Biomedical subjects

Ramesh Jasti

Publications and source records attributed to Ramesh Jasti.

4 recordsLinked to original sources

PubMed2006

Racemization in Prins cyclization reactions.

Isotopic labeling experiments were performed to elucidate a new mechanism for racemization in Prins cyclization reactions. The loss in optical activity for these reactions was shown to occur by 2-oxonia-Cope rearrangements by way of a (Z)-oxocarbenium ion intermediate. Reaction conditions such as solvent, temperature, and the nucleophile employed played a critical role in whether an erosion in enantiomeric excess was observed. Additionally, certain structural features of Prins cyclization precursors were also shown to be important for preserving optical purity in these reactions.

Acetates↗

PubMed2006

Solvolysis of a tetrahydropyranyl mesylate: mechanistic implications for the Prins cyclization, 2-oxonia-cope rearrangement, and Grob fragmentation.

[reaction: see text] A solvolysis reaction is used to demonstrate that a tetrahydropyranyl cation is a common intermediate for Prins cyclizations, 2-oxonia-Cope rearrangements, and Grob fragmentations of tetrahydropyran rings.

Cyclization↗

PubMed2005

Utilization of an oxonia-Cope rearrangement as a mechanistic probe for Prins cyclizations.

An oxonia-Cope rearrangement was used as an internal clock reaction to probe the mechanism of the Prins cyclization reaction and the subsequent nucleophilic capture of the resultant tetrahydropyranyl cation. The oxonia-Cope rearrangement was shown to occur rapidly under typical Prins cyclization conditions when the oxocarbenium ion resulting from the rearrangement is similar to or lower in energy than the starting oxocarbenium ion. Oxonia-Cope rearrangements can be disfavored by destabilizing the resultant oxocarbenium ion or by stabilizing an intermediate tetrahydropyranyl cation. Stereoselectivity in the nucleophilic capture was dramatically affected by the reactivity of the nucleophile and electrophile. More reactive partners combined rapidly to give axial-substituted Prins products through a least-motion pathway. High selectivity for the equatorial-substituted tetrahydropyran was observed for less reactive nucleophiles and electrophiles.

Acetates↗

PubMed2004

Axial-selective prins cyclizations by solvolysis of alpha-bromo ethers.

Prins cyclizations are intramolecular electrophilic additions of oxocarbenium ions. They lead to tetrahydropyrans with a heteroatom at the 4-position, and usually show moderate-to-high selectivity for equatorial substitution. We have found that Prins cyclizations carried out under specific conditions produce tetrahydropyrans with almost exclusive formation of the axial 4-substituent. TMSBr, AcBr, and TMSI all lead to axial-selective Prins cyclizations with alpha-acetoxy ether substrates in the presence lutidine. The mechanism appears to involve solvolysis of the intermediate alpha-bromo ether rather than specific or Lewis acid-catalyzed rearrangement. The scope of the reaction, the high yields, and the stereoselectivity make this a valuable new method for tetrahydropyran formation.

Allyl Compounds↗