Benzocyclobutenes rearrangement

Benzocyclobutenes form a special class of cyclobutenes because the double bond is part of an aromatic ring. Perfluoro(3-alkyl-l,2-benzocyclobutenes) rearrange in the presence of anti-mony(V) fluoride to indanes via styrene intermediates.27-31 Thus, perfluorobenzocyclobutenes 34 give perfluoroindanes 35 in moderate yields.27... 

Both thermal and photochemical rearrangement of (667) affords (668) and (669). Conversion of (667) into (669) may be regarded as a bishomo analogue of the o-quinone-dimethide-benzocyclobutene rearrangement. ... 

A similar synthesis starts from commercially available 1,5-hexadiyne and 2-methyl-cyclopent-2-enone. The benzocyclobutene is obtained from a bis-acetylene in a cobalt-catalyzed reaction. It rearranges regio- and stereoselectively to a 3-deoxy steroid derivative. The overall yield from the cyclopentenone was 40% (R.L. Funk, 1977). 

The thermal reaction of the compound (318) may be chosen as an example to show the usefulness of the benzocyclobutene — o-xylylene rearrangement in alkaloid synthesis U1). On heating (318) at 120 °C, an intermediate product (319) was formed, which cyclized to give the compound (320). This method formulated the key step in the total synthesis of the alkaloid chelidonine (321) ni). 

Radical anion reactions have been treated less thoroughly than radical cations, although the same principles should apply. Bauld et al. [13] discussed the benzocyclobutene to o-quinodimethane (3 4) rearrangement because... 

These cycloadditions with o-quinodimethanes provide a broad variety of useful fullerene functionalizations, since o-quinodimethanes can be prepared using several routes and the resulting cycloadducts are thermally stable [42], There exist several alternatives to the iodide-induced bromine 1,4-elimination of 1,2-bis (bromomethyl)-benzenes [44-47]. o-Quinodimethanes have been prepared by thermolysis of 3-isochromanone (42) [43], benzocyclobutenes (43) [48-50], isobenzothiophene 2,2-dioxides (44) [42] and sultines [51,52] or by photolysis of o-alkylphenones such as 45 [53-55] and could be added to Cjq in good yields (Scheme 4.7). Indene, thermally rearranged to isoindene, also adds to Cjq in similar fashion to quinodimethanes [56]. 

A significant acceleration of the electrocyclic ring opening of benzocyclobutene derivatives has been disclosed under the influence of a -silicon atom.55 This effect has been associated with the adjacent anion-driven electrocyclic reactions such as oxy-Cope rearrangement. 

The cobalt(I)-mediated [2 + 2 + 2] cycloaddition of 1,5-diynes with mono-alkynes provides access to benzocyclobutene derivatives (Scheme 24). Thermal rearrangement of benzocyclobutenes into o-quinodimethane and subsequent Diels-Alder reaction with an alkene moiety allow the formation of a tricyclic compound. 

Fukumoto et al have taken advantage of the propensity for inward rotation of an acyl group to prepare the isochroman-3-one (36), an intermediate in their synthesis of geneserine, a constituent of the calabar bean. The conversion of the benzocyclobutene (37) to (36) proceeds via ring opening to the o-quinodimethane (38) followed by cyclization and a 3,3-sigmatropic rearrangement. 

The same sequence when applied to benzocyclobutene-l,2-diones such as (118) afforded the adduct (119 80%), which was rearranged upon heating at 160 °C and air oxidation to the anthraquinone... 

Thus, on the basis of these results the experimental data reported earlier77c> g) with cis-104 and trans-105 do not support the theoretical conclusion of an allowed conro-tatory rearrangement of the benzocyclobutene radical anion 110. 

The earlier reported rearrangement of the benzocyclobutene radical anion 110, prepared from benzocyclobutene 121 with Na/K alloy, to give the o-xylylene radical anion 111 (M+ = K+)77a), was later retracted 77b). 

One of the early unsuccessful approaches to benzocyclopropenes consisted of the ring contraction of an appropriately substituted benzocyclobutene. When 2-bromobenzocyclobutene was reacted under the conditions of the Favorskii rearrangement (sodium methoxide), ring opening to methyl 2-(methoxymethyl)benzoate occurred rather than ring contraction to methyl benzocyclopropene-1 -carboxylate (1). ... 

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