Diastereofacial bias

The next key step, the second dihydroxylation, was deferred until the lactone 82 had been formed from compound 80 (Scheme 20). This tactic would alleviate some of the steric hindrance around the C3-C4 double bond, and would create a cyclic molecule which was predicted to have a greater diastereofacial bias. The lactone can be made by first protecting the diol 80 as the acetonide 81 (88 % yield), followed by oxidative cleavage of the two PMB groups with DDQ (86% yield).43 Dihydroxylation of 82 with the standard Upjohn conditions17 furnishes, not unexpectedly, a quantitative yield of the triol 84 as a single diastereoisomer. The triol 84 is presumably fashioned from the initially formed triol 83 by a spontaneous translactonization (see Scheme 20), an event which proved to be a substantial piece of luck, as it simultaneously freed the C-8 hydroxyl from the lactone and protected the C-3 hydroxyl in the alcohol oxidation state. 

As a result of the combination of intra- and extra-annular chirality transfer, a productive approach has been developed for the design of chiral enolate systems in which a structurally organized diastereofacial bias is established, as illustrated in the equations in Scheme 2-11. A lithium-coordinated five-membered or six-membered ring fixes the orientation between the inducing asymmetric center and the enolate20 ... 

Alternatively, it was expected that chelation with the P-nitrogen in the enolates resulting from oxazolines (182) would impose a diastereofacial bias opposite to that found in the lactones (181). Deprotonation of (182) afforded enolates which were... 

Glucose-derived diene (451) displays a notable diastereofacial bias on addition to dienophiles. Its preparation relies on the alkylation of tetraacetylglucopyranosyl brcmiide (450) with sodium salt (449) followed by 0-silylation of the resulting enone (Scheme 106). "... 

In their synthesis of (-i-)-damavaricin D (Fig. 11-24), Ronsh and co-workers used crotylboronate methodology three times in the assembly of the C(I)-C(13) polypropionate segment 250 [178, 204, 205]. The synthesis of 250 was designed so that chain growth occurs from C(13) to C(l) and such that the mismatched reaction necessary to install the C(10)-C(12) anp. anfl-dipropionate stereotriad could be dealt with early in the synthetic sequence, when the aldehyde substrate had a relatively modest diastereofacial bias (Scheme 11-9). 

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