Cyclobutane derivatives, hydrogenation

Enamines having a hydrogen on the enamine carbon also undergo cycloaddition to give cyclobutane derivatives. The latter are less stable, so that the reaction must be carried out under milder conditions in order to obtain... 

The addition of p-quinone to enamines normally produces furan derivatives, especially when the enamine possesses a 3 hydrogen (see Section III. A). 1,2 Cycloaddition is claimed to take place to give a cyclobutane derivative when p-quinone and an enamine with no jS hydrogens are allowed to react at low temperatures (51). However, little evidence is reported to verify this structural assignment, and the actual structure probably is a benzofuranol (52). Reaction of a dienamine (formed in situ) with p-quinone in the presence... 

The overall reaction includes allylic transposition of a double bond, migration of the allylic hydrogen and formation of a bond between ene and enophile. Experimental findings suggest a concerted mechanism. Alternatively a diradical species 4 might be formed as intermediate however such a species should also give rise to formation of a cyclobutane derivative 5 as a side-product. If such a by-product is not observed, one might exclude the diradical pathway ... 

The cyclobutane derivative (55) gave an optical yield of 91% in the hydrogenation of a-acetamidocinnamic acid. The catalyst was here prepared in situ from [RhCl(l,5-hexadiene)]2. The corresponding 1,2-derivative of cyclopentane gave an optical yield of only 73% and the cyclopropane and cyclohexane derivatives gave 15% and 36% respectively.235 [RhCl(cod)2] in presence of the norbomadiene-based ligand NORPHOS (56) gave up to 96% optical yields with a-acetamidocinnamic acid as substrate.236... 

Oxovinylidene- and iminovinylidene-triphenylphosphoranes react with hydrogen chloride in the molar ratio 2 1, leading to 1,3-dioxo- and 1,3-diimino-cyclobutane derivatives, which can be converted into stable exocyclic bisalkylidenephosphoranes by sodium bis(trimethylsilyl)amide (equation 102). 

Photoreactions of Thymines, etc. - Irradiation at 254 nm of the pyrimidine derivative (153) induces a Norrish Type II hydrogen abstraction from a methyl group of the t-butyl substituent. The resultant 1,4-biradical (153a) undergoes cyclization to afford an unstable cyclobutanol. Elimination of water from this species affords the final product identified as the cyclobutane derivative (154). The structure of this product was verified by X-ray diffraction techniques. The Norrish type II reactivity of the pyrimidine derivative (155) at 254 nm in water follows the analogous path to that observed for (153) and yields the cyclized product (156) in 52 % yield. - °... 

Addition of methoxycarbonylnitrene generated photochemically from methyl azidoformate to methylenecyclopropane gave methyl l-azaspiro[2.2]pentane-l-carboxylate 1, which cannot be isomerized to the cyclobutane derivative like the isosteric oxaspiropentanes. " Treatment with acids such as hydrogen chloride or methanesulfonic acid cleaved the aziridine ring and gave l-(chloromethyl)-l-(methoxycarbonylamino)cyclopropane and l-(mesyloxymethyl)-l-(methoxycarbonylamino)cyclopropane. ... 

When heated with diazabicycloundecene in DMSO, 19-iodotabersonine (278), prepared from 197 -vindolinine (109), gives mainly the expected 18,19-didehydrotabersonine (279), together with the cyclobutane derivative 280, and an optically inactive, nonbasic indole derivative, for which the structure 281 has been proposed (208). A much improved yield of 281 can be obtained if the reaction is conducted in DMF in the presence of sodium acetate. The formation of the cyclobutane derivative 280 would seem to involve a straightforward elimination of hydrogen iodide from 278, with assistance from the aniiinoacrylate function, followed by hydration of an Ai 2-double bond. The alternative fission of the 7,21- and 20,21-bonds to give the neutral product 281 may well proceed via an iminium ion 282, as illustrated in Scheme 4 (208). If correct, this mechanism requires the... 

In application of the above to hydrogen shift in the corresponding cyclobutane derivative 22, one will expect 22a as the most reactive conformer on account of better orbital overlap of the breaking cr( n bond and the cyclobutane cjc c bond, as shown by parallel dotted lines in red color required for n bond formation, to generate 23. The conformer 22b will be the least reactive to generate 24. The difference in the geometries of the donor-derived double bonds and also the reversal in the configuration on the acceptor-derived carbon must specifically be noted. 

In the literature the PE spectra of a few bicyclic and polycyclic cyclobutane and cyclobutane derivatives have been studied in order to elucidate the through space and through bond interaction of rr-orbitals. Examples are 722 to 725 and the corresponding hydrogenated products. In case of 123 the observed split between the... 

The first iron-catalysed 2-I-2-cycloaddition of enimides with alkylidene mal-onates yielded -amino acid cyclobutane derivatives with excellent yields and diastereoselectivities. The enantioselective intermolecular 2-i-2-photocycloaddition of isoquinolone with alkenes, in the presence of a chiral hydrogen-bonding template (11), produced functionalized cyclobutane cycloadducts in high yields (86-98%) and excellent regio-, diastereo-, and enantio-selectivity. ... 

The photochemical cycloaddition of acetylene to anhydrodeoxyhexenulose (71) gave, after hydrogenation, the cyclobutane derivative (72). ... 

By oxidation with permanganate it forms pinonic acid, C,oH,<503, a monobasic acid derived from cyclobutane. With strong sulphuric acid it forms a mixture of limonene, dipentene, terpinolene, terpinene, camphene and p-cymene. Hydrogen chloride reacts with turpentine oil to give CioHijCl, bomyl chloride, artificial camphor . 

Methane ethane and cyclobutane share the common feature that each one can give only a single monochloro derivative All the hydrogens of cyclobutane for example are equivalent and substitution of any one gives the same product as substitution of any other Chlorination of alkanes m which the hydrogens are not all equivalent is more com plicated m that a mixture of every possible monochloro derivative is formed as the chlo rmation of butane illustrates... 

Cyclobutanes can be conveniently prepared from cyclobutene derivatives through electrophilic addition, catalytic hydrogenation, nucleophilic addition, cycloaddition as well as light-induced addition reactions. 

The preparation of cyclobutanols by a homoallyl rearrangement has been described previously (see Houben-Weyl. Vol. 4/4, p 62), e.g. treatment of the but-3-enyl p-toluenesulfonate 1 with potassium methoxide in dioxane yielded the dicyanomethoxycyclobutane derivative 2 (61 %). It is also possible to introduce hydrogen into the cyclobutane product by the use of hydrides, for example, borohydrides. Methoxy- or ethoxy-substituted cyclobutanes are formed with alkoxides, while cyano compounds are obtained with potassium cyanide (Table 1). Electronegative substituents in the 1-position of the starting alkene are necessary for a good result in this preparative method. 

The preferred conformation of intermediates appears to be decisive in selecting which cyclobutane bond will be broken.4 In many cases the iodo compounds gave better results than the 3-bromopropyl derivatives.5 If the substrates contained iodo or bromo as well as chloro substituents, the halogen atoms reacted selectively. With one equivalent of tin hydride only the iodo or bromo substituent was removed, with an excess of tin hydride the chlorine atoms were also substituted by hydrogen atoms. 

Hydrogenative ring opening of cycloalkanes is also a well-studied area.16 252 253 289-292 Mainly cyclopropanes and cyclopentanes were studied, since three- and five-membered adsorbed carbocyclic species are believed to be intermediates in metal-catalyzed isomerization of alkanes (see Section 4.3.1). Ring-opening reactivity of different ring systems decreases in the order cyclopropane > cyclobutane > cyclopentane > cyclohexane.251 Cyclopropane and its substituted derivatives usually react below 100°C. 

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