关键词： pentalenolactones E and F   asymmetric synthesis   selenide elimination   Kauffmann methylenation   natural products  

摘要： A formal asymmetric synthesis of pentalenolactone E (1b) and pentalenolactone F (1a) has been accomplished. Ozonolysis of the diphenyl-substituted triquinane 3 and Kauffmann methylenation of ketone 5 with WOCl3/2 MeLi yielded the unsubstituted triquinane 9. The crucial rearrangement of the linear triquinanoid lactone 11 to the angular triquinanoid lactone 14a was accomplished using orthoformate and acid in methanol. Subjecting triquinanes 14a/b to the selenoxide method gave triquinene 15. Homologation of gamma-lactone 15 to the angular diquinanoid delta-lactone 2 via a Horner-Wadsworth-Emmons or Peterson reaction of hemiacetals 16a/b was, however, not successful. Chemoselective reduction of 14a afforded hemiacetals 21a/b, reaction of which with the phosphonate salt 17a ultimately led to the ketene dithioacetal 22. The angular intermediates 25a/b were obtained from 22 by reduction to give the linear hemiacetals 24a/b, which rearranged to the dithio ortholactones 25a/b in the presence of acid. Introduction of the double bond and deprotection were accomplished via selenation of 25a/b with N,N-diethylbenzeneselenylamide and treatment of selenides 30a/b with silver nitrate. The unsaturated aldehydes 28 and 29 thus obtained were converted to 2 and 31, respectively, by oxidation with manganese dioxide in the presence of sodium cyanide, methanol and acetic acid. Alkene 2 was isolated by crystallization.

关键词： complexes   aldol reaction   enolate   Catalysts   Complexes   Palladium   Asymmetric synthesis   X-ray crystallography  

摘要： A catalytic asymmetric aldol reaction proceeding via a chiral palladium(II)) enolate is described. The novel chiral diaquo palladium(II) complexes of (R)-BINAP (9a), and (R)-Tol-BINAP (9b) were prepared in good yield, and their structures were determined by X-ray crystallography. These air- and moisture-stable complexes were found to be excellent catalysts for the asymmetric aldol reaction, and up to 89% ee of asymmetric induction was achieved using these catalysts.

关键词： SAMP-Hydrazone   Diastereoselective alkylation   3-Substituted 4-oxo esters   Umpolung   1,4-Dicarbonyl compounds  

摘要： Based on aldehydes 1a—f or ketone 1g, 3-substituted 4-oxo esters 6a—g were synthesized in three steps in moderate to good overall yield (12—50%) and in excellent enantiomeric excesses (ee >90—>95%) by an Umpolung-strategy employing the SAMP-/RAMP-hydrazone method. The key step in the synthesis is the highly diastereoselective alkylation of lithiated SAMP-hydrazones 3a—g (chiral d2-nucleophiles) with tert-butyl bromoacetate (4) (a2-electrophile) to furnish the alkylated 3-hydrazono tert-butylesters 5a—g in good yields (58—91%) and in excellent diastereomeric excesses (de >90—>98%). Regeneration of the carbonyl functionality by cleavage of the hydrazones 5a—g was accomplished either by acidic hydrolysis or ozonolysis to give the acid labile and oxidation-sensitive 3-substituted 4-oxo esters 6a—g in moderate yields (19—63%) and in excellent enantiomeric excesses (ee >90—>95%). The absolute configuration of compounds 6a—g were assigned by conversion of 4-oxo esters (S)-6d and (R)-6e into their corresponding known 3-substituted λ-butyrolactones (S)-7d and (R)-7e.

关键词： 季碳   不对称合成   天然产物   全合成   合成  

摘要： 综述了季碳的不对称构筑法及其在天然产物全合成中的应用。按照合成反应不同，分７类加以介绍。参考文献２４篇。

关键词： 手性配基   联二萘   光学活性药物   不对称合成   对映体过量百分率  

摘要： 综述了手性配基联二萘及其类似物在光学活性药物合成中的研究进展及应用，包括对Ｃ＝Ｃ、Ｃ＝Ｏ双键的不对称催化氢化，Ｄｉｅｌｓ－Ａｌｄｅｒ反应，Ａｄｏｌ缩合，加氢甲酰化等。

关键词： 固态反应   溶液反应   选择性   不对称合成   有机反应  

摘要： 总结了一些典型的固态有机反应。由于固态反应的特性,多数固态有机反应表现出较溶液中更高的反应效率和更好的选择性,在某些情况下,利用光活性主体与反应物形成包结物,可实现对映选择性的不对称固态合成。

关键词： QD431.2A6L2  

摘要： 3-Methylaspartate ammonia-lyase is a bacterial enzyme that catalyses the reversible elimination of ammonia, in the presence of monovalent and divalent metal ions, from (2S,3S)-3-methylaspartic acid to yield mesaconic acid. The catalytic mechanism of the wild type enzyme obtained from Clostridium tetanomorphum involves a concerted elimination of ammonia and is thought to operate via the intermediacy of a dehydroalanine residue at the active site. No direct evidence has been established so far concerning the existence of this dehydroalanine residue, except that 3-methylaspartate ammonia-lyase was found to be irreversibly inactivated by phenylhydrazine. Inactivation of the enzyme with labelled ¹⁵Nβ-phenylhydrazine and subsequent ¹⁵N-edited NMR spectra of the digested enzyme resulted in a proposal for the inhibition mechanism by phenylhydrazine. This mechanism involved the conjugate addition of phenylhydrazine onto the dehydroalanine moiety and the subsequent cyclisation to give a cyclic adduct. A synthetic model containing a ¹⁵N atom of this adduct was synthesised and the NMR data obtained were compared with the spectra obtained earlier from the enzyme experiment. In order to carry out mapping studies of the active site it was highly desirable to prepare molecules capable of reacting specifically and irreversibly at the active site. To this end, the synthesis of aminofumaric acid and (2S,3S)-3-methyl N-amino aziridine dicarboxylic acid were attempted. (2S,3R) and (2R,3R)-3-Methylsuccinic acids were prepared in 5 steps from (2S,3S)-3-methyl aspartic acid. These compounds were found to behave as noncompetitive inhibitors of β-methylaspartase. (2S)-3,3-Dimethylaspartic acid was also synthesised and found to be a competitive inhibitor having the same binding affinity as the natural substrate. In order to prepare a β-electrophilic amino acid analogue of the physiological substrate, an asymmetric synthesis was designed that involved the stereoselective alkylation and acylati

摘要： Asymmetric Diels-Alder reaction of the pentadienoic and hexadienoic acids 2a,b with the chiral chloronitroso derivative 3 gave the primary adducts 4a,b with good-to-excellent enantioselectivity. Subsequent cis- or Irans-dihydroxylation and hydrogenolytic cleavage of the N-O bond led to the 5-amino-5-deoxy-pentono-delta-lactams 13a, 14, 15a, and 16 in the D-ribose, L-arabinose, D-xylose, and L-lyxose series, respectively, and to the 5-amino-5,6-dideoxyhexono-delta-lactams 13b and 15b in the D-allose and D-glucose series, respectively.

摘要： The cyclopropanation reactions of (1a) with ethyl and t-butyl (dimethylsulfuranylidene) acetate proceeded with good diastereoselectivity and resulted in the formation of three diastereoisomeric products. The major diastereoisomeric product ((2a) and (2b), respectively) could be isolated in pure form by simple recrystallization. The stereochemistry of the major cyclopropane product (2b) has been determined by single-crystal X-ray structural analysis. These cyclopropanation products were susceptible to ring opening of the cyclopropane ring upon reduction with sodium borohydride or acid hydrolysis. The reaction of (1b) with ethyl (dimethylsulfuranylidene) acetate gave a mixture of four diastereoisomeric cyclopropanation products. The reactions of (1a) and (1b) with the sulfur ylide derived from 3-methoxycarbonylallyldimethylsulfonium bromide were less successful in terms of product diastereoselectivities.

关键词： alkylations   asymmetric synthesis   chiral auxiliaries   chirotopicity   deprotonation  

摘要： Pseudo-C 2 -symmetrical ligands have been prepared efficiently: The attachment of the chiral alkyl group to the heteroatom (P or N) through a nonstereogenic, chirotopic carbon center facilitates their synthesis as the configuration at this carbon atom no longer needs to be controlled. Two such ligands were combined, for example, in the base 1, which is especially useful for asymmetric deprotonation of prochiral ketones [Eq. (a)].◆ (© 1998 WILEY-VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany.)