Enantiopure methyl

A sequence of transformations involving diastereoselective singlet oxygen photooxygenation of TBS-protected 3,5-cycloheptadienol 525 to predominantly 5yw-endoperoxide syn-526 and some anti-526, followed by reduction into the all-ci5 triol 527 as described in Scheme 146, was used by Johnson for synthesis of enantiopure methyl 2,4-dideoxyhexa-pyranosides D-528 and L-5284°5.406... 

In the benzyl ether series, enantiopure methylated amino alcohols were activated with Cr(CO)3 (e.g., 39) [81]. The diastereoselectivity can be very high to give the formylated products 40. The epimer at the carbon bearing the dimeth-ylamino group leads to the same selectivity. A rationale based on internal coordination of the methoxy and dimethylamino groups accounts for the selectivity [81]. 

The structure of the racemic /0-sheets was confirmed when the polymerization was initiated by enantiopure methyl esters of a-amino acids (Phe-OMe and Val-OMe), or dipeptides (Phe-Phe-OMe or Val-Val-OMe) of either absolute configuration. The homochiral initiator (Js), which is linked at the C terminus of the chains of both handedness at the two rims of the /0-sheet, converts the enan-tiotopic pockets into diastereotopic (different) pockets as a consequence, this... 

The first use of chiral oxazolines as activating groups for nucleophilic additions to arenes was described by Meyers in 1984. " Reaction of naphthyloxazoline 3 with phenyllithium followed by alkylation of the resulting anion with iodomethane afforded dihydronaphthalene 10 in 99% yield as an 83 17 mixture of separable diastereomers. Reductive cleavage of 10 by sequential treatment with methyl fluorosulfonate, NaBKi, and aqueous oxalic acid afforded the corresponding enantiopure aldehyde 11 in 88% yield. 

The stereoselective synthesis of hexacoordinated phosphate anions was also reported by the same group. A general one-pot process was developed for the preparation of C2-symmetric anions 15,16 and 17 containing enantiopure BINOL, hydrobenzoin, and tartrate-derived ligands respectively [38-40] Cpsymmetric anion 18 being prepared similarly in two steps from methyl-a-... 

An enantiopure bicyclic triazolium salt was prepared by a three step conversion of the oxazolidinone 31 (Scheme 18). The first step was a methylation affording an iminoether which was transformed into the phenylhydrazone 32. A final cyclization gave the salt 33 as a cristalline solid [25]. 

The importance of proper immobilization of enzymes can be shown in the kinetic resolution of racemic a-acetoxyamides. This group of compounds is an important class of chemicals since they can be readily transformed into a-amino acids [17], N-methylated amino acids, and tripeptide mimetics [18], amino alcohols [19], 1,2-diols [20], 1,2-diamines [21], and enantiopure l,4-dihydro-4-phenyl isoquinolinones [22]. 

Tridachiahydropyrone belongs to the family of marine polypropionates [69]. Efforts towards its total synthesis have recently led to a revision of the structure with the new proposal 2-147 [70]. The construction of the highly substituted cyclohex-enone moiety 2-146 which could be incorporated into this natural product [71] has been described by Perkins and coworkers (Scheme 2.33) [70, 72]. The conjugate addition/ Dieckmann-type cydization utilizing organocopper species as Michael donors afforded the enantiopure 2-145 in 68% yield. A further methylation of the (3-ketoester moiety in 2-145 followed by an elimination led to the desired cydohex-enone 2-146. 

Yeste et al.7 have developed chiral derivatization protocols for determination of enantiopurity of chiral diols by the H and 19F NMR spectroscopy. Three-component method has required the treatment of diol with 2-formylphenylboronic acid and a-methyl-4-fluorobenzylamine [4],... 

Similarly, the Tietze group has described an intramolecular microwave-promoted Heck reaction for the construction of the B ring in the synthesis of enantiopure B-nor-estradiol analogues (Scheme 6.10 a) [29]. The Heck coupling took place from below, anti to the angular methyl group, to form a single diastereoisomer. The best... 

Efficient oxidation of imines into nitrones can be achieved by using methyl (trilluoromethyl)dioxirane as an oxidant. This method provides enantiopure nitrones derived from 2H -pyrrole 1-oxide (23, 24). 

The chirality source in the synthesis of optically active nitrones (71) and (72) are known to be enantiopure chiral benzyl type hydroxylamines, (f )-a-methyl-benzylhydroxylamine (70a) and (f )-a-(hydroxymethyl)-benzylhydroxylamine (70b) (Scheme 2.25) (221). 

An ( )-TolBINAP-Ir complex with a protic amine such as benzylzmine as the additive has been applied for the hydrogenation of 2-phenyl-3,4,5,6-tetrahydropyridine, and enantioselectivity up to 90% ee is obtained.314 An ortho-metallated Ir dihydrid complex has been used in the hydrogenation of 2-methylquinoxaline, and enantiopurity up to 90% ee has been obtained for the 2-methyl-1,2,3,4-tetrahydroquinoxaline product (Scheme 16).327... 

Structurally novel /3-lactams were obtained using enantiopure 4-oxoazetidine-2-carbaldehydes and methylene cyclohexane and a-methyl styrene (Equation (4)).7 Boron trifluoride diethyletherate and tin(iv) chloride produced the products in the highest yields, and all ene products possessed yy/z-stereochemistry. 

In the total synthesis of cerorubenic acid-III methyl ester (105), diene 102 was converted to enantiopure tricyclic ketone 103 through an anionic oxy-Cope rearrangement (equation 56)82. Conversion of 102 to 103 afforded the entire ABC substructure of 104 and 105, most notably the double bond occupying a bridgehead site. 

Lotrafiban, a nonpeptidic glycoprotein llb/llla receptor anagonist that was under development as a treatment for the prevention of platelet aggregation and thrombus formation, was initially prepared using an 11-step linear sequence starting from methyl Cbz-L-aspartate (Scheme 1.35). An overall yield of 9 % and issues with obtaining the product in sufficient enantiopurity led the group to look for an alternative route via the enzymatic... 

The hydroxynitrile lyase (HNL)-catalysed cyanohydrin reaction is a useful method to synthesize enantiopure a-hydroxy nitriles and the corresponding a-hydroxy acids. However, small ketones, such as 2-butanone, are converted with low selectivities, due to the poor discrimination between methyl and ethyl. ... 

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