title: Intramolecular Diels-Alder reactions of brominated masked o-benzoquinones. A detour method to synthesize highly functionalized oxatricyclic [m.3.1.0] ring systems from 2-methoxyphenols.
abstract: Intramolecular Diels-Alder (IMDA) reactions of masked o-benzoquinones (MOBs) 5a-d to 7a-d and 17a-d to 19a-d generated in situ from 2-methoxyphenols 2-4 and 14-16, respectively, in the presence of alkenols 1a-d, resulting in highly functionalized oxatricyclic [m.3.1.0] ring systems are described. The MOBs 5a-d to 7a-d underwent the IMDA reactions to furnish the adducts 8a-d, 10a-d, and 12a-d (direct method) in poor yields with the concomitant formation of considerable amounts of unexpected byproducts 9a-d, 11a-d, and 13a-d, respectively. To avoid the formation of byproducts and to improve the yields of the desired cycloadducts, a detour method comprising sequential bromination of 2-methoxyphenols 2-4, tandem oxidative acetalization-Diels-Alder reaction, and debromination has been developed. The oxidation of bromophenols 14-16 in the presence of alkenols 1a-d produced the corresponding MOBs 17a-d to 19a-d, which underwent cycloaddition to afford the cycloadducts 20a-d to 22a-d, respectively, as sole products in good to high yields in a highly regio- and stereoselective manner. Treatment of the bromoadducts 20a-d to 22a-d with tributylammonium formate-palladium reagent produced the corresponding debrominated products 8a-d, 10a-d, and 12a-d in high to excellent yields. In general, the latter oxatricycles were obtained in higher overall yields via the detour method than those via the direct method.
authors.affiliation: Department of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 300.
authors.forename: Ken-Ching
authors.initials: KC
authors.lastname: Lin
authors.forename: Yi-Ling
authors.initials: YL
authors.lastname: Shen
authors.forename: N S Kameswara
authors.initials: NS
authors.lastname: Rao
authors.forename: Chun-Chen
authors.initials: CC
authors.lastname: Liao
mesh_terms: D000470:Alkaloids / Q000138:chemical synthesis; D016227:Benzoquinones / Q000737:chemistry; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000138:chemical synthesis*; D001966:Bromine; D010636:Phenols / Q000737:chemistry; D013237:Stereoisomerism
chemical_list: D000470:Alkaloids; D016227:Benzoquinones; D019086:Bridged Bicyclo Compounds, Heterocyclic; D010636:Phenols; D001966:Bromine