Preparation and Use of a General Solid-Phase Intermediate to Biomimetic Scaffolds and Peptide Condensations

The Distributed Drug Discovery (D3) program develops simple, powerful, and reproducible procedures to enable the distributed synthesis of large numbers of potential drugs for neglected diseases. The synthetic protocols are solid-phase based and inspired by published work. One promising article reported that many biomimetic molecules based on diverse scaffolds with three or more sites of variable substitution can be synthesized in one or two steps from a common key aldehyde intermediate. This intermediate was prepared by the ozonolysis of a precursor functionalized at two variable sites, restricting their presence in the subsequently formed scaffolds to ozone compatible functional groups. To broaden the scope of the groups available at one of these variable sites, we developed a synthetic route to an alternative, orthogonally protected key intermediate that allows the incorporation of ozone sensitive groups after the ozonolysis step. The utility of this orthogonally protected intermediate is demonstrated in the synthesis of several representative biomimetic scaffolds containing ozonolytically labile functional groups. It is compatible with traditional Fmoc peptide chemistry, permitting it to incorporate peptide fragments for use in fragment condensations with peptides containing cysteine at the N-terminus. Overall yields for its synthesis and utilization (as many as 13 steps) indicate good conversions at each step.


Introduction
Our Distributed Drug Discovery (D3) program seeks simple and powerful synthetic methodologies to make large numbers of biomimetic molecules from diverse scaffolds. Solid-phase synthetic protocols are ideal, because they enable multi-step syntheses to be carried out efficiently, on a small or large scale, with quick, simple work-ups and a minimal loss of material. They are the most powerful when a single protocol allows access through more than one variable step to multiple molecules with potential for biological activity. The first dramatic example was Merrifield's solid-phase synthesis of peptides [1,2], where each variable position in the growing peptide chain provided an opportunity to substitute any of 20 amino acids, and the ultimate peptide could be many amino acids long. We developed a variation on solid-phase peptide synthesis termed "unnatural peptide synthesis" [3][4][5], in which at a particular step of a small peptide synthesis, an N-terminal glycine could be converted on-resin into an unnatural amino acid that then became part of the growing sequence. Later, we extended this work with the solid-phase conversion of natural amino acids (including glycine) into the key Merrifield resin intermediate (including glycine) into the key Merrifield resin intermediate 7 [6], which was subsequently transformed into multiple peptidomimetic and biomimetic scaffolds 1-5 (Scheme 1). The final release of product in all of these cases was by cyclitive cleavage. This permits the use of acidic or basic conditions at any intermediate step [6]. To make these scaffolds, our previously reported [6] synthesis of 7 (Scheme 2) required an ozonolysis of precursor 10 that already contained the acylated residue R 2 . A major limitation of this route is its incompatibility with any R 2 groups that are ozonolytically labile, such as electron-rich (hetero)aromatics, alkenes, and a number of amino acid side chains. In this report, we describe the synthesis and utilization of the modified key intermediate 13 (Scheme 3), which is compatible with both the introduction of ozone sensitive R 2 (for example, the side chains on some amino acids such as tyrosine or tryptophan) and classic solid-phase peptide synthesis. In addition to giving broader access to the biomimetic scaffolds shown in Scheme 1, it also permits peptide fragments to be coupled with N-terminal cysteine peptides, resulting in thiazolidine lactam scaffolds 3, in which either or both R 2 and R 3 are amino acids or peptide derived. The latter case represents an example of peptide fragment condensation. Scheme 2. Previous synthesis of 7 (route is incompatible with unstable R 2 during ozonolysis). To make these scaffolds, our previously reported [6] synthesis of 7 (Scheme 2) required an ozonolysis of precursor 10 that already contained the acylated residue R 2 . A major limitation of this route is its incompatibility with any R 2 groups that are ozonolytically labile, such as electron-rich (hetero)aromatics, alkenes, and a number of amino acid side chains. In this report, we describe the synthesis and utilization of the modified key intermediate 13 (Scheme 3), which is compatible with both the introduction of ozone sensitive R 2 (for example, the side chains on some amino acids such as tyrosine or tryptophan) and classic solid-phase peptide synthesis. In addition to giving broader access to the biomimetic scaffolds shown in Scheme 1, it also permits peptide fragments to be coupled with N-terminal cysteine peptides, resulting in thiazolidine lactam scaffolds 3, in which either or both R 2 and R 3 are amino acids or peptide derived. The latter case represents an example of peptide fragment condensation. (including glycine) into the key Merrifield resin intermediate 7 [6], which was subsequently transformed into multiple peptidomimetic and biomimetic scaffolds 1-5 (Scheme 1). The final release of product in all of these cases was by cyclitive cleavage. This permits the use of acidic or basic conditions at any intermediate step [6]. To make these scaffolds, our previously reported [6] synthesis of 7 (Scheme 2) required an ozonolysis of precursor 10 that already contained the acylated residue R 2 . A major limitation of this route is its incompatibility with any R 2 groups that are ozonolytically labile, such as electron-rich (hetero)aromatics, alkenes, and a number of amino acid side chains. In this report, we describe the synthesis and utilization of the modified key intermediate 13 (Scheme 3), which is compatible with both the introduction of ozone sensitive R 2 (for example, the side chains on some amino acids such as tyrosine or tryptophan) and classic solid-phase peptide synthesis. In addition to giving broader access to the biomimetic scaffolds shown in Scheme 1, it also permits peptide fragments to be coupled with N-terminal cysteine peptides, resulting in thiazolidine lactam scaffolds 3, in which either or both R 2 and R 3 are amino acids or peptide derived. The latter case represents an example of peptide fragment condensation.
The strategy used to overcome the limitation represented by Scheme 2 was to perform the ozonolysis on the N-protected fluorenylmethyloxycarbonyl (Fmoc) derivative 11 (Scheme 3) prior to introduction of the ozone-sensitive acyl group R 2 CO. The newly formed aldehyde 12 could then be orthogonally protected as the key acetal intermediate 13 [7]. Now, the latent amino and aldehyde groups present in 13 can be selectively unmasked and functionalized, providing access to 7 with ozonolytically labile compounds, and by a process compatible with Fmoc-based solid-phase peptide synthesis (SPPS), a route to diverse unnatural peptides and peptidomimetics 1-5 (Scheme 1), as well as peptide fragment condensations. The potential for this methodology is illustrated in this report for representative scaffolds 1 and 3.
Scaffold 1 compounds, N-acyl homoserine lactones (AHLs), have received considerable attention over the last 10-15 years. Certain natural Scaffold 1 compounds, such as N-acyl homoserine lactones, are known to be used by Gram-negative bacteria as signaling molecules to initiate quorum sensing [8,9]. In early efforts designed to mimic endogenous signaling lactones, non-native AHLs have been identified as inhibitors of quorum sensing [9,10], and this has stimulated a growing search for inhibitors of biofilm formation in recent years [11][12][13].
Scaffold 3 compounds, γ-bicyclic thiazolidine lactams, have been of particular interest and can be traced back to the years of the Second World War when the various structure-activity relationships associated with the penicillins were being defined [14]. Subsequently, it was recognized that fused bicyclic structures such as 3 can mimic conformationally-restricted peptides [15][16][17][18][19]. Often, they adopt a β-turn conformation [20][21][22]: a secondary, reverse-turn structure that has been associated with various biological activities [17][18][19]23,24]. The replacement of dipeptide residues with a conformationally-restricting bicyclic thiazolidine lactam core has found application in its incorporation into the antimicrobial gramicidin S [25] and the hypertensive angiotensin II [26].

Preparation of Fmoc Acetal Intermediate Resins 13
Resin 9c (R 1 = Bn) [6] was converted to its Fmoc derivative 11c using Fmoc-Cl according to Scheme 3. Although amine 9c represents a potentially problematic or difficult coupling [27][28][29][30] by virtue of its quaternary α-carbon, there was no evidence in the subsequently released products of incomplete acylation with the reactive Fmoc-Cl. The Fmoc functional group did not present any complications during ozonolysis of 11c followed by reductive work-up [31]. The resultant aldehyde functional group in 12c was then converted to its dimethylacetal 13c using methanol, trimethylsilyl chloride, and trimethylorthoformate [7,32]. Similarly, 9a,b were converted to 13a,b. As a demonstration of the usefulness of resins 13a-c, examples of scaffolds 1 and 3 bearing ozone-labile substructures are presented.

Scaffold 1
The approach to the preparation of homoserine lactone scaffolds 1 is illustrated with the furanyl derivatives 19a-c (Scheme 4). Treatment of 13a-c with 20% piperidine followed by acylation of the deprotected amine resin with 2-furoyl chloride gave the acylated acetal resins 16a-c. The latent aldehyde was regenerated by a brief exposure (35 min) to aqueous trifluoroacetic acid, providing resins 17a-c. Reduction to the alcohol resins 18a-c was accomplished with sodium triacetoxyborohydride in acetic acid, and this was followed by cyclitive cleavage at elevated temperatures in the presence of diisopropylethylamine (DIEA, Hünig's base) to give the desired lactones 19a-c bearing the ozone-labile furan ring [33,34]  As a demonstration of the usefulness of resins 13a-c, examples of scaffolds 1 and 3 bearing ozone-labile substructures are presented.

Scaffold 1
The approach to the preparation of homoserine lactone scaffolds 1 is illustrated with the furanyl derivatives 19a-c (Scheme 4). Treatment of 13a-c with 20% piperidine followed by acylation of the deprotected amine resin with 2-furoyl chloride gave the acylated acetal resins 16a-c. The latent aldehyde was regenerated by a brief exposure (35 min) to aqueous trifluoroacetic acid, providing resins 17a-c. Reduction to the alcohol resins 18a-c was accomplished with sodium triacetoxyborohydride in acetic acid, and this was followed by cyclitive cleavage at elevated temperatures in the presence of diisopropylethylamine (DIEA, Hünig's base) to give the desired lactones 19a-c bearing the ozone-labile furan ring [33,34] in 14-25% overall yield (80-86% average yield per step over 11 steps from starting substituted Merrifield resins).

Scaffold 3
The synthesis of a scaffold 3 example featuring an ozone-sensitive group is illustrated in Scheme 5. Fmoc acetal resin 13c was deprotected and acylated with the electron-rich, ozone-sensitive, activated ester of the trimethoxybenzoic acid generated with 1-hydroxybenzotriazole (HOBt) and diisopropylcarbodiimide (DIC) to give acetal resin 20c. Cyclitive cleavage (step b) at room temperature failed, resulting in the recovery of starting material, but did proceed at elevated temperature to give a 27% overall purified yield of 21c as a 2:1 mixture of diastereomers. The required thiazolidine intermediate was formed through in situ acetic acid activation of the dimethylacetal and reaction with cysteine ethyl ester.

Demonstrating Compatibility with Peptide Chemistry
As a demonstration of the usefulness of resins 13a-c as advanced intermediates in solid-phase peptide synthesis and fragment condensation, examples of scaffolds 1 and 3 bearing amino acid residues are presented.

Scaffold 3
The synthesis of a scaffold 3 example featuring an ozone-sensitive group is illustrated in Scheme 5. Fmoc acetal resin 13c was deprotected and acylated with the electron-rich, ozone-sensitive, activated ester of the trimethoxybenzoic acid generated with 1-hydroxybenzotriazole (HOBt) and diisopropylcarbodiimide (DIC) to give acetal resin 20c. Cyclitive cleavage (step b) at room temperature failed, resulting in the recovery of starting material, but did proceed at elevated temperature to give a 27% overall purified yield of 21c as a 2:1 mixture of diastereomers. The required thiazolidine intermediate was formed through in situ acetic acid activation of the dimethylacetal and reaction with cysteine ethyl ester.

Scaffold 1
Using standard Fmoc-based peptide synthetic procedures, advanced resins 13a-c also provide the opportunity to install amino acid or peptide residues into scaffold 1 structures. Scheme 6 depicts the successful incorporation of an N-capped amino acid residue at the N-terminus of the homoserine lactone scaffold 1. After deprotection of 13c, the amine was acylated using the anhydride of Fmoc-Ala-OH. Subsequent deprotection and acylation with 4-chlorobenzoyl chloride gave the N-capped, N-terminal alanine acetal resin 23c. Acetal hydrolysis, cyanoborohydride reduction, and cyclitive cleavage then gave, in nearly equal quantities, the stereoisomers of 24c (54:46 ratio), which were separated by reverse-phase chromatography to give the 3R and 3S diastereomers. Scheme 6. Introduction of amino acid residue into Scaffold 1.

Scheme 5.
Introduction of an electron-rich aromatic ring.

Scaffold 1
Using standard Fmoc-based peptide synthetic procedures, advanced resins 13a-c also provide the opportunity to install amino acid or peptide residues into scaffold 1 structures. Scheme 6 depicts the successful incorporation of an N-capped amino acid residue at the N-terminus of the homoserine lactone scaffold 1. After deprotection of 13c, the amine was acylated using the anhydride of Fmoc-Ala-OH. Subsequent deprotection and acylation with 4-chlorobenzoyl chloride gave the N-capped, N-terminal alanine acetal resin 23c. Acetal hydrolysis, cyanoborohydride reduction, and cyclitive cleavage then gave, in nearly equal quantities, the stereoisomers of 24c (54:46 ratio), which were separated by reverse-phase chromatography to give the 3R and 3S diastereomers. Using standard Fmoc-based peptide synthetic procedures, advanced resins 13a-c also provide the opportunity to install amino acid or peptide residues into scaffold 1 structures. Scheme 6 depicts the successful incorporation of an N-capped amino acid residue at the N-terminus of the homoserine lactone scaffold 1. After deprotection of 13c, the amine was acylated using the anhydride of Fmoc-Ala-OH. Subsequent deprotection and acylation with 4-chlorobenzoyl chloride gave the N-capped, N-terminal alanine acetal resin 23c. Acetal hydrolysis, cyanoborohydride reduction, and cyclitive cleavage then gave, in nearly equal quantities, the stereoisomers of 24c (54:46 ratio), which were separated by reverse-phase chromatography to give the 3R and 3S diastereomers. Again, using standard Fmoc-based peptide synthetic procedures, amino acid or peptide residues can be incorporated into advanced resins 13a-c at (1) the N-terminus of the bicyclic framework of 3, and at (2) the C-terminus of the bicyclic framework of 3 during the bicyclization using N-terminal cysteine peptides. This second opportunity was first fulfilled with the incorporation of a glycine residue at the C-terminus of scaffold 3 (Scheme 7). Following deprotection of 13c, the amine resin was acylated with p-toluoyl chloride. Cyclitive cleavage using commercially available Cys-Gly-OH at elevated temperature then afforded a 3:1 mixture of diastereomers, which when triturated with dichloromethane and recrystallized from ethanol resulted in the isolation of a single diastereomer, as evidenced by proton NMR spectroscopy. The structure of this stereoisomer, which was solved by x-ray crystallography ( Table 1), was found to be the 2R,5S,7S isomer β-26c; this bears the identical configuration pattern as the previously reported β-27c ( Figure 1) [6]. Compound β-26c represents our first example of a fragment condensation [35][36][37][38] in which the C-terminal fragment is Cys-Gly, and the N-terminal fragment is an N-acylated modified, resin-bound phenylalanine. Again, using standard Fmoc-based peptide synthetic procedures, amino acid or peptide residues can be incorporated into advanced resins 13a-c at (1) the N-terminus of the bicyclic framework of 3, and at (2) the C-terminus of the bicyclic framework of 3 during the bicyclization using N-terminal cysteine peptides. This second opportunity was first fulfilled with the incorporation of a glycine residue at the C-terminus of scaffold 3 (Scheme 7). Following deprotection of 13c, the amine resin was acylated with p-toluoyl chloride. Cyclitive cleavage using commercially available Cys-Gly-OH at elevated temperature then afforded a 3:1 mixture of diastereomers, which when triturated with dichloromethane and recrystallized from ethanol resulted in the isolation of a single diastereomer, as evidenced by proton NMR spectroscopy. The structure of this stereoisomer, which was solved by xray crystallography (Table 1), was found to be the 2R,5S,7S isomer β-26c; this bears the identical configuration pattern as the previously reported β-27c ( Figure 1) [6]. Compound β-26c represents our first example of a fragment condensation [35][36][37][38] in which the C-terminal fragment is Cys-Gly, and the N-terminal fragment is an N-acylated modified, resin-bound phenylalanine.
As demonstrated with the two examples in Schemes 5 and 7, cyclitive cleavage of dimethylacetal resins can be executed without initial conversion to the aldehyde resins. However, elevated temperatures (90 °C) were required. We later sought milder cyclitive cleavage conditions in an effort to improve upon the diastereoselectivity. Consequently, all of the subsequent examples reported below illustrate the cyclitive cleavage to Scaffold 3 compounds occurring from the aldehyde resins.

Scaffold 3
Again, using standard Fmoc-based peptide synthetic procedures, amino acid or peptide residues can be incorporated into advanced resins 13a-c at (1) the N-terminus of the bicyclic framework of 3, and at (2) the C-terminus of the bicyclic framework of 3 during the bicyclization using N-terminal cysteine peptides. This second opportunity was first fulfilled with the incorporation of a glycine residue at the C-terminus of scaffold 3 (Scheme 7). Following deprotection of 13c, the amine resin was acylated with p-toluoyl chloride. Cyclitive cleavage using commercially available Cys-Gly-OH at elevated temperature then afforded a 3:1 mixture of diastereomers, which when triturated with dichloromethane and recrystallized from ethanol resulted in the isolation of a single diastereomer, as evidenced by proton NMR spectroscopy. The structure of this stereoisomer, which was solved by xray crystallography (Table 1), was found to be the 2R,5S,7S isomer β-26c; this bears the identical configuration pattern as the previously reported β-27c ( Figure 1) [6]. Compound β-26c represents our first example of a fragment condensation [35][36][37][38] in which the C-terminal fragment is Cys-Gly, and the N-terminal fragment is an N-acylated modified, resin-bound phenylalanine.
As demonstrated with the two examples in Schemes 5 and 7, cyclitive cleavage of dimethylacetal resins can be executed without initial conversion to the aldehyde resins. However, elevated temperatures (90 °C) were required. We later sought milder cyclitive cleavage conditions in an effort to improve upon the diastereoselectivity. Consequently, all of the subsequent examples reported below illustrate the cyclitive cleavage to Scaffold 3 compounds occurring from the aldehyde resins.    As demonstrated with the two examples in Schemes 5 and 7, cyclitive cleavage of dimethylacetal resins can be executed without initial conversion to the aldehyde resins. However, elevated temperatures (90 • C) were required. We later sought milder cyclitive cleavage conditions in an effort to improve upon the diastereoselectivity. Consequently, all of the subsequent examples reported below illustrate the cyclitive cleavage to Scaffold 3 compounds occurring from the aldehyde resins.
Compound 30b (Scheme 8) serves as an example of a scaffold 3 structure bearing alanine residues at both the R 2 and R 3 positions, and represents another fragment coupling illustration (dipeptide + dipeptide). The required Cys-Ala-OMe 32 was prepared using the mixed anhydride method (Scheme 9). Originally 31, which is the precursor to 32, was prepared by a solid-phase synthetic route in which cleavage from Wang resin was carried out in methanol/triethylamine at 55-60 • C for 42 h. However, concern for epimerization resulting from prolonged exposure to mild base at elevated temperature was substantiated as 31, which when prepared by this method was found to be a 95/5 mixture of diastereomers. When 31 was prepared by the mixed anhydride method (Scheme 9), it was stereochemically pure.

Variation of R 1 , R 2 , and YR 3 of Scaffold 3 Structures
Variations of the three substituents R 1 , R 2 , and YR 3 of scaffold 3 are defined in Table 2 and include R 1 = H, Me, and Bn; R 2 = aryl and aroyl-Ala; YR 3 = NH-Gly-OH, NH-Leu-OMe, NH-Ala-OMe, and OEt. The synthetic routes used to prepare these compounds are described in Schemes 10-12. Bicyclic thiazolidine lactams 3 with ozone-incompatible groups are represented in Scheme 10 (amino acid residues present) and Scheme 11 (acyl groups R 2 CO present), whereas 3, with ozone-compatible R 2 CO, was prepared using Scheme 12. As Scheme 11 indicates, the bicyclic thiazolidine lactam targets can be accessed in a single step (step d) from acetal resin 15 (vide supra). If milder conditions are required, the two-step process (steps b and c) allows cyclitive cleavage to proceed at slightly lower temperatures. Table 2 compounds are listed below each target structure in the three schemes. can be accessed in a single step (step d) from acetal resin 15 (vide supra). If milder conditions are required, the two-step process (steps b and c) allows cyclitive cleavage to proceed at slightly lower temperatures. Table 2 compounds are listed below each target structure in the three schemes. can be accessed in a single step (step d) from acetal resin 15 (vide supra). If milder conditions are required, the two-step process (steps b and c) allows cyclitive cleavage to proceed at slightly lower temperatures. Table 2 compounds are listed below each target structure in the three schemes. can be accessed in a single step (step d) from acetal resin 15 (vide supra). If milder conditions are required, the two-step process (steps b and c) allows cyclitive cleavage to proceed at slightly lower temperatures. Table 2 compounds are listed below each target structure in the three schemes.    Preparation of all the above scaffold 3 compounds and the others presented in Table 2 from Fmoc acetal resins 13a-c resulted in the formation of two predominating diastereomers in each case, and their diastereomeric ratios (dr) are given. The isolation of two major stereoisomers is a consequence of the achiral alkylation of 39; this introduces the allyl side chain, which is the precursor to the acetal moiety of 13a-c (Scheme 13). Although two additional diastereomers were observed in cases where R 1 = H and Me, they accounted for less than 10% of the product mixture. These two additional diastereomers are not represented in Table 2, and no effort was made to isolate and characterize them. Standard reverse-phase chromatography was used to separate the diastereomers. The overall yields (1-29%) from the commercially available starting resins, based on advertised 16 β a Material released from resin was collected after 24 h at rt, then after successive 24 h/55 • C periods. The diastereomeric ratio (dr) cited is that from the most productive, the first 24 h/55 • C period; β is defined as R 1 "up" (wedge bond), b in methanol-d 4 , c number of steps from Boc-Gly(allyl)-Merrifield, d an additional 3% was obtained as a mixture of the two diastereomers. Preparation of all the above scaffold 3 compounds and the others presented in Table 2 from Fmoc acetal resins 13a-c resulted in the formation of two predominating diastereomers in each case, and their diastereomeric ratios (dr) are given. The isolation of two major stereoisomers is a consequence of the achiral alkylation of 39; this introduces the allyl side chain, which is the precursor to the acetal moiety of 13a-c (Scheme 13). Although two additional diastereomers were observed in cases where R 1 = H and Me, they accounted for less than 10% of the product mixture. These two additional diastereomers are not represented in Table 2, and no effort was made to isolate and characterize them. Standard reverse-phase chromatography was used to separate the diastereomers. The overall yields (1-29%) from the commercially available starting resins, based on advertised loadings, are also provided with the number of steps (5-13) recorded in parentheses. The overall yields represent an average of 76-87% per step. Scheme 12. Synthesis of Scaffold 3 compounds with ozone-compatible R 2 .

Use of the Nuclear Overhauser Enhancement to Assign Stereochemistry
It is assumed that epimerization at the alpha carbon originating with the cysteine reagent had not taken place under the mildly acidic conditions for cyclitive cleavage [46]. To determine the configurations at the other two stereocenters of 3, both one-dimensional and two-dimensional (2D-NOESY) nuclear Overhauser enhancement (nOe) experiments were performed. The results from diastereomers α-30b and β-30b are illustrative. Irradiation of the methyl protons (H-18) of β-30b resulted in the enhancement of the bridgehead proton H-10, and the enhancement was reciprocated upon irradiation of H-10 ( Figure 2). In addition, the irradiation of H-10 gave an enhancement at amide proton H-21. These enhancements were observed in both the one-dimensional and two-dimensional experiments, and are consistent with the methyl group H-18, the bridgehead proton H-10, and the Scheme 14. Problematic coupling leading to the deletion of peptide 35b.

Use of the Nuclear Overhauser Enhancement to Assign Stereochemistry
It is assumed that epimerization at the alpha carbon originating with the cysteine reagent had not taken place under the mildly acidic conditions for cyclitive cleavage [46]. To determine the configurations at the other two stereocenters of 3, both one-dimensional and two-dimensional (2D-NOESY) nuclear Overhauser enhancement (nOe) experiments were performed. The results from diastereomers α-30b and β-30b are illustrative. Irradiation of the methyl protons (H-18) of β-30b resulted in the enhancement of the bridgehead proton H-10, and the enhancement was reciprocated upon irradiation of H-10 ( Figure 2). In addition, the irradiation of H-10 gave an enhancement at amide proton H-21. These enhancements were observed in both the one-dimensional and two-dimensional experiments, and are consistent with the methyl group H-18, the bridgehead proton H-10, and the amide proton H-21 oriented syn to one another. In contrast, no nOe to H-10 is observed upon irradiation of the methyl group of α-30b, nor in the reverse direction. Also, for α-30b, reciprocal enhancements are observed between protons H-10 and H-22 upon irradiation of either proton establishing their syn relationship to one another.   Table 3 compiles the chemical shifts for the ring fusion proton and the two diastereotopic protons on the adjacent carbon atom (labeled as Hx, Hy, and Hz, respectively) for the Scaffold 3 compounds described in Table 2, and illustrates a predictive aspect of stereoisomer assignment. The compounds are arranged in three groups according to their R 1 substituent (H, Me, or Bn). Also provided as ∆δyz (ppm) is the difference in chemical shift between the two diastereotopic protons of each pair of diastereomers. Listed next to this column is the nOe-assigned stereochemistry of the R 1 group, which is described as α (R 1 anti to cysteine carbonyl group or down) or β (R 1 syn to cysteine carbonyl group or up). Comparison of the R 1 = benzyl series shows without exception that the ∆δ is significantly larger for the β isomer. The ∆δ is even larger with the β isomer for the R 1 = H series; however, it is the α isomer in the R 1 = Me series that displays the larger ∆δ values. Baldwin et al. [47] observed the same trend in the R 1 = H series with diastereomers 42 and 43 (Figure 3), with the β isomer 43 showing the larger ∆δ value between the chemical shifts of Ha and Hb.

Use of the Nuclear Overhauser Enhancement to Assess Molecular Shape
The γ-bicyclic thiazolidine lactam core imparts conformational restriction [15][16][17][18][19] and when suitably substituted with amino acid residues, the resulting peptidomimetic can adopt a β-turn secondary structure [20][21][22]. Typically, this reverse-turn involves a tetrapeptide segment and is observed in optical rotatory dispersion (ORD) and circular dichroism (CD) spectra. From onedimensional nOe analyses of the diastereomers of 30b, a small enhancement (0.1%) was observed at the aromatic protons H-8 and H-9 upon irradiation of the methyl protons, H-13, of the α-isomer, α-30b (Figure 4). A reciprocated enhancement was observed at the ester methyl protons upon  Table 3 compiles the chemical shifts for the ring fusion proton and the two diastereotopic protons on the adjacent carbon atom (labeled as H x , H y , and H z , respectively) for the Scaffold 3 compounds described in Table 2, and illustrates a predictive aspect of stereoisomer assignment. The compounds are arranged in three groups according to their R 1 substituent (H, Me, or Bn). Also provided as ∆δ yz (ppm) is the difference in chemical shift between the two diastereotopic protons of each pair of diastereomers. Listed next to this column is the nOe-assigned stereochemistry of the R 1 group, which is described as α (R 1 anti to cysteine carbonyl group or down) or β (R 1 syn to cysteine carbonyl group or up). Comparison of the R 1 = benzyl series shows without exception that the ∆δ is significantly larger for the β isomer. The ∆δ is even larger with the β isomer for the R 1 = H series; however, it is the α isomer in the R 1 = Me series that displays the larger ∆δ values. Baldwin et al. [47] Table 3 compiles the chemical shifts for the ring fusion proton and the two diastereotopic protons on the adjacent carbon atom (labeled as Hx, Hy, and Hz, respectively) for the Scaffold 3 compounds described in Table 2, and illustrates a predictive aspect of stereoisomer assignment. The compounds are arranged in three groups according to their R 1 substituent (H, Me, or Bn). Also provided as ∆δyz (ppm) is the difference in chemical shift between the two diastereotopic protons of each pair of diastereomers. Listed next to this column is the nOe-assigned stereochemistry of the R 1 group, which is described as α (R 1 anti to cysteine carbonyl group or down) or β (R 1 syn to cysteine carbonyl group or up). Comparison of the R 1 = benzyl series shows without exception that the ∆δ is significantly larger for the β isomer. The ∆δ is even larger with the β isomer for the R 1 = H series; however, it is the α isomer in the R 1 = Me series that displays the larger ∆δ values. Baldwin et al. [47] observed the same trend in the R 1 = H series with diastereomers 42 and 43 (Figure 3), with the β isomer 43 showing the larger ∆δ value between the chemical shifts of Ha and Hb.

Use of the Nuclear Overhauser Enhancement to Assess Molecular Shape
The γ-bicyclic thiazolidine lactam core imparts conformational restriction [15][16][17][18][19] and when suitably substituted with amino acid residues, the resulting peptidomimetic can adopt a β-turn secondary structure [20][21][22]. Typically, this reverse-turn involves a tetrapeptide segment and is observed in optical rotatory dispersion (ORD) and circular dichroism (CD) spectra. From onedimensional nOe analyses of the diastereomers of 30b, a small enhancement (0.1%) was observed at the aromatic protons H-8 and H-9 upon irradiation of the methyl protons, H-13, of the α-isomer, α-30b (Figure 4). A reciprocated enhancement was observed at the ester methyl protons upon   The γ-bicyclic thiazolidine lactam core imparts conformational restriction [15][16][17][18][19] and when suitably substituted with amino acid residues, the resulting peptidomimetic can adopt a β-turn secondary structure [20][21][22]. Typically, this reverse-turn involves a tetrapeptide segment and is observed in optical rotatory dispersion (ORD) and circular dichroism (CD) spectra. From one-dimensional nOe analyses of the diastereomers of 30b, a small enhancement (0.1%) was observed at the aromatic protons H-8 and H-9 upon irradiation of the methyl protons, H-13, of the α-isomer, α-30b (Figure 4). A reciprocated enhancement was observed at the ester methyl protons upon irradiation of H-8. No such enhancements were observed with β-30b. These results are consistent with an approach of the termini of α-30b, and may reflect the adoption of a β-turn secondary structure by this stereoisomer in chloroform solution.

Materials and Methods
All of the reagents were purchased from Acros Organics (Geel, Belgium), Advanced Chemtech  [3], contained in a 500-mL SPPS vessel, was washed with 4 × 10 mL of N-methyl pyrrolidinone (NMP). To the resin was then added 40 mL of NMP, followed by 3.8 equivalents of diisopropylethylamine (DIEA). The vessel was swirled gently to mix the contents, and 3.0 equivalents of Fmoc chloride in 60-70 mL of NMP was added in one portion. The vessel was rocked on an orbital shaker and after 24 h the vessel was drained, and the resin was washed 3 × 45 mL each with NMP, 1:1 THF:EtOH, THF, and dichloromethane (DCM) to give resins 11a-c (R 1 = H, Me, Bn). The resin was then dried under a slow stream of dry nitrogen gas overnight.

α-(2-Oxoethyl)-α-R 1 -N-(fluorenylmethyloxycarbonyl)glycine on Merrifield resins (12a-c, R 1 = H, Me, Bn).
A 250-mL, three-neck, round-bottomed flask was charged with 1.80-3.20 mmol of resin 11a-c (R 1 = H, Me, Bn), a trace of Sudan III red dye, and 40 mL of DCM under dry argon gas. The contents were cooled in a dry ice/acetone bath, and the argon flow was replaced with a subsurface flow of oxygen gas at a rate of 0.6-0.8 L/min. Ozonolysis using an ozone generator was performed at a current of 1.0 ampere until the red dye was rendered colorless (2-3 h). The current was then reduced to zero while oxygen bubbling continued for 10 min. Diethyl sulfide (1.0 mL, 9.3 mmol) was added, and the solution was allowed to gradually warm to room temperature and stir overnight. The resin was collected by filtration into a 50-mL SPPS vessel rinsing over with DCM and THF. The resin was dried under a slow stream of dry argon gas and then under high vacuum to give 12a-c (R 1 = H, Me, Bn).

N-(3-R 1 -2-oxotetrahydrofuran-3-yl)furan-2-carboxamide (19a-c, R 1 = H, Me, Bn).
Resin 18a-c (100-157 µmol, R 1 = H, Me, Bn) was washed with 2 × 2 mL of chlorobenzene, and was then treated with 2 mL of chlorobenzene followed by 8 eq of DIEA. The mixture was heated at 75 • C for 16 h. After cooling, the vessel was drained, and the resin was washed with 2 × 2 mL of DCM. The combined filtrates were evaporated to dryness to give a residue that was purified by silica gel chromatography:

Ethyl(3R,6R,S,7aS)-6-benzyl-5-oxo-6-(3,4,5-trimethoxybenzamido)hexahydropyrrolo[2,1-b]thiazole-3-carboxylate (21c).
To 0.298 mmol of resin 20c, a solution of KOAc (147.5 mg, 1.5 mmol, 5 eq) and L-Cys-OEt-HCl (111.4 mg, 0.6 mmol, 2 eq) in glacial AcOH (3 mL) was added. The vessel was capped, rotated for 15 min, and then transferred to an oven preheated to 90 • C. After 24 h, the mixture was cooled to RT, the solution was drained from the resin, and the resin was washed with THF (2 × 3 mL) and then with CH 2 Cl 2 (3 × 3 mL). The combined filtrates were transferred to a separatory funnel containing brine (50 mL), water (50 mL), and CH 2 Cl 2 (60 mL). After extraction, the phases were separated, and the organic phase was extracted with 10% KHCO 3 /H 2 O (80 mL). The organic layer was dried over Na 2 SO 4 , filtered, and concentrated in vacuo to afford a yellow oil (206 mg). This sample was purified by flash chromatography on silica gel (4.5 g; 45% EtOAc/hexanes). This gave the product diastereomeric thiazolidines 21c as a colorless solidifying oil (41 mg; 27%).   Resin 13c (87.1 µmol) was treated with 2.5 mL of 20% piperidine in NMP for 30 min with gentle agitation. The vessel was drained, and the deprotected resin was washed with 5 × 3 mL of NMP. To this resin 262 mg (5.00 eq) of Fmoc-Ala anhydride in NMP in 3 mL of NMP was added. After 48 h, the vessel was drained, and the resin was washed with 4 × 3 mL each of NMP, THF, and NMP to give resin 22c. Resin 22c (87.1 µmol) was treated with 2.5 mL of 20% piperidine in NMP for 30 min with gentle agitation. The vessel was drained, and the deprotected resin was washed with 6 × 3 mL of NMP. The resin was then treated with 4.4 eq of 1.0 M solutions of 4-chlorobenzoyl chloride followed by 5.2 eq of DIEA. After 18 h, the vessels were drained, and the resin was washed with 4 × 3 mL each of NMP, 1:1 THF:MeOH, THF, and DCM to give resin 23c. Resin 23c (87.1 µmol) was treated with 3 mL of 4:4:1 TFA:DCM:water for 35 min with gentle agitation. The vessel was drained, and the resin was washed with 5 × 3 mL of DCM and 3 × 3 mL of THF to afford the aldehyde resin. The resin was then treated with 3 mL of 0.50 M acetic acid in THF for 10-15 min. The vessel was drained, and the resin was treated with 1 mL of 0.50 M acetic acid in THF followed by 1.04 mL (6.3 eq) of a 0.53-M solution of sodium cyanoborohydride in 0.50 M of acetic acid in THF. The vessel was gently agitated for 6 h at room temperature; then, it was drained, and the resin was washed with 3 × 3 mL each of THF, 30% aqueous THF, and 4 × 3 mL of THF. The resin was dried under a stream of nitrogen gas, and was then washed with 3 mL of chlorobenzene. Chlorobenzene (3 mL) was then added, followed by 7.9 eq (690 µmol) of DIEA. The resin was then heated at 75 • C for 16 h. After cooling, the vessel was drained, and the filtrate was evaporated to dryness to yield 4.6 mg of crude 24c.

2-((S)-2-((((9H-Fluoren-9-yl)methoxy)carbonyl)amino)propanamido)-4,4-dimethoxy-2-methylbutanoic acid on Merrifield resin (28b).
A 25-mL SPPS vessel was charged with 773 mg (363 µmol) of resin 13b, and was swelled in NMP for 30 minutes. The vessel was drained, and the resin was treated with 5 mL of 20% piperidine in NMP for 5 min. The vessel was drained, the resin was treated with 10 mL of 20% piperidine, and the vessel was rocked on an orbital shaker for 40 min. Then, it was drained, and the resin was washed with 5 × 10 mL of NMP. The deprotected resin was then treated with 859 mg (1.11 mmol, 3.05 eq) of Fmoc-Ala anhydride in 4.5 mL of NMP. The vessel was rocked for 42 h, drained, and the resin was washed with 6 × 15 mL of NMP to give resin 28b.
Fmoc-Ala Anhydride, (Fmoc-Ala) 2 O. A variation of the method of Izdebski and Pawlak [48] is described here. A 50-mL, three-neck, round-bottomed flask under argon was charged with 1.25 g (4.00 mmol) of Fmoc-Ala-OH. The flask was fitted with a thermometer and two rubber septa, and 11 mL of DCM was added via syringe. The mixture was treated with 1 mL of anhydrous DMF, and then cooled to 3 • C. To the mixture, 1.69 mL (252 mg, 2.00 mmol) of 1.0 M diisopropylcarbodiimide (DIC) in DCM was added dropwise via syringe over a two-minute period. After stirring at 3 • C for 30 minutes, the mixture was allowed to warm to room temperature, and was stirred an additional 10 minutes. The contents were filtered to afford 469 mg of crude anhydride (70 mol%) containing 30 mol% diisopropylurea (DIU), 1 6 Hz). Further drying gave 288 mg. The mother liquor was concentrated to a small volume and afforded an additional 463 mg (81 mol% anhydride) after drying. The combined yield of (Fmoc-Ala) 2 O was 56%.

Methyl((3R,7aS)-6-methyl-6-((S)-2-(4-nitrobenzamido)propanamido)-5-oxohexahydropyrrolo [2,1-b]thiazole-3-carbonyl)-L-alaninate (30b).
Hydrolysis of dimethyl acetal functional group: Resin 29b (363 µmol) was treated with 13 mL of TFA:DCM:water (4:4:1) for 35 min at room temperature. The SPPS vessel was drained, and the resin was washed with 6 × 15 mL of DCM to give the aldehyde resin. Preparation of Cys-Ala-OMe·TFA: Boc-Cys(Trt)-Ala-OMe (262 mg, 478 µmol, 1.3 eq) was treated with 5 mL of trifluoroacetic acid (TFA): triethylsilane (TES) (97.5:2.5) for two hours at room temperature. The volatiles were removed in vacuo, and the residue was treated with 5 mL of 1:1 diethyl ether:hexanes, and then decanted. This was repeated again with 5 mL of 1:1 diethyl ether: hexanes. The residue was treated with 5 mL of diethyl ether to induce solidification, and then the ether was evaporated to give the TFA salt of Cys-Ala-OMe. This material was dissolved in 3 mL of acetic acid, and was added to the pre-formed aldehyde resin described above followed by 294 mg (3.00 mmol, 8.3 eq) of potassium acetate. The vessel was rocked overnight at room temperature and then drained. The resin was then washed with 2 × 5 mL of acetic acid. Acetic acid (5 mL) was added, and the vessel was heated at 55-60 • C for 48 h. After cooling, the vessel was drained, and the resin was washed with 2 × 5 mL of acetic acid. The three filtrates were combined and evaporated to dryness to give 65.6 mg of crude 30b. The crude material was chromatographed on 2.0 g of normal phase silica gel 60 slurried in DCM. Elution with DCM, 98/2 DCM/MeOH, and 95/5 DCM/MeOH afforded 43.4 mg, which was then separated into its two major diastereomers by reverse-phase chromatography on a 5-micron, 21.4 × 250 mm, C18 column using 50/50 1:1 MeOH/MeCN (5 mM NH 4 OAc)/water (5 mM NH 4 OAc) to give 12.5 mg (7% over 13 steps) of α-30b: 1  anhydrous DMF cooled in ice/acetone was added via syringe. The mixture was stirred at −9 • C for one hour, and then at room temperature for one hour. The reaction mixture was transferred to a 250-mL beaker and was evaporated with a stream of nitrogen overnight. The residue was partitioned between 75 mL of ethyl acetate and 30 mL of pH 2 buffer. The layers were separated, and the organic phase was washed with pH 2 buffer, saturated sodium bicarbonate, twice with water (to pH 7), and then dried over sodium sulfate. Cys-Ala-OMe, trifluoroacetic acid salt (32). Boc-Cys(Trt)-Ala-OMe (411 mg, 750 µmol) was treated with 6 mL of trifluoroacetic acid/triethylsilane (97.5/2.5) solution. The mixture was stirred at room temperature for two hours, and was then concentrated to a residue that was treated with 8 mL of 1:1 hexane:diethyl ether, and then decanted from the insoluble oil. This decantation was performed two additional times using 4 mL of 1:1 hexane:diethyl ether. The oil was then triturated under diethyl ether to induce solidification. The mixture was then evaporated to give 32 as a white solid, which was immediately used in the cyclitive cleavage.
Compounds 33a-c, 34a, and 35a-c were prepared according to the methods outlined in Schemes 11 and 12: (3R)-Ethyl 6-(4-chlorobenzamido)-5-oxohexahydropyrrolo[2,1-b]thiazole-3-carboxylate (α-33a and β-33a): A mixture of 242 µmol of 15 (R 1 = H, R 2 = 4-ClPh) in 10 mL of dichloromethane contained in a 25-mL glass reaction vessel for 15 minutes was drained, and the resin was then treated with 10 mL of 4:4:1 TFA:CH 2 Cl 2 :water. The vessel was rocked for 35 minutes, drained, and the resin was washed with 5 × 3 mL of dichloromethane, and then with 2 × 3 mL of acetic acid. The now-formed aldehyde resin 7 (R 1 = H, R 2 = 4-ClPh) was then converted to 33a using Method C by treatment with 340 µmol (1.40 equiv.) of polyvinylpyridine, 193 µmol of cysteine ethyl ester hydrochloride, and 5 mL of acetic acid. The vessel was rocked overnight at room temperature. LC/MS analysis indicated product formation. The contents were then heated/rocked at 55 • C for 24 h. The vessel was drained, and the filtrate was evaporated to dryness affording 41.5 mg, which was chromatographed on 3.0 g of silica gel eluting with toluene and 80/20 toluene/ethyl acetate to afford 5.9 mg (8% over 11 steps) of α-33a as an oil; 1 H-NMR  (3R)-Ethyl6-(4-chlorobenzamido)-6-methyl-5-oxohexahydropyrrolo[2,1-b]thiazole-3-carboxylate (β-33b and α-33b): A mixture of 133.5 µmol of 15 (R 1 = Me, R 2 = 4-ClPh) and 2.4 mL of 4:4:1 TFA:CH 2 Cl 2 :water contained in a 5-mL glass reaction vessel was rotated for 35 minutes, drained, and the resin was washed with 6 × 1.5 mL of dichloromethane. The resin was then dried under a stream of nitrogen, and then under vacuum. The now-formed aldehyde resin 7 (R 1 = Me, R 2 = 4-ClPh) was then converted to 33b using Method B by treatment with 270 µmol (2.0 equiv) of cysteine ethyl ester hydrochloride in 2 mL of NMP, followed by 927 µmol (6.9 equiv.) of potassium acetate in 0.6 mL of acetic acid. The vessel was rotated for 24 h at room temperature, drained, and the resin was washed with 2 × 3 mL each of NMP, 5% DIEA in NMP, 5% DIEA in dichloromethane, and 3 × 3 mL of dichloromethane. The resin was dried under a stream of argon, and then treated with 2 mL of chlorobenzene. The contents were heated at 60 • C for 67 h, cooled, the vessel was drained, and the resin was washed with 3 × 4 mL of dichloromethane. The combined filtrates were evaporated to afford 1.7 mg. The resin was then heated at 75 • C in 3 mL of chlorobenzene for 50 h. The vessel was drained, and the resin was washed with 3 × 4 mL of dichloromethane. The combined filtrates were evaporated to afford 4.1 mg. This process was repeated at 75 • C for 40 h to afford 2.2 mg. The combined materials (1.7 mg, 4.1 mg, and 2.2 mg) were chromatographed on a Dynamax Microsorb 5-micron C18 column (21.4 × 250 mm) using a step gradient beginning with 6/4 1:1 MeCN/MeOH with 5.0 mM ammonium acetate/water with 5.0 mM ammonium acetate to afford 2.5 mg (5% over 11 steps) of β-33b as an oil; 1 H-NMR found 405.0642. The resin was subjected to methoxide cleavage conditions by treating with 1.75 mL of anhydrous THF followed by 700 µL (660 mg, 3.00 mmol, 22 equiv.) of 25% sodium methoxide in methanol for 3 h. The vessel was drained under positive argon pressure while the resin was washed with 4 mL of absolute methanol. The combined filtrates were added to a rapidly stirred, cold mixture of 20 mL of dichloromethane and 20 mL of 1 N HCl. The layers were separated, the aqueous phase was extracted once with 10 mL of dichloromethane, and the combined organics were dried (Na 2 SO 4 ). Concentration gave 13.9 mg of a mixture consisting primarily of the carboxylic acid of 33b, which was esterified with ethyl iodide (300 mg) and 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) (648 µmol) overnight at room temperature. The mixture was concentrated to a residue that was partitioned between ethyl acetate/5% citric acid/brine. The organic phase was washed with 5% citric acid and dried (MgSO 4 ). Concentration gave a wet residue that was diluted with dichloromethane and then dried with sodium sulfate. Concentration gave 8.7 mg, which was chromatographed on a Dynamax Microsorb 5-micron C18 column (21.4 × 250 mm) using a step gradient beginning with 1/1 1:1 MeCN/MeOH with 5.0 mM ammonium acetate/water with 5.0 mM ammonium acetate to afford 2.1 mg (4% over 13 steps) of α-33b as an oil; 1 H-NMR δ 1. 33  (3R)-Ethyl6-benzyl-6-(4-chlorobenzamido)-5-oxohexahydropyrrolo[2,1-b]thiazole-3-carboxylate (β-33c and α-33c): A mixture of 260 µmol of 15 (R 1 = benzyl, R 2 = 4-ClPh) and 2.4 mL of 4:4:1 TFA:CH 2 Cl 2 :water contained in a 5-mL glass reaction vessel was rotated for 35 minutes, drained, and the resin was washed with 6 × 1.5 mL of dichloromethane. The resin was then dried under a stream of nitrogen, and then under vacuum. The now-formed aldehyde resin 7 (R 1 = benzyl, R 2 = 4-ClPh) was then converted to 33c using Method F by treatment with 1.56 mL (1.56 mmol, 6.00 equivalent) of 1.0 M potassium acetate in acetic acid followed by 1.04 mL (0.520 mmol, 2.00 equivalent) of cysteine ethyl ester hydrochloride in acetic acid. After rotation at room temperature for 18 h, the vessel was drained, and the resin was washed once with 3 mL of THF, and then with 4 × 2.5 mL of 5% DIEA in dichloromethane, and then with 4 × 2 mL of dichloromethane. The resin was dried in vacuo, and then treated with 3 mL of chlorobenzene followed by 0.270 mL (200 mg, 1.55 mmol, 6.0 equiv.) of DIEA. The mixture was heated at 55 • C for 24 h, the vessel was drained, and the resin washed with 2 × 3 mL of dichloromethane. Evaporation of the combined filtrates gave only 5.0 mg. The resin was then treated with 3 mL of acetic acid, and the vessel was heated at 75 • C for 40 h. After cooling, the vessel was drained, and the resin was washed with 2 × 2 mL of acetic acid. The combined filtrates were evaporated to give 46.6 mg, which was triturated under 500 µL of warm acetonitrile to afford 15.6 mg (13% over 11 steps) of β-33c as a white solid; 1 H-NMR

Synthesis and Use of Key Intermediate 13
The synthesis of Fmoc acetal resin 13 from the advanced intermediate amino resin 9 is described (Scheme 3). Since ozonolysis of the allyl group precedes introduction of the R 2 CO group, the strategy enables the incorporation of ozone-labile moieties such as furan and trimethoxyphenyl at R 2 for a variety of peptidomimetic and biomimetic scaffolds 1-5. Resin 13 also features two orthogonally-related Fmoc and acetal protecting groups, which can be selectively removed under basic and acidic conditions, respectively, thereby providing an Fmoc-based, SPPS approach to unnatural peptides and peptidomimetics 1-5. The versatility of Fmoc acetal resin 13 is illustrated with the syntheses of homoserine lactones 19a-c (scaffold 1) and the bicyclic thiazolidine 21c (scaffold 3), which are examples that contain ozone-labile substructures. Homoserine lactone 24c and many of the bicyclic thiazolidine compounds listed in Table 2 offer examples of scaffolds 1 and 3 containing amino acid residues. Most of the bicyclic thiazolidines 3 were synthesized in 10-13 steps from commercially available Boc-protected glycine, alanine, and phenylalanine on Merrifield resin. However, access to the multiple scaffolds from the key orthogonally-protected intermediate 13 was often accomplished in four or fewer steps. Several examples (36a-c, 30b-c, and 37b) represent the successful application of peptide fragment condensation in which Cys-Ala-OMe is condensed with the Fmoc-Ala extended analog of 13a-c.

NMR Characterization: Stereochemistry and Possible Secondary Structure
The cyclitive cleavage process in acetic acid at elevated temperatures using cysteine-based nucleophiles to generate scaffold 3 compounds proceeds to afford primarily two diastereomers in ratios from 1:1 to 4:1. These stereoisomers were separated by normal-phase or reverse-phase chromatography, and their relative configurations were determined by one and two-dimensional nOe studies. The difference in chemical shifts (∆δ) between diastereotopic protons at C-6 (the methylene group of the lactam ring) is seen to be diagnostic. For the R 1 = Bn and H series, ∆δ is significantly larger for the β isomer (major diastereoisomer), whereas for the R 1 = Me series, it is the α isomer that displays the larger values. Thus, knowledge of the C-6 proton ∆δ values of the two major diastereomers in the three series (R 1 = H, Me, Bn) affords a predictive value in the assignment of the stereochemistry of future compounds. Nuclear Overhauser enhancement studies also reveal a small enhancement of signals from remote protons located on opposite ends of the four-residue peptidomimetic sequence of α-30b, and may suggest that the molecule adopts a β-turn secondary structure in chloroform.