Lithium dimethylcuprate regioselectivity

It was already noted that activated enynes bearing an acceptor substituent at the double bond react with organocuprates under 1,6-addition to provide functionalized allenes (see Section III.A)38. Interestingly, the preference of these reagents for triple bonds persists even when the distance between the acceptor group and the triple bond is increased by the introduction of further double bonds. For example, lithium dimethylcuprate attacked ethyl 8,8-dimethyl-2,4-nonadien-6-ynoate at the triple bond exclusively, and regioselective... 

Elaboration of 248 to 257 is depicted in Scheme 32. Acidic deketalization, basic epoxide formation, and silylation of the remaining hydroxyl group led to 249, which was subjected to ring opening with lithium dimethylcuprate in a regioselective (78 15) fashion to provide the desired alcohol 250 as the major... 

The regioselectivity of Grignard addition to (V-alkoxycarbon y I pyridinium salts can often be controlled by changing the conditions. For example, C(4) addition can be enhanced by the presence of catalytic amounts of copper salts reaction of salt 288 (R = Me) with lithium dimethylcuprate gives also almost exclusively the 1,4-dihydro product 290 (R = Me, R Me). 

The regioselectivity of the addition of copper-activated alkanes to activated cyclopropanes is not always maintained. An exception is the cleavage of the cyclopropyl bond in a very complex polycycle upon treatment with lithium dimethylcuprate which adds in a 1,5-fashion across the cyclopropane bond. ° ... 

The regioselectivity of nucleophilic attack for the formation of G-G bonds depends on the nature of the nucleophile as well as the substituents attached to the dienyl ligand. Reactions of 1,2-disubstituted 42a and 42b and 1,4-disubstituted 43a and 43b cations with methanol, triphenylphosphine, lithium dimethylcuprate, and sodium dimethylmalonate nucleophiles have been investigated. It was found that cations 42a and 42b direct nucleophilic attack regiospecifically to the unsubstituted C5 position whereas cations 43a and 43b undergo G1 addition, although in a less predictable manner (Scheme 3). Weak nucleophiles such as methanol, allyltrimethylsilane, furan, and allylic organozirconiums are found to attack exclusively at the less substituted pentadienyl terminus and on the less stable, but more reactive, transoid form. Malonate anion attack on 1-substituted acyclic (77 -pentadienyl)-... 

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