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Regio- versus Non-Regioselectivity of Haloenolcyclization of 2-Allylcyclo-1,3-hexanediones Using Iodine in Methanol and Pyridinium tribromide in Dichloromethane

Malose J. Mphahlele ,  Thwanthwadi B. Moekwa 

University of South Africa (UNISA), Preller street, Mucleneuk, Pretoria 0003, South Africa


We have previously employed iodine in refluxing methanol and pyridnium tribromide (PTB) in acetic acid to convert the 3-(phosphonoalkyl)cyclohexenones to a series of benzylphosphonic esters substituted in the aromatic ring with methoxy or hydroxyl group, respectively [1]. In a recent communication [2], we have demonstrated that iodocyclization of α-allylcyclohexane-1,3-diones using iodine-methanol mixture, leads exclusively to exo-cyclized 2-iodomethyltetrahydrobenzofuran-4-ones (minor) and 2-iodomethyl-4-methoxydihydrobenzofuran derivatives (major) according to Baldwin's rules [3]. On the other hand, bromoenolcyclization using PTB in dichloromethane was found to afford both the 2-bromomethyltetrahydrobenzofuran-4-ones (major) and 3-bromomethyltetrahydrobenzopyran-5-ones (minor) through exocyclization and endocyclization, respectively [2]. These observed results demonstrate the combined electrophilic and oxidative potentials of I2-MeoH and PTB. Experimental evidence in the form of isolated intermediates will also be presented to support the proposed mechanisms.

[1] M. J. Mphahlele et al., J. Chem. Soc. Perkin Trans. 2, 1996, 1455.

[2] M. J. Mphahlele, T. B. Moekwa, Org. Biomol. Chem. 2005, 3, 2469.

[3] J. E. Baldwin, J. Chem. Soc. Chem. Comm. 1976, 734.

We are grateful to the National Research Foundation (NRF) and the University of South Africa (UNISA) for financial assistance.


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Presentation: oral at 18th Conference on Physical Organic Chemistry, Symposium 1, by Malose J. Mphahlele
See On-line Journal of 18th Conference on Physical Organic Chemistry

Submitted: 2006-05-12 12:37
Revised:   2009-06-07 00:44