Ring contraction decarbonylation

The photolytic cleavage of cyclic ketones 14 leads to formation of a diradical species, that can undergo analogously the various reactions outlined above. The decarbonylation followed by intramolecular recombination yields a ring-contracted cycloalkane 15 ... 

Analogously, the tetrabromotetrapropylporphycene 9 furnishes the corresponding isocorroie-carbaldehyde 10 in good yields, but in this case no decarbonylated product was observed. The mechanism of these interesting ring contractions of porphycenes into isocorroles still needs to be determined. 

Increasing use is being made of pyran syntheses based upon [4 + 2] cycloadditions of carbonyl compounds. The appropriate unsaturated aldehyde with ethyl vinyl ether yields 53 with peracids this affords an epoxide that undergoes ring contraction to the aldehyde 54 (Scheme 23) and rhodium catalyzed decarbonylation affords the required 3-alkylfuran with the optical center intact.116 Acetoxybutadiene derivatives add active carbonyl compounds giving pyrans that contract under the influence of acids to give... 

When 3-diazobenzofuranone (226) is irradiated in an argon matrix at 8 K, a ketene 227 results as the primary product. This loses CO, and the resulting carbene 228 ring contracts to a species which exhibits an infrared (IR) frequency at 1838 cm", which was tentatively assigned to intermediate 219. Upon further irradiation, benzyne (3) is formed. The benzocyclopropenone 219 is also generated upon irradiation of benzocyclobutanedione (229) in an Ar matrix at 8 K. " An analogous sequence occurs upon irradiation of 230 at 16 K, and leads initially to cyclo-propaacenaphthenone 231, which decarbonylates to acenaphthyne (232). ... 

On the other hand, in bi- or polycyclic molecules a decarbonylation reaction such as a [5 -> 4 + 1] fragmentation of a five-membered ring is quite common. However, in these cases the reaction is not accompanied by ring contraction. As an example, norbornen-7-one (16) affords cyclohexadiene (86CP307 90JA5089). 

In a similar way 1,6 anhydro 2-ketopyranoses63 of the xylo, ribo and psico series underwent stereoselective ring contraction by decarbonylation (Scheme 33). 

They found that at room temperature using benzene as the solvent only 3% consisted of the diketone 25 and mainly the ring-contracted products were obtained 33% of keto aldehyde 24 and 28% of ketone 26. From an industrial point of view the desired compound is the keto aldehyde 24, which is an interesting intermediate for the synthesis of other cyclopentanone derivatives with floral and fruity smells. The acid catalyzed reaction mechanism leading to the synthesis of keto aldehyde 24 had been discussed earlier (25). Therefore, it is of interest whether the product distribution changes in the presence of a heterogeneous catalyst system and also whether the decarbonylation of the compound 24 to compound 26 can be suppressed. 

An interesting ring-contraction biogenetic pathway from puupuhenone (45) to haterumadienone (46) might proceed via benzylic acid rearrangement of the diketone form of 45, followed by oxidative decarbonylation. Haterumadienone (46) is the first ring-contracted... 

Ring contraction by decarbonylation Benzoxazoles from 2H-l,4-benzoxazin-2-ones... 

Ring contraction by decarbonylation 6-Azapurines (Imidazo[4,5-e][l,2,4]triazines) from 7-azapteridines... 

Decarbonylation of o-benzoquinones 374 can be considered as a method of synthesis of CPDNs 375. Photodecarbonylation of o-quinones under UV radiation has a long history, and many mechanistic studies have been performed and described in the literature (Scheme 6.91a) [222]. Another ring contraction to an indenone 376 from o-naphthoquinone 377 in aqueous H2O2 (30%) in the presence of catalytic amount of iodine in acetonitrile has also been reported (Scheme 6.91b) [223]. 

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