Enamines hydroboration

A carbonyl transposition based on enamine hydroboration has been described by Gore and coworkers167-169. The same reaction attempted on enamines of 5a-3-keto-steroids resulted in an unusual reactivity of diborane, leading to reduction of the enamine double bond. The 3 a-derivative prevailed over the 3/2-derivative as a consequence of the presence of the 19-methyl group, as demonstrated successively for the 19-nor-steroids170 (Scheme 117). 

Hydrogenation of olefins, enols, or enamines with chiral tVilkinson type catalysts, e.g., Noyort hydrogenation. Hydroboration of olefins with chiral boranes. Sharpless epoxi-dation of allylic alcohols. 

Such functional groups as OR, OH, NH2, SMe, halogen, and COOR may be present in the molecule," but not groups that are reducible by borane. Hydroboration of enamines with 9-BBN provides an indirect method for reducing an aldehyde or ketone to an alkene, for example. 

Enamines can also be converted to amino alcohols via hydroboration. Allene-boranes react with aldehydes to give alkyne-alcohols. ... 

A biomimetic synthesis of benzo[c]phenanthridine alkaloids from a protoberberine via the equivalent of a hypothetical aldehyde enamine intermediate has been developed (130,131). The enamide 230 derived from berberine (15) was subjected to hydroboration-oxidation to give alcohol 231, oxidation of which with pyridinium chlorochromate afforded directly oxyche-lerythrine (232) instead of the expected aldehyde enamide 233. However, the formation of oxychelerythrine can be rationalized in terms of the intermediacy of 233 as shown in Scheme 41. An alternative and more efficient... 

The oxidation of hexahydroindolo[2,3-a]quinolizin (574) with benzoyl peroxide, followed by reduction and removal of the benzoyl group, gave the cis alcohol (575), while hydroboration-oxidation of the same enamine (574) yielded... 

Asymmetric hydroboration of enamines with chiral diboranes, followed by oxidation with hydrogen peroxide, in aqueous sodium hydroxide, gives /1-amino alcohols in good yields and high ee (equation 29)153. The products of this reaction are useful in medicinal applications and as synthons for further synthetic elaboration. 

Enamines - alkenes.1 Hydroboration of (E)-enamines of aldehydes or ke- ones with 9-BBN followed by methanolysis (retention) affords alkenes in 60-80%... 

Hydroboration-oxidation of enamines has been reported by Borowitz and Williams171 to furnish the corresponding trans amino alcohols (Scheme 119). 

HCN, 193, 442 de-conjugation, 159 dehydrogenation, 189 enamine formation, 185-186 enolisation, 156 ei seq. epoxidation, 18, 201, 442 formation, 109, 189, 248, 337 hydroboration. So Mannich reaction (aminometh-ylation), 172 methylation, 168-171 methylene addition, 202 oxidation, Baeyer-VUliger, 349, 445... 

A number of atypical reactions of enamines have been developed. For example, hydroboration-oxida-tion affords stereochemically defined, vicinal amino alcohols (equation 26). Similarly, enamines have been shown to undergo reaction with diazomethane to form substituted cyclopropanes. These materials can be further transformed to a alkylated ketones by thermolysis followed by hydrolysis (equation 27). ... 

The structure of (24) was established by the chemical correlation shown in Scheme 3. Reduction of (24) with NaBH4 gave a mixture of (26), O-to-N acyl migration product, and (27) the latter was methylated to (29) which was identical with the product formed from the enamine (28) by oxidative hydroboration. 

The ketone 629 has been isolated from peony flowers. It had already been synthesized several times for example, by hydroboration of the enamine 630 of the isomeric ketone 627, then Cope elimination of the iV-oxide 631 of the amino alcohol. Chromyl acetate oxidation of 1,8-cineole (553) led first to 629 further oxidation gave diketones. Aspergillus niger also gave some 629, although the main products were the corresponding alcohols. ... 

Enamines of unsaturated aldehydes can be converted into the corresponding dienes for example, citronellal, a chiral aldehyde, gives the chiral non-conjugated diene, /f-citronellene, in 92% enantiomeric excess - (Scheme 172). The combination of the Lewis hydroboration procedure with the thallium acetate acetoxylation of enamines provides a means of converting enamines into acetoxycycloalkenes (Scheme 173). The hydroboration oxidation of enamines to give -aminoalcohols was first reported by Borowitz and Williams . Cw-l,2-addition of BH3 occurs, to give the trans-ji-ammocy-... 

The stereochemistry needed for swainsonine 192 is introduced by hydroboration - borane adds to the face of the alkene opposite the acetal in 186 and the regioselectivity is determined by the nitrogen atom since the alkene 186 is an enamine. 

Keywords A(1,2) strain A(1,3) strain Enamine Epimerization [2,3] and [3,3] sigmatropic shifts Intramolecular SN2 reaction Iodolactonization Hydroboration Conjugate addition 1,3-oxazolidinones Diastereoselectivity... 

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