Regioselective Superacid-Catalyzed Electrocyclization of Diphenylmethyl Cations to Fluorenes, Phenanthrols and Benzofurans

Abstract

Cationic electrocyclization of α-benzoyldiphenylmethanols in the presence of superacid provides fluorenes, phenanthrols and benzofurans in good to moderate yields. A single substitution leads to regioselective cationic electrocyclizations.

Key words

carbocations - electrocyclizations - regioselectivity - superacid - diphenylmethyl cations

References

• 1a Creary X. Chem. Rev.  1991,  91:  1625 
• 1b Creary X. Hopkinson AC. Lee-Ruff E. Advances in Carbocation Chemistry   Vol. 1:  Creary X. JAI Press; Greenwich, CT: 1989.  p.45 
• 1c Gassman PG. Tidwell TT. Acc. Chem. Res.  1983,  16:  279 
• 1d Tidwell TT. Angew. Chem., Int. Ed. Engl.  1984,  23:  20 
• 2a Dao LH. Maleki M. Hopkinson AC. Lee-Ruff E. J. Am. Chem. Soc.  1986,  108:  5237 
• 2b Hopkinson AC. Lee-Ruff E. Maleki M. Synthesis  1986,  366 
• 3 Johnston LJ. Kwong P. Shelemay A. Lee-Ruff E. J. Am. Chem. Soc.  1993,  115:  1664 
• 4 Takeuchi K. Kitagawa T. Okamato K. J. Chem. Soc., Chem. Commun.  1983,  7 
• 5 Photochemical cyclizations of methoxy-substituted benzoin esters (e. g., 4"-methoxybenzoin acetate) to give benzofurans (2-phenyl-6-methoxybenzofuran) was reported by Sheehan JC. Wilson RM. Oxford AW. J. Am. Chem. Soc.  1971,  93:  7222 
• 7a Vorlander D. Pritzsche A. Ber. Dtsch. Chem. Ges.  1913,  46:  1793 
• 7b Britzrzycki A. Herbst C. Ber. Dtsch. Chem. Ges.  1903,  36:  145 
• 7c Dobeneck HV. Kiefer R. Liebigs Ann. Chem.  1965,  684:  115 
• 7d Arnold RT. Parham WE. Dodson RM. J. Am. Chem. Soc.  1949,  71:  2439 
• 7e Hopkinson AC. Khazanie PG. Dao LH. J. Chem. Soc., Perkin Trans. 2  1979,  1395 
• 7f Delacre M. Bull. Soc. Chim. Fr.  1918,  23:  229 
• 8a Ohwada T. Shudo K. J. Am. Chem. Soc.  1988,  110:  1862 
• 8b Ohwada T. Shudo K. J. Org. Chem.  1989,  54:  5227 
• 9 Ohwada T. Suzuki T. Shudo K. J. Am. Chem. Soc.  1998,  120:  4629 
• 10 Hopkinson AC. Dao LH. Duperrouzel P. Maleki M. Lee-Ruff E. J. Chem. Soc., Chem. Commun.  1983,  727 
• 11 Maleki M. Hopkinson AC. Lee-Ruff E. Tetrahedron Lett.  1983,  24:  4911 
• 12a Olah GA. Wu A.-H. J. Org. Chem.  1991,  56:  2531 
• 12b For a review of superelectrophiles, see: Olah GA. Angew. Chem., Int. Ed. Engl.  1993,  32:  767 
• 12c See also: Olah GA. Klumpp DA. Nlyer G. Wang Q. Synthesis  1996,  321 
• 13a Krepski LR. Heilmann SM. Rasmussen JK. Tetrahedron Lett.  1983,  24:  4075 
• 13b Gassman PG. Talley JJ. Tetrahedron Lett.  1978,  3773 
• 14 Johnson CD. Acc. Chem. Res.  1993,  26:  476 
• 15 Chambers RD. Clark M. J. Chem. Soc., Perkin Trans. 1  1972,  2469 

6

It was reported that α-acetyldiphenylmethanol (2b, R = CH₃) and α-benzoyldiphenylmethanol (2d, R₁ = R₂ = H) cyclized in H₂SO₄-CHCl₃ by an electrocyclization mechanism between the benzene ring and the carbonyl group, resulting in the formation of the corresponding benzofurans, [2] [10] although the formation of the fluorene derivative 4d from 2d has also been reported under similar conditions. [11]