Spirocyclic keto esters

As with other intramolecular ene reactions, this reaction is best suited to the preparation of cyclopentanes, but can also be used for the preparation of cyclohexanes. The reaction cannot be used for the formation of cyclopropanes or cyclobutanes since the unsaturated carbonyl compound is more stable than the ene adduct. 8,e-Unsaturated ketones (167) do not give cyclobutanes (171) by enolization to give (170) followed by a type I reaction but instead give cyclohexanones (169) by enolization to give (168) followed by a type II reaction. Alkynes can replace alkenes as the enophile. Enols can be prepared from pyrolysis of enol esters, enol ethers and acetals and from -keto esters and 1,3-dicaibonyl compounds. Tlie reaction is well suited to the preparation of fused or bridged bicyclic and spirocyclic compounds. Tandem ene reactions in which two rings are formed in one pot from dienones have also been described. The examples discussed below 2-i63 restricted to those published since Conia and Le Perchec s 1975... 

Spiroannelation. A new method for intramolecular spirocyclization involves decarboxylation of eo-halogeno-/5-keto esters with lithium chloride in HMPT at 125— 140°. The method appears to be fairly general.1 Examples ... 

Keto aldehyde 6 should be easily formed from lactam acetal ester 7, which should be readily available from isoxazolidine 8. The latter can be constructed by a convergent 1,3-dipolar cycloaddition of nitrone 9 with ethyl acrylate. This strategy to spirocyclic lactams is precedented with nitrones derived from A -benzylhydroxylamine [15]. 

The synthesis of the naturally occurring spirocyclic sesquiterpene (-l-)-acorenone B 28 will exemplify the use of substrate control in the [2,3]-sigmatropic rearrangement of a sulfonium ylide. The p-keto diazo ester 25 was obtained from commercially available (—)-perillaldehyde 24 in... 

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