Type Rearrangements

Note The McLafferty rearrangement and the RDA reaction have several features in common i) both belong to the rearrangement type of fragmentations, although the name conceals this fact in case of the latter, ii) both represent pathways for alkene loss from molecular ions, and iii) both are highly versatile in structure elucidation. 

Kouno, I. Hirai, A. Fukushige, A. Jiang, Z.-H. Tanaka, T. (1999) A novel rearranged type of secoeudesmane from the roots of Lindera strychnifolia, Chem. Pharm. Bull., 47, 1056-7. 

The question of intermediates in thiophene substitutions may not always be so straightforward as portrayed so far. The sulfur atom itself can be the site of attack by an electrophile, i.e. (14) and also Section 3.13.2.4. One can postulate that selectivity in thiophenes could result from attack of an electrophile, for example a halonium ion, at sulfur followed by 1,5-sigmatropic rearrangement and deprotonation as shown in equation (6). A determination of the relative stabilities of (23) and (17) for different substituents X is needed for assessment of the validity of this possibility. Note that the rearrangement type represented by (23) to (17) is known in a photochemical variant (equation 7) (73TL3929). From ab initio calculations on thiophene it also appears that initial attack of an electrophile on sulfur is the pathway of higher electron density (72MI31302). 

Rearrangement of the glycosylamine by an Amadori rearrangement type of reaction to yield an aldoseamine or ketoseamine. 

In the dibenzoylperoxide decompositions the effects are caused in pairs which are themselves part of a minor reaction pathway. The same may be true for other reactions, especially of the rearrangement type where ionic and radical intermediates may be present simultaneously. CIDNP effects are evidence for radical intermediates, though others may be present as well, and for quantitative studies CIDNP has to be combined with other techniques to elucidate the relative importance of various possible pathways. 

An alternative mechanism involves nucleophilic attack of the C-3 of indole on the distal oxygen atom of the bound O2 coupled with deprotonation. Two pathways to the Al-formylkynureiune product have been proposed, one involves a Criegee rearrangement-type mechaiusm and the other involves an endoperoxide intermediate Further studies are required to estabhsh the exact mechaiusm of product formation. 

Moreover, most of the material covered by this review was published in 1990 or later, with a strong focus on newer developments in sulfonium ylide chemistry (Type M) and applications of the sulfenate-sulfoxide rearrangement (Type K) in the context of stereoselective, target molecule oriented synthetic work. Stressing these applications is a natural consequence of the fact that, despite the overwhelming multitude of possible processes, only very few are important from a synthetic point of view (especially Types K, L, M) and others have, to the best of the author s knowledge, never been described in the literature ( grayed out structures Types A, B2, D). The synthetic chemist s interest in some of these transformations can be traced back mainly to their beneficial application in stereoselective transformations, or to the fas-... 

The unsaturated aldehyde end groups can further decompose in reactions of Claisen rearrangement type ... 

Table 28.2 Diterpenoid alkaloids (compound 1-2 ranaconitine-type Cig-diterpenoid alkaloids, 3-13 aconitine-type Cig-diterpenoid alkaloids, 14—24 lycoctonine-type Cig-diterpenoid alkaloids, 25 Cig-rearranged-type Cig-diterpenoid alkaloid, 26-27 atisine-type C2o-diterpenoid alkaloids, 28 hetidine-type C2o-diterpenoid alkaloid, 29-36 hetidine-type C2o-diterpenoid alkaloids, 37-41 denudatine-type C2o-diterpenoid alkaloids, 42-51 vakognavine-type C2o-diterpenoid alkaloids, 52 atisine-denudatine-type C2o-diterpenoid alkaloid)...

The Ci9-diterpenoid alkaloids are the largest class of naturally occurring diterpenoid alkaloids with more than 700 compounds so far isolated and characterized from around 315 species of plants mainly from the generz. Aconitum and Delphinium. This class of alkaloids is divided into six types aconitine type (B-1), lycoctonine type (B-2), pyro type (B-3), lactone type (B-4), l, l-seco type (B-5), and rearranged type (B-6) (Fig. 28.2). The order of types from B-1 to B-6 is based upon the higher to lower number of alkaloids obtained so far. In between 2009 and Feb. 2012, only aconitine, lycoctonine, and rearranged-type Cig-diterpenoid alkaloids have been isolated and are discussed herein. 

The methods described above are applicable to the preparation of block copolymers from vinyl and diene monomers and, in the case of anionic polymerization, to certain cyclic monomers such as ethylene oxide. These methods are not in general appropriate to the preparation of condensation-type and rearrangement-type block copolymers. For such materials a number of methods may be envisaged. As an example a polymer with carboxyl end-groups may be allowed to react with a polymer containing hydroxyl end-groups ... 

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