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Exam 26: The Chemistry of the Aromatic Heterocycles and Nucleic Acids

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Question 1

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Purine is a bicyclic ring found in DNA. The structure for purine is shown below. For each lone pair of nitrogen, determine whether the lone pair resides on a 2p orbital or sp² orbital.

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The lone pairs on the nitrogens labeled ...

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Question 2

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Draw a structure for each of the compounds. a. 4-methoxyindole b. thiophene-2-carboxylic acid

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Question 3

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Give the complementary sequence to the DNA strand. 5ʹ-TATGAC-3ʹ

For each heterocycle, label as aromatic, anti-aromatic, or nonaromatic.

In biological systems, pyridoxal phosphate is a cofactor that aids in the decarboxylation of glutamic acid to 4-aminobutanoic acid. The key intermediate is shown below. Draw curved arrows to show the decarboxylation step.

Shown below are the two types of base pairs in DNA, with the bases in the same relative alignments that they are in DNA. a. Show all the relevant hydrogen bonds with dotted lines and include the unshared pairs on the acceptor atoms, within each pair. b. Which base pair has the strongest intermolecular interaction and why?

Thiophene undergoes electrophilic aromatic substitution (EAS) with bromine to give two possible regioisomers, A and

The structure of indole is shown below. Unlike most amines, indole is difficult to protonate. Its conjugate acid has a pK_a of -3.5. Explain why indole is difficult to protonate.

Identify the missing reagent(s) for the transformation:

2-Halopyridines undergo substitution under milder conditions than the Chichibabin reaction. Write a curved-arrow mechanism for this substitution.

An imidazole ring is an important heterocycle found in the amino acid histidine. Draw an orbital configuration of imidazole, showing the 2p orbitals and electrons in each orbital. Use the diagram to explain which electrons are part of the aromatic system and which can be protonated in the presence of an acid.

Draw a structure for each of the compounds. a. oxazole-5-carbaldehyde b. 4-methoxypyridine

The heterocyclic base 7-aminopropargyl-7-deaza-2,6-diaminopurine has been incorporated into a short sequence of nucleotides. The structure is shown below. Compare it to the other nucleotides and determine which nucleotide base is it likely to pair with. Draw the matching nucleotide with hydrogen bonds as dotted lines.

Name the heterocycle.

Rank the reactivity of these compounds towards electrophilic aromatic substitution, from most reactive to least. benzene, furan, thiophene, pyrrole, pyridine

Name the heterocycle.

Outline a synthesis for the transformation:

Circle the nitrogen that is more basic and explain why.

Identify the correct name or abbreviation of the given nucleoside or nucleotide:

Predict the major organic product for the reaction.