Study of stereoselectivity of reduction of 18-oxo des-E triterpenoids by sodium borohydride in the presence of cerium chloride

Structure–Activity Relationships for Withanolides as Inducers of the Cellular Heat-Shock Response

“…The use of lanthanoid cations (such as Ce 3+ ) in reactions of enones with NaBH 4 is known to cause 1,2-reduction of the carbonyl group with high selectivity compared to 1,4-reduction caused by NaBH 4 in the absence of these cations, 34 with the ratio of epimeric alcohols formed being determined by steric factors. 35 Conversion of 27 to 28, however, constitutes the first report of a steroselective reduction of only one carbonyl group of an ene-dione with NaBH 4 /CeCl 3 /MeOH/THF. The high degree of regio-and stereoselectivity observed for 27 yielding 28 may be explained as being due to the chelation of the boron atom of the reducing species [NaBH 4−n (OMe) n ] 34 to the oxygen atom of the ring-B oxirane of 27, delivering the hydride from the β-phase.…”

“…Semisynthesis of 4- epi -withaferin A ( 28 ) was efficiently achieved by the MnO 2 oxidation of 1 to 4-dehydrowithaferin A ( 27 ) followed by regio- and stereoselective reduction of its C-4 carbonyl group with NaBH 4 /CeCl 3 . The use of lanthanoid cations (such as Ce 3+ ) in reactions of enones with NaBH 4 is known to cause 1,2-reduction of the carbonyl group with high selectivity compared to 1,4-reduction caused by NaBH 4 in the absence of these cations, with the ratio of epimeric alcohols formed being determined by steric factors . Conversion of 27 to 28 , however, constitutes the first report of a steroselective reduction of only one carbonyl group of an ene-dione with NaBH 4 /CeCl 3 /MeOH/THF.…”

Oxaspirol B with p97 Inhibitory Activity and Other Oxaspirols from Lecythophora sp. FL1375 and FL1031, Endolichenic Fungi Inhabiting Parmotrema tinctorum and Cladonia evansii

“…1:1 as indicated by the 1 H NMR spectrum (Figure S16). As noted above, regiospecific 1,2reduction of enones with NaBH 4 in the presence of CeCl 3 has been reported to be nonstereospecific when the CC bond of the enone is exocyclic; 26 oxaspirol B (2a and 2b) had influenced the hydride addition to the carbonyl carbon during this reduction, similar to that observed for 5a and 5b (see above). The mechanism of this reaction presumably involved a boron complex formed between the hydroxy group β to the carbonyl carbon and delivering the hydride ion from the same side of the boron complex (Figure S17).…”

“…Regiospecific reduction of the carbonyl group of the enone moieties of the diacetates 5a and 5b with NaBH 4 /CeCl 3 yielded 9,10-di­acetyl­di­hydro­oxaspirol B isomers 6a and 6b as the major products, which on acetylation (Ac 2 O/pyridine) afforded 4,9,10-tri­acetyl­di­hydro­oxaspirol B isomers 7a and 7b , respectively (Scheme ). Because 1,2-reduction of enones with NaBH 4 /CeCl 3 has been reported to be nonstereospecific when the CC bond of the enone is exocyclic, each of the diacetates ( 5a and 5b ) should have yielded two products. The formation of only one enol from each of these suggested that the configuration of the spiro-center (C-5) may have had some influence on the hydride addition to the carbonyl group at C-4 during this reduction.…”

“…1:1 as indicated by the 1 H NMR spectrum (Figure S16). As noted above, regiospecific 1,2-reduction of enones with NaBH 4 in the presence of CeCl 3 has been reported to be nonstereospecific when the CC bond of the enone is exocyclic; this reduction should have yielded four products, two alcohols from each oxaspirol B isomer. Formation of only two products in this reaction suggested that configuration of the spiro center (C-5) of each isomer of oxaspirol B ( 2a and 2b ) had influenced the hydride addition to the carbonyl carbon during this reduction, similar to that observed for 5a and 5b (see above).…”