Cyclobutene ring acetylene derivatives

In addition, Swenton and cowm kers have reported that the dianion derivatives of certain pyrimidine-acetylene photoadducts undergo an electrocyclic reaction which may involve an anion-accelerated cyclobutene ring opening see R. N. Comber, J. S. Swenton and A. J. Wexler, /. Am. Chem. Soc., 1979,101, 5411. 

We also consider new achievements in some classical cycloaddition reactions such as the Diels-Alder condensation with acetylenic dienophiles, [2-1-2] cycloadditions with acetylene component leading to creation of cyclobutene ring, and new results in cyclobutene syntheses by [2-1-2] acyclization of phosphorus containing 1,3-butadiene derivatives synthesized starting with propargyl-type alcohols. 

The addition of substituted alkenes with electron-withdrawing groups to ruthenium acetylide complexes results in the formal [2 + 2] cycloaddition of the olefin to the acetylene moiety. Facile ring opening of the resultant ruthenium cyclobutene complex (103) generates the ruthenium butadienyl species (104). Subsequent displacement of a phosphine ligand leads to the Tj3-allylic product (105) [Eq. (93)] (90-92). The intermediate cyclobutene complex has been isolated in one instance for the monocarbonyl derivative 106 [Eq. (94)] (92). 

Cyclic active-methylene compounds react in Michael reactions not only with a,/J-unsaturated acceptors, but also with acetylene dicarboxylates, propiolates, and their derivatives (Scheme IV/8). If, for instance, acetylene carboxylic esters are added to a variety of cyclic-active methylene compounds, ring-expanded products generally are formed directly. In other cases, it is possible to isolate the intermediate cyclobutene derivatives, which can be converted to the ring enlarged compounds in a second reaction step. 

A useful reaction for the synthesis of unsaturated seven-membered heterocycles is the (2 + 2)-cycloaddition of heteroaromatic compounds, e.g., 1 //-pyrrole, furan, or thiophene derivatives, with acetylenes. In combination with a subsequent intramolecular (2 + 2)-cycloreversion (Section IV,B,2) of the annulated cyclobutene moiety, ring enlargement with two carbon atoms can be achieved. 1-Heterocycloheptatrienes, such as benzol6]azepines,26,27,65,66 benzo[fc]oxepins,67,68 benzo[6]-thiepins,69,70 and thiepins,18,71 have been successfully prepared in this way other routes are either nonexistent or laborious.72 In these compounds the reacting carbon-carbon double bond constitutes part of a (4n + 2)7r-electron system and in the (2 + 2)-cycloaddition the resonance energy of the aromatic nucleus is lost. Just like the nonaromatic heterocycles, heteroaromatic compounds have been reported to undergo (2 + 2)-cycloaddition reactions both with electron-deficient and with electron-rich acetylenes. 

In contrast to furan, the benzene ring in benzo[ ]furan (because of its large resonance energy) is dominant to such an extent that [4+2] cycloadditions are not possible. On the other hand, photochemical [2+2] cycloaddition occurs readily on the C-2/C-3 double bond leading, e.g. with an acetylene di-carboxylic ester, to the cyclobutene derivative 1 ... 

Cycloaddition reactions of the indole 2,3-double bond are not limited to alkenes as partners. Acetylenic compounds have also been used in photochemical cycloaddition reactions with indoles to produce cyclobutenone derivatives. There have been extensive studies on the reaction of indoles with dimethyl acetylenedi-carboxylate (61, DMAD), which produce a number of structurally distinct products [31]. By devising a photosensitized cycloaddition reaction of DMAD to activated indoles 60 in the presence of acetophenone, Neckers and Davis were able to produce the cyclobutene derivatives 62 in good yields (Scheme 14) [32]. The resulting cyclobutenes can then be converted to the corresponding benzazepines 63 via thermal ring-opening reactions. 

The action of zinc on the same cyclobutene derivative was also studied [6]. When the reaction was carried out in the presence of either dimethylacetylene or dimethyl acetylenedicarboxylate, hexa-methylbenzene and dimethyl tetramethylohthalate were isolated from the respective reaction mixtures. These products could arise by trapping of the cyclobutadiene by the acetylenes, and ring-opening of the adducts to give benzene derivatives. 

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