Double Wagner-Meerwein

An elimination/double Wagner-Meerwein rearrangement process has recently been developed by Langer and coworkers [39]. Treatment of compound 1-136, obtained by reaction of 1-134 and 1-135, with trifluoroacetic acid (TFA) led to the cationic species 1-137, which then underwent a twofold Wagner-Meerwein rearrangement to give the bicydic compound 1-139 via 1-138 (Scheme 1.34). 

Scheme 1.34. Domino elimination/double Wagner-Meerwein rearrangement reaction.

Under more vigorous conditions (0.2-0.5 eq. of the catalyst), the dehydration of the 3 (3-hydroxyl group occurred and the resulting compounds underwent a double Wagner-Meerwein rearrangement, originating A-neo-18a-oleanene compounds in high yields (Scheme 40). 

Scheme 40 Bi(OTf)3-xH20-promoted dehydration and double Wagner-Meerwein rearrangement of betulin (Rj = H2) and betulinic acid (Ri = O)...

Expecting that acidic isomerization involving a double Wagner-Meerwein rearrangement would transform the bicyclic olefin 74 into supposedly stable [10.10]betweena-nene and its (Z)[10.10] isomer, they treated 74 with H2S04-AcOH in benzene only to find that the product was an 85 15 mixture of 75 and 76. Solvolysis of the spiro compound 77 was also found to yield a 60 40 mixture of 75 and 76 which was totally free from the fused (Z)[10.10] and ( )[10.10] olefins. 

Prior to the advent of simple methods for radical deoxygenation, a double Wagner-Meerwein rearrangement (Scheme 23) was used as a method for removing a bridgehead hydroxy group in this series. The success of this sequence indicates the subtle balance of stabilities in this series. 

An additional feature of the protosteryl cation is that the C-10 methyl and H-5 also share an anti-axial relationship, and are also susceptible to Wagner-Meerwein rearrangements, so that the C-9 cation formed in the cycloartenol sequence may then initiate further migrations. This can be terminated by formation of a 5,6-double... 

Fig. 14.7. Wagner-Meerwein rearrangement as part of an HCl addition to a C=C double bond ...

The sulfuric acid catalyzed transformation of pinanic acid into abietic acid shown in Figure 11.6 includes a Wagner-Meerwein shift of an alkyl group. The initially formed carbenium ion, the secondary carbenium ion A, which is a localized carbenium ion, is generated by protonation of one of the C=C double bonds. A [l,2]-sigmatropic shift... 

An interesting double ring expansion sequence developed by Vogel involves the initial pinacol coupling of cyclobutanone, isomerization to ketone 79 in acid, followed by reduction and Lewis acid-promoted Wagner-Meerwein rearrangement with dehydration (Scheme 19).129) For preparative purposes, 80 can be more... 

At higher temperatures (—60 °C) the Wagner-Meerwein shifts become thermodynamically controlled and, as a consequence, ions 28 and 29 are formed (Scheme 6). These ions contain carbon-carbon double bonds with a maximum number of alkyl substituents. 

The fact that we obtain ion 34-d4 after ionization of alcohol 72-<74 is in line with the proposed mechanism. As shown in Scheme 21 some additional 1,2 Wagner-Meerwein shifts are necessary to account for the observed ion. This is reasonable because ion 36a-d4 is destabilized relative to ion 42-d3 by the lack of a methyl substituent on the double bond. 

The reaction of 4-cyclooctene-l-tosylate (and brosylate) has been studied6. After breaking the tosylate carbon bond a carbocation remains which is transannularly attacked by the nucleophilic double bond. The resulting secondary cation can be stabilized by nonstereoconlrolled attack of water or acetic acid, by proton elimination or by Wagner-Meerwein rearrangement. In the quoted example, the main product is the bicyclo[3.3.0]octane skeleton and only a small amount of rearranged bicyclo[3.2.1]octane is found as a byproduct. 

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