Superior substrate control on diastereoselection in boric Lewis acid-promoted aldol reactions. Asymmetric synthesis of a 3,4-syn homologous series of ethyl 3,5-dihydroxy-2,4-dimethyl-5-phenylpentanoates

Stereoselective Construction of Entire Diastereomeric Stereotetrads Based on an Asymmetric Morita–Baylis–Hillman Reaction

“…The stereostructures of these products were unambiguously determined as shown in Scheme . Compounds 17a and 17b were converted into the known diols 19a and 19b , respectively, the spectroscopic data and specific rotations of which were in good agreement with those reported . For the other isomers 17c – 17h , the stereochemistries of the C‐3 hydroxyl groups were determined by using Rychnovsky's method, after converting 17d , 17f , and 17h into the corresponding acetonides 20d , 20f , and 20h .…”

Diastereoselective radical debromination approach toward divergent syntheses of syn- and anti-propionate units, coupled with enantioselective and/or diastereoselective Lewis acid-promoted aldol reactions

“…However, the enantioselective aldol reaction did not work sufficiently well on the enantioselective diastereoselection in the iterative construction of the bulky propionate system, having alternative protectedoxy and methyl groups. 9 We then planned a diastereoselective approach using the classical Mukaiyama aldol reaction as an alternative to the enantioselective aldol reaction. Excellent syndiastereoselection (Felkin-Anh control) has been recognized in the BF 3 ·OEt 2 -mediated aldol reaction of a-methyl-b-siloxy aldehydes with a silylketene acetal from ethyl propionate.…”

“…After evaporation of the solvent, the residue was purified by flash-column chromatography (15% AcOEt in hexane) to afford 42 (91 mg, 85%). [h] 28 D =−44.6 (c 0.56, CHCl 3 ); IR (neat): 3489, 2980, 1730 cm −1 ; 1 H NMR (CDCl 3 ): l (ppm)=0.84 (d, J=7.0 Hz, 3H), 1.22 (t, J=7.3 Hz, 3H), 1.29 (d,J=6.8 Hz,3H),1.73 (ddq,J=2.2,3.4,7.0 Hz,1H),2.61 (dq,J=6.8,9.0 Hz,1H), 3.30 (s, 3H), 3.46 (d,J=1.7 Hz,1H),3.98 (ddd,J=1.7,2.2,9.2 Hz, 1H), 4.10 (ABq in ABX 3 , J=7. 3,10.8 Hz,Dw=9.3 Hz,2H),4.43 (d,J=3.4 Hz,1H), (m, 5H); 13 C NMR (CDCl 3 ): l (ppm)=5.…”

“…[h] 24 D =+1.4 (c 1.44, CHCl 3 ); IR (neat): 3529, 2935, 1732 cm −1 ; 1 H NMR (CDCl 3 ): l (ppm)=0.97 (d, J=6.8 Hz, 3H), 1.07 (s, 9H), 1.25 (t, J=7.1 Hz, 3H), 1.89 (s, 3H), 1.87-2.07 (m, 1H), 2.65 (d, J=4 9. Hz, 1H),3.52 (dd, J=9.8, 5.9 Hz, 1H) [one of ABq in ABX],3.58 (dd, J=9.8, 6.8 Hz, 1H) [one of ABq in ABX],4.18 (dq, J=9.7, 7.3 Hz, 2H), 4.28 (dd, J=4.9, 2.5 Hz, 1H), (m, 6H), 7.63-7.66 (m, 4H); 13 C NMR (CDCl 3 ): l (ppm)=11.0,…”

Highly π‐Facial Stereoselective Aldol Reaction of (S)‐Proline‐Derived Amide Enolate with Benzaldehydes.

“…The stereocontrol at the exocyclic stereogenic site (the C-3Ј position in the aldol) is high, due to the chelating effect of the boron functionality, resulting in only one isomer being found. [10,11] Presumably, the π-facial stereoselectivity of the aldol reaction with a cyclic enolate may be improved if the six-membered ring of the enolate is made conformationally more rigid and substituents with effective steric interactions are introduced. Evidently, for this aldol process, the two aryl groups are not sufficiently effective in preferentially shielding one face of the double bond in the boron enolate.…”