Cyclobutanes fragmentation

The theory of concerted cycloadditions may be extended to other reactions involving G bonds. For instance, let us consider the cyclobutane fragmentation into two molecules of ethylene. 

The first total synthesis of a racemic dolastane (rac-llO) was apparently published by Pattenden as a short communication 1986 followed in 1988 by a full paper [77, 78]. Pattenden s strategy is based on an intramolecular [2+2]-photocycloaddition/cyclobutane fragmentation to transform the diene 133 into the hydroazulene 136 (Scheme 20). In between the cycloaddition... 

Cyclobutane Fragmentations, irradiation of oxidative electron transfer sensitizers in the presence of aryl cyclobutanes causes ring fragmentation, in a formal retrocyclo-addition sense. For example, Mukai has shown, eq. 33 (98),... 

Paquette, L.A. and Leichter, L.M. (1971) Pyrolysis of anti-tricyclo[3.2.0.02,4] heptanes. The role of 2,4 substitution in the operation of a bond assisted cyclobutane fragmentations. Journal of the American Chemical Society, 93, 4922 -924. 

Shipe, W.D. and Sorensen, E.J. (2006) Convergent, enantioselective syntheses of guanacastepenes A and E featuring a selective cyclobutane fragmentation. Journal of the American Chemical Society, 128, 7025-7035. 

The variety of methods for accomplishing the [2 + 2] cycloaddition and the reliability of this reaction secures its position as one of the most versatile tools for the creation of the cyclobutane fragment in a variety of structures. Additional attractive dimensions to the scope of this protocol come in the ease of further transformations through ring opening and skeletal rearrangements, typical for cyclobutane fragments " (this aspect will be considered later in this chapter). 

The interpretation of these two schematic views confronts some difficulties. Option (1) requires the extrusion of a hydrogen from a methylene hardly active in acidic medium (acetic acid). Under basic conditions, it would be necessary first to convert the sulfide to a sulfoxide such as V to achieve sufficient activation of this methylene. This oxidation may be accomplished by LTA as indicated previously. Nevertheless, the ensuing cyclobutane fragmentation in VI— the actual oxidative step—by concurrent attack of acetate and departure of Pb(II) diacetate (X = PbOAc2 in Scheme 27.1) has no precedent in LTA-amine chemistry, although it is electronically balanced. Besides, the final product of this sequence would be sulfoxide VII. Having no reductive work-up procedure, this sulfoxide should survive until the isolation step. Since this is not the experimental fact, option (1) must then be discarded. 

Although the relative stereochemistry of the B,C ring fusion of compound II was not discussed in the original paper, it is always instructive to treat this point in cycloaddition reactions, and some valid predictions may be advanced on theoretical grounds only. Assuming that a transoid C=C bond is produced during the destruction of the cyclobutane fragment, the only model of inter-... 

The sequence of intramolecular photocycloaddition-cyclobutane fragmentation has been used by Crimmins in an excellent manner for the synthesis of pentalenenes and the even more sophisticated lauren-l-ene (27). The latter synthesis was accomplished in 27 steps from cyclopentenone U66). One of the key steps is the reductive cleavage of the tetracyclic cyclobutane (167), followed by hydrogenation of the resulting P,7-unsaturated ester to give keto ester (168 Scheme 57). 

TABLE XL Products from the Reactions of Cyclobutane Fragment Ions with Cyclobutane- g Observed in the Tandem Mass Spectrometer at a Collision Chamber Pressure of 5/im... 

In contrast to bis-SiMey-norbornene derivatives, disubstituted tricyclononenes turned out to be active monomers in AP [238, 240]. In tricyclononene molecule both MesSi-groups are moved by an additional one C-C bond away from the double bond and therefore from the reaction catalytic center. Synthesis of bis-MesSi-substituted tricyclononene was carried out from quadricyclane and fran5-l,2-bis(trichlorosilyl)ethylene. This route of synthesis provided formation of norbomene-type monomers with 100% exo-configuration of cyclobutane fragment that reduced steric hindrances in AP. That is why this monomer was active in AP catalyzed with common Ni- and Pd-catalyst systems. As a result, the formation of highly molecular weight polymer (Af up to 500,000 was observed [196]. 

The [2 + 2] cycloaddition reaction continues to dominate the synthesis of cyclobutanes, and a timely review on the intramolecular [2 -t- 2] photoaddition-cyclobutane fragmentation sequence has appeared. The first stable crystalline /rarts-fused cyclobutanone has been reported to be the ketone (16), which does not readily epimerize to the cis-fused isomer. The product is formed by photolysis of the enone (14) and the ketene acetal (15) followed by mild hydrolysis. ... 

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