Bridged lactones

Treatment of bicyclic lactones 66, derived from Diels-Alder reaction of 3-carboxy-2-pyrone under standard radical conditions using (TMSlsSiH, leads to bridged lactones 67, which can smoothly be converted to bicyclo[3.3.0]-lactones 68 (Scheme 10). For X = CHaOMe, this cascade of rearrangements took place in a 78% overall yield, providing 68 in diastereomerically pure form. Three additional steps provided a novel route toward Corey s lactone 69. 

Reaction of this ketone with a stannyl-lithium reagent gives one diastereo isomer of a bridged lactone. Again, give a mechanism for this step and explain the stereochemistry. Make a good conformational drawing of the lactone. 

A simple reaction that creates both chiral centres in one step was already in the literature.3 It is a cross between a Mannich and a Prins reaction involving the condensation of an unsaturated amine 24 with glyoxylic acid (25 available as an aqueous solution - fortunately the reaction works in water) to give a heterocyclic bridged lactone 26 with the right relative stereochemistry (but of course racemic). 

Some limited work has been reported by the Weinreb group on hydride reductions of N-sulfonyl imines. The ketone group of bridged lactone 150, which was... 

Homoallylic alcohols are also suitable for controlling the introduction of an alkyl chain, as shown by the synthesis of the diterpenoid atractyligenin61, The starting tricyclic alcohol is converted to the selenocarbonate, a suitable radical precursor. Treatment with tributyltin hydride initiates the stereoselective cyclization which affords the bridged lactone system, an intermediate in the diterpenoid synthesis. Due to the low cyclization rate, tributyltin hydride is added slowly to suppress the formation of noncyclized reduction products. 

Bridged /rauj-decalin-2-yl radicals are formed in cyclization reactions of bicyclic iodolac-tones55. The stereoselectivity in hydrogen-abstraction reactions is very similar to reactions of unbridged, monocyclic radicals with equivalent a- and /Fsubstituents. The bridging lactone unit, therefore, seems to have little influence on the sleric course of the reaction, and preferential attack from the axial side occurs. Axial substituents in / -position to the radical center further increase the selectivity and only the product arising from axial attack is found. 

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