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A benzene-fused five-membered lactone containing a gem-dimethyl-substituted saturated ring carbon. It can result from double methyl addition to one carbonyl of dimethyl phthalate followed by ring closure.

Insertion of oxygen beside an aldehyde or ketone carbonyl, commonly with a peroxy acid. Cyclic ketones give ring-expanded lactones. The migrating group retains its configuration, but competing migrations can give different constitutional products.

$(\mathrm{iBu}{)}_2\mathrm{AlH}$, 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.

Formation of an ester from an acyl component and an alcohol. Intramolecular esterification of a hydroxy acid makes a lactone. Fischer esterification is the acid-catalysed carboxylic-acid/alcohol case.

A cyclic ester. Intramolecular alcohol-to-carbonyl attack followed by elimination can close a lactone ring.

Formation of a lactone by intramolecular esterification or acyl substitution. Favourable ring sizes can help trap an intermediate, but do not make the product universally unreactive.

The new ring is formed by intramolecular Friedel-Crafts addition to the ketone. titanium tetrachloride coordinates the ketone oxygen of the cyclic beta-ketoester, increasing electrophilicity at that carbonyl carbon. The nearby electron-rich aryl ring attacks this activated ketone at the available position ortho to its ethyl tether.

The electron-flow sequence is: an aromatic $\pi$ bond forms the new aryl–carbonyl-carbon bond, the ketone $\pi$ bond moves onto oxygen, and loss of the aromatic proton restores aromaticity. The two-carbon tether and the shared quaternary carbon make the newly closed carbocycle six-membered. Hydrolytic work-up converts the titanium-bound alkoxide into the tertiary alcohol.

The lactone carbonyl survives. This is an addition to the ketone with retention of its oxygen as OH, rather than an acyl substitution that removes that oxygen. The pendant terminal alkene is not involved. These bond changes account for the fused aromatic ring system and the alcohol in C without needing to assign the new stereochemistry.

First hydrolyze the benzylidene acetal. Protonate one acetal oxygen and break its bond to the Ph-CH centre, giving one free alcohol and a benzylic oxocarbenium ion. Water attacks that centre; proton transfers give a hemiacetal. Protonate the remaining acetal-derived OR group and cleave it to release the second alcohol and benzaldehyde.

The resulting unsaturated hydroxy ester isThe OH on the allyl-substituted methine is gamma to the ester carbonyl. Protonation of that carbonyl enables intramolecular OH attack to give a five-membered tetrahedral intermediate. Proton transfer to methoxy and collapse expel methanol, followed by deprotonation to give the alpha,beta-unsaturated gamma-lactone.

The other liberated OH remains CH${}_2$OH. Both C=C units survive: one is the allyl substituent and the other becomes the lactone's conjugated double bond. Hydrolysis to a hydroxy acid followed by lactonization is an alternative sequence under the aqueous acid; direct acid-catalysed intramolecular ester exchange gives the same atom mapping.

The two starting ester groups are adjacent on the benzene ring. Select one ester carbon as the site receiving both incoming methyl groups. A first methyl addition gives a tetrahedral intermediate; its collapse expels methoxide to give the ortho-substituted ketone $\mathrm{o}-!\mathrm{Me}{\mathrm{O}}_2\mathrm{C}-{\mathrm{C}}_6{\mathrm{H}}_4-\mathrm{C}(=\mathrm{O})\mathrm{Me}$. A second methyl addition produces $\mathrm{o}-!\mathrm{Me}{\mathrm{O}}_2\mathrm{C}-{\mathrm{C}}_6{\mathrm{H}}_4-\mathrm{C}({\mathrm{O}}^{-})\mathrm{M}{\mathrm{e}}_2$, with the alkoxide bound to magnesium.

The newly generated oxygen is now positioned to attack the remaining ester intramolecularly. Draw an arrow from its lone pair to that ester carbonyl and another from the C=O pi bond to its oxygen. Collapse of the resulting tetrahedral intermediate reforms the carbonyl and expels methoxide, closing a five-membered lactone. Protonation of magnesium-bound species in the work-up gives 3,3-Dimethylphthalide. Equivalently, protonation can first give the ortho hydroxy ester, followed by acid-assisted lactonization with loss of methanol; both descriptions show the same required bond formation.

The oxygen in the new ring is the oxygen of the first ester's original carbonyl; the surviving lactone carbonyl comes from the second ester. The product retains the six ring carbons and both original acyl carbons, loses the two ester methoxy carbons, and gains two carbons from methylmagnesium bromide. The favourable intramolecular five-membered closure accounts for the displayed product. It does not make the isolated lactone universally inert to a large excess of grignard reagent; stoichiometry and trapping/work-up conditions matter.

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 $\mathrm{C}{\mathrm{H}}_2\mathrm{CO}$ segment.

Reduce the lactone selectively with DIBAL to its lactol. Ring opening of that lactol gives the aldehyde equivalent cis-3-hydroxycyclopentyl-$\mathrm{C}{\mathrm{H}}_2\mathrm{CHO}$. 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 $(\mathrm{EtO}{)}_2\mathrm{P}(\mathrm{O})\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_2\mathrm{R}$, where $R=\mathrm{CH}(\mathrm{Me})\mathrm{C}{\mathrm{H}}_2\mathrm{CH}(\mathrm{Me}{)}_2$. Preform the phosphonate anion and allow for consumption of base by the lactol OH. Standard stabilized-phosphonate conditions favour the required E-$\mathrm{CH}=\mathrm{CHC}{\mathrm{O}}_2\mathrm{R}$ 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.

Open the lactone formally at its acyl–oxygen bond, then replace the resulting secondary alcohol by its ketone precursor. With $R=\mathrm{CH}(\mathrm{Me})\mathrm{CH}(\mathrm{Me}{)}_2$, the useful sequence of retrosynthetic intermediates isDisconnect the last compound as a Michael addition product of the methyl-side enolate of $\mathrm{RCOC}{\mathrm{H}}_3$ and ethyl methacrylate:The substituted carbon of the methacrylate becomes the methyl-bearing carbon alpha to the lactone carbonyl. The ketone carbon becomes the oxygen-bearing ring carbon. This checks the six-membered ring connectivity.

The required branched methyl ketone itself comes from butan-2-one by ethyl-side aldol addition to acetone, followed by dehydration and selective hydrogenation:Choose and isolate the aldol regioisomer shown. For the subsequent Michael step, use a bulky base at low temperature to favour its less hindered methyl-side kinetic enolate. Reduce the resulting ketone selectively with $\mathrm{NaB}{\mathrm{H}}_4$ while retaining the ester, hydrolyse the ester, then acidify and promote intramolecular esterification with water removal. The hydroxy acid closes to the required six-membered lactone. There is no specified stereochemical relationship to enforce, but a stereochemically pure target would require an additional selectivity or resolution step.

A consistent proposed sequence is desilylation, five-membered hemiketal formation, retro hetero-Diels–Alder opening, intramolecular aldol addition and hemiacetal closure. The essential carbon rearrangement is loss of the original ketone–side-chain bond and formation of a bond between its two neighbouring carbons; it is not a simple Michael addition.

For an explicit atom map, label the starting six-membered ring $A-B=C-D-E-O_r-A$, with the ketone at $D$, methyl substituents at $A$ and $B$, the hydroxy group at $A$, and the side chain $E-X-T-\mathrm{Et}$. Here $X$ is the methylene and $T$ is the carbon bearing the protected alcohol. Let $O_D$ be the original ketone oxygen and $O_T$ the protected alcohol oxygen. These labels identify atoms, not new substituents.

First, Fluoride removes the silyl protecting group from $O_T$. The resulting alcohol attacks the protonated ketone at $D$, forming the five-membered hemiketal $D-E-X-T-O_T-D$. The original oxygen $O_D$ is now a hydroxy group; the original pyran ring remains intact at this stage.

Next, The original pyran ring can undergo a retro hetero-Diels-Alder reaction. The three electron-pair movements are $B=C\to C=D$, $D-E\to E=O_r$ and $O_r-A\to A=B$: break $D-E$ and $O_r-A$, and form the three indicated double bonds. The result is the tethered aldehyde/dienol esterThe temporary $D-O_T$ bond keeps both reacting ends within the same molecule. At $D$, both oxygen substituents remain attached, and the new $D-O_T$ tether prevents separation into independent fragments.

Then The $C=D$ enol attacks the tethered aldehyde at $E$. Formation of $C-E$, restoration of $D=O_D$, and proton transfer give the six-membered lactone ring $C-D-O_T-T-X-E-C$, with $E-O_{r}H$. This is an intramolecular aldol reaction, and it supplies the crucial new carbon–carbon bond without losing any carbon atom.

Finally, Keto-enol tautomerism at $A=B$ gives the methyl ketone at $A$ and introduces the hydrogen at $B$. The $E-O_{r}H$ alcohol then attacks that ketone, closing the five-membered hemiacetal ring $A-B-C-E-O_r-A$. The resulting fused rings have exactly the product's connectivity: $D$ is the lactone carbonyl, $O_T$ is its ring oxygen, and $O_r$ is the five-membered-ring oxygen.

The stereocentre at $T$ survives throughout this sequence. The configuration initially at $E$ is erased at the aldehyde stage, and those at $A$, $B$, $C$ and $E$ arise during the later cyclization and proton-transfer steps. A folded intramolecular aldol reaction followed by reversible hemiacetal formation is consistent with the drawn cis ring junction, but connectivity and orbital bookkeeping alone do not prove that it is the unique selected diastereomer. In particular, a claim of simple retention at $E$ would contradict this atom map.

A related pyranone-to-fused-lactone rearrangement under aqueous hydrogen fluoride is experimentally reported; that account also explicitly qualifies the relative stereochemistry as assigned on mechanistic grounds. The sequence above is a proposed explanation of the examination structure, rather than a mechanism claimed to have been experimentally established in that paper.^[ref]

Primary synthetic study. Pages 5220–5221, Schemes II–III, report aqueous hydrogen fluoride conversion of related pyranones into a fused lactone. The accompanying discussion qualifies the relative stereochemistry as a mechanistic assignment rather than an unambiguous assignment from the reported NMR spectroscopy.