Dihydropyran ring

Lewis acids can be used to initiate this cyclization. Cyclization to a tricyclic systems that included formation of a dihydropyran ring was reported using mercuric... 

Tricyclic lactone systems were synthesized by the analogous intramolecular [4+ 2]-reaction followed by hydration and oxidation of the resulting dihydropyran ring. This isomerization-cycloaddition sequence was successfully applied to the synthesis of (+)-platyphyllide [134]. 

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). 

The third adduct (669) was an ether after similar functionalization of the dihydropyran ring to the L-gulo system, 3-0-(benzyl 4-deoxy-/3-L-gulopyranosid-4-yl)- l,2 5,6-di-0- isopropylidene-a-D- glucofuranose (672) was obtained. Cleavage of the ether bond between the sugar units was possible35 by alkaline -elimination. 

Functionalization of the dihydropyran ring to a hexose system allowed the preparation of a remarkable variety of di- and tri-saccharides for example, hexopyranosyl-(1 3)-D-glucoses of the a-D-... 

Except for the disaccharide synthesis, in which introduction of natural sugars produced optically active diastereoisomers at the dihydropyran ring stage, most transformations leading to sugars was done with racemic substrates. Alcohol 3 was resolved into enantiomers by crystallization of its 6-O-camphanyl ester and subsequent hydrolysis. Recent discoveries of enantioselective catalysts and reagents permit one to obtain basic... 

Disaccharide precursor 9 was converted into 3-(oi-L-altropyranosyl)-l,2 5,6-di-0-isopropylidene-a-D-glncofuranose by functionalization of the dihydropyran ring [22],... 

Analysis of the 13C chemical shifts of C-2 and C-6 in a series of 2-substituted 6-methoxy-3,6-dihydropyran-3-ones (82) allows the assignment of configuration to the isomeric pairs. C-2 is shielded by ca. 5 p.p.m. and C-6 by ca. 2.5 p.p.m. in the trans isomer compared with the corresponding signals of the cis compound (79MI22200). The upheld shift is associated with the 1,3-diaxial disposition of the substituent and the proton on the y-carbon atom. This y-effect has also been noted in the dihydropyran ring system (74MI22200). 

In the case of the gem diester (121), the major pathway results in aromatization of the dihydropyran ring with production of the pyrylium species (121a). This is derived from the [M - C02Et]+ ion with subsequent loss of carbon monoxide and elimination of ethanol. It was shown that RDA fragmentation of (121), a minor pathway only, is a thermally induced process (72CJC1539). 

An anomeric effect is also shown by the hydroxy group in the dihydropyran ring of the anticoagulant drug warfarin (165 R = Ph). The related trans-methylketal (165 R = Me) exists as the all-staggered half-chair conformation in which the alkoxy group is axial. However, the cis isomer interconverts between the two half-chair conformations (79JOC798). 

Hot mineral acids open dihydropyran rings and recyclization to a benzene ring is feasible when a suitable substituent is present, as for example in 6-ethyl-3-vinyl-2,3-dihydro-4H-pyran (605) (74T1015). 

Presumably the ketone experiences less rotational restriction than the amide, but any enhancement in flexibility had little apparent effect on efficacy in this case. The relatively poor inhibitory capacity of the ether 109 was explained in terms of the energy penalty created by differences between the bound and solution conformation of the dihydropyran ring. 

The a-oriented alkynyl sugar 166, formed originally, is transformed to the /1-isomer 169 by forming the Co complex 167. Due to the stable propargylic cation of the Co complex, facile opening and closing of the dihydropyran ring in 167 yield the thermodynamically stable //-isomer 168 [43]. 

With the exception of the MPT precursor, all of the isolated pterins (Eq. 3) are fully oxidized and all lack the dihydropyran ring found ciystallographically in the active cofactor. This observation is not surprising, given the reversible nature of pterin alkoxylation and the oxygen sensitivity of reduced pterins. 

Two syntheses of (+ )-lycoramine (153) have been reported from the same group. The first approach required the construction of rings A and B, then of the seven-membered ring C, followed by the formation of the dihydropyran ring. Thus, the starting material was aldehyde... 

Treatment of cycloprop [c]pyrans 520 with acid leads to chromans 521 with a 5,8-substitution pattern, which is difficult to obtain by other synthetic methods (Scheme 114). This novel reaction is believed to proceed via a retro-hDA opening of the dihydropyran ring to afford 522 followed by an acid-catalyzed aldol-type cyclization and dehydration (Scheme 114) <20040L3191>. 

The Cobra motif (Fig. 27) is characterized by the bending of the side chain such that its dihydropyran ring lies over the center of the face of the macrolide ring. All but one of the conformers of this class possess unreasonably high energies according to... 

From a study of the i.r. spectra of a series of furoquinoline alkaloids containing 4-methoxy-groups, it was concluded that the intensity of ring skeletal vibrations varies with the position of other methoxy-groups and with the nature of the substituents at C-7.17 The conformational mobility of the dihydropyran ring of... 

At 150 °C, (+)-mahanimbine (5) undergoes racemization, presumably by opening and re-closure of the dihydropyran ring. At higher temperatures (200 °C), curryanine (cyclomahanimbine) (6) and curryangine (7) are formed 5 the structure... 

Platinum-catalyzed cyclization of a 2,3-dihydrofuran to the tethered alkyne provided the fused tricyclic compound 238, as shown in Scheme 71. Acid-promoted benzannulation of 238 then produced the dihydrobenzofuran, presumably via a retro-hetero-Diels-Alder opening of the dihydropyran ring <20040L3191>. 

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