An Optimized Preparation of 1,1-Dimethylallyl Esters and Their

Jun 25, 2018 - An Optimized Preparation of 1,1-Dimethylallyl Esters and Their Application to Solid-Phase Peptide Synthesis. Matthew Alexander Hostetle...
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Article Cite This: J. Org. Chem. 2018, 83, 7762−7770

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An Optimized Preparation of 1,1-Dimethylallyl Esters and Their Application to Solid-Phase Peptide Synthesis Matthew A. Hostetler and Mark A. Lipton* Department of Chemistry and Cancer Center, Purdue University 560 Oval Drive, West Lafayette, Indiana 47907-2084, United States

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S Supporting Information *

ABSTRACT: A one-step preparation of 1,1-dimethylallyl (DMA) esters was optimized for the C-terminal protection of a range of Fmocprotected amino acids. This preparation is not sensitive to the scale of reaction and affords the corresponding DMA esters in 70−99% yield with high regioselectivity. Additionally, these DMA-protected amino acids were used with the backbone amide linker (BAL) of Albericio and Barany and found to resist diketopiperazine formation during the synthesis of a series of tripeptide esters. C-terminal DMA protection is compatible with the BAL linkage and allows for standard Fmoc-based methods to be used throughout the synthesis.

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esters.21,22 DMA esters are compatible with BAL-based strategies and allow for standard Fmoc-based SPPS throughout the synthesis. In two previous communications, we reported two-step21 and one-step22 preparations of DMA-protected amino acids. While both of these original reports focused on DMA ester preparation, the ability of DMA esters to resist DKP formation during BAL-based SPPS was not studied. In the present work, we report the use of DMA esters as a means of reducing DKP formation in the BAL system while using standard SPPS protocols throughout the synthesis (Figure 2). In the process of exploring these studies, it was found that the original, onestep DMA ester preparation suffered from scale-dependent sensitivities. These observations motivated us to explore improvements to the original procedure, and these improvements are also discussed herein.

INTRODUCTION Cyclic and C-terminal-modified peptides are important classes of synthetic targets involved in drug discovery,1−5 natural product synthesis,6−8 and chemical ligation studies.9−12 A major advance in preparing such targets was achieved when the backbone amide linker (BAL) was introduced by Albericio, Barany, and co-workers in 1998.13 This strategy allows for free manipulation of both termini during solid-phase peptide synthesis (SPPS) by anchoring the growing peptide to a backbone amide nitrogen atom. This added degree of flexibility facilitates the preparation of cyclic peptides, peptide aldehydes, alcohols, thioesters, and p-nitroanilides.13−16 In the preparation of cyclic peptides and in instances where the modified Cterminus is susceptible to nucleophilic hydrolysis (thioesters and p-nitroanilides), the C-terminus must be orthogonally protected as an allyl ester throughout Fmoc-based SPPS. However, allyl protection results in near-quantitative formation of diketopiperazine (DKP) upon piperidine-promoted Fmoc cleavage at the dipeptide stage (Figure 1).13 In general, steric bulk is known to reduce this unwanted side reaction, but such C-terminal protecting groups (e.g., tert-butyl ester) are not compatible with the BAL strategy, as the peptide is cleaved from the linker under the highly acidic conditions needed to deprotect the ester.13,17−20 Methods have been developed to reduce DKP formation when allyl esters are employed with BAL, but they require unconventional synthetic steps before standard Fmoc-based SPPS protocols can be used (Figure 2).13−15 In an effort to contribute a more direct approach to remedying this issue, work in our lab has focused on developing the 1,1-dimethylallyl (DMA) C-terminal protecting group. This group combines the steric bulk of tert-butyl esters with the mild deprotection conditions of allyl © 2018 American Chemical Society

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RESULTS AND DISCUSSION The previously described one-step DMA ester preparation22 was not optimized and was adapted directly from earlier work reported by Julia and co-workers.23 In that work, Julia and coworkers found that catalytic quantities of copper(I) salts promoted reverse prenylation (γ-substitution) of potassium carboxylates with various allylic sulfonium salts, including prenyldimethylsulfonium tetrafluoroborate (1).23 When these conditions were used, the DMA ester of Fmoc-Thr(tBu)-OH was obtained on a 0.25 mmol scale in 27% yield, whereas on a 1.0 mmol scale the ester was obtained in 74% yield (Table 1, entry 1). Similar scale-dependent inconsistencies in yield and Received: March 14, 2018 Published: June 25, 2018 7762

DOI: 10.1021/acs.joc.8b00658 J. Org. Chem. 2018, 83, 7762−7770

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Figure 1. Undesired DKP formation in the BAL system (ref 13).

Figure 2. Methods to reduce DKP formation in the BAL system. (A) References 13 and 14. (B) Reference 15.

Table 1. Scale-Dependent Variations in Yield and Regioselectivity Using Original DMA Ester Preparationa

0.25 mmol scale entry

AA

yield (%)

1 2 3 4 5

Thr(tBu) Ala Val Tyr(tBu) Glu(tBu)

27 89 40 41 89

1.0 mmol scale γ/α

b

c

100/0 65/35 94/6 92/8 32/68

yield (%)

γ/α

74 98 94d 90d 84d

94/6 89/11 88/12 100/0 100/0

Conditions reported in ref 22 for DMA ester preparation; dry conditions. bIsolated yield of combined ester products after filtration through silica pad. cγ/α product ratio was determined by 1H NMR integration. dYield and γ/α ratio obtained from ref 22. a

regioselectivity were observed with other amino acids as well (Table 1). This anomaly was especially severe in the case of Fmoc-Glu(tBu)-OH (Table 1, entry 5) where an almost complete reversal of regioselectivity (in favor of the undesired, α-substituted isomer) was observed on the 0.25 mmol scale. On the basis of these results, it became evident that conditions

adapted from Julia and co-workers may not be as general for the preparation of DMA-protected amino acids as we had previously thought.22 We therefore sought to develop a more robust set of conditions that were less sensitive to variations in scale and were compatible with a range of amino acids and side-chain protecting groups. 7763

DOI: 10.1021/acs.joc.8b00658 J. Org. Chem. 2018, 83, 7762−7770

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The Journal of Organic Chemistry Improved Preparation of Sulfonium Salt (1). To begin the optimization process, we first focused our attention on the preparation of prenyldimethylsulfonium tetrafluoroborate (1). In our earlier work we reported a procedure to prepare 1 as a pale brown solid.22 Although this material was found to be stable for a period of months at −20 °C (by NMR), it was reported that freshly prepared material afforded the best yields in the formation of DMA-protected amino acids. We hypothesized that the colored impurity may have been responsible for the salt’s decomposition over time. In an effort to improve the shelf life of the sulfonium salt, we have developed a new and convenient preparation that affords 1 as a colorless, crystalline solid in 94% yield (Scheme 1). As before,

substrate to probe optimal DMA esterification conditions on the 0.5 mmol scale. Initially, the copper source was explored (Table 2, entries 1−3). While CuI (Table 2, entry 3) showed the most promise in terms of yield, an erosion in γ-selectivity was observed. Despite its lower yield (17%), CuBr promoted perfect γ-selectivity (Table 2, entry 2). These results correlated well with the initial observations of Julia and co-workers,23 and for this reason CuBr was chosen as the copper source for the remainder of the optimization process. Though the mechanism for this transformation has not been established, it has been proposed to proceed through the intermediacy of a reactive anionic copper(I) complex.23 We hypothesized that the counterion to this complex might play a role in its reactivity. To probe this effect, we replaced potassium carbonate with lithium, sodium, and cesium carbonate, respectively (Table 2, entries 2, 4−6). The use of sodium carbonate afforded a substantial increase in the yield of 4a from 17% (Table 2, entry 2) to 80% and proceeded without loss of γ-selectivity (Table 2, entry 5). The use of cesium carbonate afforded 4a in 90% yield but with substantial loss of γ-selectivity (Table 2, entry 6). The observed erosion in γselectivity can likely be attributed to the cesium effect,24,25 whereby the cesium ion promotes a direct SN2 reaction between the cesium carboxylate and sulfonium salt. This SN2 process could compete with the desired, copper-catalyzed reverse prenylation and lead to the observed erosion in γselectivity. The use of an inert atmosphere (N2) and dry solvent with sodium carbonate as the base further improved the yield of 4a to 92% (Table 2, entry 8). Notably, no erosion of yield or γ-selectivity was observed upon scaling down (0.25 mmol) or up (1.0 mmol) under these conditions (Table 2, entry 8). With the optimized DMA esterification conditions in hand, we next examined the substrate scope by subjecting a range of Fmoc-protected amino acids to the optimized conditions on the 0.25 mmol scale (Table 3). The conditions furnished the corresponding DMA esters in 70−99% yield with high γselectivity. The reactions were generally complete in 15 to 24 h, and in most cases the resulting esters could be purified by silica pad filtration. As previously observed,22 amino acids containing nucleophilic side chains require additional 1 to afford acceptable yields (3n). Additionally, the reaction was tested on the 2.0 mmol scale with Ala and Val. In both instances, the reaction showed no sensitivity to scale and resulted in high yields and regioselectivities (3c and 3d). DMA as a C-Terminal Protecting Group for Use in SPPS with the Backbone Amide Linker (BAL).13 Having developed a robust preparation of DMA-protected amino acids, we next sought to explore their use in conjunction with SPPS using the BAL system. More specifically, we wished to study the ability for DMA esters to resist undesired diketopiperazine (DKP) formation upon piperidine-promoted Fmoc deprotection at the dipeptide stage. To accomplish this, we employed a “dual-linker” system shown in Figure 3 where the peptide−BAL linkage would remain intact upon brief exposure to the dilute acid cleavage conditions required for the Sieber amide26,27 linkage. This strategy, which has been described previously,28 allows for rapid reaction analysis of resin-bound synthetic intermediates by UPLC-MS. All the products being detected possess the same UV-active (254 nm) BAL chromophore which facilitated uniform reaction component determination and cross-comparison to integration at 214 nm.29

Scheme 1. Preparation of Prenyldimethylsulfonium Tetrafluoroborate (1)

prenyl alcohol (2) is converted to 1 using a mixture of dimethyl sulfide and fluoroboric acid. The previously used aqueous workup was replaced by the removal of volatile impurities from the crude reaction mixture under reduced pressure followed by precipitation of crystalline 1 using cold diethyl ether. Material prepared by this method has proven to be stable at −20 °C for at least 8 months (by NMR) with only a slight degradation of yield in the preparation of DMA esters observed over a minimum of 5 months (Supporting Information Table S1). It is noteworthy that this revised method can be used to produce 1 on a multigram scale without loss of yield or purity. Optimized Preparation of DMA-Protected Amino Acids. With 1 in hand, we next explored optimization of the preparation of DMA-protected amino acids. As indicated in Table 2, Fmoc-Thr(tBu)-OH (3a) was chosen as the model Table 2. Optimization of the DMA Esterification of FmocThr(tBu)-OHa

entry

CuX

base

atmb

yield of 4a (%)c

γ/αd

1 2 3 4 5 6 7 8

CuCl CuBr CuI CuBr CuBr CuBr CuBr CuBr

K2CO3 K2CO3 K2CO3 Li2CO3 Na2CO3 Cs2CO3 K2CO3 Na2CO3

air air air air air air N2 N2

15 17 33 20 80 90 33 92 (95)e (93)f

87/13 100/0 91/9 94/6 100/0 28/72 100/0 100/0

a

Reactions were carried out on the 0.5 mmol scale (unless otherwise noted) with 1.1 equiv of prenyl sulfonium salt (1), 1.1 equiv of base, 10 mol % CuX, and 5 mL (0.1 M) of CH2Cl2, 18−21 h. bDry CH2Cl2 was used for reactions carried out under N2 atm. cIsolated yield of product after filtration through silica pad. dγ/α product ratio was determined by 1H NMR integration. eYield of reaction carried out on the 0.25 mmol scale, 0.08 M CH2Cl2, γ/α: (99/1). fYield of reaction carried out on 1 mmol scale, 0.08 M CH2Cl2, γ/α: (96/4). 7764

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fulvene or its addition product proved difficult. To circumvent these issues, we utilized a two-step reductive amination procedure starting with Fmoc-protected DMA esters. In this sequence, diethylamine-promoted Fmoc deprotection was completed in the presence of 1-octanethiol to scavenge the dibenzofulvene byproduct. The reaction was routinely complete within 2 h and, following evaporation of excess diethylamine, afforded the corresponding amino acid DMA esters. The resulting crude material was directly subjected to NaBH(OAc)3-promoted reductive amination conditions with solid supported o-Paldehyde 11 to afford the corresponding BAL-linked benzylamines. The resulting benzylamines were then acylated with preformed amino acid symmetric anhydrides to afford resin-bound, Fmoc-protected dipeptides (12a−d). Fmoc deprotection with 20% piperidine-DMF (1 × 5 min, 1 × 10 min) followed by acylation with a preformed symmetric anhydride of Fmoc-Glu(tBu)-OH afforded Fmocprotected tripeptides. Fmoc deprotection followed by Sieber amide cleavage with TFA−H2O−CH2Cl2 (2:1:97) provided BAL-linked tripeptides (13a−e) which were analyzed by UPLC-MS (Table 4). It is of note that we found DMA esters to be partially labile to Sieber amide cleavage conditions. For this reason, both the DMA intact and the free acid were combined to compute the yields of tripeptide products. In their original report, Albericio and Barany found piperidine-promoted Fmoc deprotection of Fmoc-Ala-BALGly-OAllyl to result in quantitative DKP formation.13 We were pleased to find that by replacing allyl ester with DMA ester (13a), an 83/17 tripeptide to DKP ratio was obtained. Intriguingly, the slightly more hindered Fmoc-Ala-BAL-AlaODMA sequence (13b) was found to afford the most DKP of all examples explored with a tripeptide to DKP ratio of 70/30. We were pleased to find that the ratio of desired tripeptide 13b to DKP could be increased to 83/17 by reducing the duration of piperidine treatment to 3 min at the dipeptide stage. While the purpose of this report was to test the limits of the ability of C-terminal DMA protection to resist DKP formation, we recommend shorter (2−3 min) piperidine treatments during Fmoc deprotection at the dipeptide stage. To obtain a direct comparison between tert-butyl and DMA, the Ala-BAL-Ala tert-butyl variant (13e) was also explored and found to afford a 94/6 tripeptide to DKP ratio. Although tert-butyl remains the most effective C-terminal protecting group for reducing DKP formation, it is not orthogonal to BAL and consequently not applicable to the problem in question. As was shown in our previous report,21 DMA esters can be cleanly removed by palladium(0) which has been well-established13−15 as an orthogonal deprotection strategy for use with the BAL. Furthermore, DMA esters are effective at resisting DKP formation and offer an alternative to the available methods for

Table 3. Synthesis of DMA Esters from Fmoc-Protected Amino Acids Using Optimized Conditionsa

AA

yield of 4 (%)b

γ/αc

Thr(tBu) (3a) Gly (3b) Ala (3c) Val (3d) Leu (3e) Tyr(tBu) (3f) Ser(tBu) (3g) Asn(Trt) (3h) Cys(Trt) (3i) Cys(PMB) (3j) Pro (3k) Glu(tBu) (3l) Lys(Boc) (3m) Arg(Pbf) (3n)

95 77 88 (93)d 84 (84)d 90 96 99 89 87 90 77 80 79 56 (70)e

99/1 95/5 97/3 (98/2)d 98/2 (97/3)d 93/7 98/2 96/4 96/4 96/4 96/4 92/8 90/10 94/6 93/7 (96/4)e

a

Reactions were carried out on the 0.25 mmol scale (unless otherwise noted) with 1.1 equiv of prenyl sulfonium salt (1), 1.1 equiv of Na2CO3, 10 mol % CuBr, and 3.1 mL (0.08 M) of dry CH2Cl2, 15− 24 h. bIsolated yield of product after filtration through silica pad or column chromatography. cγ/α product ratio was determined by 1H NMR integration. dReaction carried out on the 2.0 mmol scale. e Results obtained using 3.3 equiv of 1.

The ortho backbone amide linker (o-BAL)30 was chosen for this study due to its ease of preparation from commercially available starting material. The synthesis of o-BAL is shown in Scheme 2 and follows a protocol similar to that originally described by Jensen and co-workers.30 Ortho-selective demethylation of 2,4,6-trimethoxybenzaldehyde (5) was accomplished using 1.0 M BBr3 in CH2Cl2 to afford o-phenol 6 in 86% yield. Cesium carbonate-promoted alkylation of 6 with commercially available ethyl 5-bromovalerate (7) furnished the corresponding ethyl ester 8 in 92% yield. Finally, saponification of 8 with 4.0 M aqueous NaOH−MeOH (1:1) provided o-PALdehyde (9) in 83% yield. Acid 9 was then loaded onto the solid support via HATU-mediated coupling with Sieber amide PS resin (10). After two rounds of coupling, the remaining free amino groups were capped with Ac2O− pyridine (3:2) to afford “dual-linker” 11 (Scheme 3). Linker 11 was used to synthesize a series of tripeptides (Table 4) by the sequence of steps outlined in Scheme 4. In each instance, the tripeptide−DKP ratio was determined by integration of UPLC-MS chromatograms at 214 and 254 nm. Despite many attempts,31 the isolation and separation of Nterminal deprotected amino acid DMA esters from dibenzo-

Figure 3. “Dual-linker” strategy showing compatible dilute acidic cleavage of Sieber amide linkage. 7765

DOI: 10.1021/acs.joc.8b00658 J. Org. Chem. 2018, 83, 7762−7770

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The Journal of Organic Chemistry Scheme 2. Synthesis of o-PALdehyde 930

Scheme 3. Synthesis of Solid-Supported “Dual-Linker” 11

avoiding this undesired byproduct during BAL-based SPPS.13−15

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CONCLUSION In summary, we have developed a robust preparation of DMAprotected amino acids that is tolerant to changes in scale and compatible with a wide range of Fmoc-protected amino acids. These N- and C-terminal-protected amino acids can be directly loaded onto the solid support by a two-step Fmoc deprotection−reductive amination sequence with resin-bound PALdehyde. C-Terminal DMA protection introduces a second semipermanent protecting group that is orthogonal to Fmocbased SPPS, compatible with the backbone amide linker and resistant to DKP formation.

Table 4. Tripeptide/DKP Ratio during Synthesis of BALLinked DMA-Protected Tripeptides tripeptide

AA1

AA2

C-terminal PG

tripeptide/DKPa,b

13a 13b 13c 13d 13ed

Gly Ala Leu Ala Ala

Ala Ala Ala Leu Ala

DMA DMA DMA DMA tBud

83/17 70/30 (83/17)c 93/7 82/18 94/6

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EXPERIMENTAL SECTION

General Information. Solvents were purified and dried on activated alumina columns prior to use. Reactions where anhydrous conditions are noted were performed in glassware that was flamedried under vacuum. Copper(I) bromide was purchased from SigmaAldrich and purified by recrystallization from hydrobromic acid and water. Carbonate salts were flame-dried under vacuum prior to use. 1 H NMR and 13C NMR spectra were recorded on Bruker DRX-500, Bruker AV-III-400, and Varian Mercury-300 spectrometers in CDCl3. Chemical shift (δ values) are reported in parts per million (ppm) and coupling constants (J values) are reported in hertz (Hz). Infrared spectra were recorded on a Thermo Nicolet Nexus FTIR using an

a

Product ratios are relative, were determined by UPLC integration at 214 nm, and were cross-compared with those at 254 nm. bDMA ester was partially labile to Sieber amide cleavage conditions, and the combination of these two products are represented in tripeptide values. cTripeptide/DKP ratio obtained after 3 min piperidine−DMF Fmoc deprotection at dipeptide stage. dStarting dipeptide was prepared using commercially available H-Ala-OtBu·HCl.

Scheme 4. Synthesis of BAL-Linked, DMA-Protected Tripeptides 13a−d

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DOI: 10.1021/acs.joc.8b00658 J. Org. Chem. 2018, 83, 7762−7770

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The Journal of Organic Chemistry

(dd, J = 17.5, 10.9 Hz, 1H), 5.21 (d, J = 17.5 Hz, 1H), 5.17 (d, J = 8.7 Hz, 1H), 5.11 (d, J = 10.9 Hz, 1H), 4.39 (d, J = 7.2 Hz, 2H), 4.33 (td, J = 9.0, 5.3 Hz, 1H), 4.23 (t, J = 7.1 Hz, 1H), 1.79−1.67 (m, 1H), 1.69−1.60 (m, 1H), 1.55 (s, 7H), 0.97 (dd, J = 6.6, 1.9 Hz, 6H); 13C NMR (125 MHz, CDCl3) δ 172.0, 155.9, 144.0, 143.8*, 141.9, 141.3, 127.7, 127.0, 125.1, 120.0, 113.2, 82.1, 66.9, 52.9, 47.2, 42.04, 26.4, 26.3*, 24.8, 22.9, 22.0*; IR (neat) 3334, 2957, 2158, 1715, 1525, 1468, 1450, 1413, 1381, 1366, 1334, 1260, 1221, 1162, 1119, 1046, 989, 927, 848; HRMS (nESI-TOF) m/z: [M + Na]+ Calcd for C26H31NO4Na 444.2151, found 444.2165. Fmoc-Tyr(tBu) Dimethylallyl Ester (4f).22 Yield 96% (126 mg), colorless glassy solid; Rf = 0.35 (20% EA/Hex); γ/α: (98/2). Fmoc-Ser(tBu) Dimethylallyl Ester (4g). Yield 99% (112 mg, corrected), glass; Rf = 0.38 (20% EtOAc/hexanes); γ/α: (96/4); 1H NMR (500 MHz, CDCl3) δ 7.77 (d, J = 7.6 Hz, 2H), 7.63 (dd, J = 7.6, 4.2 Hz, 2H), 7.40 (t, J = 7.5 Hz, 2H), 7.32 (t, J = 7.5 Hz, 2H), 6.08 (dd, J = 17.5, 10.9 Hz, 1H), 5.65 (d, J = 8.8 Hz, 1H), 5.22 (d, J = 17.5 Hz, 1H), 5.10 (d, J = 10.9 Hz, 1H), 4.43 (dt, J = 9.0, 3.0 Hz, 1H), 4.42−4.34 (m, 2H), 4.25 (t, J = 7.3 Hz, 1H), 3.84 (dd, J = 8.8, 2.9 Hz, 1H), 3.60 (dd, J = 8.7, 3.0 Hz, 1H), 1.56 (d, J = 2.7 Hz, 6H), 1.18 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 169.3, 156.1, 144.0, 143.9*, 142.0, 141.3, 127.7, 127.1, 125.2, 119.9, 113.0, 82.2, 73.2, 67.1, 62.4, 54.9, 47.2, 27.4, 26.5, 26.4*; IR (neat) 3444, 2975, 2159, 1724, 1505, 1477, 1450, 1392, 1381, 1364, 1339, 1296, 1234, 1197, 1163, 1127, 1102, 1083, 1059, 1022, 987, 922, 879, 850; HRMS (nESI-TOF) m/z: [M + Na]+ Calcd for C27H33NO5Na 474.2256, found 474.2280. Fmoc-Asn(Trt) Dimethylallyl Ester (4h). Yield 89% (147 mg, corrected), white glassy solid; Rf = 0.38 (30% EtOAc/hexanes); γ/α: (96/4); 1H NMR (500 MHz, CDCl3) δ 7.77 (d, J = 7.6 Hz, 2H), 7.60 (d, J = 7.5 Hz, 2H), 7.41 (t, J = 7.5 Hz, 2H), 7.35−7.22 (m, 13H), 7.21−7.15 (m, 6H), 6.67 (s, 1H), 6.12 (d, J = 8.7 Hz, 1H), 5.95 (dd, J = 17.5, 10.9 Hz, 1H), 5.14 (d, J = 17.4 Hz, 1H), 5.02 (dd, J = 10.9, 0.8 Hz, 1H), 4.53 (dt, J = 8.9, 4.3 Hz, 1H), 4.41 (dd, J = 10.4, 7.1 Hz, 1H), 4.28 (dd, J = 10.4, 7.5 Hz, 1H), 4.21 (t, J = 7.2 Hz, 1H), 3.09 (dd, J = 16.1, 4.4 Hz, 1H), 2.84 (dd, J = 16.1, 4.2 Hz, 1H), 1.45 (d, J = 9.1 Hz, 6H); 13C NMR (125 MHz, CDCl3) δ 169.6, 169.2, 156.3, 144.3, 144.0, 143.8*, 142.1, 141.3, 128.7, 128.0, 127.7, 127.2, 127.1, 125.3, 125.2*, 119.9, 112.9, 82.6, 70.9, 67.2, 51.2, 47.1, 38.6, 26.3, 26.1*; IR (neat) 3328, 3058, 1728, 1491, 1448, 1413, 1380, 1365, 1331, 1215, 1157, 1125, 1081, 1035, 1002, 934, 903, 848; HRMS (nESI-TOF) m/z: [M + Na]+ Calcd for C43H40N2O5Na 687.2835, found 687.2879. Fmoc-Cys(Trt) Dimethylallyl Ester (4i). Yield 87% (143 mg), colorless, glassy solid; Rf = 0.32 (15% EtOAc/hexanes); γ/α: (96/4); 1 H NMR (500 MHz, CDCl3) δ 7.78 (dd, J = 7.7, 3.5 Hz, 2H), 7.62 (d, J = 7.5 Hz, 2H), 7.44−7.38 (m, 8H), 7.34−7.27 (m, 8H), 7.24− 7.18 (m, 3H), 6.04 (dd, J = 17.5, 10.9 Hz, 1H), 5.31 (d, J = 8.5 Hz, 1H), 5.17 (d, J = 17.5 Hz, 1H), 5.09 (d, J = 10.9 Hz, 1H), 4.36 (d, J = 7.4 Hz, 2H), 4.32 (dt, J = 9.5, 5.2 Hz, 1H), 4.24 (t, J = 7.3 Hz, 1H), 2.65 (dd, J = 12.2, 6.0 Hz, 1H), 2.58 (dd, J = 12.2, 4.6 Hz, 1H), 1.52 (s, 6H); 13C NMR (125 MHz, CDCl3) δ 169.2, 155.5, 144.3, 143.9, 143.8*, 141.7, 141.3, 129.5, 128.0, 127.7, 127.1, 126.8, 125.2, 120.0, 113.3, 82.9, 67.1, 66.8, 53.3, 47.1, 34.4, 26.3; IR (neat,) 3409, 3059, 2162, 1980, 1723, 1596, 1492, 1447, 1413, 1380, 1365, 1342, 1207, 1157, 1123, 1081, 1049, 1001, 927, 846; HRMS (nESI-TOF) m/z: [M + Na]+ Calcd for C42H39NO4SNa 676.2498, found 676.2522. Fmoc-Cys(PMB) Dimethylallyl Ester (4j). Yield 90% (120 mg, corrected), colorless glass; Rf = 0.36 (20% EtOAc/hexanes); γ/α: (96/4); 1H NMR (500 MHz, CDCl3) δ 7.77 (dd, J = 7.6, 2.6 Hz, 2H), 7.62 (dd, J = 7.6, 4.0 Hz, 2H), 7.40 (td, J = 7.5, 3.5 Hz, 2H), 7.31 (tdd, J = 7.5, 2.7, 1.1 Hz, 2H), 7.22 (d, J = 8.6 Hz, 2H), 6.84 (d, J = 8.6 Hz, 2H), 6.07 (dd, J = 17.5, 10.9 Hz, 1H), 5.54 (d, J = 7.9 Hz, 1H), 5.20 (d, J = 17.5 Hz, 1H), 5.11 (d, J = 10.9 Hz, 1H), 4.52 (dt, J = 7.9, 5.2 Hz, 1H), 4.45−4.35 (m, 2H), 4.24 (t, J = 7.2 Hz, 1H), 3.77 (s, 3H), 3.70 (s, 2H), 2.92 (dd, J = 13.8, 4.7 Hz, 1H), 2.83 (dd, J = 13.8, 5.8 Hz, 1H), 1.55 (s, 6H); 13C NMR (125 MHz, CDCl3) δ 169.5, 158.8, 155.7, 143.9, 143.8*, 141.6, 141.3, 130.1, 129.6, 127.7, 127.1, 125.1, 120.0, 114.0, 113.5, 83.0, 67.2, 55.3, 54.0, 47.1, 36.2, 33.7, 26.3; IR (neat) 3335, 2934, 2159, 1720, 1610, 1584, 1511, 1465,

ATR attachment. Thin-layer chromatography (TLC) was performed using EMD Millipore 60 Å silica gel-coated, glass-backed plates with F254 indicator. The silica gel used for flash chromatography was SiliaFlash silica 230−400 mesh, 60 Å particle size. Melting points were measured using a Thomas−Hoover melting point apparatus and are uncorrected. UPLC-MS was performed on a Waters Acquity UPLC in conjunction with a Waters single quadrupole mass spectrometer using electrospray ionization (ESI) and a photodiode array detector (measuring at 214 and 254 nm). A UPLC BEH C18 reverse phase, 1.7 μm, 2.1 × 50 mm column was used for separation. Low resolution mass spectrometry (MS) was recorded on an Advion Expression CMS single quadrupole mass spectrometer, using a TLC plate reader and electrospray ionization (ESI). High resolution mass spectra (HRMS) were recorded on a Sciex TripleTOF 5600 quadrupole time-of-flight (TOF) mass spectrometer using nanoelectrospray ionization (nESI) and was performed independently by the McLuckey laboratory at Purdue University. Prenyldimethylsulfonium Tetrafluoroborate (1).23 A solution of 3-methyl-2-buten-1-ol (2.4 mL, 23 mmol) and dimethyl sulfide (5.2 mL, 71 mmol) in anhydrous CH2Cl2 (23 mL) was cooled in an ice/ salt water bath (−10 °C) for 10 min. To this solution was added HBF4 (55 wt % in diethyl ether, 3.1 mL, 23 mmol) dropwise under a N2 atmosphere. The reaction mixture was allowed to slowly warm to room temperature and stirred for a total of 23 h, after which volatiles were removed under reduced pressure. The resultant colorless oil was suspended in Et2O and cooled to 0 °C for 15 min to afford a colorless precipitate. Solids were vacuum filtered and washed with excess Et2O to afford 1 (4.76 g, 94%) as a colorless, crystalline solid. mp 64−66 °C. General Preparation of 1,1-Dimethylallyl Esters (4a−n). A mixture of carboxylic acid (0.250 mmol) and Na2CO3 (29.2 mg, 0.275 mmol) was taken up in dry CH2Cl2 (3.1 mL) and stirred for 5 min. To the resulting mixture was added CuBr (3.6 mg, 0.025 mmol) followed by 1 (60.0 mg, 0.275 mmol) in portions at rt. The mixture was stirred at rt, under an inert atmosphere. Following reaction completion (15−24 h), solids were removed by filtration and the filtrate was evaporated under reduced pressure. The crude residue was purified by short SiO2 pad filtration (15−30% EtOAc/hexanes). Solvent was removed under reduced pressure, and the resulting residue was lyophilized twice to remove residual solvent and afford pure 1,1-dimethylallyl esters. Note: Compounds which afforded byproducts (4i, 4j, 4l, and 4n) were purified by flash column chromatography in 20−60% EtOAc/hexanes. *Corrected weight values factor trace solvent impurities trapped in sample and were calculated via 1H NMR integration. Fmoc-Thr(tBu) Dimethylallyl Ester (4a). Yield 95% (111 mg, corrected), colorless glass; Rf = 0.37 (15% EA/Hex); γ/α: (99/1); 1H NMR (500 MHz, CDCl3) δ 7.77 (d, J = 7.5 Hz, 2H), 7.67−7.61 (m, 2H), 7.40 (tt, J = 7.5, 1.5 Hz, 2H), 7.32 (tt, J = 7.5, 1.5 Hz, 2H), 6.09 (dd, J = 17.5, 10.9 Hz, 1H), 5.54 (d, J = 9.5 Hz, 1H), 5.25−5.17 (m, 1H), 5.09 (dd, J = 10.9, 0.8 Hz, 1H), 4.44−4.33 (m, 2H), 4.30−4.18 (m, 2H), 4.16 (dd, J = 9.5, 2.3 Hz, 1H), 1.55 (s, 6H), 1.22 (d, J = 6.2 Hz, 3H), 1.19 (s, 9H); 13C NMR (125 MHz, CDCl3) δ 169.8, 156.7, 144.0, 143.9*, 142.0, 141.3, 127.7, 127.1, 125.2, 119.9, 113.2, 82.2, 73.9, 67.3, 67.1, 60.3, 47.2, 28.8, 26.4, 26.3*, 20.80 ppm; IR (neat) 3440, 2977, 2163, 1980, 1725, 1503, 1450, 1379, 1364, 1345, 1310, 1278, 1199, 1124, 1092, 1068, 986, 951, 920, 849; HRMS (nESITOF) m/Z: [M + Na]+ Calcd for C28H35NO5Na 488.2413, found 488.2414. Fmoc-Gly Dimethylallyl Ester (4b).22 Yield 77% (71 mg), colorless solid; Rf = 0.41 (30% EtOAc/hexanes); mp 49−53 °C; γ/α: (95/5). Fmoc-Ala Dimethylallyl Ester (4c).22 Yield 88% (84 mg), colorless solid; Rf = 0.33 (20% EtOAc/hexanes); mp 76−77 °C; γ/α: (97/3). Fmoc-Val Dimethylallyl Ester (4d).22 Yield 84% (86 mg, corrected), colorless glass; Rf = 0.37 (15% EtOAac/hexanes); γ/α: (98/2). Fmoc-Leu Dimethylallyl Ester (4e). Yield 90% (95 mg, corrected), colorless glass; Rf = 0.34 (15% EtOAc/hexanes); γ/α: (93/7); 1H NMR (500 MHz, CDCl3) δ 7.77 (d, J = 7.5 Hz, 2H), 7.61 (d, J = 7.6 Hz, 2H), 7.40 (t, J = 7.5 Hz, 2H), 7.32 (td, J = 7.5, 1.1 Hz, 2H), 6.07 7767

DOI: 10.1021/acs.joc.8b00658 J. Org. Chem. 2018, 83, 7762−7770

Article

The Journal of Organic Chemistry

Cleavage Condition A (Sieber amide cleavage). An aliquot (∼1 mg) of resin-bound peptide was treated with 200 μL of TFA/H2O/ CH2Cl2 (2:1:97) and the mixture agitated for 5 min at rt. Following this time period, volatiles were removed by N2 stream. The resulting residue was dissolved in 200 μL MeOH-H2O (1:1), filtered and analyzed by UPLC-MS. Note: Product/DKP ratio was determined by integrating 214 and 254 nm peaks which are reported as relative percentages. Standard Wash Protocol. DMF (3 × 1 mL), CH2Cl2 (3 × 1 mL), MeOH (3 × 1 mL) then CH2Cl2 (3 × 1 mL). o-PALdehyde-Sieber-PS “Dual-Linker” (11). Fmoc-Sieber amide resin (10) (200 mg, 0.7 mmol/g, 1% DVB) was swelled in DMF for 30 min, drained, and washed with piperidine−DMF (1:4, 1 × 1 mL, 1 × 1.5 mL 30 min). The mixture was drained then washed with the standard wash protocol, affording a positive Kaiser ninhydrin test.34 The resin was then washed with DMF (3 × 1 mL), and to a mixture of o-PALdehyde (9) (158 mg, 0.560 mmol) and HATU (213 mg, 0.560 mmol) in DMF (1.4 mL) was added DIEA (200 μL, 1.12 mmol). This mixture added to the resin after a 5 min preactivation period. After 2 h, resin was washed by the standard wash protocol and coupling repeated. After the coupling was repeated for a second time, the resin afforded a slightly positive Kaiser ninhydrin test.34 Remaining free amino groups were capped by agitating resin with 1.5 mL of Ac2O−pyridine (3:2) mixture for 30 min, after which the Kaiser ninhydrin test34 was negative. The resin was washed using the standard wash protocol, washed with MeOH, and dried under reduced pressure. General Procedure for SPPS of DMA-Protected Tripeptides (13a−d). To a solution of Fmoc-AA1-ODMA (0.14 mmol) in CH3CN (0.28 mL) were added 1-octanethiol (120 μL, 0.70 mmol) and then diethylamine (140 μL, 1.4 mmol) dropwise. The reaction was stirred at rt for 2 h, at which point volatiles were removed under reduced pressure. The crude residue was coevaporated with CH3CN (3 × 5 mL) to afford crude free amines of DMA esters that were used without further purification. NaBH(OAc)3 (29.7 mg, 0.140 mmol) was added to the crude amino DMA esters and the mixture taken up in AcOH−DMF (1% solution, 0.3 mL). The resultant mixture was agitated to promote solubility and added directly to o-PALdehyde-Sieber-PS resin (11) (20.0 mg, 0.7 mmol/g, 0.014 mmol) that was prewashed with DMF. The reaction mixture was agitated for 18.5 h, after which the solvent was removed and the resin washed with water, followed by the standard wash protocol, and then DMF (3 × 1 mL). The reaction was agitated with piperidine−DMF (1:4, 1.5 mL, 5 min) and washed once more using the standard wash protocol to give a positive chloranil test,35 affording H-(o-BAL-Sieber-PS)-AA1-ODMA. A separate flame-dried flask was charged with Fmoc-AA2-OH (0.14 mmol) and dry CH2Cl2 (0.14 mL), and symmetric anhydride formation was initiated by addition of DIPCDI (11 μL, 0.07 mmol) dropwise at rt. The mixture was allowed to stand for 15 min, and the resulting solid was solubilized by addition of 0.14 mL of DMF. This solution was added directly to the resin and the reaction agitated for 2 h followed by the standard wash protocol. This procedure was repeated twice to afford Fmoc-AA2-(o-BAL-Sieber-PS)-AA1-ODMA, 12a−d (negative chloranil test).35 The N-terminal Fmoc protecting group was removed from the resin-bound peptide by agitation with piperidine−DMF (1:4; 1 × 5 min, 1 × 10 min) followed by the standard wash protocol to afford a positive Kaiser ninhydrin test.34 Acylation of the resulting amine intermediate was achieved by treatment with a preformed symmetric anhydride of Fmoc-Glu(tBu)-OH (5 equiv), prepared using the procedure described above, to afford Fmoc-Glu(tBu)-AA2-(o-BALSieber-PS)-AA1-ODMA. The N-terminal Fmoc group was removed by agitation with piperidine−DMF (1:4; 1 × 5 min, 1 × 20 min) to afford H-Glu(tBu)-AA2-(o-BAL-Sieber-PS)-AA1-ODMA and a positive Kaiser ninhydrin test.34 Resin cleavage was achieved using cleavage condition A to afford 13a−d, and product ratio was determined by UPLC-MS. H-Glu(tBu)-Ala-(o-BAL-NH2)-Gly-ODMA (13a). UPLC-MS (5 → 95, CH3CN-H2O, 10 min gradient): Rt 3.15 min, 17% DKP, MS

1450, 1414, 1380, 1342, 1302, 1244, 1209, 1176, 1158, 1123, 1034, 996, 926, 832; HRMS (nESI-TOF) m/z: [M + Na]+ Calcd for C31H33NO5SNa 554.1977, found 554.1992. Fmoc-Pro Dimethylallyl Ester (4k).22 Yield 77% (78 mg, corrected), pale yellow glass; Rf = 0.31 (25% EtOAc/hexanes); γ/α: (92/8). Fmoc-Glu(tBu) Dimethylallyl Ester (4l).22 Yield 80% (100 mg), colorless glass; Rf = 0.34 (20% EtOAc/hexanes); γ/α: (90/10). Fmoc-Lys(Boc) Dimethylallyl Ester (4m). Yield 79% (106 mg), colorless, amorphous solid; Rf = 0.37 (30% EtOAc/hexanes); mp 42− 43 °C; γ/α: (94/6); 1H NMR (500 MHz, CDCl30) δ 7.77 (d, J = 7.5 Hz, 2H), 7.60 (d, J = 7.5 Hz, 2H), 7.40 (t, J = 7.5 Hz, 2H), 7.32 (tt, J = 7.4, 1.3 Hz, 2H), 6.07 (dd, J = 17.5, 10.9 Hz, 1H), 5.37 (d, J = 8.4 Hz, 1H), 5.21 (d, J = 17.5 Hz, 1H), 5.11 (d, J = 10.9 Hz, 1H), 4.56 (s, 1H), 4.44−4.34 (m, 2H), 4.29 (td, J = 7.9, 5.0 Hz, 1H), 4.22 (t, J = 7.0 Hz, 1H), 3.12 (d, J = 7.9 Hz, 2H), 1.91−1.80 (m, 1H), 1.73−1.65 (m, 1H), 1.64 (d, J = 2.0 Hz, 1H), 1.54 (s, 6H), 1.54−1.48 (m, 2H), 1.43 (s, 9H), 1.43−1.30 (m, 1H); 13C NMR (125 MHz, CDCl3) δ 171.2, 156.0, 155.9, 143.9, 143.8*, 141.8, 141.3, 127.7, 127.1, 125.1, 120.0, 113.4, 82.4, 79.2, 67.0, 54.0, 47.2, 40.2, 32.4, 29.6, 28.4, 26.4, 26.3, 22.3*; IR (neat) 3339, 2977, 2934, 2161, 1694, 1514, 1479, 1450, 1381, 1365, 1248, 1165, 1126, 1082, 1043, 928, 853; HRMS (nESI-TOF) m/z: [M + Na]+ Calcd for C31H40N2O6Na 559.2784, found 559.2801. Fmoc-Arg(Pbf) Dimethylallyl Ester (4n).22 The general procedure for the synthesis of 1,1-dimethylallyl esters was followed with the use of Fmoc-Arg(Pbf)-OH (162 mg, 0.250 mmol), Na2CO3 (29.2 mg, 0.275 mmol), CuBr (3.6 mg, 0.025 mmol), and sulfonium salt 1 (180 mg, 0.83 mmol). Flash column chromatography with 60% EtOAc/ hexanes afforded 4n as a colorless glassy solid (126 mg, 70%). Rf = 0.43 (60% EtOAc/hexanes); γ/α: (96/4). 2-Hydroxy-4,6-dimethoxybenzaldehyde30 (6). A solution of 2,4,6trimethoxybenzaldehyde (5) (2.00 g, 10.2 mmol) in dry CH2Cl2 (51 mL) was cooled in an ice bath under an inert atmosphere for 10 min. To this mixture was added BBr3 (1.0 M in CH2Cl2, 10.2 mL, 10.2 mmol) dropwise. The resulting deep red solution was stirred at 0 °C for 1 h and then warmed to rt and stirred until reaction completion was shown by TLC (2 h). H2O (51 mL) was slowly added, and the resulting mixture stirred for an additional 30 min. The organic layer was removed, and the aqueous layer was extracted with CH2Cl2 (3 × 50 mL). The combined organic extracts were washed with brine (100 mL), dried over Na2SO4, filtered, and evaporated under reduced pressure. Colored impurities were removed by short SiO2 pad filtration (30% EtOAc/hexanes) to afford 6 as an off-white solid (1.61 g, 86%). Rf = 0.38 (25% EtOAc/hexanes); mp 68−70 °C (lit.32 63− 66 °C). Ethyl 5-(2-Formyl-3,5-dimethoxyphenoxy)pentanoate (8).30 Cs2CO3 (4.20 g, 12.8 mmol) was added to a solution of 6 (1.17 g, 6.42 mmol) in dry DMF (4.3 mL) at rt. Ethyl 5-bromovalerate (2.00 mL, 12.8 mmol) was added dropwise, and this mixture was heated to 60 °C until reaction completion was shown by TLC (4 h). The reaction was allowed to cool to rt and then diluted with H2O (8 mL). The mixture was extracted with CH2Cl2 (4 × 15 mL), and the combined organic extracts were washed with brine (5 × 50 mL), dried over Na2SO4, filtered, and evaporated under reduced pressure. Purification by flash chromatography (60% EA/Hex) afforded 8 as a pale yellow solid (1.84 g, 92%). Rf = 0.35 (60% EA/Hex); mp 44− 45 °C (lit.30 44−45 °C). 5-(2-Formyl-3,5-dimethoxyphenoxy)pentanoic Acid (o-PALdehyde) (9)..30,33 To a solution of 8 (1.74 g, 5.61 mmol) in MeOH (11.2 mL) was added 4.0 M NaOH (11.2 mL, 44.9 mmol) dropwise, and the resulting mixture was stirred for 1.5 h at room temperature. MeOH was removed under reduced pressure and the resulting aqueous phase extracted with EtOAc (2 × 15 mL) and then acidified to pH ∼ 1 with concentrated HCl. The acidified aqueous phase was extracted with EtOAc (2 × 50 mL), and the combined organic extracts were washed with brine (2 × 100 mL), dried over Na2SO4, filtered, and evaporated under reduced pressure to afford 9 as a yellow solid (1.31 g, 83%). mp 102−102.5 °C (lit.33 103−104 °C). 7768

DOI: 10.1021/acs.joc.8b00658 J. Org. Chem. 2018, 83, 7762−7770

Article

The Journal of Organic Chemistry (ESI) calcd for C19H27N3O6: 393.2, found 394.4 [M + H]+; Rt 3.58 min, 61% DMA deprotected product, MS (ESI) calcd for C28H44N4O10: 596.3, found 597.6 [M + H]+; Rt 4.82 min, 22% DMA esterified product, MS (ESI) calcd for C33H52N4O10: 664.4, found 665.6 [M + H]+. 83% Desired product, 17% DKP. H-Glu(tBu)-Ala-(o-BAL-NH2)-Ala-ODMA (13b). UPLC-MS (25 → 40, CH3CN-H2O, 5 min gradient): Rt 1.02 min, 30% DKP, MS (ESI) calcd for C20H29N3O6: 407.2, found 408.3 [M + H]+; Rt 1.42 and 1.67 min, 50% (total) DMA deprotected product, MS (ESI) calcd for C29H46N4O10: 610.3, found 611.4 [M + H]+; Rt 4.42 and 4.60 min, 20% (total) DMA esterified product, MS (ESI) calcd for C34H54N4O10: 678.4, found 679.4 [M + H]+. 70% desired product, 30% DKP. Note: an 83/17 tripeptide to DKP ratio was obtained from short (3 min) 20% piperidine−DMF Fmoc deprotection at dipeptide stage. This deprotection was followed by a brief washing cycle (3 × DMF, 3 × CH2Cl2, 1 × MeOH, 1 × CH2Cl2, 1 × DMF), and the remaining procedure for tripeptide formation and analysis was repeated as described in the general procedure above (Supporting Information Figure S35). H-Glu(tBu)-Ala-(o-BAL-NH2)-Leu-ODMA (13c). UPLC-MS (30 iso 1 min → 50, CH3CN−H2O, 5 min gradient): Rt 1.85 min, 7% DKP, MS (ESI) calcd for C23H35N3O6: 449.3, found 450.5 [M + H]+; Rt 2.00 and 2.25 min, 56% (total) DMA deprotected product, MS (ESI) calcd for C32H52N4O10: 652.4, found 653.6 [M + H]+; Rt 4.53, 37% DMA esterified product, MS (ESI) calcd for C37H60N4O10: 720.4, found 721.6 [M + H]+. 93% desired product, 7% DKP. H-Glu(tBu)-Leu-(o-BAL-NH2)-Ala-ODMA (13d). UPLC-MS (25 iso 1 min → 50, CH3CN−H2O, 5 min gradient): Rt 2.78 min, 18% DKP, MS (ESI) calcd for C23H35N3O6: 449.3, found 450.4 [M + H]+; Rt 2.95 and 3.25 min, 48.3% (total) DMA deprotected product, MS (ESI) calcd for C32H52N4O10: 652.4, found 653.5 [M + H]+; Rt 4.50, 4.65, 4.77, 4.87 min, 33.4% DMA esterified product, MS (ESI) calcd for C37H60N4O10: 720.4, found 721.5 [M + H]+. 82% desired product, 18% DKP. H-Glu(tBu)-Ala-(o-BAL-NH2)-Ala-OtBu (13e). A mixture of H-AlaOtBu × HCl (25.4 mg, 0.14 mmol) and NaBH(OAc)3 (29.7 mg, 0.14 mmol) was taken up in 0.3 mL of 1% AcOH−DMF, and this mixture was added directly to o-PALdehyde-Sieber-PS resin (11) (20 mg, 0.7 mmol/g, 0.014 mmol). The reaction mixture was agitated for 18 h, after which the resin was filtered and washed with water, followed by the standard wash protocol, and then DMF (3 × 1 mL). The reaction was agitated with piperidine−DMF (1:4, 1.5 mL, 5 min) and then washed once more with the standard wash protocol to give a positive chloranil test35 and afford H-(o-BAL-Sieber-PS)-Ala-OtBu. The remaining synthetic steps were repeated as described in the general procedure above. UPLC-MS (15 → 95, CH3CN-H2O, 5 min gradient): Rt 1.82 min, 6% DKP, MS (ESI) calcd for C20H29N3O6: 407.2, found 408.4 [M + H]+. Rt 2.50 min, 94% desired product, MS (ESI) calcd for C33H54N4O10: 666.4, found 667.7 [M + H]+. 94% desired product, 6% DKP.

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Mark A. Lipton: 0000-0003-0034-0243 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank David Foreman of the McLuckey Laboratory at Purdue for providing HRMS data for this project.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b00658. 1 H NMR stability studies for compound 1, 1H and 13C NMR spectra for DMA esters 4a−n, 1H NMR spectra for compounds 6, 8, and 9, and UPLC-MS chromatograms for compounds 13a−e. Table S1: The effect of sulfonium salt (1) age on the yield of DMA esterification (PDF)

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*E-mail: [email protected]. ORCID

Matthew A. Hostetler: 0000-0002-3189-6618 7769

DOI: 10.1021/acs.joc.8b00658 J. Org. Chem. 2018, 83, 7762−7770

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DOI: 10.1021/acs.joc.8b00658 J. Org. Chem. 2018, 83, 7762−7770