Synthesis of (-)-swainsonine and optically active 3,4-dihydroxy-2-hydroxymethylpyrrolidines

Efficient synthesis for each of the eight stereoisomers of the iminosugars lentiginosine and 1,4-dideoxy-1,4-imino-D-arabinitol (DAB)

Herein, we describe the efficient, diastereoselective syntheses of the iminosugars 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) 1b, lentiginosine 3a, and the seven stereoisomers of each of these iminosugars starting from 4-benzoyl-6-deoxy-6-iodoglycopyranosides 47 with yields ranging from 38 % to 68 % for the DAB and isomers 1a-1h and from 44 % to 89 % for the lentiginosine and isomers 3a-3h. We also report the syntheses of the eight stereoisomers of the 4-benzoyl-6-deoxy-6-iodoglycopyranosides 47 from commercially available sugars. Key to the iminosugar syntheses is a single multistep reaction that converts the 4-benzoyl-6-deoxy-6-iodoglycopyranosides 47 to a vinyl pyrrolidine through a one-pot zinc mediated reductive elimination, followed by a reductive amination and finally an intramolecular nucleophilic substitution. Strategic selection of the amine utilized in the reductive amination and the functionalization of the intermediate carbon-carbon double bond provides access to a vast array of iminosugars.

Recent advances on the synthesis of natural pyrrolizidine alkaloids: alexine, and its stereoisomers

Natural products have attracted the interest of the scientific community due to their importance and application. Alexine is a naturally polyhydroxylated pyrrolizidine alkaloid that is broadly found in plant sources and isolated from Alexa leiopetala. The biological properties such as glycosidase inhibitors, anti-virus, and anti-HIV activities, makes it interesting target for synthetical studies. This review reports different approaches and methodologies to the synthesis of alexine, and its stereoisomers as the target compounds in numerous studies.

Synthesis of naturally occurring iminosugars from d-fructose by the use of a zinc-mediated fragmentation reaction

A short synthesis of 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) and a formal synthesis of australine are described. In both cases, D-fructose is employed as the starting material and converted into a protected methyl 6-deoxy-6-iodo-furanoside. Zinc-mediated fragmentation produces an unsaturated ketone which serves as a key building block for both syntheses. Ozonolysis, reductive amination with benzylamine and deprotection affords 1,4-dideoxy-1,4-imino-D-arabinitol in only 7 steps and 11% overall yield from D-fructose. Alternatively, reductive amination with homoallylamine, ring-closing metathesis and protecting group manipulations give rise to an intermediate which can be converted into australine in 3 steps. The intermediate is prepared by two different strategies both of which use a total of 9 steps. The first strategy utilizes benzyl ethers for protection of fructose while the second and more effective strategy employs an isopropylidene acetal.

Ring-closing metathesis of chiral allylamines. Enantioselective synthesis of (2S,3R,4S)-3,4-dihydroxyproline

The ring-closing metathesis (RCM) of two types of unsaturated chiral allylamines III, easily available from enantiomerically enriched epoxy alcohols, has been studied. Fully protected allylamines IIIa [(1)R = CH(2)-(CH(2))(n)()-CH=CH(2); (2)R = Boc; (3)R = PMB] have been prepared from unsaturated epoxy alcohols, whereas bis-allylamines IIIb ((1)R = Ph, (2)R = allyl,(3)R = Boc or PMB) have been prepared from 2,3-epoxy-3-phenylpropanol. Both types have been subjected to RCM to provide either cyclic allylamine I or II. The synthetic potential of these intermediates has been demonstrated by the enantioselective synthesis of (2S,3R,4S)-3,4-dihydroxyproline.

Enantiospecific and Stereoselective Synthesis of Polyhydroxylated Pyrrolidines and Indolizidines from trans-4-Hydroxy-L-proline

We have developed a short, efficient, and stereoselective synthesis of polyhydroxylated pyrrolidine and indolizidine glycosidase inhibitors starting from 4-hydroxy-L-proline. The regio- and stereoselective hydroxylation of an N-Pf-4-oxoproline enolate and the stereoselective reduction of the resulting keto alcohol allowed us to introduce the cis diol present in the target compounds. The different side chains needed to complete the syntheses of the target compounds were introduced by reduction of the ester group of a substituted proline or by reaction of organolithium or organomagnesium reagents with the same group followed by stereoselective reduction of the resulting ketones. Hydrogenolysis of the alcohols thus obtained gave the hydrochlorides of the desired pyrrolidine glycosidase inhibitors, which were obtained in nine steps in overall yields greater than 50%. The indolizidine glycosidase inhibitor 8-epi-swainsonine was also prepared using this approach.

Synthesis of (2R,3R,4S)-2-hydroxymethylpyrrolidine-3,4-diol from (2S)-3,4-dehydroproline derivatives

(2R,3R,4S)-2-Hydroxymethylpyrrolidine-3,4-diol (1,4-dideoxy-1,4-imino-D-ribitol) was synthesized in five steps from N-protected (2S)-3,4-dehydroproline methyl esters. The stereoselective reaction of osmium tetraoxide with dehydroproline derivatives gave high yields of (2S,3R,4S)-3,4-dihydroxyprolines (2,3-trans-3,4-cis-3,4-dihydroxy-L-prolines) accompanied by small amounts (< 15%) of the diastereomeric (2S,3S,4R)-3,4-dihydroxyprolines (2,3-cis-3,4-cis-3,4-dihydroxy-L-prolines). The mixture of the diastereomeric glycols was converted into the isopropylidene acetals, and the isomers separated efficiently on a preparative scale. The resulting protected (2S,3R,4S)-3,4-dihydroxyproline methyl ester was reduced (LiBH4) to the 2-hydroxymethylpyrrolidine and deprotected, resulting in the production of (2R,3R,4S)-2-hydroxymethylpyrrolidine-3,4-diol in high yield and in high purity. The 1H and 13C NMR signals of the product have been unambiguously assigned using two-dimensional NMR techniques, and the identity of the title pyrrolidine confirmed by comparisons of its spectra with those reported for the authentic material.