Sammendrag
A potential intermediate in the synthesis of several trans-clerodane natural products has been constructed employing a Diels Alder reaction as a key-step. Two epimeric exo adducts were formed in a 4:3 ratio in an EtAlCl2 mediated cycloaddition of O-silylated 2-vinylcyclohex-2-enol and N-tigloylisoxazolidinone. Both isomers were converted to the trans-clerodane intermediate in four steps; reductive removal of the oxazolidinone, followed by O-benzylation, removal of the silyl protecting group and finally a Dess-Martin oxidation of the deprotected alcohol to the corresponding ketone. It was possible to transform both isomers from the cycloaddition into the final target, but the major isomer was converted at greater yields. An interesting discovery made during the work was that the desilylation demanded significantly different conditions depending on which isomer was deprotected. The fact that the cycloaddition not only resulted in an excellent exo selectivity, but also that the alkene to a large extent approached from the least hindered side, opens the possibility for enantioselective synthesis of the target compound from (R) or (S) diene starting materials in the future.
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