Dihydropyran pyrones

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... 

Fig. 6 RCM-based formation and synthetic potential of dihydropyrans VIII and a-pyrones II...

All reactions listed in Tables 5-7 were carried out under a nitrogen atmosphere, but with the rhodium or palladium catalysts no noticeable or only minor reduction in cyclopropane yields was observed when air was present. In contrast, air clearly had a yield-diminishing effect in the CuCl P(0-/-Pr)3-catalyzed reactions, especially with cyclohexene and 3,4-dihydropyran. Cyclohexene was oxidized to 2-cyclohexen-l-one, and 3,4-dihydropyran gave 5,6-dihydro-4-pyrone and 5,6-dihydro-2-pyrone, albeit in yields below 8 % 59). 

The naming of these compounds is a bit odd. Pyran refers to the six-membered oxygen-containing heterocyclic ring system with two double bonds. It is not aromatic though compounds like pyrones are. The compound with only one double bond is therefore dihydropyran, and the saturated ring system istetrahydropyran. 

A simple two-step synthesis of the a-pyrone ring system209 has been applied to the synthesis of bufadienolides.210 Reaction of the known a/3-unsaturated aldehyde (416) with the dimethyl acetal of chloroketen [ClCH=C(OMe)2] gave a 57% yield of the intermediate 2,2-dimethoxy-3 -chloro-3,4-dihydropyran (417) which on reaction with sodium methoxide in DMSO afforded the a-pyrone (418) directly in 64% yield. 

The problem was later clarified with the finding that methylation of styryl-6-dihydropyran-2,4-dione, an analogue of yangonolactone, by diazomethane yielded a mixture of isomeric CC-pyrone and y-pyrone compounds as shown in Figure 5.4. These compounds were separated on the basis of the difference in solubilities of their hydrochloride oxonium salts in ether. The compound which formed an ether insoluble salt... 

Rotenone 30, the toxic principle from the roots of derris elliptica, is a natural insecticide. Its structure shows an unique combination of benzannelated dihydropyran, dihydro-4-pyrone and dihydrofuran ring systems. 

The lactonization of MBH derivatives to give 3-arylidene-3,4-dihydropyran-2-one derivatives has been realized by treatment with TFAA in CH2CI2 at room temperature.As shown in Scheme 4.102, ot-arylidene-8-lactones 317 were obtained in 50 83% yields. Subsequent oxidation of 317 with PCC afforded the desired a-pyrones 318 in 51-64% yields. By application of this synthetic strategy, tricyclic compound 320, previously reported by Basavaiah and Satyanarayana, " and bicyclic compound 319 could be prepared from easily available MBH adducts. 

Substituted derivatives of 5,6-dihydro-a-pyrones (dihydropyran-2-ones or more specifically 2H-dihydropyran-2-ones) occur widely in nature, particularly in plants and bacteria. They possess an a,P-unsaturated-5-lactone ring (1) with an alkyl, alkenyl or aryl substituent at C-6 and occasionally a varied substitution pattern around the ring. Many of these compounds are biologically active, exhibiting phytotoxi-... 

Various types of commercially available chiral precursors, generally members of the chiral pool, have been used for the stereoselective synthesis of chiral 5,6-dihydropyran-2-ones [6] (i) carbohydrates, mainly monosaccharides 7, 19, 30] (ii) chiral hydroxy acids [8, 20, 31] (iii) chiral epoxides [9, 21, 32] and (iv) other various types of chitons, including those prepared with the aid of microorganisms or enzymes [10, 22, 33]. As discussed above, there are cases in which one or two of the stereogenic carbons of the chiral precursor are transmitted intact to C-5/C-6 of the pyrone ring of the target molecule. In other cases, however, these carbons are not transferred as such and may even disappear, but only after they have influenced the formation of other stereocenters via internal induction. 

In this section, we will discuss a few other types of asymmetric reactions that have been used for the synthesis of 5,6-dihydropyran-2-ones in an enantioenriched form. One example is the recent synthesis of a pyrone isolated from R. crassifolia where organocatalytic a-hydroxylation of aldehydes via a-aminooxylation is used (Scheme 2.24) [39b]. Treatment of 5-phenyUiexanal with nitrosobenzene and l-proline at room temperature gave a-aminooxyhexanal 125, which was treated in situ... 

Dihydropyrans from 2-pyrones via 2-alkoxy- and 2-hydroxy-tetrahydropyrans s. 13, 218... 

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