The Smelling Principle of Vetiver Oil, Unveiled by Chemical Synthesis.

Vetiver oil, produced on a multiton-scale from the roots of vetiver grass, is one of the finest and most popular perfumery materials, appearing in over a third of all fragrances. It is a complex mixture of hundreds of molecules and the specific odorant, responsible for its characteristic suave and sweet transparent, woody-ambery smell, has remained a mystery until today. Herein, we prove by an eleven-step chemical synthesis, employing a novel asymmetric organocatalytic Mukaiyama-Michael addition, that (+)-2-epi-ziza-6(13)en-3-one is the active smelling principle of vetiver oil. Its olfactory evaluation reveals a remarkable odor threshold of 29 picograms per liter air, responsible for the special sensuous aura it lends to perfumes and the quasi-pheromone-like effect it has on perfumers and consumers alike.

Total Synthesis of Disciformycin†A and B: Unusually Exigent Targets of Biological Significance.

The first total synthesis of the potent antibiotic disciformycin B (2) is described, which is exceptionally isomerization-prone and transforms into disciformycin A (1) even under notably mild conditions. To outweigh this bias, the approach to 2 hinged on the use of a silyl residue at C4 to lock the critical double bond in place and hence insure the integrity of the synthetic intermediates en route to 2. This tactic was instrumental for the preparation of the building blocks and formation of the macrocyclic ring via ring closing alkyne metathesis (RCAM). To make the end game successful, however, it proved necessary to cleave the C-silyl protecting group off; it was at this stage that the exceptional sensitivity of the target became fully apparent.

A Frustrated and Confused Lewis Pair.

We report a new class of frustrated Lewis pairs (FLPs) by the hydroboration of bulky isocyanates iPr2 ArNCO (iPr2 Ar=2,6-iPr2 C6 H3 ) and Ph2tBu ArNCO (Ph2tBu Ar=2,6-Ph2 -4-tBuC6 H2 ) with Piers' borane (HB(C6 F5 )2 ). While hydroboration of smaller isocyanates such as iPr2 ArNCO leads to isocyanate-N/B FLP adducts, hydroboration of the bulkier Ph2tBu ArNCO allows isolation of the substrate-free aminoborane with a short, covalent N-B bond. This confused FLP reversibly binds unsaturated substrates such as isocyanates and isocyanides, suggesting the intermediacy of a "normal" FLP along the reaction pathway, supported by high-level DFT studies and variable-temperature NMR spectroscopy. These results underscore the possibility of FLP behavior in systems that possess no obvious frustrated Lewis acid-base interaction.

Splitting of dihydrogen by five-membered zirconacycloallenoids: a novel pathway to conjugated diene zirconocene complexes.

The five-membered zirconacycloallenoids 2 react rapidly with dihydrogen under mild conditions to yield the corresponding (s-cis-conjugated diene)zirconocenes 3. The reaction involves splitting of the H(2) molecule between the metal center and a ligand carbon atom.