Pyran ring

One-component syntheses, in which a pyran ring (type B,l) or a 1,5-dione structure (type B,2) is already present. 

B. One-Component Syntheses 1. From Compounds with Preformed Pyran Ring... 

A closely related oxygenated heterocyclic system devoid of acidic groups interestingly shows quite different biological activity. Thus, condensation of the benzofuran hydroxyketone 66 with ethyl thiomethyl acetate (67) probably proceeds initially by formation of the acylation product 68. Intramolecular dehydration leads to formation of a pyran ring. There is thus obtained the hypo-cholesterolemic agent timefurone (69) [14],... 

The 6-endo activated epoxy alcohol cyclization process was also expected to play a central role in the annulation of pyran ring G of the natural product (see Scheme 22). Silylation of the free secondary hydroxyl group in compound 131 furnishes, after hydrobora-tion/oxidation of the double bond, compound 132. Swern oxidation of alcohol 132 produces an aldehyde which reacts efficiently with (ethoxycarbonylethylidene)triphenylphosphorane in the presence of a catalytic amount of benzoic acid in benzene at 50 °C, furnishing... 

Diynes 60 react with aldehydes 61 in the presence of [ (CODj l/SIPr catalytic system to afford dienones 63 upon electrolytic ring opening of the expected pyrans 62 (Scheme 5.18) [20]. When cyclohexanone 64 is employed the pyran ring 65 is obtained in good yield. 

The term non-classical cannabinoids is applied to a group of bicyclic compounds identified by researchers at Pfizer in the 1980s [129], These compounds lack the pyran ring of the classical cannabinoids and the second phenolic hydroxyl group of the cannabidiols, resulting in a simplified substructure represented by CP 47,497 (192) [130, 131], The non-classical cannabinoids still retain the three main pharmacophoric elements described above for the classical cannabinoids and the SAR in these regions parallels that of the classical cannabinoids [132]. 

The stereochemistry at C(4) and C(6) was then established. The cuprate addition in Step C occurred anti to the substituent at C(2) of the pyran ring. After a Wittig... 

Treatment of the tetrahydropyrano[l,6]naphthyridine 303 with phosphorus oxychloride results both in closure of the imidazole ring and rearrangement of the pyran ring to give the spiro-cyclopropano compound 304 (Equation 104) <1996JHC49>. 

Both natural and non-natural compounds with a 2ff,5ff-pyrano[4,3-fc]pyran-5-one skeleton are of interest in medicinal chemistry. Several natural products, such as the pyripyropenes, incorporate this bicyclic ring system. The group of Beifuss has described an efficient microwave-promoted domino synthesis of the 2ff,5H-pyr-ano[4,3-fo]pyran-5-one skeleton by condensation of a,/3-unsaturated aldehydes with 4-hydroxy-6-methyl-2]-f-pyran-2-one (Scheme 6.244) [428]. It is assumed that in the presence of an amino acid catalyst a Knoevenagel condensation occurs first, which is then followed by a 6jr-electron electrocyclization to the pyran ring. While the conventional thermal protocol required a reaction time of up to 25 h (refluxing ethyl... 

A pyran ring is formed in the intramolecular Diels-Alder cycloaddition of alkene-tethered enantiopure (lS,2R)-l,2-dihydroxycyclohexa-3,5-diene-l-carboxylic acid derivatives (derived from the biodihydroxylation of benzoic acid). For the three cases illustrated in Scheme 6.246, Mihovilovic and colleagues found that moderate to high yields of the desired cycloadducts could be obtained by exposing a solution of the precursor to microwave irradiation at 135-210 °C for extended periods of time... 

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