Cyclobutanes ethylene derivatives

The classic example is the butadiene system, which can rearrange photochemi-cally to either cyclobutene or bicyclobutane. The spin pairing diagrams are shown in Figure 13. The stereochemical properties of this reaction were discussed in Section III (see Fig. 8). A related reaction is the addition of two ethylene derivatives to form cyclobutanes. In this system, there are also three possible spin pairing options. 

Highly enantiomerically enriched cyclobutane derivatives are prepared via dias-tereoselective [2+2] cycloaddition reactions of chiral ethylene derivatives for instance, by photochemical (Scheme 11 and Scheme 12) [38, 39] and thermal [2+2] cycloaddition (Scheme 13) [40,41] reactions of chiral alkenes and by the [2+2] cycloaddition of chiral keteneiminium ions (Scheme 14) [42]. 

A number of cases are known where ethylene derivatives undergo n cycloaddition to form cyclobutanes. Tetracycanoethylene [TCNE (NC)2C=C(CN)2] and fluorinated ethylenes are particularly prone to behave in this way. These reactions were discovered at the laboratories of duPont de Nemours Co. and have been studied in great detail by Bartlett and his collaborators. As we have already seen, concerted cis dimerizations of this kind should be less favorable than an path via an intermediate 1,4-butadiyl biradical all these reactions in fact take place in this way. An interesting example is the reaction of TCNE with bismethylenecyclobutene (230). The normal Diels-Alder reaction to form (231) is inhibited because this would be an antiaromatic cyclobutadiene derivative and because the transition state leading to it would be isoconjugate with benzocyclobutadiene. The product is therefore the spiran (232), formed as indicated by an process. 

Irradiation (s. a. under CuBr, R B, (CH )fiO, (CHfiOOH, and HCl) Hi Cyclobutane ring from ethylene derivs. ... 

Ethylene deriv. molecules from cyclobutane ring... 

Acetic acid/irradiation Cyclobutane ring by dimerization of ethylene derivs. 

Cyclobutane derivatives are formed after exposing a mixture of alkenes and maleic anhydride to light. Photoadducts are formed by reaction of maleic anhydride with ethylene [74-85-1] and benzene (50). 

The reaction of ethylene with steroidal a,/ -unsaturated ketones yields both cis and trans cyclobutane derivatives as illustrated in Eq. 55. 145)... 

Regioselective stannylcupration of terminal alkynes and allenes, followed by quenching of the cuprate intermediate with ethylene oxide, provides a facile synthesis of cyclobutene and alkylidine cyclobutane derivatives, respectively (Scheme 3.23) [15c]. A number of total syntheses have exploited regioselective stannylcup-... 

Reduced pyrimidines are much less stable toward hydrolysis than the fully conjugated analogs, and this is often used synthetically to produce amino acids and diamines. The BH3 reduction of cyclic amidines (1,4,5,6-tetrahydropyr-imidines) to hexahydropyrimidines, and their subsequent hydrolysis was mentioned above <1999JFIC105>, but there are many more examples. For instance, m-cyclobutane /5-amino acids 544 can be prepared from the cyclobutane derivatives 542 formed by the [2-F2] photocycloaddition reaction between uracil and ethylene <2002TL6177, 2004TL7095, 2006SL1394>. 

The decay of the tetramethylene diradical derived from 2,2,5,5-t/4-cyclopenta-none is much slower than seen for the C4Hg diradical. Both principal decay modes, fragmentation to two ethylenes and ring-closure to cyclobutane, may be dependent dynamically on torsional motions of the terminal methylene groups. 

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