Homoallylic rearrangements

Furthei-more, the cyclization of the iododiene 225 affords the si.x-membered product 228. In this case too, complete inversion of the alkene stereochemistry is observed. The (Z)-allylic alcohol 229 is not the product. Therefore, the cyclization cannot be explained by a simple endo mode cyclization to form 229. This cyclization is explained by a sequence of (i) e.vo-mode carbopallada-tion to form the intermediate 226, (ii) cydopropanation to form 227. and (iii) cyclopropylcarbinyl to homoallyl rearrangement to afford the (F3-allylic alcohol 228[166]. (For further examples of cydopropanation and endo versus e o cyclization. see Section 1.1.2.2.)... 

In the alkylative cyclization of the 1,6-enyne 372 with vinyl bromide, formation of both the five-membered ring 373 by exn mode carbopalladation and isomerization of the double bonds and the six-membered ring 374 by endo mode carbopalladation are observed[269]. Their ratio depends on the catalytic species. Also, the cyclization of the 1,6-enyne 375 with /i-bromostyrene (376) affords the endo product 377. The exo mode cyclization is commonly observed in many cases, and there are two possible mechanistic explanations for that observed in these examples. One is direct endo mode carbopalladation. The other is the exo mode carbopalladation to give 378 followed by cyclopropana-tion to form 379, and the subsequent cyclopropylcarbinyl-homoallyl rearrangement affords the six-membered ring 380. Careful determination of the E or Z structure of the double bond in the cyclized product 380 is crucial for the mechanistic discussion. 

Hecogenin p-toluenesulfonylhydrazone, 402 Hofmann-Loffler reaction, 257 Homoallylic rearrangements, 379 A-homo-5a-cholestan-3-one, 356, 358, 362 A-homo-5a-cholestan-4-one, 359, 360, 368 A-homo-choIest-4a-en-3-one, 366 A-homo-estra-1(10), 2,4a-triene-4,17-dione, 367,370... 

A second convergent synthesis of haliclamine A (64) was achieved in a stepwise sequence from cyclopropyl(thiophen-2-yl)methanone (76) (Scheme 7) [37]. The protected thiophene 77 was condensed with formyl-piperidine to give 78, suitable for a Wittig olefination with 79. After desulfurization of the product 80, the deprotected alcohol 82 was subjected to homoallylic rearrangement using MesSiBr in the presence of ZnBr2. The re-... 

Coordination of the Rh(I) to the vinyl group and homoallylic rearrangement gives a rhodacyclohexene. Insertion of the alkyne into a Rh-C bond and reductive elimination completes the catalytic cycle. 

Finally, in the case of the geometrical isomers 164a,b, only products from an exo addition to the endocyclic double bond followed by homoallylic rearrangement are observed140, both in methanol and in methylene chloride. The electrophilic attack is exo specific, while the subsequent nucleophilic trapping by methanol or chloride proceeds non-stereospecifically giving equal amounts of 165 and 166 (equation 133). 

Triethylamine as the electron donor was also used by Mattay and co-workers in tandem fragmentation cyclization reactions of a-cyclopropylketones. The initial electron transfer on the ketone moiety is followed by the fast cyclopropyl-carbinyl-homoallyl rearrangement, yielding a distonic radical anion. With an appropriate unsaturated side chain within the molecule both annealated and spi-rocyclic ring systems are accessable in moderate yields (Scheme 41) [62]. 

With the appropriate precursor rac-35 in hand, the rearrangement key step was examined. As expected, basic treatment of y-keto-tosylate rac-35 resulted in the formation of a 56/44 mixture of the formal substitution product rac-43 and rac-44 as the product of homoallylic rearrangement (Scheme 12), in a combined yield of 95%. 

Furthermore, the chloro ester 1-Me also readily reacted with the nitrile ylide 37 at room temperature, however, the pyrrole derivative 40 was the only isolated product (Scheme 9) [26 a]. The latter was obviously formed from the primary cycloadduct 38 by a cyclopropylcarbinyl to homoallyl rearrangement [1]. 

Cycloguanil, 281 Cyclomethycaine, 14 Cyclopal, 269, 270 Cyclopentamine, 37 Cyclopenthiazide, 358 Cyclopentolate, 92 Cyclopropylcarbynyl-homoallyl rearrangement, 31, 151, 182 Cyclopyrazolate, 92 3,5-Cyclosteroid, 182 Cy do thiazide, 358 Cycrimine, 47 Cyproheptadine, 151... 

A homoallyl rearrangement comprises the interconversion of the but-3-enyl (homoallyl), cyclobutyl and cyclopropylmethyl systems1,2 (see Houben-Weyl, Vol. 4/4, p 61). 

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. 

Solvolysis of l-ethenylidene-3-tosyloxycycloalkanes 13 gives cyclobutanols of the bicy-clo[n.l.l]alkanol and bicyclo[n.2.0]alkanol series.14 In this reaction, the competing 1234-1243 rearrangement leads to the bicyclo[n.2.0]alkanols 15. The yield for the homoallyl rearrangement product 14, after hydrolysis, is 9% when n = 3 and 37% when n = 4. 

The reaction does not involve a simple [2k + 2n] addition, rather a [[Pg.232]

There are many examples of 1234-1243 rearrangements. In particular, the reaction provides a useful route to bridgehead-substituted bicyclo[3.2.0]heptanes, but is normally in competition with the homoallyl rearrangement (see Section 2.1.). There are three main possibilities for the preparation of cyclobutanes ... 

As well as the homoallyl rearrangement products, the action of base on y-oxo p-toluenesul-fonates 3 (X = OTs) yields cyclobutanones 5 by a 1234 -1243 rearrangement. 

Besides products from homoallyl rearrangement (see Section 2.1.), solvolysis of 5-tosyloxypen-ta-1,2-dienes also gives products from 1234-1243 rearrangement. It is possible to discriminate between the different rearrangement products if the / -allenic p-toluenesulfonates 1 arc substituted at Cl or C2. Thus, an apparent exchange of the positions of R1 and R2 occurs in the solvolysis. 

A base-induced y-homoallylic rearrangement is defined by the following scheme 170-171... 

The cyclobutyl to homoallyl rearrangement was studied above all for mechanistic reasons, especially to differentiate between different kinds of cationic intermediates.7 In general, mixtures of cyclopropylmethyl, cyclobutyl, and homoallyl derivatives are formed, depending on the type of substitution in the substrate and stability of the precursor ions. 

Thus, the ring-opening reaction of cyclobutyl derivatives is usually not used for preparative purposes. It is, on the contrary, an undesirable back reaction in the synthesis of cyclobutanes by homoallyl rearrangement (see Section 7.A.2.I.).3... 

Solvolytic rearrangement of formic acid gave quantitatively the most stable product of the homoallyl rearrangement, e.vu-bicyclo[3.2.1]oct-2-en-7-yl formate (10). In 70% aqueous dioxane buffered with triethyl-amine at 80°C, the same tricyclic dinitrobenzoate 9 furnished a mixture of 78% tricy-clo[3.2.1.02 7]octan-3-ol (11) and 22% c.w-bicyclo[3.2.1]oct-2-en-7-ol (12).21... 

As a consequence of the cyclobutyl to homoallyl rearrangement, reactions of 1-chloro-, 1-bro-mo-l,2-dimethylcyclobutane, 1-chloro- or l-bromo-l,3-dimethylcyclobutane with silver(I) te-trafluoroborate in diethyl ether gave in each case the same open-chain compound 2-fluoro-4-mcthylpent-4-ene (14) as the major product. Thus, l-bromo-l,3-dimethylcyclobutane (13) was treated with silver(l) tetrafluoroborate in diethyl ether at 0 >C to give 2-fluoro-4-methylpent-4-ene (14).22... 

Ring-cleavage reactions of cyclobutylmethyl compounds via bomoallylic carbanions have been reported.23,24 The reaction was denoted as a y-homoallylic rearrangement 25 and occurs with H-shift. 

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