New Light-Driven Molecular Motor
A R T I C L E S
of the cis-3 olefin, multiple recrystallizations from ethyl acetate and
ethanol were required before obtaining it pure as slightly yellow crystals
(mp 178.6-179.9 °C); m/z (EI, %) ) 360 (M+, 100). HRMS (EI):
calcd. for C28H24 360.1878, found 360.1866. Resolution of cis-3 was
performed by chiral HPLC using a Daicel Chiralcel OD column as the
stationary phase and a mixture of heptane:2-propanol in a ratio of 99.5:
0.5 as the eluent at a rate of 1 mL/min. The first eluted fraction (t )
4.89 min) of cis-3 contained (2R,2′R)-(P,P)-(Z)-3, and the second
fraction (t ) 6.12 min) contained (2S,2′S)-(M,M)-(Z)-3. Both trans-3
isomers have the same retention time (t ) 5.54 min) using heptane:
2-propanol in a ratio of 99.5:0.5 as the eluent. The trans-3 isomer could
be separated using a Chiralcel OD column as the stationary phase and
a mixture of heptane:2-propanol in a ratio of 99.9:0.1 as the eluent at
a rate of 1 mL/min. The retention times of the enantiomers were t )
12.2 and 14.4 min, respectively. The first eluted fraction of trans-3
contained (2R,2′R)-(P,P)-(E)-3, and the second fraction contained
(2S,2′S)-(M,M)-(E)-3. By irradiation of an enantiomerically pure
solution of (2R,2′R)-(P,P)-(Z)-3 in n-hexane at -40 °C and subsequent
heating, a mixture of (2R,2′R)-(P,P)-(Z)-3 and (2R,2′R)-(P,P)-(E)-3 was
obtained, which was separated using preparative HPLC (Daicel
Chiralcel OD column as the stationary phase and a mixture of heptane:
2-propanol in a ratio of 99.9:0.1 as the eluent at a rate of 1 mL/min)
into the individual compounds. Racemic stable trans-3: 1H NMR (300
MHz, CDCl3) δ: 1.29-1.32 (d, J ) 6.2 Hz, 6H, Me2ax), 2.32-2.37
(d, J ) 14.7 Hz, 2H, H3eq), 2.93-3.00 (dd, J ) 14.7, 5.5 Hz, 2H,
H3ax), 3.03-3.09 (m, 2H, H2eq), 7.39-7.42 (d, J ) 8.1 Hz, 2H, H4),
7.44-7.49 (m, 2H, H7), 7.52-7.57 (m, 2H, H8), 7.75-7.78 (d, J )
8.1 Hz, 2H, H5), 7.90-7.93 (d, J ) 8.4 Hz, 2H, H6), 8.25-8.27 (d, J
the ratio of both compounds and hence their concentrations were
determined (HPLC, isosbestic point: 273 nm). Since both concentra-
tions and the ꢀ of the stable trans compound are known, the UV-vis
spectrum of the unstable cis compound can be calculated according to
A ) ꢀstable trans·cstable trans·l + ꢀunstable cis·cunstable cis·l. The UV-vis spectrum
of the unstable trans compound was obtained in a similar way. (2R,2′R)-
(M,M)-cis-3: UV-vis (calculated, n-hexane) λmax(ꢀ): 221 (80 700),
264 (27 100), 305 (8800), 317 (10 400), 397 (16 900). (2R,2′R)-(M,M)-
trans-3: UV-vis (calculated, n-hexane): λmax(ꢀ) 218 (95 600), 260
(27 900), 392 (23 400). The CD spectra of the unstable compounds
have been calculated in a similar fashion as the UV-vis spectra. A
sample of known concentration of the stable trans compound was
irradiated at -40 °C with λ g 280 nm. The CD spectrum taken after
the irradiation is the sum of the CD signal of the stable trans compound
and the unstable cis compound. After conversion of the unstable cis
compound to the stable cis compound, vide supra, the ratio of both
compounds and hence their concentrations can be determined (HPLC,
isosbestic point: 273 nm). Since both concentrations and the ∆ꢀ of
the stable trans compound are known, the CD spectrum of the unstable
cis compound can be calculated according to Θ ) (3.3 × 105
×
∆ꢀstable trans·cstable trans·l)/M + (3.3 × 105 × ∆ꢀunstable cis·cunstable cis·l)/M. The
CD spectrum of the unstable trans compound was obtained in a similar
way. (2R,2′R)-(P,P)-cis-3: CD (n-hexane) λ (∆ꢀ): 209.4 (30.4), 214.8
(65.6), 224.6 (-28.4), 231.4 (99.5), 269.8 (-172.5), 373.0 (-7.3).
(2R,2′R)-(M,M)-cis-3: CD (calculated, n-hexane) λ (∆ꢀ): 222.2 (133.8),
231.8 (-161.9), 251.2 (-48.5), 259.8 (-78.6), 279.2 (96.4), 343.4 (0.9),
395.8 (11.7). (2R,2′R)-(P,P)-trans-3: CD (n-hexane) λ (∆ꢀ): 214.0
(-156.4), 220.6 (36.0), 226.8 (-154.8), 236.6 (13.0), 248.4 (-85.7),
251.6 (-59.1), 258.0 (-236.5), 266.4 (8.5), 280.8 (-12.1), 285.8
(-8.9), 298.2 (-15.3), 330.0 (10.6), 351.8 (21.5), 366.6 (20.6).
(2R,2′R)-(M,M)-trans-3: CD (calculated, n-hexane) λ (∆ꢀ): 215.6
(106.0), 221.2 (-18.1), 230.4 (98.6), 247.0 (-10.4), 253.4 (20.9), 258.0
(5.8), 269.2 (55.7), 297.6 (-3.9), 317.2 (-0.3), 388.4 (-24.2).
1
) 8.4 Hz, 2H, H9). H NMR (500 MHz, C7D8, -20 °C) δ: 1.26-
1.27 (d, J ) 6.4 Hz, 6H), 2.04-2.07 (d, J ) 14.6 Hz, 2H), 2.72-2.76
(dd, J ) 14.6, 5.6 Hz, 2H), 3.07-3.10 (m, 2H), 7.18-7.20 (d, J ) 8.2
Hz, 2H), 7.30-7.33 (m, 2H), 7.38-7.41 (m, 2H), 7.56-7.58 (d, J )
8.1 Hz, 2H), 7.72-7.74 (d, J ) 8.1, 2H), 8.38-8.39 (d, J ) 8.1 Hz,
2H). 13C NMR (50.32 MHz, CDCl3) δ: 19.3 (q), 41.3 (t), 43.1 (d),
124.1 (d), 124.7 (d), 125.0 (d), 126.7 (d), 127.7 (d), 128.4 (d), 130.2
(s), 132.9 (s), 138.5 (s), 141.4 (s), 141.8 (s). UV-vis (n-hexane)
2-Methyl-2,3-dihydro-1H-cyclopenta[a]naphthalen-1-one 4. Ke-
tone 4 was prepared in a similar fashion as that decribed below for the
enantiomerically pure ketone (R)-4 from acid chloride 8. However, for
the Friedel-Crafts reaction toluene was used instead of dichloroethane
as the solvent.
λ
max(ꢀ): 217 (103 600), 245 (42 900), 352 (28 700), 368 (29 000).
Racemic stable cis-3: 1H NMR (300 MHz, CDCl3) δ: 1.22-1.24 (d,
J ) 6.6 Hz, 6H, Me2ax), 2.66-2.71 (d, J ) 14.7 Hz, 2H, H3eq), 3.56-
3.65 (m, 4H, H2eq, H3ax), 6.34-6.39 (m, 2H, H8), 6.57-6.60 (d, J
) 8.4 Hz, 2H, H9), 6.94-6.99 (m, 2H, H7), 7.47-7.50 (d, J ) 8.1
Hz, 2H, H4), 7.64-7.66 (d, J ) 8.1 Hz, 2H, H6), 7.70-7.72 (d, J )
(2R)-2-Methyl-2,3-dihydro-1H-cyclopenta[a]naphthalen-1-one (R)-
4. A mixture of acid (R)-5 (0.55 g, 2.6 mmol) in a solution of SOCl2
(1.3 mL), DMF (2 drops), and CH2Cl2 (50 mL) was refluxed for 1 h.
The volatiles were removed under reduced pressure, and the remaining
yellow oil was dissolved in dichloroethane (100 mL) and the solution
cooled to 0 °C. There was added quickly AlCl3 (0.69 g, 5.2 mmol, 2
equiv), and the reaction mixture was stirred for 45 min. Quenching
with a saturated aqueous solution of NaHCO3 (200 mL), extraction
with CH2Cl2 (3 × 50 mL), and drying over MgSO4 gave the ketone
(R)-4 as a slighly yellow oil (0.44 g, 2.2 mmol, 86%). HPLC: ee )
99%. [R]D20 ) -95.7° (c ) 0.65, CHCl3). 1H NMR (300 MHz, CDCl3)
δ: 1.37-1.39 (d, J ) 7.0 Hz, 3H), 2.77-2.87 (m, 2H), 3.44-3.53
(dd, J ) 16.0, 8.1 Hz, 1H), 7.49-7.52 (d, J ) 8.4 Hz, 1H), 7.53-7.58
(m, 1H), 7.65-7.70 (m, 1H), 7.88-7.91 (d, J ) 8.1 Hz, 1H), 8.03-
8.06 (d, J ) 8.4 Hz, 1H), 9.14-9.17 (d, J ) 8.4 Hz, 1H). 13C NMR
(75.48 MHz, CDCl3) δ: 16.1 (q), 34.7 (t), 41.8 (d), 123.38 (d), 123.43
(d), 126.0 (d), 127.6 (d), 128.2 (d), 129.0 (s), 129.5 (s), 132.1 (s), 135.1
(d), 156.1 (s), 209.2 (s). m/z (EI, %) ) 196 (M+, 100), 181 (83). HRMS
(EI): calcd. for C14H12O 196.0888, found 196.0898. Determination of
ee by HPLC using a Daicel Chiralcel OB-H column, heptane:i-
propanol ) 99:1, tret(S)-4: 19.3 min, tret(R)-4: 22.1 min.
1
8.1 Hz, 2H, H5). H NMR (500 MHz, C7D8, -20 °C) δ: 1.17-1.18
(d, J ) 6.5 Hz, 6H), 2.46-2.49 (d, J ) 15.0 Hz, 2H), 3.38-3.42 (dd,
J ) 15.0, 6.0 Hz, 2H), 3.45-3.48 (m, 2H), 6.35-6.38 (m, 2H), 6.80-
6.83 (m, 2H), 7.34-7.36 (d, J ) 8.1 Hz, 2H), 7.50-7.51 (d, J ) 8.1
Hz, 2H), 7.57-7.59 (d, J ) 7.7 Hz, 2H); one aromatic (d) was not
observed due to overlap with the solvent. 13C NMR (75.48 MHz,
CDCl3) δ: 20.7 (q), 40.6 (t), 42.2 (d), 123.5 (d), 124.0 (2xd), 126.7
(d), 127.6 (d), 128.4 (d), 129.9 (s), 132.2 (s), 136.9 (s), 140.0 (s), 144.0
(s). UV-vis (n-hexane) λmax(ꢀ): 222 (91 900), 255 (36 000), 294 (8100),
306 (10 100), 332 (8600), 370 (19 600). Racemic unstable trans-3:1H
NMR (500 MHz, C7D8, -20 °C) δ: 0.68-0.69 (d, J ) 6.0 Hz, 6H),
2.76-2.81 (dd, J ) 15.0, 7.7 Hz, 2H), 2.94-2.98 (dd, J ) 15.0, 7.7
Hz, 2H), 3.45-3.48 (m, 2H), 7.24-7.26 (d, J ) 7.7 Hz, 2H), 7.28-
7.31 (m, 2H), 7.36-7.38 (m, 2H), 7.59-7.61 (d, J ) 8.1 Hz, 2H),
7.71-7.73 (d, J ) 7.7 Hz, 2H), 7.95-7.97 (d, J ) 8.6 Hz, 2H). Racemic
unstable cis-3: 1H NMR (500 MHz, C7D8, -20 °C) δ: 1.46-1.47 (d,
J ) 6.0 Hz, 6H), 2.93-2.97 (dd, J ) 15.6, 7.2 Hz, 2H), 3.07-3.14
(m, 2H), 3.58-3.63 (m, 2H), 6.49-6.52 (m, 2H), 6.75-6.78 (m, 2H),
7.26-7.29 (m, 4H), 7.41-7.43 (d, J ) 8.1 Hz, 2H), 7.52-7.53 (d, J
) 8.1 Hz, 2H). A sample of known concentration of the stable trans
compound was irradiated at -40 °C with λ g 280 nm. The UV-vis
spectrum taken after the irradiation is the sum of the absorption of the
stable trans compound and the unstable cis compound. After conversion
of the unstable cis compound to the stable cis compound, vide supra,
2-Methyl-3-naphthalen-2-yl-propionic Acid 5. A mixture of ester
7 (1.72 g, 7.5 mmol), KOH (6.0 g), EtOH (30 mL), and water (30 mL)
was heated at reflux overnight. Acidification of the reaction mixture
with 30% HCl and extraction with Et2O (3 × 100 mL), washing of the
combined organic layers with water (3 × 100 mL), followed by drying
over MgSO4 gave after removal of the solvent the acid 5 as a white
solid (1.4 g, 6.5 mmol, 87%). mp 87.4-88.4 °C.
9
J. AM. CHEM. SOC. VOL. 125, NO. 49, 2003 15085