The piperidine ring is the commonest heterocyclic subunit of natural products, and is a key part of numerous drugs. This article identifies the best asymmetric routes to the piperidine ring system, emphasising methods that are efficient, reliable and flexible. The cover picture shows the plant poison hemlock, which is a source of many piperidine alkaloids such as coniine.

HIV attachment via the CD4 receptor is an important target for developing novel approaches to HIV chemotherapy. Cyclotriazadisulfonamide (CADA) inhibits HIV at submicromolar levels by specifically downmodulating cell-surface and intracellular CD4. An effective five-step synthesis of CADA in 30% overall yield is reported. This synthesis has also been modified to produce more than 50 analogues. Many tailgroup analogues have been made by removing the benzyl tail of CADA and replacing it with various alkyl, acyl, alkoxycarbonyl and aminocarbonyl substituents. A series of sidearm analogues, including two unsymmetrical compounds, have also been prepared by modifying the CADA synthesis, replacing the toluenesulfonyl sidearms with other sulfonyl groups. Testing 30 of these compounds in MT-4 cells shows a wide range of CD4 down-modulation potency, which correlates with ability to inhibit HIV-1. Threedimensional quantitative structure-activity relationship (3D-QSAR) models were constructed using comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) approaches. The X-ray crystal structures of four compounds, including CADA, show the same major conformation of the central 12-membered ring. The solid-state structure of CADA was energy minimized and used to generate the remaining 29 structures, which were similarly minimized and aligned to produce the 3D-QSAR models. Both models indicate that steric bulk of the tail group, and, to a lesser extent, the sidearms mainly determine CD4 down-modulation potency in this series of compounds.

By building key structural features into hydrophilic drugs, they can be recognized by the PepT1 transporter system of the small intestine and rendered orally active. The model shown provides, for the first time, a 3D template for all known substrates of PepT1.

The binding affinities of a number of amino-acid and peptide derivatives by the mammalian intestinal peptide transporter PepT1 were investigated, using the Xenopus laevis expression system. A series of blocked amino acids, namely N-acetyl-Phe (Ac-Phe), phe-amide (Phe-NH 2 ), N-acetyl-Phe-amide (Ac-Phe-NH 2 ) and the parent compound Phe, was compared for efficacy in inhibiting the uptake of the peptideIn an equivalent set of experiments, the blocked peptides Ac-Phe-Tyr, Phe-Tyr-NH 2 and Ac-Phe-Tyr-NH 2 were compared with the parent compound Phe-Tyr. Comparing amino acids and derivatives, only Ac-Phe was an effective inhibitor of peptide uptake (K i 1.81^0.37 mm). Ac-Phe-NH 2 had a very weak interaction with PepT1 (K i 16.8^5.64 mm); neither Phe nor Phe-NH 2 interacted with PepT1 with measurable affinity. With the dipeptide and derivatives, unsurprisingly the highest affinity interaction was with Phe-Tyr (K i 0.10^0.04 mm). The blocked C-terminal peptide Phe-Tyr-NH 2 also interacted with PepT1 with a relatively high affinity (K i 0.94^0.38 mm). Both Ac-Phe-Tyr and Ac-Phe-Tyr-NH 2 interacted weakly with PepT1 (K i 8.41^0.11 and 9.97^4.01 mm, respectively). The results suggest that the N-terminus is the primary binding site for both dipeptides and tripeptides. Additional experiments with four stereoisomers of Ala-Ala-Ala support this conclusion, and lead us to propose that a histidine residue is involved in binding the C-terminus of dipeptides. In addition, a substrate binding model for PepT1 is proposed.

The factors that control the relative and absolute stereochemistry of 1.3-disubstituted and 1,2,3trisubstituted tetrahydro-P-carbolines formed via the Pictet-Spengler reaction are discussed. in particular, the stereochemical factors that lead to the predominance of cis-1.3-disubstituted products under conditions of kinetic control are presented, with the aid of X-ray crystallographic data on a number of compounds; methods for assigning relative stereochemistry on the basis of NMR data are given; the mechanism by which racemisation can occur during the Pictet-Spengler reaction has also been studied, and procedures for eliminating this problem are given.The Pictet-Spengler reaction' is the most direct method of forming the tetrahydro-P-carboline system 1, which is the commonest structural unit of indole alkaloids. For members 1 7 3 Paper 2/05 1691

4-Aminophenylacetic acid (4-APAA), a peptide mimic lacking a peptide bond, has been shown to interact with a proton-coupled oligopeptide transporter using a number of different experimental approaches. In addition to inhibiting transport of labeled peptides, these studies show that 4-APAA is itself translocated.4-APAA transport across the rat intact intestine was stimulated 18-fold by luminal acidification (to pH 6.8) as determined by high performance liquid chromatography (HPLC); in enterocytes isolated from mouse small intestine the intracellular pH was reduced on application of 4-APAA, as shown fluorimetrically with the pH indicator carboxy-SNARF; 4-APAA trans-stimulated radiolabeled peptide transport in brush-border membrane vesicles isolated from rat renal cortex; and in Xenopus oocytes expressing PepT1, 4-APAA produced trans-stimulation of radiolabeled peptide efflux, and as determined by HPLC, was a substrate for translocation by this transporter.These results with 4-APAA show for the first time that the presence of a peptide bond is not a requirement for rapid translocation through the proton-linked oligopeptide transporter (PepT1). Further investigation will be needed to determine the minimal structural requirements for a molecule to be a substrate for this transporter.The rapid uptake of intact small peptides across the brushborder membrane of the small intestinal epithelium is the major route for absorption of dietary protein ␣-amino nitrogen (1). Hitherto, it has been thought that a number of chemical features, for example free amino and carboxyl termini, are essential in contributing to substrate interaction with, and translocation through, the intestinal peptide transporter.These features include the presence of a peptide bond within the substrate molecules. Indeed a major review (1) states that "it is the presence of peptide bonds which make di-and tripeptide acceptable to the peptide transport systems." Although previous work (e.g. Ref.2) has shown that molecules lacking this feature can inhibit transport of peptides (presumably by substrate binding), we describe here for the first time rapid transport of a small totally non-peptidic substrate through the intestinal peptide transporter. The substrate, 4-aminophenylacetic acid (4-APAA), 1 was selected on the basis of its chemical structure, it being a potential mimic of a dipeptide (D-Phe-LAla) (Fig. 1) which previously we have shown to be an excellent substrate for epithelial peptide transport (3, 4). EXPERIMENTAL PROCEDURESRat renal brush-border membrane vesicles were prepared as described previously (5), and initial rates of labeled peptide transport (influx, efflux) were determined by rapid filtration (4, 6). Rat intestinal loops in vitro and vascularly perfused small intestine in situ were used to measure transepithelial fluxes in the intact small intestine as described previously (3, 7). Luminal pH was changed using a previously published protocol (8). Isolated murine enterocytes were prepared by enzymatic digestion using haluronidase, and i...

1. 4-Aminomethylbenzoic acid, a molecule which mimics the special configuration of a dipeptide, competitively inhibits peptide influx in both Xenopus Laevis oocytes expressing rabbit PepT1 and through PepT1 in rat renal brush border membrane vesicles. 2. This molecule is not translocated through PepT1 as measured both by direct HPLC analysis in PepT1-exp ressing oocytes and indirectly by its failure to trans-stimulate labelle d peptide efflux through PepT1 in oocytes and in renal membrane vessicle s. 3. However 4-aminiomethylbenzoic acid does reverse trans-stimulation through expressed PepT1 of labelled peptid efflux induced by unlabelled peptide. Quantitatively this reversal is compatible with 4-aminomethyl benzoic acid competitively binding to the external surface of PepT1. 4. 4-Aminomethylbenzoic acid (the first molecule discovered to be a non-translocated competitive inhibitor of proton-coupled oligopeptide transport) and its derivatives may thus be particularly useful as experimental tools.

A thiodipeptide carrier system is shown to be effective at enabling a range of covalently bound molecules, including benzyl, benzoyl and ibuprofen conjugates, to be transported via the intestinal peptide transporter PepT1, demonstrating its potential as a rational drug delivery target.