Preparation of diversely protected 2-azido-2-deoxyglycopyranoses from glycals

A new and efficient preparation of diversely protected 2-azido-2-deoxyglycopyranosides from the corresponding glycals is described. The glycals are first transformed into protected phenyl 2-azido-2-deoxy-selenoglycopyranosides by azido-phenylselenylation. Two procedures were employed according to the protecting groups present: sodium azide and diphenyldiselenide in the presence of (diacetoxyiodo)benzene for peracetylated glycals (Procedure A) or trimethylsilyl azide and tetra-n-butylammonium fluoride in the presence of N-phenylselenophthalimide for perbenzylated glycals (Procedure B). A gluco–manno mixture (90%) is obtained from protected d-glucal whereas only the galacto isomer is formed from protected d-galactal (75%). The compatibility of the second procedure with one free hydroxyl group and a variety of protecting groups was verified with 1,5-anhydro-2-deoxy-3,4-O-isopropylidene-d-lyxo-hex-1-enitol and its 6-O-acetyl, 6-O-allyl, 6-O-benzyl, and 6-O-tert-butyldimethylsilyl derivatives as well as with 1,5-anhydro-4,6-O-benzylidene-2-deoxy-d-lyxo-hex-1-enitol and its 3-O-acetyl and 3-O-benzyl derivatives, which were transformed into phenyl 2-azido-2-deoxy-α-d-selenogalactopyranoside derivatives in good yield. In the second step, hydrolysis of these selenoglycosides afforded diversely protected glycopyranoses in high yield. Peracetylated derivatives were hydrolyzed in the presence of N-iodosuccinimide, whereas mercury trifluoroacetate was employed for 3,4-O-isopropylidene, 4,6-O-benzylidene, and perbenzylated derivatives. In some cases the two steps can be carried out without isolation of the intermediate selenoglycoside. Keywords: glycals, 2-azido-2-deoxygalactopyranose, 2-azido-2-deoxyglucopyranose, selenoglycosides.

Synthèse du (±)-loliolide ou hydroxy-6 tétrahydro-5,6,7,7a triméthyl-4,4,7a(4H) benzofurannone-2 (6S,7aR) par cyclisation de l'acide homogéranique

Starting from pure homogeranic acid 2, in a stereoselective fashion, we describe a new synthesis of (±)-loliolide 1. Cyclization of 2 with mercury trifluoroacetate or phenylselenenyl chloride provides the isodihydroactinidiolide 6 which was converted to the racemate of loliolide after six steps.[Formula: see text]