Phenyloxazoline derivatives of amino-sugars. Part 3. Preparation of intermediates for the synthesis of sphingoglycolipids

Abstract

1,2-Dideoxy-5,6-O-isopropylidene-2′-phenyl-α-D-glucofuranoso[2,1-d]-Δ^2′-oxazoline (1) was readily converted into allyl 2-benzamido-2-deoxy-5,6-O-isopropylidene-βD-glucofuranoside (4) by the action of the pyridine salt of toluene-p-sulphonic acid in allyl alcohol–pyridine at reflux. This method is superior to acid alcoholysis for the conversion. The mesylate of the alcohol (4) was converted into allyl 2,3-dideoxy-5,6-O-isopropylidene-2′-phenyl-β-D-allofuranosido[2,3-d]-Δ^2′-oxazoline (9) by hot aqueous triethylamine. The isopropylidene group of compound (9) was hydrolysed preferentially in anhydrous methanol and the allo-diol (12) produced was converted into the talo-anhydride (17), which was hydrolysed to give the talo-diol (18). The allo-diol and the talo-anhydride are potential intermediates for the synthesis of sphingosine. The isomerisation of the allyl group of compound (9) to a prop-1-enyl group, without affecting the phenyloxazoline group, was accomplished by the action of commercial potassium t-butoxide in dimethyl sulphoxide containing t-butyl alcohol or preferably by the action of chloro(tristriphenylphosphine) rhodium (I).

The action of acidic benzyl alcohol, containing benzaldehyde, on 3-O-benzyl-1,2-dideoxy-5,6-O-isopropylidene-2′-phenyl-α-D-glucofuranoso[2,1-d]-Δ^2′-oxazoline (2) at 20 °C led to the separation of the highly crystalline (m.p. 300°) benzyl 2-benzamido-3-O-benzyl-4,6-O-benzylidene-2-deoxy-β-D-glucopyranoside (30) in high yield from the reaction mixture. This compound was converted, by two routes involving allyl ethers, into benzyl 2-acetamido-3,6-di-O-benzyl-2-deoxy-β-D-glucopyranoside (40), an intermediate required for the synthesis of the immunological determinant of the P′-antigen. The conversion of benzamido- into acetamido-groups was accomplished with acetic anhydride–acetic acid containing sodium acetate at reflux. It was shown that allyl ethers and O-benzylidene groups were stable to the conditions of this conversion. Benzyl 2-acetamido-6-O-allyl-3-O-benzyl-2-deoxy-β-D-glucopyranoside (48) was also prepared.