Among the several Mn-salen type catalysts studied, a rationally designed Mnsalen catalyst (14) was found to be the most effective for asymmetric epoxidation of simple olefins, especially cis-disubstituted and trisubstituted olefins conjugated with aryl, alkenyl and alkynyl groups. The mechanism of asymmetrc-inducition of this reaction has been well rationalized by the proposal that olefins approach oxo-metal from the side-on to give a metallaoxetane intermediate, wherein steric and electronic repulsions between the salen ligand and the olefinic substituent dictate the orientation of the incoming olefins and, the intermediate is transformed into epoxides via a radical intermediate. Asymmetric oxidation of sulfides was also found to be effectively catalyzed by Mn-salen catalyst (21).
Accomplishing the syntheses of enantiomerically pure isoptenoids and comparing their structural and physical properties with that of their natural sources reveals a surprising number of natural products to be enantiomerically impure. A deeper understanding of the relationship between the absolute configuration of bioactive molecules and their activity could be obtained by the synthesis and bioassay of their pure enantiomers. This has been exemplified in syntheses of a triterpene limatulone and pine bast scale pheromones which are discussed in detail.
Current methodology provides ready access to the bowl-shaped calix[n]arenes. Further chemical manipulation of these unique molecular structures provided a means to modifying their basic skeleton. These molecules were found to be extremely useful in host-guest chem istly due to their propensity to complex the ions and/or neutral molecules inside their cavities. They are thus ideal building blocks for the construction of various types of more sophisticated host molecules, sensors, and larger molecular systems with well-predefined structures. Some recent advances made in this field concerning the usage of calix [n]arenes in supramolecular chemistry are summarized in this review.
Absolute stereostructure of alternaric acid (1) has been determined by the synthesis of four diastereomers of the degradation product, which had been obtained in the course of structural studies. The key synthetic steps incorporated into the total synthesis of 1 were the Julia olefination of tertiary aldehyde 6 and phenylsulfone 7 and novel one pot construction of 3-acyl-4-hydroxy-5, 6-dehydro-2-pyrone 1b via Fries-type rearrangement of the O-enol acyl group of β-keto-δ-valerolactone toward the α-position of the δ-lactone. The structure-activity relationship of analogues of 1 and synthetic intermediates has been investigated.
We have demonstrated that the configuration of the hydroxyl groups in the starting materials has some influence in controlling the stereochemical outcome of the SmI2-promoted reductive couplings. Generally the reaction proceeds in good yields and provides an efficient route to polyoxygenated molecules with high and reproducible stereochemical control. In all cases of the hydroxyl-directed reductive couplings, the observed stereochemistry in the reaction products can be predicted by assuming a cyclic ketyl radical intermediate on the basis of chelation to the Sm (III) cation generated during the initial single-electron reduction of the carbonyl group with the hydroxyl group. The impact which this new methodology on synthetic organic chemistry has is clearly evident from its applicability to total synthesis, one notable example being (-) -grayanotoxin III, a unique tetracyclic diterpene.
Organometallic polycondensation mediated by organotransition metal complexes represents an efficient and versatile method for obtaining various π-conjugated polymers. For example, nickel complex-mediated dehalogenation polycondensation of dihaloaromatic compounds XArX affords poly(arylene)s (Ar)n, whereas palladium-catalyzed polycondensation between XArX and HC≡CAr'C≡CH yields poly (aryleneethynylene)s (ArC≡CAr'C≡C)n. These polymers exhibit interesting electronic and optical properties and have already found several usuful applications in their respective industrial areas.
Intramolecular cyclizations involving oxypalladation as a key step provide useful entries to oxygen-atom containing heterocycles. The oxypalladation process is highly indispensable rout to synthesizing acetals or ketones. Surveyed in this article is recent progress relevant to these oxidations from both synthetic and mechanistic viewpoints, and particular attention will be paid to the role of molecular oxygen and copper salts in palladium catalysis.
Studies have been made to explore the use of chiral chelated lithium amides or their corresponding amines for enantioselective reactions. Some of our results are discussed in terms of enantioselective deprotonation of prochiral cyclic ketones, kinetic resolution of racemic 2-substituted cyclohexanones, regioselective deprotonation of optically active 3-keto steroids, enantioselective alkylation and protonation of achiral lithium enolates. Examples of catalytic asymmetric deprotonation and alkylation by the present strategy are also discussed.
A generally applicable method for the synthesis of the putative, ultimate active metabolites, the bay region syn-and anti-diol epoxides of carcinogenic polycyclic aromatic hydrocarbons has been established. The salient features of this novel and highly efficient method include a cycloaddition between an aryne and 3, 4-dibenzyloxyfuran and the stereo-and regioselective formation of bromo or chloro alcohols, where the halogen atom is placed in the bay region, from the ether-bridge opening of 1β, 4β-epoxy-2α, 3α-carbonyldioxy-1, 2, 3, 4-tetrahydrobenzo ring systems with boron tribromide or trichloride. Mechanistic considerations for the, latter step have been rationalized in terms of the relative stabilities of the tetrahydrobenzo bay and non-bay region benzylic carbocations which have been estimated by the calculations of the heats of formation through the use of the AM1 method. Moreover, these halo alcohol intermediates have been readily resolved into enatiomerically pure form through silica gel flash column chromatography of their chiral derivatives such as (S) -O-methylmandelate esters.
This account describes our synthesis of the gilvocarcin-class aryl C-glycoside antibiotics. Particular attention has been given to the strategies and tactics developed during the study, which include (1) regio-and stereocontrolled aryl C-glycoside formation, and (2) regioselective [4 + 2] cycloaddition of 2-methoxyfuran with a sugar-bearing benzyne. A new method for the efficient generation of benzyne species at low temperature is also discussed.