Dysiherbaine (DH, 1) and its congener neodysiherbaine A (2), isolated from the Micronesian marine sponge, Dysidea herbacea, are novel excitatory amino acids with potent convulsant activity. DH activates non-NMDA type glutamate receptors [AMPA and kainic acid (KA) receptors] with considerable preference over KA receptors. Moreover, it has been shown that DH binds to the GluR5 and GluR6 KA receptor subunits with high affinity. Due to these intriguing pharmacological properties to KA receptors, DH and its designed analogues are anticipated to serve as useful tools for understanding the structure and functions of glutamate receptors in the central nervous system. In order to reveal the detailed structure-activity relationship profiles of DH, we undertook a diverted synthesis of structural analogues of DH. In this paper, we report the synthesis of simplified analogue 3,8,9-epi-neodysiherbaine A (4a), 8-epi-neodysiherbaine A (4b), 8-deoxyneodysiherbaine A (4c) and 9-deoxyneodysiherbaine A (4d) from a common intermediate 5. The synthesis of 5 started with C-glycosylation of allylsilane 9 with diacetyl-L-arabinal (10), which led to C-glycoside 8 as the sole product. Chemo- and stereoselective dihydroxylation using (DHQD)_2AQN and subsequent epoxidation delivered epoxy alcohol 7, which underwent epoxide opening/5-exo ring-closure during chromatography on silica gel, leading to bicyclic ether core 12. Stereoselective construction of the amino acid chain was efficiently realized by DuPHOS-mediated asymmetric hydrogenation of enamido ester 17 to generate key intermediate 5 via 18. Global deprotection of 5 by acid hydrolysis furnished analogue 4a. Selective deprotection of the acetonide group of 5 was achieved by using DDQ to afford diol 19, which was further converted to other analogues (3 and 4b-d) via cyclic sulfate 20 and thiocarbonate 24. The toxicity of analogues 4a-d were tested on mice. Intracerebral injection of analogue 4a did not induce any convulsant behavior even at higher dose (20μg/mouse), whereas that of 4b induced a sleeper effect. Analogues 4c and 4d were found to show convulsant activity although the potency was much weaker than that of the natural DH (1). In the radioligand binding assay using rat synaptic membrane preparation, 4b displaced [^3H]KA and [^3H]AMPA with IC_<50> values of 24.1±6.8μM and 9.7±2.3μM, respectively. In contrast, analogue 4a did not displace these radioligand from receptors even at 100μM. In addition, detailed pharmacological studies revealed that simplified analogue 3 is a selective antagonist for GluR5 KA receptors. Further neurophysiological studies of analogues are underway and will be described.

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