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Incoming links: Lactol
Achmatowicz rearrangement 2026-10-20
Diisobutylaluminium hydride 2026-10-20
, a hydride reducing reagent. Low-temperature, controlled-stoichiometry reduction can convert an ester into an aldehyde or a lactone into a lactol; conditions must prevent further reduction.
Past exam of the chemistry B course of the University of Cambridge 2023 Paper 1 B4 b i Solution 2026-10-20
The lactol is a cyclic hemiacetal and can open to , retaining its pre-existing alkoxy-bearing stereocentre. Its two anomers lose their anomeric configuration on opening to the planar aldehyde, so both can feed the same addition pathway. A first equivalent of decylmagnesium bromide can be consumed by the free OH, producing decane and a magnesium alkoxide; the excess reagent supplies the carbon nucleophile for addition.
magnesium can coordinate both the aldehyde oxygen and the adjacent benzyloxy oxygen. This produces a five-membered chelate, with ring sequence Mg–O(carbonyl)–C(aldehyde)–C(alpha)–O(benzyloxy), that restricts rotation about the aldehyde–alpha-carbon bond. Decyl attack on the less crowded face, away from the alpha-carbon's hydroxyethyl chain in the chelated arrangement, forms the new C–C bond and a magnesium alkoxide at the former aldehyde carbon. Acidic workup protonates both alkoxides to give G. The OBn bond is retained; it is not displaced by the Grignard reagent.
The product configuration is the one with OBn and the newly formed OH both projecting forward in the specified zigzag chain, while the opposite configuration at the new alcohol carbon is disfavoured. This is the Cram chelation model for an alpha-alkoxy aldehyde, rather than stereochemical memory of the starting lactol OH. Coordination is a mechanistic explanation consistent with MgBr2 and the product; the scheme does not quantify individual organomagnesium aggregates.[ref]
Felkin-Anh and Cram Chelate by University of Windsor. p. 3, section 2, Chelation and Addition–Cram Chelate Rule
The relevant primary synthetic sequence is documented independently.[ref]Highly Stereoselective Asymmetric Total Synthesis of (−)-Jimenezin via Sequential Intramolecular Amide Enolate Alkylation by Thien Nhan Lu and others. 2022, synthetic sequence leading through the protected oxygen heterocycle
The key stereochemical disconnection is the cis-1,3-disubstituted cyclopentane, not a direct functionalization of two arbitrary cyclopentane positions. A Diels-Alder reaction of cyclopentadiene with nitroethylene fixes both ends of the new two-carbon bridge on the same face of the original five-membered ring.
Isolate the useful nitronorbornene adduct, convert its secondary nitro group to a ketone by a Nef reaction, and then hydrogenate the remaining ring alkene to norcamphor. Performing the nitro-to-ketone conversion before catalytic hydrogenation avoids simply reducing the nitro group to an amine.
Use a Baeyer-Villiger oxidation to insert oxygen between the carbonyl carbon and its adjacent bridgehead carbon. Bridgehead migration is favoured over migration of the competing methylene; isolate the required lactone regioisomer if necessary. This opens the way to a hydroxy cyclopentyl side chain without breaking the original five-carbon ring. In the resulting bridged lactone, one original cyclopentadiene terminus is bonded to O and the other bears the segment.
Reduce the lactone selectively with DIBAL to its lactol. Ring opening of that lactol gives the aldehyde equivalent cis-3-hydroxycyclopentyl-. The ring OH and the two-carbon aldehyde side chain are cis because their original attachments were the two ends of the Diels–Alder bridge; neither the migration nor lactol opening inverts those cyclopentane carbons.
Perform a Horner-Wadsworth-Emmons reaction on the lactol/aldehyde equilibrium using , where . Preform the phosphonate anion and allow for consumption of base by the lactol OH. Standard stabilized-phosphonate conditions favour the required E- unit. No further hydrogenation should follow it.
The required ester substituent can be prepared by reducing 4-methylpentan-2-one to 4-methylpentan-2-ol, esterifying with bromoacetic acid, and performing an Arbuzov reaction with triethyl phosphite to make the phosphonoacetate. No absolute configuration is indicated at that alcohol-derived centre.
Bridge-derived cis ring stereochemistry plus E-selective phosphonate olefination gives both specified relationships. Achiral cycloaddition and subsequent reagents give a racemic pair with this relative configuration; isolation of one absolute enantiomer would require resolution or an asymmetric variant. The acid fragment contains nine carbons: five from cyclopentadiene, two from nitroethylene and two from the phosphonoacetate. The ester alkyl group adds six, giving fifteen overall. The atom tracing in the original schematic in question 8 shows why oxygen insertion retains the five-membered ring rather than producing a six-membered carbocycle.
The first transformation is an Achmatowicz rearrangement, followed by protection and selective reduction; the last is a Johnson-Claisen rearrangement.
- Butyllithium removes the furan 2-proton, giving a 2-lithiofuran. Its carbon attacks the aldehyde carbonyl; protonation of the resulting alkoxide gives the furfuryl alcohol. Carbon-carbon bond formation occurs at the lithiated position.
- Electrophilic oxygen transfer from mCPBA to the electron-rich furan gives an oxidized, ring-opened intermediate. Opening the oxygenated furan gives a hydroxy enedicarbonyl: the original furfuryl hydroxyl then attacks the remote aldehyde intramolecularly. This expands the five-membered furan into a six-membered lactol containing an enone carbonyl. The original furfuryl oxygen becomes the pyran ring oxygen; oxidation of the furan supplies the new carbonyl and lactol functionality.
- AgO/MeI converts the lactol OH into its methyl acetal: oxygen attacks methyl iodide by SN2 reaction, with iodide removal and proton transfer. This protects the anomeric position.
- LiAlH delivers hydride to the remaining ketone carbonyl, followed by protonation. The methyl acetal survives these basic reducing conditions, leaving the depicted allylic alcohol.
- Under weak-acid catalysis, that allylic OH replaces an ethoxy group of triethyl orthoacetate. Loss of another ethanol molecule, with removal of a methyl proton, gives the mixed ketene acetal . A six-electron, chair-like Claisen rearrangement breaks the allyl-O bond, forms a bond between the terminal allyl carbon and the ketene-acetal CH, and moves the ring double bond one position. Carbonyl formation gives the substituent.
The acetal protects the other oxygenated position during hydride reduction. The orthoester transfers an acetate-derived two-carbon unit through a ketene acetal; it does not directly substitute the allylic OH with an intact acetate ion.
