Total synthesis of (+)-castanospermine from D-mannose

The chemistry of castanospermine. Direct oxidation of the tetraacetate to the corresponding γ-lactam

From the products of oxidation of tetra-O-acetylcastanospermine (2) with N-bromosuccinimide, only the gamma-lactam 11 was isolated (19%). Treatment of this lactam with DBU in dichloromethane afforded the elimination product 12, while deacetylation with sodium methoxide in methanol gave tetraol 13 together with diene 14. The X-ray crystal structure of compound 13 is reported.

Synthesis of 5-azacastanospermine, a conformationally restricted azafagomine analogue

The 5-aza-6-deoxy analogue of castanospermine (+/-)-5a and its 1-epimer (+/-)-5b was synthesized. The synthesis started from the known compound 5-benzyloxy-7-hydroxyhepta-1,3-diene, which was protected and subjected to Diels-Alder reaction with 4-phenyl-1,2,4-triazoline-3,5-dione to give two epimeric adducts. One of these was transformed through epoxidation, acetolysis, a series of side-chain transformations that converted it into a terminally protected aldehyde, deprotection, and hydrogenolysis/reductive amination into 5a. By a similar set of reactions the other adduct epimer was converted into 5b. The castanospermine analogue 5a was a weaker inhibitor of almond beta-glucosidase and rice alpha-glucosidase than castanospermine (2) or 1-azafagomine (4), but was considerably more potent than its epimer 5b. This suggests that these enzymes have a strong preference for binding substrates or azasugars with the 6-OH in an axial conformation.

Synthesis of (-)-7-epiaustraline and (-)-1-epicastanospermine

Highly efficient and selective syntheses of the title compounds are described. The cornerstone of the synthetic plan is the tandem inter [4 + 2]/inter [3 + 2] cycloaddition process. These syntheses differ from previous applications of this strategy in that they incorporate an alkylation in the hydrogenolysis step to close the second ring of the azabicyclic systems. Notable features of the sequence are (1) the highly regio- and stereoselective [3 + 2] cycloaddition of nitronate 15 with siloxymethyl (Z)-beta-silylvinyl ketone (Z)-22b and (2) the highly selective reduction of the resulting ketone 24a with L-Selectride. A single-crystal X-ray structure analysis of synthetic (-)-7-epiaustraline confirmed that the targeted structure was successfully synthesized. This stimulated a reexamination of the structural assignment of the natural product. (-)-1-Epicastanospermine was synthesized in four steps from the common intermediate 27a. The absolute configuration of (-)-1-epicastanospermine was assured by single-crystal X-ray structure analysis of intermediate (-)-27a. Thus, the sign of the optical rotation had to be revised. The overall efficiency of these syntheses were 9 steps and 23% yield for (-)-7-epiaustraline and 10 steps and 20% yield for (-)-1-epicastanospermine

Synthesis of the enantiomers of 6-epicastanospermine and 1,6-diepicastanospermine from D- and L-gulonolactone

The synthesis of the enantiomers of 6-epicastanospermine and of 1,6-diepicastanospermine from the enantiomeric gulonolactones is reported and the structure of the former is established as (1S,6R,7R,8R,8aR)-1,6,7,8-tetrahydroxyoctahydroindolizine. The inhibitory activities of the diastereomers against the amyloglucosidase-catalysed hydrolysis of p-nitrophenyl alpha-D-glucopyranoside were investigated, and the effects of 6-epicastanospermine and of 1,6-diepicastanospermine on 14 human liver glycosidases are reported.

Effect on human liver glycosidases and short syntheses of 1 alpha,2 alpha,6 alpha,7 alpha,7a beta-1,2,6,7-tetrahydroxypyrrolizidine from D-glycero-D-gulo-heptono-1,4-lactone

The synthesis of 1 alpha,2 alpha,6 alpha,7 alpha,7a beta-1,2,6,7-tetrahydroxypyrrolizidine (1) from D-glycero-D-gulo-heptono-1,4-lactone (6) by two different routes is reported. The effects of 1 on the inhibition of 15 human liver glycosidases are described.