bulnesol is a key ingredient of oil of guaiac, a fragrance.
ratcliffe, heathcock, j. am. chem. soc. 1971, 93, 1746.
- (2 points each) fill in the missing reagents for the synthesis of bulnesol.

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{ "quality": "clear", "question_count": 10, "skills_matched": [ "step_cot", "explain_then_answer", "tutor" ], "needs_drawing": false, "concepts_used": [ "Esterification", "Acetal Protection", "Carbonyl Reduction", "Tosylation" ], "new_concepts": [ "Meerwein-Wagner Rearrangement", "Enone Reduction", "Wittig Reaction" ], "current_concepts": [ "Esterification", "Acetal Protection", "Carbonyl Reduction", "Tosylation", "Meerwein-Wagner Rearrangement", "Enone Reduction", "Wittig Reaction" ] } This multi-step organic synthesis pathway outlines the total synthesis of bulnesol, starting from a bicyclic enone precursor. We need to identify the missing reagents for each of the 10 blank boxes. ## Reduce the non-conjugated ketone selectively The starting material contains a conjugated enone system and a non-conjugated ketone. To selectively reduce the non-conjugated ketone to a secondary alcohol while leaving the enone intact, we can use a mild reducing agent. Using the **Carbonyl Reduction** knowledge point: Sodium borohydride ($\text{NaBH}_3\text{CN}$ or $\text{NaBH}_4$ at low temperature) can selectively reduce the more reactive, non-conjugated ketone. Reagent: $\text{NaBH}_4$ ## Acetylate the secondary alcohol The secondary alcohol is converted into an acetate ester ($-\text{OAc}$). Using the **Esterification** knowledge point: This transformation is achieved using acetic anhydride and a base. Reagents: $\text{Ac}_2\text{O}$, $\text{pyridine}$ (or $\text{NEt}_3$) ## Protect the remaining ketone as an acetal The ketone in the enone system needs to be protected as a cyclic acetal (ethylene glycol acetal) to prevent reaction in subsequent steps. Using the **Acetal Protection** knowledge point: This is accomplished using ethylene glycol in the presence of an acid catalyst with removal of water. Reagents: $\text{HOCH}_2\text{CH}_2\text{OH}$, $\text{TsOH}$ (or $\text{H}^+$) ## Deprotect the acetate to regenerate the alcohol The acetate group ($-\text{OAc}$) is hydrolyzed back to a secondary alcohol ($-\text{OH}$) under basic conditions. Reagents: 1. $\text{NaOH}$ (or $\text{KOH}$) 2. $\text{H}_2\text{O}$ ## Convert the alcohol to a benzyl ether The secondary alcohol is protected as a benzyl ether ($-\text{OBn}$). Using the **Ether Synthesis** knowledge point: This is typically done via a Williamson ether synthesis using a strong base to deprotonate the alcohol followed by benzyl bromide. Reagents: 1. $\text{NaH}$ 2. $\text{BnBr}$ ## Hydrolyze the acetal protecting group The cyclic acetal protecting group is cleaved to regenerate the ketone. Using the **Acetal Protection** knowledge point: This is achieved via acid-catalyzed hydrolysis. Reagents: $\text{H}_3\text{O}^+$ (or $\text{HCl}$, $\text{H}_2\text{O}$) ## Isomerize the exocyclic double bond to endocyclic The exocyclic alkene formed by the Wittig reaction ($\text{Ph}_3\text{P}=\text{CH}_2$) is isomerized to the more stable endocyclic tetrasubstituted double bond. Reagents: $\text{TsOH}$ (or $\text{H}^+$, heat) ## Convert the methyl ketone to a tertiary alcohol The methyl ketone is converted to a tertiary alcohol containing a dimethyl carbinol group ($-\text{C(OH)(CH}_3)_2$). Using the **Grignard Reaction** knowledge point: This is achieved by adding a methyl nucleophile to the ketone. Reagents: 1. $\text{MeMgBr}$ (or $\text{MeLi}$) 2. $\text{H}_3\text{O}^+$ ## Deprotect the benzyl ether to regenerate the alcohol The benzyl ether ($-\text{OBn}$) is cleaved to regenerate the secondary alcohol. Reagents: $\text{H}_2$, $\text{Pd/C}$ (already shown as $\text{H}_2/\text{Pd}$ on the arrow, so the box represents the conversion of the ketone to the secondary alcohol, or vice versa. Looking closely at the scheme, the step with $\text{H}_2/\text{Pd}$ goes from the right structure to the left structure, which is the deprotection of the benzyl ether. The box pointing from the secondary alcohol to the tosylate is the tosylation step). Using the **Tosylation** knowledge point: The secondary alcohol is converted to a tosylate ($-\text{OTs}$) using tosyl chloride and a base. Reagents: $\text{TsCl}$, $\text{pyridine}$ The missing reagents for the synthesis of bulnesol (ordered from top-left to bottom-right following the synthetic sequence) are: 1. **First box (top-left):** $\text{NaBH}_4$ 2. **Second box (top-middle):** $\text{Ac}_2\text{O}$, $\text{pyridine}$ 3. **Third box (top-right):** $\text{HOCH}_2\text{CH}_2\text{OH}$, $\text{TsOH}$ 4. **Fourth box (middle-right, step 1 & 2):** 1. $\text{NaOH}$, $\text{H}_2\text{O}$ 2. $\text{NaH}$, $\text{BnBr}$ 5. **Fifth box (middle-middle):** $\text{H}_3\text{O}^+$ (or $\text{HCl}$, $\text{H}_2\text{O}$) 6. **Sixth box (middle-left):** $\text{TsOH}$ (or $\text{H}^+$, heat) 7. **Seventh box (middle-right, below Wittig):** $\text{MeMgBr}$ (then $\text{H}_3\text{O}^+$) 8. **Eighth box (bottom-right):** $\text{H}_2$, $\text{Pd/C}$ 9. **Ninth box (bottom-left):** $\text{TsCl}$, $\text{pyridine}$ { "subject": "Natural Science", "question_type": "Fill-in-the-blank", "knowledge_point": [ "Natural Science", "Chemistry", "Organic Synthesis" ] }

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Explanation:

Reduce the non-conjugated ketone selectively

Acetylate the secondary alcohol

Protect the remaining ketone as an acetal

Deprotect the acetate to regenerate the alcohol

Convert the alcohol to a benzyl ether

Hydrolyze the acetal protecting group

Isomerize the exocyclic double bond to endocyclic

Convert the methyl ketone to a tertiary alcohol

Deprotect the benzyl ether to regenerat…

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