Chemo- and diastereoselective tandem dual oxidation of B(pin)-substituted allylic alcohols: synthesis of B(pin)-substituted epoxy alcohols, 2-keto-anti-1,3-diols and dihydroxy-tetrahydrofuran-3-ones

Abstract

A novel retrosynthetic disconnection for the stereoselective preparation of α,α′-dioxygenated carbonyl compounds is disclosed. Herein we report a method to divert the oxidation of vinyl boronate esters from the B–C bond to the CC bond, resulting in a new stereoselective class of oxidation products from vinyl boronate esters. Treatment of 2-B(pin)-substituted allylic alcohols with catalytic OV(acac)2 and TBHP resulted in a highly chemo- and diastereoselective directed epoxidation to provide B(pin)-substituted epoxy alcohols (55–96% yield, dr > 20 : 1). In the case of B(pin)-substituted bis-allylic alcohols, highly substituted bis-epoxy alcohols with five contiguous stereocenters were obtained (dr > 20 : 1). Furthermore, the difference in reactivity between allylic alcohols and 2-B(pin)-substituted allylic alcohols towards epoxidation enabled the selective oxidation of the allylic alcohol in the presence of TBHP and VO(acac)2. The reactivity difference between the two allylic alcohols suggests CCB(pin) to be more electron deficient than CC(alkyl). The B(pin)-substituted epoxy alcohols are also useful synthetic intermediates. Tandem vanadium catalyzed epoxidation of the 2-B(pin)-substituted allylic and bis-allylic alcohols with excess TBHP generated the intermediate epoxides and bis-epoxides, respectively. Subsequent addition of NaOH resulted in the oxidation of the B–C bond of the B(pin)-substituted epoxides to afford 2-keto-anti-1,3-diols (60–83% yield) and epoxide-substituted 2-keto-anti-1,3-diols (60–78% yield, dr > 20 : 1). The latter underwent a novel facile acid-mediated cyclization to furnish fully substituted dihydroxy-tetrahydrofuran-3-ones (65–91% yield, dr > 20 : 1). Such compounds are difficult to efficiently access via conventional synthetic methods.