α-Bromo Spiroketals: Stereochemistry and Elimination Reactions

Article abstract of DOI:10.1021/jo00061a025

The simple spiroketals, 1,6-dioxaspiro<4.4>nonane (2), 1,6-dioxaspiro<4.5>decane (5), 1,7-dioxaspiro<5.5>undecane (6), and (E,E)-2,8-dimethyl-1,7-dioxaspiro<5.5>undecane (7), have been brominated by bromine in carbon tetrachloride/calcium carbonate or acetic acid, and a number of mono-, di-, and tribromo derivatives have been characterized.The relative stereochemistries have been established by correlated 1H and 13C NMR spectroscopy and X-ray crystal structure determinations.Dehydrobromination with potassium tert-butoxide in either dimethyl sulfoxide or tetrahydrofuran is facile for the axial monobromides, although both axial and equatorial bromides derived from 1,6-dioxaspiro<4.5>decane (5) and 1,7-dioxaspiro<5.5>undecane (6) dehydrobromate to provide 1,6-dioxaspiro<4.5>dec-9-ene (35) and 1,7-dioxaspiro<5.5>undec-4-ene (26), respectively.Hydration of these readily acquired alkenes furnishes the corresponding 9- and 4-ols, respectively, with the latter being components of the rectal glandular secretion of Bactrocera oleae (olive fly), Bactrocera cacumintas, and Bactrocera distincta.These studies indicate that α-bromination of suitable spiroketals may be a viable later step in the synthesis of α-bromine-containing spiroketal metabolites such as obtusin and neoobtusin.