Cyclobutanes oxidative rearrangement

Carbon-Oxygen Bond Formation. CAN is an efficient reagent for the conversion of epoxides into /3-nitrato alcohols. 1,2-cA-Diols can be prepared from alkenes by reaction with CAN/I2 followed by hydrolysis with KOH. Of particular interest is the high-yield synthesis of various a-hydroxy ketones and a-amino ketones from oxiranes and aziridines, respectively. The reactions are operated under mild conditions with the use of NBS and a catalytic amount of CAN as the reagents (eq 25). In another case, N-(silylmethyl)amides can be converted to A-(methoxymethyl)amides by CAN in methanol (eq 26). This chemistry has found application in the removal of electroauxiliaries from peptide substrates. Other CAN-mediated C-0 bondforming reactions include the oxidative rearrangement of aryl cyclobutanes and oxetanes, the conversion of allylic and tertiary benzylic alcohols into their corresponding ethers, and the alkoxylation of cephem sulfoxides at the position a to the ester moiety. 

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). 

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... 

Cyclic diones or silylmethyl-protected diones react with olefines in a [2+2] fashion. Addition of these compounds to Cjq [341] leads initially to the cyclobutane fused fullerenes, which are not stable and are readily oxidized and rearranged to the furanylfullerenes 300 and 301 (Scheme 4.56). The intermediate 299 probably reacts by either intermolecular oxidation with 2 to yield 300 or by an intramolecular oxidation with the triplet fullerene moiety to yield 301. 

A cyclobutane with a carbonyl and a vinyl group in a cis-, 2-position readily undergoes a retro-Claisen rearrangement as soon as it is formed at room temperature by oxidation of the corresponding alcohol, e.g. formation of l.183... 

Three isomers of the substituted cyclobutanes 33, 34 and 35 are obtained by the [2+2] cycloaddition of isoprene, and separated at low temperature from other cooligomers without undergoing Cope rearrangement. When the mixture was subjected to hydroboration and oxidation, the alcohol 37 was obtained from the isomer 34, and easily separated from 35 and the diol 36. The alcohol 37 is a pheromone called grandisol [11]. Although overall yield was 15%, this is the shortest synthetic route to this pheromone. 

As was discussed in Section 7.2.2.3, cyclopropanes and cyclobutanes form a special group, with behavior distinct in many ways from that of other cycloalkanes. Several examples of oxidative skeletal rearrangements of these strained ring compounds are presented here. 

As discussed by Ruckel et al., (14) the catalytic cationic polymerizations of a-plnene oxide and B-plnene oxide both Involve the concomitant opening of the epoxide and cyclobutane rings In the propagation step. This rearrangement mechanism also clearly operates In the radlatlon-lnduced polymerization of these monomers, and Is depicted below for a-plnene oxide (Scheme ). 

The spiro-dihydrofuran (61) is converted into the cyclobutane derivative (62) under the influence of trifluoroacetic acid. The lithium dienolate (64), derived from the furanone (63), yields solely y-alkylated products on treatment with alkyl halides. Thermolysis of the t-butylperoxybutenolide (65) produces about equal amounts of the hydroxy-furanone (67) and the indenone (68), presumably via the oxide radical (66). Attack of iodide ion on the salt (69) results in the formation of methyl iodide, butanolide, and (surprisingly) methyl 4-iodobutanoate. A description of a study of the photochemical rearrangement of the tetrahydrofurans (70) to the bicyclic oxetans (71) has been presented. ... 

This sequence has been shown because there are examples known of substituted olefins being cyclized to cyclobutane derivatives by certain transition metal complexes (Schrauzer Heimbach, Jolly and Wilke °W). In addition, metal complexes will catalyze the opening of cyclobutane rings to olefins and other rearranged products, as shown particularly by Cassar, Eaton and Halpern f). It is likely that the sequence shown in Fig. 15, both forward and reverse, is the mechanism for many of these reactions. Metal ions such as silver(I) which do not undergo oxidation readily probably involve a different mechanism (Paquette 2a) Gassmann 2b)). 

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