Cyclopropylcarbinyl-homoallyl reactions

Mattay et al. used triethylamine as electron donor in tandem fragmentation/cyclization reactions of a-cyclopropylketones (49) [48]. The initial electron transfer to the ketone moiety is followed by subsequent cyclopropylcarbinyl-homoallyl rearrangement yielding a distonic radical anion (50). With an appropriate unsaturated side chain attached to the molecule both annelated and spirocyclic ring systems are accessible in moderate yields. Scheme 26 shows some representative examples. 

In several cases, cyclopropylcarbinyl-homoallyl rearrangements involving zirconium species were observed (Scheme 16), similar to those reported for some other transition metals [23]. Examples of reactions are depicted in Table 3. The lack of rearrangement for the homoallylic ethers in entries 3-5 was attributed to the Thorpe-Ingold effect, i.e., angle compression at the substituted carbon [24]. 

The ring-expansion rearrangement to the strobane nucleus has been observed by Herz and Hall in a study of the reactions of 15-substituted pimarenes 320). Thus, hydroxy ester 263-OH affords the strobane-like isomer (264) upon reaction with tosyl chloride in pyridine. The (263- 264) rearrangement apparently proceeds by way of a stereospecific homoallyl- cyclopropylcarbinyl- homoallyl pathway since substitution reactions at Cl5 occurred with complete retention of configuration in all cases. Hydroxy ester (264) reverted stereospecifically to 263-0Ac (78%) in refluxing acetic acid. These transformations resemble those of the sesquiterpenes thujopsene and widdrol (section C.6). Since (264) has the Ci4 stereochemistry opposite to that of strobic acid (261), the C15 epimer of 263-OH might be expected to furnish products with the proper C14 orientation. 

In support of a radical pathway for such reactions, both cyclopropylcarbinyl bromide and iodide gave rise to appreciable homoallylic substitution product with the foregoing metallostannanes. In contrast, the corresponding chloride and tosylate gave only the unrearranged product. 

In solvolytic reactions it was observed that cyclopropylcarbinyl cations are formed as transient intermediates from the homoallylic alcohols 4 (equation 5). 

In suitable cases, allylic alcohols can be converted to the cyclopropylcarbinyl cations by reaction with superacids. The reaction involves the rearrangement of the initially formed ally] cation to the homoallyl cation by a 1,2-hydride transfer followed by its cyclization to the cyclopropylcarbinyl cation19 (equation 9). 

Cyclopropyl-substituted allenes open the door to yet another reaction mode. When treated with aryl iodides in the presence of a typical Heck-catalyst system and a dienophile, cyclohexene derivatives 77 were obtained (Scheme 11) [53,54]. Thus, the initially formed arylpalladium iodide car-bopalladates 72 to form a a-allylpalladium intermediate 73. It swiftly undergoes the cyclopropylcarbinyl to homoallyl rearrangement yielding the ho-moallylpalladium species 74 which finally suffers /1-hydride elimination. The thus formed 2-aryl-1,3,5-hexatrienes 75 are prone to undergo polymerization, but can be efficiently trapped by an appropriate dienophile at the least steri-... 

Carbon shifts 228 U-, 1,3-, 1,4- and 1,5-Hydride shifts 229 Homoallylic-, cyclopropylcarbinyl- and homocyclopropylcarbinyl rearrangements 230 Other degenerate reactions 231... 

However, more rapid events, such as 5-exo cyclization and cyclopropylcarbinyl radical ring opening, can lead to radical formylation (Scheme 4-7). Thus, 1,6-dienes, when exposed to tin hydride/CO conditions, give fair yields of slannylfor-mylation products via cyclization (Scheme 4-7) [20J. On the other hand, treatment of vinylcyclopropanes with CO under similar reaction conditions leads to stannylformylation products via a cyclopropylcarbinyl radical opening to a homoallylic radical (Scheme 4-7) [21]. 

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