Diastereoselectivity hydrogenation

Diastereoselective Hydrogenation since -OH directs the H2, there is a possibility for control of stereochemistry - sensitive to H2 pressure catalyst cone, substrate cone, solvent. 

This thoroughly revised and updated new edition is a must for every synthetic organic chemist. New material has been added on homogene- ous diastereoselective hydrogenations, enantioselective oxidations, and novel chiral auxiliaries. 

The diastereoselective hydrogenation of 2,5-disubstituted furans on a Raney nickel contact provides an easy access to tetrahydroilirylcarbinols. Due to the alcohol used as the solvent it is possible to influence the direction of the stereo selection process (erythro vs. threo). The highest diastereoselectivities reached till now are in the range of 70% <96S349>. The (ij-Cij-Cjj portion of the venturicidins was synthesized stereo selectively in 17 steps from 2-furaldehyde in an overall yield of 7% <96SL135>. 

Prior literature indicated that olefins substituted with chiral sulfoxides could indeed be reduced by hydride or hydrogen with modest stereoselectivity, as summarized in Scheme 5.10. Ogura et al. reported that borane reduction of the unsaturated sulfoxide 42 gave product 43 in 87 13 diastereomer ratio and D20 quench of the borane reduction mixture gave the product 43 deuterated at the a-position to the sulfoxide, consistent with the hydroboration mechanism [10a]. In another paper, Price et al. reported diastereoselective hydrogenation of gem-disubstituted olefin rac-44 to 45 with excellent diastereoselectivity using a rhodium catalyst [10b],... 

The third member of the tandem inter [4+2]/intra [3+2] cycloaddition family is classified as the bridge mode, in which the dipolarophile is attached to the dienophile. Simple, 1,4-pentadi-enes as well as 2-alkoxy-l,4-pentadienes can function effectively as dienophiles and dipolarophile combinations with excellent chemical selectivity and regio- and diastereoselectivity. Hydrogenation of the bridged nitroso acetals produces hydroxymethylated derivatives in high diastereo- and enantioselectivity (Eq. 8.116).185... 

Table 21.5 Diastereoselective hydrogenation of alcohols with exocyclic double bond. 

Table 21.6 Diastereoselective hydrogenation of double bond in cyclic esters and carboxylic acids. 

The configuration of the product in diastereoselective hydrogenation -whether 1,2-syn or 1,2-anti - is related to the substitution pattern of the starting alkene. The allyl alcohol with a 1,1-disubstituted olefin unit affords the antiproduct, while the syn-product is formed from the allyl alcohol with a trisubsti-tuted olefmic bond (Table 21.8, entries 6-9). The complementarity in diastereoselective hydrogenation of di- and tri-substituted olefins may be rationalized based on the conformation analysis of the intermediary complex (Scheme 21.1)... 

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