Cyclobutane structure

Figure 9. Protonated cyclopropane and cyclobutane structures calculated using the 6-31G basis set.

Benzothiophene-1,1-dioxide (VI) in benzene solution, is converted in sunlight into a stable photodimer (Vila or VHb), which is considered to contain a central cyclobutane structure.56 196 Even though the monomer cannot be isolated on thermal decomposition of the dimer, the latter in boiling ethyl phthalate decomposes, forming sulfur dioxide and... 

The analogy of these dimerization processes to thermal Diels-Alder type reactions which sometimes also yield cyclobutane structures is worth noting and may be taken as one of the arguments for a diradical structure of the transition in the latter process. Also, it may be pointed out that the photoexcited state involved is presumably the same one involved in the well-known photochemical trans-cis interconversion of such olefins. 

C-3, since coupling between the vicinal protons in a cyclobutane structure should be greater than 6 Hz (82TL603). 

The yellow, nonfluorescent, hydrocarbon acenaphthylene in solution dimerizes on irradiation to form a product with a cyclobutane structure ... 

A pairwise mechanism involving a quasi-cyclobutane structure was suggested first, Eq. (3) [18], but cross metathesis between cyclopentene and 2-pentene produced a statistical ratio of cross-products 1-3 (1 2 3 = 1 2 1), Eq. (4) [19]... 

Fig. 16-19 Structure of a thymine dimer. The dimer is attached at two positions to the sugar, deoxyribose, and note its cyclobutane structure.

An ingenious method of preparing radical cations involves irradiating photodimers having a cyclobutane structure in the presence of semiconductors... 

For other polymers, similar IR and NMR spectroscopic results have been obtained, and a linear polymer with a cyclobutane structure is concluded as well. 

The [2 + 2] cycloaddition represents the most general and direct pathway for the formation of a cyclobutane structure from two alkene moieties, as outlined in Scheme 2.126. This process may occur as a concerted reaction via a cyclic transition state (mechanism a), as a stepwise reaction involving the formation of an acyclic biradical (mechanism b), or through bipolar (mechanism c) intermediates. Depending upon the structure of the reactants, cycloaddition may occur by any of these mechanisms. 

Meerwein rearrangements, a i7a-hydroxyandrostane (21) is apparently dehydrated easily with migration of the Cps) methyl group from C(i3> to Cd ) [124]. The stereoelectronic requirements in the I7 3 epimer (22) would be satisfied by migration of the C(i4)- C(ig) bond to C(i4>"-C(i7), but the contraction of ring D to a highly strained tmns fused cyclobutane structure must be less favourable than the observed cis ... 

Thermal [2 + 2] cycloadditions of alkenes are both well known and often overlooked or underutilized in synthetic practice. Yet a maturing mechanistic understanding of these reactions, and the growing importance of cyclobutane structures and of cyclobutanes as synthetic intermediates, may soon lead to more widespread applications of this synthetic method. 

There have been a number of unsuccessful attempts to trap 1,4-zwitterionic intermediates, - but there have also been some reported successes. At best, then, such trapping experiments are not uniformly diagnostic for zwitterionic intermediates. truns-Propenyl ethyl ether reacts with TONE in ethanol solvent at 25 C to give product (140). " The normal [2 + 2] adduct (139) gives this same product when allowed to react in ethanol, but at a slower rate. Just how fast this solvolysis might take place under the conditions of the cycloaddition reaction, with enol ether and TCNE present, is not clear. Acetone does not interfere with the addition of ethyl vinyl ether to TCNE, but the adduct (141) is converted over one week at room temperature to the six-membered ring product (142), which may be postulated as being derived from ci ture of a 1,4-zwitterionic intermediate in equilibrium with the cyclobutane structure. Alternatively, since acetone like ethanol is known to react with TCNE, it may be a case of acid-catalyzed solvent-assisted solvolysis. 

With this background of information, Alder and Stein were surprised to find that phenyl azide reacts exothermally at room temperature with dicyclopentadiene to form a monoadduct, m.p. 130-131°, in quantitative yield. Two formulas had been advanced for dicyclopentadiene. Kramer and Spilker, discoverers of cyclopen-tadiene, assigned to the dimer the symmetrical cyclobutane structure (1), resulting from dimerization analogous to the formation of the truxillic acids from cinnamic acid. Wieland suggested the unsyrametrical formula (2), resulting from 1,4-addition of one molecule to the double bond of another. Two lines of evidence favored the... 

The CX-type cyclobutane structure of the dimer was determined by NMR and Mass spectroscopies. The NMR spectra of the monomer, dimer, and polymer are shown in Figure 1. The dimer, recrystallized from methanol, shows essentially the same X-ray diffraction pattern as that of the dimer as-prepared, and is also photoreactive to give the polymer. The resulting polymer is highly crystalline. As a result, it is concluded that the polymerization of the I crystal ( R Me, R Et) proceeds through one type of intermediate dimer to give the homo-adduct polymer. 

S-type) cyclobutane ring Thus, the polymer structure should have a unique repeating unit in which three types of cyclobutane structures are incorporated in a sequence of [aPEPaP eP ] (or [Q SjSa/3 S/3 ]), where /3 and are of opposite absolute configuration. Considering the stereochemistry of the cyclobutane rings, the polymer should be double syndiotactic ... 

For the reaction of diamond, a nonconcerted, radical mechanism is assumed as well, yet the formation of an intermediate it-complex is not observed as the symmetric surface double bond is not polarized (Figure 6.43). The lack of polarity becomes obvious for acrylonitrile in comparison to the addition to silicon, another regioselectivity is found for diamond. In the first case, the reaction takes place on the C=N-triple bond, while on a diamond surface it is the C=C-double bond forming a cyclobutane structure. 

The previously proposed structure for the photodimer (46) of 2-phenylmethylene -methyl-S-oxazolin-S-one11 has been confirmed by X-ray crystallographic analysis,119 which indicated the centrosym-metric structure originally postulated. The 4-phenyl analog (35) also dimerizes to the cyclobutane structure when irradiated in benzene.120... 

The photodimerization of cinnamic derivatives to form cyclobutane structures is a very well-known photochemical reaction. 

C,oH,80,Mr 154.25, oil, bp. 120°C(1.2 kPa). Mono-terpene with cyclobutane structure, C-1 epimer of grandisol, known as the sexual pheromone of the male boll weevil. It is not known if F. possesses the same biological activity. F. occurs in the roots of the mug-wort species Artemisia fragrans (Asteraceae), content 0.0033%. ... 

The pyrimidine bases thymine (T) and cytosine (C) form dimers at sites with adjacent pyrimidine moieties, so-called dipyrimidine sites, in the DNA chain, which have been well characterized with respect to chemical structure and mutagenic potential. The dimerization presented in Scheme 8.1 is a [In+ln] cycloaddition (see Section 7.3) involving the two C(5)=C(6) double bonds, leading to cyclobutane structures denoted by the symbol T< >T, or generally Pyr< >Pyr. 

Although the M-H,0 ion is represented here by a cyclobutane structure, this need not be its actual structure no evidence is available to confirm such structures, but it is felt that new bond formation in the product ion lowers the energy requirements of the reaction. The fact that hydrogen is abstracted from the 8-position has been conclusively danonstrated by extensive deuterium-labeling studies. That is, replacement of the hydrogens on the 5-carbon by deuterium results in an almost-exclusive... 

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