Anti-1,3-diols by Addition of Dialkylzinc Reagents to 4-Acetoxy-1,3-dioxanes

Versatile process for the stereodiverse construction of 1,3-polyols: iterative chain elongation with chiral building blocks

A versatile process for the construction of 1,3-polyols, a key structural element of polyketide-type natural products, is presented. The modular synthesis strategy involves the iterative chain elongation with novel four-carbon building blocks to access all possible stereoisomers of a growing 1,3-polyol chain. These chiral building blocks are designed to install four carbon atoms with two stereogenic centres by performing only four experimentally simple steps per elongation cycle, thus making these building blocks attractive for the realization of a universal platform from which to access a diverse range of polyketidic molecules.

Electrostatic Effects on the Reactions of Cyclohexanone Oxocarbenium Ions

Nucleophilic substitution reactions of 4-substituted cyclohexanone acetals display different selectivities depending upon the electronic nature of the substituent. Alkyl groups favor equatorial positions in the oxocarbenium ions, but alkoxy groups prefer axial conformers. The reactions of acetals with alkoxy groups constrained to either equatorial or axial positions suggest that the presence of an axial alkoxy group distorts the oxocarbenium ion, changing its inherent preferences for facial attack.

Stereochemistry of Nucleophilic Substitution Reactions Depending upon Substituent: Evidence for Electrostatic Stabilization of Pseudoaxial Conformers of Oxocarbenium Ions by Heteroatom Substituents

Lewis acid-mediated nucleophilic substitution reactions of substituted tetrahydropyran acetates reveal that the conformational preferences of six-membered-ring cations depend significantly upon the electronic nature of the substituent. Nucleophilic substitutions of C-3 and C-4 alkyl-substituted tetrahydropyran acetates proceeded via pseudoequatorially substituted oxocarbenium ions, as would be expected by consideration of steric effects. Substitutions of C-3 and C-4 alkoxy-substituted tetrahydropyran acetates, however, proceeded via pseudoaxially oriented oxocarbenium ions. The unusual selectivities controlled by the alkoxy groups were demonstrated for a range of other heteroatom substituents, including nitrogen, fluorine, chlorine, and bromine. It is believed that the pseudoaxial conformation is preferred in the ground state of the cation because of an electrostatic attraction between the cationic carbon center of the oxocarbenium ion and the heteroatom substituent. This analysis is supported by the observation that selectivity diminishes down the halogen series, which is inconsistent with electron donation as might be expected during anchimeric assistance. The C-2 heteroatom-substituted systems gave moderately high 1,2-cis selectivity, while small alkyl substituents showed no selectivity. Only in the case of the tert-butyl group at C-2 was high 1,2-trans selectivity observed. These studies reinforce the idea that ground-state conformational effects need to be considered along with steric approach considerations.

Asymmetric synthesis of beta-hydroxy acid via stereoselective dirhodium(II)-catalyzed C-H insertion of alpha-alkoxydiazoketone

A new methodology for the asymmetric synthesis of beta-hydroxy acid was developed. Dirhodium(II)-catalyzed C-H insertion of alpha-alkoxydiazoketone (3), which was prepared from primary alkyl halide (1) and readily available chiral alpha-hydroxy acid (2), gave stereoselectively 2,5-cis-disubstituted 3(2H)-furanone (4). The Baeyer-Villiger reaction of 4 followed by treatment with an acid afforded chiral beta-hydroxy acid (6) with high optical purity.

Enantioselective Syntheses of 2-Deoxyxylono-1,4-lactone and 2-Deoxyribono-1,4-lactone from 1,3-Dioxan-5-yl Diazoacetates

1,3-Dioxan-5-yl diazoacetates are valuable substrates for highly diastereoselective and enantioselective carbon-hydrogen insertion reactions. trans-2-(tert-Butyl)-1,3-dioxan-5-yl diazoacetate is a direct precursor to 2-deoxyribono-1,4-lactone in up to 81% ee, whereas cis-2-(tert-butyl)-1,3-dioxan-5-yl diazoacetate yields only the protected 2-deoxyxylono-1,4-lactone in up to 96% ee. However, trans-2-aryl-1,3-dioxan-5-yl diazoacetate (aryl = phenyl or 2-naphthyl) forms the precursor to 2-deoxyxylono-1,4-lactone in up to 95% ee but with the mirror image configuration of that produced from the trans-2-(tert-butyl) analogue. The catalysts that are most suitable for these carbon-hydrogen insertion reactions are chiral dirhodium(II) carboxamidates. 1,3-Dialkoxy-2-propyl diazoacetates give mainly 2-deoxyxylono-1,4-lactone derivatives (>90:10) with generally high enantiocontrol, but replacement of hydrogen at the 2-position of these 2-propyl diazoacetates led to a mixture of products.