For the first time, nucleophilic ring-openings of cyclopropanated 7-oxabenzonorbornadiene were investigated, providing a novel approach to the preparation of 2-methyl-1,2-dihydronaphthalen-1-ols. Satisfactory yields (up to 95%) were achieved using n-Bu2CuCNLi2 as the nucleophile and Et2O as the solvent. The reaction demonstrated successful incorporation of primary, secondary, tertiary and aromatic nucleophiles, as well as ring-openings of substrates bearing arene substituents and C1-bridgehead substituents. A generalized mechanism for these transformations is also proposed.
NotesVol. 69 with water, and dried over anhydrous sodium sulfate. Removal of the solvent gave 20.1 g. of crude product. This was distilled through a three-inch Vigreux column and the fraction boiling from 74-92°( 17 mm.) (mostly 75°( 17 mm.)) collected. The yield was 11.6 g. (70%). The residue weighed 3.6 g. On redistillation the b. p. was 76-79°( 17 mm.). The boiling point and density (¿2725 0.939) agree with literature values4 obtained for the product prepared by different methods. Refractive index m21d 1.4929.Attempted Asymmetric Syntheses.-Attempts were made to impart an asymmetric bias to the above reaction by conducting it, in the same manner, in the presence of various optically active agents. The quantities of reagents and results are given in Table I. The amount of phenyl magnesium bromide used in each case was sufficient to react completely with the optically active agent and the ethyl -chloroethyl ether, plus a 100%, excess of the quantity required for the latter reaction. All rotations were taken in 95% ethanol.
Symmetrical and unsymmetrical 7-oxabenzonorbornadienes have been cyclopropanated using diazomethane gas under palladium catalysis. Good to excellent isolated yields (64-96%) of cyclopropanated oxabenzonorbornadienes were obtained, and excellent stereoselectivities were observed with exo-cyclopropanes as the only stereoisomeric products. 7-Oxabenzonorbornadiene (1a) is known to undergo a variety of transformations leading to highly substituted cyclic systems. 1,2 In particular, asymmetric ring openings of 1a and its analogues have been shown to produce several stereocenters in a single step. 2-5 Studies by our research group have demonstrated different modes of transitionmetal-catalyzed reactions of 1a under varied conditions, giving rise to several products 2-7 depending on the prescribed treatment (Scheme 1). For example, when 1a was treated with an alkyne in the presence of Cp * Ru(COD)Cl, [2+2] cycloaddition provided cyclobutene adduct 2. 6,7 Furthermore, we found that this reaction also proceeds in the presence of alkynyl halides 8 and ynamides. 9 When the alkyne was changed to secondary propargylic alcohol 8 and the reaction was carried out in MeOH, isochromene 3 was formed; this reaction was also found to proceed using a cationic Ru catalyst (e.g., [CpRu(MeCN) 3 ] + PF 6 -). 10,11 On the other hand, if the former catalyst were treated with 1a alongside 8 in THF, cyclopropane 4 was observed. 12 Our group has also found that in the absence of alkyne, Cp * Ru(COD)Cl catalyzes the isomerization of oxabenzonorbornadiene 1a to the corresponding naphthalene oxide 5 or naphthol 6. 13,14 Furthermore, we have reported the asymmetric cyclodimerization of 1a to produce dimers 7 in excellent enantioselectivity (up to 99% ee) using a cationic Rh(I) catalyst. 15As cyclopropanes are reported to have comparable reactivities to alkenes, 16 our group decided to explore the chemistry of cyclopropanated derivatives of 1a. To this end, the addition of a methylene equivalent across the olefin of 1a was envisaged to produce cyclopropane 9a (Scheme 2), which could later be subjected to various transition-metal-catalyzed reactions giving rise to different types of useful frameworks.Scheme 1 Transition-metal-catalyzed reactions of oxabenzonorbornadiene 1a
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