The tandem ylide generation and rearrangement or cycloaddition sequence triggered by copper (I) or dirhodium (II) complex-catalyzed decomposition of α-diazo carbonyl compounds has recently found numerous applications in organic synthesis. While exceptionally high levels of enantiocontrol in C-H insertions and cyclopropanations have already been achieved using welldesigned chiral metal catalysts, the development of an enantioselective version of this process has not been so straightforward as it was believed until several years ago that the reactions of ylides would occur from the free ylide detached from the catalyst rather than from the metal complexassociated ylide. The rapid recent developments in this area reviewed here have shown that high levels of asymmetric induction are possible with rearrangements of oxonium ylides and with 1, 3-dipolar cycloadditions of carbonyl ylides where the chiral metal catalyst can remain bound to the ylides in the product-forming step.
Development of water-soluble synthetic receptors is described, particularly focusing on the mechanistic aspects of binding. Binding of guest by receptor is driven by (1) enthalpic stabilization owing to the host-guest attractive forces and (2) entropic stabilization upon desolvation. A number of thermodynamic data collected for synthetic receptors indicate that the binding in organic solvent is enthalpically driven while that in water both entropically and enthalpically driven. The average binding constant of cyclodextrins is 102.4M-1, while that of proteins 109.3M-1, indicating that understanding of the large difference in affinity between synthetic and native receptors will help further development of synthetic receptor chemistry. One important aspect of biological recognition is the switching between the R state and the T state of the receptor as suggested by the induced-fit binding model. Design of the receptor with a conformationally or solvationally unstable state will be the key to the higher-order functions of the synthetic receptors. The important role of synthetic organic chemistry in the development of these functional molecules is emphasized.
Various halogenated organic compounds, originated from marine livings, have been well-known over five decades. Particularly, cyclic bromo-ether have attracted many chemists to study their existence, biogenetic syntheses and artificial syntheses, owing to their unique structures. Laurencia species produce eight-membered ethers as major products, although their syntheses by chemical procedures are generally difficult. These compounds are divided into laurenan and lauthisan groups based on the type of ring formation. The laurenan compounds are characteristic in respect of C6-S and C7-S configurations, while the lauthisan series haveR and R at the requisite positions. It has been proposed without any proof that the bromine atoms in the marine compounds could be introduced via bromo cationic species, generated by the two-electron oxidation of bromide ion with bromoperoxidase (BPO) and hydrogen peroxide. We studied on the enzymatic formation of these bromo-ethers, and found that both series of laurenan and lauthisan compounds could be biosynthesized directly from linear C15 compounds, laurediols. Herein, we mention our study of these natural products, and the recent studies around this field.
Palladium (0) -catalyzed tandem cyclization of 6-allyl-2, 7-octadienyl acetate 4 afforded the trans-fused bicyclo [3.3.0] octane 5 by intramolecular alkene insertion to a π-allylpalladium intermediate with 5-exo mode, followed by another intramolecular alkene insertion to the resulting σ-alkylpalladium intermediate. The reaction of 6, a methyl substituted derivative at the 7 position, also provided the novel trans-fused 7 and 8 whereas that of 11 which has dimethyl groups at the 2 and 7 position lead to 12 via 6-exo cyclization. Intramolecular reactions of the π-allylpalladium intermediates to allene moieties afforded two modes of cyclization (15→16 and 32→33). The former reaction was applied to a synthesis of isoiridomyrmecin (31). A domino cyclization of acyclic 41 was achieved via five and six consecutive carbon-carbon bond formations to successfully provide tricyclic 44 and tetracyclic 43.
Introduction of amino acid units into a ferrocene or 2, 6-pyridinedicarboxamide scaffold has been demonstrated to form an ordered structure in both solid and solution states. Conformational enantiomerization through chirality organization was achieved. The single-crystal X-ray structure determination of the ferrocene bearing the podand chiral dipeptide chains (-L-Ala-L-Pro-OEt) and the D-derivative revealed two C2-symmetrical intramolecular interchain hydrogen bondings to induce the chirality organized structures. The molecular structures of the L- and D-isomer are in a good mirror image relationship based on conformational enantiomers. The single-crystal X-ray structure determination of the chiral 2, 6-pyridinedicarboxamide bearing the podand L-histidyl moieties and the D-derivative confirmed a left- and right-handed helical conformation, respectively, through intramolecular hydrogen bonding and chirality of the podand histidyl moieties. The propensity to form the chiral helicity appears to be controlled by the configuration of the histidyl α-carbon atoms. These chiral 2, 6-pyridinedicarboxamides were found to induce the chiral complexation.
Enantio-enriched axially chiral allenyltitanium compounds were synthesized from optically active propargyl alcohol derivatives by the reaction with a divalent titanium reagent, Ti(O-i-Pr)4/2i-PrMgX. The reaction of the allenyltitaniums with a variety of electrophiles such as carbonyl compounds, H+, D+, Br2, NCS, dialkyl azodicarboxylates, trialkylstannyl chlorides and alkylidenemalonates proceeded with high regio- and stereoselectivities to give the corresponding propargylated compounds with a high optical purity. Based on the stereochemical outcome of the reaction, the reaction mechanisms are discussed.