Preparation of perfluoroalkane-tagged chiral auxiliaries and their application to stereoselective synthesis of a β 2 -amino acid building block

Both enantiomers of novel perfluoroalkane-tagged 5,5-diphenyl-2-oxazolidinone (DIOZ) auxiliaries have been prepared using a two-step one-pot synthesis, starting from commercially available N -( tert -butoxycarbonyl)- L -valine methyl ester. The key feature of this approach is the efficient generation of a suitably active perfluoroalkyl-aryllithium species. By use of this protocol, the perfluoroalkane-tagged DIOZ auxiliaries are obtained in high enantiomeric purity and on multigram scales with overall yields exceeding 50%. The perfluoroalkane-tagged auxiliaries enable the use of fluorous solid-phase extraction, allowing efficient purification of tagged intermediates from crude reaction mixtures. The new auxiliaries have been applied in an asymmetric synthesis of β 2 - N -Fmoc-phenylalanine via a stereoselective conjugate addition.


Introduction
In the field of stoichiometric asymmetric synthesis, fluorous solid-phase extraction (FSPE) techniques have been applied to many chemical transformations, offering a powerful alternative to standard separation and purification methods.Initially proposed as a method of separating catalysts from process streams, perfluoroalkane chains have been developed as general supports for organic reagents and auxiliaries.Perfluoroalkane-tagged chiral auxiliaries are soluble in typical organic reaction solvents, and the perfluoroalkane chains are impervious to most common reagents.
Generally, minor or no modifications to the classical reaction conditions are necessary.After a reaction is complete, selective recovery of the tagged material is easily accomplished by FSPE.Several research groups from academia and industry have demonstrated that fluorous synthesis with chiral auxiliary methods is both viable and broadly applicable.[6]

Results and Discussion
Our initial approach to the synthesis was aimed at a late stage introduction of the perfluoroalkane tag to enable the use of a common intermediate in the preparation of a diverse range of auxiliaries in terms of the perfluoroalkane chain length and the manner of attachment of the tag (Scheme 1).
The oxazolidinone 3a was synthesized using a modification of the procedure described by Seebach. 4 para-Bromophenol 1a was protected with TIPS or TBDMS groups, via a standard protocol (silyl chloride, imidazole) to provide compounds 1b and 1c respectively, in nearly quantitative yields.The protected phenols were then converted into the corresponding Grignard reagents.The thus formed organo-magnesium halide species were immediately reacted with N-(tert-butoxycarbonyl)-L-valine methyl ester to deliver the tertiary alcohol intermediates 2b and 2c in good yields (87-95%).Treatment of these open-chain intermediates with potassium tertbutoxide in THF at room temperature gave the oxazolidinones 3b and 3c in 81% and 78% yields, respectively.(i) Mg, THF, 17 h, 0 °C→RT, 87-95%.(ii) KO t Bu, THF, 16 h, 0 °C-RT, 78-81%.
Deprotection with HF•pyridine afforded the free bis-phenol 3a (84-90%), which could be converted into the desired perfluoroalkane-tagged DIOZ auxiliaries either by re-protection with a freshly prepared perfluoroalkylsilyl chloride affording DIOZ 3d (59%) or alkylation with hexadecafluoro-nonane triflate to provide perfluoro-ether DIOZ 3e (43%).An alternative DIOZ auxiliary, where attachment of the perfluoroalkane tag to the phenyl rings is via a carbon-carbon bond, was synthesized from the commercially available perfluoroalkylaryl bromide 4 (R f = C 2 H 4 C 6 F 13 , Scheme 2).Compound 4 was treated with n-BuLi at low temperature to afford the corresponding metallated species and immediately allowed to react with N-(tert-butoxycarbonyl)-L-valine methyl ester, providing the tertiary alcohol 5 (82%).Surprisingly, the application of the Grignard protocol resulted in absolutely no conversion, presumably as a result of failure to initiate formation of the Grignard reagent.Treatment of the perfluoroalkane-tagged open-chain intermediate 5 with potassium tert-butoxide in THF gave the DIOZ compound 6 in 57% yield.A more expeditious version of this route involved the synthesis of DIOZ auxiliary 6 from the commercially available aryl bromide 4 via metallation, addition and cyclisation in a one-pot process.The overall yield using this most convenient route was 50%.Using the same one-pot protocol but starting with N-(tert-butoxycarbonyl)-D-valine methyl ester, afforded the optical antipode of the auxiliary, namely DIOZ ent-6.
The novel perfluoroalkane tagged chiral auxiliaries were then applied to stereoselective syntheses of a β 2 -amino acid as depicted in Scheme 3. 6,7 A key aspect of this protocol was the application of FSPE purification techniques.As the fluorous tagged chiral auxiliaries 3d, 3e and 6 showed similar chromatographic behavior in several elution and separation experiments we choose 6 and ent-6 as auxiliaries in an asymmetric synthesis on larger scale due to the ease of synthesis.Both enantiomers of the required α,β-unsaturated imide starting material, 7 and ent-7, were prepared using a standard acylation protocol (DMAP, DIPCD, 2 eq.2-benzylacrylic acid) in good yield (78%).The imide intermediates 7 and ent-7 were exposed to a twofold excess of O-benzyl hydroxylamine at 72 °C for 24 h in the key stereoselective conjugate addition reaction.Lower temperatures required significantly longer reaction times.The resultant aza-Michael adducts 8 and ent-8 were purified using FSPE: A standard glass flash chromatography column was packed with commercially available Fluoroflash™ and pre-conditioned by passing through 80:20 MeOH-H 2 O (v/v) under positive pressure.The residue was loaded on to the top of the column, using DMF as the loading solvent.The column was then eluted with MeOH/H 2 O.The resulting crude products 8 and ent-8 were a ca.8.3:1 mixture of diastereomers, judged by HPLC, at the stereogenic centre introduced by the conjugate addition reaction.Formation of the tosylate salt of compound 8 and subsequent recrystallization thereof, improved the diastereomeric excess to 93% as determined by 1 H-NMR and HPLC.Removal of the auxiliary 6 and concomitant release of the β 2 -aminoxy acid 9 was facilitated with LiOH/H 2 O 2 in THF/H 2 O (74%).The auxiliary was conveniently recovered (96%) using the same FSPE procedure.The previously reported β 2 -aminoxy acid 9 was converted to the corresponding β 2 -amino acid 10 via hydrogenation (H 2 , Pd/C, MeOH, NH 3 , H 2 O) and Fmoc-protected using standard conditions (FmocOSu, NaHCO 3 , THF) to afford compound 11 in 30% over the two steps.
This sample of the Fmoc-β 2 -amino acid 11 was identical, as judged by 1 H-NMR and low resolution mass spectrometric analysis, to previously reported authentic material. 8,9The depicted absolute stereochemistry is correlated to the chiral HPLC retention characteristics of the authentic material.The enantiomeric excess of the Fmoc-β 2 -amino acid 11 resulting from both isomers of the auxiliary was analyzed via HPLC and found to exceed 95%.

Conclusions
The experiments described in this publication have enabled production of perfluoroalkane-tagged DIOZ chiral auxiliaries from commercially available starting materials in a one-pot operation.Application of the perfluoroalkane-tagged DIOZ auxiliaries provided a short and efficient synthesis of both enantiomeric forms of β 2 -N-Fmoc-phenylalanine (described in the Experimental Section). 14,15This route is distinguished by its succinctness and ease of recovery of the DIOZ auxiliary.Purification of products was achieved through silica and fluorous solidphase extractions, with only one recrystallization being required.We have performed the complete sequence on various scales, up to the formation of 25 g of the target compound in a single batch and the process requires no equipment other than typical laboratory glassware.We believe that the reported perfluoroalkane-tagged DIOZ auxiliaries are truly powerful tools for drug discovery and high-throughput chemistry and are viable alternatives to conventional or solid supported auxiliaries.

Experimental Section
General Procedures.All reagents were obtained commercially and used as received unless otherwise noted.TLC was performed on Merck silica gel plates 60 F-254, Art.no.5729.Reverse-phase HPLC analyses were performed on an Agilent-1100 using a Macherey-Nagel CC 70/4 Nucleosil 100-3 C18 HD column, acetonitrile and water both containing 0.05% TFA, a column temperature of 35 °C, with a flow rate of 1.0 mL/min and measuring at 216 nm.The standard gradient used was 5-100% MeCN over 6 min, 100% MeCN for 1.5 min followed by 100-5% MeCN over 0.5 min.NMR was performed using a 400 MHz Varian spectrometer, AS 400 Oxford. 1 H shifts were referenced to CDCl 3 at 7.25 ppm with tetramethylsilane as internal standard for 1 H NMR. MS was measured using VG Platform (Fisons Instruments), Spectraflow 783 Detector, HP 1100 Series HPLC.Melting points were measured using a Büchi, B-545 apparatus.
(4-Bromo-phenoxy)triisopropylsilane (1b).A solution of 4-bromophenol (1a, 43.96 g, 254 mmol) and TIPS-Cl (54.95 mL, 257 mmol) in DCM, was cooled, on an ice bath to 0 ºC whilst being stirred under nitrogen.The solution was treated with imidazole (43.3 g, 636 mmol) added gradually with caution, and then allowed to warm to RT, with stirring, over the course of 16 h.The resulting suspension was washed sequentially with HCl (2 x 200 mL of a 0.5 M aqueous solution), sat.aq.NaHCO 3 (200 mL) and brine (200 mL), then dried (MgSO 4 ) and concentrated under reduced pressure.The resulting residue was taken up into hexane (400 mL) and adsorbed onto silica gel (200 g) under reduced pressure.The resulting free-flowing solid was then added to the top of a short plug of silica (70 g) and eluted with hexane (3000 mL).Concentration of the appropriate fractions (R f = 0.6-0.8)under reduced pressure afforded compound 1b (81.7 g, 248 mmol, 98 %) as a colourless oil.The following protocol was adapted from a previously reported procedure. 4A dry 250 mL round-bottomed flask fitted with a condenser, dropping funnel, thermometer and magnetic stirrer, was charged with Mg turnings (6.42 g, 264 mmol) and stirred, dry, under an atmosphere of nitrogen, overnight.A solution of compound 1b (8.0 g, 24.3 mmol) in THF (20 mL) was added and stirring continued, until the reaction started spontaneously as evidenced by refluxing of the THF.At this stage, more of compound 1b (74.82 g, 227.2 mmol) in THF (60 mL) was added, dropwise, at a rate sufficient to maintain reflux.The resultant mixture was heated at reflux, for 2 h, then cooled to below 6 ºC on an ice bath.A solution of N-(tert-butoxycarbonyl)-L-valine methyl ester (17.46 g, 75.5 mmol) in THF (20 mL) was added dropwise, at such a rate as to maintain the internal reaction temperature below 6 ºC, then the mixture was allowed to warm to RT, with stirring, over 18 h.The mixture was poured into icecold sat.aq.NH 4 Cl (200), the aqueous phase was separated and extracted with EtOAc (2 x 150 mL).The combined organic phases were washed with brine (150 mL) and dried (MgSO 4 ), concentrated under reduced pressure and purified via flash chromatography.Concentration of the appropriate fractions (R f = 0.

-)-(S)-5,5-Bis-(4-hydroxyphenyl)-4-isopropyloxazolidin-2-one (3a).
A dry polyethylene bottle flushed with nitrogen was charged with a magnetic stirrer and a solution of compound 3b (17.5 g, 27.9 mmol) in THF (150 mL) and cooled in an ice bath with stirring, while HF•pyridine (35 mL of a 65-70% commercial solution) was added dropwise.The reaction was allowed to warm to RT over 16 h.The resultant mixture was diluted with diethyl ether (100 mL) and sat.aq.NaHCO 3 (500 mL).Solid NaHCO 3 was then added, in portions, until the reaction mixture was neutralised as evidenced by no more CO 2 being liberated.The organic layer was separated and the aqueous layer was extracted with 50:50 THF / diethyl ether (2 x 200 mL).The pooled organic layers were washed with brine, dried (MgSO 4 ), and concentrated under reduced pressure.
The residue was purified via flash chromatography.Concentration of the appropriate fractions (R f = 0.45, 1:9 MeOH / DCM) afforded compound 3a (7.36 g, 23.5 mmol, 84%).This protocol was also used to prepare compound 3a, in similar yield, via deprotection of compound 3c. 1 12 and Cs 2 CO 3 (21.5 g, 65.9 mmol) in DMF (140 ml) was stirred, under a nitrogen atmosphere, at a temperature of 120 ºC, for 72 h.The resultant suspension was cooled to RT, then placed on an ice bath, and treated with HCl (250 mL of a 0.5 M aqueous solution).
After stirring for 5 min, the mixture was diluted with diethyl ether (500 mL) and poured into a separation funnel.The aqueous phase was separated, and the organic phase was washed with HCl (2 x 250 mL of a 0.5 M aqueous solution), then dried (MgSO 4 ) and concentrated under reduced pressure.The residue was purified via flash chromatography.Concentration of the appropriate fractions (R f = 0.2, DCM elution) afforded compound 3e (10.58 g, 9.27 mmol, 43%).9 mmol) in THF (10 mL) maintained at -78 ºC under an atmosphere of nitrogen was treated, dropwise, with n-BuLi (6.2 mL of a 1.6 M solution in hexane, 9.92 mmol) and stirring was continued at this temperature for 20 minutes.The suspension was allowed to warm to -50 ºC for 10 minutes and then cooled again to -78 ºC, before being treated, dropwise, with N-(tertbutoxycarbonyl)-L-valine methyl ester (0.93 g, 4.0 mmol) in THF (5 mL).Stirring was continued at -78 ºC for 6 h before quenching, at this temperature, with sat.aq.NH 4 Cl (25 mL).
The resultant suspension was diluted with diethyl ether (50 mL).The aqueous phase was separated and the organic phase was washed sequentially with sat.aq.NH 4 Cl (40 mL) and brine (40 mL), then dried (MgSO 4 ) and concentrated under reduced pressure.The residue was purified via flash chromatography.Concentration of the appropriate fractions (R f = 0.35, 9:1 hexane / EtOAc) yielded the desired compound 5 (3.4 g, 3.25 mmol, ~82%), albeit impure due to coeluting N-(tert-butoxycarbonyl)-L-valine methyl ester starting material.This material was not characterised but rather was employed immediately in the next step.The following protocol was adapted from a previously reported procedure. 4A magnetically stirred solution of compound 5 (3.4 g, 3.25 mmol) maintained at 0 ºC under an atmosphere of nitrogen, was treated with potassium tert-butoxide (0.44 g, 3.9 mmol) in one portion, and the resultant mixture was allowed to slowly warm to RT over the course of 16 h.The suspension was treated with NH 4 Cl (10 mL of a 10% aqueous solution).The aqueous phase was separated and washed with diethyl ether (2 x 20 mL).The pooled organic phases were then dried and concentrated under reduced pressure.The resultant crude material was then purified via flash chromatography.Concentration of the appropriate fractions (R f = 0. octyl)benzene (4, 40.83 g, 81.1 mmol) in THF (400 mL) maintained at -78 ºC under an atmosphere of nitrogen, was added n-BuLi (50.63 mL of a 1.6 M solution in hexane, 81.0 mmol) and stirring was continued at this temperature for 20 min.The resultant suspension was then treated, dropwise, with N-(tert-butoxycarbonyl)-L-valine methyl ester (7.5 g, 32.4 mmol) in THF (10 mL) and stirring was continued at -78 ºC for 6 h.After this time, the dry ice-actone bath was removed and the mixture was allowed to warm to RT with continued stirring for 16 h.The reaction was then quenched with sat.aq.NH 4 Cl (200 mL) and diluted with diethyl ether (400 mL).The aqueous layer was separated and the organic layer was washed sequentially with sat.aq.NH 4 Cl (200 mL) and brine (200 mL), then dried (MgSO 4 ) and concentrated under reduced pressure.The residue was purified via flash chromatography.Concentration of the appropriate fractions (R f = 0.3, 3:1 hexane / EtOAc) afforded compound 6 (15.3 g, 15.72 mmol, 50% over two steps).This material was identical, as judged by 1 H-NMR, 13 C-NMR, 19 F-NMR and low resolution mass spectrometric analysis, as well as chromatographic behaviour (TLC and flash-column chromatography), with the material prepared via the two-step process reported above.

(-)-(S)-3-(2-Benzylacryloyl)-4-isopropyl-5,5-bis-[4-(
phenyl]oxazolidin-2-one (7).The following protocol was adapted from a previously reported procedure. 1A magnetically stirred suspension of compound 6 (3.88 g, 3.99 mmol), DMAP (0.098 g, 0.8 mmol) and 2-benzyl-acrylic acid (1.3 g, 8.0 mmol) in DCM (20 mL), maintained at 0 ºC under an atmosphere of nitrogen, was treated with DIPCD (1.24 mL, 1.01 g, 8.0 mmol).After 10 min, the reaction was allowed to warm to RT and stirring was continued for 16 h.The diisopropyl urea formed was filtered, and the precipitate washed with DCM (10 mL).The filtrate was washed with sat.aq.NaHCO 3 (10 mL), then dried (MgSO 4 ) and concentrated under reduced pressure.The residue was purified via flash chromatography.Concentration of the appropriate fractions (R f = 0.3, 95:5 hexane / EtOAc) gave compound 7 (3.5 g, 3.13 mmol, 78%).The following protocol was adapted from a previously reported procedure. 6A dry round-bottomed flask was charged with a magnetic stirrer bead, compound 7 (3.11g, 2.78 mmol), O-benzyl hydroxylamine (0.684 g, 5.55 mmol) and THF (10 mL).The resultant solution was heated, with stirring, to 72 ºC under an atmosphere of nitrogen.The reaction was allowed to proceed at this temperature for 24 h.The mixture was then cooled to RT, concentrated under reduced pressure and purified via FSPE.A standard glass flash chromatography column was packed with commercially available Fluoroflash™ (35 g) and pre-conditioned by passing through MeOH / H 2 O (80 mL of an 80 : 20 v/v mixture) under positive pressure.The residue was loaded on to the top of the column, using DMF (4 mL) as the loading solvent.The column was then eluted with MeOH / H 2 O (80 mL of an 80 : 20 v/v mixture).Concentration of this fraction under reduced pressure afforded o-benzyl hydroxylamine (0.27 g, 2.13 mmol).The column was then eluted successively with MeOH (100 mL), THF (120 mL) and acetone (120 mL).The three organic fractions were pooled and concentrated under reduced pressure to afford a clear colourless oil containing the desired compound 8 (3.24 g, 2.61 mmol, 94%  Compound ent-8 was prepared using the same protocols as those described for the preparation of compound 8, using compound ent-7 (3.5 g, 3.13 mmol) as starting material.Purification via the same means (FSPE & precipitation of tosylate) afforded compound ent-8 (2.25 g, 1.6 mmol, 51% over the two steps).This material was identical, as judged by 1 H-NMR, 13 C-NMR, 19 F-NMR and low resolution mass spectrometric analysis, as well as chromatographic behaviour (TLC and flashcolumn chromatography), with the optical antipode compound 8 reported above.
(R)-2-(Benzyloxyaminomethyl)-3-phenylpropionic acid (9) and recovery of compound 6 via FSPE.A magnetically stirred solution of compound 8 (2.06 g, 1.66 mmol, 93% d.e), in THF (80 mL) and water (10 mL), maintained at 0 ºC under an atmosphere of nitrogen, was treated with H 2 O 2 (40 mL of a 30% aqueous solution), followed by LiOH (160 mL of a 4.5% aqueous solution).The reaction mixture was stirred at this temperature for 45 min, before being quenched via addition of cold sat.aq.sodium sulfite (40 mL), and allowed to warm to RT over the course of 30 min.The resultant suspension was extracted with EtOAc (4 x 100 mL) and the organic extracts dried (MgSO 4 ), and concentrated under reduced pressure to give a residue which was subjected to FSPE.A commercially available Fluoroflash™ cartridge (20 g) was preconditioned by passing through MeOH / H 2 O (60 mL of an 80 : 20 v/v mixture) under positive pressure.The residue was loaded on to the top of the column, using DMF (4 mL) as the loading solvent.The column was then eluted with MeOH / H 2 O (60 mL of an 80 : 20 v/v mixture).
Concentration of this fraction under reduced pressure afforded non-fluorinated organic impurities.The column was then eluted successively with MeOH (80 mL), THF (100 mL) and acetone (100 mL).The three organic fractions were pooled and concentrated under reduced pressure to afford recovered compound 6 (1.56 g, 1.60 mmol, 96%) of high purity, as determined by 1 H-NMR and LCMS.The aqueous layer from the original solvent extraction was acidified with conc.HCl to a pH of 4-5, as determined using universal indicator strips, and then extracted with EtOAc (3 x 150 mL).The combined extracts were washed with water (2 x 50 mL) then dried (MgSO 4 ), and concentrated under reduced pressure to afford compound 9 (350 mg, 1.23 mmol, 74%).This crude sample was identical, as judged by 1 H-NMR and low resolution mass spectrometric analysis with previously reported authentic material.Due to the previously reported limited stability of this compound it was immediately subjected to the following hydrogenation step. 6(R)-2-Aminomethyl-3-phenylpropionic acid (10).The crude sample of compound 9 (350 mg, 1.23 mmol) from the previous step, was dissolved in MeOH, NH 3 and H 2 O (3:1:1, 10 mL) and 10% Pd/C (100 mg) was added to the solution.The mixture was then stirred at RT under an atmosphere of hydrogen for 72 h (at which point TLC analysis indicated complete consumption of starting material), before being filtered through a plug of Celite, and concentrated under reduced pressure, to provide compound 10.This crude sample was identical, as judged by 1 H-NMR with previously reported authentic material. 8,9R)-2-[(9H-Fluoren-9-ylmethoxycarbonylamino)methyl]-3-phenylpropionic acid (11).The crude sample of compound 10 (350 mg) produced in the previous step was dissolved in THF and H 2 O (10 mL of a 1:1 mixture) and cooled to 0 ºC, whilst being stirred under nitrogen.The solution was treated with Fmoc-OSu (0.420 g, 1.25 mmol) and NaHCO 3 (1.0 g, 11.88 mmol) and allowed to warm to RT over the course of 2 h.THF was removed under reduced pressure and the aqueous residue was washed with diethyl ether (2 x 10 mL).The organic layers were discarded and the aqueous layer was acidified with sat.aq.KHSO 4 and extracted with DCM (3 x 20 mL).The combined organic extracts were dried (MgSO 4 ), filtered, and concentrated under reduced pressure.The residue was purified via flash chromatography.Concentration of the appropriate fractions (R f = 0.3, 50:1 DCM / MeOH) afforded compound 11 (122 mg, 0.36 mmol, ~30% over two steps, >95% e.e. as determined by chiral HPLC).This sample of compound 11 was identical, as judged by 1 H-NMR, low resolution mass spectrometric analysis and chromatographic retention characteristics with previously reported authentic material.
12After 5 minutes, the reaction vessel was placed on a rotary evaporator and heated to 70 ºC under a reduced pressure of ~30 mBar to remove tert-butanol.To this vessel was then added a solution of compound 3a (1.22 g, 3.89 mmol) in THF (10 mL) with stirring, at RT, under an atmosphere of nitrogen.Imidazole (1.06 g, 15.57mmol) was then added, in a single portion, and stirring was continued at this temperature for 65 h.The mixture was diluted with diethyl ether (50 mL) and washed sequentially with HCl (2 x 20 mL of a 0.5 M aqueous solution), sat.aq.NaHCO 3 (20 mL) and H 2 O (20 mL), then dried (MgSO 4 ) and concentrated under reduced pressure.The resulting crude oil was purified via flash chromatography.
). Subjection of this material to chiral HPLC analysis (Chiralpak™ AD-H 1196 analytical column, 2:98 v/v isopropanol-hexane elution, flowrate 1.0 mL/min) confirmed that it had been obtained in 80% d.e: R t 12.85 (major diastereomer), 16.96 min (minor diastereomer).A magnetically stirred solution of compound 8 (3.18 g, 2.56 mmol), in diethyl ether (250 mL) maintained at RT under an atmosphere of nitrogen, was treated with a solution of p-toluenesulfonic acid monohydrate (512 mg, 2.73 mmol) in diethyl ether (20 mL) and stirring was continued for 72 h.After this time, the resultant suspension was cooled to 0 ºC on an ice bath, and then filtered.The solid material was washed with ice cold diethyl ether (2 x 25 mL) and dried under reduced pressure to afford the toluene sulfonic acid salt of compound 8 (2.53 g, 1.79 mmol, 70%, 93% d.e. by 1 H-NMR).