The asymmetric synthesis of β-lactams. Stereocontrolled Asymmetric Tandem Michael Additions and Subsequent Alkylations of E-[(η5-C5H5)Fe(CO)(PPH3) COCH = CHME]. X-ray Crystal Structure of (RS)-E-[(η5-C5H5) Fe (CO) (PPH3) COCH = CHME]
Davies SG., Dordor-Hedgecock IM., Sutton KH., Walker JC., Jones RH., Prout K.
Michael addition of methyllithium to the E-crotonyl complex (RS)-[η5-C5H5)Fe(CO)-(PPh3 )COCHCHMe] followed by trapping of the resultant enolate with methyl iodide gives (RS)-[(η5-C5H5)Fe(CO)(PPh3 )COCH(Me)CHMe2] (d.e. > 100:1), also generated by treatment of (RS)-[(η5-C5H5)Fe(CO)(PPh3 )COCH2CH(OMe)2] with three equivalents of methyllithium and methyl iodide. Addition of n-butyllithium to the (RS)-E-crotonyl complex followed by protonation with methanol occurs with high diastereoselectivity. Quenching with methyl iodide gives (RS)-[(η5-C5H5)Fe(CO)(PPh3 )COCH(Me)CH(Me)n-Bu], also generated by treating either diastereoisomer of [(η5C5H5)Fe(CO)(PPh3)COC H2CH(Me)OMe] with two equivalents of n-butyllithium and methyl iodide. Decomplexatlon gives the known erythro-2,3-dimethyl-heptanoic acid. Similarly, Michael addition of lithium benzylamide and electrophilic quenching with methanol or methyl iodide occurs with high diastereoselectivity and gives upon decomplexation, 4-methyl- and cis-3,4-dimethyl-N-benzyl-β-lactams respectively. The stereochemical results are rationalised by addition occurring to the E-crotonyl complex in the anti (CO to CO) and cisoid conformation and subsequent alkylation of the unhindered face of the E-enolate generated. Confirmation is provided by an X-ray crystal structure analysis of (RS)-E-[(η5-C5H5)Fe(CO)(PPh3 )COCHCHMe]. When repeated with the optically pure (S)-E-crotonyl complex, decomplexation gives essentially optically pure (2R),(3R)-(-)-N-benzyl-2,3-dimethylheptanamide, (4S)-(-)-4-methyl- and (3R),(4S)-(-)-cis-3,4-dimethyl-N-benzyl-β-lactams. © 1986.