1.2- dihydropyrane carboxylic acid

Dihydropyrane carboxylic acid Cleavage Product after cleavage ... 

Dihydro-2f/-pyran-2-one has been prepared by reductive cycliza-tion of 5-hydroxy-2-pentynoic acid [2-Pentynoic acid, 5-hydroxy-], which is obtained in two steps from acetylene [Ethyne] and ethylene oxide [Oxirane] 3 and by the reaction of dihydropyran [277-Pyran, 3,4-dihydro-] with singlet oxygen [Oxygen, singlet].4,5 2ff-Pyran-2-one has been prepared by pyrolysis of heavy metal salts of coumalic acid [2//-Pyran-5-carboxylic acid, 2-oxo-],8 by pyrolysis of a-pyrone-6-carboxylic acid [211 - Pyran-6-carboxyl ic acid, 2-oxo-] over copper,7 and by pyrolysis of coumalic acid over copper (66-70% yield).8... 

Dihydropyrans may be transformed to pyrans by dehydrogenation, dehydration, elimination of an alcohol or a carboxylic acid molecule, as well as by dehydrochlorination. 

Dihydropyran is of value as a protecting group for alcohols and phenols, and to a lesser extent amines, carboxylic acids and thiols (B-67MI22403, B-81MI22404). The resulting tetrahydropyranyl ethers (736) are stable to base, but are readily cleaved under acidic conditions (Scheme 284). 

Reaction of Jt-excessive heterocycles (e.g. thiophene, indole), enol ethers (e.g. dihydropyran) and enol acetates, and carboxylic acids with chlorosulfonyl isocyanate leads in generally excellent yields to A-chlorosulfonylamides. These intermediates are converted into the corresponding nitriles by heating in DMF, although the yields can be somewhat variable. A recent reinvestigation of the N-chlorosulfonylamide to nitrile conversion revealed that treatment of the amides with one equivalent of triethylamine led to formation of the nitriles in excellent yield. Clearly, the mechanisms of the DMF and the EtsN induced transformations are different. 

Table 4 Dihydropyran, Carboxylic, and Sulfonic Acid Supports...

The lactonization of alkenes by carboxylic acids, originally effected with Mn(III) compounds, 2 was shown to occur efficiently with Ce(IV) salts. Both procedures are improved by sonication. example of this synthetic procedure is the transformation of 3,4-(2H)-dihydropyran to l,7-dioxa-2-oxo-3-carbomethoxy-cfs-bicyclo[4.3.0.]nonane 1. The procedure using acetonitrile as the solvent proves to be suitable for the lactonization of a wide range of substrates, including acid-sensitive enol ethers. 

Carboxylic acids add to olefinic linkages under acid catalysis to form esters and the method has been employed to prepare tetrahydropyranyl (reagent dihydropyran) [52] and t-butyl (reagent wo-butene) [53, 54, 55] esters. The little used isopropylidene malonate (15) is similarly produced from iso-propenyl acetate and malonic acid in the presence of sulphuric acid [56, 57]. 

The basic cycloadduct, butyl 2-methoxy-5,6-dihydro-2//-pyran-6-carboxylate 2a, can be reduced to 6-hydroxymethyl-2-methoxy-5,6-dihydro-2f/-pyran 2b. Both 5,6-dihydro- 2/f-pyrans 2a and 2b can be converted in several ways to sugars, cir-Hydroxy 1 ation or epoxidation, followed by basic (or acidic) opening of epoxides with various nucleophilic reagents leads to various 4-deoxyhexoses 3b [18], Dihydropyran 2b can be transformed in a... 

Carboxyl groups are often masked by reaction with dihydropyran, which yields esters that are stable toward base but easily hydrolyzed by dilute aqueous acids. Account in detail both for the formation of these esters and for their ease of hydrolysis. See See. 19.15.)... 

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