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(+/-)-bakkenolide-A (1) was prepared in five steps from ethyl 4-benzyloxyacetoacetate by sequential alkylations with tiglyl bromide and (Z)-5-bromo-1,3-pentadiene, followed by an intramolecular Diels-Alder reaction of (E,Z)-triene 25b as the key step. The hydrindane cycloadduct 28 was subjected to hydrogenation and spontaneous or acid-catalyzed lactonization, followed by a Witttig reaction to introduce the exocyclic methylene group of 1. The known 7-epibakkenolide-A (2) and novel 10-epi- and 7,10-diepibakkenolide-A (3 and 4, respectively) stereoisomers were obtained as minor byproducts. When (E)-5-bromo-1,3-pentadiene was used instead of the Z-isomer, the 10-epi- and 7,10-diepibakkenolides were the major products. In both cases exo cyclization was preferred over endo. An alternative approach was based on a similar intramolecular Diels-Alder cycloaddition, using dimethyl malonate instead of ethyl 4-benzyloxyacetoacetate as the starting material for the double alkylation preceding the cycloaddition step. The cycloadduct was then converted into the corresponding alpha-phenylseleno propargyl esters 16 or 22. However, attempted formation of the spiro center by a radical cyclization resulted chiefly in reductive deselenization.

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The conjugate addition of ester and ketone enolates to a-methylene /3-lactones 1 and 2 proceeds with high stereocontrol of the newly formed chiral centers. The p-lactones 3-8 with ester functionalities in the a-side chain were obtained in good yields from the addition of ester enolates, while the dispiro-/3-lactones 9-11 were formed in the addition of acetophenone and propiophenone enolates. The decarboxylation of the resulting ester-functionalized p-lactones produced quantitatively the corresponding y,d-unsaturated esters 12-17 with complete retention of the initial p-lactone geometry. This unprecedented two-step sequence, i.e. Michael addition followed by decarboxylation, constitutes the first stereoselective synthesis of ester-functionalized, (Ehconfigurated alkenes, in which the synthetic potential of the a-methylene p-lactones as allene equivalents is advantageously utilized.The ready availability of the highly functionalized a-methylene P-lactones by various methods' and their numerous reaction modes2 as masked allenes2 make these compounds attractive and valuable building blocks in organic synthesis. For example, their cycloaddition with dienes followed by stereoselective decarboxylation to the corresponding olefins with retention of the initial p-lactone geometry has demonstrated that a-methylene p-lactones are useful allene equivalents. Moreover, the recent topological resolution by Diels-Alder cycloaddition and retrocleavage at moderate temperatures5 provides convenient access to enantiomerically pure a-methylene p-lactones for asymmetric synthesis.Recently, we have reported on the stereoselective synthesis of allyl amines and sulfides by nucleophilic addition of amines and thiols to a-methylene P-lactones and subsequent stereoselective (retention) decarboxylation of the resulting amino and thio /3-lactones.6 Since no general methods for the stereoselective preparation of y,d-unsaturated esters appear to have been reported, it was of interest to extend the conjugate addition Albert, R.; Grau, N. D.; Hasemann, L.; Nestler, B.; Peters, E.-M.; Peters, K.; Prechtl, F.; von Schnering, H. G. Albert, R.; Hasemann, L.; Nava Salgado, V. 0.; Nestler, B.; Peters, E.-M.; Peters, K.; Prechtl, F.; von Schnering, H. G.methodology with a-methylene @-lactones to the Michael addition of ester enolates. Herein, we illustrate that the addition of ester enolates to a-methylene p-lactones and subsequent decarboxylation constitutes a general, stereocontrolled preparation of disubstituted y,d-unsaturated esters. Results and DiscussionThe lithium enolates were generated by treatment of the corresponding esters with lithium diisopropylamide (LDA) at -78 "C in tetrahydrofuran (THF) according to the method of Ireland.7 The conjugate addition (Michael reaction) of the ester enolate of tert-butyl 2-methylpropionate to a-methylene @-lactones 2 at -78 "C afforded the mixture of the P-lactones cis-3 and trans-3 after hydrolysis with aqueous ammonium chloride (Scheme 1). The cis:trans ratio was 16234, and as expected, the trans diastere...

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