Acetals reactions with organomagnesium

Imines 223 derived from glyoxal acetals react with various organomagnesium compounds with high diastereoselectivity (equation 152) . The 1,2-aminoalcohols 224 can be converted into the protected enantiopure aminoaldehydes 225. For these reactions toluene was found to be a superior solvent. 

Dioxopyrrolizidine was widely used by Micheel et al33,118 as starting compound in the syntheses of some pyrrolizidine derivatives. Its reaction with phosphorus pentasulfide gave rise to pyrrolizidine-3,5-dithione, and it was converted via a Reformatsky reaction into ethyl 3-oxo-5-hydroxypyrrolizidine-5-acetate (165) and ethyl 3-oxo-pyrrolizylidene-5-acetate (166). Condensation with organomagnesium... 

Catalysed reactions of organomagnesium compounds with allylic acetates are potentially useful in synthesis, but lack of regioselectivity may be a problem ... 

In acyclic systems, reductions of the ketone group of 1.155 (Y = COR) give poor selectivities. Reactions of o-substituted aldehydes 1.155 (Y = CHO) with organomagnesium reagents, perfluoroalkyllithiums or nitromethane [540] or chloracetophenone [540, 544] anions are very selective. Such is also the case for their reactions with functionalized isonitriles [540], silyl enolethers or thioketene acetals in the presence of Lewis acids [545, 546], or in B aylis-Hillmann reactions [547],... 

The versatihty of the present furanone synthetic method was demonstrated in the stereoselective synthesis of the Z-isomer of multicolanate (139) [118] (Scheme 26). The prerequisite compound 137 was prepared by successive treatment of appropriate organomagnesium and organohthium reagents, and it was transformed smoothly with lead tetraacetate to the target molecule (the incomplete acetate product 138 was subjected to elimination reaction with DBU). 

Uncatalyzed addition reactions of Grignard reagents with nonconjugated alkenes and aUcynes are of limited use in synthesis. However, carbon-carbon double bonds substituted by a leaving group, such as an acetate, are susceptible to be displaced by organomagnesium compounds presumably by an addition-elimination pathway. A few examples have been reported (equation 64). 

For the preparation of dihydrodiorganosilane which are liquid at 20 °C, gaseous Cl2SiH2 is dissolved in an ether solution of organomagnesium bromide. After 30 min refluxing, the reaction mixture is hydrolysed with aqueous acetic acid (10%). The following compounds are obtained in this way [124] ... 

Organomagnesiums frequently prove superior also in other types of reactions. They may facilitate the oxidation of a carbon-metal to a carbon-oxygen bond, secure clean monoaddition of an acetylide to an activated ester (a critical issue in a monensin synthesis X favor in the presence of a copper catalyst 1,4-addition onto a conjugated enone over 1,2-addition, reorient the attack of formaldehyde on a benzylic entitiy from the a- to the or /to-position, and provide diastereoselectivity in nucleophilic additions onto aldehydes. Furthermore organomagnesiums combine under carbon-carbon linking with a variety of organic halides, tosylates, and acetates if the process is mediated by transition elements such as palladium(O) copper(I), nickel(II) or iron(II) Organoalkalis are often less fit to enter such catalytic cycles. 

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