Enantioselective deprotection

Table 12.3-13. Enantioselective deprotection of N-protected o.L-amino acids by o-aminoacylase from Alcaligenesfaecalis DA-11961.

Using the dioxanone 40 as a synthetic dihydroxyacetone phosphate analogue, Enders and Grondal were able to synthesize several selectively protected carbohydrates in a direct and highly stereoselective fashion (57). As an example, the reaction between 40 and the aldehyde 43 catalyzed by R)-12 gave the acetonide-protected o-psicose 44 in 76% with excellent dia- and enantioselectivity. Deprotection of 44 gave o-psicose (45) quantitatively (Scheme 11). 

Kukula P, Matousek V, Mallat T, Baiker A. Structural effects in the Pd-induced enantioselective deprotection-decarboxylation of p-ketoesters. Tetrahedron Asymm. 2007 18 2859-2868. 

The Pictet-Spengler condensation has been of vital importance in the synthesis of numerous P-carboline and isoquinoline compounds in addition to its use in the formation of alkaloid natural products of complex structure. A tandem retro-aldol and Pictet-Spengler sequence was utilized in a concise and enantioselective synthesis of 18-pseudoyohimbone. Amine 49 cyclized under acidic conditions to give the condensation product 50 in good yield. Deprotection of the ketone produced the indole alkaloid 51. 

An enantioselective variant of the diene cydization reaction has been developed by application of chiral zirconocene derivatives, such as Brintzinger s catalyst (12) [10]. Mori and co-workers demonstrated that substituted dial-lylbenzylamine 25 could be cyclized to pyrrolidines 26 and 27 in a 2 1 ratio using chiral complex 12 in up to 79% yield with up to 95% ee (Eq. 4) [ 17,18]. This reaction was similarly applied to 2-substituted 1,6-dienes, which provided the analogous cyclopentane derivatives in up to 99% ee with similar diastereoselectivities [19]. When cyclic, internal olefins were used, spirocyclic compounds were isolated. The enantioselection in these reactions is thought to derive from either the ate or the transmetallation step. The stereoselectivity of this reaction has been extended to the selective reaction of enantiotopic olefin compounds to form bicyclic products such as 28, in 24% yield and 59% ee after deprotection (Eq. 5) [20]. 

Fig. 17. Chiral acetonide building blocks 36-39, protected 3rd-generation dendrimer 40 and deprotected 2nd-generation dendrimer 41, derived from enantioselectively dihydroxylated styrenes and cinnamic alcohols [74-76]...

When the peptide synthesis was complete, the phosphines were deprotected by sequential treatment with MeOTf and HMPT (Scheme 36.9). Addition of the rhodium precursor then created the catalyst library, which was screened, on the pin in the enantioselective hydrogenation of methyl-2-acetamidoacrylate (see Scheme 36.10). Unfortunately, this beautiful concept was poorly rewarded with rather low enantioselectivities. 

Scheme 6.27. Zr-catalyzed enantioselective Mannich reactions when more electron-withdrawing chiral ligands are used, enantioselectivity and reactivity levels are enhanced. Deprotection is carried out in two steps.

One of the most attractive biocatalytic options is the nitrilase-catalysed enantioselective hydrolysis of the racemic cyanohydrin. The hydroxyacid is produced directly without need for protection/deprotection steps and cyanohydrins racemize spontaneously at neutral or... 

Alternatively, enzymatic resolution of 61 by hydrolysis or of 62 by enzymatic esterification could be achieved with >99% ee and enantioselectivities of E>200, e.g. hydrolysis with common lipases like CAL-B or BCL (Amano PS) [86-88]. Wittig reaction and deprotection led to 64. Enzymatic resolution is also possible at the stage of C15-racemic 65 [86-88]. 

This enantioselective preparation of allylic alcohols has been applied to the synthesis of the side chain of prostaglandins . The addition to functionalized aldehydes, such as 483, allows the synthesis of C2-symmetrical 1,4-diols, such as 484, with excellent diastereoselectivity and enantioselectivity . An extension of this method allows the synthesis of C3-symmetrical dioF . Aldol-type products result from the catalytic enantioselective addition of functionalized dialkylzincs to 3-TIPSO-substituted aldehydes, such as 485, followed by a protection-deprotection and oxidation sequence affording 486 in 70% yield and 91% ee (Scheme 118) . The addition to a-alkoxyaldehydes provides a... 

Electroenzymatic reactions are not only important in the development of ampero-metric biosensors. They can also be very valuable for organic synthesis. The enantio- and diasteroselectivity of the redox enzymes can be used effectively for the synthesis of enantiomerically pure compounds, as, for example, in the enantioselective reduction of prochiral carbonyl compounds, or in the enantio-selective, distereoselective, or enantiomer differentiating oxidation of chiral, achiral, or mes< -polyols. The introduction of hydroxy groups into aliphatic and aromatic compounds can be just as interesting. In addition, the regioselectivity of the oxidation of a certain hydroxy function in a polyol by an enzymatic oxidation can be extremely valuable, thus avoiding a sometimes complicated protection-deprotection strategy. 

Several other practical syntheses of enantiopure amino acid derivatives have been accomplished recently from substrate 35 (Chart 10.6). The Imperiali group has used two techniques following PTC alkylations that occurred with modest enantioselectivity (50-53% ee). The first involved fractional recrystallization followed by subsequent deprotection/reprotection to give 39 (>99% ee). In the second method, enzymatic hydrolysis of the amino acid methyl ester with alkaline protease and then nitrogen acylation gave 40 (99% ee) [16]. Several other publications that deal with related purification techniques have appeared [17-19]. 

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