Synthesis and reactivity of (eta5-pentadienyl)iron(+) cations and studies towards the synthesis of heteroscyphic acid A via oxidative radical cyclization and organoiron methodology
Abstract
While (η5 -pentadienyl)iron(1+) cations were first prepared more than 40 years ago, the reactivity of these complexes is of continuing interest, particularly for the synthesis of conjugated polyenes. Tricarbonyl(1-methoxycarbonyl-5-phenylpentadienyl)iron(+1) hexaflurophosphate was prepared in two steps from tricarbonyl(methyl 6-oxo-2,4-hexadienoate) iron. While addition of carbon and heteroatom nucleophiles to this 1,5-disubstituted cation generally occurs at the phenyl-substituted dienyl carbon to afford (2,4-dienoate)iron complexes, the addition of phthalimide proceeded at C2 to afford a (pentenediyl)iron complex. The (pentenediyl)iron complex was structurally characterized by X-ray diffraction analysis. As part of our interest in the synthesis of diterpenes containing a 3-methyl-1,3Z-pentadienyl side chain we prepared two symmetrical iron(+1) cations, tricarbonyl(3-methylpentadienyl)iron(+1) and dicarbonyltriphenylphosphine(3-methylpentadienyl)iron(+1) cations. For the tricarbonyl ligated cation, a counter ion effect on the addition of malonate nucleophiles was observed. While reaction of the tricarbonyl ligated cation with lithium dimethyl malonate gave the (1,3Z-diene)iron complex via addition at C1, the reaction of same cation with sodium dimethyl malonate gave the cis -4,5-disubstituted cyclohexenone product via addition at C2. In order to synthesize the proposed structure of heteroscyphic acid A methylester, a bicyclic β-ketoester was synthesized in six steps starting from commercially available 5-hexen-1-ol. Reaction of ketoester enolate with dicarbonyltriphenylphosphine(3-methylpentadienyl)iron(+1) cation, followed by CAN mediated decomplexation gave the installation of 3-methyl-1,3Z-pentadienyl side chain with opposite relative stereochemistry of that of heteroscyphic acid A. Following the similar protocol the reaction of bicyclic esterenolate with dicarbonyltriphenylphosphine(3-methylpentadienyl)iron(+1) cation, followed by decomplexation gave the installation of 3-methyl-1,3Z-pentadienyl side chain with correct relative stereochemistry of that of heteroscyphic acid A. Reevaluation of the NMR spectral data for the heteroscyphic acids revealed that the side chain of these compounds more likely possess the E -stereochemistry. (Abstract shortened by UMI.)
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