Cyclobutanes rearrangement

Proton transfer from H3 + and CH5+ to cyclopropane yields a C3H7 + ion, which at atmospheric pressures is largely stabilized by collision (9). This ion reacts as a sec-propyl ion with an added interceptor molecule (9). Hence, the protonated cyclopropane ion undergoes ring opening to acquire the sec-propyl ion structure. Similarly, it has been shown that protonated cyclobutane rearranges to the sec-C4H9 + structure. 

Patra, D. and Ghosh, S. (1995) Photocycloaddition-cyclobutane rearrangement to spiro cyclopentanones application in a formal synthesis of ( )-a-cedrene. Journal of the Chemical Society, Perkin Transactions 1, 2635-2641. 

Yet again tryptophan is the ultimate precursor for the dictazoles, e.g., A (52) and dictazolines, e.g., C (53) (2010JOC2399). It is believed that the four-membered structures precede the constitutionally isomeric dictazolines in the biosynthesis pathway via a vinyl cyclobutane rearrangement. 

When one double bond of semibullvalene is replaced by the single bond of a cyclobutane ring, a (2 -vinylcyclopropyl)cyclobutane rearrangement can take place at 300°C (240) gives (241). ... 

Ghosh, S., Patra, D., and Samajdar, S., Intramolecular [2 +2 ]-photocyclo-addition-cyclobutane rearrangement. A novel stereocontrolled approach to highly substituted cyclopentanones. Tetrahedron Lett., 37, 2073, 1996. 

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. 

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). 

Metallacycles as intermediates in redox rearrangements of cyclopropane and cyclobutane derivatives 96MI10. 

The novel [6+2] annulation approach developed by the Takeda group has also been included in a threefold anionic/pericyclic process (Scheme 2.149) [340]. The reaction leads to functionalized eight-membered rings 2-659 in a highly stereoselective manner, starting from acylsilanes 2-656 and 3-(trimethylsilyl)vinyl-lithium (2-657). After 1,2-addition and 1,2-Brook rearrangement, the cyclobutane 2-... 

The photolysis of cyclic diazo ketones in hydroxylic solvents leads to ring contracted carboxylic acid derivatives via this ketocarbene -> ketene rearrangement. Examples of such reactions are given in (2.24)239) and (2.25) 240). In this last example a photoequilibrium between the diazo ketone and its valence isomer, a diazirine, has been observed, both products then eliminating nitrogen to afford the cyclobutane carboxylic acid. 

Vinylcyclopropanes represent particularly useful functionality. They do permit a ring expansion to cyclobutanes via the cyclopropylcarbinyl cation manifold (Eq. 9). Equally important, such systems suffer smooth thermal rearrangement to cyclopen-... 

The key step of de Mayo s approach towards methyl marasmate made use of the rearrangement of the cyclobutane (227) to afford the bieyclo[2.2.1]system (228], which upon cleavage with Pb(0Ac)4 and esterification with CH2N2 afforded the ester (229) 76). 

Another skeletal rearrangement involving the strained cyclobutane ring was recently reported 93). The formic acid catalyzed reaction converted the compound (273)... 

Tetramethylene-ethane (TME), or 2,2/-bis-allyl diradical 81, was suggested as an intermediate in the thermal dimerization of allene, as well as in the interconversions of 1,2-dimethylenecyclobutane 82, methylenespiropentane 83, bis-cyclopropylidene 84 and other bicyclic systems (equation 30)45. The isolation of two different isomeric dimethylene cyclobutanes 87 and 88 (in a ca 2 I ratio) after the thermal rearrangement of the deuteriated 1,2-dimethylene cyclobutane 85 suggests that the rearrangement proceeds via a perpendicular tetramethyleneethane diradical (2,2/-bisallyl) 86 (equation 31)45. 

ET-induced cycloadditions of polycyclic olefins and cycloreversions of cyclobutane species have been studied by ESR spectroscopy [266]. Upon chemical and electrochemical reduction, 2,2 -distyrylbiphenyl rearranges by intramolecular coupling into a bis-benzylic dihydrophenanthrene dianion (Scheme 1), which can be either protonated to a 9,10 -dibenzyl-9,10-dihydrophenanthrene or oxidatively coupled to a cyclobutane species. It is interesting to note that the intramolecular bond... 

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