References and Notes
For reviews see:
1a
Eames J.
Angew. Chem. Int. Ed.
2000,
39:
885
1b
Eames J. In
Organic Synthesis Highlights
Vol. V:
Wiley-VCH;
Weinheim:
2003.
Chap. 17.
p.151-164
1c
Dehli JR.
Gotor V.
Chem. Soc. Rev.
2002,
31:
365
1d
Dehli JR.
Gotor V.
ARKIVOC
2002,
(v):
196
1e
Liao L.
Zhang F.
Dmitrenko O.
Bach RD.
Fox JM.
J. Am. Chem. Soc.
2004,
126:
4490
2
Preston SC. D. Phil. Dissertation; Oxford University: UK, 1989; Diss. Abstr. Int. B
1990,
51:
2896
3a
Vedejs E.
Chen X.
J. Am. Chem. Soc.
1997,
119:
2584
For additional studies, see:
3b
Vedejs E.
Rozners E.
J. Am. Chem. Soc.
2001,
123:
2428
3c
Vedejs E.
Daugulis O.
J. Am. Chem. Soc.
2003,
125:
4166
4 For a comprehensive review into quasi-enantiomers, see: Zhang Q.
Curran DP.
Chem. Eur. J.
2005,
11:
4866
5a
Davies SG.
Diez D.
El Hammouni MM.
Garner AC.
Garrido NM.
Long MJ.
Morrison RM.
Smith AD.
Sweet MJ.
Withey JM.
Chem. Commun.
2003,
2410
For additional studies, see
5b
Davies SG.
Garner AC.
Long MJ.
Smith AD.
Sweet MJ.
Withey JM.
Org. Biomol. Chem.
2004,
2:
3355
5c
Davies SG.
Garner AC.
Long MJ.
Morrison RM.
Roberts PM.
Savory ED.
Smith AD.
Sweet MJ.
Withey JM.
Org. Biomol. Chem.
2005,
3:
2762
6a
Coumbarides GS.
Dingjan M.
Eames J.
Flinn A.
Motevalli M.
Northen J.
Yohannes Y.
Synlett
2006,
101
For additional studies, see:
6b
Coumbarides GS.
Eames J.
Flinn A.
Northen J.
Yohannes Y.
Tetrahedron Lett.
2005,
46:
849
6c
Coumbarides GS.
Dingjan M.
Eames J.
Flinn A.
Northen J.
Yohannes Y.
Tetrahedron Lett.
2005,
46:
2897
6d
Chavda S.
Coulbeck E.
Coumbarides GS.
Dingjan M.
Eames J.
Ghilagaber S.
Yohannes Y.
Tetrahedron: Asymmetry
2006,
17:
3386
6e
Boyd E.
Chavda S.
Eames J.
Yohannes Y.
Tetrahedron: Asymmetry
2007,
18:
476
6f
Coumbarides GS.
Dingjan M.
Eames J.
Flinn A.
Northen J.
Chirality
2007,
19:
321
6g
Chavda S.
Coumbarides GS.
Dingjan M.
Eames J.
Flinn A.
Northen J.
Chirality
2007,
19:
313
6h
Boyd E.
Coulbeck E.
Coumbarides GS.
Chavda S.
Dingjan M.
Eames J.
Flinn A.
Motevalli M.
Northen J.
Yohannes Y.
Tetrahedron: Asymmetry
2007,
18:
2515
The use of ten equivalents of racemic alcohol has been reported by:
7a
Evans DA.
Anderson JC.
Taylor MK.
Tetrahedron Lett.
1993,
34:
5563
7b
Miller SJ.
Copeland GT.
Papaioannou N.
Horstmann TE.
Ruel EM.
J. Am. Chem. Soc.
1998,
120:
1629
7c
Bull SD.
Davies SG.
Garner AC.
Kruchinin D.
Key MS.
Roberts PM.
Savory AD.
Smith AD.
Thomson JE.
Org. Biomol. Chem.
2006,
4:
2945
7d
Coulbeck E.
Eames J.
Tetrahedron: Asymmetry
2007,
18:
2313
7e See also ref. 18.
7f For our study, an excess of alcohol rac-16 was used to minimize epimerisation of the product, potential racemisation of the active ester(s) and as a competitive Lewis base.
8 Alternatively, n-BuLi (in hexanes) and PhLi (in dibutyl ether) could be used but they contained traces of lithium butoxide which can lead to the formation of an inseparable by-product, butyl 2-phenylpropionate (in 16% and 5% yields for n-BuLi and PhLi, respectively). Characterisation data for butyl 2-phenylpropionate; R
f
[light PE (40-60 °C)-Et2O, 1:1] 0.80. IR (film): 1674 (C=O) cm-1. 1H NMR (400 MHz, CDCl3): δ = 7.15-7.28 (m, 5 H, 5 × CH, Ph), 3.99 (td, J = 1.5, 6.8 Hz, 2 H, OCH2), 3.63 (q, J = 7.2 Hz, 1 H, CHMe), 1.43-1.52 (m, 2 H, CH2), 1.42 (d, J = 7.2 Hz, 3 H, CHMe), 1.18-1.28 (m, 2 H, CH2), 0.79 (t, J = 7.5 Hz, 3 H, Me). 13C NMR (100 MHz, CDCl3): δ = 174 (C=O), 140.6 (i-C, Ph), 128.4, 127.4, 126.9 (3 × CH, Ph), 64.5 (OCH2), 45.5 (PhCH), 30.4, 18.9 (2 × CH2), 18.4 (MeCH), 13.6 (MeCH2). HRMS: m/z [M+] calcd for C13H18O2: 206.1299; found: 206.1301.
9 Using racemic lithium 1-(4-methoxyphenyl)ethoxide, epimerisation of esters such as (S,S)-anti-24 and (S,R)-syn-24, has been shown to occur at a significantly faster rate than simple transesterification.
10
Boyd E.
Chavda S.
Coulbeck E.
Coumbarides GS.
Dingjan M.
Eames J.
Flinn A.
Krishnamurthy AK.
Namutebi M.
Northen J.
Yohannes Y.
Tetrahedron: Asymmetry
2006,
17:
3406 ; and references therein
11 Measured by 1H NMR (400 MHz) spectroscopy; for rac,anti-17, the methyl doublets appear at δ = 1.43 (d, J = 7.2 Hz, 3 H, MeCH) and 1.42 (d, J = 6.6 Hz, 3 H, MeCH), whereas for rac,syn-17, the methyl doublets appear at δ = 1.41 (d, J = 7.2 Hz, 3 H, MeCH) and 1.35 (d, J = 6.6 Hz, 3 H, MeCH).
12 For active esters: R
f
[light PE (40-60 °C)-Et2O, 9:1] 0.69 [for (R)-11] and 0.50 [for (S)-12]. For further information see ref. 10.
13 For a representative procedure, see ref. 7d.
For representative reviews, see:
14a
Fu G.
Acc. Chem. Res.
2000,
33:
412
14b
Miller SJ.
Acc. Chem. Res.
2004,
37:
601
14c
France S.
Guerin DJ.
Miller SJ.
Lectka T.
Chem. Rev.
2003,
103:
2985
14d
Spivey AC.
Arseniyadis S.
Angew. Chem. Int. Ed.
2004,
43:
5436
14e
Vedejs E.
Jure M.
Angew. Chem. Int. Ed.
2005,
44:
3974
14f
Pamies O.
Bäckvall J.-E.
Chem. Rev.
2003,
103:
3247
14g
Robinson EJE.
Bull SD.
Tetrahedron: Asymmetry
2003,
14:
1407
For a review, see:
15a
Ghanem A.
Aboul-Enein HY.
Chirality
2005,
17:
1
For comprehensive examples, see:
15b
Morgan B.
Oehlschlager AO.
Stokes TM.
J. Org. Chem.
1992,
57:
3231
15c
Brown SM.
Davies SG.
de Sousa JAA.
Tetrahedron: Asymmetry
1993,
4:
813
15d
Naemura K.
Murata M.
Tanake R.
Yano M.
Hirose K.
Tobe Y.
Tetrahedron: Asymmetry
1996,
7:
1581
15e
Naemura K.
Murata M.
Tanake R.
Yano M.
Hirose K.
Tobe Y.
Tetrahedron: Asymmetry
1996,
7:
3285
15f
Cordova A.
Tremblay MR.
Clapham B.
Janda KD.
J. Org. Chem.
2001,
66:
5645
15g
Swaleh SM.
Hungerhoff B.
Sonnenschein H.
Theil F.
Tetrahedron
2002,
58:
4085
15h
Hungerhoff B.
Sonnenschein H.
Theil F.
J. Org. Chem.
2002,
67:
1781
16
Eames J.
Synthesis by Resolution and Inversion, In Science of Synthesis
36:
Thomas EJ.
Clayden JC.
Georg Thieme Verlag;
Stuttgart:
2007.
p.341-421
17
Experimental Procedure for 1-Phenylethyl 2-Phenylpropionate [(
R
,
R
)-
anti
-17] and 1-Phenylethyl 2-(6-Methoxynaphthalen-2-yl)propionate [(
S
,
S
)-
anti
-24] Derived from the Parallel Kinetic Resolution of 1-Phenylethanol (
rac
-16) Using Active Esters (
R
)-11 and (
S
)-12: t-BuLi (1.81 mL, 1.7 M in pentane, 3.07 mmol) was added to a stirred solution of 1-phenylethanol (rac-16; 1.25 g, 1.24 mL, 10.25 mmol) in THF at -78 °C. A solution of ZnCl2 (3.07 mL, 1 M in Et2O, 3.07 mmol) was added and the resulting solution was stirred for 2 min. An equimolar combination of pentafluorophenyl 2-phenylpropionate [(R)-11; 0.16 g, 0.51 mmol] and pentafluorophenyl 2-(6-methoxynaphthalen-2-yl)propionate [(S)-12; 0.203 g, 0.51 mmol] in THF (20 mL) was added, and the resulting solution was stirred for 12 h. The reaction was quenched by the addition of sat. aq NH4Cl (5 mL) and H2O (10 mL). The organic layer was extracted with CH2Cl2 (3 × 50 mL), washed with H2O (10 mL), dried (over MgSO4) and evaporated under reduced pressure. The residue was purified by flash column chromatography on silica gel eluting with light PE (40-60 °C)-Et2O (9:1) to give a pair of inseparable diastereomers (anti/syn, 93:7) of 1-phenylethyl 2-phenylpropionates (R,R)-anti-17 and (R,S)-syn-17 (0.10 g, 77%) as an oil {R
F [light PE (40-60 °C)-Et2O (1:1)] 0.80} and a pair of inseparable diastereomers (anti/syn, 94:6) of 1-phenylethyl 2-(6-methoxynaphthalen-2-yl)propionates (S,S)-anti-24 and (S,R)-syn-24 (0.13 g, 76%) as an oil {R
f
[light PE (40-60 °C)-Et2O (1:1)] 0.62}. Pentafluorophenol, if present, can be removed by an aq NaOH extraction. All compounds synthesised had satisfactory 1H and 13C NMR, IR and HRM spectra with >95% purity.
Characterisation data for:
1-Phenylethyl 2-Phenylpropionate [(R,R)-anti-17]: transparent solid; mp 81-83 °C; R
f
[light PE (40-60 °C)-Et2O, 1:1] 0.80; [α]D
20 +10.53 (c = 3.0, CHCl3) {lit.19 [α]D
20 +9.9 (c = 0.87, CHCl3)}. IR (CHCl3): 1730 (C=O) cm-1. 1H NMR (400 MHz, CDCl3): δ = 7.14-7.30 (m, 8 H, 8 × CH, PhA and PhB), 7.00-7.05 (m, 2 H, 2 × CH, PhA or PhB), 5.78 (q, J = 6.6 Hz, 1 H, PhCHMeO), 3.68 (q, J = 7.2 Hz, 1 H, PhCHMe), 1.43 (d, J = 7.2 Hz, 3 H, PhCHMe), 1.42 (d, J = 6.6 Hz, 3 H, PhCHMeO). 13C NMR (100 MHz, CDCl3): δ = 173.5 (C=O), 141.6 (i-C, PhCHMeO), 140.4 (i-C, PhCHMe), 128.5,2 128.3,2 127.6,2 127.5,1 127.0,1 125.6,2 (10 × CH, PhA, PhB), 72.4 (PhCHMeO), 45.7 (PhCHMe), 22.3 (PhCHMeO), 18.3 (PhCHMe). HRMS: m/z [MNH4
+] calcd for C17H22NO2: 272.1645; found: 272.1648. MS: m/z (%) = 254 (10) [M+], 105 (100) [PhCHMe+].
1-Phenylethyl 2-(6-Methoxynaphthalene-2-yl)propionate [(S,S)-anti-24]: white solid; mp 94-96 °C; R
f
[light PE (40-60 °C)-Et2O (1:1)] 0.62; [α]D
20 +26.6 (c = 3.2, CHCl3). IR (CHCl3): 1723 (C=O) cm-1. 1H NMR (400 MHz, CDCl3): δ = 7.66 (d, J = 8.4 Hz, 1 H, CH, Ar), 7.63 (d, J = 8.4 Hz, 1 H, CH, Ar), 7.55 (br s, 1 H, CH, Ar), 7.33 (dd, J = 1.8, 8.3 Hz, 1 H, CH, Ar), 7.08-7.19 (m, 7 H, 7 × CH, Ar, Ph), 5.86 (q, J = 6.6 Hz, 1 H, PhCHMeO), 3.90 (s, 3 H, OMe, Ar), 3.88 (q, J = 7.2 Hz, 1 H, ArCHMe), 1.56 (d, J = 7.2 Hz, 3 H, ArCHMe), 1.50 (d, J = 6.6 Hz, 3 H, PhCHCMeO). 13C NMR (100 MHz, CDCl3): δ = 173.7 (C=O), 147.5 (i-CO, Ar), 141.6 (i-C, Ph), 135.6 (i-C, Ar), 133.6, 128.9 (2 × i-C, Ar), 129.3,1 127.0,1 126.4,1 126.0,1 118.8,1 105.51 (6 × CH, Ar), 128.2,2 127.5,1 125.7,2 (5 × CH, Ph), 72.5 (PhCHMeO), 55.3 (OMe), 45.6 (ArCHMe), 22.3 (PhCHMeO), 18.4 (ArCHMe). HRMS: m/z [MNH4
+] calcd for C22H26NO3: 352.1907; found: 352.1907. MS: m/z (%) = 334 (20) [M+], 185 (100) [ArCHMe+], 105 (60) [PhCHMe+].
18 Fu has reported the use of 2-methyl-2-butanol (t-amyl alcohol) as a solvent within the efficient resolution of 1-phenylethanol using a chiral DMAP equivalent. For additional information, see: Ruble JC.
Tweddell J.
Fu GC.
J. Org. Chem.
1998,
63:
2794
19
Yang H.
Henke E.
Bornscheuer UT.
Tetrahedron: Asymmetry
1999,
10:
957