Zinc enolates Subject

The addition of zinc enolates to alkenes in the intramolecular version finds several examples in recent literature. Thus, hydrazone 155, subjected to the same treatment reported for 151 (equation 80), undergoes diastereoselective 5-exo-trig (n = 1) or 6-exo-trig (n = 2) carbocyclization to yield -156, which on reaction with the electrophile E+ gives 157 (equation 81)174. 

Ireland utilized a sulfide ring contraction process for the ring-forming step of his synthesis of 47 (see Scheme 1.16) The diethyl acetal of 5-hydroxyhexanal (73) was converted to the trichloroethyl carbonate and the acetal hydrolized to produce aldehyde 74 in 86% yield. Addition of the zinc enolate of N,N-dimethylethanethioamide (75) to 74 then afforded an intennediate hydroxy thioamide which was subjected to acylation and reductive carbonate cleavage to yield 76 (53%). Acylation with chloroacetyl chloride was followed by ring closure to give the macrocycle 77 (24%). 

Alternatively, furan may also be brominated and then subjected to an exhaustive methanolysis. A zinc chloride-catalysed double enol ether condensation with 1-propenyl methyl ether (Miiller-Cunradi-Pieroh reaction) gives finally the crystaUine (aU )-Cio-dialdehyde in an overall yield of >50 %. 

In addition to metal-based catalysts, organocatalysts are also selective promoters of asymmetric Diels-Alder reactions. Several groups reported the use of cinchona alkaloid catalysts in standard Diels-Alder reactions. Deng combined 2-pyrones with a,P unsaturated ketones, while Bernard and Ricci focused on the reactions of vinylindoles with quinones and maleimides. Lectka reported enantioselective inverse electron demand hetero Diels-Alder reactions of ketene enolates and o-benzoquininone diimides catalyzed by a combination of benzoylquinidine and zinc triflate. For example, subjecting diimide 51 to the standard reaction conditions yields cycloadduct 52 as a single stereoisomer, which can be easily converted to... 

---