Efficient Asymmetric Synthesis of Long-Chain Polyketides Containing up to Ten Contiguous Stereogenic Centres by Double Chain Elongation

The 2,3-anti-3,4-syn-stereotriad (1Z,2S,3R,4S)-1-ethylidene-2,4-dimethyl-3-[(1S)-1-phenylethoxy]-5-oxopent-1-yl isobutyrate {(-)-8, obtained in a one-pot operation from the trimethylsilyl (Z)-enol ether derived from pentan-3-one and (1E,3Z)-1[(1S)-phenylethoxy]-2-methylpenta-1,3-dien-3-yl isobutyrate through SO2 umpolung} contains an ethyl ketone moiety that undergoes diastereoselective cross-aldol reactions with acetaldehyde. After subsequent reductions, various diastereomeric stereohexads (polypropionate fragments containing six contiguous stereogenic centres) in which the (Z)-enol isobutyrate units remained intact were obtained. After suitable protection, these were converted into their corresponding lithium (Z)-enolates, which could react with isopropylidene D-glyceraldehyde [(+)-9] to give aldol products that were reduced to various stereodecads (polyketides with ten contiguous stereogenic centres). The disclosed chemistry represents a quick, asymmetric, diastereoselective and stereodivergent construction of long-chain polyketides and analogues by a double-chain-elongation strategy.