Q3(b). 3-Phenyl-4-pentenal from 3-(cinnamyloxy)acrylic acid
3-Phenyl-4-pentenal is synthesised from 3-(cinnamyloxy)acrylic acid on heating [scheme given in the paper]. (i) Identify the type of pericyclic reaction involved in the formation of the unsaturated intermediate. (ii) What elements are lost when the intermediate is converted to 3-phenyl-4-pentenal? (iii) Identify the carbon atom in the starting acrylic acid that becomes the aldehyde carbon in the pentenal. (iv) Write the steps involved for this transformation.
Answer in brief
A [3,3]-sigmatropic Claisen rearrangement gives a β-aldehydo acid, which loses CO₂; C-3 of the acrylic acid (the =CH–O carbon) becomes the aldehyde carbon.
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CoversClaisen rearrangement[3,3]-sigmatropic shiftallyl vinyl etherdecarboxylation3-phenyl-4-pentenal
All parts of Question 3
- 3(a) Cycloocta-1,4-diene to a tricyclic isomer: heat or light15 marks
- 3(b) 3-Phenyl-4-pentenal from 3-(cinnamyloxy)acrylic acid15 marks
- 3(c) Regioselectivity of Hofmann elimination by Newman projections10 marks
- 3(d) Methoxybutadienes with acrolein10 marks