Enantioselective Synthesis of the Chromane Moiety of Vitamin E

Abstract

Several new approaches for the enantioselective synthesis of the chromane moiety of vitamin E are described. Sonogashira coupling of 3a with the alkyne 4 and subsequent elimination gave 6, which was bis(hydroxylated) in 93% yield and with 85% ee. Recrystallization gave enantiopure 7a, which was hydrogenated and transformed into the vitamin E precursor 11. The bis(hydroxylation) of 18 and 21 to give 9 and 22, respectively, was less than satisfactory, proceeding with ee values of 28% and 18%. In contrast, stereoselective allylation of ketone 15 followed by removal of the protecting group or ozonolysis of the allyl moiety furnished the allyl alcohol 26 and the aldehyde 27, respectively, in almost enantiopure form, which again could be used as precursors for vitamin E. Partial hydrogenation of 5a gave the alkene 32a and that of 28 the alkene 30b, both of which show interesting atropisomerism.

Palladium-catalysed reactions of alcohols. Part B: Formation of C–C and C–N bonds from unsaturated alcohols

“…5 and 6), [18][19][20][21][22] they have been mostly performed using stoichiometric amounts of ammonium halides in organic solvents (Table 1, entry 4; Eq. 7) [23][24][25][26][27][28][29][30][31][32][33][34][35] or catalytic amounts of n-Bu 4 NCl in water (Table 1, entry 5; Eq. 8).…”

α‐Oxygenated Crotyltitanium and Dyotropic Rearrangement in the Total Synthesis of Discodermolide

“…After protection of the C7 hydroxy group, regeneration of the primary alcohol at C1, and oxidation under TEMPO/PhI(OAc) 2 conditions, the crude aldehyde was treated with sodium chlorite26 to afford a carboxylic acid, which was readily converted into its methyl ester with TMS‐diazomethane 27. The partial reduction of the triple bond at C8–C9 was realized with PtO 2 28. Finally, iododestannylation gave the required vinyl iodide 18 in 30.6 % overall yield for nine steps.…”

Enantioselective transesterification of (±)-6-benzyloxy-2,5,7,8-tetramethyl-3,4-dihydro-2H-1-benzopyran-2-ylmethanol catalyzed by the Amano PS lipase in the ionic liquid [bmim]PF6

“…In the last few de cades, homochiral chromane building blocks for toco pherols and tocotrienols have been actively synthesized by asymmetric epoxidation of allylic alcohols and the Sharp less dihydroxylation of enynes in combination with cross coupling reactions of iodoarenes with chiral diols in the presence of palladium complexes as catalysts. [16][17][18][19] An ef ficient and versatile synthetic strategy for the design of homochiral chromanes that involves palladium catalyzed asymmetric allylic alkylation of phenols with allyl car bonates has been developed by various research teams. [20][21][22] With the chiral ligands and catalysts found, the target chromanes have been obtained with >97% ee.…”