Oxa-triquinane

Photochemical transformations of bicyclo[2.2.2]octenones derived from Diels-Alder reaction of spiroepoxycyclohexa-2,4-dienones 116 have been exploited by Singh s group in their synthetic studies towards triquinane 119 and oxa-triquinane 121, protoilludane 120, and oxa-sterpurane 122 frameworks (Figure 30) [120, 154]. Spiroepoxycyclohexa-2,4-... 

Singh, V. and Alam, S.Q., Structural diversity through intramolecular cycloaddition and modulation of chemical reactivity in excited state. Synthesis and photoreactions of 3-oxa-tricy-clo[5.2.2.0 [ undecenones novel stereoselective route to oxa-triquinanes and oxa-sterpuranes,Bio-org. Med. Chem. Lett., 2517, 2000. 

The Matlin group has shown that irradiation of cyclooctadienone 36 in the presence of vinyl ethers produces 7-norbornanones 41 and oxa-triquinanes 42 (Table 81.5). The reaction is envisioned as proceeding via the step-wise cycloaddition to oxyaUyl 37. The electron-rich vinyl ether stabilizes the incipient cationic center in 43, which then undergoes internal O- and C-alkylation of the enolate moiety to give 41 and 42. The product distribution and yields did not show any significant variation with the polarity of the solvent (hexanes, benzene, CHjClj or neat vinyl ether). It is interesting to note that no enone-alkene [2-1-2]-adduct was observed, even when ethyl vinyl ether was used as solvent. This is in contrast to the behavior of pyran-4-one, where irradiation in neat furan produced [2-1-2]-adducts. The photo-excited state lifetime of 36 is limited by rapid cis,trans-isomerization to 40, which does not permit intermolecular capture to be competitive. 

The vinyl ether acetal functionality in the oxa-triquinanes can be readily hydrolyzed to dicarbonyl containing diquinanes, for example, 42b —> 44. 

Acetophenone-sensitized irradiation of (83) affords an 80% yield of the triquinane derivative (84). This product is the key intermediate in a new synthesis of (— )-hirsutene. The tricyclotridecenones shown in Scheme 3 are photoreac-tive. Irradiation with acetone as the sensitizer brings about an oxa-di- jt-methane rearrangement to yield (85) while direct irradiation in benzene gives (86) by a... 

Oxa-di-7C-methane Reactions in Polycyclic-Bridged P,y-Enones Leading to the Linear Triquinane Framework ... 

Singh, V. and Jagadish, B., Synthesis of exo-annulated bicyclo[2.2.2]octenone and its oxa-di-Jt-methane rearrangement to cisisynxis triquinane, Indian J. Chem., 35B, 401,1996. 

---