Cyclopropane ring-openings thermal

It has been recently shown that the selective alkylation and strand scission of deoxytetranucleotide d(GTAG)-27 chosen as DNA model, results from the formation of covalent adducts 28 and 29 on the N-7 of guanine and N-3 of adenine with opening of the cyclopropane ring, respectively. Thermal treatment of 28 (90 °G, 5 min) afforded the d(deoxyribose-TAG) 30 with liberation of N-7 alkyl-guanine 31, while treatment of 29 provided the d(GT-deoxyribose-G) 32 and the N-3 alkyladenine 33 [27]. The stabilities of adducts 28 and 29 were tj/2 = 31 h and 3.2 h, respectively therefore, the cleavage reaction of adduct 29 proceeds much faster than that of 28, Eq. (11) [27]. 

A series of polymers (including polysulphones, polyethers) containing the monomer fra s-l,2-diphenylcyclopropane have been synthesised [49] by solution polycondensation. CP/MAS spectroscopy was used, among other techniques, to characterise the cyclopropane ring opening reaction which had occurred in the final thermally crosslinked product. One such polymer (Fig. 15.2.45) showed characteristic peaks in its CP/MAS spectrum (Fig. 15.2.46). 

Furans and some of its derivatives have been cyclopropanated with the ketocarbenoids derived from ethyl diazoacetate and copper catalysts. The 2-oxabicyclo[3.1.0]hex-3-enes thus formed are easily ring-opened to 1,4-diacylbutadienes thermally, thermo-catalytically or by proton catalysis 14,136). The method has been put to good use by Rh2(OAc)4-catalyzed cyclopropanation of furan with diazoketones 275 to bicyclic products 276. Even at room temperature, they undergo electrocyclic ring-opening and cis, trans-dienes 277a are obtained with fair selectivity 257,258). These compounds served as starting materials in the total syntheses 257 259) of some HETE s (mono-... 

In a single case, thermal treatment of enone 36 induced a 1,3-sigmatropic silyl shift which led to the formation of the rearranged product 37 (Eq. 8.3) [40], So far, only one example of a ring-opening reaction of a cyclopropane 38 has been reported which furnished tetramethoxy-substituted allene 39 (Eq. 8.4) [41],... 

Scheme 1. Preparation of l-chloro-l-(trichloroethenyl)cyclopropanes 7 by the addition of thermally ring-opened tetrachlorocyclopropene 5 onto alkenes 6 [7-10,25]...

The reaction of carbenoids with aromatic systems was first reported by Buchner and coworkers in the 1890s.6 The reaction offers a direct entry to cycloheptatrienes and has been used to synthesize tropones, tropolones and azulenes.6 Neither the thermal nor copper-catalyzed reactions, however, proceed in good yield. The problems associated with these transformations were clearly demonstrated in a recent reexamination of die thermal decomposition of ethyl diazoacetate in excess anisole (137).129 A careful analysis of the reaction mixture revealed the presence of seven components (138-144) in 34% overall yield (Scheme 29). The cycloheptatrienes (138M142) were considered to be formed by cyclopropanation followed by electrocyclic ring opening of the resulting norcaradienes. A mixture of products arose because the cyclopropanation was not regioselective and, also, the initially formed cycloheptatrienes were labile under the reaction conditions. 

Siloxy-l-vinylcyclopropanes rearrange thermally to give 1-siloxycyclopentene derivatives or may be hydrolyzed to cyclobutanones by acids (J.M. Conia, 1975 B.M. Trost, 1973 G.C. Girard, 1974). The cyclopropane ring may also be opened by base-catalyzed hydrolysis of the silyl ethers to form ethyl ketones. 

In principle, the vinylcarbene-to-cyclopropene isomerization is reversible. While this has not been reported for 167 (where ring-opening could produce not only alkenyl(sulfonyl)carbene 166, but also the isomeric l-alkenyl(silyl)carbene), it was found that 1-trimethylsilylcyclopropenone acetal 168 (equation 49) by thermal ring-opening yields the (trimethylsilyl)vinylcarbene 169 besides traces of the isomeric vinylcarbene 170. Both carbenes are obviously nucleophilic since they are able to cyclopropanate the... 

An illustration of this divergent reactivity can be observed in BanwelTs use of gem-dibromocyclopropane 7 to construct an intermediate used in an approach to the Amaryllidaceae alkaloids (Scheme 4.2).9 When 7 was heated in the absence of Lewis acid, allyl bromide 8 was produced in quantitative yield via thermal ring opening of the cyclopropane and subsequent trapping of the resultant cation by free bromide in... 

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