The absorbance spectrum of 1,3-butadiene displays an absorption in the UV region

For each of the following compounds, identify whether each CRC p system is cumulated, conjugated, or isolated: O OH OH O HO O cis-Aconitic acid Plays a role in the (a) citric acid cycle Ocimene Present in the essential oils (b) of many plants cis-Jasmone A constituent of many perfumes O (c) (R)-Carvone Responsible for the flavor of spearmint

Identify reagents necessary to convert cyclohexane into 1,3-cyclohexadiene. Notice that the starting material has no leaving groups and cannot simply be treated with a strong base. You must first introduce functionality into the starting material (for help with introducing functional groups, see Section 12.2).

Rank the three C!C bonds of 1-butene in terms of bond length (from shortest to longest).

Compare the following three isomeric dienes: (a) Which compound will liberate the least heat upon hydrogenation with 2 mol of hydrogen gas? Why? (b) Which compound will liberate the most heat upon hydrogenation with 2 mol of hydrogen gas? Why? (For help, see Section 8.5.)

Identify the most stable compound:

Draw an energy diagram showing the relative energy levels of the MOs for 1,3,5,7-octatetraene and identify the HOMO and LUMO for both the ground state and the excited state.

Predict the products for each of the following reactions and propose a mechanism that explains the formation of each product: (a) HCl ? (b) HCl ? (c) HCl ? (d) HBr ? (e) HBr ? (f) HBr ?

Consider the following two dienes. When treated with HBr, one of these dienes yields four products, while the other diene yields only two products. Explain.

Propose the structure of a conjugated diene for which 1,2-addition of HBr produces the same product as 1,4-addition of HBr.

Predict the products for each of the following reactions, and in each case, determine which product will predominate: (a) 40C HBr ? (b) 0C ? HCl (c) 0C ?

When 1,4-dimethylcyclohepta1,3-diene is treated with HBr at elevated temperature, the 1,2-adduct predominates, rather than the 1,4-adduct. Explain this result. Br 1,2-Adduct Br 1,4-Adduct + 1 2 3 4 Br 40C

When 1,3-cyclopentadiene is treated with HBr, the temperature does not have an influence on the identity of the major product. Explain.

What monomer should be used to make each of the following polymers: (b) F F F F F F C C C N N N (a)

Predict the products for each of the following reactions: N N CN CN + En O OMe O OMe (a) + COOH (b) COOH + O (c) O + O O + (d) O O (e) O + NC CN (f) + O (g) + O OH (h) + O (i) +

Predict the product(s) obtained when benzoquinone is treated with excess butadiene: O O (Excess)

Propose a mechanism for the following transformation: O O O O

Consider the following two isomers of 2,4-hexadiene. One isomer reacts rapidly as a diene in a Diels-Alder reaction, and the other does not. Identify which isomer is more reactive and explain your choice. (2E,4E)-Hexadiene (2Z,4Z)-Hexadiene

Rank the following dienes in terms of reactivity in DielsAlder reactions (from least reactive to most reactive):

Predict the products for each of the following reactions: (a) NC CN + O CN (b) NC + O H (c) + S MeO MeO COOH HOOC (d) + CN (e) + O O O O (f)

Consider the following [4+4] cycloaddition process. Would you expect this process to occur through a thermal or photochemical pathway? Justify your answer with MO theory. +

Predict the major product(s) for each of the following thermal electrocyclic reactions: (a) ? Heat (b) ? Heat ? (c) Heat

Predict the product(s) for each of the following reactions: Heat ? (c) (a) hn ? (b) hn

Draw the structure of cis-3,4-diethylcyclobutene. (a) Conrotatory ring opening produces only one product. Draw the product and determine whether its formation is best achieved under thermal conditions or photochemical conditions. (b) Disrotatory ring opening can produce two possible products, but only one of these products is obtained in practice. Identify the product that is formed and explain why the other possible product is not formed.

Identify whether the product obtained from each of the following reactions is a meso compound or a pair of enantiomers: (a) Irradiation of (2E,4Z,6Z )-4,5-dimethyl-2,4,6-octatriene with UV light (b) Subjecting (2E,4Z,6Z )-4,5-dimethyl-2,4,6-octatriene to elevated temperature (c) Irradiation of (2E,4Z,6E)-4,5-dimethyl-2,4,6-octatriene with UV light

For each of the following reactions, use brackets and two numbers to identify the type of sigmatropic rearrangement taking place: O O (a) Heat H H (b) Heat

Consider the structure of cis-1,2-divinylcyclopropane: This compound is stable at low temperature but rearranges at room temperature to produce 1,4-cycloheptadiene. (a) Draw a mechanism for this transformation. (b) Using brackets and numbers, identify the type of sigmatropic rearrangement taking place in this case. (c) This reaction proceeds to completion, which means that the concentration of the reactant is insignificant once the equilibrium has been established. Explain why this reaction proceeds to completion.

Predict the product for each of the following reactions: O (a) Heat ? (b) ? Heat (c) Heat ? O (d) He

Draw a plausible mechanism for the following transformation: Heat

Use Woodward-Fieser rules to estimate the expected lmax for each of the following compounds: (a) (b) (c) (d)

Identify which of the following compounds is expected to have a larger lmax:

For each of the following tasks, use the color wheel in Figure 17.37. (a) Identify the color of a compound that absorbs orange light. (b) Identify the color of a compound that absorbs blue-green light. (c) Identify the color of a compound that absorbs orange-yellow light.

Draw the structure of each of the following compounds: (a) 1,4-Cyclohexadiene (b) 1,3-Cyclohexadiene (c) (Z)-1,3-Pentadiene (d) (2Z,4E)-Hepta-2,4-diene (e) 2,3-Dimethyl-1,3-butadiene

Circle each compound that has a conjugated p system: O

For each of the following pairs of compounds determine whether the two compounds would be at equilibrium at room temperature or whether they could be isolated from one another: (a) (b) (c)

Identify the structure of the conjugated diene that will react with one equivalent of HBr to yield a racemic mixture of 3-bromocyclohexene.

Draw the major product expected when 1,3-butadiene is treated with one equivalent of HBr at 0C and show the mechanism of its formation.

Draw the major product expected when 1,3-butadiene is treated with one equivalent of HBr at 40C and show the mechanism of its formation.

Draw all four products obtained when 2-ethyl-3-methyl-1,3- cyclohexadiene is treated with HBr at room temperature and show the mechanism of their formation.

After performing the reaction from Problem 17.38, the reaction flask is heated to 40C and two of the products become the major products. When the flask is then cooled to 0C, no change occurs in the product distribution. Explain why an increase in temperature causes a change in the product distribution and explain why a subsequent reduction in temperature has no effect.

Each of the following compounds does not participate as a diene in a Diels-Alder reaction. Explain why in each case. (a) (b) (c) (d)

Rank the following dienophiles (from least reactive to most reactive) in terms of reactivity in a Diels-Alder reaction: O O O O

The absorbance spectrum of 1,3-butadiene displays an absorption in the UV region (lmax=217 nm), while the absorption spectrum of 1,2-butadiene does not display a similar absorption. Explain.

Predict the products for each of the following Diels-Alder reactions: COOH HOOC (a) + (b) HOOC CN + O O (c) + S O O O (d) + CN (e) NC + O O O O S MeO OMe (f)

Identify the reagents you would use to prepare each of the following compounds via a Diels-Alder reaction: COOH (a) COOH CN CN CN CN (b) CHO CHO (c) O O O O (d) O O H H (e) O O H H (f) COOH COOH En (g) + O (h)

Starting with 1,3-butadiene as your only source of carbon atoms and using any other reagents of your choice, design a synthesis of the following compound: H O CH3 + En

The following triene reacts with excess maleic anhydride to produce a compound with molecular formula C14H12O6. Draw the structure of this product (ignoring stereochemistry). C14H12O6 O O O Maleic anhydride (excess)

Chlordane is a powerful insecticide that was used in the United States during the second half of the twentieth century. Its use was discontinued in 1988 in recognition of its persistence and accumulation in the environment. Identify the reagents you would use to prepare chlordane via a Diels-Alder reaction. Cl Cl Cl H H Cl Cl Cl Cl Cl Chlordane

Cyclopentadiene reacts very rapidly in Diels-Alder reactions. In contrast, 1,3-cyclohexadiene reacts more slowly and 1,3-cycloheptadiene is practically unreactive. Can you offer an explanation for this trend?

Compound A (C7H10) exhibits a lmax of 230 nm in its UV absorption spectrum. Upon hydrogenation with a metal catalyst, compound A will react with two equivalents of hydrogen gas. Ozonolysis of compound A yields the following two compounds. Identify two possible structures for compound A. H H O O H O O Compound A H 1) O3 2) DMS

Rank the following compounds in order of increasing lmax:

Which of the following compounds do you expect to have a longer lmax?

Predict the expected lmax of the following compound:

When 5-deutero-5-methyl-1,3-cyclopentadiene is warmed to room temperature, it rapidly rearranges, giving an equilibrium mixture containing the original compound as well as two others. Propose a plausible mechanism for the formation of these other two compounds. Me D Me D Me D 25C 25C

Propose a plausible mechanism for the following transformation: D D D D Heat

Propose a method for achieving the following transformation:

Predict the product for each of the following electrocyclic reactions: (a) ? Heat (b) ?hn (c) Heat ? (d) ? hn

Predict which side of the following equilibrium is favored and explain your choice. Heat

When trans-3,4-dimethylcyclobutene is heated, conrotatory ring opening can produce two different products, yet only one is formed. Draw both products, identify which product is formed, and then explain why the other product is not formed.

Predict the product for each of the following reactions: O Heat (a) ? O Heat (b) ? Heat (c) ?

a-Terpinene is a pleasant-smelling compound present in the essential oil of marjoram, a perennial herb. Upon hydrogenation with a metal catalyst, a-terpinene reacts with two equivalents of hydrogen gas to produce 1-isopropyl-4-methylcyclohexane. Ozonolysis of a-terpinene yields the following two compounds: O H H O O O (a) How many double bonds are present in a-terpinene? (b) Identify the structure of a-terpinene. (c) Predict the wavelength of maximum absorption (lmax) in the absorption spectrum of a-terpinene.

Draw all possible conjugated dienes with molecular formula C6H10, taking special care not to draw the same compound twice.

Treatment of 1,2-dibromocycloheptane with excess potassium tert-butoxide yields a product that absorbs UV light. Identify the product.

In each of the following pairs of compounds identify the compound that liberates the most heat upon hydrogenation: (a) (b)

Would you expect nitroethylene to be more or less reactive than ethylene in a Diels-Alder reaction? (Hint: Draw the resonance structures of nitroethylene.) Ethylene Nitroethylene N O O ! @

Propose a plausible mechanism for the following transformation. (Hint: Only two sequential pericyclic reactions are required.) O O ^

When 2-methoxy-1,3-butadiene is treated with 3-buten-2- one, the resulting Diels-Alder reaction has two potential regiochemical outcomes: MeO O MeO O Heat MeO O + Nevertheless, only one of these products is obtained. Draw resonance structures for the diene and the dienophile and use these resonance structures to make an educated guess regarding which product is formed.

Propose a mechanism for the following transformation: H H H H Heat

Based on your answer to Problem 17.67, propose a mechanism for the following transformation: Heat O O O O O O CO2 + +

Compare the structures of 1,4-pentadiene and divinylamine: 1,4-Pentadiene N H Divinylamine The first compound does not absorb UV light in the region between 200 and 400 nm. The second compound does absorb light above 200 nm. Using this information, identify the hybridization state of the nitrogen atom in divinylamine and justify your answer.

The poison gelsemoxonine can be isolated from the leaves of a plant native to southeastern Asia (Gelsemium elegans). A key step in a synthesis of this natural product involves a thermally initiated (70C) sigmatropic rearrangement of the compound shown below (R = protecting group) (J. Am. Chem. Soc. 2011, 133, 1763417637). Predict the product of this reaction and propose a mechanism for its formation. N OMe O Me3SiO O RO

Upon irradiation, some organic compounds undergo a reversible change in molecular structure, such as cistrans isomerization or cyclization of p-conjugated chromophores. This property, called photochromism, has shown strong potential application in the development of optical data storage materials. For example, upon treatment with UV light, compound 1 undergoes electrocyclic ring closure to give compound 2 (J. Photochem. Photobiols. A. 2006, 184, 177183). Further treatment of 2 with visible light causes a reversible ring-opening reaction, regenerating 1. Draw the structure of 2, determine whether it is chiral, and provide a mechanism for its formation. O S S O O 1 2 hnuv hnVis

A hetero Diels-Alder reaction is a variation of the Diels-Alder reaction in which one or more of the carbon atoms of the diene and/or the dienophile are replaced by other atoms such as oxygen or nitrogen. With this in mind, propose a mechanism consistent with the following transformation in which a multifunctional acyclic reactant produces a macrocyclic (large ring) product (Org. Lett. 2001, 3, 723726): O O O O O O O O Ph Ph Ph O O O O 195C

During a recent investigation into the chemistry of oligofurans (polymers of the furan heterocycle, which show potential to be used in materials), the investigators observed an interesting reaction with dienophiles. When trifuran (below) is treated with maleimide, only the terminal furan participates in a [4+2] reaction (Org. Lett. 2012, 14, 502505). Provide a reasonable explanation for this observation. O O O HN O O O H N O O O O O O O Not observed Exclusive product N

During the first total synthesis of chelidonine, a cytotoxic natural product isolated from the root of Chelidonium majus, the investigators used only heat to achieve the following transformation (J. Am. Chem. Soc. 1971, 93, 38363837). Propose a plausible mechanism for this thermal cyclization process. O O N R O O O O N O O R 120C

During studies directed toward the synthesis of atropurpuran, a diterpene with interesting molecular architecture, the investigators used high temperature to convert an acyclic tetraene into a tricyclic compound, shown below (Org. Lett. 2010, 12, 11521155). Propose a plausible mechanism for this thermal cyclization process. R2N NTs R2N 135C NTs

When tricyclic cyclooctyne derivative A reacts with benzyl azide (C6H5CH2N3), a [3+2] cycloaddition occurs between the alkyne and the azide (called a click reaction) to install a new five-membered heterocycle (J. Am. Chem. Soc. 2012, 134, 91999208): N H3C H3C O + A N O N N N B C N N N Ph Ph @ ! (a) Propose a plausible mechanism for the formation of B. (b) Compounds B and C are constitutional isomers that are formed in roughly equal amounts. Draw the structure of compound C. (c) Replacing alkyne A with each of the following alkynes results in a significantly slower reaction. Propose one or more reasons for this decrease in rate for each alkyne D-F. O N O D Unreactive E 375X Slower than A F 1000X Slower than A

Endiandric acids are a class of natural products isolated from the Australian plant Endiandra introrsa. Natural products containing chirality centers are generally found in nature as a single enantiomeric form (optically active), but the endiandric acids are an exception, as they are isolated as racemates. It has been suggested that they are formed via a series of pericyclic reactions from achiral starting materials, such as 1 (Aust. J. Chem. 1982, 35, 22472256). H 1 H HO2C Ph CO2R Ph Intramolecular Diels-Alder Endiandric acid G Endiandric acid C (a) The first step in the conversion of compound 1 to endiandric acids is believed to be a thermal electrocyclic reaction to form compound 2, which possesses an eight-membered ring. Draw the structure of compound 2 and explain why it is formed as a racemic mixture. (b) Compound 2 can undergo another electrocyclic reaction, thereby forming the skeletal structure of endiandric acid G. Draw a mechanism for this process. (c) Endiandric acid C is formed from endiandric acid G via an intramolecular Diels-Alder reaction. Draw the structure of endiandric acid C and show a mechanism for its formation.

Roquefortine C belongs to a family of natural products first isolated from cultures of the fungus Penicillium roqueforti in the mid1970s. Since then, roquefortine C has also been found in other natural sources, including blue cheese. Curiously, isoroquefortine C (a diastereomer of roquefortine C) is not found in nature, despite studies revealing that it is more stable than roquefortine C (J. Am. Chem. Soc. 2008, 130, 62816287). Provide a rational explanation for the difference in energy between these two compounds. N N N N N H O O H H H Isoroquefortine C N N N H O O H H Roquefortine C

As shown below, compound 1 can be transformed to compound 2, which undergoes a thio-Claisen rearrangement (similar to a Claisen rearrangement, but with sulfur instead of oxygen) to give compound 3 (J. Am. Chem. Soc. 2000, 122, 190191). N Ph Ph S 1) BuLi 2) Br 1 N Ph Ph S 3 2 ThioClaisen ? (a) Identify the structure of compound 2, which contains a carboncarbon p bond in the Z configuration. (Hint: Work backward from compound 3.) (b) Propose a plausible mechanism for the transformation of 1 to 2. (c) Offer an explanation for the stereochemical outcome that is observed with respect to the newly formed chirality center. (Hint: You might find it helpful to use a molecular model set.)