Other Methods for Carbocyclic Construction: Pentalenonelactone A Methyl Ester (Chuang), Atlanticone C (Bach), Aquatolide (Takao), Incarviatone A (Sarpong), Oridonin (Luo), Illisimonin A (Rychnovsky)
Photochemical transformations can be carried out efficiently in flow. Gary Jing Chuang of the Chung Yuan Christian University set the skeleton of pentalenolactone A methyl ester (3) by the rearrangement of 1 to 2 (J. Org. Chem. 2019, 84, 10172. DOI: 10.1021/acs.joc.9b01349).
Thorsten Bach of the Technische Universität München devised a synthesis of atlanticone C (6) based on the photochemical transformation of 4 to 5 (Angew. Chem. Int. Ed. 2019, 58, 14629. DOI: 10.1002/anie.201908619). There were three steps in the conversion of 4 to 5: initial [2+2] photocycloaddition, thermal opening of that product to the cyclooctatriene, then photochemical cyclization of the cyclooctatriene to 5.
Aquatolide (9) contains five contiguous stereogenic centers. Ken-ichi Takao of Keio University assembled this complex skeleton by the crossed photochemical [2+2] cycloaddition of 7, leading to 8 (Angew. Chem. Int. Ed. 2019, 58, 9851. DOI: 10.1002/anie.201904404).
Incarviotone A (12) was isolated from Incarvillea delavayi, used in Chinese folk medicine to treat rheumatism. Richmond Sarpong of the University of California, Berkeley assembled the central aromatic ring of 12 by the cyclization of 10 to 11 (J. Am. Chem. Soc. 2019, 141, 14421. DOI: 10.1021/jacs.9b07693).
Oridonin (15) is the major active ingredient of Rabdosia rubescens, a Chinese over the counter anti-inflammatory. En route to 15, Tuoping Luo of Peking University devised the interrupted Nazarov cyclization of 13 to 14 (J. Am. Chem. Soc. 2019, 141, 20048. DOI: 10.1021/jacs.9b12034).
Illisimonin A (18), isolated in small quantity from the fruit of Illicium simonsii, showed promising neuroprotective activity. A key step in the synthesis of 18 reported by Scott D. Rychnovsky of the University of California, Irvine was the acid-mediated rearrangement of 16 to 17 (J. Am. Chem. Soc. 2019, 141, 13295. DOI: 10.1021/jacs.9b05065).