Review of synthetic approaches toward maoecrystal V

Synthetic approaches toward the complex natural product diterpenoid maoecrystal V are reviewed, including successful total syntheses, published synthetic efforts, and efforts compiled from dissertations.

Diels–Alder Cycloaddition: Nicolaioidesin B (Coster), Lycorine (Cho), Bucidirasin A (Nakada), Maoecrystal V (Thomson), Kuwanon J (Wulff/Lei), Vinigrol (Kaliappan)

β-Ocimene 2 is an inexpensive Diels–Alder diene. En route to nicolaioidesin B 4, Mark J. Coster of Griffith University showed (Tetrahedron Lett. 2014, 55, 6864) that the Weinreb amide 1 added to the E isomer of 2 with high selectivity, to give 3. The alkaloid lycorine 8 is found throughout the Amaryllidaceae. Cheon-Gyu Cho of Hanyang University developed (Org. Lett. 2014, 16, 5718) a succinct route to 8 based on the use of the boryl styrene 5 as a Diels–Alder dienophile. Masahisa Nakada of Waseda University (Org. Lett. 2014, 16, 4734) prepared the enantiomerically-pure enone 9 by way of a baker’s yeast reduction of a prochiral dik­etone. Diels–Alder addition to 10 led to 11, that was carried on to bucidirasin A 12. Regan J. Thomson of Northwestern University prepared (J. Am. Chem. Soc. 2014, 136, 17750) the triene 13 by asymmetric epoxidation of a prochiral enone. Diels–Alder addition of the very reactive nitroethylene to give 14 completed the carbon skel­eton of maoecrystal V 15. William D. Wulff of Michigan State University and Xiaoguang Lei of Peking University optimized (Angew. Chem. Int. Ed. 2014, 53, 9257) the organocatalyzed Diels–Alder cycloaddition of 17 to the diene 16. Deprotection then completed the synthesis of the prenylflavonoid kuwanon J 18. In 2012, Barriault described (OHL 20121224) the conversion of 20 to the com­plex diterpene vinigrol 21. Krishna P. Paliappan of the Indian Institute of Technology Bombay showed (Org. Lett. 2014, 16, 5540) that the triene precursor to 20 could be prepared by ring-closing metathesis of 19. In the absence of ethylene, a different product was formed.