Second order Beckmann rearrangements

The reaction that normally occurs on treatment of a ketoxime with a Lewis or proton acid is the Beckmann rearrangement (18-17) fragmentations are considered side reactions, often called abnormal or second-order Beckmann rearrangements. Obviously, the substrates mentioned are much more susceptible to fragmentation than are ordinary ketoximes, since in each case an unshared pair is available to assist in removal of the group cleaving from the carbon. However, fragmentation is a side reaction even with ordinary ketoximes and, in cases where a particularly stable carbocation can be cleaved, may be the main reaction. ... 

Fragmentation of the intermediate or concerted formation of nitriles from the activated oxime (Scheme 9, pathway 3) this is the Beckmann fragmentation. In some circumstances this pathway becomes dominant, particularly when there are quaternary carbons adjacent to the oxime. This transformation has found particular utility in ring-cleavage processes (sometimes called abnormal or second-order Beckmann rearrangements). 

Fusco and Rossi797 observed the rupture of the pyrazole ring when 4-nitrosopyrazoles (81) were treated with phosphorus pentachloride, forming isomeric chlorocyanoazines (82 and 83). The reaction is evidently similar to the second-order Beckmann rearrangement.798... 

The chemistry of such bridged naphthalenes-benzomorphans and some related compounds has been reviewed previously.<1 3) The first synthesis of a benzomorphan was achieved by Barltrop(4) however, prior to that synthesis, Schopf,(5) in his investigations of the structure of morphine, had isolated the benzomorphan, 2, from a thionyl chloride-induced, second-order Beckmann rearrangement of dihydrocodeinone oxime, 1. Reinvestigation(5a,5b) of the Beckmann rearrangement and a study of the Schmidt reaction of 6-oxomor-phinans confirmed Schopf s work and led to the isolation of the anticipated lactam (2a). 

Heating 3-nitroso-2-phenyl-l//-indole with excess phosphoryl chloride or tosyl chloride in sulfolane at 200 C for 1 h, followed by dilution with water, produces 2-phenylquinazolin-4(3//)-one in 90 or 68% yield, respectively. The ring expansion proceeds via the second-order Beckmann rearrangement of the imino-oxime tautomeric form of 3-nitroso-2-phenyl-l//-indole. °... 

Beckmann rearrangements that go with fragmentation are sometimes caiied anomaious or second-order Beckmann rearrangements You shouid not use the second of these names and, in any case, Beckmann fragmentation is much better than either. 

Beckmann rearrangement,4 Beckmann rearrangement of the oxime of ada-mantanone (1) to the lactam (2) by conventional reagents led to yields of less than 25% usually a considerable amount of (3) was formed as a result of a second-order Beckmann rearrangement. However, use of PPE in refluxing chloroform... 

Higher yields of demethylated steroids have been realised (Scheme 13) using the unsaturated nitrile (595), which was obtained by a second-order Beckmann rearrangement of the 4,4-dimethyl-3-ketoxime. 

The second order Beckmann rearrangement carried out on the C-22 oxime of strychnine is the first step of a preferred pathway for the conversion of the alkaloid into the Wieland-Gumlich aldehyde (Chart 6.2). 

Cyclobutane.—Further reports of grandisol (90) synthesis include Magnus s full paper (Vol. 6, p. 22) and an almost identical Japanese report of an earlier synthesis (Vol. 3, p. 25) based upon a dihydropyranone-ethylene cycloaddition.A third synthesis utilizes cyclopropanation of 4-methoxy-3,6,6-trimethylcyclohexa-2,4-dienone to yield (91) followed by rearrangement of the a-oxycyclopropylcarbinyl cation of (91) to (92). After reduction of the cyclobutanone, second-order Beckmann cleavage of the cyclopentanone oxime gave (93) from which grandisol (90) was readily obtained. 

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