2/f-Pyran, 3,4-dihydro

H-Pyran, 3,4-dihydro-2-methoxy-4-methyl-synthesis, 3, 772 2/f-Pyran, dimethoxy-reactions... 

Dihydro-2-methyl-2/f-pyran-3,4-diol IS,AS)-form, D-679 Galactal D-form, G-I... 

As oxygen is divalent, no strict equivalent of benzene exists, although the pyrylium cation does achieve aromaticity (see Chapter 1). Both 2/f-pyrans and 4/f-pyrans are known, but are encountered more frequently as their carbonyl analogues pyran-2-one and pyran-4-one (see Box 4.1). In addition, reduced forms such as 3,4-dihydro-2//-pyran and 3,4,5,6-... 

Six-membered oxaenoncs [e.g., 5,6-dihydro-2i/-pyran-2-one,109, 2,2-dimethyl-l,3-dioxin-4-one,110-112 2,2-dimethyl-2/f-furo[3,4- ]pyran-4,7(3//,5//)-dione113] also give cyclobutanes on irradiation in the presence of alkenes. The corresponding six-membered thiaenones usually deactivate via ZjE isomerization.114... 

When reacted with an alcohol in the presence of a catalytic amount of a strong acid, 3,4-dihydro-2/f-pyran yields a base-stable 2-alkoxytetrahydropyran (a cyclic acetal) (Scheme 4.13). The reaction is reversed if the acetal is treated with aqueous acid (see Section 3.3.4, page 54), so that this provides a simple way of protecting alcohols in syntheses where a strong base might otherwise deprotonate them. The conformational preferences of 2-alkoxytetrahydopyrans, mediated by the anomeric effect, were commented upon earlier (Section 1.5.3). 

Shaking of rac-3,4-dihydro-6-methyl-2-(l-oxopropyl)-2/f-pyran with baker s yeast resulted in initial formation of racemic 6-hydroxy-2,7-nonanedione and then by kinetic resolution/ diastereoselective reduction in the production of (67 ,7S)-6,7-dihydroxy-2-nonanone (2) where both relative and absolute configuration had to be determined (see p 470)134. 

V-Unsubstituted dihydropyridines can exist in at least five tautomeric forms (Section 2.2.5.2). At least for /V-substituted compounds 1,4-dihydropyridines (cf. 453) are generally more stable, by ca. 9 kJ mol than the 3,4-dihydro and the 1,2-dihydro isomers (cf 454). By contrast 2/f-pyrans appear to be thermodynamically more stable than 4//-pyrans. All three types of 1,3-oxazine are known. 

Hetero-Diels-Alder reaction with inverted electron demand between a, 3-unsatu-rated carbonyl compounds (1-oxa-l,3-butadienes 11 Scheme 6) and enol ethers provides an access to 6-alkoxy-3,4-dihydro-2/f-pyrans 12 [31,32]. These heterocycles are also useful... 

The C=C stretching frequencies of 3,4-dihydro-2//-pyran (13) and 5,6-dihydro-2/f-pyran (14) occur at 1630 cm-1 <58MI22200> and 1640 cm-1 (58CB1S89), respectively. In the former series, the influence of substituents at a saturated carbon is small and somewhat variable, but a methyl or methoxy substituent at C-5 or C-6 raises the frequency by ca. 40 cm-1 (55JA4571, 78JOC667, 81HCA1247). 

Analysis of the far IR-spectra of 3,4-dihydro-2//- pyran (13) (72JCP(57)2572> and 5,6-dihydro-2/f- pyran (14) (81JST(71)97> indicates that for both molecules the most stable conformation is a half-chair form. The barrier to planarity is greater for the former compound. These preferred structures are in accord with the half-chair conformation established for cyclohexene and its derivatives. The conformational mobility of cyclohexene is greater than that of the 3,4-dihydropyran. The increased stabilization of the pyran has been attributed to delocalization of the v- electrons of the alkenic carbon atoms and the oxygen lone-pairs (69TL4713). 

Cycloadditions involving the more nucleophilic vinyl ethers are easier than those above and the reaction has considerable synthetic potential. The reactants are heated at 180-190 °C in a sealed vessel and the adduct is rapidly formed in high yield (50JA3079, 51JA5267). Full experimental details have been published for the synthesis of 3,4-dihydro-2-methoxy-4-methyl-2/f-pyran from methyl vinyl ether, and the same technique was used to prepare a further 13 dihydropyrans (630SC(4)3il>. 

Naphtho[l,2-f>]pyran-4-one, 3,6-diacetyl-2-methyl-deacetylation, 3, 713 Naphtho[ 1,2-6]pyran-4-one, 5,6-dihydro-synthesis, 3, 811... 

Naphtho[2,l -f>]pyran-1 -one, 2,3-dihydro-dehydrogenation, 3, 724 halogenation, 3, 731 N aphthopyranones synthesis, 3, 805 Naphthopyran-2-ones, dihydrosynthesis, 3, 802 Naphtho[l, 2-b]pyran-2-ones synthesis, 3, 802 Naphtho[2,1 -h]pyran-3-ones synthesis, 3, 803, 856 Naphtho[2,1 -c]pyran-4-ones synthesis, 3, 831 Naphtho[2,3-c]pyran-l-ones synthesis, 3, 831 Naphthopyrans lH NMR, 3, 580 IR spectra, 3, 594 synthesis, 3, 743, 748, 750, 763 UV spectra, 3, 598 Naphthopyrans, dihydrosynthesis, 3, 778, 783 Naphtho[ 1,2-f>]pyran-4-thione, styryl-properties, 3, 708... 

AH- Pyran, 2-alkoxy-4-methyl-2,3-dihydro-conformation, 3, 630 4H-Pyran, 2-amino-IR spectra, 3, 593 synthesis, 3, 758 4/f-Pyran, 4-benzylidene-synthesis, 3, 762 4/f-Pyran, 2,3-dihydro-halogenation, 3, 723 hydroboration, 3, 723 oxepines from, 3, 725 oxidation, 3, 724 reactions, with acids, 3, 723 with carbenes, 3, 725 4/f-Pyran, 5,6-dihydro-synthesis, 2, 91 4/f-Pyran, 2,6-diphenyl-hydrogenation, 3, 777 4/f-Pyran, 6-ethyl-3-vinyl-2,3-dihydro-reactions, with acids, 3, 723 4/f-Pyran, 2-methoxy-synthesis, 3, 762 AH- Pyran, 2,4,4,6-tetramethyl-IR spectra, 3, 593 AH- Pyran, 2,4,6-triphenyl-IR spectra, 3, 593... 

Analogous photorearrangements have been reported for other cyclic peroxides,194 and the conversion of the peroxide (243) to the benzo-furo[3,2-h]naphtho[l,2-d]furan (244) is believed to involve an intermediate oxiran.195 Studies of the photodecomposition of 3,4-dihydro-2/f-pyrans,196 tetrahydropyran,197 and 1,4-dioxan198 have been reported. 

The [4 + 2 ]cycloaddition of the carbonyl group of aldehydes as well as of ketones and 1,3-butadienes is a well established method for the synthesis of 5,6-dihydropyrans which are useful substrates for the preparation of carbohydrates and many other natural products. Several excellent reviews on this topic have appeared [10-12,14,22]. The first example of this type of reaction using 2,4-dimethyl-1,3-butadiene and formaldehyde to give the 2,4-dimethyl-5,6-dihydro-2/f-pyran in 60% yield was published by Gresham and Steadmen in 1949 (Scheme 2-1, Eq. 1) [57]. 

---