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Incoming links: Cyclopentanone
Use the nitro group as a masked carbonyl and a carbon nucleophile. The most informative disconnection reverses the six-membered aldol annulation. Before that closure, the cyclopentanone beta carbon bearsThe carbon carrying will become the lower carbonyl of the final six-membered ring by a Nef reaction. It comes from the specified nitromethane. The remote methyl ketone supplies the upper methyl-substituted alkene carbon after aldol condensation.
First, use a mild base to generate a nitromethane-derived nitronate and add it conjugatively to the indicated cyclopent-2-en-1-one. Protonation gives 3-(nitromethyl)cyclopentanone. Deprotonate its nitromethyl group and perform a second Michael addition to methyl vinyl ketone, giving . The nitro-stabilized carbon is appreciably more acidic than a simple ketone alpha carbon, permitting selective nitronate formation; control the acceptor stoichiometry to avoid a further Michael alkylation of the remaining nitro-bearing H.
Form the enolate/enamine at the cyclopentanone carbon between its carbonyl and the side-chain-bearing carbon. Intramolecular attack on the remote methyl ketone closes a six-membered ring; the two shared atoms are the original enone alpha and beta carbons. Hydrolysis if required and dehydration create the alkene between the original alpha carbon and the newly formed tertiary carbinol carbon. That carbinol carbon retains its methyl substituent, placing the methyl exactly as required. The original cyclopentanone carbonyl remains the carbonyl of the five-membered ring.
Finally, form the secondary nitronate and carry out controlled acidic Nef hydrolysis to replace its unit by . The resulting fused product contains the two required ketones. Carrying out the annulation before exposing this second ketone reduces competing enolate/aldol pathways that would be available in the corresponding triketone. The nitro intermediate's competing deprotonation means that the aldol step needs conditions designed to access the ring carbonyl enolate/enamine; simply treating every carbonyl position as equally reactive would not establish the route.
The carbon ledger is from cyclopentenone, from nitromethane and from methyl vinyl ketone, giving the nine-carbon fused skeleton plus its methyl substituent. A primary-nitro Nef reaction performed before the second carbon alkylation would instead give an aldehyde and would lose this useful protected-carbonyl construction.
The left ring system is spiro, and the right-hand substituent is , not OMe directly attached to the cyclohexene ring. Let denote the right cyclohexenyl group bearing that methoxyethyl substituent cis to its linker. The connection to the left alkene carbon is . These observations suggest two Diels–Alder constructions. A left-hand dienehas the original cyclopentane carbon as one terminal diene atom; cycloaddition with an ethylene equivalent gives the spiro six-membered ring. Its residual double bond is between the two internal diene atoms, putting the linker on the required alkene carbon.
Build and its four-carbon alcohol appendage first. Protect the OH group of the indicated 4-Bromobutan-1-ol, for example as a THP ether. Convert the primary bromide to its grignard reagent and formylate with DMF, giving . olefination with an allylidenetriphenylphosphorane, , supplies a conjugated diene ; select the required diene geometry rather than assume a single E/Z outcome.
Cycloaddition with methyl vinyl ketone produces a cyclohexene with a protected four-carbon alcohol chain at one saturated ring carbon and an acetyl group at its adjacent ring carbon. Choose and isolate the adjacent-substituent cis regioisomer, matching the two dashed substituent bonds in the target. Endo preference and diene geometry can aid the selection, but an unsymmetrical diene is not guaranteed to give a unique regioisomer. Reduce the acetyl ketone to , then methylate that alcohol with sodium hydride/methyl iodide while the chain-end OH is protected. Remove the THP group under mild acid conditions to reveal , preserving the secondary methyl ether and the ring alkene.
Oxidize this primary alcohol to . Add vinylmagnesium bromide to give , then oxidize the allylic alcohol to the enone . This carbonyl carbon is the original alcohol-end carbon of 4-bromobutanol; after olefination it will be the linker-bearing carbon of the left ring alkene.
In the other branch, reduce the specified cyclopentanone to cyclopentanol, convert to bromocyclopentane, and react with triphenylphosphine to obtain the cyclopentylphosphonium salt. Deprotonation gives the cyclopentylidene phosphorane, whose ylidic carbon is part of the five-membered ring. Wittig olefination of the just-prepared enone gives the substituted diene displayed above. This carbonyl–ylide pairing keeps the ring carbon terminal in the diene and avoids accidentally constructing a fused ring instead of a spiro ring.
Finally, cycloadd an ethylene equivalent. Ethylene itself requires suitably forcing/high-pressure conditions; a more reactive alternative is phenyl vinyl sulfone, followed by reductive removal of the sulfone substituent using a method compatible with the methyl ether and retained C=C bonds. The latter installs the same two otherwise unsubstituted saturated carbons. Control or separate any additional cycloaddition isomers, and retain the previously selected cis configuration on .
The carbon count provides a strong check: cyclopentanone supplies five, bromobutanol four, DMF formylation one, the first allylidene ylide three, methyl vinyl ketone four, O-methylation one, vinylmagnesium two, and the final ethylene equivalent two, totalling 22 target carbons. Three original bromobutanol carbons form the linker; its fourth becomes the substituted carbon of the spiro ring's alkene. The proposal is a multistep constitutional/relative-stereochemical route with explicit regioisomer and diastereomer selection, not an unsupported claim of a single quantitative cycloaddition product.
The target is 5-Aminohexanoic acid, . Disconnect its terminal amine/carboxylic-acid relationship through hydrolysis of 6-methylpiperidin-2-one. This six-membered lactam follows from a regioselected Beckmann ring expansion of 2-methylcyclopentanone oxime.
Start with cyclopentanone. Form its enolate with lithium diisopropylamide in dry THF at low temperature, add methyl iodide, then quench to obtain 2-methylcyclopentanone. Control the methyl iodide/base stoichiometry to suppress a second alkylation. React the ketone with hydroxylamine to form its oxime and select the geometrical oxime isomer in which the bond to the methyl-substituted adjacent ring carbon is anti to the departing oxime oxygen group.
Activate the oxime, for example under appropriate acid-promoted Beckmann conditions. The anti methyl-bearing C–C bond migrates onto nitrogen as the N–O-derived leaving group departs, inserting nitrogen between the original carbonyl carbon and the methyl-substituted ring carbon. Hydration/tautomerization gives 6-methylpiperidin-2-one. The opposite oxime geometry would insert nitrogen on the other side and ultimately place methyl at a different position in the amino acid; oxime geometry is therefore an essential part of this proposal.
Hydrolyse the lactam with aqueous acid and heat, then adjust the work-up to the desired acid/ammonium form. Cleavage of its acyl–nitrogen bond opens the ring to after suitable neutralization. All five original cyclopentanone carbons are retained, and the one methyl carbon comes from methyl iodide. A racemic mixture product is appropriate to the stereochemically unspecified target. The reaction sequence is an organic ring-expansion synthesis and requires no biological transformation.
