BHA resins

Combinatorial solid-phase synthetic methodologies have been used extensively in drug development [8]. A new solid-phase synthesis of 2-imidazolidones has been discovered by Goff, based on a domino aminoacylation/Michael addition reaction [9]. Thus, when immobilized amine 10-26 (HMPB-BHA resin) was treated with phenylisocyanate in the presence of triethylamine, a smooth formation of 2-imida-zolidone took place. Acid-catalyzed removal from solid phase provided 10-27 in good yield (Scheme 10.6). 

FIGURE 5.16 Production of amides by cleavage of benzhydryl amides. Recognition that removal by acidolysis of benzhydryl protectors from carboxamides gave the amides (B) led to development of benzhydrylamine (BHA) resin (C).33 Treatment with HF of a peptide amide that has been assembled on a BHA resin using Boc/Bzl chemistry gives the peptide amide (D). Peptide amide is also obtainable by ammonolysis of the resin-bound benzyl ester (A), a reaction that is more efficient if gaseous NH3 is employed (see Section 8.3). 

Preparation of [Asp3(OFm),Lys12(Fmoc),Ala15]-GRF(l-29)-BHA-Resin [154l... 

The intermediate, Boc-[Ala]15-GRF(13-29)-BHA-resin was dried in vacuo (38.25 g, 6.84 mmol) and a portion (9.63 g, 1.72 mmol) was removed and stepwise solid-phase synthesis continued as described in the protocol above. After the incorporation of Lys12(Fmoc), step 8 was replaced with 0.6% DIPEA/ CH2C12. The coupling with Boc-Asn-OH (2.63 g, 6 equiv) was carried out by preactivation with HOBt (1.53 g, 6.6 equiv) and DCC (2.34 g, 6 equiv). The intermediate Boc-[Asp3(OFm),Lys12(Fmoc),Ala15]-GRF(3-29)-BHA-resin (12.0 g, 1.72 mmol) was dried in vacuo and a portion (7.75g, 1.11 mmol) was removed for the last two cycles of the synthesis. 

Asymmetric parallel P-sheet sequences incorporating template 4 (R1 = H) were synthesized on BHA resin and reacted with the bifunctional electrophile 4 (R1=Pfp). The remaining active ester on the template-terminated resin-bound peptide was reacted with the N-terminus of a different side-chain protected peptide prepared on the oxime resin, as shown in Scheme 11. 

Ikota (20) reported the first solid-phase synthesis of motilin in 1980, and the overall yield was 1.8%. Coy et al. (21) improved the synthesis and got a 10% yield. Coy s synthesis was carried out on a Beckman model 990 automatic peptide synthesizer using A -Boc chemistry. Motilin was assembled stepwise on a 1% cross-linked BHA resin with Boc-a-benzyl-Glu as the first amino acid to be incorporated. In this manner, C-terminal Gin could be directly formed on HE cleavage of the final peptide from the resin. The detailed coupling schedule is shown in Table 2 (22). The symmetric anhydride method was used for... 

Table 2 Schedule of events for assembling the peptide BHA resin...

This peptide was synthesized by Fisher et al. (44) in 1974 employing the solid-phase technique using A -Boc chemistry. BHA-type resin was used for the synthesis on a Beckman Model 990 solid-phase peptide synthesizer. Dried BHA-resin hydrochloride was neutralized with triethylamine and stirred overnight with an excess of A -Boc-Met and DCI in DCM. The remaining unreacted amino groups of the resin were acetylated with a mixture of AC2O and triethylamine in DMF overnight. Amino-acid analysis of the resin, after hydrolysis in 6 A... 

Solid-phase synthesis of a-/p-MSH were achieved in our group in 1980 (103), both using A -Boc chemistry. In the preparation of a-MSH, amino acids were coupled successively to valine-BHA resin with threefold excess of A -Boc amino acid and 2.4-fold excess of DCC in 1-15 hours. Removal of the A -Boc protecting group was achieved by a treatment of 45% TFA in DCM that contained 2% anisole. The side-chain protections were as follows Ser and Asp were protected by Bn groups ... 

These peptides were synthesized by solid-phase method using standard A -Boc chemistry (143), and the couplings were performed with a Beckman Model 990 peptide synthesizer (Beckman Instruments, Fullerton, CA). The BHA resin was used... 

More efficient cleavage of the product as observed with BHA resin. 

Observations - Compatible with Boc/Bnz SPPS strategy. - More acid-labile than BHA resin. Chemical reaction Reductive methylation [142], N-alkylation, reduction with BHVTHF, cyclization with thiophosgene and triphosgene [143], ... 

Compatible with Boc/Bnz SPPS strategy - Compared to the BHA resin, more stable against ... 

Diacid-based linkers, such as the succinic linker 21, have been described to prepare alcohols. The procedure involves the esterification of the starting alcohol with succinic anhydride and DMAP to yield the hemiester that is anchored to an amino containing-resin by means of an amide bond. The bound alcohol is then elaborated and finally released with a nucleophile. Oligosaccharides have been assembled following this approach and released with aqueous ammonia or sodium methoxide in methanol-dioxane [73, 74]. Peptide alcohols have also been prepared with the succinic linker on BHA resin and released by treatment with NH3 in MeOH for 72-96h or hydrazine in DMF for 24h [75]. Similarly, hydroquinone-0,0 -diacetic acid (linker 22) has been used to link nucleosides to polystyrene or CPG supports. Cleavage of oligonucleotides was carried out with aqueous ammonia [76]. Other diacids with a similar function have also been described [77]. 

Benzhydrylamlne resins (Yi=H, BHA resin YissOCHs, MBHA resin)... 

These results indicate that as little as 5 /imol/g peptide-resin free amino group can be detected by this method. If the resin contains no quaternary charged groups (e.g., PAM or BHA resins), the blue color will all be in the solution. 

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