Stereoselective Formation of Fully Substituted Ketone Enolates

Abstract

The application of stereochemically defined acyclic fully substituted enolates of ketones to the enantioselective synthesis of quaternary carbon stereocenters would be highly valuable.H erein, we describe an approach leading to the formation of several new stereogenic centers through ac ombined metalation-addition of acarbonyl-carbamoyl transfer to reveal in situ stereodefined a,a-disubstituted enolates of ketone as asingle stereoisomer.This approach could produce aseries of aldol and Mannich products from enol carbamate with excellent diastereomeric ratios.The carbonyl group is one of the most versatile and broadly utilized functional groups in organic synthesis.[1] Thes uccess of this widely employed starting material results from its multiple reactivities.Reactions proceed either at the electrophilic carbon or nucleophilic oxygen center of the carbonyl group,o ra lternatively at the adjacent CÀHp osition by removing an acidic hydrogen. In the latter case,anenolate for carbon-carbon and carbon-heteroatom bonds formation is generated. Taking into consideration the significance of enolates as valuable intermediates in asymmetric organic synthesis, [2] one can evaluate the consequence of developing efficient methods to the direct access of a,a-disubstituted metal enolates 1 as ar oute to the formation of challenging quaternary carbon stereocenters (Scheme 1, Path A). [3] Indeed, one structural element that invariably increases the difficulty of achemical synthesis is the presence of quaternary carbon stereocenters in the target molecule.[4] Thei mpediment to synthesis presented by such centers arises from the steric congestion imposed by the four attached carbons.I f such stereocenters could be prepared in asingle-pot operation through the formation of several CÀCb onds from common and easily accessible starting materials,i tw ould surely find numerous applications in organic synthesis. [5,6] However,t he generation of as tereodefined fully substituted enolate precursor as as ingle isomer in acyclic systems is not atrivial task, especially for ketone enolates,a nd therefore all the enolization reactions leading to stereodefined a,a-disubstituted enolates were restricted to esters and amides.F or instance, a,a-acyclic disubstituted esters require ap erfect conformational control for the abstraction of the a-hydrogen, [3] and this control can only be achieved by deprotonation of diastereomerically pure a,a-acyclic dialkylated amides, [7] enantiomerically pure a,a-acyclic dialkylated esters with chiral bases (Scheme 1, Path B), [8] or through conjugate additions.[9]As we have been involved over the last few years in the development of synthetic strategies leading to the creation of several carbon-carbon bonds in acyclicsystems and as inglepot operation, including the formation of quaternary carbon stereocenters, [5,10] we reported adirect access to the formation of a,a-disubstituted enolates of amides by carbocupration of ynamides followed by selective oxidation of the resulting alkenyl cuprates species (S...

Regio‐ and Stereoselective Synthesis of Fully Substituted Silyl Enol Ethers of Ketones and Aldehydes in Acyclic Systems

“…Attractive solutions to this problem have been reported, involving either the in situ generation and trapping of ketene intermediates (Scheme 1, path b), [13] through ac ombined metalation-addition of ac arbonyl and carbamoyl transfer (Scheme 1, path c), [14] or through aMichael addition followed by as p 2 -type Brook rearrangement (Scheme 1, path d). [15] Although these methods provide the desired stereodefined enolates of ketones,t hey intrinsically present some limitations.F or instance,t he stereochemistry of the enolate resulting from the reaction of methyllithium to ketene depends on the steric bulk between the two substituents on the ketene (R 2 versus Me).…”

Regio‐ and Stereoselective Synthesis of Fully Substituted Silyl Enol Ethers of Ketones and Aldehydes in Acyclic Systems

“…Attractive solutions to this problem have been reported, involving either the in situ generation and trapping of ketene intermediates (Scheme 1, path b), [13] through ac ombined metalation-addition of ac arbonyl and carbamoyl transfer (Scheme 1, path c), [14] or through aMichael addition followed by as p 2 -type Brook rearrangement (Scheme 1, path d). [15] Although these methods provide the desired stereodefined enolates of ketones,t hey intrinsically present some limitations.F or instance,t he stereochemistry of the enolate resulting from the reaction of methyllithium to ketene depends on the steric bulk between the two substituents on the ketene (R 2 versus Me).…”

Lithium Enolates in the Enantioselective Construction of Tetrasubstituted Carbon Centers with Chiral Lithium Amides as Noncovalent Stereodirecting Auxiliaries

“…Although enolizations of oxazolidinone- and N -alkylephedrine-based auxiliaries are highly stereoselective owing to allylic strain, 12 this same strain precludes the simple generation of the fully substituted enolates required for the formation of tetrasubstituted sp 3 carbon stereocenters. 13,14,15 …”