The stereochemistry of the Diels-Alder reaction of 1,2-dichloro-3,3-ditluorocyclopropene (Sa) to 1,4-diphenyl-1,3-butadiene (6) and 1,3-diphenylisobenzofuran (7) was unambiguously established by X-ray structure determination. In all cases known so far, tetrahalogenocyclopropenes add exo to open-chain dienes and furans. The previously reported exo-addition product (2a) of 1-bromo-2-chlorocyclopropene (5b) to 7 allows assignments of the stereochemistry of other additions of 5b to furans. exo-Addition usually predominates, but in some cases endo-adducts are also formed. This contrasts with reports in the literature that 5b adds preferentially endo to open-chain dienes.
1H-Cyclopropa[b]naphthalene (10a) and 3-methyl or dimethyl derivatives have been synthesized by interception of appropriately substituted o-quinodimethanes 3 with 1-bromo-2-chlorocyclopropene 5, and subsequent dehydrohalogenation of the adducts. The o-quinodimethane derivatives 3 in turn were obtained from the diynes 7 via base-induced isomerization to bisallenes 8 and thermal electrocyclic ring closure.One of the earliest approaches towards 1 H-cyclopropabenzene (1) consists in cycloaddition of butadiene to a di-or tetrahalogenocyclopropene followed by dehydrohalogenation with base (Scheme 1 ) [l]. In principle, the route may be extended to synthesis of the benzannellated homologues, cyclopropa[b]naphthalenes. However, only 1, l-dichloro-2,7-diphenylcyclopropa[b]naphthalene (2) has been prepared in this way [2], since the o -quinodimethane 3, required for the cycloaddition, was readily available from trans -1,2-diphenylbenzocyclobutene (4) via electrocyclic ring opening [3]. In general, however, the conditions used for generation of o-benzoquinodimethanes are not compatible with the presence of the very sensitive halogenocyclopropenes. Scheme I 4 3 2 Recently, we reported that o -naphthoquinodimethane, generated from o -dipropargylbenzene [4] is intercepted with 1,2-dihalogeno-and tetrahalogenocyclopropenes [5].The adducts can be transformed to cycloprop[b]anthracene and its 1,l-dihalogeno derivatives. Although preliminary experiments with o -benzoquinodimethane (3a) and tetrahalogenocyclopropenes afforded no cycloadducts, we found subsequently that the more reactive 1-bromo-2-chlorocyclopropene (5) [6] is moderately efficient in the cycloaddition to 3a. In this communication, we report the application of the sequence towards an alternative synthesis of 1H-cyclopropa[b]naphthalene (10) [7] and some of its methyl derivatives.
Introduction. -The chemistry of cyclopropabenzenes, the most highly strained members of the o-fused cycloalkabenzene series is now well established [l] [3]. In parallel, several halogen-stabilized cyclopropabenzenyl cations such as 3' have been prepared by ionization of the corresponding difluoro precursors such as 4' and characterized by 'H-, I9F-, and I3C-NMR
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.