Regiochemistry and stereochemistry of intramolecular [2+2] photocycloaddition of carbon-carbon double bonds to cyclohexenones

Total Synthesis of Cochlearol B via Intramolecular [2+2] Photocycloaddition

“…[12] Next, we focused on intramolecular [2+2] photocycloaddition. [13] Contrary to our expectation, irradiation of 4 by a mercury lamp did not furnish the desired pentacyclic compound 3 at À78 8C, and a complex mixture was obtained at higher temperatures. On the other hand, intramolecular [2+2] photocycloaddition of 16, which has a more electron-deficient olefin by conjugation with the dienone ring, proceeded smoothly upon irradiation at À78 8C to afford pentacyclic compound 18 in 74 % yield.…”

“…[16] Finally, cleavage of the pivaloyl ester of 24 under basic conditions furnished cochlearol B (1) in 94 % yield. The spectroscopic ( 1 H NMR, 13 C NMR, and HRMS) data for the synthetic 2 matched those reported for the natural product. Furthermore, the stereochemistry of the synthetic 2 was clearly determined by X-ray crystallographic analysis.…”

Intramolecular [2+2] Photocycloaddition of Cyclic Enones: Selectivity Control by Lewis Acids and Mechanistic Implications

“…While the stereoconvergency of the reactions 5 k → 6 k supports the intermediacy of 1,4‐diradical 14 , the high simple diastereoselectivity is remarkable if compared to the non‐existent diastereoselectivity (d.r.=50:50) observed for the racemic reaction . The finding seems to indicate that intermediate 14 remains in the coordination sphere of the catalyst which forces the methyl group in the trans position of the resulting cyclobutane ring.…”

“…An alternative explanation for the efficiency of the Lewis acid‐catalyzed process would be a rapid cyclization on the singlet hypersurface prior to ISC. In order to distinguish between the two pathways and to substantiate the fact that the catalyzed reaction proceeds indeed on the triplet hypersurface, we turned to a classical experiment that had been earlier performed with substrates 5 k by Becker and co‐workers . Upon irradiation at λ >330 nm (uranium glass filter), it had been found that both diastereoisomers cis ‐ 5 k and trans ‐ 5 k gave in separate reactions products rac ‐ 6 k as a cis / trans mixture in a ratio of 50:50.…”

Chapter 9. Synthetic methods

“…227 The photoaddition of cyclopentene to cyclohexenone gives four adducts (138)-( 141) (68 :4 1 : 7 : 25), which were isomer-b H H ized by base to the cis-isomers (138), (139) (75:25) in contradiction to an earlier report.228 [3 + 2). The [2 + 21 cycloaddition of a$-unsaturated ketones and alkenes has been extended in a novel way by placing an alkyne group at the p-position of the enone.…”