Homoallyl, from cyclopropyl

This rearrangement, which accounts for the scrambling, is completely stereospecific.The rearrangements probably take place through a nonplanar cyclobutyl cation intermediate or transition state. The formation of cyclobutyl and homoallylic products from a cyclopropyl-methyl cation is also completely stereospecific. These products may arise by direct attack of the nucleophile on 58 or on the cyclobutyl cation intermediate. A planar cyclobutyl cation is ruled out in both cases because it would be symmetrical and the stereospecificity would be lost. 

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-... 

Orthoesters of acrylic acid are converted to y.y-dialkoxyallylzirconocene ethoxides with CpjZr. These latter species serve as 2,2-dialkoxycyclopropyl anion synthon, showing y-addition reactivity toward carbonyl compounds to afford homoallylic alcohols. In the presence of MejSiOTf cyclopropyl carbinols are formed, due to a switch to the -addition mode conjugate addition to enones leads to P-cyclopropyl ketones (e.g., from 2-cyclopen tenone)... 

The striking influence of y-substitution on product formation is readily explained in terms of the relative charge densities at the cyclopropylmethyl and homoallylmethyl centers in the incipient nonclassical carbocation intermediates. Starting from y-di- and monosubstituted homoallylic iodides such carbocations do have considerable cyclopropyl carbocationic character, which, as a consequence, leads to cyclopropylmethyl derivatives on substitution. In the nonsubstituted case, the intermediate carbocation has predominant homoallylic character which under these conditions results in elimination. 

Starting from (Z)-undeca-l,5-dien-3-ol a biogenetic-type synthesis of (+ )-dictyopterene A was achieved using the homoallyl-cyclopropyl rearrangement as the key step for cyclopropyl formation. Solvolysis of (Z)-3-mesyloxyundeca-l,5-diene under a variety of conditions failed to give cyclopropylmethyl compounds due to the presence of the terminal double bond. In contrast, (Z)-3-mesyloxy-l-(phenylthio)undeca-5-ene (3) afforded a mixture of epimeric cyclopropyl-methanols 4 in 91 % yield. Appropriate functional group transformation eventually leads to the desired dictyopterene A. 

The formation of the cyclopropyl ketones can be explained by assuming considerable interaction between the C-Si tr-bond and an electron-deficient orbital at the y-carbon which arises from the regiospecific acylation to the olefinic bond. Cyclodesilylation of the intermediate electrophilic species leads to cyclopropyl ketone, whereas a 1,2-hydride transfer followed by -elimination of the trimethylsilyl group leads to the homoallylic ketone. 

The essential difference between a homoallylic ion of type 455 and an a-cyclopropylcarbinyl ion of type 461 is that in the latter case the electron delocalization involves two bonds of the cyclopropyl group while in the former, only one. The available data indicate that the conversion of a bisected ion of type 461 into a homoallylic ion requires large energy expenditure and proceeds with difficulty this is also corroborated by quantum chemical calculations. But in the ion 455 these losses are compensated by the energy gain resulting from the formation of a homoaromatic system. 

Quite interestingly the behavior of the allyl radical and its cyclized counterpart the cyclopropyl radical is completely different from that of the homoallylic-methylcyclopropyl system. It does not seem that cyclized products resulting from the allyl radical have ever been observed. The opening of the cyclopropyl radical is, however, also a very slow process. Nevertheless, opening of substituted cyclopropyl radicals is observed when the substituents highly stabilize the allylic radical. In the same way photolytic cleavage of cyclopropanes is facilitated when each of the radical centers formed is stabilized. ... 

In summary, the results discussed above reveal that an ether oxygen in a medium or a bicyclic ring system may offer significant anchimeric assistance in reactions in solvolytic displacement reactions. Because of inductive effects, steric effects, field effects, and internal return, anchimeric assistance, when present, is often difficult to demonstrate kinetically. Products formed from bicyclic and tricyclic oxonium ions probably more often result from participation after carbocation formation. Participation by oxygen apparently occurs to the exclusion of transannular hydride shifts in medium ring systems, but other types of participation such as homoallylic or formation of cyclopropyl carbinyl cations will most likely occur to the exclusion of ether oxygen participation. 

Enamines can be obtained from cyclopropyl, cyclopentyl, methyl, and ethyl ketones [the latter affording the (jE)-isomer stereoselectively] but the reaction of amines with dicyclopropyl and cyclopropyl phenyl or a-thienyl ketones results in homoallylic rearrangement to 1-substituted 1,4-diaminobut-l-enes. ... 

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