Stereoselective synthesis of highly O-functionalized enantiopure 2,3,4-trisubstituted tetrahydrofurans by tandem debenzylative cyclization of glycal derived 2,3-epoxy alcohols

Metal free synthesis of 2,3-dideoxy-α, β-unsaturated carbohydrate enals (Perlin aldehydes)

A metal free synthesis of enantiopure 2,3-dideoxy-α, β-unsaturated carbohydrate enals (Perlin aldehydes), in CH₃CN-0.02 N H₂SO₄ in water (1:1, v/v) with 0.5 equivalent additives (4-hydroxy-6-methyl-2-pyrone or 4-amino coumarin), has been reported. This efficient protocol works well for the acetylated glycals (glucal, galactal and arabinal) and afforded Perlin aldehydes and hemiacetals in acceptable to good yields. Whereas, benzylated glycals furnished respective Perlin aldehydes, hemiacetals and the 2-deoxy derivatives, under similar reaction conditions. The products yields were significantly reduced when the additives were removed from the reaction mixture, indicating that they constitute an essential component of this approach. Further the use of 0.02 N H₂SO₄ in water: acetonitrile (1:1, v/v) solvent system is essential for the formation of Perlin aldehydes. The similar reactions under neutral reaction conditions (CH₃CN:H₂O, 1:1, v/v) with additives, afforded the hemiacetals as major product. This methodology is a metal free approach to Perlin aldehyde synthesis and therefore having additional benefit of its use in preparation of bioactive drug molecules, where metal toxicity is the major concern.

Very short highly enantioselective Grignard synthesis of 2,2-disubstituted tetrahydrofurans and tetrahydropyrans

Phenones with elongated chains are shown to be excellent substrates for ligand-promoted asymmetric Grignard synthesis of tertiary alcohols. In turn this enables the simple, short and highly enantioselective (up to 96% ee) preparation of chiral 2,2-disubstituted THFs and THPs. Thus, asymmetric addition of Grignard reagents to γ-chlorobutyrophenones and δ-chlorovalerophenones takes place in the presence of a chiral diaminocyclohexyl-derived tridentate ligand and subsequent base-promoted intramolecular cyclisation occurs with complete retention of asymmetry. As examples of the methodology, we report the shortest syntheses of gossonorol, γ-ethyl-γ-phenylbutyrolactone and δ-methyl-δ-tolylvalerolactone, the joint-shortest and flexible synthesis of boivinianin A and the shortest formal syntheses of boivinianin B and yingzhaosu C.

Carbohydrates in diversity-oriented synthesis: challenges and opportunities

Carbohydrates are attractive building blocks for diversity-oriented synthesis due to their stereochemical diversity and high density of polar functional groups.

Substrate and stereocontrolled iodocycloetherification of highly functionalized enantiomerically pure allylic alcohols: application to synthesis of cytotoxic 2-epi jaspine B and its biological evaluation

A mechanistic study on iodocycloetherification of enantiopure allylic alcohols is established. Its application to synthesis of marine cytotoxic 2-epi jaspine B has been demonstrated.

Synthesis of β-C-galactosyl D- and L-alanines

Synthesis of β-C-D-galactosyl D- and L-alanines is carried out via a highly stereoselective Grignard reaction of glycosyl iodides, Sharpless dihydroxylation and S(N)2 displacement of the corresponding mesylate or tosylate. Alternatively, attempted triflation of the intermediate alcohols triggers a stereoselective debenzylative cyclization leading to interesting bicyclic trans-fused compounds.

Diastereoselective, three-component cascade synthesis of tetrahydrofurans and tetrahydropyrans employing the tandem Mukaiyama aldol-lactonization process

A full account of studies leading to the development of a cascade sequence that generates as many as two C-C bonds, one C-O bond, and three new stereocenters providing substituted tetrahydrofurans (THFs) from simple gamma-ketoaldehydes and thiopyridyl ketene acetals is described. The process involves a tandem Mukaiyama aldol-lactonization (TMAL) and accumulated evidence suggests the intermediacy of a silylated beta-lactone that is intercepted by the pendant ketone. Formation of a cyclic oxocarbenium is followed by reduction with silicon-based nucleophiles leading to a highly diastereoselective synthesis of tetrahydrofurans. This cascade process has now been extended to the synthesis of tetrahydropyrans from simple delta-ketoaldehydes. The stereochemical outcome of the cascade processes described was determined by NOE correlations, coupling constant analysis, and X-ray crystallography of the derived oxygen heterocycles and is in accord with established and recently proposed models for nucleophilic additions to cyclic 5- and 6-membered oxocarbenium ions. The utility of this process was demonstrated by the synthesis of the tetrahydrofuran fragment of colopsinol B.