Abstract
A number of variously substituted phenanthridines have been synthesized
using the newly developed methodology under mild conditions. The
Hendrickson reagent initiated cascade annulation, which is composed
of a mild conversion of stable amide precursor to highly reactive
imido-carbonium intermediate and a subsequent intramolecular Friedel-Crafts
reaction, successfully served as the key method.
Key words
phenanthridine - cascade reaction - Hendrickson
reagent - carbocation - Friedel-Crafts
reaction
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9
Representative
Procedure for the Synthesis of 6-Methyl-phenanthridine (2a)
To
a solution of Ph3 PO (260 mg, 0.9 mmol, 3 equiv) in anhyd
CH2 Cl2 (5 mL) was added Tf2 O (0.078
mL, 0.45 mmol, 1.5 equiv) dropwise under nitrogen atmosphere at 0 ˚C.
After 15 min, amide 1a (64 mg, 0.3 mmol,
1 equiv) in anhyd CH2 Cl2 (2 mL) was added.
The reaction was then warmed to r.t. and stirred until completion.
The reaction was quenched by addition of sat. aq NaHCO3 .
The mixture was extracted with CH2 Cl2 (3 × 10
mL). The combined extracts were washed with brine, dried over anhyd
Na2 SO4 , and concentrated. The crude product
was purified by column chromatography on silica gel using a mixture
of PE and EtOAc (5:1 to 2:1, v/v) as the eluent to afford
a light-yellow solid 2a (51 mg, 95%).4n ¹ H
NMR (300 MHz, CDCl3 ): δ = 8.64 (1 H,
d, J = 8.4
Hz), 8.55 (1 H, d, J = 7.8
Hz), 8.24 (1 H, d, J = 7.8
Hz), 8.11 (1 H, d, J = 7.8
Hz), 7.88-7.61 (4 H, m), 2.98 (3 H, s). MS (EI): m/z = 193 [M+ ].
