Quinocarcin (1) isolated from Streptomyces melanovinaceus in 1983, has been the subject of recent synthetic endeavors due to its unique pentacyclic structure and potent antitumor activity against various types of mammalian cancers. In this article, synthetic studies on 1 and its related compounds including three independent total syntheses of 1 are reviewed with particular focus on their strategies. Furthermore, the structure-activity relationships of 1 are summarized, disclosing their novel aspects which might be useful for both elucidating the mode of action and developing more effective anticancer agents.
Resorcinol cyclic tetramer (calix  resorcinarene) binds hydroxylic guest compounds including sugars in apolar organic media. The driving force of this complexation is a combination of hydrogen bonding and CH-πinteraction. The latter interaction gives rise to not only a 1 : 1 host : guest stoichiometry but also a sizable selectivity arizing from the apolar alkyl moiety of the guests. Sugars and related guests can also be bound in water to tetrasulfonate derivatives of the host. The CH-π interaction again provides the major driving force. It is sensitive to the π-electron density of the aromatic cavity of the host. Thus, the guest (sugar) binding ability of the host is enhanced either by introduction of additional electron-donating OH groups on the benzene rings or upon deprotonation of the OH groups giving rise to anionic host. The CH-π interaction may be regarded as a kind of hydrogen bonding between π-base and σ-acid.
Reactions of low coordinated phosphorus compounds with dichlorocarbene and successive conversions of the resulting phosphiranes to phosphacumulenes are reviewed. Sterically protected diphosphenes reacted with dichlorocarbene to give the corresponding dichlorodiphosphiranes, which reacted with methyllithium to give 1, 3-diphosphaallenes. Similarly, the reaction of (E) - or (Z) -2-phenyl-1- (2, 4, 6-tri- t-butylphenyl) -1-phosphaethenes with dichlorocarbene afforded trans-phosphirane, which was converted to 1-phosphaallene. In some cases, the reactions of dichlorocarbene with 1-phosphaallenes, 1, 3-diphosphaallenes, or 1-phosphabutatrienes resulted in the formation of the rearranged products. Reactions of dichlorocarbene with other phosphorus species in low coordination states such as 1-aza-3-phosphaallene, 1, 4-diphosphabutatriene, or phosphaalkyne are also described.
The chiral ketone 1 was treated with potassium hydride and a series of alkyl halides in the presence of 18-crown-6 to furnish chiral alkylated ketones 2 in 4873% ee without any external chiral source. These findings contravene the accepted view that chirality at the α-carbon to carbonyl groups is lost in the corresponding enolates because the enolates themselves are achiral. To explain this phenomenon, we propose the novel concept that chirality at the α-carbon to the carbonyl group is memorized as dynamic axial chirality with respect to the C1-C2 bond in the intermediate enolate, and is then regenerated as central chirality in the reaction products (memory of chirality). This mechanism was supported by trapping the enolate as a methyl enol ether 11 of 65% ee. The present concept was successfully applied to the development of the asymmetric α-alkylation reactions of α-amino acids. Thus, phenylalanine derivative 22 was found to undergo asymmetric α-alkylation in up to 88% ee when treated with lithium 2, 2, 6, 6-tetramethylpiperidide followed by alkyl halides.
We have detected the unsymmetrical π faces of the carbonyl groups of spiro [cyclopentane] fluorenes and 2-substituted and 3-substituted dibenzobicyclo [2.2.2] octadienones (2-substituted and 3-substituted 9, 10- dihydro-9, 10- (11-ketoethano) anthracenes) and of the olefin groups in spiro [cyclopentene]fluorenes and 2-substituted dibenzobicyclo [2.2.2] octatrienes (2-substituted-9, 10-dihydro-9, 10-ethenoanthracenes), wherein σ-type overlaps of the π orbitals are involved, in a similar manner to spiro and longicyclic conjugations, respectively. An intrinsically nonequivalent substituent at distal positions modulates the epoxidation and dihydroxylation of the olefin group and the reduction of the carbonyl group. Herein we interpret these biases or non-biases in terms of unsymmetrization of π lobes of the olefin and carbonyl π orbitals, arising from nonequivalent π-π interactions rather than from electron-donating or -withdrawing effect.
Propynal-Co2(CO)6 complex, prepared from the reaction of the corresponding propynal with dicobaltoctacarbonyl, has been shown to be an excellent substrate for highly syn-selective aldol reactions with silyl enol ethers and silyl ketene acetal species under the Mukaiyama condition. On the basis of these stereoselective reactions, we have completed synthesis of some of biologically active compounds such as PS-5, blastmycinone, erythro-C18-sphingosine, and bengamide E. On the other hand, a novel way for construction of tetrahydropyrans and tetrahydrofurans has been developed by regio- and stereoselective ring formation of Co2(CO)6- complexed alkyne-epoxide through the 6-endo mode and 5-endo mode type ring closure, respectively.