Synthesis and conformational study of chiral oxepines: the Baylis-Hillman reaction and RCM approach with sugar aldehyde.
The Baylis-Hillman reaction of 3-O-allyl-alpha-d-xylo-pentodialdo-1,4-furanose 3 afforded a diastereomeric mixture of d-gluco- and l-ido-configured alpha-methylene-beta-hydroxy esters 4a and 4b, respectively, in a ratio of 2:3. Reduction of the ester functionality in 4a/4b gave alcohols 5a/5b. The diene thus formed in 5a/5b was subjected to ring-closing metathesis (Grubbs' second-generation catalyst) to afford oxa-bicyclic ring system 6a/6b in high yield. Further manipulation of the acetonide functionality in 6a and 6b afforded new polyhydroxylated oxepines 1a/2a and 1b/2b, respectively. The (1)H NMR of oxepines 1a and 1b in D(2)O showed doubling of signals indicating their existence in two different rotamers/conformers. This fact was substantiated by calculating energetics of 1 and 2 conformers using the density functional theory and correlating the calculated (1)H NMR chemical shift pattern with that of the experimental spectra.
Jadhav
Vrushali H
VH
Garware Research Centre, Department of Chemistry, University ofPune, Pune 411 007, India.
Bande
Omprakash P
OP
Pinjari
Rahul V
RV
Gejji
Shridhar P
SP
Puranik
Vedavati G
VG
Dhavale
Dilip D
DD