2- Alkoxy-5,6-dihydro-2H-pyrans

Attempts were made in order to obtain adducts 198 in enantiomeric form by cyclo-addition of 1-alkoxy-l,3-butadienes to optically active esters of glyoxylic acid96 the enantiomeric purities of the adducts were, however, poor.96 Optically active butyl 2-alkoxy-5,6-dihydro-2H-pyran-6-carboxylates (198, R1 = Bu) were obtained when the R group in 197 was a carbohydrate moiety. The diastereoisomers resulting from cyclo-addition were separated by chromatography (see Section VII and Ref. 350). 

Epoxidation of 2-alkoxy-5,6-dihydro-2H-pyran (193) with peroxy acids leads to both possible epoxides, with the (i-erythro stereoisomer preponderating.65... 

Some general reactions of 2-alkoxy-5,6-dihydro-2H-pyrans have been investigated. 

Reaction of 2-alkoxy-5,6-dihydro-2H-pyrans (e.g., 309) with methyldiphen-ylsilane leads to the dealkoxylated products 322 in good yields. 

The alkoxy group in 2-alkoxy-5,6 dihydro-2H-pyrans (279) can be exchanged for another OR or NR R group when the substrate is heated at 150-200° with suitable alcohols or amines this opens the way to the nucleoside type of compound (e.g. 323). The alkoxy group in 279 can be also be exchanged for a thioalkyl group in a substitution reaction with mercaptans. The reactions are catalyzed by stannic chloride or diluted hydrochloric acid. The 2-alkylthio derivatives (324) are often accompanied by allylic rearrangement products (325). 

Extension of the side chain at position 6 in 2-alkoxy-5,6-dihydro-2H-pyrans was achieved by base-catalyzed condensation of fatty acid esters with alkyl... 

H-Pyran-6-carboxylic acid, rrans-2-alkoxy-5,6-dihydro- H NMR,3, 578 Pyrancarboxylic acids, oxo-decarboxylation, 2, 52 Pyran-5-carboxylic acids synthesis, 3, 792... 

Hydrogenation of the triple bond in 195 to a cis double-bond, followed by acid-catalyzed cyclization of the alkene, leads to 6-substi-tuted 2-alkoxy-5,6-dihydro-2//-pyrans (196). This method was used by H. Newman86 for the preparation of 2-ethoxy-5,6-dihydro-6-methyl-2H-pyran (196, R1 = Et, R = Me). 

The majority of the total syntheses of desosamine described in the literature have been based on the same reaction-sequence, namely, 2-alkoxy-5,6-dihydro-6-methyl-2H-pyran (247) was epoxidized and the epoxide (248) was then opened with aqueous dimethylamine to afford a mixture of the dimethylamino sugars 249. Interestingly, despite the simple reaction-pathway, all syntheses performed between 1962 and 1969 met with only limited success. 

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