Asymmetric dehydration of β-hydroxy esters and application to the syntheses of flavane derivatives

Dehydration reactions in polyfunctional natural products

“…This chemistry was later utilized in the synthesis of enantioenriched flavane derivatives. 139 While the asymmetric chemistry clearly exemplifies the stereochemical requirements for facile elimination, and the possibility for reagent-based facilitation of selective dehydration reactions, the kinetic resolution approach clearly provides enantio-enriched β-hydroxy esters as the output is not an expedient output of alkene products that could not have been prepared in other ways. This methodology promises the development of a site-selective dehydration by identifying a system that can recognize one specific alcohol moiety and facilitate its dehydration.…”

Kinetic Resolution of Racemic Allylic Alcohols by Catalytic Asymmetric Substitution of the OH Group with Monosubstituted Hydrazines

“…[2] Te chnically speaking, the kinetic resolution of racemica llylic alcoholsh as been accomplished by either epoxidation [3] or electrophilica ddition [4] of the C=Cb ond, and by either acylation [5] or oxidation of the OH group( Scheme 1). [7] In addition, racemic allylic alcohols having b-keto, [8] b-ester, [9] or b'-ester groups, [10] were reported to be resolved by catalytic processes involving OH-elimination or C=Cb ond hydrogenation. [7] In addition, racemic allylic alcohols having b-keto, [8] b-ester, [9] or b'-ester groups, [10] were reported to be resolved by catalytic processes involving OH-elimination or C=Cb ond hydrogenation.…”

“…[6] For racemic terminal allylic alcohols, their kinetic resolution has been developed by iridium-catalyzed asymmetrics ubstitution of the OH group. [7] In addition, racemic allylic alcohols having b-keto, [8] b-ester, [9] or b'-ester groups, [10] were reported to be resolved by catalytic processes involving OH-elimination or C=Cb ond hydrogenation. Whereas some of these methods are highly effective in resolving racemic allylic alcohols, the development of new methods is still highly desirable to expand the scope as well as improve enantioselectivity .T ot his end, we have developed an ew strategy for the efficient kinetic resolution of racemic allylic alcohols through palladium/sulfonyl-hydrazide-catalyzeda symmetric OH-substitution with monosubstituted hydrazines undera ir at room temperature (Scheme 1).…”

“…In general, the reaction with allylic alcohols bearing a-alkyl groups and g-aryl (or heteroaryl) groups proceeded with exclusive a-selectivity and complete retention of the E-alkene geometry (entries 1-8, s factors2 1t o > 400). The racemic allylic alcohols were also efficiently resolved with s factors ranging from 128 to > 400 (entries [10][11][12][13][14][15][16][17][18][19][20].I nc ontrast, lower but still acceptable enantioselectivity was achieved when the a-substituents of allylic alcohols were aryl groups (entries9 and 22). Unlike the other cases, the allylic moiety of the p-allylpalladium intermediate generated from alcohol rac-1 i is symmetrical.…”

One‐Pot Heck and Reduction: Application towards Efficient Synthesis of Flavans Promoted by Lewis Acid

“…Some of the prevalent procedures include intramolecular Mitsunobu reaction to form 2‐substituted chromans, reported by Hogetts et al . Similarly, Park and co‐workers also implemented intermolecular Mitsunobu on catalytic asymmetric dehydration of β‐hydroxy esters to generate enantiorich flavan derivatives . The research group of Masesane described the synthesis of flavans via chalcones.…”