Compound \(\mathbf{P}\) was prepared based on a four-component reaction at room temperature in methanol. The required starting materials for the synthesis are (structure of \(\mathbf{P}\) shown).
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The Ugi reaction is a powerful multicomponent method to rapidly assemble peptide-like scaffolds.
The amine provides the \(-NH\)-link, the aldehyde contributes the substituent on the \(\alpha\)-carbon, the carboxylic acid contributes the acyl group, and the isocyanide provides the distal N-substituent.
Always map each fragment of the product back to the corresponding starting material to verify correctness.
Step 1: Identify the reaction type. Compound \(\mathbf{P}\) clearly bears the hallmark structure of an Ugi four-component reaction (Ugi-4CR) product, which arises from the condensation of:
\[
\text{Amine} + \text{Aldehyde} + \text{Carboxylic Acid} + \text{Isocyanide}.
\]
Step 2: Match fragments.
The indole-ethylamine side chain in \(\mathbf{P}\) corresponds to tryptamine, i.e., 2-(1H-indol-3-yl)ethylamine.
The benzylic substituent (–CHPh) in the central scaffold must originate from benzaldehyde (PhCHO).
The acyl group in the final amide unit comes from the carboxylic acid, which is acetic acid (MeCO\(_2\)H).
The distal N-substituent is derived from the isocyanide, here cyclohexyl isocyanide.
Step 3: Rationalize the option. Although isocyanides are neutral species, the problem statement’s option (A) represents cyclohexyl isocyanide with a cationic drawing. Since the structural mapping of the fragments in compound \(\mathbf{P}\) exactly matches (A), it must be considered the correct answer in this context.
Step 4: Conclusion. Therefore, the correct set of four building blocks is given in option (A).
