Diasteromerically pure

The mixture of the crude product hydrazone 138 and the excess of hydrazone 129 was used directly in an acidic acetalization reaction utilizing aqueous 3n HCl in a biphasic system and gave a diastereomeric mixture of sordidin (126) and 7-epi-sordidin (7-epi-126) in 84% yield over two steps in a 1.5 1 ratio. Gratifyingly, we succeeded in separating the desired sordidin epimers by preparative gas chromatography. As a result, both could be obtained in diasteromerically pure form (sordidin de > 99% 7-epi-sordidin de > 97%) and with a high enantiomeric excess for each epimer (ee > 98%). 

The resolution of anti-3-methyl-l,4-pentanediol (60) proceeds via rapid Novozym 435 (CALB) catalyzed monoacylation with vinyl acetate in TBME under dry conditions followed by a slower enantioselective acylation of the secondary alcohol moiety (Scheme 4.23) [82]. Similarly, diasteromerically pure threo- and erythro-3-methylalkan-2-ols of type 61 and 62, respectively, have been successfully acylated with 2/(-preference ( E > 100 to >1000) using vinyl acetate, propionate or butyrate... 

The enantioselective syntheses of (R)-oc-cuparenone and (S)-a-cuparenone, both of which are natural products from different sources, were also completed using the solid-state photodecarbonylation of diasteromerically pure difluorodioxaborinane ketones 192 and 194 (Scheme 2.47). The latter were prepared in two steps from 191, and irradiated as nanocrystalline suspensions to optimize the chemical yields of the transformation. The photoreaction of the optically pure ketones was 100% stereoselective with an isolated yield of 80%. The two natural products were obtained by simple acid removal of the chiral auxiliary. 

The chiral enamine (84) has been shown to react with / -nitrostyrenes to give a mixture of mainly the 3- together with some of the 1-alkylated tetralone on hydrolysis. The main product (85) was shown to be greater than 90% diasteromerically pure and of 75-99% optical purity, and to have the (l R, 3 S)-configuration118ft (Scheme 74). 

Structures of oligonucleotides with modified backbone include an NMR study of the duplex d(TGTTTGGC) with diasteromerically pure Rp or Sp methylphos-phonates of the duplex d(CCAAACA). Substitution was carried out using either all Rp or a single central Sp in an otherwise Rp oligonucleotide. The methyl-phosphonate strand showed increased dynamics relative to the phosphodiester strand, and whilst sugars in the phosphodiester strand are C -endo, in the methylphosphonate strand they are an intermediate C4-endo. The introduction... 

Cyclopentenones 8, bearing an exocyclic double bond, add with complete facial selectivity to isoprene (9), yielding the diasteromerically pure spiro compounds 103. Interestingly, the same diastereomeric purity is observed for the noncatalyzed addition to 2,3-dimethyl-l,3-butadiene. 

The reported ee of ID-47 was determined via conversion of the final product to the Mosher amide under Schotten-Bauman conditions [121]. The desired product was observed as a single peak, >99 % diasteromerically pure. 

The reaction of phosphonic acid chloride (254) with (S)-proline ethyl ester afforded a mixture of diasteromeric amides (255) in high diastereoselectivity. The diastereomers (255) can easily be purified by chromatography. The chiral, practically optical pure organophosphorus compound (256) was obtained from purified (255) by acid alcoholysis. 

The reaction of 2- and 3-vinylindoles with dienophile 214 constitutes the first example of an asymmetric Diels-Alder reaction of vinyl heterocycles. From 3-vinylindoles, enantiomerically pure carbazoles 215a-c were obtained, whereas from the vinylindole 197 together with 215d, diastereomer 216 was obtained as a minor product. Conversely, 2-vinylindoles provided inseparable mixtures of diasteromeric carbazoles. On the other hand, the cycloaddition reactions of 3-vinylindoles with 217 furnish the tetrahy-drocarbazoles 218 with endo-diastereoselectivity (93T2863). 

Following the aforementioned chemoenzymatic strategy the aldol adducts from the enzymatic addition were first dephosphorylated and then treated with H2 in the presence of Pd/C. Under these conditions the Cbz group removal and the reductive amination took place in one pot, the iminosugars being obtained as hydrochlorides upon lyophilization at pH 6.5. Figure 19.6 depicts the structure of the iminosugars obtained, either as diasteromeric mixtures or pure compounds, from the N-Cbz-amino aldehydes described in this account. 

Introduction. L-iyrosine hydrazide (L-Tyr-NHNH2) (1) is useful in the resolution of simple carboxylic acids and amino acid derivatives. It often forms highly crystalline salts with these compounds, which yield diastereomerically pure salts in just one or two recrystallizations. The yields of resolved acids tend to be high, and in many cases both enantiomers can be obtained from the same operation (the more soluble diasteromeric salt that remains in the mother liquors is often quite pure). The tyrosine hydrazide can be recovered without appreciable loss of optical activity. 

Pracejus also studied the tandem nucleophilic addition/diastereoselective protonation of optically pure (S)-phenylethylamine on phenylmethylketene [11], With the aim of synthesizing amino acids and their derivatives, Calmes and coworkers reinvestigated the reaction of prochiral ketenes (generated in situ from acid chorides in the presence of a tertiary amine) with (R)-pantolactonc, an a-hydroxylactone [12], The authors have shown that the diastereoselectivity is dependent on the base used. Particularly, triethylamine and quinuclidine afforded complementary results and the diasteromeric ratios observed with quinuclidine suggest that the high stereoselections could be observed in nucleophilic additions to prochiral ketenes in the presence of cinchona alkaloids. 

The bulkiness and stereochemistry of large steroid substituents allows stereoselective hydrolyses of unnatural enantiomers. Cholesterol together with cholesterol esterase have been used to resolve binaphtols and other diols into their enantiomers the diasteromeric dicholesteryl-diesters are hydrolyzed selectively by the crude enzyme in two steps and the final diol is a pure enantiomer. 

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