Stereoselective synthesis of allyl-C-mannosyl compounds: use of a temporary silicon connection in intramolecular allylation strategies with allylsilanes.

Methyl mannoside 16 containing an allyldimethylsilyl ether at C(2) was synthesized in nine steps from D-mannose. Reaction with TMSOTf in MeCN at room-temperature effected C-glycosylation to provide the alpha-allyl-C-mannosyl product 18 with excellent stereoselectivity. Crossover experiments over a range of reaction concentrations proved that reaction was proceeding via an intermolecular pathway rather than the hoped-for intramolecular delivery route. The exceptionally high stereoselectivity of this allylation in the presence of an acid-scavenger, 2,6-DTBMP, can be attributed to the allylsilyl ether 16 behaving as the allylating agent. Geometrical constraints in the seven-membered ring transition state account for the lack of intramolecular allyl transfer. Attaching a modified allylsilane 29a-c to C(2)OH of methyl mannoside 15 improved matters. Reaction of the tethered mannosides 27a-c with TMSOTf in the presence of 2,6-DTBMP in MeCN at rt provided a range of products, which depended on the size of the alkyl substituents at the silyl ether tether. Diene products were the major compounds irrespective of the size of the alkyl substituents at the silyl ether tether. Their formation can be understood by intramolecular allylation of the allylsilane on to the activated anomeric center, followed by collapse of the intermediate carbocation by preferential attack of an external nucleophile at the silyl ether tether, rather than at the allylic silicon center. A cascade of further reactions rationalizes the formation of the 2-dienyl-substituted tetrahydrofuran 30 and dienes 39 and 40. The desired beta-allyl-C-mannosyl products 42 and 43 were obtained, albeit in low yield, when bulky ethyl and isopropyl groups were employed at the silyl ether tether. Stereospecific oxidative cleavage of the silyl tether in 42 and 43 provided the corresponding stereodefined diols 44 and 45, respectively. Attempts to improve the yield and diastereoselectivity of the desired beta-allyl-C-mannosyls by moving to a sulfoxide mannosyl donor, which could be activated at low temperature, proved unsuccessful.