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.

An efficient and straightforward ten-step asymmetric synthesis of the polyketide tarchonanthuslactone (1) in good overall yield (21% starting from chloro lactone 5) and with excellent diastereomeric and enantiomeric excesses (de, ee Ն 96%) is described. The new synthetic route is based on the α,β-unsaturated δ-lactone building block 5, available in

An Efficient Asymmetric Synthesis of Tarchonanthuslactone. -Key steps of the new asymmetric synthesis of tarchonanthuslactone (X) are the enzymatic transformation of diketo ester (I) to lactone (II) in excellent stereoselectivity, and the construction of the tarchonanthuslactone skeleton (IX) by an umpolung technique. -(ENDERS*, D.; STEINBUSCH, D.; Eur.

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