2001 cycloaddition reactions

cycloaddition reactionsO 0070

48 - 043Efficient Syntheses of Thiochromans via Cationic Cycloadditions. —Starting from α-benzotriazolyl sulfides (I) or (IX), three types of thiochromansare synthesized: 2,4-diaryl-, 2,4-diaryl-4-methyl-, and 2,3,4-trisubstitutedchromans. By treatment of the starting sulfides with ZnBr2 in the presence ofvarious alkenes, the generated thionium ions undergo cycloaddition with thealkenes to give mixtures of diastereomers which are separated in most cases togive pure major diastereomers. The reactions can be either performed at roomtemperature for 14-18 h or at reflux for ca. 4 h. The reaction temperature hasnegligible effect on yield or diastereoselectivity. — (KATRITZKY, ALAN R.;BUTTON, MARTIN A. C.; J. Org. Chem. 66 (2001) 16, 5595-5600; Cent.Heterocycl. Compd., Univ. Fla., Gainesville, FL 32611, USA; EN)

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