In the manipulation of organic compounds, oxidation reaction and reduction reaction are the most imp — Organic Chemistry Chemistry Question
Oxidation and reduction in organic synthesis
In the manipulation of organic compounds, oxidation reaction and reduction reaction are the most important reactions. Especially, a chemo–selective, region–selective or stereo–selective reduction/oxidation is very important in designing an efficient organic synthesis of a target molecule. While nature achieves such selectivity through specific design of active sites of enzymes, chemical transformation mostly relies on subtle difference in reactivity by changing the nature of reagents.
The following scheme is a good example of chemo–selective reduction and oxidation reactions starting from ethyl cyanoacetate.
[VISUAL]
When ethyl cyanoacetate was treated with a reducing agent NaBH4 in presence of FeCl3, a selective reduction of a functional group was observed. When the product A was reacted with benzoyl chloride, 1 equivalent of benzoyl chloride was consumed to form B. What are the structures of A and B?
Model Answer
A: ethyl 3-aminopropanoate (H2N-CH2-CH2-COOCH2CH3)
B: ethyl 3-(benzoylamino)propanoate (Ph-CONH-CH2-CH2-COOCH2CH3)
[VISUAL]
Dess–Martin Periodinane (DMP) is a strong but mild oxidizing agent, and can oxidize various functional groups in a selective manner. When B was oxidized with DMP a clean oxidation occurred to form C. 1H–NMR, 13C–NMR, IR and mass spectra were obtained. These spectra showed that a clean transformation occurred to form a single product. In the 1H–NMR a doublet between 5~6 ppm shows the coupling constant J = 8.8 Hz. Draw the structure of C.
Model Answer
C: ethyl (Z-3-(benzoylamino)acrylate (Ph-CONH-CH=CH-COOCH2CH3 in cis-configuration)
[VISUAL]
In the 1H–NMR, the chemical shift of one peak showed up near δ = 11.5 ppm. Assign the proton in the structure C for this chemical shift. What is the reason for the chemical shift for the assigned proton that does not appear in the region (δ = ~ 8 ppm) for ordinary protons of that functional group?
1H–NMR of C
13C–NMR of C
IR of C
Mass spectrum of C
[VISUAL]
Model Answer
The peak near δ = 11.5 ppm is assigned to the amide (NH) proton of compound C.
The cis-configuration of the olefin allows strong hydrogen bonding between the proton of the amide and the carbonyl oxygen of the ester as shown in the following structure:
[VISUAL]
The strong hydrogen bonding moved the chemical shift of the amide proton toward further down-field.