O-nitrobenzyl alcohols

The intramolecular hydrogen abstraction has been largely developed for the case of the nitro group [22,37], and is well known for some derivatives of o-nitrotoluene, for o-nitrobenzyl alcohols, ethers and esters [38-40] as well as on the acetals of o-nitrobenzaldehyde [41], With these compounds, the nn (singlet or triplet) state abstracts a hydrogen and the biradical relaxes to the aci-nitro form (the first intermediate is actually observed in bicyclic systems where formation of the nitronic acid is sterically hindered) [41], which under basic conditions dissociates to the corresponding anion [42],... 

The reaction is most useful for the preparation of olefinic, halo, and nitro alcohols from the corresponding substituted aldehydes and ketones. These substituents ate very often affected by other reduction procedures. Excellent directions are found in the preparations of crotyl alcohol (60%), l-bromo-5-hexanol (64%), l-chloco-4-pentanol (76%), /S,/S,/S-trichloroethyl alcohol (84%), methyl-p-chlorophenylcarbinol (81%), and o-nitrobenzyl alcohol (90%). The reaction has also been used in the preparation of certain tetralols and decalols as well as 9-fluo-renylcarbinol (50%). The thiophene and furan nuclei are not reduced. 

Bis(o-nitrophenyl)ethanediol (50) has been proposed as a practical photolabile protecting group for ketones and aldehydes which is superior to the monosub-stituted o-nitrophenylethanediol. The presence of a single stereocentre in the latter leads to the formation of two diastereomers when it is used with another chiral molecule, thus complicating NMR signal patterns, and often making purification difficult. In addition, the obvious alternative of ketal formation from two molecules of o-nitrobenzyl alcohol instead of a diol is usually impractical. On the other hand (50) is easily accessible as a pure enantiomer, and the ketals which it forms with aldehydes and ketones are smoothly deprotected in neutral conditions by irradiation at 350 nm. 

Anthranil is a minor product from the electrolytic reduction of the erythro or threo forms of the o-nitrobenzyl alcohol 114 (R2 = Me, R2 = N02).145 The fluorosulfonic acid-induced cyclization of (o-nitrophenyl)succinic anhydride affords a mixture of 4-hydroxy-2-quinolone and anthranil-3-acetic acid (115 R = CH2C02H), and not, as previously reported, a mixture of 1-hydroxyindole- and 2-hydroxy-A2-indoline-3-carboxylic acids.146... 

The structure of Bamberger s agnotobenzaldehyde, an intermediate in the acid-catalyzed rearrangement of o-nitrobenzyl alcohol to o-nitro-sobenzaldehyde, has been formulated as the 2,l-benzisoxazolin-3-ol [170 R1 = 0-NO2C6H4CH(OH), R2 = H, R3 = OH], Acetic anhydride converts it into the Af-acetyl derivative (170 R1 = Ac, R2 = H, R3 = OH) which with calcium hypochlorite is oxidized to o-nitrosobenzaldehyde.247... 

Reductive acylation and transesterification. Carbon monoxide acts as both reductant and acylating agent for o-substituted nitrobenzenes, delivering dihydroben-zoxazin-2-ones from o-nitrobenzyl alcohols. Pd(OAc)2 catalyzes formation of the vinyl esters of hydroxycarboxylic acids by reaction with vinyl acetate without acetylating the hydroxyl group. 

The undermentioned reductions may be carried out by simple adaptations to the procedures chloral to trichloroethjd alcohol i-nitroacetophenone of a-methyl-3-nitrobenzyl alcohol and o-nitrobenzaldehyde to o-nitrobenzyl alcohol. 

Aldehydes and ketones are reduced to primary and secondary alcohols respectively, often in high yield. The reaction owes its usefulness to the fact that carbon-carbon double bonds and many other unsaturated groups are unaffected, thus allowing selective reduction of carbonyl groups. For example, cinnamaldehyde is converted into cinnamyl alcohol, o-nitrobenzaldehyde gives o-nitrobenzyl alcohol and phenacyl bromide gives the alcohol 76 (7.57). 

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