Highly diastereoselective preparation of tert-homoallylic alcohols using organotitanium reagents is described. The reaction of allyltitanocenes, generated by the reductive titanation of allylic sulfides with titanocene(II)-1-butene complex, with phenyl and sterically hindered alkyl methyl ketones produced anti-tertiary homoallylic alcohols with complete diastereoselectivity. Even when sterically less congested alkyl methyl ketones such as methyl ethyl ketone and cyclic enones were employed, the homoallylic alcohols were obtained with unprecedented high diastereoselectivity. Using α-silyl allyl sulfides, β,δ-disubstituted tertiary homoallylic alcohols were obtained with high regio- and stereoselectivity. A range of syn-tertiary homoallylic alcohols were also obtained by the reaction of α-silylallenyltitanocenes generated by the reductive titanation of γ-silylpropargylic carbonates with titanocene(II)-triethyl phosphite complex, with ketones and following desilylation and partial hydrogenation. The construction of three consecutive stereogenic centers has been achieved by two approaches, the addition of allyltitanium reagents to α-chiral ketones and the reaction of ketones with chiral allyltitanocenes generated by the desulfurizative titanation of allylic sulfides obtained by the alkylation of allyl sulfide with secondary halides. Acyclic systems bearing adjacent two quaternary centers were also constructed by the use of allyltitanocenes, generated by the desulfurizative titanation of γ,γ-disubstituted allyl sulfides.
Asymmetric autocatalysis with amplification of enantiomeric excess is found in the enantioselective addition of diisopropylzinc to pyrimidine-5-carbaldehyde using pyrimidyl alkanol as an asymmetric autocatalyst. Asymmetric autocatalysis has been employed as a method for clarifying the origin of homochirality. Circularly polarized light, quartz and statistical fluctuation of enantiomeric imbalance act as chiral triggers for asymmetric autocatalysis to afford highly enantioenriched products. We have investigated the asymmetric autocatalysis using chiral crystals formed from achiral and racemic compounds as the origins of chirality. Absolute control of the crystal chirality of cytosine was achieved by the removal of crystal water of achiral cytosine monohydrate. Enantioselective carbon-carbon bond formation on the enantiotopic crystal face of aldehyde was established by using the vapor of diisopropylzinc. In addition, asymmetric autocatalysis triggered by chiral compounds arising from H, C and O isotopes substitution has been achieved. Reversal phenomena of enantioselectivity were observed in β-amino alcohol catalyzed dialkylzinc addition to aldehyde by using the mixture of two different β-amino alcohols as chiral ligands.
The zoanthamine alkaloids isolated from the genus Zoanthus species during the last three decades constitute a distinctive family of marine metabolites. These natural products are characterized by the densely functionalized heptacyclic framework, as exemplified by the structures of norzoanthamine (1), zoanthamine (2), and zoanthenol (3), as well as their potent spectrum of unique biological properties such as promising antiosteoporotic properties. Their distinctive biological activities, combined with novel chemical structures, make this family of alkaloids extremely attractive targets for chemical synthesis. However, because of their densely functionalized complex structures, synthetic studies of zoanthamine alkaloids had seriously been impeded until we reached the first total synthesis of norzoanthamine. We describe herein our synthetic approach toward the total synthesis of zoanthamine alkaloids (1-3), focusing on how to tackle and overcome various synthetic challenges.
This account presents our recent efforts in exploring the synthetic potential of configurationally labile chiral carbanions next to electron-withdrawing groups that have been considered to be impossible even to generate. We used epoxysilane rearrangement to generate a chiral carbanion whose chirality was transformed from epoxide. The detailed analysis of the process showed that a carbamoyloxy group plays a more prominent role in fixing a chiral lithiocarbanion than expected. This understanding has led to the successful development of a method that allows enantioselective trapping of an α-chiral carbanion of acyclic nitrile by a carbon electrophile. During the course of these studies, we have found that the configurational stability of a carbanion highly depends on the solvent and we have proposed that the dependency is attributable to the ratio of CIP and SIP associated with their solvated structures.
The rapid growth of the click chemistry concept produced a wide variety of functional polymers. In this research field, polymeric ion sensors, which can recognize specific metal cations or anions, have mainly been synthesized by the Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC). However, due to several disadvantages suggested for the CuAAC, the development of new types of click chemistry reactions has recently started. Among them, it was found that the high-yielding addition reactions between electron-rich alkynes and cyano-based acceptors offer promising application possibilities as polymeric chemosensors. The donor-acceptor type products feature intense charge-transfer bands in the visible region. The recognition of some specific metal cations or anions resulted in well-defined visible colorimetric changes in the optical spectra.
Rotaxanes are mechanically interlocked molecules, in which an axle is threaded into a wheel and trapped with bulky stopper groups at the termini. Bistable rotaxanes possessing two or more station moieties have attracted particular interests, because they can switch on/off and change the molecular lengths in response to external stimuli. In addition to the molecular-level properties of these molecules in solution, properties in nano-, micro-, and macroscale of the molecular-integrated systems recently have attracted attentions. This review highlights the development in the design and synthesis of bistable rotaxanes to integrate from molecules to the systems to show actuation and locomotion.