Alkylations of reducing sugars

Pyrazines are mainly used in roasted, peanut and chocolate flavours. Alkyl pyrazines can be obtained by the reaction in an autoclave at high temperature (120-160 °C) of reducing sugars like rhamnose and ammonia. 

Alkylation of enamines with epoxides or acetoxybromoalkanes provided intermediates for cyclic enol ethers (668) and branched chain sugars were obtained by enamine alkylation (669). Sodium enolates of vinylogous amides underwent carbon and nitrogen methylation (570), while vicinal endiamines formed bis-quaternary amonium salts (647). Reactions of enamines with a cyclopropenyl cation gave alkylated imonium products (57/), and 2-benzylidene-3-methylbenzothiazoline was shown to undergo enamine alkylation and acylation (572). A cyclic enamine was alkylated with methylbromoacetate and the product reduced with sodium borohydride to the key intermediate in a synthesis of the quebrachamine skeleton (57i). 

Radical additions to 0-alkyl aldoximes, easily accessible from reducing sugars, have been exploited for the synthesis of allosamizoline, the aglycon of aUosamidin [334]. The C ring of anguidine has also been prepared by reductive carbocyclization of an aldehyde on an unsaturated ester [335]. 

Stereochemical control in 0-alkylative /3-glycosylation relies on the kinetic anomeric effect [110] namely, that the oxyanion derived from a reducing sugar is more reactive in its 8-ori-ented form 71B than the corresponding a-anion 71A which should be more abundant. This tactic has met with partial success in cases of reactive substrate [111] (O Scheme 38). For example, reaction of 72 with a primary triflate in THF in the presence of NaH afforded the 8-glycoside in 62% yield. 

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