Organoiron complexes in organic synthesis. Part 5. Spirocyclisation reactions of tricarbonylcyclohexadienyliumiron complexes

Abstract

Attempted spirocyclisation of tricarbonyl{dimethyl[3-(4-methoxycyclohexa-2,4-dienylium)propyl]malonate}iron hexafluorophosphate (4) and tricarbonyl[methyl 5-(4-methoxycyclohexa-2,4-dieny)ium-3-oxopentanoate]iron hexafluorophosphate (11) to the desired spiro[4.5]decane derivatives (5) and (12) failed under a variety of mild conditions. Spirocyclisation of tricarbonyl[methyl 6-(4-methoxycyclohexa-2,4-dienylium)-3-oxohexanoate]iron hexafluorophosphate (25) occurs smoothly and rapidly at –78° to give the tricarbonyl[η-7–10-(methyl 9-methoxy-2-oxospiro[5.5]undeca-7,9-diene-1-carboxylate)]iron diastereoisomers (26) and (27).