N,N'-diarylureas: a new family of atropisomers exhibiting highly diastereoselective reactivity.

2,6-disubstituted N-aryl ureas rotate slowly about their Ar-N bonds and can exist as separable atropisomers. They also react remarkably diastereoselectively, with the urea axis controlling new stereogenic centers with high fidelity in a variety of nucleophilic and electrophilic addition reactions. The sense of diastereoselectivity in lateral lithiation-electrophilic quench reactions is electrophile-dependent and appears to be the result of stereospecific reaction with one of two interconvertible diastereoisomeric organolithiums.

Intramolecular hetero-Michael addition of beta-hydroxyenones for the preparation of highly substituted tetrahydropyranones.

Structurally diverse beta-hydroxyenones are shown to undergo nonoxidative 6-endo-trig ring closure to form highly substituted tetrahydropyranones. Amberlyst-15, Al(ClO(4))(3) x 9 H(2)O and [Pd(MeCN)(4)](BF(4))(2) were found to be suitable catalysts for these intramolecular conjugate additions, preventing side reactions, such as dehydration or retroaldolisation. The use of [Pd(MeCN)(4)](BF(4))(2) is particularly effective, as this palladium-mediated reaction is under kinetic control and generates tri- and tetrasubstituted tetrahydropyranones with high levels of diastereocontrol. In the presence of the Lewis acid Al(ClO(4))(3) x 9 H(2)O, the reaction proceeded with a similar level of diastereocontrol; however, in contrast to [Pd(MeCN)(4)](BF(4))(2), this catalyst can promote enolisation. The palladium-mediated reaction was also found to be compatible with an enantioenriched beta-hydroxyenone substrate, giving no loss of enantiopurity upon ring closure. The most distinctive synthetic development to emerge from this new chemistry is the possibility to access tri- and tetrasubstituted 2,6-anti-tetrahydropyranones from anti-aldol precursors. These compounds are particularly difficult to access by using alternative methodologies. Two modes of activation were envisaged for the ring closure, involving metal coordination to either the C=C or C=O functional groups. Experimental results suggest that C=O coordination was the preferred mode of activation for reactions performed in the presence of Al(ClO(4))(3) x 9 H(2)O or [Pd(MeCN)(4)](BF(4))(2).

Palladium-catalyzed oxidative cyclizations: synthesis of dihydropyranones and furanones.

A boron-mediated syn- and anti-stereoselective aldol reaction giving rise to various beta-hydroxyenones was coupled to a Pd((II))-mediated oxidative cyclization to give 2,3,6-trisubstituted syn- and anti-dihydropyranones in good yields. The Pd((II))-mediated oxidative cyclization was expanded to alpha-hydroxyenones leading to furan-3(2H)-one derivatives, which include natural product bullatenone and a known precursor of geiparvarin. The sole product of the oxidative cyclization of alpha,beta-dihydroxyenone was a five-membered furan-3(2H)-one derivative, suggesting that the ring closure of these diols is both chemo- and regioselective.

Ring-selective functionalization of N,N'-diarylureas by regioselective N-alkylation and directed ortho metalation.

[reaction: see text]. Unsymmetrical N,N'-diarylureas may be alkylated regioselectively at the more sterically congested nitrogen atom. The resulting mono-N-alkylated ureas undergo directed metalation (ortholithiation) with sec-BuLi to yield, on electrophilic quench, products functionalized regioselectively at the ring bearing the alkylated nitrogen atom.