The vacuolar H+-ATPase (V-ATPase), located on the ruffled border of the osteoclast, is a proton pump which is responsible for secreting the massive amounts of protons that are required for the bone resorption process. With the aim to identify new agents which are able to prevent the excessive bone resorption associated with osteoporosis, we have designed a novel class of potent and selective inhibitors of the osteoclast proton pump, starting from the structure of the specific V-ATPase inhibitor bafilomycin A1. Compounds 3a-d potently inhibited the V-ATPase in chicken osteoclast membranes (IC50 = 60-180 nM) and were able to prevent bone resorption by human osteoclasts in vitro at low-nanomolar concentrations. Notably, the EC50 of compound 3c in this assay was 45-fold lower than the concentration required for half-maximal inhibition of the V-ATPase from human kidney cortex. These results support the validity of the osteoclast proton pump as a useful molecular target to produce novel inhibitors of bone resorption, potentially useful as antiosteporotic agents.

Optimisation of a novel series of osteoclast ATPase inhibitors led to (2Z,4E)-5-(5,6-dichloro-2-indolyl)-2-methoxy-N-(1,2,2,6,6-pentamethylpiperidin-4-yl)-2,4-pentadienamide (1) that was the most potent compound in an in vitro osteoclast ATPase assay and in human bone resorption assays. Two of the possible geometric isomers have also been prepared and shown to be significantly less potent than 1.

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A series of racemic alpha-thio-, beta-thio-, and alpha-methyl-beta-thio-substituted aldehydes has been prepared, and their Lewis acid promoted aldol condensation with nonstereogenic and stereogenic silylketene acetals and silyl enolethers has been studied. With alpha-thio-substituted aldehydes, a high level of non-chelation-controlled diastereofacial selectivity can be easily achieved, while chelation control requires a strongly chelating catalyst and a small, aliphatic S-protecting group. Some examples of addition of stereogenic nucleophiles occurring with efficient diastereofacial syn simple stereoselection are also reported. The reactions of beta-thio-substituted aldehydes are less stereoselective, in particular when the stereocenter is in the beta-position. Models of stereoselection are presented to rationalize the results that were compared with those obtained in similar reactions with chiral alkoxy aldehydes.

Simply generated (TiCl4, Et3N, −78°C) titanium enolates of some thioesters and α-thio substituted esters undergo aldol condensations with achiral and chiral aldehydes with low to high stereoselectivity. An 1H-NMR study of the TiCl4/ester complexation and of the enolization process is presented. The relation between the enolate structure and the aldol product stereochemistry is discussed.

Treatment of 2-pyridylthioesters with triethylamine in the presence of titanium tetrachloride affords titanium enolates that add to imines to give β-lactams in fair to excellent yields with moderate to good stereoselectivity.