Palladacyclic intermediates effectively lower the high energy barrier to cleavage of a C(sp3)H bond. CH activation of benzylic gem‐dialkyl groups of bromo‐ and iodobenzenes produces olefins and cyclobutabenzenes, respectively, without homocoupling (see scheme).

A catalytic asymmetric synthesis of the axially chiral bridged biaryl (-)-2, a structural analogue of natural (-)-rhazinilam possessing original antimitotic properties, is described. The key step is an intermolecular asymmetric Suzuki coupling, furnishing the nonbridged biaryl (-)-6, precursor of (-)-2, with up to 40% ee using binaphthyl ligand 7a. Various known or new binaphthyl and ferrocenyl phosphines as well as phosphetanes were screened as ligands in this reaction, the conditions of which were optimized. The comparison with another Suzuki coupling system showed that 7a is the most versatile ligand described to date for this type of transformation. This work gives the first application of the asymmetric Suzuki coupling to a biologically relevant target.

Photolysis of cyclopentadienylmanganese tricarbonyl derivatives in the presence of a proton source leads to decomplexation and formation of the free cyclopentadiene in excellent yield. In contrast, attempted decomplexation using Ce(IV) or Fe(III) leads merely to decomposition products. It is proposed that, upon photolytic loss of a CO ligand, coordination of methanol or water facilitates an intramolecular proton transfer to the cyclopentadienyl ring. This method is applicable to polyfunctional molecules such as ethynylestradiol. In this new synthetic approach, these cymantrenes have also been used as precursors to the corresponding iron, tungsten, titanium, and rhenium complexes via the generation of the intermediate cyclopentadienes.

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A convergent asymmetric total synthesis of pironetin (1), a polyketide with immunosuppressive, antitumor, and plant-growth regulating activities is described. The synthesis was realized by coupling between the C(8)-C(14) 2 and C(7)-C(2) 15 fragments, respectively, by using a Mukaiyama-aldol reaction. The stereogenic centers of each fragment were generated by employing the SAMP/RAMP hydrazone (SAMP=(S)-1-amino-2-methoxymethylpyrrolidine, RAMP=(R)-1-amino-2-methoxymethylpyrrolidine) methodology as a key step. An asymmetric alpha-alkylation of diethyl ketone permitted the introduction of the C(10) stereogenic center of 2, whereas the stereocenters C(4) and C(5) of 15 were installed by an asymmetric aldol reaction. Finally, the formation of the alpha,beta-unsaturated delta-lactone was achieved by ring-closing metathesis in the presence of catalytic amounts of titanium tetraisopropoxide.

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Enantioselective syntheses Enantioselective syntheses O 0031Asymmetric Synthesis of an Axially Chiral Antimitotic Biaryl via an Atropo--Enantioselective Suzuki Cross-Coupling. -The first application of an atropo--enantioselective Suzuki coupling to the synthesis of a biologically relevant derivative is presented. Binaphthyl-derived P,N-ligand (LIG) is proven to be the best ligand for this reaction by detailed ligand screenings and optimization studies. -(HERRBACH, A.; MARINETTI, A.; BAUDOIN*, O.; GUENARD, D.; GUERITTE, F.; J. Org. Chem. 68 (2003) 12, 4897-4905; Inst. Chim. Subst. Nat., CNRS, F-91198 Gif-sur-Yvette, Fr.; Eng.) -Nuesgen 40-038