A new chiral titanium species for the ring opening reactions of meso epoxides

Asymmetric catalysts

“…297 Other notable catalysts which have been employed in related reactions include gallium-BINOL complexes 301 and titanium-diol complexes derived from spiroketals. 298 In addition further results of the ingenious combinatorial-modification approach to the optimisation of a titanium(IV) complex for this application have been reported. 302 Kinetic resolution of racemic epoxides using asymmetric catalysis is a known process, however the use of only 0.2 mol% of a salen-cobalt complex serves to provide an excellent catalysis for the almost perfectly enantioselective ring opening of epoxypropene to a diol (Scheme 58).…”

“…Catalytic asymmetric ring opening of meso (usually cyclic) epoxides may be achieved through the use of a number of nucleophilic reagents. [297][298][299][300][301][302] These include azide, 297-299 carboxylic acids, 300 thiols 301 and cyanide anion. 302 Using a chromium-salen complex as catalyst, 1,2-epoxycyclohex-4-ene may be ring opened in 93% yield and 92% ee to yield a key intermediate in the synthesis of balanol.…”

Total Synthesis of Callyspongiolide, Part 2: The Ynoate Metathesis/cis‐Reduction Strategy

“…[2,3] The preparation of the required sulfone 10 commencedw ith the mono-silylation of but-2-en-1,4-diol (2), which gave good results on scale when carried out at low temperature (Scheme 2). Sharpless epoxidation of the resultingp roduct furnished compound 3 (93 % ee), [24,25] which underwent ah ighly selective hydroxy-directed ring opening with MeMgBr/CuI to give multigram quantities of diol 4 in excellent yield. [25] The derived p-methoxybenzylidene acetal was readily transformed into aldehyde 5,w hich proved rather sensitivea nd prone to epimerization.…”

“…Sharpless epoxidation of the resultingp roduct furnished compound 3 (93 % ee), [24,25] which underwent ah ighly selective hydroxy-directed ring opening with MeMgBr/CuI to give multigram quantities of diol 4 in excellent yield. [25] The derived p-methoxybenzylidene acetal was readily transformed into aldehyde 5,w hich proved rather sensitivea nd prone to epimerization. [26] Although 5 could be elaborated via Ta kai olefination [27,28] into an adequate alkenyl iodide, as necessary for the later attachment of the side chain of callyspongiolide, this reaction required al arge excesso fC rCl 2 ;m oreover, the subsequent reductive acetal opening with Dibal-H to give the targetedc ompound 8 also proved surprisingly troublesome.T herefore we pursued an alternative route to this compound, in which aldehyde 5 was first transformed into the corresponding terminal alkyne 6 on treatment with Bestmann-Ohira reagent [29] which had to be preactivated with NaOMe prior to additiono ft he aldehyde to avoid epimerization.…”

Synthesis and Conformational Analysis of meso-Ter(1,3-dioxan-4-yls)

“…75%, but this is not detrimental in a synthesis of a meso compound. To obtain the 1,3-diol 36, of several methods tested, [33] a copper(I)-catalysed Grignard addition [34] gave the highest (7:1) selectivity towards the formation of a 1,3-over a 1,2-diol. The diol 36 was then converted into the p-methoxybenzylidene acetal 37 (92%).…”