Anft -aldol products

Ghosh also took advantage of the C—2 hydroxyl moiety of aminoindanols as a handle in the aldol reaction. Chiral sulfonamide 41 was O-acylated to give ester 42. The titanium enolate of ester 42 was formed as a single isomer and added to a solution of aldehyde, precomplexed with titanium tetrachloride, to yield the anft -aldol product 43 in excellent diastereoselectivities.63 One additional advantage of the ester-derived chiral auxiliaries was their ease of removal under mild conditions. Thus, hydrolysis of 43 afforded a ft -a-methyl- 3-hydroxy acid 44 as a pure enantiomer and cis-1-/ -1 o I y I s u I f on a m i do- 2 - i n da n ol was recovered without loss of optical purity (Scheme 24.7).63... 

The chiral sulfonamide 45, which can be prepared in two steps from commercially available (I R,2S)-cis-1 -amino-2-indanol (44), was introduced by Ghosh and Onishi for the synthesis of enantiomerically pure anft -aldol products via titanium enolate19a (Scheme 2.2q). 

Only 5 mol% of the highly reactive aminophosphonate 132a was needed in the aldol reaction between cyclic ketones (3, 14 and 48b, 2 equiv.) with aldehydes to afford the corresponding anft-aldol products 4, 15 and 49 in moderated yields (36-79%), diastereoselectivities (0-80% de), and excellent enantioselectivities (89-98%)... 

Acylation, Alkylation, and Aldolization (Acyl Species-+ a-, P-, or a/fi-Functionalized Acyl Product) Alkylation reactions of sodium enolates of various lV-acyl-a-methyltoluene-2,a-sultams with selected (both activated and nonactivated ) alkyl iodides and bromides proceed with good C(a)-re stereocontrol (90-99% de). Analogous acylations with various acid chlorides can also be performed, giving p-keto products (97-99% de). Selective reduction of these latter products with Zinc Borohydride (chelate controlled, 82.6-98.2% de) or N-Selectride (nonchelate controlled, 95.8-99.6% de) can provide syn- and anft-aldol derivatives, respectively. ... 

X=2-HC02CgH -l-C02) was used in aldol reaction between cyclohexanone and aromatic aldehydes aldol products 4 mainly as anft-isomer were obtained in moderate results (66-94% yield, 23 0% de, 50-95% ee) [223]. Catalyst 135a was recovered and reused six-fold obtaining similar enantiomeric excess but lower yields and diastereoselectivities. 

Diastereoselective synthesis of lactones.1 Acylation of the enolate (LDA) of the vinylogous urethane (1) results in a product (2) that on reduction with LiBH[CH(CH3)C2H5]3 (3) forms the anft-lactone (4) exclusively (equation I). This two-step synthesis of lactones is the equilvalent of an aldol condensation between... 

The directed aldol reaction in the presence of TiC found many applications in natural product synthesis. Equation (7) shows an example of the aldol reaction utilized in the synthesis of tautomycin [46], in which many sensitive functional groups survived the reaction conditions. The production of the depicted single isomer after the titanium-mediated aldol reaction could be rationalized in terms of the chelation-controlled (anft-Felkin) reaction path [37]. A stereochemical model has been presented for merged 1,2- and 1,3-asymmetric induction in diastereoselective Mukaiyama aldol reaction and related processes [47]. 

Depending on the absolute configuration of the chiral component in the aldol reaction, the prevailing enantiomer wiU be formed as a syn- or anft -product, depending on the structure of the enolate. The chiral component can be either one of the reactants or catalyst. In the first case we have non-catcdytic asymmetric aldol reaction, in the second a catalytic asymmetric aldol reaction. The configuration at the chiral center in the chiral component determines the direction of enantiose-lectivity. Both cases are discussed in the following sections. 

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