Chapter 8, Problem 8.69P

Interpretation Introduction

(a)

Interpretation:

The mechanism with appropriate arrows showing the conversion of intermediate A to B through two $\text{1, 2-hydride}$ shifts followed by two $\text{1, 2-methyl}$ shifts is to be drawn.

Concept introduction:

Carbocation intermediate can rearrange to become more stable via two possible rearrangements, $\text{1, 2-hydride}$ shift or $\text{1, 2-methyl}$ shift. Hydrogen atom or methyl group from adjacent carbon atom is shifted to the positively charged carbon to form a more stable carbocation. Each $\text{1, 2-hydride}$ shift and $\text{1, 2-methyl}$ shift requires one curved arrow originating from the electron-rich bond and pointing to the electron poor ${\text{C}}^{\text{+}}$.

Interpretation Introduction

(b)

Interpretation:

The mechanism with appropriate curved arrows showing the conversion of intermediate B to Lanosterol is to be drawn.

Concept introduction:

The Carbocation intermediate rearrangement is followed by electrophilic elimination resulting in the formation of C=C bond.

Two curved arrows are needed to show electrophilic elimination. The first curved arrow originates from the base to the electrophile, and the second arrow is drawn from the electrophile to the mid of the C-C bond where C=C bond is formed.