University of Groningen

Total syntheses of (–)-Borrelidin and (–)-Doliculide and the development of the catalyticasymmetric addition of Grignard reagents to ketonesMadduri Venkata, Ashoka Vardhan Reddy

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Chapter 8

Iridium-Catalyzed Chemoselective Epimerization of Amines and Alcohols

In this chapter a novel method for the selective epimerization of 1,2-amino-alcohols using half-sandwich cationic iridacycles is described. Furthermore, these catalysts are applied in the epimerization of hydroxy and amino groups in complex molecules.*

Parts of this chapter is submitted for publication: Madduri, A. V. R.; Jerphagnon, T.; Minnaard, A. J.; Feringa, B. L.

* The experiments described in this chapter were carried out jointly by Dr. T. Jerphagnon and me.

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8.1 Introduction

Chiral 1,2-amino-alcohols are very versatile building blocks in the synthesis of natural products1-3 and bioactive compounds such as protease inhibitors.4-8 Their structures represent prominent pharmacophores and are present in numerous precursors for the synthesis of amino acids and amino sugars.9-17 In addition, 1,2-amino alcohols function as chiral auxiliaries2,18-20 and ligands for transition metal catalysis.21-25 Stereocontrolled synthesis of 1,2-amino alcohols continues to be of considerable importance.26-32 The most frequently applied synthetic routes to these molecules involve functional group interconversions,1,3,33,34 such as reduction of amino acids35,36 or amino ketones,37-39 hydroboration of enamines,40,41 aminohydroxylation of olefins,42-44 nucleophilic additions to epoxides45-49 or aziridines3,50-52 and nucleophilic substitution of diol surrogates.53-55 However, most of these approaches involve multiple synthetic steps and provide access to either syn or anti amino alcohols exclusively. In view of their synthetic utility, both amino alcohol isomers are equally important and hence direct catalytic interconversion of 1,2-anti-amino alcohols to 1,2-syn-amino alcohols is of high practical significance. An important extension of such catalytic methodology would be the conversion of natural products, wherein epimers and analogues thereof, may be easily obtained via such transformation. Although Mitsunobu56-58 type reactions in principle provide a similar outcome, their practicality is often severely limited.

Recently, we developed the efficient racemization of chiral alcohols and amines by employing novel cationic iridium(III) half-sandwich catalysts.59 This complex is among the fastest racemization catalysts known.60-67 These iridacycle catalysts Ir1 & Ir2 were obtained from one equivalent of [IrCp*Cl2]2, and two equivalents of N-methyl benzylamine and 2-phenyl-2-imidazoline respectively, in the presence of sodium hydroxide and potassium hexafluorophosphate in acetonitrile (Scheme 1).68-70

195

Iridium catalyzed epimerization of 1,2-amino-alcohols

Scheme 1: Synthesis of the iridacycle racemization catalysts

It has been shown, that after activation with potassium tert-butoxide, iridacycle Ir1 showed high efficiency in the racemization of enantiopure secondary alcohols (Scheme 2).59

Scheme 2: Racemization of alcohols with iridacycle [Ir1]59

In combination with a double mutant of haloalcohol dehalogenase HheC (C153S W249F), in a biphasic system, activated iridacycle catalyst Ir1 allowed the chemo-enzymatic dynamic kinetic resolution of racemic -haloalcohols to enantioenriched epoxides (Scheme 3).71

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Scheme 3: Chemoezymatic DKR of -chloroalcohols to enantiopure epoxides (mutations Cys153Ser, which increases the enzyme’s stability toward oxidation and Trp249Phe, which increases its enantioselectivity especially for aromatic substrates) BSA = Bovine serum albumin HEPES = (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid) Subsequently, the remarkable observation was made that base-activated Ir1 did not racemize secondary amines, but non-activated Ir1 did so, being unable to racemize secondary alcohols! The more efficient catalyst for amine racemization Ir2, possessing a 2-phenylimidazolidine ligand, racemized (S)-N, -dimethylbenzylamine considerably faster than Ir1, and racemate was obtained in 40 min (Scheme 4).59 Furthermore it was shown that the iridacycles are faster racemization catalysts than the corresponding ruthenacycles.59

Scheme 4: Racemization of amines with iridacycle [Ir2]

We envisioned the use of this observed mutual chemoselectivity for the racemization of alcohols and amines in the direct epimerization of 1,2-amino alcohols (Scheme 5). In addition, we took the chance to explore the racemization of alcohols in more complex molecules containing other functional groups.

197

Iridium catalyzed epimerization of 1,2-amino-alcohols

Scheme 5: Iridacycle-catalyzed epimerization of 1,2-aminoalcohols

8.2 Results and Discussion

We started with attempts to epimerize chiral amino-alcohols using anti-(1R,2S)-ephedrine 1 (Figure 1) as a substrate in the presence of 5 mol% of Ir1 and 5.5 mol% of KOtBu in toluene at room temperature (Figure 1, Table 1). After 4 h, a 1:1 diastereomeric ratio of anti-(1R,2S)-ephedrine 1 and syn-(1S,2S)-pseudoephedrine 3 was observed by 1H-NMR and chiral GC. It is noteworthy that under these reaction conditions no epimerization of the amine moiety took place. After 16h, complete conversion to syn-(1S,2S)-pseudoephedrine 3 was obtained (Table 1, Entry 2). A lower activity was obtained with anti-(1R,2S)-norephedrine 2 providing syn-(1S,2S)-pseudonorephedrine 4 in a d.r. of 1:0.23 after 3 h which might be due to the presence of the primary amine; a feature previously observed in the case of racemization of chiral amines. Complete epimerization (d.r.=1:1) was obtained after 16 h (Table 1, Entry 3 and 4). Under the same reaction conditions, no reaction was observed with syn-(1R,2R)-ephedrine 5 (Table 1, Entry 5).

Figure 1: Structures of chiral amino-alcohols

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Table 1: Epimerization of chiral alcohols in amino-alcohols (Scheme 2, route (a))

Entry[a] Substrates Time (h) Products anti / syn[b,c] 1 1 4 1/3 1:1

2 1 16 3 0:1

3 2 3 2/4 1:0.23

4 2 16 2/4 1:1

5 5 16 5 0:1

6 6 16 6 1:0.1

7 7 16 7 1:0

8 8 16 8 1:0

9d 9 16 10 1:0

10d 10 16 10 1:0

(a) 0.25 mmol substrate, 5 mol% [Ir1], 5.5 mol% KOtBu, Toluene. (b) Determined by NMR. (c) Conversion measured by GC-MS, during the reaction only starting material or product is observed and no decomposition of the starting material or product is seen. (d) trans / cis ratio.

With anti-(1R,2S)-1-phenyl-2-(1-pyrrolidinyl)-1-propanol 6, 10% of epimerization to the syn-product was observed after 16 h (Entry 6, Table 1). No reaction occurred when anti-(1R,2S)-2-amino-1,2-diphenylethanol 7 or anti-(1S,2R)-2-(isopropylamino)-1,2-diphenylethanol 8 were used (Table 1, Entries 7 and 8). Starting from (1R,2S)-cis-1-amino-2-indanol 9, complete conversion to the trans-product 10 was found after 16 h whereas no reaction was observed with (1R,2R)-trans-1-amino-2-indanol 10 (Table 1, Entries 9 and 10).

For amine racemization, different reaction conditions are required compared to alcohol racemization. This implies no activation of the catalyst with KOtBu and a higher reaction temperature. Using 2 mol% of Ir2 in chlorobenzene at 100oC with anti-(1R,2S)-ephedrine 1 as substrate, epimerization of the amine moiety proceeds rapidly and a ratio anti/syn of 1:1 is obtained after 40 min (Table 2, Entry 1). The reaction rate decreased and after 16 h an anti/syn ratio of 1:1.5 was found (Table

199

Iridium catalyzed epimerization of 1,2-amino-alcohols

2, Entry 2). When substrates substituted with primary amines were used such as anti-(1R,2S)-norephedrine 2, anti-(1R,2S)-2-amino-1,2-diphenylethanol 7 or (1R,2S)-cis-1-amino-2-indanol 9, no reaction occurred (Table 2, Entries 3, 8 and 10). With anti-(1S,2R)-2-(isopropylamino)-1,2-diphenylethanol 8, the syn-product was formed in less than 5% after 16 h (Table 2, Entry 9). We were surprised to see that epimerization of tertiary cyclic amine of anti-(1R,2S)-1-phenyl-2-(1-pyrrolidinyl)-1-propanol 6 occurred very fast with a anti/syn ratio of 1:1 after 40 min starting from pure anti isomer (Table 2, Entry 5). After 16 h, no further change was detected compared to 3 h reaction time (Table 2, Entries 6 and 7).

Table 2: Epimerization of chiral amines in amino-alcohols (Scheme 2, route (b))

Entry[a] Substrates Time (h) Products anti / syn[b,c]

1 1 0.67 1/5 1:1

2 1 16 1/5 1:1.5

3 2 16 2 -

4 5 16 3 0:1

5 6 0.67 6 1:0.97

6 6 3 6 1:1.6

7 6 16 6 1:1.6

8 7 16 7 -

9 8 16 8 1:0.04

10 9 16 9 - (a) 0.25 mmol substrate, 2 mol% [Ir2], Phenyl chloride. (b) Determined by NMR (c) Conversion measured by GC-MS, during the reaction only starting material or product is observed and no decomposition of the starting material or product is seen. The natural alkaloid quinine 11, a natural alkaloid, was also employed under the epimerization conditions for the amine and alcohol moieties (Scheme 3). No reaction occurred for the epimerization of the amine using 2 mol% of Ir2 in

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chlorobenzene at 100oC, which is attributed to the rigid structure of the cyclic amine moiety of quinine 11. A mixture of isomers 11/12, with a diastereomeric ratio of 1:1 was obtained during the epimerization of the alcohol moiety using 10 mol% of Ir1 and 11 mol% of KOtBu in toluene at room temperature after 3 d. This catalytic transformation illustrates that a quinine epimer can be readily obtained.

Scheme 6: Iridacycle catalyzed epimerization alcohol moiety of quinine 11

Also multi-functional molecule 13, which is a building block in the synthesis of the lower part of Borrelidin,72 was submitted to the conditions for epimerization of the alcohol moiety (Scheme 4). Using 5 mol% of Ir1, 5.5 mol% of KOtBu and starting from pure syn-homoallylic alcohol 13, a diastereomeric ratio syn/anti of 1:0.8 was obtained after 3 d at room temperature. It is noteworthy that under these mild reaction conditions no isomerization of the terminal olefin was detected.

Scheme 7: Iridacycle catalyzed epimerization of building block of the lower part of Borrelidin

8.3 Summary and concluding remarks In conclusion, we developed novel methodology for the selective epimerization of amino-alcohols using iridium(III) half-sandwich cationic catalysts. Using base activation of the precatalyst, epimerization of an alcohol moiety can be performed without affecting the chiral amine. Without activation of the catalyst using base, only epimerization of amine occurred. Furthermore, these catalytic systems are

201

Iridium catalyzed epimerization of 1,2-amino-alcohols

suitable for the selective epimerization of complex natural products such as quinine. It allows the synthesis of other diastereomers without affecting other functionality in the molecule as illustrated with the homoallylic alcohol from the lower part of Borrelidin. This method can be applied to access analogues of natural products which can be useful in the discovery of novel biological activities.

8.4 Experimental section

General

Reagents were purchased from Aldrich, Acros, and Strem and were used as provided, unless stated otherwise. Pentane and chlorobenzene were reagent grade. Toluene was distilled over Na and acetonitrile was distilled over CaH2. All moisture sensitive reactions were performed using Schlenk technique, previously heated with a heatgun under vacuum, which were fitted with rubber septa under a positive pressure of nitrogen. Flash column chromatography was performed using silica gel (Silica-P flash silica gel from Silicycle, size 40-63 m) or activated neutral aluminium oxide (Merck aluminium oxide 90 neutral activated). TLC was performed on silica gel 60/Kieselguhr F254 or neutral aluminum oxide 60 F254. 1H and 13C NMR spectra were recorded on a Varian VXR300 (299.97 MHz for 1H, 75.5 MHz for 13C) or a Varian AMX400 (399.93 MHz for 1H, 100.6 MHz for 13C) spectrometer. Chemical shifts for protons are reported in parts per million scale ( scale) downfield from tetramethylsilane and are referenced to residual NMR solvent peak (CHCl3: = 7.26). Chemical shifts for carbon are calibrated to the middle signal of the 13C-triplet of the solvent CDCl3 ( = 77.0). HPLC spectra were obtained using a Shimadzu LC-20AD equipped with a Chiralcel OD-H and AS-H column. GC-MS data were recorded on a Hewlett Packard HP6890 equipped with a HP1 column and an HP 5973 mass selective detector. High resolution mass spectra (HRMS) were recorded on a AEI-MS-902 and FTMS orbitrap (Thermo Fisher Scientific) mass spectrometer. Elemental analyses were carried out on a EuroVector Euro EA elemental analysis. Synthesis of the iridacycle complexes The procedure described by Pfeffer et al. for analogous compounds was used.68 In a typical experiment, a 50 mL Schlenk tube was thoroughly flame-dried and put under an atmosphere of nitrogen, after which the following compounds were added, sequentially: [Cp*IrCl2]2 (120 mg, 0.15 mmol), KPF6 (110 mg, 0.60 mmol), NaOH (12 mg, 0.30 mmol), amine (0.30 mmol), and dry acetonitrile (4 mL). This mixture was stirred at 45 °C for 16 h. The mixture was then cooled down to rt, washed with hexane and filtered over neutral aluminum oxide (eluent: MeCN). The resulting solution was concentrated in vacuo. Subsequent stripping with dry Et2O yielded the iridacycle complex.

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NMR Data: [( 5-C5Me5)Ir(C6H4-2-CH2NHCH3)(NCCH3)](PF6) [Ir1]

Using 49 mg of N-methylbenzylamine affording [Ir1] as a yellow powder (233 mg, 0.37 mmol, 92%). 1H NMR (400.0 MHz, CDCl3, 298 K): 7.36 (d, 3JH-H= 6.9 Hz, 1H, Ph-H), 7.07 (d, 3JH-H= 6.9 Hz, 1H, Ph-H), 7.00 (t, 3JH-H= 7.2 Hz, 1H, Ph-H), 6.92 (t, 3JH-H= 7.0 Hz, 1H, Ph-H), 4.40 (s, 1H, NH), 4.17 (s, 1H, CH2), 3.68 (s, 1H, CH2), 3.11 (s, 3H, NCH3), 2.38 (s, 3H, NCCH3), 1.68 (s, 15H, C5(CH3)5). 13C NMR (100.6 MHz, CDCl3, 298 K): 150.2, 146.6, 135.2, 134.4, 127.4, 123.6, 121.0, 119.1, 89.8, 67.2, 44.7, 9.0. HRMS (EI+) calcd. for C18H25IrN+: (= M+ - CH3CN/PF6

-) 448.1616, found: 448.1617. [( 5-C5Me5)Ir(C6H4-2-C3H5N2)(NCCH3)](PF6) [Ir2]

Using 59 mg of 2-phenyl-2-imidazoline affording [Ir2] as a yellowish powder (237 mg, 0.36 mmol, 91%). 1H NMR (400.0 MHz, CDCl3, 298 K): 7.67 (d, 3JH-H= 7.6 Hz, 1H, Ph-H), 7.33 (d, 3JH-H= 7.6 Hz, 1H, Ph-H), 7.20 (t, 3JH-H= 7.4 Hz, 1H, Ph-H), 7.07 (t, 3JH-H= 7.6 Hz, 1H, Ph-H), 5.91 (s, 1H, NH), 3.97 (s, 4H, (CH2)2), 2.33 (s, 3H, NCCH3), 1.78 (s, 15H, C5(CH3)5). 13C NMR (100.6 MHz, CDCl3, 298 K): 177.7, 157.3, 135.3, 134.8, 131.8, 125.5, 123.2, 117.5 (CH3CN), 89.8, 51.8, 45.9, 9.2 (CH3CN). HRMS (EI+) calcd. for C19H24IrN2

+: (= M+ - CH3CN/PF6-) 473.1563, found:

473.1542. Anal. Calcd for C21H27F6N3PIr: C, 38.29; H, 4.13; N, 6.38. Found: C, 37.93; H, 4.07; N, 6.18.

General procedures for selective epimerization of chiral alcohols

In a thoroughly flame-dried Schlenk flask under an atmosphere of nitrogen, 10 mol of catalyst and 12 mol of KOtBu were dissolved in 1 mL of freshly distilled

toluene and the mixture stirred for 5 min, after which 200 mol of chiral amino-alcohol was added. The reaction was monitored by periodically taking 0.1 mL aliquots from the mixture, filtering them over silica gel (eluent: Et2O) and analyzing the resulting samples by GCMS, chiral GC and NMR. During the reaction either starting material or product are the only detectable compounds and no decomposition of the starting material or product is observed. Compounds 1 to 10 are commercially available. Isolated yields are not determined. The physical data were identical in all respects to those of commercially available samples. General procedures for selective epimerization of chiral amines

Under a nitrogen atmosphere, 0.25 mmol of amino-alcohol was added to a solution of 0.005 mmol of iridacycle (2 mol %) in 1 mL of dry and degassed toluene at the desired temperature. The reaction mixture was then stirred at the specified temperature. For GCMS and chiral GC analysis, samples were taken (50 μL), filtered through silica gel (eluent: Et2O) and diluted in 1.0 mL of dichloromethane after adding 2 drops of triethylamine and acetic anhydride. For HPLC analysis, samples were taken (50 μL), filtered through silica gel (eluent: Et2O), the solvent removed under vacuum and the residue was diluted in 1.5 mL of isopropanol.

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Iridium catalyzed epimerization of 1,2-amino-alcohols

During the reaction starting material or product are only the compounds present and no decomposition of the starting material or product are observed. Compounds 1 to 10 are commercially available. The physical data were identical in all respects to those reported for commercially available samples. Isolated yields are not determined.

New compounds NMR Data:

(1S)-(6-methoxyquinolin-4-yl)((2S,4S,5R)-5-vinylquinuclidin-2-yl)methanol (12).

12

N

MeO NOH

H

1H NMR (400 MHz, CDCl3) 8.63 (d, J = 4.4 Hz, 1H), 7.91 (d, J = 9.2 Hz, 1H), 7.53 (d, J = 4.4 Hz, 1H), 7.25 (d, J = 7.0 Hz, 1H), 7.16 (s, 1H), 5.76 (s, 0H), 5.74 – 5.47 (m, 1H), 4.95 (t, J = 12.8 Hz, 2H), 3.83 (s, 4H), 3.17 (t, J = 11.5 Hz, 2H), 2.77 (d, J = 6.6 Hz, 2H), 2.38 (s, 1H), 2.00 – 1.77 (m, 3H), 1.58 (s, 1H), 1.48 (d, J = 10.0 Hz, 2H), 1.25 (s, 1H). (S)-1-((1R,2R)-2-(((4-methoxybenzyl)oxy)methyl)cyclopentyl)but-3-en-1-ol (13).

1H NMR (400 MHz, CDCl3) 7.25 (d, J = 8.3 Hz, 2H), 6.87 (d, J = 8.0 Hz, 2H), 6.10 – 5.86 (m, 1H), 5.08 (ddd, J = 8.4, 7.8, 7.4 Hz, 2H), 4.48 (q, J = 11.8 Hz, 2H), 3.79 (s, 3H), 3.51 (dd, J = 8.7, 4.4 Hz, 1H), 3.46 – 3.36 (m, 1H), 3.18 (t, J = 9.4 Hz, 1H), 2.38 (d, J = 10.9 Hz, 1H), 2.09 (ddd, J = 19.8, 10.5, 2.3 Hz, 2H), 1.93 – 1.67 (m, 2H), 1.69 – 1.52 (m, 2H), 1.53 – 1.38 (m, 1H), 1.31-1.14 (m, 3H). 13C NMR (101 MHz, CDCl3) 159.20, 135.72, 129.48, 129.37, 116.35, 113.75, 74.91, 74.17, 72.81, 55.17, 55.14, 51.65, 43.87, 40.39, 30.94, 29.86, 24.45. HRMS (NI-) calcd. for C18H26O3: (=M-) 289.1882, found: 289.1798.

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Chapter 8

(R)-1-((1R,2R)-2-(((4-methoxybenzyl)oxy)methyl)cyclopentyl)but-3-en-1-ol (14).

1H NMR (400 MHz, CDCl3) 7.25 (d, J = 8.4 Hz, 2H), 6.88 (d, J = 8.6 Hz, 2H), 5.87 (m, 1H), 5.11 (dd, J = 9.8, 1.3 Hz, 2H), 4.46 (s, 2H), 3.80 (s, 3H), 3.75 – 3.59 (m, 1H), 3.42 (dd, J = 8.9, 6.0 Hz, 1H), 3.34 – 3.21 (m, 1H), 2.31 – 2.19 (m, 1H), 2.11 (m, 2H), 1.86 – 1.64 (m, 3H), 1.63 – 1.43 (m, 3H), 1.39 – 1.27 (m, 2H). 13C NMR (101 MHz, CDCl3) 159.13, 136.12, 130.25, 129.23, 117.09, 117.02, 113.76, 74.10, 72.77, 72.05, 55.27, 55.22, 49.47, 40.60, 39.46, 30.17, 28.00, 24.49. HRMS (NI-) calcd. for C18H26O3: (=M-) 289.1882, found: 289.1878.

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