Cyclopropyl esters rearrangement

Note that we predict ring opening of the cyclopropyl cation to require activation this at first sight seems to be at variance with evidence that rearrangement occurs as a concerted process in the solvolysis of cyclopropyl esters and indeed acts as a driving force 44). Moreover the evidence shows very clearly that this is so only for one of the possible disrotatory processes, i.e. that indicated in 25 ... 

The cyclopropyl-allyl rearrangement has been shown to proceed with nucleophilic assistance,87i232<233 and the intermediate allyl cation can be trapped by nucleophiles leading to synthetically useful derivatives. An example is the formation of an unsaturated acetal and the propiolic acid ortho ester (equations 92 and 93).232... 

The ring contraction of 1,2-cyclobutanediones is promoted either by acids or by bases in protic media". The reaction products are a-hydroxycyclopropanecarboxylic acids or the corresponding esters. Rearrangements of some 2-alkylidenecyclobutanols to cyclopropyl carbonyl compounds are also known. 1,2-Epoxycyclobutanes afford cyclopropanecarbal-dehydes or cyclopropyl ketones under the catalytic action of protic acids, lithium iodide or lithium thiocyanate (equation 107) . Under the latter conditions, the reaction... 

Halocarbene adducts 1 of enol esters rearrange readily to form (a-halo)-a,/8-unsaturated ketones 2 under all reaction conditions that remove the acyl group via intermediate 2-halocyclo-propanols. Contrary to cationic cyclopropyl to allyl rearrangements, fluoride can be extruded in these rearrangements. 

Rh (CO)i6 is a highly active catalyst in the reaction of ethyl diazoacetate with alkenes to give cyclopropyl esters. Yields obtained are better than those obtained using palladium and copper catalysts. Methyl ester derivatives of tricyclo[4.2.0.0 ]octane have been prepared by a photochemical Wolff rearrangement of a related diazoketone in methanol. ... 

The photorearrangement of a dienone was noted<4) as early as 1830 in a study of the sesquiterpene a-santonin (1). However, the structure and stereochemistry of the various photoproducts were not conclusively established until 1965.(6) Upon irradiation in neutral media, a-santonin (1) undergoes rapid rearrangement to the cyclopropyl ketone, lumisantonin (2). However, if the irradiation is not terminated after a short period of time the lumisantonin itself rearranges into a linearly conjugated dienone (3). The dienone (3) can be isolated from the photolysis of either (1) or (2) in benzene or ether. In nucleophilic solvents (alcohol or water) the dienone (3) is also photo-chemically active and is further converted into an ester or an acid (photo-santonic acid) (4). 

A different result was obtained in the cycloaddition to methylenecyclo-propanes 216-218 tearing alkoxycarbonyl substituents on the cyclopropyl ring. In this instance, 1,2,3-triazoles 220 isomeric with the triazolines 219 were formed in the reaction [57]. The formation of triazoles 220 is rationalised by the intermediate formation of triazolines 219, which are unstable under the reaction conditions and undergo a rearrangement to the aromatic triazoles via a hydrogen transfer that probably occurs with the assistance of the proximal ester carbonyl (Scheme 35). The formation of triazoles 220 also confirms the regio-chemistry of the cycloaddition for the methylene unsubstituted methylene-cyclopropanes, still leaving some doubt for the substituted ones 156 and 157. 

Chiral 2,2-disubstituted cyclobutanones have been obtained by asymmetric rearrangement of chiral sulfinyl- 177,178 and sulfanylcyclopropanes.179 Using readily available cyclopropyl 4-tolyl (/ )-sulfoxide (l),180 the requisite sulfinylcyclopropanes 3 and 3 were obtained by a sequence of lithiation, reaction with carboxylic acid esters and stereoselective addition of Grignard reagents to the ketones 2 thus formed.178 The corresponding sulfanylcyclopropanes 4 and 4 resulted from a sequence of protection, reduction and deprotection.179... 

Cyclopropyl nitrites are considerably more reactive than other alkyl nitrites and readily rearrange below ice-bath temperatures.117) Thus, the nitrite ester of tetramethylcyclopropanone methyl hemiacetal is easily converted to the (3-propionate radical 149 at — 80 °C and, in the presence... 

MO calculations have been carried out on the isomerization of cyclopropane to propene, and the MNDO method has been used to study the reaction pathway and to optimize the structure of reactant, transition structure, and product of the ring opening reaction of bicyclo[1.1.0]butane. Various methods have been employed to estimate the rate constants for ring opening of the 2-cyclopropyl-2-propyl radical. 1-Acceptor-1-sulfenyl-substituted 2-vinylcyclopropanes of the type (430) have been found to afford 6-sulfenyl-a,jS y, -unsaturated carboxylic esters and nitriles (431) upon treatment with acid, by a process which involves C(l)—C(2) bond fission and a novel 1,5-sulfenyl rearrangement (see Scheme 110). It has been shown that the benzophenone-sensitized photolysis of vinyl norcaradiene derivatives, such as 5-(2-methylprop-l-enyl)-3-oxatricyclo[4.4.0.0 ]deca-7,9-dien-4-ones (432), results in the regioselective cleavage of only one of the cyclopropyl c-bonds to afford isochroman-3-one derivatives (433). It has been reported that the major product obtained from the reaction of structurally diverse a-diazo ketones with an electron-rich alkene in the... 

Due to the higher oxidation level cyclopropylcarbinyl-homoallyl rearrangement (see also Section IV.C) of oxycyclopropanes gives, y-unsaturated carbonyl compounds. This feature has been discovered by Julia and coworkers, who reacted certain esters with Grignard reagents or LiAlH4. The corresponding cyclopropyl carbinol opens under acidic conditions to j5,y-unsaturated aldehydes (equation This conversion also works for... 

Intermediate formation of a cyclopropyl ring has also been suggested in electrochemical reductions although to a lesser extent. 2-Alkyl-substituted acetoacetic esters undergo the Tafel rearrangement upon reduction at a lead cathode in an acidic medium with complete reduction of the functional groups Analysis of the reduction products... 

Various cyclopropyl diazo ketones undergo Wollf rearrangement upon photolysis ° or exposure to silver(I) oxide to give derivatives of cyclo-propylacetic acid in fairly good yield, e.g. formation of 9, When the reaction is carried out in the presence of an alcohol, esters were obtained " whereas reactions in aqueous media gave acids. ... 

---