Erythro-threo ratios

Alkylation of aldol type educts, e.g., /3-hydroxy esters, using LDA and alkyl halides leads stereoselectively to erythro substitution. The erythro threo ratio of the products is of the order of 95 5. Allylic and benzylic bromides can also be used. The allyl groups can later be ozonolysed to gjve aldehydes, and many interesting oligofunctional products with two adjacent chiral centres become available from chiral aldol type educts (G. Prater, 1984 D. Seebach, 1984 see also M. Nakatsuka, 1990, p. 5586). 

This chemical bond between the metal and the hydroxyl group of ahyl alcohol has an important effect on stereoselectivity. Asymmetric epoxidation is weU-known. The most stereoselective catalyst is Ti(OR) which is one of the early transition metal compounds and has no 0x0 group (28). Epoxidation of isopropylvinylcarbinol [4798-45-2] (1-isopropylaHyl alcohol) using a combined chiral catalyst of Ti(OR)4 and L-(+)-diethyl tartrate and (CH2)3COOH as the oxidant, stops at 50% conversion, and the erythro threo ratio of the product is 97 3. The reason for the reaction stopping at 50% conversion is that only one enantiomer can react and the unreacted enantiomer is recovered in optically pure form (28). 

Akiyama, T. Goto, H. Nawawi, D. S. Syafii, W. Matsumoto, Y. Meshitsuka, G. Erythro/threo ratio of P-O-4-structures as an important structural characteristic of lignin. Part 4 variation in the erythro/threo ratio in softwood and hardwood lignins and its relation to syringyl/guaiacyl ratio. Holzforschung 2005, 59, 276-281. 

Amide Aldehyde Conditions Erythro-Threo Ratio Ref. 

Enolate L,LiBOTf Solvent Erythro-Threo Ratio... 

Enolate (M) R3CHO Erythro-Threo Ratio" Yield (% 65)... 

Enolsilane Aldehyde [at r (°C)] Erythro-Threo Ratio Yield (%)... 

Ester Erythro-Threo Ratio Diastereoface Selection"... 

The erythro-threo ratio of the product was greater than 9 1. 

Chiral ketone catalysts of the Yang-type (5a and 5b, see above) and of the Shi-type (10, Scheme 10.2) have been successfully used for kinetic resolution of several racemic olefins, in particular allylic ethers (Scheme 10.4) [28, 29]. Remarkable and synthetically quite useful S values of up to 100 (ketone 5b) and above 100 (ketone 10) were achieved. Epoxidation of the substrates shown in Scheme 10.4 proceeds with good diastereoselectivity. For the cyclic substrates investigated with ketone 10 the trans-epoxides are formed predominantly and cis/trans-ratios were usually better than 20 1 [29]. For the linear substrates shown in Scheme 10.4 epoxidation catalyzed by ketone 5b resulted in the predominant formation of the erythro-epoxides (erythro/threo-ratio usually better than 49 1) [28]. 

Finally, if R2 Rv the bulk of R will have little effect upon the erythro threo ratio and the erythro product will dominate. The erythro aldol is always obtained in the reaction... 

The catalyst can be prepared according to Ref. [4]. To a solution of the catalyst (49 mg, 0.12 mmol) in tetrahydrofuran (THF) (8 mL) were successively added benzaldehyde (110 pL, 1.1 mmol) and a solution of silyl enol ether (233 mg, 1.0 mmol) in THF (2 mL) at —78 °C under argon atmosphere. After stirring for 6 h, water (2 mL) was added and the mixture warmed to r.t. for 2 h. Usual workup followed by chromatographic purification gave the aldol adduct (198 mg, 74%) with an erythro threo ratio of 7 3. 

Table 6.7.3. Erythro/threo ratios of isolated and in situ lignins (Tsutsumi et al 1990 and unpubl work)...

In most cases, the Reformatsky reaction is not very stereospecific and mixtures of erythro- and threo-p-hydroxyesters are obtained when asymmetric a-haloesters are used as reagents (equation 79). The erythro threo ratio appears to depend on the solvent polarity and the reaction time. However, it is of current interest to develop highly stereocontrolled asymmetric Reformatsky reactions. To date, high diastereoselectivities could be achieved only in a few cases either by substitution of zinc with other metal... 

Epoxidation of allylic alcohols (5, 75-76). Corrected values for the erythro threo ratios obtained on epoxidation of allylic alcohols with TBHP catalyzed with VO(acac)2 or Mo(CO)6 have been reported. In general greater selectivity is observed with the vanadium catalyst. In simple systems formation of erythro-epoxy alcohols is markedly favored over the threoAsomers, but substitution of a c -alkyl group on the double bond favors formation of threo-epoxy alcohols. 

Synthetic Applications in Alicyclic Systems 3.1053 Configuration of the Newly Formed Double Bond 3.103.4 The Erythro-Threo Ratio... 

T Akiyama, T Sngrmoto, Y Matsnmoto, G Meshitsuka. Erythro/threo ratio of beta -0-4 structures as an important stmctnral characteristic of lignin I. Improvement of ozonation method for the qnantitative analysis of lignin side-chain structnre. J Wood Sci 48 210-215, 2002. 

T Sugimoto, T Akiyama, Y Matsumoto, G Meshitsuka. The erythro/threo ratio of beta-O-4 structures as an important structural characteristic of lignin Part 2. Changes in erythro/threo (E/T) ratio of beta-O-4 structures during delignification reactions. Holtforschung 56 416 21, 2002. 

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