Other Methods for Carbocyclic Construction: Grandisol (Bach), Gracilioether F (Enders/Tang), Conosilane A (She), ent-Progesterone (Jahn), Spirasine IV (Zhang), Cyclotrinol (Li)
Grandisol (3) is a component of the aggregation pheromone of the economically important cotton boll weevil. En route to 3, Thorsten Bach of the Technische Universität München achieved significant enantioselectivity in the [2+2] cycloaddition of 3-methyl cyclohexenone 1 with ethylene to give 2 (J. Am. Chem. Soc. 2018, 140, 3228. DOI: 10.1021/jacs.8b01011).
Dieter Enders of RWTH Aachen University and Yefeng Tang of Tsinghua University also used [2+2] photocycloaddition, to convert 4 to 5. Direct oxidation of the methyl ether could be stopped at the ketone, or extended, leading directly to gracilioether F (6) (J. Am. Chem. Soc. 2018, 140, 1937. DOI: 10.1021/jacs.7b12903).
Xuegong She of Lanzhou University devised the diastereoselective free radical cyclization of 7 to 8. Further oxidation carried 8 on to conosilane A (9) (Chem. Commun. 2018, 54, 912. DOI: 10.1039/C7CC09367E).
Ullrich Jahn of the Czech Academy of Sciences took advantage of the facile radical dissociation and subsequent cyclization of the alkoxy amine 10 to the alkoxy amine 11. This was the key step in the development of a practical route to ent steroids, including ent-progesterone 12 (Org. Lett. 2018, 20, 946. DOI: 10.1021/acs.orglett.7b03838).
Min Zhang of Chongqing University combined 13 and 14 to give an intermediate, that proceeded via diastereoselective intramolecular 1,3-dipolar cycloaddition to the tetracyclic amine 15. This set the stage for the elaboration of the hexacyclic diterpenoid alkaloid spirasine IV (16) (Angew. Chem. Int. Ed. 2018, 57, 937. DOI: 10.1002/anie.201711414).
The rearranged steroid cyclotrinol 19 has antibacterial acitivty. Chuang-Chuang Li of the Southern University of Science and Technology assembled 19 by the diastereoselective type II intramolecular [5+2] cycloaddition of 17 to 18 (J. Am. Chem. Soc. 2018, 140, 5365. DOI: 10.1021/jacs.8b02629).