Polyhydroxylated pyrrolizidines. Part 5: Stereoselective synthesis of 1,2-dihydroxypyrrolizidines as a model for the preparation of densely polyhydroxylated pyrrolizidines

Pyrrolizidine alkaloids

This review covers pyrrolizidine alkaloids isolated from natural sources. Topics include: aspects of structure, isolation, and biological/pharmacological studies; total syntheses of necic acids, necine bases and closely-related non-natural analogues.

Structure-guided minimalist redesign of the L-fuculose-1-phosphate aldolase active site: expedient synthesis of novel polyhydroxylated pyrrolizidines and their inhibitory properties against glycosidases and intestinal disaccharidases

A minimalist active site redesign of the L-fuculose-1-phosphate aldolase from E. coli FucA was envisaged, to extend its tolerance towards bulky and conformationally restricted N-Cbz-amino aldehyde acceptor substrates (Cbz=benzyloxycarbonyl). Various mutants at the active site of the FucA wild type were obtained and screened with seven sterically demanding N-Cbz-amino aldehydes including N-Cbz-prolinal derivatives. FucA F131A showed an aldol activity of 62 μmol h(-1) mg(-1) with (R)-N-Cbz-prolinal, whereas no detectable activity was observed with the FucA wild type. For the other substrates, the F131A mutant gave aldol activities from 4 to about 25 times higher than those observed with the FucA wild type. With regard to the stereochemistry of the reactions, the (R)-amino aldehydes gave exclusively the anti configured aldol adducts whereas their S counterparts gave variable ratios of anti/syn diastereoisomers. Interestingly, the F131A mutant was highly stereoselective both with (R)- and with (S)-N-Cbz-prolinal, exclusively producing the anti and syn aldol adducts, respectively. Molecular models suggest that this improved activity towards bulky and more rigid substrates, such as N-Cbz-prolinal, could arise from a better fit of the substrate into the hydrophobic pocket created by the F131A mutation, due to an additional π-cation interaction with the residue K205' and to efficient contact between the substrate and the mechanistically important Y113' and Y209' residues. An expedient synthesis of novel polyhydroxylated pyrrolizidines related to the hyacinthacine and alexine types was accomplished through aldol additions of dihydroxyacetone phosphate (DHAP) to hydroxyprolinal derivatives with the hyperactive FucA F131A as catalyst. The iminocyclitols obtained were fully characterised and found to be moderate to weak inhibitors (relative to 1,4-dideoxy-1,4-imino-L-arabinitol (LAB) and 1,4-dideoxy-1,4-imino-D-arabinitol (DAB)) against glycosidases and rat intestinal saccharidases.

Highly Chiral Muscarinic Ligands: The Discovery of (2S,2‘R,3‘S,5‘R)-1-Methyl-2-(2-methyl-1,3-oxathiolan-5-yl)pyrrolidine 3-sulfoxide Methyl Iodide, a Potent, Functionally Selective, M2 Partial Agonist

By further steric complication of previously studied highly chiral muscarinic agonists, we have obtained a small chiral library of enantiomeric and diasteromeric 1-methyl-2-(2-methyl-1,3-oxathiolan-5-yl)pyrrolidine 3-sulfoxides. Binding studies on cloned human muscarinic receptors expressed in CHO cells show that the introduction of a fourth stereogenic center gives undetectable affinity for hm1, hm3, hm4 and hm5 subtypes while leaving a quite modest affinity only for hm2 subtypes. However, functional studies on model M1-M4 muscarinic tissues have shown that three compounds of the series [(-)-5, (-)-7, (+)-8] are endowed with functional activity and behave as M2 selective partial agonists. Among them, compound (2S,2'R,3'S,5'R)-1-methyl-2-(2-methyl-1,3-oxathiolan-5-yl)pyrrolidine 3-sulfoxide methyl iodide [(+)-8] is particularly interesting, as it is a potent partial agonist on guinea pig atrium (force) (M2; pD2=7.65, alpha=0.41) while being a poor antagonist on M1, M3, and M4 model tissues (pKb<5).