ABSTRACT
Herein, a high-yielding cycloaddition reaction of ß-ketoesters and azides to provide 1,2,3-triazoles is described. The reactions employing 2-unsubstituted ß-ketoesters were found to provide 5-methyl-1,2,3-triazoles, whereas 2-alkyl-substituted ß-ketoesters provided 5-hydroxy-1,2,3-triazoles (shown to be relatively acidic) in high yields and as single regioisomers. Several novel compounds were reported and characterized including long-chain 5-hydroxy-1,2,3-triazoles potentially bioisosteric to hydroxamic acids.
Subject(s)
Azides , Triazoles , Cycloaddition ReactionABSTRACT
Herein, we present the first example of synthesis of 3,4-dihydropyran-2-ones from cinnamic thioesters via a stereoselective phase-transfer-catalyzed domino Michael-cyclization reaction with acetylacetone. The reaction proceeded under the catalysis of Cinchona-derived quaternary ammonium phenoxide that, in combination with inorganic bases, provided 3,4-dihydropyran-2-ones in yields of up to 93% and enantioselectivities of up to 88% enantiomeric excess.
ABSTRACT
Herein we report the first organocatalysed enantioselective synthesis of gingesulfonic acids and shogasulfonic acids via a mild and convenient aminothiourea-catalysed conjugate addition of bisulfite to the olefin moiety of α,ß-unsaturated carbonyls-a technology previously reported by us. A series of optically active naturally occurring sulfonic acids are prepared in their natural and unnatural configurations, and their absolute configurations are unequivocally confirmed by single crystal X-ray diffractometry.
ABSTRACT
An enantioselective protocol for the desymmetrization of cis-3,5- O-arylidenecyclohexanones has been developed that proceeded under the catalysis of readily available and inexpensive Cinchona-derived quaternary ammonium salts. The synthetic relevance of the methodology was exemplified by the synthesis of a key intermediate that could be used in the preparation of the active pharmaceutical ingredient, paricalcitol (Zemplar).
ABSTRACT
The energy barriers in our recently discovered Walden-type inversion of chlorophosphonium salts are similar to those for Cope rearrangements of caged cyclic hydrocarbons. Therefore, we have designed a molecular system that integrates the two processes, thereby producing the first embodiment of a chemical species that can undergo two entirely different and independent stereomutation mechanisms at the same nominal asymmetric center. Thus, the energy barrier to the rearrangement of 9-phenyl-9-phosphabarbaralane oxide, which is easily prepared by a new high-yielding synthesis, was found to be roughly 11â kcal mol-1 . This value is in contrast to the parent barbaralane (7.3â kcal mol-1 ) but in good agreement with our computational results for the rearrangement barriers. Crucially, in the corresponding chlorophosphonium derivative, two stereomutations occur simultaneously: a fast Cope rearrangement (barrier ≈12â kcal mol-1 ) and a slow Walden-type inversion of the phosphorus center (barrier ≈21â kcal mol-1 ). The computational model also revealed a relationship between the Cope rearrangement barrier and the bridgehead distance. The phenomenon of two independent and geometrically orthogonal stereomutations at a single asymmetric center provided important general insights into reaction pathway bifurcation, microscopic reversibility, and dynamic stereochemistry. This first example of coexisting alternative mechanisms that involve covalent bonds may encourage the design of new types of dynamic molecular structures.