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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1. Synthesis and Chemical Transformation of Fused Tetrazoles Derivatives from of 2-bromomethyl- and 2-iodomethyl-3,5,6,7-tetrahydrobenzofuran-4-ones