Ring contractions 3,2-sigmatropic rearrangements

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

Here the substrates are 3-silyloxy-l,5-enynes the concept was to combine a metal-induced 6-endo-trig cyclization with a ring contraction by a sigmatropic rearrangement (Scheme 12.12).27 From the cationic intermediate, elimination to the biaryl product can be observed in addition to the aldehyde. 

Sigmatropic rearrangements of cyclic sulfonium and ammonium ylides have been extensively used in ring contraction and ring enlargement methods. An example is shown in Scheme 55. 

Ring Expansion/Contraction via Sigmatropic Rearrangements - Cope Rearrangement... 

This weakening of the bond between the allylic carbon and the neighboring atom can lead to bond rupture as illustrated in the photoisomerization of 2H,6//-thiin-3-one-l-oxides to 3//,7H-l,2-Oxathiepin-4-ones [14], These seven-membered ring systems undergo further thermal ring contraction to thietanones via a 2,3-sigmatropic rearrangement followed by a Pummerer reaction (Scheme 10). 

The thermal and acid-catalysed ring-contractions of l-acyl-l,2-diazepines to pyridine iV-imides and/or 2-(acylamino)pyridines is well known. Moore has now reported a new reaction path in the thermolysis of (84), leading to the 1,3-diazepine (85) in addition to a 6-benzamidopyridine. A stepwise conversion of (84) into (85) is favoured, assisted by electron release from oxygen, rather than a sigmatropic rearrangement " [cf. the formation of (65) described above]. 

SCHEME 17.7. Substrate-controlled asymmetric s3mthesis of (+)-acorenone B 28 through a [2,3]-sigmatropic rearrangement-ring contraction of ylide 26. 

In contrast, the corresponding sulfoxides do undergo such a bond rupture between the allylic C-atom and the neighboring S-atom as a first chemical step after photoexcitation.As shown in Scheme 15, 2H,6H-thiin-3-one-l-oxides 47 photoisomerize to 3H,7H-l,2-oxathiepin-4-ones 48, which undergo a further thermal ring contraction to thietanone-1-oxides 49 via a 2,3-sigmatropic rearrangement. 

Other Rearrangement Reactions.—In addition to the above-mentioned ring expansion and contraction reactions, miscellaneous rearrangements such as the photochemical [1,3] sigmatropic shift have been used in synthesis. An early example was in the synthesis of model corrins the formation of the vinylogous amide (141) occurred in 50% yield by irradiation of the N-acyl-enamide (140) in cyclohexane.3-Acetylcyclohexanone (143) was similarly obtained by irradiating the vinyl alcohol (142) in benzene. ... 

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