Diols aromatic, oxidation with chromium

In contrast to the usual reaction of aromatic aldehydes with cyclic ketones o-nitrobenzaldehyde condenses with 17-ketones to produce good yields of seco-acids, a reaction which has been applied to the preparation of 16-oxa-steroids. Thus, 3 -hydroxy-5a-androstan-17-one or its acetate affords the seco-steroid (153), which can be oxidised either as the free acid by ozone and alkaline hydrogen peroxide to the diacid (155) or, as its methyl ester (154), with chromium trioxide to the monomethyl ester (156). Diborane reduction of the diacid (155) or lithium aluminium hydride reduction of the dimethyl ester (157) gave the trans-diol (158), cyclised with toluene-p-sulphonic acid to 16-oxa-androstan-3)5-ol (159) or, by oxidation with Jones reagent to the lactone (152) (as 3-ketone) in quantitative yield. This lactone could also be obtained by lithium borohydride reduction of the monomethyl ester (156), whilst diborane reduction of (156) and cyclisation of the resulting (151) afforded the isomeric lactone (150). The diacid (155) reacted with acetic anhydride to afford exclusively the cis-anhydride (161) which was reduced directly with lithium aluminium hydride to the cis-lactone (160) or, as its derived dimethyl ester (162) to the cis-diol (163) which cyclised to 16-oxa-14)5-androstan-3) -ol (164). 

Despite its own valuable synthetic potential, the use of [ C2]acetylene as a starting material for various building blocks is of much higher relevance. Mercury(II)-catalyzed hydration, for example, gives [ C2]acetaldehyde (Figure 8.5, Route 1) The same reaction carried out in the presence of ammonium persulfate furnishes [ 2] acetic acid (Route 2). Trapping of its mono- or dianion with formaldehyde or carbon dioxide affords [2,3- C2]propynol, [2,3- C2]butyne-l,4-diol, [2,3- C2]propiolic acid " and [2,3- C2]acetylenedicarboxylic acid, respectively (Routes 3-6). UV irradiation of a mixture of HBr and [ C2]acetylene produces l,2-dibromo[ C2]ethane (Route 8) . Reduction with chromium(II) chloride followed by a two-step epoxidation of the initially formed [ C2]ethylene converts [ 2]acetylene into [ C2]ethylene oxide (Route 7) . Finally, catalytic homotrimerization or co-trimerization with other alkynes provides [ " C ]benzene or substituted [ " C ]benzenes, respectively, the central starting materials for the vast majority of substituted benzenoid aromatic compounds (Route 9). 

Another approach to designing shape-selective heterogeneous oxidation catalysts was to use redox metal oxides as the pillaring agents in the preparation of pillared clays. These redox pillared clays have been used for a number of selective oxidations. Chromium pillared montmorillonite (Cr-PILC) is an effective catalyst for the selective oxidation of alcohols with tert-butyl hydroperoxide. 7 Primary aliphatic and aromatic alcohols are oxidized to the aldehydes in very good yields. Secondary alcohols are selectively oxidized in the presence of a primary hydroxy group of a diol to give keto alcohols in excellent yields (Eqn. 21.12). 2... 

Cheng has published a convenient and synthetically useful alternative method to the NHK reaction for the arylation of aromatic aldehydes in a mild and selective way with nickel(ll) bromide/zinc/dppe mediated protocol for the synthesis of diaryl carbinols. Durandetti reported an electrochemical coupling of aryl halides with aldehydes for the synthesis of diaryl carbinols which was catalytic in chromium and nickel salts. Comins utilized the NHK reaction to prepare 5-(l-hydroxyalkyl)-2,3-dihydro-4-pyridones, which were then explored in reductive, oxidative and substitutive reactions. " The first asymmetric catalytic synthesis of 5y -alk-l-ene-3,4-diols was developed the regio-, diastereo- and enantioselective addition of 3-chloropropenyl pivaloate to aldehydes was made possible by exploiting Salen r(II) species in a catalytic version of the NHK reaction. ... 

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