Synthesis of teasterone and typhasterol, brassinolide-related steroids with plant-growth-promoting activity
Brassinolide-related natural steroids, teasterone (4), (22R,23R,24S)-3β,22,23-trihydroxy-5α-ergostan-6-one, and typhasterol (3), 3α-isomer of (4), have been synthesized from crinosterol (5). The cyclopropyl ketone (6), obtained by solvolysis of crinosterol methanesulphonate followed by Jones oxidation, was transformed by treatment with acid and acetylation into (22E,24S)-6-oxo-5α-ergost-22-en-3β-yl acetate (7), which was epoxidized. Epoxide-ring opening of the separated (22R,23R)-epoxide (9) with 30% HBr–AcOH, followed by inversion reaction at the carbon bearing bromine, and acetylation, provided the (22R,23R,24S)-triacetate (10), which was saponified to yield teasterone (4). The sulphonate (11), derived from teasterone (4), was submitted to an improved inversion reaction with caesium acetate. Deprotection of the resulting 3α-acetate (12) provided typhasterol (3).