Asymmetric Allylation of Glycidols Mediated by Allyl Acetate via Iridium Catalyzed Hydrogen Transfer

Abstract

Glycidols prepared via Sharpless asymmetric epoxidation participate in asymmetric redox-neutral carbonyl allylation with good levels of catalyst-directed diastereoselectivity. Equally stereoselective allylations may be performed from the aldehyde oxidation level using 2-propanol as the terminal reductant. An epoxide ring opening reaction using AlMe₃-n-BuLi is used to prepare the propionate-based stereotetrad spanning C17–C23 of dictyostatin, illustrating how this method may be applied to polyketide construction.

Graphical abstract

Epoxides are important building blocks in chemical synthesis, including the construction of polyketide natural products where they also appear as native structural motifs.¹ Accordingly, several methods have been reported for the asymmetric allylation² of glycidic aldehydes using reagents based on boron,^{3, 4} tin,^{5, 6} silicon,^{7, 8} indium⁹ and magnesium.¹⁰ These methods have proven effective in certain contexts,^3–10 however, due to pronounced match-mismatch effects only one diastereomer of the secondary homoallylic glycidol is generally accessible in highly diastereomerically enriched form.¹¹ Additionally, indirect formation of secondary homoallylic glycidols via enantioselective allylation of α,β-unsaturated aldehydes followed by Sharpless asymmetric epoxidation is problematic, as modest diastereoselectivities are evident in reactions of secondary (Z)-allylic alcohols.¹²

By harnessing the native reducing ability of alcohols, we have discovered a new, redox-economic class of C-C bond formations that merge the characteristics of carbonyl addition and transfer hydrogenation.¹³ These hydrogen auto-transfer processes utilize alcohol oxidation to drive reductive generation of transient organometallic nucleophiles. The resulting carbonyl-organometal pair combines to furnish products of addition, directly converting lower alcohols to higher alcohols. Based on this pattern of reactivity, diverse enantioselective alcohol C-H functionalizations have been developed including the C-H allylation¹⁴ and crotylation^15,16 of primary alcohols to form secondary homoallylic alcohols. In the present account, this allylation method is applied to the conversion of primary glycidols to secondary homoallylic glycidols.

Initial studies were focused on the allylation of glycidol 1a, which is prepared through Sharpless asymmetric epoxidation of geraniol.¹⁷ For the sake of convenience, the cyclometalated π-allyliridium C,O-benzoate catalysts were generated in situ from commercial components. Thus, glycidol 1a was exposed to allyl acetate in the presence of [Ir(cod)Cl]₂, a series of 4-substituted-3-nitro-benzoic acids, assorted axially chiral chelating phosphine ligands, and various inorganic bases. A small set of optimization experiments quickly led to the identification of effective conditions. Thus, using iridium catalysts (R)- or (S)-Ir-I modified by SEGPHOS, the diastereomeric secondary homoallylic glycidols 2a and epi-2a were obtained in excellent yields and diastereoselectivities, respectively (Table 1). Notably, unlike the corresponding allylborations,¹¹ the enantiomeric catalysts delivered 2a and epi-2a with roughly equivalent levels of catalyst-directed diastereoselectivity (2a:epi-2a = 12:1 vs 1:11), suggesting the present iridium catalysts are insensitive to match-mismatch effects. Indeed, using an achiral iridium catalyst modified by dppf, a 1:1 mixture of diastereomeric secondary homoallylic glycidols 2b and epi-2b were obtained.

Table 1.

Iridium catalyzed C-H allylation of glycidols 1a–1f to form adducts 2a–2f and epi-2a–2f.^a

^aCited yields are of material isolated by silica gel chromatography. ArCO₂H refers to 4-cyano-3-nitrobenzoic acid. Diastereomeric ratios were determined by ¹H NMR of crude reaction mixtures. See Supporting Information for further experimental details.

^bOn 1 mmol scale, 2a is formed in 81% yield and 10:1 dr.

The identification of favorable conditions for the asymmetric C-H allylation of glycidol 1a prompted a more detailed investigation into the scope of this process (Table 1). cis-Glycidols 1b–1e were specifically selected for study as the corresponding secondary homoallylic glycidols 2b–2e and epi-2b-epi-2e are inaccessible using conventional allylation methods^2–10 due to the modest diastereoselectivities reported in Sharpless asymmetric epoxidations of secondary (Z)-allylic alcohols¹² along with the stereochemically labile nature of (Z)-α,β-unsaturated aldehydes. In the event, cis-glycidols 1b–1e were converted to the secondary homoallylic glycidols 2b–2e and epi-2b-epi-2e, respectively, in good yields and good levels of catalyst-directed diastereoselectivity. In certain cases, the iridium catalysts (R)- or (S)-Ir-II modified by Cl,MeO-BIPHEP were found to enforce higher yields and diastereoselectivities than the iridium catalysts (R)- or (S)-Ir-I modified by SEGPHOS. To complete this study, the allylation of trans-glycidol 1f was explored. Whereas secondary homoallylic glycidol 2f was formed with excellent levels of catalyst directed stereoselectivity, the isomeric glycidol epi-2f was formed in low yield with less pronounced levels of stereocontrol. These data suggest match-mismatch effects may be more important in reactions of trans-glycidols. Finally, it is worth noting that due to the Horeau principle, all major reaction products derived from 1a, 1b and 1f, are obtained in >99% enantiomeric excess.¹⁸

The direct C-H allylation of primary glycidols 1a–1e is both step-economic and redox-economic as it avoids discrete formation and isolation of less tractable glycidic aldehydes. Nevertheless, under certain circumstances it may be desirable to conduct the allylation from the aldehyde oxidation level. To assess the feasibility of utilizing glycidic aldehydes as reactants, the reductive coupling of allyl acetate with dehydro-1a was performed using 2-propanol as the terminal reductant under otherwise standard conditions (eq 1). The respective secondary homoallylic glycidols 2a and epi-2a were formed in good yields with excellent levels of catalyst-directed diastereoselectivity. The efficiencies observed in the reactions of glycidic aldehyde dehydro-1a were roughly equivalent to those observed using the corresponding glycidol 1a (Table 1).

(1)

To illustrate how the present method may be applied to polyketide construction, secondary homoallylic glycidol 2c was subjected to conditions for regioselective epoxide ring opening using AlMe₃-n-BuLi (eq 2).¹⁹ The desired adduct 3c, which was obtained in 77% yield, embodies a propionate based stereotetrad spanning C17–C23 of dictyostatin, a marine macrolide that displays antimitotic activity against multi-drug resistant cancer cell lines at nanomolar levels.^20,21

(2)

In summary, we report that glycidols prepared through Sharpless asymmetric epoxidation participate in direct carbinol C-H allylation with excellent levels of catalyst directed diastereoselectivity. This method is redox-and step-economic, as it avoids discrete formation of less tractable glycidic aldehydes. Further, this method overcomes limitations evident in corresponding allylations of glycidic aldehydes using allylboron reagents.^3a,b,11 Finally, as Sharpless asymmetric epoxidations of secondary (Z)-allylic alcohols display low levels of diastereoselectivity,¹² indirect formation of the present secondary homoallylic glycidols through an asymmetric enal allylation-epoxidation sequence is not feasible. Future studies will focus on the use of α-olefins²² as pronucleophiles in alcohol-mediated carbonyl addition.