Pyran ring, dihydro- from

Two other xanthones, have been isolated from the same species of Hypericum [72] a) 2-deprenylrheediaxanthone B (1,5,6 - trihydroxy-4 ,5 -dihydro - 4 , 4 , 5 - trimethylfurano - ( 2 , 3 3, 4 ) xanthone) and b) isojacareubin (1,5,6 - rihydroxy - 6 ,6 -dimethylpyrano - (2 3 3,4) -xanthone). They were peculiar in their structure, being conjugated with a furan and a pyran ring, respectively. 

Only little chemical and biological investigations have been conducted on acronycine derivatives modified at the 1 and/or 2 positions on the pyran ring. From a chemical point of view, previously described compounds modified at those positions only include bromo (7), nitro (8, 27), dihydro (361 (7, 27), and dihydronitro (8, 27) derivatives. 

Ring opening of aziridines formally derived from 5,6-dihydro-2A/-pyran shows good regioselectivity involving preferential attack at C-4 of the pyran ring <97T1417>. 

Startg. m. treated with 0.1 M pyridine hydrochloride in pyridine at 25° for 12 h product. Y 71%. The method allows the overall transformation of a cw-glycol to a conjugated enal via 0,0 -stannylene derivs. and subsequent Horner (or heterodiene) synthesis. F.e. and with aq. AcOH, also 2-alkylidene-l,5-dialdehydes from 2-alkoxy-3,4-dihydro-2//-pyran ring, s. C. Burnouf et al.. Tetrahedron Letters 29, 5533-4 (1988). 

In connection with studies on the ring-opening polymerization of cyclic acetals, we have undertaken investigations on the polymerization of bicyclic acetals, bicyclic oxalactone, and bicyclic oxalactam, which yield polysaccharide analogs, macrocyclic oligoesters, and a hydrophilic polyamide, respectively, some of which can be expected to be useful as novel speciality polymers. The monomers employed in the studies were prepared via synthetic routes presented in Scheme 1, starting from 3,4-dihydro-2H-pyran-2-carbaldehyde (acrolein dimer) I. 

Cyclization of the ylide derived from phosphonium salt (141) gives the 3,4-dihydro-2iT-benzoxocin (142) in low yield as a mixture with the pyran (143) (74JOC3038). Hydrogenation of (142) leads to the benzoxocin (136). The bromobenzoxocin (145) is formed on Baeyer-Villager oxidation of the seven-membered ring ketone (144) (58JCS2797). 

It has been concluded from a study of the microwave spectrum of 5,6-dihydro-2//- pyran that the ring skeleton is twisted such that the heteroatom and C-2 are on opposite sides of the plane formed by the other four carbon atoms <74JCP(60)3987>. Interatomic distances and bond angles are given in Figure 10. 

Dihydroxynaphthalene and 9,10-diacetoxyphenanthrene react with 1,2-diols and 1,2-dithiols in a one-pot synthesis of annulated 2,3-dihydro-1,4-dioxins and -1,4-dithiins (Scheme 26) <04TL1343>. The reaction of 2,3-dihydroxynaphthalene with 1,2-dihalogenated aromatic compounds leads to linearly annulated dioxins of particular interest are tri- and tetra-dioxins and various hetero-fused dioxins e.g. 62 (34%). Several examples yield cation radical salts on electrocrystallisation <04T8899>. Linear arrays of fused pyran-dioxin-cyclohexane rings as found in natural products derived from the milkweed family have been described e.g. 63 <04EJO4911>. 

Iridoids, a widely occurring class of heterobicyclic monoterpenes, are derived from 2-hydroxy-3,4-dihydro-2//-pyran with a 3,4-fused methylpentane ring. Iridodial 15 is the parent system. As an enol lactol, it is in equilibrium with a dialdehyde ... 

Chroman (3,4-dihydro-2//-l-benzopyran) is derived from 3,4-dihydro-2if-pyran by annulation of a benzene ring. Its structural isomer is the isochroman 1. 2-Phenylchroman 2 is known as... 

Regio- and enantioselective differentiation of symmetrical polyols has been accomplished with dispoke derivatives [Scheme 3.106]. Reaction of S,S)-6,6 -bis(2-methyl-3,4-dihydro-2//-pyran) (105.1) with 1,5-disilylated xylitol 106.2 in boiling chloroform containing a catalytic amount of camphorsulfonic acid gave the protected polyol 1063 as the only isolated product. The reaction was completely diastereoselective and the product formed was the most thermodynamically stable in which the two side chain methyl groups and the two hydro-xylated side chains on the dioxane ring were equatorially oriented, with the spirocentres benefiting from maximum anomeric stabilisation. 

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