A highly stereoselective preparation of syn-1,2-diols from secondary α-allenic alcohols via iodocyclization. Synthesis of (±)-threo-L-factor and (±)-disparlure

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Abstract
The iodocyclization of the N-tosyl carbamates 5 of secondary α-allenic alcohols 4 is described. The reaction sequence involves the addition of iodine to the terminal olefin of the allene moiety of 5 resulting in the formation of an isomeric mixture of Z and E diiodides 6 and 7. This initial process is followed, in the same pot, by reaction of the diiodides with silver carbonate. Hydrolysis of the resulting trans imino carbonates 8 provides syn-1,2-diols 10 in a highly diastereoselective (>50:1) and regioselective (>20:1) fashion. Oxazolidinones 9 are minor products that arise from the participation of the carbamate nitrogen atom in the cyclization reaction. The regioselectivity of the cyclization reaction from 6/7 to 8 is highly dependent upon the reaction solvent with the optimum results being obtained in ether/MeCN mixtures (10:1 to 50:1) or CH2Cl2. The synthetic utility of the vinyl iodo diols 10 is demonstrated by the synthesis of (±)-threo-5-hydroxy-4-decanolide (17) ((±)-threo-L-factor) and (±)-cis-7, 8-epoxy-2-methyloctadecane (24) ((±)-disparlure). The key step in each synthesis involves the conversion of the vinyl iodide moiety of 10 into the alkynes 15 or 18 (from 10j and 10k, respectively) via a high-yielding dehydrohalogenation reaction.

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