2-Methyl-1,3-dioxane, conformational

A number of cis/trans 4,6-dialkyl-2,2-dimethyl-l,3-dioxanes were studied by C NMR spectroscopy (93JOC5251). The C NMR shifts of C -Me groups (Scheme 8) were found to be very sensitive to the 1,3-dioxane conformation [chair form Me(ax) ca. 19 ppm and Me(eq) ca. 30 ppm— pure 30.89 ppm in the twist-boat form both methyl carbons resonate at ca. 25 ppm (pure 24.70 ppm)]. With these values, AG° of the chair to twist-boat equilibrium was calculated (Table IV). For 13a (nitrile), 13b (alkyne), and 13e (methyl ester) (Scheme 8) in CH2CI2, the temperature dependence of the AG° values was determined. Depending on the substituent, small negative or positive entropy terms were found generally the enthalpy term dominates the -AG° value. In the tram isomers 13, the cyano and alkyne substituents favor the chair conformation, but CHO, ester, alkene, and alkyl substituents, respectively, clearly favor the twist-boat conforma-... 

The preferred conformations of both 1-methyl-l-phenylcyclohexane and 2-methyl-2-phenyl-l,3-dioxane have the phenyl group in the axial orientation even though the conformational flee energy of the phenyl group (2.9 kcal) is greater than that for a methyl group (1.8 kcal). Explain. 

Cyclic Esters of Phosphorous Acid.—A large number of 2-substituted-4-methyl-l,3,2-dioxaphospholans (88) have been prepared and their stereochemistry and conformations investigated by 1H and 31P n.m.r.69 Unlike the corresponding 1,3-dioxans, the tra/w-isomer (88a) is favoured in all cases, and each isomer is best described in terms of two rapidly equilibrating half-chair conformers with the 4-alkyl group pseudo-axial or pseudo-equatorial. 

Pihlaja and Rossi [83ACSA(B)289] prepared l,3-dioxan-2-one and all of its methyl derivatives, recorded their C NMR spectra, and derived the methyl substituent shift parameters by a multiple linear regression analysis of the anancomeric and two equivalent chair conformers (Table X). With these values, the authors estimated the conformational equilibria for two unequally populated chair conformations (Nos. 2, 3, 9, 11, and 14 in Table X). A consistent picture of the predominance of the chair conformation and the corresponding chair chair equilibria in l,3-dioxan-2-ones was obtained in complete agreement with earlier H NMR results. 

For compound 22, X-ray analysis demonstrates that the dioxane ring adopts the chair conformation and that the imidoyl amino group prefers an axial conformation <2002T2621>. For (l,4-benzodioxin-2(3//)-yl)methyl sulfamic acid ester 21, the conformation of the dihydrodioxin ring is close to an ideal half-chair and for 1,2,4,6,7,9-hexafluoro-... 

The constrained bis(oxazolines) 9a and 9b can be constructed beginning with malononitrile 32 as shown by Ghosh and co-workers. Thus, treatment of 32 with anhydrous hydrochloric acid in dioxane, as shown by Lehn and co-workers, yielded imidate salt 33 (Fig. 9.9). Condensation of the imidate salt with commercially available (15,2/ )-l-aminoindan-2-ol afforded the conformationally constrained bis(oxazoline) inda-box 9a. Alkylation at the bridging methylene of 9a was carried out by Davies and co-workers.Treatment of 9a with lithium diisopropylamide followed by alkylation with methyl iodide afforded 9b. Alternatively, alkylation with diiodoalkanes incorporated ring systems at the bridging position (structures 34a-d). 

The force field calculations of the enthalpies (AAiT) of the four hemiketals of 6-methylpyrano[2,3-/ ]dioxane-8a-ols clearly indicate that the /3-m-fused link is the most favored, followed by the a-cis- and /3-/ra r-isomers, while the a-/ra r-compound, due to the pyranoid ring being forced into 4 conformation with an axial methyl group, is the least likely to be formed <1991MI235>. 

The racemic form of methyl 3,4-anhydro-/3-mycaroside (82) has been synthesized 35 it is an epoxide of a branched-chain sugar having the oxirane ring fused to the pyranose ring. Hydrolysis of the oxirane ring with hydrochloric acid in aqueous p-dioxane occurs with substitution at C-4, the (more accessible) secondary position, as would be expected from its favored °H, conformation (82) anticipated, because it is an analog of the 3,4-anhydro-D-allopyranoside derivatives considered previously (see p. 148). 

Intermediate-intensity positive n -+ r Cotton effects (all in dioxane) of the 3-oxotriterpenoids 54, 57-59 were attributed to an A-ring chair-boat conformational equilibrium. The contribution of the boat form (like in 2/3-methyl model derivatives 53 and 56) was estimated to be ca 30%140. 

From methyl a-D-glucopyranoside, 55% of 2-O-benzyl and 18% of 3-O-benzyl derivatives could be obtained using 1,4-dioxane as a solvent. No monobenzyl derivative was observed under standard DMF solvent conditions [131]. Conformational equilibria 4Ct XC4 play a role in the results of alkylation of all-equatorial methyl P-D-xylopyranoside and methyl P-D-glucopyranoside [131]. 

The X-ray structures of two anthraquinone derivates of 1,3-dioxane were published. In both 1 -methoxy-4-(2-methylprop-2-enyloxy)-2-[(2i ,6i )-4,4,6-trimethyl-1,3-dioxan-2-yl]-anthraquinone and 4-hydroxy-3-(2-methyl prop-2-enyloxy)-2-[(27 ,67 )-4,4,6-trimethyl-l,3-dioxan-2-yl]anthraquinone the 1,3-dioxane ring adopts the chair conformation and the substituents in positions 2 and 6 are in equatorial conformations (99AX(C)436). Finally, Freeman et al. (02JMS(T)43), employing both ab initio theory and density function theory, calculated the energies of chair, half-chair, sofa, twist, and boat conformers of 1,3-dioxane. 

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