sed si face at C-4' of PLP-substrate imines and the similarity of conformational changes upon substrate binding document the relatedness of these proteins but offer no obvious catalytic advantage. Examination of the cryptic stereochemistry of their reactions has given a conformational picture of the PLP-substrate complexes in the active sites and provided insight into mechanistic aspects, especially intramolecular proton transfers. With the exception of racemases,64 PLP enzymes generally force retention modes of operation at the a and ß carbons of substrates for processes as diverse as transamination, decarboxylation, retroaldolization, ß replacement, ,ß elimination, and d>7 elimination.65 This allows in many cases the mediation of multiple proton transfers in a catalytic cycle by a single base; internal proton recycling can then reduce (64) E.
The reaction between (tj4-cyclohexadiene)Mo(CO)2Cp (1) or (i74-5-methylcyclohexadiene)Mo(CO)2Cp (3) cations and a range of stable enolate nucleophiles has been studied and was found to occur with high regio-and stereoselectivity to give (ir-allyl)Mo(CO)2Cp complexes. The C-C bond formation between 1 and unsymmetrical enolates was diastereoselective: reaction with methyl l-oxo-2-sodiocyclopentanecarboxylate gave a single diastereomer, the relative stereochemistry of which was determined by X-ray methods. Decomplexation of the product (7r-allyl)Mo(CO)2Cp complexes was accomplished by treatment with iodine. Complexes containing a pendant carboxylic acid produced lactones with high regio-and stereocontrol, while complexes lacking a nucleophilic group gave substituted iodocyclohexenes which could be further manipulated. The value of this method for stereocontrol is illustrated by the preparation of an acyclic fragment having relative stereochemistry corresponding to the C(4), C(5), and C(6) centers in the macrolide antibiotics tylosin and magnamycin B.
An efficient high-yielding procedure is described for the intramolecular carbonylative coupling of , -diynes to give cyclopentadienone-Fe(CO)3 complexes. Exchanging one CO ligand with PPh3 affords control over the manipulation of , '-trimethylsilyl-substituted cyclopentadienone complexes. The preparation of , -diynes with a hydroxy group adjacent to one alkyne unit leads to modest stereocontrol upon cyclization forming the Fe(CO)3 complex. The hydroxy-substituted complex was oxidized to ketone. Borohydride reduction and Grignard addition to the ketone proceed anti to the Fe(CO)3 moiety. The X-ray crystal structure of C^H^FeOsP shows that it crystallizes in the monoclinic space group 2 / in a unit cell of dimensions a = 12.0595 ( 24), b = 13.3926 (30), c = 19.0682 (38) k, ß -93.98 (2)°, with Z = 4. moieties accompanied by the insertion of carbon monoxide.1 Historically, the iron carbonyl mediated cyclization of acetylenes to form cyclopentadienone (CPD) complexes has been, with few exceptions, an inefficient process. Early reports cite the preparation of CPD tricarbonyliron complexes from phenyl-and diphenylacetylene in very low yields (<15%).2 Only in cases where the R group is (1)
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