Stereoselectivity allylic zinc-aldehyde reaction

Homoallylic alcohols can be obtained by sonicating a mixture of allylic halides, zinc, and aldehydes or ketones in aqueous THF (Fig. 51).282 The yields are not always satisfactory, and replacement of the THF-water solvent by THF-aqueous NH4CI greatly improves the method. With these conditions, sonication is tmnecessary, and stirring at room temperature is sufficient to provide activation. Allylic bromides react faster than the chlorides and the process is chemoselective, thus enabling allylation of aldehydes in the presence of ketones.283 in all cases, the allylic group becomes attached to the substrate at the more substituted carbon atom. The stereoselectivity of this reaction was recently studied.284 Allylation can... 

In ( )-[2-(l-propenyl)-l, 3-dithian-2-yl]lithium, no problem of EjZ selectivity arises. It is easily prepared by deprotonation of the allylic dithiane87,88 with butyllithium in THF, whereas deprotonation of the 2-propylidene-l, 3-dithiane requires the assistance of HMPA. The addition to saturated aldehydes proceeds with excellent y-regioseleetivity and anti selectivity88,89. As often observed in similar cases, aldehydes which bear an, p2-carbon atom adjacent to the carbonyl group give lower selectivities. The stereoselectivity decreases with ketones (2-bu-tanone y/a 84 16, antiisyn 77 23)88. The reaction with ethyl 2-oxopropanoate is merely nonstereoselective90, but addition of zinc chloride improved the syn/anti ratio to 96 4, leading to an efficient synthesis of ( )-crobarbatic acid. 

The combined carbocupration of ynol ether followed by a zinc homologation in the presence of aldehydes has provided, in a single-pot operation, stereodefined allylic vicinal diol substmctures in good isolated yields and stereoselectivities (up to 92 8 dr) The stereochemistry of the final adducts suggests that the reaction 0 proceeds through a chairlike transition state in which the substituent of the aldehyde preferentially occupies a pseudoaxial position to avoid gauche interactions with an -configurated alkoxy-substituted aUylmetal species. A similar stereochemical outcome has been reported for the allylation reaction of 3,3-disubstituted allylzinc species with aldehydes. ... 

Homoenolate Reactivity. Since the previous e-EROS report, a number of examples have been described using the cyclopropanone acetals. Thus, the zinc homoenolate, known to undergo a highly regioselective and stereoselective Sn2 allylation reaction (eq 6), is used in the synthesis of moenomycin analogues. The activated titanium homoenolate reacts with aldehydes or ketones to give y-hydroxy esters that serve as precursors to y-lactones. ... 

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