(+)-proto-Quercitol, a natural versatile chiral building block for the synthesis of the α-glucosidase inhibitors, 5-amino-1,2,3,4-cyclohexanetetrols

An efficient synthesis of diastereomerically pure 5-amino-1,2,3,4-cyclohexanetetrols (6 and 11) and quercitol derivatives from naturally available (+)-proto-quercitol (1) is described. The stereochemistry of 1 is perfectly set up for regioselective protection of the hydroxy group which was further functionalized into the target aminocyclitol in a straightforward manner. The present approach provides a protocol for preparing aminocyclitols in large quantities. In addition, the absolute stereochemistry of (+)-proto-quercitol was addressed using the modified Mosher’s method. Of the synthesized aminocyclitols, 11 potentially inhibits α-glucosidase with an IC₅₀ value of 12.5 μM, which is 45 times greater than that of the standard antidiabetes drug, Acarbose^®.     

Cyclic tetrapeptides from marine bacteria associated with the seaweed Diginea sp. and the sponge Halisarca ectofibrosa

A Pseudomonas sp. was cultured which was associated with the Japanese seaweed Diginea sp. Crude extracts prepared from this bacterial culture were found to inhibit the growth of other marine bacterial strains. From this bacterial culture, two new peptides cyclo-[phenylalanyl-prolyl-leucyl-prolyl] (3) and cyclo-[isoleucyl-prolyl-leucyl-alanyl] (4) have been isolated together with two known peptides (1) and (2). The crude extract from a culture of Pseudoalteromonas sp. associated with the Thai sponge Halisarca ectofibrosa was found to inhibit the growth of Bacillus subtilis and Vibrio anguillarum. Isolation studies yielded a fraction containing two peptides that were identified as cyclo-[phenylalanyl-leucyl]₂ (5) and cyclo-[leucyl-isoleucyl]₂ (6) by means of LC-MS and 2D NMR data. Absolute stereochemistry was confirmed by the synthesis of cyclo-[l-phenylalanyl-l-leucyl]₂. Peptides (1)-(3) were also isolated from this bacterial strain. None of the individual peptides isolated in this study showed antibiotic activity.