Palladium-Catalyzed Construction of Quaternary Stereocenters by Enantioselective Arylation of γ-Lactams with Aryl Chlorides and Bromides.

Herein, we report the first Pd-catalyzed enantioselective arylation of α-substituted γ-lactams. Two sets of conditions were developed for this transformation, allowing for the use of either aryl chlorides or bromides as electrophiles. Utilizing a highly electron-rich dialkylphosphine ligand we have been able to construct α-quaternary centers in good yields (up to 91 % yield) and high enantioselectivities (up to 97 % ee).

Cerium-catalyzed, oxidative synthesis of annulated, tetrasubstituted dihydrofuran-derivatives.

Densely functionalized, annulated dihydrofuran derivatives are prepared by a cerium-catalyzed aerobic oxidation reaction. The operationally simple transformation is environmentally and economically benign, since the precatalyst CeCl3·7H2O is non-toxic and inexpensive and the oxidant is simply dioxygen from air. Starting materials are ß-oxoesters and silylenolethers, and the latter are derived from acetoacetate or acetylacetone. The reaction sequence is performed in one flask and consists of α-oxidation and Mukaiyama aldol reaction. Apart from tetrahydrocyclopenta[b]furan derivatives one example of a tetrahydrofuro[3,4-b]furan and one tetrahydro-3aH-furo[2,3-c]pyrrole derivative are prepared.

Formation of δ-Lactones with anti-Baeyer-Villiger Regiochemistry: Investigations into the Mechanism of the Cerium-Catalyzed Aerobic Coupling of ß-Oxoesters with Enol Acetates.

The cerium-catalyzed, aerobic coupling of ß-oxoesters with enol acetates and dioxygen yields δ-lactones with a 1,4-diketone moiety. In contrast to the Baeyer-Villiger oxidation (BVO), where the higher substituted residue migrates; in the case of this oxidative C-C coupling reaction, the less substituted alkyl residue undergoes a 1,2-shift. An endoperoxidic oxycarbenium ion comparable to the Criegee intermediate in the BVO is proposed as a reaction intermediate and submitted to conformational analysis by computational methods. As a result, the inverse regiochemistry is explained by a primary stereoelectronic effect. A Hammett analysis using different donor- and acceptor-substituted enol esters provides support for the oxycarbenium ion being the crucial intermediate in the rate determining step of the conversion. An overall mechanism is suggested with a radical chain reaction for the formation of endoperoxides from ß-oxoesters, enol acetates and dioxygen with a cerium(IV) species as initiating reagent.

Formation of δ-Lactones by Cerium-Catalyzed, Baeyer-Villiger-Type Coupling of ß-Oxoesters, Enol Acetates, and Dioxygen.

Formation of δ-lactones is observed when cyclopentanone-2-carboxylates are converted in a cerium-catalyzed reaction with α-aryl vinyl acetates under oxidative conditions. The products of this transformation possess a 1,4-dicarbonyl constitution together with a quaternary carbon center. Atmospheric oxygen is the oxidant in this process, which can be regarded as ideal from economic and ecological points of view. Further advantages of this new C-C coupling and oxidation reaction are its operational simplicity and the application of nontoxic and inexpensive CeCl3·7 H2O as precatalyst. This so far unprecedented reaction is proposed to proceed via 1,2-dioxane derivatives, which decompose under formation of an oxycarbenium cation in a Baeyer-Villiger-type pathway. This mechanistic picture is supported by the observation that electron-rich (donor substituted or heteroaromatic) enol esters give higher yields than electron deficient congeners. Apart from 1,4-diketones and α-hydroxylated ß-oxoesters formed as byproducts, the yields of δ-lactones range from moderate to good (up to 74%).

Three-component reaction toward polyannulated quinazolinones, benzoxazinones, and benzothiazinones.

An efficient conversion of readily accessible cyclic imines with acid chlorides, that are able to take part in nucleophilic aromatic substitution (SNAr) reactions is realized in a new three-component, one-pot reaction, giving at least tricyclic annulated quinazolinones, benzoxazinones, and benzothiazinones as a result of the employed nitrogen, oxygen, or sulfur nucleophiles, respectively. Especially in the case of quinazolinones, this convenient strategy enables the access to heterocycles of heightened diversity, which offer the development of efficient derivatizations of the elaborated heterocyclic scaffolds. In a subsequent Heck or Ullmann cyclization, further annulations to the mentioned quinazolinones can be carried out.