14 • Additional Problems
14 • Additional Problems
Show the structures of all possible adducts of the following diene with 1 equivalent of HCl:
Show the product of the Diels–Alder reaction of the following diene with 3-buten-2-one,
H2CCHCOCH3. Make sure you show the full stereochemistry of the reaction product.
The following diene does not undergo Diels–Alder reactions. Explain.
The following model is that of an allylic carbocation intermediate formed by protonation of a conjugated diene with HBr. Show the structure of the diene and the structures of the final reaction products.
We’ve seen that the Diels–Alder cycloaddition reaction is a one-step, pericyclic process that occurs through a cyclic transition state. Propose a mechanism for the following reaction:
Luminol, which is used by forensic scientists to find blood, fluoresces as a result of Diels–Alder-like process. The dianion of luminol reacts with O2 to form an unstable peroxide intermediate that then loses nitrogen to form a dicarboxylate and emit light. The process is similar to that in Problems 14-21 and 14-22. Propose a mechanism for this process.
A useful diene in the synthesis of many naturally occurring substances is known as Danishefsky’s diene. It’s useful because after the Diels–Alder reaction it can be converted into a product that can't be accessed by a typical Diels–Alder reaction. Show the Diels–Alder adduct and propose a mechanism that accounts for the final products.
Electrophilic addition of Br2 to isoprene (2-methyl-1,3-butadiene) yields the following product mixture:
Of the 1,2-addition products, explain why 3,4-dibromo-3-methyl-1-butene (21%) predominates over 3,4-dibromo-2-methyl-1-butene (3%).
Draw the possible products resulting from addition of 1 equivalent of HCl to 1-phenyl-1,3-butadiene. Which would you expect to predominate, and why?
2,3-Di-tert-butyl-1,3-butadiene does not undergo Diels–Alder reactions. Explain.
Show the structure, including stereochemistry, of the product from the following Diels–Alder reaction:
Reaction of isoprene (2-methyl-1,3-butadiene) with ethyl propenoate gives a mixture of two Diels–Alder adducts. Show the structure of both, and explain why a mixture is formed.
Rank the following dienophiles in order of their expected reactivity in the Diels–Alder reaction.
1,3-Pentadiene is much more reactive in Diels–Alder reactions than 2,4-pentadienal. Why might this be?
Show the product of the following reaction.
The following ultraviolet absorption maxima have been measured:
What conclusion can you draw about the effect of alkyl substitution on UV absorption maxima? Approximately what effect does each added alkyl group have?
β-Ocimene is a pleasant-smelling hydrocarbon found in the leaves of certain herbs. It has the molecular formula C10H16 and a UV absorption maximum at 232 nm. On hydrogenation with a palladium catalyst, 2,6-dimethyloctane is obtained. Ozonolysis of β-ocimene, followed by treatment with zinc and acetic acid, produces the following four fragments:
Addition of HCl to 1-methoxycyclohexene yields 1-chloro-1-methoxycyclohexane as a sole product. Use resonance structures to explain why none of the other regioisomer is formed.
Aldrin, a chlorinated insecticide now banned from use in most countries since 1990, can be made by Diels–Alder reaction of hexachloro-1,3-cyclopentadiene with norbornadiene. What is the structure of aldrin?
The triene shown here reacts with 2 equivalents of maleic anhydride to yield a product with the formula C17H16O6. Predict a structure for the product.
Myrcene, C10H16, is found in oil of bay leaves and is isomeric with β-ocimene (Problem 14-49). It has an ultraviolet absorption at 226 nm and can be hydrogenated to yield 2,6-dimethyloctane. On ozonolysis followed by zinc/acetic acid treatment, myrcene yields formaldehyde, acetone, and 2-oxopentanedial:
Propose a structure for myrcene, and write the reactions, showing starting material and products.
Hydrocarbon A, C10H14, has a UV absorption at λmax = 236 nm and gives hydrocarbon B, C10H18, on hydrogenation. Ozonolysis of A, followed by zinc/acetic acid treatment, yields the following diketo dialdehyde:
An illustration shows the structure of diketo dialdehyde. It shows two carbonyl groups single bonded to each other. Each carbonyl group is bonded to a chain of three methylene groups and then to an aldehyde group.
Adiponitrile, a starting material used in the manufacture of nylon, can be prepared in three steps from 1,3-butadiene. How would you carry out this synthesis?
Ergosterol, a precursor of vitamin D, has λmax = 282 nm and molar absorptivity ϵ = 11,900. What is the concentration of ergosterol in a solution whose absorbance A = 0.065 with a sample pathlength l = 1.00 cm?
Dimethyl butynedioate undergoes a Diels–Alder reaction with (2E,4E)-2,4-hexadiene. Show the structure and stereochemistry of the product.
How would you carry out the following synthesis (more than one step is required)? What stereochemical relationship between the –CO2CH3 group attached to the cyclohexane ring and the –CHO groups would your synthesis produce?
The double bond of an enamine (alkene + amine) is much more nucleophilic than a typical alkene double bond. Assuming that the nitrogen atom in an enamine is sp2-hybridized, draw an orbital picture of an enamine, and explain why the double bond is electron-rich.
Benzene has an ultraviolet absorption at λmax = 204 nm, and para-toluidine has λmax = 235 nm. How do you account for this difference?