Internal aldolization

A plausible mechanism involves the reaction of the dihydride precursor with t-butylethylene to the 14-e complex [Ir(C6H3-2,6 CH2P-f-Bu2 2)]> which undergoes the oxidative-addition reaction of the alcohol to afford a hydride alkoxide complex. Further /i-hydride ehmination gives the alde-hyde/ketone and regenerates the dihydride active species [55]. In the particular case of 2,5-hexanediol as the substrate, the product is the cycHc ketone 3-methyl-2-cyclopenten-l-one. The formation of this ketone involves the oxidation of both OH groups to 2,5-hexanedione followed by an internal aldol reaction and further oxidation as in the final step of a Robinson annotation reaction [56]. 

The enone lactone 410 could also be obtained in a single operation from the ozonolysis in methanol of hemi-ketal [3 (118). In this reaction, 41 produced first the diketone 14 which underwent an internal aldol condensation to 415 which is nicely set up to give K) vi a the intermediate 416. 

Internal aldol condensations (condensations where both carbonyl groups are on the same chain) lead to ring formation. 

Pentanedione is in equilibrium with two enolate ions after treatment with base. Enolate A is stable and unreactive, while enolate B can undergo internal aldol condensation to form a cyclobutenone product. But, because the aldol reaction is reversible and the cyclobutenone product is highly strained, there is little of this product present when equilibrium is reached. At equilibrium, only the stable, diketone enolate ion A is present. 

Robinson annulation reaction (Section 23.12) a multistep sequence for building a new cyclohexenone ring onto a ketone. The sequence involves an initial Michael reaction of the ketone followed by an internal aldol cyclization. 

Substituted cyclopentenones ate formed by internal aldol condensation of y-diketones in the presence of basic catalysts. 

Certain substituted cyclohexenones are made by an analogous internal aldol condensation and decarboxylation of his-estets resulting from condensation of aldehydes with acetoacetic ester. 

Two different modes of reaction have been reported for steroidal A -3-ketones 20). Potassium persulphate in sulphuric acid gave the 4 0xa"3 ketone (23), considered to arise by an initial Baeyer-Villiger oxidation to the unsaturated lactone (21) as an enol lactone of the C(g)-aldehyde (22) this could suffer further degradation through a second Baeyer-Villiger attack on the aldehyde group [64], Other. workers [6 ] used peroxytrifluoroacetic acid and obtained the 5a-carboxy--4"Oxa 3-ketone (25) and the bridged product (26), apparently derived by an internal aldol condensation of the intermediate lactone-aldehyde (24). 

A further paper elaborates upon the internal aldol condensation of an 8,9-seco-8,9,11-trione to give so-called linear steroid analogues. ... 

Alkylation of isovaleramide with 1,3-dichlorobut-2-ene yields (139) after methyl-ation acid-catalysed hydrolysis and internal aldol condensation gives ( )-piperitone. The value of piperitenone and isopiperitenone formation, probably via electrocyclic reaction of the pyrolytic acetic acid-elimination product from A - and A -isomers of (49), cannot be assessed in the absence of reaction yields. (S)-(-)-Pulegone is obtained in good yield from (- )-citronellol by oxidation with pyridinium chlorochromate followed by double-bond isomerization. Low-temperature reduction of ( —)-carvone to ( —)-cz5-carveol (140) and... 

The biological activity of the diterpenoids of the aphidicolin-stemodin series makes these compounds attractive targets. The total synthesis of stemodin (96) has been described.172 The key spiro-centre at C-9 was constructed by the internal aldol condensation of the keto-aldehyde (97) to afford (98). Several stereoselective syntheses of aphidicolin have been reported with different solutions to the problem... 

NAD+. In addition to the two redox steps (Eq. 20-2, b and d), this enzyme catalyzes both the conversion of the P anomer of glucose 6-P to the open-chain aldehyde form and the internal aldol condensation of Eq. 20-2, step The pro-R hydrogen at C-6 of glucose 1-P... 

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