The title compound 30 was synthesized starting from the endo,syn,endo Diels‐Alder adduct 3a of hexahydro‐5,6,7,8‐tetrachloro‐9,9‐dimethoxy‐5,8‐methanonaphthalene‐1,4‐diol diacetate 6 and 1,2,3,4‐tetrachloro‐5,5‐dimethoxycyclopentadiene (TDCp) in six steps, keyed upon a symmetry‐allowed [4+4] photocyclization of decahydro‐11,12‐dioxo‐[1,4;5,8]dimethanoanthracene‐9,10‐diol diacetate 22. The epimeric monoacetate 26 related to 22 was also synthesized and their thermolysis and photolysis were investigated. Oxidation of diol 30 afforded hexacyclic bridged hemiacetal 31 as a result of transannular reaction. The structure of hemiacetal 31 was analyzed by single‐crystal X‐ray crystallography. 相似文献
It is known that semigroups are Ramsey algebras. This paper is an attempt to understand the role associativity plays in a binary system being a Ramsey algebra. Specifically, we show that the nonassociative Moufang loop of octonions is not a Ramsey algebra. 相似文献
Chaotic maps have various properties that mirror the security requirements of cryptographic algorithms. As such, researchers have utilized them in the design of algorithms such as hash functions. Although there exist a wide range of chaos-based hash functions in literature, most of them are designed in an ad hoc manner rather than relying on well-established design paradigms. In addition, they are commonly implemented using floating-point operations which are inefficient as compared to their bitwise counterparts. The combination of convoluted designs and floating-point representation also leads to hash functions that are difficult to analyze; therefore, claims of security cannot be verified easily. These issues are some of the reasons why chaos-based hash functions have not seen widespread use in practice. This paper proposes a new unkeyed hash function based on a chaotic sponge construction and fixed-point arithmetic to overcome the aforementioned problems. The use of a sponge construction provides provable security justifications, whereas the use of fixed-point arithmetic allows chaotic map operations to be implemented using bitwise operations. The combination of these design elements leads to a design that is both efficient and facilitates future cryptanalysis for security verification. Security and performance evaluations indicate that the proposed hash function has near-ideal diffusion, confusion, collision resistance, and distribution properties in addition to a hashing speed that is at least on par with the current state of the art in chaos-based hash functions.
Extensive chromatographic isolation and purification of the extracts of the stem bark of Calophyllum inophyllum and Calophyllum soulattri have resulted in 11 xanthones. C. inophyllum gave inophinnin (1), inophinone (2), pyranojacareubin (5), rheediaxanthone A (6), macluraxanthone (7) and 4-hydroxyxanthone (8), while C. soulattri afforded soulattrin (3), phylattrin (4), caloxanthone C (9), brasixanthone B (10) and trapezifolixanthone (11). The structures of these compounds were determined on the basis of spectroscopic analyses such as 1D and 2D NMR, GC–MS, IR and UV. Cytotoxicity screening (MTT assay) carried out in vitro on all the xanthones using five human cancer cell lines indicated good activities for some of these xanthones. The structure–activity relationship study revealed that the inhibitory activities exhibited by these xanthone derivatives to be closely related to the existence and nature of the pyrano and the prenyl substituent groups on their skeleton. 相似文献
We investigate the class of σ-stable Poisson–Kingman random probability measures (RPMs) in the context of Bayesian nonparametric mixture modeling. This is a large class of discrete RPMs, which encompasses most of the popular discrete RPMs used in Bayesian nonparametrics, such as the Dirichlet process, Pitman–Yor process, the normalized inverse Gaussian process, and the normalized generalized Gamma process. We show how certain sampling properties and marginal characterizations of σ-stable Poisson–Kingman RPMs can be usefully exploited for devising a Markov chain Monte Carlo (MCMC) algorithm for performing posterior inference with a Bayesian nonparametric mixture model. Specifically, we introduce a novel and efficient MCMC sampling scheme in an augmented space that has a small number of auxiliary variables per iteration. We apply our sampling scheme to a density estimation and clustering tasks with unidimensional and multidimensional datasets, and compare it against competing MCMC sampling schemes. Supplementary materials for this article are available online. 相似文献
The space of totally real r-cycles of a totally real projectivevariety is embedded into the space of complex r-cycles by complexification.The holomorphic taffy argument in the proof of Lawson's suspensiontheorem is proved by using Chow forms, and this proof givesan analogous result for totally real cycle spaces. The Sturmtheorem is used to derive a criterion for a real polynomialsof degree d to have d distinct real roots, and this criterionis used to prove the openness of some subsets of real divisors.This enables us to prove that the suspension map induces a weakhomotopy equivalence between two enlarged spaces of totallyreal cycle spaces. 相似文献