Stepwise chain lengthening

The first industrial synthesis of P,P-carotene (3) by Roche [1] followed the C19 + C2 + C19 synthesis principle [1,20]. As in the vitamin A process, the polyene chain was produced by Grignard coupling, elimination and partial hydrogenation. In addition, a new effective synthesis for polyene aldehydes has now been developed in the form of the enol ether condensation and employed industrially for the first time in the production of the C19-aldehyde 27 (Scheme 6). The enol ether condensation permits specific stepwise lengthening of conjugated aldehydes by two carbon atoms each time. Use of prop-l-enyl ethyl ether (28) (Scheme 9) gives a-methyl-branched polyene aldehydes. Chemically, the chain lengthening proceeds in three steps as follows ... 

In the sequencing of bradykinin one of the proline residues was overlooked. The error was corrected both by a reexamination of the degradation study [9] and through experiments with peptides synthesized for this purpose [10]. From the various syntheses of bradykinin the scheme of Nicolaides and de Wald, stepwise chain-lengthening with active esters [11] is mentioned here. Also, bradykinin was the first biologically active peptide synthesized by the solid phase method [12] (Chap. 5). 

Stepwise Chain-lengthening Starting with the C-terminal Residue 129... 

A Bodanszky, M Bodanszky, N Chandramouli, JZ Kwei, J Martinez, JC Tolle. Active esters of 9-fluorenylmethyloxycarbonyl amino acids and their application in the stepwise lengthening of a peptide chain. J Org Chem 45, 72, 1980. 

The degree of polymerization depends on the duration of the process. After 7 min, the molecular mass is equal to 9400 (the polydispersity index is 5.30). When the reaction is carried out for 15 min, the molecular mass of the polymer increases to 37,000 and the polydispersity index reaches 7.31 (Bauld et al. 1996). Depending on whether cation-radical centers arise at the expense of intramolecular electron transfer or in a stepwise intermolecular lengthening, polymerization can occur, respectively, through a chain or a step-growth process (Bauld and Roh 2002). In the reaction depicted in Scheme 7.17, both chain and step-growth propagations are involved. 

A more recent synthetic concept is based on the reaction of diethylamino-sub-stituted silyl-lithium compounds with triflate derivatives of silanes or oligosi-lanes. It leads to stepwise lengthening of Si-Si chains which can be used to assemble polymeric structures [74]. 

The function of primers is to serve as receptors for glucose residues, which become attached stepwise at the nonreducing ends. In this way the primer chains are lengthened by repetition of the above process until their length becomes a limiting factor. Apparently at that time the chains also separate from the primer, producing long unbranched molecules. This... 

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