Zinc-mediated allylation

The zinc-mediated allylation of racemic branched a-oxoalkanoic esters proceeds with moderate stereoselectivity46, better results are obtained with (2-propenyl)trimethylsilane/titanium(IV)... 

The aqueous Barbier-Grignard-type reaction has also been used in the synthesis of natural products. Chan and Li used the zinc mediated allylation as a key step in a total synthesis of (+)-muscarine (Scheme 8.5).72 The strategy was based on the observation that the diastereoselectivity of the allylation reaction in water can be reversed through the protection of the a-hydroxyl group. 

SYNTHESIS OF 1 -CYCLOHEXYLPENT-3-EN-1 -OL USING ZINC-MEDIATED ALLYLATION... 

Two interesting features of the synthesis worthy of some discussion are the addition of an organometallic reagent to aldehyde 584 and the choice of the protecting group. The zinc-mediated allylation of 584 produces a chromatographically separable 71 29 mixture of anti diastereomer 585 and the corresponding syn diastereomer (not shown). This stereoselectivity... 

This reaction was first reported by Luche in 1985. It is the modification of the initial aqueous allylation of carbonyl compounds (in 1983) via the intermediacy of tin in the presence of a catalytic amount of hydrobromic acid (HBr), to the zinc mediated allylation of carbonyl compounds using ultrasonic irradiation in saturated aqueous NH4CI/THF solution. Thus the new reaction condition is referred to as the Luche reaction. In parallel, Li" and Chan extended the Barbier-Grignard-type reaction to successfully proceed in aqueous media and contributed to the new area called green chemistry. ... 

Unlike the metallic tin-mediated allylation of carbonyl compounds, which requires either acidic medium, aluminum additive, ultrasonic radiation or heating, the metallic zinc-mediated allylation proceeds at room temperature. 

With functionalized allyl bromides, such as ethyl (2-bromomethyl)acry-late, the zinc-mediated allylation in a saturated aqueous NH4CI/THF mixture under reflux provided a-methylene-y-butyrolactones in good yield (Mattes and Benezra, 1985) ... 

Zinc-mediated allylation in the Luche conditions (commercial zinc powder in a saturated aqueous NH4 Cl/THF mixture) was applied in a five-step synthesis of (+)-muscarine, wherein an aldehyde derived from ethyl-lactate was treated with allyl bromide. No chelation control was observed with the a-alkoxyaldehyde, as well (Chan and Li, 1992a). 

Facing these difficulties concerning the syrJanti-selectivity in carbonyl allylations, Marton et al., (1996a) extensively studied the zinc-mediated allylation of aldehydes with allyl halides in cosolvent/HaO (NH4CI) and in cosol-vent/H20 (NH4Cl)/haloorganotin media. The stereochemistry seems to be determined by the structure of the radical anions, which were presumed to be formed through an electron-transfer process. 

Of great interest is the use of Oppolzer s camphorsultam as chiral auxiliary in a zinc-mediated allylation of oximes in the Luche conditions ... 

The formation of organometallic intermediates, however, can be initiated by an allylic radical anion on the metal surface (Figure 4.3). Noteworthy is the high value of the electron affinity of allylic bromide and, to a lesser extent, chloride (Moyano et al, 1990). As a matter of fact, in tin-mediated reactions, Wurtz coupling products were sometimes detected (Kim et al, 1993), but, unlike the zinc-mediated allylation, no dimerization of the carbonyl compounds was observed in the absence of the halide (Einhom and Luche, 1987). 

Hanessian, S. Yang, R.-Y. (1996) The asymmetric synthesis of allylglycine and other unnatural a-amino acids via zinc-mediated allylation of oximes in aqueous media. Tetrahedron Lett., 37, 5273-6. 

Petrier, C. Einhorn, J. Luche, J.-L. (1985) Selective tin and zinc mediated allylations of carbonyl compounds in aqueous media. Tetrahedron Lett., 26,1449-52. 

Sjoholm, R. Rairama, R. Ahonen, M. (1994) Zinc mediated allylation of aldehydes and ketones with cinnamyl chloride in aqueous medium,/. Chem. Soc., Chem. Commm., 1217-18. 

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