A series of pericyclic reaction can be used to construct complex organic molecular structure in a st — Organic Chemistry Chemistry Question
Pericyclic Reaction
A series of pericyclic reaction can be used to construct complex organic molecular structure in a stereocontrolled manner. For example, Moore et al. (J. Org. Chem. 1998, 63, 6905.) reported the synthesis of triquinane derivatives from the corresponding cyclobutenone as shown in scheme below.
Propose structures of intermediates A and B.
[VISUAL]
In the synthesis of prostaglandin, Corey et al. (J. Am. Chem. Soc. 1969, 91, 5675.) used cycloaddition reaction as a key step along with a series of straightforward chemical transformations to set up stereochemistry at the periphery of cyclopentyl structure as shown in scheme below.
[VISUAL]
Identify structures of compounds C-G with correct relative stereochemistry.
There is a carbon, labeled with an asterisk in compound 3. Indicate the position of this labeled carbon in either compound 1 or 2 by placing an asterisk on the carbon in the given structure. [VISUAL]
Model Answer
Either position on compound 1 is correct.
If a student synthesizes compound 3 from compounds 1 and 2 by following the synthetic scheme above, how many possible stereoisomers of compound 3 can the student obtain?
Model Answer
Two. Reaction between 1 and 2 will give a pair of enantiomers which are inseparable. (Note: The two enantiomers can be resolved but this process is not mentioned in the context.) The subsequent processes, although generated a few new chiral centers, employed achiral reagent and the newly formed stereocenters were controlled by the existing stereochemistry. Therefore, stereochemical information was carried over from compound C to compound 3.
1,3-Dipolar cycloaddition is a powerful tool for the construction of heterocyclic structure. For example, upon heating, compound H undergoes intramolecular [4+2]-cycloaddition to yield compound I. Reduction of a weak N-O bond of compound I by catalytic hydrogenation gave product J.
[VISUAL]
Propose structures of compounds I and J.
If a racemic mixture of H is used in the reaction, write all possible stereo-isomer(s) of products J.
Model Answer
Racemic mixture of H will give product I which has a plane of symmetry. Therefore, reduction of I (only a stereoisomer) will give only 1 possible product J. Note: Compound I has a plane of symmetry, thus its mirror image (compound I’) is identical (meso-compound).