Cyclopropyl-allyl rearrangement

Tochtermann reported the addition of dichlorocarbene to the racemic glycal 52, whose cyclopropyl-allyl rearrangement leads to the 2//-pyran. The synthesis of the optically pure (+)-(2S,3R,7S) and (-)-(2f ,3S,7f )-glycal precursors has also been achieved. As pointed out, optically pure glycals are versatile precursors for carbohydrate synthesis <00EJO1741>. 

The silver ion assisted carbon-halogen bond cleavage and the unraveling of the cyclopropane ring by the cyclopropyl-allyl rearrangement was first noted in the formation of 2-bromocyclohexen-l-ol from dibromobicyclo[3.l.0]hexane under solvolytic conditions (equation 86).220 The silver ion assisted solvolysis of the dihalocyclopropane adduct (43), derived from a Birch reduction product, smoothly rearranges to the tropone (equation 87).221 A number of other synthetic applications222-226 have beien reported... 

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 process is believed to occur by addition to form the more stable cation, (242), which undergoes a cyclopropyl-allyl rearrangement followed by a 1,2-shift of an R-group to produce an allyl-substituted dialkyl aluminium, (243), as in the above case. This is trapped by added water, or undergoes an allylic rearrangement before being trapped I72>. 

Reaction of 1,2,3-trimethylcyclopropene with trichloromethyl lithium generated from bromotrichloromethane and methyl lithium at —110 °C produces the cyclobutene (279). This may be explained in terms of intermediate formation of the bicyclobutane (280), followed by cyclopropyl-allyl rearrangement, though the dichloro-cyclopropane could not be detected even at —73 °C 209). 

The double bonds in tetraphenylallene are inert to dichlorocarbene therefore, it underwent addition to the phenyl ring in low yield. The product 34 thus formed underwent cyclopropyl-allyl rearrangement to give 35. ... 

On the other hand, when tropone (41) is allowed to react with the phosphaalkynes 9a, 9b, or 9 e at a somewhat raised temperature in the molar ratio 1 1 in toluene, the reaction sequence ends with the formation of the diphosphapentacyclic compounds 45a-c [45]. In other words, this means that the initially formed 43 must have undergone a stereoselective, [8 + 2]-cycloaddition with a second equivalent of tropone (41). The hexacyclic species 44 thus formed experience a cyclopropyl-allyl rearrangement with subsequent [1,5]-H shift and ring inversion, finally to furnish the isolated pentacyclic compounds 45 [45]. The semiempirically... 

Although electron-transfer-promoted reactions of 89 are unknown, cation radical 95 has been generated by y-irradiation and its matrix-dependent reactivity was recently reported by Gerson and his co-workers. In a CF3CCI3 matrix at 77 K, 95 rearranges rapidly to the bisected tetramethyleneethane cation radical 96. However, deprotonation occurs at 120 K in a CFjClCFClj matrix to form 97 (Scheme 26). It is reasonable to assume that cation radical 96 is formed by the cyclopropyl-allyl rearrangement of 94 itself generated from 95. ... 

Difluorocarbene can be generated by thermolysis of a trifluoromethylmercurial in the presence of sodium iodide, and its cycloaddition to norbornadiene has been investigated. A mixture of two products (79) and (80) is obtained, the latter evidently arising by a linear cheleotropic reaction. Similar primary products are obtained from the addition of chlorofluorocarbene to norbornadiene, but here the reaction is complicated by secondary cyclopropyl-allyl rearrangements. 

Cyclopropyl-allyl rearrangement has been employed as a synthetic tool and studies of homocyclopropenyl cations have led to the stable dication (319)... 

Attempted generation of a dicarbocation from (363 R = Me) leads only to (364) or ring-cleaved cations. However, (363 R = Ph) readily gives rise to (365) and thus the cyclopropane ring, of itself offers insufficient stabilization for these species. " Solvolysis of the tosylate (366 R = Me) results in almost quantitative cyclopropyl-allyl rearrangement. However, with (366 R = Ph or cyclopropyl) mesomeric stabilization of the cation intermediate results in ca. 90% of products derived from the unopened ion. ... 

The ability of cyclopropabenzene to undergo [2 -I- 4] or [6 + 4] cycloaddition has been employed in the synthesis of (326) by initial conversion into (325). Under the conditions employed, (327) and tetrachlorocyclopropene afford products of cyclopropyl-allyl rearrangement of (presumably) initially formed endo-adduct. Other routine additions of cyclopropenes to cyclopentadiene and its derivatives have been presented. ... 

Whereas the base cleavage of 2-arylcyclopropanone hemiacetals proceeds with exclusive C-1—C-2 bond heterolysis, acid-catalysed decomposition occurs by way of C-2—C-3 cleavage to give 1-arylpropanones, a process explicable by cyclopropyl-allyl rearrangement. The rearrangement is also implicated in the acylation of 2-cyclopropylpyridine 1-oxide and in the decomposition of dichlorocarbene adducts of polymethylnaphthalenes. ... 

The thermal isomerization of 5-acetyl-5-methylbicyclo[2,l,0]pentane (494) has been the subject of a detailed kinetic study. Rates and activation parameters have been determined for all the processes. The exo-endo interconversion and the rearrangement to the cyclopentene (495) evidently do not involve a common intermediate. The activation parameters for the rearrangement to cyclopentenyl ketone, AH = 22.2 kcal mol and AS = - 33.0 cal K mol are indicative of a highly ordered transition state and suggest that the cyclopropyl-allylic rearrangement is an electrocyclic process. 

A tris-adduct (48), the structure of which was assigned on spectroscopic evidence, has been obtained from hexamethyl(Dewar)benzene. The reaction apparently involves a cyclopropyl-allyl rearrangement and HCl elimination as shown in Scheme 6. The bis-adduct (47) was isolated when the dichloro-carbene was generated by the action of potassium t-butoxide on chloroform. 

Rearrangements. "—Cyclopropyl-Allyl Rearrangement. This is one of the molecular rearrangements which has been investigated by calculations based upon the principle of least motion (PLM). ° In agreement with more sophisticated theoretical methods, PLM leads to the correct prediction of which of the two disrotatory openings will occur in the cyclopropyl-allyl transformation. 

The silver-nitrate-catalysed cyclopropyl-allyl rearrangement of (451) has been used in the preparation of benzazocines and, by ring expansion of (452) with silver acetate-acetic acid, the two substituted trflns,cis-cyclonona-dienes have been made. The trans,trflns-cyclodecadienes (454), which are involved in a synthesis of cyclodeca l,6-diyne-3,8-diol, have been prepared by... 

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