- Asymmetric aldol additions: Use of titanium tetrachloride and (-)-sparteine for the soft enolization of N-acyl oxazolidinones, oxazolidinethiones, and thiazolidinethiones
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Asymmetric aldol additions using chlorotitanium enolates of N-acyloxazolidinone, oxazolidinethione, and thiazolidinethione propionates proceed with high diastereoselectivity for the Evans or non-Evans syn product depending on the nature and amount of the base used. With 1 equiv of titanium tetrachloride and 2 equiv of (-)-sparteine as the base or 1 equiv of (-)-sparteine and 1 equiv of N-methyl-2-pyrrolidinone, selectivities of 97:3 to >99:1 were obtained for the Evans syn aldol products using N-propionyl oxazolidinones, oxazolidinethiones, and thiazolidinethiones. The non-Evans syn aldol adducts are available with the oxazolidinethione and thiazolidinethiones by altering the Lewis acid/amine base ratios. The change in facial selectivity in the aldol additions is proposed to be a result of switching of mechanistic pathways between chelated and nonchelated transition states. The auxiliaries can be reductively removed or cleaved by nucleophilic acyl substitution. Iterative aldol sequences with high diastereoselectivity can also be accomplished.
- Crimmins,King,Tabet,Chaudhary
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p. 894 - 902
(2007/10/03)
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- Total synthesis of (+)-crocacin C
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The total synthesis of potent antifungal and cytotoxic agent (+)-crocacin C in optically pure form following a convergent strategy is described here in detail. The synthesis also established the absolute stereochemistry of the molecule having a (6S,7S,8R,9S) configuration. While C8 and C9 stereocenters were built following a Ti(IV)-mediated diastereoselective 'non-Evans' aldol reaction based on 2-oxazolidinethione chiral auxiliary, lithium dimethylcuprate opening of a C6-C7 chiral epoxide prepared by Sharpless epoxidation method established the remaining two stereocentres.
- Chakraborty, Tushar K,Jayaprakash, Sarva,Laxman, Pasunoori
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p. 9461 - 9467
(2007/10/03)
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- Synthesis of optically active α-phenylselenyl carbonyl derivatives
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Homochiral silyl enol ethers derived from N-acyl oxazolidinones can be phenyselenylated in high diastereoselectivity and good yield with phenylselenyl chloride. Removal of the chiral auxiliary is accompanied by some epimerisation, leading to isolation of
- Holmes,Nadin,O'Hanlon,Pearson
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p. 1289 - 1302
(2007/10/02)
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