Thermolysis, benzocyclobutenes

The C-8 methylated tetrahydroprotoberberine alkaloids coralydine and 0-methylcorytenchirine have also been synthesized via benzocyclobutene thermolysis followed by reduction. ... 

For a recent review of the benzocyclobutene thermolysis reaction in alkaloid synthesis, see T. Kametani and K. Fukumoto, Heterocycles, 3, 29 (1975). 

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]. 

The thermolysis of benzocyclobutene either in the condensed or vapor phase has been shown by a variety of techniques to initially yield an o-quinodimethane (Fig. 9). The fate of the o-quinodimethane in the absence of other coreactive species appears to depend to a large extent upon the conditions under which it is generated. Thus for example, the thermolysis of benzocyclobutene hydrocarbon itself in a sealed flask at 200 °C gave a 24% yield of 1,2,5,6-dibenzocycloocta-diene 20 along with a mixture of unidentified oligomeric materials (Fig. 12) [70]. 

Fig. 12. Condensed phase thermolysis products of benzocyclobutene hydrocarbon 1...

Cyclization of side chain nitriles has found extensive use in the synthesis of benzocyclobutenes (70 n = 2),104 the versatile synthons which open on mild thermolysis to give o-quinodimethanes for inter- and intra-molecular [4 + 2] trapping.108 The nitrile group in (70) can be manipulated into a variety of functionalities for appending the dienophile portion. For example, in the synthesis of chelidonine, the nitrile (71) was converted, by hydrolysis followed by Curtius degradation and reaction of the formed isocyanate with benzyl alcohol, to a urethane (72). The latter was then condensed with a benzyl bromide to get the compound (73), which was elaborated further as shown in Scheme 14.109... 

In a recent re-examination of the thermolysis of benzocyclobutenes for the in situ generation of o-quinodimethanes, the resultant IMDA diastereoselectivity was highly dependent on the nature of the hydroxyl protective group.90 The intramolecular 4 + 2-cycloaddition of o-quinodimethanes (83), derived from ene-bis(sulfinylallenes) (82), with electron-deficient and electron-rich alkenes produced the corresponding polycyclic aromatic compounds (84) (Scheme 22).91 The enantioselective Diels-Alder... 

The next three reactions (Scheme 3.38), cotrimerization of enediyne 146 with bis(trimethylsilyl)acetylene 150, thermolysis of the benzocyclobutene derivative 151, and the intramolecular Diels-Alder reaction of the intermediate 152, were... 

Flash vacuum thermolysis of o-methylbenzyl halides (140) at 500-600 C gives benzocyclobutenes. This reaction has recently been extended to the preparation of cyano and chloro sub-... 

This approach to cyclohexadienes can be considered an extension of that described for the precursor to type I cyclohexadiene in which the vinyl group is part of a cyclic stmcture. An example is tte low-temperature photochemical generation of the bis-exomethylene compound (196) from the benzocyclobutene (195), which was followed by an electrocyclic reaction to give compound (197). Thermolysis of (198) also leads to an assumed bis-exo-methylene intermediate (199), wldch electrocyclizes with involvement of die pyridinium ring to afford die alkaloid ellipticene (200) after dehydrogenation. ... 

A synthetic route to asymmetric tetralins applies the cyclization of chiral acrylates 28 or fumarates 38 to 2-quinodimethanes, which must be generated in situ. Different procedures lead to a-hydroxy 2-quinodimethanes 49 and 56, for instance the irradiation of 2-methylbenzalde-hyde (48)10 or thermolysis of the benzocyclobutene 53 or the cyclic sulfone 541... 

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