Intermediate coupling procedure, limitations

Vaska s complex ([IrCl(CO)(PPh3)2]) also catalyzed the carbonylative coupling of diynes, which provided bicyclic cyclopentadienones (Scheme 11.23) [35]. Due to the instability of the products, the substrates are limited to symmetrical diynes with aromatic groups on their termini nonetheless, this reaction still serves as the catalytic and practical procedure for the synthesis of cyclopentadienones, which are anti-aromatic with a 47t system and serve as active synthetic intermediates. 

A method utilizing the Dess-Martin periodinane[12 for the conversion of a peptide a-hy-droxy ester into the corresponding a-oxo ester was reported by Burkhart et al.[8l The final product, peptide a-oxo ester, obtained in this process contains a mixture of enantiomers at C2 in PI of the peptide. The optical impurity arises not from the oxidation reaction but the synthesis of one of the intermediates, 2-hydroxy-3-nitro-4-phenylbutanoic acid, which generates four diastereomers at two adjacent chiral carbons. This procedure is limited to the synthesis of peptide a-oxo esters with the phenylalanine residue at the PI position. A more diversified approach is achieved by using a-hydroxy- 3-amino acids 14 as the key intermediate that permits selective introduction of an amino acid residue at PI of the peptide it can also be coupled to N-protected amino acids or N-protected peptides and further transformations give a-oxo esters 19, a-oxo acids 20, and a-oxoamides 22 (Scheme 4)J3 61... 

A number of improvements have also been described in the other methods used for synthesizing unsymmetrical porphyrins from open chain tetrapyrroles. However, the direct coupling of two dipyrrolic precursors to give porphyrins is still often the method of choice as a convergent procedure it minimizes the number of stages involved and is usually only limited by the need for one of the dipyrrolic intermediates to be symmetrical (to avoid the formation of more than one porphyrin). 

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