Highly Stereoselective Aldol Reaction Based on Titanium Enolates from (<i>S</i>)-1-Benzyloxy-2-methyl-3-pentanone

Solsona, Joan G.; Nebot, Joaquim; Romea, Pedro; Urpı́, Fèlix

doi:10.1021/jo050792l

Cited by 44 publications

(21 citation statements)

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“…Table 6.3, entries 12-28). The latter are more commonly employed, in part because of the increased expense and technical challenge associated with the use of Sn(II) enolates [75]. Judging from the results obtained with Paterson's dipropionate equivalent 20 (Scheme 6.14 and Table 6.3), it can be anticipated that reactions of (S)-syn-(Z)-8a (e.g., ML n = SnOTf) with (S)-12 should be ''matched'' and give the 49ssa adduct.…”

Section: Tin Enolatesmentioning

confidence: 99%

“…reactions of the Ti(IV) (Z)-enolate (Z)-21a (ML n = TiCl 3 ), prepared by reaction of 20a with TiCl 4 /Et 3 N, were much less stereoselective [73c],[75], perhaps because of less favorable intramolecular coordination in the Ti(IV) enolate compared to the Sn(II) enolate. Interestingly, Urpí et al[75] found that the analogous Ti(IV) enolate (Z)-21a (ML n = Ti(Oi-Pr)Cl 2 ), prepared by reaction of 20a with Ti(Oi-Pr)Cl 3 /Et 3 N, showed excellent stereoselectivity.…”

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confidence: 99%

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Polypropionate Synthesis via Substrate‐Controlled Stereoselective Aldol Couplings of Chiral Fragments

Ward

2013

Modern Methods in Stereoselective Aldol Reactions

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Section: Tin Enolatesmentioning

confidence: 99%

mentioning

confidence: 99%

Polypropionate Synthesis via Substrate‐Controlled Stereoselective Aldol Couplings of Chiral Fragments

Ward

2013

Modern Methods in Stereoselective Aldol Reactions

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“…The authors used different titanium reagents and evaluated the selectivity of the aldol reactions between ethyl ketones ent- 15,11, and 20 and isobutyraldehyde (2). The best results were obtained for ethyl ketone ent-15 (R = Bn) using Ti(iPrO)Cl 3 as the Lewis acid (Table 5.1, entry 3) [19].…”

Section: 4-asymmetric Induction Using α-Alkyl Ketonesmentioning

confidence: 99%

“…The partially matched reaction involving aldehyde ent-253 (case C) was the most interesting of the stereochemical permutations because the enolate facial bias established a matched relationship with the aldehyde β-heteroatom and a mismatched relationship with respect to the normally dominant aldehyde α-stereocenter. Felkin control was overridden in this reaction to produce the anti-Felkin aldol adduct 331 (dr = 81 :19). It is difficult to explain the stereochemical outcome of this reaction without invoking the β-heteroatom as a stereochemical determinant.…”

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Asymmetric Induction in Aldol Additions

Dias¹,

Polo²,

Lucca³

et al. 2013

Modern Methods in Stereoselective Aldol Reactions

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The selectivity of aldol reactions involving kinetic enolates is related to various factors such as the nature of the Lewis acid (Li, Mg, B, Al, Ti, Sn, etc.), the presence of stereogenic centers in both the substrates and reagents, the nature of their substituents and the reaction conditions. Another factor of major importance is the nature of the enolate double bond (Z or E) [1].In general, it is well known that in aldol reactions involving metal enolates, which proceed through a six-membered cyclic transition state, the Z-enolate leads to the formation of 1,2-syn aldol adducts and the E-enolate affords aldol adducts with the 1,2-anti configuration. A rationalization for the observed selectivities of these aldol reactions was proposed by Zimmerman and Traxler (Scheme 5.1) [2]. According to this model, the nature of the double bond geometry plays a primary role in determining the energies of the competing transition states. For Z-enolates, the transition state TS2 is more destabilized than transition state TS1 because of the 1,3-diaxial interactions between the R 1 and R 3 substituents as well as between these substituents and one of the metal ligands (L). Therefore, the Z-enolate preferentially leads to the formation of the 1,2-syn aldol adduct.For the E-enolate, 1,3-diaxial interactions between the R 1 and R 3 substituents and between both R 1 and R 3 and one of the metal ligands are present in transition state TS4. Therefore, the 1,2-anti aldol adduct is obtained from the lower energy transition state TS3.Modern aldol reactions use preformed enolates to obtain good yields by preventing side reactions. This methodology has proven to be very effective for various Lewis acids derived from lithium, titanium, tin, and boron. By using preformed enolates with chiral substrates and reagents, it is possible to obtain aldol adducts with an excellent degree of asymmetric induction.The IUPAC defines asymmetric induction as the traditional term describing the preferential formation in a chemical reaction of one enantiomer (or diastereomer) over the other as a result of a chiral feature present in the substrate, reagent, catalyst, or the environment [3].

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Trichlorotitanium Isopropoxide

Romea

Urpı́

2008

Encyclopedia of Reagents for Organic Synthesis

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Highly Stereoselective Aldol Reaction Based on Titanium Enolates from (S)-1-Benzyloxy-2-methyl-3-pentanone

Cited by 44 publications

References 25 publications

Polypropionate Synthesis via Substrate‐Controlled Stereoselective Aldol Couplings of Chiral Fragments

Polypropionate Synthesis via Substrate‐Controlled Stereoselective Aldol Couplings of Chiral Fragments

Asymmetric Induction in Aldol Additions

Trichlorotitanium Isopropoxide

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