Esters solid phase synthesis

More recently, Tietze and Steinmetz (96SL667) used the patented polystyrene-resin methodology for for the solid-phase synthesis of a large number of diverse )3-keto esters 40a-h. These were reacted with phenylhydrazine in THF at room temperature to give hydrazones 41a-h that were then cychzed into 2-phenyl 5-substituted pyrazol-3-ones 42a-h by heating in toluene at 1(X)°C (Scheme 12). 

Albericio F, Barany G. Hypersensitive acid-labile (HAL) tris(alkoxy)benzyl ester anchoring for solid-phase synthesis of protected peptide segments. Tetrahedron Lett 1991 32 1015-1018. 

Solution-phase enantioselective synthesis of 437 and 438 thus achieved was also translated into solid-phase synthesis <2002TL8981>. The oxazolidinone 441 prepared from L-tyrosine methyl ester via 440 was attached to Merrifield resin to produce 442. Resin-bound 442 was converted to 443 (Scheme 98). 

Rink, H. (1987) Solid-phase synthesis of protected peptide fragments using a trialkoxy-diphenyl-methyl-ester resin. Tetrahedron Lett. 28, 3787-3790. 

Acylketenes, derived from Meldrum s acid, add to esters of (R)-4,5-dihydrothiazolin-4-catboxylic acid to give 81 <00OL2065>. A solid-phase synthesis of 2P-methyl substituted penam derivatives utilises a tether through an ester group <99TA3893>. 

In 1983, Prasad et al.12 first reported the condensation of chloromethyl polystyrene with /V-hydroxyphthalimide to give the ester, hydrazinolysis of which yielded the desired resin-bound hydroxylamine. However, the sole purpose of this reagent was to react with, and hence extract ketones from, a complex steroidal mixture, and its use for the solid-phase synthesis of hydroxamic acids was not explored. Recently, the exploitation of the above solid-phase approach for the synthesis of hydroxamic acids was independently reported by three groups,7-9 all of which differ only in the method for the initial anchoring of TV-hydroxyphtha-limide to an 4-alkoxybenzyl alcohol functionalized polystyrene or trityl chloride polystyrene. Subsequent /V-deprotection was... 

The solid-phase synthesis of glycopeptides was first realized by applying the polymeric benzyl ester principle of Merrifield. According to this methodology, Lavielle and associates (50) used 7V-(tm-butyloxycarbonyl)-<9-glycosyl serine derivative 153 for condensation with resin-linked alanine 154. 

Otvos et al. (27) adopted the Fmoc-pentafluorophenyl ester methodology and have applied O-deacetylated glycopeptide intermediates (170 and 171) for solid-phase synthesis of glycopeptides. In this way, the A-glycopep-tide (26) H-Gly-L-Lys-L-Ala-L-Tyr-L-Thr-L-Ile-L-Phe-L-... 

Scheme 1.3 Solid-phase synthesis of (3-(l—y3)-linked glucosamine dimers employing ester linkages to the support.

Scheme 1.4 Solid-phase synthesis of P-(l —> 6)-linked gentiotetraose employing ester groups for permanent protection, for temporary protection, and as linkage to the support.

Activated esters (see Section 2.9) Activated esters of peptides are rarely used because there is no general method available for converting an (V -protected peptide into the ester with a guarantee that it will be a single isomer. Attempts have been made to overcome this obstacle (see Section 7.8). However, solid phase synthesis allows the preparation of thioesters of segments (see Section 7.10). Once the ester is in hand, it can be aminolyzed without generation of a second isomer if suitable conditions are employed. 

Use me thio esters (-C02SR ) obtained from me ROCO-Peptide-SR -linker-resins assembled by solid-phase synthesis (see Section 7.10).74... 

The first peptide amides prepared by solid-phase synthesis were obtained by ammonolysis of resin-bound benzyl esters of peptides in solvents containing methanol (Figure 5.16, A). The method was occasionally employed but was not popular because it was inefficient, producing some ester in addition to the amide. A new variant employing gaseous ammonia will likely rekindle this approach (see Section 8.3). During the early developments of solid-phase synthesis, it was known that the... 

Once it is part of a cyclic dipeptide, the prolyl residue becomes susceptible to enantiomerization by base (see Section 7.22). The implication of the tendency of dipeptide esters to form piperazine-2,5-diones is that their amino groups cannot be left unprotonated for any length of time. The problem arises during neutralization after acidolysis of a Boc-dipeptide ester and after removal of an Fmoc group from an Fmoc-dipeptide ester by piperidine or other secondary amine. The problem is so severe with proline that a synthesis involving deprotection of Fmoc-Lys(Z)-Pro-OBzl produced only the cyclic dipeptide and no linear tripeptide. The problem surfaces in solid-phase synthesis after incorporation of the second residue of a chain that is bound to the support by a benzyl-ester type linkage. There is also the added difficulty that hydroxymethyl groups are liberated, and they can be the source of other side reactions. 

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