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SUMMARYDeuterium and tritium 5,7,7-tris-labelled 24-epibrassinolide were prepared by base catalyzed exchange reaction using 24-epicastasterone tetraacetate I or bis-isopropylidenedioxy-24-epicastasterone 8 and labelled water. Baeyer-Villiger oxidation of the obtained labelled 6-ketones 2 and 3 with CF3C03H gave after alkaline deacetylation of the resulting 4 and 5 the desired tris-labelled 24-epibrassinolides 6 and 2, respectively, or starting from 9 under simultaneous oxidation and deprotection in one step the same final products.

The 'H and I3C NMR spectra of the two phytohormones, brassinolide and 24-epi-brassinolide, and the two synthetic analogues 22,23,24trisepi-brassinolide and (22S,23S)-2&homo-brassinolide, were assigned using 1 D aod 2D NMR techniques. Independent assignment strategies are presented for the 'H and "C chemical shifts of the steroidal side-chain.

Since the discovery of brassinolide, the first member of a new class of steroidal plant growth regulators, the synthesis of such compounds has represented an area of tremendous activity. The synthetic pathways to the most important member brassinolide and other native brassinosteroids as well as to structural analogs including aza and thio compounds are reviewed. Besides the various methods for A/B ring functionalization especially the different principles for a stereoelective side‐chain construction, starting from 22‐aldehydes, Δ22‐sterols or pregnane derivatives as steroidal precursors, are discussed.