Elucidation of Racemization Process of Azaspirene Skeleton in Neutral Aqueous Media.

Azaspirene and related congeners, which possess various biological activities, have a unique spirocyclic core structure. However, there are few studies on the chemical properties of (-)-azaspirene, despite the fact that it may provide important insights into unveiling the biosynthetic pathway. Here, we report a nine-step chemical synthesis of an azaspirene analogue with a new finding that the natural (-)-azaspirene skeleton easily racemizes in neutral aqueous media.

Photoinduced electron transfer in photorobust coumarins linked with electron donors affording long lifetimes of triplet charge-separated states.

Electron donor (D) substituted 3-ethoxycarbonylcoumarin (CM) derivatives [D-CM: D = 4-diphenylaminophenyl (DPA), 4-diethylaminophenyl (DEA), 4-dimethylaminophenyl (DMA), and 2-methyl-4-dimethylaminophenyl (MeDMA)] are synthesized and characterized. Photoinduced electron transfer (ET) from the D moiety to the acceptor (CM) and back electron transfer (BET) are investigated by femtosecond and nanosecond laser flash photolysis measurements. Femtosecond laser excitation at 355 nm of a deaerated acetonitrile (MeCN) solution of D-CM shows generation of the singlet charge-separated (CS) state [(1)(D(.+)-CM(.-))] by ET from D to the singlet excited state of the CM moiety ((1)CM*), and this is followed by rapid decay within 3 ns to afford the triplet excited state (D-(3)CM*). Nanosecond laser excitation of a deaerated MeCN solution of D-CM results in formation of the triplet CS state by ET from D to (3)CM*. The quantum yield of formation of the triplet CS state [(3)(DPA(.+)-CM(.-))] in the presence of iodobenzene (PhI) in deaerated MeCN increases with increasing concentration of PhI to reach 27 % at 0.5 M PhI. The triplet CS state decays by bimolecular BET because of the long CS lifetime by unimolecular BET. Formation of the long-lived triplet CS state was confirmed by electron spin resonance (ESR) measurements. The photorobust nature of DPA-CM is demonstrated by multiple laser pulse excitation (>1000 times) at 355 nm. The photoinduced ET and BET rate constants of a series of D-CM are thoroughly analyzed in light of the Marcus theory of electron transfer.

Simultaneous production of p-tolualdehyde and hydrogen peroxide in photocatalytic oxygenation of p-xylene and reduction of oxygen with 9-mesityl-10-methylacridinium ion derivatives.

Photooxygenation of p-xylene by oxygen occurs efficiently under photoirradiation of 9-mesityl-2,7,10-trimethylacridinium ion (Me(2)Acr(+)-Mes) to yield p-tolualdehyde and hydrogen peroxide, which is initiated via photoinduced electron transfer of Me(2)Acr(+)-Mes to produce the electron-transfer state.

Synthetic studies on macroviracin A: a rapid construction of C(42) macrocyclic dilactone corresponding to the core.

The C(2)-symmetric macrodiolide core 2 of an antiviral agent, macroviracin A (1), was constructed in a single step by the intermolecular macrodimerization of C(22)-hydroxy carboxylic acid 3 with 2-chloro-1,3-dimethylimidazolinium chloride and DMAP in the presence of sodium hydride (NaH). The use of potassium hydride instead of NaH caused the intramolecular cyclization, predominantly providing the corresponding monomer 26. The acid 3 was synthesized through a series of reactions such as the coupling reaction of acetylene 5 and oxirane 6, stereoselective glycosidation with the trichloroacetimidate method, and Jones oxidation.