The endogenous cannabinoid system (ECS) regulates a broad number of physiological processes, and perturbations in its normal functioning are linked to many disorders.[1] In this respect, the development of high-sensitivity and highthroughput analytical tools that afforded a broader view of the ECS would be highly valuable. As such, small-molecule fluorescent probes could provide dynamic information concerning the direct spatial and temporal expression levels of cannabinoid receptors as has been recently reported for some specific classes of enzymes.[2] Accordingly, the development of small-molecule probes that are able to recognize cannabinoid receptors is an area of current interest, because they could complement and even overcome some of the drawbacks of the available antibodies.[3] Recent attempts toward this goal have been mainly focused on the CB2 receptor (CB2R= cannabinoid receptor type2). However, these probes show moderate affinities [Ki=(260–387) nm], and their application in native systems is limited.[4] Similarly, endocannabinoidbased probes display modest affinities for CB1 and CB2 receptors [(84.7–450) nm],[5] a fact that could limit their use in complex systems. Therefore, we focused our efforts on the synthetic high-affinity cannabinoid ligands HU210 [Ki (CB1R)= 0.061 nm, Ki (CB2R)= 0.52 nm],[6] and HU308 [Ki (CB1R)> 10 000 nm, Ki (CB2R)= 22.7 nm][7](Scheme 1). Among the different tags, biotin was selected owing to its versatility for detection by a variety of readily available (strept) avidin conjugates. A closer look to the structure of these ligands revealed that the most straightforward possibility was to attach the tag at the free hydroxy groups (Scheme 1).

Ligand HU210 was prepared as previously described.[8] For the introduction of the tag, we made some attempts to selectively acylate the allylic hydroxy group using either Mitsunobu conditions [9] or the HfCl4· 2 THF catalyst.[10] However, none of them gave good yields in our hands and we carried out the reaction between ligand HU210 and N-(+)-biotinyl-6-aminohexanoic acid in the presence of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and 4-dimethylaminopyridine. Under these conditions, formation of bisacylated product was not observed, starting material was partly recovered, and compounds 1 and 2 were obtained after separation by column chromatography with 25% and 14% yields, respectively. Ligand HU308 was obtained as previously described [7] from commercially available α-pinene. Esterification of ligand HU308 with N-(+)-biotinyl-6-aminohexanoic acid gave derivative 3 in 55% yield. Compounds 1–3 were