Dicyclohexylcarbodiimide , amide bond formation with

Dicyclohexylcarbodiimide (DCC) and similar condensing agents are used extensively for amide bond formation in peptide synthesis. DCC also is used in esterifications and as a component in the Moflfatt oxidation. One of the major difficulties with DCC is that its by-product, N,iV -dicyclo-hexylurea, frequently cocrystallizes or cochromatographs with the desired product. The PS-diimide (39) overcomes this recovery problem since the product urea (40) is attached to the polymer, is readily removed by filtration and is recycled to (39), as shown in Scheme... 

Thus unsubstituted (R=H) and substituted (R = alkyl) non-stabilized diyiides 1 react with phenylisocyanate and dicyclohexylcarbodiimide (R NCX), leading to the formation of new monoylide type intermediates. These last ones react in situ with carbonyl compounds through a Wittig type reaction leading respectively to a,)8-unsaturated amides 2 and amidines 3, with a high E stereoselectivity, the double bond being di- or tri-substituted [48,49]. By a similar reactional pathway, diyiides also react with carbonic acid derivatives, with the synthesis as final products of -a,/l-unsaturated esters 4 and acids 5 [50]. 

Diphenylphosphinic mixed anhydrides have been utilized to form peptide bonds. Peptides are easier to isolate by this method than by employing 1,3-Dicyclohexylcarbodiimide. These anhydrides are the method of choice for the formation of amides of 2-alkenoic acids (eq 1 ). Carbodiimide and acyl carbonate methods proved to be inferior. Primary amines result in better yields than secondary amines. This activation protocol can be employed to form thiol esters (eq 2) p-Amino acids are readily converted to p-lactams with chlorodiphenylphosphine oxide (eq 3). Secondary amines work best. This activation protocol has been utilized to convert acids to amines via a Curtius rearrangement. Phenols have been generated from diene acids, presumably via base-induced elimination of diphenylphosphinic acid from the mixed anhydrides to form ketenes which spontaneously cyclize. Acids have been converted to ketones via activation followed by reaction with organometallic reagents (eq 4)."... 

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