22R,23R,24R)-24-Azido-26,27,28-trinorcastasterone (3) was synthesized from stigmasterol in 12 steps via the reaction of Ti(O-iPr) 2 (N 3 ) 2 with the corresponding suitably protected threo-22-hydroxy-23,24-epoxide 4.Brassinolide (1) is a powerful plant growth-promoter that was discovered in 1979. 1 Since then, numerous other brassinosteroids have been reported and their synthesis, biosynthesis and metabolism, structure-activity relationships, biological activity and field applications have been intensely investigated. 2 The molecular biology of brassinosteroids has recently come under scrutiny, 2a,2d,2e,2g,3 but much remains to be learned about how brassinosteroids regulate gene expression. The existence of one or more brassinosteroid receptors in plants seems probable, but to date such receptors have proved to be elusive, and their isolation and characterization remains an important task for the future.The technique of photoaffinity labelling 4 has been successfully employed in the characterization of other types of steroid (and other) receptors and requires the incorporation into a suitable ligand of a photolabile functional group that can form covalent bonds with the receptor. To our knowledge, no affinity labels for the putative brassinosteroid receptor have yet been reported.Since the introduction of an affinity label into a ligand can disrupt its recognition by a receptor, the site at which the label is incorporated can be critical. The design of an affinity-labelled brassinosteroid was guided by the following considerations of known structure-activity relationships. 2a,2e,2f,2h,2i,5 Castasterone (2) is a biosynthetic precursor of 1 in some plants, 6 but displays significant biological activity of its own in certain other species. 7 Other brassinosteroids containing B-rings with 6-keto groups are also known to be bioactive. Moreover, brassinosteroids tolerate significant structural variation of the side chain, with a variety of homo, nor, epi and unsaturated analogues of the C-24 to C-28 segment showing significant activity. However, the vicinal diol moiety, or the corresponding methyl ethers 5f at C-22 and C-23, as well as the correct configuration of these stereocenters, is required for strong bioactivity. We therefore chose to prepare (22R,23R,24R)-24-azido-26,27,28-trinorcastasterone (3) as a potential affinity label, in which a B-ring ketone is present and the C-22,23 diol unit remains intact.The synthesis of the azidocastasterone analogue 3 was achieved by a variation of the procedure that we recently devised for the synthesis of brassinolide (1) and castasterone (2). 8 Thus, the mixture of threo-and erythro-epoxy alcohols 4 and 5 is readily available from stigmasterol (Scheme). These diastereomers are difficult to separate, but 4 reacts considerably faster than 5 with both carbon and various heteroatom nucleophiles, resulting in attack at C-24 with inversion of configuration. When 4 and 5 were treated with i-PrMgCl and CuCN, followed by deprotection, 2 was obtained from 4, while 5 was recovered intac...