Lactones, preparation from alkenyl acids

The enantiomerically-pure intermediate 1 was prepared from the dioxolanone 4, available in three steps from L-malic acid. Lewis acid-mediated homologation converted 4, a 4 1 mixture of diastereomers, into 5 as a single diastereomer. After establishment of the alkenyl iodide, it necessary to maintain the lactone in its open form. A solution was found in the formation of the Weinreb amide. The final stereogenic center was established by Brown allylation of the derived aldehyde. The alkene metathesis to form 1 was carried out with the commercially-available Schrock Mo catalyst. The authors did not comment on the relative efficacy of alternative alkene metathesis catalysts. 

Ley reported that selenium promoted carbocyclization reactions can also be effected by the enolic olefinic bonds of -dicarbonyl compounds [111]. These reactions occur with N-PSP in the presence of zinc iodide, tin tetrachloride or aluminium trichloride. An example is reported in Scheme 33. In the intermediate 219, derived from the -ketoester 218, cyclization through the oxygen atom to afford 220 is kinetically favoured. This reaction, however, is reversible, and upon prolonged reaction times and in the presence of strong acids the carbocyclization product 221 is formed. This procedure has been recently employed by Ley to effect the conversion of the alkenyl p-keto lactone 222 into the tricyclic selenide 223 (Scheme 33) which is a key intermediate in the preparation of model compounds with antifeedant activity [112]. 

The 5-keto acid 11 is supposed to be formed by hydroboration and oxidation of the terminal CC double bond in 12, feasible by allylation of the enone 13. Enone 13, once again, originates from an intramolecular Knoevenagel alkenylation of the a,5-diketone 14 prepared by addition of ethylmagnesium halide to the enol lactone 15 of the 5-keto acid 16. Preceding this keto acid, the allyl compound 17 is formed by nucleophilic opening of the cyclopropane ring with hydride and mefliylation in a position of the carbonyl in the cyclopropyl ketone 18. 

A method for preparing alkenyl triflates from lactones and one for preparing a,P-unsaturated aldonic acids by chain extending D-glyceraldehyde derivatives are covered in Chapter 16, and the preparation of sugar vinyltin derivatives from sugar acetylenes is mentioned in Chapter 17. 

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