Alcohols aromatic, reductive cleavage

The rigidity of the hexacyclic cage structure of koumine (18) renders some of its chemical behavior quite unusual, for instance, the resistance to Hofmann degradation shown by /Va-acetyldihydrokoumine methyl hydroxide (27). However, owing to the presence of a /J-aromatic imino system in 18, reductive cleavage by sodium-alcohol to yield dihydrokouminol (39) proceeds smoothly. This reaction has been considered to occur through a radical-anion mechanism as indicated in Scheme 12 (27). 

Reductive cleavage of O—O, O—N, O—S and S—N bonds is also possible. For example, the peroxide bond in the dioxazine (152) is cleaved by treatment with zinc in ethanolic potassium hydroxide, or by catalytic hydrogenolysis (64JOC291). Zinc in acid cleaves the S—N bond in (153) (68LA(715)223). Concomitant reduction of the sulfoxide also occurs, so that subsequent recyclization and aromatization with loss of ammonia leads to the ben-zothiazole (154). The O—S bond in (3) is reductively cleaved by sodium borohydride to give the alcohol (155) (77TL4245). 

Bicyclic nitroso acetals were able to be synthesised by employing ethyl vinyl ether (dienophile), styrene (dipolarophile) and the previously discussed resin-bound ni-troalkenes in a one-pot tandem [4+2]/[3+2]. As illustrated in Scheme 7.30, several aromatic and aliphatic substituents could be introduced to the bicyclic scaffold. Reductive cleavage of the cycloadducts with lithium aluminium hydride (LLAIH4) gave rise to the 3a-methyl alcohol substituted nitroso acetals in moderate overall yields. All these examples demonstrate that resin-bound nitroalkenes can be readily synthesised by microwave synthesis and thereafter can be used as starting materials, in a variety of high pressure-promoted cycloadditions. 

Reduction may also be brought about by sodium and alcohol, although extensive cleavage, of the cyanide group may occur, viz., RCN— RH + NaCN. Lithium aluminum hydride has been successfully employed for the reduction of aliphatic and aromatic nitriles as well as several cyanides in the thiophene series. ... 

Partial reduction of the aromatic ring is especially easy in anthracene-9-carboxylic acid which was reduced to 9,10-dihydroanthracene-9-carboxylic acid with 2.5% sodium amalgam in aqueous sodium carbonate at 10° in 80% yield [987]. Aromatic carboxylic acids with hydroxyl groups in the ortho positions suffer ring cleavage during reductions with sodium in alcohols and are converted to dicarboxylic acids after fission of the intermediate j8-keto acids. 

The method of choice for the prefixation of the aromatic halves is the very practicable esterification reaction. This type of bridge ideally fulfills the requirements demanded easy formation, high thermal stability, good electronic coupling conditions, and, finally, facile cleavage by saponification or reduction. The chiral biaryls are thus obtained as biscarboxylic acids or bisbenzyl alcohols, respectively. 

Solutions of these metals in liquid ammonia effect (i) the reduction of a range of functional groups such as carbonyl and acetylenic and also conjugated and aromatic systems, and (ii) cleavage of benzyl and allyl ethers and thioethers. These reactions are usually carried out by the general procedure of adding the metal to a solution of the substrate in liquid ammonia to which dry methanol or ethanol or t-butanol has been added to provide a ready proton source (alcohols are more acidic than ammonia).34... 

The following reactions proceed with the participation of the allylic boron system (i) allylboration and protolytic cleavage of organic compounds with multiple bonds, (ii) allylboron-alkyne condensation,598 599 (iii) reductive mono-and trans-a,a -diallylation of nitrogen aromatic compounds, (iv) disproportionation processes between tribut-2-enylborane and BX3 (X = C1, Br, OR, SR). Allylboration of carbonyl compounds, thioketones, imines, or nitriles leads to the homoallylic alcohols, thiols, or amines (Equations (136) and (137). It is most important that 1,2-addition to aldehydes and imines proceeds with high diastereoselectivity so that ( )-allylic boranes and boronates give the anti-products, while -products are formed preferentially from (Z)-isomers. 

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