Cyclobutyl carbonium ion

This rearrangement is analogous to the homoallylcyclopropylcarbinyl-cyclobutyl-carbonium-ion transformations involved in squalene biogenesis, ... 

An isomer of this carbonium ion has the primary carbon bearing the positive charge. This arrangement is called the cyclopropylmethyl carbon skeleton. If this carbonium ion is used, then it may rearrange to form the cyclobutyl carbonium ion. Suggest how this may occur. 

The Nature of the Intermediate in Carbonium-Ion Type Interconversion Reactions of Cyclobutyl, Cyclopropylcarbinyl and Allylcarbinyl Derivatives. J. Amer. chem. Soc. 73, 3542 (1951). 

Without rearranging, the carbonium ion could react with water to form the protonated form of the corresponding alcohol (XXX), or a proton could be lost from the next carbon atom to form the olefin (XXXI). An alternative path of reaction would be the interchange of one of the groups on the adjacent carbon atom with the carbonium ion, followed by one of the reactions shown above. With the cyclobutyl-methyl carbonium ion, then, there are three possibilities A hydrogen atom with its pair of electrons might move, producing the new carbonium ion (XXXII), which could then become stabilized by a reaction with water (XXXITI), or by the elimination of a proton from an adjacent carbon atom (XXXIV). 0... 

Alkoxy-l-methylpyridinium salts (XII-729) have been used to study the behavior of carbonium ions in solvolyses. They are easy to prepare and purify, and in solvolyses the leaving group does not undergo internal return. The solvolysis of 4-(cyclopropylcarbinyloxy)-l-methylpyridinium iodide (XII-729, r = ch2-c1) in 80% ethanol gives some cyclopropylcarbinol, cyclobutanol, and allylcarbinol but mainly the corresponding ethyl ethers (XII-730 R = cyclo-propylcarbinyl, cyclobutyl, allylcarbinyl R = CjHs) in a ratio of 3.6 1.5 1.0. 

If the initial cyclization of trans trans- or cis, /raw -farnesyl pyrophosphates (52-OPP and 51-OPP) occurs upon the terminal (10, 11) double in an anti-Markovnikov orientation, eleven-membered cyclic dienyl carbonium ions are produced. Deprotonation of the trans, trans-intermediate affords humulene (97) while bis-homoallyl- cyclobutyl carbinyl cyclization furnishes, after elimination, caryophyllene (99). Although the all traw -configuration of the three double bonds in humulene would appear to require trans, rra -farnesyl pyrophosphate as precursor, the possibility exists that the immediate predecessor of caryophyllene is cis, /rtz 5-farnesyl (or cisoid nerolidyl) pyrophosphate. 

---