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General Method for the Organocatalytic Aldol Condensation of Ketones 5 and 6.
Organocatalyst 3 or 4 (0.05 mmol) was added to a stirred solution of triketone 5 or 6 (1.0 mmol) in anhyd MeCN (2.0 mL). The reaction mixture was refluxed for an appropriate
time. The reaction was monitored by capillary GC. After completion of the reaction,
toluene (5 mL) was added, the mixture was concentrated under vacuum and the crude
product was purified by chromatography on silica gel (30% EtOAc in PE). The obtained
ketones 1 and 2 are known compounds and our spectroscopic and chromatographic data are in agreement
with published data. The ee was determined by HPLC (Daicel Chiralcel OD-H (250 × 4.6
mm), detection at λ = 254 nm, eluent: 4% of i-PrOH in hexane, flow rate 0.8 mL/min); t
R (S)-1 = 19.4 min, t
R (R)-1 = 21.6 min, t
R (S)-2 = 25.6 min, t
R (R)-2 = 28.3 min.
<A NAME="RG08106ST-18">18</A> Converting stereogenic N atoms via chelation is described by:
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(4a
S
*,8a
S
*)-6,8a-Dimethylhexahydro-2
H
-chromene-2,5 (3
H
)-dione (7).
A 2:1 mixture of two isomers with differing orientations of the 6-methyl group. Main
isomer with 6R* configuration: 1H NMR (500 MHz, CDCl3): δ = 2.73 (m, 1 H, H-3), 2.54 (m, 1 H, H-4a), 2.49 (m, 1 H, H-3), 2.43 (m, 1 H,
H-6), 2.42 (m, 1 H, H-4), 2.15 (m, 1 H, H-8), 1.97 (m, 1 H, H-8), 1.94 (m, 1 H, H-7),
1.93 (m, 1 H, H-4), 1.71 (m, 1 H, H-7), 1.54 (s, 3 H, 8a-Me), 1.03 (d, J = 6.5 Hz, 3 H, 6-Me). 13C NMR (125 MHz, CDCl3): δ = 209.33 (C-5), 171.24 (C-2), 86.19 (C-8a), 49.52 (C-4a), 44.32 (C-6), 37.48
(C-8), 29.83 (C-7), 28.39 (C-8a-Me), 25.54 (C-3), 15.69 (C-4), 14.19 (C-6 Me).
Minor isomer with 6S* configuration: 1H NMR (500 MHz, CDCl3,): δ = 2.68 (m, 1 H, H-3), 2.66 (m, 1 H, H-4a), 2.57 (m, 1 H, H-6), 2.45 (m, 1 H,
H-3), 2.27 (m, 1 H, H-4), 2.10 (m, 2 H, H-7), 2.08 (m, 2 H, H-8), 1.95 (m, 1 H, H-4),
1.44 (s, 3 H, 8a-Me), 1.13 (d, J = 6.5 Hz, 3 H, 6-Me). 13C NMR (125 MHz, CDCl3): δ = 211.33 (C-5), 169.97 (C-2), 85.27 (C-8a), 51.49 (C-4a), 41.53 (C-6), 33.14
(C-8), 27.90 (C-7), 27.81 (C-8a-Me), 27.56 (C-3), 19.53 (C-4), 15.23 (C-6 Me). MS
(EI): m/z (%) = 196 (3) [M+], 181 (2), 168 (4), 112 (86), 84 (29), 43 (100).
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