CH}_{2}=\mathrm{C}\left(\mathrm{CH}_{3}\right) \mathrm{CH}=\mathrm{CH}_{2}\right]\) with one equivalent of \(\mathrm{mCPBA}\) forms \(\mathrm{A}\) as the major product. A gives a molecular ion at 84 in its mass spectrum, and peaks at \(2850-3150 \mathrm{~cm}^{-1}\) in its IR spectrum. The 'H NMR spectrum of \(\mathbf{A}\) is given below. What is the structure of \(A\) ?

Explain why ferulic acid, a natural product found in rice, oats, and other plants, is both an antioxidant and a sunscreen.