Pyrans ring-opening

The metal is chelated by an ene-1,2-dithiolate (dithiolene) of the dihydropyran ring which is fused to a partially reduced pteridine 178. The syntheses of these molybdopterin-related proligands were described as model compounds for verification of the stability in vitro <2001CC 123>. The proligand 179 is a relatively stable compound however, after several weeks, the pyran ring opened and oxidation to the diol derivatives 180 was observed (Scheme 35). 

Pyran decomposes in contact with moist air or adds but its tetrasubstituted derivatives, e.g. (155), are stable and are usefully converted into naphthalenes (156) by treatment with perchloric acid. A 4-benzyl or 4-naphthylmethyl substituent is essential. Pyran ring opening occurs and is followed by cyclization and elimination of a simple ketone which is formed from the 2- or 6-substituent. Alkyl groups at C-2 or C-6 are converted into the ketone more easily than a phenyl group (64LA(678)202). 

Nucleophiles usually react with 47/-benzopyran-4-ones at C-2, which results in pyran ring opening. The condensation of dilithiooximes onto 4/7-chroman-4-ones occurs by a 1,2-addition process to afford 47/-chromene 4-spiro-5 -isoxazolines 130 in good yield (Equation 61) <2004TL7351>. 

Pterins are redox active in their own right and can adopt one of several oxidation levels, for example, fully oxidized, dihydro, or tetrahydro (Fig. 18). The stereochemical nature of MPT observed in each protein crystallographic study (13) is equivalent to that of the fully reduced, or tetrahydropterin, state. This argues against the existence of dihydropterin states such as 5,6-dihydrop-terin or 7,8-dihydropterin, and one of the various quininoid forms of the dihydropterin, in the crystalline forms of these enzymes that have been characterized by X-ray crystallography. However, it is important to note that, for the tricyclic structure of MPT, the tetrahydropterin state is equivalent to that of a dihydropterin, as manifest in the pyran ring-opened form of the bicyclic... 

Nucleophilic attack of hydroxylamine at position 6 of 2//-pyran-2-ones 105 affords stereoselectively, through pyran ring opening and subsequent cyclization, (-isoxazol-4-yl)-,-dehydroamino acids 106 mainly as (Z)-isomers (Scheme 57) <2002J(P1)675>. 

Quantum mechanical calculations on the 2//-pyran ring opening and closing agree with the different mechanisms for photocoloration and photodecoloration experimentally observed in spiropyrans. The photobleaching proceeds from the 7 state, and mechanistically is very different from thermal bleaching, which proceeds from the S0 state and has a much lower activation of energy barrier. 

Miranda, L.D., Marrero, J.G., Bantista, E., Maldonado, E., and Ortega, A. 2009. Microwave-assisted gold (I) catalyzed pyran ring opening in brevifloralactone Synthesis of the haw-triwaic acid core. Tetrahedron letters, 50 633-35. 

Naphthyridin-2(1 //)-ones 66 were prepared from pyrano[4,3-/ ]pyridine-2,7-diones 67 by successive treatment with NH3, HBr and Zn in acetic acid (1993H1). It was believed that the attack of the ammonia molecule on the C(5) atom led to pyran ring opening. Subsequent decarboxylation afforded a tautomeric mixture of substituted tetrahydropyridines 68a,b. Dinitrile 69 generated by elimination of ammonia underwent cyclization under the action of HBr to form 1,6-naphthyridine derivative 70. Dehydrobromination with Zn in acetic acid gave the final products 66. 

A mixture of cis- or rr zn5-3-chloro-2-methyltetrahydrofuran and Sml2 in THE refluxed for 5-165 h (E)-3-pentenol. Y 75-90% (>95% E). With Na (cf. Synth. Meth. 6, 6), a mixture of geoisomers are normally obtained from tetrahydrofuran derivs. There was no over-reduction by this method, which was also applied to pyran ring opening. F.e. inch 1,3-enyne and styrene derivs., and effect of additives, s. L. Crombie, L.J. Rainbow, Tetrahedron Letters 29, 6517-20 (1988). 

Trifluoromethylpyranopyrazole 121 reacted at C(6) atom with excess of methyl mercaptoacetate in the presence of triethylamine to form a derivative 122 of trifluo-romethyl thiophene bonded with a pyrazole fragment. The reaction took place via pyran ring opening and intramolecular aldol-type condensation [71],... 

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