Enantioselective conjugate allylation of cyclic enones

Article abstract of DOI:10.1021/jo2013753

Enantioselective organocatalytic 1,2-allylation of a cyclic enone followed by anionic oxy-Cope rearrangement delivered the ketone as a mixture of diastereomers. This appears to be a general method for the net enantioselective conjugate allylation of cycli

Full text of DOI:10.1021/jo2013753

NOTE
pubs.acs.org/joc
Enantioselective Conjugate Allylation of Cyclic Enones
Douglass F. Taber,* David A. Gerstenhaber, and James F. Berry
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
S Supporting Information
b
ABSTRACT: Enantioselective organocatalytic 1,2-allylation of a cyclic enone followed
by anionic oxy-Cope rearrangement delivered the ketone as a mixture of diastereomers.
This appears to be a general method for the net enantioselective conjugate allylation of
cyclic enones.
everal procedures have been put forward in recent years1 for
enantioselective conjugate addition to prochiral cyclic en-
rearrangement of the aryl-substituted allylated alcohols (3dAr,
3cAr) proceeded efficiently without 18-crown-6, but the yields
were slightly higher when it was included.
S
ones. To date, however, there has been only one report1b of
enantioselective conjugate addition to an α-substituted cyclic
enone such as 1a. It occurred to us that catalytic enantioselective
1,2-allylation2 followed by oxy-Cope rearrangement3 could offer
a solution4,5 to this long-standing problem (eq 1).
Of the substances reported here, only 4b had previously been
reported, in racemic form and without characterization.11 We
expect that the net catalytic enantioselective conjugate allylation
of cyclic enones introduced here will have many applications in
target-directed synthesis. The practicality of the Schaus organo-
catalytic allylation (room temperature in t-BuOH, 5 mol %
catalyst, commercial, and easily recoverable) is particularly note-
worthy.
’ EXPERIMENTAL SECTION
General Procedures. 1H NMR and 13C NMR spectra were
recorded, as solutions in deuteriochloroform (CDCl3) unless otherwise
indicated, at 400 and 100 MHz, respectively. 13C multiplicities were
determined with the aid of a JVERT pulse sequence, differentiating
the signals for methyl and methine carbons as “d” from methylene and
quaternary carbons as “u”. The infrared (IR) spectra were determined as
neat oils. Rf values indicated refer to thin-layer chromatography (TLC)
on 2.5 ꢀ 10 cm, 250 μm analytical plates coated with silica gel GF and
developed in the solvent system indicated. All glassware was oven-dried
and rinsed with dry solvent before use. THF was distilled from sodium
metal/benzophenone ketyl under dry nitrogen. Toluene, dichloro-
methane, and acetonitrile were distilled from calcium hydride under
dry nitrogen. CH2Cl2 is dichloromethane, MTBE is methyl tert-butyl
ether, and PE is petroleum ether. All reactions were conducted under N2
and stirred magnetically. We prepared KH in paraffin, but it is now
commercially available.
(R)-1-(2-Propenyl)-2-(3-phenoxymethoxypropyl)-2-cyclo-
hexenol (3a). To a dry 10 mL round-bottom flask was charged
(S)-(ꢁ)-3,30-dibromo-1,10-bi-2-naphthol (73 mg, 0.165 mmol), fol-
lowed by tert-butyl alcohol (362 mg, 8.23 mmol) and allylborane 2
(778 mg, 6.17 mmol). This suspension was stirred until all of the BINOL
was dissolved. To this clear solution was charged enone 1a (1.00 g, 4.12
mmol), and the reaction was stirred at room temperature overnight.
α-Iodo and α-alkyl6 cyclic enones (Table 1) are easily
prepared.7 Of the several methods2 that have been put forward
for the catalytic enantioselective allylation of ketones, we were
attracted to that of Schaus,2g,8 which employed the easily
prepared allylboronate 2 as the allyl donor and the commercially
available 3,30-dibromo-BINOL as the enantioselective catalyst.
We initiated our studies with the enone 1a6a,f (Table 1).
Following the updated Schaus protocol,8 stirring the enone with
2 in concentrated t-BuOH solution with a catalytic amount of
(S)-3,30-dibromo-1,10-bi-2-naphthol at room temperature for
24 h, we found that the 1,2-addition proceeded smoothly. It
was gratifying that 5 mol % of the organocatalyst was sufficient
and that >90% of the catalyst could be recovered by extraction.
We found that this protocol worked equally well for 5-, 6-, and
7-membered rings and with 2-alkyl and 2-iodo substitution. The
enantiomeric excess for each of the 1,2-allylations was established
by chiral HPLC, except for 3b, the ee of which was secured by
comparison of the optical rotation of the chromatographed and
distilled product with that of the same substance prepared by
methyl coupling of 3d.
For the oxy-Cope rearrangements (Table 2), we found it con-
venient to use KH in paraffin.9 With the alkyl enones, it was also
necessary to include an equivalent of 18-crown-6.10 The oxy-Cope
Received: July 6, 2011
Published: August 10, 2011
r
2011 American Chemical Society
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|

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NOTE
Table 1. Enantioselective Allylation of Cyclic Enones
Table 2. KH-Mediated Oxy-Cope Rearrangement
a Additions were carried out using 5 mol % of (S)-3,30-dibromo-BINOL.
c
b Yields are for pure isolated substances. Addition was effected using
5 mol % of (R)-3,30-dibromo-BINOL. d Both the starting material and
the product were volatile.
The reaction was concentrated directly to silica and chromatographed to
give 3a (938 mg, 84% yield, 98% ee) as a yellow oil: enantioselective
HPLC (3% i-PrOH/hexanes, Chiralpak IA 4.6 mm ꢀ 250 mm, UV
detection at 254 nm, 0.08 mL/min) tR = 6.74 (major) tR = 4.68 (minor);
TLC Rf = 0.24 (MTBE/PE, 20:80); [α]D +23.3 (DCM, 20 °C); IR
(neat, cmꢁ1) 3439 (s), 2928 (s), 2850 (m), 1668 (m), 1633.8 (s), 1442.1
(s), 1363.5 (s); 1H NMR (400 MHz, CDCl3) δ ppm 1.5ꢁ2.5 (m, 13H),
3.5 (m, 2H), 4.5 (s, 2H), 5.1 (m, 1H), 5.2 (m, 1H), 5.8 (m, 1H), 7.3ꢁ7.4
(m, 5H); 13C NMR (400 MHz, CDCl3) δ ppm u: 139.9, 138.0, 117.6,
77.0, 76.7, 76.3, 72.5, 71.4, 69.9, 43.5, 35.7, 28.4, 27.0, 25.3, 18.5; d:
133.9, 127.9, 127.3, 127.1, 124.9; HRMS calcd for C19H25O (M+ ꢁOH)
269.1905, found 269.1909.
a Oxy-Cope rearrangement was carried out with dicyclohexyl-18-crown-
6 and KH in THF. b Products were a mixture of α-epimers. c Yields
are for pure isolated substances. d t-BuOK was used for the rearrange-
ment. e Prepared by Stille coupling of the corresponding iodoalkene.
f Prepared by Kumada coupling of the corresponding alkene.
33.0, 32.8 d: 142.1, 132.7; HRMS calcd for C8H10I (M+ ꢁ OH)
232.9827, found 232.9826.
(S)-1-(2-Propenyl)-2-methyl-2-cyclohexenol (3b): light yel-
low oil; [α]20D = ꢁ34.9 (c = 1.00, CH2Cl2); TLC Rf (MTBE/PE, 1:4) =
0.46; 1H NMR δ 5.78 (m, 1H), 5.64 (m, 1H), 5.2 (m, 2H), 2.40 (dt, J =
7.2, 1.2 Hz, 2H), 1.98 (m, 2H), 1.77 (m, 1H), 1.76 (s, 3H), 1.64 (m, 4H);
13C NMR δ u: 137.0, 118.2, 71.6, 43.6, 35.7, 25.6, 19.1; d: 134.1, 126.6,
17.8; IR 1639, 1440, 1174, 973, 912 cmꢁ1; HRMS calcd for C10H15
(M ꢁ OH) 135.1174, obsd 135.1173.
(R)-1-(2-Propenyl)-2-iodo-2-cyclohexenol (3d): clear oil
(95% yield, 92% ee); TLC Rf = 0.29 (MTBE/PE 20:80); [α]D ꢁ34.9
(DCM, 20°C); chiral HPLC (2% i-PrOH/hexanes, Chirapak IA 4.6 mm ꢀ
250 mm; UV detection at 254 nm, 0.08 mL/min) tR = 17.34 (major),
tR = 16.80 (minor); IR (neat, cmꢁ1) 3436 (s), 3074 (m), 2937 (m), 1638
(m), 1435 (s); 1H NMR (400 MHz, CDCl3) d ppm 1.6ꢁ1.8 (m, 2H),
1.8ꢁ2.2 (m, 5H), 2.4ꢁ2.5 (d, 2H), 5.1ꢁ5.2 (m, 2H), 5.7ꢁ5.9 (m, 1H),
6.6 (m, 1H); 13C NMR (400 MHz, CDCl3) d ppm u: 118.9, 111.7, 73.0,
47.1, 34.1, 29.6, 18.9; d: 141.9, 132.9; HRMS calcd for C9H12I
(M+ ꢁOH) 246.9992, found 246.9984
(R)-1-(2-Propenyl)-2-iodo-2-cyclopentenol (3c): yellow solid
(mp 32ꢁ35 °C, 92% yield, 93% ee); TLC Rf = 0.29 (DCM/MTBE/PE
10:20:70); chiral HPLC (3% i-PrOH/hexanes, Chirapak IA 4.6 mm ꢀ
250 mm, UV detection at 254 nm, 0.08 mL/min) tR = 8.420 (major),
tR = 9.205 (minor); [α]D +52.4 (DCM, 20 °C); IR (neat, cmꢁ1) 3400
(s), 3066 (m), 2918 (s), 2840 (m), 1638 (m), 1599 (m), 1427 (m), 1373
(R)-1-(2-Propenyl)-2-iodo-2-cycloheptenol (3e): yellow oil
(89% yield, 85% ee); TLC Rf = 0.50 (MTBE/PE, 20:80); [α]D ꢁ46
(DCM, 20 °C); chiral HPLC (0.1% i-PrOH/hexanes, Chiracel OJH 4.6
mm ꢀ 250 mm, UV detection at 254 nm, 0.08 mL/min) tR = 10.721
(major), tR = 12.399 (minor); IR (neat, cmꢁ1) 3459 (b), 3076 (m), 2918
(s), 2859 (m), 1682 (m), 1643 (m), 1609 (m), 1442 (m), 1343 (m) 1H
1
(m), 1309 (m) H NMR (400 MHz, CDCl3) δ ppm 1.9 (m, 2H),
2.2ꢁ2.6 (m, 5H), 5.1ꢁ5.2 (dd, 2H), 5.6ꢁ5.9 (m, 1H), 6.3 (s, 1H) 13C
NMR (400 MHz, CDCl3) δ ppm u: 119, 134.0, 106.5, 86.0, 123.5, 44.5,
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NOTE
NMR (400 MHz, CDCl3) δ ppm 1.6ꢁ1.9,(m, 5H), 2.0ꢁ2.2 (m, 4H),
2.5 (m, 2H), 5.2 (ds, 2H), 5.8ꢁ5.9 (m, 1H), 6.7 (t, 1H); 13C NMR (400
MHz, CDCl3) δ ppm 141.5, 132.5, 118.6, 118.0, 77.7, 44.3, 33.0, 28.5,
24.7, 20.7; HRMS calcd for C10H14I (M+ ꢁ OH) 261.0141, found
261.0198.
(m), 2939 (s), 2836 (s), 1740 (s), 1640 (m), 1584 (m), 1478 (s), 1268
(s); 1H NMR (400 MHz, CDCl3) δ ppm 1.4ꢁ1.6 (m, 1H) 2.0 (m, 1H)
2.2ꢁ2.4 (m, 2H) 2.4ꢁ2.5 (m, 2H) 3.0 (d, 1H, J = 10.8 Hz), 3.7 (s, 3H),
3.9 (s, 3H), 5.0 (m, 2H), 5.7 (m, 1H), 6.6 (d, 1H, J = 8.2 Hz), 6.8ꢁ6.9 (d,
1H, J = 8.2 Hz), 7.0 (t, 1H, J = 8.2 Hz); 13C NMR (400 MHz, CDCl3)
δ ppm u: 218.4, 152.7, 146.9, 132.7, 116.6, 38.2, 38.0, 27.3 d: 135.7,
123.7, 122.8, 111.8, 59.8, 58.4, 55.7, 44.0; HRMS calcd for C16H20O3
(M+) 260.1412, found 260.1401.
(1S,2S)- 2-(3-Phenoxymethoxypropyl)- 3-(2-propenyl)cyclo-
hexanone (4a): To a 100 mL round-bottom flask was charged 3a
(1.07 g, 3.77 mmol), followed by 18-crown-6 (996 mg, 3.77 mmol) and
50 mL of THF. The reaction was sparged with N2 for 10 min, and
KH(P) (452 mg 5.65 mmol) was added portionwise. The reaction was
heated to reflux for 1 h and then quenched with saturated aqueous
ammonium chloride. The organic layer was partitioned between Et2O
and water. The organic layer was dried (anhydrous Na2SO4) and con-
centrated. The residue was chromatographed giving 4a (749 mg, 70%
yield) as a yellow oil: TLC Rf = 0.20 (DCM/MTBE/PE, 10:20:70);
[α]D +18 (c = 0.02, CH2Cl2, 20 °C) IR (neat, cmꢁ1) 3029 (m), 2931
(s), 1709 (m), 1639 (m), 1495 (s), 1453 (s), 1360 (s); major
(1S,2S)-2-Methyl-3-(2-propenyl)cycloheptanone (4f): yel-
low oil (52% yield) as a mixture of diastereomers; TLC Rf = 0.30
(MTBE/PE, 5:95); [α]20 ꢁ45.0 (c = 0.03; CH2Cl2, 20 °C); IR
D
(neat, cmꢁ1) 3074 (m), 2927 (s), 2862 (s), 1700 (s), 1640 (m), 1451
(m), 1374 (m), 1H NMR (400 MHz, CDCl3) δ ppm 1.0ꢁ1.1 (d, 2H,
J=8.2 Hz) 1.1ꢁ1.2 (d, 1H, J=8.2 Hz) 1.5ꢁ2.0 (m, 8H) 2.1 (m, 1H)
2.3ꢁ2.4 (m, 2H), 2.6 (m, 1H), 2.8ꢁ2.9 (m, 1H), 5.0 (m, 2H), 5.0ꢁ5.1
(m, 2H), 5.6ꢁ5.8 (m, 1H); major 13C NMR (400 MHz, CDCl3)
δ ppm u: 215.7, 116.2, 43.3, 34.5, 32.3, 25.9, 24.2 d: 137.2, 49.3, 40.1,
12.9; HRMS calcd for C11H19O (M + H) 167.1436, found 167.1435.
1
diastereomer: H NMR (400 MHz, CDCl3) δ ppm 1.49 - 2.45 (m,
14H) 3.5 (t, 2 H) 4.5 (s, 2 H) 5.1 (m, 2 H) 5.8 (m, 1 H) 7.21 - 7.4 (m,
5H); 13C NMR (400 MHz, CDCl3) δ ppm u: 213.3, 138.6, 117.0, 72.7,
70.3, 37.8, 28.7, 27.3, 26.2, 24.7, 24.1, 22.0 d: 135.7, 128.3, 127.5, 54.6,
41.9; HRMS calcd for C19H27O2 (M + H) 287.2011, found 287.2002.
(1R,2R)-2-Methyl-3-(2-propenyl)cyclohexanone (4b). To a
500 mL round-bottom flask were charged toluene (140 mL), 18-crown-
6 (6.48 g, 24.5 mmol), and potassium tert-butoxide (5.49 g, 49 mmol).
Ketone 3b (5.33 g, 35 mmol) was charged in 30 mL of toluene. The
reaction was heated to 84 °C for 1 h and was then quenched with
saturated aqueous NH4Cl. The toluene/PE solution was then diluted
with more PE and placed directly on a column for purification by silica
gel chromatography to afford ketone 4b (2.162 g, 64% yield, mixture of
trans/cis diastereomers) as a light yellow oil: [α]20D = +10.0 (c = 1.00,
CH2Cl2, 2:1 mixture of trans:cis diastereomers); TLC Rf (MTBE/PE,
1:4) = 0.67; 1H NMR δ 5.85ꢁ5.73 (m, 1H, trans diastereomer),
5.75ꢁ5.63 (m, 1H, cis diastereomer), 5.11ꢁ5.03 (m, 2H, trans dia-
stereomer), 5.03ꢁ4.98 (m, 2H, cis diastereomer), 2.66ꢁ2.00 (m, 6H),
1.93ꢁ1.41 (m, 4H), 1.05 (d, J = 12.8 Hz, 3H, trans diastereomer), 1.02
(d, J = 12.8 Hz, 3H, cis diastereomer); 13C NMR (trans diastereomer)
δ u: 213.4, 117.1, 41.5, 38.2, 30.3, 25.7; d: 135.5, 49.4, 45.2, 11.9; (cis
diastereomer) δ u: 214.5, 116.3, 39.8, 33.7, 26.7, 23.7; d: 136.5, 48.7,
41.9, 11.5; IR: 1711, 1640, 1447, 1221, 999, 914 cmꢁ1; HRMS calcd for
C10H17O (M + H) 153.1279, obsd 153.1283.
’ ASSOCIATED CONTENT
Supporting Information. 1H and 13C NMR spectra for
S
b
all new compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: taberdf@udel.edu.
’ ACKNOWLEDGMENT
We thank Dr. John Dykins for mass spectrometric measure-
ments, supported by the NSF (0541775), Dr. Shi Bai for NMR
assistance (NSF CRIF:MU, CHE 0840401), and the NIH
(GM42056) for financial support. We thank the Schaus group
for sharing their results prior to publication.
’ REFERENCES
(1) (a) Catalytic enantioselective conjugate allylation is effective
with doubly activated cycloalkenones: Shizuka, M.; Snapper, M. L.
Angew. Chem., Int. Ed. 2008, 47, 5049. (b) Despite excellent progress
with catalytic enantioselective conjugate addition, there has been only
one report of addition to an α-alkyl or α-aryl cyclic enone: Vuagnoux-
d’Augustin, M.; Alexakis, A. Chem.—Eur. J. 2007, 13, 9647. For other
leading references, see: (c) Alexakis, A.; Li, K. Angew. Chem., Int. Ed.
2006, 45, 7600. (d) Wencel, J.; Rix, D.; Jennequin, T.; Labat, S.; Crꢀevisy,
C.; Mauduit, M. Tetrahedron: Asymmetry 2008, 19, 1804. (e) Malmgren,
M.; Granander, J.; Amedjkouh, M. Tetrahedron: Asymmetry 2008,
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Asymm. 2009, 20, 2866.
(2) Several methods have been developed for the enantioselective
1,2-allylation of ketones: (a) Tietze, L. F.; Schiemann, K.; Wegner, C.
J. Am. Chem. Soc. 1995, 117, 5851. (b) Tagliavini, E.; Casolari, S.;
D’Addario, D. Org. Lett. 1999, 1, 1061. (c) Wada, R.; Oisaki, K.; Kanai,
M.; Shibasaki, M. J. Am. Chem. Soc. 2004, 126, 8910. (d) Walsh, P. J.;
Kim, J. G.; Waltz, K. M.; Garcia, I. F.; Kwiatkowski, D. J. Am. Chem. Soc.
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(1R,2S)-2-(2,3-Dimethoxyphenyl)-3-(2-propenyl)cyclohe-
xanone (4c): white solid (89% yield, mp = 70ꢁ75 °C); TLC Rf = 0.5
(MTBE/PE 20:80); [α]20 ꢁ64 (c = 0.2 CH2Cl2, 20 °C); IR
D
(neat, cmꢁ1) 2934 (s), 2931 (s), 1710 (s), 1584 (m), 1475 (s), 1267
1
(s), H NMR (400 MHz, CDCl3) δ ppm 1.5 (m, 2H) 1.8 (m, 2 H)
2.0ꢁ2.2 (m, 4 H) 2.4 (m, 1H) 2.6 (m, 1 H), 3.6 (d, 1H, J = 9.2 Hz), 3.7
(s, 3H), 3.9 (s, 3H), 4.9 (m, 2H), 5.6ꢁ5.8 (m, 1H), 6.6 (d, 1H, J = 8.2
Hz) 6.8 (d, 1H, J = 8.2 Hz) 7.1 (t, 1H, J = 8.2 Hz) 13C NMR (400 MHz,
CDCl3) δ ppm u: 210.0, 152.6, 147.4, 131.6, 116.8, 41.8, 39.0, 30.6, 25.1
d: 135.8, 123.7, 121.7, 111.0, 60.4, 56.8, 55.6, 43.5; HRMS calcd for
C17H23O3 (M + H) 275.1647, found 275.1659.
(1S,2S)-2-(3-Phenoxymethoxypropyl)-3-(2-propenyl)cyclo-
pentanone (4d): yellow oil (52% yield); TLC Rf = 0.35 (MTBE/PE,
20:80); [α]20D ꢁ72 (c = 0.1 CH2Cl2, 20 °C); IR (neat, cmꢁ1) 2909 (m),
2857 (s), 1735 (s), 1700 (s), 1638 (m), 1450 (m); 1H NMR (400 MHz,
CDCl3) δ ppm 1.3ꢁ2.5 (m, 13H) 3.4ꢁ3.5 (m, 2H) 4.5 (m, 2H), 2.1 (m,
1H) 5.0ꢁ5.1 (m, 2H), 5.7ꢁ5.9 (m, 1H), 7.2ꢁ7.4 (m, 5H), 13C NMR
(400 MHz, CDCl3) δ ppm u: 220.2, 138.1, 116.2, 72.4, 69.9, 38.1, 37.2,
27.3, 26.5, 21.6 d: 137.5, 127.9, 126.5, 126.0, 52.1, 41.9; HRMS calcd for
C18H25O2 (M + H) 273.1854, found 273.1850.
(1R,2S)-2-(2,3-Dimethoxyphenyl)-3-(2-propenyl)cyclopen-
tanone (4e): yellow oil (67% yield); TLC Rf = 0.67 (MTBE/PE,
20:80);); [α]20D +38 (c = 0.1 CH2Cl2, 20 °C); IR (neat, cmꢁ1) 3073
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NOTE
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Toll, L.; Asplund, M. C. J. Org. Chem. 2009, 74, 1187. (b) Li, F.;
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introduced by alkylation. For an overview, see: (a) Stork, G.; Rosen,
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Chem. Soc. 1993, 115, 11014. (c) Armitage, M. A.; Lathbury, D. C.;
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Tunoori, A. R.; Niestro, A. J.; Gronwald, O.; Maier, M. E. Tetrahedron
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