Q58 · CSIR-NET Chemistry, June 2012
Paper: CSIR-NET June 2012 · Subject: Organic Chemistry · Chapter: Pericyclic Reactions · Topic: Electrocyclic · Marks: 2 · Difficulty: Medium
The two step conversion of 7-dehydrocholesterol to vitamin $\mathrm{D_3}$ proceeds through 
$\rightarrow$ 
$\rightarrow$ (a)Photochemical electrocyclic disrotatory ring opening; and thermal antarafacial [1, 7]-H shift.
(b)Photochemical electrocyclic conrotatory ring opening; and thermal antarafacial [1, 7]-H shift.
(c)Thermal electrocyclic conrotatory ring opening; and photochemical superafacial [1, 7]-H shift.
(d)Thermal electrocyclic disrotatory ring opening; and thermal suprafacial [1, 7]-H shift.
Answer
Answer: B ✓ checked by 4AB · confidence high
Explanation
Ring B of 7-dehydrocholesterol opens photochemically (6π, conrotatory) to previtamin D₃, which isomerises thermally by an antarafacial [1,7]-hydrogen shift to vitamin D₃.
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