1,1-dibromoalkenes reduction

Alkenylstannanes react with 1,1-dibromoalkenes to give enynes.204 These reactions are thought to involve elimination of the elements of HBr prior to reductive elimination. 

Applying the procedure just described, Rouzaud and Sinay (65) described a stereospecific synthesis of a C-disaccharide in which a methylene group takes the place of the interunit oxygen atom. Starting from 51a and the dibromoalkene 59, the hemiacetal 60 was obtained. Reduction of 60, followed by catalytic hydrogenolysis, gave the /J-(l — 6)- C-disaccharide 61. 

The dibromoalkene S-40 can be prepared from S-ethyl lactate by introduction of the MEM (methoxyethoxymethyl) protecting group, reduction to the O-protected lactaldehyde and Corey-Fuchs carbonyl olefination (Scheme 19). The l -enantiomer of 40 is available analogously from f -isobutyl lactate and serves as the reagent in the enantiomeric series. The lithium carbenoid S-41 is generated from S-40 by treatment with n-butyllithium in diethyl ether and reacted with aliphatic and aromatic aldehydes in tetrahydrofuran. High diastereoselectivities are reached, as shown in Scheme 19 . ... 

Reduction of gem-dibromides. Triethyl phosphite and triethylamine reduces gent-dibromocyclopropanes and gem-dibromoalkenes to the corresponding monobromides. 

Reduction of bromides. gem-Dibromocyclopropanes and g m-dibromoalkenes are reduced to the monobromides by (C,H50)2P(0)H and N(CiHdv Under these conditions gem-bromochlorocyclopropancs are reduced to chlorocyclopropanes.- l,l-Dibromo-2-tri-methylsilyloxycyclopropanes or a-bromo-a, 3-enones are reduced in this way to [3,7-cnones. Other examples indicate that only activated halogen atoms are subject to this reduction. 

Vicinal dibromoalkenes and diiodoalkenes undergo reduction, with loss of both halide ions, to yield alkynes that (depending on the presence of activating substituents) may be further reduced to olefins and saturated species work by Rosenthal and coworkers... 

Reduction of gem-dibromoalkenes to monobromoalkenes with diethyl phosphite and triethylamine. 

The C-disaccharide 43 (2-deaminotunicmnycin) and its 7-epimer have been synthesized by addition of the 6-deoxy-6-diazo-D-galactose derivative 40 to aldehyde 29, followed by LiBHEta-reduction of the ketone 41 and epoxide 42 thus obtained as a separable mixture (Scheme 6). The preparation of the first pseudo-C-disacchande by use of the alkenyllithium compound derived from dibromoalkene 44 is covered in Chapter 18. 

The radical reduction (Bu3SnH, AIBN) of tetra-O-acetyl-p-D-glucopyranosyl bromide is known to afford predominantly the product of acetyl migration i.e. l,3,4,6-tetra-0-acetyl-2-deoxy-a-D-glucopyranose. In the presence of diphenyl diselenide (or PhSeH) this migration is inhibited and the direct reduction product is present in >95%. Treatment of dibromoalkene 9 with butyl lithium generated the corresponding alkynyl lithium which was successfully added to a cyclohexanone derivative. ... 

Use of Sml2 in benzene-HMPA mixture in reduction reactions of vicinal dibromoalkenes... 

---