Structure elucidation of and synthetic approaches to monatin, a metabolite from Schlerochiton ilicifolius

Ackerman, Louis Gabriel Jozua (1990-12)

Thesis (PhD)--Stellenbosch University , 1990.

Thesis

ENGLISH ABSTRACT: Monatin, or 4-hydroxy-4-(3-indolylmethyl) glutamic acid is a high-intensity sweet tasting amino acid found in the root bark of the indigenous plant Schlerochiton ilicifolius. In the thesis the isolation, structure elucidation and a number of synthetic approaches toward the total synthesis of monatin are described. Extensive fractionation of an aqueous extract of the root bark of S. ilicifolius using AG50WX8 strong acid cation exchange resin followed by successive gel filtration procedures using Biogel P2- and Sephadex G10 gels and guided by the intense sweet taste resulted in the isolation of monatin as a mixture of salts in which the sodium salt predominates. Potassium and calcium are the two other cations present. The structure elucidation is based mainly on the analysis of data obtained for monatin and the lactone ester of the N-2,4-dinitrophenyl derivative, prepared by reaction of monatin with Sangers reagent and diazomethane, by 1H and 13C n.m.r. techniques. The X-ray crystallographic study of both monatin and the derivative proved disappointing in that the reflections measured were weak and as a consequence refinement of the data was severely curtailed. However the resultant structures do show the skeletal atoms of the two compounds and in each case the relative stereochemistry of the two chiral centres could be deduced. A comparison of the specific rotation of monatin with those of related 4-hydroxy-4-methylglutamic acids indicates that monatin could have the (2S,4S) configuration. Retrosynthetic analysis of the monatin molecule identified a number of routes which could be utilized for the synthesis of monatin and analogues in which the indole moiety is replaced by a phenyl or aryl group. Seven approaches toward the synthesis of monatin were investigated using in most instances model compounds to establish optimum reaction conditions. Only the last approach, based on a 1,3-dipolar cyclo-addition reaction met with a measure of success: reaction of the 1,3-dipolar compound formed by reaction of N-t-Boc-indole-3-aldehyde with methyl N-benzylglycinate, with the dipolarophile methyl 2-acetoxyacrylate, generated a pyrrolidine with the requisite substituents needed for the monatin structure. In the event the final step, the cleavage of the C-2--N bond of the substituted pyrrolidine ring to give monatin, failed. During the investigation of the 1,3-dipolar cycloaddition reaction several substituted pyrrolidines were prepared. In each case several racemic stereoisomers were formed. In addition a number of substituted oxazolines and pyrrolizidines were obtained as minor by-products during these reactions. The stereochemistry of these compounds was deduced from the proton-proton nuclear Overhauser effect studies and X-ray crystallographic data.

AFRIKAANSE OPSOMMING: Monatien, of 4-hidroksi-4-(3-indolielmetiel) glutamicnsuur is 'n aminosuur met 'n hoëintensiteit soet smaak, wat in die wortelbas van die inheemse plant Schlerochiton ilicifolius voorkom. In die proefskrif word die isolasie, struktuuropklaring en 'n aantal sintetiese benaderings tot die totaalsintese daarvan beskryf. Omvattende fraksionering van 'n waterige ekstrak van die wortelbas van S. ilicifolius met behulp van AG50WX8 sterksuur-katioonuitruilhars gevolg deur opeenvolgende gelfiltrasies op Biogel P2 en Sephadex G10 filtrasiegel met die intense soet smaak as indikator, het gelei tot die isolasie van monatien as 'n mengsel van soute waarin die natriumsout oorheers het. Kalium en kalsium is die ander twee katione wat ook voorgekom het. Die struktuuropklaring is hoofsaaklik gebaseer op 1H en 13C k.m.r.-data wat vir monatien en die laktoonester van die N-2,4-dinitrofeniel derivaat verkry is. Laasgenoemde verbinding is berei deur reaksie van monatien met Sanger se reagens en diasometaan. Die X-straalkristallografiese studie van beide monatien en die derivaat het teleurstellende resultate gelewer aangesien swak refleksie-intensiteite die daaruitspruitende verfyning beperk het. Die verkreë strukture toon egter weI die raamwerkatome van beide verbindings waaruit die relatiewe stereochemie van die twee chirale sentra afgelei kon word. Vergelyking van die spesifieke rotasie van monatien met die van die verwante 4-hidroksi-4-metielglutamiensure dui daarop dat monatien die (2S,4S) konfigurasie mag besit. Retrosintetiese analises van monatien het 'n aantal sintetiese roetes aangedui wat gevolg kon word vir die sintese van monatien en/of monatienanaloë waarin die indoolbrokstuk denr 'n feniel of arielgroep vervang is. Sewe benaderings waarin modelverbindings gebruik is om die gunstigste reaksietoestande te bepaal, is ondersoek. Slegs die laaste benadering, gebaseer op 'n 1,3-dipolêre siklo-addisiereaksie was gedeeltelik suksesvol, naamlik reaksie van die 1,3-dipolêre verbinding gevorm uit N-t-Boc-indool-3-aldehied en metiel N-bensielglisinaat met 'n dipolarofiel, metiel-2-asetoksi-akrilaat, wat 'n pirrolidien gelewer het met die vereiste substituente benodig vir die monatienstruktuur. Die laaste stap, naamlik splyting van die C-2--N binding van die gesubstitueerde pirrolidienring was onsuksesvol. Die ondersoek van die 1,3-dipolêre siklo-addisiereaksie het verskeie gesubstitueerde pirrolidiene as rasemiese sterioisomere gelewer. Hiermee saam het 'n aantal oksasoliene en pirrolisidiene as byprodukte gevorm en hulle strukture is met behulp van proton/proton kern-Overhauser-effekstudies en X-straalkristallografis afgelei.

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