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Addition of an enolate or enol to an aldehyde or ketone, giving a beta-hydroxy carbonyl compound. Subsequent dehydration is a distinct step and gives an aldol condensation.

An allylic organotitanium species that can add its allyl group to a carbonyl compound, furnishing a homoallylic alcohol after hydrolysis.

A 1,2-carbon migration interconverts adjacent tertiary alpha-hydroxy carbonyl compounds. Formation of a new carbonyl at the original carbinol carbon accompanies conversion of the original carbonyl into an alcohol. Unlike dehydration, the net process preserves both oxygens.

An ether with oxygen directly attached to an alkene carbon. Electrophilic activation and hydrolysis can convert an appropriate enol ether back to a carbonyl compound.

Interconversion of a carbonyl compound with its enol or enolate-derived forms. Reprotonation of a planar intermediate can epimerize an alpha stereocentre.

oxidation of an alkyl halide or a related electrophile by dimethyl sulfoxide. An alkoxysulfonium intermediate undergoes base-promoted elimination to a carbonyl compound and dimethyl sulfide.

An enamine formed reversibly between proline and an enolizable carbonyl compound attacks an electrophilic carbonyl. The carboxylic acid group can orient and activate the electrophile; hydrolysis releases the aldol product and regenerates proline.

A grignard reagent in which magnesium is attached directly to an alkenyl carbon, $\mathrm{RCH}=\mathrm{CHMgX}$. Addition to a carbonyl compound can form an allylic alcohol while retaining the geometry of the pre-existing alkene.