摘要：

Functionalised bicyclic exo -glycals are readily obtained by base-catalysed (typically MeONa in MeOH) alkynol cycloisomerisation of ethynylated cyclic saccharides. Thus, base treatment of the phenylethynyl- and halogenoethynylated 1- O -acetyl-ribofuranoses 22 - 24 and the 4-ethynylated 1-thioglucopyranosides 30 - 33 gave - after deacetylation - selectively the ( Z )-configured exocyclic enol ethers 26 - 28 (84-91%) and 34 - 37 (63-76%), respectively, resulting from a trans -5- exo-dig cyclisation. The ring closure to the trans -dioxahexahydroindans 34 - 37 is favoured by a concerted intramolecular protonation of the intermediate vinyl anion by the neighbouring HOC(3). Cycloisomerisation of the 6- O -acetyl-4-(phenylethynyl)-1-thio- - D -glucopyranoside 39 occurred via the corresponding phenylethynylated allenes to provide the galacto -configured ( Z )- and ( E )- cis -dioxahexahydroindans 40 (30%) and 41 (51%). Surprisingly, the HOC(4) unprotected -d- galactopyranosyl-buta-1,3-diyne 15 and the - D -glucopyranosyl-buta-1,3-diyne 51 (and its 2-bromoethynyl analogue) undergo a 6- exo-dig ring closure to the 2,5-dioxabicyclo[2.2.2]octanes 16 - 19 and 52 / 53 , respectively, the ring closure requiring a boat conformation ( B 1,4 for 15 , 1,4 B for 51 ). Ring strain ( anti -reflex effect) prevents an alkynol cycloisomerisation of 4-(phenylbuta-1,3-diynyl, bromoethynyl, or iodoethynyl)levoglucosan 56 - 59 , and 56 reacted by elimination to the hex-1-ene-3,5-diyne 59 (82%), while isomerisation of 57 and 58 led to epimeric mixtures of the haloallenes 60 (82%) and 61 (68%).

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