Ring expansion and rearrangement

Barluenga et al. reacted l-alkyl-3-phenyl-3-(JV-alkylamino)-2-propen-l-oneimines with dichlorodiphenylsilane and triethylamine and obtained di-azasilacyclohexadienes 94. The sila compounds underwent ring expansion and rearrangement with dialkylacetylenedicarboxylates to give silyl-substituted 1,5-diazocines 95 (86AG190). 

The initial dichlorocarbene adducts of 1-methoxybiphenylene undergo ring-expansion and rearrangement reactions via norcaradien-7-ol intermediates (Scheme... 

Chapter 13, by Narayanaswamy and coworkers, provides a general overview of the potential applications of ring expansion and rearrangement reactions in the synthesis of highly valuable, septanoses as bioactive, carbohydrate-based ligands with multiple biomedical applications. 

In most cases, the cleavage of a carbon-carbon bond causes rearrangements of the carbon skeleton Ring contraction, ring expansion, and alkyl group migration are observed under different conditions These transformations proceed in most cases in the presence of catalysts at elevated temperatures Examples where only temperature causes rearrangements will be discussed m the next section... 

Nitrones derived from 2-azabicyclo[5.3.0]decane give quinolizidine compounds by photochemical Beckmann rearrangement which implies simultaneous ring expansion and ring contraction reactions. Intramolecular Schmidt reactions in 2(4-azidobutyl)-cyclopentanones also give quinolizidinone derivatives by ring expansion. Examples of both types of reactions are given in Sections 12.01.11.1 and 12.01.11.3, respectively. 

Reaction of atomic carbon with alkenes generally involves both DBA and vinyl C—H insertion. An interesting example is the reaction of C atoms with styrene in which the major products are phenylallene (21) and indene (22). The synthesis of a number of specifically deuterated styrenes and the measurement of the deuterium isotope effects on the 21/22 ratio led to the conclusion that 21 was formed by DBA followed by ring expansion and by C—H(D) insertion into and followed by rearrangement of the resultant frawi-vinylcarbene (23). The indene was formed by C—H(D) insertion into Xb followed by cyclization of the resultant cw-vinylcarbene (24) (Eq. 18). An examination of the product ratios and their label distributions when atoms are used leads to the conclusion that the ratio of C=C addition to C—H insertion is 0.72 1 in this case. 

Azepin-2-ones are prepared by the ring expansion of 2,6-dialkylphenolates with ethereal chloramine at -70 °C (Scheme 28) (B-69MI51600), and by the mechanistically similar base-promoted ring opening and rearrangement of spiroquinol ethers (234) (81JOC4077). 

Four oxazocine rings and 15 benzoxazocine systems are possible a compilation of the literature on these compounds has been prepared (82H(19)709). Many of these ring systems have been described in patents and rather obscure journals, and in a number of cases the evidence for structural assignments is inadequate. Methods of preparation include cyclization of polyfunctional chains, and ring expansions or rearrangement of several types. Some representative examples of oxazocines and benzoxazocines that have been well characterized are compounds (305)-(310). 

Beckmann rearrangement of quinuclidin-3-one oxime (103) in the presence of polyphosphoric acid or oleum proceeds with quinuelidine ring expansion and the formation of 3-oxo-l,4-diazabicyclo[3.2.2]-nonane (146).m,m... 

From Other heterocycles. See Sections 3.5.2.Z (ring expansion and ring contraction) and 3.4.3.1.9 (mono-cyclic rearrangement) for further preparations by the types of reaction indicated. 

By suitable substitution the enaminones can often serve as precursors for heterocycles and preparation of indoles, carbazoles, quinolines, acridines and phenaNthridines can be achieved easily. However, this part of enaminone chemistry can lead to surprising and unexpected reactions if the multifunctional properties of the enaminones are ignored, e.g. ring contraction, ring expansion and other rearrangements are observed. In some cases jft-ketoenamines react as the ene-component in cycloaddition. Enaminones are even suitable synthones for building aromatic rings. 

The initial intermediate 33 is the normal bis-(N — Si)-coordinated complex, which is obtained in analogy to numerous other neutral hexacoordinate complexes 30-38 (Eq. 15), prepared from SiC and various trichlorosilanes and substituted O-trimethylsilylated hydrazides. The final product 70 is the result of a novel molecular rearrangement, by which a chloride has been displaced by the dimethylamino-nitrogen and has migrated from carbon to silicon, accompanied by ring expansion and conversion of the N -> Si to O -> Si coordination. 

With some polycyclic substrates, a tandem ring expansion and ring contraction can take place under conditions of oxidative rearrangement. The 11-oxolanostanyl acetate (48 equation 46) undergoes such a reaction, in which ring c is contracted and ring d is expand and aromatized. The yield is poor though, and such a transformation would seem to have limited synthetic potential. 

Insertion of nitrenes occurs readily and may lead to ring expansion and a variety of rearrangements [195, 196] (Figure 9.82). 

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