Synthesis and bioactivity of natural and C-3 fluorinated biosynthetic precursors of 28-homobrassinolide

In this paper we describe the synthesis of two new fluorinated brassinoster oids (22R,23R)-22,23-dihydroxy-3 beta -fluorostigmastan-6-one and (22R,23R) -22,23-ditaydroxy-3 beta -fluorostigmastan-6-one. Their bioactivities were evaluated in the rice lamina inclination test and compared with that of 28- homocastasterone, 28-homotyphasterol and 28-homoteasterone, possible biosyn thetic precursors of 28-homobrassinolide. Results confirmed expected simila rities between the biosynthesis of 24-ethylbrassinosteroids (named as the 2 8-homo series) and that described for 24-methylbrassinosteroids, and also i ndicated that these precursors might exhibit per se activities. (C) 2001 Pu blished by Elsevier Science Ltd.