Double Wagner-Meerwein rearrangement

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 ...

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... 

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. 

Double bond formation can sometimes be less predictable when the resulting carbocation is highly stabilized. For example, the substrate 12, when exposed to Burgess reagent, produced 13 via a Wagner-Meerwein rearrangement. 

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