Wu, Yikang published the artcileEnantioselective Total Synthesis of (+)-Brefeldin A and 7-epi-Brefeldin A, Application In Synthesis of 171877-39-7, the main research area is asym synthesis brefeldin a epibrefeldin stereoselective aldol chiral auxiliary; chiral auxiliary oxazolidinone thiazolidinethione stereoselective aldol reaction protecting group; stereoselective reduction elimination solvent effect asym synthesis brefeldin a; intramol Mukaiyama aldol condensation stereoselective Michael asym synthesis brefeldin.
A convergent enantioselective route to brefeldin A (I) and 7-epi-BFA was developed. The key C-4/C-5 chiral centers were established by using chiral auxiliary induced intermol. asym. aldolization in the presence of TiCl4 and TMEDA. The results with the thiazolidinethione/TiCl4 mediated intermol. asym. aldolization added some new information about the scope and limitations to the existing knowledge of that type of reactions (which so far was essentially limited to the reactions with N-propionyl thiazolidinethiones). During the course a method for protecting the liable aldol hydroxyl groups by using inexpensive TBSCl in DMF with 2,6-lutidine as the base was developed to replace the otherwise unavoidable TBSOTf procedure. Due to the excessive steric hindrance, removal of the auxiliary was much more difficult than most literature cases. Cleavage of the oxazolidinone by reduction was almost impossible. The thiazolidinethione auxiliary was relatively easier to remove. However, several reactions reported for facile removal of thiazolidinethione auxiliaries in the literature still failed. Reductive removal of the thiazolidinethione auxiliary was most effectively realized with LiBH4 in di-Et ether in the presence of 1 equiv of MeOH (a modification of a literature procedure for removal of oxazolidinone auxiliaries in less hindered substrates). Apart from the auxiliary removal, oxidation of the alc. into aldehyde and the deprotection of the dithiolane protecting group were also rather difficult in the present context. A range of methods were screened before final solutions were found. The five-membered ring was constructed by employing an intramol. Mukaiyama reaction after many attempts with the intramol. aldolization under a variety of conditions failed. The rate of elimination of the alkoxyl to form the α,β-double bond of a key intermediate cyclopentenone with DBU was highly solvent dependent (very sluggish in CH2Cl2 but rather fast in MeOH). Introduction of the lower chain (which was synthesized by using a Jacobsen KHR to establish the C-15 chirality) was achieved through a Michael addition similar to the precedents in the literature. It has not been noticed before that the yield of this Michael reaction could be dramatically raised by using 3 equiv of the copper-lithium reagent. Reduction of the C-7 carbonyl was apparently more difficult than similar cases in the literature. After examination of many reagents under various conditions, it was found that the best reagent for yielding the α-isomer was (S)-2-methyl-CBS-borolidine/BH3 and that for the β-isomer was L-Selectride. The α- and β-isomers were then further elaborated into (+)-brefeldin A and 7-epi-BFA, resp. An unexpected yet very interesting solubility difference between BFA and 7-epi-BFA was also observed
Journal of Organic Chemistry published new progress about Aldol condensation, stereoselective. 171877-39-7 belongs to class thiazole, name is (S)-4-Benzylthiazolidine-2-thione, and the molecular formula is C10H11NS2, Application In Synthesis of 171877-39-7.
Referemce:
Thiazole | C3H3NS – PubChem,
Thiazole | chemical compound | Britannica