Rearrangement benzils

Studies by Levisalles and Tkatchenko on the mechanism and stereochemistry of the steroidal benzilic rearrangement have demonstrated that the configuration of the product obtained in about 85 % yield from 5a-cholestane-3,4-dione (27) is as shown (28). When redistilled ethylene glycol mono-... 

In practice, the scope of the benzilic rearrangement as a tool for the ring contraction of steroids is defined by the availability of a-diketones, or the corresponding diosphenols. 

Benzilic rearrangement of diosphenol (44) gives the A-norsteroid (45) in 90 % yield. 

The generality of the rearrangement is further illustrated by the reaction of 2,2 -furil with hydroxide ion in dry ether (Table 1). Likewise, 2,2 -pyridil is rearranged in hot methanol solution (40 min) to give the sodium salt of 2,2 -pyridilic acid (86%). Acidification, however, affords bis(2-pyridyl)methanol by decarboxylation since 2,2 -pyridilic acid (16) is structurally similar to a -keto acid. Benzilic rearrangement of 2,2 -pyridil with methanolic nickel(ll) and cobalt(II) acetates results in the formation of metal complexes of 2,2 -pyridilic acid (17 92%). A plausible mechanism is summarized in Scheme 4. Rearrangement is also observed with 2,2 -quinaldil, but benzil, 2,2 -furil or 1-phenyl-2-(2 -pyridyl)ethane-1,2-dione are not susceptible to these metal template reactions. 

The benzilic rearrangement of an o-quinone results in ring contraction. Among the earliest recorded examples is the synthetically useful conversion of phenanthroquinones into 9-hydroxyfluorenecarboxylic acids (equation As with the benzilic acids, compounds such as (28) are susceptible to oxidative... 

TTie rearrangements of hydroxy-1,4-benzoquinones in alkali are more complex. Under conditions in which polyporic acid (34) is converted, by way of retro-aldol ring cleavage and benzilic rearrangement, into a-benzyl-P-phenylsuccinic acid, cis- and tranr-a-benzylcinnamic acid and oxalic acid, atromentin... 

Among the many interconversions which occur readily in related cyclobutane, cyclopropane and open-chain frameworks, the stereospecific rearrangement of a-halo- or a-tosyloxycy-clobutanones into cyclopropanecarboxylic acid derivatives, has been shown to follow the semi-benzilic rearrangement mechanism, i. e. addition of nucleophiles to the carbonyl carbon atom and concerted displacement of halide (or tosyloxy) ions with 1,2-migration of the Ca — C carbonyl bond (see Section 4.1.2.2.5.). °... 

Aromatic and Cyclopentanoid Carotenoids.—Violerythrin (40) is a blue carotenoid formed by hydrolysis and mild oxidation of the sea anemone diester actinioerythrin (41). The proposed biogenesis involving a benzilic rearrange-... 

An elaborate but effective method of expanding the isoquinoline ring has been reported by Chazerain.28 The dihydroisoquinoline 100 (Rl = R2 = OEt) was converted by benzoyl chloride into the benzil 175 (R = OEt) which by a benzilic rearrangement gave 176 (R = OEt) debenzoyla-... 

First example Ogg and Bergstrom (1931) published a series of papers designed to demonstrate possible analogies between heterocyclic systems and their acyclic and alicyclic counterparts. Quinoxaline, for example, was described as an ammono glyoxal and 2,3-diphenylquinoxaline 1 was considered to be the heterocyclic equivalent of benzyl. In an attempt to justify this hypothesis, the authors carried out the reaction of 2,3-diphenylquinoxaline 1 with potassium amide in liquid ammonia, anticipating a reaction similar to the benzyl —> benzilic rearrangement, which would lead to the formation of 2,2-diphenyl-3-aminoquinoxaline 2 (Scheme 6.1). The reaction did, in fact, lead to a new product-2-phenylbenzimidazole 3 in an approximately 30 % yield and the recovery of about 60 % of unchanged 2,3-diphenylquinoxaline 1 (Taylor and McKillop 1965). 

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