Rhodium-catalyzed cyclopropanations of 2-aryl-2H-chromenes with dialkyl malonate esters. A comparison of α-diazo derivatives and phenyliodonium ylides

Asymmetric Chemoenzymatic One-Pot Synthesis of α-Hydroxy Half-Esters

A new chemoenzymatic one-pot strategy has been developed for the synthesis of α-hydroxy half-esters containing consecutive quaternary and tertiary stereocenters using asymmetric cascade catalysis. In this study, an asymmetric Ca²⁺-catalyzed [2,3]-Wittig rearrangement reaction was proven to be suitable for a combination with porcine liver esterase-mediated hydrolysis resulting in the enhanced enantiomeric purity of the obtained products in a one-pot synthesis compared to the stepwise method.

Lewis acid catalyzed [3 + 2] annulation of ketenimines with donor–acceptor cyclopropanes: an approach to 2-alkylidenepyrrolidine derivatives

C,C,N-Trisubstituted ketenimines undergo [3 + 2] annulation reactions via its CN bond with donor–acceptor cyclopropanes triggered by Sc(OTf)₃ to yield a diversity of 2-alkylidene pyrrolidines.

Synthesis of 3-benzylidene-dihydrofurochromen-2-ones: promising intermediates for biflavonoid synthesis

A route to 3-benzylidene dihydrofurochromen-2-ones from 2H-chromenes is described. Lactonization of 2H-chromenes was achieved using a two-step cyclopropanation-rearrangement sequence. Subsequent conversion of these intermediates to the corresponding α-benzylidene lactones was achieved by lithium enolate Aldol reaction, followed by base-promoted elimination of the aldolate mesylates. The alkene geometry was found to be base-dependent. While KO t Bu favored formation of the E-isomer, DBU showed a slight preference for the Z-isomer. In further studies, these 3-benzylidene dihydrofurochromen-2-ones were converted to polyaromatic structures possessing all the required functionality for biflavonoid synthesis.