Cyclopropylcarbinyl-homoallyl rearrangements

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. 

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. 

The reduction of methyl 2-siloxycyclopropanecarboxylates can also be started at the ester function when lithium aluminum hydride in ether is the reagent. The resulting alcohols undergo the wellknown cyclopropylcarbinyl/homoallyl rearrangement upon treatment with acid to provide P/y-unsaturated carbonyl compounds 117. These are synthesized isomerically pure and in good yields in a number of cases, if the two-phase-system 2N hydrochloric acid/pentane is employed 78). Otherwise the very easy isomerization to the conjugated a,p-unsaturated compounds 118 occurs to some extend, which can intentionally be completed by base catalysis. 

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]. 

A unique use of the cyclopropane moiety as a protecting group for the 5,6-double bond in steroids and its deprotection by an acid-induced cyclopropylcarbinyl-homoallyl rearrangement is shown in equation 17 ... 

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... 

Feldman has developed a new version of the TOCO process [97-98] that incorporates a cyclopropylcarbinyl-homoallyl rearrangement, as illustrated by the example in Scheme 54 [97a[. The stereochemical outcome of cyclization can be... 

Deamination of 19-amino-A Manosten-3p-ol provides a 9(10- 19)-<3Z t -lanostene derivative analogous to (341) (Cio methyl ether in place of Cio hydroxyl), presumably by way of a homoallyl-cyclopropylcarbinyl-homoallyl rearrangement 435). The initial product was considered to be the 3,10-diol, which underwent methanolysis to the Cio methyl ether during recrystallization. 

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