References and Notes
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For benanomicin-pradimicin, see:
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1a
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1c For ohioensin B, see: Dijoux-Franca MG.
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1d For parviflorenes, see: Toume K.
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1e For compound 4, see: Zheng G.
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15a
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15b For leading references, see: De Lucchi O.
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- 17 For preparation of SmI2 in THF, see: Girard P.
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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 1H NMR and diagnostic NOE interactions (Figure
[2]
).
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 K2CO3 and the products were extracted with EtOAc (3 ×). The combined organic extracts were washed with brine, dried (Na2SO4) 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.
11 The coupling constant between Ha and Hb in 17 was 4.6 Hz, indicating a cis relationship. The diagnostic NOE interactions between HaŽHb and HbŽHc in 17 suggested the conformation shown below (Figure
[3]
).
12 Compound 16 was conformationally stable at room temperature. However, it underwent isomerization in toluene in 5 h at 90 °C. The equilibrium ratio was 36:64, diequatorial/diaxial (Figure
[4]
).
13 The stereostructure of trans-7b was determined by X-ray crystal structure analysis (Figure
[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]
). In a similar manner, cis isomers of 7c and 7c′ were both converted into cis-19.
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.