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Part I: An anti-Selective Four-Component Direct Aldol Addition Involving Chemoselective Thioester Enolate Formation & Part II: Reductive Enolization of α-Halo Thioesters for Use in Direct Aldol Addition
- Abstract:
The aldol reaction is a versatile and powerful reaction in organic chemistry. Two projects have been undertaken in which a direct approach to the aldol reaction is investigated. The first project, which was already underway upon my arrival in the Coltart lab, sought to exploit special properties of thioesters and thiolates in the context of the aldol addition reaction. It was found that thiolate conjugate addition to an α,β-unsaturated thioester could be used to initiate a one-pot cascade reaction, in the presence of an aldehyde and MgBr2•OEt2, that affords the aldol addition product. This method yields a facile and chemoselective route to the aldol condensation with high anti¬ stereoselectivity. The yields found in this reaction generally exceed 70% after only 30 minutes, even when the reaction is conducted open to the air.
Another method for the aldol addition reaction was found when an α-halothioester was reacted with triphenylphosphine in the presence of MgI2 and an aldehyde. The aldol product is generated by reductive enolization of the α-halothioester, which does not require an α-substituted enolate precursor, unlike the conjugate addition based aldol reaction. This allows for a broader scope of aldol products to be synthesized and generates the predominantly syn stereoisomer. Moderate to good yields are achieved in 1 hour with this reductive enolization approach.
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