C diastereoselectivity

Scheme 14.2 (a) Enantio- vs. (b) Concept of the diastereoselective synthesis, (c) diastereoselective epoxidation. 

Rodrigues,]. A. R. Abramovitch, R. A. de Sousa,]. D. F. Leiva, G. C. Diastereoselective Synthesis of Cularine Alkaloids via Enium Ions and an Easy Entry to Isoquinolines by Aza-Wittig Electrocyclic Ring Closure.]. Org. Chan. 2004, 69, 2920-2928. 

Sharov, V. S., Ferrington, D. A., Squier, T. C., and Schoneich, C., Diastereoselective reduction of protein-bound methionine sulfoxide by methionine sulfoxide reductase. FEBS Lett. 455, 247-250(1999). 

Collado, L, Ezquerra, J., Jose-Vaquero, L, and Pedregal, C., Diastereoselective functionalization of... 

Jeffrey pine in America. Figure 4.49 summarizes our frontalin synthesis, which successfully provided over 10g of (LS ,5ft)-9889 The first and important step in our synthesis was the asymmetric reduction of p-kclo ester A with baker s yeast to give hydroxy ester B of 97.7% ee. Methylation of the dianion derived from B gave C diastereoselectively, which was oxidized to give D, Baeyer-Villiger oxidation of D afforded E with retention of configuration. Subsequent conversion of E to 98 proceeded smoothly to give 10 g of the sample in an overall yield of 7.8% based on A (10 steps). 

In 2001, p-lactams have been reported to be obtained via Staudinger reaction with complete c -diastereoselection starting from prochiral imine chromium complexes (Scheme 14), [63]. 

Compound 16 is formed in 89% yield starting from 12 with clean retention of configuration at the phosphorus atom as determined by X-ray diffraction. This compound is the product of two consecutive (first P-O to P-C followed by P-N to P-C) diastereoselective [1,3] rearrangements. The fact that 16 can also be prepared from 13 suggests that the P-N to P-C migration only occurs once the P-0 to P-C step has been completed. 

The reaction is c -diastereoselective, yielding preferentially the most thermodynamically stable m-isomer favored by an intramolecular hydrogen bond between the hydroxyl group and the pyridine nitrogen atom. The mechanism for the synthesis of these compounds, similar to mechanism in Scheme 5.15, is shown in Scheme 5.18 (represented for furan(thiophene)-3,4-dicarbaldehyde). 

Besson M, Pinel C. Diastereoselective heterogeneous catalytic hydrogenation of aromatic or heteroaromatic compounds. Top. CataL 2003 25 43-61. 

Ibrahim-Ouali, M., Sinibaldi, M.-E., Troin, Y, GuiUaume, D., and Gramain, J.-C., Diastereoselective photochemical synthesis of 3,3 -disubstuted indohnes. Tetrahedron, 53,16083-16096,1997. 

A highly diastereoselective alkcnylation of c/s-4-cyclopentene-l,3>diols has been achieved with 0-protected (Z)-l-iodo-l-octen-3-ols and palladium catalyst (S. Torii, 1989). The ( )-isomers yielded 1 1 mixtures of diastcrcomcric products. The (Z)-alkenylpalladium intermediate is thought to undergo sy/i-addition to the less crowded face of the prochiral cyclopentene followed by syn-elimination of a hydropalladium intermediate. 

Cram erythro-products" (G.E. Keck, 1984 A, B, C). [3-(Silyloxy)allyl]stannanes and O-pro-tected a- or y -hydroxy aldehydes yield 1,2,3- or 1,2,4-triols with three chiral centres with high regio- and diastereoselectivity (G.E. Keck, 1987). 

The (partial) description of the synthesis and coupling of the five fragments starts with the cyclohexyl moiety C —C. The first step involved the enantio- and diastereoselective harpless epoxidation of l,4-pentadien-3-ol described on p. 126f. The epoxide was converted in four steps to a d-vinyl d-lactone which gave a 3-cyclohexenecarboxylate via Ireland-CIaisen rearrangement (cf. p. 87). Uncatalysed hydroboration and oxidation (cf. p. 131) yielded the desired trans-2-methoxycyclohexanol which was protected as a silyl ether. The methyl car-... 

The dicarboxylation of cyclic alkenes is a useful reaction. All-c.vo-methyl-7-oxabicyclo(2.2.1]heptane-2,3,5,6-tetracarboxylate (233) was prepared from the cyclic alkene 232 using Pd on carbon and CuCh in MeOH at room temperature with high diastereoselectivity[216]. The dicarbonylation of cyclopentene... 

The (racemic) tmns disulfoxide of 1,3-dithiolane 59 is readily deprotonated at C2 by lithium hexamethyldisilazide, and the resulting anion reacts with aldehydes at -78°C with moderate to excellent diastereoselectivity to give mainly the products 60, although subsequent cleavage of these to give the a-hydroxyaldehydes was not described (97JOC1139). 

Danishefsky et al. were probably the first to observe that lanthanide complexes can catalyze the cycloaddition reaction of aldehydes with activated dienes [24]. The reaction of benzaldehyde la with activated conjugated dienes such as 2d was found to be catalyzed by Eu(hfc)3 16 giving up to 58% ee (Scheme 4.16). The ee of the cycloaddition products for other substrates was in the range 20-40% with 1 mol% loading of 16. Catalyst 16 has also been used for diastereoselective cycloaddition reactions using chiral 0-menthoxy-activated dienes derived from (-)-menthol, giving up to 84% de [24b,c] it has also been used for the synthesis of optically pure saccharides. 

An expedient and stereoselective synthesis of bicyclic ketone 30 exemplifies the utility and elegance of Corey s new catalytic system (see Scheme 8). Reaction of the (R)-tryptophan-derived oxazaboro-lidine 42 (5 mol %), 5-(benzyloxymethyl)-l,3-cyclopentadiene 26, and 2-bromoacrolein (43) at -78 °C in methylene chloride gives, after eight hours, diastereomeric adducts 44 in a yield of 83 % (95 5 exo.endo diastereoselectivity 96 4 enantioselectivity for the exo isomer). After reaction, the /V-tosyltryptophan can be recovered for reuse. The basic premise is that oxazaborolidine 42 induces the Diels-Alder reaction between intermediates 26 and 43 to proceed through a transition state geometry that maximizes attractive donor-acceptor interactions. Coordination of the dienophile at the face of boron that is cis to the 3-indolylmethyl substituent is thus favored.19d f Treatment of the 95 5 mixture of exo/endo diastereo-mers with 5 mol % aqueous AgNC>3 selectively converts the minor, but more reactive, endo aldehyde diastereomer into water-soluble... 

Removal of the unsaturated side-chain appendage from C-8 in 22 provides diol lactone 23 and allylic bromide 24 as potential precursors. In the synthetic direction, a diastereoselective alkylation of a hydroxyl-protected lactone enolate derived from 23 with allylic bromide 24 could accomplish the assembly of 22, an intermediate that possesses all of the carbon atoms of PGF2o- It was anticipated that preexisting asymmetry in the lactone enolate would induce the... 

An important task remaining is the stereocontrolled introduction of a methyl group at C-8. When a cold (-78 °C) solution of 14 in THF is treated successively with LDA and methyl iodide and then warmed to -45 °C, intermediate 24 admixed with minor amounts of the C-8 epimer is formed in a yield of 95 %. The action of LDA on 14 generates a lactone enolate which is alkylated on carbon in a diastereoselective fashion with methyl iodide to give 24. It is of no consequence that 24 is contaminated with small amounts of the unwanted C-8 epimer because hydrolysis of the mixture with lithium hydroxide affords, after Jones oxidation of the secondary alcohol, a single keto acid (13) in an overall yield of 80%. Apparently, the undesired diastereoisomer is epimerized to the desired one under the basic conditions of the saponification step. 

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