Olefinic stereocenters, removal

The retrosynthetic elimination of olefinic stereocenters (E or Z) was illustrated above by the conversion 147 => 148 under substrate spatial control. It is also possible to remove olefinic stereocenters under transform mechanism control. Examples of such processes are the retrosynthetie generation of acetylenes from olefins by transforms such as trans-hydroalumination (LiAlH4), ci5-hydroboration (R2BH), or ci -carbometallation... 

The completion of the synthesis of the polyol glycoside subunit 7 requires construction of the fully substituted stereocenter at C-10 and a stereocontrolled dihydroxylation of the C3-C4 geminally-disub-stituted olefin (see Scheme 10). The action of methyllithium on Af-methoxy-Af-methylamide 50) furnishes a methyl ketone which is subsequently converted into intermediate 10 through oxidative removal of the /j-methoxybenzyl protecting group with DDQ. Intermediate 10 is produced in an overall yield of 83 % from 50) , and is a suitable substrate for an a-chelation-controlled carbonyl addition reaction.18 When intermediate 10 is exposed to three equivalents of... 

Red-Al) or the Z-olefin 341 after benzylation. Stereoselective dihydroxylation and regiose-lective protection of the diol provides protected polyol 342 that is transformed in three steps to terminal epoxide 343. Removal of the p-methoxybenzyl group followed by acid-catalyzed epoxide opening yields exclusively tetrahydropyran 344 after protection of the primary alcohol and deacetylation. Inversion of the C2 stereocenter by an oxidation-reduction sequence and deprotection furnishes the C-mimic of a,a-trehalose 345. 

Protection of the 3-OH with a benzyl group, then removal of the THP group followed by a Mitsunobu reaction affords 976, in which the hydroxyl stereocenter at C-2 is inverted. Conversion of the olefin to an aldehyde results in lactol formation, and subsequent oxidation furnishes lactone 977. Removal of the benzyl protecting group by hydrogenolysis gives diastereomer 978. 

---