Samarium(II)-mediated reaction of allylic phosphate esters with carbonyl compounds: A new method for “Umpolung” of allylic phosphates

Chemistry of the Secondary Metabolites of Termites

Isolation, structure determination, synthesis, and biochemistry of the low-molecular-weight compounds of the secretion of exocrine glands of termites are described, with an emphasis on pheromones and defensive compounds.

Enantiospecific synthesis of the cubitane skeleton

The fully substituted 12-membered macrocycle of the cubitane-type diterpenoids has been assembled in an enantioselective manner following a novel "bridge-and-cut" strategy. Hydroxyalkylation of (S)-carvone afforded a carvonylgeraniol, which underwent transannular cyclization on treatment with samarium diiodide in THF. Fragmentation of one of the shorter bridges of the resulting [8.2.2]bicycle liberated the 12-membered ring with the desired cis-arrangement of the isopropenyl side chains.

Diastereoselective addition of gamma-substituted allylic nucleophiles to ketones: highly stereoselective synthesis of tertiary homoallylic alcohols using an allylic tributylstannane/stannous chloride system

The diastereoselective addition of gamma-substituted allylic nucleophiles to ketones has been accomplished to give tertiary homoallylic alcohols. The reaction of tributylcinnamyltin 1a with simple ketones 2 in the presence of stannous chloride (SnCl(2)) gave the tertiary homoallylic alcohols 3, which include the anti form (based on Ph and OH), with high diastereoselectivity. In the reaction course, transmetalation of tributylcinnamyltin 1a with SnCl(2) proceeds to form an active nucleophile which is tentatively considered to be a cinnamyltin(II) species. A cyclic transition state A is favorable because the chlorinated tin(II) center is highly capable of accepting ligands. The other diastereomers (syn form) 4 were obtained in the reaction of tributylcinnamyltin 1a with ketones 2 by the use of BF(3) x OEt(2) instead of SnCl(2). This reaction proceeds through an acyclic transition state in which BF(3) acts as a Lewis acid for activation of ketones. When 3-tributylstannylcyclohexene 1b or 3-tributylstannylcyclopentene 1c was used with SnCl(2), high diastereoselective formation of the corresponding homoallylic alcohols 6 which have the syn form (based on ring chain and OH) was observed. The selectivity was also explained by the cyclic transition state B. When tributylcrotyltin 1d or 1e was used, the stereochemistry of the products depends on the additives (SnCl(2) or BF(3) x OEt(2)), substituents of ketones, and reaction temperature. It is interesting that those additives compensate for each other in terms of diastereoselective alkylation. The alkylation of alpha-alkoxy, aryloxy, or hydroxyketones 16 was achieved in extremely high selectivity using an allylic tributyltin 1a-c/SnCl(2) system. The chelation by carbonyl and beta-oxygens provides a rigid transition state (E or F) for selective reactions. It is noted that the hydroxyketone can be used without protection in this reaction system. The relative stereochemistry of the produced tertiary homoallylic alcohols was determined on the basis of X-ray analyses.