Some reactions of 2,4-dimethoxytetrahydropyrans

Abstract

Acid-catalyzed elimination of methanol from 2,4-dimethoxytetrahydropyran (1) produces 2-methoxy-5,6-dihydro-2H-pyran (3) rather than the expected olefin 4-methoxy-3,4-dihydro-2H-pyran (2).The reaction of 1,3-dibromo-5,5-dimethylhydantoin with 3 in ether -methanol gives a 2:l mixture of the isomers 3~-bromo-2cr,4cr-dimethoxytetrahydropyran (5a) and 3cr-bromo-2cr,4P-dimethoxytetrahydropyran (5b) respectively. A rationale is given to explain the preponderance of 5a over 5b and the highly selective attack of the bromine of the hydantoin and the methanol on C-3 and C-4 respectively of the double bond of 3. Reduction of 5ab with zinc in ethanol provides only compound 3.The room temperature reaction of 1 in a mixture of water and 1,2-dimethoxyethane containing Amberlite IR-120, produces 2-hydroxy-4-methoxytetrahydropyran (6) as an equilibrium mixture of cis and trans isomers in the ratio 1 :l. This gave a value of 0.9 kcal/mole for the anomeric effect in 6. Pyrolysis of the derivative, 2-acetoxy-4-methoxytetrahydropyran failed to produce the olefin 2 and resulted only in extensive decomposition.

Preparation of cis- and trans-2,5-dimethoxytetrahydropyran and cis- and trans-2,5-dimethoxy-6-methyltetrahydropyran

“…= 1.0:0.3:0.18 respectively in order of increasing retention time) under the same conditions showed a 50% decrease in the last component and a slight increase in the first component, it is clear that under our conditions of bromomethoxylation (-50°, short reaction time) the products 7, X, and Y obtained can not be the result of isomerization, but must be the products stemming directly from the reaction of 1,3-dibromo-5,5-dimethylhydantoin and methanol on 6. This is of interest from a theoretical standpoint (7). The reason why one isomer did isomerize slowly is no doubt due to the fact that the methoxy and bromo substituents are trans to each other and thus anchimeric assistance by the bromine atom could assist the anomeric methoxy group to leave.…”

“…crystalline deposit occurred (46 % yield) which proved to be the major product. Spin decoupling experiments indicated its structure to be 3a-bromo-2a,5a-dimetl~oxytetrahydropyran (7).…”

“…ref. 7), and also a mixture of7, X, and Y (area ratio on g.1.c. = 1.0:0.3:0.18 respectively in order of increasing retention time) under the same conditions showed a 50% decrease in the last component and a slight increase in the first component, it is clear that under our conditions of bromomethoxylation (-50°, short reaction time) the products 7, X, and Y obtained can not be the result of isomerization, but must be the products stemming directly from the reaction of 1,3-dibromo-5,5-dimethylhydantoin and methanol on 6.…”

The Total Synthesis of Carbohydrates

“…The synthesis of methyl 2,3-anhydro-4-deoxy-6-O-methyl-a-~~-lyxohexopyranoside (91) has been achievedlO* from 2-methoxymethyl-3,4-dihydro-2Wpyran (87). The sequence of reactions for this synthesis is shown in Scheme 35.…”