Prins cyclizations: labeling studies and application to natural product synthesis.

[GRAPHICS] The first syntheses of two natural products, catechols 1 and 2, isolated from Plectranthus sylvestris (labiatae), are reported. Oxygen-18 labeling studies support the proposed intermediacy of a stabilized benzylic cation in the acid-promoted cyclization of an aldehyde and benzylic homoallylic alcohol possessing an electron-rich aromatic ring, In contrast, with an electron-deficient aromatic ring the pathway via a benzylic cation is only minor.