M-Dioxane ring

In ethylidenation reactions (R = R " = H R = R =Me), compound 47 preponderates over compound 49, whereas in acetonation reactions (R-R" = Me), the reverse is true. This fact indicates that the m-dioxane ring is more stable in the diethylidene acetal than in the diisopropylidene acetal, a fact that can be attributed to the absence of an axial methyl group in an ethylidene acetal having a m-dioxane moiety in the chair conformation. 

The furanoid part of 2,3 4,6-di-O-isopropylidene-a-L-sorbofuranose (3) was also assigned a Tl conformation this compound was considered to be particularly stable because the conformation assigned to the furanoid part is stabilized by both the 1,3-dioxolane and the m-dioxane rings. More-detailed conformational data are needed for... 

An enormous number of different 1,3-dioxane structures have been reported since 1996 in Figure 3, mono-, bicyclic and spiro variants are presented, while Figure 4 contains examples of tricyclic structures with the 1,3-dioxane moiety. The conformations, bond lengths, bond and dihedral angles of the 1,3-dioxane rings are determined by the ring fusion, the attached substituents, and the presence of exocyclic double bonds. Thus, published structures are classified as either monocyclic (mono), spiro-substituted (spiro), bicyclic (bi), or tricyclic (tri). The well-known Meldrum s acid derivatives (M) have been most intensively studied. 

Fusions of two ra-dioxane rings are found in the 1,3 2,4- and 2,4 3,5-diacetals of glycitols, and fusions of a m-dioxane and a pyranose ring in the 4,6-acetals of glycopyranosides. If the constituent rings retain the chair forms, the bicyclic systems with trans junctions will be rigid and completely analogous to [Pg.19]

It follows from the H-n.m.r. spectra of the 1,6-anhydrohexofuranose peracetates (see Table IV) that their 1,3-dioxane ring exists in the alCa4 conformation99,113,119,121,793 and their oxolane ring favors the E0 conformation,119 although, for the D-g alacto triacetate (271), the E4 conformation of the oxolane ring was assumed.119,783 For l,6-anhydro-/3-... 

Dichloro- and -diphenyl-1,4-dioxan The so-called cis 2,3-dichloro-l,4-dioxan, m.p. 52°, is actually the irtms-isomer the 2,3-diphenyl-l,4-dioxans with m.ps. 46° and 132° should be designated cis and trans respectively the dioxan ring has a chair conformation throughout Chen and Le Ffevre, 1965... 

In British practice the locant (or locants) is placed immediately before the syllable that it qualifies (see p. 35), as in hexan-3-one, hexane-2,3-dione, 1-chloro-hexan-3-one, hex-3-en-l-yne, pyrazol-3-yl, carbazole-2-carboxylic acid. In USA, one locant (or one set of locants) for a suffix is placed in front of the stem name, as in 3-hexanone, 2,3-hexanedione, l-chloro-3-hexanone, 3-hexene, 3-pyrazolyl but only one numeral (or set of numerals) is so placed any others precede the syUable that they qualify, 3-hexen-l-yne, 3-cyclohexen-l-one (the 1 maybe omitted in the second name, giving 3-cyclohexenone, where the 3 refers to the unsaturation en ). There are, however, many cases where parent names carry locants in front of them, e.g., m-dioxan, 1,7-naphthyridine (where the heteroatoms in the ring are cited by the locants 1,7 or m) in such cases the additional locants for suffixes are all placed after the parent name, as in m-dioxan-4-carboxylic or l,7-naphthyridin-2(lfl )-one. The tendency in Chemical Abstracts is to place the locants after the parent name whenever ambiguity might arise it is matched by a tendency in Great Britain to place one numeral in front of the parent name when no ambiguity can arise. A compromise seems to be working itself out unofficially. 

The axial equatorial ratio for oxygen in 1,3-oxathian 3-oxide in [ He]acetone is greater than 8 1 at -95 C however, in CDCI3 at 38 °C, no preference exists. In 1,4-oxathian 4-oxide the conformation with oxygen axial is preferred by 0.68 kcal mol at - 80 °C. Equilibrations and 220 MHz n.m.r. studies of various 2-alkyl-4-methyl- and 2-alkyl-4,4-dimethyl-l,3-oxathiolans confirmed earlier conclusions that the 1,3-oxathiolan ring is less flexible than the 1,3-dioxan ring, and somewhat more puckered. The data indicated a preferred envelope structure, in which either the oxygen or C-5 was the flap atom. 

When two heteroatoms are present in a saturated six-membered ring their effects are approximately additive. Apart from the case of two a oxygen atoms—in 1,3-dioxane (48) the shift of C-2 is S 95.4 instead of 5 108 which a double shift of 40 p.p.m. would require—predictions of shift made on this basis are usually accurate to within 5 p.p.m. and are generally much closer than this. Observed shifts for a few representative examples are shown in structures (48)-(52). 

The -position of the cyclopropane ring was established by the observed negative circular dichroism curve (Ae — 2.03 in dioxane at 298 m/i) and consideration of the inverse octant rule for cyclopropyl ketones. ... 

Early efforts to effect the photoinduced ring expansion of aryl azides to 3H-azepines in the presence of other nucleophiles met with only limited success. For example, irradiation of phenyl azide in hydrogen sulfide-diethyl ether, or in methanol, gave 17/-azepine-2(3//)-thione35 (5% mp 106—107 " O and 2-methoxy-3//-azepine (11 %),2 3 respectively. Later workers194 failed to reproduce this latter result, but found that in strongly basic media (3 M potassium hydroxide in methanol/dioxane) and in the presence of 18-crown-6, 17/-azepin-2(3//)-one was produced in 48% yield. In the absence of the crown ether the yield of azepinone falls to 35%. 

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