Menthyl esters synthesis

Asymmetric synthesis of 2,5-dimethyl-2,4-hexadiene (28) and /-menthyl diazoacetate (29) with chiral copper complexes (30) was successfully conducted by Aratani et al. [13] to afford the (1 A)-chrysanthem ic acid /-menthyl ester (31) in high optical and chemical yield. Since this finding, a lot of chiral copper complexes have been reported and applied to the asymmetric synthesis of (IR)-chrysanthemate. However, these copper complexes required more than 1 mol% of the catalyst and the cis/trans ratio still remains unsatisfactory. Moreover, /-menthyl ester was crucial for the high enantioselectivity. Given an industrial production of... 

This sequence was obviously not amenable to a synthesis of optically active a-allokainic acid given the fact that an aminomalonate group was necessary. After unfruitful assays with menthyl esters, the Swiss group was rewarded by the discovery that the phenylmenthyl group (180) brings sufficient asymmetry to the reaction intermediate to afford products with a high percentage of favorable diastereoisomer (Scheme 35) (181). 

In 1958 Herbrandson and Cusano (103) prepared menthyl esters of p-iodobenzenesulfinic acid 62. Darwish and McLearen (104) described the synthesis and separation of diastereomeric esters 63 from optically active a-methylbenzyl alchols. Similarly, methane-sulfinyl chloride 64 was found to react with cholesterol to give a... 

In extension of this work, the synthesis of enantiomeric and diastereomeric sulfinimidamides was also described. Treatment of the diastereomerically pure menthyl ester 90 with the lithium salt of AT-methylaniline was found to give the corresponding sulfinimidamide... 

Optically pure (M)-(—)-2-formylhexahelicene (76, R = 2-formyl), prepared from the corresponding resolved (—)-carboxy-menthyl ester, was used as a common precursor for the photochemical synthesis of optically pure (M)-[8]-, (M)-[9]-, (M)-... 

Furyl)glycolic acid (605), a substrate for the synthesis of hex-oses (see Section IV) was resolved119 into both enantiomeric forms by recrystallization of its 2(R)-menthyl ester. From the R enantiomer of 605, methyl a-D-glucopyranoside320 and methyl a-D-mannopyrano-side227 were obtained by a sequence of reactions discussed in Section IV. No racemization occurred at any stage of the synthesis. 

Diastereoselective complexation of the latter cases (144a) proceeded with only modest selectivity for the former cases (144b) selectivity varied with solvent and temperatnre. The best case (dr, 97 3) ntilized a matched enantiopnre allyl tosylate with a menthyl ester anxiliary. Finally, in a few cases the addition of HFe(CO)3(NO) across diene has also proved snccesshil in the synthesis of (143). ... 

A non-biomimetic synthesis of /J-(-)-horsfiline (57) has also been recently reported which was based on a thermal intermolecular 1,3-dipolar cycloaddition reaction as outlined in Scheme 7 [63J. The reaction of the optically active menthyl ester 67 acting as a dipolarophile, with the JV-methylazomethine ylide 68 (thermally generated in situ from sarcosine and formaldehyde) proceeded with n-facial diastereoselectivity to produce a chromatographically separable mixture of 69 and the unwanted diastereomer. Subsequent cleavage of the chiral auxiliary, followed by removal of the carboxylic acid group by the Barton radical method provided J7-(-)-horsfiline. 

The synthesis of L-methyl DOPA 80 by the Strecker reaction was straightforward and of course produced racemic material. A conventional resolution by crystallising the menthyl ester 83 from hexane and hydrolysis of acetal, ester and amide in 48% HBr (note that no racemisation by enolisation can occur) gives good yields.19... 

Reactions exhibiting diastereofacial selectivity, which occur when the imine or the enolate contains an endogenous stereocenter or a chiral auxiliary, have important applications for the synthesis of optically active 3-l ctams and 3-amino carboxylic acid derivatives. Early work by Furukawa et al. has demonstrated the viability of preparing optically active 3-amino acids from chiral imines. For example, the Schiff base derived from (5)-a-methylbenzylamine (110) reacts with Reformatsky reagent (111) to give, after hydrolysis and removal of the chiral auxiliary, 3-amino-2,2-dimethyl-3-phenylpropionic acid (112) in 33% ee (Scheme 21). Similar Reformatsky reactions have been performed using (-)-menthyl esters but the enantiomeric excess values are lower. ... 

Chiral sulfoxides with a pyridine substituent 16 are useful chiral dienophiles (Section D. 1.6.1.1.1.1.2.2.). The synthesis starts with the (—)-menthyl ester of propynoic acid which undergoes addition of 2-pyridinethiol to the triple bond. The oxidation of the sulfur occurs with some diastereoselectivity and the isomers are separated by crystallization from hexane16. 

Details of Szantay s synthesis of vincamine have been published. A significant stereoselectivity was introduced into the synthesis by a modification in which the enamine (212) was alkylated by means of the (-)-menthyl ester of a-acetoxyacrylic acid (Scheme 35). Methanolysis of the product gave a methyl... 

The previously developed coupling of the dianions of diesters with 1, u)-dihalides has now been made into an efficient asymmetric process by the use of menthyl esters (Scheme 1). Interestingly the use of a homochiral acetal such as (4) allows a diastereo-selective cyclopropane-forming reaction to be performed in a reaction which could be widely applicable in natural product synthesis. ... 

Shknada, Y, et ah, Enzymatic synthesis of L-menthyl esters in organic solvent-free system, J. Amer. Oil Chem. Soc., 76, 10, p. 1139, 1999. 

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