Chemoselective reductions with sodium borohydride and lithium borohydride were examined in mixed solvents containing methanol. Ester, lactone, epoxide, diaryl disulfide, azide, and carboxylic anhydride were reduced chemoselectively in the presence of other reducible groups with sodium borohydride in refluxing t -butyl alcohol or tetrahydrofuran with dropwise addition of methanol. Chemoselectivity of these reductions were better than that of lithium aluminum hydride. On the other hand, ester, lactone, and epoxide were reduced rapidly and chemoselectively with lithium borohydride in mixed solvent containing methanol. Primary amides were reduced chemoselectively in the presence of secondary amide and metal carboxylate with lithium borohydride in MeOH-diglyme. This chemoselectivity is unprecedented.
This article describes a compilation of recent development in the practical preparation of versatile chiral building blocks carried out in the authors' laboratories using D-mannitol as common chiral starting material. The described examples are accompanied by full experimental details for the practical purpose.
Indole-2-carboxylic acid and its derivatives (2) are much more stable than usual indoles (1) toward acid and oxidation condition, while still reactive at the 3-position. As the 2-carboxyl group of 2 can easily be removed by decarboxylation, 2 can be considered to be stable equivalent of 1. The expected role of the carboxyl (or ester) group at the 2-position is i) stabilization of the pyrrole ring, ii) to give 2 different reactivities from 1, iii) conversion itself to other functional group, and iv) use as a blocking group. On the basis of this concept reported syntheses, reactivities, and synthetic application of 2 are reviewed.
Functions of noncyclic polythers have been paid attention in recent years. Their molecular structures were designed with the change of chain structures, terminal groups, kinds and numbers of hetero atoms, etc. They were demonstrated to be available as carriers for ion-transport through liquid membranes, solvent extraction, ion-selective electrodes, and so on. It is in detail described that noncyclic polyethers exhibiting Li+ -selectivity which have been newly prepared can be useful not only for ion-separation but also for ion-analysis. In addition, the relationship between their structures and functions has been discussed based on spectroscopic behaviors, X-ray analysis, and also the inspection of CPK molecular model.
Amarolide, one of the representative quassinoids, is a tetracyclic terpenoid having a picrasane skeleton with 10 chiral centers. The first total synthesis of (±) -amarolide has been accomplished stereoselectively from a known tricyclic compound. Orthoester Claisen rearrangement and lead tetraacetate oxidation were utilized as key reactions to prepare 12β-hydroxypicrasan-3-one, a compound which has the correct relative stereochemistry of all the six ring-juncture chiral centers of the picrasane skeleton. This 12β-hydroxypicrasan-3-one was transformed into (±) -amarolide by 18 steps reactions, including the 1, 3-carbonyl transposition reactions, introduction of two hydroxyl groups at C-2 and C-11 positions, respectively, and an oxidation of the ether ring to afford δ-lactone. The NMR spectral data of synthetic amarolide and its diacetate were completely identical with those of natural ones. Overall yield of this synthesis of (±) -amarolide from the known tricyclic compound by 35-step reactions was 0.2%. As the transformation of amarolide into quassin has already been known, this synthesis constitutes also a formal synthesis of (±) -quassin.