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The α-ketol rearrangement of tertiary α-hydroxy esters induced by LiAlH 4 /acid treatment provides α-hydroxy ketones in good yields and high diastereoselectivity. A mechanism is proposed for this reaction as well as a model to explain the diastereoselectivity of the rearrangement.

A diastereodivergent addition of allenylzincs to aryl glyoxylates was observed depending on the method used for the preparation of the allenylzinc reagent. The allenylzincs were prepared from propargylic benzoates in the presence of a palladium catalyst or by metalation of alkynes.

[1,2]-Wittig rearrangement of (benzyloxy)acetamides can lead to substituted α-hydroxyamides in good yields and good diastereoselectivity.

Depending on the steric hindrance of the reaction centers, hemiacetal vinylogs 1 in the presence of BF3.OEt2 can, with enol ethers, lead to a Michael-type addition (4) or an aldolisation-type reaction (3). Hemiacetal vinylog 1b always yields the second reaction leading to β-methoxy-γ,δ-ethylenic carbonyl compounds 4 precursors of polyenic carbonyl compounds 5. Crotonaldehyde and methanol can be used instead of compound 1b.

The enolate of prenal 1 prepared from the corresponding silyl enol ether 2 or enol acetate 3 led to a γ-regiospecific reaction with polyunsaturated aldehydes 4 yielding dihydropyrans 5 leading after hydrolysis to polyenals 7 . This process allows the introduction of the isoprenyl skeleton. A synthesis of retinal. from β-ionylidenacetaldehyde is reported.

A γ-condensation of the enolate of prenal 1 is observed with aldehydes and α,β-unsaturated aldehydes. In this last case, a conjugate addition is obtained. Intermediate dihydropyrans 4 yield dienals 5 by hydrolysis.

The α-ketol rearrangement of tertiary α-hydroxy esters induced by LiAlH 4 /acid treatment provides α-hydroxy ketones in good yields and high diastereoselectivity. A mechanism is proposed for this reaction as well as a model to explain the diastereoselectivity of the rearrangement.

Bromination of enamines of prenal and crotonaldehyde yields the corresponding ω-bromoaldehydes via the simple hydrolysis of iminium salts. The carbonyl function of these aldehydes can be protected as acetals or dioxolane. The overall process can be realized in a one pot procedure. The dioxolane is the starting material of a phosphonate which allows,by condensation with β-ionylidenacetaldehyde a direct access to the retinal with a 27.5% overall yield from the enamine 2

A diastereodivergent addition of allenylzincs to aryl glyoxylates was observed depending on the method used for the preparation of the allenylzinc reagent. The allenylzincs were prepared from propargylic benzoates in the presence of a palladium catalyst or by metalation of alkynes.

A γ-condensation of the enolate of prenal 1 is observed with aldehydes and α,β-unsaturated aldehydes. In this last case, a conjugate addition is obtained. Intermediate dihydropyrans 4 yield dienals 5 by hydrolysis.

The enolate of prenal 1 prepared from the corresponding silyl enol ether 2 or enol acetate 3 led to a γ-regiospecific reaction with polyunsaturated aldehydes 4 yielding dihydropyrans 5 leading after hydrolysis to polyenals 7 . This process allows the introduction of the isoprenyl skeleton. A synthesis of retinal. from β-ionylidenacetaldehyde is reported.