Side Wang resins

A second side reaction suffered by Ar(4)-unfunctionalized quinoxali-nones leading to racemization at the a-carbon was revealed in the closely related studies of Morales and co-workers, who assembled the [6,6] ring system from 4-fluoro-3-nitrobenzoic acid coupled as an ester to Wang resin (lb) (Scheme 12).9 This undesirable side reaction could also be prevented by functionalization of iV(4) prior to cleavage, which in this instance was accomplished by acylation with chloroformates and thiochloroformates. Derivatization at (V(l) was again effected using the Ellman alkylation protocol and provided optically pure samples of the quinoxalinones (72). 

Amino acids suitable for side-chain attachment to supports that have been used to prepare cyclic peptides on insoluble supports include tyrosine (etherified with Wang resin [60]), histidine (N-alkylated with trityl resin [79]), aspartic acid [66,67,80], asparagine [80], glutamic acid [80,81], lysine [65], and serine [65],... 

Fmoc protected amino acids, 2-chlorotrityl(Clt) resins. Rink amide MBHA resin, Wang resins, HBTU and HOBt are commercially available from AnaSpec, Inc. The side chain protecting groups for the amino acids are t-butyl for Asp, Glu, Ser, Thr, Tyr trityl for Asn, Cys, Gin, His Pmc for Arg and Boc for Lys. N-methylpyrrolidinone(NMP), Omnisolv grade, was purchased from VWR. Piperidine, diisopropylethylamine (DIEA)were purchased from Aldrich. 

The same authors have broadened the scope of the solid-phase application of this reaction by employing different cyclic amidines instead of only the 2-aminopyridine nucleus and using a Wang resin-bound isonitrile ester [357]. In this paper the authors justified the use of catalytic PTS A instead of a carboxylic acid such as AcOH, because the latter would trigger a Passerini [342, 358] side reaction (Scheme 93). 

Figure 1.2 Dual cleavage on a Wang resin as an example of this side-reaction in other (poly)alkoxybenzyl resins. Adapted from reference [20].

This acylation strategy was used to prepare pol5mier-supported chromium carbenes. Microwave irradiation on Wang resin shows the same regioselectivity as solution chemistry but fewer side products. The resin-bound phenol 21 is simultaneously released and oxidized to the benzoquinone 22 with ceric ammonium nitrate. Microwave irradiation has been shown to accelerate the DBR and provide high yields of benzannulated products in short reaction times (ca. 5 min). ... 

---