Sequential rearrangements

Stable derivatives of the quusi-fullerene 40 were obtained in a one-pot reaction upon treating C50 with the tetrazine 41 (Scheme 11.10) [40]. The sequential rearrangement and fragmentation of the primary adducts, involving intermediates 42 and 43, afforded 43 in yields between 14 and 22%. Unambiguous structural proof was obtained by X-ray crystallography. 

Sequential rearrangements have been observed in the acetolysis of l -bishomo-cubyl methanesulfonate (855) which produce, inter alia, acetates (848), (851), and ( 52)569. 61% of (855) undergo rearrangement with internal return to slower sol-volyzing mesylates. The stereochemistry of the internal-returned mesylates suggests ionization to bridged-ion pairs, followed by frontside collapse. 

Many of the sequential rearrangement oxidations (Section 2.4.1.3.1.1.2.) may be based on this mechanism. 

Many other reaction sequences which are referred to as tandem rearrangements are already described in several sections before, c.g.. tandem Claisen rearrangement/aldol reactions, cyclopropanation/Cope rearrangements, etc. Iterative as well as sequential rearrangements have also been discussed. 

An interesting case consistent with this model is the rearrangement of 1-phenyl-l-trimethylsilyl-2-re/-t-butyldimethylsilylhydrazine (3) (26). Because this compound contains two different organosilyl substituents, multiple anionic rearrangements are possible. Loss of a proton from 3 produces the anion 3a, which can retu range to 4a. The latter can become protonated to 4 or, alternatively, to 5a, which can give upon protonation 5 [Eq. (10)]. Note that two sequential rearrangements are necessary to form 5. 

The in comparison to steroids atypical epoxysqualene cychsation, followed by a series of sequential rearrangements within the A,B-ring system, produces first the monocyclic iridal. Addition of a methyl group from S-adenosylmeth-ionine at the terminal double bond of the side-chain initiates the second cyclisa-tion. Cycloiridal results from dehydrogenation of the side-chain. Oxidative cleavage of the central double bond finally produces the desired scent molecule. [57]... 

On the other hand, the preparation of secondary amides from aldoximes has been recently achieved through the one-pot sequential rearrangement/A -alkylation process depicted in Scheme 18 [93]. The reactions were carried out heating a toluene solution of the aldoxime with a mixture of dimers [ RuCl(p-Cl)(r -/7-cymene) 2]... 

Scheme 18 Synthesis of secondary amides from aldoximes through sequential rearrangement/ iV-alkylation reactions...

Kim et al. found that two cycloaddition pathways, [4 + 2] and [3 + 2], are possible in the reaction of 61, having a gem diester on the carbon tether, and they are dependent on the reaction conditions. The gold-catalyzed reaction of 61 proceeded at room temperature to give benzocycloheptenone derivative 68 exclusively. The reaction probably proceeded through intramolecular [3 -I- 2] cycloaddition of Au-pyrylium intermediate 69, followed by a sequential rearrangement, as shown in 70. On the other hand, a 1 1 mixture of 62 and 68 was obtained when the reaction was conducted at no °C (Scheme 15.28) [40],... 

In this reaction, carbonylation of di-lithio reagents 1 was assumed to give an acyclic carbonyUithium species 29 as the first reaction intermediate, which immediately tmdergoes cycloaddition reaction followed by sequential rearrangement to afford cyclic dianions, such as 31-A to 31-F (Fig. 4). We isolated and characterized the... 

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