Abstract
Stereoselective access to 9-alkyl-10-hydroxy-9,10-dihydrophenanthrenes has been developed via the samarium(II) iodide mediated reductive cyclization of an ene-aldehyde appended to a biphenyl scaffold.
Key words
ene-aldehyde - samarium(II) iodide - reductive cyclization
References and Notes
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6 Although a mixture of dl - and meso -isomers of 2,4-pentanediol was employed, the latter preferentially took part in the reaction, giving the diastereomer 12 in 81% yield with a small amount of other diastereomers (<6% yields). The structure of 12 was assigned by 1 H NMR and diagnostic NOE interactions (Figure
[2 ]
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7 This reaction also proceeded in the absence of a proton source, giving the product in 10% yield with a sizable amount of unidentified byproducts.
8 Typical procedure for the reductive cyclization: SmI2 (0.10 M in THF,
[17 ]
4.4 mL, 0.44 mmol) was added to a solution of the ene-aldehyde 8a (49.9 mg, 0.178 mmol) and MeOH (0.160 mL 3.92 mmol) in degassed THF (1.6 mL) at 0 °C. After stirring for 5 min at 0 °C, the reaction was stopped by adding aqueous K2 CO3 and the products were extracted with EtOAc (3 ×). The combined organic extracts were washed with brine, dried (Na2 SO4 ) and concentrated in vacuo. The residue was purified by preparative TLC (silica gel 60 PF254 ) to give 7a (trans /cis = 10:1) in quantitative yield as a colorless oil.
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11 The coupling constant between Ha and Hb in 17 was 4.6 Hz, indicating a cis relationship. The diagnostic NOE interactions between Ha ŽHb and Hb ŽHc in 17 suggested the conformation shown below (Figure
[3 ]
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[4 ]
).
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[5 ]
).
14 The PTLC purification gave four separable diastereomers associated with relation between the 9,10-stereogenic centers in the phenanthrene skeleton and the sulfur chiral center. Oxidation of trans -7c and trans -7c ′ by MCPBA led to the same compound 18 . The stereostructure of 18 was determined by X-ray analysis (Scheme
[9 ]
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16 Isolation of the Pummerer product was attempted; however, it was prone to undergo hydrolysis to form the corresponding aldehyde. This aldehyde was easily dehydrated to afford 9-phenanthrene carboxaldehyde.
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