We show that a very large enhancement of third-order optical nonlinearity ͑␥͒ of -conjugated molecules can be realized without a major redshift of the absorption spectrum that disturbs optical transparency in the visible region. By changing the number ͑n͒ of CϭC bonds of ␤ carotene ͑n =11͒ from 7 to 15, a remarkable 3.4-fold increase in the ␥ value was observed when n = 15 relative to that of ␤ carotene. This enhancement of ␥ mainly originates from three-photon resonance of a lowest optically allowed excited state. The controversial higher-lying essential state is not important for generating the large value of ␥.

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The enantioselective total syntheses of lepadiformine marine alkaloids, azatricyclic natural products isolated from marine tunicates, were completed. These alkaloids have a unique chemical structure characterized by the trans-1-azadecalin (AB ring system) fused with the spirocyclic ring (AC ring system). Here we found that a cycloisomerization reaction from functionalized linear substrates to a 1-azaspiro[4.5]decane framework corresponding to the AC ring in lepadiformines is promoted by a catalytic amount of mercury(II) triflate (Hg(OTf) ). The total syntheses of (-)-lepadiformines A and B were achieved in 28 % and 21 % overall yields, respectively, through the novel cycloisomerization reaction. The syntheses of (+)- and (-)-lepadiformine C hydrochloride salts also enabled us to determine the absolute configuration of natural lepadiformine C. It has been found that a phenomenon of enantiodivergence occurs in lepadiformine alkaloids from a single species of marine tunicate, Clavelina moluccensis. The cytotoxic activities of synthesized lepadiformine hydrochloride salts and their synthetic intermediates were evaluated.

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Synthesis and Biological Activities of Degraded Limonoids, (±)-Fraxinellonone and Its Related Compounds.-The first synthesis of title compound (II) is achieved starting from keto lactone (I) and transformed to its analogues isofraxinellone (III) and fraxinellone (IV) (no experimental details given). The products are found to exhibit moderate insect-antifeeding activity and ichthyotoxic effects. -(OKAMURA, H.; YAMAUCHI, K.; MIYAWAKI, K.; IWAGAWA, T.; NAKATANI, M.; Tetrahedron Lett. 38 (1997) 2, 263-266; Dep. Chem., Fac. Sci., Kagoshima Univ., Kagoshima 890, Japan; EN)