Dienolates monoalkylation

As shown in Scheme 49, the cyclohexylimine of 10-methyl-l(9)-octalin-2-one has been methylated at C-3 via the kinetically formed cross-conjugated dienamine anion (95) or monomethylated at C-1 via the thermodynamically more stable extended anion (96). Steroidal and simpler enones have also been monoalkylated at the a-position via their corresponding hetero- or homo-annular extended dienamine anions. Likewise, a-alkylations are the rule for liAiated a,3-unsaturated aldimines. The thermodynamically controlled procedure for the synthesis of a-methyl a,3-unsaturated ketones is a vast improvement over conventional methodology using extended metal dienolates where a,a-dimethylation is a severe... 

In this event, the addition of 2.1 equivalents of NaHMDS in THF at —78°C converted 17 into a prostereogenic cyclic enolate which then attacked the tartrate-derived dielectrophile 18 with a high level of facial selectivity to generate an intermediate of type 19. Subsequent cyclization was then biased to provide the desired pseudo meso product 20, a compound that was ultimately obtained in 92% yield. In contrast, when the conditions were changed to prevent chelation by using LiHMDS instead of NaHMDS in a solvent mixture of THF and HMPA (hexamethyl-phosphoramide), the C2-symmetric variant of this product (27, see column figures) was formed in 58 % yield. This alternate outcome presumably reflects the preliminary formation of an acyclic lithium dienolate which led to a monoalkylated product corresponding to 26. ... 

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