1,3-Dioxolane ring conformation

Ci9H2408Ss Ethyl 3,7-anhydro-6,8-0-benzylidene-4-deoxy-2-(ethyl-enedithio)-D-talo-2-octulosonate (ABTOET)265 P212121 Z = 4 Dx = 1.359 R = 0.045 for 1,520 intensities. The structure contains a fused pyranoid ring and a dioxolane ring. The conformation of the pyranoid part is a distorted 6C3, with Q = 57 pm, 6= 20°,

chair conformations of the dioxolane and the dithiene rings... 

P2j Z = 4 Dx = 1.357 R = 0.048 for 1,989 intensities. The crystal structure contains two symmetry-independent molecules having almost the same conformation. The lactone rings are planar the conformation of the dioxolane ring in one is close to an envelope, and that in the other has a twist conformation. The linkage between the rings, as defined by 0-4-C-4-C-5-0-5, is -65 and —75°. The two independent molecules are hydrogen-bonded into dimers by the two N—... 

More recently McDonald et al. reported a rare example of 7-membered oxacyde formation based on the same concept [21]. Thus, polyoxygenated terminal alkynes 60 containing the dioxolane structure in the tether gave 7-endo cyclized glycals 61 in good yield. It is necessary to have the dioxolane ring in the tether for efficient reaction, probably favoring the suitable conformation for cyclization (Scheme 5.21). 

The X-ray crystal structure of fZ)-4-[(5)-2,2-dimethyl-l,3-dioxolan -ylmethyl-ene]-2-phenyl-5(4//)-oxazolone has been determined. " The analysis shows an almost planar disposition for the entire molecule with the exception of the dioxolane ring that adopts an envelope conformation. As such, the dioxolane ring is mainly situated on the si,si diastereotopic face of the olefinic bond, a situation that accounts for the observed diastereoselectivity in Diels-Alder reactions. 

P212121 Z = 4 Dx = 1.365 R = 0.047 for 736 intensities. The pyrano-side has a distorted 2S0 or B2i5 conformation the dioxolane ring is 0-4E (52°), such that all substituents are equatorially or quasi-equa-torially attached. 

P212121 Z = 4 Dx = 1.25 R = 0.066 for 1,234 intensities. The conformation of the septanose is a 5,6C2 (d) twist-chair. Both dioxolane rings are envelopes, 2E (17°) and lE (16°). The chloroacetate group is almost planar, with the largest deviation (of 11°) in the C-7-0-5 torsion-angle. There is no hydrogen-bonding, which accounts for the relatively low density. 

From the previous sections on theoretical and spectral aspects of 1,3-dioxolanes, it is clear that the ring is conformationally labile and the data are best explained in terms of a freely pseudorotating system with a small energy barrier between conformers (B-80MI43001). The half-chair (39) and envelope (40) forms are the two lowest-energy conformers and it is the interconversion of these structures which best describes the pseudorotation (75JA1358). The X-ray structure of (1) shows the dioxolane ring has a conformation mid-way between these forms. 

The presence of additives in the reaction mixture, however, can significantly enhance the selectivity of nucleophilic addition to (13 equation 8). Very high ratios (>95 5) are observed when the addition of furylHAium to (13) is carried out in the presence of Zn or Sn salts (Table 7). Hie stereochemistry of the product is explained by a conformation in which the zinc or tin atom coordinates with the carbonyl oxygen and the 3-oxygen of the dioxolane ring (Figure 8). Nucleophilic attack from the less-hinder face selectively produces (16). At present, the applicability of this reaction to other substrates is not known. 

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