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<A NAME="RD32304ST-12">12</A> For a recently reported route to an indenol using a RCM approach see:
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Typical Experimental Procedure for Forming Indenols:
Grubbs catalyst 1 (5 mol%) was added to a degassed solution (N2) of 5a (262 mg, 0.62 mmol) in CH2Cl2 (25 mL). The solution was then stirred under N2 at r.t. for 3 h. After evaporation of the solvent and column chromatographic purification
of the residue (5-10% EtOAc-hexane), 4-isopropoxy-5-methoxy-1H-inden-1-ol (6a) was obtained as a pale yellow oil (191 mg, 87%). 1H NMR (200 MHz, CDCl3, assignments with the same superscript may be interchanged): δ = 7.12 (d, 1 H, J = 7.9 Hz, 7-H), 6.73 (d, 1 H, J = 5.6 Hz, 3-H), 6.66 (d, 1 H, J = 7.9 Hz, 2-H), 6.27 (dd, 1 H, J = 5.6 and 2.0 Hz, 6-H), 5.04 (br s, 1 H, 1-H), 4.33 [br sept, 1 H, J = 6.2 Hz, CH(CH3)2], 3.80 (s, 3 H, OCH3), 2.08 (br s, 1 H, OH), 1.28 [d, 3 H, J = 6.4 Hz, CH(CH
3)CH3], 1.25 [d, 3 H, J = 6.5 Hz, CH(CH3)CH
3]. 13C NMR (50 MHz, CDCl3): δ = 153.3 (C), 140.1 (C), 138.4 (C), 137.8 (CH), 136.7 (C), 129.2 (CH), 118.9 (CH),
109.4 (CH), 77.1 (OCH), 75.6 (C-O), 55.9 (OCH3), 22.5 (CH3), 22.4 (CH3).
IR: 3400(br), 1674, 1617, 1596, 1556 cm-1. HRMS (CI):
m/z calcd for C13H16O3: 220.10995; found: 220.10990. MS: m/z (%) = 220 (69) [M
+
], 179 (18), 178 (100), 177 (28), 176 (16), 163 (41), 161 (12), 149 (16), 147 (18),
146 (18), 135 (10), 118 (10), 77 (10), 69 (11), 57 (14), 55 (12), 43 (28), 41 (19).
<A NAME="RD32304ST-18">18</A>
All new compounds were identified using routine spectroscopy and gave satisfactory
data.
<A NAME="RD32304ST-19">19</A>
In an attempt to improve the yields of indenols 6d and 6e we protected the alcohol functionalities of 5d and 5e with acetyl groups. However, much to our surprise, the RCM reactions of these substrates,
at r.t. or at 110 °C, did not proceed to afford the acetyl-protected cyclic products
and only starting material was recovered.
<A NAME="RD32304ST-20">20</A>
Typical Experimental Procedure for Forming Indenones:
Grubbs catalyst 1 (8 mol%) was added to a degassed solution (N2) of 5a (141 mg, 0.54 mmol) in toluene (10 mL). The solution was then heated at 80 °C under
N2 for 2 h. Evaporation of the solvent and column chromatographic purification of the
residue (20% EtOAc-hexane) then afforded 4-isopropoxy-5-methoxyinden-1-one (7a) as a yellow semi-solid (72 mg, 62%). 1H NMR (200 MHz, CDCl3): δ = 7.64 (d, 1 H, J = 5.8 Hz, H-3), 7.38 (d, 1 H, J = 7.8 Hz, H-7), 6.60 (d, 1 H, J = 7.8 Hz, H-6), 5.86 (d, 1 H, J = 5.8 Hz, H-2), 4.40 [sept, 1 H, J = 6.1 Hz, CH(CH3)2], 3.87 (s, 3 H, OCH3), 1.31 [d, 6 H, J = 6.1 Hz, CH(CH
3)2]. 13C NMR (50 MHz, CDCl3): δ = 197.2 (C=O), 158.8 (C), 145.5 (CH), 142.0 (C), 137.5 (C), 127.9 (CH), 123.9
(C), 119.7 (CH), 109.8 (CH), 75.9 (OCH), 56.1 (OCH3), 22.4 (CH3). IR: 1708, 1604 cm-1. HRMS (CI): m/z calcd for C13H14O3: 218.09430; found: 218.09433. MS: m/z (%) = 218 (29) [M
+
], 193 (21), 178 (20), 177 (40), 176 (100), 175 (25), 163 (17), 161 (23), 149 (37),
147 (20), 105 (21), 91 (17), 77 (26), 73 (23), 69 (22), 57 (33), 55 (26), 43 (49),
41 (46).
<A NAME="RD32304ST-21">21</A>
X-ray crystal structure of 4-isopropoxy-5-methoxy-2-methyl-3-phenylinden-1-one (7f): crystallized from CHCl3, formula: C20H20O3, M = 308.36, color of crystal: yellow, prism, crystal size 0.31 × 0.20 × 0.02 mm, a = 6.3298 (11) Å, b = 18.133 (3) Å, c = 7.7645 (13) Å, β = 110.094 (3)°, V = 836.9 (2) Å3, ρcalc = 1.224 Mg/m3, µ = 0.081 mm-1, F(000) = 328, Z = 2, monoclinic, space group P21, T = 293 K, 5132 reflections collected, 2444 independent reflections, θmax = 26.50°, 212 refined parameters, maximum residual electron density 0.129 and -0.161
e·Å-3. R1 = 0.0421, wR2 = 0.0813. Crystallographic data for the structure have been deposited with the Cambridge
Crystallographic Data Centre as deposition No. CCDC-250072.
<A NAME="RD32304ST-22">22</A> Storage of the indenols (as oils) without the exclusion of air resulted in a
slow conversion into indenones (ca. 5% over six months)
A literature search also revealed that mild oxidants such a MnO2,23a chromium(III)(salen) with PhI(OAc)2
23b or PhIO23c have been successfully used to accomplish the indenol to indenone transformation.
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in more detail. 4-Isopropoxy-5-methoxyindan-1-one (8): 1H NMR (200 MHz, CDCl3): δ = 7.48 (d, 1 H, J = 8.3 Hz, 7-H), 6.95 (d, 1 H, J = 8.3 Hz, 6-H), 4.51 [sept, 1 H, J = 6.1 Hz, CH(CH3)2], 3.90 (s, 3 H, OCH3), 3.08-3.02 (m, 2 H, 2-H), 2.66-2.60 (m, 2 H, 3-H), 1.29 [d, 6 H, J = 6.1 Hz, CH(CH
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