Transformation of a Four-membered Ring

Ring synthesis by transformation of a four-membered ring... 

FIGURE 5. Transformation of a four-membered ring zinc phosphate (SBU) to a linear chain phosphate and the transformation of the latter to a 3D sodalite-related structure with a 4688 cage. 

FIGURE 8. Transformation of a four-membered ring aluminophos-phate (S4R) to a layered phosphate, which then undergoes transformation to another layered structure by the loss of water molecules, Note that the 2D component contains four-membered... 

The driving force for this reaction seems to be related to the steric repulsion between the two PPh3 fragments in the chelating bis(ylide) group and to the transformation of a four-membered ring into a five-membered ring. 

This chapter deals with [2 + 2]cycloadditions of various chromophors to an olefinic double bond with formation of a four-membered ring, with reactions proceeding as well in an intermolecular as in an intramolecular pattern. Due to the variety of the starting materials available (ketones, enones, olefins, imines, thioketones, etc.. . .), due to the diversity of products obtained, and last but not least, due to the fact that cyclobutanes and oxetanes are not accessible by such a simple one-step transformation in a non-photo-chemical reaction, the [2+2]photocycloaddition has become equivalent to the (thermal) Diels-Alder reaction in importance as for ring construction in organic synthesis. 

Cyclobntane. A few examples of cyclobutane derivatives have been described in the carbohydrate series. Formation of this type of ring involves a 2+2 cycloaddition. Relevant examples of cycloaddition of dichloroketene on glucals, explored by Redlich [195] and Lallemand [196,197], and significant transformations of the four-membered ring, such as ketone 165a, are given in Scheme 56. 

The authors proposed the following mechanism to explain the unusual organic transformations. The /z-peroxo-/i-oxo complex, 15, reacts in a biradical fashion with the protons on a methylenic carbon to generate a carbene and two hydroxy radicals that recombine to give a gem-diol. This diol will in turn collapse to the ketone product. The diketonization of acetylenes was explained in a similar fashion with the formation of a four-member ring intermediate (Scheme 2) (36, 37). 

A prototypical reaction involving oxidative addition of a four-membered ring to a transition metal is seen in the Cr(CO)6 mediated transformation of biphe-nylene to 9-fluorenone [24]. The product can be viewed as arising from the insertion of chromium into the central C-C bond bridging the two aromatic rings, subsequent carbonylation, and reductive elimination. 

Several stereochemical facets show up on Diels-Alder reactions of 7,8-disubstituted o-quinodimeth-anes. A seminal study of addition reactions of 7,S-diphenylbenzocyclobutenes provided strong evidence for a conrotatory opening of the four-membered ring, followed by a supra-supra-facial endo-selective Diels-Alder reaction with TV-phenylmaleimide, as exemplified by the transformation (607) - (609) (Scheme 137). Based on this result, it is now generally agreed that the trans relation of substituents at C-7 and C-8 in benzocyclobutenes translates, via the transient ( , )-o-quinodimethanes, e.g. (608), into the cis disposition in the corresponding cycloadducts. 

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